A muscle stem cell for every muscle: variability of satellite cell biology among different muscle groups

Interplay between Pitx2 and Pax7 temporally governs specification of extraocular muscle stem cells

Gene regulatory networks that act upstream of skeletal muscle fate determinants are distinct in different anatomical locations. Despite recent efforts, a clear understanding of the cascade of events underlying the emergence and maintenance of the stem cell pool in specific muscle groups remains unresolved and debated. Here, we invalidated Pitx2 with multiple Cre-driver mice prenatally, postnatally, and during lineage progression. We showed that this gene becomes progressively dispensable for specification and maintenance of the muscle stem (MuSC) cell pool in extraocular muscles (EOMs) despite being, together with Myf5, a major upstream regulator during early development. Moreover, constitutive inactivation of Pax7 postnatally led to a greater loss of MuSCs in the EOMs compared to the limb. Thus, we propose a relay between Pitx2, Myf5 and Pax7 for EOM stem cell maintenance. We demonstrate also that MuSCs in the EOMs adopt a quiescent state earlier that those in limb muscles and do not spontaneously proliferate in the adult, yet EOMs have a significantly higher content of Pax7+ MuSCs per area pre- and post-natally. Finally, while limb MuSCs proliferate in the mdx mouse model for Duchenne muscular dystrophy, significantly less MuSCs were present in the EOMs of the mdx mouse model compared to controls, and they were not proliferative. Overall, our study provides a comprehensive in vivo characterisation of MuSC heterogeneity along the body axis and brings further insights into the unusual sparing of EOMs during muscular dystrophy.

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.

CRIPTO-based micro-heterogeneity of mouse muscle satellite cells enables adaptive response to regenerative microenvironment

Skeletal muscle repair relies on heterogeneous populations of satellite cells (SCs). The mechanisms that regulate SC homeostasis and state transition during activation are currently unknown. Here, we investigated the emerging role of non-genetic micro-heterogeneity, i.e., intrinsic cell-to-cell variability of a population, in this process. We demonstrate that micro-heterogeneity of the membrane protein CRIPTO in mouse-activated SCs (ASCs) identifies metastable cell states that allow a rapid response of the population to environmental changes. Mechanistically, CRIPTO micro-heterogeneity is generated and maintained through a process of intracellular trafficking coupled with active shedding of CRIPTO from the plasma membrane. Irreversible perturbation of CRIPTO micro-heterogeneity affects the balance of proliferation, self-renewal, and myogenic commitment in ASCs, resulting in increased self-renewal in vivo. Our findings demonstrate that CRIPTO micro-heterogeneity regulates the adaptative response of ASCs to microenvironmental changes, providing insights into the role of intrinsic heterogeneity in preserving stem cell population diversity during tissue repair.

Androgen action on myogenesis throughout the lifespan; comparison with neurogenesis

Androgens' pleiotropic actions in promoting sex differences present not only a challenge to providing a comprehensive account of their function, but also an opportunity to gain insights by comparing androgenic actions across organ systems. Although often overlooked by neuroscientists, skeletal muscle is another androgen-responsive organ system which shares with the nervous system properties of electrochemical excitability, behavioral relevance, and remarkable capacity for adaptive plasticity. Here we review androgenic regulation of mitogenic plasticity in skeletal muscle with the goal of identifying areas of interest to those researching androgenic mechanisms mediating sexual differentiation of neurogenesis. We use an organizational-activational framework to relate broad areas of similarity and difference between androgen effects on mitogenesis in muscle and brain throughout the lifespan, from early organogenesis, through pubertal organization, adult activation, and aging. The focus of the review is androgenic regulation of muscle-specific stem cells (satellite cells), which share with neural stem cells essential functions in development, plasticity, and repair, albeit with distinct, muscle-specific features. Also considered are areas of paracrine and endocrine interaction between androgen action on muscle and nervous system, including mediation of neural plasticity of innervating and distal neural populations by muscle-produced trophic factors.

