Harnessing the potential of mesenchymal stem cells–derived exosomes in degenerative diseases

Abstract

Mesenchymal stem cells (MSCs) have gained attention as a promising therapeutic approach in both preclinical and clinical osteoarthritis (OA) settings. Various joint cell types, such as chondrocytes, synovial fibroblasts, osteoblasts, and tenocytes, can produce and release extracellular vesicles (EVs), which subsequently influence the biological activities of recipient cells. Recently, extracellular vesicles derived from mesenchymal stem cells (MSC-EVs) have shown the potential to modulate various physiological and pathological processes through the modulation of cellular differentiation, immune responses, and tissue repair. This review explores the roles and therapeutic potential of MSC-EVs in OA and rheumatoid arthritis, cardiovascular disease, age-related macular degeneration, Alzheimer's disease, and other degenerative diseases. Notably, we provide a comprehensive summary of exosome biogenesis, microRNA composition, mechanisms of intercellular transfer, and their evolving role in the highlight of exosome-based treatments in both preclinical and clinical avenues.

1. Introduction

1.1. The urgency of developing innovative approaches to address degenerative diseases

The global population is aging. By 2050, over 80% of older people are predicted to reside in low- and middle-income countries; this demographic shift will have considerable social and economic implications [1]. Therefore, prioritizing the development of new treatments and interventions for degenerative diseases is imperative to ensure enhanced health outcomes and improved quality of life for the aging global population (see Table 1).

Table 1.

Current applications of MSC-EVs in treating degenerative conditions. In vitro and in vivo efficacy and promotion of cellular functions to facilitate tissue repair.

