Calcineurin activity is required for the initiation of skeletal muscle differentiation

Mitochondria as a potential regulator of myogenesis

Recent studies have shown that mitochondria play a role in the regulation of myogenesis. Indeed, the abundance, morphology, and functional properties of mitochondria become altered when the myoblasts differentiate into myotubes. For example, mitochondrial mass/volume, mtDNA copy number, and mitochondrial respiration are markedly increased after the onset of myogenic differentiation. Besides, mitochondrial enzyme activity is also increased, suggesting that the metabolic shift from glycolysis to oxidative phosphorylation as the major energy source occurs during myogenic differentiation. Several lines of evidence suggest that impairment of mitochondrial function and activity blocks myogenic differentiation. However, yet little is known about the molecular mechanisms underlying the regulation of myogenesis by mitochondria. Understanding how mitochondria are involved in myogenesis will provide a valuable insight into the underlying mechanisms that regulate the maintenance of cellular homeostasis. Here, we will summarize the current knowledge regarding the role of mitochondria as a potential regulator of myogenesis.

Regulation of MEF2 transcriptional activity by calcineurin/mAKAP complexes

The calcium/calmodulin-dependent protein phosphatase calcineurin is required for the induction of transcriptional events that initiate and promote myogenic differentiation. An important effector for calcineurin in striated muscle is the transcription factor myocyte enhancer factor 2 (MEF2). The targeting of the enzyme and substrate to specific intracellular compartments by scaffold proteins often confers specificity in phosphatase activity. We now show that the scaffolding protein mAKAP organizes a calcineurin/MEF2 signaling complex in myocytes, regulating gene transcription. A calcineurin/mAKAP/MEF2 complex can be isolated from C2C12 cells and cardiac myocytes, and the calcineurin/MEF2 association is dependent on mAKAP expression. We have identified a peptide comprising the calcineurin binding domain in mAKAP that can disrupt the binding of the phosphatase to the scaffold in vivo. Dominant interference of calcineurin/mAKAP binding blunts the increase in MEF2 transcriptional activity seen during myoblast differentiation, as well as the expression of endogenous MEF2-target genes. Furthermore, disruption of calcineurin binding to mAKAP in cardiac myocytes inhibits adrenergic-induced cellular hypertrophy. Together these data illustrate the importance of calcineurin anchoring by the mAKAP scaffold for MEF2 regulation.

G9a mediates Sharp-1-dependent inhibition of skeletal muscle differentiation

Sharp-1, a basic helix-loop-helix transcription factor, is a potent repressor of skeletal muscle differentiation and is dysregulated in muscle pathologies. However, the mechanisms by which it inhibits myogenesis are not fully understood. Here we show that G9a, a lysine methyltransferase, is involved in Sharp-1-mediated inhibition of muscle differentiation. We demonstrate that G9a directly interacts with Sharp-1 and enhances its ability to transcriptionally repress the myogenin promoter. Concomitant with a differentiation block, G9a-dependent histone H3 lysine 9 dimethylation (H3K9me2) and MyoD methylation are apparent upon Sharp-1 overexpression in muscle cells. RNA interference-mediated reduction of G9a or pharmacological inhibition of its activity erases these repressive marks and rescues the differentiation defect imposed by Sharp-1. Our findings provide new insights into Sharp-1-dependent regulation of myogenesis and identify epigenetic mechanisms that could be targeted in myopathies characterized by elevated Sharp-1 levels.

Muscle development and obesity: Is there a relationship?

The formation of skeletal muscle from the epithelial somites involves a series of events triggered by temporally and spatially discrete signals resulting in the generation of muscle fibers which vary in their contractile and metabolic nature. The fiber type composition of muscles varies between individuals and it has now been found that there are differences in fiber type proportions between lean and obese animals and humans. Amongst the possible causes of obesity, it has been suggested that inappropriate prenatal environments may 'program' the fetus and may lead to increased risks for disease in adult life. The characteristics of muscle are both heritable and plastic, giving the tissue some ability to adapt to signals and stimuli both pre and postnatally. Given that muscle is a site of fatty acid oxidation and carbohydrate metabolism and that its development can be changed by prenatal events, it is interesting to examine the possible relationship between muscle development and the risk of obesity.

Ran and calcineurin can participate collaboratively in the regulation of spermatogenesis in scallop

Calcineurin is a calcium/calmodulin-dependent protein phosphatase that plays important roles in the transduction of calcium signals in a variety of tissues. In addition, calcineurin has been implicated in the process of spermatogenesis. A novel calcineurin-binding protein, CaNBP75, has been identified in scallop testis. The C-terminal region of CaNBP75 is homologous to the C-terminal region of RanBP3, a Ran-binding domain-containing protein. A small G protein Ran has been involved in spermiogenesis by virtue of the fact that its localization in spermatids changes during spermiogenesis. The current study was performed to investigate the functions of Ran and CaNBP75 in the regulation of calcineurin in testis to further understand the basic functions of calcineurin during spermatogenesis. First, cloning and sequencing of a scallop Ran cDNA isolated from testis revealed that scallop Ran is well-conserved at the amino acid level. Secondly, direct binding of Ran to CaNBP75 was demonstrated in an in vitro pull-down assay. Thirdly, analysis of the tissue distribution of Ran, CaNBP75, and calcineurin showed that these proteins are abundantly expressed in testis. Fourthly, comparison of the expression profiles of Ran and CaNBP75 with that of calcineurin in scallop testis during the maturation cycle revealed that Ran and CaNBP75 mRNA levels increase during meiosis and spermiogenesis, similar to calcineurin. Finally, co-immunoprecipitation analysis suggests that Ran, CaNBP75, and calcineurin interact in scallop testis during maturation. These results suggest that Ran, CaNBP75, and calcineurin may act in a coordinated manner to regulate spermatogenesis.

Mechanosensitive mechanisms in transcriptional regulation

Transcriptional regulation contributes to the maintenance of pluripotency, self-renewal and differentiation in embryonic cells and in stem cells. Therefore, control of gene expression at the level of transcription is crucial for embryonic development, as well as for organogenesis, functional adaptation, and regeneration in adult tissues and organs. In the past, most work has focused on how transcriptional regulation results from the complex interplay between chemical cues, adhesion signals, transcription factors and their co-regulators during development. However, chemical signaling alone is not sufficient to explain how three-dimensional (3D) tissues and organs are constructed and maintained through the spatiotemporal control of transcriptional activities. Accumulated evidence indicates that mechanical cues, which include physical forces (e.g. tension, compression or shear stress), alterations in extracellular matrix (ECM) mechanics and changes in cell shape, are transmitted to the nucleus directly or indirectly to orchestrate transcriptional activities that are crucial for embryogenesis and organogenesis. In this Commentary, we review how the mechanical control of gene transcription contributes to the maintenance of pluripotency, determination of cell fate, pattern formation and organogenesis, as well as how it is involved in the control of cell and tissue function throughout embryogenesis and adult life. A deeper understanding of these mechanosensitive transcriptional control mechanisms should lead to new approaches to tissue engineering and regenerative medicine.

