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Background: Type 2 diabetes mellitus (T2DM) is a chronic metabolic disease that commonly results from a high-calorie diet and sedentary lifestyle, leading to insulin resistance and glucose homeostasis perturbation. Physical activity is recommended as one first-line treatment in T2DM, but it leads to contrasted results. We hypothesized that, instead of applying standard exercise protocols, the prescription of personalized exercise programs specifically designed to reverse the potential metabolic alterations in skeletal muscle could result in better results., Methods: To test this hypothesis, we drew the metabolic signature of the fast-twitch quadriceps muscle, based on a combined unbiased NMR spectroscopy and RT-qPCR study, in several T2DM mouse models of different genetic background (129S1/SvImJ, C57Bl/6J), sex and aetiology (high-fat diet (HFD) or HFD/Streptozotocin (STZ) induction or transgenic MKR (FVB-Tg Ckm-IGF1R*K1003R)1Dlr/J) mice. Three selected mouse models with unique muscular metabolic signatures were submitted to three different swimming-based programs, designed to address each metabolic specificity., Results: We found that depending on the genetic background, the sex, and the mode of T2DM induction, specific muscular adaptations occurred, including depressed glycolysis associated with elevated PDK4 expression, shift to β-oxidation, or deregulation of amino-acid homeostasis. Interestingly, dedicated swimming-based exercises designed to restore specific metabolic alterations in muscle were found optimal in improving systemic T2DM hallmarks, including a significant reduction in insulin resistance, the improvement of glucose homeostasis, and a delay in sensorimotor function alterations., Conclusion: The muscle metabolism constitutes an important clue for the design of precision exercises with potential clinical implications for T2DM patients., (© 2024. The Author(s).)

The calcineurin/NFAT (nuclear factor of activated T-cells) signaling pathway is involved in the modulation of the adult muscle fiber type, but its role in the establishment of the muscle phenotype remains elusive. Here, we show that the NFAT member NFATc2 cooperates with the basic helix-loop-helix transcription factor MyoD to induce the expression of a specific myosin heavy chain (MHC) isoform, the neonatal one, during embryogenesis. We found this cooperation to be crucial, as Myod/Nfatc2 double-null mice die at birth, with a dramatic reduction of the major neonatal MHC isoform normally expressed at birth in skeletal muscles, such as limb and intercostal muscles, whereas its expression is unaffected in myofibers mutated for either factor alone. Using gel shift and chromatin immunoprecipitation assays, we identified NFATc2 bound to the neonatal Mhc gene, whereas NFATc1 and NFATc3 would preferentially bind the embryonic Mhc gene. We provide evidence that MyoD synergistically cooperates with NFATc2 at the neonatal Mhc promoter. Altogether, our findings demonstrate that the calcineurin/NFAT pathway plays a new role in establishing the early muscle fiber type in immature myofibers during embryogenesis.

Unlabelled: Although Xenopus is a key model organism in developmental biology, little is known about the myotome formation in this species. Here, we assessed the expression of myogenic regulatory factors of the Myod family (MRFs) during embryonic development and revealed distinct MRF programs., Results: The expression pattern of each MRF during embryonic development highlights three successive myogenic waves. We showed that a first median and lateral myogenesis initiates before dermomyotome formation: the median cell population expresses Myf5, Myod, and Mrf4, whereas the lateral one expresses Myod, moderate levels of Myogenin and Mrf4. The second wave of myoblasts arising from the dermomyotome is characterized by the full MRF program expression, with high levels of Myogenin. The third wave is revealed by Myf5 expression in the myotome and could contribute to the formation of plurinucleated fibers at larval stages. Furthermore, Myf5- or Myod-expressing anlagen are identified in craniofacial myogenesis., Conclusions: The first median and lateral myogenesis and their associated MRF programs have probably disappeared in mammals. However, some aspects of Xenopus myogenesis have been conserved such as the development of somitic muscles by successive myogenic waves and the existence of Myf5-dependent and -independent lineages., (Copyright © 2012 Wiley Periodicals, Inc.)

