Mechanisms and clinical prospects of mesenchymal stem cells and their derivatives in treating hypothyroidism-induced injuries: A focus on cardiovascular,skeletal muscle,and nervous systems

Cardiovascular injury: Multiple pathomechanisms and clinical risks mediated by THs deficiency

Bioactive T₃ has significant effects on cardiomyocytes. Hypothyroidism increases the risk of ischemic heart disease, myocardial infarction, and cardiovascular mortality through multiple mechanisms, including metabolic disorders, endothelial damage, atherosclerosis, and myocardial remodeling ³⁸ (Figure 2). This condition directly regulates the cardiovascular system via both genomic and nongenomic pathways while mediating systemic metabolic‒cardiovascular dysregulation.

OPEN IN VIEWER

Figure 2.

Pathophysiological mechanisms of cardiovascular manifestations in hypothyroidism. The diagram links thyroid hormone deficiency to cardiovascular disease through interconnected pathways. This includes impaired vasodilation and reduced vascular compliance, promoting hypertension and atherosclerosis via inflammation and arterial stiffening. Concurrent metabolic dysregulation elevates cholesterol and triglycerides. Clinically, this manifests as electrocardiographic abnormalities (e.g. bradycardia, prolonged QT interval). Ultimately, these changes impair cardiac relaxation and filling, leading to diastolic dysfunction, reduced output, and increased vascular resistance. Created with BioRender.com.

At the metabolic level, hypothyroidism activates HMG-CoA reductase to promote cholesterol biosynthesis. Simultaneously, it downregulates hepatic low-density lipoprotein (LDL) receptor expression and inhibits the CYP7A1-mediated clearance of low-density lipoprotein cholesterol (LDL-C), leading to elevated total cholesterol (TC), LDL-C, and triglyceride (TG) levels while compensating for the reduction in high-density lipoprotein cholesterol (HDL-C) ³ . Reduced lipoprotein lipase (LPL) activity further hinders the hydrolysis of TG-rich chylomicrons, resulting in a characteristic lipid metabolism imbalance. Hypothyroidism also causes glucose transporter 4 (GLUT4) translocation disorders in peripheral tissues and increases hepatic phosphoenolpyruvate carboxykinase and glucose-6-phosphatase expression, promoting gluconeogenesis and increasing type 2 diabetes risk ³⁸ . Through mechanisms such as disrupting lipid/glycogen metabolism homeostasis, inducing insulin resistance, and enhancing oxidative stress, hypothyroidism exacerbates the pathological process of metabolic dysfunction. This amplifies metabolic-related cardiovascular risks—including accelerated atherosclerosis, deteriorated endothelial function, and imbalance in myocardial energy metabolism—ultimately significantly increasing the incidence of adverse cardiovascular events.

Cardiac changes associated with hypothyroidism manifest through electrocardiogram abnormalities and echocardiographic findings. These alterations result primarily from the direct regulation of myocardial cell electrophysiological properties by THs (particularly T₃) and vascular dysfunction. During disease progression, early-stage myocardial injury predominantly involves left ventricular diastolic dysfunction, whereas advanced stages progress to dual impairment of both systolic and diastolic functions, accompanied by significant electrophysiological abnormalities that substantially increase the risk of malignant arrhythmias ⁴⁵ . Even in subclinical hypothyroidism (SCH), clinical observations reveal increased carotid intima‒media thickness, diminished flow-mediated dilation (FMD), and elevated arterial stiffness—pathological changes collectively promoting inflammatory responses, oxidative stress, and endothelial dysfunction^46,47. Within the pathological context of hypothyroidism, these vascular abnormalities accelerate atherosclerosis through mechanisms including impaired synthesis of vascular endothelial nitric oxide (NO), leading to peripheral resistance elevation, coagulation‒fibrinolysis system imbalance, and abnormal levels of inflammatory markers such as lipoprotein A and homocysteine. Clinical studies have demonstrated an independent correlation between elevated serum TSH levels and increased cardiovascular risk in young SCH patients ⁴⁸ ; hypothyroidism significantly increases hospitalization risk due to heart failure exacerbation ⁴⁷ . In addition, hypothyroidism-associated thyroxine deficiency syndrome occurs in 20%–30% of chronic heart failure patients and is strongly associated with all-cause mortality ⁴⁷ . Severe hypothyroidism may cause hyperproteinous pericardial effusion due to increased capillary permeability and impaired lymphatic drainage, with rare cases progressing to cardiac tamponade. Therefore, early identification and intervention are critical to effectively reduce the risk of cardiovascular events.

Cardiovascular protection by LT4: Controversy over improved surrogate markers and the absence of hard endpoints

Vascular endothelial function serves as a critical biomarker for cardiovascular risk. Small-scale studies indicate that THT may improve endothelial function, with preliminary evidence supporting its potential cardiocerebrovascular protective effects (e.g. enhanced cardiac function and vascular endothelial function) ⁴⁹ . Experimental data show that LT4 treatment not only corrects thyroid dysfunction but also significantly reduces TC, LDL-C, C-reactive protein (CRP), and bradykinin B1 (BNP) levels, suggesting improvements in metabolic and inflammatory states ⁴⁹ . More importantly, LT4 treatment has been proven to markedly increase blood flow–mediated FMD and decrease pulse wave velocity (PWV), indicating its ability to improve endothelial function and arterial stiffness ⁴⁹ . These synergistic hemodynamic improvements support the hypothesis that LT4 exerts cardiovascular protective effects through multiple pathways, including metabolic regulation, anti-inflammatory effects, and vascular repair. Although limited in sample size, these studies provide new perspectives on the pathological link between thyroid function and cardiovascular risk, highlighting the potential value of LT4 in the secondary prevention of cardiovascular diseases.

