Collagen VI regulates satellite cell self-renewal and muscle regeneration

Glycations on Decellularized Muscle Matrix Reduce Muscle Regeneration and Increase Inflammation

Volumetric muscle loss (VML) due to traumatic injury results in the abrupt loss of contractile units, stem cells, and connective tissue, leading to long-term muscle dysfunction and reduced regenerative potential. Muscle connective tissue contains a proregenerative extracellular matrix (ECM), and our lab harnesses the regenerative capacity of decellularized muscle matrix (DMM) to treat VML, a condition with limited treatment options. However, a major limitation is that muscle often comes from aged donors. Previous work from our lab showed that aged donor muscle contains higher levels of advanced glycation end-product (AGE) cross-links compared to muscle from younger donors. This study aimed to determine whether increased AGE cross-links reduce the regenerative capacity of DMM. To test this, we first generated AGEs in DMM with direct D-ribose incubation. We then removed ∼35% of the gastrocnemius muscle in a model and treated it with either AGE-DMM or standard DMM (no AGEs), comparing results to controls. Although muscle force results remained unchanged between AGE-DMM and DMM, AGEs led to reduced muscle mass in histological sections, fewer fibers, and smaller fiber diameters. AGEs also increased collagen levels in histology, but protein assays showed reduced collagen production. We investigated the canonical receptor for AGEs, the receptor for AGEs (RAGE), and found elevated levels in AGE-treated VML compared to DMM alone, along with increased levels of the noncanonical receptor galectin-3. Both RAGE and galectin-3 are associated with inflammation, and proteomics revealed higher inflammatory markers in AGE-treated muscle than in DMM alone. In conclusion, our data suggest that AGEs impair the regenerative potential of DMM, highlighting the importance of considering donor age when sourcing muscle for DMM therapies. Impact Statement This study investigates advanced glycation end-product cross-links in skeletal muscle extracellular matrix (ECM) as a way to model its deleterious effects on muscle regeneration in vivo. We demonstrate here that ECM glycations reduce muscle regeneration, enhance inflammatory markers, reduce ECM protein production, and proteomic analysis identified unique targets that could be explored in future research endeavors.

Mitochondrial fatty acid oxidation regulates adult muscle stem cell function through modulating metabolic flux and protein acetylation

During homeostasis and regeneration, satellite cells, the resident stem cells of skeletal muscle, have distinct metabolic requirements for fate transitions between quiescence, proliferation and differentiation. However, the contribution of distinct energy sources to satellite cell metabolism and function remains largely unexplored. Here, we uncover a role of mitochondrial fatty acid oxidation (FAO) in satellite cell integrity and function. Single-cell RNA sequencing revealed progressive enrichment of mitochondrial FAO and downstream pathways during activation, proliferation and myogenic commitment of satellite cells. Deletion of Carnitine palmitoyltransferase 2 (Cpt2), the rate-limiting enzyme in FAO, hampered muscle stem cell expansion and differentiation upon acute muscle injury, markedly delaying regeneration. Cpt2 deficiency reduces acetyl-CoA levels in satellite cells, impeding the metabolic flux and acetylation of selective proteins including Pax7, the central transcriptional regulator of satellite cells. Notably, acetate supplementation restored cellular metabolic flux and partially rescued the regenerative defects of Cpt2-null satellite cells. These findings highlight an essential role of fatty acid oxidation in controlling satellite cell function and suggest an integration of lipid metabolism and protein acetylation in adult stem cells.

Extracellular Matrix-Mediated Crosslinking of Adhesive Hyaluronic Acid Patch for Treating Volumetric Muscle Injury

The treatment of volumetric muscle loss (VML) is challenging owing to the deficiency of appropriate bioscaffolds and ineffective therapeutic outcomes with conventional approaches. Adhesive hydrogels have emerged as potential tools for promoting tissue regeneration, offering effective integration with damaged tissues. However, technical procedures requiring non-biocompatible crosslinking methods, mechanical mismatches, or the absence of tissue-specific microenvironments in conventional adhesive hydrogels have hindered their successful application in tissue reconstruction. In this study, a patch-type adhesive hydrogel is developed by combining a muscle tissue-derived extracellular matrix (MEM) and catechol-conjugated hyaluronic acid (HA-CA). This MEM-containing HA-CA (HCM) patch hydrogel utilizes MEM as both a biocompatible crosslinker and a therapeutic substance. The HCM patch hydrogel, crosslinked through a combination of covalent and non-covalent interactions via MEM and catechol in the composite, not only provides stable physical support but also develops muscle tissue-specific biochemical cues, mediating the recruitment and maturation of muscular cells. In animal models of VML, the HCM patch hydrogel effectively stimulates satellite cells and supports de novo muscle regeneration with functional restoration. This study emphasizes the efficacy and ready-to-use convenience of HCM patch hydrogels for muscle regeneration.

Collagen type VI regulates TGF-β bioavailability in skeletal muscle in mice

Collagen VI-related disorders (COL6-RDs) are a group of rare muscular dystrophies caused by pathogenic variants in collagen VI genes (COL6A1, COL6A2, and COL6A3). Collagen type VI is a heterotrimeric, microfibrillar component of the muscle extracellular matrix (ECM), predominantly secreted by resident fibroadipogenic precursor cells in skeletal muscle. The absence or mislocalization of collagen VI in the ECM underlies the noncell-autonomous dysfunction and dystrophic changes in skeletal muscle with a yet elusive direct mechanistic link between the ECM and myofiber dysfunction. Here, we conducted a comprehensive natural history and outcome study in a mouse model of COL6-RDs (Col6a2-/- mice) using standardized (TREAT-NMD) functional, histological, and physiological parameters. Notably, we identify a conspicuous dysregulation of the TGF-β pathway early in the disease process and propose that the collagen VI-deficient matrix is not capable of regulating the dynamic TGF-β bioavailability both at baseline and in response to muscle injury. Thus, we propose a new mechanism for pathogenesis of the disease that links the ECM regulation of TGF-β with downstream skeletal muscle abnormalities, paving the way for the development and validation of therapeutics that target this pathway.

Cell-Instructive Biomaterials with Native-Like Biochemical Complexity

Biochemical signals in native tissue microenvironments instruct cell behavior during many biological processes ranging from developmental morphogenesis and tissue regeneration to tumor metastasis and disease progression. The detection and characterization of these signals using spatial and highly resolved quantitative methods have revealed their existence as matricellular proteins in the matrisome, some of which are bound to the extracellular matrix while others are freely diffusing. Including these biochemical signals in engineered biomaterials can impart enhanced functionality and native-like complexity, ultimately benefiting efforts to understand, model, and treat various diseases. In this review, we discuss advances in characterizing, mimicking, and harnessing biochemical signals in developing advanced engineered biomaterials. An overview of the diverse forms in which these biochemical signals exist and their effects on intracellular signal transduction is also provided. Finally, we highlight the application of biochemically complex biomaterials in the three broadly defined areas of tissue regeneration, immunoengineering, and organoid morphogenesis.

Extracellular Matrix Stiffness: Mechanotransduction and Mechanobiological Response-Driven Strategies for Biomedical Applications Targeting Fibroblast Inflammation

The extracellular matrix (ECM) is a dynamic network providing mechanical and biochemical cues that regulate cellular behavior. ECM stiffness critically influences fibroblasts, the primary ECM producers, particularly in inflammation and fibrosis. This review explores the role of ECM stiffness in fibroblast-driven inflammation and tissue remodeling, focusing on the physicochemical and biological mechanisms involved. Engineered materials, hydrogels, and polydimethylsiloxane (PDMS) are highlighted for replicating tissue-specific stiffness, enabling precise control over cell-matrix interactions. The surface functionalization of substrate materials, including collagen, polydopamine, and fibronectin, enhances bioactivity and fibroblast adhesion. Key mechanotransduction pathways, such as integrin signaling and YAP/TAZ activation, are related to regulating fibroblast behaviors and inflammatory responses. The role of fibroblasts in driving chronic inflammatory diseases emphasizes their therapeutic potentials. Advances in ECM-modifying strategies, including tunable biomaterials and hydrogel-based therapies, are explored for applications in tissue engineering, drug delivery, anti-inflammatory treatments, and diagnostic tools for the accurate diagnosis and prognosis of ECM stiffness-related inflammatory diseases. This review integrates mechanobiology with biomedical innovations, providing a comprehensive prognosis of fibroblast responses to ECM stiffness and outlining future directions for targeted therapies.

