Muscle satellite cells are a functionally heterogeneous population in both somite-derived and branchiomeric muscles

Optimizing Confocal Imaging Protocols for Muscle Fiber Typing in the Mouse Masseter Muscle

The masseter muscle, a key orofacial muscle, demonstrates unique anatomical and functional properties, including sexual dimorphism in myosin heavy chain (MyHC) expression and complex fiber architecture. Despite its importance in mastication and relevance to various disorders, phenotypic characterization of the masseter remains limited. Conventional fluorescence microscopy has been a cornerstone in muscle fiber typing, reliably identifying MyHC isoforms and measuring fiber cross-sectional areas. Building on this foundation, confocal microscopy offers complementary advantages, such as enhanced resolution, increased flexibility for multiplexing, and the ability to visualize complex structures in three dimensions. This study presents a detailed protocol for using confocal microscopy to achieve high-resolution imaging and molecular characterization of masseter muscle cryosections. By leveraging advanced technologies such as white light lasers and extended z-length imaging, this method ensures precise spectral separation, simultaneous multichannel fluorescence detection, and the ability to capture muscle architecture in three dimensions. The protocol includes tissue preparation, immunostaining for MyHC isoforms, and postprocessing for fiber segmentation and quantification. The imaging setup was optimized for minimizing signal bleed through, improving the signal-to-noise ratio, and enabling detailed visualization of muscle fibers and molecular markers. Image postprocessing allows for quantification of the cross-sectional area of individual fibers, nuclei location measurements, and identification of MyHC isoforms within each fiber. This confocal microscopy-based protocol provides similar resolution and contrast compared to conventional techniques, enabling robust multiplexed imaging and 3D reconstruction of muscle structures. These advantages make it a valuable tool for studying complex muscle architecture, offering broad applications in muscle physiology and pathology research. Key features • Enables high-resolution imaging of muscle fiber architecture, capturing detailed spatial relationships using extended z-length and advanced spectral separation techniques. • Supports simultaneous detection of multiple molecular markers for robust muscle fiber typing and molecular localization. • Allows for the generation of three-dimensional models to analyze muscle structures such as neuromuscular junctions, extracellular matrix, and mitochondrial organization. • Adaptable to various skeletal muscles and species, providing valuable insights into muscle physiology, regeneration, and disease processes. Graphical overview Analyzing muscle fiber composition and morphology in mice's masseter muscle using confocal microscopy. Workflow for characterizing rodent masseter muscle fibers using advanced confocal microscopy. Confocal microscopy, equipped with white light laser technology and optimized z-stack imaging, allows precise spectral unmixing to reduce bleed through and enhance signal detection. The z-length is extended beyond the physical thickness of the sample to account for potential variations in tissue flatness and ensure complete imaging of all focal planes. The resulting high-resolution images provide detailed insights into fiber architecture, molecular composition, and cross-sectional areas, ensuring robust and reproducible data for analyzing the complex phenotypic characteristics of the masseter and other muscles.

Optimizing the Host Niche to Fuel Cleft Lip Muscle Regeneration

OBJECTIVE

Lip muscle reconstruction is a key strategy for cleft lip repair, but the outcome of muscle regeneration is suboptimal. The lack of relevant models restricts studies on cleft lip muscle regeneration.

DESIGN

In this study, we developed an animal xenograft model by transplanting muscle samples from patients with cleft lip into different host sites in immunodeficient mice. The grafted muscles were harvested after 1, 2, and 5 months to investigate the temporal dynamics of myofiber growth and maturation. Comparisons were made among muscle biopsies, xenografts in the masseter (MAS) muscle of the mice, and xenografts in the tibialis anterior (TA) muscle of the mice to determine the optimal host niche.

Histological analysis of myofiber number and size, fiber type switching, innervation, and blood supply was performed to evaluate the cleft lip muscle regeneration process.

RESULTS

The muscles from cleft lip patients underwent an active degeneration-regeneration cycle. The fiber diameter in the MAS niche was significantly larger than that in the TA niche and was comparable to the fiber diameter of the donor biopsy from which it originated. The innervation and blood supply of the muscle grafts at the MAS host site were also superior to those of the grafts at the TA host site.

CONCLUSIONS

The MAS muscles of mice provided the most favorable microenvironment for cleft lip muscle regeneration. This patient-centered xenograft model offers a platform for cleft lip muscle regeneration studies.

Molecular characterization of the heterogeneity of satellite cell populations isolated from an individual Turkey pectoralis major muscle

Satellite cells (SCs) are myogenic stem cells responsible for post hatch muscle growth and the regeneration of muscle fibers. Satellite cells are not a homogenous population of cells within a muscle and have variable rates of proliferation and differentiation even within a single fiber type muscle like the turkey pectoralis major muscle. In this study, the single satellite cell clones derived from the same turkey pectoralis major muscle with different proliferation rates were compared. The clones were classified as either fast-growing (early clone) or slow-growing (late clone) SCs. To thoroughly examine the molecular differences between these two groups, RNA sequencing was conducted to compare their transcriptomes following 72 h of proliferation. Principal Component Analysis confirmed that the transcriptomic profiles of early- and late-clones are markedly distinct. Differential gene expression analysis identified over 5,300 genes that were significantly differentially expressed between the two groups of cells. Gene ontology analysis showed that genes highly expressed in early clones are responsible for the fundamental aspects of muscle biology, including muscle tissue development and structural maturation. Conversely, genes upregulated in late clones are involved in cell-cell communication, extracellular matrix interactions, signal ligand activity, and cytokine activity-key components for forming an extracellular niche essential for functional satellite cells maintenance. Further examination of specific gene ontology categories such as muscle structure development and extracellular matrix components indicated significant differences in gene expression patterns between early- and late-clones. These findings highlight the genetic and functional diversity of SCs in turkeys. The distinct roles of these satellite cell populations indicate that a balance between them is necessary for preserving the normal physiological functions of SCs.

Achieving myoblast engraftment into intact skeletal muscle via extracellular matrix

Cell therapy of skeletal muscles is a promising approach for the prevention of muscular diseases and age-related muscle atrophy. However, cell transplantation to treat muscle atrophy that does not involve disease, such as sarcopenia, is considered impossible because externally injected cells rarely engraft into non-injured muscle tissue. Additionally, skeletal muscle-specific somatic stem cells, called satellite cells, lose their ability to adhere to tissue after being cultured in vitro and transforming into myoblasts. To overcome these hurdles, we explored using extracellular matrix (ECM) components to create a niche environment conducive for myoblasts during transplantation. We demonstrated that myoblasts mixed with ECM components can be engrafted into intact skeletal muscle and significantly increase muscle mass in a mouse model. These findings implicate cell transplantation therapy as a viable option for the treatment of sarcopenia. The findings will inform advancements in regenerative medicine for skeletal muscles.

The Hox-based positional memory in muscle stem cells

The skeletal muscle is a contractile tissue distributed throughout the body with various anatomical sizes, shapes and functions. In pathological conditions, such as muscular dystrophy, age-related sarcopenia and cancer cachexia, skeletal muscles are not uniformly affected throughout the body. This region-specific vulnerability cannot be fully explained by known physiological classifications, including muscle fiber types. Accumulating evidence indicates that the expression patterns of topographic homeobox (Hox) genes provide a molecular signature of positional memory, reflecting the anatomical locations and embryonic history of muscles and their associated muscle stem cells in adult mice and humans. Hox-based positional memory is not merely a remnant of embryonic development but is expected to be an intrinsic determinant controlling muscle function because recent studies have shown that aberrant Hox genes affect muscle stem cells. In this review, we discuss the concept of Hox-based positional memory, which may offer a new perspective on the region-specific pathophysiology of muscle disorders.

Effect of Chicken Age on Proliferation and Differentiation Abilities of Muscle Stem Cells and Nutritional Characteristics of Cultured Meat Tissue

This study aimed to investigate effects of chicken age on proliferation and differentiation capacity of muscle satellite cells (MSCs) and to determine total amino acid contents of cultured meat (CM) produced. Chicken MSCs (cMSCs) were isolated from hindlimb muscles of broiler chickens at 5-week-old (5W) and 19-embryonic-day (19ED), respectively. Proliferation abilities (population doubling time and cell counting kit 8) of cMSCs from 19ED were significantly higher than those from 5W (p<0.05). Likewise, both myotube formation area and expression of myosin heavy chain heavy of cMSCs from 19ED were significantly higher than those from 5W (p<0.05). After cMSCs were serially subcultured for long-term cultivation in 2D flasks to produce cultured meat tissue (CMT), total amino acid contents of CMT showed no significant difference between 5W and 19ED chickens (p>0.05). This finding suggests that cMSCs from chicken embryos are more suitable for improving the production efficiency of CM than those derived from young chickens.

