Signaling Pathways in Skeletal Muscle Remodeling

Thyroid hormone is required for the phenotype transitions induced by the pharmacological inhibition of calcineurin in adult soleus muscle of rats

The present experiment was designed to examine the effects of hypothyroidism and calcineurin inhibition induced by cyclosporin A (CsA) administration on both contractile and metabolic soleus muscle phenotypes, with a novel approach to the signaling pathway controlling mitochondrial biogenesis. Twenty-eight rats were randomly assigned to four groups, normothyroid, hypothyroid, and orally treated with either CsA (25 mg/kg, N-CsA and H-CsA) or vehicle (N-Vh and H-Vh), for 3 wk. Muscle phenotype was estimated by the MHC profile and activities of oxidative and glycolytic enzymes. We measured mRNA levels of the peroxisome proliferator-activated receptor-gamma coactivator-1 alpha (PGC-1 alpha), the major regulator of mitochondrial content. We also studied the expression of the catalytic A-subunit of calcineurin (CnA) both at protein and transcript levels and mRNA levels of modulatory calcineurin inhibitor proteins (MCIP)-1 and -2, which are differentially regulated by calcineurin activity and thyroid hormone, respectively. CsA-administration induced a slow-to-fast MHC transition limited to the type IIA isoform, which is associated with increased oxidative capacities. Hypothyroidism strongly decreased both the expression of fast MHC isoforms and oxidative capacities. Effects of CsA administration on muscle phenotype were blocked in conditions of thyroid hormone deficiency. Changes in the oxidative profile were strongly related to PGC-1 alpha changes and associated with phosphorylation of p38 MAPK. Calcineurin and MCIPs mRNA levels were decreased by both hypothyroidism and CsA without additive effects. Taken together, these results suggest that adult muscle phenotype is primarily under the control of thyroid state. Physiological levels of thyroid hormone are required for the effects of calcineurin inhibition on slow oxidative muscle phenotype.

Cooperative control of striated muscle mass and metabolism by MuRF1 and MuRF2

The muscle-specific RING finger proteins MuRF1 and MuRF2 have been proposed to regulate protein degradation and gene expression in muscle tissues. We have tested the in vivo roles of MuRF1 and MuRF2 for muscle metabolism by using knockout (KO) mouse models. Single MuRF1 and MuRF2 KO mice are healthy and have normal muscles. Double knockout (dKO) mice obtained by the inactivation of all four MuRF1 and MuRF2 alleles developed extreme cardiac and milder skeletal muscle hypertrophy. Muscle hypertrophy in dKO mice was maintained throughout the murine life span and was associated with chronically activated muscle protein synthesis. During ageing (months 4–18), skeletal muscle mass remained stable, whereas body fat content did not increase in dKO mice as compared with wild-type controls. Other catabolic factors such as MAFbox/atrogin1 were expressed at normal levels and did not respond to or prevent muscle hypertrophy in dKO mice. Thus, combined inhibition of MuRF1/MuRF2 could provide a potent strategy to stimulate striated muscles anabolically and to protect muscles from sarcopenia during ageing.

Insights into function and regulation of small heat shock protein 25 (HSPB1) in a mouse model with targeted gene disruption

The mammalian small heat shock protein (sHSPs) family is comprised of 10 members and includes HSPB1, which is proposed to play an essential role in cellular physiology, acting as a molecular chaperone to regulate diverse cellular processes. Whilst differential roles for sHSPs are suggested for specific tissues, the relative contribution of individual sHSP family members in cellular and organ physiology remains unclear. To address the function of HSPB1 in vivo and determine its tissue-specific expression during development and in the adult, we generated knock-in mice where the coding sequence of hspb1 is replaced by a lacZ reporter gene. Hspb1 expression marks myogenic differentiation with specific expression first confined to developing cardiac muscles and the vascular system, and later in skeletal muscles with specific expression at advanced stages of myoblast differentiation. In the adult, hspb1 expression was observed in other tissues, such as stratified squamous epithelium of skin, oronasal cavity, tongue, esophagus, and uterine cervix but its expression was most prominent in the musculature. Interestingly, in cardiac muscle hsbp1 expression was down-regulated during the neonatal period and maintained to a relatively low steady-level throughout adulthood. Despite this widespread expression, hspb1-/- mice were viable and fertile with no apparent morphological abnormalities in tissues under physiological conditions. However, at the cellular level and under stress conditions (heat challenge), HSPB1 act synergistically with the stress-induced HSPA1 (HSP70) in thermotolerance development, protecting cells from apoptosis. Our data thus indicate a nonessential role for HSPB1 in embryonic development and for maintenance of tissues under physiological conditions, but also shows that it plays an important role by acting synergistically with other HSPs during stress conditions to exert cytoprotection and anti-apoptotic effects.

