Skeletal muscle cell hypertrophy induced by inhibitors of metalloproteases; myostatin as a potential mediator

Review: myogenic and muscle toxicity targets of environmental methylmercury exposure

A number of environmental toxicants are noted for their activity that leads to declined motor function. However, the role of muscle as a proximal toxicity target organ for environmental agents has received considerably less attention than the toxicity targets in the nervous system. Nonetheless, the effects of conventional neurotoxicants on processes of myogenesis and muscle maintenance are beginning to resolve a concerted role of muscle as a susceptible toxicity target. A large body of evidence from epidemiological, animal, and in vitro studies has established that methylmercury (MeHg) is a potent developmental toxicant, with the nervous system being a preferred target. Despite its well-recognized status as a neurotoxicant, there is accumulating evidence that MeHg also targets muscle and neuromuscular development as well as contributes to the etiology of motor defects with prenatal MeHg exposure. Here, we summarize evidence for targets of MeHg in the morphogenesis and maintenance of skeletal muscle that reveal effects on MeHg distribution, myogenesis, myotube formation, myotendinous junction formation, neuromuscular junction formation, and satellite cell-mediated muscle repair. We briefly recapitulate the molecular and cellular mechanisms of skeletal muscle development and highlight the pragmatic role of alternative model organisms, Drosophila and zebrafish, in delineating the molecular underpinnings of muscle development and MeHg-mediated myotoxicity. Finally, we discuss how toxicity targets in muscle development may inform the developmental origins of health and disease theory to explain the etiology of environmentally induced adult motor deficits and accelerated decline in muscle fitness with aging.

Context-dependent regulation of pectoralis myostatin and lipid transporters by temperature and photoperiod in dark-eyed juncos

A prominent example of seasonal phenotypic flexibility is the winter increase in thermogenic capacity (=summit metabolism, [Formula: see text]) in small birds, which is often accompanied by increases in pectoralis muscle mass and lipid catabolic capacity. Temperature or photoperiod may be drivers of the winter phenotype, but their relative impacts on muscle remodeling or lipid transport pathways are little known. We examined photoperiod and temperature effects on pectoralis muscle expression of myostatin, a muscle growth inhibitor, and its tolloid-like protein activators (TLL-1 and TLL-2), and sarcolemmal and intracellular lipid transporters in dark-eyed juncos Junco hyemalis. We acclimated winter juncos to four temperature (3 °C or 24 °C) and photoperiod [short-day (SD) = 8L:16D; long-day (LD) = 16L:8D] treatments. We found that myostatin, TLL-1, TLL-2, and lipid transporter mRNA expression and myostatin protein expression did not differ among treatments, but treatments interacted to influence lipid transporter protein expression. Fatty acid translocase (FAT/CD36) levels were higher for cold SD than for other treatments. Membrane-bound fatty acid binding protein (FABPpm) levels, however, were higher for the cold LD treatment than for cold SD and warm LD treatments. Cytosolic fatty acid binding protein (FABP_c) levels were higher on LD than on SD at 3 °C, but higher on SD than on LD at 24 °C. Cold temperature groups showed upregulation of these lipid transporters, which could contribute to elevated M_sum compared to warm groups on the same photoperiod. However, interactions of temperature or photoperiod effects on muscle remodeling and lipid transport pathways suggest that these effects are context-dependent.

Within-Winter Flexibility in Muscle Masses, Myostatin, and Cellular Aerobic Metabolic Intensity in Passerine Birds

Metabolic rates of passerine birds are flexible traits that vary both seasonally and among and within winters. Seasonal variation in summit metabolic rates (M_sum = maximum thermoregulatory metabolism) in birds is consistently correlated with changes in pectoralis muscle and heart masses and sometimes with variation in cellular aerobic metabolic intensity, so these traits might also be associated with shorter-term, within-winter variation in metabolic rates. To determine whether these mechanisms are associated with within-winter variation in M_sum, we examined the effects of short-term (ST; 0-7 d), medium-term (MT; 14-30 d), and long-term (LT; 30-yr means) temperature variables on pectoralis muscle and heart masses, pectoralis expression of the muscle-growth inhibitor myostatin and its metalloproteinase activators TLL-1 and TLL-2, and pectoralis and heart citrate synthase (CS; an indicator of cellular aerobic metabolic intensity) activities for two temperate-zone resident passerines, house sparrows (Passer domesticus) and dark-eyed juncos (Junco hyemalis). For both species, pectoralis mass residuals were positively correlated with ST temperature variables, suggesting that cold temperatures resulted in increased turnover of pectoralis muscle, but heart mass showed little within-winter variation for either species. Pectoralis mRNA and protein expression of myostatin and the TLLs were only weakly correlated with ST and MT temperature variables, which is largely consistent with trends in muscle masses for both species. Pectoralis and heart CS activities showed weak and variable trends with ST temperature variables in both species, suggesting only minor effects of temperature variation on cellular aerobic metabolic intensity. Thus, neither muscle or heart masses, regulation by the myostatin system, nor cellular aerobic metabolic intensity varied consistently with winter temperature, suggesting that other factors regulate within-winter metabolic variation in these birds.

