Myogenic Progenitor Cells Control Extracellular Matrix Production by Fibroblasts during Skeletal Muscle Hypertrophy

Single-cell transcriptomic analysis reveals alterations to cellular dynamics and paracrine signaling in radiation-induced muscle pathology

Radiation therapy causes long-term skeletal muscle atrophy and fibrosis in juvenile cancer survivors. The mechanisms responsible for the skeletal muscle late effects of radiation therapy are not well-understood and have prevented the development of effective treatments. Using single-cell RNA sequencing (scRNA-seq), we characterize cellular dynamics and communication in a murine model of therapeutic radiation at 24 h and 56 days post-irradiation (post-IR). We detected changes in muscle stem (satellite) cells (MuSCs) characterized by an acute preservation of committed MuSCs and long-term relative depletion of deep quiescent MuSCs. A conserved senescence Cdkn1a signature was observed in all muscle-resident cells post-IR. Genes related to fibroblast proliferation were upregulated and a fibrotic and senescent transcriptome persisted in fibro-adipogenic progenitors (FAPs) post-IR. Intercellular communication analysis revealed FAPs as the primary contributor of extracellular matrix (ECM) and target of monocyte/macrophage-derived transforming growth factor (TGF)-β signaling post-IR through TGF-βR2 on FAPs. Together, our findings provide insights into the potential mechanisms and intercellular communication responsible for radiation-induced muscle atrophy and fibrosis.NEW & NOTEWORTHY This work describes, for the first time, the transcriptional changes occurring following radiation exposure in the skeletal muscle microenvironment using scRNA-seq technology. We revelated that FAPs exhibited a profibrotic and senescent transcriptome. Radiation exposure led to a conserved and persistent Cdkn1a gene signature and impairs intercellular communication, increasing TGF-βR2 signaling in FAPs. These findings uncover potential mechanisms and intercellular communication responsible for long-term muscle impairments post-radiation, offering new targets for therapeutic intervention.

Nutritional strategies targeting age-related skeletal muscle fibrosis: underlying mechanisms

Aging is associated with a reduced number and function of muscle stem cells (MuSC). This results in a decreased muscle regenerative capacity and increased formation of fibrotic tissue, impairing skeletal muscle function. This review provides an overview of in vitro and in vivo animal studies investigating nutritional interventions with the potential to inhibit pathophysiological mechanisms involved in the development of skeletal muscle fibrosis. Mechanism targets include 1) MuSC function and myogenic differentiation, 2) M1 to M2 macrophage polarization, 3) myofibroblast activity or extracellular matrix (ECM) deposition, and 4) reactive oxygen species (ROS) mediated pathways, such as NOX2/4 activity. Most promising nutrients described in this review are phytonutrients, vitamins and amino acids. Quercetin targets multiple pathways (showing decreased inflammation, ECM expression and NOX2/4 activity) in various cell types and tissues (kidney, aorta, liver and (heart) muscle) of rodents and rabbits, which could contribute to fibrosis development. Additionally, sulforaphane is a promising candidate as it inhibits inflammation, ECM expression, and ROS production in mouse skeletal muscle. After validation of the effects in human skeletal muscle, supplementation with these nutrients could be implemented in a multifaceted intervention (including exercise and adequate protein intake) targeting age-related skeletal muscle fibrosis.

The Outcome of Cell Therapy Treating Urinary Incontinence Correlates with Precise Cell Localization in the Sphincter Complex

Background/Objectives: Urethral sphincter muscle deficiency is the leading cause of stress urinary incontinence. Preclinical and clinical studies suggested that cell therapy may improve the situation. However, the overall efficacy of cell therapies did often not satisfy the patient's needs. We, therefore, investigated in a large animal model of incontinence if the localization of injected regenerative cells in the deficient urethral sphincter muscle correlated with the outcome. Methods: Urethral sphincter insufficiency was induced in three cohorts of pigs and confirmed by urodynamics. Then, either myogenic progenitor cells (MPCs) or adipose tissue-derived stromal cells (ADSCs) were injected into the injured sphincter complex by Williams needle under visual using a cystoscope. Sham-treated animals served as controls. Functional sphincter muscle regeneration was monitored by urodynamics over 5 weeks of follow-up. The localization of the injected cells was investigated by histology of cryosections of the tissue targeted. Results: Injection of MPCs near the sphincter muscle yielded better functional recovery when compared to MPC injections in adjacent sides. By contrast, injection of ADSCs in the submucosal tissue adjacent to the muscle led to better regeneration when compared to ADSC injections into the sphincter muscle. After five weeks of follow-up, MPCs yielded an overall robust but not significant improvement when compared to mock-treated controls, while ADSC injections reached significance. Conclusions: This small proof-of-principle study suggests that the clinical outcome of cell therapy for urinary incontinence depends on the choice of therapeutic cells and the precise localization of the cells in the tissue targeted as well.

Cardiac Fibroblasts: Helping or Hurting

Cardiac fibroblasts (CFs) are the essential cell type for heart morphogenesis and homeostasis. In addition to maintaining the structural integrity of the heart tissue, muscle fibroblasts are involved in complex signaling cascades that regulate cardiomyocyte proliferation, migration, and maturation. While CFs serve as the primary source of extracellular matrix proteins (ECM), tissue repair, and paracrine signaling, they are also responsible for adverse pathological changes associated with cardiovascular disease. Following activation, fibroblasts produce excessive ECM components that ultimately lead to fibrosis and cardiac dysfunction. Decades of research have led to a much deeper understanding of the role of CFs in cardiogenesis. Recent studies using the single-cell genomic approach have focused on advancing the role of CFs in cellular interactions, and the mechanistic implications involved during cardiovascular development and disease. Arguably, the unique role of fibroblasts in development, tissue repair, and disease progression categorizes them into the friend or foe category. This brief review summarizes the current understanding of cardiac fibroblast biology and discusses the key findings in the context of development and pathophysiological conditions.

Mapping Organism-wide Single Cell mRNA Expression Linked to Extracellular Vesicle Biogenesis, Secretion, and Cargo

Extracellular vesicles (EVs) are functional lipid-bound nanoparticles trafficked between cells and found in every biofluid. It is widely claimed that EVs can be secreted by every cell, but the quantity and composition of these EVs can differ greatly among cell types and tissues. Defining this heterogeneity has broad implications for EV-based communication in health and disease. Recent discoveries have linked single-cell EV secretion to the expression of genes encoding EV machinery and cargo. To gain insight at single-cell resolution across an entire organism, we compared the abundance, variance, and co-expression of 67 genes involved in EV biogenesis and secretion, or carried as cargo, across >44 000 cells obtained from 117 cell populations in the Tabula Muris. Our analysis provides both novel holistic and cell population-specific insight into EV biology. The highest overall expression of EV genes occurs in secretory cells of the pancreas and perhaps more surprisingly, multiple non-neuronal cell populations of the brain. We find that the most abundant EV genes encode the most abundant EV cargo proteins (tetraspanins and syndecans), but these genes are highly differentially expressed across functionally distinct cell populations. Expression variance identifies dynamic and constitutively expressed EV genes while co-expression analysis reveals novel insights into cell population-specific coordination of expression. Results of our analysis illustrate the diverse transcriptional regulation of EV genes which could be useful for predicting how individual cell populations might communicate via EVs to influence health and disease.

A satellite cell-dependent epigenetic fingerprint in skeletal muscle identity genes after lifelong physical activity

Satellite cells comprise a small proportion of mononuclear cells in adult skeletal muscle. Despite their relative rarity, satellite cells have critical functions in muscle adaptation, particularly during prolonged exercise training. The mechanisms by which satellite cells mediate skeletal muscle responsiveness to physical activity throughout the lifespan are still being defined, but epigenetic regulation may play a role. To explore this possibility, we analyzed global DNA methylation patterns in muscle tissue from female mice that engaged in lifelong voluntary unweighted wheel running with or without satellite cells. Satellite cells were ablated in adulthood using the tamoxifen-inducible Pax7-DTA model. Compared to sedentary mice, wheel running for 13 months caused muscle DNA methylation differences in the promoter regions of numerous muscle fiber-enriched genes-Cacgn1, Dnm2, Mlip, Myl1, Myom2, Mstn, Sgca, Sgcg, Tnnc1, Tnni2, Tpm1, and Ttn-only when satellite cells were present. These genes relate to muscle fiber identity, cytoarchitecture, and size as well as overall muscle function. Epigenetic alterations to such genes are consistent with previously observed histological and in vivo impairments to running adaptation after satellite cell depletion in these same mice. Musk promoter region methylation was affected only in the absence of satellite cells with lifelong running relative to sedentary; this dovetails with work showing that satellite cells influence skeletal muscle innervation. Defining the epigenetic effects of satellite cells on identity genes in muscle fibers after lifelong physical activity provides new directions for how these rare stem cells can promote muscle adaptation and function throughout the lifespan.