Slow or fast: Implications of myofibre type and associated differences for manifestation of neuromuscular disorders

Many neuromuscular disorders can have a differential impact on a specific myofibre type, forming the central premise of this review. The many different skeletal muscles in mammals contain a spectrum of slow- to fast-twitch myofibres with varying levels of protein isoforms that determine their distinctive contractile, metabolic, and other properties. The variations in functional properties across the range of classic 'slow' to 'fast' myofibres are outlined, combined with exemplars of the predominantly slow-twitch soleus and fast-twitch extensor digitorum longus muscles, species comparisons, and techniques used to study these properties. Other intrinsic and extrinsic differences are discussed in the context of slow and fast myofibres. These include inherent susceptibility to damage, myonecrosis, and regeneration, plus extrinsic nerves, extracellular matrix, and vasculature, examined in the context of growth, ageing, metabolic syndrome, and sexual dimorphism. These many differences emphasise the importance of carefully considering the influence of myofibre-type composition on manifestation of various neuromuscular disorders across the lifespan for both sexes. Equally, understanding the different responses of slow and fast myofibres due to intrinsic and extrinsic factors can provide deep insight into the precise molecular mechanisms that initiate and exacerbate various neuromuscular disorders. This focus on the influence of different myofibre types is of fundamental importance to enhance translation for clinical management and therapies for many skeletal muscle disorders.

Bovine HOXA11 Gene Identified from RNA-Seq: mRNA Profile Analysis and Genetic Variation Detection Using ME Method and Their Associations with Carcass Traits

The Homeobox A11 (HOXA11) gene regulates limb skeletal development and muscle growth, thus, it was selected as a candidate gene for bovine carcass traits. In this study, we analyzed the mRNA expression level of HOXA11 in various tissues and cells, and determined the genetic variations in the HOXA11 gene, which might be used as molecular markers for cattle breeding. The mRNA expression profiles of HOXA11 in bovine different tissues showed that HOXA11 was highly expressed in both fat and muscle. The gene expression trend of HOXA11 in myoblasts and adipocytes indicated that HOXA11 might be involved in the differentiation of bovine myoblasts and adipocytes. The data in the Ensembl database showed that there are two putative insertion/deletion (InDel) polymorphisms in the bovine HOXA11 gene. The insertion site (rs515880802) was located in the upstream region (NC_037331.1: g. 68853364-68853365) and named as P1-Ins-4-bp, and the deletion site (rs517582703) was located in the intronic region (NC_037331.1: g. 68859510-68859517) and named as P2-Del-8-bp. These polymorphisms within the HOXA11 gene were identified and genotyped by PCR amplification, agarose gel electrophoresis and DNA sequencing in the 640 Shandong Black Cattle Genetic Resource (SDBCGR) population. Moreover, the mutation frequency was very low after detection, so the mathematical expectation (ME) method was used for detection. Statistical analysis demonstrated that P1-Ins-4-bp was significantly correlated with the beef shoulder (p = 0.012) and tongue root (p = 0.004). Meanwhile, P2-Del-8-bp displayed a significant correlation with the back tendon (p = 0.008), money tendon (p = 2.84 × 10^-4), thick flank (p = 0.034), beef shin (p = 9.09 × 10^-7), triangle thick flank (p = 0.04), triangle flank (p = 1.00 × 10^-6), rump (p = 0.018) and small tenderloin (p = 0.043) in the female SDBCGR population. In summary, these outcomes may provide a new perspective for accelerating the molecular breeding of cattle through marker-assisted selection (MAS) strategies.

Macroglossia and less advanced dystrophic change in the tongue muscle of the Duchenne muscular dystrophy rat

Background

Duchenne muscular dystrophy (DMD) is an X-linked muscle disease caused by a complete lack of dystrophin, which stabilizes the plasma membrane of myofibers. The orofacial function is affected in an advanced stage of DMD and this often leads to an eating disorder such as dysphagia. Dysphagia is caused by multiple etiologies including decreased mastication and swallowing. Therefore, preventing the functional declines of mastication and swallowing in DMD is important to improve the patient’s quality of life. In the present study, using a rat model of DMD we generated previously, we performed analyses on the masseter and tongue muscles, both are required for proper eating function.

Methods

Age-related changes of the masseter and tongue muscle of DMD rats were analyzed morphometrically, histologically, and immunohistochemically. Also, transcription of cellular senescent markers, and utrophin (Utrn), a functional analog of dystrophin, was examined.

Results

The masseter muscle of DMD rats showed progressive dystrophic changes as observed in their hindlimb muscle, accompanied by increased transcription of p16 and p19. On the other hand, the tongue of DMD rats showed macroglossia due to hypertrophy of myofibers with less dystrophic changes. Proliferative activity was preserved in the satellite cells from the tongue muscle but was perturbed severely in those from the masseter muscle. While Utrn transcription was increased in the masseter muscle of DMD rats compared to WT rats, probably due to a compensatory mechanism, its level in the tongue muscle was comparable between WT and DMD rats and was similar to that in the masseter muscle of DMD rats.

