Restricting calcium currents is required for correct fiber type specification in skeletal muscle.

Nasreen Sultana,Johannes Rainer,Christoph Schwarzer,Peter Szentesi,Ariane Benedetti,Gerald J Obermair,Beatrix Dienes,Petronel Tuluc,Monika Sztretye,Laszlo Csernoch,Bernhard E Flucher,Michael W Hess

doi:10.1242/dev.129676

Abstract

ABSTRACTSkeletal muscle excitation-contraction (EC) coupling is independent of calcium influx. In fact, alternative splicing of the voltage-gated calcium channel CaV1.1 actively suppresses calcium currents in mature muscle. Whether this is necessary for normal development and function of muscle is not known. However, splicing defects that cause aberrant expression of the calcium-conducting developmental CaV1.1e splice variant correlate with muscle weakness in myotonic dystrophy. Here, we deleted CaV1.1 (Cacna1s) exon 29 in mice. These mice displayed normal overall motor performance, although grip force and voluntary running were reduced. Continued expression of the developmental CaV1.1e splice variant in adult mice caused increased calcium influx during EC coupling, altered calcium homeostasis, and spontaneous calcium sparklets in isolated muscle fibers. Contractile force was reduced and endurance enhanced. Key regulators of fiber type specification were dysregulated and the fiber type composition was shifted toward slower fibers. However, oxidative enzyme activity and mitochondrial content declined. These findings indicate that limiting calcium influx during skeletal muscle EC coupling is important for the secondary function of the calcium signal in the activity-dependent regulation of fiber type composition and to prevent muscle disease.

Highlights

Calcium is the principal second messenger regulating skeletal muscle contraction, growth and differentiation
We reasoned that because the short transcript CaV1.1e is predominant during fetal development, CaV1.1ΔE29 mice would develop normally up to birth, but that the aberrant continuing expression of the high-conductance developmental calcium channel splice variant throughout postnatal development and adult life would reveal any influence of the extra calcium influx on EC coupling and/or other calciummediated signaling processes regulating muscle growth and differentiation
In mature mice the increased slow fiber content and the increased expression of PGC1α were no longer accompanied by the expected increase in oxidative enzyme activity, most likely because the increased calcium influx in CaV1.1ΔE29/ΔE29 muscles caused a severe loss of mitochondria

Summary

Introduction

Calcium is the principal second messenger regulating skeletal muscle contraction, growth and differentiation. In excitationcontraction (EC) coupling, cytoplasmic calcium levels are rapidly increased in response to action potentials and the magnitude of these calcium signals regulates the force of contraction. Voltage-gated calcium channels (CaV1.1) and calcium release channels (type 1 ryanodine receptors, RyR1) are physically coupled to one another so that voltage-dependent activation of. Calcium currents through the major CaV1.1a splice variant are small and activate slowly, only at strong membrane depolarizations

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Conclusion