Polymorphisms in Wnt signaling pathway genes are significantly associated with chicken carcass traits

The Wnt signaling pathway plays a crucial role during embryogenesis in vertebrates. In this study, 124 SNP in 31 Wnt signaling pathway genes were selected to genotype 764 individuals in an F(2) resource population by reciprocally crossing Silkie fowls and Cornish broilers, and 102 SNP were polymorphic. Pairwise linkage disequilibrium among the SNP within each gene was calculated. Haplotypes were reconstructed from the SNP in strong linkage disequilibrium. The associations of SNP and haplotypes with carcass traits were analyzed respectively, and the SNP contributions to phenotypic variance were estimated. The present study showed that 58 SNP in 24 genes and 8 haplotype blocks within 7 genes were significantly (P < 0.05) associated with at least one carcass trait. Fourteen SNP (among the 58 SNP) explained >2% phenotypic variance, 12 of which had significantly (P < 0.01) additive or dominant effects. Furthermore, both rs15865526 (Wnt9A) and rs14066777 (MAPK9) as well as their corresponding haplotype blocks were significantly associated with shank circumference and wing weight, respectively. In addition, 5 muscle-weight-related SNP explained >7% phenotypic variance, which was much higher than those of others. It was found that the Wnt signaling pathway was strongly associated with chicken carcass traits, and 7 genes were particularly important, namely RHOA and CHP for breast muscle weight, Wnt3A for breast muscle weight percentage over carcass weight, RAC1 for thigh weight percentage and thigh muscle weight percentage over carcass weight, Wnt11 for thigh weight percentage over carcass weight, Wnt9A for shank length, and MAPK9 for shank circumference. It is evident that Wnt signaling plays a major role in regulating carcass characteristics important for production traits in chickens.

Phosphorylation of Stim1 at serine 575 via netrin-2/Cdo-activated ERK1/2 is critical for the promyogenic function of Stim1

The promyogenic cell surface molecule Cdo is required for activation of extracellular signal-regulated kinase (ERK) and nuclear factor of activated T cells c3 (NFATc3) induced by netrin-2 in myogenic differentiation. However, the molecular mechanism leading to NFATc3 activation is unknown. Stromal interaction molecule 1 (Stim1), an internal calcium sensor of the endoplasmic reticulum store, promotes myogenesis via activation of NFATc3. In this study we investigated the functional interaction between Cdo and Stim1 in myogenic differentiation. Overexpression and depletion of Stim1 enhanced or decreased myotube formation, respectively. Of interest, Stim1 protein levels were decreased in Cdo-deficient perinatal hindlimb muscles or primary myoblasts; this correlates with defective NFATc3 activation in Cdo(-/-) myoblasts upon differentiation. Forced activation of NFATc3 by overexpression of calcineurin restored differentiation of Cdo-depleted C2C12 myoblasts. Furthermore, Cdo and Stim1 formed a complex in 293T cells or in differentiating C2C12 myoblasts. The netrin-2-mediated NFATc3 activation was coincident with robust interactions between Cdo and Stim1 in myoblasts and the ERK-mediated Stim1 phosphorylation at serine 575. The serine 575 phosphorylation was enhanced in C2C12 cells upon differentiation, and the alanine substitution of serine 575 failed to restore differentiation of Stim1-depleted myoblasts. Taken together, the results indicate that cell adhesion signaling triggered by netrin-2/Cdo induces Stim1 phosphorylation at serine 575 by ERK, which promotes myoblast differentiation.

Expression and association of TRPC1 with TRPC3 during skeletal myogenesis

INTRODUCTION

TRPC1 and TRPC3 proteins are widely expressed in skeletal muscles in forming calcium-permeable channels. Herein we characterize the expression pattern of TRPC transcripts during skeletal myogenesis in C2C12 myoblasts.

METHODS

We used polymerase chain reaction and Western blotting to detect expression levels, immunohistochemistry for subcellular localization, and co-immunoprecipitation techniques to assess interaction.

RESULTS

TRPC1 localizes to the cytoplasm and is enriched in the perinuclear region in undifferentiated myoblasts. Expression of TRPC1 increases significantly during myogenesis and resides mainly in differentiated myocytes and myotubes. TRPC3 is absent in undifferentiated myoblasts, is dramatically upregulated in differentiated culture, and is preferentially expressed in myotubes. Physical interaction of TRPC1-TRPC3 was observed, suggesting the possible existence of heteromers.

CONCLUSIONS

Expression of TRPC1 and TRPC3 is tightly regulated during myogensis. Evidence of TRPC1-TRPC3 interaction was first demonstrated in a muscle cell line. The functional consequences of this interaction remain to be established.

Lysine methyltransferase G9a methylates the transcription factor MyoD and regulates skeletal muscle differentiation

Skeletal muscle cells have served as a paradigm for understanding mechanisms leading to cellular differentiation. The proliferation and differentiation of muscle precursor cells require the concerted activity of myogenic regulatory factors including MyoD. In addition, chromatin modifiers mediate dynamic modifications of histone tails that are vital to reprogramming cells toward terminal differentiation. Here, we provide evidence for a unique dimension to epigenetic regulation of skeletal myogenesis. We demonstrate that the lysine methyltransferase G9a is dynamically expressed in myoblasts and impedes differentiation in a methyltransferase activity-dependent manner. In addition to mediating histone H3 lysine-9 di-methylation (H3K9me2) on MyoD target promoters, endogenous G9a interacts with MyoD in precursor cells and directly methylates it at lysine 104 (K104) to constrain its transcriptional activity. Mutation of K104 renders MyoD refractory to inhibition by G9a and enhances its myogenic activity. Interestingly, MyoD methylation is critical for G9a-mediated inhibition of myogenesis. These findings provide evidence of an unanticipated role for methyltransferases in cellular differentiation states by direct posttranslational modification of a transcription factor.

P43-dependent mitochondrial activity regulates myoblast differentiation and slow myosin isoform expression by control of Calcineurin expression

We have previously shown that mitochondrial protein synthesis regulates myoblast differentiation, partly through the control of c-Myc expression, a cellular oncogene regulating myogenin expression and myoblast withdrawal from the cell cycle. In this study we provide evidence of the involvement of Calcineurin in this regulation. In C2C12 myoblasts, inhibition of mitochondrial protein synthesis by chloramphenicol decreases Calcineurin expression. Conversely, stimulation of this process by overexpressing the T3 mitochondrial receptor (p43) increases Calcineurin expression. Moreover, expression of a constitutively active Calcineurin (ΔCN) stimulates myoblast differentiation, whereas a Calcineurin antisense has the opposite effect. Lastly, ΔCN expression or stimulation of mitochondrial protein synthesis specifically increases slow myosin heavy chain expression. In conclusion, these data clearly suggest that, partly via Calcineurin expression, mitochondrial protein synthesis is involved in muscle development through the control of myoblast differentiation and probably the acquisition of the contractile and metabolic phenotype of muscle fibres.

Skeletal myosin light chain kinase regulates skeletal myogenesis by phosphorylation of MEF2C

The MEF2 factors regulate transcription during cardiac and skeletal myogenesis. MEF2 factors establish skeletal muscle commitment by amplifying and synergizing with MyoD. While phosphorylation is known to regulate MEF2 function, lineage-specific regulation is unknown. Here, we show that phosphorylation of MEF2C on T(80) by skeletal myosin light chain kinase (skMLCK) enhances skeletal and not cardiac myogenesis. A phosphorylation-deficient MEF2C mutant (MEFT80A) enhanced cardiac, but not skeletal myogenesis in P19 stem cells. Further, MEFT80A was deficient in recruitment of p300 to skeletal but not cardiac muscle promoters. In gain-of-function studies, skMLCK upregulated myogenic regulatory factor (MRF) expression, leading to enhanced skeletal myogenesis in P19 cells and more efficient myogenic conversion. In loss-of-function studies, MLCK was essential for efficient MRF expression and subsequent myogenesis in embryonic stem (ES) and P19 cells as well as for proper activation of quiescent satellite cells. Thus, skMLCK regulates MRF expression by controlling the MEF2C-dependent recruitment of histone acetyltransferases to skeletal muscle promoters. This work identifies the first kinase that regulates MyoD and Myf5 expression in ES or satellite cells.

