Calcineurin Is Required for Skeletal Muscle Hypertrophy

Abstract

Molecular signaling pathways linking increases in skeletal muscle usage to alterations in muscle size have not been identified. In the present study, we tested the hypothesis that calcineurin, a calcium-regulated phosphatase recently implicated in the signaling of some forms of cardiomyopathic growth, is required to induce skeletal muscle hypertrophy and muscle fiber type conversions associated with functional overload in vivo. Administration of the specific calcineurin inhibitors cyclosporin (CsA) or FK506 to mice, for which the fast plantaris muscle was overloaded for 1-4 weeks, prevented the rapid doubling of mass and individual fiber size and the 4 -20-fold increase in the number of slow fibers that characterize this condition. CsA treatment influenced the expression of muscle myofibrillar protein genes in a way reflective of fiber phenotype transformations but only in the long term of the overload condition, suggesting that the control of this growth response by calcineurin is not limited to the transcriptional activation of these muscle-specific genes. Clinically, these results provide insight to the post-surgical muscle wasting and weakness observed in recovering transplant recipients administered therapeutic dosages of these immunosuppressants.The amount and type of contractile proteins incorporated into the myofibrils of skeletal muscle fibers are major determinants of the size, strength, and speed of these cells (1). To date, the molecular events linking muscle usage to the cellular expression and accumulation of these proteins are unknown. Recently, calcineurin, a cytoplasmic calcium-regulated phosphatase implicated in the pathogenesis of hypertrophic cardiomyopathy (2, 3), has emerged as a possible candidate in the signaling of skeletal muscle cellular growth and the fiber type transformations (4) of these cells. Calcineurin is an enticing prospect as a regulatory enzyme in this signaling because its selective activation of NF-AT (nuclear factor of activated T cells) transcription factors in response to sustained increases in intracellular calcium concentrations (5) is reminiscent of calcium fluctuations provoked by the activation of muscle cells during extensive contractile work (4, 6).Typically, a fast weight-bearing muscle subjected to the functional loss of its synergists will compensate by displaying within 2-4 weeks a doubling of mass and individual fiber sizes and an increase in strength (7,8). A muscle overloaded in this manner will also contract more slowly as a result of rapid fiber type transformations characterized by an increase in the number of fibers exhibiting slower, more energy-efficient contractile and calcium-handling proteins (7-9). In rodent fast muscle, the myosin heavy chain (MHC) 1 enzyme component of the major contractile protein myosin displays a conversion pattern in response to overload that follows from the fastest to the slowest isoform in the order MHC IIb 3 IIx 3 IIa 3 I/slow. The signaling of these adaptations may well be mediated by calcineurin since most of the fun...

A Protein Encoded within the Down Syndrome Critical Region Is Enriched in Striated Muscles and Inhibits Calcineurin Signaling

“…MCIP itself may be subject to regulation by protein phosphorylation and dephosphorylation, and thereby may serve to integrate other signaling inputs with the calcineurin pathway. We speculate that these proteins may serve as negative regulators to prevent adverse consequences of unrestrained calcineurin activity in muscle tissues (7,8,18,19). However, it is also possible that the interaction with MCIP directs calcineurin to specific intracellular locales or to specific phosphoprotein substrates.…”

“…Moreover, hypertrophy and heart failure in these animals, and in certain other animal models of cardiomyopathy, are prevented by administration of the calcineurin antagonist drugs cyclosporin A or FK-506 (17). In skeletal muscles, calcineurin signaling is implicated both in hypertrophic growth stimulated by insulin-like growth factor-1 (8,18), and in the control of specialized programs of gene expression that establish distinctive myofiber subtypes (9,19). These observations have stimulated interest in the therapeutic potential of modifying calcineurin activity selectively in muscle cells while avoiding unwanted consequences of altered calcineurin signaling in other cell types (20).…”

A protein kinase B-dependent and rapamycin-sensitive pathway controls skeletal muscle growth but not fiber type specification

“…The physiological role of calcineurin has been investigated in vivo by using pharmacological inhibition with cyclosporin A (CsA) and FK506. These inhibitors were found to induce a slow-to-fast fiber type switch in adult muscle (4) and prevent the fast-to-slow conversion induced by functional overload (5), but this finding was not confirmed in another study (6). The effect of pharmacological inhibition of calcineurin on skeletal muscle growth is also controversial.…”

“…The effect of pharmacological inhibition of calcineurin on skeletal muscle growth is also controversial. Some researchers have reported that muscle hypertrophy induced by functional overload and muscle weight recovery after atrophy is inhibited by CsA and FK506 (5,7), although this hypertrophy may vary according to muscle type and stage of muscle growth (8), whereas others found no inhibition (6,9). A genetic inhibitory strategy was recently used to show that overexpression of the calcineurin peptide inhibitor cain͞ cabin-1 blocks, like CsA and FK506, the expression of the slow fiber phenotype but not fiber growth during muscle regeneration (10).…”

Effect of calcineurin inhibitors on myotoxic activity of crotoxin and Bothrops asper phospholipase A2 myotoxins in vivo and in vitro

“…FK506 has distinct molecular structure as compared to CsA, however it binds to the same surface of calcineurin (Huai et al., 2002) and efficiently inhibits calcineurin with 100-fold lower concentration as compared to CsA (Liu et al, 1991). The doses of CsA and FK506 utilized in the present work in vivo (20 and 2 mg/kg/day b.w., respectively) and in vitro (10 − 6 M) have been previously studied and are able to significantly reduce calcineurin activity (∼ 60%) (Lai et al, 1998;Dunn et al, 1999;Friday et al, 2000;Dunn et al, 2001;Serrano et al, 2001;Miyabara et al, 2005). Calcineurin inhibition by CsA (Fig.…”