Mitochondrial homeostasis in bone aging: Unraveling dysfunctional pathways and developing precision therapies

Abstract

Mitochondria have complex functional and information-processing networks that play key roles in both health regulation and disease progression. However, the multiple properties and complex thresholds of mitochondrial dysfunction and quality control make the contribution of mitochondria to bone aging elusive. These factors prevent mitochondria from being among the most important precision therapies. Currently, many strategies that target mitochondrial homeostasis have entered clinical trials. In mitochondria, mitochondrial DNA (mtDNA) and its associated proteins are potential therapeutic agents for immunometabolic diseases and tissue injury, with the aim of enhancing mitochondrial function. Here, we comprehensively review the intrinsic mechanisms of mitochondrial dysfunction and quality control leading to bone aging and summarize current strategies for the treatment of skeletal aging disorders and the clinical translation of relevant agents in terms of unraveling dysfunctional pathways and developing precision therapies. In this review, we offer a general overview of the progress of clinical application in the treatment of skeletal senescence diseases, and we also provide prospects for the challenges associated with the role of mitochondrial dysfunction in bone senescence in clinical application and future trends in this field.

1. Introduction

Bone plays a vital role in accomplishing bodily movements as a scaffolding that supports the entire body. Skeletal homeostasis is an important regulatory mechanism that ensures the physiological functions of bone. Understanding the mechanisms involved in the regulation of skeletal homeostasis plays an essential role in maintaining skeletal stability and is critical to the body’s ability to accomplish a variety of sports [1]. Bone aging is a complex process that involves various metabolic decreases that ultimately lead to bone aging-related diseases. The main cellular components in bone tissue include osteoblasts, osteocytes, osteoclasts, and mesenchymal stem cells. The normal maintenance of cellular metabolism is highly important for the regulation of skeletal homeostasis [2].

Mitochondria, recognized as cellular “powerhouses”, are often thought to be organelles in which eukaryotic cells engulf α proteobacteria and change through endosymbiotic evolution [3]. These organelles have the same double-layer membrane structure as their bacterial ancestors and can regulate cell signal transduction, metabolism, and cellular senescence and death. Aberrant kinetics and quality control, dysfunctional protein homeostasis, inhibition of Adenosine triphosphate (ATP) production, dysregulation of calcium homeostasis, and metabolic reprogramming frequently occur concomitantly and interact with each other under pathological conditions, thereby affecting the ability of mitochondria to serve as hubs for biosynthesis and signal transduction [4,5]. Therefore, mitochondrial dysfunction acts as a central driver of the bone aging process by impairing energy production, amplifying oxidative stress, and disrupting the metabolic homeostasis of bone cells [6]. These findings also indicate that understanding mitochondrial activity during bone aging and its potential regulatory mechanisms is crucial for the prevention and treatment of bone aging-related diseases in the future.

Numerous studies have shown that alterations in mitochondrial function are key aspects of aging. The basic energy conversion process in the cell is facilitated by mitochondria. Senescence causes a decrease in oxidative phosphorylation efficiency, which alters ATP production and increases Reactive Oxygen Species (ROS) levels [7]. The accumulation of ROS leads to the oxidation of mitochondrial DNA (mtDNA), proteins, and lipids, which further decreases mitochondrial viability, hinders mitophagy, and eventually leads to mitochondrial dysfunction [7]. Bone aging is related mainly to mitochondrial dysfunction and mitochondrial quality control. Impaired function is inextricably linked to ROS overaccumulation, which in turn leads to mitochondrial oxidative stress. In addition, signaling pathways such as inflammasomes triggered by mtDNA can also lead to mitochondrial dysfunction. The strong energy supply capacity of mitochondria allows them to play an indispensable role in cellular metabolism. Other mitochondrial properties, including mitophagy, mitochondrial dynamics, and mitochondrial protein homeostasis, are also important for regulation in bone. Studies have shown that the regulation of mitophagy by the PTEN-induced kinase 1 (PINK1)/Parkin pathway, the Adenosine 5′-monophosphate (AMP)-activated protein kinase (AMPK) signaling pathway and the Silent information regulator (SIRT) signaling pathway plays important roles in bone diseases such as osteoarthritis and degenerative disc disease [8]. Nevertheless, the study of the mitochondria-related mechanisms involved in osteopathy is in its early stages. A better understanding of the concrete mechanisms by which mitochondria regulate bone aging-related diseases will contribute to the development of more efficacious therapeutic strategies.

In this review, we comprehensively summarize and elaborate the mechanism leading to bone aging from the perspectives of mitochondrial dysfunction and mitochondrial quality control disorders. We discuss interventions involving mitochondria to target bone aging-related diseases. In addition, these studies may reveal the link between mitochondrial dysfunction and osteoporosis and provide new ideas for osteoporosis treatment. Ultimately, these efforts are expected to revolutionize the prevention and treatment of bone diseases, improve the quality of life of elderly individuals, and potentially combat aging-related degenerative diseases (Fig. 1).

Fig. 1.

The mechanisms related to mitochondria include mitophagy, oxidative stress, proteostasis and mitochondrial dynamics. Abnormalities in these signaling pathways can lead to bone-related diseases. We then reviewed some therapeutic directions for bone aging diseases based on the alterations in mitochondrial structures and functions.

2. Overview of mitochondrial functions and behaviors

As an evolutionarily highly conserved organelle, mitochondria extend their function far beyond ATP production; they are also involved in the biosynthesis of nucleotides, lipids, and iron-sulfur clusters, and they regulate calcium signaling, redox homeostasis, as well as influence the innate immune response and programmed cell death [9]. Their functional versatility is underpinned by their morphological behaviors, constantly undergoing fusion and fission processes collectively referred to as mitochondrial dynamics. Fusion allows mitochondria to elongate and share contents such as DNA and proteins, promoting functional integrity, whereas fission facilitates the segregation of damaged components for mitophagy [10]. These behaviors are not only essential for maintaining cellular health but also responsive to metabolic demands, stress conditions, and developmental cues. Dysregulation of mitochondrial dynamics has been implicated in various diseases, including neurodegenerative disorders, cancer, and osteoporosis, highlighting their central role in cellular physiology.

In terms of cellular architecture, the mitochondrial network is influenced by quality control systems (biogenesis, dynamics, mitophagy, and proteolysis) [10]. The induction of mitochondrial biogenesis is associated with the activation of transcription factors that act on mitochondrial genes and with the upregulation of the local translation of mitochondrial proteins [11]. The upregulation of proliferator-activated receptor-γ coactivator 1-α (PGC-1α) is thought to be a major regulator of mitochondrial biogenesis. PGC-1α initiates a transcriptional cascade by activating PGC-1α, which stimulates nuclear respiratory factor-1 (NRF-1), nuclear respiratory factor-1 (NRF-2), and estrogen-related receptor-α (ERR-α), ultimately expressing mitochondrial transcription factor A (TFAM), the final effector of mtDNA transcription and replication, thereby promoting mtDNA transcription and replication [6,11]. Notably, nearly 99% of the proteins in mitochondria are regulated by nuclear proteins, which enter the mitochondrial compartment via different screening and entry mechanisms, which, when discussed in greater depth, are monitored by cytoplasmic nucleosomes, the ubiquitin-proteasome system, chaperone proteins, mitochondrial proteases, etc. [6]. Mitochondria contains approximately 1500 different proteins in humans, of which over 99% are encoded by nuclear DNA and must be imported post-translationally. This import process is tightly regulated and involves multiple quality control systems, including cytosolic chaperones, the ubiquitin–proteasome system (UPS), mitochondrial-derived vesicles, and matrix-localized proteases, which collectively ensure proper protein homeostasis [12].

In addition, the processes of mitochondrial fission and fusion are closely related to the shape and distribution of mitochondria, and we found that both processes are regulated by highly conserved proteins and organelles, thus ensuring the maximization of mitochondrial functions [6]. Mitochondrial morphology is dynamically regulated by the opposing processes of fission and fusion, which are essential for maintaining organelle integrity and function. These processes are orchestrated by evolutionarily conserved proteins, such as DRP1, MFN1/2, and OPA1, and are further modulated by interactions with other cellular structures, including the endoplasmic reticulum and lysosomes. Another important function of mitochondria is mitophagy, a special form of autophagy that works to selectively remove aging or damaged mitochondria. Many signaling pathways and proteins are tightly regulated in this process, with the PINK1-Parkin pathway playing a prominent major role. When external damage to mitochondria occurs, PINK1 accumulates on the outer mitochondrial membrane, and at this time, the E3 ubiquitin ligase Parkin is also recruited to the outer mitochondrial membrane. This results in the ubiquitination of mitochondrial proteins and the subsequent binding of phagosomes to lysosomes, which promotes the process of systematic degradation and recirculation of the mitochondrial contents and facilitates the removal of damaged components. Within mitochondria, the collapsed inner mitochondrial membrane (IMM) delivers a vast surface area for protein input and efficient oxidative phosphorylation. The mitochondrial oxidative phosphorylation system is central to cellular metabolism [13]. In contrast, a series of ATP synthases and protein complexes (I through IV) together form the respiratory chain embedded in the IMM, which becomes a central component of mitochondrial energy production [6]. In contrast, the mitochondrial respiratory chain, composed of four multi-subunit complexes (I–IV) and ATP synthase (complex V), is embedded within the inner mitochondrial membrane and plays a central role in oxidative phosphorylation (OXPHOS), driving ATP production through electron transport. Recent studies have shown that a fifth mechanism exists: the release of TCA (tricarboxylic acid) cycle metabolites by mitochondria to control cellular function. During this process, pyruvate, fatty acids and glutamine, the main metabolic substrates, are metabolically degraded to acetyl-CoA and α-ketoglutarate (α-kG), respectively [14]. They enter the TCA cycle, which further drives electron transport and ATP production.

3. Mitochondrial dysfunction in bone aging

Bone aging is a chronic process of decline, and multiple biological disorders can interfere with this process [7]. Mitochondrial dysfunction can be used as a classical marker of bone aging. Mitochondrial dysfunction has been found to be the most closely associated marker of aging. During this aging process, a number of factors contribute to the decline in mitochondrial function, such as elevated ROS, mutations in mtDNA, oxidation of proteins, aberrant energy metabolism in vivo, and decreased mitochondrial production.

From one perspective, mitochondrial dysfunction leads to elevated levels of ROS, which can result in damage to the mitochondrial electron transport chain, further exacerbating mitochondrial energy metabolism disorders and oxidative damage to mtDNA. And when mtDNA is damaged and released into the cytoplasm, it is in turn recognized by cGAS (cyclic GMP-AMP synthase) and activates the STING (interferon gene-stimulating factor) signaling pathway, which induces the expression of type I interferons and inflammatory factors involved in the innate immune response [15]. Thus, while ROS, cGAS-STING, and inflammation can each initiate mitochondrial dysfunction independently, they also operate within a tightly regulated network. Understanding this interplay is crucial for dissecting the molecular basis of mitochondrial-associated diseases, especially in metabolically active tissues such as bone.

3.1. Mitochondrial oxidative stress

During the process of cellular senescence, mitochondria undergoes great changes in morphology and function, and defective mitochondria produces large amounts of ROS, which triggers oxidative stress in the mitochondria. ROS also attack other organelles, which eventually leads to the occurrence of related diseases. ROS are byproducts of cellular metabolism, produced mainly by the oxidative phosphorylation of mitochondria, and have a variety of key functions in cell signaling pathways. Notably, reactive oxygen species in osteoblasts can also originate from non-mitochondrial pathways, such as NADPH oxidase. In osteoblasts, NOX2 is one of the major isozymes that is activated through redox-dependent NF-κB and MAPK pathways and is essential for RANKL-mediated osteoclast formation [16]. Meanwhile, NOX4, a novel target for regulating osteoblast function and antioxidant defense, has been shown to exacerbates oxidative stressand impairs bone formation when absent under metabolic stress conditions. Cells typically have a defense mechanism that eliminates ROS through the antioxidant system and keeps them within safe concentrations. These biodefense systems can be divided into enzyme-based systems, such as superoxide dismutase (SOD), glutathione (GSH), and peroxide rexin, and enzyme-based nonenzyme systems, including multivitamins, polyphenols, and coenzyme Q10. Therefore, once the rate of ROS production and clearance is disproportionate, it can lead to oxidative stress, which in turn can lead to mitochondrial dysfunction.