A critical size volumetric muscle loss model in mouse masseter with impaired mastication on nutrition

Orofacial muscle defect due to congenital anomalies, tumour ablation or traumatic accident that exceeds endogenous regeneration capacity may lead to sustained deficits in masticatory function and nutrition intake. Functional recovery has always been the goal of muscle tissue repair, but currently, there is no suitable model for quantitative analyses of either functional consequences or treatment efficacy of orofacial muscle defect. This study proposed a critical size volumetric muscle loss (VML) model in mouse masseter with impaired mastication on nutrition. Full-thickness VML defects in diameter of 1.0, 1.5, 2.0 and 3.0 mm were generated in the centre of the mouse masseter using a biopsy punch to determine the critical size for functional impairment. In the VML region, myogenesis was dampened but fibrogenesis was activated, as long with a reduction in the density of the neuromuscular junction and an increase in vascular density. Accordingly, persistent fibrosis was observed in the centre region of VML in all diameters. The 2.0 mm diameter was the critical threshold to masticatory function impairment after VML in the masseter. VML of 3.0 mm diameter led to a significant impact on nutrition intake and body weight gain. Autologous muscle graft effectively relieved the fibrosis and functional deficit after VML injury in the masseter. This model serves as a reliable tool in studying functional recovery strategies for orofacial muscle defects.

A Novel Rat Model for Muscle Regeneration and Fibrosis Studies in Surgical Lip Repair

OBJECTIVE

Lip muscle undergoes suboptimal regeneration after surgical repair, but the mechanism underlying this observation remains obscure. This study provided a rat model to investigate lip muscle regeneration after surgical intervention.

DESIGN

This work provided a detailed description of the rat orbicularis oris muscle anatomy, and a surgically injured model was established based on the muscle anatomy.

MAIN OUTCOME MEASURES

Morphological and histological features of the rat orbicularis oris muscle were characterized. The processes of myogenesis and fibrogenesis were examined between the untreated and surgically injured groups.

RESULTS

Rat orbicularis oris muscle is encapsulated by the vermilion and oral mucosa. Although it remains a thin layer of flat muscle with tight myocutaneous and myomucosal junctions, if accessed properly, the rat orbicularis oris muscle could be isolated as a cylindrical muscle bundle with considerable size, facilitating further surgical manipulations of the muscle fibers. Muscles in steady state and after surgical intervention demonstrated distinct molecular features in the myogenesis and fibrogenesis processes, which were quantifiable in tissue section analysis.

CONCLUSION

The orbicularis oris muscle dissection procedures and injury model provided in this work clarify the rat lip muscle anatomy. The injury model offered a platform to analyze the effects of surgical interventions commonly used in lip repair on orbicularis oris muscle regeneration.

Ex vivo two-photon imaging of whole-mount skeletal muscles to visualize stem cell behavior

Quiescent skeletal muscle stem cells (MuSCs) are morphologically and functionally heterogeneous and exhibit different lengths of cellular extensions, which we call protrusions. Here, we present a protocol for ex vivo two-photon imaging of MuSCs in their native environment. We describe steps for muscle dissection, fixation, embedding, imaging, and analysis of datasets. This protocol allows the examination of MuSC morphology and protrusions at the single-cell level as well as stem cell numbers. For complete details on the use and execution of this protocol, please refer to Ma et al. (2022).1.

Effect of muscle fibre types and carnosine levels on the expression of carnosine-related genes in pig skeletal muscle

It is generally accepted that carnosine (β-alanyl-L-histidine) content is higher in glycolytic than in oxidative muscle fibres, but the underlying mechanisms responsible for this difference remain to be elucidated. A first study to better understand potential mechanisms involved was undertaken (1) to determine whether differences in the expression of carnosine-related enzymes (CARNS1, CNDP2) and transporters (SLC6A6, SLC15A3, SLC15A4, SLC36A1) exist between oxidative and glycolytic myofibres and (2) to study the effect of carnosine on myoblast proliferative growth and on carnosine-related gene expression in cultured myoblasts isolated from glycolytic and oxidative muscles. Immunohistochemistry analyses were conducted to determine the cellular localization of carnosine-related proteins. Laser-capture microdissection and qPCR analyses were performed to measure the expression of carnosine-related genes in different myofibres isolated from the longissimus dorsi muscle of ten crossbred pigs. Myogenic cells originating from glycolytic and oxidative muscles were cultured to assess the effect of carnosine (0, 10, 25 and 50 mM) on their proliferative growth and on carnosine-related gene expression. The mRNA abundance of CNDP2 and of the studied carnosine transporters was higher in oxidative than in glycolytic myofibres. Since carnosine synthase (CARNS1) mRNA abundance was not affected by either the fibre type or the addition of carnosine to myoblasts, its transcriptional regulation would not be the main process by which carnosine content differences are determined in oxidative and glycolytic muscles. The addition of carnosine to myoblasts leading to a dose-dependent increase in SLC15A3 transcripts, however, suggests a role for this transporter in carnosine uptake and/or efflux to maintain cellular homeostasis.

Macrophages and stem/progenitor cells interplay in adipose tissue and skeletal muscle: a review

Like all immune cells, macrophages do not act autonomously but in unison with other immune cells, surrounding tissues, and the niche they occupy. Constant exchange of information between cellular and noncellular participants within a tissue allows for preserving homeostasis and defining responses in a pathologic environment. Although molecular mechanisms and pathways involved in reciprocal signaling between macrophages and other immune cells have been known for decades, much less is known about interactions between macrophages and stem/progenitor cells. Based on the time when stem cells form, there are two stem cell types: embryonic stem cells existing only in an early embryo, which are pluripotent and can differentiate into any cell type present in an adult, and somatic (adult) stem cells formed in fetus and persisting for whole adult life. Tissues and organs have their own (tissue-specific and organ-specific) adult stem cells, which serve as a reserve for tissue homeostasis and regeneration after injury. It is still uncertain whether organ- and tissue-specific stem cells are actual stem cells or just progenitor cells. The important question is how stem/progenitor cells can sculpt macrophage phenotype and functions. Even less is known if or how macrophages can shape stem/progenitor cell functions, their divisions, and fate. We describe here examples from recent studies of how stem/progenitor cells can affect macrophages and how macrophages can influence stem/progenitor cell properties, functions, and destiny.

Gαi2 regulates the adult myogenesis of masticatory muscle satellite cells

Although similar to trunk and limb skeletal muscles, masticatory muscles are believed as unique in both developmental origins and myogenesis. Gαi2 has been demonstrated to promote muscle hypertrophy and muscle satellite cell differentiation in limb muscles. However, the effect of Gαi2 on masticatory muscles is still unexplored. This study aimed to identify the role of Gαi2 in the proliferation and differentiation of masticatory muscle satellite cells, further exploring the metabolic mechanism of masticatory muscles. The proliferation rate, myotube size, fusion index of masticatory muscle satellite cells and Pax7, Myf5, MyoD, Tcf21 and Musculin expressions were significantly decreased by Gαi2 knockdown, while in cells infected with AdV4-Gαi2, the proliferation rate, myotube size, fusion index and Tbx1 expression were significantly increased. Masticatory muscle satellite cells also displayed phenotype transformation as Gαi2 changed. In addition, Gαi2 altered myosin heavy chain (MyHC) isoforms of myotubes with less MyHC-2A expression in siGαi2 group and more MyHC-slow expression in AdV4-Gαi2 group. In conclusion, Gαi2 could positively affect the adult myogenesis of masticatory muscle satellite cells and maintain the superiority of MyHC-slow. Masticatory muscle satellite cells may have their unique Gαi2-regulated myogenic transcriptional networks, although they may share some common characteristics with trunk and limb muscles.

