Cyclic tensile loading regulates nucleus pulposus cell autophagy through mitochondrial dynamics: molecular mechanisms and implications

Abstract

Background

Chronic low back pain is predominantly driven by intervertebral disc degeneration (IVDD), a process rooted in the dysregulation of extracellular matrix (ECM) homeostasis within nucleus pulposus (NP) cells. These cells reside in a harsh microenvironment characterized by hypoxia, nutrient scarcity, and mechanical stress, making the regulatory pathways of autophagy and mitochondrial dynamics critical for their survival and function.

Main Body

This review synthesizes current evidence demonstrating that cyclic tensile loading (CTL) is a decisive factor directing NP cell fate through the coupling of mitochondrial fission–fusion dynamics and autophagic flux. We delineate a dual mechano-response: moderate, physiological CTL (approximately 5–10% strain, based primarily on in-vitro models) promotes cytoprotective autophagy and mitochondrial fusion via AMPK/mTOR and integrin–FAK signaling, supporting ECM synthesis. This involves activation of TFEB-driven lysosomal biogenesis and PINK1–Parkin-mediated mitophagy. Conversely, supraphysiological, pathological CTL (> 15–20% strain) triggers DRP1-dependent mitochondrial fission, activates PINK1–Parkin pathways alongside ROS/JNK signaling, and induces BNIP3-associated autophagic dysfunction. This cascade leads to inflammasome activation, cellular senescence, apoptosis, and ultimately ECM catabolism. We further dissect key molecular transducers, including Piezo1, HIF-1α/BNIP3, and the cytoskeleton, which convert mechanical stimuli into autophagic responses. The pivotal duality of autophagy—protective versus cytotoxic—is shown to hinge on the maintenance of mitochondrial dynamic equilibrium, the disruption of which accelerates IVDD.

Conclusions

The integration of biomechanical, mitochondrial, and autophagic axes provides a novel framework for understanding IVDD pathogenesis. This synthesis identifies promising therapeutic targets, such as DRP1, mitophagy regulators, and SIRT3, which have shown potential in preclinical models to decouple pathological mechano-signaling and preserve NP cell function. The review establishes a mechanistic rationale for developing interventions aimed at halting IVDD progression by modulating the cellular response to mechanical stress.

Method

We performed a focused, reproducible search of PubMed/MEDLINE, Embase, Scopus, and Web of Science for literature published through October 1, 2025 using terms such as “nucleus pulposus,” “intervertebral disc,” “cyclic tensile loading,” “mitochondria,” “mitophagy,” “autophagy,” “DRP1,” “PINK1,” and “BNIP3.” Reference lists of retrieved articles and recent reviews were hand-searched. We included original experimental work (in vitro, ex vivo, animal, and human tissue studies) and narrative/systematic reviews when NP-specific data were limited. We excluded non-English articles without English abstracts, conference abstracts without accessible full text, and purely computational studies lacking experimental validation. Because this is a narrative review we did not perform PRISMA-style quantitative meta-analyses or formal quality grading.

FACTS

• The precise strain thresholds that separate adaptive from pathological responses remain undefined. While 5–10% strain is broadly considered physiological and > 15% strain is commonly used to denote pathological loading, these numeric thresholds are mostly derived from in vitro cyclic tensile/stretch experiments (e.g., Flexcell systems and isolated human/rodent NP cell stretch studies) and are only partially corroborated by in vivo compression/overload models. Differences in loading mode (tension vs. compression), frequency, duration and species make direct translation to human in vivo thresholds uncertain.

• The role of HIF-1α in mechanotransduction is contextually paradoxical. HIF-1α is essential for NP cell survival in hypoxia and can promote protective mitophagy via BNIP3. However, under mechanical overload, its sustained activation may contribute to a metabolic shift and apoptotic signaling. The factors determining this dual outcome are a critical unknown.

• The specificity and off-target effects of key pharmacological tools, like the DRP1 inhibitor Mdivi-1, complicate data interpretation. Mdivi-1’s additional inhibition of mitochondrial complex I confounds the separation of DRP1-mediated fission effects from general bioenergetic suppression, necessitating more specific inhibitors or genetic models.

• The spatiotemporal coordination between mitochondrial fission/fusion dynamics and autophagosome-lysosome fusion is poorly resolved. It is unclear how the cell prioritizes the clearance of fragmented mitochondria via mitophagy versus their re-integration via fusion following the cessation of mechanical stress.

• The in vivo transition from acute mechanical injury to chronic disc degeneration is not fully mapped. While in vitro models show clear sequences of events, the timeline and key drivers of the perpetuation of pathology, specifically, how senescent cells and the SASP create a self-sustaining degenerative microenvironment, require further elucidation.

Introduction

Chronic low back pain is a major global health problem, commonly driven by intervertebral disc degeneration (IVDD). In the United States the combined economic burden from treatment and lost productivity exceeds US$100 billion annually, and radiographic signs of disc degeneration are present in most older adults. These facts underscore the need for mechanistic therapies that go beyond symptom control and fusion surgery [1–3]. The intervertebral disc (IVD) is an avascular, load-bearing organ with three principal regions: (i) the central, gelatinous nucleus pulposus (NP); (ii) the surrounding lamellar annulus fibrosus (AF); and (iii) the cartilaginous endplates (CEPs) that contact vertebral bodies. These regions act together to distribute spinal loads [1, 2]. NP cells (notochordal lineage) live in a harsh microenvironment: low oxygen (≈ 2–5% O₂), mildly acidic pH (~ 6.8), poor nutrient diffusion, and variable osmotic pressure. Under these conditions NP cells rely mainly on glycolysis for ATP production [1, 3]. NP cells maintain ECM homeostasis by producing key macromolecules, chiefly the aggregating proteoglycan aggrecan and type II collagen. These molecules provide hydration-dependent viscoelasticity that is essential for load distribution and normal spinal mobility [2, 4]. However, NP cells exhibit heightened vulnerability to deviations from physiological mechanical loading, triggering ECM catabolism through upregulated matrix metalloproteinases (MMPs) and A disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS) enzymes, thereby initiating the degenerative cascade [2, 4, 5].

Autophagy, an evolutionarily conserved lysosomal degradation pathway, plays a context-dependent role in IVDD pathophysiology. Basal autophagy protects NP cells by removing damaged organelles and misfolded proteins. However, when autophagy is excessive or blocked at late stages it can become harmful and contribute to cell death and disc degeneration [3, 6, 7]. Molecular evidence from human degenerate discs reveals altered expression profiles of autophagy markers, including elevated LC3-II/LC3-I ratios (indicating autophagosome formation) and dysregulated p62/SQSTM1 levels (reflecting impaired autophagic flux), underscoring autophagy’s clinical relevance in IVDD [6–8]. The functional duality of autophagy, cytoprotective versus cytotoxic, depends critically on its integration with mitochondrial quality control pathways, particularly mitochondrial dynamics, which govern organelle morphology, distribution, and function [7, 9, 10].

Mitochondrial dynamics encompass the opposing processes of fission and fusion, finely tuned to metabolic demands and cellular stress. Fission, mediated by dynamin-related protein 1 (DRP1) recruitment to mitochondrial membranes via adaptors (Mitochondrial fission 1 protein (FIS1), Mitochondrial fission factor (MFF), MID49/51), generates fragmented mitochondria that facilitate the segregation of damaged components for mitophagic clearance [9–11]. Fusion, orchestrated by mitofusins (MFN1/2) at the outer mitochondrial membrane and optic atrophy protein 1 (OPA1) at the inner membrane, promotes mitochondrial network connectivity, enabling content mixing, mitochondrial DNA (mtDNA) repair, and optimization of ATP synthesis and redox balance [9, 10]. In the physiologically hypoxic NP, mitochondrial dynamics assume heightened importance, as dysregulated fission-fusion equilibrium exacerbates reactive oxygen species (ROS) generation, metabolic dysfunction, and inflammasome activation, culminating in NP cell senescence or apoptosis [1, 3, 7].

Cyclic tensile loading (CTL), a physiologically relevant mechanical stimulus, modulates NP cell viability and metabolism in a parameter-dependent manner. Routine spinal movements subject NP cells to variable tensile strains. In experimental systems, strains of ~ 5–10% are often treated as ‘physiological’, whereas > 15% is used to model overload, but these numeric cut-offs depend on the model, frequency and duration of loading [2, 4, 12]. Moderate-magnitude CTL (e.g., 5–10% strain) promotes ECM anabolism and cellular homeostasis via mechanotransductive pathways involving integrin-mediated activation of FAK-MEK5-ERK5 signaling [2, 4]. In stark contrast, supraphysiological CTL (> 15–20% strain) induces pathological responses, including cytoskeletal disassembly, ROS overproduction, and sustained endoplasmic reticulum (ERS) stress, ultimately triggering mitochondrial dysfunction and apoptosis [4, 6, 12]. Recent studies demonstrate that high-magnitude CTL (e.g., 20% strain at 1 Hz) disrupts mitochondrial dynamics, inducing DRP1-dependent fission, loss of membrane potential (ΔΨm), and PINK1-Parkin-mediated mitophagy in NP cells [6, 7, 13]. Mechanical overload activates mechanosensitive channels (e.g., Piezo1) and triggers Ca²⁺ influx; Piezo1 involvement is now reported in NP/IVD contexts but much of the mechanistic detail (downstream Ca²⁺→DRP1→fission→NLRP3) has been characterized in cartilage or other cell types (cartilage-related / cross-tissue inference). We therefore distinguish NP-specific reports (cited below) from cartilage/other-tissue studies when discussing downstream kinase and inflammasome activation [2, 4]. Key molecular regulators mediating the interactions between CTL, mitochondrial dynamics, and autophagic outcomes in NP cells are summarized in Table 1.

Table 1.