Conclusions

Muscular dystrophy is less advanced in the tongue muscle compared to the masseter muscle in the DMD rat.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13395-022-00307-7.

All for One and One for All: Regenerating Skeletal Muscle

Despite the evolutionary loss of tissue regenerative potential, robust skeletal muscle repair processes are largely retained even in higher vertebrates. In mammals, the skeletal muscle regeneration program is driven by resident stem cells termed satellite cells, guided by the coordinated activity of multiple intrinsic and extrinsic factors and other cell types. A thorough understanding of muscle repair mechanisms is crucial not only for combating skeletal myopathies, but for its prospective aid in devising therapeutic strategies to endow regenerative potential on otherwise regeneration-deficient organs. In this review, we discuss skeletal muscle regeneration from an evolutionary perspective, summarize the current knowledge of cellular and molecular mechanisms, and highlight novel paradigms of muscle repair revealed by explorations of the recent decade.

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.

Reduced growth rate of aged muscle stem cells is associated with impaired mechanosensitivity

Aging-associated muscle wasting and impaired regeneration are caused by deficiencies in muscle stem cell (MuSC) number and function. We postulated that aged MuSCs are intrinsically impaired in their responsiveness to omnipresent mechanical cues through alterations in MuSC morphology, mechanical properties, and number of integrins, culminating in impaired proliferative capacity. Here we show that aged MuSCs exhibited significantly lower growth rate and reduced integrin-α7 expression as well as lower number of phospho-paxillin clusters than young MuSCs. Moreover, aged MuSCs were less firmly attached to matrigel-coated glass substrates compared to young MuSCs, as 43% of the cells detached in response to pulsating fluid shear stress (1 Pa). YAP nuclear localization was 59% higher than in young MuSCs, yet YAP target genes Cyr61 and Ctgf were substantially downregulated. When subjected to pulsating fluid shear stress, aged MuSCs exhibited reduced upregulation of proliferation-related genes. Together these results indicate that aged MuSCs exhibit impaired mechanosensitivity and growth potential, accompanied by altered morphology and mechanical properties as well as reduced integrin-α7 expression. Aging-associated impaired muscle regenerative capacity and muscle wasting is likely due to aging-induced intrinsic MuSC alterations and dysfunctional mechanosensitivity.

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.

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.

Neuromuscular fatigue and recovery after strenuous exercise depends on skeletal muscle size and stem cell characteristics

Hamstring muscle injury is highly prevalent in sports involving repeated maximal sprinting. Although neuromuscular fatigue is thought to be a risk factor, the mechanisms underlying the fatigue response to repeated maximal sprints are unclear. Here, we show that repeated maximal sprints induce neuromuscular fatigue accompanied with a prolonged strength loss in hamstring muscles. The immediate hamstring strength loss was linked to both central and peripheral fatigue, while prolonged strength loss was associated with indicators of muscle damage. The kinematic changes immediately after sprinting likely protected fatigued hamstrings from excess elongation stress, while larger hamstring muscle physiological cross-sectional area and lower myoblast:fibroblast ratio appeared to protect against fatigue/damage and improve muscle recovery within the first 48 h after sprinting. We have therefore identified novel mechanisms that likely regulate the fatigue/damage response and initial recovery following repeated maximal sprinting in humans.

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.

Differential effects of right and left heart failure on skeletal muscle in rats

BACKGROUND

Exercise intolerance is a cardinal symptom in right (RV) and left ventricular (LV) failure. The underlying skeletal muscle contributes to increased morbidity in patients. Here, we compared skeletal muscle sarcopenia in a novel two-stage model of RV failure to an established model of LV failure.

METHODS

Pulmonary artery banding (PAB) or aortic banding (AOB) was performed in weanling rats, inducing a transition from compensated cardiac hypertrophy (after 7 weeks) to heart failure (after 22-26 weeks). Cardiac function was characterized by echocardiography. Skeletal muscle catabolic/anabolic balance and energy metabolism were analysed by histological and biochemical methods, real-time PCR, and western blot.