STIM1 as a key regulator for Ca2+ homeostasis in skeletal-muscle development and function

Stromal interaction molecules (STIM) were identified as the endoplasmic-reticulum (ER) Ca2+ sensor controlling store-operated Ca2+ entry (SOCE) and Ca2+-release-activated Ca2+ (CRAC) channels in non-excitable cells. STIM proteins target Orai1-3, tetrameric Ca2+-permeable channels in the plasma membrane. Structure-function analysis revealed the molecular determinants and the key steps in the activation process of Orai by STIM. Recently, STIM1 was found to be expressed at high levels in skeletal muscle controlling muscle function and properties. Novel STIM targets besides Orai channels are emerging.Here, we will focus on the role of STIM1 in skeletal-muscle structure, development and function. The molecular mechanism underpinning skeletal-muscle physiology points toward an essential role for STIM1-controlled SOCE to drive Ca2+/calcineurin/nuclear factor of activated T cells (NFAT)-dependent morphogenetic remodeling programs and to support adequate sarcoplasmic-reticulum (SR) Ca2+-store filling. Also in our hands, STIM1 is transiently up-regulated during the initial phase of in vitro myogenesis of C2C12 cells. The molecular targets of STIM1 in these cells likely involve Orai channels and canonical transient receptor potential (TRPC) channels TRPC1 and TRPC3. The fast kinetics of SOCE activation in skeletal muscle seem to depend on the triad-junction formation, favoring a pre-localization and/or pre-formation of STIM1-protein complexes with the plasma-membrane Ca2+-influx channels. Moreover, Orai1-mediated Ca2+ influx seems to be essential for controlling the resting Ca2+ concentration and for proper SR Ca2+ filling. Hence, Ca2+ influx through STIM1-dependent activation of SOCE from the T-tubule system may recycle extracellular Ca2+ losses during muscle stimulation, thereby maintaining proper filling of the SR Ca2+ stores and muscle function. Importantly, mouse models for dystrophic pathologies, like Duchenne muscular dystrophy, point towards an enhanced Ca2+ influx through Orai1 and/or TRPC channels, leading to Ca2+-dependent apoptosis and muscle degeneration. In addition, human myopathies have been associated with dysfunctional SOCE. Immunodeficient patients harboring loss-of-function Orai1 mutations develop myopathies, while patients suffering from Duchenne muscular dystrophy display alterations in their Ca2+-handling proteins, including STIM proteins. In any case, the molecular determinants responsible for SOCE in human skeletal muscle and for dysregulated SOCE in patients of muscular dystrophy require further examination.

Biphasic regulation of intracellular calcium by gemfibrozil contributes to inhibiting L6 myoblast differentiation: implications for clinical myotoxicity

Gemfibrozil is the most myotoxic fibrate drug commonly used for dyslipidemia, but the mechanism is poorly understood. The current study revealed that gemfibrozil inhibits myoblast differentiation through the regulation of intracellular calcium ([Ca(2+)]i) as revealed in L6 myoblasts by use of laser scan confocal microscopy and flow cytometry using Fluo-4 AM as a probe. Gemfibrozil at 20-400 μM, could regulate [Ca(2+)]i in L6 cells in a biphasic manner, and sustained reduction was observed when the concentration reached 200 μM. Inhibition of L6 differentiation by gemfibrozil was concentration-dependent with maximal effect noted between 200 and 400 μM, as indicated by creatine kinase activities and the differentiation index, respectively. In differentiating L6 myoblasts, gemfibrozil at concentrations below 400 μM led to no significant signs of apoptosis or cytotoxicity, whereas differentiation, inhibited by 200 μM gemfibrozil, was only partially recovered. A good correlation was noted between gemfibrozil concentrations that regulate [Ca(2+)]i and inhibit L6 myoblasts differentiation, and both are within the range of total serum concentrations found in the clinic. These data suggest a potential pharmacodynamic effect of gemfibrozil on myogenesis as a warning sign, in addition to the complex pharmacokinetic interactions. It is also noteworthy that mobilization of [Ca(2+)]i by gemfibrozil may trigger complex biological responses besides myocyte differentiation. Information revealed in this study explores the mechanism of gemfibrozil-induced myotoxicity through the regulation of intracellular calcium.

The role of store-operated calcium influx in skeletal muscle signaling

In cardiac and skeletal muscle Ca(2+) release from intracellular stores triggers actomyosin cross-bridge formation and the generation of contractile force. In the face of large fluctuations of intracellular calcium ([Ca(2+)](i)) that occur with contractile activity, myocytes are able to sense and respond to changes in workload and patterns of activation through calcium signaling pathways which modulate gene expression and cellular metabolism. Store-operated calcium influx has emerged as a mechanism by which calcium signaling pathways are activated in order to respond to the changing demands of the myocyte. Abnormalities of store-operated calcium influx may contribute to maladaptive muscle remodeling in multiple disease states. The importance of store-operated calcium influx in muscle is confirmed in mice lacking STIM1 which die perinatally and in patients with mutations on STIM1 or Orai1 who exhibit a myopathy exhibited by hypotonia. In this review, we consider the role of store-operated Ca(2+) entry into skeletal muscle as a critical mediator of Ca(2+) dependent gene expression and how alterations in Ca(2+) influx may influence muscle development and disease.

Debio-025 is more effective than prednisone in reducing muscular pathology in mdx mice

Muscular dystrophy results in the progressive wasting and necrosis of skeletal muscle. Glucocorticoids such as prednisone have emerged as a front-line treatment for many forms of this disease. Recently, Debio-025, a cyclophilin inhibitor that desensitizes the mitochondrial permeability pore and subsequent cellular necrosis, was shown to improve pathology in three different mouse models of muscular dystrophy. However it is not known if Debio-025 can work in conjunction with prednisone, or how it compares against prednisone in mitigating disease in dystrophic mouse models. Here we show that Debio-025 reduced the variations in myofiber cross-sectional areas, decreased fibrosis, and decreased infiltration of activated macrophages more efficiently than prednisone. However the use of prednisone and Debio-025 together had no additional effect on these histopathological indexes. Orally administered Debio-025 also reduced creatine kinase blood levels and improved grip strength in mdx mice after 6 weeks of treatment, and the combination of Debio-025 with prednisone increased muscle function slightly better than prednisone alone. Thus, our results suggest that Debio-025 is as, effective as or slightly better than, prednisone in mitigating muscular dystrophy in the mdx mouse model of disease.

Myoferlin regulation by NFAT in muscle injury, regeneration and repair

Ferlin proteins mediate membrane-fusion events in response to Ca(2+). Myoferlin, a member of the ferlin family, is required for normal muscle development, during which it mediates myoblast fusion. We isolated both damaged and intact myofibers from a mouse model of muscular dystrophy using laser-capture microdissection and found that the levels of myoferlin mRNA and protein were increased in damaged myofibers. To better define the components of the muscle-injury response, we identified a discreet 1543-bp fragment of the myoferlin promoter, containing multiple NFAT-binding sites, and found that this was sufficient to drive high-level myoferlin expression in cells and in vivo. This promoter recapitulated normal myoferlin expression in that it was downregulated in healthy myofibers and was upregulated in response to myofiber damage. Transgenic mice expressing GFP under the control of the myoferlin promoter were generated and GFP expression in this model was used to track muscle damage in vivo after muscle injury and in muscle disease. Myoferlin modulates the response to muscle injury through its activity in both myoblasts and mature myofibers.