Key molecular regulators linking CTL, mitochondrial dynamics, and autophagy in NP cells

Mechanotransducer	Mitochondrial target	Autophagic outcome	Functional consequence
Piezo1 Channel	Calcium influx → DRP1 phosphorylation	BNIP3L/NIX-mediated mitophagy	ECM catabolism under high strain [2, 4]
Integrin-FAK Axis	ERK5-dependent MFN2 expression	Suppression of excessive mitophagy	ECM preservation under moderate strain [2, 4]
ROS/ERS Pathways	MFF recruitment of DRP1	PINK1-Parkin mitophagy initiation	Senescence/apoptosis acceleration [3, 6]
RhoA/ROCK Signaling	F-actin remodeling → mitochondrial trafficking	Modulation of autophagosome-lysosome fusion	Strain magnitude-dependent cell fate [12, 13]
HIF-1α Stabilization	Metabolic shift to glycolysis	Mitophagy suppression in hypoxia	NP cell survival in avascular niche [1, 3]

Despite these advances, a critical knowledge gap persists regarding how mitochondrial dynamics serve as a biomechanical transducer, converting CTL signals into graded autophagic responses in NP cells. No comprehensive review has integrated the biomechanical, organelle biological, and molecular dimensions of this regulation, leaving unresolved questions about, The spatiotemporal coordination of fission/fusion machinery in response to varying CTL parameters (magnitude, frequency, duration) [6, 12, 13], Strain-sensitive regulators of mitophagy selectivity (e.g., FUNDC1, BCL2L13) in the hypoxic disc niche [1, 7], Crosstalk between cytoskeletal remodeling (e.g., F-actin reorganization via RhoA/ROCK) and mitochondrial-lysosomal networks in autophagic flux completion [10, 12, 13], and the paradoxical role of autophagy in IVDD, whereby physiological CTL promotes cytoprotective autophagy, but pathological CTL switches autophagy toward self-digestive pathways [6, 7].

By clarifying the mechanobiological interactions between CTL, mitochondrial dynamics, and autophagy, this review seeks to synthesize recent developments in our understanding of how CTL regulates NP cell autophagy through mitochondrial dynamics. This will lay the groundwork for new regenerative strategies that target the molecular intersection of mechanical signaling and organelle homeostasis in order to slow or even reverse the progression of IVDD. Table 2 flags the tissue origin of the key supporting data, assigns a conservative evidence strength for NP (Strong/Moderate/Weak) and lists main limitations that underlie the cross-tissue inference.

Table 2.

Evidence map for key mechanistic claims

Mechanistic claim	NP/IVD-specific studies	Cartilage / other-tissue studies	Evidence strength (NP)	Key limitations
CTL magnitude → DRP1 activation → mitochondrial fission	Some in-vitro NP stretch studies [14]	Multiple studies in chondrocytes and cardiomyocytes show ERK/DRP1 axis [15, 16]	Moderate	Many mechanistic phosphorylation / kinase links come from non-NP models; in-vivo NP data are limited.
Piezo1 activation → Ca²⁺ influx → DRP1 activation	Emerging NP/IVD papers show Piezo1 upregulation in degenerate discs [17, 18]	Robust evidence in chondrocytes showing Piezo1/TRPV4 crosstalk and Ca²⁺ signaling.	Moderate	Downstream steps (CaMKII → DRP1 Ser616) often inferred from non-NP cell mechanistic studies.
Mdivi-1 reverses CTL-induced fission / protects NP	Some rodent compression models report benefit but are often systemic or non-NP endpoints.	Beneficial/contradictory outcomes reported in cochlea, kidney, cancer models. (Reports vary.)	Weak–Moderate	Mdivi-1 has known off-target complex-I [19]
FUNDC1 Ser13 dephosphorylation promotes mitophagy under CTL	Little direct NP data for CTL-triggered FUNDC1 regulation	Strong mechanistic biochemical data in cell biology (PGAM5→FUNDC1 S13 dephosphorylation) [20].	Weak (NP)	Evidence based on non-NP mechanistic work; NP validation needed.
PINK1–Parkin mitophagy active under moderate CTL	Some NP in-vitro studies report PINK1/Parkin recruitment after depolarization.	Well-studied in many tissues (neurons, heart).	Moderate	Mechanistic confirmation in intact, loaded discs (in vivo) is sparse.

Autophagy & mitochondrial dynamics in NP homeostasis

Autophagy machinery in NP cells

Autophagy, a lysosome-dependent degradation pathway, is indispensable for maintaining NP cell homeostasis in the hypoxic, nutrient-poor IVD microenvironment. The process initiates with ULK1/AMPK complex activation following energy stress or Mechanistic Target Of Rapamycin complex 1 (mTORC1) inhibition. Under nutrient deprivation, Adenosine Monophosphate-activated Protein Kinase (AMPK) phosphorylates Unc-51 Like Autophagy Activating Kinase 1 (ULK1) at Ser⁷⁷⁷, bypassing mTOR-mediated suppression (Ser⁷⁵⁷ phosphorylation) and triggering phagophore formation [21, 22]. Nucleation requires the Beclin-1/VPS34 class III PI3K complex, where AMPK-dependent phosphorylation of Beclin-1 at BECN1-Ser⁹³/⁹⁶ enhances lipid kinase activity, promoting phosphatidylinositol-3-phosphate (PI3P) generation and phagophore membrane assembly [1, 22]. During elongation, the ubiquitin-like conjugation system facilitates LC3-II lipidation via ATG5-ATG12 complexes, enabling autophagosomal membrane expansion and cargo sequestration [1]. Terminally, lysosomal fusion involves Lysosomal-associated membrane protein 1 (LAMP1)-positive lysosomes docking to autophagosomes through SNARE proteins (e.g., STX17-SNAP29-VAMP8), culminating in autolysosome formation and cargo degradation [22, 23].

Key regulators fine-tune this cascade in NP cells such as mTORC1 integrates nutrient and mechanical signals, suppressing autophagy under physiological loading but becoming hyperactivated in degenerative discs, exacerbating ECM catabolism [1, 24], AMPK serves as an energy sensor activated by LKB1 during glucose deprivation; it stimulates autophagy via ULK1 phosphorylation and inhibits mTOR through TSC2/Raptor signaling [21, 25], and TFEB, the master transcriptional regulator of lysosomal biogenesis, translocates to the nucleus upon dephosphorylation by calcineurin, upregulating genes like BECN1, MAP1LC3B, and LAMP1 under oxidative stress [22, 24, 26]. Markers such as LC3-II and Beclin-1 report autophagosome biogenesis (initiation/elongation) while PINK1 reports mitophagy initiation; these markers alone do not demonstrate completion of autophagic/mitophagic flux. Robust evidence of impaired flux requires assays that measure cargo degradation or lysosomal competence, for example, p62/SQSTM1 turnover measured ± lysosomal inhibition, tandem mCherry-GFP-LC3 or mito-QC reporters, pH-sensitive mitophagy probes (mt-Keima), lysosomal pH/acidification measurements, or direct autolysosome counts by TEM [27–29]. The core regulators orchestrating these stages of autophagy in NP cells are detailed in Table 3.

Table 3.

Core autophagy regulators in NP cells

Stage	Key components	Function	Regulatory inputs
Initiation	ULK1 complex (ULK1, ATG13, FIP200)	Phagophore nucleation	AMPK (activator), mTOR (inhibitor)
Nucleation	Beclin-1/VPS34/PI3KC3	PI3P generation for membrane scaffolding	AMPK phosphorylation, Bcl-2 dissociation
Elongation	ATG5-ATG12/LC3-II	Cargo conjugation and membrane expansion	ATG7-mediated lipidation
Fusion	LAMP1/STX17/SNAP29	Autophagosome-lysosome docking	Ca²⁺ signaling, Rab GTPases

Key stages of autophagy initiation, nucleation, elongation, and fusion in NP cells are schematically illustrated in Fig. 1.

Fig. 1.

Schematic representation of the autophagy machinery in NP cells. The figure illustrates the stages of autophagy, initiation (ULK1 complex activation), nucleation (Beclin-1/VPS34/PI3K complex), elongation (ATG5–ATG12/LC3-II conjugation), and fusion (lysosome–autophagosome docking)—and highlights regulatory inputs such as AMPK, mTOR, and TFEB

Mitochondrial dynamics in IVD homeostasis and degeneration

Mitochondrial dynamics, fission and fusion, orchestrate mitochondrial morphology, distribution, and function in NP cells. Fission is driven by DRP1 recruitment to mitochondrial membranes via adaptors (FIS1, MFF, MID49/51), culminating in GTPase-dependent membrane scission. This process segregates damaged mitochondrial segments for mitophagic clearance [24, 30]. Conversely, fusion maintains network integrity: MFN1/2 mediates outer membrane tethering, while OPA1 stabilizes inner membrane cristae, facilitating mitochondrial content mixing and ATP/ROS balance [22, 24]. Physiological CTL at 5–10% strain sustains fusion-fission equilibrium, supporting mitochondrial metabolism and redox homeostasis [1].

In IVD degeneration (IDD), chronic overloading (≥ 20% strain) or inflammatory stress (e.g., TNF-α/IL-1β) dysregulates dynamics, skewing toward excessive fission. DRP1 is the principal GTPase driving mitochondrial fission; bursting of the fission–fusion balance toward fission contributes to fragmentation, bioenergetic decline, ROS production and downstream apoptotic or inflammasome signaling in degenerate discs [24, 30, 31]. Mechanistic regulation of DRP1 (kinase phosphorylation events, AMPK-dependent recruitment via the MFF receptor, and context-dependent pharmacology) is discussed in detail in Sect.  “Integrated signaling model, how CTL controls mitochondrial dynamics and autophagy”–“Experimental evidence from in vitro and in vivo models” below. The balance between mitochondrial fission and fusion under physiological and pathological conditions is shown in Fig. 2.