Role of nuclear receptor coregulators in metabolism

Understanding the molecular regulation of metabolism will lead to a better understanding of the pathogenesis and treatment of common metabolic conditions, including obesity and diabetes. Nuclear receptors are a family of transcription factors, many of which play major roles in regulating metabolic genes in key tissues. They function by recruiting coregulators to the promoters of metabolic genes that can either activate or repress transcription. This review examines the roles of these coregulators in the control of metabolism in adipose tissue and skeletal muscle, and discusses how they result in coordinated and regulated control of metabolic pathways. In particular, the ligand-dependent recruitment of both coactivators and corepressors has potential implications for the treatment of obesity and diabetes.

A Foxo/Notch pathway controls myogenic differentiation and fiber type specification

Forkhead box O (Foxo) transcription factors govern metabolism and cellular differentiation. Unlike Foxo-dependent metabolic pathways and target genes, the mechanisms by which these proteins regulate differentiation have not been explored. Activation of Notch signaling mimics the effects of Foxo gain of function on cellular differentiation. Using muscle differentiation as a model system, we show that Foxo physically and functionally interacts with Notch by promoting corepressor clearance from the Notch effector Csl, leading to activation of Notch target genes. Inhibition of myoblast differentiation by constitutively active Foxo1 is partly rescued by inhibition of Notch signaling while Foxo1 loss of function precludes Notch inhibition of myogenesis and increases myogenic determination gene (MyoD) expression. Accordingly, conditional Foxo1 ablation in skeletal muscle results in increased formation of MyoD-containing (fast-twitch) muscle fibers and altered fiber type distribution at the expense of myogenin-containing (slow-twitch) fibers. Notch/Foxo1 cooperation may integrate environmental cues through Notch with metabolic cues through Foxo1 to regulate progenitor cell maintenance and differentiation.

Histone deacetylase degradation and MEF2 activation promote the formation of slow-twitch myofibers

Skeletal muscle is composed of heterogeneous myofibers with distinctive rates of contraction, metabolic properties, and susceptibility to fatigue. We show that class II histone deacetylase (HDAC) proteins, which function as transcriptional repressors of the myocyte enhancer factor 2 (MEF2) transcription factor, fail to accumulate in the soleus, a slow muscle, compared with fast muscles (e.g., white vastus lateralis). Accordingly, pharmacological blockade of proteasome function specifically increases expression of class II HDAC proteins in the soleus in vivo. Using gain- and loss-of-function approaches in mice, we discovered that class II HDAC proteins suppress the formation of slow twitch, oxidative myofibers through the repression of MEF2 activity. Conversely, expression of a hyperactive form of MEF2 in skeletal muscle of transgenic mice promotes the formation of slow fibers and enhances running endurance, enabling mice to run almost twice the distance of WT littermates. Thus, the selective degradation of class II HDACs in slow skeletal muscle provides a mechanism for enhancing physical performance and resistance to fatigue by augmenting the transcriptional activity of MEF2. These findings provide what we believe are new insights into the molecular basis of skeletal muscle function and have important implications for possible therapeutic interventions into muscular diseases.

Activity-dependent signaling pathways controlling muscle diversity and plasticity

A variety of fiber types with different contractile and metabolic properties is present in mammalian skeletal muscle. The fiber-type profile is controlled by nerve activity via specific signaling pathways, whose identification may provide potential therapeutic targets for the prevention and treatment of metabolic and neuromuscular diseases.