The functional role of calcineurin in hypertrophy, regeneration, and disorders of skeletal muscle

Skeletal muscle uses calcium as a second messenger to respond and adapt to environmental stimuli. Elevations in intracellular calcium levels activate calcineurin, a serine/threonine phosphatase, resulting in the expression of a set of genes involved in the maintenance, growth, and remodeling of skeletal muscle. In this review, we discuss the effects of calcineurin activity on hypertrophy, regeneration, and disorders of skeletal muscle. Calcineurin is a potent regulator of muscle remodeling, enhancing the differentiation through upregulation of myogenin or MEF2A and downregulation of the Id1 family and myostatin. Foxo may also be a downstream candidate for a calcineurin signaling molecule during muscle regeneration. The strategy of controlling the amount of calcineurin may be effective for the treatment of muscular disorders such as DMD, UCMD, and LGMD. Activation of calcineurin produces muscular hypertrophy of the slow-twitch soleus muscle but not fast-twitch muscles.

Heat stress increases myonuclear number and fiber size via satellite cell activation in rat regenerating soleus fibers

To investigate the effects of heat stress (hyperthermia) on muscle degeneration-regeneration, the soleus muscles of adult male Wistar rats were injected bilaterally with a single injection of bupivacaine. The rats were assigned to a sedentary control (Con), heat stress (Heat), bupivacaine-injected (BPVC), or bupivacaine-injected plus heat stress (BPVC+Heat) group. Heat stress was induced in the Heat and BPVC+Heat groups by immersion of the lower half of the body into water maintained at 42 ± 1°C for 30 min 48 h after the injection of bupivacaine and every other day during the following 1 or 2 wk. The soleus muscles in all groups were excised 24 h after the final bout of heat stress. Mean muscle weight, fiber cross-sectional area, myonuclear number, and heat shock protein 72 (Hsp72) and calcineurin protein levels were lower in the BPVC than in the Con or Heat groups at both time points. In contrast, several of these parameters in the BPVC+Heat group were not different or higher than in the Con or Heat groups at the 1- and/or 2-wk time points. The number of total and activated satellite cells, estimated by analyses of Pax7-negative, M-cadherin-negative, and MyoD-positive nuclei, was greater in BPVC+Heat than in all other groups. Combined, the results indicate that heat stress-related activation of satellite cells and upregulation of Hsp72 and calcineurin expression played important roles in the regeneration of the soleus fibers after bupivacaine injection.

Powerful signals for weak muscles

The aim of the present review is to summarise, evaluate and critique the different mechanisms involved in anabolic growth of skeletal muscle and the catabolic processes involved in cancer cachexia and sarcopenia of ageing. This is highly relevant, since they represent targets for future promising clinical investigations. Sarcopenia is an inevitable process associated with a gradual reduction in muscle mass and strength, associated with a reduction in motor unit number and atrophy of muscle fibres, especially the fast type IIa fibres. The loss of muscle mass with ageing is clinically important because it leads to diminished functional ability and associated complications. Cachexia is widely recognised as severe and rapid wasting accompanying disease states such as cancer or immunodeficiency disease. One of the main characteristics of cancer cachexia is asthenia or lack of strength, which is directly related to the muscle loss. Indeed, apart from the speed of loss, muscle wasting during cancer and ageing share many common metabolic pathways and mediators. In healthy young individuals, muscles maintain their mass and function because of a balance between protein synthesis and protein degradation associated with rates of anabolic and catabolic processes, respectively. Muscles grow (hypertrophy) when protein synthesis exceeds protein degradation. Conversely, muscles shrink (atrophy) when protein degradation dominates. These processes are not occurring independently of each other, but are finely coordinated by a web of intricate signalling networks. Such signalling networks are in charge of executing environmental and cellular cues that ultimately determine whether muscle proteins are synthesised or degraded. Increasing our understanding for the pathways involved in hypertrophy and atrophy and particularly the interaction of these pathways is essential in designing therapeutic strategies for both prevention and treatment of muscle wasting conditions with age and with disease.

Ca2+/calmodulin-dependent transcriptional pathways: potential mediators of skeletal muscle growth and development

The loss of muscle mass with age and disuse has a significant impact on the physiological and social well-being of the aged; this is an increasingly important problem as the population becomes skewed towards older age. Exercise has psychological benefits but it also impacts on muscle protein synthesis and degradation, increasing muscle tissue volume in both young and older individuals. Skeletal muscle hypertrophy involves an increase in muscle mass and cross-sectional area and associated increased myofibrillar protein content. Attempts to understand the molecular mechanisms that underlie muscle growth, development and maintenance, have focused on characterising the molecular pathways that initiate, maintain and regenerate skeletal muscle. Such understanding may aid in improving targeted interventional therapies for age-related muscle loss and muscle wasting associated with diseases. Two major routes through which skeletal muscle development and growth are regulated are insulin-like growth factor I (IGF-I) and Ca(2+)/calmodulin-dependent transcriptional pathways. Many reviews have focused on understanding the signalling pathways of IGF-I and its receptor, which govern skeletal muscle hypertrophy. However, alternative molecular signalling pathways such as the Ca(2+)/calmodulin-dependent transcriptional pathways should also be considered as potential mediators of muscle growth. These latter pathways have received relatively little attention and the purpose herein is to highlight the progress being made in the understanding of these pathways and associated molecules: calmodulin, calmodulin kinases (CaMKs), calcineurin and nuclear factor of activated T-cell (NFAT), which are involved in skeletal muscle regulation. We describe: (1) how conformational changes in the Ca(2+) sensor calmodulin result in the exposure of binding pockets for the target proteins (CaMKs and calcineurin). (2) How Calmodulin consequently activates either the Ca(2+)/calmodulin-dependent kinases pathways (via CaMKs) or calmodulin-dependent serine/threonine phosphatases (via calcineurin). (3) How calmodulin kinases alter transcription in the nucleus through the phosphorylation, deactivation and translocation of histone deacetylase 4 (HDAC4) from the nucleus to the cytoplasm. (4) How calcineurin transmits signals to the nucleus through the dephosphorylation and translocation of NFAT from the cytoplasm to the nucleus.

Foxo3-/- mice demonstrate reduced numbers of pre-B and recirculating B cells but normal splenic B cell sub-population distribution

B cell antigen receptor (BCR) cross-linking promotes proliferation and survival of mature B cells. Phosphoinositide-3-kinase-mediated down-regulation of pro-apoptotic and anti-mitogenic genes such as the Foxo family of transcription factors is an important component of this process. Previously, we demonstrated that BCR signaling decreases expression of transcripts for Foxo1, Foxo3 and Foxo4. We now show that BCR-induced down-regulation of Foxo3 and Foxo4 mRNA expression occurs via distinct mechanisms from those established for Foxo1. While Foxo1, Foxo3 and Foxo4 bind the same DNA sequence, the differential control of their expression upon B cell activation suggests that they may have unique functions in the B lineage. To begin to address this issue, we evaluated B cell development and function in Foxo3-/- mice. No effect of Foxo3 deficiency was observed with respect to the following parameters in the splenic B cell compartment: sub-population distribution, proliferation, in vitro differentiation and expression of the Foxo target genes cyclin G2 and B cell translocation gene 1. However, Foxo3-/- mice demonstrated increased basal levels of IgG2a, IgG3 and IgA. A significant reduction in pre-B cell numbers was also observed in Foxo3-/- bone marrow. Finally, recirculating B cells in the bone marrow and peripheral blood were decreased in Foxo3-/- mice, perhaps due to lower than normal expression of receptor for sphingosine-1 phosphate, which mediates egress from lymphoid organs. Thus, Foxo3 makes a unique contribution to B cell development, B cell localization and control of Ig levels.