Senescence can be defined as a time-dependent decline in function that is closely related to cellular damage in critical organs. Numerous studies have shown that the function of mitochondria affects the phenotype of bones, which also indicates that there is always a dynamic equilibrium relationship between mitochondria and bones. Numerous studies have shown that the most obvious manifestation of dysfunction is the overproduction and accumulation of reactive oxygen species. Limited amounts of ROS can activate mitophagy to prevent or reduce mitochondrial damage; however, when excessive amounts of ROS are present, dysfunctional mitochondria accumulate, and ROS production further increases [17,18]. As a result, mitochondrial dysfunction due to abnormal ROS content can disrupt this balance, which may be the cause of bone aging and related diseases.

Mitochondria are closely associated with a wide range of processes associated with aging, including aging, inflammation and, more generally, the age-dependent decline in tissue and organ function. It can control the aging of cells by regulating metabolism. The production of ATP in mitochondria involves a series of reactions called OXPHOS, which includes four ETC complexes and ATP synthases that are involved in the reaction [19]. When mitochondria rely less on oxygen catabolism and more on glycolysis, they signal the onset of aging. For example, p53 plays an essential role in aging. Studies have shown that it can inhibit the expression of mitochondrial malate (ME2), which in turn reacts to p53, and the two regulate each other to regulate metabolism and aging.

In addition, mitochondria regulate cell aging and death through oxidative stress, and ROS play a major role in this process. Studies have shown that the mitochondria-associated proteins SOD2 and Sirt3 play key roles in the regulation of ROS during osteoclast differentiation. NF-κB ligand (RANKL) induces the expression of SOD2 and Sirt3 to meet the need to control oxidative stress due to increased mitochondrial biogenesis during osteoclastogenesis [20]. However, the research team has not further investigated its role in the pathogenesis of osteoporosis and other bone-destructive diseases.

Furthermore, Kim et al. utilized the deficiency of the vital CI subunit Ndufs4 to serve as a model of mitochondrial dysfunction, revealing that mitochondrial CI is a key regulator of both innate immune and bone homeostasis [21]. Therefore, it is not difficult to speculate that the removal of mitochondrial oxidants delays bone aging. Aging also causes a decrease in mitochondrial membrane potentiation. The reason for this decrease has been explained differently by different teams, but the latest evidence suggests that the lower ∆Ψm observed in senescent cells is due to enhanced activation of the mPTP, which increases the proportion of depolarized mitochondria [22]. The correctness of this conclusion has yet to be proven.

The occurrence of oxidative stress is related to the accumulation of reactive oxygen species in cells. The accumulation of ROS in osteoblasts can lead to the breakage, degeneration and inactivation of intracellular lipids and proteins, which in turn causes irreversible oxidative stress damage and leads to the aggravation of osteoporosis. Environmental toxins represent a significant source of such oxidative stress. For instance, Cyclohexyl phthalate (BCP), a common contaminant, has been found to inhibit mitochondrial complexes and reduce the oxygen consumption rate of exposed osteoblasts and can also induce oxidative damage by increasing oxidative markers and inhibiting antioxidant enzymes, thereby causing oxidative stress in human osteoblasts and significantly inhibiting osteoblast secretion activity [23]. Therefore, antioxidant supplementation and mitochondrial support therapy may be useful adjuncts to counteract the effects on skeletal integrity. Because the accumulation of ROS and very few raw osteogenic materials in the osteoporotic bone microenvironment can greatly inhibit the activity of osteoblasts, Wu et al. constructed a biomatrix with a multifaceted bone microenvironment modifier, mineralized zipper G4-heme DNase hydrogel (MDH), to improve osteoporotic osteogenesis and promote high-quality bone defect repair (Fig. 2). Another study revealed that the trait hypoxia-inducible factor-1α (HIF-1α), a transcription factor, mediates adaptive responses to oxidative stress through its nuclear translocation and gene expression [24].

Fig. 2.

Oxidative stress and bone aging mechanism diagram. This diagram depicts the major sources of reactive oxygen species (ROS), including mitochondrial oxidative phosphorylation (OXPHOS) and NADPH oxidase (NOX2/NOX4) systems. The balance between ROS production and antioxidant defense systems (SOD, GSH, peroxiredoxin, vitamins) is shown, with disruption leading to oxidative stress. Key consequences include mitochondrial dysfunction (decreased membrane potential ΔΨm, mPTP activation, impaired mitophagy), activation of signaling pathways (NF-κB, MAPK, SIRT3/SOD2, HIF-1α), and subsequent bone cell dysfunction affecting osteoblasts (reduced activity and differentiation), osteoclasts (enhanced activity), and bone marrow stromal cells (BMSCs).

Moreover, excessive oxidative stress in osteoporosis can lead to abnormal bone remodeling. Zhou et al. conducted experiments on resected ovarian rats and reported that melatonin ameliorated mitochondrial oxidative stress through the SIRT3/SOD2 signaling pathway, thereby promoting osteogenesis, improving bone mass around the prosthesis, and increasing initial stability [25]. Therefore, melatonin may be a suitable drug to reduce the rate of implant failure and prolong the life of the prosthesis after total joint replacement. Interestingly, MnTBAP, a novel superoxide dismutase mimic, can protect osteoblasts from the cytotoxicity and dysfunction caused by oxidative stress. Cao et al. experimentally demonstrated that MnTBAP can inhibit the development of postmenopausal osteoporosis by reducing mitochondrial oxidative stress in osteoblasts and may be a potential drug for the treatment of postmenopausal osteoporosis [26]. Zhao et al. reported that motherwort protects BMSC proliferation and differentiation from oxidative stress by activating mitochondrial autophagy, which relies on the PI3K/Akt/mTOR pathway, suggesting that motherwort may have potential uses in osteoporosis and bone defect repair [27].

In summary, mitochondrial oxidative stress acts as a primary instigator in bone aging. The age-related accumulation of ROS directly damages osteoblasts, leading to their dysfunction and apoptosis, while simultaneously promoting osteoclast activity. This dual assault disrupts the delicate balance of bone remodeling. In BMSCs, oxidative stress skews differentiation away from osteogenesis and toward adipogenesis, contributing to marrow fat accumulation. Collectively, these cellular dysfunctions, driven by persistent oxidative stress, manifest as the hallmark features of aged bone: low bone mass, deteriorated microarchitecture, and increased fragility.

3.2. mtDNA-related pathways

Aging is a major risk factor for the development of skeletal disorders. Accumulating evidence indicates that age-related mitochondrial DNA (mtDNA) mutations contribute to the decline in bone homeostasis by compromising mitochondrial function in bone cells, especially osteoblasts and osteocytes. These mutations impair ATP production and elevate reactive oxygen species (ROS) levels, promoting oxidative stress—a key driver of osteoblast dysfunction, increased osteoclast activity, and ultimately, bone loss. Excessive accumulation of ROS attacks proteins and nucleotides, and mitochondria are more susceptible to damage because mitochondrial DNA is more susceptible to mutation than is nuclear DNA [28]. Damaged mitochondria produce more than 10 times more ROS than healthy mitochondria do, and large amounts of ROS can damage mitochondria and aggravate mitochondrial dysfunction; thus, timely removal of damaged mitochondria plays an important role in maintaining cellular homeostasis.

In addition, mtDNA can also contribute to the occurrence of inflammation. Bone aging can also occur through the development of chronic inflammation, such as osteoarthritis. Typically, immune and nonimmune cells in the body express pattern recognition receptors (PRRs), which are then activated to trigger inflammation [29]. Importantly, viral and bacterial molecules associated with infection and endogenous molecules known as damage-associated molecular patterns (DAMPs) activate PRRs [29]. Many mitochondrial components and metabolites can function as damage-associated molecular patterns (DAMPs) and promote inflammation when released into the cytosol or the extracellular environment.

According to existing studies, mitochondrial damage-associated molecular pattern (mtDAMP) signaling pathways include the following: intracellular signaling via cyclic GMP-AMP synthase and interferon response stimulator 1 (STING1); activation by mtDNA; and induction of inflammasomes.

3.2.1. cGAS‒STING pathway

Cyclic GMP-AMP synthase (cGAS) is a cytosolic DNA sensor that plays an important role in skeletal biology. Recent studies have shown that in aging or damaged bone tissues, mtDNA or nuclear DNA fragments can be released into the cytoplasm, leading to activation of the cGAS-STING signaling pathway [30]. cGAS uses ATP and GTP as substrates and then forms cyclic GMP-AMP (cGAMP), which is part of the nucleotidyl transferase family. cGAMP is also known as a second messenger. In contrast, a transmembrane protein called cGAMP Interacting Factor 1 (STING1), which is located primarily in the endoplasmic reticulum and is a stimulator of the interferon response, is expressed in a variety of endothelial and epithelial cells, as well as in T cells, B cells, and myeloid cells [31].

UXT is a small chaperone-like protein that is essential for preventing STING1-mediated overactivation of type I IFN signaling through the autophagic degradation of STING1 by SQSTM1 (chelate 1). When stimulated by DNA mimics or circular cGAMP, UXT specifically interacts with STING1 and promotes STING1 degradation through selective macroautophagy/autophagy.

During infection with several RNA viruses, STING1 interacts with mitochondrial antiviral signaling proteins (MAVS) and becomes an immunoconjugate protein located on the mitochondrial outer membrane [31]. Recruited to the STING1-MAVS complex on the mitochondria-associated membrane (MAM) is a viral RNA sensor called DExD/H-box helicase 58 (DDX58), which is followed by the IRF3 transcription factor and TBK1, which connects to MAVS to deliver DDX58-MAVS-mediated signaling [32]. Upon conjugation with mitochondrial fusion regulators (i.e., mitomycin, including mitomycin 1 [MFN1] and mitomycin 2 [MFN2]), mitochondrial STING1 leads to an increase in mitochondrial fusion-mediated ROS formation and lipid hydroperoxidation, which promotes iron toxicity susceptibility [31]. However, the research team did not clearly explain the relationships among mitochondrial DNA damage, cell death, and the STING1 pathway in controlling bacteriophage inflammation and tolerance to tissue damage (Fig. 3).

Fig. 3.

Pathway of action of the cGAS-STING pathway, which drives the inflammatory response in mitochondria. Initiate an immune response by recognizing cell membrane DNA from bacteria, viruses, or the cell itself. Upon recognition of cell membrane DNA, cGAS catalyzes the formation of cGAMP, which activates STING. The activation of STING leads to the activation of TBK1, which in turn phosphorylates IRF3. Phosphorylated IRF3 promotes IFN expression in the nucleus. STING also activates NF-κB by phosphorylating the kinase IKK, which in turn promotes the transcription of proinflammatory cytokine genes.

As an inflammatory activator, mtDNA is oxidized by ROS, released into the cytoplasm, and can activate the cGAS-STING pathway, resulting in a large upregulation of interferon genes, thereby conferring an antiviral status on the cell. In contrast, during programmed cell death, the apoptotic proteins BAX and BAK permeabilize the outer mitochondrial membrane, allowing proapoptotic molecules to enter the cytosol from the inner membrane space, thereby initiating the caspase cascade and resulting in rapid cell death [33]. However, in the presence of caspase inhibition, BAX and BAK can permeabilize the outer mitochondrial membrane, allowing mtDNA to be released, thereby activating cGAS. In 2015, West et al. published a pioneering study in Nature that systematically clarified that mtDNA leakage can act as an endogenous damage-associated molecular pattern (DAMP), directly activating the cGAS-STING signaling axis. This study found that when mitochondrial homeostasis is impaired (e.g., TFAM deletion leading to mtDNA instability) or the nuclease TREX1 is functionally defective, mtDNA escapes to the cytoplasm, where it is recognized by the cytoplasmic DNA sensor cGAS, which catalyzes the synthesis of cGAMP, which then activates a STING-dependent type I interferon response. This finding not only reveals the molecular bridge between mitochondrial stress and innate immunity, but also provides a new explanation for the pathogenesis of autoimmune diseases such as systemic lupus erythematosus [34]. And the cGAS/STING signaling pathway has been shown to play a key role in regulating the invasion of rheumatoid synovial tissue.

