Emerging role of mesenchymal stem/stromal cells (MSCs) and MSCs-derived exosomes in bone- and joint-associated musculoskeletal disorders: a new frontier

Inflammation has been displayed that contributes to the various aspects of the physiological and pathological procedures of musculoskeletal conditions [39]. For example, physiological inflammation is required for tissue repair and regeneration, like bone and fracture recovery, and is also an alarm for bone and joint infection (BJI) [40]. Nonetheless, many results have proven that deregulated inflammation may ease chronic inflammation in degenerative musculoskeletal diseases, like OA and intervertebral disc (IVD) degeneration [41, 42]. Notably, host biomolecules produced by deteriorated or stressed cells, such as damage-associated molecular patterns (DAMPs), bring about and endure a non-infectious inflammatory reaction [43]. DAMPs are biomolecules that possess a physiological role inside the cell, whereas they attain other activities when exposed to the extracellular environment [43]. Indeed, they alert the body about danger and ultimately arouse an inflammatory reaction. Following their detection by specific sensor-bearing cells, activation of inflammatory responses occurs and, in turn, causes the enhanced local generation of pro-inflammatory cytokines/chemokines by innate immune cells [44]. Mechanistically, upgraded levels of DAMPs activate inflammasome to induce caspase-1 activation, leading to the cleavage of immature precursors of interleukin (IL)-1β and IL-18 into their mature releasable forms [45, 46]. Accordingly, abrogating the DAMP-induced inflammatory responses is an excellent plan to facilitate the clinical management of inflammatory conditions. Active forms of inflammatory diseases, such as RA, are a direct consequence of the discrepancy in the distribution of functional pro-inflammatory T helper 17 cells (Th17) and anti-inflammatory regulatory T cells (Tregs). Thus, targeting immune response through therapeutic modalities is a putative strategy to manage such conditions [47].

Researchers have sought different strategies to alleviate inflammatory responses in the target tissue to enable tissue recovery. In this light, studies have shown that MSCs can moderate inflammatory response by increasing anti-inflammatory processes during tissue repair, thus offering appropriate milieu to cartilage, muscle, or bone regeneration [48]. Both cell-to-cell and paracrine contact typically exert MSCs-elicited immunomodulation through the secretion of a myriad of soluble mediators [49, 50]. They regulate the adaptive and innate immune reactions in musculoskeletal conditions by attenuating Th1 and Th17 cells in vivo [10] and, conversely, improving Th2 and Tregs cell proliferation and activation [51, 52]. It has been shown that mouse BM-MSCs could deter the induction of pro-inflammatory macrophages (M1) while provoking anti-inflammatory macrophages (M2) activity in vitro [53]. The MSCs transplantation also diminishes IL-1, IL-6, IL-8, 1L-17, tumor necrosis factor (TNF)‑α and interferon (IFN)‑γ levels and improves IL-10 and transforming growth factor (TGF)-β levels in inflamed tissue. A broad spectrum of reports shows that MSCs-secreted biomolecules, including TGF-β, IL-10, cyclooxygenase-2 (COX-2), prostaglandin E2 (PGE2), hepatocyte growth factor (HGF), indoleamine-pyrrole 2, 3-dioxygenase (IDO), TNFα-stimulated gene-6 (TSG6), and nitric oxide (NO), play fundamental roles in this regard [51, 52, 54]. For instance, Manferdini et al. (2013) indicated that human adipose tissue (AT)-derived MSCs induce anti-inflammatory influences on chondrocytes and synoviocytes derived from OA patients in a PGE2-dependent manner [55]. IL-6-dependent PGE2 secretion by BM-MSCs from C57BL/6 or DBA/1 mice could avert local inflammation in experimental arthritis in vivo [56]. Another study showed that healthy rat AT-MSCs-derived exosomes ameliorate diabetic osteoporosis in vivo by alleviating NLR family pyrin domain containing 3 (NLRP3) inflammasome activation [57]. Negative regulation of NLRP3 inflammasome activation hinders the secretion of IL-1β and IL-18 in osteoclasts in vitro and leads to restored bone after bone loss in animal models of streptozotocin-induced diabetic osteoporosis in vivo [57]. Interestingly, Gu et al. (2016) reports presented that MSCs derived from gingival tissue (GMSCs) could ameliorate collagen-induced arthritis (CIA) in DBA/1 mice by facilitating the apoptosis of activated T cells through the Fas ligand (FasL)/Fas axis [58].