Identification of FHL1 as a regulator of skeletal muscle mass: implications for human myopathy

Regulators of skeletal muscle mass are of interest, given the morbidity and mortality of muscle atrophy and myopathy. Four-and-a-half LIM protein 1 (FHL1) is mutated in several human myopathies, including reducing-body myopathy (RBM). The normal function of FHL1 in muscle and how it causes myopathy remains unknown. We find that FHL1 transgenic expression in mouse skeletal muscle promotes hypertrophy and an oxidative fiber-type switch, leading to increased whole-body strength and fatigue resistance. Additionally, FHL1 overexpression enhances myoblast fusion, resulting in hypertrophic myotubes in C2C12 cells, (a phenotype rescued by calcineurin inhibition). In FHL1-RBM C2C12 cells, there are no hypertrophic myotubes. FHL1 binds with the calcineurin-regulated transcription factor NFATc1 (nuclear factor of activated T cells, cytoplasmic, calcineurin-dependent 1), enhancing NFATc1 transcriptional activity. Mutant RBM-FHL1 forms aggregate bodies in C2C12 cells, sequestering NFATc1 and resulting in reduced NFAT nuclear translocation and transcriptional activity. NFATc1 also colocalizes with mutant FHL1 to reducing bodies in RBM-afflicted skeletal muscle. Therefore, via NFATc1 signaling regulation, FHL1 appears to modulate muscle mass and strength enhancement.

Chapter 2. Calcineurin signaling and the slow oxidative skeletal muscle fiber type

Calcineurin, also known as protein phosphatase 2B (PP2B), is a calcium-calmodulin-dependent phosphatase. It couples intracellular calcium to dephosphorylate selected substrates resulting in diverse biological consequences depending on cell type. In mammals, calcineurin's functions include neuronal growth, development of cardiac valves and hypertrophy, activation of lymphocytes, and the regulation of ion channels and enzymes. This chapter focuses on the key roles of calcineurin in skeletal muscle differentiation, regeneration, and fiber type conversion to an oxidative state, all of which are crucial to muscle development, metabolism, and functional adaptations. It seeks to integrate the current knowledge of calcineurin signaling in skeletal muscle and its interactions with other prominent regulatory pathways and their signaling intermediates to form a molecular overview that could provide directions for possible future exploitations in human metabolic health.

Interleukin-1beta-dependent signaling between astrocytes and neurons depends critically on astrocytic calcineurin/NFAT activity

Interleukin-1beta (IL-1beta) and the Ca(2+)/calmodulin-dependent protein phosphatase, calcineurin, have each been shown to play an important role in neuroinflammation. However, whether these signaling molecules interact to coordinate immune/inflammatory processes and neurodegeneration has not been investigated. Here, we show that exogenous application of IL-1beta (10 ng/ml) recruited calcineurin/NFAT (nuclear factor of activated T cells) activation in primary astrocyte-enriched cultures within minutes, through a pathway involving IL-1 receptors and L-type Ca(2+) channels. Adenovirus-mediated delivery of the NFAT inhibitor, VIVIT, suppressed the IL-1beta-dependent induction of several inflammatory mediators and/or markers of astrocyte activation, including tumor necrosis factor alpha, granulocyte/macrophage colony-stimulating factor, and vimentin. Expression of an activated form of calcineurin in one set of astrocyte cultures also triggered the release of factors that, in turn, stimulated NFAT activity in a second set of "naive" astrocytes. This effect was prevented when calcineurin-expressing cultures co-expressed VIVIT, suggesting that the calcineurin/NFAT pathway coordinates positive feedback signaling between astrocytes. In the presence of astrocytes and neurons, 48-h delivery of IL-1beta was associated with several excitotoxic effects, including NMDA receptor-dependent neuronal death, elevated extracellular glutamate, and hyperexcitable synaptic activity. Each of these effects were reversed or ameliorated by targeted delivery of VIVIT to astrocytes. IL-1beta also caused an NFAT-dependent reduction in excitatory amino acid transporter levels, indicating a possible mechanism for IL-1beta-mediated excitotoxicity. Taken together, the results have potentially important implications for the propagation and maintenance of neuroinflammatory signaling processes associated with many neurodegenerative conditions and diseases.

Investigation of Debio 025, a cyclophilin inhibitor, in the dystrophic mdx mouse, a model for Duchenne muscular dystrophy

BACKGROUND AND PURPOSE

Duchenne muscular dystrophy (DMD) is a severe muscle wasting disorder caused by the absence of the cytoskeletal protein dystrophin. This leads to muscle cell death accompanied by chronic inflammation. Cyclosporin A (CsA) is a powerful immunosuppressive drug, which has been proposed for DMD treatment. CsA also directly regulates the mitochondrial permeability transition pore (mPTP), which participates in cell death pathways through the inhibition of cyclophilin D. Here, we evaluated whether Debio 025, a cyclophilin inhibitor with no immunosuppressive activity, improves the dystrophic condition in a mouse model of DMD, through regulation of mPTP.

EXPERIMENTAL APPROACH

The potency of Debio 025 to protect mouse dystrophic cells against mitochondria-mediated death was assessed by caspase-3 activity and calcium retention capacity assays. Mdx(5Cv) mice (3-week-old) were treated daily by gavage for 2 weeks with Debio 025 (10, 30 or 100 mg kg(-1)), CsA (10 mg kg(-1)) or placebo. The effects on muscle necrosis and function were measured.

KEY RESULTS

In vitro investigations showed protective effect of low concentrations of Debio 025 against cell death. Histology demonstrated that Debio 025 partially protected the diaphragm and soleus muscles against necrosis (10 and 100 mg kg(-1), respectively). Hindlimb muscles from mice receiving Debio 025 at 10 mg kg(-1) relaxed faster, showed alteration in the stimulation frequency-dependent recruitment of muscle fibres and displayed a higher resistance to mechanical stress.

CONCLUSIONS AND IMPLICATIONS

Debio 025 partially improved the structure and the function of the dystrophic mouse muscle, suggesting that therapies targeting the mPTP may be helpful to DMD patients.

Cyclosporin A modulates cellular localization of MEF2C protein and blocks fiber hypertrophy in the overloaded soleus muscle of mice

The molecular signaling pathway linked to hypertrophy of the anti-gravity/postural soleus muscle after mechanical overloading has not been identified. Using reverse transcription-polymerase chain reaction (RT-PCR), Western blot, and immunohistochemical analyses, we investigated whether the amounts of myocyte enhancer factor (MEF)2C, MEF2D, and myogenin change in the mechanically overloaded soleus muscle after treatment with the calcineurin inhibitor cyclosporine A (CsA). Adult male ICR mice were subjected to a surgical ablation of the gastrocnemius muscle and treated with either CsA (25 mg/kg) or vehicle, once daily. They were killed at 2, 4, 7, 10, and 14 days post-injury. Mechanical overloading resulted in a significant increase in the wet weight and the cross-sectional area of slow and fast fibers of the soleus muscle in placebo-treated mice but not CsA-treated mice. RT-PCR analysis did not show a marked difference in MEF2C and MEF2D mRNA levels in the overloaded soleus muscle in placebo- or CsA-administered mice. After 2 days of mechanical overloading, we observed co-localization of MEF2C and myogenin in several mononuclear cells under both conditions. These MEF2C-positive mononuclear cells also possessed immunoreactivity for c-Met, a satellite cell marker. At 4 days, mechanical overloading induced marked expression of MEF2C but not MEF2D in the subsarcolemmal region in a group of myotubes and/or myofibers. Such a MEF2C-positive region emerged less often in the hypertrophied soleus muscle subjected to the treatment with CsA. At 7 days, we observed many mononuclear cells possessing both MEF2C and myogenin protein in mice treated with CsA, but not the placebo. Our results demonstrated that CsA treatment modulates the amount and cellular localization of MEF2C protein. The modulation of MEF2C by CsA treatment may inhibit the hypertrophic process in the soleus muscle after mechanical overloading.

