Berdeaux R, Stewart R. cAMP signaling in skeletal muscle adaptation: hypertrophy, metabolism, and regeneration. Am J Physiol Endocrinol Metab 303: E1-E17, 2012. First published February 21, 2012 doi:10.1152/ajpendo.00555.2011.-Among organ systems, skeletal muscle is perhaps the most structurally specialized. The remarkable subcellular architecture of this tissue allows it to empower movement with instructions from motor neurons. Despite this high degree of specialization, skeletal muscle also has intrinsic signaling mechanisms that allow adaptation to long-term changes in demand and regeneration after acute damage. The second messenger adenosine 3=,5=-monophosphate (cAMP) not only elicits acute changes within myofibers during exercise but also contributes to myofiber size and metabolic phenotype in the long term. Strikingly, sustained activation of cAMP signaling leads to pronounced hypertrophic responses in skeletal myofibers through largely elusive molecular mechanisms. These pathways can promote hypertrophy and combat atrophy in animal models of disorders including muscular dystrophy, agerelated atrophy, denervation injury, disuse atrophy, cancer cachexia, and sepsis. cAMP also participates in muscle development and regeneration mediated by muscle precursor cells; thus, downstream signaling pathways may potentially be harnessed to promote muscle regeneration in patients with acute damage or muscular dystrophy. In this review, we summarize studies implicating cAMP signaling in skeletal muscle adaptation. We also highlight ligands that induce cAMP signaling and downstream effectors that are promising pharmacological targets. cyclic AMP; skeletal muscle; cell signaling; muscle regeneration; atrophy; protein kinase A ORIGINALLY DISCOVERED by Sutherland and Rall in liver homogenates in 1958 (218), adenosine 3=,5=-monophosphate (cyclic AMP, or cAMP) has since been intensively studied and is one of the best-characterized signaling molecules. In skeletal muscle, acute cAMP signaling has been implicated in regulation of glycogenolysis (213), contractility (32,83,84,88,138,234), sarcoplasmic calcium dynamics (58,147,184,204), and recovery from sustained contractile activity (44, 162). The net result of acute cAMP action on the order of minutes in skeletal muscle can generally be described as increased contractile force and rapid recovery of ion balance, especially during prolonged contractions. These acute changes are most pertinent to muscle contraction and energy utilization during exercise, when epinephrine is rapidly released into the circulation (92) and cAMP accumulates in muscle (72).Many studies have shown, however, that cAMP-inducing agents or genetic modification of proteins involved in cAMP signaling can also have adaptive effects on skeletal muscle by increasing myofiber size and promoting fiber-type transitions to glycolytic fibers (9,18,42,43,57,63,86,91,101,121,128,154,155,163,190,193,194,215). The prohypertrophic actions of ␤-adrenergic receptor (␤-AR) agonists and corticotropin-releasing factor recept...

The cAMP response element binding protein (CREB) plays key roles in differentiation of embryonic skeletal muscle progenitors and survival of adult skeletal muscle. However, little is known about the physiologic signals that activate CREB in normal muscle. Here we show that CREB phosphorylation and target genes are induced after acute muscle injury and during regeneration due to genetic mutation. Activated CREB localizes to both myogenic precursor cells and newly regenerating myofibers within regenerating areas. Moreover, we found that signals from damaged skeletal muscle tissue induce CREB phosphorylation and target gene expression in primary mouse myoblasts. An activated CREB mutant (CREBY134F) potentiates myoblast proliferation as well as expression of early myogenic transcription factors in cultured primary myocytes. Consistently, activated CREB-YF promotes myoblast proliferation after acute muscle injury in vivo and enhances muscle regeneration in dystrophic mdx mice. Our findings reveal a new physiologic function for CREB in contributing to skeletal muscle regeneration.

