SUMMARYAn important unresolved question in skeletal muscle plasticity is whether satellite cells are necessary for muscle fiber hypertrophy. To address this issue, a novel mouse strain (Pax7-DTA) was created which enabled the conditional ablation of >90% of satellite cells in mature skeletal muscle following tamoxifen administration. To test the hypothesis that satellite cells are necessary for skeletal muscle hypertrophy, the plantaris muscle of adult Pax7-DTA mice was subjected to mechanical overload by surgical removal of the synergist muscle. Following two weeks of overload, satellite cell-depleted muscle showed the same increases in muscle mass (approximately twofold) and fiber cross-sectional area with hypertrophy as observed in the vehicle-treated group. The typical increase in myonuclei with hypertrophy was absent in satellite cell-depleted fibers, resulting in expansion of the myonuclear domain. Consistent with lack of nuclear addition to enlarged fibers, long-term BrdU labeling showed a significant reduction in the number of BrdU-positive myonuclei in satellite cell-depleted muscle compared with vehicle-treated muscle. Single fiber functional analyses showed no difference in specific force, Ca 2+ sensitivity, rate of cross-bridge cycling and cooperativity between hypertrophied fibers from vehicle and tamoxifen-treated groups. Although a small component of the hypertrophic response, both fiber hyperplasia and regeneration were significantly blunted following satellite cell depletion, indicating a distinct requirement for satellite cells during these processes. These results provide convincing evidence that skeletal muscle fibers are capable of mounting a robust hypertrophic response to mechanical overload that is not dependent on satellite cells.

High-resistance exercise training results in an increase in muscle wet mass and protein content. To begin to address the acute changes following a single bout of high-resistance exercise, a new model has been developed. Training rats twice a week for 6 wk resulted in 13.9 and 14.4% hypertrophy in the extensor digitorum longus (EDL) and tibialis anterior (TA) muscles, respectively. Polysome profiles after high-resistance lengthening contractions suggest that the rate of initiation is increased. The activity of the 70-kDa S6 protein kinase (p70S6k), a regulator of translation initiation, is also increased following high-resistance lengthening contractions (TA, 363 ± 29%; EDL, 353 ± 39%). Furthermore, the increase in p70S6k activity 6 h after exercise correlates with the percent change in muscle mass after 6 wk of training ( r = 0.998). The tight correlation between the activation of p70S6k and the long-term increase in muscle mass suggests that p70S6k phosphorylation may be a good marker for the phenotypic changes that characterize muscle hypertrophy and may play a role in load-induced skeletal muscle growth.

Circadian rhythms of cell and organismal physiology are controlled by an autoregulatory transcription-translation feedback loop that regulates the expression of rhythmic genes in a tissue-specific manner. Recent studies have suggested that components of the circadian pacemaker, such as the Clock and Per2 gene products, regulate a wide variety of processes, including obesity, sensitization to cocaine, cancer susceptibility, and morbidity to chemotherapeutic agents. To identify a more complete cohort of genes that are transcriptionally regulated by CLOCK and/or circadian rhythms, we used a DNA array interrogating the mouse protein-encoding transcriptome to measure gene expression in liver and skeletal muscle from WT and Clock mutant mice. In WT tissue, we found that a large percentage of expressed genes were transcription factors that were rhythmic in either muscle or liver, but not in both, suggesting that tissue-specific output of the pacemaker is regulated in part by a transcriptional cascade. In comparing tissues from WT and Clock mutant mice, we found that the Clock mutation affects the expression of many genes that are rhythmic in WT tissue, but also profoundly affects many nonrhythmic genes. In both liver and skeletal muscle, a significant number of CLOCKregulated genes were associated with the cell cycle and cell proliferation. To determine whether the observed patterns in cell-cycle gene expression in Clock mutants resulted in functional dysregulation, we compared proliferation rates of fibroblasts derived from WT or Clock mutant embryos and found that the Clock mutation significantly inhibits cell growth and proliferation.cell cycle ͉ circadian rhythms ͉ Clock mutation ͉ gene expression ͉ protein-encoding transcriptome M any organisms have Ϸ24-h rhythms in metabolism, physiology, and behavior that are driven by cell autonomous circadian pacemakers (1). These circadian rhythms allow organisms to coordinate a myriad of physiological processes with the changing environment. In mammals, the circadian pacemaker is composed of interlocked transcription-translation feedback loops: the primary loop is composed of the basic helix-loophelix transcription factors CLOCK and BMAL1, which drive transcription of the Period (Per1, Per2) and Cryptochrome (Cry1, Cry2) genes (1, 2). PER and CRY proteins form the negative limb of the feedback loop by inhibiting their own CLOCK: BMAL1-induced transcription; turnover of PER and CRY allows the cycle to begin anew. The interlocked loop consists of REV-ERB-␣ and ROR␣, which repress and activate the Bmal1 gene, thereby modulating its function (3, 4). Mutation or deletion of Clock (5), Bmal1 (6), Per1/2 genes (7, 8), or Cry1/2 (9, 10) genes results in behavioral arrhythmicity and disruption of the autoregulatory loop, whereas disruption of components of the secondary loop results in short period-length phenotypes (3, 4).The molecular components of the circadian clock are present in the majority of neurons in the suprachiasmatic nucleus (SCN), a bilateral body in the anterior hypot...