Hematopoietic cell transplantation provides an immune-tolerant platform for myoblast transplantation in dystrophic dogs

Duchenne Muscular Dystrophy (DMD) is the most common and severe form of muscular dystrophy in humans. The goal of myogenic stem cell transplant therapy for DMD is to increase dystrophin expression in existing muscle fibers and to provide a source of stem cells for future muscle generation. Although syngeneic myogenic stem cell transplants have been successful in mice, allogeneic transplants of myogenic stem cells were ineffective in several human trials. To determine whether allogeneic muscle progenitor cells can be successfully transplanted in an immune-tolerant recipient, we induced immune tolerance in two DMD-affected (cxmd) dogs through hematopoietic cell transplantation (HCT). Injection of freshly isolated muscle-derived cells from the HCT donor into either fully or partially chimeric xmd recipients restored dystrophin expression up to 6.48% of wild-type levels, reduced the number of centrally located nuclei, and improved muscle structure. Dystrophin expression was maintained for at least 24 weeks. Taken together, these data indicate that immune tolerance to donor myoblasts provides an important platform from which to further improve myoblast transplantation, with the goal of restoring dystrophin expression to patients with DMD.

STIM1 signalling controls store-operated calcium entry required for development and contractile function in skeletal muscle

It is now well established that stromal interaction molecule 1 (STIM1) is the calcium sensor of endoplasmic reticulum stores required to activate store-operated calcium entry (SOC) channels at the surface of non-excitable cells. However, little is known about STIM1 in excitable cells, such as striated muscle, where the complement of calcium regulatory molecules is rather disparate from that of non-excitable cells. Here, we show that STIM1 is expressed in both myotubes and adult skeletal muscle. Myotubes lacking functional STIM1 fail to show SOC and fatigue rapidly. Moreover, mice lacking functional STIM1 die perinatally from a skeletal myopathy. In addition, STIM1 haploinsufficiency confers a contractile defect only under conditions where rapid refilling of stores would be needed. These findings provide insight into the role of STIM1 in skeletal muscle and suggest that STIM1 has a universal role as an ER/SR calcium sensor in both excitable and non-excitable cells.

Myocyte enhancer factor 2A is transcriptionally autoregulated

MEF2 (myocyte enhancer factor 2) proteins are a small family of transcription factors that play pivotal roles in striated muscle differentiation, development, and metabolism, in neuron survival and synaptic formation, and in lymphocyte selection and activation. Products of the four mammalian MEF2 genes, MEF2A, MEF2B, MEF2C, and MEF2D, are expressed with overlapping but distinct temporospatial patterns. Toward analysis of MEF2A functions and the determinants of its regulated expression, we have mapped and begun studies of the transcriptional control regions of this gene. Heterogeneous 5'-untranslated regions of MEF2A mRNAs result from use of alternative promoters and splicing patterns. The two closely approximated TATA-less promoters are approximately 65 kb upstream of the exon containing the sole initiation codon. Ribonuclease protection and primer extension assays show that each promoter is active in various adult tissues. A canonical MEF2 site overlies the major promoter 1 transcription start site. This element specifically binds MEF2 factors, including endogenous nuclear MEF2A according to chromatin immunoprecipitation studies, and is critical to MEF2A transcription in myocytes. The site exerts reciprocal control of the alternative promoters, silencing promoter 1 and activating promoter 2 under some conditions. Erk5 and p38 MAPK signaling stimulate MEF2A expression by activating both promoters from the MEF2 element. MEF2A transcription is therefore subject to positive or negative regulation by its protein products, depending on signaling activities that influence MEF2 factor trans-activity. The sole MEF2 gene of the cephalochordate amphioxus has a similar regulatory region structure, suggesting that this mode of autoregulatory control is conserved among higher metazoan MEF2 genes.

Genetic and pharmacologic inhibition of mitochondrial-dependent necrosis attenuates muscular dystrophy

Muscular dystrophies comprise a diverse group of genetic disorders that lead to muscle wasting and, in many instances, premature death. Many mutations that cause muscular dystrophy compromise the support network that connects myofilament proteins within the cell to the basal lamina outside the cell, rendering the sarcolemma more permeable or leaky. Here we show that deletion of the gene encoding cyclophilin D (Ppif) rendered mitochondria largely insensitive to the calcium overload-induced swelling associated with a defective sarcolemma, thus reducing myofiber necrosis in two distinct models of muscular dystrophy. Mice lacking delta-sarcoglycan (Scgd(-/-) mice) showed markedly less dystrophic disease in both skeletal muscle and heart in the absence of Ppif. Moreover, the premature lethality associated with deletion of Lama2, encoding the alpha-2 chain of laminin-2, was rescued, as were other indices of dystrophic disease. Treatment with the cyclophilin inhibitor Debio-025 similarly reduced mitochondrial swelling and necrotic disease manifestations in mdx mice, a model of Duchenne muscular dystrophy, and in Scgd(-/-) mice. Thus, mitochondrial-dependent necrosis represents a prominent disease mechanism in muscular dystrophy, suggesting that inhibition of cyclophilin D could provide a new pharmacologic treatment strategy for these diseases.

alpha-Dystrobrevin isoforms differ in their colocalization with and stabilization of agrin-induced acetylcholine receptor clusters

In skeletal muscle, alpha-dystrobrevin (alphaDB) is expressed throughout the sarcolemma with high concentrations at the neuromuscular junction. Mice lacking alphaDB display a mild muscular dystrophy and perturbations at the neuromuscular junction that include disruptions to acetylcholine receptor (AChR) cluster stability and patterning. In adult skeletal muscle, three alternatively spliced isoforms (alphaDB1, alphaDB2, alphaDB3) are expressed, while two other splice variants (alphaDB1(-) and alphaDB2(-)) are expressed only during early development. alphaDB is clearly important in AChR stabilization; however, the degree to which individual alphaDB isoforms and their specific functional domains contribute to AChR cluster stability is not fully understood. To investigate this, we established a primary muscle cell culture system from alphaDB knockout mice and stably expressed individual alphaDB isoforms using retroviral infection. A comparison between wild-type and alphaDB knockout muscle cells showed that in the absence of alphaDB, fewer AChR clusters formed in response to agrin treatment, and these AChR clusters were very unstable. Retroviral expression studies revealed that the largest isoforms (alphaDB1, alphaDB1(-), alphaDB2, alphaDB2(-)) colocalized with agrin-induced AChR clusters and rescued the AChR cluster formation defects back to wild-type levels, while only the first three isoforms fully rescued AChR cluster stability back to wild-type levels. alphaDB2(-) conferred an intermediate level of stability to the AChR clusters. In contrast, alphaDB3 showed no specific colocalization with AChR clusters and little effect on AChR cluster formation or stabilization. Twice as much syntrophin was found associated with alphaDB2 compared with alphaDB2(-) in myotubes suggesting that increased recruitment of syntrophin by alphaDB may enhance the stability of AChR clusters. Taken together, these data demonstrate that different alphaDB isoforms have different functional capabilities in the formation and maintenance of AChR clusters in muscle cells, and that these differences are likely due to the presence of different functional domains in each isoform.