However, large-scale research findings remain controversial. Studies by Feller et al. ⁵⁰ indicated that LT4 therapy failed to significantly reduce all-cause mortality, cardiovascular events, atrial fibrillation, or heart failure risk in elderly patients with SCH regardless of their cardiovascular history or age. Consistently, a meta-analysis revealed that LT4 treatment did not substantially lower systolic blood pressure in SCH patients ³ . McAninch et al. noted that among multiple lipid profile indicators (TC, LDL-C, HDL-C, and TG), only TC significantly improved after LT4 therapy. They further emphasized that while such therapy corrects hypothyroidism, it cannot resolve tissue-specific T₃/T₄ conversion disorders or reverse the progression of myocardial fibrosis ⁵¹ (Figure 3).

OPEN IN VIEWER

Figure 3.

Observed therapeutic outcomes and clinical endpoint paradox in thyroid hormone therapy (THT) for hypothyroidism. The diagram illustrates the disconnect between biomarker improvement and clinical outcomes. While THT effectively improves cardiovascular biomarkers—such as lowering cholesterol, CRP, and NTproBNP, and enhancing endothelial function—these changes do not translate into reduced mortality, fewer cardiovascular events, or reversal of myocardial fibrosis. This highlights a mechanistic gap between intermediate functional benefits and meaningful clinical endpoints. Created with BioRender.com.

Multidimensional damage to the nervous system during development and selective brain region mechanisms

THs regulate brain development and function throughout the lifespan, with homeostatic imbalance causing multidimensional neurological damage (Figure 5). Studies on rodent models of hypothyroidism revealed structural brain damage characterized by significant atrophy, including whole-brain volume reduction, cortical thinning, decreased vascular density, and reduced myelin sheathing in white matter regions such as the corpus callosum^57,58. These effects exhibit notable sex-specific differences: male mice, owing to high thyroid hormone receptor alpha (TRα) expression in the hippocampus, show greater sensitivity of the neural stem cells (NSCs) to THs fluctuations, whereas females demonstrate greater functional compensatory capacity ⁵⁹ . The combined effects of these structural changes and sex-specific mechanisms ultimately lead to cognitive deficits, enhanced anxiety-like behaviors, and white matter developmental abnormalities in model animals^60,61.

OPEN IN VIEWER

Figure 5.

The characteristics and clinical manifestations of the effects of hypothyroidism on the nervous system. The diagram illustrates the progressive damage to the central nervous system resulting from impaired thyroid hormone signaling. Dysfunction of nuclear receptors (TRα/TRβ), along with reduced hormone transport (MCT8) and local activation (DIO2↓), suppresses expression of key genes such as BDNF and MBP. This suppression triggers glial cell damage and neuronal injury, characterized by mitochondrial dysfunction and impaired synaptic plasticity. These combined cellular deficits drive extensive neurostructural remodeling, including gray and white matter atrophy, neuronal migration disorders, blood–brain barrier dysfunction, and vascular network rarefaction. Created with BioRender.com.

Although rodent models provide valuable insights into the neuropathological mechanisms of hypothyroidism, significant differences in brain cell types, neural network topology, and spatiotemporal myelination patterns between rodents and humans limit the clinical translation of findings ⁶² . Human neuroimaging evidence indicates that hypothyroidism causes cortical volume reduction, decreased white matter myelination, and vascular network thinning ⁶³ , with a critical developmental window for the brain’s sensitivity to THs. Hypothyroidism occurring during pregnancy or early life can significantly impair neuronal density in the cerebral cortex, hippocampal synaptic plasticity, and the signal integration function of cerebellar Purkinje cells. Children with SCH exhibit atrophy of the frontal/parietal cortex, abnormal elevation of cholinergic compounds and N-acetyl-aspartate levels in the hippocampus and cerebellum, accompanied by disordered white matter fiber structures such as the corpus callosum^64,65. Owing to impaired myelin integrity in major fiber tracts such as the corpus callosum and corticospinal tracts in hypothyroid patients, axonal conduction delays occur^66,67, leading to language, cognitive, memory, and motor coordination disorders^59,64,68. In addition, in hereditary THs signaling defects, TRα mutations cause resistance to TH alpha, which is characterized by reduced nonverbal IQ and sensory‒motor integration disorders; thyroid hormone receptor beta (TRβ) dominant‒negative mutations result in resistance to TH beta, which is characterized by severe intellectual disability and attention deficit symptoms^69,70; and SLC16A2 mutations cause monocarboxylate transporter 8 (MCT8) dysfunction, triggering Allan–Herndon–Dudley syndrome (AHDS), which manifests as progressive myelin degeneration, abnormal neuronal migration, and oligodendrocyte maturation arrest^71,72.