Decellularized Extracellular Matrix-Derived Hydrogels: a Powerful Class of Biomaterials for Skeletal Muscle Regenerative Engineering Applications

Purpose

The extracellular matrix (ECM) is a complicated milieu consisting of structural and functional molecules secreted by the resident cells that provides an optimal microenvironmental niche for enhanced cell adhesion, growth, differentiation, and tissue formation and maturation. For decades, ECM bio-scaffolds prepared from decellularized tissues have been used to promote skeletal muscle regeneration; however, it was recently discovered that these decellularized ECM (dECM) materials can be further processed into hydrogels, thus expanding the potential applications of dECM materials in skeletal muscle regenerative engineerisng (SMRE). This review article highlights the recent advances in dECM-derived hydrogels toward skeletal muscle regeneration and repair.

Method

We screened articles in PubMed and bibliographic search using a combination of keywords. Relevant and high-cited articles were chosen for inclusion in this narrative review.

Results

Here, we discuss the skeletal muscle ECM's structure, function, and biochemical composition with emphasis on the role of the ECM during skeletal muscle embryogenesis, growth, development, and repair. Furthermore, we review various hydrogels used to promote skeletal muscle regeneration. We also review the current applications of dECM-derived hydrogels toward SMRE. Finally, we discuss the clinical translation potential of dECM-derived hydrogels for skeletal muscle regeneration and repair and their potential clinical considerations in the future.

Conclusion

Although much progress has been made in the field of dECM-derived hydrogels toward SMRE, it is still in its nascent stage. We believe improving and standardizing the methods of decellularization, lowering the immunogenicity of dECMs, and carrying out in vivo investigations in large animal models would advance their future clinical applications.

Lay Summary

Researchers have discovered an effective way to turn tissue materials into jelly-like substances known as extracellular matrix (ECM)-derived hydrogels. These ECM-derived hydrogels can help muscles heal better after serious injuries. They can be injected into gaps or used to guide muscle growth in the lab or body. This review article explains how these ECM-derived hydrogels are made and how they can be used to improve muscle healing. It also discusses their possible use in clinics and what needs to be considered before using them for medical treatments.

Nanomechanics of cell-derived matrices as a functional read-out in collagen VI-related congenital muscular dystrophies

Changes in the mechanical properties of the extracellular matrix (ECM) are a hallmark of disease. Due to its relevance, several in vitro models have been developed for the ECM, including cell-derived matrices (CDMs). CDMs are decellularized natural ECMs assembled by cells that closely mimic the in vivo stromal fibre organization and molecular content. Here, we applied atomic force microscopy-force spectroscopy (AFM-FS) to evaluate the nanomechanical properties of CDMs obtained from patients diagnosed with collagen VI-related congenital muscular dystrophies (COL6-RDs). COL6-RDs are a set of neuromuscular conditions caused by pathogenic variants in any of the three major COL6 genes, which result in deficiency or dysfunction of the COL6 incorporated into the ECM of connective tissues. Current diagnosis includes the genetic confirmation of the disease and categorization of the phenotype based on maximum motor ability, as no direct correlation exists between genotype and phenotype of COL6-RDs. We describe differences in the elastic modulus (E) among CDMs from patients with different clinical phenotypes, as well as the restoration of E in CDMs obtained from genetically edited cells. Results anticipate the potential of the nanomechanical analysis of CDMs as a complementary clinical tool, providing phenotypic information about COL6-RDs and their response to gene therapies.

Biomaterial-Based Regenerative Strategies for Volumetric Muscle Loss: Challenges and Solutions

Significance: Volumetric muscle loss (VML) is caused by the loss of significant amounts of skeletal muscle tissue. VML cannot be repaired by intrinsic regenerative processes, resulting in permanent loss of muscle function and disability. Current rehabilitative-focused treatment strategies lack efficacy and do not restore muscle function, indicating the need for the development of effective regenerative strategies. Recent Advances: Recent developments implicate biomaterial-based approaches for promoting muscle repair and functional restoration post-VML. Specifically, bioscaffolds transplanted in the injury site have been utilized to mimic endogenous cues of the ablated tissue to promote myogenic pathways, increase neo-myofiber synthesis, and ultimately restore contractile function to the injured unit. Critical Issues: Despite the development and preclinical testing of various biomaterial-based regenerative strategies, effective therapies for patients are not available. The unique challenges posed for biomaterial-based treatments of VML injuries, including its scalability and clinical applicability beyond small-animal models, impede progress. Furthermore, production of tissue-engineered constructs is technically demanding, with reproducibility issues at scale and complexities in achieving vascularization and innervation of large constructs. Future Directions: Biomaterial-based regenerative strategies designed to comprehensively address the pathophysiology of VML are needed. Considerations for clinical translation, including scalability and regulatory compliance, should also be considered when developing such strategies. In addition, an integrated approach that combines regenerative and rehabilitative strategies is essential for ensuring functional improvement.

An Efficient Method for Enrichment and In Vitro Propagation of Muscle Stem Cells Derived from Black Sea Bream (Acanthopagrus schlegelii) Skeletal Muscle

Muscle stem cells (MSCs) play a crucial role in muscle growth, repair, and regeneration, offering potential applications in cell-mediated therapy, tissue engineering, and alternative food production. Despite significant advancements in isolating and enriching MSCs from mammalian tissues, research on fish MSCs remains limited. This study aimed to establish an optimized protocol for isolating, enriching, and propagating black sea bream (Acanthopagrus schlegelii) MSCs for potential biotechnological applications. Skeletal muscle tissues were enzymatically dissociated using various enzymes, with collagenase type II and pronase identified as the most effective combination for cell isolation and tissue debris removal. Differential plating (DP) on collagen type I effectively enriched MSCs, as evidenced by a significant increase in Pax7 expression in non-adhesive cells. Among several adhesion substrates tested, Matrigel-coated dishes best supported the maintenance and differentiation potential of enriched MSCs, enabling robust myotube formation. To mitigate the high cost of Matrigel, cells were transitioned to laminin- or gelatin-coated dishes after the early passages. Notably, Matrigel-conditioned cells maintained their survival and differentiation capacities on these more cost-effective substrates. After long-term culture on gelatin-coated dishes, the cell lines were stably maintained for more than 25 passages, and their myogenic differentiation potentials were well preserved, with variations observed between the cell lines. These findings provide a foundational framework for the efficient isolation, enrichment, and culture of fish MSCs, contributing to the development of scalable and cost-effective protocols for their application in muscle biology and biotechnology.

Effects of Aging on Intramuscular Collagen-Related Factors After Injury to Mouse Tibialis Anterior Muscle

Collagen I is the most abundant type of intramuscular collagen. Lysyl oxidase promotes collagen cross-link formation, which helps stabilize the extracellular matrix. Furthermore, matrix metalloproteinases, responsible for collagen degradation, maintain typical muscle structure and function through remodeling. Although it is well known that aging leads to delayed recovery of muscle fibers, the impact of aging on the remodeling of intramuscular collagen is not well understood. In this study, we investigated the impact of aging on collagen remodeling during muscle injury recovery using young and old mouse models. Muscle injury was induced in the right tibialis anterior (TA) muscle of male C57BL/6J mice [aged 21 weeks (young) and 92 weeks (old)] using intramuscular cardiotoxin injection, with the left TA serving as a sham with saline injection. Following a one-week recovery period, aging was found to delay the recovery of the fiber cross-sectional area. The intensity and area of immunoreactivity for collagen I were significantly increased in old mice compared to young mice post-injury. Additionally, Lox expression and the number of LOX (+) cells in the extracellular matrix significantly increased in old mice compared to young mice post-injury. Furthermore, Mmp9 and MMP9 expression levels after muscle injury were higher in old mice than in young mice. These results suggest that muscle injury in old mice can lead to increased collagen I accumulation, enhanced collagen cross-link formation, and elevated MMP9 expression compared to young mice.