Xenopus myotome is formed by a first medial and lateral myogenesis directly arising from the presomitic mesoderm followed by a second myogenic wave emanating from the dermomyotome. Here, by a series of gain and loss of function experiments, we showed that Mef2d, a member of the Mef2 family of MADS-box transcription factors, appeared as an upstream regulator of lateral myogenesis, and as an inducer of dermomyotome formation at the beginning of neurulation. In the lateral presomitic cells, we showed that Mef2d transactivates Myod expression which is necessary for lateral myogenesis. In the most lateral cells of the presomitic mesoderm, we showed that Mef2d and Paraxis (Tcf15), a member of the Twist family of transcription factors, were co-localized and activate directly the expression of Meox2, which acts upstream of Pax3 expression during dermomyotome formation. Cell tracing experiments confirm that the most lateral Meox2 expressing cells of the presomitic mesoderm correspond to the dermomyotome progenitors since they give rise to the most dorsal cells of the somitic mesoderm. Thus, Xenopus Mef2d couples lateral myogenesis to dermomyotome formation before somite segmentation. These results together with our previous works reveal striking similarities between dermomyotome and tendon formation in Xenopus: both develop in association with myogenic cells and both involve a gene transactivation pathway where one member of the Mef2 family, Mef2d or Mef2c, cooperates with a bHLH protein of the Twist family, Paraxis or Scx (Scleraxis) respectively. We propose that these shared characteristics in Xenopus laevis reflect the existence of a vertebrate ancestral mechanism which has coupled the development of the myogenic cells to the formation of associated tissues during somite compartmentalization.

Sprouty (Spry) proteins were identified as negative regulators of fibroblast growth factor (FGF) signaling in vertebrates and invertebrates. Given the importance of the FGFs in myogenesis, we performed cardiotoxin injury-induced regeneration experiments on soleus muscles of both, adult control and FGF6 ( - / - ) mutant mice and analyzed the accumulation of Spry (1, 2 and 4) transcripts using semi-quantitative and real-time RT-PCR assays and in situ hybridization. We also analyzed the effects of muscle denervation on the accumulation of Spry transcripts. The three Spry genes begin to be expressed as early as the first stages of muscle regeneration and are characterized by distinct expression patterns. Moreover, Spry gene expression was highly and differentially up-regulated, precociously by the lack of FGF6, and belatedly by muscle denervation strongly suggesting that the transient rise of Spry mRNA accumulation was associated to muscle differentiation. Rescue experiments supported the idea of a specific relationship between FGF6 and Spry 2, both being known for their particular involvement in myogenesis.

Important functions in myogenesis have been proposed for FGF6, a member of the fibroblast growth factor family accumulating almost exclusively in the myogenic lineage, but its precise role in vivo remains mostly unclear. Here, using FGF6 (-/-) mice and rescue experiments by injection of recombinant FGF6, we dissected the functional role of FGF6 during in vivo myogenesis. We found that the appearance of myotubes was accelerated during regeneration of the soleus of FGF6 (-/-) mice versus wild type mice. This accelerated differentiation was correlated with increased expression of differentiation markers such as CdkIs and calcineurin, as well as structural markers such as MHCI and slow TnI. We showed that an elevated transcript level for calcineurin Aalpha subunit correlated with a positive regulation of calcineurin A activity in regenerating soleus of the FGF6 (-/-) mice. Cyclin D1 and calcineurin were up- and down-regulated, respectively in a dose-dependent manner upon injection of rhFGF6 in regenerating soleus of the mutant mice. We showed an increase of the number of slow oxidative (type I) myofibers, whereas fast oxidative (type IIa) myofibers were decreased in number in regenerating soleus of FGF6 (-/-) mice versus that of wild type mice. In adult soleus, the number of type I myofibers was also higher in FGF6 (-/-) mice than in wild type mice. Taken together these results evidenced a specific phenotype for soleus of the FGF6 (-/-) mice and led us to propose a model accounting for a specific dose-dependent effect of FGF6 in muscle regeneration. At high doses, FGF6 stimulates the proliferation of the myogenic stem cells, whereas at lower doses it regulates both muscle differentiation and muscle phenotype via a calcineurin-signaling pathway., ((c) 2004 Wiley-Liss, Inc.)