From the above, it is clear that mtDNA leakage is a key initiating event for the activation of the cGAS-STING pathway; however, this process is highly dependent on the regulation of mitochondrial membrane permeability. In recent years, several important studies published in Science have systematically revealed that mPTP (mitochondrial permeability transition pore), BAX/BAK-mediated rupture of the outer membrane, and oligomerization of VDAC (voltage-dependent anion channel) collectively constitute the core molecular mechanisms of mtDNA leakage under different forms of mitochondrial stress. Rongvaux et al. demonstrated in Science in 2014 that activation of the pro-apoptotic proteins BAX and BAK induces mitochondrial outer membrane permeabilization (MOMP), which not only mediates apoptosis, but also leads to the fragmentation and release of mtDNA into the cytoplasm, which triggers a strong type I interferon response [35]. In addition, VDACs, especially VDAC1 and VDAC3, have been demonstrated to be involved in the mtDNA release process. Giorgi et al. found that under conditions of oxidative stress or calcium overload, VDACs oligomerize and synergize with BAX/BAK to promote increased mitochondrial outer membrane permeability and the formation of a pore structure that permits the escape of mtDNA. It has been reported that in a myocardial ischemia/reperfusion injury model, oxidative stress induces a sustained opening of mPTP, leading to mitochondrial matrix swelling, rupture of the inner membrane, and concomitant release of mtDNA into the cytoplasm through the ruptured inner and outer membranes, which then activates a cGAS-STING-dependent inflammatory response. This finding establishes the role of mPTP as a key channel for mtDNA release. These pathways not only play traditional roles in cell death, but also provide a new mechanistic basis for understanding inflammatory diseases, aging and autoimmune diseases.

Overall, The cGAS-STING-driven senescence-associated secretory phenotype (SASP) in bone marrow mesenchymal stem cells (BMSCs) biases their differentiation away from osteoblasts and towards adipocytes, a hallmark of an aged bone marrow niche, while also creating a pro-inflammatory milieu that further disrupts osteoblast-osteoclast cross-talk (Fig. 3).

With advancing age, mitochondrial function in bone marrow mesenchymal stem cells (BMSCs) and osteoblasts decreases and ROS levels increase, leading to increased mtDNA damage and release. These cytoplasmic mtDNAs activate the cGAS-STING signaling axis, which in turn induces the production of type I interferon and various pro-inflammatory factors (e.g., IL-6 and TNF-α), creating a persistent state of low-grade inflammation that can disrupt bone homeostasis. This low-grade inflammatory state inhibits osteogenic differentiation, promotes adipogenic differentiation, and ultimately leads to the development of bone loss and osteoporosis [36]. Collectively, the persistent low-grade inflammation driven by the cGAS-STING axis, as detailed above, represents a critical consequence of failed mitochondrial quality control.

In addition, sustained activation of STING1 induces iron death and exacerbates mitochondrial damage and oxidative stress in bone progenitor cells, which further amplifies the release of mtDNA, which activates the cGAS-STING signaling axis and never creates a cycle of low-grade inflammatory states [37]. The cGAS-STING pathway has been established as a promising therapeutic target for bone aging diseases, and targeting the cGAS-STING signaling axis may be a new strategy for delaying bone aging and treating osteoporosis.

3.2.2. Inflammasomes

In addition to the cGAS-STING pathway, mtDNA can also drive inflammasome activation. Inflammasomes are often thought of as intracellular protein complexes used in response to infection, injury, etc. [38]. A review of the literature revealed that most studies of inflammasomes have focused on the NLR family 3 (NLRP3) of nucleotide-binding oligomeric domain-like receptors (NLRs) [38]. Its activation leads to caspase-1-dependent secretion of proinflammatory cytokines such as interleukin-1β (IL-1β) and IL-18, as well as a form of inflammatory cell death called pyroptosis. The activators of NLRP3 are diverse and include reactive oxygen species (ROS), potassium efflux, changes in cell volume, calcium signaling, and lysosomal disruption, which are considered key upstream signals required for NLRP3 activation.

A research team reported that, compared with healthy cartilage and chondrocytes, OA cartilage biopsies and chondrocytes presented increased inflammasome component protein expression. Researchers have shown that ROS increase the expression levels of Caspase-1 and IL-1β/IL-18 in osteoblasts, synovial cells, and other cells of OA mice, suggesting that leptin-induced NLRP3 inflammasome formation/activation in OA chondrocytes is mediated by nicotinamide adenine dinucleotide phosphate (NOX4)-dependent ROS production [39]. The results of this study also shed light on the pathway by which ROS activate inflammasomes. Moreover, we know that inflammasomes are cytoplasmic multiprotein complexes of NLR and caspase-1 that process and activate IL-1β and IL-18 upon assembly. Studies have demonstrated that the activation of NALP3 can be blocked by inhibiting cellular potassium efflux, and low intracellular potassium concentrations are also necessary for the activation of the NALP1 inflammasome by the lethal toxin Bacillus anthracis. In vitro, NALP inflammasome assembly and caspase-1 recruitment occur spontaneously at potassium concentrations below 90 mM but are prevented at higher concentrations. Therefore, a low intracellular potassium concentration may be one of the triggers for NALP3 inflammasome activation (Fig. 4).

Fig. 4.

Two-step activation of the NLRP3 inflammasome: NF-κB transcription and mtDNA/ROS regulation. Activation of this inflammasome requires two steps: initiation and activation. The first is to activate the transcription of the NF-κB gene through multiple pattern recognition receptors (PRRs) to produce NLRP3, IL-1β, and IL-1α. During this second step of activation, both mitochondrial DNA (mtDNA) and reactive oxygen species (ROS) act on NLRP3, which is also regulated by Ca²⁺ and K⁺Cl^-. When NLRP3 is activated, it produces caspase-1, which in turn produces proinflammatory cytokines.

4. Imbalanced mitochondrial quality control during bone aging

Mitochondrial quality control (MQC) is essential for maintaining mitochondrial integrity and cellular homeostasis. When MQC mechanisms—such as mitophagy, mitochondrial fission/fusion dynamics, and proteolytic pathways—become impaired, defective mitochondria accumulate, leading to increased oxidative stress, bioenergetic failure, and the activation of pro-inflammatory signaling cascades, such as the cGAS-STING pathway described in the previous section. This mitochondrial dysfunction is a hallmark of cellular senescence and contributes to the degeneration of post-mitotic tissues, including bone, ultimately promoting age-related pathologies [40].

From the perspective of mitochondrial autophagy, the reduced scavenging capacity of damaged mitochondria,often achieved through the PINK1-Parkin pathway or receptor-mediated mechanisms, is widely recognized as a major factor in reactive oxygen species (ROS) accumulation, mitochondrial DNA (mtDNA) release and the development of chronic inflammation. This is particularly critical in the pathogenesis of aging and degenerative diseases. In addition, some researchers have suggested that the balance between mitochondrial fusion and fission plays a more upstream regulatory role in mitochondrial quality control. Kandul et al. showed that Drp1-mediated excessive division impairs mitochondrial energy metabolism and makes cells more sensitive to apoptotic signals, and this phenomenon is particularly prominent in neurodegenerative and metabolic diseases [41]. As for disturbances in mitochondrial proteostasis, these mainly include dysfunction of the mitochondrial unfolded protein response (UPR^mt) as well as matrix proteases such as LONP1 and CLPP. When we seek to understand these mechanisms, it is important to note that they constitute a highly integrated network, and we need to pay special attention to their interactions with each other, which will help to reveal the molecular basis of mitochondrial dysfunction, as well as provide a theoretical basis for future targeted intervention strategies in bone aging diseases.

4.1. Mitochondrial autophagy

Recently, the regulation of mitophagy, as a new research hotspot, has been shown to play a positive role in the treatment of a variety of diseases, which also shows that bone aging is closely related to mitophagy [42]. Mitophagy is the response of mitochondria to various external stresses. Therefore, abnormalities in mitophagy play a key role in bone disease and cannot be ignored. The aim of this review was to summarize the potential mechanisms involved in mitophagy and bone aging.

Mitophagy can generally be classified into two major types: the ubiquitin protein ligase(PRKN)-dependent pathway and the ubiquitin protein ligase(PRKN)-independent pathway, both of which play essential roles in mitochondrial quality control, among which PRKN-dependent mitophagy represents the most extensively investigated pathway. Together, PTEN-induced putative kinase 1 (PINK1) and the E3-ubiquitin ligase PRKN mediate PRKN-dependent mitophagy. Under physiological conditions, erythropoietin associated with premature aging proteins cleaves PINK1 transferred to the inner mitochondrial membrane, which in turn leads to the degradation of N-terminally truncated PINK1. Nevertheless, the loss of mitochondrial translocation of the transmembrane potential leads to an as yet unspecified accumulation of PINK1 by disrupting the translocation of PINK1. In addition, the PRKN-independent mitotic pathway is largely dependent on revenue proteins, including BNIP3, NIX, and FUNDC1, which cooperate directly with LC3 and GABARAP through the LIR motif to eliminate mitochondria. Disruption of either of these two autophagy pathways can lead to the accumulation of damaged mitochondria, which in turn can lead to a number of undesirable consequences.

Autophagy, a membrane-dependent mechanism of subcellular component turnover, is a highly conserved process of catabolism in eukaryotic cells. One of the main functions of autophagy is to remove unwanted macromolecules or pathogens from the body, such as proteins that undergo structural misfolding, damaged mitochondria, and peroxisomes, and to provide energy and nutrients to the cell through the lysosomal mechanism [43]. Appropriate levels of autophagy are beneficial because they protect cells against pathological or physiological injuries, but excessive levels of autophagy can trigger apoptosis and cause irreversible damage.

Typically, autophagy is considered to be nonspecific. Because of this, we name the selective autophagy process that occurs in mitochondria, called mitochondrial phagocytosis. This is one of the important processes involved in completing mitochondrial quality control. To maintain mitochondrial homeostasis, mitochondrial autophagy, a specialized type of autophagy, removes damaged organelles and excess proteins and reduces stress levels caused by noxious stimuli. A growing body of research suggests that mitochondrial autophagy plays a key role in bone-related diseases. When the level of mitochondrial autophagy is abnormal, the metabolic homeostasis of bone is also disturbed [44] (Fig. 5).

Fig. 5.

PINK1/Parkin-mediated and receptor-mediated mitophagy pathways and their role in aging and cellular senescence. During mitophagy and aging in healthy mitochondria, PINK1 can be constitutively abolished upon entry into the proteasome endosome. When mitochondria undergo oxidative stress, PINK1 import is inhibited, and PINK1 accumulates on the outer membrane. As soon as it docks on the external membrane, PINK1 recruits and activates Parkin by phosphorylating S65 via ubiquitin and the ubiquitin-like structural domain of Parkin. Mitochondrial substrates on the OMM are then ubiquitinated via the Parkin pathway, thereby recruiting LC3 bound to the phagosome via autophagy adapters (e.g., p62, OPTN, and NDP52). The phagosome swells and engulfs the mitochondria, forming an autophagosome that is disassembled by fusion with the lysosome. Receptor-mediated mitophagy: The mitogenic receptors NIX, FUNDC1 and BNIP3 are vigorously activated during the course of reticulocyte maturation. This leads to elevated levels of receptors on OMMs, which can bind to LC3 bound to phagosomes via the LIR. After being phagocytized by autophagosomes and merging with lysosomes, mitochondria are damaged by lysosomal hydrolases. Sustained accumulation of misfolded proteins and aggregates, however, may overactivate or even shut down mitophagy at advanced ages. In addition, insufficient mitophagy also induces mitochondrial dysfunction and cellular senescence. PINK1, Phosphatase and tensin homolog-induced putative kinase 1; Own mother machine, mitochondrial outer membrane; LC3, String light 3; OPTN, optic protein; NDP52, nuclear spiking protein 52 kDa; BNIP3, Bcl-2 interacting protein 3; FUNDC1, FUN14 structural domain-containing1.

The interplay between mitophagy and bone cell fate is critical. Adequate mitophagy serves as a quality-control mechanism, preserving the function of osteoblasts and chondrocytes by removing damaged mitochondria. However, in aging, both insufficient mitophagy (leading to the accumulation of dysfunctional mitochondria and amplified ROS production) and excessive mitophagy (potentially triggering cell death) can occur. In osteocytes, impaired mitophagy promotes the production of cathepsin K, a key enzyme in bone resorption. In chondrocytes, its dysregulation accelerates extracellular matrix degradation. Thus, the loss of mitophagy homeostasis directly contributes to the net bone loss and cartilage degeneration characteristic of skeletal aging.

4.1.1. PRKN-dependent pathway

PRKN-dependent mitophagy is mediated by PINK1 (a PTEN-induced putative kinase 1) and PRKN (a cytosolic E3-ubiquitin ligase). As a serine/threonine protein kinase, PINK1 has mitochondrial target sequences whose primary function is proteasomal degradation. Parkin is an E3 ubiquitin-conjugating enzyme with an Ubl structural domain at its N-terminal end, and an uncharacteristic RING structural domain, RING0, is newly recognized in the junction region between the Ubl and RBR structural domains [45]. Mitochondrial phagocytosis mediated by PINK1/Parkin is a prominent mitochondrial stress signaling pathway that mediates specialized ubiquitination and scavenges damaged mitochondria through selective autophagy mechanisms.