Distinctive patterns of microRNA expression in primary muscular disorders

The primary muscle disorders are a diverse group of diseases caused by various defective structural proteins, abnormal signaling molecules, enzymes and proteins involved in posttranslational modifications, and other mechanisms. Although there is increasing clarification of the primary aberrant cellular processes responsible for these conditions, the decisive factors involved in the secondary pathogenic cascades are still mainly obscure. Given the emerging roles of microRNAs (miRNAs) in modulation of cellular phenotypes, we searched for miRNAs regulated during the degenerative process of muscle to gain insight into the specific regulation of genes that are disrupted in pathological muscle conditions. We describe 185 miRNAs that are up- or down-regulated in 10 major muscular disorders in humans [Duchenne muscular dystrophy (DMD), Becker muscular dystrophy, facioscapulohumeral muscular dystrophy, limb-girdle muscular dystrophies types 2A and 2B, Miyoshi myopathy, nemaline myopathy, polymyositis, dermatomyositis, and inclusion body myositis]. Although five miRNAs were found to be consistently regulated in almost all samples analyzed, pointing to possible involvement of a common regulatory mechanism, others were dysregulated only in one disease and not at all in the other disorders. Functional correlation between the predicted targets of these miRNAs and mRNA expression demonstrated tight posttranscriptional regulation at the mRNA level in DMD and Miyoshi myopathy. Together with direct mRNA-miRNA predicted interactions demonstrated in DMD, some of which are involved in known secondary response functions and others that are involved in muscle regeneration, these findings suggest an important role of miRNAs in specific physiological pathways underlying the disease pathology.

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.

The histone deacetylase HDAC4 connects neural activity to muscle transcriptional reprogramming

Neural activity actively regulates muscle gene expression. This regulation is crucial for specifying muscle functionality and synaptic protein expression. How neural activity is relayed into nuclei and connected to the muscle transcriptional machinery, however, is not known. Here we identify the histone deacetylase HDAC4 as the critical linker connecting neural activity to muscle transcription. We found that HDAC4 is normally concentrated at the neuromuscular junction (NMJ), where nerve innervates muscle. Remarkably, reduced neural input by surgical denervation or neuromuscular diseases dissociates HDAC4 from the NMJ and dramatically induces its expression, leading to robust HDAC4 nuclear accumulation. We present evidence that nuclear accumulated HDAC4 is responsible for the coordinated induction of synaptic genes upon denervation. Inactivation of HDAC4 prevents denervation-induced synaptic acetyl-choline receptor (nAChR) and MUSK transcription whereas forced expression of HDAC4 mimics denervation and activates ectopic nAChR transcription throughout myofibers. We determined that HDAC4 executes activity-dependent transcription by regulating the Dach2-myogenin transcriptional cascade where inhibition of the repressor Dach2 by HDAC4 permits the induction of the transcription factor myogenin, which in turn activates synaptic gene expression. Our findings establish HDAC4 as a neural activity-regulated deacetylase and a key signaling component that relays neural activity to the muscle transcriptional machinery.

The transcriptional corepressor RIP140 regulates oxidative metabolism in skeletal muscle

Nuclear receptor signaling plays an important role in energy metabolism. In this study we demonstrate that the nuclear receptor corepressor RIP140 is a key regulator of metabolism in skeletal muscle. RIP140 is expressed in a fiber type-specific manner, and manipulation of its levels in null, heterozygous, and transgenic mice demonstrate that low levels promote while increased expression suppresses the formation of oxidative fibers. Expression profiling reveals global changes in the expression of genes implicated in both myofiber phenotype and metabolic functions. Genes involved in fatty-acid oxidation, oxidative phosphorylation, and mitochondrial biogenesis are upregulated in the absence of RIP140. Analysis of cultured myofibers demonstrates that the changes in expression are intrinsic to muscle cells and that nuclear receptor-regulated genes are direct targets for repression by RIP140. Therefore RIP140 is an important signaling factor in the regulation of skeletal muscle function and physiology.

De-phosphorylation of MyoD is linking nerve-evoked activity to fast myosin heavy chain expression in rodent adult skeletal muscle

Elucidating the molecular pathways linking electrical activity to gene expression is necessary for understanding the effects of exercise on muscle. Fast muscles express higher levels of MyoD and lower levels of myogenin than slow muscles, and we have previously linked myogenin to expression of oxidative enzymes. We here report that in slow muscles, compared with fast, 6 times as much of the MyoD is in an inactive form phosphorylated at T115. In fast muscles, 10 h of slow electrical stimulation had no effect on the total MyoD protein level, but the fraction of phosphorylated MyoD was increased 4-fold. Longer stimulation also decreased the total level of MyoD mRNA and protein, while the level of myogenin protein was increased. Fast patterned stimulation did not have any of these effects. Overexpression of wild type MyoD had variable effects in active slow muscles, but increased expression of fast myosin heavy chain in denervated muscles. In normally active soleus muscles, MyoD mutated at T115 (but not at S200) increased the number of fibres containing fast myosin from 50% to 85% in mice and from 13% to 62% in rats. These data establish de-phosphorylated active MyoD as a link between the pattern of electrical activity and fast fibre type in adult muscles.