Fig. 2.

Mitochondrial dynamics in IVD homeostasis and degeneration. Physiological CTL maintains a fusion–fission balance via MFN1/2 and OPA1, whereas pathological stress drives excessive DRP1-mediated fission, cytochrome c release, ROS production, and apoptosis

Autophagy-mitochondrial crosstalk in mechanotransduction

Mitochondria and autophagy engage in bidirectional crosstalk critical for NP cell adaptation to mechanical stimuli. Mitophagy, the selective autophagic clearance of damaged mitochondria, is governed by receptor-specific pathways, (1) PINK1/Parkin axis: Depolarized mitochondria accumulate PINK1 on the outer membrane, phosphorylating ubiquitin (Ser⁶⁵) and recruiting Parkin. Parkin ubiquitinates VDAC1 and MFN, recruiting optineurin and p62 to initiate LC3-mediated engulfment [22]. PINK1 or Parkin accumulation is an initiation signal and does not by itself demonstrate completed mitophagic flux. Confirmation requires assays that detect delivery of mitochondrial cargo to lysosomes (for example, mt-Keima / mito-QC reporter readouts, tandem reporters or p62 turnover with lysosomal blockade). Authors citing PINK1/Parkin studies should indicate whether the primary data measured initiation only (PINK1 accumulation, ubiquitination, LC3 co-localisation) or actual cargo degradation [29, 32]. (2) Receptor-mediated pathways: Hypoxia-inducible receptors like BNIP3 and FUNDC1 directly bind LC3 via LIR motifs. BNIP3 displaces Bcl-2 from Beclin-1, concurrently activating mitophagy and non-selective autophagy [25, 30, 33]. CTL modulates these pathways: moderate strain (5–10%) enhances PINK1 stabilization and FUNDC1 phosphorylation (Ser¹⁷), promoting protective mitophagy, whereas excessive strain suppresses Parkin translocation via cytoskeletal disruption [1, 30].

Beyond degradation, mitochondria supply membranes for autophagosome biogenesis. Mitochondria-derived vesicles (MDVs), generated under mild oxidative stress, deliver phospholipids (e.g., phosphatidylserine) to expanding phagophores via ATG2B-VPS13D complexes [24, 30]. Additionally, metabolic feedback occurs: mitophagy removes ROS-generating mitochondria, reducing oxidative inhibition of ATG4B and sustaining LC3 processing. Conversely, autophagy deficiency impairs mitochondrial turnover, leading to mtDNA release and cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING)-driven inflammation in degenerated discs [1, 34]. The major mitophagy receptors and their regulation in NP cells under various triggers are outlined in Table 4.

Table 4.

Mitophagy receptors and their regulation in NP cells

Receptor	Activation trigger	Mechanism	Functional outcome
PINK1/Parkin	Mitochondrial depolarization	PINK1 phosphorylates ubiquitin → Parkin recruitment → Ubiquitination of OMM proteins	Clearance of severely damaged mitochondria
BNIP3	Hypoxia/ROS	Displaces Bcl-2 from Beclin-1 → Autophagy activation; Binds LC3 via LIR domain	Mitophagy and autophagy synergy
FUNDC1	Cyclic strain	CTL-induced dephosphorylation (Ser¹³) → Enhanced LC3 interaction	Mechanical stress adaptation
BNIP3L/NIX	Nutrient stress	Recruits GABARAP via LIR motif; Cooperates with PINK1	Erythropoietic remnant clearance

Biomechanical regulation of NP Cells

IVD mechanobiology: dual roles of mechanical loading

IVDs experience complex biomechanical forces, including compression, torsion, shear, and tensile strain, which profoundly influence NP cell viability and ECM homeostasis. Physiological loading, moderate hydrostatic pressure (≈ 0.2–0.5 MPa) with low tensile strain (≤ 10%), promotes tissue anabolism. Under these conditions SOX9 expression and aggrecan (ACAN) synthesis increase via FAK–MEK5–ERK5–AP-1 signaling, which reinforces collagen II networks and proteoglycan retention and thus preserves disc hydration and viscoelasticity [2, 6]. Low-frequency cyclic stretch (0.1 Hz, 5% strain) additionally suppresses pro-inflammatory pathways by inhibiting resistin-induced nuclear factor-κB (NF-κB) activation and interleukin-20 (IL-20) expression, thereby mitigating inflammatory cascades implicated in early degeneration [35]. Conversely, pathologic overloading, including high-magnitude compression (> 1.0 MPa) or tensile strain (> 15%), induces catabolic degeneration. Sustained compression triggers p38 MAPK-ROS-mediated senescence, elevating senescence-associated β-galactosidase (SA-β-gal), p16, and p53 expression while suppressing proliferating cell nuclear antigen (PCNA) [36–38]. Similarly, hyperphysiological tensile strain (> 20% elongation) upregulates matrix-degrading enzymes MMP-13 and ADAMTS-5 via calcium-dependent Piezo1 activation and RhoA/ROCK pathway stimulation, disrupting collagen-proteoglycan architecture and accelerating ECM degradation [2, 39]. This mechanical overload concurrently induces ERS and ROS overproduction, creating a self-perpetuating cycle of matrix catabolism and cellular dysfunction [2, 36]. The distinct cellular outcomes resulting from physiological versus pathological biomechanical loading parameters on NP cells are summarized in Table 5.

Table 5.

Biomechanical loading parameters and their effects on NP cells

Loading type	Parameters	Signaling pathways	Cellular outcomes
Physiologic loading	Hydrostatic pressure (0.2–0.5 MPa); Tension (5–10% strain, 0.1–1 Hz)	FAK-MEK5-ERK5; SOX9 upregulation	↑ ECM synthesis (aggrecan, collagen II); ↓ Inflammation
Pathologic loading	Compression (> 1.0 MPa); Tension (> 15% strain, ≥ 2 Hz)	p38 MAPK-ROS; RhoA/ROCK; NF-κB	↑ Senescence markers (p16, p53); ↑ Catabolic enzymes (MMP13, ADAMTS5)
Low CTL	≤ 10% strain, 0.5–1 Hz	AMPK-mTOR autophagy flux; Integrin-FAK	↑ Autophagy; ↓ Apoptosis; ↑ Mitochondrial fusion
High CTL	> 15% strain, ≥ 2 Hz	DRP1 fission; PINK1-Parkin; NLRP3	↑ Mitochondrial fragmentation; ↑ ROS; ↑ Inflammasome activation

Strain magnitude-dependent responses to CTL

CTL exerts dichotomous effects on NP cells contingent on strain magnitude, frequency, and duration. Low-magnitude CTL (5–10% elongation at 0.5–1 Hz) enhances cellular adaptability through autophagy induction and mitochondrial protection. At 5% strain and 0.1 Hz, CTL upregulates LC3-II lipidation and Beclin-1 expression while suppressing p62/SQSTM1 accumulation, indicating augmented autophagic flux [6, 37]. This proteolytic clearance of damaged organelles coincides with reduced caspase-3 activation and diminished apoptosis, preserving NP cell viability. Mechanistically, AMPK-dependent ULK1 phosphorylation inhibits mTORC1, relieving its suppression on autophagosome formation [6]. Furthermore, physiologic strain promotes mitochondrial network integrity by upregulating MFN2 and OPA1 expression, optimizing oxidative phosphorylation and reducing ROS leakage [2, 37]. In stark contrast, high-magnitude CTL (> 15% strain at ≥ 2 Hz) instigates mitochondrial fission and inflammatory cascades. High mechanical strain (e.g., 20% elongation in some in-vitro systems) associates with DRP1 Ser616 phosphorylation and mitochondrial recruitment; however, much of the mechanistic mapping of the ERK→DRP1(Ser616) axis derives from non-NP cell types (tumor, chondrocyte and cardiac studies). We therefore label this link as ‘cross-tissue inference (evidence from cartilage/cell models)’ and note that direct demonstration of ERK2→DRP1 Ser616 in primary human NP cells under CTL remains limited [2, 16, 36, 40]. This fission precipitates mPTP opening and cytochrome-c release, activating caspase-9-dependent apoptosis. Concurrently, fragmented mitochondria overproduce superoxide (O₂•⁻) through ETC uncoupling, which stabilizes Hypoxia-inducible factor 1 alpha (HIF-1α) and activates the NLRP3 inflammasome [2, 36, 37]. Consequently, NP cells secrete Interleukin-1 beta (IL-1β), Tumor necrosis factor alpha (TNF-α), and IL-6, which further suppress aggrecan synthesis and stimulate ADAMTS-5-mediated proteoglycan degradation [2, 35]. Prolonged high CTL (≥ 24 h) additionally reinforces DNA damage and activates the p53-p21-Rb pathway, inducing irreversible senescence characterized by SA-β-gal positivity and cell cycle arrest [36, 37].