Following recognition by PINK1, defective mitochondria can be phagocytized by autophagosomes during the initial phase of mitophagy and then hydrolytically degraded by fusion with lysosomes. Mitochondrial damage and depolarization increase the presence of PINK1 on the surface of mitochondria, thus preventing the mitochondrial import of PINK1. PINK1 phosphorylates Parkin, which cooperates in the conversion of PINK1 to an active ubiquitin phosphorylation-dependent E3 ligase to abrogate MMP damage to mitochondria [46]. Wang et al. reported that gold nanoparticles (AuNPs) increased the proliferation and osteogenic differentiation of periodontal ligament stem cells (PDLSCs) by activating PINK1-mediated mitophagy, revealing the role of AuNPs in regulating the osteogenic differentiation of PDLSCs [47]. In addition, PINK1/Parkin-mediated mitophagy reduced plasma late oxidized protein product (AOPP) levels and inhibited AOPP-induced apoptosis of osteoblasts, thus ameliorating bone loss, bone microstructural disruption, and loss of bone mineral density associated with AOPP accumulation. Therapeutic strategies that increase osteoblastic mitophagy and safeguard mitochondrial function may have the potential to treat senile osteoporosis.

Prominent on the list of mitophagy-specific modulators, the PINK1-Parkin signaling pathway is a major mitotic regulator. The research group established a fluoride-exposed Parkin knockout mouse model and a fluoride-exposed calcium-supplemented mouse model and used various experimental methods to emphasize the essential role of the PINK1/Parkin signaling pathway in inhibiting mitophagy and apoptosis and attenuating skeletal damage. The brilliance of this study is that the relationship between mitophagy and apoptosis has been preliminarily clarified, but the disadvantage is that comparative tests have not been conducted to determine exactly how much calcium is most effective in reducing fluoride accumulation in bone. Furthermore, in an attempt to reduce IL-1β-mediated mitophagy and autophagosome formation in chondrocytes, irisin also acts through the expression of a number of reversal signaling molecules, including SIRT3, which is known to increase the mitochondrial membrane potential, ATP production, and peroxisomal enzyme activity [48]. Another research team demonstrated that mitophagy damage can induce pyroptosis in macrophages, stimulating high concentrations of CoPs through the NLRP3/caspase-1/GSDMD signaling pathway and resulting in the release of inflammatory cytokines and, subsequently, osteoclasts [49].

Parkin signaling plays a key role in inducing mitochondrial phagocytosis by regulating mitochondrial outer membrane protein ubiquitination and promoting its degradation in the context of mitochondrial dysfunction. PINK1 expression is required for Parkin recruitment to depolarized mitochondria and Parkin-induced mitochondrial autophagy. Richard Youle and his team made groundbreaking contributions to the field of mitophagy, particularly in elucidating the mechanism by which PINK1 responds to mitochondrial damage and activates PARKIN. Miratul Muqit and his team validated the critical role of Ser65 phosphorylation in enabling Parkin activation of its E3 ligase activity and reveals new mechanistic insights into how disease-associated mutations of Parkin may impact on E3 ligase activity [50]. These findings provided mechanistic insight into how the PINK1-PARKIN signaling axis is precisely regulated and underscored the central role of Ser65 phosphorylation in mitophagy and Parkinson’s disease pathogenesis. Luo et al. speculated that C2S may modulate macrophage inflammation by inducing mitochondrial dysfunction and significantly upregulating the expression of mitochondrial PINK1/Parkin in macrophages to trigger autophagy, a process that potently promotes the osteogenic differentiation of BMSCs. This finding also suggests that the substance has potential applications in the fabrication of immunomodulatory biomaterials/implants with osteoinductive properties, but the team did not strongly support the above hypothesis (Fig. 6).

Fig. 6.

Schematic diagram of the mitochondrial PRKN-dependent pathway. (a) The key proteins in this pathway are PINK1, a serine/threonine kinase located on the depolarized outer mitochondrial membrane, and parkin, an E3 ubiquitin ligase. (b) When an abnormal change in the mitochondrial membrane potential occurs, PINK1 accumulates on the outer mitochondrial membrane, leading to the phosphorylation and activation of parkin, which results in the attachment of ubiquitin (Ub) to the mitochondrial substrate, (c) which is subsequently encapsulated by the lysosome and ultimately degraded by the mitochondria.

The PINK1/Parkin signaling pathway can also indirectly lead to autophagy and apoptosis. Chondrocyte basal autophagy levels were greater in the HIF-1α-elevated environment than in the control environment, suggesting that chondrocyte autophagy is dependent on HIF-1α expression and has greater resistance to IL-1β-induced inflammatory damage, which highlights the importance of HIF-1α in the function of hypoxic chondrocytes in OA. Increased expression of autophagy proteins leads to better repair of chondrocytes, thereby decreasing the level of reactive oxygen species production and reducing damage to chondrocyte integrity. Therefore, we can conclude that the silencing of HIF-1α activates PINK1/Parkin and BNIP3 mitochondrial autophagy proteins in cells, leading to the mobilization of the Caspase/Cleaved Caspase 3 apoptosis cascade [51].

The application of the PINK1/Parkin signaling pathway is more obviously reflected in the treatment of bone-related diseases. Al exposure has been shown to activate PINK1/Parkin-mediated mitophagy, and PINK1/Parkin-mediated mitophagy plays a protective role in Al-induced bone damage. In addition, a recent study revealed that the mitochondrion-targeted antioxidant mitochondrial quinone (MitoQ) prevents IVDD by ameliorating mitochondrial dysfunction and redox imbalance through the promotion of PINK1/Parkin-mediated mitophagy and the restoration of mitochondrial autophagic flux. Ma et al. reported that SIRT1 ameliorates IL-1β-induced thermoapoptosis in nasopharyngeal carcinoma by decreasing ROS production in mitochondria and alleviating mitochondrial dysfunction via mitophagy mediated by the PINK1/Parkin pathway [52]. A previous study revealed that the activation of NLRP3 inflammatory vesicles was stimulated by IL-1β via mitochondrial oxidative stress damage and mitochondrial ROS generation [53]. Hence, modulating the inflamed body and mitophagy of NLRP3 could be a potential and prospective tactic for the future treatment of inflammation-associated IVDDs.

4.1.2. PRKN-independent pathway

A substantial body of research on mitophagy has focused on the PINK1/Parkin signaling pathway. However, increasing evidence indicates that many proteins and lipids are able to complete the autophagy mechanism independently of the PINK1/Parkin signaling pathway. For example, mitochondrial herniation leads to the induction of IMM exposure and ubiquitination, which initiates the induction of PINK1/Parkin-independent mitochondrial segregation pathways. This process is dependent on BAK/BAX regardless of downstream STING activation. IMM-induced mitophagy is an alternative mechanism by which cells prevent unwanted responses induced by damaged mitochondria [54].

The PRKN-independent mitochondrial autophagy pathway exclusively interacts with different LC3 and GABARAP proteins to specifically transport dysfunctional mitochondria into autophagosomes. This mainly includes BNIP3L/BNIP3 and AMBRA1.

BNIP3L was originally identified as a dimeric proapoptotic mitochondrial protein that physically interacts with the adenovirus E1B 19K and the antiapoptotic protein BCL2. BNIP3 is an autophagy regulator similar to BNIP3L in that it contains a BH3 domain, which regulates autophagy through pRB/E2F. The cell death activity of BNIP3 and BNIP3L is mediated by either the BH3 structural domain or the C-terminal TM structural domain. The TM structural domain of BNIP3 is distinctive in that it can autonomously and stably dimerize and contributes to the mitochondrial localization of BNIP3 [55].

BNIP3L affects mitophagy under hypoxic conditions. The combined mechanism of LIR phosphorylation and receptor dimerization is necessary for the initiation and progression of appropriate BNIP3L-dependent mitophagy. Under hypoxic conditions, the expression levels of both BNIP3 and BNIP3L are increased, resulting in hypoxia-induced cell death. Moreover, Lu’s research team discovered the effect of HIF-1α on interleukin-1β-induced autophagy in human chondrocytes. Silencing of hypoxia-inducible factor-1α activates the mitochondrial autophagy proteins PINK1/Parkin and BNIP3/BAX in chondrocytes, which in turn initiates apoptosis via the caspase 3 cascade, indicating that aberrant expression of HIF-1α contributes to mitophagy activation [56].

When there is a high demand for mitochondrial OXPHOS, BNIP3L also participates in the induced mitotic process. Accelerated recovery of the mitochondrial pool is thought to help improve OXPHOS efficiency. In this case, the small GTPase RHEB is recruited to the OMM, where it is proposed to stimulate mitophagy by binding BNIP3L and recruiting LC3 [57].

Furthermore, Hu et al. demonstrated that HIF-1α was increased in a dose-dependent manner after dimethyloxopropionylglycine (DMOG)-induced HIF-1α expression through enhancement of the mitogenic HIF-1α/BNIP3 signaling pathway in vivo and in vitro. si-HIF-1α significantly reduced BNIP3 expression, indicating that the HIF-1α/BNIP3 pathway is an important mechanism involved in DMOG protection. In addition, HIF-1α is essential for promoting the synthesis of the extracellular matrix and suppressing hypoxia-induced ECM degradation at the transcriptional level [[58], [59]]. Research has shown that HIF-1α-mediated autophagy exerts a preventative effect on hypoxia-stimulated chondrocytes. Nevertheless, the exact mechanism by which HIF-1α inhibits apoptosis and senescence is unclear. The protective effects of HIF-1α through the induction of the transcription of antiaging and antiapoptotic genes are unknown. To determine how autophagy reduces senescence and promotes apoptosis in hypoxic environments, further studies are needed [58]. In another study, peroxisome proliferator-activated receptor-γ coactivator 1-α (PGC1-α) expression was markedly reduced in OA, whereas knockdown of PGC1-α activated selective mitotic pathways independent of PRKN by upregulating BCL2 and BNIP3. Thus, the modulation of the PGC1-α/BNIP3 mitochondrial autophagy axis is likely to have a curative effect on OA cartilage degeneration.

Another central pathway of mitochondrial autophagy unrelated to PARKIN is the Beclin-1-regulated autophagy (Ambra1) activation pathway, which is also known as autophagy/Beclin-1 regulator 1. This regulator is an adaptor protein that promotes autophagosome core complex formation in mammalian target of rapamycin complex 1 (mTORC1)-dependent autophagy [60]. Recent studies have shown that Ambra1 also orchestrates cellular responses to starvation or other stresses, including the transfer of the autophagosome nuclear component to the endoplasmic reticulum, the regulation of ubiquitination and the stability of the kinase ULK1, the selective clearance of mitochondria, and the downregulation of the cell cycle [60].

In the process of mitochondrial selective autophagy, the autophagy receptor functions by directly interacting with LC3 and thereby tethering the mitochondria to the site of phagocytosis. BNIP3L mediates mitochondrial removal during reticulocyte differentiation, whereas FUNDC1 mediates mitochondrial clearance after hypoxia. Through the generation and expression of organelle-targeted mutants of Ambra1, we found that Ambra1 is capable of inducing perinuclear mitochondrial relocalization, mitochondrial depolarization, and ubiquitination, as well as the recruitment of molecular platforms required for functional mitophagy through a Parkin-independent pathway [61]. In this case, Ambra1-ActA contributes to mitochondrial clearance by translocating damaged mitochondria to autophagosomes via an interaction from its LC3 interaction region motif to LC3 [60]. Furthermore, Strappazzon et al. confirmed the existence of an Ambra1-dependent mitotic pathway by confirming that wild-type Ambra1 is also adequate to restore mitotic induction in PINK1- or Parkin-deficient cells [61].

4.2. Mitochondrial dynamics

As dynamic organelles, mitochondria are subject to changing patterns of biogenesis, fusion, and division regulation [41]. Mitochondrial dynamics include fusion, fission, selective degradation, and transport processes. Mitochondrial dynamics change the quality and quantity of mitochondria by regulating their morphology and remodeling the mitochondrial network so that they can respond quickly to cellular energy demands. Mitochondria undergo the continuous processes of fission, fusion, mitophagy and transport cycles, which determine the morphology, quality, quantity and distribution of mitochondria within cells, as well as the mitochondrial function [62]. In addition, mitochondrial dynamic imbalance is increasingly recognized as a central mechanism underlying age-related declines in bone mass and skeletal function. Therefore, understanding the role of mitochondrial dynamics in bone-related diseases is highly important. Changes in mitochondrial conformation require protein drive, and control of protein quantification and quality is key to maintaining the balance of the mitochondrial fission and fusion processes.

