Ca²⁺/calmodulin-based signalling in the regulation of the muscle fibre phenotype and its therapeutic potential via modulation of utrophin A and myostatin expression

Abstract

Ca2+ signalling plays an important role in excitation-contraction coupling and the resultant force output of skeletal muscle. It is also known to play a crucial role in modulating both short- and long-term muscle cellular phenotypic adaptations associated with these events. Ca2+ signalling via the Ca2+/calmodulin (CaM)-dependent phosphatase calcineurin (CnA) and via Ca2+/CaM-dependent kinases, such as CaMKI and CaMKII, is known to regulate hypertrophic growth in response to overload, to direct slow versus fast fibre gene expression, and to contribute to mitochondrial biogenesis. The CnA- and CaMK-dependent regulation of the downstream transcription factors nuclear factor of activated T cells (NFAT) and myocyte-specific enhancer factor 2 are known to activate muscle-specific genes associated with a slower, more oxidative fibre phenotype. We have also recently shown the expression of utrophin A, a cytoskeletal protein that accumulates at the neuromuscular junction and plays a role in maturation of the postsynaptic apparatus, to be regulated by CnA-NFAT and Ca2+/CaM signalling. This regulation is fibre-type specific and potentiated by interactions with the transcriptional regulators and coactivators GA binding protein (also known as nuclear respiratory factor 2) and peroxisome proliferator-activated receptor-gamma coactivator 1 alpha. Another downstream target of CnA signalling may be myostatin, a transforming growth factor-beta family member that is a negative regulator of muscle growth. While the list of the downstream targets of CnA/NFAT- and Ca2+/CaM-dependent signalling is emerging, the precise interaction of these pathways with the Ca2+-independent pathways p38 mitogen-activated protein kinase, extracellular signal-regulated kinases 1 and 2, phosphoinositide-3 kinase, and protein kinase B (Akt/PKB) must also be considered when deciphering fibre responses and plasticity to altered contractile load.

Slow myosin heavy chain expression in the absence of muscle activity

“…It has been proposed that the nerve plays an important role in the specification of the slow MHC phenotype via a calcineurin-dependent signaling pathway and the downstream transcription factors, nuclear factor of activated T cells (NFAT) and myocyte-specific enhancer factor 2 (MEF-2) (for review, Refs. 40,49). This was confirmed by the fact that calcineurin inhibitors decrease the relative expression of MHC-1 protein in the rat soleus muscles (9,50).…”

Molecular and Cellular Mechanism of Muscle Regeneration

“…We found that intraperitoneal CsA treatment daily at 25 mg/Kg/day enhanced the expression of myostatin and Smad3 mRNA in regeneration-defective tibialis anterior muscle after an injection of bupivacaine [89]. The possibility that myostatin is a downstream mediator of calcineurin signaling has been indicated by experiments with two different transgenic mice [94]. In addition, calcineurin's pharmacological inhibition caused a decline in the transcription and activation of myogenin and MyoD during myogenic differentiation by a downregulation of MyoD expression [95].…”

“…Activated calcineurin inhibit the functional role of Egr-1 and Smad2/3 [87,89], and promotes myogenic differentiation. Calcineurin signaling is markedly inhibited by myostatain [94] and FOXO [96,97]. IGF-I produced by the regenerating muscle activates PI3-K-Akt-mTOR signaling resulting in a positive protein balance.…”

Recent research developments in regeneration of skeletal muscle

“…The authors found that intraperitoneal CsA daily treatment (25 mg/Kg/day) enhanced the expression of myostatin and Smad3 mRNA in regenerationdefective tibialis anterior muscle after an injection of bupivacaine 61) . The possibility that myostatin is a downstream mediator of calcineurin signaling has been indicated by experiments with two different types of transgenic mice 65) . In addition, calcineurin's pharmacological inhibition caused a decline in the transcription and activation of myogenin and MyoD, during myogenic differentiation, by a downregulation of MyoD expression 66) .…”

“…During muscle regeneration, the defect of IGF-I expression may affect downstream molecules such as calcineurin 85) and Akt 86) , although Charvet et al 84) did not investigate whether HAS-Cre:Sf/Sf mice had defective calcineurin-and/or Akt-signaling in the skeletal muscles. However, several downstream candidates of these signaling molecules, the NFATc2 87) , MyoD 58) , myogenin 59) , and myostatin 61,65) , were modulated by the SRF mutation 84) . Therefore, the impaired calcineurin-and/or Aktdependent signaling elicited by the reduction in IGF-I may regulate the regenerative defect recognized in HASCre:Sf/Sf mouse muscles.…”