A primer on global molecular responses to exercise in skeletal muscle: Omics in focus

Advances in skeletal muscle omics has expanded our understanding of exercise-induced adaptations at the molecular level. Over the past 2 decades, transcriptome studies in muscle have detailed acute and chronic responses to resistance, endurance, and concurrent exercise, focusing on variables such as training status, nutrition, age, sex, and metabolic health profile. Multi-omics approaches, such as the integration of transcriptomic and epigenetic data, along with emerging ribosomal RNA sequencing advancements, have further provided insights into how skeletal muscle adapts to exercise across the lifespan. Downstream of the transcriptome, proteomic and phosphoproteomic studies have identified novel regulators of exercise adaptations, while single-cell/nucleus and spatial sequencing technologies promise to evolve our understanding of cellular specialization and communication in and around skeletal muscle cells. This narrative review highlights (a) the historical foundations of exercise omics in skeletal muscle, (b) current research at 3 layers of the omics cascade (DNA, RNA, and protein), and (c) applications of single-cell omics and spatial sequencing technologies to study skeletal muscle adaptation to exercise. Further elaboration of muscle's global molecular footprint using multi-omics methods will help researchers and practitioners develop more effective and targeted approaches to improve skeletal muscle health as well as athletic performance.

Fork in the road: therapeutic and pathological actions for fibro-adipogenic progenitors following musculoskeletal injury

Musculoskeletal injuries are a substantial source of global disability through weakness and loss of function, which can be attributable, in part, to deficits in skeletal muscle quality. Poor muscle quality, resulting from fibrotic pathology or fatty infiltration, strongly predicts lower rates of patient recovery following injury and higher rates of re-injury. The cellular sources of fibrosis and fatty infiltration within skeletal muscle are mesenchymal fibro-adipogenic progenitors (FAPs), which are central effectors to support muscle homeostasis, regeneration and growth. However, following acute or chronic musculoskeletal injury, FAPs can promote fibro/fatty pathology within muscle that is likely to limit recovery and repair. Given their indispensable role within skeletal muscle, FAPs have emerged as a compelling cellular target to promote tissue recovery following acute and chronic injury. This review provides insight into the aetiology of FAP activity following various musculoskeletal injuries, in addition to signalling components that effect FAP differentiation. Contrasting pathology with therapeutic potential, insight into disease- and injury-specific FAP activation further cements their role as crucial effectors to improve muscle function and enhance patient outcomes.

Muscle regeneration and muscle stem cells in metabolic disease

Skeletal muscle has a high regenerative capacity due to its resident adult muscle stem cells (MuSCs), which can repair damaged tissue by forming myofibres de novo. Stem cell dependent regeneration is critical for maintaining skeletal muscle health, and different conditions can draw heavily on MuSC support to preserve muscle function, including metabolic diseases such as diabetes. The global incidence and burden of diabetes is increasing, and skeletal muscle is critical for maintaining systemic metabolic homeostasis and improving outcomes for diabetic patients. Thus, poor muscle health in diabetes, termed diabetic myopathy, is an important complication that must be addressed. The health of MuSCs is also affected by diabetes, responsible for the poor muscle regenerative capacity and contributing to the functional decline in diabetic patients. Here, we review the impact of diabetes and metabolic disease on MuSCs and skeletal muscle, including potential mechanisms for impaired muscle regeneration and MuSC dysfunction, and how these deficits could be addressed.

Effects of leg immobilization and recovery resistance training on skeletal muscle-molecular markers in previously resistance-trained versus untrained adults

We sought to examine how resistance training (RT) status in young healthy individuals, either well resistance trained (T, n = 10) or untrained (UT, n = 11), affected molecular markers with leg immobilization followed by recovery RT. All participants underwent 2 wk of left leg immobilization via a locking leg brace. Afterward, all participants underwent 8 wk (3 days/wk) of knee extensor-focused progressive RT. Vastus lateralis (VL) ultrasound-derived thickness and muscle cross-sectional area were measured at baseline (PRE), immediately after disuse (MID), and after RT (POST) with VL muscle biopsies also being collected at these time points. Both groups presented lower ultrasound-derived VL size metrics at MID versus PRE (P ≤ 0.001), and values increased in both groups from MID to POST (P < 0.05); however, VL size increased from PRE to POST in UT only (P < 0.001). Mean and type II myofiber cross-sectional area values were greater at PRE and POST versus MID (P < 0.05), with T being greater than UT throughout (P ≤ 0.012). In both groups, satellite cell number was not affected by leg immobilization but increased in response to RT (P ≤ 0.014), with T being greater than UT throughout (P = 0.004). Total RNA (ribosome content) decreased (P = 0.010) from PRE to MID while total RNA and certain endoplasmic reticulum stress proteins increased from MID to POST regardless of training status. Immobilization-induced muscle atrophy and recovery RT hypertrophy outcomes are similar between UT and T participants, and the lack of molecular signature differences between groups supports these findings. However, results are limited to younger adults undergoing noncomplicated disuse.NEW & NOTEWORTHY Formerly trained and untrained individuals demonstrate similar atrophic responses to disuse while untrained individuals exhibited a greater hypertrophic response to subsequent resistance training. The molecular responses accompanying these changes were largely similar between groups and included increases in satellite cell content with resistance training and increases in ribosome biogenesis, which was largely driven by the formerly trained group.

Adipose mesenchymal stem cell-derived extracellular vesicles alleviate renal fibrosis by reducing epithelial-mesenchymal transition via the FOXS1/Wnt/β-catenin signaling pathway

BACKGROUND

Adipose mesenchymal stem cells (ADSCs) exert beneficial effects on kidney disease through a paracrine mechanism. However, the specific molecular mechanisms by which ADSCs treat renal fibrosis are not yet fully understood. Therefore, it is crucial to clarify the therapeutic effects of ADSC-derived extracellular vesicles (ADSC-EVs) on the progression of renal fibrosis and their underlying mechanisms.

METHODS

We investigated the therapeutic effects of ADSC-EVs on renal fibrosis both in vivo and in vitro. Key genes and signaling pathways were identified with RNA sequencing analysis of HK-2 cells. The role and underlying mechanism of the FOXS1/Wnt/β-catenin pathway in mediating antifibrotic effects were also verified.

RESULTS

In vivo, We found that ADSC-EV treatment significantly improves renal fibrosis in unilateral ureteral obstruction (UUO)-induced renal fibrosis mice models. And in vitro, our data suggested that ADSC-EVs can reduce epithelial-mesenchymal transition (EMT) to inhibit fibrosis in transforming growth factor-β1 (TGF-β1)-treated HK-2 cells. The RNA sequencing results showed that FOXS1 was the primary gene involved in ADSC-EV treatment of renal fibrosis. RT-qPCR suggested that ADSC-EV treatment reversed elevated FOXS1 level both in TGF-β1-treated HK-2 cells and UUO-induced renal fibrosis mice models. Moreover, Western blot analysis confirmed that ADSC-EVs alleviate renal fibrosis and EMT by inhibiting the expression of FOXS1 in HK-2 cells treated with TGF-β1. Furthermore, overexpression of FOXS1 in HK-2 cells could promote the occurrence of fibrosis and knockdown of FOXS1 could reduce the occurrence of fibrosis. Finally, ADSC-EVs may exert these effects via FOXS1-mediated activation of the Wnt/β-catenin pathway.

CONCLUSION

Taken together, we have confirmed that ADSC-EVs alleviate renal fibrosis by reducing EMT via the FOXS1/Wnt/β-catenin signaling pathway.

Effect of human myoblasts on tenogenic progression in 2D and 3D culture models

Tendon injuries and disorders associated with mechanical tendon overuse are common musculoskeletal problems. Even though tendons play a central role in human movement, the intrinsic healing process of tendon is very slow. So far, it is known that tendon cell activity is supported by several interstitial cells within the tendon. However, the interplay between the tendon and the adjacent muscle for tendon regeneration and development processes has not been fully investigated. Here, we tested whether factors released from muscle derived myogenic cells (myoblasts) enhance tenogenic progressions of human tendon derived cells (tendon fibroblasts) using two-dimensional (2D) culture model and a three-dimensional (3D)-engineered tendon construct culture model, which mimics tendon regeneration and development. The conditioned media from myoblasts and unconditioned media as control were applied to tendon fibroblasts. In 2D, immunofluorescence analysis revealed increased collagen type I expressing area and increased migration potential when conditioned media from myoblasts were applied. In the 3D-engineered human tendon construct model, wet weight, diameter, and cross-sectional area of the tendon constructs were increased in response to the application of conditioned media from myoblasts, whereas the collagen density was lower and mechanical function was reduced both at the functional level (maximum stiffness) and the material level (maximum stress and modulus). These results indicate that myoblast-derived factors extend collagen expressing area and enhance migration of tendon fibroblasts, while factors involved in the robustness of extra-cellular matrix deposition of tissue-engineered tendon constructs are lacking. Our findings suggest that adjacent muscle affects the signaling interplay in tendons.