Ageing Skeletal Muscle: The Ubiquitous Muscle Stem Cell

In 1999, in a review by Beardsley, the potential of adult stem cells, in repair and regeneration was heralded (Beardsley Sci Am 281:30-31, 1999). Since then, the field of regenerative medicine has grown exponentially, with the capability of restoring or regenerating the function of damaged, diseased or aged human tissues being an underpinning motivation. If successful, stem cell therapies offer the potential to treat, for example degenerative diseases. In the subsequent 20 years, extensive progress has been made in the arena of adult stem cells (for a recent review see (Zakrzewski et al. Stem Cell Res Ther 10:68, 2019)). Prior to the growth of the adult stem cell research arena, much focus had been placed on the potential of embryonic stem cells (ESCs). The first research revealing the potential of these cells was published in 1981, when scientists reported the ability of cultured stem cells from murine embryos, to not only self-renew, but to also become all cells of the three germ layers of the developing embryo (Evans and Kaufman Nature 292:154-156, 1981), (Martin Proc Natl Acad Sci U S A 78:7634-7638, 1981). It took almost 20 years, following these discoveries, for this technology to translate to human ESCs, using donated human embryos. In 1998, Thomson et al. reported the creation of the first human embryonic cell line (Thomson et al. Science 282:1145-1147, 1998). However, research utilising human ESCs was hampered by ethical and religious constraints and indeed in 2001 George W. Bush restricted US research funding to human ESCs, which had already been banked. The contentious nature of this arena perhaps facilitated the use of and the research potential for adult stem cells. It is beyond the scope of this review to focus on ESCs, although their potential for enhancing our understanding of human development is huge (for a recent review see (Cyranoski Nature 555:428-430, 2018)). Rather, although ESCs and their epigenetic regulation will be introduced for background understanding, the focus will be on stem cells more generally, the role of epigenetics in stem cell fate, skeletal muscle, skeletal muscle stem cells, the impact of ageing on muscle wasting and the mechanisms underpinning loss, with a focus on epigenetic adaptation.

Sox6 Differentially Regulates Inherited Myogenic Abilities and Muscle Fiber Types of Satellite Cells Derived from Fast- and Slow-Type Muscles

Adult skeletal muscle is primarily divided into fast and slow-type muscles, which have distinct capacities for regeneration, metabolism and contractibility. Satellite cells plays an important role in adult skeletal muscle. However, the underlying mechanisms of satellite cell myogenesis are poorly understood. We previously found that Sox6 was highly expressed in adult fast-type muscle. Therefore, we aimed to validate the satellite cell myogenesis from different muscle fiber types and investigate the regulation of Sox6 on satellite cell myogenesis. First, we isolated satellite cells from fast- and slow-type muscles individually. We found that satellite cells derived from different muscle fiber types generated myotubes similar to their origin types. Further, we observed that cells derived from fast muscles had a higher efficiency to proliferate but lower potential to self-renew compared to the cells derived from slow muscles. Then we demonstrated that Sox6 facilitated the development of satellite cells-derived myotubes toward their inherent muscle fiber types. We revealed that higher expression of Nfix during the differentiation of fast-type muscle-derived myogenic cells inhibited the transcription of slow-type isoforms (MyH7B, Tnnc1) by binding to Sox6. On the other hand, Sox6 activated Mef2C to promote the slow fiber formation in slow-type muscle-derived myogenic cells with Nfix low expression, showing a different effect of Sox6 on the regulation of satellite cell development. Our findings demonstrated that satellite cells, the myogenic progenitor cells, tend to develop towards the fiber type similar to where they originated. The expression of Sox6 and Nfix partially explain the developmental differences of myogenic cells derived from fast- and slow-type muscles.

Involvement of muscle satellite cell dysfunction in neuromuscular disorders: Expanding the portfolio of satellite cell-opathies

Neuromuscular disorders are a heterogeneous group of acquired or hereditary conditions that affect striated muscle function. The resulting decrease in muscle strength and motility irreversibly impacts quality of life. In addition to directly affecting skeletal muscle, pathogenesis can also arise from dysfunctional crosstalk between nerves and muscles, and may include cardiac impairment. Muscular weakness is often progressive and paralleled by continuous decline in the ability of skeletal muscle to functionally adapt and regenerate. Normally, the skeletal muscle resident stem cells, named satellite cells, ensure tissue homeostasis by providing myoblasts for growth, maintenance, repair and regeneration. We recently defined 'Satellite Cell-opathies' as those inherited neuromuscular conditions presenting satellite cell dysfunction in muscular dystrophies and myopathies (doi:10.1016/j.yexcr.2021.112906). Here, we expand the portfolio of Satellite Cell-opathies by evaluating the potential impairment of satellite cell function across all 16 categories of neuromuscular disorders, including those with mainly neurogenic and cardiac involvement. We explore the expression dynamics of myopathogenes, genes whose mutation leads to skeletal muscle pathogenesis, using transcriptomic analysis. This revealed that 45% of myopathogenes are differentially expressed during early satellite cell activation (0 - 5 hours). Of these 271 myopathogenes, 83 respond to Pax7, a master regulator of satellite cells. Our analysis suggests possible perturbation of satellite cell function in many neuromuscular disorders across all categories, including those where skeletal muscle pathology is not predominant. This characterisation further aids understanding of pathomechanisms and informs on development of prognostic and diagnostic tools, and ultimately, new therapeutics.

Tongue Muscle for the Analysis of Head Muscle Regeneration Dynamics

Tongue muscle damage impairs speaking and eating, thereby degrading overall health and quality of life. Skeletal muscles of the body are diverse in embryonic origin, anatomic location, and gene expression profiles. Responses to disease, atrophy, aging, or drugs vary among different muscles. Currently, most muscle studies are focused on limb muscles and the tongue is neglected. The regenerative ability of tongue muscle remains unknown, and thus there is need for tongue muscle research models. Here, we present a comprehensive characterization of the spatiotemporal dynamics in a mouse model of tongue muscle regeneration and establish a method for the isolation of primary tongue-derived satellite cells. We compare and contrast our observations with the tibialis anterior (TA) limb muscle. Acute injury was induced by intramuscular injection of cardiotoxin, a cytolytic agent, and examined at multiple timepoints. Initially, necrotic myofibers with fragmented sarcoplasm became infiltrated with inflammatory cells. Concomitantly, satellite cells expanded rapidly. Seven days postinjury, regenerated myofibers with centralized nuclei appeared. Full regeneration, as well as an absence of fibrosis, was evident 21 d postinjury. Primary tongue-derived satellite cells were isolated by enzymatic separation of tongue epithelium from mesenchyme followed by magnetic-activated cell sorting. We observed that tongue displays an efficient regenerative response similar to TA but with slightly faster kinetics. In vitro, tongue-derived satellite cells differentiated robustly into mature myotubes with spontaneous contractile behavior and myogenic marker expression. Comparison of gene expression signatures between tongue and TA-derived satellite cells revealed differences in the expression of positional-identity genes, including the HOX family. In conclusion, we have established a model for tongue regeneration useful for investigations of orofacial muscle biology. Furthermore, we showed that tongue is a viable source of satellite cells with unique properties and inherited positional memory.

Defining and identifying satellite cell-opathies within muscular dystrophies and myopathies

Muscular dystrophies and congenital myopathies arise from specific genetic mutations causing skeletal muscle weakness that reduces quality of life. Muscle health relies on resident muscle stem cells called satellite cells, which enable life-course muscle growth, maintenance, repair and regeneration. Such tuned plasticity gradually diminishes in muscle diseases, suggesting compromised satellite cell function. A central issue however, is whether the pathogenic mutation perturbs satellite cell function directly and/or indirectly via an increasingly hostile microenvironment as disease progresses. Here, we explore the effects on satellite cell function of pathogenic mutations in genes (myopathogenes) that associate with muscle disorders, to evaluate clinical and muscle pathological hallmarks that define dysfunctional satellite cells. We deploy transcriptomic analysis and comparison between muscular dystrophies and myopathies to determine the contribution of satellite cell dysfunction using literature, expression dynamics of myopathogenes and their response to the satellite cell regulator PAX7. Our multimodal approach extends current pathological classifications to define Satellite Cell-opathies: muscle disorders in which satellite cell dysfunction contributes to pathology. Primary Satellite Cell-opathies are conditions where mutations in a myopathogene directly affect satellite cell function, such as in Progressive Congenital Myopathy with Scoliosis (MYOSCO) and Carey-Fineman-Ziter Syndrome (CFZS). Primary satellite cell-opathies are generally characterised as being congenital with general hypotonia, and specific involvement of respiratory, trunk and facial muscles, although serum CK levels are usually within the normal range. Secondary Satellite Cell-opathies have mutations in myopathogenes that affect both satellite cells and muscle fibres. Such classification aids diagnosis and predicting probable disease course, as well as informing on treatment and therapeutic development.