Myofibers cultured in viscoelastic hydrogels reveal the effects of integrin-binding and mechanosensing on muscle satellite cells

Quiescent skeletal muscle satellite cells (SCs) located on myofibers activate in response to muscle injury to regenerate muscle; however, identifying the role of specific matrix signals on SC behavior in vivo is difficult. Therefore, we developed a viscoelastic hydrogel with tunable properties to encapsulate myofibers while maintaining stem cell niche polarity and SC-myofiber interactions to investigate how matrix signals, including viscoelasticity and the integrin-binding ligand arginyl-glycyl-aspartic acid (RGD), influence SC behavior during muscle regeneration. Viscoelastic hydrogels support myofiber culture while preserving SC stemness for up to 72 hours post-encapsulation, minimizing myofiber hypercontraction and SC hyperproliferation compared to Matrigel. Pax7 is continuously expressed in SCs on myofibers embedded in hydrogels with higher stress relaxation while SCs differentiate when embedded in elastic hydrogels. Increasing RGD concentrations activates SCs and translocates YAP/TAZ to the nucleus as revealed by photo-expansion microscopy. Deleting YAP/TAZ abrogates RGD-mediated activation of SCs, and thus, YAP/TAZ mediates RGD ligand-induced SC activation and subsequent proliferation. STATEMENT OF SIGNIFICANCE: Satellite cells (SCs) are responsible for muscle maintenance and regeneration, but how the extracellular matrix regulates SC function is less understood and would benefit from new biomaterial models that can recapitulate the complexity of SC niche in vitro. Upon isolation of myofibers, SCs exit quiescence, becoming activated. To circumvent this issue, we developed a viscoelastic hydrogel for encapsulating myofibers, which maintains SC quiescence and limits differentiation, allowing the study of RGD effects. We showed that increasing RGD concentration promotes activation and suppresses differentiation. Finally, to allow high resolution imaging for resolving the subcellular localization of YAP/TAZ transcriptional co-activators, we applied photo-expansion microscopy and gel-to-gel transfer techniques to quantify YAP/TAZ nuclear-cytoplasmic ratio, revealing that RGD-mediated activation relies on YAP/TAZ nuclear translocation.

[Ultrastructural changes of skeletal muscle tissue of patients with dysferlinopathy]

Dysferlinopathy represents an orphan disease within the spectrum of progressive muscular dystrophies, occurring at a frequency of 1 to 9 cases per 1.000.000 individuals (Orphanet, 2024). It arises from mutations in the DYSF gene (OMIM 603009, 2p13, NM_003494.4), which is responsible for coding the transmembrane protein dysferlin. Dysferlin plays a critical role in the repair of muscle fiber membranes and the cellular processes of skeletal muscle regeneration. Although the molecular mechanisms of dysferlin-mediated repair are under active investigation, reports on the ultrastructural alterations in human skeletal muscles due to dysferlin deficiency are sparse.

OBJECTIVE

To identify the ultrastructural pathomorphological features of skeletal muscles in 6 patients with dysferlinopathy.

MATERIAL AND METHODS

This study presents pathomorphological, immunohistochemical, and ultrastructural data from skeletal muscle biopsies of 6 patients with molecularly confirmed dysferlinopathy.

RESULTS

Examination of paraffin-embedded sections of the anterior tibialis and vastus lateralis muscles, stained with hematoxylin and eosin, identified a primarily myopathic pattern of skeletal muscle injury. Immunohistochemical staining with dysferlin antibodies revealed the absence of the protein in muscle tissue compared to the positive control. Transmission electron microscopy has revealed ultrastructural alterations characteristic of dysferlinopathy, although not specific, including thickening and fragmentation of the basal membrane, thinning and lysis of myofibrils, folding and disruptions of the sarcolemma, destruction of mitochondria, and, newly described in this disease, necrosis of myosatellite cells and telocytes in skeletal muscles.

CONCLUSION

Despite the non-specificity of the identified ultrastructural alterations, electron microscopy of skeletal muscle biopsies in dysferlinopathy can provide additional information about the mechanisms underlying the disease development. The finding of myosatellite cell and telocyte necrosis indicates the impairment of skeletal muscle regenerative capacity, which may be a novel link in the pathogenesis of dysferlinopathy.

FAPs orchestrate homeostasis of muscle physiology and pathophysiology

As a common clinical manifestation, muscle weakness is prevalent in people with mobility disorders. Further studies of muscle weakness have found that patients with muscle weakness present with persistent muscle inflammation, loss of muscle fibers, fat infiltration, and interstitial fibrosis. Therefore, we propose the concept of muscle microenvironment homeostasis, which explains the abnormal pathological changes in muscles through the imbalance of muscle microenvironment homeostasis. And we identified an interstitial progenitor cell FAP during the transition from normal muscle microenvironment homeostasis to muscle microenvironment imbalance caused by muscle damage diseases. As a kind of pluripotent stem cell, FAPs do not participate in myogenic differentiation, but can differentiate into fibroblasts, adipocytes, osteoblasts, and chondrocytes. As a kind of mesenchymal progenitor cell, it is involved in the generation of extracellular matrix, regulate muscle regeneration, and maintain neuromuscular junction. However, the muscle microenvironment is disrupted by the causative factors, and the abnormal activities of FAPs eventually contribute to the complex pathological changes in muscles. Targeting the mechanisms of these muscle pathological changes, we have identified appropriate signaling targets for FAPs to improve and even treat muscle damage diseases. In this review, we propose the construction of muscle microenvironmental homeostasis and find the key cells that cause pathological changes in muscle after homeostasis is broken. By studying the mechanism of abnormal differentiation and apoptosis of FAPs, we found a strategy to inhibit the abnormal pathological changes in muscle damage diseases and improve muscle regeneration.

Comparative Analysis of Different Extracellular Matrices for the Maintenance of Bovine Satellite Cells

Cultured meat produced using satellite cells has emerged to address issues such as overpopulation, the ethical conundrums associated with the breeding environment, and the methane gas emissions associated with factory farming. To date, however, the challenges of maintaining satellite cells in vitro and reducing the costs of the culture media are still substantial. Gelatin, collagen, and fibronectin are commonly used extracellular matrices (ECMs) that facilitate signal integration with the cells and promote cell adhesion. In this study, we compared the proliferation, cell cycle, immunocytochemistry, and expression levels of Pax7, Pax3, Myf5, MyoD1, and MyoG genes in bovine satellite cells (BSCs) cultured on gelatin-, collagen- and fibronectin-coated dishes as part of short- and long-term cultures. We observed that BSCs cultured on gelatin-coated dishes showed higher levels of Pax7 expression than BSCs cultured on collagen- and fibronectin-coated dishes in both short- and long-term cultures, indicating that BSCs cultured on gelatin effectively maintained the satellite cell population in both the short- and long-term cultures. Our study highlights that gelatin is an effective ECM for the maintenance of BSCs and the production of cultured meat.

Mechanisms of muscle cells alterations and regeneration decline during aging

Skeletal muscles are essential for locomotion and body metabolism regulation. As muscles age, they lose strength, elasticity, and metabolic capability, leading to ineffective motion and metabolic derangement. Both cellular and extracellular alterations significantly influence muscle aging. Satellite cells (SCs), the primary muscle stem cells responsible for muscle regeneration, become exhausted, resulting in diminished population and functionality during aging. This decline in SC function impairs intercellular interactions as well as extracellular matrix production, further hindering muscle regeneration. Other muscle-resident cells, such as fibro-adipogenic progenitors (FAPs), pericytes, and immune cells, also deteriorate with age, reducing local growth factor activities and responsiveness to stress or injury. Systemic signaling, including hormonal changes, contributes to muscle cellular catabolism and disrupts muscle homeostasis. Collectively, these cellular and environmental components interact, disrupting muscle homeostasis and regeneration in advancing age. Understanding these complex interactions offers insights into potential regenerative strategies to mitigate age-related muscle degeneration.