The MSCs-induced modulatory functions strongly rely on the environmental stimuli. Under certain conditions, MSCs can induce immune responses by producing the pro-inflammatory cytokines and performing as antigen-presenting cells. Their immunostimulatory capacities can be shifted into an immunosuppressive phenotype through a process referred “licensing.” This phenotypic and functional shift is mediated by inflammatory cytokines, in particular, IFN-γ or TNF-α [59]. The dual role of MSCs should be considered once evaluating their immunomodulatory attributes and their clinical applications [60].

Unlike avascular cartilage, bone endothelial cells, such as bone microvascular endothelial cells (BMECs) and also endothelial progenitor cells (EPCs), are involved mainly in the maintenance of vascular homeostasis [38, 61, 62]. The vasculature serves a paramount role in the evolvement of musculoskeletal structures [63]. Vascularization is crucial in the cartilages’ differentiation and mineralization, leading to normal bone formation. By supporting the delivery of nutrients, oxygen, and cells, blood vessels sustain joints and soft tissue's structural and functional integrity and thus promote tissue recovery [64]. Angiogenesis could elicit beneficial effect for treating the various musculoskeletal disorders, such as osteonecrosis. In contrast, angiogenesis inhibitors may enhance the risk of osteonecrosis of the jaw [65]. Of course, dysregulated vascular turnover is supposed to participate in the progression of some disorders, like RA [66]. Pathological analyses signify that aberrant vascularization can stimulate and continue the inflammatory and hyperproliferative milieu of joint [66].

Data signifies the anatomical position of MSC as residing in the “perivascular” space of blood vessels disseminated across the whole body and thus proposes that MSCs may participate in the production of the new blood vessels in vivo [67, 68]. MSCs can secret angiogenic factors and protease to enable blood vessel formation and promote angiogenesis. They release various soluble regulators of angiogenesis, such as matrix metalloprotease 2 (MMP-2), TGF-β1, basic-fibroblast growth factor (b-FGF), IL-6, and vascular endothelial growth factor (VEGF) [69, 70]. Liu et al. (2017) exhibited that MSCs-secreted exosomes can deter bone loss and enhance microvessel density in the femoral head in a rodent model of osteonecrosis of the femoral head (ONFH) by transducing phosphatidylinositol 3-kinase (PI3K)/Akt axis in ECs [71]. It has previously been found that PI3K/Akt signaling pathway is induced by a diversity of stimuli in ECs and adjusts several critical steps in angiogenesis, comprising ECs survival, migration, and capillary-like structure formation [72]. Likewise, Li et al. (2013) indicated that intravenous administration of allogeneic MSCs induced vascular and bone regeneration in the necrotic region of the femoral head in a rabbit model of avascular necrosis of the femoral head (ANFH). This effect was probably caused by improving the target tissue's bone morphogenetic proteins (BMPs), VEGF, and osteopontin (OPN) levels [73]. MSCs-mediated pro-angiogenic products facilitate fracture healing [74] and segmental bone defect [75] in vivo.

MSCs release various cytokines, performing as trophic mediators to regulate neighboring cells. They can potentiate chondrocyte proliferation and abrogate their apoptosis by secreting multiple mediators, such as FGF-1, VEGF-A, and platelet-derived growth factor (PDGF) [76]. Indeed, the augmented cartilage formation found in pellet cocultures of MSCs and chondrocytes primarily relies on the trophic effects of the MSCs, leading to promoting chondrocyte growth and matrix deposition rather than MSCs' trans-differentiation into mature and functional chondrocyte [77]. Like parental MSCs, MSCs-derived exosomes trigger chondrocyte proliferation by inducing Akt and ERK axis in chondrocytes [78, 79]. In addition to the supposed growth factors, miR-135b-enriched exosomes can enhance TGF-β1 expression, increase chondrocyte proliferation, and sustain cartilage repair [80]. Also, long non-coding RNAs (lncRNA) KLF3-AS1-enriched exosomes induces cartilage repair and chondrocyte proliferation in vivo [20] by positive regulation of the G-protein-coupled receptor kinase-interacting protein 1 (GIT1) expression [81]. The GIT1 protein typically enhances the proliferation of chondrocytes and hurdles their apoptosis [82], while its ablation fences chondrocyte differentiation and survival [83]. In addition to the desired effect on chondrocyte viability, proliferation, and differentiation, MSCs-derived exosomes provoke osteogenesis and prohibits osteoporosis in vivo [84]. Meanwhile, Liu et al. (2018) showed that umbilical cord (UC)-derived MSC transplantation ameliorated the joint damage and osteoporosis in collagen-induced arthritic (CIA) mice by improving osteogenic differentiation of CIA mainly via inhibiting TNF-α [85]. MSCs can also release BMP-2 in the defect site, which eventually supports new bone tissue formation, enhances osteoblast function, and sustains the newly synthesized bone tissue's dynamic balance. Such effects are exerted by transducing Smad-mediated pathways MAPK pathway, thereby eliciting osteogenesis [86, 87]. The analysis also revealed that miR-935-enriched exosomes induce osteoblast proliferation and differentiation in osteoporotic rats by down-regulation of signal transducer and activator of transcription 1 (STAT1), operating as a negative regulator of alkaline phosphatase (ALP) expression and activity [88]. ALP is an initial marker of osteoblast differentiation; its improved levels suggest enhanced mineralization. In vitro results also signify that UC-MSCs-derived exosomes may serve as a critical regulator of bone metabolism by transporting C-type lectin domain family 11 member A (CLEC11A) and may denote a putative strategy for averting and treatment of osteoporosis [89]. CLEC11A-carrying exosomes also can increase the change from adipogenic to osteogenic differentiation of bone marrow (BM)-derived MSCs in vitro and down-regulates osteoclast formation [89].