Head muscle fibro-adipogenic progenitors account for the tilted regeneration towards fibrosis

Branchiomeric head muscle is ontogenetically and phylogenetically distinct from somitic limb muscle, and they exhibit different regenerative capacity. Unique satellite cell property of head muscle could explain the impaired myofiber formation, but the underlying mechanism for fibrosis is still elusive. In this work, we first established a freezing-induced skeletal muscle regeneration model and made comparisons between the regeneration characteristics in tibialis anterior (TA) muscle and masseter (MAS) muscle. The process of myogenesis and fibrogenesis were investigated by histological, immunohistochemical and cellular analysis, to characterize the role of muscle satellite cell (MuSCs) and fibro-adipogenic progenitors (FAPs) in TA and MAS muscle regeneration. Our results revealed that FAPs infiltrated the fibrotic area during MAS muscle regeneration. In contrast to the rapid rise and fall of FAPs number at the early regeneration stages in TA muscle, the number of MAS FAPs increased to a plateau without descending till 14 days after injury. It is the first time that the pivotal role of FAPs in head muscle regeneration was characterized. The persistence of FAPs without timely clearance in the first two weeks of regeneration could be accountable for the head muscle fibrosis.

Tumor necrosis factor alpha regulates myogenesis to inhibit differentiation and promote proliferation in satellite cells

TNF-α and NF-κB signaling is involved in the wasting of skeletal muscle in various conditions, in addition to cancer cachexia. TNF-α and NF-κB signaling promotes the expression level of muscle RING finger protein 1, a ubiquitin ligase, causing muscle degradation. Several studies have indicated that of TNF-α and NF-κB signaling suppresses muscle differentiation by reducing the levels of MyoD protein. On the other hand, TNF-α and NF-κB is required for myoblast proliferation. Thus, the role of TNF-α and NF-κB signaling in the process of myogenesis and regeneration of skeletal muscle is not completely elucidated. Here, we reported that TNF-α reduced the width of single fibers of skeletal muscle in an organ culture model. TNF-α and p65 repressed the transactivation of MyoD and suppressed myoblast differentiation. In addition, TNF-α increased the number of satellite cells, and NF-κB signaling was promoted at the proliferation stage during skeletal muscle regeneration in vivo. TNF-α and NF-κB signaling regulate myogenesis to inhibit differentiation and promote proliferation in satellite cells.

Emergence of heart and branchiomeric muscles in cardiopharyngeal mesoderm

Branchiomeric muscles of the head and neck originate in a population of cranial mesoderm termed cardiopharyngeal mesoderm that also contains progenitor cells contributing to growth of the embryonic heart. Retrospective lineage analysis has shown that branchiomeric muscles share a clonal origin with parts of the heart, indicating the presence of common heart and head muscle progenitor cells in the early embryo. Genetic lineage tracing and functional studies in the mouse, as well as in Ciona and zebrafish, together with recent experiments using single cell transcriptomics and multipotent stem cells, have provided further support for the existence of bipotent head and heart muscle progenitor cells. Current challenges concern defining where and when such common progenitor cells exist in mammalian embryos and how alternative myogenic derivatives emerge in cardiopharyngeal mesoderm. Addressing these questions will provide insights into mechanisms of cell fate acquisition and the evolution of vertebrate musculature, as well as clinical insights into the origins of muscle restricted myopathies and congenital defects affecting craniofacial and cardiac development.

Hox Proteins in the Regulation of Muscle Development

Hox genes encode evolutionary conserved transcription factors that specify the anterior-posterior axis in all bilaterians. Being well known for their role in patterning ectoderm-derivatives, such as CNS and spinal cord, Hox protein function is also crucial in mesodermal patterning. While well described in the case of the vertebrate skeleton, much less is known about Hox functions in the development of different muscle types. In contrast to vertebrates however, studies in the fruit fly, Drosophila melanogaster, have provided precious insights into the requirement of Hox at multiple stages of the myogenic process. Here, we provide a comprehensive overview of Hox protein function in Drosophila and vertebrate muscle development, with a focus on the molecular mechanisms underlying target gene regulation in this process. Emphasizing a tight ectoderm/mesoderm cross talk for proper locomotion, we discuss shared principles between CNS and muscle lineage specification and the emerging role of Hox in neuromuscular circuit establishment.

It takes all kinds: heterogeneity among satellite cells and fibro-adipogenic progenitors during skeletal muscle regeneration

Vertebrate skeletal muscle is composed of multinucleate myofibers that are surrounded by muscle connective tissue. Following injury, muscle is able to robustly regenerate because of tissue-resident muscle stem cells, called satellite cells. In addition, efficient and complete regeneration depends on other cells resident in muscle - including fibro-adipogenic progenitors (FAPs). Increasing evidence from single-cell analyses and genetic and transplantation experiments suggests that satellite cells and FAPs are heterogeneous cell populations. Here, we review our current understanding of the heterogeneity of satellite cells, their myogenic derivatives and FAPs in terms of gene expression, anatomical location, age and timing during the regenerative process - each of which have potentially important functional consequences.

An Evaluation of Muscle Repair Techniques: Implications in Musculoskeletal Healing and Corollaries in Oral-Facial Clefting

We performed an animal study to identify the techniques associated with the best muscle healing outcomes in cleft lip/palate surgery. The right triceps of thirty adult male Sprague-Dawley rats were cut and repaired by three different suture techniques: simple (n = 10), overlapping (n = 10), and splitting sutures (n = 10). Muscle tissues were isolated from 5 rats per group 1 and 8 weeks postoperation. The inflammatory response and muscle fiber healing were evaluated by hematoxylin and eosin (H&E) staining, Western blotting, immunohistochemistry for TNF-α and IL-1β, and immunofluorescence for laminin and MyoD. Grip strength (N/100 g) and spatial gait symmetry were evaluated before surgery and 1, 2, 4 and 8 weeks postoperation. Eight weeks postoperation, grip force per weight was significantly higher in the simple suture (median, 3.49; IQR, 3.28-3.66) and overlapping groups (median, 3.3; IQR, 3.17-3.47) than the splitting group (median, 2.91; IQR, 2.76-3.05). There was no significant difference in range of motion between groups. The simple group exhibited significant remission of inflammation by H&E staining and lower expression of TNF-α and IL-1β than the other groups by Western blotting and immunohistochemistry. Immunofluorescence revealed stronger expression of MyoD and weaker expression of laminin in the splitting group than in the other groups at week 8, indicating prolonged inflammation and healing followed by poor muscle fiber remodeling. Simple and overlapping sutures demonstrated similar functional healing, although greater inflammation and failure to maintain a thicker muscle belly were observed in the overlapping suture group compared with the simple suture group. Therefore, reconstruction of the philtral column with overlapping sutures alone may result in limited long-term fullness, and additional procedures may be needed.

Control of satellite cell function in muscle regeneration and its disruption in ageing

Skeletal muscle contains a designated population of adult stem cells, called satellite cells, which are generally quiescent. In homeostasis, satellite cells proliferate only sporadically and usually by asymmetric cell division to replace myofibres damaged by daily activity and maintain the stem cell pool. However, satellite cells can also be robustly activated upon tissue injury, after which they undergo symmetric divisions to generate new stem cells and numerous proliferating myoblasts that later differentiate to muscle cells (myocytes) to rebuild the muscle fibre, thereby supporting skeletal muscle regeneration. Recent discoveries show that satellite cells have a great degree of population heterogeneity, and that their cell fate choices during the regeneration process are dictated by both intrinsic and extrinsic mechanisms. Extrinsic cues come largely from communication with the numerous distinct stromal cell types in their niche, creating a dynamically interactive microenvironment. This Review discusses the role and regulation of satellite cells in skeletal muscle homeostasis and regeneration. In particular, we highlight the cell-intrinsic control of quiescence versus activation, the importance of satellite cell-niche communication, and deregulation of these mechanisms associated with ageing. The increasing understanding of how satellite cells are regulated will help to advance muscle regeneration and rejuvenation therapies.