The discovery, in the late 1980s, of the MyoD gene family of muscle transcription factors has proved to be a milestone in understanding the molecular events controlling the specification and differentiation of the muscle lineage. From gene knock-out mice experiments progressively emerged the idea that each myogenic regulatory factor (MRF) has evolved a specialized as well as a redundant role in muscle differentiation. To date, MyoD serves as a paradigm for the MRF mode of function. The features of gene regulation by MyoD support a model in which subprograms of gene expression are achieved by the combination of promoter-specific regulation of MyoD binding and MyoD-mediated binding of various ancillary proteins. This binding likely includes site-specific chromatin reorganization by means of direct or indirect interaction with remodeling enzymes. In this cascade of molecular events leading to the proper and reproducible activation of muscle gene expression, the role and mode of function of other MRFs still remains largely unclear. Recent in vivo findings using the Xenopus embryo model strongly support the concept that a single MRF can specifically control a subset of muscle genes and, thus, can be substituted by other MRFs albeit with dramatically lower efficiency. The topic of this review is to summarize the molecular data accounting for a redundant and/or specific involvement of each member of the MyoD family in myogenesis in the light of recent studies on the Xenopus model., (Copyright (c) 2004 Wiley-Liss, Inc.)

Important functions in myogenesis have been proposed for FGF6, a member of the fibroblast growth factor family accumulating almost exclusively in the myogenic lineage. However, the use of FGF6(-/-) mutant mice gave contradictory results and the role of FGF6 during myogenesis remains largely unclear. Using FGF6(-/-) mice, we first analysed the morphology of the regenerated soleus following cardiotoxin injection and showed hypertrophied myofibres in soleus of the mutant mice as compared to wild-type mice. Secondly, to examine the function of the IGF family in the hypertrophy process, we used semiquantitative and real-time RT-PCR assays and Western blots to monitor the expression of the insulin-like growth factors (IGF-I and IGF-II), their receptors [type I IGF receptor (IGF1R) and IGF-II receptor (IGF2R)], and of a binding protein IGFBP-5 in regenerating soleus muscles of FGF6(-/-) knockout mice vs. wild-type mice. In the mutant, both IGF-II and IGF2R, but not IGF-I and IGF1R, were strongly up-regulated, whereas IGFBP5 was down-regulated, strongly suggesting that, in the absence of FGF6, the mechanisms leading to myofibre hypertrophy were mediated specifically by an IGF-II/IGF2R signalling pathway distinct from the classic mechanism involving IGF-I and IGF1R previously described for skeletal muscle hypertrophy. The potential regulating role of IGFBP5 on IGF-II expression is also discussed. This report shows for the first time a specific role for FGF6 in the regulation of myofibre size during a process of in vivo myogenesis.

FGF6, a member of the fibroblast growth factor (FGF) family, accumulated almost exclusively in the myogenic lineage, supporting the finding that FGF6 could specifically regulate myogenesis. Using FGF6 (-/-) mutant mice, important functions in muscle regeneration have been proposed for FGF6 but remain largely controversial. Here, we examined the effect of a single injection of recombinant FGF6 (rhFGF6) on the regeneration of mouse soleus subjected to cardiotoxin injection, specifically looking for molecular and morphological phenotypes. The injection of rhFGF6 has two effects. First, there is an up-regulation of cyclin D1 mRNA, accounting for the regulating role of a high FGF6 concentration on proliferation, and second, differentiation markers such as CdkIs and MHC I and Tn I increase and cellular differentiation is accelerated. We also show a down-regulation of endogenous FGF6, acceleration of FGFR1 receptor expression and deceleration of the FGFR4 receptor expression, possibly accounting for biphasic effects of exogenous FGF6 on muscle regeneration.