Cross-training in birds: cold and exercise training produce similar changes in maximal metabolic output, muscle masses and myostatin expression in house sparrows (Passer domesticus)

Maximal metabolic outputs for exercise and thermogenesis in birds presumably influence fitness through effects on flight and shivering performance. Because both summit (Msum, maximum thermoregulatory metabolic rate) and maximum (MMR, maximum exercise metabolic rate) metabolic rates are functions of skeletal muscle activity, correlations between these measurements and their mechanistic underpinnings might occur. To examine whether such correlations occur, we measured the effects of experimental cold and exercise training protocols for 3 weeks on body (Mb) and muscle (Mpec) masses, basal metabolic rate (BMR), Msum, MMR, pectoralis mRNA and protein expression for myostatin, and mRNA expression of TLL-1 and TLL-2 (metalloproteinase activators of myostatin) in house sparrows (Passer domesticus). Both training protocols increased Msum, MMR, Mb and Mpec, but BMR increased with cold training and decreased with exercise training. No significant differences occurred for pectoralis myostatin mRNA expression, but cold and exercise increased the expression of TLL-1 and TLL-2. Pectoralis myostatin protein levels were generally reduced for both training groups. These data clearly demonstrate cross-training effects of cold and exercise in birds, and are consistent with a role for myostatin in increasing pectoralis muscle mass and driving organismal increases in metabolic capacities.

Phenotypic flexibility of skeletal muscle and heart masses and expression of myostatin and tolloid-like proteinases in migrating passerine birds

Migrant birds require large flight muscles and hearts to enhance aerobic capacity and support sustained flight. A potential mechanism for increasing muscle and heart masses during migration in birds is the muscle growth inhibitor myostatin and its metalloproteinase activators, tolloid-like proteinases (TLL-1 and TLL-2). We hypothesized that myostatin, TLL-1 and TLL-2 are downregulated during migration in pectoralis and hearts of migratory passerines to promote hypertrophy. We measured seasonal variation of tissue masses, mRNA expression of myostatin, TLL-1, and TLL-2, and myostatin protein levels in pectoralis muscle and heart for yellow warblers (Setophaga petechia), warbling vireos (Vireo gilvus), and yellow-rumped warblers (Setophaga coronata). Pectoralis mass was greatest in spring for warbling vireos and yellow warblers, but was stable between spring and fall for yellow-rumped warblers. Heart mass was higher in spring than in fall for yellow-rumped warblers, lowest in fall for warbling vireos, and seasonally stable for yellow warblers. Pectoralis and heart mRNA expression of myostatin and the TLLs did not differ significantly for any of the three species, offering little support for our hypothesis for a prominent role for myostatin in regulating migration-induced variation in pectoralis and heart masses. In contrast, pectoralis myostatin protein levels were lowest in spring for all three species, consistent with our hypothesis. Myostatin protein levels in heart, however, were seasonally stable for warbling vireos and yellow warblers, and increased in spring relative to fall for yellow-rumped warblers. These data offer mixed support for our hypothesis for the pectoralis, but suggest that myostatin is not a prominent regulator of migration-induced heart hypertrophy. Moreover, the different seasonal patterns for pectoralis mRNA and protein expression suggest that post-transcriptional modification of myostatin may contribute to pectoralis mass regulation during migration.

Seasonal variation in pectoralis muscle and heart myostatin and tolloid-like proteinases in small birds: a regulatory role for seasonal phenotypic flexibility?

Seasonally variable environments produce seasonal phenotypes in small birds such that winter birds have higher thermogenic capacities and pectoralis and heart masses. One potential regulator of these seasonal phenotypes is myostatin, a muscle growth inhibitor, which may be downregulated under conditions promoting increased energy demand. We examined summer-to-winter variation in skeletal muscle and heart masses and used qPCR and Western blots to measure levels of myostatin and its metalloproteinase activators TLL-1 and TLL-2 for two small temperate-zone resident birds, American goldfinches (Spinus tristis) and black-capped chickadees (Poecile atricapillus). Winter pectoralis and heart masses were significantly greater than in summer for American goldfinches. Neither myostatin expression nor protein levels differed significantly between seasons for goldfinch pectoralis. However, myostatin levels in goldfinch heart were significantly greater in summer than in winter, although heart myostatin expression was seasonally stable. In addition, expression of both metalloproteinase activators was greater in summer than in winter goldfinches for both pectoralis and heart, significantly so except for heart TLL-2 (P = 0.083). Black-capped chickadees showed no significant seasonal variation in muscle or heart masses. Seasonal patterns of pectoralis and heart expression and/or protein levels for myostatin and its metalloproteinase activators in chickadees showed no consistent seasonal trends, which may help explain the absence of significant seasonal variation in muscle or heart masses for chickadees in this study. These data are partially consistent with a regulatory role for myostatin, and especially myostatin processing capacity, in mediating seasonal metabolic phenotypes of small birds.