In adults and elderly individuals, thyroid dysfunction is closely associated with neuropsychiatric disorders: reduced gray matter volume in the frontal/occipital regions and decreased GABA concentration in the medial prefrontal cortex lead to an imbalance in the inhibitory–excitatory network, causing depression and executive dysfunction ⁷¹ . Notably, elderly hypothyroid patients have a significantly increased risk of cognitive decline, involving mechanisms related to impaired β-amyloid clearance and excessive Tau protein phosphorylation ⁷³ . Intervention studies have shown that early initiation of LT4 therapy can reverse GABA deficiency and improve memory function ⁷² .

In conclusion, hypothyroidism exhibits distinct developmental stage specificity and regional selectivity in neurodegenerative processes. THs regulate myelin formation, synaptic plasticity, and neurotransmitter homeostasis to maintain brain function. Deficiencies in the spatiotemporal specificity of THs signaling pathways—manifested as developmental transport disorders, metabolic dysregulation in adulthood, and clearance impairment in old age—constitute a potential common mechanism underlying neuropsychiatric disorders.

Neuroregulation of THs: Imbalance of two pathways and glial-immune injury

At the molecular level, THs regulate neural function through dual genomic and nongenomic pathways. In the genomic pathway, THs bind to the nuclear receptors TRα/TRβ to form complexes that directly activate the expression of key genes such as brain-derived neurotrophic factor (BDNF) and myelin basic protein (MBP). Hypothyroidism disrupts this signaling pathway, significantly inhibiting neurogenesis and myelination, thereby affecting memory formation and anxiety-like behaviors^74–76. Research has revealed that T₃ specifically suppresses Sox2 expression via the nuclear receptor TRα1, driving the differentiation of the NSCs into neurons. THs deficiency during development impairs Sox2 suppression while simultaneously downregulating the EGFR pathway (which promotes glial differentiation), ultimately hindering neurogenesis and disrupting the neuron/glia output balance, impacting nervous system function ⁷⁷ . In the nongenomic pathway, THs primarily mediates the rapid regulation of cellular energy homeostasis, including the modulation of mitochondrial respiratory chain complex activity and reactive oxygen species (ROS) metabolic balance. In the central nervous system, approximately 80% of active T₃ rely on local conversion from T₄ by type 2 deiodinase (Dio2) within glial cells, such as astrocytes, followed by transport into neurons via MCT8. The hypothyroid state inhibits Dio2 activity, reducing intracellular active T₃ generation and exacerbating energy metabolism dysregulation^78–80.

Notably, hypothyroidism causes multifaceted damage to glial cells: Dysfunction of the TH transporter (MCT8/OATP1C1) in astrocytes leads to autometabolic abnormalities and mitochondrial respiratory defects, thereby weakening their neurotrophic support capacity ⁷⁸ . During developmental hypothyroidism, the Sonic Hedgehog signaling pathway is inhibited, reducing the number of parvalbumin-positive GABAergic interneurons in the cortex and disrupting inhibitory neural microcirculation ⁸¹ . Immune interactions further amplify neural injury: thyroid peroxidase antibodies (TPO-Ab) bind to astrocytes, triggering autoimmune inflammation. Pro-inflammatory factors such as interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) enhance neuronal excitability by downregulating GABA receptor expression ⁷⁸ . This triple dysfunction involving the glial-metabolic-immune systems creates a cascading damage cascade, ultimately manifesting as neuromotor imbalance, synaptic plasticity impairment, and neural network remodeling disorders. Clinically, this results in cognitive impairment, psychiatric disorders, and increased susceptibility to epilepsy ⁸² .

Improving hypothyroidism and its multisystem complications

The MSCs and their derivatives have demonstrated remarkable efficacy in treating hypothyroidism and related organ damage (Table S2). Transplantation of human amniotic mesenchymal stem cells (hAMSCs) significantly improved thyroid function and multiorgan damage in a mouse model of age-related of subclinical hypothyroidism (AR-SCH). Starting from day 10 of treatment, serum TSH levels decreased markedly, and thyroid autoantibodies (TPO-Ab, TgAb) were reduced, alleviating the destruction of thyroid follicular structures and inflammatory infiltration. In addition, the hAMSCs effectively regulated lipid metabolism, improved hepatic steatosis, and enhanced cardiac function. Mechanistically, the hAMSCs restored the immune balance and inhibited apoptosis in the thyroid, liver, and cardiomyocytes. This therapy reverses core pathological changes in AR-SCH through the dual effects of immunomodulation and anti-apoptosis, effectively improving thyroid function, metabolic disorders, and tissue damage, providing a novel cellular therapeutic strategy for patients unresponsive to conventional treatments ⁸³ . Hypothyroidism significantly impairs bone repair capacity in rats, as manifested by decreased biomechanical properties and degeneration of the bone microstructure. The combined application of low-intensity laser therapy and MSC-conditioned medium (MSC-CM) effectively reversed the hypothyroidism-induced inhibition of bone regeneration. In the treatment group, the bone biomechanical parameters returned to normal levels, with increased trabecular volume and osteoblast count, demonstrating that this synergistic therapy improved bone formation metabolism and mechanical strength, suggesting a new therapeutic approach for hypothyroidism-related bone healing disorders. The use of the MSC-CM alone also effectively improved the biomechanical properties and bone morphology of hypothyroidism rats with bone defects ⁸⁴ . Furthermore, bone marrow–derived mesenchymal stem cells (BM-MSCs) transplantation can repair hypothyroidism-induced pancreatic injury by improving hyperglycemia and insulin secretion, increasing total serum antioxidant capacity, and reducing the levels of the oxidative stress marker malondialdehyde (MDA). Histological evidence confirmed the recovery of the exocrine acinar structure in the pancreas, reducing inflammatory infiltration, improving the islet structure in the endocrine system, and decreasing apoptosis ⁸⁵ .