Striated muscle: an inadequate soil for cancers

Many organs of the body are susceptible to cancer development. However, striated muscles-which include skeletal and cardiac muscles-are rarely the sites of primary cancers. Most deaths from cancer arise due to complications associated with the development of secondary metastatic tumours, for which there are few effective therapies. However, as with primary cancers, the establishment of metastatic tumours in striated muscle accounts for a disproportionately small fraction of secondary tumours, relative to the proportion of body composition. Examining why primary and metastatic cancers are comparatively rare in striated muscle presents an opportunity to better understand mechanisms that can influence cancer cell biology. To gain insights into the incidence and distribution of muscle metastases, this review presents a definitive summary of the 210 case studies of metastasis in muscle published since 2010. To examine why metastases rarely form in muscles, this review considers the mechanisms currently proposed to render muscle an inhospitable environment for cancers. The "seed and soil" hypothesis proposes that tissues' differences in susceptibility to metastatic colonization are due to differing host microenvironments that promote or suppress metastatic growth to varying degrees. As such, the "soil" within muscle may not be conducive to cancer growth. Gaining a greater understanding of the mechanisms that underpin the resistance of muscles to cancer may provide new insights into mechanisms of tumour growth and progression, and offer opportunities to leverage insights into the development of interventions with the potential to inhibit metastasis in susceptible tissues.

Composite bioink incorporating cell-laden liver decellularized extracellular matrix for bioprinting of scaffolds for bone tissue engineering

The field of bone tissue engineering (BTE) has witnessed a revolutionary breakthrough with the advent of three-dimensional (3D) bioprinting technology, which is considered an ideal choice for constructing scaffolds for bone regeneration. The key to realizing scaffold biofunctions is the selection and design of an appropriate bioink, and existing bioinks have significant limitations. In this study, a composite bioink based on natural polymers (gelatin and alginate) and liver decellularized extracellular matrix (LdECM) was developed and used to fabricate scaffolds for BTE using 3D bioprinting. Through in vitro studies, the concentration of LdECM incorporated into the bioink was optimized to achieve printability and stability and to improve the proliferation and osteogenic differentiation of loaded rat bone mesenchymal stem cells (rBMSCs). Furthermore, in vivo experiments were conducted using a Sprague Dawley rat model of critical-sized calvarial defects. The proposed rBMSC-laden LdECM-gelatin-alginate scaffold, bioprinted layer-by-layer, was implanted in the rat calvarial defect and the development of new bone growth was studied for four weeks. The findings showed that the proposed bioactive scaffolds facilitated angiogenesis and osteogenesis at the defect site. The findings of this study suggest that the developed rBMSC-laden LdECM-gelatin-alginate bioink has great potential for clinical translation and application in solving bone regeneration problems.

Durable Muscle Extracellular Matrix Engineered with Adhesive Phenolic Moieties for Effective Skeletal Muscle Regeneration in Muscle Atrophy

Muscle atrophy detrimentally impacts health and exacerbates physical disability, leading to increased mortality. In particular, sarcopenia, aging-related degenerative muscle loss, necessitates urgent remedies. Current approaches for treating muscle atrophy include exercise and nutrition, while drug exploration remains in its early stages. Cell therapy, focusing on satellite cells, faces significant challenge due to poor engraftment, safety issue, and high cost. Cell-free approach using extracellular matrix (ECM) shows a regenerative potential, but a lack of mechanical and adhesive properties hinders prolonged efficacy of ECM therapy. Here, durable muscle ECM (MEM) hydrogels for muscle atrophy by fortifying MEM with adhesive phenolic moieties including catechol and pyrogallol are demonstrated. The resultant phenolic MEM hydrogels exhibit enhanced mechanical and adhesive properties and provide sustained muscle-like microenvironments to address muscle atrophy. No local and systemic toxicities are observed after phenolic MEM injection into tibialis anterior muscle. Notably, these engineered MEM hydrogels, devoid of cells or drugs, induce tissue rejuvenation by promoting muscle protein synthesis and facilitating functional muscle recovery in mouse models of disuse- and age-induced atrophy. This study introduces cell-free, ECM-based therapeutics with translational potential for muscle atrophy by reversing muscle loss and restoring function.

Distinct muscle regenerative capacity of human induced pluripotent stem cell-derived mesenchymal stromal cells in Ullrich congenital muscular dystrophy model mice

BACKGROUND

Ullrich congenital muscular dystrophy (UCMD) is caused by a deficiency in type 6 collagen (COL6) due to mutations in COL6A1, COL6A2, or COL6A3. COL6 deficiency alters the extracellular matrix structure and biomechanical properties, leading to mitochondrial defects and impaired muscle regeneration. Therefore, mesenchymal stromal cells (MSCs) that secrete COL6 have attracted attention as potential therapeutic targets. Various tissue-derived MSCs exert therapeutic effects in various diseases. However, no reports have compared the effects of MSCs of different origins on UCMD pathology.

METHODS

To evaluate which MSC population has the highest therapeutic efficacy for UCMD, in vivo (transplantation of MSCs to Col6a1-KO/NSG mice) and in vitro experiments (muscle stem cell [MuSCs] co-culture with MSCs) were conducted using adipose tissue-derived MSCs, bone marrow-derived MSCs, and xeno-free-induced iPSC-derived MSCs (XF-iMSCs).

RESULTS

In transplantation experiments on Col6a1-KO/NSG mice, the group transplanted with XF-iMSCs showed significantly enhanced muscle fiber regeneration compared to the other groups 1 week after transplantation. At 12 weeks after transplantation, only the XF-iMSCs transplantation group showed a significantly larger muscle fiber diameter than the other groups without inducing fibrosis, which was observed in the other transplantation groups. Similarly, in co-culture experiments, XF-iMSCs were found to more effectively promote the fusion and differentiation of MuSCs derived from Col6a1-KO/NSG mice than the other primary MSCs investigated in this study. Additionally, in vitro knockdown and supplementation experiments suggested that the IGF2 secreted by XF-iMSCs promoted MuSC differentiation.

CONCLUSION

XF-iMSCs are promising candidates for promoting muscle regeneration while avoiding fibrosis, offering a safer and more effective therapeutic approach for UCMD than other potential therapies.

Granular Hydrogels Improve Myogenic Invasion and Repair after Volumetric Muscle Loss

Skeletal muscle injuries including volumetric muscle loss (VML) lead to excessive tissue scarring and permanent functional disability. Despite its high prevalence, there is currently no effective treatment for VML. Bioengineering interventions such as biomaterials that fill the VML defect to support cell and tissue growth are a promising therapeutic strategy. However, traditional biomaterials developed for this purpose lack the pore features needed to support cell infiltration. The present study investigates for the first time, the impact of granular hydrogels on muscle repair - hypothesizing that their flowability will permit conformable filling of the defect site and their inherent porosity will support the invasion of native myogenic cells, leading to effective muscle repair. Small and large microparticle fragments are prepared from photocurable hyaluronic acid polymer via extrusion fragmentation and facile size sorting. In assembled granular hydrogels, particle size and degree of packing significantly influence pore features, rheological behavior, and injectability. Using a mouse model of VML, it is demonstrated that, in contrast to bulk hydrogels, granular hydrogels support early-stage (satellite cell invasion) and late-stage (myofiber regeneration) muscle repair processes. Together, these results highlight the promising potential of injectable and porous granular hydrogels in supporting endogenous repair after severe muscle injury.

Beyond the bulk: overview and novel insights into the dynamics of muscle satellite cells during muscle regeneration

Skeletal muscle possesses remarkable regenerative capabilities, fully recovering within a month following severe acute damage. Central to this process are muscle satellite cells (MuSCs), a resident population of somatic stem cells capable of self-renewal and differentiation. Despite the highly predictable course of muscle regeneration, evaluating this process has been challenging due to the heterogeneous nature of myogenic precursors and the limited insight provided by traditional markers with overlapping expression patterns. Notably, recent advancements in single-cell technologies, such as single-cell (scRNA-seq) and single-nucleus RNA sequencing (snRNA-seq), have revolutionized muscle research. These approaches allow for comprehensive profiling of individual cells, unveiling dynamic heterogeneity among myogenic precursors and their contributions to regeneration. Through single-cell transcriptome analyses, researchers gain valuable insights into cellular diversity and functional dynamics of MuSCs post-injury. This review aims to consolidate classical and new insights into the heterogeneity of myogenic precursors, including the latest discoveries from novel single-cell technologies.