Aim: The aim of this report is to show that eccentric exercise under well-controlled conditions is an alternative model, to chemical and mechanical analyses, and analyse the process of degeneration/regeneration in mouse soleus., Methods: For this, mice were submitted to a single bout of eccentric exercise on a treadmill down a 14 degrees decline for 150 min and the soleus muscle was analysed at different times following exercise by histology and in situ hybridization in comparison with cardiotoxin-injured muscles., Results: We analyse the regenerative process by detection of the accumulation of transcripts coding for the two myogenic regulatory factors, Myf-5 and MyoD, which are good markers of the activated satellite cells. From 24 h post-exercise (P-E), clusters of mononucleated Myf-5/MyoD-positive cells were detected. Their number increased up to 96 h P-E when young MyoD-positive myotubes with central nuclei began to appear. From 96 to 168 h P-E the number of myotubes increased, about 10-fold, the new myotubes representing 58% of the muscle cells (168 h P-E)., Conclusion: These results show that this protocol of eccentric exercise is able to induce a drastic degeneration/regeneration process in the soleus muscle. This offers the opportunity to perform biochemical and molecular analyses of a process of regeneration without muscle environment defects. The advantages of this model are discussed in the context of fundamental and therapeutical perspectives.

Follistatin and myostatin are two secreted proteins involved in the control of muscle mass during development. These two proteins have opposite effects on muscle growth, as documented by genetic models. The aims of this work were to analyze in mouse, by using in situ hybridization, the spatial and temporal expression patterns of follistatin and myostatin mRNAs during soleus regeneration after cardiotoxin injury, and to investigate the influence of innervation on the accumulation of these two transcripts. Follistatin transcripts could be detected in activated satellite cells as early as the first stages of regeneration and were transiently expressed in forming myotubes. In contrast, myostatin mRNAs accumulated persistently throughout the regeneration process as well as in adult control soleus. Denervation significantly affected both follistatin and myostatin transcript accumulation, but in opposite ways. Muscle denervation persistently reduced the levels of myostatin transcripts as early as the young myotube stage, whereas the levels of follistatin mRNA were strongly increased in the small myotubes in the late stages of regeneration. These results are discussed with regard to the potential functions of both follistatin, as a positive regulator of muscle differentiation, and myostatin, as a negative regulator of skeletal muscle growth. We suggest that the belated up-regulation of the follistatin mRNA level in the small myotubes of the regenerating soleus as well as the down-regulation of the myostatin transcript level after denervation contribute to the differentiation process in denervated regenerating muscle., (Copyright 2003 Wiley-Liss, Inc.)

Given the importance of the myogenic regulatory factors (MRFs) for myoblast differentiation during development, the aims of this work were to clarify the spatial and temporal expression pattern of the four MRF mRNAs during soleus regeneration in mouse after cardiotoxin injury, using in situ hybridization, and to investigate the influence of innervation on the expression of each MRF during a complete degeneration/regeneration process. For this, we performed cardiotoxin injury-induced regeneration experiments on denervated soleus muscle. Myf-5, MyoD, and MRF4 mRNAs were detected in satellite cell-derived myoblasts in the first stages of muscle regeneration analyzed (2--3 days P-I). The Myf-5 transcript level dramatically decreased in young multinucleated myotubes, whereas MyoD and MRF4 transcripts were expressed persistently throughout the regeneration process. Myogenin mRNA was transiently expressed in forming myotubes. These results are discussed with regard to the potential relationships between MyoD and MRF4 in the satellite cell differentiation pathway. Muscle denervation precociously (at 8 days P-I) upregulated both the Myf-5 and the MRF4 mRNA levels, whereas the increase of both MyoD and myogenin mRNA levels was observed later, in the late stages of regeneration (30 days P-I). This significant accumulation of each differentially upregulated MRF during soleus regeneration after denervation suggests that each myogenic factor might have a distinct role in the regulatory control of muscle gene expression. This role is discussed in relation to the expression of the nerve-regulated genes, such as the nAChR subunit gene family. (J Histochem Cytochem 49:887-899, 2001)

SPOCK is prevalent in developing synaptic fields of the central nervous system (Charbonnier et al., 2000. Mech. Dev. 90, 317-321). The expression of SPOCK during neuromuscular junction (NMJ) formation was compared to agrin and acetylcholine receptor (AChR) distribution. SPOCK is detected within the myogenic masses during the early steps of embryonic development, and distributed in the cytoplasm of myotubes before coclustering with AChRs. In the adult, SPOCK is present in axons and is highly expressed by Schwann cells. SPOCK altered expression pattern after nerve lesioning, or cholinergic transmission blockade, strongly indicate that its cellular distribution at the NMJ depends on innervation.