Collectively, the studies above demonstrate the therapeutic potential of MSC-based strategies. It is important to distinguish the maturity of this evidence. The capacity of the hAMSCs or BM-MSCs to improve functional and histological outcomes in specific hypothyroid animal models represents a well-established and reproducible finding across independent studies^83,85. Similarly, the anti-apoptotic and general antioxidant effects of the MSCs in these contexts are strongly supported. However, other aspects remain in the early-stage or mechanistic hypothesis phase. For instance, the specific contribution of metabolic reprogramming versus direct differentiation to tissue repair, and the long-term stability of immune modulation beyond the experimental period, are inferences based on compelling but preliminary data. The translation of bone repair synergy using laser therapy ⁸⁴ to clinical settings is also a promising yet speculative avenue requiring validation in more complex models. Thus, while the therapeutic foundation is solid, the precise mechanistic hierarchy and optimal application protocols constitute the current frontier of investigation.

Molecular mechanism regulating the immune microenvironment in the treatment of AIT

The core mechanism by which the MSCs treat AIT lies in restoring immune homeostasis and suppressing inflammatory damage. Specifically, in both AIT patients and experimental autoimmune thyroiditis (EAT) models, thyroid tissue exhibits significant oxidative stress and abnormal activation of inflammatory pathways, including ROS accumulation, overactivation of the STING pathway, NLRP3 inflammatory body activation, infiltration of M1 macrophages (iNOS+), and endoplasmic reticulum stress ⁸⁶ . Notably, the MSCs effectively ameliorate thyroid pathological damage in EAT. The key mechanism involves inhibiting the STING pathway, thereby suppressing downstream NLRP3 inflammatory body activation and M1 macrophage polarization while significantly alleviating endoplasmic reticulum stress. This pathway has been identified as a critical target in the EAT model. Although its primacy in human AIT and across different MSC sources awaits further confirmation ⁸⁷ .

In addition to the aforementioned mechanisms, another study revealed the importance of regulating T-cell signaling pathways. Specifically, the human umbilical cord mesenchymal stem cells (hUC-MSCs) can suppress the phosphorylation of signal transduction and activation factor 3 (STAT3) ⁸⁶ by increasing the expression of protein tyrosine phosphatase non-receptor type 2 (PTPN2) in T cells. This regulatory effect directly leads to a reduction in the proportion of IL-17α+ Th17 cells and an increase in CD25⁺FOXP3⁺ Treg cells in the spleen, effectively improving the Th17/Treg imbalance. Concurrently, it is accompanied by decreased serum levels of the Th1-type cytokine interferon-gamma (IFN-γ) and an increased IFN-γ/IL-4 ratio. Crucially, shRNA-mediated knockout of PTPN2 in T cells partially reversed the immunomodulatory effects of the hUCMSCs, strongly confirming that the PTPN2/STAT3 signaling axis plays a central role in hUCMSCs-mediated immune homeostasis reconstruction ⁸⁶ . Finally, these findings were robustly validated in large animal models. In the canine EAT model, both the adipose-derived stem cells (ADSCs) and CTLA4Ig-ASCs effectively reduce TgAA titers, alleviate thyroid inflammatory infiltration, and increase the proportion of regulatory T cells in the peripheral blood and lymph nodes ⁸⁵ . In addition, its secretory group can significantly inhibit the proliferation of peripheral blood mononuclear cells, and the transplanted cells are located mainly in immune-related organs such as the spleen, thyroid, and lymph nodes ⁸⁸ , which provides direct evidence for its mode of action.

The MSC therapy effectively reverses the pathological progression of AIT by synergistically inhibiting inflammatory pathways and reshaping immune homeostasis (Figure 6). Breakthroughs in key mechanistic studies, combined with the application of genetic engineering strategies and successful validation in large animal models, mark pivotal advancements in breakthrough treatments for refractory AIT, accelerating its clinical translation.

OPEN IN VIEWER

Figure 6.

Study on the mechanism and pathway of stem cells and their derivatives in treating hypothyroidism and Hashimoto’s thyroiditis. They target key drivers such as immune dysregulation and endoplasmic reticulum stress, which activate harmful inflammatory pathways like the NLRP3 inflammasome. Through immunomodulation (e.g. promoting Tregs and M2 macrophages), cytoprotection (anti-apoptotic, antioxidant), and specific pathway inhibition (notably suppressing STING overactivation), MSCs alleviate inflammation and cellular stress. This multi-targeted action restores tissue homeostasis. Created with BioRender.com.