Hydrogel biomaterials that stiffen and soften on demand reveal that skeletal muscle stem cells harbor a mechanical memory

Muscle stem cells (MuSCs) are specialized cells that reside in adult skeletal muscle poised to repair muscle tissue. The ability of MuSCs to regenerate damaged tissues declines markedly with aging and in diseases such as Duchenne muscular dystrophy, but the underlying causes of MuSC dysfunction remain poorly understood. Both aging and disease result in dramatic increases in the stiffness of the muscle tissue microenvironment from fibrosis. MuSCs are known to lose their regenerative potential if cultured on stiff plastic substrates. We sought to determine whether MuSCs harbor a memory of their past microenvironment and if it can be overcome. We tested MuSCs in situ using dynamic hydrogel biomaterials that soften or stiffen on demand in response to light and found that freshly isolated MuSCs develop a persistent memory of substrate stiffness characterized by loss of proliferative progenitors within the first three days of culture on stiff substrates. MuSCs cultured on soft hydrogels had altered cytoskeletal organization and activity of Rho and Rac guanosine triphosphate hydrolase (GTPase) and Yes-associated protein mechanotransduction pathways compared to those on stiff hydrogels. Pharmacologic inhibition identified RhoA activation as responsible for the mechanical memory phenotype, and single-cell RNA sequencing revealed a molecular signature of the mechanical memory. These studies highlight that microenvironmental stiffness regulates MuSC fate and leads to MuSC dysfunction that is not readily reversed by changing stiffness. Our results suggest that stiffness can be circumvented by targeting downstream signaling pathways to overcome stem cell dysfunction in aged and disease states with aberrant fibrotic tissue mechanics.

Deoxynivalenol triggers mitotic catastrophe and apoptosis in C2C12 myoblasts

Deoxynivalenol (DON), commonly known as vomitoxin, is a mycotoxin produced by fungi and is frequently found as a contaminant in various cereal-based food worldwide. While the harmful effects of DON have been extensively studied in different tissues, its specific impact on the proliferation of skeletal muscle cells remains unclear. In this study, we utilized murine C2C12 myoblasts as a model to explore the influence of DON on their proliferation. Our observations indicated that DON exhibits dose-dependent toxicity, significantly inhibiting the proliferation of C2C12 cells. Through the application of RNA-seq analysis combined with gene set enrichment analysis, we identified a noteworthy downregulation of genes linked to the extracellular matrix (ECM) and condensed chromosome. Concurrently with the reduced expression of ECM genes, immunostaining analysis revealed notable changes in the distribution of fibronectin, a vital ECM component, condensing into clusters and punctate formations. Remarkably, the exposure to DON induced the formation of multipolar spindles, leading to the disruption of the normal cell cycle. This, in turn, activated the p53-p21 signaling pathway and ultimately resulted in apoptosis. These findings contribute significant insights into the mechanisms through which DON induces toxicity within skeletal muscle cells.

Type VI Collagen Deficiency Causes Enhanced Periodontal Tissue Destruction

The periodontal ligament (PDL) is a fibrillar connective tissue that lies between the alveolar bone and the tooth and is composed of highly specialized extracellular matrix (ECM) molecules and a heterogeneous population of cells that are responsible for collagen formation, immune response, bone formation, and chewing force sensation. Type VI collagen (COL6), a widely distributed ECM molecule, plays a critical role in the structural integrity and mechanical properties of various tissues including muscle, tendon, bone, cartilage, and skin. However, its role in the PDL remains largely unknown. Our study shows that deficiency of COL6 impairs PDL fibrillogenesis and exacerbates tissue destruction in ligature-induced periodontitis (LIP). We found that COL6-deficient mice exhibited increased bone loss and degraded PDL in LIP and that fibroblasts expressing high levels of Col6α2 are pivotal in ECM organization and cell-ECM interactions. Moreover, COL6 deficiency in the PDL led to an increased number of fibroblasts geared toward the inflammatory response. We also observed that cultured COL6-deficient fibroblasts from the PDL exhibited decreased expression of genes related to collagen fiber turnover and ECM organization as well as migration and proliferation. Our findings suggest that COL6 plays a crucial role in the PDL, influencing fibroblast function in fibrillogenesis and affecting the immune response in periodontitis. These insights advance our understanding of the molecular mechanisms underlying PDL maturation and periodontal disease.

We need to talk-how muscle stem cells communicate

Skeletal muscle is one of the tissues with the highest ability to regenerate, a finely controlled process which is critically depending on muscle stem cells. Muscle stem cell functionality depends on intrinsic signaling pathways and interaction with their immediate niche. Upon injury quiescent muscle stem cells get activated, proliferate and fuse to form new myofibers, a process involving the interaction of multiple cell types in regenerating skeletal muscle. Receptors in muscle stem cells receive the respective signals through direct cell-cell interaction, signaling via secreted factors or cell-matrix interactions thereby regulating responses of muscle stem cells to external stimuli. Here, we discuss how muscle stem cells interact with their immediate niche focusing on how this controls their quiescence, activation and self-renewal and how these processes are altered in age and disease.

Characterization of Proteome Changes in Aged and Collagen VI-Deficient Human Pericyte Cultures

Pericytes are a distinct type of cells interacting with endothelial cells in blood vessels and contributing to endothelial barrier integrity. Furthermore, pericytes show mesenchymal stem cell properties. Muscle-derived pericytes can demonstrate both angiogenic and myogenic capabilities. It is well known that regenerative abilities and muscle stem cell potential decline during aging, leading to sarcopenia. Therefore, this study aimed to investigate the potential of pericytes in supporting muscle differentiation and angiogenesis in elderly individuals and in patients affected by Ullrich congenital muscular dystrophy or by Bethlem myopathy, two inherited conditions caused by mutations in collagen VI genes and sharing similarities with the progressive skeletal muscle changes observed during aging. The study characterized pericytes from different age groups and from individuals with collagen VI deficiency by mass spectrometry-based proteomic and bioinformatic analyses. The findings revealed that aged pericytes display metabolic changes comparable to those seen in aging skeletal muscle, as well as a decline in their stem potential, reduced protein synthesis, and alterations in focal adhesion and contractility, pointing to a decrease in their ability to form blood vessels. Strikingly, pericytes from young patients with collagen VI deficiency showed similar characteristics to aged pericytes, but were found to still handle oxidative stress effectively together with an enhanced angiogenic capacity.

Fibro-adipogenic progenitors in physiological adipogenesis and intermuscular adipose tissue remodeling

Excessive accumulation of intermuscular adipose tissue (IMAT) is a common pathological feature in various metabolic and health conditions and can cause muscle atrophy, reduced function, inflammation, insulin resistance, cardiovascular issues, and unhealthy aging. Although IMAT results from fat accumulation in muscle, the mechanisms underlying its onset, development, cellular components, and functions remain unclear. IMAT levels are influenced by several factors, such as changes in the tissue environment, muscle type and origin, extent and duration of trauma, and persistent activation of fibro-adipogenic progenitors (FAPs). FAPs are a diverse and transcriptionally heterogeneous population of stromal cells essential for tissue maintenance, neuromuscular stability, and tissue regeneration. However, in cases of chronic inflammation and pathological conditions, FAPs expand and differentiate into adipocytes, resulting in the development of abnormal and ectopic IMAT. This review discusses the role of FAPs in adipogenesis and how they remodel IMAT. It highlights evidence supporting FAPs and FAP-derived adipocytes as constituents of IMAT, emphasizing their significance in adipose tissue maintenance and development, as well as their involvement in metabolic disorders, chronic pathologies and diseases. We also investigated the intricate molecular pathways and cell interactions governing FAP behavior, adipogenesis, and IMAT accumulation in chronic diseases and muscle deconditioning. Finally, we hypothesize that impaired cellular metabolic flexibility in dysfunctional muscles impacts FAPs, leading to IMAT. A deeper understanding of the biology of IMAT accumulation and the mechanisms regulating FAP behavior and fate are essential for the development of new therapeutic strategies for several debilitating conditions.