Skeletal muscle fiber-type switching, exercise intolerance, and myopathy in PGC-1alpha muscle-specific knock-out animals

The transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha) is a key integrator of neuromuscular activity in skeletal muscle. Ectopic expression of PGC-1alpha in muscle results in increased mitochondrial number and function as well as an increase in oxidative, fatigue-resistant muscle fibers. Whole body PGC-1alpha knock-out mice have a very complex phenotype but do not have a marked skeletal muscle phenotype. We thus analyzed skeletal muscle-specific PGC-1alpha knock-out mice to identify a specific role for PGC-1alpha in skeletal muscle function. These mice exhibit a shift from oxidative type I and IIa toward type IIx and IIb muscle fibers. Moreover, skeletal muscle-specific PGC-1alpha knock-out animals have reduced endurance capacity and exhibit fiber damage and elevated markers of inflammation following treadmill running. Our data demonstrate a critical role for PGC-1alpha in maintenance of normal fiber type composition and of muscle fiber integrity following exertion.

The dual effects of Cdh1/APC in myogenesis

Ubiquitin-dependent proteolysis plays an important role in regulating fundamental biological functions, including cell division and cellular differentiation. Previous studies implicate the ubiquitin-proteasome system (UPS) in myogenic differentiation through regulating cell cycle progression and modulating myogenic factors such as MyoD and Myf5. Certain ubiquitin protein ligases, including the SCF complex and APC, have been suggested to govern terminal muscle differentiation. However, the underlying mechanism of regulation of both the cell cycle and myogenic factors by the UPS during this process remains unclear. We have dissected the role of the UPS in myogenic differentiation using an in vitro muscle differentiation system based on C2C12 cells. We demonstrate that Cdh1-APC regulates two critical proteins, Skp2 and Myf5, for proteolysis during muscle differentiation. The targeting of Skp2 by Cdh1-APC for destruction results in elevation of p21 and p27, which are crucial for coordinating cellular division and differentiation. Degradation of Myf5 by Cdh1-APC facilitates myogenic fusion. Knockdown of Cdh1 by siRNA significantly attenuates muscle differentiation. Taken together, Cdh1-APC is an important ubiquitin E3 ligase that modulates muscle differentiation through coordinating cell cycle progression and initiating the myogenic differentiation program.

Upregulation of myocellular DGAT1 augments triglyceride synthesis in skeletal muscle and protects against fat-induced insulin resistance

Increased fat deposition in skeletal muscle is associated with insulin resistance. However, exercise increases both intramyocellular fat stores and insulin sensitivity, a phenomenon referred to as "the athlete's paradox". In this study, we provide evidence that augmenting triglyceride synthesis in skeletal muscle is intrinsically connected with increased insulin sensitivity. Exercise increased diacylglycerol (DAG) acyltransferase (DGAT) activity in skeletal muscle. Channeling fatty acid substrates into TG resulted in decreased DAG and ceramide levels. Transgenic overexpression of DGAT1 in mouse skeletal muscle replicated these findings and protected mice against high-fat diet-induced insulin resistance. Moreover, in isolated muscle, DGAT1 deficiency exacerbated insulin resistance caused by fatty acids, whereas DGAT1 overexpression mitigated the detrimental effect of fatty acids. The heightened insulin sensitivity in the transgenic mice was associated with attenuated fat-induced activation of DAG-responsive PKCs and the stress mediator JNK1. Consistent with these changes, serine phosphorylation of insulin receptor substrate 1 was reduced, and Akt activation and glucose 4 membrane translocation were increased. In conclusion, upregulation of DGAT1 in skeletal muscle is sufficient to recreate the athlete's paradox and illustrates a mechanism of exercise-induced enhancement of muscle insulin sensitivity. Thus, increasing muscle DGAT activity may offer a new approach to prevent and treat insulin resistance and type 2 diabetes mellitus.