Multisource MSCs and derivatives: Heart regeneration is driven by synergistic, paracrine, and targeted communication networks

The MSCs and their derivatives serve as core therapeutic agents in cardiac regeneration, promoting myocardial repair through multiple synergistic mechanisms (Figure 7). The MSCs derived from adipose tissue, bone marrow, and the umbilical cord can differentiate into cardiomyocyte-like cells both in vitro and in vivo; these cells express cardiac-specific genes and induce angiogenesis ⁸⁹ . Their paracrine effects enhance cardiac function by regulating cardiomyocyte and vascular smooth muscle functions, reducing inflammatory fibrosis, and inhibiting apoptosis ⁹⁰ . Exos involved in paracrine signaling show promising potential for cardiovascular disease–related cardiac repair. The therapeutic potential stems from multiple beneficial mechanisms, including antifibrotic, anti-inflammatory, proangiogenic, antioxidant, and antiapoptotic effects, which collectively improve cardiac function ⁹¹ . MicroRNAs (miRNAs) regulate angiogenesis through three mechanisms: in situ transcriptional suppression (targeted degradation of endothelial/pericyte-derived angiogenic mRNAs), paracrine-mediated delivery (Exos-transmitted miRNA to distal tissues), and microenvironmental reprogramming, ultimately dynamically shaping the tissue microenvironment and demonstrating significant potential in cardiovascular diseases ⁹² . The precise regulation mediated by the MSC-Exos content is particularly crucial: miR-223-targeted inhibition of inflammatory genes such as IL-6 and IL-1β reduces myocardial injury^93,94. As carriers of bioactive molecules (TIMP4), the MSC-Exos achieve functional reprogramming of distal target cells (T cells) through myocardial Exos, regulating the Th17/Treg immune balance to ultimately improve heart failure progression ⁹⁵ . The overexpression of HIF2α enhances the angiogenic effects of the hUC-MSC-EVs by upregulating miR-146a to inhibit Spred1, thereby activating the AKT/ERK pathway to promote endothelial cell proliferation, migration, and tubule formation ⁹⁶ .

OPEN IN VIEWER

Figure 7.

An integrated framework for cardiac regeneration: synergistic MSC networks, animal model validation, and biomaterial-based strategies to overcome therapeutic bottlenecks. This schematic summarizes the integrated therapeutic profile of MSCs derived from various tissues. The platform possesses inherent differentiation potential and operates primarily through a potent paracrine network, which provides anti-apoptotic, anti-fibrotic, and pro-regenerative effects. Its secretome contains key regulatory miRNAs and proteins that modulate inflammation, activate pro-angiogenic pathways (e.g. via VEGF), and deliver protective factors. To amplify these effects, synergistic strategies—including biomaterial encapsulation (e.g. in alginate or PLLA matrices) and preconditioning (e.g. with GDF15)—are employed to enhance the platform’s therapeutic efficacy. Created with BioRender.com.

Mechanistic exploration and translational value of animal models in cardiac regeneration research

Owing to their high similarity to human hearts, large animal models provide crucial platforms for the clinical translation of the MSC and MSC-Exo therapies. Current technological optimizations in this field focus on improving cell sourcing, exploring combination therapy strategies, and applying gene-editing technologies to reduce the risk of immune rejection. For instance, Seibt et al. ⁹⁷ demonstrated that intramyocardial injection of the ADSCs effectively reduced the incidence of ventricular arrhythmia in a myocardial infarction rat model. The cardioprotective mechanisms include the upregulation of Cx43 expression in infarcted areas, the suppression of the levels of the pro-inflammatory cytokine interleukin-1 beta (IL-1β), and the reduction in the levels of oxidative stress markers such as 4-HNE, indicating that the ADSCs exert therapeutic effects through anti-inflammatory, antioxidant, and electrophysiological coupling improvements. However, the harsh microenvironment following myocardial infarction severely limits the survival and efficacy of the transplanted MSCs. To address this, researchers have developed multiple strategies. Zhuo Wang et al.’s ⁹⁸ team created an ultrasound-responsive phase-change biomimetic nanoparticle that integrates the MSC membranes with macrophage membranes and carries the key molecule miRNA-125b to regulate cardiomyocyte apoptosis and fibroblast activity. Through ultrasound-targeted microbubble disruption technology, the nanoparticles achieved precise delivery to inflamed myocardial regions, effectively overcoming microenvironmental constraints and significantly promoting cardiac function recovery. In a rat model of myocardial infarction, pretreatment of the MSCs with 2,4-dinitrophenol (DNP) before transplantation significantly increased cell survival in infarcted areas. This treatment also improved cellular homing, adhesion, and differentiation potential, resulting in reduced scar tissue formation, better preservation of left ventricular wall thickness, enhanced angiogenic activity, and improved cardiac function. In addition, the MSC-Exos themselves serve as therapeutic agents through dual mechanisms: upregulating the long noncoding RNA HAND2-AS1 and inhibiting miR-17-5p to activate the Mfn2 pathway, thereby mitigating myocardial injury, apoptosis, oxidative stress, and inflammation induced by hypoxia/reoxygenation ⁹⁹ . Moreover, it delivers the key molecule miR-21-5p to macrophages, promoting their polarization from the pro-inflammatory M1 phenotype to the anti-inflammatory M2 phenotype. This transformation is characterized by decreased expression of M1 markers (iNOS, IL-1β, IL-6, and TNF-α) and increased M2 marker levels, accompanied by reduced inflammatory factor levels and elevated anti-inflammatory agent levels, effectively reducing inflammatory responses and promoting cardiac repair ¹⁰⁰ . Notably, this protective effect is miR-21-5p dependent.

In summary, researchers have effectively overcome microenvironmental constraints through multiple approaches, significantly improving transplant cell survival rates, targeting efficiency, and repairing efficacy, ultimately facilitating the restoration of cardiac structure and function.