Spatiotemporal dynamics exhibited by horizontal basal cells reveal a pro-neurogenic pathway during injury-induced olfactory epithelium regeneration

Horizontal basal cells (HBCs) mediate olfactory epithelium (OE) regeneration following severe tissue injury. The dynamism of the post-injury environment is well illustrated by in silico modeling of RNA sequencing data that demonstrate an evolving HBC transcriptome. Unfortunately, spatiotemporally dynamic processes occurring within HBCs in situ remain poorly understood. Here, we show that HBCs at 24 h post-OE injury spatially redistribute a constellation of proteins, which, in turn, helped to nominate Rac1 as a regulator of HBC differentiation during OE regeneration. Using our primary culture model to activate HBCs pharmacologically, we demonstrate that concurrent Rac1 inhibition attenuates HBC differentiation potential. This in vitro functional impairment manifested in vivo as decreased HBC differentiation into olfactory sensory neurons following HBC-specific Rac1 conditional knockout. Taken together, our data potentiate the design of hyposmia-alleviating therapies and highlight aspects of in situ HBC spatiotemporal dynamics that deserve further investigation.

Therapeutic applications of biological macromolecules and scaffolds for skeletal muscle regeneration: A review

Skeletal muscle (SM) mass and strength maintenance are important requirements for human well-being. SM regeneration to repair minor injuries depends upon the myogenic activities of muscle satellite (stem) cells. However, losses of regenerative properties following volumetric muscle loss or severe trauma or due to congenital muscular abnormalities are not self-restorable, and thus, these conditions have major healthcare implications and pose clinical challenges. In this context, tissue engineering based on different types of biomaterials and scaffolds provides an encouraging means of structural and functional SM reconstruction. In particular, biomimetic (able to transmit biological signals) and several porous scaffolds are rapidly evolving. Several biological macromolecules/biomaterials (collagen, gelatin, alginate, chitosan, and fibrin etc.) are being widely used for SM regeneration. However, available alternatives for SM regeneration must be redesigned to make them more user-friendly and economically feasible with longer shelf lives. This review aimed to explore the biological aspects of SM regeneration and the roles played by several biological macromolecules and scaffolds in SM regeneration in cases of volumetric muscle loss.

Islr regulates satellite cells asymmetric division through the SPARC/p-ERK1/2 signaling pathway

Satellite cells (SCs) are adult muscle stem cells responsible for muscle regeneration after acute and chronic muscle injuries. The balance between stem cell self-renewal and differentiation determines the kinetics and efficiency of skeletal muscle regeneration. This study assessed the function of Islr in SC asymmetric division. The deletion of Islr reduced muscle regeneration in adult mice by decreasing the SC pool. Islr is pivotal for SC proliferation, and its deletion promoted the asymmetric division of SCs. A mechanistic search revealed that Islr bound to and degraded secreted protein acidic and rich in cysteine (SPARC), which activated p-ERK1/2 signaling required for asymmetric division. These findings demonstrate that Islr is a key regulator of SC division through the SPARC/p-ERK1/2 signaling pathway. These data provide a basis for treating myopathy.

The coordinated activities of collagen VI and XII in maintenance of tissue structure, function and repair: evidence for a physical interaction

Collagen VI and collagen XII are structurally complex collagens of the extracellular matrix (ECM). Like all collagens, type VI and XII both possess triple-helical components that facilitate participation in the ECM network, but collagen VI and XII are distinct from the more abundant fibrillar collagens in that they also possess arrays of structurally globular modules with the capacity to propagate signaling to attached cells. Cell attachment to collagen VI and XII is known to regulate protective, proliferative or developmental processes through a variety of mechanisms, but a growing body of genetic and biochemical evidence suggests that at least some of these phenomena may be potentiated through mechanisms that require coordinated interaction between the two collagens. For example, genetic studies in humans have identified forms of myopathic Ehlers-Danlos syndrome with overlapping phenotypes that result from mutations in either collagen VI or XII, and biochemical and cell-based studies have identified accessory molecules that could form bridging interactions between the two collagens. However, the demonstration of a direct or ternary structural interaction between collagen VI or XII has not yet been reported. This Hypothesis and Theory review article examines the evidence that supports the existence of a functional complex between type VI and XII collagen in the ECM and discusses potential biological implications.

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.

Apelin stimulation of the vascular skeletal muscle stem cell niche enhances endogenous repair in dystrophic mice

Impaired skeletal muscle stem cell (MuSC) function has long been suspected to contribute to the pathogenesis of muscular dystrophy (MD). Here, we showed that defects in the endothelial cell (EC) compartment of the vascular stem cell niche in mouse models of Duchenne MD, laminin α2-related MD, and collagen VI-related myopathy were associated with inefficient mobilization of MuSCs after tissue damage. Using chemoinformatic analysis, we identified the 13-amino acid form of the peptide hormone apelin (AP-13) as a candidate for systemic stimulation of skeletal muscle ECs. Systemic administration of AP-13 using osmotic pumps generated a pro-proliferative EC-rich niche that supported MuSC function through angiocrine factors and markedly improved tissue regeneration and muscle strength in all three dystrophic mouse models. Moreover, EC-specific knockout of the apelin receptor led to regenerative defects that phenocopied key pathological features of MD, including vascular defects, fibrosis, muscle fiber necrosis, impaired MuSC function, and reduced force generation. Together, these studies provide in vivo proof of concept that enhancing endogenous skeletal muscle repair by targeting the vascular niche is a viable therapeutic avenue for MD and characterized AP-13 as a candidate for further study for the systemic treatment of MuSC dysfunction.

Culture substrate stiffness impacts human myoblast contractility-dependent proliferation and nuclear envelope wrinkling

Understanding how biophysical and biochemical microenvironmental cues together influence the regenerative activities of muscle stem cells and their progeny is crucial in strategizing remedies for pathological dysregulation of these cues in aging and disease. In this study, we investigated the cell-level influences of extracellular matrix (ECM) ligands and culture substrate stiffness on primary human myoblast contractility and proliferation within 16 h of plating and found that tethered fibronectin led to stronger stiffness-dependent responses compared to laminin and collagen. A proteome-wide analysis further uncovered cell metabolism, cytoskeletal and nuclear component regulation distinctions between cells cultured on soft and stiff substrates. Interestingly, we found that softer substrates increased the incidence of myoblasts with a wrinkled nucleus, and that the extent of wrinkling could predict Ki67 (also known as MKI67) expression. Nuclear wrinkling and Ki67 expression could be controlled by pharmacological manipulation of cellular contractility, offering a potential cellular mechanism. These results provide new insights into the regulation of human myoblast stiffness-dependent contractility response by ECM ligands and highlight a link between myoblast contractility and proliferation.

Role of microenvironment on muscle stem cell function in health, adaptation, and disease

The role of the cellular microenvironment has recently gained attention in the context of muscle health, adaption, and disease. Emerging evidence supports major roles for the extracellular matrix (ECM) in regeneration and the dynamic regulation of the satellite cell niche. Satellite cells normally reside in a quiescent state in healthy muscle, but upon muscle injury, they activate, proliferate, and fuse to the damaged fibers to restore muscle function and architecture. This chapter reviews the composition and mechanical properties of skeletal muscle ECM and the role of these factors in contributing to the satellite cell niche that impact muscle regeneration. In addition, the chapter details the effects of satellite cell-matrix interactions and provides evidence that there is bidirectional regulation affecting both the cellular and extracellular microenvironment within skeletal muscle. Lastly, emerging methods to investigate satellite cell-matrix interactions will be presented.

Decoding the forces that shape muscle stem cell function

Skeletal muscle is a force-producing organ composed of muscle tissues, connective tissues, blood vessels, and nerves, all working in synergy to enable movement and provide support to the body. While robust biomechanical descriptions of skeletal muscle force production at the body or tissue level exist, little is known about force application on microstructures within the muscles, such as cells. Among various cell types, skeletal muscle stem cells reside in the muscle tissue environment and play a crucial role in driving the self-repair process when muscle damage occurs. Early evidence indicates that the fate and function of skeletal muscle stem cells are controlled by both biophysical and biochemical factors in their microenvironments, but much remains to accomplish in quantitatively describing the biophysical muscle stem cell microenvironment. This book chapter aims to review current knowledge on the influence of biophysical stresses and landscape properties on muscle stem cells in heath, aging, and diseases.