The permanent devastation of the cartilage, tendon, and bone that include synovial joints is the main pathological symptom of RA and OA [90, 91]. Cartilage comprises proteoglycans and type II collagen, while tendon and bone are made up primarily of type I collagen [92]. Mechanistically, inflammatory cytokines, in particular, IL-1β and TNF-α, excite the generation of MMPs, which degrade all constituents of the ECM [93]. The collagenases, MMP-1 and MMP-13, have principal roles in RA and OA since they act as rate-limiting ingredients in the collagen degradation process [94]. MMP-1 is created primarily by the synovial cells that line the joints [95], while MMP-13 is constructed by cartilage-resident chondrocytes [96]. MMP-13 degrades the proteoglycan molecule aggrecan and plays a dual role in matrix destruction [97]. In arthritis, the expression of other MMPs (e.g., MMP-2, MMP-3, and MMP-9) that degrade non-collagen matrix ingredients of the joints is also raised [98]. MSC-derived exosomes could evoke the expression of chondrocyte markers (e.g., type II collagen and aggrecan) while constraining catabolics, such as MMP-13, and a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS5) in an animal model of arthritis [99, 100]. Thus, scientists have focused on their unique ability to prohibit matrix degradation during arthritis progress. Interestingly, synovial explants exposed to MSC-exosomes exhibit down-regulated expression of MMP1 and MMP13, shedding light on the potential of exosomes to influence matrix turnover in synovium and cartilage explants [101].

Although there is no general approach to separating exosomes from other nano- and micro-particles [102, 103], several universally documented techniques have been developed to facilitate the efficient isolation of exosomes from culture media. They encompass differential ultracentrifugation (DUC), density gradients, size exclusion chromatography (SEC), precipitation, and filtration/ultrafiltration (UF) [104]. Meanwhile, DUC and UF are the most dependable method (Table 1). In DUC, a run of centrifugation cycles with dissimilar centrifugal force and time enables exosomes isolation according to alterations in their density and size [105]. Although DUC is simple and cost-effective, this approach has some drawbacks, such as low output and specificity. Another issue is the possibility of contamination with other vesicles and damage to the exosomes due to high-speed centrifuges. Interestingly, the combination of DUC with a sucrose density gradient increased the yield and purity of the exosomes [106, 107]. UF separates exosomes based on the pore size of the filter and is faster and much less troublesome than DUC. However, isolation of the exosomes is challenging when it is contaminated with other vesicles of the same size because the principle of UF is according to the size of vesicles [108, 109]. Similarly, SEC separates high-purity exosomes based on particle size by utilizing columns filled with pore beads. However, this process is time consuming and unsuitable for use in high volume samples, thus barricading its widespread application [108, 110]. Each isolation method has special edges and flaws; its drawbacks could be compromised by merging two or more purification methods and potentiating purity and quantity.