With advancing age, osteoblasts exhibit a shift toward excessive mitochondrial fission, primarily mediated by increased expression and activation of dynamin-related protein 1 (DRP1). Smirnova et al. demonstrated that overup-regulation of Drp1 and its abnormal distribution eventually led to abnormal mitochondrial fission, which is harmful to cells. Gan et al. Also demonstrate that blockade of Drp1 attenuates oxidative stress-induced osteoblast dysfunction [63]. However, the diversity of the mitochondrial network, i.e., its number and morphology, depends on the balance between the two opposite processes of fusion and fission. Fusion between OMMs is mediated by membrane-anchored dynamin superfamily members, mitofusin 1 and 2, whereas fusion between IMMs is mediated by a single dynamin superfamily member, optic atrophy 1 (OPA1). Meanwhile, the expression of fusion-associated proteins such as MFN2 (mitochondrial fusionin 2) and OPA1 (optic atrophy protein 1) undergoes significant down-regulation during bone aging, which further exacerbates the instability and dysfunction of the mitochondrial network.

In addition to osteoblasts, recent studies have revealed that mitochondrial dynamics also play an important role in the regulation of osteoclast function. Research has shown that the expression levels of MFN2 and OPA1 decrease during osteoclast differentiation, suggesting that reduced mitochondrial fusion capacity may favor the transition of osteoclasts toward a highly metabolically active state [64]. In mitochondrial dynamics, in addition to the influence of fusion molecules on osteoclast differentiation and activity, mitochondrial mitogen-like protein DRP1 can down-regulate the key transcription factors of osteoclast formation, c-Fos and NFATc1, to inhibit osteoclast differentiation by knocking down or using DRP1 inhibitor Mdivi1 [63]. Studies have shown that inhibition of DRP1 significantly reduces osteoclastogenesis and bone-resorbing activity [65], suggesting that mitochondrial fission plays a promoting role in this process (Table 1).

Table 1.

The relationship between mitochondrial dynamics and osteoblast/osteoclast function.

Mitochondrial dynamic process	Key regulatory proteins	Role in osteoblasts	Role in osteoclasts
Mitochondrial fusion	MFN1,MFN2 (outer membrane fusion); OPA1 (inner membrane fusion)	Maintains mitochondrial network stability and function; Reduced fusion leads to mitochondrial dysfunction and decreased osteogenesis; MFN2 and OPA1 expression declines during bone aging;	Reduced fusion capacity may promote a highly metabolically active state; MFN2 and OPA1 expression decreases during osteoclast differentiation;
Mitochondrial fission	DRP1	Age-related upregulation of DRP1 causes excessive fission; Leads to mitochondrial fragmentation and increased oxidative stress; Inhibition of DRP1 improves osteoblast function;	DRP1-mediated fission promotes osteoclastogenesis and bone-resorbing activity; Inhibition of DRP1 (e.g., with Mdivi-1) reduces osteoclast formation;

In essence, the balance of mitochondrial dynamics is crucial for bone homeostasis. The age-related shift towards excessive fission, mediated by elevated DRP1 activity, fragments the mitochondrial network in osteoblasts, compromising their energy production and biosynthetic capacity, thereby directly contributing to impaired bone formation. Conversely, in osteoclasts, DRP1-mediated fission is necessary for their bone-resorbing activity. The concomitant decline in fusion proteins (MFN2, OPA1) further exacerbates mitochondrial dysfunction. This imbalance in dynamics across different bone cell types disrupts the coordination between bone formation and resorption, ultimately leading to the net bone loss and microarchitectural deterioration that define the aged skeleton.

Mitochondrial cleavage is modulated by the production of pDrp1 and Fis-1, and mitochondrial fusion is regulated by the mitoproteins MFN1, MFN2, and Opa1, which are located on the outer mitochondrial membrane. The stem cell microenvironment and mitochondria influence the osteogenic differentiation of mesenchymal stem cells, which play essential roles in the proliferation and differentiation of stem cells. Rigid ECMs have been shown to facilitate mitochondrial fusion by increasing the expression of MFN1 and MFN2 and inhibiting DRP1 activity, which in turn contributes to the osteogenic differentiation of MSCs. In addition, MNF2 in skeletal muscle is particularly important for maintaining insulin sensitivity and limiting oxidative stress [66]. The adjustment of mitochondrial dynamics by zinc ions and Sr2+ contributes to bone incorporation and bone mineralization, which provides a new therapeutic paradigm for diabetic patients with novel bone implants with mitochondrial modulation capabilities.

Moreover, the connection between the endoplasmic reticulum and mitochondria is important for mitochondrial fission. The research team successfully constructed a bone-seeking nanomedicine, BTZ@ZnPc-ALN, which was codelivered with BTZ and ZnPc by detecting in vitro stability and bone affinity. Under irradiation, the drug prevents bone metastasis of breast cancer by producing a large amount of ROS to induce mitochondrial damage, which in turn causes an endoplasmic reticulum stress response, providing a new idea for the treatment of this disease [67]. Mitochondrial dynamics play an extremely important role in bone-related diseases, and the regulation of related proteins also provides a new direction for the development of therapeutic options (Fig. 7).

Fig. 7.

Mitochondrial dynamics and aging. Mitochondrial dynamics are critical for maintaining mitochondrial homeostasis and regulating the number, shape, function, and subcellular distribution of mitochondria. Many GTPases play kinetic regulatory roles in mitochondria. The DRP1 protein guides mitochondrial fission, and MFN1 and MFN2 on the external membrane, with OPA1 on the internal membrane, mediate fusion. Damaged parts of mitochondria are separated from healthy parts by DRP1 and engulfed by phagocytes for mitophagy. Moreover, healthy strands of mitochondria fuse with other mitochondria to fulfill their antiaging function. Inadequate or excessive fission can lead to mitochondrial autophagy or fragmentation.

4.3. Mitochondrial proteostasis

Mitochondria are the powerhouses of cells, and many human diseases are associated with mitochondrial dysfunction. Therefore, maintaining the homeostasis of mitochondrial proteins is essential for the performance of cellular functions. Mitochondrial protein homeostasis is the equilibrium state of the proteome within the mitochondria during synthesis, folding, modification and degradation.

It is governed by a variety of factors. First, mitochondria are coated by two membranes, the inner and outer membranes, and mitochondrial DNA (mtDNA) is also present inside, whereas most mitochondrial proteins are encoded by nuclear genes. Proteins must remain unfolded during their entry into the mitochondria, and differences in the internal mitochondrial and cytoplasmic environments affect the structure and function of the protein. Therefore, maintaining mitochondrial protein homeostasis is also important for the regulation of mitochondrial homeostasis. In addition, the main targets of our oxidative modifications during aging are proteins. Mitochondria, in turn, are the main site of ROS production, making mitochondrial proteins particularly vulnerable to oxidative modification [68]. Both unfolded mitochondrial proteins and aging can lead to an imbalance in mitochondrial quality control. As a molecular protective mechanism, UPRMT helps rearrange misfolded or misfolded proteins in the mitochondria, thus creating a stabilized mitochondrial ecosystem [7]. Bone homeostasis refers to the state of equilibrium of the bone system under normal physiological conditions. This balance is regulated and maintained by a variety of cells in bone, such as osteoblasts, osteoclasts, and mesenchymal stem cells, and the function of these cells is closely related to that of mitochondria.

4.4. Mitochondrial biogenesis

Mitochondrial biogenesis is a key biological process by which cells coordinate the expression of the nuclear and mitochondrial genomes to synthesize new mitochondria and maintain their functional integrity. This process plays a central role in energy metabolism, oxidative stress response, cell differentiation, and aging regulation [69]. With aging, mitochondrial function in bone tissue gradually declines, and the capacity for mitochondrial biogenesis diminishes. This leads to insufficient energy supply in osteoblasts, increased accumulation of reactive oxygen species (ROS), and accelerated cellular senescence, thereby promoting the development and progression of age-related skeletal disorders such as osteoporosis.

Recent studies have shown that peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) is a key regulatory factor for mitochondrial biogenesis, which drives mitochondrial DNA replication and the assembly of respiratory chain complexes by activating transcription factors such as nuclear respiratory factors 1 and 2 (NRF-1/NRF-2) and mitochondrial transcription factor A (TFAM). In neurodegenerative diseases, the AMPK-SIRT1-PGC-1α signaling axis has been shown to play a critical role in exercise-induced mitochondrial biogenesis, and activation of this pathway can significantly ameliorate mitochondrial dysfunction in metabolic disorders. In aged mice, PGC-1α expression is significantly downregulated in osteoblasts, leading to reduced mitochondrial mass and impaired oxidative phosphorylation function. However, exercise or activation of the AMPK signaling pathway can restore PGC-1α expression, enhance mitochondrial biogenesis, and significantly improve bone mineral density and bone microstructure [70]. Moreover, studies have shown that the mitochondrial unfolded protein response (UPRmt), through ATFS-1-mediated nuclear transcriptional reprogramming, can coordinately promote mitochondrial biogenesis and quality control, providing a novel target for anti-aging interventions. SIRT3 regulates mitochondrial enzyme activity through deacetylation and enhances the stability of TFAM, thereby maintaining mitochondrial DNA copy number and osteoblast function. Studies have revealed that SIRT3-deficient mice exhibit premature bone loss, whereas its overexpression can delay the progression of skeletal aging [71]. In summary, defects in mitochondrial biogenesis are a key driver of skeletal aging, and this process is highly dynamic and regulated at multiple levels. A deeper understanding of its molecular mechanisms not only advances our knowledge of cellular energy homeostasis, but also provides potential therapeutic strategies for aging-related diseases.

The decline in mitochondrial biogenesis with age is a fundamental defect that underpins the energetic crisis in bone cells. The downregulation of master regulators like PGC-1α directly leads to a reduction in mitochondrial mass and functional capacity within osteoblasts. This bioenergetic impairment compromises the anabolic activities of bone-forming cells, limiting matrix synthesis and mineralization. In BMSCs, insufficient biogenesis contributes to their senescence and alters their differentiation potential. Therefore, the failure to generate new, healthy mitochondria is not a passive consequence but an active driver of the hypo-metabolic state and functional decline of bone cells, which directly translates into the reduced bone formation rates and overall loss of bone mass observed in aging.

5. Therapeutics of mitochondrial dysfunction in bone aging

5.1. Targeting mitochondrial oxidative stress

As mentioned earlier, mitochondria are a major source of ROS, which are generated mainly through oxidative stress. Oxidative stress plays an instrumental role in the aging process, leading to degenerative diseases as we age. Endogenous enzyme species, comprising transmembrane NADPH oxidases and the mitochondrial electron transport chain (ETC), are primarily responsible for the production of O₂^- and H₂O₂. Therefore, mitochondrial ROS levels can be used as a stress monitoring system to assess mitochondrial dysfunction and the state of cellular health. In addition, the mitochondria themselves are candidates for oxidative stimulation, causing liposome and mtDNA damage or posttranslational modification of proteins [72]. ROS production is an essential signal in organisms that regulates cellular function, mediates inflammation, and influences tissue pathophysiology in bone tissue. Excessive ROS directly promote osteoclastogenesis by activating RANKL signaling and simultaneously suppress osteoblast function by inducing mitochondrial membrane permeabilization and caspase activation, thereby disrupting the balance between bone resorption and formation. ROS appear to play an essential role in the skeleton by affecting both of these cell types. These discoveries also suggest promising therapeutic strategies for bone disease that target ROS [73].

The oxidative stress that occurs in bone defects can lead to regeneration delays, particularly in the elderly population and in patients who have undergone cancer treatment. This delay can be attributed to the build-up of senescent cells, which results in elevated levels of reactive oxygen species (ROS) in these cell populations. A variety of scaffolds, hydrogels, nanocarriers and drugs are being developed for the treatment of bone-associated diseases. Biomaterials, either synthetic or natural, can facilitate the structural and functional restoration of bone. The implant material is in contact with the patient’s tissue and can interact with endogenous bone. In a variety of therapeutic approaches, the discovery of novel biomaterials and drugs that target high concentrations of ROS has demonstrated significant potential for treating bone-related diseases. Antioxidants or selected therapeutic compounds can be added to biomaterials and released in response to oxidative stimuli. As a result, tissue-engineered scaffolds are emerging as alternative treatments for skeletal defects. Researchers have utilized the ROS-scavenging characteristics of cerium oxide nanoparticles to mitigate the high oxidative stress microenvironment created as a result of the presence of senescent cells. The presence of cerium oxide can modulate ROS levels, thereby reducing aging levels and promoting osteogenesis [74]. In addition, oxidants are involved in the bidirectional modulation of mitochondrial dynamics, such as the remodeling of mitochondrial cristae and networks.