Aptamer-conjugated gold nanoparticles enable oligonucleotide delivery into muscle stem cells to promote regeneration of dystrophic muscles

Inefficient targeting of muscle stem cells (MuSCs), also called satellite cells, represents a major bottleneck of current therapeutic strategies for muscular dystrophies, as it precludes the possibility of promoting compensatory regeneration. Here we describe a muscle-targeting delivery platform, based on gold nanoparticles, that enables the release of therapeutic oligonucleotides into MuSCs. We demonstrate that AuNPs conjugation to an aptamer against α7/β1 integrin dimers directs either local or systemic delivery of microRNA-206 to MuSCs, thereby promoting muscle regeneration and improving muscle functionality, in a mouse model of Duchenne Muscular Dystrophy. We show here that this platform is biocompatible, non-toxic, and non-immunogenic, and it can be easily adapted for the release of a wide range of therapeutic oligonucleotides into diseased muscles.

Muscle-derived extracellular vesicles mediate crosstalk between skeletal muscle and other organs

Skeletal muscle (SKM) has crucial roles in locomotor activity and posture within the body and also functions have been recognized as an actively secretory organ. Numerous bioactive molecules are secreted by SKM and transported by extracellular vesicles (EVs), a novel class of mediators of communication between cells and organs that contain various types of cargo molecules including lipids, proteins and nucleic acids. SKM-derived EVs (SKM-EVs) are intercellular communicators with significant roles in the crosstalk between SKM and other organs. In this review, we briefly describe the biological characteristics, composition, and uptake mechanisms of EVs, particularly exosomes, comprehensively summarize the regulatory effects of SKM-EVs on the function of, which include myogenesis, muscle repair and regeneration, as well as metabolic regulation. Furthermore, we explore the impact of SKM- EVs on various organs including bone, the cardiovascular system, adipose tissue, and nervous system. As emerging evidence suggests that SKM-EVs are involved in the development and regulation of type 2 diabetes (T2D), systemic inflammation, and other chronic diseases, we also highlight the potential of SKM-EVs as therapeutic targets and diagnostic biomarkers, emphasizing the need for further research to elucidate the complex mechanisms underlying intercellular communication in physiological and pathological contexts.

miRNA-1 regulation is necessary for mechanical overload-induced muscle hypertrophy in male mice

MicroRNAs (miRNAs) are small, noncoding RNAs that play a critical role in regulating gene expression post-transcriptionally. They are involved in various developmental and physiological processes, and their dysregulation is linked to various diseases. Skeletal muscle-specific miRNAs, including miR-1, play a crucial role in the development and maintenance of skeletal muscle. It has been demonstrated that the expression of miR-1 decreases by approximately 50% in response to hypertrophic stimuli, suggesting its potential involvement in muscle hypertrophy. In our study, we hypothesize that reduction of miR-1 levels is necessary for skeletal muscle growth due to its interaction to essential pro-growth genes. Promoting a smaller reduction of miR-1 levels, we observed a blunted hypertrophic response in mice undergoing a murine model of muscle hypertrophy. In addition, our results suggest that miR-1 inhibits the expression of Itm2a, a membrane-related protein, as potential miR-1-related candidate for skeletal muscle hypertrophy. While the exact mechanism in muscle hypertrophy has not been identified, our results suggest that miR-1-regulated membrane proteins are important for skeletal muscle hypertrophy.

Exploring the molecular basis of efficient feed utilization in low residual feed intake slow-growing ducks based on breast muscle transcriptome

Residual feed intake (RFI) has recently gained attention as a key indicator of feed efficiency in poultry. In this study, 800 slow-growing ducks with similar initial body weights were reared in an experimental facility until they were culled at 42 d of age. Thirty high RFI (HRFI) and 30 low RFI (LRFI) birds were selected to evaluate their growth performance, carcass characteristics, and muscle development. Transcriptome and weighted gene co-expression correlation network analyses of pectoral muscles were conducted on six LRFI and six HRFI ducks. The results revealed that selecting for LRFI significantly reduced feed consumption (P < 0.05) and improved feed efficiency without affecting the growth performance, slaughter rate, or meat quality of ducks (P > 0.05). Moreover, compared with HRFI ducks, LRFI ducks had a lower pectoral muscle fat content (P < 0.05), larger muscle fiber diameter and area (P < 0.05), and lower muscle fiber density (P < 0.05). There were significant differences in gene expression between LRFI and HRFI ducks, with 102 upregulated and 258 downregulated genes, which were enriched in the PPAR signaling pathway, adipocytokine signaling pathway, actin cytoskeleton regulation, ECM-receptor interaction, and focal adhesion. The expression of genes associated with fat and energy metabolism, including ACSL6, PCK1, APOC3, HMGCS2, PRKAG3, and G6PC1, was downregulated in LRFI ducks, and weighted gene co-expression correlation network analysis identified PRKAG3 as a hub gene. Our findings indicate that reduced mitochondrial energy metabolism may contribute to the RFI of slow-growing ducks, with PRKAG3 playing a pivotal role in this biological process. These findings provide novel insights into the molecular changes underlying RFI variation in slow-growing ducks.

Daily blood flow restriction does not affect muscle fiber capillarization and satellite cell content during 2 wk of bed rest in healthy young men

The present study assessed whether single-leg daily blood flow restriction (BFR) treatment attenuates the decline in muscle fiber size, capillarization, and satellite cell (SC) content during 2 wk of bed rest in healthy, young men. Twelve healthy, young men (age: 24 ± 3 yr; BMI: 23.7 ± 3.1 kg/m2) were subjected to 2 wk of bed rest, during which one leg was exposed to three times daily 5 min of BFR, whereas the contralateral leg received sham treatment [control (CON)]. Muscle biopsies were obtained from the m. vastus lateralis from both the BFR and CON legs before and immediately after 2 wk of bed rest. Types I and II muscle fiber size, myonuclear content, capillarization, and SC content were assessed by immunohistochemistry. No significant decline in either type I or type II muscle fiber size was observed following bed rest, with no differences between the CON and BFR legs (P > 0.05). Type I muscle fiber capillary density increased in response to bed rest in both legs (P < 0.05), whereas other muscle fiber capillarization measures remained unaltered. SC content decreased in both type I (from 7.4 ± 3.2 to 5.9 ± 2.7 per 100 fibers) and type II (from 7.2 ± 3.4 to 6.5 ± 3.2 per 100 fibers) muscle fibers (main effect of time P = 0.018), with no significant differences between the BFR and CON legs (P > 0.05). In conclusion, 2 wk of bed rest has no effect on muscle capillarization and decreases the SC content, and daily BFR treatment does not affect skeletal muscle fiber size and SC content in healthy, young men.NEW & NOTEWORTHY We recently reported that the application of daily blood flow restriction (BFR) treatment does not preserve muscle mass or strength and does not modulate daily muscle protein synthesis rates during 2 wk of bed rest. Here, we show that 2 wk of bed rest resulted in a decrease in satellite cell (SC) content. In addition, the BFR treatment did not affect muscle fiber size, capillarization, and SC content during 2 wk of bed rest.

The expanding roles of myonuclei in adult skeletal muscle health and function

Skeletal muscle cells (myofibers) require multiple nuclei to support a cytoplasmic volume that is larger than other mononuclear cell types. It is dogmatic that mammalian resident myonuclei rely on stem cells (specifically satellite cells) for adding new DNA to muscle fibers to facilitate cytoplasmic expansion that occurs during muscle growth. In this review, we discuss the relationship between cell size and supporting genetic material. We present evidence that myonuclei may undergo DNA synthesis as a strategy to increase genetic material in myofibers independent from satellite cells. We then describe the details of our experiments that demonstrated that mammalian myonuclei can replicate DNA in vivo. Finally, we present our findings in the context of expanding knowledge about myonuclear heterogeneity, myonuclear mobility and shape. We also address why myonuclear replication is potentially important and provide future directions for remaining unknowns. Myonuclear DNA replication, coupled with new discoveries about myonuclear transcription, morphology, and behavior in response to stress, may provide opportunities to leverage previously unappreciated skeletal muscle biological processes for therapeutic targets that support muscle mass, function, and plasticity.