RESULTS

Two clearly distinguishable stages of left or right heart disease with a comparable severity were reached. However, skeletal muscle impairment was significantly more pronounced in LV failure. While the compensatory stage resulted only in minor changes, soleus and gastrocnemius muscle of AOB rats at the decompensated stage demonstrated reduced weight and fibre diameter, higher proteasome activity and expression of the muscle-specific ubiquitin E3 ligases muscle-specific RING finger 1 and atrogin-1, increased expression of the atrophy marker myostatin, increased autophagy activation, and impaired mitochondrial function and respiratory chain gene expression. Soleus and gastrocnemius muscle of PAB rats did not show significant changes in muscle weight and proteasome or autophagy activation, but mitochondrial function was mildly impaired as well. The diaphragm did not demonstrate differences in any model or disease stage except for myostatin expression, which was altered at the decompensated stage in both models. Plasma interleukin (IL)-6 and angiotensin II were strongly increased at the decompensated stage (AOB > > PAB). Soleus and gastrocnemius muscle itself demonstrated an increase in IL-6 expression independent from blood-derived cytokines only in AOB animals. In vitro experiments in rat skeletal muscle cells suggested a direct impact of IL-6 and angiotensin II on distinctive atrophic changes.

CONCLUSIONS

Manifold skeletal muscle alterations are more pronounced in LV failure compared with RV failure despite a similar ventricular impairment. Most of the catabolic changes were observed in soleus or gastrocnemius muscle rather than in the constantly active diaphragm. Mitochondrial dysfunction and up-regulation of myostatin were identified as the earliest signs of skeletal muscle impairment.

Myogenic marker expression as a function of age and exercise-based therapy in the tongue

Degeneration of tongue muscles with aging may contribute to swallowing deficits observed in elderly people. However, the capacity for tongue muscle stem cells (SCs) to regenerate and repair the aged tongue and improve tongue strength following tongue exercise (a current clinical treatment) has never been examined. We found that the expression of regenerative, myogenic markers were impaired with age and may be related to increased expression of senescent marker p16INK4a. Tongue strength increased in young adult and old rats following exercise and was related to the expression of Pax7, MyoD, myogenin, and p16INK4a. Our study also suggests that strengthening of tongue muscles via clinical rehabilitation strategies also increased the expression of SC regenerative markers in the tongue throughout the exercise duration.

Impaired Regeneration in Dystrophic Muscle-New Target for Therapy

Muscle stem cells (MuSCs), known as satellite cells (SCs) have an incredible ability to regenerate, which enables the maintenance and growth of muscle tissue. In response to damaging stimuli, SCs are activated, proliferate, differentiate, and fuse to repair or generate a new muscle fiber. However, dystrophic muscles are characterized by poor muscle regeneration along with chronic inflammation and fibrosis. Indications for SC involvement in muscular dystrophy pathologies are accumulating, but their contribution to muscle pathophysiology is not precisely understood. In congenital muscular dystrophy type 1A (LAMA2-CMD), mutations in Lama2 gene cause either complete or partial absence in laminin-211 protein. Laminin-211 functions as a link between muscle extracellular matrix (ECM) and two adhesion systems in the sarcolemma; one is the well-known dystrophin-glycoprotein complex (DGC), and the second is the integrin complex. Because of its protein interactions and location, laminin-211 has a crucial role in muscle function and survival by maintaining sarcolemma integrity. In addition, laminin-211 is expressed in SCs and suggested to have a role in SC proliferation and differentiation. Downstream to the primary defect in laminin-211, several secondary genes and pathways accelerate disease mechanism, while at the same time there are unsuccessful attempts to regenerate as compensation for the dystrophic process. Lately, next-generation sequencing platforms have advanced our knowledge about the secondary events occurring in various diseases, elucidate the pathophysiology, and characterize new essential targets for development of new treatment strategies. This review will mainly focus on SC contribution to impaired regeneration in muscular dystrophies and specifically new findings suggesting SC involvement in LAMA2-CMD pathology.

Generation of craniofacial myogenic progenitor cells from human induced pluripotent stem cells for skeletal muscle tissue regeneration

Craniofacial skeletal muscle is composed of approximately 60 muscles, which have critical functions including food uptake, eye movements and facial expressions. Although craniofacial muscles have significantly different embryonic origin, most current skeletal muscle differentiation protocols using human induced pluripotent stem cells (iPSCs) are based on somite-derived limb and trunk muscle developmental pathways. Since the lack of a protocol for craniofacial muscles is a significant gap in the iPSC-derived muscle field, we have developed an optimized protocol to generate craniofacial myogenic precursor cells (cMPCs) from human iPSCs by mimicking key signaling pathways during craniofacial embryonic myogenesis. At each different stage, human iPSC-derived cMPCs mirror the transcription factor expression profiles seen in their counterparts during embryo development. After the bi-potential cranial pharyngeal mesoderm is established, cells are committed to cranial skeletal muscle lineages with inhibition of cardiac lineages and are purified by flow cytometry. Furthermore, identities of Ipsc-derived cMPCs are verified with human primary myoblasts from craniofacial muscles using RNA sequencing. These data suggest that our new method could provide not only in vitro research tools to study muscle specificity of muscular dystrophy but also abundant and reliable cellular resources for tissue engineering to support craniofacial reconstruction surgery.