EV Source	Model	Results	Reference
Osteoarthritis
Chondrocyte-derived exosomes (CC-Exos)	In vitro, chondrocyte	1.Enhanced cartilage progenitor cell expansion and increased expression of chondrogenesis-related factors.	[28]
		2.Promoted collagen deposition, reduced vascular ingrowth, and developed into cartilage.
BMSC-derived exosomes (BMSC-Exos)	In vivo, anterior cruciate ligament transection (ACLT) + destabilization of the medial meniscus (DMM) OA model	1.Mitigated cartilage damage by targeting proinflammatory factors with miR-135b.	[29,30]
		2.Inhibited chondrocyte apoptosis and MMP expression by modulating Drp1-mediated mitophagy.
Embryonic MSC-derived exosomes (EMSC-Exos)	In vitro, chondrocyte	1.Maintained chondrocyte phenotype with increased collagen type II synthesis and reduced ADAMTS5 expression.	[31]
		2.Effects linked to adenosine-triggered protein kinases, TGF-β, and IGF activation.
Human synovial MSC-derived exosomes (hSMSC-Exos)	In vitro, chondrocyte	Promoted chondrocyte proliferation and migration by upregulating Wnt5a, activating YAP signaling pathways, and suppressing extracellular matrix formation.	[32,33]
ADSC-derived exosomes (ADSC-Exos)	In vitro, OA chondrocyte	Reduced senescence-associated β-galactosidase activity and the secretion of inflammatory mediators from OA osteoblasts and catabolic mediators from OA chondrocytes.	[34]
IPFP-derived exosomes (IPFP-Exos)	In vitro, chondrocyte	Elevated levels of Sox-9, aggrecan, and type II collagen expression, more effective than IPFP-Exos pretreated with kartogenin.	[35]
ADSC-Exos	In vitro, chondrocyte	Increased periosteal cell proliferation and chondrogenic capacity linked to miR-145 and miR-221, respectively.	[36]
IPFP-Exos	In vivo, DMM-induced OA animal model	1.Enhanced chondrocyte autophagy through miR-100-5p inhibition of mTOR.	[37]
		2.Intra-articular administration of antagomir-100-5p protected cartilage from deterioration and improved gait by repressing chondrocyte apoptosis through the mTOR-autophagy pathway.
hSMSC-Exos	In vivo, ACLT-induced OA model	hSMSC-Exos overexpressing miR-140-5p augmented cartilage regeneration and slowed knee OA progression in a rat model.	[38]
BMSC-Exos	In vivo, collagenase-induced OA mouse model	BMSC-Exos overexpressing miR-92a-3p suppressed cartilage degradation by directly targeting WNT5A and preserving articular chondrocyte function.	[39]
BMSC-Exos	In vivo, ACLT + DMM OA surgery model	TGF-β1 promoted chondrocyte proliferation by modulating Sp1 through miR-135b sourced from BMSC-Exos, aiding cartilage restoration.	[40]
Cardiovascular disease
HIF-1α engineered MSC-derived EVs (HIF-1α-EVs)	In vitro: cardiomyocytes and endothelial cells under hypoxia and serum deprivation (H/SD); In vivo: Sprague Dawley rats with acute myocardial infarction (AMI)	1.Reduced cardiomyocyte apoptosis and enhanced endothelial cell angiogenesis.	[81]
		2.Reduced fibrosis and improved cardiac function in rats.
		3.Enhanced effects with RGD-biotin hydrogels.
Human bone marrow MSC-derived EVs (MSC-EVs)	In vitro: human umbilical vein endothelial cells; In vivo: rat MI model	1.Promoted endothelial cell proliferation, migration, and tube formation in vitro.	[84]
		2.Enhanced blood flow recovery, reduced infarct size, and preserved cardiac performance in vivo.
MSC-Exos derived from MSCs pretreated with ischemic rat heart extract (MSCE-Exos)	In vitro: human umbilical vein endothelial cells (HUVECs)	1.Enhanced HUVEC proliferation and migration.	[85]
		2.Proteomic analysis revealed upregulation of angiogenesis-related proteins, including DMBT1.
		3.DMBT1 delivery via MSCE-Exos was crucial for angiogenesis, with silencing of DMBT1 impairing HUVEC activity.
		4.Ischemic heart extracts revealed increased levels of IL-22 and subsequent upregulation of VEGF and DMBT1 in MSCs, which enhanced the angiogenic effects of the derived exosomes.
Human umbilical cord MSC-derived exosomes (hucMSC-Exos)	In vivo: AMI rats; In vitro: hypoxic H9C2 cells	1.miR-19a was transferred, which offered protection for cardiomyocytes through the reduction of apoptosis and infarct size.	[86]
		2.miR-19a targeted SOX6; inhibition of SOX6 reduced hypoxic damage.
		3.Enhanced cardioprotection through the activation of AKT and inhibition of JNK3/caspase-3 pathway.
Mesenchymal stem cell–derived exosomes (MSC-Exos)	In vivo: mouse model of myocardial ischemia/reperfusion (I/R); In vitro: macrophage polarization studies	1.Reduced infarct size and inflammation postmyocardial I/R.	[87]
		2.Facilitated macrophage polarization from M1 to M2, improving cardiac recovery.
		3.miR-182 targeted TLR4, influencing macrophage polarization and reducing inflammation.
MicroRNA-1-transduced MSCs (MSC(miR-1))	In vivo: C57BL/6 mice with MI	1.Enhanced differentiation of transplanted MSCs into cardiomyocytes in the infarcted zone.	[88]
		2.Improved cardiac function.
		3.Increased cell survival and cardiomyogenic differentiation.
Age-related macular degeneration
Human umbilical cord MSC-derived exosomes (hucMSC-Exos)	In vitro: RPE cells; In vivo: laser-induced CNV and subretinal fibrosis model in mice	1.Intravitreal injection of hucMSC-Exo reduced subretinal fibrosis and CNV.	[94]
		2.Suppressed RPE cell migration and promoted mesenchymal–epithelial transition via miR-27b.
		3.miR-27b targeted HOXC6, inhibiting the EMT process induced by TGF-β2.
Adipose-derived MSC exosomes (Ad-MSC-Exos)	In vivo: streptozotocin-induced diabetes in rabbits	1.Improved retinal structure, with SC and IO routes showing well-defined retinal layers similar to normal retina.	[95]
		2.IV route resulted in less organized retinal layers.
		3.Significant increase in micRNA-222 expression associated with retinal repair and regeneration.
MSC-derived exosomes (MSC-Exos)	In vivo: mouse models of photoreceptor loss (MNU-induced and Pde6bmut)	1.Intravitreal MSC transplantation and exosomal transplantation counteracted photoreceptor apoptosis and alleviated retinal degeneration.	[101]
		2.Effects sustained for 1–2 months after a single injection.
		3.miR-21 targeted Pdcd4, protecting photoreceptors and preventing retinal dysfunction.
Human bone marrow–derived MSCs (hBMSCs)	In vitro: Cultured hBMSCs	1.Overexpression of the miR-183/96/182 cluster upregulated neuroretinal genes such as OTX2, NRL, PKCα, and recoverin.	[102]
		2.Ectopic expression of the miR-183 cluster increased CRX and rhodopsin levels at mRNA and protein levels, suggesting initiation of photoreceptor cell differentiation.
		3.No morphological changes in cells despite gene expression alterations.
Alzheimer's disease
Human adipose tissue-derived MSC exosomes (ADSC-Exos)	In vitro: N2a neuroblastoma cells	1.Carried enzymatically active neprilysin (NEP), a pivotal β-amyloid-degrading enzyme.	[118]
		2.Transferred NEP into N2a cells, significantly reducing both secreted and intracellular Aβ levels.
		3.More effective than bone marrow–derived MSC exosomes.
Mesenchymal stem cell–derived EVs (MSC-EVs)	In vivo: APP/PS1 mouse model of Alzheimer's disease	1.Reduced inducible nitric oxide synthase (iNOS) mRNA and protein levels in primary cultured neurons and APP/PS1 mice.	[119]
		2.Improved cognitive behaviors and rescued synaptic transmission and long-term potentiation in the hippocampal CA1 region.
Mesenchymal stem cells and cell–derived EVs (MSC-EVs)	In vitro: Transwell cocultures with rat hippocampal neurons	1.Protected hippocampal neurons from amyloid-beta oligomer (AβO)-induced oxidative stress and synapse damage.	[120]
		2.Protection involved internalization and degradation of AβOs, release of catalase-containing EVs, and secretion of anti-inflammatory cytokines and growth factors.
Human Wharton's jelly MSC-derived EVs (hMSC-EVs)	In vitro: Primary hippocampal cultures exposed to AβOs	1.Internalized by hippocampal neurons, enhanced in the presence of AβOs.	[121]
		2.Protected neurons from oxidative stress and synaptic damage induced by AβOs.
		3.Neuroprotection mediated by catalase EVs, abolished by catalase inhibition.
Mesenchymal stem cell–derived exosomes (MSC-Exos)	In vivo: mouse model of Alzheimer's disease	1.Stimulated neurogenesis in the subventricular zone.	[123]
		2.Alleviated beta-amyloid 1-42-induced cognitive impairment in Morris water maze and novel object recognition tests.
Human umbilical cord MSC-derived exosomes (hucMSC-Exos)	In vivo: AD mouse model; In vitro: BV2 microglial cells	1.Alleviated neuroinflammation and reduced amyloid-beta deposition in AD mouse models.	[125].
		2.Improved cognitive function and modulated microglial activation.
		3.Regulated inflammatory cytokine levels both in vivo and in vitro.
Allogenic human adipose MSC-derived exosomes (ahaMSCs-Exos)	Phase I/II clinical trial in patients with mild to moderate Alzheimer's disease	1.No adverse events were reported during the trial.	[129]
		2.The medium-dose arm exhibited significant improvement in cognitive function, as measured using ADAS-cog and Montreal Cognitive Assessment scores.
		3.Although no significant changes in amyloid or tau levels were observed, a reduction in hippocampal volume loss was noted in the medium-dose arm.