Excessive ROS in OA cause DNA and chondrocyte damage and affect ECM formation and turnover through the stimulation of matrix metalloproteinases (MMPs). Furthermore, overproduction of the ECM stimulates immune cells to produce more ROS. A positive correlation has been shown between high ROS levels and OA, so many antioxidants (including vitamin C, polysaccharides, or drug-free polymers) that scavenge lower ROS levels are used to inhibit the inflammatory response caused by OA. Polyphenols extracted from plants such as tea or grapes have strong antioxidant properties, so we encapsulated them layer by layer in gelatin nanoparticles and used them extensively, mainly for bone regeneration [75]. In the treatment of osteoarthritis, traditional OA therapy requires frequent administration to maintain the concentration of the drug due to its rapid removal. In addition, several research teams have utilized the high level of ROS in OA as a trigger for the release of drugs from ROS-responsive biomaterials. Hollow poly(lactide-co-glycolide) (PLGA) microspheres for the treatment of osteoarthritis have been designed for the treatment of OA and are capable of delivering the anti-inflammatory drugs dexamethasone, FeCl₂, sodium bicarbonate and ethanol. This biomaterial has shown significant therapeutic effect.

With further research on cartilage metabolism and mitochondrial regulation, new targeted drugs, including the inhibitor amibarbital, the antioxidant N-acetylcysteine (NAC) and some Chinese herbal medicines, such as rhine (RH) nanoprodrugs and curcumin, which reduce inflammation and relieve oxidative stress, and the targeted drug DMAR Ds, such as tofacitinib, have good therapeutic effects in alleviating pain and reducing disease progression in both moderate to severe osteoarthritis and rheumatoid arthritis. Notably, although arthritis is characterized by inflammation-induced cartilage damage [76], overproduction of mitochondrial ROS, changes in the mitochondrial membrane potential, and dysfunction of the mitochondrial electron transport chain (ETC) may also contribute to the development of arthritis. The therapeutic potential of targeting the subchondral microenvironment via mitochondria has been demonstrated through preclinical studies, and several trials have been carried out to investigate its efficacy, which is likely to provide new insights into the treatment of arthritis.

Additionally, mitochondrial oxidative stress has long been the focus of research, pharmacological agents with antioxidant properties are being investigated. 3-Hydroxy-3-methylglutaryl-CoA reductase inhibitors, commonly known as statins, are cholesterol-lowering drugs used to treat various conditions, including hypercholesterolemia [77]. Statins have a variety of beneficial effects, ranging from anti-inflammatory, antiproliferative and antithrombotic effects to profound cellular antioxidant effects. Similarly, the use of statins in the treatment of osteoporosis has led to a series of trials. In a clinical trial (NCT06359353), researchers studied the impact of pitavastatin on bone metabolism in women with postmenopausal osteoporosis, but the results of the trial have not yet been published. Statins may have side effects that cause muscle aches and nerve disorders [78].

Osteoporosis (OP) is a metabolic bone disease characterized by imbalances in bone homeostasis, low bone mass, and altered microstructure, which seriously affects the life span of patients. The disease is age-related, and the balance between bone formation and bone resorption becomes imbalanced with age; therefore, cellular senescence is thought to be a factor in the imbalance of bone homeostasis [79]. The emphasis of osteoporosis-related treatments is restoring skeletal balance and preventing fractures. However, numerous risk factors have been associated with osteoporotic fractures, including low core bone mass, hormonal factors, the use of certain medications (e.g., glucocorticoids), smoking, low physical activity, low calcium and vitamin D intake, race, short stature, and personal or family history of fracture [80]. Therefore, in addition to vitamin D and calcium supplements, drugs that can treat osteoporosis extensively are difficult to develop. Available drugs fall into two categories: antiresorptive drugs (i.e., estrogen, selective estrogen receptor modulators, bisphosphonates, and denosumab) and anabolic drugs (i.e., teriparatide, abaparatide, and romolizumab); the former are mostly suitable for osteoporosis in postmenopausal women. Here, we focus on trials of new osteoporosis drugs.

Currently available antiresorptive drugs, such as bisphosphonates, are inextricably associated with serious side effects, such as bisphosphonate-associated osteonecrosis of the jaw (BRONJ), which can cause extreme pain in patients. For this purpose, researchers have derived a novel carbon point, ALEN—CD, from polyethylene glycol (PEG) and alendronate (ALEN), which has fewer side effects, good biocompatibility and bone-targeting ability. It can modulate the bone immune microenvironment by impacting mitochondrial metabolism, particularly oxidative phosphorylation (OXPHOS). This drug has been validated in animal models, but further clinical trials are needed (Table 2).

Table 2.

Interference with Bone aging by targeting mitochondria.

Target	Interventions	Mechanism of action
Mitochondrial biogenesis	Resveratrol,	Increase mitochondrial mass and elevating Nrf1 and TFAM
	Calorie Restriction,	Activation of AMPK
	Metformin,	Activation of AMPK
	Rhizoma Coptidis,	Activation of AMPK
	Rapamycin	Inhibition of mTOR
Mitophagy	Metformin,	Enhancing the SIRT1/PINK1/Parkin pathway
	Rapamycin,	Inhibiting mTOR Elevation of PINK1, Parkin and BECN1
	Urolithin A,	Activation of PINK1/Parkin pathway
	Oleanolic Acid,	Modulating of FUNDC1, LC3B, p62, TOM20
	Kaempferol Rhapontigenin	Activation of PINK1/Parkin pathway; Modulating FUNDC1, LC3-II, p62
Mitochondrial dynamics	Resveratrol,	Promotion of fission and suppression of fusion via PINK1/Parkin pathway; Inhibition of both fusion and fission
	Exercise	Regulation of mitochondrial fission, fusion AMPK activation
Mitochondrial oxidative stress	Cerium oxide Nanoparticles,	Scavenges ROS and alleviates the microenvironment of high oxidative stress
	Polyphenols	Antioxidant

5.2. Targeting mitophagy

Pharmacological treatments targeting mitophagy are of great interest and may have enormous translational potential. In the study of mitophagy and bone aging, cathepsin K is worth noting.

Cathepsin K is expressed mainly in osteoclasts and cleaves type II collagen and proteoglycans; studies have shown that mitophagy in osteocytes contributes to GC-induced cathepsin K production and may play a role in GC-induced skeletal loss. On the other hand, MIV-711 is a highly potent and selective cathepsin K inhibitor, and Erik et al. reported that MIV-711 alleviated related conditions in a dog model of partial meniscectomy and a rabbit ACLT model through animal models, showing great potential for clinical translation [81]; however, owing to the rapid degeneration of animal models, preclinical research on MIV-711 still needs many experiments. In a double-blind, randomized, phase 2 controlled clinical trial (NCT02705625), MIV-711 demonstrated excellent efficacy, safety, and tolerability in treating osteoarthritis. This finding reflects its great value in clinical application. Notably, dysregulation of mitophagy in chondrocytes also accelerates the development of OA. In chondrocytes under inflammatory stress, irisin can increase mitochondrial fusion, decrease mitochondrial division, and increase mitophagy to remove damaged mitochondria to increase chondrocyte survival in inflammatory states [48]. Kang et al. used Parkin-mediated mitophagy to prevent ROS overproduction, mitochondrial damage, and apoptosis in endplate chondrocytes under oxidative stress [82].

Moreover, cathepsin K inhibitors have been used in clinical studies of osteoporosis to increase bone mineral density and improve skeletal strength in the spine and hips. Unfortunately, however, in a randomized, double-blind phase III study that included postmenopausal women with osteoporosis, a clinically relevant reduction in fractures at multiple sites was demonstrated, but the risk of cardiovascular and cerebrovascular accidents was greatly increased, and the drug was eventually withdrawn from the regulatory approval process [83]. In preclinical studies, long-term alendronate therapy induced an increase in the number of uniquely giant, hypernuclear, isolated osteoclasts undergoing long-term apoptosis. Of course, as age-related diseases, antiaging drugs are also being studied for the treatment of osteoporosis, with the goal of identifying potential therapeutic agents to reduce the aging phenotype of bone and cartilage. Several studies have shown that the combination of senescent cells (SnCs), the senescence-associated secretory phenotype (SASP) and dasatinib plus quercetin (DQ) is a newly developed treatment for a variety of age-related diseases [84]. Dasatinib can induce apoptosis through the related pathways of mitochondrial autophagy, and DQ has advantages in improving the bone microenvironment and contributing to bone regeneration and repair. Notably, dasatinib and quercetin have also been shown to ameliorate age-dependent disc degeneration in mice, and the combination of the two can promote anabolism in human cartilage.

Importantly, mitophagy is essential for mitochondrial quality control. PINK1 triggers mitochondria-selective autophagy and is involved in mitochondrial regeneration. Immunohistochemical staining revealed that the expression of PINK1 in the bones of osteoporosis patients was reduced, which supports the practical role of PINK1 in human osteoporosis. These studies provide new clues for elucidating the molecular mechanisms of mitochondrial dynamics and the regulation of osteogenic differentiation. As a natural SIRT1-activating complement, metformin can treat age-related diseases such as osteoporosis and bone loss by activating PINK1/Parkin-mediated mitophagy and by inhibiting the PDK4/oxidative stress-mediated apoptosis pathway, attenuating the stereotypic transition of VSMCs to an osteogenic phenotype, and mitochondrial homeostasis is involved in this process. Rapamycin ultimately reduces cellular senescence by inhibiting mTOR and increasing the expression of PINK1, Parkin, and the autophagic protein BECN1.

Allantoin A is a polyphenol that is present in pomegranates, cherries and walnuts; it is also a metabolite of postbiosis. Allantoin A has been consistently shown to activate mitophagy and enhance mitochondrial function in preclinical studies on preclinical models of aging and age-related diseases and plays an essential role in the repair of bone defects and skeletal mineral density [85]. These findings also suggest that urolithin A can be used to treat diseases related to bone aging. Another natural substance, oleanolic acid, is a naturally occurring pentacyclic triterpenoid that has been shown to have antioxidant and cyclooxygenase (P450)-modulating effects. Oleanolic acid alleviates senescence-associated changes in mitophagy by adjusting the levels of LC3B, p62, TOM20, and FUNDC1, as opposed to BNIP3 or Parkin [86]. In summary, maintaining proper modulation of mitophagy is essential for regulating the aging process.

5.3. Stem cell-based therapy that targets mitophagy

Physiologically, bones are metabolically active and undergo lifelong remodeling, which requires the interaction of various types of cells. Bone is composed of two main stem cell lineages: mesenchymal stem cell lineages (e.g., MSCs, osteoblasts, and osteocytes) and hematopoietic stem cell lineages (e.g., HSCs, osteoclasts, and immune cells). The transplantation of healthy mesenchymal stem cells for regeneration in osteocyte therapy, as well as drugs that can help restore mesenchymal stem cell-mediated osteoblastogenesis and inhibit hematopoietic stem cell-derived osteoclasts, has been shown to significantly improve bone aging and disease. Considering the central role of mitochondrial metabolism and oxidative signaling in regulating backbone cell behavior, reagents capable of regulating mitochondrial metabolism, as well as drugs known as antioxidants for oxidative inhibition, are increasingly showing efficacy in counteracting bone aging and pathology by modulating stem cell specifications [87]. For example, resistin counteracts skeletal loss through mitogen-mediated acceleration of osteogenic amelioration in mouse mesenchymal stem cells.

Bone marrow mesenchymal stem cells have regenerative and immunomodulatory properties, and impaired cartilage differentiation is a key factor in cartilage degeneration. Therefore, promoting cartilage production and regeneration has been shown in clinical studies to improve the development of osteoarthritis. Uzieline et al. demonstrated in preclinical experiments that the antihypertensive drug nifedipine promotes cartilage production [88]. Nifedipine is a calcium-blocking drug that operates through L-shaped voltages and is mainly used to treat hypertension, but its impact on cartilage homeostasis and the development of osteoarthritis remains unclear. Thus, an analysis of the effects of the agonist BayK8644 indicated that nifedipine inhibited mitochondrial respiration in cartilage cells and bone marrow mesenchymal stem cells, shifting the cells from oxidative phosphorylation to glycolysis. Additionally, nifedipine promotes the production of type II collagen as well as proteoglycans, suggesting that the drug promotes the synthesis of articular cartilage.

In addition to the chondrogenesis of bone marrow mesenchymal stem cells, osteoclast-mediated subchondral bone lesions and cartilage degeneration are also key targets in osteoarthritis. In the pathogenesis of OA, the loss of chondrocyte function is the main causative factor. Reactive oxygen species (ROS)-induced oxidative stress disrupts cartilage homeostasis and induces cell death. Moreover, the inflammatory cytokines IL-1β and TNF-α also increase osteoclast activity, leading to chondrocyte death.