MicroRNAs (miRNAs) are a class of highly conserved, noncoding RNAs involved in posttranscriptional gene regulation. A small number of muscle-specific miRNAs have been identified and shown to have a role in myoblast proliferation and differentiation as well as embryonic muscle growth. The primary objective of the present study was to determine the expression level of the muscle-specific miRNAs in the soleus and plantaris muscles and whether their expression in the plantaris was altered in response to functional overload. Of the miRNAs examined, only miRNA-206 was differentially expressed between soleus and plantaris muscles, as reflected by the sevenfold higher expression in the soleus for both the primary miRNA (pri-miRNA) and mature miRNA (miR). Following 7 days of functional overload, transcript levels for both pri-miRNA-1-2 and pri-miRNA-133a-2 increased by approximately 2-fold, whereas pri-miRNA-206 levels were elevated 18.3-fold. In contrast, expression of miR-1 and miR-133a were downregulated by approximately 50% following overload. The discrepancy between pri-miRNA and miR expression following overload was not explained by a change in the expression of components of the miRNA biogenesis pathway, since Drosha and Exportin-5 transcript levels were significantly increased by 50% in response to functional overload, whereas Dicer expression remained unchanged. These results are the first to report alterations in expression of muscle-specific miRNAs in adult skeletal muscle and suggest miRNAs may have a role in the adaptation to functional overload.

MyoD, a master regulator of myogenesis, exhibits a circadian rhythm in its mRNA and protein levels, suggesting a possible role in the daily maintenance of muscle phenotype and function. We report that MyoD is a direct target of the circadian transcriptional activators CLOCK and BMAL1, which bind in a rhythmic manner to the core enhancer of the MyoD promoter. Skeletal muscle of Clock Δ19 and Bmal1 −/− mutant mice exhibited ∼30% reductions in normalized maximal force. A similar reduction in force was observed at the single-fiber level. Electron microscopy (EM) showed that the myofilament architecture was disrupted in skeletal muscle of Clock Δ19 , Bmal1 −/− , and MyoD −/− mice. The alteration in myofilament organization was associated with decreased expression of actin, myosins, titin, and several MyoD target genes. EM analysis also demonstrated that muscle from both Clock Δ19 and Bmal1 −/− mice had a 40% reduction in mitochondrial volume. The remaining mitochondria in these mutant mice displayed aberrant morphology and increased uncoupling of respiration. This mitochondrial pathology was not seen in muscle of MyoD −/− mice. We suggest that altered expression of both Pgc-1α and Pgc-1β in Clock Δ19 and Bmal1 −/− mice may underlie this pathology. Taken together, our results demonstrate that disruption of CLOCK or BMAL1 leads to structural and functional alterations at the cellular level in skeletal muscle. The identification of MyoD as a clock-controlled gene provides a mechanism by which the circadian clock may generate a muscle-specific circadian transcriptome in an adaptive role for the daily maintenance of adult skeletal muscle.circadian clock | myofilaments | mitochondria A fundamental, evolutionarily conserved property of most organisms, from cyanobacteria to plants and animals, is the daily cycling of their internal physiology as well as certain behaviors in animals, such as sleep and feeding (1). The timing of these circadian rhythms is synchronized to the environment by external cues, with light and nutrient availability being two of the most salient entrainment cues (2). The synchronization of endogenous circadian rhythms with the daily cycles in the environment provides an adaptive mechanism for organisms to anticipate cyclic demands on cellular physiology and behavior (3, 4). At the molecular level, the circadian clock represents a well-defined gene regulatory network composed of transcriptional-translational feedback loops (5). The positive arm of the loop is composed of the transcription factors CLOCK and BMAL1, which heterodimerize and bind to E-box elements on target genes such as Per1 to drive the negative part of the feedback loop (5). More recently, the same molecular clock factors that have been identified in the central clock in the suprachiasmatic nucleus have been found to exist in most peripheral tissues (reviewed in ref. 6).We recently characterized the circadian transcriptome of adult skeletal muscle. These mRNAs exhibit significant oscillation in gene expression with a repeating period len...