Choroidal Neovascularization (CNV); cone-rod homeobox (CRX); subconjunctival (SC); and intraocular (IO); yes-associated protein (YAP).

Thus, innovative therapies and technologies for degenerative diseases must be explored. This involves genetics, regenerative medicine, and personalized treatment [[2], [3], [4]]. This review article explores new approaches in regenerative medicine, with a particular focus on the therapeutic applications of exosomes derived from MSCs. It further examines how MSC-derived exosomes (MSC-Exos) could potentially manage and reverse the effects of degenerative diseases, thereby opening new pathways to enhance the health and longevity of the aging population.

1.2. Mesenchymal stem cells and extracellular vesicles

Stem cells with multipotent differentiation potential and regenerative capacity can be broadly categorized into two groups: embryonic and adult stem cells [5]. Embryonic stem cells are derived from sources such as the term placenta, amniotic fluid, and umbilical cord. Adult stem cells are found in various tissues or organs, including the bone marrow (BM) [6], trabecular bone [7], adipose tissue [8], synovial fluid [9], synovium [10], and peripheral blood [11]. These cells are often used in treating knee osteoarthritis (OA). Stem cells from different sources exhibit varying differentiation capacities, clinical benefits, and culture characteristics. Therefore, selecting the appropriate cell source is crucial for successful mesenchymal stem cell therapies. Common sources include the bone marrow, adipose tissue, synovial fluid, and synovium. Several studies have indicated that bone marrow–derived MSCs (BM-MSCs) are often the primary cell source, followed by adipose tissue-derived MSCs (ADSCs).

Exosomes are a type of extracellular vesicle (EV) that range from 30 to 150 nm in diameter [12]. They are present in various cell types [13] and extracellular fluids, such as plasma [14], synovial fluid [15], urine [16], amniotic fluid [17], saliva [18], cerebrospinal fluid [19], breast milk [20], and tears [21]. MSC-Exos facilitate the transfer of bioactive lipids, nucleic acids (including DNA, mRNAs, and noncoding RNAs) [22], and proteins between cells. This transfer elicits biological responses such as gene regulation [23], proliferation, apoptosis [24], and immunomodulation [25] in recipient cells [26]. MSC-Exos exhibit heterogeneity, with extracellular RNA extracted from exosomes and nonvesicles derived from the same cell also displaying heterogeneity [27].

2. Osteoarthritis

Research on the effects of exosomes from various cell types on OA has yielded significant findings. For example, exosomes derived from chondrocytes (CC-Exos) have been demonstrated to enhance the proliferation of cartilage progenitor cells and significantly promote chondrogenesis-related factors. They also increase collagen deposition, reduce vascular ingrowth, and consistently develop into cartilage [28]. Additionally, studies have indicated that exosomes from bone marrow–derived mesenchymal stem cells (BMSC-Exos) alleviate cartilage damage in rats with OA by carrying high levels of miR-135b, which targets proinflammatory factors elevated in the serum and reduces cartilage tissue damage [29]. These exosomes also inhibit chondrocyte apoptosis and the expression of matrix metalloproteinases (MMPs) by regulating Drp1-mediated mitophagy [30]. Exosomes from embryonic MSCs (EMSC-Exos) contribute to maintaining the chondrocyte phenotype by increasing the synthesis of collagen type II and reducing the expression of ADAMTS5. The beneficial effects of EMSC-Exos may be linked to adenosine activation of protein kinases, transforming growth factor-β (TGF-β), and insulin-like growth factor (IGF) [31].

A key finding in recent research is the role of human synovial mesenchymal stem cell–derived exosomes (hSMSC-Exos) in stimulating chondrocyte proliferation and migration. This effect is mediated by the upregulation of Wnt5a, which activates yes-associated protein (YAP) signaling pathways and suppresses extracellular matrix formation [32,33]. YAP is a transcriptional co-activator in the Hippo signaling pathway, is crucial for promoting cell growth and inhibiting apoptosis when activated. The activation of YAP signaling by Wnt5a is pivotal for enhancing tissue regeneration processes. This discovery has significant therapeutic implications for treating osteoarthritis, offering a potential route to enhance joint repair and functionality.

Recent research has underscored the chondroprotective role of exosomes from subcutaneous adipose-derived stem cells (ADSC-Exos). These exosomes reduce senescence-associated β-galactosidase activity and the production of inflammatory mediators from OA osteoblasts and catabolic mediators from OA chondrocytes [34]. Furthermore, chondrocytes treated with exosomes isolated from infrapatellar fat pad mesenchymal stem cells (IPFP-Exos) were observed to exhibit upregulated Sox-9, aggrecan, and type II collagen expressions, outperforming exosomes derived from IPFP-MSCs pretreated with kartogenin [35]. These findings highlight the potential of ADSC-Exos in OA treatment.