Valuation of the physicochemical properties of exosomes, including size, shape, surface charge, and density, is urgently required for specifying their biological interfaces [111]. Thus, several strategies, including biophysical, molecular, and microfluidic methods, are currently being developed to characterize exosomes [106]. Biophysical methods are mainly applied to determine the exosomal size range. They include nanoparticle tracking analysis (NTA), dynamic light scattering (DLS), tunable resistive pulse sensing (TRPS), flow cytometry (FACS), transmission electron microscopy (TEM), and atomic force microscopy (AFM) [112]. NTA is one of the most critical biophysical approaches, determining the exosomes concentration and size distribution in the 10 nm to 2 µm range [113]. In addition to the biophysical methods, some other molecular approaches, like Raman spectroscopy, a non-destructive chemical analysis system, have been exploited to characterize exosomes [114]. A microfluidic-based tool is also employed to determine exosomes' binding to specific antibodies on microfluidics channels and subsequently to bound vesicles elution. Finally, exosomes may be characterized by determining the presence of their load molecules, more importantly, RNA, using next-generation sequencing (NGS) [115], microarray analysis [116], and digital droplet PCR [117]. Further discussion on the methods of exosomes characterizing is beyond the scope of this paper, and thereby we referred the audience to some excellent articles in this regard [118‐120].

Upon isolation and characterizing, procured exosomes can be applied to ameliorate musculoskeletal diseases by multiple mechanisms listed in previous sections (Fig. 1).

Based on the literature, the restricted amounts of isolated exosomes from parental MSCs hurdle its large-scale production and thus barricade its medical utility. The MSCs experience replicative senescence after a few passages, and therefore, their innate capability to assemble and release exosomes is compromised. Accordingly, detecting or designing strategies or biomolecules to dodge the restricted amounts of produced vesicles are of paramount importance. Kim et al. (2021), for the first time, demonstrated that tangential flow filtration (TFF) system-based strategy may result in more significant numbers of exosomes in comparison to the conventional UCF [121]. Interestingly, ultrasonication of ultracentrifuged MSC-exosomes followed by centrifugation and filtration permits enhancing exosomes’ yield about 20-fold, based on Wang et al. (2019) reports [122]. A hollow fiber three-dimensional (3D) culture system can also enable continuous production of MSC-derived exosomes [123, 124] and leads to exosomes yields 20-fold more than two-dimensional (2D) cultures [125]. Additionally, a 3D mechanical microenvironment can improve the osteogenic activity of MSCs-derived exosomes and alter exosomal miRNA content [126]. Meanwhile, a widely recognized biomaterial, 45S5 Bioglass^® (BG), enhanced the exosomes release from MSCs by promoting the expression of neutral sphingomyelinase-2 (nSMase2) and Rab27a, which up-regulated the nSMases and Rab GTPases axis [127] respectively. Such exosomes also elicited better pro-angiogenic activity and neovascularization by improved levels of the miR-1290 [127]. It should be noted that any strategy and combination used to increase the number of exosomes should not have any adverse effect on the cells and the released exosomes.

OA is the most common painful condition with chronic articular cartilage deterioration. The pathophysiology of OA is complicated and described by the disparity between the synthesis and catabolism of chondrocytes and ECM in association with deregulated inflammation, causing the progressive destruction of articular cartilage [128]. Because of the self-renewal and differentiation properties of MSCs and the secretion of miscellaneous biomolecules, several exogenous MSC-based cell treatments have been designated to alleviate OA [129, 130]. The MSCs-elicited effects substantially rely on the paracrine release of cytokines, growth factors, and exosomes [131].