Biological materials and pretreatment strategies work together to overcome the efficacy bottleneck of heart regeneration therapy

While stem cells and Exos therapies show significant potential in myocardial repair, their therapeutic efficacy is constrained by the low differentiation efficiency of stem cells into cardiomyocytes. There is an urgent need to develop novel biomaterials and pretreatment strategies to synergistically optimize transplantation outcomes. In biomaterial development, natural polymers such as collagen and alginate, along with synthetic degradable materials such as poly-lactic-co-glycolic acid (PLGA) and polylactic acid, have been widely applied to reshape the extracellular matrix microenvironment, regulate paracrine signaling, and serve as delivery carriers ¹⁰¹ . Moreover, nanomaterials enhance repair efficacy through multiple pathways: gold nanoparticles improve cardiac troponin detection sensitivity; injectable hydrogels loaded with vascular endothelial growth factor (VEGF) and other factors promote angiogenesis; and PLGA nanoparticles target macrophage polarization to restore immune homeostasis. Notably, alginate hydrogels encapsulating miR-126/146a-loaded Exos synergistically promoted angiogenesis, enhanced component retention, and achieved multitarget antifibrotic effects. This significantly reduces the infarcted myocardial area and increases CD31/Connexin43 expression, providing a novel strategy for preventing and treating postmyocardial infarction heart failure^102,103. In response to stem cells pretreatment, the GDF15-pretreated MSCs inhibited mitochondrial division and ROS generation by activating the AMPK pathway, significantly improving cells survival under ischemic conditions while enhancing paracrine functions. This approach improved cardiac function and fibrosis ¹⁰⁴ in a mouse model of myocardial infarction. In addition, the hAMSCs combined with PGS-co-PCL films synergistically enhanced cardiac function parameters (left ventricular ejection fraction, fractional shortening, and stroke volume) and increased VEGF expression in myocardial ischemia‒reperfusion injury models ¹⁰⁵ (Table S3). Furthermore, HSP20-mediated myocardial Exos effectively improved cardiac function in myocardial infarction model mice by activating the AKT signaling pathway ¹⁰⁶ .

The BM-MSCs and the multidimensional mechanism and optimization scheme of Exos regulating skeletal muscle regeneration

Muscle and bone tissue injuries with degenerative changes are common pathological conditions leading to patient disability, where endogenous regeneration is limited by injury severity and the inflammatory microenvironment. The BM-MSCs promote skeletal muscle regeneration through paracrine effects, cellular interactions, and microenvironment regulation. Kinnaird et al. ¹⁰⁷ demonstrated significant enrichment of VEGF, basic fibroblast growth factor (bFGF), placental growth factor (PlGF), and monocyte chemotactic protein-1 (MCP-1) in the BM-MSCs-conditioned medium. Sassoli Chiara’s team further confirmed that key secreted factors such as VEGF and sphingosine 1-phosphate (S1P) synergistically activate the Notch-1/AKT signaling pathway in myoblasts, driving myoblast proliferation and myotubule formation ¹⁰⁸ . In BALB/c mouse ischemic limb models, the BM-MSCs transplantation significantly increased the mid-thigh conduit vessel density and cross-sectional area, enhanced blood flow velocity and distal limb perfusion, reduced the muscle atrophy index and fibrotic area, and improved limb function scores ¹⁰⁹ . Molecular validation revealed upregulated bFGF/VEGF expression at transplantation sites, confirming that paracrine mechanisms achieve functional remodeling through angiogenesis promotion and antifibrosis effects.

The Exos act as core signaling mediators for precise regulation. The miR-210-3p molecules they carry target and inhibit KLF7 gene expression, activating the PI3K/AKT pathway to increase myoblast satellite cell activity ¹¹⁰ . Moreover, miR-215 suppresses oxidative stress-induced myocyte apoptosis through the FABP3 axis ¹¹¹ . However, the regenerative capacity of the BM-MSCs is limited by donor age. In contrast, the hDE-MSCs exhibit enhanced myogenic differentiation capability and improved in vivo implantation efficiency during early development because of their high serum response factor expression level ¹¹² .

Engineering and combined therapies can effectively overcome therapeutic bottlenecks: Hypoxia pretreatment enhances Exos’ regenerative efficacy ¹¹⁰ , whereas coadministration with platelet-rich plasma synergistically activates mitochondrial biosynthesis and promotes myogenic marker expression ¹¹³ . The BM-MSCs and their Exos inhibit the ubiquitin‒proteasome system by activating the Sirt1/PGC-1α pathway, thereby reducing weight loss-induced skeletal muscle atrophy through the suppression of oxidative stress and inflammation ¹¹⁴ . Cross-tissue comparisons demonstrated that the BM-MSCs, with their strong osteogenic differentiation capacity, are suitable for bone‒muscle interface repair, whereas skeletal muscle‒derived stromal cells have greater advantages in tendon regeneration because of their high expression of tendon regulatory proteins ¹¹⁵ . In summary, the BM-MSCs provide a translational foundation for muscle degenerative diseases and trauma repair through three-tier networks: paracrine factors, Exos-miRNAs, and microenvironment reprogramming.