3. MSC exosomal miRNA therapy for cartilage protection

Studies on periosteal cells treated with exosomes have revealed a correlation between elevated levels of miR-145 and miR-221 and enhanced proliferation and chondrogenic potential of these cells, respectively [36]. Additionally, miR-100-5p derived from IPFP-MSCs significantly promotes chondrocyte autophagy by inhibiting mTOR. The intra-articular injection of antagomir-100-5p has been demonstrated to protect cartilage from damage and improve gait patterns in mice with destabilization of the medial meniscus (DMM)-induced OA by suppressing chondrocyte apoptosis through the mTOR-autophagy pathway [37]. Furthermore, exosomes derived from human synovial MSCs overexpressing miR-140-5p were demonstrated to enhance cartilage regeneration and delay the progression of knee OA in a rat OA model [38]. Exosomes from human bone marrow–derived MSCs overexpressing miR-92a-3p inhibited cartilage degradation in a collagenase-induced OA mouse model by directly targeting WNT5A, thereby preserving articular chondrocyte the function [39]. Furthermore, in an animal study, TGF-β1 was demonstrated to promote chondrocyte proliferation and facilitate cartilage repair in a rat OA model by regulating Sp1 through miR-135b derived from BMSC exosomes [40]. These findings highlight the critical role of miRNA regulation in modulating gene expression during chondrogenic differentiation. Validating these miRNAs and their targets could support further research into safe and effective delivery systems, enhancing the therapeutic potential of miRNAs in OA treatment.

4. MSC-derived exosomal miRNA therapy for rheumatoid arthritis

Rheumatoid arthritis (RA) is a long-term inflammatory autoimmune disorder that mainly targets the joints. It is characterized by persistent synovial inflammation, which results in the deterioration of cartilage and bon [41]. Numerous genetic and environmental factors have been linked to a heightened risk of developing RA [42]. Along with synovial membrane hyperplasia and overactivation of osteoclasts, increased bone degradation is another key hallmark of RA [43,44]. Research has demonstrated that a range of immune cells, including T cells, B cells, and macrophages, play a role in the development of arthritis [45,46]. Despite the availability of conventional synthetic DMARDs (csDMARDs), biologic agents (bDMARDs), and targeted synthetic DMARDs (tsDMARDs), these treatments often encounter challenges such as side effects [47]. Additionally, up to 40% of patients experience inadequate responses to these therapies (primary inefficacy) or lose their effectiveness over time (secondary inefficacy) [48]. Consequently, the potent immunomodulatory properties of MSC-Exos present a promising new approach for treating joint swelling and cartilage erosion.

While several clinical trials have demonstrated the effects of MSC-based therapy in RA patients, an optimal MSC-based therapeutic protocol has yet to be established [49]. Thus, utilizing MSCs for RA treatment remains challenging. Most studies have employed allogeneic MSCs, as obtaining and cultivating a sufficient quantity of autologous MSCs from RA patients can be difficult. Additionally, autologous MSCs from RA patients may possess intrinsic genetic defects that could impair their anti-inflammatory capabilities. MSC-Exos have not yet been utilized in the treatment of RA. However, the effectiveness of MSC-Exos-based therapy has been demonstrated in experimental animal models of RA. In these studies, MSC-Exos-based therapy has been shown to significantly reduce the onset and progression of experimental arthritis.

Li et al. reported that MSC-derived Exos expressing miRNA-150-5p reduced the secretion of inflammatory cytokines, such as TNF-α and IL-1β, in CIA mice, thereby suppressing RA progression in vivo [50]. Furthermore, MSC-derived exosomal circFBXW7 was shown to suppress the proliferation, migration, and inflammatory responses of rheumatoid fibroblast-like synoviocytes and mitigate RA in rats by sponging miR-216a-3p and activating HDAC4 [51]. Tavasolian et al. reported that in CIA mice, miR-146a-transduced ADSC-Exos increased the expression of FoxP3, TGFβ, and IL-10, while miR-155-transduced ADSC-Exos elevated levels of RORγt, IL-17, and IL-6 [52]. This indicates that Exos can serve as vehicles for the intracellular transfer of miRNAs, presenting a potential therapeutic strategy for RA. Moreover, other in vitro studies demonstrated that Exos derived from bone marrow stem cells contained increased levels of target miRNAs, such as miR-150-5p, miR-548e, miR-34a, miR-320a, miR-124a, miR-216a-3p, miR-192-5p, and miR-143-3p, which inhibited inflammation in mice with RA [51,[53], [54], [55], [56], [57], [58]]. Therefore, MSC-Exos could potentially serve as innovative therapeutic agents for cell-free or cell-component-based treatment of RA.

5. Cardiovascular diseases

Cardiovascular diseases (CVDs) are the leading cause of death worldwide, accounting for approximately 17.9 million deaths annually. These diseases include various heart and vascular conditions such as coronary and cerebrovascular diseases [59]. More than 80% of these deaths result from heart attacks and strokes, with a significant number occurring prematurely in individuals under 70 years of age. Inflammation plays a crucial role in their development of CVDs, and anti-inflammatory therapies have been demonstrated to be effective in managing these conditions [60]. Myocardial infarction (MI) and ischemic heart disease are particularly influenced by inflammation and fibrosis. Various cell types produce EVs that influence these processes, making EVs valuable tools for diagnosis, prognosis, and treatment.