A naturally occurring calcineurin variant inhibits FoxO activity and enhances skeletal muscle regeneration

The calcium-activated phosphatase calcineurin (Cn) transduces physiological signals through intracellular pathways to influence the expression of specific genes. Here, we characterize a naturally occurring splicing variant of the CnAβ catalytic subunit (CnAβ1) in which the autoinhibitory domain that controls enzyme activation is replaced with a unique C-terminal region. The CnAβ1 enzyme is constitutively active and dephosphorylates its NFAT target in a cyclosporine-resistant manner. CnAβ1 is highly expressed in proliferating myoblasts and regenerating skeletal muscle fibers. In myoblasts, CnAβ1 knockdown activates FoxO-regulated genes, reduces proliferation, and induces myoblast differentiation. Conversely, CnAβ1 overexpression inhibits FoxO and prevents myotube atrophy. Supplemental CnAβ1 transgene expression in skeletal muscle leads to enhanced regeneration, reduced scar formation, and accelerated resolution of inflammation. This unique mode of action distinguishes the CnAβ1 isoform as a candidate for interventional strategies in muscle wasting treatment.

Molecular control of mammalian myoblast fusion

The fusion of postmitotic mononucleated myoblasts to form syncytial myofibers is a critical step in the formation of skeletal muscle. Myoblast fusion occurs both during development and throughout adulthood, as skeletal muscle growth and regeneration require the accumulation of additional nuclei within myofibers. Myoblasts must undergo a complex series of molecular and morphological changes prior to fusing with one another. Although many molecules regulating myoblast fusion have been identified, the precise mechanism by which these molecules act in concert to control fusion remains to be elucidated. A comprehensive understanding of how myo-blast fusion is controlled may contribute to the treatment of various disorders associated with loss of muscle mass. In this chapter, we examine progress made toward elucidating the cellular and molecular pathways involved in mammalian myoblast fusion. Special emphasis is placed on the molecules that regulate myofiber formation without discernibly affecting biochemical differentiation.

Muscle-specific overexpression of the type 1 IGF receptor results in myoblast-independent muscle hypertrophy via PI3K, and not calcineurin, signaling

The insulin-like growth factors (IGF-I and IGF-II), working through the type 1 IGF receptor (IGF-1R), are key mediators of skeletal muscle fiber growth and hypertrophy. These processes are largely dependent on stimulation of proliferation and differentiation of muscle precursor cells, termed myoblasts. It has not been rigorously determined whether the IGFs can also mediate skeletal muscle hypertrophy in a myoblast-independent fashion. Similarly, although the phosphatidylinositol 3-kinase (PI3K) and calcineurin signaling pathways have been implicated in skeletal muscle hypertrophy, these pathways are also involved in skeletal myoblast differentiation. To determine whether the IGFs can stimulate skeletal muscle hypertrophy in a myoblast-independent fashion, we developed and validated a retroviral expression vector that mediated overexpression of the human IGF-1R in rat L6 skeletal myotubes (immature muscle fibers), but not in myoblasts. L6 myotubes transduced with this vector accumulated significantly higher amounts of myofibrillar proteins, in a ligand- and receptor-dependent manner, than controls and demonstrated significantly increased rates of protein synthesis. Stimulation of myotube hypertrophy was independent of myoblast contributions, inasmuch as these cultures did not exhibit increased levels of myoblast proliferation or differentiation. Experiments with PI3K and calcineurin inhibitors indicated that myoblast-independent myotube hypertrophy was mediated by PI3K, but not calcineurin, signaling. This study demonstrates that IGF can mediate skeletal muscle hypertrophy in a myoblast-independent fashion and suggests that muscle-specific overexpression of the IGF-1R or stimulation of its signaling pathways could be used to develop strategies to ameliorate muscle wasting without stimulating proliferative pathways leading to carcinogenesis or other pathological sequelae.

[Cellular regulation of anabolism and catabolism in skeletal muscle during immobilisation, aging and critical illness]

Skeletal muscle atrophy is associated with situations of acute and chronical illness, such as sepsis, surgery, trauma and immobility. Additionally, it is a common problem during the physiological process of aging. The myofibrillar proteins myosin and actin, which are essential for muscle contraction, are the major targets during the process of protein degradation. This leads to a general loss of muscle mass, muscle strength and to increased muscle fatigue. In critically ill or immobile patients skeletal muscle atrophy is accompanied by enhanced inflammation, reduced wound healing, weaning complications and difficulties in mobilisation. During aging it results in falls, fractures, physical injuries and loss of mobility. Relating to the primary stimulators - hormones, muscle lengthening, stress, inflammation, neuronal activity - research is now focusing on the investigation of the signal transduction pathways, which influence protein synthesis and protein degradation during skeletal muscle atrophy.

The Ca(V) 1.2 Ca(2+) channel is expressed in sarcolemma of type I and IIa myofibers of adult skeletal muscle

Although Ca(2+)-dependent signaling pathways are important for skeletal muscle plasticity, the sources of Ca(2+) that activate these signaling pathways are not completely understood. Influx of Ca(2+) through surface membrane Ca(2+) channels may activate these pathways. We examined expression of two L-type Ca(2+) channels in adult skeletal muscle, the Ca(V) 1.1 and Ca(V) 1.2, with isoform-specific antibodies in Western blots and immunocytochemistry assays. Consistent with a large body of work, expression of the Ca(V) 1.1 was restricted to skeletal muscle where it was expressed in T-tubules. Ca(V) 1.2 was also expressed in skeletal muscle, in the sarcolemma of type I and IIa myofibers. Exercise-induced alterations in muscle fiber types cause a concomitant increase in the number of both Ca(V) 1.2 and type IIa-positive fibers. Taken together, these data suggest that the Ca(V) 1.2 Ca(2+) channel is expressed in adult skeletal muscle in a fiber type-specific manner, which may help to maintain oxidative muscle phenotype.

JAK1-STAT1-STAT3, a key pathway promoting proliferation and preventing premature differentiation of myoblasts

Skeletal muscle stem cell-derived myoblasts are mainly responsible for postnatal muscle growth and injury-induced muscle regeneration. However, the cellular signaling pathways controlling the proliferation and differentiation of myoblasts are not fully understood. We demonstrate that Janus kinase 1 (JAK1) is required for myoblast proliferation and that it also functions as a checkpoint to prevent myoblasts from premature differentiation. Deliberate knockdown of JAK1 in both primary and immortalized myoblasts induces precocious myogenic differentiation with a concomitant reduction in cell proliferation. This is caused, in part, by an accelerated induction of MyoD, myocyte enhancer-binding factor 2 (MEF2), p21Cip1, and p27Kip1, a faster down-regulation of Id1, and an increase in MEF2-dependent gene transcription. Downstream of JAK1, of all the signal transducer and activator of transcriptions (STATs) present in myoblasts, we find that only STAT1 knockdown promotes myogenic differentiation in both primary and immortalized myoblasts. Leukemia inhibitory factor stimulates myoblast proliferation and represses differentiation via JAK1-STAT1-STAT3. Thus, JAK1-STAT1-STAT3 constitutes a signaling pathway that promotes myoblast proliferation and prevents premature myoblast differentiation.