The ADSCs through the study of multimodal paracrine networks, immune metabolism, and engineering enhancement

The ADSCs demonstrate multimodal regulatory mechanisms and significant therapeutic potential in muscle regeneration. With respect to differentiation regulation, specific growth factors can induce the ADSCs to express smooth muscle/skeletal muscle markers and functional ion channels, enabling them to acquire contractile function ¹¹⁶ . IL-1β-activated macrophages drive ADSCs toward angiogenic smooth muscle cell transformation by secreting prostaglandin F2α ¹¹⁷ . In vitro myogenic differentiation models using human adipose-derived stem cells (hADSCs) confirmed that insulin-like growth factor 1 (IGF1) and fibroblast growth factor 2 (FGF2) synergistically activate myogenic differentiation signaling pathways, characterized by significantly upregulated expression of the key regulator MyoD1 (MyoD1↑) ¹¹⁸ . Their paracrine effects, particularly through secreted matrices and EVs, where miRNAs in EVs and proteins in soluble proteins each play unique roles, collectively regulate cell proliferation, differentiation, anti-inflammatory responses, and angiogenesis, thereby accelerating impaired muscle repair ¹¹⁹ . Mouse hindlimb muscle defect models demonstrate that Exos deliver bioactive molecules (e.g. miRNAs) to significantly promote myocyte proliferation and differentiation, which is marked by elevated expression levels of the key myogenic regulators MyoD1 and myoblastin (MyoD1, myoblastin↑), ultimately accelerating muscle tissue regeneration ¹²⁰ . In the SD rat hindlimb ischemia model, adipose-derived stromal vascular components (AD-SVFs) and ADSCs were demonstrated to significantly promote angiogenesis and blood flow recovery in ischemic tissues by secreting key angiogenic factors such as VEGF ¹²¹ . The ADSCs also exhibit significant immunomodulatory properties, inducing macrophages to polarize into the repair-promoting M2 phenotype through the HIF-1α/IL-10/Stat3/ARG-1 axis while coordinating neutrophil and bone marrow progenitor cell infiltration to reshape the ischemic microenvironment and accelerate vascular maturation ¹²² . Clinical translational studies revealed that, compared with the BM-MSCs, ADSCs demonstrate superior efficacy in reducing collagen deposition, promoting myelination, and facilitating reinnervation ¹²³ . Combined strategies involving hypoxic environments—such as the use of Se@SiO2 nanomaterials to enhance antioxidant and mitochondrial functions or the integration of exercise interventions to synergistically regulate AMPK pathways—can further optimize the therapeutic outcomes of the ADSCs ¹²⁴ . Lee and Kemp ¹²⁵ confirmed that a 2% hypoxic environment inhibits hADSC osteoblast and adipocyte differentiation but promotes their myogenic differentiation potential. By integrating multiple targets, including the inflammatory‒oxidative stress balance, angiogenesis, and autonomous cell differentiation, cross-model studies have demonstrated that the ADSCs effectively improve the structural and functional integrity of damaged muscle tissue.

The umbilical cord–derived mesenchymal stromal cells dominate skeletal muscle repair through a paracrine mechanism

The role of umbilical cord–derived mesenchymal stromal cells (UC-MSCs), including umbilical cord blood CD133⁺ hematopoietic stem cells and the Wharton’s Jelly–derived mesenchymal stem cells (WJ-MSCs) from Wollaston’s gel, in skeletal muscle regeneration is mainly dependent on paracrine effects rather than direct differentiation. Studies have shown that although the UC-MSCs can express some myogenic markers, their direct participation in muscle regeneration is low: Koponen et al. ¹²⁶ found that transplanted UCB progenitor cells with VEGF-D were unable to achieve long-term colonization, and the gene modification did not significantly improve angiogenesis or muscle regeneration in the hindlimb ischemia model, indicating that the repair effect was mainly derived from paracrine mechanism. Similarly, Grabowska’s team demonstrated that the transplanted WJ-MSCs can be infused into new muscle fibers only at a low frequency in vivo but still significantly increase muscle mass posttransplantation, a mechanism likely related to the secretion of regenerative-promoting factors ¹²⁷ .

Notably, pretreatment strategies can partially enhance therapeutic efficacy. For example, SDF-1 pretreatment did not alter the myofiber incorporation rate in the WJ-MSCs but significantly increased regenerative muscle mass ¹²⁷ . Han et al. ¹²⁸ further demonstrated that a 1% oxygen concentration activated antiapoptotic pathways and enhanced angiogenic factor secretion, thereby markedly improving capillary neovascularization in the UC-MSCs and significantly improving ischemic injury in mouse hindlimbs. These findings collectively indicate that the UC-MSCs primarily promote muscle repair through microenvironment regulation. The limited myogenic differentiation potential of these materials may be overcome by optimizing the combination of paracrine factors or integrating bioengineering strategies.

In summary, the BM-MSCs, ADSCs, and UC-MSCs primarily regulate inflammation, angiogenesis, and cellular activity through paracrine mechanisms (secreted factors and Exos) to promote muscle regeneration. Among these, the BM-MSCs excel in repairing the bone‒muscle interface, whereas the ADSCs demonstrate multimodal regulatory capabilities (immunomodulation and differentiation) and engineered enhancement potential. In contrast, the UC-MSCs rely mainly on paracrine mechanisms with relatively lower myogenic differentiation efficiency (Figure 8) (Table S4).

OPEN IN VIEWER

Figure 8.

Mechanisms underlying the promotion of skeletal muscle regeneration by MSCs. This schematic outlines how MSCs promote muscle repair through integrated mechanisms. They stimulate satellite cells, modulate the immune response toward regeneration (e.g. promoting M2 macrophages), and secrete key factors (VEGFs, bFGF) that reduce fibrosis and enhance re-innervation. Central to this process is the activation of the PI3K/AKT pathway, which inhibits protein degradation. This potent paracrine action leads to measurable improvements in muscle mass, vascularization, and functional recovery. Created with BioRender.com.