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.

Dosage effect of multiple genes accounts for multisystem disorder of myotonic dystrophy type 1

Multisystem manifestations in myotonic dystrophy type 1 (DM1) may be due to dosage reduction in multiple genes induced by aberrant expansion of CTG repeats in DMPK, including DMPK, its neighboring genes (SIX5 or DMWD) and downstream MBNL1. However, direct evidence is lacking. Here, we develop a new strategy to generate mice carrying multigene heterozygous mutations to mimic dosage reduction in one step by injection of haploid embryonic stem cells with mutant Dmpk, Six5 and Mbnl1 into oocytes. The triple heterozygous mutant mice exhibit adult-onset DM1 phenotypes. With the additional mutation in Dmwd, the quadruple heterozygous mutant mice recapitulate many major manifestations in congenital DM1. Moreover, muscle stem cells in both models display reduced stemness, providing a unique model for screening small molecules for treatment of DM1. Our results suggest that the complex symptoms of DM1 result from the reduced dosage of multiple genes.

A study of chewing muscles: Age-related changes in type I collagen and matrix metalloproteinase-2 expression

OBJECTIVE

In this study, the aim was to investigate the biochemical, physiological and histological changes that occur in masticatory muscles of the masticatory system with aging.

DESIGN

In this study, 14 BALB/c mice were used. Animals were divided into two equal groups of seven. Group I was organized as the group of young animals (n = 7) and Group II as the group of adult animals (n = 7). After routine histological follow-up was performed, the tissues were embedded in paraffin. 4-5 μm thick cross-sections were taken from paraffin-embedded tissues and they were stained with Haemotoxylin and Eosin Type I collagen and Matrix metalloproteinase-2 (MMP-2) immunohistochemically.

RESULTS

It was observed that there was a decrease and shrinking in blood vessels due to aging. In young mice, Type I collagen and MMP-2 immunoreactivity in the masseter muscle tissue showed low staining, while Type I collagen and MMP-2 immunoreactivity in the temporal muscle tissue showed moderate staining. Type I collagen and MMP-2 immunoreactivity were significantly higher in the masseter and temporal muscles of elderly mice (p = 0.001). In the H-score evaluation, MMP-2 immune reactivity was significantly lower in young mice than in older mice (p = 0.001).

CONCLUSION

It was determined that severe pain complications and functional losses are likely to occur with the increase of degeneration due to aging of masticator muscles.

The panniculus carnosus muscle: A novel model of striated muscle regeneration that exhibits sex differences in the mdx mouse

The dermal striated muscle panniculus carnosus (PC), prevalent in lower mammals with remnants in humans, is highly regenerative, and whose function is purported to be linked to defence and shivering thermogenesis. Given the heterogeneity of responses of different muscles to disease, we set out to characterize the PC in wild-type and muscular dystrophic mdx mice. The mouse PC contained mainly fast-twitch type IIB myofibers showing body wide distribution. The PC exemplified heterogeneity in myofiber sizes and a prevalence of central nucleated fibres (CNFs), hallmarks of regeneration, in wild-type and mdx muscles, which increased with age. PC myofibers were hypertrophic in mdx compared to wild-type mice. Sexual dimorphism was apparent with a two-fold increase in CNFs in PC from male versus female mdx mice. To evaluate myogenic potential, PC muscle progenitors were isolated from 8-week old wild-type and mdx mice, grown and differentiated for 7-days. Myogenic profiling of PC-derived myocytes suggested that male mdx satellite cells (SCs) were more myogenic than female counterparts, independent of SC density in PC muscles. Muscle regenerative differences in the PC were associated with alterations in expression of calcium handling regulatory proteins. These studies highlight unique aspects of the PC muscle and its potential as a model to study mechanisms of striated muscle regeneration in health and disease.