Real-Time Functional Assay of Volumetric Muscle Loss Injured Mouse Masseter Muscles via Nanomembrane Electronics

Skeletal muscle has a remarkable regeneration capacity to recover its structure and function after injury, except for the traumatic loss of critical muscle volume, called volumetric muscle loss (VML). Although many extremity VML models have been conducted, craniofacial VML has not been well-studied due to unavailable in vivo assay tools. Here, this paper reports a wireless, noninvasive nanomembrane system that integrates skin-wearable printed sensors and electronics for real-time, continuous monitoring of VML on craniofacial muscles. The craniofacial VML model, using biopsy punch-induced masseter muscle injury, shows impaired muscle regeneration. To measure the electrophysiology of small and round masseter muscles of active mice during mastication, a wearable nanomembrane system with stretchable graphene sensors that can be laminated to the skin over target muscles is utilized. The noninvasive system provides highly sensitive electromyogram detection on masseter muscles with or without VML injury. Furthermore, it is demonstrated that the wireless sensor can monitor the recovery after transplantation surgery for craniofacial VML. Overall, the presented study shows the enormous potential of the masseter muscle VML injury model and wearable assay tool for the mechanism study and the therapeutic development of craniofacial VML.

Natural Compounds Attenuate Denervation-Induced Skeletal Muscle Atrophy

The weight of skeletal muscle accounts for approximately 40% of the whole weight in a healthy individual, and the normal metabolism and motor function of the muscle are indispensable for healthy life. In addition, the skeletal muscle of the maxillofacial region plays an important role not only in eating and swallowing, but also in communication, such as facial expressions and conversations. In recent years, skeletal muscle atrophy has received worldwide attention as a serious health problem. However, the mechanism of skeletal muscle atrophy that has been clarified at present is insufficient, and a therapeutic method against skeletal muscle atrophy has not been established. This review provides views on the importance of skeletal muscle in the maxillofacial region and explains the differences between skeletal muscles in the maxillofacial region and other regions. We summarize the findings to change in gene expression in muscle remodeling and emphasize the advantages and disadvantages of denervation-induced skeletal muscle atrophy model. Finally, we discuss the newly discovered beneficial effects of natural compounds on skeletal muscle atrophy.

Distinct Embryonic Origin and Injury Response of Resident Stem Cells in Craniofacial Muscles

Craniofacial muscles emerge as a developmental novelty during the evolution from invertebrates to vertebrates, facilitating diversified modes of predation, feeding and communication. In contrast to the well-studied limb muscles, knowledge about craniofacial muscle stem cell biology has only recently starts to be gathered. Craniofacial muscles are distinct from their counterparts in other regions in terms of both their embryonic origin and their injury response. Compared with somite-derived limb muscles, pharyngeal arch-derived craniofacial muscles demonstrate delayed myofiber reconstitution and prolonged fibrosis during repair. The regeneration of muscle is orchestrated by a blended source of stem/progenitor cells, including myogenic muscle satellite cells (MuSCs), mesenchymal fibro-adipogenic progenitors (FAPs) and other interstitial progenitors. Limb muscles host MuSCs of the Pax3 lineage, and FAPs from the mesoderm, while craniofacial muscles have MuSCs of the Mesp1 lineage and FAPs from the ectoderm-derived neural crest. Both in vivo and in vitro data revealed distinct patterns of proliferation and differentiation in these craniofacial muscle stem/progenitor cells. Additionally, the proportion of cells of different embryonic origins changes throughout postnatal development in the craniofacial muscles, creating a more dynamic niche environment than in other muscles. In-depth comparative studies of the stem cell biology of craniofacial and limb muscles might inspire the development of novel therapeutics to improve the management of myopathic diseases. Based on the most up-to-date literature, we delineated the pivotal cell populations regulating craniofacial muscle repair and identified clues that might elucidate the distinct embryonic origin and injury response in craniofacial muscle cells.

Wnt7a promotes muscle regeneration in branchiomeric orbicularis oris muscle

BACKGROUND

The orbicularis oris muscle exhibits a deficiency in cleft lip patients. Compared with the somite-derived limb muscles, the regeneration performance of the branchiomeric orofacial muscle has seldom been investigated.

OBJECTIVE

This study aims to explore the possibility of augmenting the orbicularis oris muscle through the stimulus of Wnt7a.

METHODS

Adult rat orbicularis oris muscle and tibialis anterior muscle were injected with recombinant human Wnt7a protein. The muscles were harvested at different time points after Wnt7a delivery. Muscle regeneration-related activity, including cell proliferation, stem cell proportion, myofiber plasticity, and total fiber number, was examined.

RESULTS

Adult rat orbicularis oris muscle and tibialis anterior muscle exhibit similar regeneration-related activities after Wnt7a administration. Recombinant human Wnt7a administration resulted in enhanced cell proliferation, stem cell expansion, and fiber type remodelling in rat orbicularis oris muscle. In addition, newly formed myofibers were detected, contributing to an increased total fiber number.

CONCLUSION

Wnt7a induces vigorous regeneration in rat orbicularis oris muscle. This study helps lay a foundation for developing biotherapies to combat orofacial muscle deficiency.

Hoxa10 mediates positional memory to govern stem cell function in adult skeletal muscle

Muscle stem cells (satellite cells) are distributed throughout the body and have heterogeneous properties among muscles. However, functional topographical genes in satellite cells of adult muscle remain unidentified. Here, we show that expression of Homeobox-A (Hox-A) cluster genes accompanied with DNA hypermethylation of the Hox-A locus was robustly maintained in both somite-derived muscles and their associated satellite cells in adult mice, which recapitulates their embryonic origin. Somite-derived satellite cells were clearly separated from cells derived from cranial mesoderm in Hoxa10 expression. Hoxa10 inactivation led to genomic instability and mitotic catastrophe in somite-derived satellite cells in mice and human. Satellite cell-specific Hoxa10 ablation in mice resulted in a decline in the regenerative ability of somite-derived muscles, which were unobserved in cranial mesoderm-derived muscles. Thus, our results show that Hox gene expression profiles instill the embryonic history in satellite cells as positional memory, potentially modulating region-specific pathophysiology in adult muscles.

Abundant Synthesis of Netrin-1 in Satellite Cell-Derived Myoblasts Isolated from EDL Rather Than Soleus Muscle Regulates Fast-Type Myotube Formation

Resident myogenic stem cells (satellite cells) are attracting attention for their novel roles in myofiber type regulation. In the myogenic differentiation phase, satellite cells from soleus muscle (slow fiber-abundant) synthesize and secrete higher levels of semaphorin 3A (Sema3A, a multifunctional modulator) than those derived from extensor digitorum longus (EDL; fast fiber-abundant), suggesting the role of Sema3A in forming slow-twitch myofibers. However, the regulatory mechanisms underlying fast-twitch myotube commitment remain unclear. Herein, we focused on netrin family members (netrin-1, -3, and -4) that compete with Sema3A in neurogenesis and osteogenesis. We examined whether netrins affect fast-twitch myotube generation by evaluating their expression in primary satellite cell cultures. Initially, netrins are upregulated during myogenic differentiation. Next, we compared the expression levels of netrins and their cell membrane receptors between soleus- and EDL-derived satellite cells; only netrin-1 showed higher expression in EDL-derived satellite cells than in soleus-derived satellite cells. We also performed netrin-1 knockdown experiments and additional experiments with recombinant netrin-1 in differentiated satellite cell-derived myoblasts. Netrin-1 knockdown in myoblasts substantially reduced fast-type myosin heavy chain (MyHC) expression; exogenous netrin-1 upregulated fast-type MyHC in satellite cells. Thus, netrin-1 synthesized in EDL-derived satellite cells may promote myofiber type commitment of fast muscles.

Heterogeneity of Satellite Cells Implicates DELTA1/NOTCH2 Signaling in Self-Renewal

How satellite cells and their progenitors balance differentiation and self-renewal to achieve sustainable tissue regeneration is not well understood. A major roadblock to understanding satellite cell fate decisions has been the difficulty of studying this process in vivo. By visualizing expression dynamics of myogenic transcription factors during early regeneration in vivo, we identify the time point at which cells undergo decisions to differentiate or self-renew. Single-cell RNA sequencing reveals heterogeneity of satellite cells, including a subpopulation enriched in Notch2 receptor expression, during both muscle homeostasis and regeneration. Furthermore, we reveal that differentiating cells express the Dll1 ligand. Using antagonistic antibodies, we demonstrate that the DLL1 and NOTCH2 signaling pair is required for satellite cell self-renewal. Thus, differentiating cells provide the self-renewing signal during regeneration, enabling proportional regeneration in response to injury while maintaining the satellite cell pool. These findings have implications for therapeutic control of muscle regeneration.