5.4. Targeting mitochondrial dynamics

Mitochondria are not stationary within the cell but are constantly fusing and dividing, a process known as mitochondrial dynamics. Fusion helps mix mitochondrial contents, maintaining the integrity of mitochondrial function; on the other hand, it allows mitochondria to adapt better to changes in energy requirements within the cell and removes damaged mitochondria through the autophagic pathway when the cell is damaged.

As a metabolic bone disease, osteoporosis is caused mainly by an imbalance in skeletal remodeling, in which the balance between osteoblasts and osteoclasts is disrupted, in which mitochondrial dynamics play an essential role. In terms of mitochondrial biogenesis, mitochondrial enzyme Mtu1 deficiency leads to a decrease in the efficiency of 2-thiolidine modification of mitochondrial tRNA in mouse BMSCs, which in turn impairs the translation of mtDNA, resulting in an imbalance in mitochondrial dynamics and osteoporosis in mice. In terms of mitochondrial fusion and division, several studies have elucidated the vital role of mitochondrial dynamics in the progression of osteoporosis. As a glucocorticoid that can cause osteoporosis, dexamethasone has been shown to disturb mitochondrial dynamics in BMSCs, resulting in increased mitochondrial division and decreased fusion, resulting in the inhibition of osteogenic differentiation. In vivo studies focused on PKM2, a key protein in glycolysis, and C3k significantly promoted the osteogenic differentiation of BMSCs by promoting mitochondrial fusion via the inhibition of PKM2 expression. In vivo experiments revealed that C3k significantly attenuated bone loss in the femurs of ovariectomized (OVX) rats, demonstrating that PKM2 is a target for intervention in osteoporosis treatment through mitochondrial dynamics [89]. Therefore, elucidating the role of this altered mitochondrial dynamic pattern and exploring whether its modulation could restore bone homeostasis warrants further investigation.

Mitochondrial dynamics in osteoclasts also affect the occurrence and development of osteoporosis. Glutathione (GSH) is one of the most important intracellular ROS scavengers, and GSH can reduce mitochondrial development and OC differentiation in OVX mice by inhibiting the CREB signaling pathway, thereby improving osteoporosis. MFN2-mediated mitochondrial fusion has also been found to be essential for osteoclast (OC) production, and the knockout of MFN2, which targets OCs, can significantly improve bone analysis parameters and age-related osteoporosis in aged female mice compared with controls [90]. These studies suggest that bone resorption can be reduced by targeting mitochondrial dynamics during osteoclast formation, providing a new perspective for pharmacological intervention in osteoporosis.

As one of the most common joint diseases, osteoarthritis (OA) is characterized by a series of pathological changes in cartilage, bone, and surrounding tissues that eventually lead to joint dysfunction, and an intact mitochondrial structure is considered a prerequisite for the normal survival of chondrocytes. Defects in mitochondrial function have also been found in cartilage-related diseases such as OA, so the relationship between mitochondrial dynamics and OA has gradually attracted attention. The onset and progression of OA are associated with the accumulation of oxygenated stress and senescent chondrocytes (SnCs), and endothelin-1 (ET-1) overexpression leads to an increase in the number of SnCs and an increase in intracellular mitochondrial division [91]; however, there is an overfusion phenomenon in senescent and OA chondrocytes with increased MFN2 expression, and this uncoupling phenomenon of mitochondrial dynamics can lead to cartilage damage. Moreover, moderate mechanical stress promotes mitochondrial dynamics by increasing the expression of MFN1/2 and OPA1 and the translocation of Drp1 from the cytoplasm to mitochondria, which can ameliorate the adverse effects on the apoptosis of chondrocytes in OA. Mitochondrial kinetic uncoupling can promote the occurrence and development of OA by increasing mitochondrial division or fusion alone, and strengthening the mitochondrial kinetic cycle as a whole may be an effective way to resist OA.

In recent years, a new target, transforming growth factor-β3 (TGF-β3), has been considered promising for the treatment of osteoarthritis. Kapetanakis et al. reported significantly higher serum TGF-β3 protein levels in patients with knee OA than in controls via enzyme-linked immunosorbent assay. An increase in TGF-β3 is positively correlated with pain, function, and the imaging stage of OA. A no-intervention cross-sectional trial is being conducted to assess the altered expression of the TGF-β/SMAD signaling pathway in patients with OA (NCT05218122). Notably, TGF-β3-mediated Smad2/3 signaling has antiproliferative and anti-inflammatory effects on young robust chondrons. In contrast, TGF-β3-induced Smad1/5/8 signaling has been implicated in controlling proliferation in pathological cartilage. The difference between the two mainly depends on the concentration of this growth factor. Low levels of TGF-β3 stimulate chondrocyte proliferation and induce type II collagen and proteoglycan deposition via the Smad2/3 signaling pathway [92]. These results indicate the therapeutic potential of targeted TGF-β therapy for arthritis. In addition, the utilization of TGF-β3 in chondrogenic tissue engineering should not be overlooked. An immobilized biodegradable TGF-β3 scaffold can be used to construct tissue-engineered cartilage effectively. It has been validated in animal models. Although a preclinical model of TGF-β3 has been established, it still needs to be perfected by later animal experiments. In addition, we cannot ignore the potential complications and adverse side effects of TGF-β3, such as problems caused by high doses of TGF-β3, including fibrosis of cartilage and noncartilaginous tissues, synovitis, abscess formation, cartilage erosion, and osteomalacia [93]. In a parallel, randomized, placebo-controlled, double-blind clinical trial (NCT03562429), TGF was shown to reliably significantly alleviate bone and cartilage progression by measuring changes from baseline and mitochondrial DNA in various organisms(Table 3).

Table 3.

Pharmacological mitochondrial therapeutics for skeletal diseases.

Reagent	Condition	Mechanism	Function	Effects
Resveratrol	Mice, intraperitoneal injection	Metabolic regulation, oxidation inhibition	Promotes osteogenic differentiation and mitochondrial biogenesis	Treatment of osteoporosis and its associated fractures
Metformin	Mice, intraperitoneal injection	Metabolic regulation, oxidation inhibition	anabolism	Promotes osteogenesis and treats diabetic bone injuries
Rapamycin	Mice, subcutaneous injection, intraperitoneal injection	Metabolic regulation	Regulates osteogenic differentiation	Treatment of osteoporosis
NAC	mammal, take orally, subcutaneous injection	oxidation inhibition	anabolism, Anti-inflammatory effect	Delay bone loss and treat osteoarthritis
a-LA	Mice, take orally	oxidation inhibition	anabolism, Anti-inflammatory effect, Anti-osteoclasts	Prevention and treatment of multiple bone loss
Vitamin C	human, take orally or Autosecretion	oxidation inhibition	Anti-bone resorption and anabolism	Prevents bone loss
Peroxidase	Mice, targeting mitochondria only	oxidation inhibition	Anti-bone resorption	Protects osteoblast function and improves osteogenesis inhibited by oxidative stress
MOTS-c	Mice, intraperitoneal injection	Metabolic regulation	Antiresorptive	Treats bone loss
SRT2104	Mice, take orally	Metabolic regulation	anabolism	Delay bone aging

5.5. Targeting mitochondrial biogenesis

We typically approach therapeutic strategies for mitochondrial biogenesis from two primary perspectives: biochemistry and energy metabolism [7], and the key pathway for mitochondrial biogenesis is PGC-1α. Regular physical activity enhances the biogenesis of mitochondria in a wide variety of tissues, thereby meeting the demand for ATP; in response to the “signal” the cell receives, more mitochondria are produced to fulfill more energy requirements.

High-intensity interval training improves glucose utilization, contributes to the prevention of various metabolic diseases, and increases the levels of cytosolic energy-converting factors such as PGC-1α, AMPK, SIRT1, and ROS in skeletal muscle. Exercise can redeem age-related fragmentation of the mitochondrial network, as exercise increases the expression and activity of PGC-1α, which promotes mitochondrial biogenesis and functional upregulation. AMPK is another key factor that regulates energy homeostasis in cells and is a high-level regulator of PGC-1α. Since AMPK is a core regulator of the metabolism of both lipids and glucose, it represents a promising therapeutic goal for the treatment of age-related diseases, including osteoarthritis and osteoporosis, which are associated with bone aging. The overexpression of serine/threonine kinase 11 (STK11) has been shown to prevent glucocorticoid-induced osteoporosis by activating the AMPK/SIRT1/PGC1α axis [94].

Research has shown that intermittent fasting inhibits mTOR activity, diminishes Sirt3 expression, and increases the ratio of NAD/NADH in cells, thereby activating AMPK/PGC-1α. In addition, calorie-restricted (CR) diets have been shown to improve mitochondrial biogenesis through the activation of AMPK. Dietary support and prospective drugs called caloric restriction mimetics or energetic restriction mimetics are a hypothetical class of drugs that could mimic the significant antiaging efficacy of CR. Studies have shown that copper can be used as a dietary supplement for bone metabolism, with favorable results in slowing bone mineral loss and reducing the levels of resorption markers [95]. As a CR mimetic, reservoir resveratrol improves muscle performance in elderly individuals via increased mitochondrial mass and enhancement of Nrf-1 and TFAM. Further investigations revealed that resveratrol treatment (5 μM) during the osteogenic differentiation of PO-MSCs increased the mitochondrial mass and mtDNA copy number, suggesting that resveratrol promotes mitochondrial biogenesis during the osteogenic differentiation of PO-MSCs and can be used as an adjunctive therapeutic agent for osteoporosis and/or osteoporotic fractures [96] (Fig. 8).

Fig. 8.

Advanced summary of signaling pathways and regulatory mechanisms in osteoporosis. The regulatory mechanism of osteoporosis is relatively complex, and representative substances of different signaling pathways have been identified. The signaling molecule AMPK can mediate or directly act on mitochondria while also inhibiting the activity of mTOR. HIF-1α-BNIP3 can also participate in the regulation of osteoporosis, and the intracellular protein PGC-1α can inhibit the activity of BNIP3, thereby participating in the regulation of osteoporosis. In addition, the SIRT family of proteins plays an important role. SIRT6 acts directly on mitochondria, and SIRT3 works by regulating the PINK1/parkin signaling pathway.

As a cellular mechanism that generates new mitochondria, mitochondrial biogenesis helps ensure tissue homeostasis. Pharmacological triggers that activate mitochondrial biogenesis have emerged as effective tactics to alleviate diseases characterized by mitochondrial dysfunction [86]. As a nodal regulator of mitochondrial biogenesis, PGC-1α integrates upstairs signals and initiates the progression of downstream mitochondrial genes, thereby promoting mitochondrial biogenesis, and has also become a direct target of drug intervention. Arthritis includes osteoarthritis (OA), a chronic musculoskeletal disease, and rheumatoid arthritis (an autoimmune disease), both of which are closely related to the structure and organization of the extracellular matrix (ECM) of chondrocytes. Approximately 0.5–1.0% of the world’s population suffers from RA, which can lead to chronic joint damage and complications, and OA affects mainly older people, leading to joint degeneration and severe pain. Existing medications, such as nonsteroidal anti-inflammatory drugs (NSAIDs) and corticosteroids, do little to ease the progression of arthritis and can even lead to systemic side effects, such as gastrointestinal adverse effects caused by long-term use of NSAIDs [97]. Moreover, we need to pay attention to the crucial role of mitochondria in chondrocyte metabolism: mitochondria not only provide chondrocytes with the necessary ATP but also participate in the regulation of cellular homeostasis, such as redox, the balance of intracellular calcium ions and mitochondrial biogenesis.

Upstream receptors react to disturbances in the cellular energy profile and nutrient availability [6]. For example, AMPK is known to be an important modulator of the metabolism of both glucose and fatty acids; it becomes activated under energy-deficient conditions and is recognized for its high AMP/ATP ratio. This stimulation facilitates the biogenesis of mitochondria by promoting the activity of PGC-1α through its translocation to the core of the cell [98]. Within the downstream spectrum, various translational factors and coreceptors respond to these upper-stream signals, coordinating the expression of nuclear and mitochondrial genes and thus regulating mitochondrial biogenesis [6]. Some of these include nuclear respiratory factors 1 and 2 (NRF1 and NRF2), which determine turnover in many nuclear-encoded mitochondrial genes, as well as the peroxisome proliferator-activated receptor (PPAR), which controls lipid metabolism and mitochondrial function. Estrogen-related receptors (ERRs), which are required for oxidative phosphorylation, also play a vital role in synchronizing gene expression.