A distinct cardiopharyngeal mesoderm genetic hierarchy establishes antero-posterior patterning of esophagus striated muscle

In most vertebrates, the upper digestive tract is composed of muscularized jaws linked to the esophagus that permits food ingestion and swallowing. Masticatory and esophagus striated muscles (ESM) share a common cardiopharyngeal mesoderm (CPM) origin, however ESM are unusual among striated muscles as they are established in the absence of a primary skeletal muscle scaffold. Using mouse chimeras, we show that the transcription factors Tbx1 and Isl1 are required cell-autonomously for myogenic specification of ESM progenitors. Further, genetic loss-of-function and pharmacological studies point to MET/HGF signaling for antero-posterior migration of esophagus muscle progenitors, where Hgf ligand is expressed in adjacent smooth muscle cells. These observations highlight the functional relevance of a smooth and striated muscle progenitor dialogue for ESM patterning. Our findings establish a Tbx1-Isl1-Met genetic hierarchy that uniquely regulates esophagus myogenesis and identify distinct genetic signatures that can be used as framework to interpret pathologies arising within CPM derivatives.

Manganese influx and expression of ZIP8 is essential in primary myoblasts and contributes to activation of SOD2

Trace elements such as copper (Cu), zinc (Zn), iron (Fe), and manganese (Mn) function as enzyme cofactors and second messengers in cell signaling. Trace elements are emerging as key regulators of differentiation and development of mammalian tissues including blood, brain, and skeletal muscle. We previously reported an influx of Cu and dynamic expression of metal transporters during differentiation of skeletal muscle cells. Here, we demonstrate that during differentiation of skeletal myoblasts an increase of Mn, Fe and Zn also occurs. Interestingly the Mn increase is concomitant with increased Mn-dependent SOD2 levels. To better understand the Mn import pathway in skeletal muscle cells, we probed the functional relevance of the closely related proteins ZIP8 and ZIP14, which are implicated in Zn, Mn, and Fe transport. Partial depletion of ZIP8 severely impaired growth of myoblasts and led to cell death under differentiation conditions, indicating that ZIP8-mediated metal transport is essential in skeletal muscle cells. Moreover, knockdown of Zip8 impaired activity of the Mn-dependent SOD2. Growth defects were partially rescued only by Mn supplementation to the medium, suggesting additional functions for ZIP8 in the skeletal muscle lineage. Restoring wild type Zip8 into the knockdown cells rescued the proliferation and differentiation phenotypes. On the other hand, knockdown of Zip14, had only a mild effect on myotube size, consistent with a role for ZIP14 in muscle hypertrophy. Simultaneous knockdown of both Zip8 and Zip14 further impaired differentiation and led cell death. This is the first report on the functional relevance of two members of the ZIP family of metal transporters in the skeletal muscle lineage, and further supports the paradigm that trace metal transporters are important modulators of mammalian tissue development.

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.

Unique morphogenetic signatures define mammalian neck muscles and associated connective tissues

In vertebrates, head and trunk muscles develop from different mesodermal populations and are regulated by distinct genetic networks. Neck muscles at the head-trunk interface remain poorly defined due to their complex morphogenesis and dual mesodermal origins. Here, we use genetically modified mice to establish a 3D model that integrates regulatory genes, cell populations and morphogenetic events that define this transition zone. We show that the evolutionary conserved cucullaris-derived muscles originate from posterior cardiopharyngeal mesoderm, not lateral plate mesoderm, and we define new boundaries for neural crest and mesodermal contributions to neck connective tissue. Furthermore, lineage studies and functional analysis of Tbx1- and Pax3-null mice reveal a unique developmental program for somitic neck muscles that is distinct from that of somitic trunk muscles. Our findings unveil the embryological and developmental requirements underlying tetrapod neck myogenesis and provide a blueprint to investigate how muscle subsets are selectively affected in some human myopathies.

Botulinum Toxin Induced Atrophy: An Uncharted Territory

Botulinum neurotoxins (BoNTs) produce local chemo-denervation by cleaving soluble N-ethylmaleimide-sensitive factor activating protein receptor (SNARE) proteins. Botulinum neurotoxins are therapeutically indicated in several neurological disorders and have been in use for three decades. The long-term efficacy, safety, and side effects of BoNTs have been well documented in the literature. However, the development of muscle atrophy following chronic exposure to BoNTs has not received sufficient attention. Muscle atrophy is not only cosmetically distressing, but also has an impact on future injections. An extensive literature search was conducted on atrophy and mechanisms of atrophy. Five hundred and four relevant articles in the English language were reviewed. This review revealed the surprising lack of documentation of atrophy within the literature. In addition, as demonstrated in this review, the mechanisms and the clinical factors that may lead to atrophy have also been poorly studied. However, even with this limited information it is possible to indicate factors that could modify the clinical approach to botulinum toxin injections. This review highlights the need for further study of atrophy following BoNT injections.