Gelatinases impart susceptibility to high-fat diet-induced obesity in mice

Gelatinases play a role in adipose and muscle hypertrophy and could be involved in tissue remodeling in response to high-fat diet (HFD) intake. This study tested potential roles of gelatinases (matrix metalloproteinses-2 and -9 [MMP-2 and -9]) in relationship to an antigrowth factor [myostatin (MSTN)] known to be dysregulated in relation to HFD-induced obesity (HFDIO) propensity. In vitro and ex vivo analyses demonstrated that MMP-9 increased mature MSTN levels, indicating a potential role of gelatinases in MSTN activation in vivo. HFD intake resulted in increased body weight and circulating blood glucose values in C57BL/6J and MMP-9 null mice, with no changes observed in SWR/J mice. HFD intake attenuated MMP-9 and MMP-2 mRNA levels in SWR/J mice while elevating MMP-2 levels in skeletal muscle in C57BL/6J mice. In MMP-9 null mice, the effects of HFD intake were muted. Consistent with changes in mRNA levels, HFD intake increased MMP-9 activity in muscle tissue of C57BL/6J mice, demonstrating a strong relationship between HFDIO susceptibility and local MMP regulation. Overall, resistance to HFDIO appears to correspond to low MMP-9 and MSTN levels, suggesting a role of MMP-9 in MSTN activation in local tissue responses to HFD intake.

An image analysis method for the precise selection and quantitation of fluorescently labeled cellular constituents: application to the measurement of human muscle cells in culture

The accurate measurement of the morphological characteristics of cells with nonuniform conformations presents difficulties. We report here a straightforward method using immunofluorescent staining and the commercially available imaging program Adobe Photoshop, which allows objective and precise information to be gathered on irregularly shaped cells. We have applied this measurement technique to the analysis of human muscle cells and their immunologically marked intracellular constituents, as these cells are prone to adopting a highly branched phenotype in culture. Use of this method can be used to overcome many of the long-standing limitations of conventional approaches for quantifying muscle cell size in vitro. In addition, wider applications of Photoshop as a quantitative and semiquantitative tool in immunocytochemistry are explored.

Analysis of the effects of androgens and training on myostatin propeptide and follistatin concentrations in blood and skeletal muscle using highly sensitive immuno PCR

Myostatin propeptide (MYOPRO) and follistatin (FOLLI) are potent myostatin inhibitors. In this study we analysed effects of training and androgens on MYOPRO and FOLLI concentrations in blood and skeletal muscle using Immuno PCR. Young healthy males performed either a 3-month endurance training or a strength training. Blood and biopsy samples were analysed. Training did not significantly affect MYOPRO and FOLLI concentrations in serum and muscle. To investigate whether total skeletal muscle mass may affect circulating MYOPRO and FOLLI levels, blood samples of tetraplegic patients, untrained volunteers and bodybuilders were analysed. MYOPRO was significantly increased exclusively in the bodybuilder group. In orchiectomised rats MYOPRO increased in blood and muscle after treatment with testosterone. In summary our data demonstrate that moderate training does not affect the concentrations of MYOPRO to FOLLI. In contrast androgen treatment results in a significant increase of MYOPRO in skeletal muscle and serum.

Bowman-Birk inhibitor attenuates dystrophic pathology in mdx mice

Bowman-Birk inhibitor concentrate (BBIC), a serine protease inhibitor, has been shown to diminish disuse atrophy of skeletal muscle. Duchenne muscular dystrophy (DMD) results from a loss of dystrophin protein and involves an ongoing inflammatory response, with matrix remodeling and activation of transforming growth factor (TGF)-β(1) leading to tissue fibrosis. Inflammatory-mediated increases in extracellular protease activity may drive much of this pathological tissue remodeling. Hence, we evaluated the ability of BBIC, an extracellular serine protease inhibitor, to impact pathology in the mouse model of DMD (mdx mouse). Mdx mice fed 1% BBIC in their diet had increased skeletal muscle mass and tetanic force and improved muscle integrity (less Evans blue dye uptake). Importantly, mdx mice treated with BBIC were less susceptible to contraction-induced injury. Changes consistent with decreased degeneration/regeneration, as well as reduced TGF-β(1) and fibrosis, were observed in the BBIC-treated mdx mice. While Akt signaling was unchanged, myostatin activitation and Smad signaling were reduced. Given that BBIC treatment increases mass and strength, while decreasing fibrosis in skeletal muscles of the mdx mouse, it should be evaluated as a possible therapeutic to slow the progression of disease in human DMD patients.