Myoblast-derived ADAMTS-like 2 promotes skeletal muscle regeneration after injury

Skeletal muscle regeneration and functional recovery after minor injuries requires the activation of muscle-resident myogenic muscle stem cells (i.e. satellite cells) and their subsequent differentiation into myoblasts, myocytes, and ultimately myofibers. We recently identified secreted ADAMTS-like 2 (ADAMTSL2) as a pro-myogenic regulator of muscle development, where it promoted myoblast differentiation. Since myoblast differentiation is a key process in skeletal muscle regeneration, we here examined the role of ADAMTSL2 during muscle regeneration after BaCl2 injury. Specifically, we found that muscle regeneration was delayed after ablation of ADAMTSL2 in myogenic precursor cells and accelerated following injection of pro-myogenic ADAMTSL2 protein domains. Mechanistically, ADAMTSL2 regulated the number of committed myoblasts, which are the precursors for myocytes and regenerating myofibers. Collectively, our data support a role for myoblast-derived ADAMTSL2 as a positive regulator of muscle regeneration and provide a proof-of-concept for potential therapeutic applications.

Age-Related ECM Stiffness Mediates TRAIL Activation in Muscle Stem Cell Differentiation

The stiffening of the extracellular matrix (ECM) with age hinders muscle regeneration by causing intrinsic muscle stem cell (MuSC) dysfunction through a poorly understood mechanism. Here, the study aims to study those age-related molecular changes in the differentiation of MuSCs due to age and/or stiffness. Hence, young and aged MuSCs are seeded onto substrates engineered to mimic a soft and stiff ECM microenvironment to study those molecular changes using single-cell RNA sequencing (scRNA). The trajectory of scRNA data of the MuSCs under four different conditions undergoing differentiation is analyzed as well as the active molecular pathways and transcription factors driving those differentiation fates. Data revealed the presence of a branching point within the trajectory leading to the emergence of an age-related fibroblastic population characterized by activation of the TNF-related apoptosis-inducing ligand (TRAIL) pathway, which is significantly activated in aged cells cultured on stiff substrates. Next, using the collagen cross-linking inhibitor β-aminopropionitrile (BAPN) in vivo, the study elucidates stiffness changes on TRAIL downstream apoptotic targets (caspase 8 and caspase 3) using immunostaining. TRAIL activity is significantly inhibited by BAPN in aged animals, indicating a complex mechanism of age-related declines in muscle function through inflammatory and apoptotic mediators.

Extracellular vesicle miR-206 improves chronic binge alcohol-mediated decreased myoblast differentiation in SIV-infected female macaques

Alcohol misuse in people with human immunodeficiency virus (HIV) (PWH) and chronic binge alcohol (CBA) administration in simian immunodeficiency virus (SIV)-infected macaques are associated with increased physical frailty and impaired functional skeletal muscle mass, respectively. Previous studies by our group demonstrate that muscle-enriched microRNAs (myomiRs) are differentially expressed in skeletal muscle (SKM) from CBA-administered SIV-infected male macaques and their altered expression contributes to impaired differentiation of SKM stem cells or myoblasts. MicroRNAs can be transported in extracellular vesicles (EVs) to mediate numerous cellular responses through intercellular communication. The present study tested the hypothesis that EV-mediated delivery of miR-206 can ameliorate CBA-mediated decreases in myoblast differentiation. Myoblasts were isolated from SKM of female SIV-infected, antiretroviral therapy-treated macaques that received either CBA (2.5 g/kg/day, CBA/SIV) or water (VEH/SIV) for 14.5 mo. Myotube and myotube-derived EV myomiR expression, including miR-206, was lower in the CBA/SIV group. Overexpression of miR-206 decreased histone deacetylase 4 (HDAC4) and paired box 7 (PAX7) expression in myotubes and increased fusion index, a differentiation index, in CBA/SIV-derived myotubes. Similarly, EV-mediated delivery of miR-206 increased both fusion index and myotube density of CBA/SIV-derived myoblasts. These results support the potential therapeutic utility of EVs in delivering myomiRs to improve SKM stem cell differentiation.NEW & NOTEWORTHY Alcohol decreases skeletal muscle myoblast differentiation into myotubes, which is associated with decreased expression of microRNA-206. We show that delivering exogenous miR-206 in plasma-derived extracellular vesicles (EVs) to myoblasts derived from alcohol-administered animals increases myotube differentiation. These results support the potential therapeutic utility of EVs in delivering muscle-enriched microRNAs to improve skeletal muscle stem cell differentiation.

miR-1, miR-133a, miR-29b and skeletal muscle fibrosis in chronic limb-threatening ischaemia

Chronic limb-threatening ischaemia (CLTI), the most severe manifestation of peripheral arterial disease (PAD), is associated with a poor prognosis and high amputation rates. Despite novel therapeutic approaches being investigated, no significant clinical benefits have been observed yet. Understanding the molecular pathways of skeletal muscle dysfunction in CLTI is crucial for designing successful treatments. This study aimed to identify miRNAs dysregulated in muscle biopsies from PAD cohorts. Using MIcroRNA ENrichment TURned NETwork (MIENTURNET) on a publicly accessible RNA-sequencing dataset of PAD cohorts, we identified a list of miRNAs that were over-represented among the upregulated differentially expressed genes (DEGs) in CLTI. Next, we validated the altered expression of these miRNAs and their targets in mice with hindlimb ischaemia (HLI). Our results showed a significant downregulation of miR-1, miR-133a, and miR-29b levels in the ischaemic limbs versus the contralateral non-ischaemic limb. A miRNA target protein-protein interaction network identified extracellular matrix components, including collagen-1a1, -3a1, and -4a1, fibronectin-1, fibrin-1, matrix metalloproteinase-2 and -14, and Sparc, which were upregulated in the ischaemic muscle of mice. This is the first study to identify miR-1, miR-133a, and miR-29b as potential contributors to fibrosis and vascular pathology in CLTI muscle, which supports their potential as novel therapeutic agents for this condition.

Recent Advances of Exosomes Derived from Skeletal Muscle and Crosstalk with Other Tissues

Skeletal muscle plays a crucial role in movement, metabolism, and energy homeostasis. As the most metabolically active endocrine organ in the body, it has recently attracted widespread attention. Skeletal muscle possesses the ability to release adipocytokines, bioactive peptides, small molecular metabolites, nucleotides, and other myogenic cell factors; some of which have been shown to be encapsulated within small vesicles, particularly exosomes. These skeletal muscle exosomes (SKM-Exos) are released into the bloodstream and subsequently interact with receptor cell membranes to modulate the physiological and pathological characteristics of various tissues. Therefore, SKM-Exos may facilitate diverse interactions between skeletal muscle and other tissues while also serving as biomarkers that reflect the physiological and pathological states of muscle function. This review delves into the pivotal role and intricate molecular mechanisms of SKM-Exos and its derived miRNAs in the maturation and rejuvenation of skeletal muscle, along with their intercellular signaling dynamics and physiological significance in interfacing with other tissues.

mRNA expression profiles in muscle-derived extracellular vesicles of Large White and wild boar piglets reveal their potential roles in immunity and muscle phenotype

Introduction

Extracellular vesicles (EVs) known for their pivotal role in intercellular communication through RNA delivery, hold paramount implications for understanding muscle phenotypic variations in diverse pig breeds.

Methods

In this study, we compared the mRNA expression profiles of longissimus dorsi muscles and muscle-derived extracellular vesicles (M-EVs), and also examined the diversity of enriched genes in M-EVs between weaned wild boars and commercial Large White pigs with respect to their numbers and biological functions.

Results

The results of the study showed that the variation in the expression profiles of mRNAs between muscles and M-EVs was much greater than the variability between the respective breeds. Meanwhile, the enrichment trend of low-expressed genes (ranked <1,000) was significantly (p-value ≤ 0.05) powerful in M-EVs compared to highly expressed genes in muscles. In addition, M-EVs carried a smaller proportion of coding sequences and a larger proportion of untranslated region sequences compared to muscles. There were 2,110 genes enriched in M-EVs (MEGs) in Large White pigs and 2,322 MEGs in wild boars, with 1,490 MEGs shared interbreeds including cyclin D2 (CCND2), which inhibits myogenic differentiation. Of the 89 KEGG pathways that were significantly enriched (p-value ≤ 0.05) for these MEGs, 13 unique to Large White pigs were mainly related to immunity, 27 unique to wild boars were functionally diverse but included cell fate regulation such as the Notch signaling pathway and the TGF-beta signaling pathway, and 49 were common to both breeds were also functionally complex but partially related to innate immunity, such as the Complement and coagulation cascades and the Fc gamma R-mediated phagocytosis.

Discussion

These findings suggest that mRNAs in M-EVs have the potential to serve as indicators of muscle phenotype differences between the two pig breeds, highlighting the need for further exploration into the role of EV-RNAs in pig phenotype formation.