Wnt7a induces satellite cell expansion, myofiber hyperplasia and hypertrophy in rat craniofacial muscle

Craniofacial muscles drive critical functions in the head, including speech, feeding and expression. Compared with their counterparts in trunk and limbs, craniofacial muscles are of distinct embryonic origins, which might consequently lead to different growth patterns and regenerative potential. In this study, rat levator veli palatini muscle and masseter muscle were compared with tibialis anterior muscle in their response to exogenous Wnt7a stimulus, which has been proved effective in promoting muscle regeneration in the limbs. Histological, cellular and molecular analyses were performed both under basal condition and after a single dose injection of recombinant human Wnt7a. Under basal condition, levator veli palatini muscle demonstrated considerably more satellite cells than the others. After Wnt7a administration, regeneration-related activities, including satellite cell expansion, myofiber hyperplasia and hypertrophy were generally observed in all three muscles, but with obvious differences in the extent. The composition of fast/slow myofibers underwent substantial alterations, and the pattern varied among the three muscles. Location-specific alterations in the expression level of core components in planar cell polarity pathway, Akt/mTOR pathway and myostatin pathway were also observed. In conclusion, both craniofacial and limb muscles could be effectively expanded by exogenous Wnt7a stimulus, but muscle-to-muscle variations in response patterns existed.

Dynamic changes in copper homeostasis and post-transcriptional regulation of Atp7a during myogenic differentiation

Copper (Cu) is an essential metal required for activity of a number of redox active enzymes that participate in critical cellular pathways such as metabolism and cell signaling. Because it is also a toxic metal, Cu must be tightly controlled by a series of transporters and chaperone proteins that regulate Cu homeostasis. The critical nature of Cu is highlighted by the fact that mutations in Cu homeostasis genes cause pathologic conditions such as Menkes and Wilson diseases. While Cu homeostasis in highly affected tissues like the liver and brain is well understood, no study has probed the role of Cu in development of skeletal muscle, another tissue that often shows pathology in these conditions. Here, we found an increase in whole cell Cu content during differentiation of cultured immortalized or primary myoblasts derived from mouse satellite cells. We demonstrate that Cu is required for both proliferation and differentiation of primary myoblasts. We also show that a key Cu homeostasis gene, Atp7a, undergoes dynamic changes in expression during myogenic differentiation. Alternative polyadenylation and stability of Atp7a mRNA fluctuates with differentiation stage of the myoblasts, indicating post-transcriptional regulation of Atp7a that depends on the differentiation state. This is the first report of a requirement for Cu during myogenic differentiation and provides the basis for understanding the network of Cu transport associated with myogenesis.

Disturbed Ca2+ Homeostasis in Muscle-Wasting Disorders

Ca²⁺ is essential for proper structure and function of skeletal muscle. It not only activates contraction and force development but also participates in multiple signaling pathways. Low levels of Ca²⁺ restrain muscle regeneration by limiting the fusion of satellite cells. Ironically, sustained elevations of Ca²⁺ also result in muscle degeneration as this ion promotes high rates of protein breakdown. Moreover, transforming growth factors (TGFs) which are well known for controlling muscle growth also regulate Ca²⁺ channels. Thus, therapies focused on changing levels of Ca²⁺ and TGFs are promising for treating muscle-wasting disorders. Three principal systems govern the homeostasis of Ca²⁺, namely, excitation-contraction (EC) coupling, excitation-coupled Ca²⁺ entry (ECCE), and store-operated Ca²⁺ entry (SOCE). Accordingly, alterations in these systems can lead to weakness and atrophy in many hereditary diseases, such as Brody disease, central core disease (CCD), tubular aggregate myopathy (TAM), myotonic dystrophy type 1 (MD1), oculopharyngeal muscular dystrophy (OPMD), and Duchenne muscular dystrophy (DMD). Here, the interrelationship between all these molecules and processes is reviewed.