The extracellular matrix niche of muscle stem cells

Preserving the potency of stem cells in adult tissues is very demanding and relies on the concerted action of various cellular and non-cellular elements in a precise stoichiometry. This balanced microenvironment is found in specific anatomical "pockets" within the tissue, known as the stem cell niche. In this review, we explore the interplay between stem cells and their niches, with a primary focus on skeletal muscle stem cells and the extracellular matrix (ECM). Quiescent muscle stem cells, known as satellite cells are active producers of a diverse array of ECM molecules, encompassing major constituents like collagens, laminins, and integrins, some of which are explored in this review. The conventional perception of ECM as merely a structural scaffold is evolving. Collagens can directly interact as ligands with receptors on satellite cells, while other ECM proteins have the capacity to sequester growth factors and regulate their release, especially relevant during satellite cell turnover in homeostasis or activation upon injury. Additionally, we explore an evolutionary perspective on the ECM across a range of multicellular organisms and discuss a model wherein satellite cells are self-sustained by generating their own niche. Considering the prevalence of ECM proteins in the connective tissue of various organs it is not surprising that mutations in ECM genes have pathological implications, including in muscle, where they can lead to myopathies. However, the particular role of certain disease-related ECM proteins in stem cell maintenance highlights the potential contribution of stem cell deregulation to the progression of these disorders.

Muscle stem cell dysfunction in rhabdomyosarcoma and muscular dystrophy

Muscle stem cells (MuSCs) are crucial to the repair and homeostasis of mature skeletal muscle. MuSC dysfunction and dysregulation of the myogenic program can contribute to the development of pathology ranging from cancers like rhabdomyosarcoma (RMS) or muscle degenerative diseases such as Duchenne muscular dystrophy (DMD). Both diseases exhibit dysregulation at nearly all steps of myogenesis. For instance, MuSC self-renewal processes are altered. In RMS, this leads to the creation of tumor propagating cells. In DMD, impaired asymmetric stem cell division creates a bias towards producing self-renewing stem cells instead of committing to differentiation. Hyperproliferation of these cells contribute to tumorigenesis in RMS and symmetric expansion of the self-renewing MuSC population in DMD. Both diseases also exhibit a repression of factors involved in terminal differentiation, halting RMS cells in the proliferative stage and thus driving tumor growth. Conversely, the MuSCs in DMD exhibit impaired differentiation and fuse prematurely, affecting myonuclei maturation and the integrity of the dystrophic muscle fiber. Finally, both disease states cause alterations to the MuSC niche. Various elements of the niche such as inflammatory and migratory signaling that impact MuSC behavior are dysregulated. Here we show how these seemingly distantly related diseases indeed have similarities in MuSC dysfunction, underlying the importance of considering MuSCs when studying the pathophysiology of muscle diseases.

The satellite cell in skeletal muscle: A story of heterogeneity

Skeletal muscle is a highly represented tissue in mammals and is composed of fibers that are extremely adaptable and capable of regeneration. This characteristic of muscle fibers is made possible by a cell type called satellite cells. Adjacent to the fibers, satellite cells are found in a quiescent state and located between the muscle fibers membrane and the basal lamina. These cells are required for the growth and regeneration of skeletal muscle through myogenesis. This process is known to be tightly sequenced from the activation to the differentiation/fusion of myofibers. However, for the past fifteen years, researchers have been interested in examining satellite cell heterogeneity and have identified different subpopulations displaying distinct characteristics based on localization, quiescence state, stemness capacity, cell-cycle progression or gene expression. A small subset of satellite cells appears to represent multipotent long-term self-renewing muscle stem cells (MuSC). All these distinctions led us to the hypothesis that the characteristics of myogenesis might not be linear and therefore may be more permissive based on the evidence that satellite cells are a heterogeneous population. In this review, we discuss the different subpopulations that exist within the satellite cell pool to highlight the heterogeneity and to gain further understanding of the myogenesis progress. Finally, we discuss the long term self-renewing MuSC subpopulation that is capable of dividing asymmetrically and discuss the molecular mechanisms regulating MuSC polarization during health and disease.

Extraocular muscle stem cells exhibit distinct cellular properties associated with non-muscle molecular signatures

Skeletal muscle stem cells (MuSCs) are recognised as functionally heterogeneous. Cranial MuSCs are reported to have greater proliferative and regenerative capacity when compared with those in the limb. A comprehensive understanding of the mechanisms underlying this functional heterogeneity is lacking. Here, we have used clonal analysis, live imaging and single cell transcriptomic analysis to identify crucial features that distinguish extraocular muscle (EOM) from limb muscle stem cell populations. A MyogeninntdTom reporter showed that the increased proliferation capacity of EOM MuSCs correlates with deferred differentiation and lower expression of the myogenic commitment gene Myod. Unexpectedly, EOM MuSCs activated in vitro expressed a large array of extracellular matrix components typical of mesenchymal non-muscle cells. Computational analysis underscored a distinct co-regulatory module, which is absent in limb MuSCs, as driver of these features. The EOM transcription factor network, with Foxc1 as key player, appears to be hardwired to EOM identity as it persists during growth, disease and in vitro after several passages. Our findings shed light on how high-performing MuSCs regulate myogenic commitment by remodelling their local environment and adopting properties not generally associated with myogenic cells.

Trisomy 21 Alters Cell Proliferation and Migration of iPSC-Derived Cardiomyocytes on Type VI Collagen

Purpose

Individuals with Down syndrome (DS) are 2000 times more likely to develop a congenital heart defect (CHD) than the typical population Freeman et al. in Am J Med Genet 80:213-217 (1998). The majority of CHDs in individuals with DS characteristically involve the atrioventricular (AV) canal, including the valves and the atrial or ventricular septum. Type VI collagen (COLVI) is the primary structural component in the developing septa and endocardial cushions, with two of the three genes encoding for COLVI located on human chromosome 21 and upregulated in Down syndrome (von Kaisenberg et al. in Obstet Gynecol 91:319-323, 1998; Gittenberger-De Groot et al. in Anatom Rec Part A 275:1109-1116, 2023).

Methods

To investigate the effect of COLVI dosage on cardiomyocytes with trisomy 21, induced pluripotent stem cells (iPSC) from individuals with DS and age- and sex-matched controls were differentiated into cardiomyocytes (iPSC-CM) and plated on varying concentrations of COLVI.

Results

Real time quantitative PCR showed decreased expression of cardiac-specific genes of DS iPSC-CM lines compared to control iPSC-CM. As expected, DS iPSC-CM had increased expression of genes on chromosome 21, including COL6A1, COL6A2, as well as genes not located on chromosome 21, namely COL6A3, HAS2 and HYAL2. We found that higher concentrations of COLVI result in decreased proliferation and migration of DS iPSC-CM, but not control iPSC-CM.

Conclusions

These results suggest that the increased expression of COLVI in DS may result in lower migration-driven elongation of endocardial cushions stemming from lower cell proliferation and migration, possibly contributing to the high incidence of CHD in the DS population.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12195-023-00791-x.

Native collagen VI delays early muscle stem cell differentiation

Adult muscle stem cells (MuSCs) are critical for muscle homeostasis and regeneration, and their behavior relies on a finely regulated niche made of specific extracellular matrix (ECM) components and soluble factors. Among ECM proteins, collagen VI (Col6) influences the mechanical properties of the niche and, in turn, MuSC self-renewal capabilities. Here, we investigated whether Col6 can exert a direct function as a biochemical signal for regulating the stemness and differentiation of murine MuSCs and myoblasts. Native Col6, but not its pepsin-resistant fragment, counteracts the early differentiation of myogenic cells by reducing the expression of differentiation marker genes and preserving stemness features, with inhibition of the canonical Wnt pathway. Our data indicate that extracellular Col6 acts as a soluble ligand in delaying early myogenic differentiation by regulating intracellular signals involved in adult myogenesis.

Affected cell types for hundreds of Mendelian diseases revealed by analysis of human and mouse single-cell data

Mendelian diseases tend to manifest clinically in certain tissues, yet their affected cell types typically remain elusive. Single-cell expression studies showed that overexpression of disease-associated genes may point to the affected cell types. Here, we developed a method that infers disease-affected cell types from the preferential expression of disease-associated genes in cell types (PrEDiCT). We applied PrEDiCT to single-cell expression data of six human tissues, to infer the cell types affected in Mendelian diseases. Overall, we inferred the likely affected cell types for 328 diseases. We corroborated our findings by literature text-mining, expert validation, and recapitulation in mouse corresponding tissues. Based on these findings, we explored characteristics of disease-affected cell types, showed that diseases manifesting in multiple tissues tend to affect similar cell types, and highlighted cases where gene functions could be used to refine inference. Together, these findings expand the molecular understanding of disease mechanisms and cellular vulnerability.