In 2019, Zhang et al. evaluated the effects of MSCs-derived exosomes in the modification of inflammatory response, nociceptive behavior, and condylar cartilage and subchondral bone healing in a rat model of temporomandibular joint osteoarthritis (TMJ-OA) [132]. They showed that exosomes administration enhanced glycosaminoglycans (GAGs) synthesis and down-regulated NO and MMP13 production in damaged cartilage by transducing Akt, ERK, and AMPK signaling in resident chondrocytes [132]. As demonstrated in both human and experimental OA models, loss of GAG chains of proteoglycans is a primary incident of OA leading to cartilage destruction [133]. Also, MMP-13 is crucial for OA progression, and suppression of MMP13 is an operative approach to decelerate articular cartilage degeneration [134]. Thus, improving GAGs production and negative regulation of MMP-13 by exosomes can elicit both chondroinductive and chondroprotective effects in vivo [134]. Exosomes also support the chondrocyte phenotype by enhancing collagen type II synthesis and reducing the expression of central aggrecanase-degrading articular cartilage matrix, ADAMTS5, which ultimately alleviates cartilage destruction in vivo [135, 136]. Other in vivo results exhibited that exosomes derived from amniotic fluid-(AF)-derived MSCs can improve pain tolerance levels and ameliorate histological scores more evidently than direct administration of the AF-MSC [137]. The effects were mainly attributed to the exosomal TGF-β, which induces chondrogenesis and down-regulates inflammation by inducing anti-inflammatory M2 macrophage polarization [137]. The inequality of M1/M2 macrophages happens in knee OA, and the levels of inequality are related to various degrees of knee OA [138]. M2 macrophages produce anti-inflammatory mediators, such as IL-10, TGF-β, C-C motif chemokine ligand (CCL) 1, CCL17, CCL18, and CCL22 [139]. Accordingly, normalizing this proportion by improving percentages of anti-inflammatory M2 macrophages by MSCs-exosomes therapy averts inflammatory response and exerts chondroprotective effects. Besides, MSC-derived exosomes can potentiate proliferation and abrogate apoptosis of chondrocytes by affecting the lncRNA-KLF3-AS1/miR-206/ G-protein-coupled receptor kinase-interacting protein 1 (GIT1) signaling pathway in OA. The lncRNA-KLF3-AS1-carrying exosomes down-regulates miR-206 to facilitate GIT1 expression, a downstream target of miR-206 [20, 81]. GIT1 is a downstream target of various growth factors, such as PDGF [140] and integrin-β1 [82], and its activation increases chondrocyte proliferation and migration while prohibiting its apoptosis. There is clear evince indicating that GIT1 contributes to the positive regulation of type II collagen expression in chondrocytes [83]. In addition, miR-100-5p-enriched exosomes protected articular cartilage and ameliorated gait abnormalities by suppressing the mammalian target of rapamycin (mTOR)-autophagy pathway in chondrocytes [141], while up-regulation of mTOR expression resulted in increased chondrocyte apoptosis [142]. Targeting cadherin-11 (CDH11) by exosomal miR-127-3p in chondrocytes blocks the Wnt/β-catenin pathway and ameliorates chondrocyte damage in OA pre-clinical models, while CDH11 overexpression in chondrocytes drops exosomes efficacy [143]. In joints, CDH11 is primarily expressed in fibroblast-like synoviocytes (FLS) and participates in adjusting migration, invasion, and degradation of joint tissue. The IL-17 mediated the expression of CDH11 in FLS aggravates synovitis and bone devastation; thus, inhibition of its expression and activity could efficiently ameliorate cartilage damages [144]. Abolishment of the inhibitory effect of MSCs on CDH11 expression by FLS by inhibition of IL-10 activity highlights the intimate association between CDH11 and IL-10 activities [145].

RA is a chronic, symmetrical, inflammatory autoimmune disease that primarily influences small joints, continuing to larger joints, and ultimately the skin, eyes, heart, kidneys, and lungs [146]. Mainly, bone, cartilage, tendons, and ligaments of joints are destructed. Therapeutic modalities have focused on attenuating joint inflammation and pain, potentiating joint function, and bypassing joint deterioration and deformity [147].