SIK1 is a class II HDAC kinase that promotes survival of skeletal myocytes

During physical exercise, increases in motor neuron activity stimulate the expression of muscle-specific genes through the myocyte enhancer factor 2 (MEF2) family of transcription factors. Elevations in intracellular calcium increase MEF2 activity via the phosphorylation-dependent inactivation of class II histone deacetylases (HDACs). In studies to determine the role of the cAMP responsive element binding protein (CREB) in skeletal muscle, we found that mice expressing a dominant-negative CREB transgene (M-ACREB mice) exhibited a dystrophic phenotype along with reduced MEF2 activity. Class II HDAC phosphorylation was decreased in M-ACREB myofibers due to a reduction in amounts of Snf1lk (encoding salt inducible kinase, SIK1), a CREB target gene that functions as a class II HDAC kinase. Inhibiting class II HDAC activity either by viral expression of Snf1lk or by the administration of a small molecule antagonist improved the dystrophic phenotype in M-ACREB mice, pointing to an important role for the SIK1-HDAC pathway in regulating muscle function.

PPARdelta expression is influenced by muscle activity and induces slow muscle properties in adult rat muscles after somatic gene transfer

The effects of exercise on skeletal muscle are mediated by a coupling between muscle electrical activity and gene expression. Several activity correlates, such as intracellular Ca(2+), hypoxia and metabolites like free fatty acids (FFAs), might initiate signalling pathways regulating fibre-type-specific genes. FFAs can be sensed by lipid-dependent transcription factors of the peroxisome proliferator-activated receptor (PPAR) family. We found that the mRNA for the predominant muscle isoform, PPARdelta, was three-fold higher in the slow/oxidative soleus compared to the fast/glycolytic extensor digitorum longus (EDL) muscle. In histological sections of the soleus, the most oxidative fibres display the highest levels of PPARdelta protein. When the soleus muscle was stimulated electrically by a pattern mimicking fast/glycolytic IIb motor units, the mRNA level of PPARdelta was reduced to less than half within 24 h. In the EDL, a three-fold increase was observed after slow type I-like electrical stimulation. When a constitutively active form of PPARdelta was overexpressed for 14 days in normally active adult fibres after somatic gene transfer, the number of I/IIa hybrids in the EDL more than tripled, IIa fibres increased from 14% to 25%, and IIb fibres decreased from 55% to 45%. The level of succinate dehydrogenase activity increased and size decreased, also when compared to normal fibres of the same type. Thus PPARdelta can change myosin heavy chain, oxidative enzymes and size locally in muscle cells in the absence of general exercise. Previous studies on PPARdelta in muscle have been performed in transgenic animals where the transgene has been present during muscle development. Our data suggest that PPARdelta can mediate activity effects acutely in pre-existing adult fibres, and thus is an important link in excitation-transcription coupling.

Therapeutic approaches for muscle wasting disorders

Muscle wasting and weakness are common in many disease states and conditions including aging, cancer cachexia, sepsis, denervation, disuse, inactivity, burns, HIV-acquired immunodeficiency syndrome (AIDS), chronic kidney or heart failure, unloading/microgravity, and muscular dystrophies. Although the maintenance of muscle mass is generally regarded as a simple balance between protein synthesis and protein degradation, these mechanisms are not strictly independent, but in fact they are coordinated by a number of different and sometimes complementary signaling pathways. Clearer details are now emerging about these different molecular pathways and the extent to which these pathways contribute to the etiology of various muscle wasting disorders. Therapeutic strategies for attenuating muscle wasting and improving muscle function vary in efficacy. Exercise and nutritional interventions have merit for slowing the rate of muscle atrophy in some muscle wasting conditions, but in most cases they cannot halt or reverse the wasting process. Hormonal and/or other drug strategies that can target key steps in the molecular pathways that regulate protein synthesis and protein degradation are needed. This review describes the signaling pathways that maintain muscle mass and provides an overview of some of the major conditions where muscle wasting and weakness are indicated. The review provides details on some therapeutic strategies that could potentially attenuate muscle atrophy, promote muscle growth, and ultimately improve muscle function. The emphasis is on therapies that can increase muscle mass and improve functional outcomes that will ultimately lead to improvement in the quality of life for affected patients.