Magnetic-field-driven targeting of exosomes modulates immune and metabolic changes in dystrophic muscle

Exosomes are promising therapeutics for tissue repair and regeneration to induce and guide appropriate immune responses in dystrophic pathologies. However, manipulating exosomes to control their biodistribution and targeting them in vivo to achieve adequate therapeutic benefits still poses a major challenge. Here we overcome this limitation by developing an externally controlled delivery system for primed annexin A1 myo-exosomes (Exomyo). Effective nanocarriers are realized by immobilizing the Exomyo onto ferromagnetic nanotubes to achieve controlled delivery and localization of Exomyo to skeletal muscles by systemic injection using an external magnetic field. Quantitative muscle-level analyses revealed that macrophages dominate the uptake of Exomyo from these ferromagnetic nanotubes in vivo to synergistically promote beneficial muscle responses in a murine animal model of Duchenne muscular dystrophy. Our findings provide insights into the development of exosome-based therapies for muscle diseases and, in general, highlight the formulation of effective functional nanocarriers aimed at optimizing exosome biodistribution.

Aerobic exercise suppresses CCN2 secretion from senescent muscle stem cells and boosts muscle regeneration in aged mice

BACKGROUND

Aging negatively impacts tissue repair, particularly in skeletal muscle, where the regenerative capacity of muscle stem cells (MuSCs) diminishes with age. Although aerobic exercise is known to attenuate skeletal muscle atrophy, its specific impact on the regenerative and repair capacity of MuSCs remains unclear.

METHODS

Mice underwent moderate-intensity continuous training (MICT) from 9 months (aged + Ex-9M) or 20 months (aged + Ex-20M) to 25 months, with age-matched (aged) and adult controls. Histological examinations and MuSC transplantation assays assessed aerobic exercise effects on MuSC function and muscle regeneration. CCN2/connective tissue growth factor modulation (overexpression and knockdown) in MuSCs and AICAR supplementation effects were explored.

RESULTS

Aged mice displayed significantly reduced running duration (65.33 ± 4.32 vs. 161.9 ± 1.29 min, mean ± SD, P < 0.001) and distance (659.17 ± 103.64 vs. 3058.28 ± 46.26 m, P < 0.001) compared with adults. This reduction was accompanied by skeletal muscle weight loss and decreased myofiber cross-sectional area (CSA). However, MICT initiated at 9 or 20 months led to a marked increase in running duration (142.75 ± 3.14 and 133.86 ± 20.47 min, respectively, P < 0.001 compared with aged mice) and distance (2347.58 ± 145.11 and 2263 ± 643.87 m, respectively, P < 0.001). Additionally, MICT resulted in increased skeletal muscle weight and enhanced CSA. In a muscle injury model, aged mice exhibited fewer central nuclear fibres (CNFs; 266.35 ± 68.66/mm2), while adult, aged + Ex-9M and aged + Ex-20M groups showed significantly higher CNF counts (610.82 ± 46.76, 513.42 ± 47.19 and 548.29 ± 71.82/mm2, respectively; P < 0.001 compared with aged mice). MuSCs isolated from aged mice displayed increased CCN2 expression, which was effectively suppressed by MICT. Transplantation of MuSCs overexpressing CCN2 (Lenti-CCN2, Lenti-CON as control) into injured tibialis anterior muscle compromised regeneration capacity, resulting in significantly fewer CNFs in the Lenti-CCN2 group compared with Lenti-CON (488.07 ± 27.63 vs. 173.99 ± 14.28/mm2, P < 0.001) at 7 days post-injury (dpi). Conversely, knockdown of CCN2 (Lenti-CCN2shR, Lenti-NegsiR as control) in aged MuSCs improved regeneration capacity, significantly increasing the CNF count from 254.5 ± 26.36 to 560.39 ± 48.71/mm2. Lenti-CCN2 MuSCs also increased fibroblast proliferation and exacerbated skeletal muscle fibrosis, while knockdown of CCN2 in aged MuSCs mitigated this pattern. AICAR supplementation, mimicking exercise, replicated the beneficial effects of aerobic exercise by mitigating muscle weight decline, enhancing satellite cell activity and reducing fibrosis.

CONCLUSIONS

Aerobic exercise effectively reverses the decline in endurance capacity and mitigates muscle atrophy in aged mice. It inhibits CCN2 secretion from senescent MuSCs, thereby enhancing skeletal muscle regeneration and preventing fibrosis in aged mice. AICAR supplementation mimics the beneficial effects of aerobic exercise.

Direct and cell-mediated EV-ECM interplay

Extracellular vesicles (EV) are a heterogeneous group of lipid particles excreted by cells. They play an important role in regeneration, development, inflammation, and cancer progression, together with the extracellular matrix (ECM), which they constantly interact with. In this review, we discuss direct and indirect interactions of EVs and the ECM and their impact on different physiological processes. The ECM affects the secretion of EVs, and the properties of the ECM and EVs modulate EVs' diffusion and adhesion. On the other hand, EVs can affect the ECM both directly through enzymes and indirectly through the modulation of the ECM synthesis and remodeling by cells. This review emphasizes recently discovered types of EVs bound to the ECM and isolated by enzymatic digestion, including matrix-bound nanovesicles (MBV) and tissue-derived EV (TiEV). In addition to the experimental studies, computer models of the EV-ECM-cell interactions, from all-atom models to quantitative pharmacology models aiming to improve our understanding of the interaction mechanisms, are also considered. STATEMENT OF SIGNIFICANCE: Application of extracellular vesicles in tissue engineering is an actively developing area. Vesicles not only affect cells themselves but also interact with the matrix and change it. The matrix also influences both cells and vesicles. In this review, different possible types of interactions between vesicles, matrix, and cells are discussed. Furthermore, the united EV-ECM system and its regulation through the cellular activity are presented.

Satellite cell-derived TRIM28 is pivotal for mechanical load- and injury-induced myogenesis

Satellite cells are skeletal muscle stem cells that contribute to postnatal muscle growth, and they endow skeletal muscle with the ability to regenerate after a severe injury. Here we discover that this myogenic potential of satellite cells requires a protein called tripartite motif-containing 28 (TRIM28). Interestingly, different from the role reported in a previous study based on C2C12 myoblasts, multiple lines of both in vitro and in vivo evidence reveal that the myogenic function of TRIM28 is not dependent on changes in the phosphorylation of its serine 473 residue. Moreover, the functions of TRIM28 are not mediated through the regulation of satellite cell proliferation or differentiation. Instead, our findings indicate that TRIM28 regulates the ability of satellite cells to progress through the process of fusion. Specifically, we discover that TRIM28 controls the expression of a fusogenic protein called myomixer and concomitant fusion pore formation. Collectively, the outcomes of this study expose the framework of a novel regulatory pathway that is essential for myogenesis.

microRNA-1 Regulates Metabolic Flexibility in Skeletal Muscle via Pyruvate Metabolism

MicroRNA-1 (miR-1) is the most abundant miRNA in adult skeletal muscle. To determine the function of miR-1 in adult skeletal muscle, we generated an inducible, skeletal muscle-specific miR-1 knockout (KO) mouse. Integration of RNA-sequencing (RNA-seq) data from miR-1 KO muscle with Argonaute 2 enhanced crosslinking and immunoprecipitation sequencing (AGO2 eCLIP-seq) from human skeletal muscle identified miR-1 target genes involved with glycolysis and pyruvate metabolism. The loss of miR-1 in skeletal muscle induced cancer-like metabolic reprogramming, as shown by higher pyruvate kinase muscle isozyme M2 (PKM2) protein levels, which promoted glycolysis. Comprehensive bioenergetic and metabolic phenotyping combined with skeletal muscle proteomics and metabolomics further demonstrated that miR-1 KO induced metabolic inflexibility as a result of pyruvate oxidation resistance. While the genetic loss of miR-1 reduced endurance exercise performance in mice and in C. elegans, the physiological down-regulation of miR-1 expression in response to a hypertrophic stimulus in both humans and mice causes a similar metabolic reprogramming that supports muscle cell growth. Taken together, these data identify a novel post-translational mechanism of adult skeletal muscle metabolism regulation mediated by miR-1.

Extracellular vesicles may provide an alternative detoxification pathway during skeletal muscle myoblast ageing

Skeletal muscle (SM) acts as a secretory organ, capable of releasing myokines and extracellular vesicles (SM-EVs) that impact myogenesis and homeostasis. While age-related changes have been previously reported in murine SM-EVs, no study has comprehensively profiled SM-EV in human models. To this end, we provide the first comprehensive comparison of SM-EVs from young and old human primary skeletal muscle cells (HPMCs) to map changes associated with SM ageing. HPMCs, isolated from young (24 ± 1.7 years old) and older (69 ± 2.6 years old) participants, were immunomagnetically sorted based on the presence of the myogenic marker CD56 (N-CAM) and cultured as pure (100% CD56+) or mixed populations (MP: 90% CD56+). SM-EVs were isolated using an optimised protocol combining ultrafiltration and size exclusion chromatography (UF + SEC) and their biological content was extensively characterised using Raman spectroscopy (RS) and liquid chromatography mass spectrometry (LC-MS). Minimal variations in basic EV parameters (particle number, size, protein markers) were observed between young and old populations. However, biochemical fingerprinting by RS highlighted increased protein (amide I), lipid (phospholipids and phosphatidylcholine) and hypoxanthine signatures for older SM-EVs. Through LC-MS, we identified 84 shared proteins with functions principally related to cell homeostasis, muscle maintenance and transcriptional regulation. Significantly, SM-EVs from older participants were comparatively enriched in proteins involved in oxidative stress and DNA/RNA mutagenesis, such as E3 ubiquitin-protein ligase TTC3 (TTC3), little elongation complex subunit 1 (ICE1) and Acetyl-CoA carboxylase 1 (ACACA). These data suggest SM-EVs could provide an alternative pathway for homeostasis and detoxification during SM ageing.