A myriad of studies have exhibited that the administration of MSCs-derived exosomes could be an effective strategy to reduce RA pathological symptoms [148‐150]. Meanwhile, Zheng et al. (2020) found that miR-192-5p-enriched exosomes delayed inflammatory response in CIA rat models of RA substantially by negative regulation of the as-related C3 botulinum toxin substrate 2 (RAC2) [151]. RAC2 is often up-regulated in the RA synovium and macrophages and induces inflammation by various mechanisms, more importantly, interacting and activating inducible nitric oxide synthase (iNOS) [152]. The enzyme iNOS inspires NO's formation, triggering deregulated inflammation [152, 153]. In addition, exosomal lncRNA heart and neural crest derivatives expressed 2-antisense RNA 1 (HAND2-AS1) could avert undesired biological behavior of RA-FLS [154]. FLSs are the leading cell type encompassing the structure of the synovial intima. They induce joint inflammation and devastation in RA by secreting pro-inflammatory mediators, like IL-15 and dickkopf-related protein 1 (DKK1) [155]. Exosomal HAND2-AS1 inhibits the proliferation, motility, and inflammation and concurrently stimulates apoptosis in RA-FLSs by down-regulation of the nuclear factor kappa B (NF-κB) pathway [154]. Indeed, HAND2-AS1 directly bypasses miR-143-3p, which acts as a positive regulator of the NF-κB pathway [154]. Likewise, miR-320a-carrying MSCs-derived exosomes abrogated FLS activation through inhibiting C-X-C motif chemokine ligand 9 (CXCL9) expression [156]. CXCL9 and its receptor, C-X-C motif chemokine receptor 3 (CXCR3), are highly expressed in the synovial tissue of RA patients and are thought to contribute to RA pathophysiology [157]. Exosomal miR-320a targets CXCL9 into RA-FLS and suppresses the activation, migration, and invasion of RA-FLSs [156]. The miR-320a-enriched exosomes also attenuates the serum levels of IL-1β, IL-6, and IL-8 in CIA mice, conferring potent anti-inflammatory activities [156]. Apart from the non-manipulated MSCs-derived exosomes, genetically modifying parental MSCs or exosomes (post-isolation) may heighten their anti-inflammatory and pro-regenerative capacities [158]. For instance, Tavasolian et al. (2020) corroborated that miRNA-146a overexpression may enhance the immunomodulatory influences of MSC-derived exosomes in mice models of RA [159]. Higher immunomodulatory competencies were dependent on the increased Tregs population in the spleen of treated mice in association with up-regulated forkhead box P3 (Fox-P3), TGFβ, and IL-10 gene expression [159]. In addition to the plummeting inflammation, MSCs-derived exosomes inhibit deregulated angiogenesis in rodent models of RA [26, 160]. As described, deregulated angiogenesis plays a pathological role in RA as it enables the migration and homing of large numbers of inflammatory cells and molecules [161, 162]. In 2018, Chen et al. found that miR-150-enriched exosomes inhibited tube formation in human umbilical vein endothelial cells (HUVECs) by targeting MMP14 and VEGF in vitro and alleviated hind paw thickness and the clinical arthritic scores by inhibiting synoviocyte hyperplasia and angiogenesis [26]. Similarly, synovial (S)-MSCs-derived exosomes impaired VEGF expression and angiogenic activity in vitro and in CIA mice [160]. Mechanistically, exosomal circular RNAs (circRNAs) EDIL3 can down-regulate miR-485-3p, targeting protein inhibitor of activated STAT3 (PIAS3). PIAS3 is recognized to inhibit STAT3 activity and thus decrease downstream VEGF [160]. Accordingly, circEDIL3-carrying exosomes reduced synovial VEGF and subsequently attenuated arthritis severity in the CIA mouse model [160].

A summary of pre-clinical studies based on MSCs-derived exosomes therapy in OA and RA is listed in Table 2.

Osteoporosis is a mutual age-related condition described by reduced bone mass and weakening bone microarchitecture, causing enhanced skeletal fragility and fracture risk [163, 164]. Although its pathophysiology is complicated, inequality between osteoblasts and osteoclasts, diminished bone volume, and raised adipogenesis in the bone marrow play critical roles [165]. Moreover, inflammatory responses and miRNAs contribute to osteoporosis [166].

In 2021, Yahao et al. exhibited that human UC-MSC-derived exosomes trigger osteogenesis and avert osteoporosis in vivo mainly by transporting various miRNAs, such as miR-2110 and miR-328-3p [84]. These miRNAs support bone development and inhibit osteoclast activities, giving them a dual role in alleviating osteoporosis. As known, osteoclasts sustain the balance of bone metabolism by collaborating with osteoblasts [167]. A deregulated function of osteoclasts brings about several diseases, including osteoporosis, periprosthetic osteolysis, bone tumors, and Paget's disease [168, 169]. Molecular analysis implies that improved receptor activator of nuclear factor-κB ligand (RANKL) levels overactivates osteoclasts by up-regulating inflammasome activation and leads to the loss of bone mass [170]. In contrast, osteoclast deficiency results in osteopetrosis. Thereby, targeting osteoclast activities by cell-based therapeutic or small molecules showed great capacity to lessen the pathological symptoms of osteoporosis. Interestingly, Zhang et al. (2021) revealed that AT-MSCs-derived exosomes could lower diabetic osteoporosis by interfering with the NLRP3 inflammasome activation in osteoclasts, thereby inhibiting the IL-1β and IL-18 secretion [171]. In streptozotocin-induced diabetic osteoporosis rats, administration of miR-146a-enriched exosomes inhibited the TNF-α, IL-18, and IL-1β expression, reduced inflammasome activation, and ultimately attenuated bone resorption and improved bone mass [172]. The miR-146a also inhibits osteoclast transformation by negative regulation of the critical regulators of NF-κβ signaling, thus suggesting that miR-146a can be a therapeutic target for treating inflammation-associated bone loss [173].