Activation of mammalian Tolloid-like 1 expression by hypoxia in human neuroblastoma SH-SY5Y cells

Mammalian Tolloid-like 1 (mTll-1) is an astacin metalloprotease that is a member of the Tolloid family of proteins. mTll-1 cleaves chordin, an inhibitor of bone morphogenetic proteins (BMPs) and potentiates activity of the BMPs. Prenatal stress and glucocorticoids decrease mTll-1 expression whereas voluntary exercise increase mTll-1 gene expression in the mouse hippocampus. Here, we studied the underlying molecular mechanisms by which hypoxia regulates human mTll-1 gene expression. When cells were subjected to hypoxia, the expression of endogenous mTll-1 was upregulated in SH-SY5Y human neuroblastoma cells. Dual-luciferase assay and site-directed mutagenesis showed the presence of hypoxia responsive elements (HREs) at position 625 that was essential for activation of mTll-1 expression under hypoxic conditions. The binding of hypoxia-inducible factor (HIF-1) protein to the HREs was confirmed by gel shift assay. These results indicate that the HRE motif is directly involved in the activation of the mTll-1 transcription under hypoxic conditions.

Seasonal variation of myostatin gene expression in pectoralis muscle of house sparrows (Passer domesticus) is consistent with a role in regulating thermogenic capacity and cold tolerance

Winter acclimatization in small birds overwintering in cold climates, including house sparrows (Passer domesticus), is associated with improved cold tolerance, elevated summit metabolic rates (M(sum) = maximum cold-induced metabolic rate), and increased pectoralis muscle mass compared to summer birds. Myostatin is a potent autocrine/paracrine inhibitor of skeletal muscle growth in mammals and birds and is a potential candidate for regulation of seasonal phenotypic flexibility in birds. As a first step toward examining such a role for myostatin in small birds, we measured summer and winter gene expression of myostatin and its potential metalloproteinase activators TLL-1 and TLL-2 in house sparrows from southeastern South Dakota. Gene expression of myostatin decreased significantly in winter, with summer values exceeding winter values by 1.52-fold. Moreover, gene expression of TLL-1 was also significantly reduced in winter, with summer values exceeding winter values by 1.55-fold. These data are consistent with the hypothesis that the winter increases in pectoralis muscle mass, M(sum), and cold tolerance in house sparrows are mediated by reduced levels of myostatin and its activator TLL-1, and they suggest the possibility that myostatin may be a common mediator of phenotypic flexibility of muscle mass in birds.

Multifunctional roles of MT1-MMP in myofiber formation and morphostatic maintenance of skeletal muscle

Sequential activation of muscle-specific transcription factors is the critical basis for myogenic differentiation. However, the complexity of this process does not exclude the possibility that other molecules and systems are regulatory as well. We observed that myogenic differentiation proceeded through three distinct stages of proliferation, elongation and fusion, which are distinguishable by their cellular morphologies and gene expression patterns of proliferation- and differentiation-specific markers. Treatment of the differentiating myoblasts with inhibitors of matrix metalloproteinases (MMPs) revealed that MMP activity at the elongation stage is a critical prerequisite to complete the successive myoblast cell fusion. The MMP regulated the myogenic differentiation independently from the genetic program that governs expression of the myogenic genes. Membrane-type 1 matrix metalloproteinase (MT1-MMP) was identified as a major contributor to this checkpoint for morphological differentiation and degraded fibronectin, a possible inhibitory factor for myogenic cell fusion. A MT1-MMP deficiency caused similar myogenic impediments forming smaller myofibers in situ. Additionally, the mutant mice demonstrated some central nucleation of the myofibers typically found in muscular dystrophy and MT1-MMP was found to cleave laminin-2/4 in the basement membrane. Thus, MT1-MMP is a new multilateral regulator for muscle differentiation and maintenance through processing of stage-specific distinct ECM substrates.

(Make) stick and cut loose--disintegrin metalloproteases in development and disease