Kaempferol-Enhanced Migration and Differentiation of C2C12 Myoblasts via ITG1B/FAK/Paxillin and IGF1R/AKT/mTOR Signaling Pathways

SCOPE

Kaempferol (KMP), a bioactive flavonoid compound found in fruits and vegetables, contributes to human health in many ways but little is known about its relationship with muscle mass. The effect of KMP on C2C12 myoblast differentiation and the mechanisms that might underlie that effect are studied.

METHODS AND RESULTS

This study finds that KMP (1, 10 µM) increases the migration and differentiation of C2C12 myoblasts in vitro. Studying the possible mechanism underlying its effect on migration, the study finds that KMP activates Integrin Subunit Beta 1 (ITGB1) in C2C12 myoblasts, increasing p-FAK (Tyr398) and its downstream cell division cycle 42 (CDC42), a protein previously associated with cell migration. Regarding differentiation, KMP upregulates the expression of myosin heavy chain (MHC) and activates IGF1/AKT/mTOR/P70S6K. Interestingly, pretreatment with an AKT inhibitor (LY294002) and siRNA knockdown of IGF1R leads to a decrease in cell differentiation, suggesting that IGF1/AKT activation is required for KMP to induce C2C12 myoblast differentiation.

CONCLUSION

Together, the findings suggest that KMP enhances the migration and differentiation of C2C12 myoblasts through the ITG1B/FAK/paxillin and IGF1R/AKT/mTOR pathways. Thus, KMP supplementation might potentially be used to prevent or delay age-related loss of muscle mass and help maintain muscle health.

Advanced Analysis and Validation of a microRNA Signature for Fanconi Anemia

Some years ago, we reported the generation of a Fanconi anemia (FA) microRNA signature. This study aims to develop an analytical strategy to select a smaller and more reliable set of molecules that could be tested for potential benefits for the FA phenotype, elucidate its biochemical and molecular mechanisms, address experimental activity, and evaluate its possible impact on FA therapy. In silico analyses of the data obtained in the original study were thoroughly processed and anenrichment analysis was employed to identify the classes of genes that are over-represented in the FA-miRNA population under study. Primary bone marrow mononuclear cells (MNCs) from sixFA patients and sixhealthy donors as control samples were employed in the study. RNAs containing the small RNA fractions were reverse-transcribed and real-time PCR was performed in triplicate using the specific primers. Experiments were performed in triplicate.The in-silico analysis reported six miRNAs as likely contributors to the complex pathological spectrum of FA. Among these, three miRNAs were validated by real-time PCR. Primary bone marrow mononuclear cells (MNCs) reported a significant reduction in the expression level of miRNA-1246 and miRNA-206 in the FA samples in comparison to controls.This study highlights several biochemical pathways as culprits in the phenotypic manifestations and the pathophysiological mechanisms acting in FA. A relatively low number of miRNAs appear involved in all these different phenotypes, demonstrating the extreme plasticity of the gene expression modulation. This study further highlights miR-206 as a pivotal player in regulatory functions and signaling in the bone marrow mesenchymal stem cell (BMSC) process in FA. Due to this evidence, the activity of miR-206 in FA deserves specific experimental scrutiny. The results, here presented, might be relevant in the management of FA.

The Neuromuscular Disorder Mediated by Extracellular Vesicles in Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) represents a neurodegenerative disorder characterized by the progressive loss of both upper and lower motor neurons, resulting in muscular atrophy and eventual paralysis. While much research has concentrated on investigating the impact of major mutations associated with ALS on motor neurons and central nervous system (CNS) cells, recent studies have unveiled that ALS pathogenesis extends beyond CNS imbalances, encompassing dysregulation in other tissues such as skeletal muscle. Evidence from animal models and patients supports this broader perspective. Skeletal muscle, once considered solely as an effector organ, is now recognized as possessing significant secretory activity capable of influencing motor neuron survival. However, the precise cellular and molecular mechanisms underlying the detrimental effects observed in muscle and its associated structures in ALS remain poorly understood. Additionally, emerging data suggest that extracellular vesicles (EVs) may play a role in the establishment and function of the neuromuscular junction (NMJ) under both physiological and pathological conditions and in wasting and regeneration of skeletal muscles, particularly in neurodegenerative diseases like ALS. This review aims to explore the key findings about skeletal muscle involvement in ALS, shedding light on the potential underlying mechanisms and contributions of EVs and their possible application for the design of biosensors.

Post-natal muscle growth and protein turnover: a narrative review of current understanding

A model explaining the dietary-protein-driven post-natal skeletal muscle growth and protein turnover in the rat is updated, and the mechanisms involved are described, in this narrative review. Dietary protein controls both bone length and muscle growth, which are interrelated through mechanotransduction mechanisms with muscle growth induced both from stretching subsequent to bone length growth and from internal work against gravity. This induces satellite cell activation, myogenesis and remodelling of the extracellular matrix, establishing a growth capacity for myofibre length and cross-sectional area. Protein deposition within this capacity is enabled by adequate dietary protein and other key nutrients. After briefly reviewing the experimental animal origins of the growth model, key concepts and processes important for growth are reviewed. These include the growth in number and size of the myonuclear domain, satellite cell activity during post-natal development and the autocrine/paracrine action of IGF-1. Regulatory and signalling pathways reviewed include developmental mechanotransduction, signalling through the insulin/IGF-1-PI3K-Akt and the Ras-MAPK pathways in the myofibre and during mechanotransduction of satellite cells. Likely pathways activated by maximal-intensity muscle contractions are highlighted and the regulation of the capacity for protein synthesis in terms of ribosome assembly and the translational regulation of 5-TOPmRNA classes by mTORC1 and LARP1 are discussed. Evidence for and potential mechanisms by which volume limitation of muscle growth can occur which would limit protein deposition within the myofibre are reviewed. An understanding of how muscle growth is achieved allows better nutritional management of its growth in health and disease.

Stem cell secretome treatment improves whole-body metabolism, reduces adiposity, and promotes skeletal muscle function in aged mice

Aging coincides with the progressive loss of muscle mass and strength, increased adiposity, and diminished physical function. Accordingly, interventions aimed at improving muscle, metabolic, and/or physical health are of interest to mitigate the adverse effects of aging. In this study, we tested a stem cell secretome product, which contains extracellular vesicles and growth, cytoskeletal remodeling, and immunomodulatory factors. We examined the effects of 4 weeks of 2×/week unilateral intramuscular secretome injections (quadriceps) in ambulatory aged male C57BL/6 mice (22-24 months) compared to saline-injected aged-matched controls. Secretome delivery substantially increased whole-body lean mass and decreased fat mass, corresponding to higher myofiber cross-sectional area and smaller adipocyte size, respectively. Secretome-treated mice also had greater whole-body physical function (grip strength and rotarod performance) and had higher energy expenditure and physical activity levels compared to control mice. Furthermore, secretome-treated mice had greater skeletal muscle Pax7+ cell abundance, capillary density, collagen IV turnover, reduced intramuscular lipids, and greater Akt and hormone sensitive lipase phosphorylation in adipose tissue. Finally, secretome treatment in vitro directly enhanced muscle cell growth and IL-6 production, and in adipocytes, it reduced lipid content and improved insulin sensitivity. Moreover, indirect treatment with secretome-treated myotube culture media also enhanced muscle cell growth and adipocyte size reduction. Together, these data suggest that intramuscular treatment with a stem cell secretome improves whole-body metabolism, physical function, and remodels skeletal muscle and adipose tissue in aged mice.

Tracking of Nascent Matrix Deposition during Muscle Stem Cell Activation across Lifespan Using Engineered Hydrogels

Adult stem cells occupy a niche that contributes to their function, but how stem cells rebuild their microenvironment after injury remains an open-ended question. Herein, biomaterial-based systems and metabolic labeling are utilized to evaluate how skeletal muscle stem cells deposit extracellular matrix. Muscle stem cells and committed myoblasts are observed to generate less nascent matrix than muscle resident fibro-adipogenic progenitors. When cultured on substrates that matched the stiffness of physiological uninjured and injured muscles, muscle stem cells increased nascent matrix deposition with activation kinetics. Reducing the ability to deposit nascent matrix by an inhibitor of vesicle trafficking (Exo-1) attenuated muscle stem cell function and mimicked impairments observed from muscle stem cells isolated from old muscles. Old muscle stem cells are observed to deposit less nascent matrix than young muscle stem cells, which is rescued with therapeutic supplementation of insulin-like growth factors. These results highlight the role of nascent matrix production with muscle stem cell activation.