Following an acute MI, the body swiftly initiates immune responses at both cellular and humoral levels [61]. A notable reaction involves the differentiation of monocytes into macrophages [62]. A study indicated a substantial increase in EV release in the heart within 15–24 h post-MI [63]. These EVs, originating from cardiomyocytes and endothelial cells, trigger the release of chemokines and cytokines from monocytes and modulate macrophage inflammatory responses, depending on the donor cell's condition (ischemic vs. nonischemic) [64]. Post-MI, the interaction between cardiomyocytes extends beyond the heart, with myocardial micro-RNAs (miRNAs) such as miR-1, miR-208, and miR-499 transported via EVs to distant organs such as the bone marrow, where they suppress the expression of CXC chemokine receptor 4 (CXCR4) in mononuclear cells. This facilitates their release into the bloodstream [65]. Additionally, EVs originating from endothelial cells mobilize and activate monocytes from the spleen [66]. These EVs, regardless of their cell origin, recruit and alter the phenotype of peripheral mononuclear cells. Moreover, the quantity of EVs is strongly correlated with the severity of myocardial damage, indicating their potential as diagnostic and prognostic markers in MI [66]. Interestingly, EVs produced by endothelial cells overexpressing Krüppel-like factor 2 (KLF2) mitigate the recruitment of Ly6C^high monocytes and reduce ischemia-reperfusion injury post-MI [67]. EVs released from cardiac stromal-progenitor cells across different species, including mice, rats, and humans, demonstrate immunomodulatory actions by influencing macrophage polarization in MI models, underscoring their therapeutic potential [68,69]. EVs from noncardiomyocyte sources also shape inflammatory and fibrotic responses post-MI. Macrophages release various noncoding RNAs via EVs, which influence the behavior of cardiac fibroblasts. Notably, EVs carrying circular RNA circUbe3a exacerbate myocardial fibrosis by altering cardiac fibroblast proliferation, migration, and phenotype [70]. Activated macrophages transport miR-155-rich EVs to cardiac fibroblasts, inhibiting their growth and enhancing inflammation, thereby increasing the risk of cardiac rupture [71]. Furthermore, the delivery of miR-155-loaded EVs from macrophages to endothelial cells exacerbates ischemic damage by inducing antiangiogenic effects [72]. Despite causing cardiac damage, EVs from innate immune cells offer beneficial effects post-MI. For instance, EVs from dendritic cells activate CD4⁺ T cells, thereby improving cardiac function post-MI [73]. Recent research has increasingly focused on the therapeutic modulation of T cells in MI, mainly using regulatory T cells (Tregs) and their EVs. Studies have demonstrated that EVs from cardiac stromal-progenitor cells enhance Treg functions, promoting their proliferation and IL-10 production and contributing to cardioprotective outcomes in inflammation models [74]. The dynamic interplay between EVs and various cell types is critical in immune and repair responses post-MI. EVs contribute to inflammatory and fibrotic processes and have promising diagnostic and therapeutic applications, mainly through immune cell modulation. Approaches that leverage EV properties, particularly those derived from regulatory T cells and cardiac progenitor cells, are a promising avenue for improving outcomes in patients with cardiac injuries.

Heart failure (HF) is a chronic and progressive condition resulting from structural or functional cardiac irregularities, presenting primarily in two forms: heart failure with reduced ejection fraction and heart failure with preserved ejection fraction (HFpEF) [75]. Characterized by compromised ventricular blood ejection or filling, HF typically manifests as symptoms including fatigue, dyspnea, and edema, posing a considerable public health challenge globally and resulting in considerable morbidity and mortality. The mechanisms underlying HF are diverse and often linked to its root causes. EVs, extensively implicated in numerous aspects of HF pathophysiology, particularly chronic inflammation [76], play a critical role in intercellular communication in the context of HF. By transporting miRNAs and proinflammatory cytokines, EVs influence cardiac function and repair mechanisms. For instance, EVs derived from cardiac cells carrying miR-21-5p and miR-378 have been demonstrated to regulate angiogenesis, cardiomyocyte survival, and fibrosis, indicating their potential as modulators of disease progression [77]. Additionally, the bidirectional communication facilitated by specific miRNAs in EVs, such as miR-155 and miR-217, among cardiomyocytes, macrophages, and fibroblasts, underscores their role in either exacerbating or mitigating cardiac hypertrophy and ischemia [78]. Furtheremore, exosomes have been investigated as potential biomarkers for CVDs due to their ability to circulate in body fluids stably and because they encode information on a variety of disease status indicators [79]. Exosomes derived from cardiomyocytes, endothelial cells, and fibroblasts play a role in intercellular communication under both physiological and pathological conditions [80]. Their ability to mirror the cellular origin and the pathological disturbance renders them suitable for noninvasive diagnosis and prognosis of CVDs [79].

MSCs secrete exosomes that possess properties that boost cardiac repair, underscoring their therapeutic potential in CVDs [68,[81], [82], [83]]. Preclinical studies have revealed that MSC-Exos can interact with vascular endothelial cells, promoting angiogenesis, a pivotal process in heart tissue repair [84,85]. These exosomes, mainly originating from human umbilical cord mesenchymal stem cells (hucMSCs), have been observed to transport microRNA-19a (miR-19a) to cardiomyocytes, thereby promoting cell survival by targeting the SOX6 gene and modulating the AKT/JNK3/caspase-3 signaling pathway [86]. Furthermore, MSC-Exos can modulate the immune response in the heart by promoting the polarization of macrophages toward the anti-inflammatory M2 phenotype as opposed to the proinflammatory M1 phenotype, thereby mitigating inflammation and reducing infarct size in mouse models [87]. Additionally, studies have indicated that the transplantation of MSCs into the infarcted region enhances cardiac function, with further enhancement achieved by genetically modifying MSCs to overexpress miR-1, which enhances their survival and differentiation into cardiomyocytes [88]. Despite these promising findings, the practical application of MSC-Exos in CVD treatment faces challenges due to the limited expansion and survival of natural MSCs posttransplantation. Therefore, ongoing efforts are directed toward optimizing and engineering MSCs to enhance their therapeutic efficacy in CVD treatment [89].