"A disintegrin and metalloprotease" (ADAM) proteases form a still growing family of about 40 type 1 transmembrane proteins. They are defined by a common modular ectodomain architecture that combines cell deadhesion/adhesion and fusion motifs (disintegrin and cysteine-rich domains), with a Zn-protease domain capped by a large prodomain. Their ectodomain thus strikingly resembles snake venom disintegrin proteases, which by combined integrin blocking and extracellular proteolysis, can cause extensive tissue damage after snake bites. A surprisingly large proportion (13 ADAMs) is exclusively expressed in the male gonads, and only a minority can be found throughout all tissues. As predicted by their amino acid sequence, a major proportion of this family has not maintained a functional protease domain, most probably rendering them into pure adhesion and/or fusion proteins. For most ADAMs, the respective key function has remained elusive. Despite their overall conserved ectodomain structure, ADAMs appear to be subdivided into those with a predominant role in direct adhesion (e.g., ADAMs 1, 2, and 3) and those mainly acting as proteases (e.g., ADAMs 10 and 17). Only for a few of them are functions of more than one domain documented (e.g., ADAM9 in cell fusion and proteolysis). Several ADAMs exist in both membrane-resident and secreted isoforms; the functional significance of this dichotomy is in most cases still unclear. Knockout phenotypes have been informative only in a few cases (ADAMs 1, 2, 10, 12, 15, 17, and 19) and are mainly related to their protease function. A common denominator of ADAM-mediated proteolysis is the ectodomain shedding of a broad spectrum of substrates, including paracrine growth factors like epidermal growth factor receptor (EGFR) ligands, cell adhesion molecules like CD44 or cadherins, and the initiation of regulated intramembrane proteolysis (RIP), whereby the transmembrane fragment of the respective substrate is further cleaved by an intramembrane cleaving protease to release an intracellular domain acting as a nuclear transcription regulator. Most ADAMs feature a significant overlap of substrate specificities, explaining why an inactivation of individual ADAMs only rarely causes major phenotypes.

The possible role of myostatin in skeletal muscle atrophy and cachexia

The presence of sufficient skeletal muscle is of paramount importance for body function. Cachexia can be defined as a wasting syndrome describing the progressive loss of both adipose and skeletal muscle tissue in concert with severe injury, chronic or end-stage malignant and infectious diseases. Generally, cachexia predisposes to poor prognosis, co-morbidities and death. One signaling pathway possibly involved in muscle atrophy and cachexia is the myostatin cascade. This transforming growth factor-beta superfamily member myostatin is a strong candidate for regulating muscle mass, and is shown to inhibit muscle growth in different in vivo mammalian models. Overall, the modulation of the myostatin pathway seems interesting from the perspective of both pathology and sports medicine. Hence, myostatin signaling components and post-translational modulators are possible targets of pharmacological and other treatments against muscle loss, thus potentially contributing to the understanding and mitigation of muscle atrophies associated with inactivity, senescence and disease.

Role of serum myostatin during the lactation period

Myostatin, also known as GDF-8 (Growth/Differentiation Factor-8), is a member of the TGF-beta superfamily that negatively regulates skeletal muscle mass in mammals. Mutation of the myostatin gene in mice, cattle, and humans causes a massively developed skeletal muscle, characterized by muscle hypertrophy and hyperplasia. Although myostatin is predominantly expressed in skeletal muscle tissue, several recent studies have shown the presence of myostatin protein in blood and suggested a possible role for circulating myostatin in the regulation of skeletal muscle mass. In the present study, we examined changes in the levels of active form myostatin (13 kDa) in serum after birth by Western blot analysis to predict the role of serum myostatin in early postnatal muscle growth in the rat. Interestingly, the amount of active form myostatin in serum increased after birth and then decreased along with ageing after weaning. To clarify the role of increased serum myostatin during the postnatal period, we administrated follistatin, an inhibitor of myostatin activity, into postnatal rats intraperitoneally just after birth. Follistatin-administration during the postnatal period caused selective hypertrophy of type II muscle fibers in the soleus muscle. These results demonstrate that myostatin in serum acts on skeletal muscle and negatively regulates early postnatal muscle growth.

Short-term endurance training results in a muscle-specific decrease of myostatin mRNA content in the rat

AIM

Myostatin has been characterized as a negative regulator of skeletal muscle growth. To examine a probable function of myostatin during the adaptation of skeletal muscle in response to training, we analysed the effect of short-term endurance training on myostatin and insulin-like growth factor-I (IGF-I) mRNA contents in rat skeletal muscles. To assess the impact of the training stimulus, mRNA levels of metabolic genes were analysed simultaneously.

METHODS

Male Wistar rats were trained for 5 days by swimming, while another group remained untrained. Myostatin, IGF-I, glucose transporter 4 (GLUT4), hexokinase II (HK II) and hydroxyacyl-CoA dehydrogenase (HAD) mRNA levels were determined by real-time reverse transcriptase-polymerase chain reaction (RT-PCR) in gastrocnemius, vastus lateralis and soleus muscles. A time course experiment was conducted, in order to examine transient changes of myostatin mRNA contents in gastrocnemius 7 and 24 h after one-swimming session as well as 24 h after a 3-day swimming training.

RESULTS

No significant changes in IGF-I and GLUT4 mRNA levels were found in any of the muscles analysed. mRNA contents of myostatin were significantly reduced in gastrocnemius and vastus lateralis but not in soleus. In agreement to this pattern, we found significantly higher mRNA levels of HK II and HAD in the trained group. The time course experiment revealed significantly reduced myostatin mRNA contents in gastrocnemius 7 but not 24 h post-exercise. The 3-day swimming training resulted also in significantly lower myostatin mRNA levels in the trained group.