Mechanosensors transducing tensile forces

NP cells convert biomechanical stimuli into biochemical signals through specialized mechanosensors embedded in the plasma membrane and cytoskeleton. Integrin receptors, particularly α5β1 heterodimers, serve as primary anchors to ECM components like fibronectin and laminin. Under tensile strain, integrin clustering activates focal adhesion kinase (FAK), which recruits PI3K and catalyzes PIP3 synthesis. This cascade stimulates AKT and ERK5 phosphorylation, promoting SOX9 nuclear translocation and downstream aggrecan/collagen II transcription [2, 6]. Pharmacologic inhibition of integrin-ECM binding abolishes CTL-induced ERK5 activation, confirming its indispensability for mechanotransduction [2]. Mechanosensitive ion channels, notably Piezo1 and TRPV4, mediate rapid calcium influx in response to membrane deformation. High-magnitude CTL (≥ 20% strain) activates Piezo1 within seconds, eliciting Ca²⁺ currents that activate calcineurin. This phosphatase dephosphorylates nuclear factor of activated T-cells (NFAT), enabling its nuclear translocation and transcriptional upregulation of pro-inflammatory cytokines (IL-6, IL-8) and catabolic enzymes [39, 41–43]. TRPV4 activation similarly elevates intracellular Ca²⁺ but demonstrates crosstalk with Piezo1; agonist co-stimulation experiments reveal mutual inhibition, suggesting compensatory interplay in regulating NP cell mechanoresponses [39, 41]. Primary cilia, solitary microtubule-based organelles projecting from NP cells, sense tensile strain via Hedgehog (Hh) signaling. Mechanical deformation promotes Smoothened (SMO) accumulation within ciliary shafts, activating GLI transcription factors that modulate ECM gene expression [44, 45]. Ciliary disruption in NP cells, via IFT88 depletion, abrogates strain-induced Hh signaling and impairs collagen II synthesis, underscoring their role in mechanotransduction [44, 45]. Finally, Hippo pathway effectors YAP/TAZ shuttle to the nucleus under high CTL (> 15% strain), bind TEAD transcription factors and drive proliferative and fibrotic programs (e.g., CYR61, CTGF). YAP also controls mechanoresponsive non-coding RNAs, notably, a YAP→miR-130a→VGLL4 positive feedback loop has been demonstrated that amplifies YAP activity in mechanobiological contexts [46]. Persistent nuclear YAP signaling therefore has the capacity to reprogram gene and miRNA networks toward fibrosis and senescence under chronic overload [2, 36, 37, 46].

Mechanotransduction pathways linking CTL to autophagy via mitochondria (overview)

NP cells translate tensile loading into metabolic and organellar responses. Physiological CTL supports mitochondrial health and adaptive autophagy, whereas excessive CTL shifts mitochondria toward fragmentation and mitophagy failure, outcomes that are determined by a network of mechanosensors (integrins/FAK, Piezo1/TRPV4, primary cilia), downstream kinases and phosphatases, and hypoxia-sensitive regulators (HIF-1α/BNIP3). To avoid redundancy, the detailed molecular mechanisms are presented in Sect.  “Integrated signaling model, how CTL controls mitochondrial dynamics and autophagy” here we briefly summarize CTL-induced mitochondrial remodeling (4.1) and the supporting experimental evidence (4.3). A schematic overview of CTL-regulated signaling pathways converging on mitochondrial remodeling and autophagy is depicted in Fig. 3.

Fig. 3.

Mechanotransduction pathways linking CTL to autophagy via mitochondrial remodeling. The diagram summarizes key sensors (integrins/FAK, Piezo1, TRPV4, YAP/TAZ), downstream signaling (AMPK/mTOR, ROS/JNK, HIF-1α/BNIP3), and outcomes on mitochondrial fission/fusion and autophagic flux

CTL-induced mitochondrial remodeling

Mitochondrial architecture dynamically adapts to mechanical stimuli, with low-magnitude CTL promoting fusion and high-magnitude CTL inducing fission, thereby differentially regulating autophagy. Under low-frequency, physiological CTL (e.g., 5% strain at 0.1 Hz), NP cells exhibit upregulation of fusion mediators MFN2 and OPA1, fostering elongated, interconnected mitochondrial networks [35]. These fused mitochondria enhance oxidative phosphorylation efficiency, elevating ATP production and reducing the AMP: ATP ratio. Consequently, AMPK activity diminishes, suppressing its inhibitory phosphorylation of mTORC1 and shifting the balance toward reduced autophagic flux [35, 47]. This metabolic state supports matrix synthesis and cellular homeostasis, as observed in healthy IVDs where nutrient supply is limited but sufficient under physiological loading.

High-magnitude CTL (e.g., > 10% strain) rapidly activates DRP1 by post-translational modification. Kinases such as ERK1/2 or CaMKII phosphorylate DRP1 at Ser616, which promotes its translocation to mitochondria, oligomerization, and membrane constriction [48, 49]. This fission produces fragmented mitochondria; many become depolarized and overproduce Mitochondrial Reactive Oxygen Species (mtROS) because of ETC instability [50, 51]. mtROS and released mtDNA act as DAMPs that activate inflammasomes and mitophagy receptors. Depolarized mitochondria also stabilize PINK1, recruit Parkin, and promote ubiquitination of outer-membrane proteins to recruit LC3-adaptors (for example, OPTN and NDP52) [49, 52]. Moderate fission supports quality control, but excessive DRP1 activation under pathological CTL can overwhelm repair pathways and cause cytotoxic mitophagy. It is important to note that fission can also be adaptive: DRP1-mediated fragmentation is required to segregate damaged mitochondrial fragments for mitophagy and, in some contexts, supports cell survival and reprogramming [40, 53]. Table 6 summarizes mitochondrial responses to differential CTL regimes (low vs. high). Note on assay interpretation: LC3-II and Beclin-1 report autophagosome biogenesis; PINK1 reports mitophagy initiation. True autophagic/mitophagic flux should be confirmed by assays that detect lysosomal degradation (for example, p62 turnover ± lysosomal inhibitor, tandem mCherry-GFP-LC3/mito-QC, mt-Keima, lysosomal pH measurements, or autolysosome counts by TEM). Table annotations label entries as ‘flux’ only where primary studies used such assays; otherwise the entries indicate marker abundance (initiation) [27, 28].

Table 6.

Mitochondrial responses to differential CTL

CTL regime	Mitochondrial dynamics	Key molecular changes	Metabolic/Autophagic outcome	Autophagic flux status
Low CTL (5% strain, 0.1 Hz)	Fusion dominant	↑ MFN2/OPA1; ↓ AMPK; ↑ ATP synthesis	Reduced autophagy; Enhanced matrix synthesis	↑ LC3-II turnover; ↑ Lysosomal acidification
High CTL (> 10% strain)	Fission dominant	↑ pDRP1Ser616; ↑ mtROS/mtDNA; ↑ PINK1/Parkin	BNIP3/NIX-mediated mitophagy; Inflammasome activation	↓ Autophagosome-lysosome fusion; p62 accumulation

Integrated signaling model, how CTL controls mitochondrial dynamics and autophagy

Mechanotransduction converges on a small set of nodal regulators that set mitochondrial morphology and autophagic flux. We summarize the essential causal links here and refer to this subsection where detailed mechanisms are discussed elsewhere in the manuscript.

Energy sensor node (AMPK–mTOR–ULK1)

Under energy stress (e.g., prolonged or supraphysiological CTL) AMPK is activated and phosphorylates ULK1 to initiate autophagy while phosphorylating mitochondrial fission factor (MFF) to recruit DRP1 and promote fission. Conversely, under physiological CTL AMPK activity is lower, allowing mTORC1 to restrain excessive autophagy and maintain anabolic programs. The AMPK→MFF→DRP1 axis therefore links cellular energetic state to mitochondrial fragmentation and selective mitophagy [54–56].

Redox/stress node (mtROS → JNK / MAPK)

Mitochondrial fragmentation increases mtROS which activates stress kinases (JNK, ERK). JNK-mediated phosphorylation of Bcl-2 disrupts Bcl-2–Beclin-1 binding and promotes Beclin-1–VPS34 nucleation of autophagy [49, 50, 57]. ERK/JNK also contribute to DRP1 Ser616 phosphorylation that enhances DRP1 mitochondrial translocation. This node therefore converts organelle dysfunction into enhanced autophagic initiation, which can be adaptive or, if lysosomal flux fails, maladaptive [15, 16, 40].

Hypoxia / receptor-mediated mitophagy node (HIF-1α → BNIP3 / FUNDC1; PINK1–Parkin)

In the hypoxic NP niche, HIF-1α upregulates BNIP3 which can recruit LC3 directly and displace Bcl-2 from Beclin-1; severe mitochondrial depolarization stabilizes PINK1 and recruits Parkin to ubiquitylate OMM proteins and recruit autophagy adaptors. The balance between receptor-mediated mitophagy (e.g., BNIP3, FUNDC1) and ubiquitin-dependent mitophagy (PINK1–Parkin) determines whether clearance is efficient [47–49]. Key signaling pathways transducing mechanical stress into mitochondrial autophagy are summarized in Table 7.

Table 7.

Key signaling pathways in CTL-induced mitochondrial autophagy

Pathway	Mechanical trigger	Mitochondrial effect	Autophagy outcome
AMPK/mTOR	ATP depletion (High CTL)	Phosphorylates MFF → DRP1 recruitment; Fission	AMPK → ULK1 activation → Autophagy initiation
ROS/JNK	mtROS from fission	JNK → pDRP1Ser616 → Enhanced fission	JNK → Bcl-2/Beclin-1 dissociation → Autophagy
HIF-1α/BNIP3	Hypoxia + High CTL	BNIP3 binds LC3 on damaged mitochondria	Mitophagy via receptor-mediated engulfment

Calcium / mechanosensor node (Piezo1 / TRPV4 → Ca²⁺ → CaMKII / calcineurin)

Rapid Ca²⁺ influx through Piezo1/TRPV4 activates CaMKII (promotes DRP1 Ser616) and calcineurin (which dephosphorylates DRP1 Ser637), jointly favoring DRP1 activation and fission under high strain. Integrin–FAK pathways further modulate these nodes via ERK and PI3K/Akt signaling [35, 50, 51, 58, 59].