Excess Glucose Impedes the Proliferation of Skeletal Muscle Satellite Cells Under Adherent Culture Conditions

Glucose is a major energy source consumed by proliferating mammalian cells. Therefore, in general, proliferating cells have the preference of high glucose contents in extracellular environment. Here, we showed that high glucose concentrations impede the proliferation of satellite cells, which are muscle-specific stem cells, under adherent culture conditions. We found that the proliferation activity of satellite cells was higher in glucose-free DMEM growth medium (low-glucose medium with a glucose concentration of 2 mM) than in standard glucose DMEM (high-glucose medium with a glucose concentration of 19 mM). Satellite cells cultured in the high-glucose medium showed a decreased population of reserve cells, identified by staining for Pax7 expression, suggesting that glucose concentration affects cell fate determination. In conclusion, glucose is a factor that decides the cell fate of skeletal muscle-specific stem cells. Due to this unique feature of satellite cells, hyperglycemia may negatively affect the regenerative capability of skeletal muscle myofibers and thus facilitate sarcopenia.

Myosin heavy chains in extraocular muscle fibres: Distribution, regulation and function

This review examines kinetic properties and distribution of the 11 isoforms of myosin heavy chain (MyHC) expressed in extraocular muscle (EOM) fibre types and the regulation and function of these MyHCs. Although recruitment and discharge characteristics of ocular motoneurons during fixation and eye movements are well documented, work directly linking these properties with motor unit contractile speed and MyHC composition is lacking. Recruitment of motor units according to Henneman's size principle has some support in EOMs but needs consolidation. Both neurogenic and myogenic mechanisms regulate MyHC expression as in other muscle allotypes. Developmentally, multiply-innervated (MIFs) and singly-innervated fibres (SIFs) are derived presumably from distinct myoblast lineages, ending up expressing MyHCs in the slow and fast ends of the kinetic spectrum respectively. They modulate the synaptic inputs of their motoneurons through different retrogradely transported neurotrophins, thereby specifying their tonic and phasic impulse patterns. Immunohistochemical analyses of EOMs regenerating in situ and in limb muscle beds suggest that the very impulse patterns driving various ocular movements equip effectors with appropriate MyHC compositions and speeds to accomplish their tasks. These experiments also suggest that satellite cells of SIFs and MIFs are distinct lineages expressing different MyHCs during regeneration. MyHC compositions and functional characteristics of orbital fibres show longitudinal variations that facilitate linear ocular rotation during saccades. Palisade endings on global MIFs are postulated to respond to active and passive tensions by triggering axon reflexes that play important roles during fixation, saccades and vergence. How EOMs implement Listings law during ocular rotation is discussed.

The body region specificity in murine models of muscle regeneration and atrophy

AIM

Skeletal muscles are distributed throughout the body, presenting a variety of sizes, shapes and functions. Here, we examined whether muscle regeneration and atrophy occurred homogeneously throughout the body in mouse models.

METHODS

Acute muscle regeneration was induced by a single intramuscular injection of cardiotoxin in adult mice. Chronic muscle regeneration was assessed in mdx mice. Muscle atrophy in different muscles was evaluated by cancer cachexia, ageing and castration mouse models.

RESULTS

We found that, in the cardiotoxin-injected acute muscle injury model, head muscles slowly regenerated, while limb muscles exhibited a rapid regeneration and even overgrowth. This overgrowth was also observed in limb muscles alone (but not in head muscles) in mdx mice as chronic injury models. We described the body region-specific decline in the muscle mass in muscle atrophy models: cancer cachexia-induced, aged and castrated mice. The positional identities, including gene expression profiles and hormone sensitivity, were robustly preserved in the ectopically engrafted satellite cell-derived muscles in the castrated model.

CONCLUSION

Our results indicate that positional identities in muscles should be considered for the development of efficient regenerative therapies for muscle weakness, such as muscular dystrophy and age-related sarcopenia.

A tissue engineering approach for repairing craniofacial volumetric muscle loss in a sheep following a 2, 4, and 6-month recovery

Volumetric muscle loss (VML) is the loss of skeletal muscle that results in significant and persistent impairment of function. The unique characteristics of craniofacial muscle compared trunk and limb skeletal muscle, including differences in gene expression, satellite cell phenotype, and regenerative capacity, suggest that VML injuries may affect craniofacial muscle more severely. However, despite these notable differences, there are currently no animal models of craniofacial VML. In a previous sheep hindlimb VML study, we showed that our lab’s tissue engineered skeletal muscle units (SMUs) were able to restore muscle force production to a level that was statistically indistinguishable from the uninjured contralateral muscle. Thus, the goals of this study were to: 1) develop a model of craniofacial VML in a large animal model and 2) to evaluate the efficacy of our SMUs in repairing a 30% VML in the ovine zygomaticus major muscle. Overall, there was no significant difference in functional recovery between the SMU-treated group and the unrepaired control. Despite the use of the same injury and repair model used in our previous study, results showed differences in pathophysiology between craniofacial and hindlimb VML. Specifically, the craniofacial model was affected by concomitant denervation and ischemia injuries that were not exhibited in the hindlimb model. While clinically realistic, the additional ischemia and denervation likely created an injury that was too severe for our SMUs to repair. This study highlights the importance of balancing the use of a clinically realistic model while also maintaining control over variables related to the severity of the injury. These variables include the volume of muscle removed, the location of the VML injury, and the geometry of the injury, as these affect both the muscle’s ability to self-regenerate as well as the probability of success of the treatment.

Damaged Myofiber-Derived Metabolic Enzymes Act as Activators of Muscle Satellite Cells

Muscle satellite cells are normally quiescent but are rapidly activated following muscle damage. Here, we investigated whether damaged myofibers influence the activation of satellite cells. Our findings revealed that satellite cells are directly activated by damaged-myofiber-derived factors (DMDFs). DMDFs induced satellite cells to enter the cell cycle; however, the cells stayed at the G1 phase and did not undergo S phase, and these cells were reversible to the quiescent-like state. Proteome analysis identified metabolic enzymes, including GAPDH, as DMDFs, whose recombinant proteins stimulated the activation of satellite cells. Satellite cells pre-exposed to the DMDFs demonstrated accelerated proliferation ex vivo. Treatment with recombinant GAPDH prior to muscle injury promoted expansion of the satellite cell population in vivo. Thus, our results indicate that DMDFs are not only a set of biomarkers for muscle damage, but also act as moonlighting proteins involved in satellite cell activation at the initial step of muscle regeneration.

Can Wharton jelly derived or adipose tissue derived mesenchymal stem cell can be a treatment option for duchenne muscular dystrophy? Answers as transcriptomic aspect

INTRODUCTION

Mesenchymal stem cells (MSCs) are able to differentiate into several cell lineages including skeletal muscle. In addition to their differentiation capacities, they have the ability to transfer their content genomic information horizontally through their exosomes and fusion abilities, as we have shown in our previous clinic study on Duchenne Muscular Dystrophy (DMD) patients, dystrophin expression increased after MSC treatment. Therefore, this study aimed to compare the transcriptomic properties of Wharton's jelly derived (WJ-) MSC and Adipose tissue (AT-) derived MSC, which are the two most preferred sources in MSC treatments applied in DMD.

METHODS

Both MSC cell lines obtained from ATCC (PCS-500-010; PCS-500-011) were characterized by flow cytometry then WJ-MSC and AT-MSC cell lines were sequenced via RNA-SEQ. R language was used to obtain the differentially expressed genes (DEGs) and differentially expressed miRNAs, respectively. Additionally, in order to support the results of our study, a gene expression profile data set of DMD patients (GSE1004) were acquired from Gene Expression Omnibus (GEO) database.

RESULTS

Here, we demonstrated that activated WNT signaling and downregulated TGF-β pathways under the control of decreased mir-24 which are involved in myogenic differentiation are differentially expressed in WJ-MSC. We have shown that the expression of mir-199a-5p, which is known to increase in exosomes of DMD patients, is less in WJ-MSC. Additionally, we have shown activated PI3K/Akt pathway, which is controlling mitochondria transfer via Tunnelling Nanotube as a new perspective in cellular therapies in myodegenerative diseases, in WJ-MSC more than in AT-MSCs.

CONCLUSION

Summing up, WJ-MSC, which we recommend as an appropriate source candidate due to its immune-regulation properties, stands forward as a preferable source in the cellular treatment of DMD patients due to its transcriptomic aspect.