Matrisome Transcriptome Dynamics during Tissue Aging

The extracellular matrix (ECM) is a complex three-dimensional network of macromolecules that provides structural support for the cells and plays a significant role in tissue homeostasis and repair. Growing evidence indicates that dysregulation of ECM remodeling contributes to various pathological conditions in the body, including age-associated diseases. In this work, gene expression data of normal human tissues obtained from the Genotype-Tissue Expression project, as well as data from MatrisomeDB 2.0, the ECM-protein knowledge database, are used to estimate the age-dependent matrisome transcriptome dynamics in the blood, heart, brain, liver, kidneys, lungs, and muscle. Differential gene expression (DE) analysis revealed dozens of matrisome genes encoding both structural elements of the ECM and ECM-associated proteins, which had a tissue-specific expression profile with age. Among common DE genes that changed expression with age in at least three tissues, COL18A1, MFAP1, IGFBP7, AEBP1, LTBP2, LTBP4, LG14, EFEMP1, PRELP, BGN, FAM20B, CTSC, CTSS, and CLEC2B were observed. The findings of the study also reveal that there are sex-specific alterations during aging in the matrisome gene expression. Taken together, the results obtained in this work may help in understanding the role of the ECM in tissue aging and might prove valuable for the future development of the field of ECM research in general.

The effects of resistance training on denervated myofibers, senescent cells, and associated protein markers in middle-aged adults

Denervated myofibers and senescent cells are hallmarks of skeletal muscle aging. However, sparse research has examined how resistance training affects these outcomes. We investigated the effects of unilateral leg extensor resistance training (2 days/week for 8 weeks) on denervated myofibers, senescent cells, and associated protein markers in apparently healthy middle-aged participants (MA, 55 ± 8 years old, 17 females, 9 males). We obtained dual-leg vastus lateralis (VL) muscle cross-sectional area (mCSA), VL biopsies, and strength assessments before and after training. Fiber cross-sectional area (fCSA), satellite cells (Pax7+), denervated myofibers (NCAM+), senescent cells (p16+ or p21+), proteins associated with denervation and senescence, and senescence-associated secretory phenotype (SASP) proteins were analyzed from biopsy specimens. Leg extensor peak torque increased after training (p < .001), while VL mCSA trended upward (interaction p = .082). No significant changes were observed for Type I/II fCSAs, NCAM+ myofibers, or senescent (p16+ or p21+) cells, albeit satellite cells increased after training (p = .037). While >90% satellite cells were not p16+ or p21+, most p16+ and p21+ cells were Pax7+ (>90% on average). Training altered 13 out of 46 proteins related to muscle-nerve communication (all upregulated, p < .05) and 10 out of 19 proteins related to cellular senescence (9 upregulated, p < .05). Only 1 out of 17 SASP protein increased with training (IGFBP-3, p = .031). In conclusion, resistance training upregulates proteins associated with muscle-nerve communication in MA participants but does not alter NCAM+ myofibers. Moreover, while training increased senescence-related proteins, this coincided with an increase in satellite cells but not alterations in senescent cell content or SASP proteins. These latter findings suggest shorter term resistance training is an unlikely inducer of cellular senescence in apparently healthy middle-aged participants. However, similar study designs are needed in older and diseased populations before definitive conclusions can be drawn.

Effects of Paraspinal Intramuscular Injection of Atelocollagen in Patients with Chronic Low Back Pain: A Retrospective Observational Study

Background/Objectives: Atelocollagen is used for soft tissue repair and reconstruction by replacing defective or damaged muscles, membranes, ligaments, and tendons. This study aimed to evaluate the clinical efficacy and safety of additional paraspinal intramuscular injection of atelocollagen on lumbar epidural steroid injection for reducing pain and improving functional capacity of patients with chronic low back pain (CLBP). Methods: We retrospectively enrolled 608 consecutive patients with CLBP who received lumbar epidural steroid injection with or without additional paraspinal intramuscular injection of atelocollagen. The Numerical Rating Scale and the Oswestry Disability Index were used to assess pain and functional capacity, respectively, before the procedure, and three months after the injection. Also, we analyzed the relationship between the additional paraspinal intramuscular injection of atelocollagen and the success rate. Results: Both Numerical Rating Scale and the Oswestry Disability Index scores were significantly reduced in both groups at three months after injection. However, there was a significant difference between the two groups. Furthermore, the success rate was significantly higher in the additional paraspinal intramuscular injection of atelocollagen group. Conclusions: This study's results showed that additional paraspinal intramuscular injection of atelocollagen on lumbar epidural steroid injection reduced pain and improved functional capacity for patients with CLBP. Therefore, the paraspinal intramuscular injection of atelocollagen may be a promising option for the treatment of patients with CLBP.

Semaphorins: Missing Signals in Age-dependent Alteration of Neuromuscular Junctions and Skeletal Muscle Regeneration

Skeletal muscle is characterized by a remarkable capacity to rearrange after physiological changes and efficiently regenerate. However, during aging, extensive injury, or pathological conditions, the complete regenerative program is severely affected, with a progressive loss of muscle mass and function, a condition known as sarcopenia. The compromised tissue repair program is attributable to the gradual depletion of stem cells and to altered regulatory signals. Defective muscle regeneration can severely affect re-innervation by motor axons, and neuromuscular junctions (NMJs) development, ultimately leading to skeletal muscle atrophy. Defects in NMJ formation and maintenance occur physiologically during aging and are responsible for the pathogenesis of several neuromuscular disorders. However, it is still largely unknown how neuromuscular connections are restored on regenerating fibers. It has been suggested that attractive and repelling signals used for axon guidance could be implicated in this process; in particular, guidance molecules called semaphorins play a key role. Semaphorins are a wide family of extracellular regulatory signals with a multifaceted role in cell-cell communication. Originally discovered as axon guidance factors, they have been implicated in cancer progression, embryonal organogenesis, skeletal muscle innervation, and other physiological and developmental functions in different tissues. In particular, in skeletal muscle, specific semaphorin molecules are involved in the restoration and remodeling of the nerve-muscle connections, thus emphasizing their plausible role to ensure the success of muscle regeneration. This review article aims to discuss the impact of aging on skeletal muscle regeneration and NMJs remodeling and will highlight the most recent insights about the role of semaphorins in this context.

Muscle stem cell niche dynamics during muscle homeostasis and regeneration

The process of skeletal muscle regeneration involves a coordinated interplay of specific cellular and molecular interactions within the injury site. This review provides an overview of the cellular and molecular components in regenerating skeletal muscle, focusing on how these cells or molecules in the niche regulate muscle stem cell functions. Dysfunctions of muscle stem cell-to-niche cell communications during aging and disease will also be discussed. A better understanding of how niche cells coordinate with muscle stem cells for muscle repair will greatly aid the development of therapeutic strategies for treating muscle-related disorders.

YAP/TAZ as Molecular Targets in Skeletal Muscle Atrophy and Osteoporosis

Skeletal muscles and bones are closely connected anatomically and functionally. Age-related degeneration in these tissues is associated with physical disability in the elderly and significantly impacts their quality of life. Understanding the mechanisms of age-related musculoskeletal tissue degeneration is crucial for identifying molecular targets for therapeutic interventions for skeletal muscle atrophy and osteoporosis. The Hippo pathway is a recently identified signaling pathway that plays critical roles in development, tissue homeostasis, and regeneration. The Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) are key downstream effectors of the mammalian Hippo signaling pathway. This review highlights the fundamental roles of YAP and TAZ in the homeostatic maintenance and regeneration of skeletal muscles and bones. YAP/TAZ play a significant role in stem cell function by relaying various environmental signals to stem cells. Skeletal muscle atrophy and osteoporosis are related to stem cell dysfunction or senescence triggered by YAP/TAZ dysregulation resulting from reduced mechanosensing and mitochondrial function in stem cells. In contrast, the maintenance of YAP/TAZ activation can suppress stem cell senescence and tissue dysfunction and may be used as a basis for the development of potential therapeutic strategies. Thus, targeting YAP/TAZ holds significant therapeutic potential for alleviating age-related muscle and bone dysfunction and improving the quality of life in the elderly.