Integrated outcome

Low/physiological CTL biases the network toward mitochondrial fusion and efficient mitophagy (preserving ATP, reducing mtROS) [60, 61], whereas high/supraphysiological CTL drives the AMPK–MFF–DRP1 and Ca²⁺–CaMKII/calcineurin axes to fragment mitochondria [54, 62, 63], producing mtROS and mtDNA that activate inflammasomes (NLRP3) and the cGAS–STING pathway [64–66]. If lysosomal function is preserved, mitophagy clears damaged mitochondria [67]; if lysosomal flux collapses (lysosomal alkalinization, TFEB dysregulation), autophagosomes accumulate and the cell shifts from adaptive autophagy to cytotoxic autophagy, senescence and apoptosis [26, 68]. Practically, we recommend the following diagnostic rules to decide whether to enhance or limit mitophagy in NP models: measure PINK1-dependent phospho-ubiquitin (p-Ub Ser65) as an index of mitophagy initiation [69, 70]; pair LC3-II/LC3-I ratio with p62 turnover ± lysosomal blockade to distinguish flux from stalled autophagosome accumulation [28]; and assess lysosomal pH (e.g., pHLARE / LysoSensor) because lysosomal alkalinization (pH ≳ 5.5) predicts failed cargo degradation and argues for restoring lysosomal acidification before stimulating mitophagy. These combined readouts provide a conservative decision matrix to avoid promoting maladaptive mitophagy [69–71].

Experimental evidence from in vitro and in vivo models

In vitro studies (most commonly using Flexcell^® and similar uniaxial/biaxial stretch platforms) demonstrate that high CTL (often 10–20% strain at ~ 1 Hz in these cell culture systems) induces DRP1 translocation to mitochondria within minutes and drives changes in autophagy markers in isolated NP cells; these in vitro thresholds are useful for mechanistic dissection but do not necessarily equate directly to in vivo disc strains. Complementary in vivo models (rat tail static/dynamic compression and large-animal dynamic/compression models) corroborate the qualitative sequence, mechanical overload → mitochondrial fragmentation → autophagic impairment, but report different absolute magnitudes and temporal dynamics because of tissue boundary conditions, ECM buffering and systemic responses [36, 47, 72, 73]. Pharmacologic inhibition of DRP1 (most commonly using mdivi-1) remains a valuable experimental approach, but conclusions must be balanced by two facts. First, mitochondrial fission itself is frequently adaptive, DRP1-dependent fission is required for physiologic processes including mitophagy, cell reprogramming and stress adaptation; genetic loss or functional blockade of DRP1 can impair mitophagic clearance and reduce cell survival in multiple models. For example, DRP1-dependent fission is necessary for early stages of cell reprogramming [40], and multiple reviews and primary studies document contexts in which fission supports mitophagy and cellular recovery rather than only causing injury [40, 53]. Second, Mdivi-1 has off-target and context-dependent effects that confound a simple ‘fission-is-bad’ interpretation: Mdivi-1 can inhibit mitochondrial complex I and thereby alter respiration and ROS independently of DRP1, and mechanistic re-analysis has shown this bioenergetic action at experimental doses in mammalian cells. These pharmacologic caveats mean that both apparent protective and apparent deleterious effects of Mdivi-1 can reflect complex I inhibition or downstream metabolic changes rather than selective DRP1 blockade [19, 74, 75]. Taken together, the literature supports a contextual model: transient, regulated fission promotes mitophagy and adaptation in many settings, whereas chronic or excessive DRP1 activation drives fragmentation, bioenergetic collapse and inflammation. Therefore, mechanotransduction studies should (i) present both genetic (siRNA/conditional knockout) and pharmacologic evidence where possible, (ii) report concentrations/exposure times and respiration/ETC readouts when using mdivi-1, and (iii) where possible, validate findings with newer, more selective DRP1 inhibitors (e.g., DRP1i27) or genetic tools [19, 53].

In vivo models corroborate these findings. Rat tail static compression (1.3 MPa) induces progressive disc degeneration, characterized by reduced autophagy markers (LC3-II, Beclin-1) in NP tissue and increased apoptosis. Immunohistochemistry reveals elevated pDRP1Ser616 and decreased MFN2/OPA1 in compressed discs, correlating with histopathological degeneration scores [47]. Notably, notochordal cells, critical for NP homeostasis, exhibit large vacuoles with robust basal autophagy under physiological conditions. Compression downregulates their autophagic flux, accelerating their disappearance and chondrocyte-like cell apoptosis [47]. Several studies report protective effects of the small molecule Mdivi-1 in animal models; however, many of these data come from non-NP tissues (e.g., cochlea, kidney) or systemic models. Importantly, Mdivi-1 has known off-target effects (reversible inhibition of mitochondrial complex I) and therefore its protective actions cannot be ascribed solely to DRP1 inhibition without orthogonal genetic evidence. We therefore flag Mdivi-1 results as ‘cross-tissue / pharmacology-limited’ and recommend interpreting such findings with caution until NP-specific genetic or highly selective DRP1 inhibitors are tested [19, 49, 51]. However, contrasting reports indicate Mdivi-1 exacerbates renal tubular damage in contrast-induced nephropathy by blocking protective mitophagy, underscoring context-dependent outcomes [51].

These models validate the DRP1/LC3-II/BNIP3 axis as a critical effector in load-induced disc degeneration. Future studies should employ genetic models (e.g., cell-specific DRP1 knockouts) and advanced imaging (e.g., mito-Keima reporters) to dissect spatial and temporal dynamics of mitophagy in the loaded disc.

Key controversies and unresolved questions

Mdivi-1 has been influential in demonstrating the role of fission in disease models, but two caveats must be emphasized. (1) Several robust studies show that DRP1-dependent fission is physiologically required for processes such as mitophagy and cellular reprogramming; genetic DRP1 deficiency or forced elongation can impair clearance of damaged mitochondria and increase susceptibility to stress in neurons, renal tubule cells and other cell types [40, 53]. (2) Mdivi-1 has off-target actions: mechanistic work demonstrated reversible complex-I inhibition at concentrations commonly used experimentally, altering respiration and ROS independent of DRP1 and thereby complicating interpretation [19, 75]. Collectively, these findings argue that apparent benefit from mdivi-1 requires validation by orthogonal genetic/chemical approaches and by direct measurement of mitochondrial respiration and ROS in the same experiments.

Dual role of fission, Moderate fission facilitates mitophagy and quality control, but excessive fission causes bioenergetic collapse. The strain thresholds differentiating adaptive vs. pathological responses remain ill-defined. HIF-1α’s paradoxical effects, HIF-1α stabilizes under hypoxia and promotes BNIP3-mediated mitophagy. However, excessive BNIP3 may induce apoptosis. How CTL modulates this balance is unclear.

In summary, CTL magnitude and frequency dictate mitochondrial dynamics via conserved mechanotransduction pathways, with DRP1-mediated fission acting as a pivotal switch for mitophagy induction in NP cells. Targeting these pathways, particularly DRP1 activation or BNIP3 expression, holds therapeutic potential for mitigating mechanical overload-induced disc degeneration.

Pathological implications: from mechanobiology to disc degeneration

Supraphysiological CTL shifts NP cells into a maladaptive state characterized by mitochondrial dysfunction, oxidative stress, and defective organelle clearance, processes that together drive inflammation, cellular senescence and extracellular matrix loss. The molecular cascade that converts mechanical strain into mtDNA release, NLRP3 inflammasome activation and cGAS–STING signaling is described in detail in Sect.  “Integrated signaling model, how CTL controls mitochondrial dynamics and autophagy” here we emphasize the pathophysiological consequences for tissue structure and therapeutic opportunities to restore homeostasis.

Dysregulated autophagy serves as a critical nexus between mechanical stress and NP cell pathology. While physiological tensile loading (5–10% elongation) promotes cytoprotective autophagy through AMPK/PGC-1α-mediated pathways, excessive loading overwhelms this adaptive response, triggering autophagy failure. Studies demonstrate that supraphysiological strain (20% at 1 Hz) initially upregulates autophagic markers (LC3-II, ATG5, and BECN1) but ultimately causes autophagosome-lysosome fusion defects due to lysosomal alkalinization [12, 76]. This impairment prevents efficient clearance of damaged mitochondria, leading to the accumulation of dysfunctional organelles that propagate oxidative stress and metabolic dysfunction [1]. The resulting mitophagy insufficiency creates a permissive environment for NP cell senescence, characterized by p53-p21-Rb pathway activation, irreversible cell cycle arrest, and telomere-associated DNA damage foci [36]. Senescent NP cells exhibit a 4.3-fold increase in SA-β-galactosidase activity and diminished proliferative capacity, compromising tissue repair mechanisms. Furthermore, mitochondrial dysfunction lowers the apoptotic threshold, with Bax/Bak oligomerization promoting cytochrome c release and caspase-3 activation. This dual pathology of senescence and apoptosis depletes functional NP cells, reducing proteoglycan synthesis by > 60% and accelerating disc dehydration [36, 77, 78]. The pathological consequences stemming from dysregulated mitophagy in mechanically overloaded NP cells are synthesized in Table 8.

Table 8.