In addition to targeting osteoclast differentiation and activity, MSCs-derived exosomes promotes osteoblast activity and proliferation [21, 22]. In vitro, MSC-derived exosomes potentiates the expansion of an osteoblast cell line hFOB 1.19 by up-regulation of the glucose transporter 3 (or GLUT3) levels and triggering the MAPK signaling pathway [22]. The hFOB1.19 cells exposed with MSCs-derived exosomes experienced attenuated apoptosis mainly achieved by down-regulation of apoptosis-related genes, such as caspase-3 and -9 [21]. Such positive effects could be strengthened by raised exosomal miR-150-3p [174]. Wang et al. (2016) have proposed that the miR-150-3p combines inflammation signaling and osteogenesis and participates in the inhibition of effects of inflammation on bone formation [175]. As well, a diversity of miRNAs, such as miR-21, miR-126, miR-29a, miR-142, miR-218, and miR-451, have manifested an excellent capability to trigger osteogenesis [176]. Meanwhile, Zhang et al. (2021) revealed that exosomal miR-935 targets STAT1 and up-regulates ALP activity in osteoporotic rats [88]. STAT1, in fact, acts as a cytoplasmic attenuator of the RUNX family transcription factor 2 (RUNX2) and inhibits the proliferation and differentiation of osteoblasts [177]. Importantly, STAT1–/– osteoblasts demonstrate improved ALP activity and enable better mineralization of bone [177]. Accordingly, negative regulation of STAT1 expression and activities as achieved by exosomal miR-935 is a rational therapeutic plan to improve bone mass in vivo.

Improving angiogenesis is another mechanism by which MSCs-derived exosomes enable bone defect repair [178, 179]. Evidence points that declined angiogenesis results in osteoporosis, and the improved local angiogenesis can relieve osteoporosis [180]. New blood vessels bring oxygen and nutrients to the highly metabolically active regenerating callus. In ovariectomized rats, MSC-derived exosomes intensely inspired bone regeneration and angiogenesis in critical-sized calvarial defects [181]. Two studies showed that exosomal miR-29a [182] and miR-146a [178] serve a crucial role in the MSCs-exosomes-mediated pro-angiogenic effects in osteoporotic rodents. Such miRNAs regulate EC's biological activities, like viability, proliferation, migration, and differentiation. Although up-regulated levels of the miR-29a and miR-146a in tumor tissue have a worse prognosis [183, 184], they play a preferred role in bone repair by positively affecting ECs proliferation.

Osteonecrosis, also identified as avascular necrosis (AVN), aseptic necrosis, or ischemic bone necrosis, is described as bone cell loss resulting from impaired blood flow to the bone from a traumatic or non-traumatic source [185, 186]. Although osteonecrosis usually ensues in the hip joint (femoral head), termed osteonecrosis of the femoral head (ONFH), it can be may also happen in other anatomical regions, such as the shoulder, knee, and ankle [187]. In ONFH, the inadequate blood supply brings about subchondral bone loss and often marked damage to BM [188].

In 2019, Liao et al. revealed that bone marrow (BM)-MSCs-derived exosomes carrying miR-122-5p enhanced the proliferation and differentiation of osteoblasts in vitro [189]. The miR-122-5p-enriched exosomes reduced ONFH progress by down-regulating Sprouty2 (SPRY2), directing the activation of the receptor tyrosine kinases (RTKs)/Ras/ MAPK signaling pathway [189]. In ONFH rabbit models, exosomes administration enhanced bone mineral density (BMD), trabecular bone volume (TBV), and mean trabecular plate thickness (MTPT) of the femoral head, indicating amelioration of ONFH in vivo [189]. Besides, induced pluripotent stem cell (iPSC)-derived MSCs-derived exosomes restricted bone loss and augmented microvessel density in the femoral head of treated ONFH rodents [71]. Additionally, iPS-MSC-exosomes elicited the proliferation, migration, and tube-forming potential of ECs in vitro by transducing the PI3K/Akt signaling pathway in ECs [71]. Thereby, in addition to promoting ontogenesis, angiogenesis fosters exosomes-mediated recovery in the animal model of ONFH. Other reports exhibited that hypoxia-primed MSCs-derived exosomes may show superiority over normoxia MSCs-derived exosomes in terms of exerting pro-angiogenic activity in steroid-induced ONFH in rats [190]. Meanwhile, Yuan et al. (2021) found that hypoxia-primed BM-MSCs-derived exosomes can induce proliferation, migration, and VEGF expression of ECs more prominently than those derived from BM-MSCs cultured under normoxia [190]. Such exosomes inhibited bone loss and high vessel volume in the femoral head in ONFH in rats mainly by provoking angiogenesis [190]. In addition to the MSCs priming with hypoxia, exosomes derived from genetically modified MSCs to overexpress hypoxia-inducible factor 1 alpha (HIF-1α) boosted bone regeneration and angiogenesis, as evidenced by improved trabecular reconstruction and microvascular density in ONFH rabbits [191]. A combination therapy with exosomes and other modalities has also authenticated a more favored therapeutic effect than monotherapy in vivo. Zhang et al. (2020) demonstrated that co-administration of iPS-MSC-exosomes and miR-135b reduced bone loss in ONFH rats mainly by improving proliferation and inhibiting apoptosis of osteoblast cells [192]. Molecular analysis disclosed that miR-135b could intensify the influences of iPS-MSC-exosomes by negative regulation of programmed cell death protein 4 (PDCD4) [192]. PDCD4 down-regulates various survival and proliferation involved signaling axis, like the MAPK axis, thus compromising the expansion and growth of the osteoblast [193]. Although it acts as a tumor suppressor in osteosarcoma [194], elevated levels of the PDCD4 may stall bone repair due to its suppressive effects on multiple axes.