Reciprocal communication between FAPs and muscle cells via distinct extracellular vesicle miRNAs in muscle regeneration

Muscle regeneration is a complex process relying on precise teamwork between multiple cell types, including muscle stem cells (MuSCs) and fibroadipogenic progenitors (FAPs). FAPs are also the main source of intramuscular adipose tissue (IMAT). Muscles without FAPs exhibit decreased IMAT infiltration but also deficient muscle regeneration, indicating the importance of FAPs in the repair process. Here, we demonstrate the presence of bidirectional crosstalk between FAPs and MuSCs via their secretion of extracellular vesicles (EVs) containing distinct clusters of miRNAs that is crucial for normal muscle regeneration. Thus, after acute muscle injury, there is activation of FAPs leading to a transient rise in IMAT. These FAPs also release EVs enriched with a selected group of miRNAs, a number of which come from an imprinted region on chromosome 12. The most abundant of these is miR-127-3p, which targets the sphingosine-1-phosphate receptor S1pr3 and activates myogenesis. Indeed, intramuscular injection of EVs from immortalized FAPs speeds regeneration of injured muscle. In late stages of muscle repair, in a feedback loop, MuSCs and their derived myoblasts/myotubes secrete EVs enriched in miR-206-3p and miR-27a/b-3p. The miRNAs repress FAP adipogenesis, allowing full muscle regeneration. Together, the reciprocal communication between FAPs and muscle cells via miRNAs in their secreted EVs plays a critical role in limiting IMAT infiltration while stimulating muscle regeneration, hence providing an important mechanism for skeletal muscle repair and homeostasis.

The potential role of stem cells-derived extracellular vesicles in the treatment of musculoskeletal system diseases

The therapeutic potential of stem cells-derived extracellular vesicles (EVs) has shown a great progress in the regenerative medicine. EVs are rich in a variety of bioactive substances, which are important carriers of signal transmission and interactions between cells, and they play an important role in the processes of tissue repair and regeneration. Several studies have shown that stem cells-derived EVs regulate immunity, promote cell proliferation and differentiation, enhance bone and vascular regeneration, and play an increasingly important role in musculoskeletal system. This review aimed to describe the biological characteristics of stem cells-derived EVs and discuss their potential role in the therapy of musculoskeletal system diseases.

Myogenic exosome miR-140-5p modulates skeletal muscle regeneration and injury repair by regulating muscle satellite cells

Muscle satellite cells (SCs) play a crucial role in the regeneration and repair of skeletal muscle injuries. Previous studies have shown that myogenic exosomes can enhance satellite cell proliferation, while the expression of miR-140-5p is significantly reduced during the repair process of mouse skeletal muscle injuries induced by BaCl2. This study aims to investigate the potential of myogenic exosomes carrying miR-140-5p inhibitors to activate SCs and influence the regeneration of injured muscles. Myogenic progenitor cell exosomes (MPC-Exo) and contained miR-140-5p mimics/inhibitors myogenic exosomes (MPC-Exo140+ and MPC-Exo140-) were employed to treat SCs and use the model. The results demonstrate that miR-140-5p regulates SC proliferation by targeting Pax7. Upon the addition of MPC-Exo and MPC-Exo140-, Pax7 expression in SCs significantly increased, leading to the transition of the cell cycle from G1 to S phase and an enhancement in cell proliferation. Furthermore, the therapeutic effect of MPC-Exo140- was validated in animal model, where the expression of muscle growth-related genes substantially increased in the gastrocnemius muscle. Our research demonstrates that MPC-Exo140- can effectively activate dormant muscle satellite cells, initiating their proliferation and differentiation processes, ultimately leading to the formation of new skeletal muscle cells and promoting skeletal muscle repair and remodeling.

Age-Associated Differences in Recovery from Exercise-Induced Muscle Damage

Understanding the intricate mechanisms governing the cellular response to resistance exercise is paramount for promoting healthy aging. This narrative review explored the age-related alterations in recovery from resistance exercise, focusing on the nuanced aspects of exercise-induced muscle damage in older adults. Due to the limited number of studies in older adults that attempt to delineate age differences in muscle discovery, we delve into the multifaceted cellular influences of chronic low-grade inflammation, modifications in the extracellular matrix, and the role of lipid mediators in shaping the recovery landscape in aging skeletal muscle. From our literature search, it is evident that aged muscle displays delayed, prolonged, and inefficient recovery. These changes can be attributed to anabolic resistance, the stiffening of the extracellular matrix, mitochondrial dysfunction, and unresolved inflammation as well as alterations in satellite cell function. Collectively, these age-related impairments may impact subsequent adaptations to resistance exercise. Insights gleaned from this exploration may inform targeted interventions aimed at enhancing the efficacy of resistance training programs tailored to the specific needs of older adults, ultimately fostering healthy aging and preserving functional independence.

Roles of Myokines and Muscle-Derived Extracellular Vesicles in Musculoskeletal Deterioration under Disuse Conditions

Prolonged inactivity and disuse conditions, such as those experienced during spaceflight and prolonged bedrest, are frequently accompanied by detrimental effects on the motor system, including skeletal muscle atrophy and bone loss, which greatly increase the risk of osteoporosis and fractures. Moreover, the decrease in glucose and lipid utilization in skeletal muscles, a consequence of muscle atrophy, also contributes to the development of metabolic syndrome. Clarifying the mechanisms involved in disuse-induced musculoskeletal deterioration is important, providing therapeutic targets and a scientific foundation for the treatment of musculoskeletal disorders under disuse conditions. Skeletal muscle, as a powerful endocrine organ, participates in the regulation of physiological and biochemical functions of local or distal tissues and organs, including itself, in endocrine, autocrine, or paracrine manners. As a motor organ adjacent to muscle, bone tissue exhibits a relative lag in degenerative changes compared to skeletal muscle under disuse conditions. Based on this phenomenon, roles and mechanisms involved in the communication between skeletal muscle and bone, especially from muscle to bone, under disuse conditions have attracted widespread attention. In this review, we summarize the roles and regulatory mechanisms of muscle-derived myokines and extracellular vesicles (EVs) in the occurrence of muscle atrophy and bone loss under disuse conditions, as well as discuss future perspectives based on existing research.

Leveraging Biomaterial Platforms to Study Aging-Related Neural and Muscular Degeneration

Aging is a complex multifactorial process that results in tissue function impairment across the whole organism. One of the common consequences of this process is the loss of muscle mass and the associated decline in muscle function, known as sarcopenia. Aging also presents with an increased risk of developing other pathological conditions such as neurodegeneration. Muscular and neuronal degeneration cause mobility issues and cognitive impairment, hence having a major impact on the quality of life of the older population. The development of novel therapies that can ameliorate the effects of aging is currently hindered by our limited knowledge of the underlying mechanisms and the use of models that fail to recapitulate the structure and composition of the cell microenvironment. The emergence of bioengineering techniques based on the use of biomimetic materials and biofabrication methods has opened the possibility of generating 3D models of muscular and nervous tissues that better mimic the native extracellular matrix. These platforms are particularly advantageous for drug testing and mechanistic studies. In this review, we discuss the developments made in the creation of 3D models of aging-related neuronal and muscular degeneration and we provide a perspective on the future directions for the field.

Downregulation of Genes for Skeletal Muscle Extracellular Matrix Components by Cisplatin

The extracellular matrix (ECM) in skeletal muscle is involved in a variety of physiological functions beyond the mechanical support of muscle tissue, nerves, and blood vessels; however, the role of the ECM in skeletal muscle remains unclear. There is little information regarding changes in the expression of factors comprising the ECM during cisplatin-induced muscle atrophy. In the present study, we examined the changes in gene expressions for skeletal muscle extracellular matrix components in skeletal muscle during cisplatin-induced muscle atrophy. Intraperitoneal administration of cisplatin caused muscle atrophy in mice and during this cisplatin-induced muscle atrophy, the expression of many procollagen genes (Col1a1, Col1a2, Col3a1, Col4a1, Col5a1, and Col5a2), elastin (Eln), fibronectin (Fn1), Laminin (Lama1, Lama2, and Lamb1) decorin (Dcn), heparan sulphate proteoglycans (Hspg2) and integrin (Itgb1) constituting the ECM was suppressed. Additional studies are needed to elucidate the pathological significance and mechanisms of reduced gene expression of ECM components associated with cisplatin-induced muscle atrophy.

The role of satellite cell-derived TRIM28 in mechanical load- and injury-induced myogenesis

Satellite cells are skeletal muscle stem cells that contribute to postnatal muscle growth, and they endow skeletal muscle with the ability to regenerate after a severe injury. Here we discovered that this myogenic potential of satellite cells requires a protein called tripartite motif-containing 28 (TRIM28). Unexpectedly, multiple lines of both in vitro and in vivo evidence revealed that the myogenic function of TRIM28 is not dependent on changes in the phosphorylation of its serine 473 residue. Moreover, the functions of TRIM28 were not mediated through the regulation of satellite cell proliferation or differentiation. Instead, our findings indicate that TRIM28 regulates the ability of satellite cells to progress through the process of fusion. Specifically, we discovered that TRIM28 controls the expression of a fusogenic protein called myomixer and concomitant fusion pore formation. Collectively, the outcomes of this study expose the framework of a novel regulatory pathway that is essential for myogenesis.