Uremic Sarcopenia: Clinical Evidence and Basic Experimental Approach

Sustained physical activity extends healthy life years while a lower activity due to sarcopenia can reduce them. Sarcopenia is defined as a decrease in skeletal muscle mass and strength due not only to aging, but also from a variety of debilitating chronic illnesses such as cancer and heart failure. Patients with chronic kidney disease (CKD), who tend to be cachexic and in frail health, may develop uremic sarcopenia or uremic myopathy due to an imbalance between muscle protein synthesis and catabolism. Here, we review clinical evidence indicating reduced physical activity as renal function deteriorates and explore evidence-supported therapeutic options focusing on nutrition and physical training. In addition, although sarcopenia is a clinical concept and difficult to recapitulate in basic research, several in vivo approaches have been attempted, such as rodent subtotal nephrectomy representing both renal dysfunction and muscle weakness. This review highlights molecular mechanisms and promising interventions for uremic sarcopenia that were revealed through basic research. Extensive study is still needed to cast light on the many aspects of locomotive organ impairments in CKD and explore the ways that diet and exercise therapies can improve both outcomes and quality of life at every level.

Skeletal muscle in health and disease

Skeletal muscle fibres are multinucleated cells that contain postmitotic nuclei (i.e. they are no longer able to divide) and perform muscle contraction. They are formed by fusion of muscle precursor cells, and grow into elongating myofibres by the addition of further precursor cells, called satellite cells, which are also responsible for regeneration following injury. Skeletal muscle regeneration occurs in most muscular dystrophies in response to necrosis of muscle fibres. However, the complex environment within dystrophic skeletal muscle, which includes inflammatory cells, fibroblasts and fibro-adipogenic cells, together with the genetic background of the in vivo model and the muscle being studied, complicates the interpretation of laboratory studies on muscular dystrophies. Many genes are expressed in satellite cells and in other tissues, which makes it difficult to determine the molecular cause of various types of muscular dystrophies. Here, and in the accompanying poster, we discuss our current knowledge of the cellular mechanisms that govern the growth and regeneration of skeletal muscle, and highlight the defects in satellite cell function that give rise to muscular dystrophies.

Summary: The mechanisms of skeletal muscle development, growth and regeneration are described. We discuss whether these processes are dysregulated in inherited muscle diseases and identify pathways that may represent therapeutic targets.

Anti-Aging Effects of Schisandrae chinensis Fructus Extract: Improvement of Insulin Sensitivity and Muscle Function in Aged Mice

Schisandrae chinensis Fructus has a long history of medicinal use as a tonic, a sedative, and an antitussive drug. In this study, we investigated the beneficial effects of Schisandrae chinensis Fructus ethanol extract (SFe) on metabolism in an aged mouse model. Sixteen-month-old C57BL/6J mice were fed with a diet supplemented with SFe for 4 months. Insulin sensitivity was lower at 20 months of age than at 16 months of age; however, the decrease in insulin sensitivity was less in SFe-fed mice. SFe supplementation also appeared to improve glucose tolerance. Body weight gain was lower in SFe-fed mice than in mice fed the control diet. Body fat mass was lower and the lean mass was higher in SFe-fed mice. In addition, the grip strength was enhanced in SFe-fed mice. Histological analysis of the tibialis anterior muscle showed that the size of myofiber and slow-twitch red muscle was increased by SFe supplementation. The expression of proteins related to muscle protein synthesis such as phospho-Erk1 and phospho-S6K1 was increased by SFe supplementation. The mRNA expression of genes related to myogenesis and their encoded proteins such as MyoD, Myf5, MRF4, myogenin, and myosin heavy chain, was increased, whereas that of genes related to muscle degradation, such as atrogin-1, MuRF-1, and myostatin, were decreased relative to control mice. These results suggest that SFe supplementation might have beneficial effects for the improvement of insulin sensitivity and inhibition of muscle loss that occur with aging.

Orofacial Muscles: Embryonic Development and Regeneration after Injury

Orofacial congenital defects such as cleft lip and/or palate are associated with impaired muscle regeneration and fibrosis after surgery. Also, other orofacial reconstructions or trauma may end up in defective muscle regeneration and fibrosis. The aim of this review is to discuss current knowledge on the development and regeneration of orofacial muscles in comparison to trunk and limb muscles. The orofacial muscles include the tongue muscles and the branchiomeric muscles in the lower face. Their main functions are chewing, swallowing, and speech. All orofacial muscles originate from the mesoderm of the pharyngeal arches under the control of cranial neural crest cells. Research in vertebrate models indicates that the molecular regulation of orofacial muscle development is different from that of trunk and limb muscles. In addition, the regenerative ability of orofacial muscles is lower, and they develop more fibrosis than other skeletal muscles. Therefore, specific approaches need to be developed to stimulate orofacial muscle regeneration. Regeneration may be stimulated by growth factors such fibroblast growth factors and hepatocyte growth factor, while fibrosis may be reduced by targeting the transforming growth factor β1 (TGFβ1)/myofibroblast axis. New approaches that combine these 2 aspects will improve the surgical treatment of orofacial muscle defects.

A Comparison of Ovine Facial and Limb Muscle as a Primary Cell Source for Engineered Skeletal Muscle

Volumetric muscle loss (VML) contributes to the number of soft tissue injuries that necessitate reconstructive surgery, but treatment options are often limited by tissue availability and donor site morbidity. To combat these issues, our laboratory has developed scaffold-free tissue-engineered skeletal muscle units (SMUs) as a novel treatment for VML injuries. Recently, we have begun experiments addressing VML in facial muscle, and the optimal starting cell population for engineered skeletal muscle tissue for this application may not be cells derived from hindlimb muscles due to reported heterogeneity of cell populations. Thus, the purpose of this study was to compare SMUs fabricated from both craniofacial and hindlimb sources to determine which cell source is best suited for the engineering of skeletal muscle. Herein, we assessed the development, structure, and function of SMUs derived from four muscle sources, including two hindlimb muscles (i.e., soleus and semimembranosus [SM]) and two craniofacial muscles (i.e., zygomaticus major and masseter). Overall, the zygomaticus major exhibited the least efficient digestion, and SMUs fabricated from this muscle exhibited the least aligned myosin heavy chain staining and consequently, the lowest average force production. Conversely, the SM muscle exhibited the most efficient digestion and the highest number of myotubes/mm²; however, the SM, masseter, and soleus groups were roughly equivalent in terms of force production and histological structure. Impact Statement An empirical comparison of the development, structure, and function of engineered skeletal muscle tissue fabricated from different muscles, including both craniofacial and hindlimb sources, will not only provide insight into innate regenerative mechanisms of skeletal muscle but also will give our team and other researchers the information necessary to determine which cell sources are best suited for the skeletal muscle tissue engineering.

Gene expression profiling of skeletal myogenesis in human embryonic stem cells reveals a potential cascade of transcription factors regulating stages of myogenesis, including quiescent/activated satellite cell-like gene expression

Human embryonic stem cell (hESC)-derived skeletal muscle progenitors (SMP)—defined as PAX7-expressing cells with myogenic potential—can provide an abundant source of donor material for muscle stem cell therapy. As in vitro myogenesis is decoupled from in vivo timing and 3D-embryo structure, it is important to characterize what stage or type of muscle is modeled in culture. Here, gene expression profiling is analyzed in hESCs over a 50 day skeletal myogenesis protocol and compared to datasets of other hESC-derived skeletal muscle and adult murine satellite cells. Furthermore, day 2 cultures differentiated with high or lower concentrations of CHIR99021, a GSK3A/GSK3B inhibitor, were contrasted. Expression profiling of the 50 day time course identified successively expressed gene subsets involved in mesoderm/paraxial mesoderm induction, somitogenesis, and skeletal muscle commitment/formation which could be regulated by a putative cascade of transcription factors. Initiating differentiation with higher CHIR99021 concentrations significantly increased expression of MSGN1 and TGFB-superfamily genes, notably NODAL, resulting in enhanced paraxial mesoderm and reduced ectoderm/neuronal gene expression. Comparison to adult satellite cells revealed that genes expressed in 50-day cultures correlated better with those expressed by quiescent or early activated satellite cells, which have the greatest therapeutic potential. Day 50 cultures were similar to other hESC-derived skeletal muscle and both expressed known and novel SMP surface proteins. Overall, a putative cascade of transcription factors has been identified which regulates four stages of myogenesis. Subsets of these factors were upregulated by high CHIR99021 or their binding sites were significantly over-represented during SMP activation, ranging from quiescent to late-activated stages. This analysis serves as a resource to further study the progression of in vitro skeletal myogenesis and could be mined to identify novel markers of pluripotent-derived SMPs or regulatory transcription/growth factors. Finally, 50-day hESC-derived SMPs appear similar to quiescent/early activated satellite cells, suggesting they possess therapeutic potential.