CONCLUSION

This study demonstrated that short-term endurance training may modulate myostatin mRNA levels, implying a probable role of myostatin in remodelling of skeletal muscle in response to training.

ADAM 33 and its association with airway remodeling and hyperresponsiveness in asthma

Asthma is known to be a Th2 inflammatory syndrome that leads to intermittent airway obstruction. However, the mechanisms involved in development of the clinical features remain enigmatic, although genetic elements clearly are involved. Recently, based on a large genome wide screen involving families in the United Kingdom and the United States with at least two siblings with asthma, a locus was identified that encoded for a family of proteases. This group of proteins is now known as the ADAM superfamily. In this review, we discuss the ADAM superfamily and, in particular, ADAM 33, a member of a family of genes which encode a subgroup of zinc dependent metalloproteinase (metzincin). The potential for therapeutic intervention with ADAM 33 is extremely attractive and further work will not only focus on the specific domains of ADAM 33, but also the mechanisms by which they lead to bronchial hyperreactivity.

Regulation of muscle mass by myostatin

Myostatin is a secreted protein that acts as a negative regulator of skeletal muscle mass. During embryogenesis, myostatin is expressed by cells in the myotome and in developing skeletal muscle and acts to regulate the final number of muscle fibers that are formed. During adult life, myostatin protein is produced by skeletal muscle, circulates in the blood, and acts to limit muscle fiber growth. The existence of circulating tissue-specific growth inhibitors of this type was hypothesized over 40 years ago to explain how sizes of individual tissues are controlled. Skeletal muscle appears to be the first example of a tissue whose size is controlled by this type of regulatory mechanism, and myostatin appears to be the first example of the long-sought chalone.

Long Term Adrenal Insufficiency Induces Skeletal Muscle Atrophy and Increases the Serum Levels of Active Form Myostatin in Rat Serum

Skeletal muscle wasting is a common symptom in the adrenal insufficiency such as Addison's disease. Although it has been suspected that several cytokines and/or growth factors are responsible for the manifestation of the symptom, the precise mechanisms underlying the phenomenon have so far been poorly understood. Myostatin is predominantly expressed in skeletal muscles and involved in the regulation of skeletal muscle mass. Recently, several reports indicated that myostatin is secreted into the circulation and the increased levels of circulating myostatin is associated with the induction of skeletal muscle wasting in adult animals. We, therefore, hypothesized that the increased levels of circulating myostatin may account for the development of skeletal muscle wasting in adrenal insufficiency. To test the validity of this hypothesis, we compared the serum levels of myostatin in normal with those in bilaterally adrenalectomized (ADX) rats, a model of Addison's disease, by Western blot analysis. The active form of myostatin (13 kDa) was barely detectable in the sera collected either 1 month or 2 month after adrenalectomy, but present at conspicuously detectable levels in those obtained 3 month after the operation, while the total amounts of myostatin proteins (sum of the precursor and the active forms) remained constant at all the time points examined post-operatively. These results are consistent with the hypothesis that the increased serum levels of active form of myostatin protein, induced yet unknown post-translational control mechanisms may be responsible, at least in part, for the muscle wasting associated with the adrenal insufficiency syndromes.

Transgenic salmon overexpressing growth hormone exhibit decreased myostatin transcript and protein expression

To characterize the role of myostatin (MSTN) in fast growing animals and to examine the relationship between MSTN and growth hormone (GH), MSTN transcript and protein expression were measured in coho salmon overexpressing GH and in wild-type coho salmon. Quantitative real-time RT-PCR and western analysis were used to measure RNA expression of the two salmonid MSTN genes (1 and 2) and levels of MSTN immunoreactive protein (MIP) in developing embryos and adult coho salmon tissues. In transgenic and control coho embryos, MSTN1 and MSTN2 RNA expression were initially observed at about the time of eying, and a 42 kDa MIP was just detected prior to hatching. Expression of the MSTN1 transcript in transgenic salmon was not different from that in wild-type adult coho salmon muscle and brain tissue. However, expression of the MSTN2 transcript was less in white muscle, and greater in red muscle, from transgenic fish compared to wild-type salmon of the same size. Northern analysis revealed that expression of the MSTN2 transcript was less in white muscle from wild-type, age-matched salmon than in transgenic fish. In addition, there was less presumed bioactive MIP in muscle taken from adult transgenic fish compared to controls and evidence of differential protein processing. Decreased MSTN expression in faster growing fish suggests that MSTN does act as a negative regulator of muscle growth in fish, as it does in mammals. The results of this study also suggest that the anabolic effects of GH could be mediated through MSTN.