cAMP signaling can both promote and inhibit myogenic differentiation, but little is known about the mechanisms mediating promyogenic effects of cAMP. We previously demonstrated that the cAMP response element-binding protein (CREB) transcriptional target salt-inducible kinase 1 (SIK1) promotes MEF2 activity in myocytes via phosphorylation of class II histone deacetylase proteins (HDACs). However, it was unknown whether SIK1 couples cAMP signaling to the HDAC-MEF2 pathway during myogenesis and how this response could specifically occur in differentiating muscle cells. To address these questions, we explored SIK1 regulation and function in muscle precursor cells before and during myogenic differentiation. We found that in primary myogenic progenitor cells exposed to cAMP-inducing agents, Sik1 transcription is induced, but the protein is rapidly degraded by the proteasome. By contrast, sustained cAMP signaling extends the half-life of SIK1 in part by phosphorylation of Thr475, a previously uncharacterized site that we show can be phosphorylated by PKA in cell-free assays. We also identified a functional PEST domain near Thr475 that contributes to SIK1 degradation. During differentiation of primary myogenic progenitor cells, when PKA activity has been shown to increase, we observe elevated Sik1 transcripts as well as marked accumulation and stabilization of SIK1 protein. Depletion of Sik1 in primary muscle precursor cells profoundly impairs MEF2 protein accumulation and myogenic differentiation. Our findings support an emerging model in which SIK1 integrates cAMP signaling with the myogenic program to support appropriate timing of differentiation. M yoblast differentiation is driven by a transcriptional cascade that is subject to precise temporal control during development as well as after acute muscle injury (1). The second messenger cAMP and its primary effector PKA are known to contribute to myoblast proliferation in the developing dermomyotome (2), to promote migration and fusion of muscle precursor cells (3, 4), and to exert potent anabolic effects on adult skeletal muscle (5). During differentiation of myoblasts ex vivo, intracellular cAMP peaks transiently before cell-cell fusion (6, 7), and PKA signaling is required for expression of differentiation markers in cultured myoblasts (8) and in mouse embryos (2). However, sustained cAMP-PKA signaling prevents myogenic differentiation, in part through inhibition of basic helix-loop-helix and MADS transcription factors (9-11). The mechanisms by which a transient peak of cAMP promotes myoblast differentiation are still poorly understood. Because ligands that induce cAMP production promote muscle regeneration (5), it is important to identify promyogenic cAMP signaling effectors.The transcription factor CREB (cAMP response elementbinding protein) is a key effector of cAMP-PKA in many cell types (12). CREB phosphorylation is induced during early stages of vertebrate myogenesis (2, 13), and CREB activity is required in mice for dermomyotome development (2). We previously...

The transient receptor potential vanilloid 3 channel (TRPV3) is abundantly expressed in epidermal keratinocytes and plays important roles in sensory biology and skin health. Mg2+ deficiency causes skin disorders under certain pathological conditions such as type 2 diabetes mellitus. In this study, we investigated the effect of Mg2+ on TRPV3 in primary epidermal keratinocytes. Extracellular Mg2+ ([Mg2+]o) inhibited TRPV3-mediated membrane current and calcium influx. TRPV3 activation induced a calcium signaling pathway culminating in activation of the cAMP response element binding (CREB). TRPV3 inhibition by [Mg2+]o, the TRPV3 blocker ruthenium red or TRPV3 siRNA suppressed this response. In TRPV3-expressing Chinese hamster ovary (CHO) cells, both extracellular and intracellular Mg2+ inhibited TRPV3 single-channel conductance but not open probability. Neutralization of an aspartic acid residue (D641) in the extracellular pore loop or two acidic residues (E679, E682) in the inner pore region significantly attenuated the inhibitory effect of extracellular or intracellular Mg2+ on TRPV3-mediated signaling, respectively. Our findings suggest that epidermal TRPV3 is tonically inhibited by both extracellular and intracellular Mg2+, which act on both sides of the channel pore loop. Mg2+ deficiency may promote the function of TRPV3 and contribute to the pathogenesis of skin diseases.