Ablation of the carboxy-terminal end of MAMDC2 causes a distinct muscular dystrophy

The extracellular matrix (ECM) has an important role in the development and maintenance of skeletal muscle, and several muscle diseases are associated with the dysfunction of ECM elements. MAMDC2 is a putative ECM protein and its role in cell proliferation has been investigated in certain cancer types. However, its participation in skeletal muscle physiology has not been previously studied. We describe 17 individuals with an autosomal dominant muscular dystrophy belonging to two unrelated families in which different heterozygous truncating variants in the last exon of MAMDC2 co-segregate correctly with the disease. The radiological aspect of muscle involvement resembles that of COL6 myopathies with fat replacement at the peripheral rim of vastii muscles. In this cohort, a subfascial and peri-tendinous pattern is observed in upper and lower limb muscles. Here we show that MAMDC2 is expressed in adult skeletal muscle and differentiating muscle cells, where it appears to localize to the sarcoplasm and myonuclei. In addition, we show it is secreted by myoblasts and differentiating myotubes into to the extracellular compartment. The last exon encodes a disordered region with a polar residue compositional bias loss of which likely induces a toxic effect of the mutant protein. The precise mechanisms by which the altered MAMDC2 proteins cause disease remains to be determined. MAMDC2 is a skeletal muscle disease-associated protein. Its role in muscle development and ECM-muscle communication remains to be fully elucidated. Screening of the last exon of MAMDC2 should be considered in patients presenting with autosomal dominant muscular dystrophy, particularly in those with a subfascial radiological pattern of muscle involvement.

Regenerating human skeletal muscle forms an emerging niche in vivo to support PAX7 cells

Skeletal muscle stem and progenitor cells including those derived from human pluripotent stem cells (hPSCs) offer an avenue towards personalized therapies and readily fuse to form human-mouse myofibres in vivo. However, skeletal muscle progenitor cells (SMPCs) inefficiently colonize chimeric stem cell niches and instead associate with human myofibres resembling foetal niches. We hypothesized competition with mouse satellite cells (SCs) prevented SMPC engraftment into the SC niche and thus generated an SC ablation mouse compatible with human engraftment. Single-nucleus RNA sequencing of SC-ablated mice identified the absence of a transient myofibre subtype during regeneration expressing Actc1. Similarly, ACTC1+ human myofibres supporting PAX7+ SMPCs increased in SC-ablated mice, and after re-injury we found SMPCs could now repopulate into chimeric niches. To demonstrate ACTC1+ myofibres are essential to supporting PAX7 SMPCs, we generated caspase-inducible ACTC1 depletion human pluripotent stem cells, and upon SMPC engraftment we found a 90% reduction in ACTC1+ myofibres and a 100-fold decrease in PAX7 cell numbers compared with non-induced controls. We used spatial RNA sequencing to identify key factors driving emerging human niche formation between ACTC1+ myofibres and PAX7+ SMPCs in vivo. This revealed that transient regenerating human myofibres are essential for emerging niche formation in vivo to support PAX7 SMPCs.

Extracellular matrix: the critical contributor to skeletal muscle regeneration-a comprehensive review

The regenerative ability of skeletal muscle (SM) in response to damage, injury, or disease is a highly intricate process that involves the coordinated activities of multiple cell types and biomolecular factors. Of these, extracellular matrix (ECM) is considered a fundamental component of SM regenerative ability. This review briefly discusses SM myogenesis and regeneration, the roles played by muscle satellite cells (MSCs), other cells, and ECM components, and the effects of their dysregulations on these processes. In addition, we review the various types of ECM scaffolds and biomaterials used for SM regeneration, their applications, recent advances in ECM scaffold research, and their impacts on tissue engineering and SM regeneration, especially in the context of severe muscle injury, which frequently results in substantial muscle loss and impaired regenerative capacity. This review was undertaken to provide a comprehensive overview of SM myogenesis and regeneration, the stem cells used for muscle regeneration, the significance of ECM in SM regeneration, and to enhance understanding of the essential role of the ECM scaffold during SM regeneration.

The dorsal portion of the bovine diaphragm as a useful tissue for producing a 3D muscle scaffold

Three-dimensional (3D) organoids are an innovative approach to obtain an in vitro model for ex vivo studies to overcome the limitations of monolayer cell culture and reduce the use of animal models. An organoid of skeletal muscle requires the presence of the extracellular matrix to represent a functional muscle in vitro, which is why decellularized tissue is an optimal choice. Various muscles have been considered to produce a muscle organoid, most from rodents or small animals, and only recently some studies have been reported on the muscles of large animals. This work presents a muscular organoid produced from the bovine diaphragm, which has a peculiar multilayered structure with different fibre orientations depending on the considered area. This paper analyses the anatomical structure of the bovine diaphragm, selects the most appropriate portion, and presents a decellularization protocol for a multilayered muscle. In addition, a preliminary test of recellularization with primary bovine myocytes was presented with the future aim of obtaining a 3D muscle allogenic organoid, completely bovine-derived. The results demonstrate that the dorsal portion of bovine diaphragm presents a regular alternation of muscular and fibrous layers and that the complete decellularization does not affect the biocompatibility. These results provide a strong foundation for the potential application of this portion of tissue as a scaffold for in vitro studies of muscle organoids.

Contextual fear memory impairment in Angelman syndrome model mice is associated with altered transcriptional responses

Angelman syndrome (AS) is a rare neurogenetic disorder caused by UBE3A deficiency and characterized by severe developmental delay, cognitive impairment, and motor dysfunction. In the present study, we performed RNA-seq on hippocampal samples from both wildtype (WT) and AS male mice, with or without contextual fear memory recall. There were 281 recall-associated differentially expressed genes (DEGs) in WT mice and 268 DEGs in AS mice, with 129 shared by the two genotypes. Gene ontology analysis showed that extracellular matrix and stimulation-induced response genes were prominently enriched in recall-associated DEGs in WT mice, while nuclear acid metabolism and tissue development genes were highly enriched in those from AS mice. Further analyses showed that the 129 shared DEGs belonged to nuclear acid metabolism and tissue development genes. Unique recall DEGs in WT mice were enriched in biological processes critical for synaptic plasticity and learning and memory, including the extracellular matrix network clustered around fibronectin 1 and collagens. In contrast, AS-specific DEGs were not enriched in any known pathways. These results suggest that memory recall in AS mice, while altering the transcriptome, fails to recruit memory-associated transcriptional programs, which could be responsible for the memory impairment in AS mice.

The multifaceted role of macrophages in homeostatic and injured skeletal muscle

Skeletal muscle is essential for body physical activity, energy metabolism, and temperature maintenance. It has excellent capabilities to maintain homeostasis and to regenerate after injury, which indispensably relies on muscle stem cells, satellite cells (MuSCs). The quiescence, activation, and differentiation of MuSCs are tightly regulated in homeostatic and regenerating muscles. Among the important regulators are intramuscular macrophages, which are functionally heterogeneous with different subtypes present in a spatiotemporal manner to regulate the balance of different MuSC statuses. During chronic injury and aging, intramuscular macrophages often undergo aberrant activation, which in turn disrupts muscle homeostasis and regenerative repair. Growing evidence suggests that the aberrant activation is mainly triggered by altered muscle microenvironment. The trained immunity that affects myeloid progenitors during hematopoiesis may also contribute. Aged immune system may contribute, in part, to the aging-related sarcopenia and compromised skeletal muscle injury repair. As macrophages are actively involved in the progression of many muscle diseases, manipulating their functional activation has become a promising therapeutic approach, which requires comprehensive knowledge of the cellular and molecular mechanisms underlying the diverse activation. To this end, we discuss here the current knowledge of multifaceted role of macrophages in skeletal muscle homeostasis, injury, and repair.