Morphological and molecular comparisons between tibialis anterior muscle and levator veli palatini muscle: A preliminary study on their augmentation potential

Tibialis anterior (TA) muscle and other somite-derived limb muscles remain the prototype in skeletal muscle study. The majority of head muscles, however, develop from branchial arches and maintain a number of heterogeneities in comparison with their limb counterparts. Levator veli palatini (LVP) muscle is a deep-located head muscle responsible for breathing, swallowing and speech, and is central to cleft palate surgery, yet lacks morphological and molecular investigation. In the present study, multiscale in vivo analyses were performed to compare TA and LVP muscle in terms of their myofiber composition, in-situ stem cell population and augmentation potential. TA muscle was identified to be primarily composed of type 2B myofibers while LVP muscle primarily consisted of type 2A and 2X myofibers. In addition, LVP muscle maintained a higher percentage of centrally-nucleated myofibers and a greater population of satellite cells. Notably, TA and LVP muscle responded to exogenous Wnt7a stimulus in different ways. Three weeks after Wnt7a administration, TA muscle exhibited an increase in myofiber number and a decrease in myofiber size, while LVP muscle demonstrated no significant changes in myofiber number or myofiber size. These results suggested that LVP muscle exhibits obvious differences in comparison with TA muscle. Therefore, knowledge acquired from TA muscle studies requires further testing before being applied to LVP muscle.

Targeted Lipidomic Analysis of Myoblasts by GC-MS and LC-MS/MS

Lipids represent ∼10% of the cell dry mass and play essential roles in membrane composition and physical properties, energy storage, and signaling pathways. In the developing or the regenerating skeletal muscle, modifications in the content or the flipping between leaflets of membrane lipid components can modulate the fusion capacity of myoblasts, thus constituting one of the regulatory mechanisms underlying myofiber growth. Recently, few genes controlling these qualitative and quantitative modifications have started to be unraveled. The precise functional characterization of these genes requires both qualitative and quantitative evaluations of a global lipid profile. Here, we describe a lipidomic protocol using mass spectrometry, allowing assessing the content of fatty acids, glycerophospholipids, and cholesterol in the routinely used C2C12 mouse myoblast cell line, or in primary cultures of mouse myoblasts.

Cellular Hypoxia Promotes Heterotopic Ossification by Amplifying BMP Signaling

Hypoxia and inflammation are implicated in the episodic induction of heterotopic endochondral ossification (HEO); however, the molecular mechanisms are unknown. HIF-1α integrates the cellular response to both hypoxia and inflammation and is a prime candidate for regulating HEO. We investigated the role of hypoxia and HIF-1α in fibrodysplasia ossificans progressiva (FOP), the most catastrophic form of HEO in humans. We found that HIF-1α increases the intensity and duration of canonical bone morphogenetic protein (BMP) signaling through Rabaptin 5 (RABEP1)-mediated retention of Activin A receptor, type I (ACVR1), a BMP receptor, in the endosomal compartment of hypoxic connective tissue progenitor cells from patients with FOP. We further show that early inflammatory FOP lesions in humans and in a mouse model are markedly hypoxic, and inhibition of HIF-1α by genetic or pharmacologic means restores canonical BMP signaling to normoxic levels in human FOP cells and profoundly reduces HEO in a constitutively active Acvr1(Q207D/+) mouse model of FOP. Thus, an inflammation and cellular oxygen-sensing mechanism that modulates intracellular retention of a mutant BMP receptor determines, in part, its pathologic activity in FOP. Our study provides critical insight into a previously unrecognized role of HIF-1α in the hypoxic amplification of BMP signaling and in the episodic induction of HEO in FOP and further identifies HIF-1α as a therapeutic target for FOP and perhaps nongenetic forms of HEO. © 2016 American Society for Bone and Mineral Research.

Identification and Characterization of the Dermal Panniculus Carnosus Muscle Stem Cells

The dermal Panniculus carnosus (PC) muscle is important for wound contraction in lower mammals and represents an interesting model of muscle regeneration due to its high cell turnover. The resident satellite cells (the bona fide muscle stem cells) remain poorly characterized. Here we analyzed PC satellite cells with regard to developmental origin and purported function. Lineage tracing shows that they originate in Myf5⁺, Pax3/Pax7⁺ cell populations. Skin and muscle wounding increased PC myofiber turnover, with the satellite cell progeny being involved in muscle regeneration but with no detectable contribution to the wound-bed myofibroblasts. Since hematopoietic stem cells fuse to PC myofibers in the absence of injury, we also studied the contribution of bone marrow-derived cells to the PC satellite cell compartment, demonstrating that cells of donor origin are capable of repopulating the PC muscle stem cell niche after irradiation and bone marrow transplantation but may not fully acquire the relevant myogenic commitment.

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PC satellite cells originate from Myf5⁺, Pax3/Pax7⁺ cell lineages

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Skin and muscle wounding increase PC myofiber turnover

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Donor bone marrow cells repopulate the PC satellite niche after BMT

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Dermis-derived myogenesis originates from the PC satellite cell population