6. Age-related macular degeneration

Age-related macular degeneration (AMD), a leading cause of vision loss in older adults, has both dry and wet forms and is characterized by the accumulation of lysosomal lipofuscin, the formation of drusen, and the degeneration of retinal pigment epithelium (RPE) [90]. In wet AMD, neovascularization originating from the choriocapillaris leads to detrimental swelling due to increased VEGF expression. Despite the availability of current treatments such as anti-VEGF drugs, their efficacy varies, particularly in the advanced stages of the diseases. EVs, particularly those derived from aged RPEs and marked by lysosomal associated membrane protein 2 (LAMP2), CD63, and CD81, are implicated in drusen formation and may influence RPE function by enhancing exocytosis and releasing proteins that contribute to dysfunction through oxidative stress. Oxidative stress prompts EV release from RPEs, transferring stress signals to healthy cells and inducing apoptosis and inflammation [91]. In response to blue-light stimulation, EVs containing inflammasome mRNA contribute to AMD progression by stimulating angiogenesis and choroidal neovascularization. To address this problem, researchers have explored the therapeutic potential of ADSCs and MSCs [92]. Studies have indicated that ADSCs enhance their migratory capacity in response to conditioned medium derived from stressed RPE cells and effectively protect RPE cells from oxidative damage. Similarly, MSCs have been demonstrated to shield RPE cells from sodium iodate–induced death by suppressing the NF-kB pathway, which activates the NLRP3 inflammasome, while also preserving mitochondrial integrity [93].

These interventions significantly reduce the levels of inflammatory markers such as IL-6, iNOS, IFN-γ, and IL-17, but increase the levels of anti-inflammatory factor TGF-β, thereby alleviating inflammatory conditions in the eye and improve visual functions in AMD models. Such enhancements encompass improvements in visual acuity, visual field, and multifocal photopic electroretinogram (mf-ERG) findings in clinical settings. These results underscore the potential of MSC-Exos to complement RPE replacement therapies for patients with AMD.

The therapeutic and diagnostic potential of MSC exosomes in AMD has garnered considerable attention for their ability to regulate vital pathological processes in degenerative ocular diseases. Studies, such as those conducted by Li et al. and Safwat et al., have underscored the efficacy of exosomes derived from human umbilical cord and adipose tissue MSCs in addressing subretinal fibrosis, choroidal neovascularization, and retinal layer restoration. These exosomes transport essential miRNAs—such as miR-27b and proteins that promote cellular repair and inhibit processes, for example, the epithelial–mesenchymal transition (EMT)—effectively addressing the complex pathophysiology of neovascular AMD [94,95]. Moreover, MSC-Exos protect against degenerative retinal diseases through various pathways. These include the inhibition of retinal tissue inflammation [96,97], downregulation of VEGF expression to suppress choroidal neovascularization [98,99], amelioration of subretinal fibrosis [97], inhibition of microglial activation [100], and protection against photoreceptor apoptosis [[101], [102], [103]]. These multifaceted range of effects highlight the potential of MSC-Exos in AMD treatment in their targeting of the mechanisms underlying disease progression and enhancing of regenerative processes that are crucial for ocular tissue recovery and healing.

The role of exosomal cargos, such as miRNAs and proteins, extends to their potential as biomarkers for AMD diagnosis. Proteins released from RPE-derived EVs, including cathepsin D and Hsp70, detected in the aqueous humor of patients with AMD, suggest their use as biomarkers and therapeutic targets [104]. Exosomal miRNAs such as miR-410 and miR-19a play a role in critical VEGF signaling pathways, influencing apoptosis and angiogenesis, which are crucial for the development of choroidal neovascularization [105]. Studies have suggested that miRNA-410 may reduce VEGF expression and inhibit retinal angiogenesis. Furthermore, EVs released by retinal astrocytes exhibit antiangiogenic properties, inhibiting laser-induced choroidal neovascularization [106]. These findings underscore the dual potential of MSC-Exos in mitigating AMD symptoms and serving as diagnostic tools. However, despite promising preclinical results, more extensive studies on the safety, efficacy, and mechanisms of action of EVs must be conducted before they can be brought from the lab to the clinic. Continued research and clinical trials are imperative to establish the applicability and effectiveness of EV-based therapies in AMD, paving the way for new advancements in managing this debilitating condition.

7. Alzheimer's disease

Alzheimer's disease (AD) is the leading cause of dementia and accounts for 60%–80% of cases of dementia. AD is characterized by the accumulation of amyloid-beta plaques and neurofibrillary tangles, leading to a progressive decline in neurological function [107]. The clinical symptoms of AD include memory loss and worsening cognitive impairment, severely affecting daily functioning and increasing dependency [108]. The FDA has approved several medications for AD treatment, including cholinesterase inhibitors (donepezil, rivastigmine, and galantamine) and memantine, an NMDA receptor modulator [109]. Although these treatments aim to alleviate symptoms and enhance the patient's quality of life, they cannot effectively halt disease progression. This emphasizes the urgent need for identifying innovative therapeutic approaches that can directly target the underlying causes of AD.