A few pharmacological agents that can modulate these regulatory nodes, thus potentiating mitochondrial biogenesis, have been identified. For example, 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) and PXL770 can act as activators of AMPK. AICAR stimulates AMPK through mimicking the action of AMP, thereby promoting the transcriptional activity of the coactivator PGC-1α and ultimately enhancing mitochondrial biogenesis. In addition to its effectiveness in treating diabetes, an additional compound, metformin, also increases mitochondrial biogenesis through the activation of AMPK, leading to a surge in PGC-1α exposure and mobilization, thereby treating osteoporosis. In addition, sirtuin-activating compounds (STACs), including resveratrol, increase the activity of the SIRT1-PGC-1α axis, and appropriate doses of resveratrol increase ALP activity and calcium deposition, suggesting that resveratrol enhances osteogenesis [96]. These compounds highlight the use of strategically placed pharmacological interventions to augment mitochondrial biogenesis in the presence of mitochondrial dysfunction through actions on specific targets.

5.6. Mitochondrial supplementation therapy

In recent years, mitochondrial supplementation therapy has emerged as a promising therapeutic strategy for a variety of diseases, particularly those associated with mitochondrial dysfunction and cellular aging.

Mitochondrial supplementation therapy primarily consists of two forms: direct mitochondrial supplementation and intercellular mitochondrial transfer therapy. It has been shown that exogenous mitochondria can be taken up by receptor cells through a variety of pathways and restore their function within the cell. Thus, inspired by this, mitochondrial supplementation therapy helps to restore mitochondrial energy metabolism and alleviate cellular dysfunction by delivering functional mitochondria to target cells or tissues. At the core of this therapy is the selection of an appropriate mitochondrial source, which not only relates to the functional integrity of the mitochondria, but also has a direct impact on delivery efficiency, immunocompatibility, and feasibility of clinical translation [99].

Osteolineage cells, including osteoprogenitor cells, pre-osteoblasts, and mature osteoblasts, play a central role in bone formation and remodeling. In recent years, increasing evidence has revealed that these cells not only serve as the primary drivers of osteogenesis but also possess the ability to transfer functional mitochondria to other cell types, particularly in the regulation of bone resorption within the bone microenvironment. Recent studies have shown the existence of mitochondrial transfer between bone cells. A study published in Nature reveals for the first time that osteoblastic lineage cells can regulate myeloid-mediated bone resorption through mitochondrial transfer [100]. The study found that during the process of bone remodeling, bone progenitor cells were able to transfer functional mitochondria to myeloid cells (such as osteoclast precursor cells), thereby inhibiting their overactivation and reducing bone resorption. This discovery not only unveiled a novel intercellular metabolic communication mechanism in bone homeostasis but also laid a solid theoretical foundation for the application of mitochondrial supplementation therapy in bone metabolic diseases [100,101].

Of course, there will be other sources of mitochondria, such as the common stem cell sources, platelet sources, and artificially engineered mitochondrial sources. MSCs are one of the most widely studied and promising mitochondrial donors, and MSCs are known for their immunomodulatory properties and their ability to donate mitochondria to damaged cells under stressful conditions [102]. The advantage of platelet-derived mitochondria, on the other hand, is that they are abundant and readily available in the blood. Recent studies have shown that the platelet contains high-quality mitochondria, but the shorter lifespan and lower metabolic activity of its mitochondria after transfer are still current problems [103]. To overcome the limitations of natural mitochondrial sources, researchers have begun to explore artificially engineered mitochondrial delivery systems, including liposomes, nanoparticles, bionic vesicles and exosomes [104]. Currently, exosomes loaded with mitochondria or their components have been found to significantly attenuate the inflammatory response in models of myocardial infarction and neurodegenerative diseases.

Although mitochondrial complementary therapies have shown therapeutic potential in many diseases, many challenges still exist before they can be truly applied in the clinic. Existing studies have shown that only a small amount of mitochondria can be taken up and function by the recipient cells after direct mitochondrial injection. In addition to the lack of delivery efficiency, the difficulty in achieving precise targeting is not a small issue. Furthermore, mitochondria contain their own DNA (mtDNA) which is antigenic in nature. Especially when using mitochondria from allogeneic or xenogeneic sources, an immune response may be triggered, leading to the release of inflammatory factors or even acute rejection [15]. In the future, we believe that with the optimization of the mitochondrial delivery system and precise target modulation, this therapy will provide a new intervention strategy for age-related bone diseases.

6. Conclusion and perspective

Bone aging is not merely a chronic functional decline but a multifaceted process driven by a cascade of mitochondrial dysfunctions that disrupt cellular homeostasis and tissue integrity. As the major source of power for skeletal cells, the quality control of mitochondria is of utmost importance. In addition, mitochondria, as the power source of cells, play important roles in bone aging, bone remodeling, and bone disease. As detailed throughout this review, mitochondrial dysfunction initiates a cascade of pathogenic events—including oxidative damage, chronic inflammation via mtDNA release, and failed quality control—that directly disrupt bone homeostasis and drive the progression of age-related skeletal diseases. Here, we summarize a series of mechanisms of mitochondrial dysfunction and mitochondrial quality control by reviewing the literature on the relationship between mitochondria and bone aging in recent years and summarize the clinical drug development and treatment options for bone-related diseases involving mitochondria with the aim of developing precision therapies.

Mitochondrial dysfunction is a hallmark of aging. There are two aspects of mitochondrial dysfunction: mitochondrial oxidative stress, the mitochondrial DNA-related cGAS‒STING pathway, and inflammasomes. Among them, excess ROS produced by mitochondrial oxidative stress play important roles in many bone aging diseases. Moreover, both pathways involving mtDNA are involved in the development of inflammation. Here, we describe the mechanism of action that produces inflammation. From the perspective of mitochondrial quality control, we subsequently explored three mechanisms of action in the bone aging process: mitophagy, mitochondrial dynamics and mitochondrial protein homeostasis. A review of the relevant literature revealed that there are signaling pathways associated with the control of mitophagy in bone aging diseases that can be classified into PRKN-dependent and PRKN-nondependent pathways. In particular, the PINK1/Parkin pathway is the most widely studied pathway for mitophagy signaling. Nevertheless, other mitophagy-related signaling pathways may exist that need to be further investigated in the context of bone-related diseases. Mitochondrial dynamics has been a key research direction in recent years, and here, we discuss the effects and mechanisms of mitochondrial biogenesis, fusion and fission on bone aging diseases. Finally, we analyze a range of mechanisms of action for mitochondrial protein homeostasis.

Finally, we focus on the clinical application of these methods. While some drugs have been approved for the treatment of bone aging-related diseases, our findings suggest that their targets can be further improved to reduce bone and extraosseous side effects. Many drugs, while therapeutic, are prohibitive owing to their substantial side effects and high cost of use. From the perspectives of mitophagy, mitochondrial biogenesis, mitochondrial dynamics, oxidative stress, etc., we reviewed the existing clinical trials and literature and summarized the new therapeutic targets developed in recent years, with the goal of facilitating clinical applications. We believe that more bone-targeted therapies will be utilized as clinical treatments for skeletal diseases under the impetus of precision medicine.

In conclusion, we summarize the mechanism of action of mitochondrial dysfunction and mitochondrial quality control in bone aging and summarize the latest progress in the preclinical development and clinical application of drugs for the treatment of osteoarthritis and osteoporosis. We hope that this work contributes to the development of new targeted therapeutic strategies for the treatment of bone aging diseases.

However, although the therapeutic strategies targeting mitochondrial dysfunction that we have discussed show some potential to improve mitochondrial function, they still face many limitations and challenges in clinical translation. Firstly, many mitochondria-targeted drugs lack sufficient specificity, making it difficult to act precisely on the receptor to make a difference. Secondly, in the previous presentations, delivery systems for mitochondria-targeted therapies are still incomplete, and there is a technical bottleneck especially in the effective introduction of large molecules such as RNA or proteins into mitochondria. In addition, mitochondrial diseases are often genetically and phenotypically highly heterogeneous. This diversity complicates the development of “one-size-fits-all” therapies and requires personalized treatment regimens. Meanwhile, the lack of reliable biomarkers for early diagnosis and efficacy assessment also limits the precision and monitorability of treatment.

Therefore, in the future, we need to push mitochondrial therapeutic strategies toward greater efficiency, precision, and safety. Firstly, mitochondria-targeted delivery systems should be vigorously developed, and priority should be given to the research and design of novel delivery vehicles (such as membrane-penetrating peptides, nanoparticles, or lipid carriers) to deliver therapeutic substances to mitochondria in different tissues in an efficient and selective manner. Secondly, advancing mitochondrial gene editing technology is a key breakthrough in the treatment of inherited mitochondrial diseases. Exploring precision editing tools for mitochondrial DNA (mtDNA) to enable root cause therapy for inherited mitochondrial diseases. In addition, advances in genomics, transcriptomics, and metabolomics are being utilized to tailor therapeutic regimens to the genetic and metabolic profiles of individuals in order to improve the treatment of heterogeneous mitochondrial diseases. Finally, exploring combined therapeutic strategies to address the complexity of mitochondrial dysfunction. A growing amount of research suggests that it is often difficult to fully restore mitochondrial function with a single intervention. Multi-targeted combination therapies combining antioxidants, metabolic precursors, mitochondrial autophagy activators, and gene therapy may demonstrate superior therapeutic efficacy in neurodegenerative diseases, aging-related disorders, and cancer. We believe that through interdisciplinary collaborative innovation, mitochondrial therapy will move from basic research to clinical translation, providing new therapeutic hope for a variety of major diseases (Table 4).

Table 4.

Mitochondrial therapeutic strategies: challenges and future perspectives.

Category	Key challenges	Future directions
Therapeutic specificity	Many mitochondria-targeted drugs lack sufficient specificity, leading to off-target effects and reduced efficacy	Develop highly specific delivery systems to precisely target mitochondrial components across different tissues
Delivery systems	Current delivery methods are inefficient, especially for large molecules (e.g., RNA, proteins); limited ability to cross mitochondrial membranes	Advance novel delivery vehicles, to enable efficient and selective mitochondrial delivery
Genetic and phenotypic heterogeneity	Mitochondrial diseases exhibit high genetic and clinical diversity, making standardized therapies ineffective	Combining genomics, transcriptomics and metabolomics to develop personalized treatment plans based on individual genetic and metabolic profiles
Gene editing	No effective tools for precise editing of mitochondrial DNA (mtDNA); inherited mutations remain difficult to correct	Accelerate the research and development of mitochondrial gene editing technology, to realize the root cause treatment of hereditary mitochondrial diseases
Diagnosis & monitoring	Lack of reliable biomarkers for early detection and treatment response assessment	Discover and validate non-invasive biomarkers to improve diagnostic accuracy and therapeutic monitoring
Therapeutic complexity clinical translation	Single-target interventions often fail to restore mitochondrial function due to the multifaceted nature of dysfunction Gap between preclinical success and clinical application remains wide.	Explore combination therapies Foster interdisciplinary collaborative innovation

CRediT authorship contribution statement

Yu Zhang: Writing – original draft, Methodology, Data curation. Xishui Liu: Writing – review & editing, Visualization, Validation, Supervision, Data curation. Zijie Xiang: Writing – review & editing, Investigation, Formal analysis. Yuqing Yang: Visualization, Formal analysis. Lei Xing: Methodology, Data curation. Yu Chen: Methodology, Formal analysis. Siming Zhang: Supervision, Formal analysis. Shixiang Zhao: Supervision, Resources, Methodology, Funding acquisition. Youzhi Hong: Visualization, Validation, Supervision, Formal analysis, Conceptualization. Yusen Qiao: Writing – review & editing, Visualization, Validation, Supervision, Funding acquisition, Formal analysis. Jiaxiang Bai: Validation, Supervision, Methodology, Investigation, Funding acquisition.

Declaration of competing interest

The authors declare that they have no conflicts of interest in this work.

Biographies

Yusen Qiao earned his Ph.D. degree at Soochow University. He then pursued post-doctoral research at the First Affiliated Hospital of Soochow University. He serves on the Editorial Boards of Innovation Medicine, Exploration, Cell Proliferation, Biomaterials Translational. Dr. Qiao’s investigations focus on elucidating the mechanisms governing bone defect repair within inflammatory milieus and on devising innovative strategies to amplify therapeutic efficacy.

Jiaxiang Bai obtained his Ph.D. degree at Soochow University. Dr. Bai is a Managing Editor for Smart Materials in Medicine (Elsevier), and serves on the Editorial Boards of Research (Science Partner Journal), Journal of Orthopedic Translation, BMC Musculoskeletal Disorders, and VIEW. Prof. Bai’s research focuses on developing advanced biomaterials for orthopedic applications and regenerative medicine.