The molecular basis of skeletal muscle atrophy

Skeletal muscle atrophy attributable to muscular inactivity has significant adverse functional consequences. While the initiating physiological event leading to atrophy seems to be the loss of muscle tension and a good deal of the physiology of muscle atrophy has been characterized, little is known about the triggers or the molecular signaling events underlying this process. Decreases in protein synthesis and increases in protein degradation both have been shown to contribute to muscle protein loss due to disuse, and recent work has delineated elements of both synthetic and proteolytic processes underlying muscle atrophy. It is also becoming evident that interactions among known proteolytic pathways (ubiquitin-proteasome, lysosomal, and calpain) are involved in muscle proteolysis during atrophy. Factors such as TNF-alpha, glucocorticoids, myostatin, and reactive oxygen species can induce muscle protein loss under specified conditions. Also, it is now apparent that the transcription factor NF-kappaB is a key intracellular signal transducer in disuse atrophy. Transcriptional profiles of atrophying muscle show both up- and downregulation of various genes over time, thus providing further evidence that there are multiple concurrent processes involved in muscle atrophy. The purpose of this review is to synthesize our current understanding of the molecular regulation of muscle atrophy. We also discuss how ongoing work should uncover more about the molecular underpinnings of muscle wasting, particularly that due to disuse.

Role of myostatin in metabolism

PURPOSE OF REVIEW

To review papers on myostatin published in 2003 and early 2004. Myostatin is a negative regulator of skeletal muscle mass produced in this tissue. Inactivating mutations of the myostatin gene or interaction of myostatin protein with follistatin and other inhibitory proteins induce a hypermuscular phenotype in cattle and mice; this is assumed to result from inhibition of muscle cell proliferation and DNA and protein synthesis (antianabolic effects). Myostatin also controls muscle mass in other animals, and appears to affect adipose tissue mass.

RECENT FINDINGS

New protein interactions inhibiting myostatin that lead to double muscling, as well as the induction of hypermuscularity with myostatin antibodies, or the generation of a myostatin conditional knockout mouse, have been reported. Conversely, a transgenic mouse over-expressing myostatin and exhibiting reduced muscle mass in a gender-specific process has been obtained. In addition, novel inactivating mutations in the myostatin gene and genetic loci regulating myostatin effects, and the characterization of the myostatin gene and its effects on metabolism in fish and chicken have been described. Finally, the regulation of myostatin levels by growth hormone, glucorticoids, anabolic agents, nutritional status and exercise, the characterization of myostatin signaling pathways, and the clarification of myostatin effects on cell replication and differentiation, are other important recent findings.

SUMMARY

These studies suggest that proteins and drugs that inactivate myostatin, or interfere with its binding to its receptor, may be useful for the therapy of wasting and degenerative muscle diseases and for the food industry. Other promising approaches may derive from new insights into the biochemical cascade that mediates myostatin effects, and into the role of myostatin in the regulation of fat metabolism and of heart and muscle regeneration after injury.

Role of metalloprotease disintegrin ADAM12 in determination of quiescent reserve cells during myogenic differentiation in vitro

Skeletal myoblasts grown in vitro and induced to differentiate either form differentiated multinucleated myotubes or give rise to quiescent, undifferentiated "reserve cells" that share several characteristics with muscle satellite cells. The mechanism of determination of reserve cells is poorly understood. We find that the expression level of the metalloprotease disintegrin ADAM12 is much higher in proliferating C2C12 myoblasts and in reserve cells than in myotubes. Inhibition of ADAM12 expression in differentiating C2C12 cultures by small interfering RNA is accompanied by lower expression levels of both quiescence markers (retinoblastoma-related protein p130 and cell cycle inhibitor p27) and differentiation markers (myogenin and integrin alpha7A isoform). Overexpression of ADAM12 in C2C12 cells under conditions that promote cell cycle progression leads to upregulation of p130 and p27, cell cycle arrest, and downregulation of MyoD. Thus, enhanced expression of ADAM12 induces a quiescence-like phenotype and does not stimulate differentiation. We also show that the region extending from the disintegrin to the transmembrane domain of ADAM12 and containing cell adhesion activity as well as the cytoplasmic domain of ADAM12 are required for ADAM12-mediated cell cycle arrest, while the metalloprotease domain is not essential. Our results suggest that ADAM12-mediated adhesion and/or signaling may play a role in determination of the pool of reserve cells during myoblast differentiation.