Pathological consequences of dysregulated mitophagy in NP cells

Mitochondrial defect	Downstream effects	Structural consequences
mtDNA release via miMOMP	cGAS-STING activation → IFN-α/β, TNF-α, IL-6 production	Inflammatory niche formation
ROS overproduction	Oxidative damage to collagen II and aggrecan	Loss of hydration and disc height
Reduced OXPHOS capacity	ATP depletion → impaired anabolic activity	Decreased proteoglycan synthesis
Apoptosome assembly	Caspase-3 activation → cell death	Cellular depletion and matrix thinning

The intersection of mitochondrial dysfunction, autophagy failure, and cellular senescence creates a self-amplifying catabolic microenvironment. Senescent NP cells develop a potent (SASP), releasing pro-inflammatory cytokines (IL-1β, IL-6, TNF-α), matrix metalloproteinases (MMP-1, MMP-3, MMP-13), and ADAMTS-4/5 at levels 5–8 times higher than healthy cells [76, 77, 79]. This protease storm selectively degrades critical ECM components, including collagen II and aggrecan, while simultaneously inhibiting new matrix synthesis through TGF-β suppression. Particularly significant is the mechanical downregulation of SIRT3, a mitochondrial deacetylase essential for antioxidant defense. Reduced SIRT3 activity under excessive load leads to hyperacetylation and inhibition of SOD2, exacerbating oxidative stress and creating a feedforward loop that perpetuates ECM degradation [36, 80, 81]. Additionally, mtDNA released through minority-MOMP (miMOMP) or MOMP-linked inner-membrane permeabilization can enter the cytosol and activate intracellular DNA sensors (cGAS-STING) or endosomal TLR9, inducing NF-κB and type I interferon responses that amplify inflammation and the SASP. Mechanistically, minority-MOMP (miMOMP) was shown to release mtDNA via BAX/BAK macropores and to drive cGAS-STING signaling [77], and mitochondrial inner-membrane extrusion/permeabilization enables mtDNA escape during permeabilization events [82]. In nucleus-pulposus cells, mtDNA-driven TLR9 and cGAS-STING activation has been reported to induce inflammasome activation and pyroptosis [1, 77, 82–85]. Promising molecular targets for interrupting the pathological cascade from mechanical stress to IVDD are highlighted in Table 9.

Table 9.

Key molecular mediators linking mechanical stress to IVDD pathology

Molecular target	Function	Therapeutic potential
PINK1/Parkin	Mitophagy initiation	Gene therapy to enhance clearance
cGAS-STING	Cytosolic DNA sensing	Inhibitors (H-151, RU.521)
SIRT3	Mitochondrial deacetylase	Activators (Honokiol, NAD⁺ boosters)
NLRP3 inflammasome	IL-1β processing	MCC950 suppression
DRP1	Mitochondrial fission	Mdivi-1 inhibition

Animal models and clinical evidence robustly corroborate the mechanobiological pathway from tensile overload to disc degeneration. Bipedal rodent models subjected to upright posture exhibit accelerated NP cell senescence, mitochondrial dysfunction, and matrix degradation compared to quadrupedal controls, confirming abnormal mechanical loading as a primary instigator of IVDD [72, 86]. Flexercell tension systems applying controlled cyclic tensile strain (20% at 1 Hz) to human NP cells precisely replicate key degenerative features, including SA-β-gal positivity, γH2AX foci, and reduced lamin B1 expression [36]. Crucially, large animal models with biomechanical similarity to humans, including chondrodystrophoid canines and ovine models, demonstrate that dynamic compression equivalent to 7.5% disc height reduction induces mitochondrial fission within NP cells within 14 days, followed by autophagic impairment and collagen I/III deposition [72, 86]. Clinical and human-tissue studies increasingly support the translational relevance of mitochondrial damage, cytosolic mtDNA signalling, inflammasome activation and dysregulated autophagy in human IVDD. In surgically obtained human NP specimens, Zhang et al. showed higher protein expression of cGAS, STING and NLRP3 in more-degenerate discs and reported significant positive correlations between these protein levels and Pfirrmann grade (linear regression on n = 16 patient samples; p < 0.05) [85]. Lu et al. (human NP tissue and primary human NP cells) similarly reported increased TLR9, NF-κB and NLRP3 expression with degeneration (western blotting: representative n = 5 per Pfirrmann grade; correlation analyses reported on n = 20), and linked mtDNA release via mPTP opening to inflammasome-dependent pyroptosis in human NP cells [87]. Quantitative analyses of autophagy markers in human NP (Quan et al.) (n = 16 surgical specimens) documented that LC3-II expression peaked in Pfirrmann grade III (P < 0.05 versus other grades) whereas p62/SQSTM1 increased progressively with higher Pfirrmann grades, consistent with impaired autophagic flux in advanced degeneration [88].

Evidence from the cartilage endplate (CEP) is emerging: Ma et al. reported stage-dependent changes in Nrf2 and related mitochondrial/iron-handling signatures in human CEP tissue, supporting CEP involvement in mitochondrial stress and ferroptosis pathways in human specimens [89]. Region-specific reviews synthesize available NP/AF/CEP human and preclinical data and note that mitophagy and mitochondrial signalling differ between NP, AF and CEP tissue compartments, a point of translational importance when selecting biomarkers or delivery strategies [90]. On the clinical-imaging side, meta-analytic evidence links radiological degeneration to symptoms (Brinjikji et al.: disc degeneration OR 2.24, 95% CI 1.21–4.15; disc bulge OR 7.54, 95% CI 1.28–44.56 in the pooled analyses), providing population-level effect sizes that tie structural degeneration to clinical presentation [91].

Taken together, these human-tissue and imaging reports (human NP: Zhang 2022, Lu 2023, Quan 2020; human CEP: Ma 2023/2024; synthesis: Feng 2025) substantiate the mechanistic axis linking mitochondrial dysfunction → mtDNA signalling → inflammasome activation and autophagy dysregulation in human IVDD and justify prioritizing mitochondria/autophagy biomarkers (e.g., cytosolic mtDNA, DRP1 pSer616, LC3-II/p62 ratios) in translational workflows [85, 87–90].

Clinical perspective: Human tissue analyses and imaging meta-data indicate that mitochondrial stress and autophagy dysregulation are measurable in clinical specimens (NP and CEP) and track with degeneration severity, supporting immediate translational priorities: (1) validate minimally-invasive biomarkers (cytosolic mtDNA in serum/synovial fluid, DRP1 pSer616, LC3-II/p62 and STING/NLRP3 protein panels) across Pfirrmann grades in existing surgical biobanks; (2) link molecular endotypes to imaging phenotypes (Pfirrmann grade, Modic changes, disc height index) and patient-reported outcomes to enable stratified enrolment in early-phase trials; (3) prioritize local (intradiscal) delivery strategies or short-course local modulation (to avoid systemic mTOR inhibition) and report feasibility endpoints (leakage, tissue histology, local immune response) in large-animal models before first-in-human microdosing. The clinical readouts for early trials should include objective imaging endpoints (DHI, Pfirrmann/MRI T2 metrics), standard pain/function measures (VAS/ODI) and molecular biomarkers described above. The human evidence summarized here supports a realistic translational path but highlights the need for standardized tissue-banked assays and reporting of effect sizes for the candidate biomarkers to enable power calculations for clinical studies [85, 87, 88].

Therapeutic opportunities

The molecular interplay between CTL, mitochondrial dynamics, and autophagy in NP cells presents multiple druggable targets for mitigating IVDD. Mitochondrial stabilizers offer a primary intervention strategy. Mdivi-1, commonly used as a DRP1 inhibitor, reduces pathological mitochondrial fission and can preserve cristae structure and limit oxidative stress in models of mechanical overload. Preclinical studies report beneficial effects of Mdivi-1 (reported dosing ranges vary; e.g., 5–20 mg/kg/day in some rodent studies) including reduced NLRP3 inflammasome activation and less mitochondria-driven apoptosis in compressed discs. Because Mdivi-1 may have additional effects on mitochondrial complex I, these results should be interpreted with caution and ideally confirmed with more selective inhibitors or genetic models [92–94]. Complementary to this, the mitochondria-targeted antioxidant SS-31 (elamipretide) accumulates 5,000-fold in the inner mitochondrial membrane, scavenges mtROS, and prevents cardiolipin peroxidation [95–97]. SS-31 (1–10 mg/kg) maintains mitochondrial bioenergetics by stabilizing respiratory chain supercomplexes, thereby inhibiting NF-κB-driven inflammation and NLRP3-mediated pyroptosis in NP cells exposed to lipopolysaccharide-induced stress [95, 96, 98]. Its rapid renal clearance and minimal off-target effects support clinical translatability [96].

Autophagy modulators provide temporal precision for restoring cellular homeostasis. Rapamycin (0.1–1 µM) induces protective autophagy during compressive stress by inhibiting mTORC1, thereby promoting LC3-II conversion and p62 degradation in NP cells [99]. This enhances mitophagy flux, clearing fragmented mitochondria and suppressing caspase-3 activation [99, 100]. Conversely, spautin-1 (5–10 µM) inhibits excessive autophagy by promoting proteasomal degradation of Vps34 complexes, preventing autophagosome overaccumulation during prolonged tensile strain [99]. This dual modulation strategy, enhancing autophagy initiation while limiting its hyperactivity, optimizes cytoprotection against biomechanical injury.

Anti-inflammatory biologics target downstream effectors of mitochondrial dysfunction. Resolvin D1 (RvD1; 100 nM) disrupts the mtROS/NF-κB positive feedback loop by binding to GPR32 receptors, reducing TNF-α and IL-6 synthesis in mechanically stressed NP cells [95, 101]. It synergizes with SS-31 by augmenting antioxidant gene expression (e.g., SOD2) and suppressing NADPH oxidase activity, thus decoupling inflammation from oxidative damage [93, 95].

Biomaterial strategies enable site-specific, mechanoresponsive drug delivery. Injectable thermosensitive hydrogels e.g., poly(N-isopropylacrylamide-co-acrylic acid) loaded with Mdivi-1 release the drug selectively under pathological strain magnitudes (> 15% tensile strain) via strain-dependent polymer relaxation [92, 93]. In rat IVDD models, this system reduced endplate sclerosis and preserved glycosaminoglycan content by 40% compared to systemic delivery [92]. Three-dimensional decellularized NP-matrix scaffolds tuned to physiological stiffness (0.2–1 kPa) regulate YAP nuclear translocation and can thereby prevent downstream YAP-mediated inhibition of autophagy observed in several cell types [102, 103]. This mechanoregulation promotes TFEB-driven lysosomal biogenesis and PINK1/Parkin-mediated mitophagy, replicating aspects of healthy NP mechanotransductive signaling [99, 102, 103]. These scaffolds enhance TFEB-driven lysosomal biogenesis and promote PINK1/Parkin-mediated mitophagy, replicating the mechanotransductive signaling of healthy NP tissue [95, 99].