Bex1 knock out mice show altered skeletal muscle regeneration

Bex1 and Calmodulin (CaM) are upregulated during skeletal muscle regeneration. We confirm this finding and demonstrate the novel finding that they interact in a calcium-dependent manner. To study the role of Bex1 and its interaction with CaM in skeletal muscle regeneration, we generated Bex1 knock out (Bex1-KO) mice. These mice appeared to develop normally and are fertile, but displayed a functional deficit in exercise performance compared to wild type (WT) mice. After intramuscular injection of cardiotoxin, which causes extensive and reproducible myotrauma followed by recovery, regenerating muscles of Bex1-KO mice exhibited elevated and prolonged cell proliferation, as well as delayed cell differentiation, compared to WT mice. Thus, our results provide the first evidence that Bex1-KO mice show altered muscle regeneration, and allow us to propose that the interaction of Bex1 with Ca(2+)/CaM may be involved in skeletal muscle regeneration.

A new calcineurin inhibition domain in Cabin1

Calcineurin (CN), a calcium-activated phosphatase, plays a critical role in various biological processes including T cell activation. Cabin1, a calcineurin binding protein 1, has been shown to bind directly to CN using its C-terminal region and inhibit CN activity. However, no increase in CN activity has been found in Cabin1DeltaC T cells, which produce a truncated Cabin1 lacking the C-terminal CN binding region. Here, we report that Cabin1 has additional CN binding domain in its 701-900 amino acid residues. Cabin1 (701-900) blocked both CN-mediated dephosphorylation and nuclear import of NFAT and thus inhibited IL-2 production in response to PMA/ionomycin stimulation. This fact may explain why Cabin1DeltaC mice previously showed no significant defect in CN-mediated signaling pathway.

Calcineurin differentially regulates fast myosin heavy chain genes in oxidative muscle fibre type conversion

In skeletal muscle, calcineurin is crucial for myocyte differentiation and in the determination of the slow oxidative fibre phenotype, both processes being important determinants of muscle performance, metabolic health and meat-animal production. Fibre type is defined by the isoform identity of the skeletal myosin heavy chain (MyHC). We have examined the responses of the major MyHC genes to calcineurin signalling during fibre formation of muscle C2C12 cells. We have found that calcineurin acts as a signal to up-regulate the fast-oxidative MyHC2a gene and to down-regulate the faster MyHC2x and MyHC2b genes in a manner that appears to be NFAT-independent. Contrary to expectation, the up-regulation of MyHCslow by calcineurin seems to be time-dependent and is only detectable once the initial differential expression of the post-natal fast MyHC genes has been established. The simultaneous elevated expression of MyHC2a and the repression of MyHC2x and MyHC2b expression indicate that both processes (elevation and repression) are actively coordinated during oxidative fibre conversion. We have further determined that muscle LIM protein (MLP), a calcineurin-binding Z-line co-factor, is induced by calcineurin and that its co-expression with calcineurin has an additive effect on MyHCslow expression. Hence, post-natal fast MyHCs are important early effector targets of calcineurin, whereas MyHCslow up-regulation is mediated in part by calcineurin-induced MLP.

Identification of a new hybrid serum response factor and myocyte enhancer factor 2-binding element in MyoD enhancer required for MyoD expression during myogenesis

MyoD is a critical myogenic factor induced rapidly upon activation of quiescent satellite cells, and required for their differentiation during muscle regeneration. One of the two enhancers of MyoD, the distal regulatory region, is essential for MyoD expression in postnatal muscle. This enhancer contains a functional divergent serum response factor (SRF)-binding CArG element required for MyoD expression during myoblast growth and muscle regeneration in vivo. Electrophoretic mobility shift assay, chromatin immunoprecipitation, and microinjection analyses show this element is a hybrid SRF- and MEF2 Binding (SMB) sequence where myocyte enhancer factor 2 (MEF2) complexes can compete out binding of SRF at the onset of differentiation. As cells differentiate into postmitotic myotubes, MyoD expression no longer requires SRF but instead MEF2 binding to this dual-specificity element. As such, the MyoD enhancer SMB element is the site for a molecular relay where MyoD expression is first initiated in activated satellite cells in an SRF-dependent manner and then increased and maintained by MEF2 binding in differentiated myotubes. Therefore, SMB is a DNA element with dual and stage-specific binding activity, which modulates the effects of regulatory proteins critical in controlling the balance between proliferation and differentiation.

Calcineurin-A alpha activation enhances the structure and function of regenerating muscles after myotoxic injury

Calcineurin signaling is essential for successful muscle regeneration. Although calcineurin inhibition compromises muscle repair, it is not known whether calcineurin activation can enhance muscle repair after injury. Tibialis anterior (TA) muscles from adult wild-type (WT) and transgenic mice overexpressing the constitutively active calcineurin-A alpha transgene under the control of the mitochondrial creatine kinase promoter (MCK-CnA alpha*) were injected with the myotoxic snake venom Notexin to destroy all muscle fibers. The TA muscle of the contralateral limb served as the uninjured control. Muscle structure was assessed at 5 and 9 days postinjury, and muscle function was tested in situ at 9 days postinjury. Calcineurin stimulation enhanced muscle regeneration and altered levels of myoregulatory factors (MRFs). Recovery of myofiber size and force-producing capacity was hastened in injured muscles of MCK-CnA alpha* mice compared with control. Myogenin levels were greater 5 days postinjury and myocyte enhancer factor 2a (MEF2a) expression was greater 9 days postinjury in muscles of MCK-CnA alpha* mice compared with WT mice. Higher MEF2a expression in regenerating muscles of MCK-CnA alpha* mice 9 days postinjury may be related to an increase of slow fiber genes. Calcineurin activation in uninjured and injured TA muscles slowed muscle contractile properties, reduced fatigability, and enhanced force recovery after 4 min of intermittent maximal stimulation. Therefore, calcineurin activation can confer structural and functional benefits to regenerating skeletal muscles, which may be mediated in part by differential expression of MRFs.

Developmental regulation of the mouse IGF-I exon 1 promoter region by calcineurin activation of NFAT in skeletal muscle

Skeletal muscle development and growth are regulated through multiple signaling pathways that include insulin-like growth factor I (IGF-I) and calcineurin activation of nuclear factor of activated T cell (NFAT) transcription factors. The developmental regulation and molecular mechanisms that control IGF-I gene expression in murine embryos and in differentiating C2C12 skeletal myocytes were examined. IGF-I is expressed in developing skeletal muscle, and its embryonic expression is significantly reduced in embryos lacking both NFATc3 and NFATc4. During development, the IGF-I exon 1 promoter is active in multiple organ systems, including skeletal muscle, whereas the alternative exon 2 promoter is expressed predominantly in the liver. The IGF-I exon 1 promoter flanking sequence includes two highly conserved regions that contain NFAT consensus binding sequences. One of these conserved regions contains a calcineurin/NFAT-responsive regulatory region that is preferentially activated by NFATc3 in C2C12 skeletal muscle cells and NIH3T3 fibroblasts. This NFAT-responsive region contains three clustered NFAT consensus binding sequences, and mutagenesis experiments demonstrated the requirement for two of these in calcineurin or NFATc3 responsiveness. Chromatin immunoprecipitation analyses demonstrated that endogenous IGF-I genomic sequences containing these conserved NFAT binding sequences interact preferentially with NFATc3 in C2C12 cells. Together, these experiments demonstrated that a NFAT-rich regulatory element in the IGF-I exon 1 promoter flanking region is responsive to calcineurin signaling and NFAT activation in skeletal muscle cells. The identification of a calcineurin/NFAT-responsive element in the IGF-I gene represents a potential mechanism of intersection of these signaling pathways in the control of muscle development and homeostasis.