Circadian rhythms are approximate 24-h behavioral and physiological cycles that function to prepare an organism for daily environmental changes. The basic clock mechanism is a network of transcriptional-translational feedback loops that drive rhythmic expression of genes over a 24-h period. The objectives of this study were to identify transcripts with a circadian pattern of expression in adult skeletal muscle and to determine the effect of the Clock mutation on gene expression. Expression profiling on muscle samples collected every 4 h for 48 h was performed. Using COSOPT, we identified a total of 215 transcripts as having a circadian pattern of expression. Real-time PCR results verified the circadian expression of the core clock genes, Bmal1, Per2, and Cry2. Annotation revealed cycling genes were involved in a range of biological processes including transcription, lipid metabolism, protein degradation, ion transport, and vesicular trafficking. The tissue specificity of the skeletal muscle circadian transcriptome was highlighted by the presence of known muscle-specific genes such as Myod1, Ucp3, Atrogin1 (Fbxo32), and Myh1 (myosin heavy chain IIX). Expression profiling was also performed on muscle from the Clock mutant mouse and sarcomeric genes such as actin and titin, and many mitochondrial genes were significantly downregulated in the muscle of Clock mutant mice. Defining the circadian transcriptome in adult skeletal muscle and identifying the significant alterations in gene expression that occur in muscle of the Clock mutant mouse provide the basis for understanding the role of circadian rhythms in the daily maintenance of skeletal muscle.

The aim of this study was to understand better the specific signaling events resulting from different modes of exercise. Three different exercise protocols were employed based on their well-characterized, long-term training effects on either muscle hypertrophy or endurance phenotypes. Rats were subjected to a single bout of either a high-frequency electrical stimulation, a low-frequency electrical stimulation, or a running exercise protocol. Postexercise intracellular signaling was analyzed in the tibialis anterior and soleus muscles at 0, 3, and 6 h. A prolonged increase in p70(S6k) and a transient increase in protein kinase B phosphorylation were only observed in response to a growth-inducing stimulus (e.g., tibialis anterior in high-frequency electrical stimulation). In contrast, extracellular regulated kinase and 38-kDa stress-activated protein kinase were activated in response to all forms of exercise at 0 h, but only extracellular regulated kinase phosphorylation was found significantly elevated at 6 h after running exercise. These results demonstrate that different exercise protocols resulted in the selective activation of specific intracellular signaling pathways, which may determine the specific adaptations induced by different forms of exercise.

In response to growth factors, mTOR (mammalian target of rapamycin) has been identified as a central component of the signalling pathways that control the translational machinery and cell growth. Signalling through mTOR has also been shown to be necessary for the mechanical load-induced growth of cardiac and skeletal muscles. Although the mechanisms involved for mechanically induced activation of mTOR are not known, it has been suggested that activation of PI3K (phosphoinositide 3-kinase) and protein kinase B (Akt), via the release of locally acting growth factors, underlies this process. In the present study, we show that mechanically stimulating (passive stretch) the skeletal muscle ex vivo results in the activation of mTOR-dependent signalling events. The activation of mTOR-dependent signalling events was necessary for an increase in translational efficiency, demonstrating the physiological significance of this pathway. Using pharmacological inhibitors, we show that activation of mTOR-dependent signalling occurs through a PI3K-independent pathway. Consistent with these results, mechanically induced signalling through mTOR was not disrupted in muscles from Akt1-/- mice. In addition, ex vivo co-incubation experiments, along with in vitro conditioned-media experiments, demonstrate that a mechanically induced release of locally acting autocrine/paracrine growth factors was not sufficient for the activation of the mTOR pathway. Taken together, our results demonstrate that mechanical stimuli can activate the mTOR pathway independent of PI3K/Akt1 and locally acting growth factors. Thus mechanical stimuli and growth factors provide distinct inputs through which mTOR co-ordinates an increase in the translational efficiency.