Pharmacological synergies amplify therapeutic efficacy. Low-dose rapamycin (0.1 µM) combined with Mdivi-1 (10 µM) enhances mitophagy and mitochondrial fusion in human NP cells, reducing IL-1β secretion by 60% under cyclic stretch [99]. Resveratrol (5–20 µM) activates SIRT1, which deacetylates LC3 and Atg7 to facilitate autophagosome formation while stabilizing PGC-1α-dependent mitochondrial biogenesis [96, 99]. Metformin (1–2 mM) indirectly inhibits DRP1 via AMPK phosphorylation at Ser637, improving mitochondrial networking and reducing lactate production in nutrient-deprived NP cells [93, 94].

Biomechanical rehabilitation strategies exploit physiological loading to reactivate homeostatic pathways. Cyclic tensile strain at 0.5 Hz and 5% magnitude promotes HSP70/SIRT3 signaling, which stabilizes OPA1-mediated mitochondrial fusion and suppresses FIS1-dependent fission [99]. This regimen enhances parkin-dependent mitophagy, clearing damaged organelles while sparing functional mitochondria, effects abolished by SIRT3 knockout [99]. In contrast, static compression or strains > 10% fragment mitochondria and impair autolysosome acidification, highlighting the therapeutic window for controlled dynamic loading [99, 100].

Emerging tools refine therapeutic precision. CRISPR/Cas9-mediated knockout of Drp1 in NP cells confirms that mitochondrial elongation per se is insufficient to rescue oxidative metabolism; Mdivi-1’s efficacy requires concomitant complex I modulation [94, 104]. Mitochondriotropic nanocarriers (e.g., triphenylphosphonium-conjugated liposomes) enhance SS-31 delivery, increasing its accumulation in NP tissue by 8-fold compared to free peptide [95, 96]. Mechanosensitive miR-29a inhibitors embedded in shear-thinning hydrogels silence catabolic genes (e.g., MMP13) selectively under pathological strain, preserving aggrecan synthesis [99, 105, 106].

For personalized spine therapies, patient stratification using mitochondrial biomarkers (e.g., circulating mtDNA levels or DRP1 Ser616 phosphorylation) could guide Mdivi-1 dosing [93]. Injectable biomaterials releasing autophagy inducers in response to strain-sensitive miRNAs (e.g., miR-155) may enable early intervention in degenerate discs. Integrating tensile loading protocols with mitochondrial rejuvenation represents a paradigm shift from symptomatic management to mechanobiological restoration in IVDD. Promising therapeutic targets, their rationale, delivery challenges, and innovative strategies are compiled in Table 10.

Table 10.

Promising therapeutic targets and modalities

Therapeutic target	Rationale	Delivery challenges	Innovative strategies
DRP1 phosphorylation	Reduces excessive fission under high strain	Off-target effects on neuronal DRP1	Peptide inhibitors targeting the mitochondrial fission factor (Mff)-DRP1 interface
STING pathway inhibition	Blocks inflammation from cytosolic mtDNA	Systemic immunosuppression	ECM-binding STING antagonists (e.g., H-151 nanoparticles)
TFEB activation	Enhances lysosomal biogenesis and autophagosome clearance	Risk of lysosomal membrane permeabilization	Ultrasound-responsive TFEB gene actuators
SIRT3 potentiation	Restores SOD2 activity and redox balance	Poor bioavailability	Mitochondriotropic SIRT3 activators (e.g., triphenylphosphonium-conjugated honokiol)

A concise pathway map summarizing the mechanistic links between CTL, mitochondrial dysfunction, autophagy dysregulation, and the sites of action of Mdivi-1, SS-31 and rapamycin is shown in Fig. 4.

Fig. 4.

Schematic pathway map summarizing how CTL alters mitochondrial dynamics and autophagy in NP cells, and where the three therapeutic agents act. High CTL promotes DRP1-mediated mitochondrial fission, mtROS and inflammasome activation, mitophagy failure and ECM catabolism. Mdivi-1 (blue): Inhibits DRP1 activation/mitochondrial translocation to reduce excessive fission and limit mtROS release. SS-31 (Elamipretide) (yellow): Targets the inner mitochondrial membrane to stabilize cardiolipin, preserve respiratory supercomplexes and scavenge mtROS. Rapamycin (green): Inhibits mTORC1 to restore adaptive autophagy/mitophagy flux and improve clearance of damaged mitochondria

Challenges and future directions

The investigation of CTL in regulating NP cell autophagy through mitochondrial dynamics faces several conceptual and technical challenges that impede therapeutic translation. A primary limitation lies in the incomplete understanding of mechanotransduction networks linking cytoskeletal deformation to mitochondrial responses. While cytoskeletal elements like F-actin are known to mediate CTL-induced mitochondrial fission, as evidenced by cytochalasin D blocking fission and preserving bioenergetics under 20% strain [12, 107], the identity of specific mechanosensors (e.g., integrins, primary cilia, or Piezo channels) and their crosstalk with organellar contact sites remains poorly characterized. Recent work suggests mitochondrial-ER contact sites may facilitate calcium flux during mechanical stress, but their role in NP cells is unexplored [108–110]. Furthermore, the temporal dynamics of autophagy induction under CTL lack resolution: studies using 20% elongation at 1 Hz show autophagic flux peaks at 6 h but collapses by 24 h due to lysosomal alkalinization [6], yet the precise tipping point where protective autophagy transitions to failure remains undefined. This knowledge gap is exacerbated by the context-dependent outcomes of mitophagy, where moderate strain (5–10%) enhances mitochondrial quality control via PINK1/Parkin, while supraphysiological strain (≥ 20%) induces DRP1-mediated pathological fission without compensatory clearance [111, 112].

Methodological constraints significantly hinder progress, particularly the reliance on oversimplified in vitro models. Most studies employ 2D Flexercell systems delivering uniaxial tensile strain, neglecting the multiaxial loading and complex ECM interactions present in vivo [6, 112]. Consequently, findings may not replicate the physiological strain distributions experienced by NP cells embedded in 3D collagen-proteoglycan matrices. Additionally, current models fail to capture the metabolic gradients (e.g., oxygen, glucose) inherent to disc tissue, which critically modulate autophagy and mitochondrial function. While novel bioreactors incorporating hypoxia and osmolarity gradients have emerged [112], their integration with mechanical loading remains limited. Animal models also present challenges: bipedal rodents and dynamic compression ovine studies demonstrate mitochondrial fragmentation and autophagic impairment preceding disc degeneration [112], but interspecies variations in disc size, cellular metabolism, and loading patterns complicate extrapolation to human pathophysiology.

The translational gap remains formidable, as no current therapies specifically target mitochondrial dynamics or autophagy in disc degeneration. While DRP1 inhibitors (e.g., Mdivi-1) and SIRT3 activators (e.g., honokiol) show promise in preclinical studies for reducing oxidative stress and matrix catabolism, three pragmatic concerns temper enthusiasm: uncertain inhibitor specificity (notably Mdivi-1’s reported complex-I effects), the systemic safety profile of autophagy modulators such as rapamycin, and the technical and biological challenges of delivering therapeutics reliably to the avascular NP. Each of these translational barriers is discussed below and should be explicitly acknowledged when interpreting preclinical efficacy data [19, 113, 114]. Nanoparticle-based delivery systems have achieved transient mitochondrial targeting in articular cartilage, but similar approaches for NP cells require optimization for disc penetration and retention [109].

Translational barriers, delivery feasibility and safety considerations

While mechanistic targets (DRP1, mTOR, PINK1/Parkin, SIRT3) are attractive in preclinical models, several translational barriers, including drug specificity/off-target actions, systemic safety of autophagy modulators, and the technical difficulties of targeted intradiscal delivery, require candid discussion before clinical translation. Below we summarize the main practical and safety issues and their implications for future preclinical and clinical work [19, 113, 115].

Mdivi-1 specificity and off-target pharmacology. Mdivi-1 is commonly used as a pharmacologic DRP1 inhibitor in mechanobiology studies; however, it has been shown to reversibly inhibit mitochondrial complex I at concentrations used experimentally and to modulate ROS independently of DRP1 genetic deletion, raising important interpretive and safety concerns. Thus, conclusions based on Mdivi-1 alone should be framed as provisional and validated with orthogonal approaches (isoform-selective small molecules, genetic knockdown/conditional Drp1 knockout, or newly described DRP1 inhibitors). When discussing Mdivi-1 results we therefore highlight its ambiguous mechanism and recommend reporting concentrations, exposure duration, and any respiration/ETC readouts to help readers judge whether effects reflect fission inhibition or bioenergetic modulation [19, 116].

Rapamycin / mTOR inhibition: systemic impacts and safety trade-offs. Systemic mTOR inhibitors (rapamycin/sirolimus and rapalogs) exert potent immunosuppression and metabolic effects (e.g., cytopenias, dyslipidemia, impaired wound healing and increased infection risk) that limit chronic systemic use and complicate peri-operative application. Local delivery could reduce systemic exposure but may still alter local immune responses and tissue repair; therefore, safety assessment must include local histology (inflammation, fibrosis), wound healing assays, and systemic monitoring in large-animal models before clinical translation. Where proposing rapamycin-based strategies, state explicitly the safety trade-offs and the need for local or mechano-responsive delivery approaches to minimize systemic exposure [113, 117].