Although bone tissue is capable of natural healing following injuries, the regenerative aptitude of bone tissue is restricted by several factors, including age, type of fracture, and genetic bone disorder [195, 196]. Moreover, about 13% of tibial shaft fractures are associated with fracture non-union or delayed union, characterized as the most intense complication of traumatic fractures [197].

In 2020, Jiang et al. showed that BM-MSCs-derived exosomes promoted fracture healing in mice, as evidenced by X-ray imaging, in part by the transfer of miR-25 [198]. The miR-25 targets Smad ubiquitination regulatory factor-1 (SMURF1) and improves osteoblast differentiation, proliferation, and migration [198]. SMURF1 typically suppresses Runx2 protein expression by stimulating ubiquitination degradation of Runx2 and thus hurdles ontogenesis in vivo. Accordingly, targeting SMURF1 expression and activity by exosomal miR-25 resulted in up-regulated Runx2 levels [198]. Runx2 contributes to the expression of multiple osteogenic genes, including collagen I, osteopontin (OPN), ALP, bone sialoprotein, and osteocalcin (OCN) [199]. As a result, Runx2 overexpression induced by exosomal miR-25 can enable fracture repair. Exosomes also enriched the expression of VEGF and HIF-1α in a rat model of stabilized fracture, thus accelerating fracture healing by triggering angiogenesis [200]. Such effect can be potentiated by hypoxic precondition of MSCs, according to Liu et al. (2020) reports [23]. They demonstrated for the first time that exosomes derived from MSCs under hypoxia could induce more prominent effects on bone fracture healing compared with those under normoxia [23]. Mechanistically, hypoxia preconditioning caused improved production of exosomal miR-126 through up-regulating the HIF-1α axis [23]. The miR-126 intensifies VEGF signaling, angiogenesis, and vascular integrity by suppressing protein production of endogenous VEGF repressors [201]. These results indicated that hypoxia preconditioning could be a putative approach to maximize the actions of MSC-derived exosomes to offer better bone fracture healing. Like activating the VEGF and HIF-1 signaling axis by exosomal miRNAs, miR-335-carrying BM-MSCs-derived exosomes can induce the Wnt/β-catenin pathway in osteoblasts-like cells in vitro [202]. Wnt/β-catenin signaling plays a fundamental role in attaining peak bone mass, influencing the mesenchymal progenitors' commitment to the osteoblast lineage and the anabolic capability of osteoblasts depositing bone matrix. In contrast, Wnt/β-catenin signaling abnormalities have been reported in cartilage and bone defects [203]. Regardless of inducing Wnt/β-catenin signaling, miR-335 can provoke osteoblast cells differentiation via down-regulating the expression of dickkopf‑1 (DKK1) and lessening their apoptosis by down-regulating caspase-3, conferring an excellent capacity for accelerating bone fracture [204]. Additionally, BM-MSCs-derived exosomes hampered IL-1β-mediated inflammation and apoptosis and improved cell proliferation by activating the PI3K/AKT/mTOR signaling pathway and concealing autophagy [205].

A summary of pre-clinical studies based on MSCs-derived exosomes therapy in common bone-associated musculoskeletal conditions is listed in Table 3.