Myogenic and nonmyogenic cells differentially express proteinases, Hsc/Hsp70, and BAG-1 during skeletal muscle regeneration

Skeletal muscle has a remarkable capacity to regenerate after injury. To determine whether changes in the expression of proteinases, 73-kDa constitutive heat shock cognate protein (Hsc70) and stress-inducible 72-kDa heat shock protein (Hsp70) (Hsc/Hsp70), and Bcl-2-associated gene product-1 (BAG-1) contribute to the remodeling response of muscle tissue, tibialis anterior muscles of male Sprague-Dawley rats were injected with 0.75% bupivacaine and removed at 3, 5, 7, 10, 14, 21, or 35 days postinjection (n = 5-7/group). The immunohistochemical analysis of desmin, alpha-actin, and developmental/neonatal myosin heavy chain expressions indicated the presence of myoblasts (days 3-7), inflammatory cells (days 3-7), degenerating myofibers (days 3-7), regenerating myofibers (days 5-10), and growing mature myofibers (days 10-21) in regenerating muscles. Our biochemical analysis documented profound adaptations in proteolytic metabolism characterized by significant increases in the enzyme activities of matrix metalloproteinases 2 and 9 and plasminogen activators (days 3-14), calpains 1 and 2 (days 3-7), cathepsins B and L(days 3-10), and proteasome (days 3-14). Proteasome activity was strongly correlated with proliferating cell nuclear antigen protein level, suggesting that proteasome played a key role in myoblast proliferation. The expression pattern of BAG-1, a regulatory cofactor of Hsc/Hsp70 at the interface between protein folding and proteasomal proteolysis, did not corroborate the changes in proteasome enzyme activity, suggesting that BAG-1 may promote other functions, such as the folding capacity of Hsc/Hsp70. Altogether, the diversity of functions attributed to proteinases in the present study was strongly supported by the relative changes in the proportion of myogenic and nonmyogenic cells over the time course of regeneration.

Regulation of myostatin in vivo by growth and differentiation factor-associated serum protein-1: a novel protein with protease inhibitor and follistatin domains

Myostatin, a member of the TGFbeta superfamily, is a potent and specific negative regulator of skeletal muscle mass. In serum, myostatin circulates as part of a latent complex containing myostatin propeptide and/or follistatin-related gene (FLRG). Here, we report the identification of an additional protein associated with endogenous myostatin in normal mouse and human serum, discovered by affinity purification and mass spectrometry. This protein, which we have named growth and differentiation factor-associated serum protein-1 (GASP-1), contains multiple domains associated with protease-inhibitory proteins, including a whey acidic protein domain, a Kazal domain, two Kunitz domains, and a netrin domain. GASP-1 also contains a domain homologous to the 10-cysteine repeat found in follistatin, a protein that binds and inhibits activin, another member of the TGFbeta superfamily. We have cloned mouse GASP-1 and shown that it inhibits the biological activity of mature myostatin, but not activin, in a luciferase reporter gene assay. Surprisingly, recombinant GASP-1 binds directly not only to mature myostatin, but also to the myostatin propeptide. Thus, GASP-1 represents a novel class of inhibitory TGFbeta binding proteins.

Myostatin precursor is present in several tissues in teleost fish: a comparative immunolocalization study

In this study, the distribution of myostatin was investigated during larval and postlarval developmental stages of Sparus aurata(sea bream), Solea solea(sole) and Brachydanio rerio(zebrafish) by immunohistochemistry using antisera raised against a synthetic peptide located within the precursor region of sea bream myostatin. All the three species examined showed the strongest immunoreactivity in red skeletal muscle in juveniles and adults. During larval development of sea bream, strong staining was detected in skin and brain. Immunoreactivity was also found in muscle, pharynx, gills, pancreas and liver. From metamorphosis, immunoreactivity was identifiable in the oesophagus, in the apical portion of the stomach epithelium, in the intestinal epithelium and in renal tubules. In larval zebrafish at hatching, the most intense myostatin immunoreactivity was evident in the skin epithelium. Immunoreactivity was also found in the retina and brain. In the adult, an intense immunostaining occurred in the gastrointestinal tract as well as in the ovary. In sole larvae, immunoreactivity was found in liver and intestine. Our results support the hypothesis suggested earlier that myostatins in fish have retained a different partition (compared with mammals) of the expression patterns and functions which characterized the ancestral gene before the duplication event that gave rise to growth differentiation factor-11 (GDF-11) and GDF-8 (myostatin).

Increased MAPK signaling during in vitro muscle wounding

Regeneration of skeletal muscle upon injury is a complex process, involving activation of satellite cells, followed by migration, fusion, and regeneration of damaged myofibers. Previous work concerning the role of the mitogen activated protein (MAP) kinase signaling pathways in muscle injury comes primarily from studies using chemically induced wounding. The purpose of this study was to test the hypothesis that physical injury to skeletal muscle cells in vitro activates the MAP kinase signaling pathways. We demonstrate that extracellular signal regulated kinases (ERKs) 1, 2, and p38 are rapidly and transiently activated in response to injury in C2C12 cells, and are primarily localized to cells adjacent to the wound bed. Culture medium from wounded cells is able to stimulate activation of p38 but not ERK in unwounded cells. These results suggest that both ERK and p38 are involved in the response of muscle cells to physical injury in culture, and reflect what is seen in whole tissues in vivo.