New factors contributing to dynamic calcium regulation in the skeletal muscle triad-a crowded place

Proteomic profiling of impaired excitation-contraction coupling and abnormal calcium handling in muscular dystrophy

The X-linked inherited neuromuscular disorder Duchenne muscular dystrophy is characterised by primary abnormalities in the membrane cytoskeletal component dystrophin. The almost complete absence of the Dp427-M isoform of dystrophin in skeletal muscles renders contractile fibres more susceptible to progressive degeneration and a leaky sarcolemma membrane. This in turn results in abnormal calcium homeostasis, enhanced proteolysis and impaired excitation-contraction coupling. Biochemical and mass spectrometry-based proteomic studies of both patient biopsy specimens and genetic animal models of dystrophinopathy have demonstrated significant changes in the concentration and/or physiological function of essential calcium-regulatory proteins in dystrophin-lacking voluntary muscles. Abnormalities include dystrophinopathy-associated changes in voltage sensing receptors, calcium release channels, calcium pumps and calcium binding proteins. This review article provides an overview of the importance of the sarcolemmal dystrophin-glycoprotein complex and the wider dystrophin complexome in skeletal muscle and its linkage to depolarisation-induced calcium-release mechanisms and the excitation-contraction-relaxation cycle. Besides chronic inflammation, fat substitution and reactive myofibrosis, a major pathobiochemical hallmark of X-linked muscular dystrophy is represented by the chronic influx of calcium ions through the damaged plasmalemma in conjunction with abnormal intracellular calcium fluxes and buffering. Impaired calcium handling proteins should therefore be included in an improved biomarker signature of Duchenne muscular dystrophy.

Excitation-contraction coupling in mammalian skeletal muscle: Blending old and last-decade research

The excitation–contraction coupling (ECC) in skeletal muscle refers to the Ca²⁺-mediated link between the membrane excitation and the mechanical contraction. The initiation and propagation of an action potential through the membranous system of the sarcolemma and the tubular network lead to the activation of the Ca²⁺-release units (CRU): tightly coupled dihydropyridine and ryanodine (RyR) receptors. The RyR gating allows a rapid, massive, and highly regulated release of Ca²⁺ from the sarcoplasmic reticulum (SR). The release from triadic places generates a sarcomeric gradient of Ca²⁺ concentrations ([Ca²⁺]) depending on the distance of a subcellular region from the CRU. Upon release, the diffusing Ca²⁺ has multiple fates: binds to troponin C thus activating the contractile machinery, binds to classical sarcoplasmic Ca²⁺ buffers such as parvalbumin, adenosine triphosphate and, experimentally, fluorescent dyes, enters the mitochondria and the SR, or is recycled through the Na⁺/Ca²⁺ exchanger and store-operated Ca²⁺ entry (SOCE) mechanisms. To commemorate the 7^th decade after being coined, we comprehensively and critically reviewed “old”, historical landmarks and well-established concepts, and blended them with recent advances to have a complete, quantitative-focused landscape of the ECC. We discuss the: 1) elucidation of the CRU structures at near-atomic resolution and its implications for functional coupling; 2) reliable quantification of peak sarcoplasmic [Ca²⁺] using fast, low affinity Ca²⁺ dyes and the relative contributions of the Ca²⁺-binding mechanisms to the whole concert of Ca²⁺ fluxes inside the fibre; 3) articulation of this novel quantitative information with the unveiled structural details of the molecular machinery involved in mitochondrial Ca²⁺ handing to understand how and how much Ca²⁺ enters the mitochondria; 4) presence of the SOCE machinery and its different modes of activation, which awaits understanding of its magnitude and relevance in situ; 5) pharmacology of the ECC, and 6) emerging topics such as the use and potential applications of super-resolution and induced pluripotent stem cells (iPSC) in ECC. Blending the old with the new works better!

RGK protein-mediated impairment of slow depolarization- dependent Ca2+ entry into developing myotubes

Three physiological functions have been described for the skeletal muscle 1,4-dihydropyridine receptor (Ca(V)1.1):(1) voltage-sensor for excitation-contraction (EC) coupling, (2) L-type Ca(2+) channel, and (3) voltage-sensor for slow depolarization-dependent Ca(2+) entry. Members of the RGK (Rad, Rem, Rem2, Gem/Kir) family of monomeric GTP-binding proteins are potent inhibitors of the former two functions of Ca(V)1.1. However, it is not known whether the latter function that has been attributed to Ca(V)1.1 is subject to modulation by RGK proteins. Thus, the purpose of this study was to determine whether Rad, Gem and/or Rem inhibit the slowly developing, persistent Ca(2+) entry that is dependent on the voltage-sensing capability of Ca(V)1.1. As a means to investigate this question, Venus fluorescent protein-fused RGK proteins(V-Rad, V-Rem and V-Gem) were overexpressed in “normal” mouse myotubes. We observed that such overexpression of V-Rad, V-Rem or V-Gem in myotubes caused marked changes in morphology of the cells. As shown previously for YFPRem,both L-type current and EC coupling were also impaired greatly in myotubes expressing either V-Rad or V-Gem. There ductions in L-type current and EC coupling were paralleled by reductions in depolarization-induced Ca(2+) entry. Our observations provide the first evidence of modulation of this enigmatic Ca(2+) entry pathway peculiar to skeletal muscle.

Ca(V)1.1: The atypical prototypical voltage-gated Ca²⁺ channel

Ca(V)1.1 is the prototype for the other nine known Ca(V) channel isoforms, yet it has functional properties that make it truly atypical of this group. Specifically, Ca(V)1.1 is expressed solely in skeletal muscle where it serves multiple purposes; it is the voltage sensor for excitation-contraction coupling and it is an L-type Ca²⁺ channel which contributes to a form of activity-dependent Ca²⁺ entry that has been termed Excitation-coupled Ca²⁺ entry. The ability of Ca(V)1.1 to serve as voltage-sensor for excitation-contraction coupling appears to be unique among Ca(V) channels, whereas the physiological role of its more conventional function as a Ca²⁺ channel has been a matter of uncertainty for nearly 50 years. In this chapter, we discuss how Ca(V)1.1 supports excitation-contraction coupling, the possible relevance of Ca²⁺ entry through Ca(V)1.1 and how alterations of Ca(V)1.1 function can have pathophysiological consequences. This article is part of a Special Issue entitled: Calcium channels.