Protein kinase B/Akt: a nexus of growth factor and cytokine signaling in determining muscle mass

Although the boundaries of skeletal muscle size are fundamentally determined by genetics, this dynamic tissue also demonstrates great plasticity in response to environmental and hormonal factors. Recent work indicates that contractile activity, nutrients, growth factors, and cytokines all contribute to determining muscle mass. Muscle responds not only to endocrine hormones but also to the autocrine production of growth factors and cytokines. Skeletal muscle synthesizes anabolic growth factors such as insulin-like growth factor (IGF)-I and potentially inhibitory cytokines such as interleukin (IL)-6, tumor necrosis factor (TNF)-alpha, and myostatin. These self-regulating inputs in turn influence muscle metabolism, including the use of nutrients such as glucose and amino acids. These changes are principally achieved by altering the activity of the protein kinase known as protein kinase B or Akt. Akt plays a central role in integrating anabolic and catabolic responses by transducing growth factor and cytokine signals via changes in the phosphorylation of its numerous substrates. Activation of Akt stimulates muscle hypertrophy and antagonizes the loss of muscle protein. Here we review the many signals that funnel through Akt to alter muscle mass.

Molecular determinants for the recruitment of the ubiquitin-ligase MuRF-1 onto M-line titin

Titin forms an intrasarcomeric filament system in vertebrate striated muscle, which has elastic and signaling properties and is thereby central to mechanotransduction. Near its C-terminus and directly preceding a kinase domain, titin contains a conserved pattern of Ig and FnIII modules (Ig(A168)-Ig(A169)-FnIII(A170), hereby A168-A170) that recruits the E3 ubiquitin-ligase MuRF-1 to the filament. This interaction is thought to regulate myofibril turnover and the trophic state of muscle. We have elucidated the crystal structure of A168-A170, characterized MuRF-1 variants by circular dichroism (CD) and SEC-MALS, and studied the interaction of both components by isothermal calorimetry, SPOTS blots, and pull-down assays. This has led to the identification of the molecular determinants of the binding. A168-A170 shows an extended, rigid architecture, which is characterized by a shallow surface groove that spans its full length and a distinct loop protrusion in its middle point. In MuRF-1, a C-terminal helical domain is sufficient to bind A168-A170 with high affinity. This helical region predictably docks into the surface groove of A168-A170. Furthermore, pull-down assays demonstrate that the loop protrusion in A168-A170 is a key mediator of MuRF-1 recognition. Our findings indicate that this region of titin could serve as a target to attempt therapeutic inhibition of MuRF-1-mediated muscle turnover, where binding of small molecules to its distinctive structural features could block MuRF-1 access.

The transcriptional coactivator PGC-1beta drives the formation of oxidative type IIX fibers in skeletal muscle

Skeletal muscle must perform different kinds of work, and distinct fiber types have evolved to accommodate these. Previous work had shown that the transcriptional coactivator PGC-1alpha drives the formation of type I and IIA muscle fibers, which are "slow-twitch" and highly oxidative. We show here that transgenic expression of PGC-1beta, a coactivator functionally similar to but distinct from PGC-1alpha, causes a marked induction of IIX fibers, which are oxidative but have "fast-twitch" biophysical properties. PGC-1beta coactivates the MEF2 family of transcription factors to stimulate the type IIX myosin heavy chain (MHC) promoter. PGC-1beta transgenic muscle fibers are rich in mitochondria and are highly oxidative, at least in part due to coactivation by PGC-1beta of ERRalpha and PPARalpha. Consequently, these transgenic animals can run for longer and at higher work loads than wild-type animals. Together, these data indicate that PGC-1beta drives the formation of highly oxidative fibers containing type IIX MHC.

Of bears, frogs, meat, mice and men: complexity of factors affecting skeletal muscle mass and fat

Extreme loss of skeletal muscle mass (atrophy) occurs in human muscles that are not used. In striking contrast, skeletal muscles do not rapidly waste away in hibernating mammals such as bears, or aestivating frogs, subjected to many months of inactivity and starvation. What factors regulate skeletal muscle mass and what mechanisms protect against muscle atrophy in some species? Severe atrophy also occurs with ageing and there is much clinical interest in reducing such loss of muscle mass and strength (sarcopenia). In the meat industry, a key aim is optimizing the control of skeletal muscle growth and meat quality. The impaired response of muscle to insulin resulting in diabetes, that is a consequence of the metabolic impact of increasing obesity and fat deposition in humans, is also of increasing clinical concern. Intensive research in these fields, combined with mouse models, is reviewed with respect to the molecular control of muscle growth (myogenesis) and atrophy/hypertrophy and fat deposition (adipogenesis) in skeletal muscle, with a focus on IGF-1/insulin signaling.