Challenges in targeted hydrogel / intradiscal placement and retention. Injectable hydrogels and mechanoresponsive delivery vehicles are promising for localizing drugs to the avascular NP, but they face several practical hurdles: (i) needle puncture and injection volume can itself accelerate degeneration if not optimized; (ii) leakage into the annulus fibrosus or epidural space can reduce local concentration and cause off-target effects; (iii) avascular disc transport constraints limit diffusion and require materials with long retention or active targeting; and (iv) sterilization, manufacturing scale-up, and regulatory pathway for combination products are nontrivial. Preclinical evaluation should therefore include tests for: injection-related damage (needle gauge and volume), leakage mapping (imaging/histology), residence time and drug release kinetics under physiologic loading, and local tissue responses (immune cell infiltration, fibrosis). Reporting these feasibility and safety endpoints will substantially strengthen translational claims [115, 118, 119].

To bridge this gap, we recommend coordinated use of human resources: (1) systematically analyzing existing surgical human NP tissue biobanks for mitochondrial biomarkers (cytosolic mtDNA, DRP1 pSer616, SIRT3 expression) and autophagy flux markers (LC3-II/I ratio, p62/LAMP2) across Pfirrmann grades; (2) performing MRI–molecular correlation studies to link imaging phenotypes (e.g., Modic changes, Pfirrmann grade) with mitochondrial/autophagy signatures; and (3) integrating available epidemiologic meta-analyses (e.g., on occupational whole-body vibration and symptomatic LBP) to prioritize clinically relevant exposures for mechanistic validation. Such human-centered workflows will accelerate biomarker validation and patient stratification for early mechanobiological interventions [120, 121]. Clinical trial design also faces hurdles: patient stratification based on mechanobiological endotypes (e.g., “fission-dominant” vs. “autophagy-incompetent” phenotypes) is nonexistent, and conventional imaging cannot resolve mitochondrial fragmentation or autophagosome accumulation. Emerging techniques like Raman spectroscopy and mito-targeted contrast agents may enable non-invasive monitoring of mitochondrial health in future studies [109, 112].

Future research priorities should focus on four axes: First, advanced mechanobiological models integrating organ-on-chip technology with human iPSC-derived NP cells to simulate physiologically relevant loading and nutrient conditions. These systems must incorporate immune components (e.g., macrophage-NP cell cocultures) given inflammation’s role in modulating autophagy [107, 122]. Second, high-resolution temporal mapping of mitochondrial-lysosomal crosstalk using biosensors (e.g., mt-Keima, MitoQC) to quantify mitophagic flux in live NP cells under cyclic strain, capturing subcellular spatial dynamics [108, 111]. Third, multi-omics approaches to delineate strain-specific molecular signatures: single-cell RNA sequencing of NP cells from loaded discs has identified mechanoresponsive non-coding RNAs regulating ATG5 expression, suggesting novel therapeutic targets [112]. Finally, precision mechanotherapy leveraging computational modeling to design patient-specific loading regimens. For instance, motion preservation devices could be optimized using finite element analysis calibrated to individual disc mechanics, potentially activating cytoprotective autophagy while avoiding pathological thresholds [109].

Addressing these challenges requires interdisciplinary collaboration between mechanobiologists, mitochondrial physiologists, and clinical researchers. Only through integrated approaches can we bridge the gap between bench insights and effective therapies for disc degeneration.

Conclusion

CTL is a key regulator of NP cell fate because it links mechanical input to mitochondrial dynamics and autophagic flux. Physiological CTL (≈ 5–10% strain at ≤ 1 Hz) supports disc homeostasis by favoring mitochondrial fusion (MFN2/OPA1) and cytoprotective autophagy, which helps NP cells survive in a hypoxic niche. By contrast, supraphysiological CTL (> 15% strain) promotes DRP1-dependent fission, elevates ROS and inflammasome signaling, and can convert autophagy from a protective to a self-destructive process that speeds senescence and ECM breakdown. Thus, mitochondria act as a mechanotransductive hub that translates biomechanical signals into graded autophagic responses via pathways such as AMPK/mTOR, ROS/JNK and HIF-1α/BNIP3.

Therapeutically, this mechanistic understanding illuminates promising strategies to halt disc degeneration: DRP1 inhibitors (e.g., Mdivi-1), mitochondrial antioxidants (SS-31), and autophagy modulators (rapamycin) show efficacy in preclinical models by decoupling mechanical overload from cellular demise. Biomaterial innovations, such as strain-responsive hydrogels and decellularized scaffolds, offer targeted delivery to restore mechano-metabolic equilibrium. Future success hinges on resolving key knowledge gaps, including context-specific HIF-1α regulation, in vivo mitophagy dynamics, and patient stratification using mitochondrial biomarkers. Importantly, recent human histopathology and meta-analytic imaging data corroborate the key mechanistic links between mitochondrial dysfunction, impaired autophagy and clinically relevant disc degeneration, thereby underscoring the translational potential of mitochondrial–autophagy–targeted therapies for symptomatic low back pain.

By bridging mechanobiology, organelle dynamics, and autophagy, this work redefines IVDD not merely as structural failure but as a disorder of cellular mechanotransduction. Harnessing the “Goldilocks zone” of physiological loading while pharmacologically targeting mitochondrial-autophagy crosstalk represents a transformative frontier for disc regeneration, shifting from symptom management to root-cause intervention in low back pain.

Abbreviations

CTL

Cyclic tensile loading

NP

Nucleus pulposus

IVDD

Intervertebral disc degeneration

IVD

Intervertebral disc

ECM

Extracellular matrix

AF

Annulus fibrosus

CEP(s)

Cartilaginous endplate(s)

MMP(s)

Matrix metalloproteinase(s)

ADAMTS

A disintegrin and metalloproteinase with thrombospondin motifs

LC3 / LC3

II—Microtubule—associated proteins 1 A/1B light chain 3 (LC3; lipidated form LC3—II)

p62 / SQSTM1

Sequestosome 1

ULK1

Unc—51 Like Autophagy activating kinase 1

AMPK

Adenosine monophosphate—activated protein kinase

mTOR / mTORC1

Mechanistic target of rapamycin (complex 1)

BECN1 / Beclin

1—Beclin 1

VPS34 / PI3KC3

Class III phosphatidylinositol 3—kinase (VPS34)

ATG5 / ATG12 / ATG7

Autophagy related proteins

LAMP1

Lysosomal—associated membrane protein 1

SNARE / STX17 / SNAP29 / VAMP8

SNARE proteins involved in autophagosome—lysosome fusion

TFEB

Transcription factor EB (lysosomal biogenesis regulator)

PINK1

PTEN—induced putative kinase 1

Parkin

E3 ubiquitin ligase parkin (PARK2)

DRP1 (Drp1)

Dynamin—related protein 1 (mitochondrial fission GTPase)

BNIP3

BCL2/adenovirus E1B 19 kDa protein—interacting protein 3 (mitophagy receptor)

BNIP3L / NIX

BNIP3—like (NIX), mitophagy receptor

FUNDC1

FUN14 domain containing 1 (mitophagy receptor)

MFN1 / MFN2

Mitofusin 1 / Mitofusin 2 (mitochondrial outer membrane fusion proteins)

OPA1

Optic atrophy 1 (inner membrane fusion protein)

FIS1

Mitochondrial fission 1 protein

MFF

Mitochondrial fission factor

MID49 / MID51

Mitochondrial dynamics proteins (Drp1 receptors)

ROS

Reactive oxygen species

mtROS

Mitochondrial reactive oxygen species

mtDNA

Mitochondrial DNA

NLRP3

NOD—like receptor family pyrin domain containing 3 (inflammasome)

cGAS

STING—Cyclic GMP—AMP synthase—stimulator of interferon genes (cytosolic DNA sensing pathway)

TLR9

Toll—like receptor 9

SASP

Senescence—associated secretory phenotype

SA

β—gal—Senescence—Associatedβ—galactosidase

PCNA

Proliferating cell nuclear antigen

HIF

1α—Hypoxia—inducible factor 1 alpha

Piezo1

Mechanosensitive piezo type ion channel 1 (mechanotransducer)

TRPV4

Transient receptor potential vanilloid 4 (mechanosensitive channel)

FAK

Focal adhesion kinase

ERS / ER

Endoplasmic reticulum stress / endoplasmic reticulum

JNK

c—Jun N—terminal kinase

ERK / ERK1/2 / ERK5

Extracellular signal—regulated kinase family

p53 / p21 / p16 (p16INK4a)

Tumor suppressor / cell—cycle regulators

SIRT3 / SIRT1

Sirtuin 3 / Sirtuin 1 (mitochondrial / nuclear deacetylases)

PGC

1α—Peroxisome proliferator—activated receptor gamma coactivator 1—alpha

SS

31 (Elamipretide)—Mitochondria—targeted antioxidant peptide

Mdivi

1—Mitochondrial division inhibitor—1 (Drp1 inhibitor)

RvD1

Resolvin D1 (pro—resolving lipid mediator)

NF

κB—Nuclear factor kappa—light—chain—enhancer of activated B cells

IL

1β, IL—6, IL—8—Interleukin—1 beta, Interleukin—6, Interleukin—8

TNF

α—Tumor Necrosis Factor alpha

YAP / TAZ

Yes—associated protein / transcriptional co—activator with PDZ—binding motif

MDVs

Mitochondria—derived vesicles

mt

Keima / MitoQC—Mitophagy biosensors mentioned as experimental tools

CRISPR/Cas9

Clustered regularly interspaced short palindromic repeats / CRISPR associated protein 9

OXPHOS

Oxidative phosphorylation

GPR32

G protein—coupled receptor 32

MCC950

Small molecule NLRP3 inhibitor

H

151, RU.521—STING pathway inhibitors

NAD⁺

Nicotinamide adenine dinucleotide

miR / microRNA

microRNA

Author contributions

Hamed Soleimani Samarkhazan conceptualization, wrote and visualization the main manuscript text and Shaodong Xue Revise the manuscript. All authors reviewed the manuscript.

Funding

This research did not receive any financial support from public, commercial, or nonprofit organizations.

Data availability

No datasets were generated or analysed during the current study.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare that they have no competing interest.