R3hdml regulates satellite cell proliferation and differentiation

In this study, we identified a previously uncharacterized skeletal satellite cell-secreted protein, R3h domain containing-like (R3hdml). Expression of R3hdml increases during skeletal muscle development and differentiation in mice. Body weight and skeletal muscle mass of R3hdml knockout (KO) mice are lower compared to control mice. Expression levels of cell cycle-related markers, phosphorylation of Akt, and expression of insulin-like growth factor within the skeletal muscle are reduced in R3hdml KO mice compared to control mice. Expression of R3hdml increases during muscle regeneration in response to cardiotoxin (CTX)-induced muscle injury. Recovery of handgrip strength after CTX injection was significantly impaired in R3hdml KO mice, which is rescued by R3hdml. Our results indicate that R3hdml is required for skeletal muscle development, regeneration, and, in particular, satellite cell proliferation and differentiation.

Osteopontin-derived synthetic peptide SVVYGLR has potent utility in the functional regeneration of oral and maxillofacial skeletal muscles

Oral and maxillofacial skeletal muscles are critical for oral motor functions, and severe damage to these muscles by trauma or surgery may lead to persistent functional impairment. This study investigated the effects of SVVYGLR (SV) peptide, a thrombin-cleaved osteopontin-derived motif, on histopathological wound healing and functional repair after severe injury of skeletal muscles. A rat model of volumetric muscle loss bilateral masseter muscle was developed. A single dose of SV-peptide or phosphate-buffered saline (PBS) was separately injected into the injured muscle belly. Histopathological and functional analyses were performed 1-8 weeks after the treatment. Behavioral analysis during free-feeding revealed that the feeding rate markedly increased in the SV-peptide group, in contrast, the PBS group showed fewer changes after the injury. Electromyogram recordings from injured muscles demonstrated amplification of rectified burst activity over time accompanied by increased maximal amplitude and duration in the SV-peptide group, in contrast, the PBS group showed moderate changes. A lissajous figure for bilateral masseter muscle activities also revealed superior functional recovery by the SV-peptide treatment. The SV-peptide also facilitated regeneration of muscles composed of matured myofibers with a greater diameter compared to the PBS group. In addition, granulation in the earlier period and fibrosis in the later period of wound healing were significantly inhibited by the SV-peptide treatment but not by the PBS treatment. Therefore, local application of the SV-peptide could help facilitate regeneration of muscles, inhibition of fibrosis, and improvement of functional impairment of oral and maxillofacial skeletal muscles damaged by severe trauma or surgery.

Muscle fibrosis in the soft palate: Delivery of cells, growth factors and anti-fibrotics

The healing of skeletal muscle injuries after major trauma or surgical reconstruction is often complicated by the development of fibrosis leading to impaired function. Research in the field of muscle regeneration is mainly focused on the restoration of muscle mass while far less attention is paid to the prevention of fibrosis. In this review, we take as an example the reconstruction of the muscles in the soft palate of cleft palate patients. After surgical closure of the soft palate, muscle function during speech is often impaired by a shortage of muscle tissue as well as the development of fibrosis. We will give a short overview of the most common approaches to generate muscle mass and then focus on strategies to prevent fibrosis. These include anti-fibrotic strategies that have been developed for muscle and other organs by the delivery of small molecules, decorin and miRNAs. Anti-fibrotic compounds should be delivered in aligned constructs in order to obtain the organized architecture of muscle tissue. The available techniques for the preparation of aligned muscle constructs will be discussed. The combination of approaches to generate muscle mass with anti-fibrotic components in an aligned muscle construct may greatly improve the functional outcome of regenerative therapies for muscle injuries.

Tissue engineering strategies combining molecular targets against inflammation and fibrosis, and umbilical cord blood stem cells to improve hampered muscle and skin regeneration following cleft repair

Cleft lip with or without cleft palate is a congenital deformity that occurs in about 1 of 700 newborns, affecting the dentition, bone, skin, muscles and mucosa in the orofacial region. A cleft can give rise to problems with maxillofacial growth, dental development, speech, and eating, and can also cause hearing impairment. Surgical repair of the lip may lead to impaired regeneration of muscle and skin, fibrosis, and scar formation. This may result in hampered facial growth and dental development affecting oral function and lip and nose esthetics. Therefore, secondary surgery to correct the scar is often indicated. We will discuss the molecular and cellular pathways involved in facial and lip myogenesis, muscle anatomy in the normal and cleft lip, and complications following surgery. The aim of this review is to outline a novel molecular and cellular strategy to improve musculature and skin regeneration and to reduce scar formation following cleft repair. Orofacial clefting can be diagnosed in the fetus through prenatal ultrasound screening and allows planning for the harvesting of umbilical cord blood stem cells upon birth. Tissue engineering techniques using these cord blood stem cells and molecular targeting of inflammation and fibrosis during surgery may promote tissue regeneration. We expect that this novel strategy improves both muscle and skin regeneration, resulting in better function and esthetics after cleft repair.

Scavenger Receptor Class A1 Mediates Uptake of Morpholino Antisense Oligonucleotide into Dystrophic Skeletal Muscle

Exon skipping using phosphorodiamidate morpholino oligomers (PMOs) is a promising treatment strategy for Duchenne muscular dystrophy (DMD). The most significant limitation of these clinically used compounds is their lack of delivery systems that target muscles; thus, cell-penetrating peptides are being developed to enhance uptake into muscles. Recently, we reported that uptake of peptide-conjugated PMOs into myofibers was mediated by scavenger receptor class A (SR-A), which binds negatively charged ligands. However, the mechanism by which the naked PMOs are taken up into fibers is poorly understood. In this study, we found that PMO uptake and exon-skipping efficiency were promoted in dystrophin-deficient myotubes via endocytosis through a caveolin-dependent pathway. Interestingly, SR-A1 was upregulated and localized in juxtaposition with caveolin-3 in these myotubes and promoted PMO-induced exon skipping. SR-A1 was also upregulated in the skeletal muscle of mdx52 mice and mediated PMO uptake. In addition, PMOs with neutral backbones had negative zeta potentials owing to their nucleobase compositions and interacted with SR-A1. In conclusion, PMOs with negative zeta potential were taken up into dystrophin-deficient skeletal muscle by upregulated SR-A1. Therefore, the development of a drug delivery system targeting SR-A1 could lead to highly efficient exon-skipping therapies for DMD.

Age-related alterations in swallowing biomechanics

BACKGROUND

Aging rodent models allow for the discovery of underlying mechanisms of cranial muscle dysfunction. Methods are needed to allow quantification of complex, multivariate biomechanical movements during swallowing. Videofluoroscopic swallow studies (VSS) are the standard of care in assessment of swallowing disorders in patients and validated quantitative, kinematic, and morphometric analysis methods have been developed. Our purpose was to adapt validated morphometric techniques to the rodent to computationally analyze swallowing dysfunction in the aging rodent.

METHODS

VSS, quantitative analyses (bolus area, bolus velocity, mastication rate) and a rodent specific multivariate, morphometric computational analysis of swallowing biomechanics were performed on 20 swallows from 5 young adult and 5 old Fischer 344/Brown Norway rats. Eight anatomical landmarks were used to track the relative change in position of skeletal levers (cranial base, vertebral column, mandible) and soft tissue landmarks (upper esophageal sphincter, base of tongue).

RESULTS

Bolus area significantly increased and mastication rate significantly decreased with age. Aging accounted for 77.1% of the variance in swallow biomechanics, and 18.7% of the variance was associated with swallow phase (oral vs pharyngeal). Post hoc analyses identified age-related alterations in tongue base retraction, mastication, and head posture during the swallow.

CONCLUSION

Geometric morphometric analysis of rodent swallows suggests that swallow biomechanics are altered with age. When used in combination with biological assays of age-related adaptations in neuromuscular systems, this multivariate analysis may increase our understanding of underlying musculoskeletal dysfunction that contributes to swallowing disorders with aging.