PGC-1alpha protects skeletal muscle from atrophy by suppressing FoxO3 action and atrophy-specific gene transcription

Maintaining muscle size and fiber composition requires contractile activity. Increased activity stimulates expression of the transcriptional coactivator PGC-1alpha (peroxisome proliferator-activated receptor gamma coactivator 1alpha), which promotes fiber-type switching from glycolytic toward more oxidative fibers. In response to disuse or denervation, but also in fasting and many systemic diseases, muscles undergo marked atrophy through a common set of transcriptional changes. FoxO family transcription factors play a critical role in this loss of cell protein, and when activated, FoxO3 causes expression of the atrophy-related ubiquitin ligases atrogin-1 and MuRF-1 and profound loss of muscle mass. To understand how exercise might retard muscle atrophy, we investigated the possible interplay between PGC-1alpha and the FoxO family in regulation of muscle size. Rodent muscles showed a large decrease in PGC-1alpha mRNA during atrophy induced by denervation as well as by cancer cachexia, diabetes, and renal failure. Furthermore, in transgenic mice overexpressing PGC-1alpha, denervation and fasting caused a much smaller decrease in muscle fiber diameter and a smaller induction of atrogin-1 and MuRF-1 than in control mice. Increased expression of PGC-1alpha also increased mRNA for several genes involved in energy metabolism whose expression decreases during atrophy. Transfection of PGC-1alpha into adult fibers reduced the capacity of FoxO3 to cause fiber atrophy and to bind to and transcribe from the atrogin-1 promoter. Thus, the high levels of PGC-1alpha in dark and exercising muscles can explain their resistance to atrophy, and the rapid fall in PGC-1alpha during atrophy should enhance the FoxO-dependent loss of muscle mass.

Myomaxin is a novel transcriptional target of MEF2A that encodes a Xin-related alpha-actinin-interacting protein

The physiological targets regulated by MEF2 in striated muscle are not completely known. Several recent studies have identified novel downstream target genes and shed light on the global transcriptional network regulated by MEF2 in muscle. In our continuing effort to identify novel, downstream pathways controlled by MEF2, we have used mef2a knock-out mice to find those genes dependent on MEF2A transcriptional activity. Here, we describe the characterization of a direct, downstream target gene for the MEF2A transcription factor encoding a large, muscle-specific protein that localizes to the Z-disc/costameric region in striated muscle. This gene, called myomaxin, was identified as a gene markedly down-regulated in MEF2A knock-out hearts. Myomaxin is the mouse ortholog of a partial human cDNA of unknown function named cardiomyopathy associated gene 3 (CMYA3). Myomaxin is expressed as a single, large transcript of approximately 11 kilobases in adult heart and skeletal muscle with an open reading frame of 3,283 amino acids. The protein encoded by the myomaxin gene is related to the actin-binding protein Xin and interacts with the sarcomeric Z-disc protein, alpha-actinin-2. Our findings demonstrate that Myomaxin functions directly downstream of MEF2A at the peripheral Z-disc complex in striated muscle potentially playing a role in regulating cytoarchitectural integrity.

Functional alpha 1 protease inhibitor produced by a human hepatoma cell line

Alpha 1 protease inhibitor antigen was identified in the culture medium of the human ascites hepatoma cell line SK-HEP-1. Trypsin inhibitory activity and alpha 1 Pl antigen accumulated in serum-free medium concomitantly over a period of several days. Radioactive alpha 1 Pl antigen was detected in conditioned medium from cultures supplemented with 35S-L-methionine, indicating a synthesis and release of the protein. Alpha 1 Pl antigen in conditioned medium appeared to be antigenically identical to that in human plasma, and the newly synthesized (radiolabeled) antigen co-migrated with plasma, alpha 1 Pl after immunoelectrophoresis or SDS-polyacrylamide gel electrophoresis. Moreover, evidence is presented that the synthesized inhibitor exhibits functional activity, since the 35S-labeled alpha 1 Pl in conditioned medium complexes with trypsin. We conclude that SK-HEP-1 cells in culture produce functionally active alpha 1 Pl which may be identical to that in plasma.