The innovative use of MSC-Exos in treating AD highlights their remarkable multifunctionality and immense potential in modern medicine. MSC-Exos are ushering in a new era of medical advancements, offering hope to millions with AD through their extraordinary versatility. First, MSC-Exos are an ideal tool for “liquid biopsy.” They can be extracted from peripheral blood and enriched using immunoprecipitation techniques, providing a highly sensitive and specific method for early AD diagnosis [[110], [111], [112]]. Additionally, MSC-Exos serve as effective drug delivery vehicles. Their excellent biocompatibility and ability to cross the blood–brain barrier allow them to transport drugs directly to targeted brain areas [113,114]. MSC-Exos also play a crucial role in clearing pathogenic proteins. They facilitate the removal of beta-amyloid and tau proteins, which are associated with neuronal damage and death, leading to progressive memory and cognitive decline [[115], [116], [117], [118], [119], [120], [121]]. Furthermore, glycosphingolipids on the surface of exosomes can bind to Aβ, accelerating its clearance [122]. In terms of therapeutic applications, MSC-Exos are highly promising as a means to treat AD. Research conducted by Reza-Zaldivar et al. demonstrated that MSC-Exos can promote neurogenesis and alleviate cognitive impairments in an AD mouse model, indicating their potential in developing cell-free therapies [123]. These exosomes are particularly appealing due to their minimal immunogenicity. Guo et al. reviewed their ability to modify the progression of AD, emphasizing their suitability as a therapeutic tool [124]. Moreover, MSC-Exos derived from human umbilical cord mesenchymal stem cells can alleviate neuroinflammation and reduce amyloid-beta deposition in AD models by modulating microglial activation [125]. They regulate miRNA content and neuronal excitability, contribute to synaptic plasticity [126,127], enhance neuronal activity, and regulate oxidative stress, thereby aiding in neural recovery [128]. Overall, MSC-Exos offer promising new diagnostic and therapeutic approaches for AD treatment strategies, making them a powerful tool against this increasingly severe neurodegenerative disease.

A clinical trial (NCT04388982), conducted in phases I and II, investigated the efficacy and safety of intranasally administered allogenic human adipose MSCs-derived exosomes (ahaMSCs-Exos) in patients with mild to moderate AD [129]. Over a period of 12 weeks, participants received the treatment twice weekly, with no adverse events reported, indicating the treatment's safety and tolerability. Notably, the group receiving a medium dose exhibited significant cognitive improvement. By the 12th week, a reduction in the Alzheimer's Disease Assessment Scale-Cognitive section (ADAS-cog) scores by 2.33 points and an increase in the Montreal Cognitive Assessment scores by 2.38 points relative to the baseline were observed. These findings suggest enhanced cognitive function, with continued improvement in ADAS-cog scores observed up to week 36. Additionally, this group exhibited less hippocampal volume shrinkage, indicating potential benefits in preserving brain volume. These findings support further investigation into the use of ahaMSCs-Exos for AD treatment, particularly at dosages of at least 4 × 10⁸ particles, given their potential to serve as a novel therapeutic approach.

Exosomes have also been explored as potential biomarkers for neurological disorders such as AD, Parkinson's disease, and stroke [130]. Exosomes originating from the central nervous system are detectable in cerebrospinal and peripheral bodily fluids, and the contents of these exosomes vary with disease status [130]. For example, exosomes isolated from plasma or cerebrospinal fluid samples of patients with AD have been observed to carry disease-related proteins, suggesting their utility as biomarkers for AD [130].

Exosomes derived from human umbilical cord mesenchymal stem cells (huc-MSCs) have shown promising therapeutic effects in a mouse model designed to mimic AD (AβPP/PS1 double transgenic mice). These exosomes improved cognitive abilities in the mice, particularly in spatial learning and memory. Furthermore, they led to a significant decrease in amyloid-beta plaques within the cortex and hippocampus. Additionally, the activity of enzymes responsible for degrading the Aβ peptide, namely neprilysin and insulin-degrading enzymes (IDE), increased upon exosome administration. The treatment also effectively reduced neuroinflammation, as evidenced by decreased levels of proinflammatory cytokines and increased production of anti-inflammatory cytokines. However, the precise mechanism underlying this inflammatory response remains unknown. In another study, the administration of exosomes secreted from MSC cells to an AD mouse model improved cognitive function by promoting neurogenesis in the subventricular zone. Exosomes derived from adipose stem cells also hold promise in protecting against AD and may serve as a novel therapeutic approach. These exosomes were found to reduce levels of Aβ, alter the Aβ1-42/1–40 ratio, and reduce neuronal apoptosis, all characteristic features of AD. Additionally, they were shown to promote neurite outgrowth. Neprilysin and IDE, enzymes responsible for degrading Aβ, were detected in these exosomes, further supporting their therapeutic potential in AD. Exosomes carrying neprilysin contribute to the reduction of Aβ levels when internalized, and statins have been found to increase the secretion of exosomes carrying IDE protein, aiding in Aβ clearance. However, the mechanisms involved remain unclear. Furthermore, other enzymes capable of degrading Aβ, such as endothelin-converting enzymes 1/2 and metalloproteinases, as well as cystatin C, a protein that inhibits cysteine protease and found to be imbalanced in AD, are released by exosomes. Studies on primary cortical neurons overexpressing PS2 mutations related to familial AD have demonstrated reduced levels of various forms of exosomal cystatin and Aβ1-40 concentrations.

Cystatin may offer therapeutic benefits in treating AD due to its potential positive effects on the brain, prompting considerable interest in regulating its levels for medical purposes. Exosomes, known for their ability to traverse the blood–brain barrier, offer a promising avenue for AD therapy, particularly in gene therapy applications, where they can accurately deliver nucleic acids. A notable strategy involves the use of exosomes to transport short interfering RNA molecules that target and suppress the BACE1 enzyme responsible for cleaving APP and generating Aβ peptide. Studies have indicated that neuronal exosomes carry Aβ, contributing to the formation of amyloid plaques. Strategies for reducing the secretion or uptake of exosomes by neural cells might prove beneficial. For instance, inhibiting exosome uptake, which relies on the dynamin protein, has shown promise. In experimental models, blocking the enzyme-neutral sphingomyelinase 2, which is involved in ceramide production, may reduce exosome levels, plaque formation, and neuronal cell death. However, caution is warranted when considering the elimination of exosome secretion due to their beneficial roles in AD.