Neural repair potential: Multilineage characteristics, Exo-mediated mechanisms, and advantages of clinical transformation

The developmental origins of the MSCs involve the mesoderm and cranial neural crest ²⁹ . These cells express multiple lineage markers, including α-smooth muscle actin (α-SMA) and smooth muscle myosin heavy chain from the mesenchymal lineage; neuro-associated markers, such as Nestin and Tuj-1; and endothelial molecules, such as CD146 and CD105, along with key signaling receptors, including transforming growth factor-beta receptor (TGF-β receptor) and integrins. This marker profile suggests their potential to participate in tissue repair processes through integrin-mediated extracellular matrix adhesion and paracrine signals (e.g. the TGF-β pathway). The MSCs possess multidirectional differentiation capabilities, not only differentiating into mesodermal cells (such as bone, adipose, and cartilage cells) but also demonstrating potential for endodermal and neuroectodermal differentiation ¹²⁹ . In nervous system repair, the MSCs and their derivatives function primarily through paracrine mechanisms, particularly by secreting EVs ¹³⁰ containing bioactive molecules such as proteins, miRNAs, and mRNAs. These vesicles enable cellular replacement to repair damaged tissues, reshape regenerative microenvironments, and protect existing neurons/glioblasts. These vesicles, with their specific membrane structures, can be precisely targeted and fused with cells. By delivering critical cargo molecules such as amyloid-β (Aβ), tau proteins (Tau), and miRNAs, they exert bidirectional regulatory effects on brain diseases ¹³¹ , modulating neuronal functions and behaviors while suppressing pro-inflammatory cytokine release, reducing neuroinflammation, promoting the NSCs proliferation and differentiation, and enhancing the nervous system’s self-repair capacity—providing new avenues for neurological disease diagnosis and treatment ¹³⁰ . Although the NSCs are considered ideal candidates for central nervous system repair because of their neural differentiation potential, their clinical applications remain limited. In contrast, the MSCs, the more commonly used clinical counterparts, primarily exert therapeutic effects through secretome-mediated immune regulation and nutritional support. Strategies such as promoting bidirectional interactions between the MSCs and NSCs, inducing the MSCs (particularly neural crest-derived subpopulations) to differentiate into neurons, and coculturing with biological scaffolds or reconstructing neural ecological niches can further increase the potential for MSCs-mediated neurorepair ¹³² .

Mediated neurotrophic multitarget repair: Myelin, neurons, glial cells, and neurons

Clinical transformation path: Treatment challenges and precise control strategies for neurological diseases

In addition to animal studies, research has further revealed the therapeutic potential of the MSCs and EVs in neurological disorders. The MSCs deliver neurotrophic compounds via secreted EVs, while the MSC-EVs can penetrate biological barriers to target central nervous system injury sites, demonstrating promise for treating neurodegenerative diseases ¹⁵¹ . In addition, both the MSCs and their released Exos promote neural repair and alleviate neuropathic pain through mechanisms including immune regulation, anti-inflammatory effects, and axonal/myelination regeneration^130,152. Preliminary clinical trials have evaluated the safety and feasibility of EVs in neurological conditions. In an acute ischemic stroke trial, five male patients with malignant middle cerebral artery occlusion treated with craniectomy received intracerebral injections of allogeneic placental MSC-derived EVs within 48 h of symptom onset, with no reported adverse events. In a mild-to-moderate AD trial (NCT04388982), phase I/Ⅱ clinical data revealed good tolerability and safety of intranasal administration of the allogeneic human adipose-derived MSC-Exos (hADMSC-Exos) in nine patients. The study established a minimum effective dose of 4 × 10⁸ particles for subsequent trials and suggested comparable therapeutic potential to existing AD medications such as GV-971 or donepezil. Together, these findings demonstrate that the MSC-EVs-based cell-free therapies have promising clinical applications in the field of neurological diseases, and their preliminary safety and feasibility data lay the foundation for deeper clinical translation.

In summary, the MSCs and their Exos, among other EVs, as “cell-free” therapies, deliver bioactive substances to regulate neuroinflammation at multiple targets, promote myelin regeneration, protect neurons, and improve cognitive function. These therapies have unique advantages in treating various neurological diseases, such as stroke and Alzheimer’s disease (Figure 9, Table S5). They avoid the risks associated with cell transplantation, have shown safety and efficacy in preliminary clinical trials, and represent a breakthrough in the field of neural repair. Future efforts should focus on engineering optimization and large-scale clinical validation to accelerate their application.

OPEN IN VIEWER

Figure 9.

Mechanisms of remyelination and neuroprotection mediated by MSCs. This diagram illustrates the multi-target strategy of MSCs for promoting neural repair. By inhibiting key inflammatory pathways (e.g. NF-κB, JAK2/STAT3), MSCs reduce neuroinflammation, promote an anti-inflammatory glial phenotype, and help preserve blood–brain barrier integrity, thereby fostering remyelination and synaptic repair. In addition, certain MSCs (e.g. WJ-MSCs) can directly differentiate into neural cells to support structural regeneration. Through these combined actions, MSCs enhance neuroprotection and facilitate functional recovery. Created with BioRender.com.