Exosomes have been used as vehicles for delivering therapeutic agents for AD. An innovative strategy entailed loading exosomes with curcumin to target disease mechanisms in an AD mouse model treated with okadaic acid. These curcumin-loaded exosomes effectively countered tau hyperphosphorylation, leading to enhanced cognitive functions. This improvement was attributed to the activation of AKT and inhibition of GSK-3β, a key player in neurofibrillary tangle formation. Although these results are promising, further research is necessary to confirm the therapeutic potential of exosomes in AD treatment.

8. Conclusion

Exosomes derived from stem cells play a pivotal role in transferring their cargo, including miRNA, to parenchymal cells under various conditions such as cartilage degradation, CVDs, brain disorders, and eye diseases. This ability makes exosomes essential for promoting plasticity and functional recovery in degenerative diseases. Given the necessity for complex paracrine signaling, exosomes offer a promising therapeutic approach for managing intricate conditions such as degenerative diseases. The diverse miRNA content in stem cell–derived exosomes allow for tailored therapeutic responses, enhancing their efficacy. Furthermore, exosomes exhibit considerable potential as diagnostic markers for these diseases.

Exosomes offer numerous advantages over traditional cell-based therapies for treating degenerative diseases, as evidenced by clinical trials. Unlike the systemic injection of cells into the bloodstream, which can cause vessel blockages, exosomes with diameters at the nanometer scale can swiftly penetrate the blood–brain barrier and reach the brain without obstructing small vessels. Moreover, exosomes can be effectively retained within joints to treat joint damage. Overall, exosome therapy offers a more targeted and efficient approach for tissue regeneration compared with MSC therapies.

Ongoing research explores the advantages of using stem cell–derived exosome therapy for degenerative diseases. These exosomes, whether natural or engineered, offer therapeutic potential. Although some studies have shown positive results in acute injury disease models, there remains a gap in research concerning chronic degenerative diseases such as OA, CVD, AMD, and AD and. Further investigations are necessary to understand the pathogenesis of these degenerative diseases and to assess the potential benefits of exosomes derived from various sources of MSCs, different preconditioning statuses, doses, and therapeutic regimens (Fig. 1).

Fig. 1.

Overview of the protective, regenerative, and immunomodulatory abilities of MSC-Exos in experimental models of various degenerative diseases. Exosomes have demonstrated potential across a range of degenerative diseases, offering a versatile platform for therapeutic intervention. These tiny vesicles, originating from various cell types, have a diverse range of functions depending on environmental cues and their source. They play crucial roles in intercellular signaling, immune regulation, and tissue repair mechanisms. Notably, MSC-Exos demonstrate the ability to modulate immune responses within cartilage, cardiac, ocular, and brain tissues. Furthermore, the miRNAs or proteomic carried by exosomes can reflect disease status, making them potential disease-specific biomarkers. Their inherent qualities as efficient drug carriers further enhance their appeal as optimal delivery systems. Therefore, the integration of exosome-based therapies and diagnostics presents a promising avenue for addressing age-related degenerative conditions in clinical settings.

9. Perspectives

Exosomes play crucial roles in intercellular communication and hold immense potential for advanced therapeutics. Their diverse origins and inherent targeting abilities make them effective in treating various diseases, including cancer, neurodegenerative disorders, and CVDs. Through the modulation of target cells, exosomes influence proliferation, differentiation, and immune responses as well as serve as carriers for therapeutic payloads, promoting tissue repair and enhancing treatment efficacy. This review explored the multifaceted role of exosomes, emphasizing their diverse sourcing from various cell types and organs, and their pivotal role in developing clinical treatments for cancers and regenerative medicine. Additionally, exosome biogenesis, molecular composition, and current advancements in exosome-based therapies were explored, and challenges and future directions in exosome research were addressed with the aim of translating these therapies into clinical practice. Understanding the complex molecular landscapes of exosomes is crucial for harnessing their diagnostic and therapeutic potentials, leading to innovative strategies in regenerative medicine and disease treatment.

Recent research has increasingly focused on leveraging the therapeutic potential of exosomes for the treatment of various diseases, including cancer, neurodegenerative disorders, CVDs, and regenerative medicine. Exosomes have unique properties, such as remarkable stability and inherent targeting capabilities, which make them promising candidates for therapeutic applications, particularly in cell therapy. Studies have demonstrated that exosomes differ in their biological activities depending on the cell source they are derived from and can effectively modulate various cellular processes, underscoring their potential in targeted drug delivery and therapy. Despite challenges such as their limited circulation lifetime and relatively weak targeting capacity, researchers are working to improve the performance and clinical utility of exosome engineering and therapies.

In conclusion, exosomes are promising therapeutic vehicles in cell therapy given their distinct properties and abilities. However, additional research is necessary to address current challenges and fully harness their potential in this field. This review offers a comprehensive overview of exosomes and their evolving role in intercellular communication, with a particular focus on their utility in cell therapy, along with discussions on exosome biogenesis, composition, mechanisms of intercellular transfer, and the current landscape of exosome-based treatments in both preclinical and clinical settings.

Funding

This research was funded by the Ministry of Science and Technology (MOST 111-2314-B-418-004, MOST 111-2321-B-418-002-, NSTC 112-2321-B-418-004- and NSTC 112-2314-B-418-010-MY3), Far Eastern Memorial Hospital (FEMH; 2022-C-042, 2022-C-123, 2023-C-124 and 2024-C-105), National Yang Ming Chiao Tung University–FEMH Cooperation (111DN12, 111DN22, 112DN13, 112DN22, 113DN09 and 113DN16), and National Taiwan University Hospital–FEMH Cooperation (111-FTN0012).

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.