Mechanistic models for muscle diseases and disorders originating in the sarcoplasmic reticulum

AbstractThis review focuses on muscle disorders and diseases caused by defects in the Ca2+ release channels of the sarcoplasmic reticulum, the ryanodine receptors, and in the luminal, low affinity, high capacity Ca2+-binding proteins, calsequestrins. It provides a time line over the past half century of the highlights of research on malignant hyperthermia (MH), central core disease (CCD) and catecholaminergic polymorphic ventricular tachycardia (CPVT), that resulted in the identification of the ryanodine receptor (RYR), calsequestrin (CASQ) and dihydropyridine receptor (CACNA1S) genes as sites of disease-causing mutations. This is followed by a description of approaches to functional analysis of the effects of disease-causing mutations on protein function, focusing on studies of how mutations affect spontaneous (store overload-induced) Ca2+-release from the sarcoplasmic reticulum, the underlying cause of MH and CPVT. Subsequent sections describe results obtained by analysis of knockin mouse lines carrying MH- and CCD-causing mutations, including a Casq1 knockout. The review concludes with the presentation of two mechanistic models. The first shows how dysregulation of Ca2+ homeostasis can lead to muscle diseases involving both RyR and Casq proteins. The second describes a theory of central core formation wherein non-uniformity of Ca2+ release, resulting in non-uniformity of muscle contraction, is presented as an intrinsic property of the specific tertiary structure of mutant heterotetrameric ryanodine receptors and as the underlying cause of core formation in skeletal muscle. This article is part of a Special Issue entitled: 11th European Symposium on Calcium

Muscle weakness in Ryr1 I4895T/WT knock-in mice as a result of reduced ryanodine receptor Ca 2+ ion permeation and release from the sarcoplasmic reticulum

The type 1 isoform of the ryanodine receptor (RYR1) is the Ca 2+ release channel of the sarcoplasmic reticulum (SR) that is activated during skeletal muscle excitation–contraction (EC) coupling. Mutations in the RYR1 gene cause several rare inherited skeletal muscle disorders, including malignant hyperthermia and central core disease (CCD). The human RYR1 I4898T mutation is one of the most common CCD mutations. To elucidate the mechanism by which RYR1 function is altered by this mutation, we characterized in vivo muscle strength, EC coupling, SR Ca 2+ content, and RYR1 Ca 2+ release channel function using adult heterozygous Ryr1 I4895T/+ knock-in mice (IT/+). Compared with age-matched wild-type (WT) mice, IT/+ mice exhibited significantly reduced upper body and grip strength. In spite of normal total SR Ca 2+ content, both electrically evoked and 4-chloro- m -cresol–induced Ca 2+ release were significantly reduced and slowed in single intact flexor digitorum brevis fibers isolated from 4–6-mo-old IT/+ mice. The sensitivity of the SR Ca 2+ release mechanism to activation was not enhanced in fibers of IT/+ mice. Single-channel measurements of purified recombinant channels incorporated in planar lipid bilayers revealed that Ca 2+ permeation was abolished for homotetrameric IT channels and significantly reduced for heterotetrameric WT:IT channels. Collectively, these findings indicate that in vivo muscle weakness observed in IT/+ knock-in mice arises from a reduction in the magnitude and rate of RYR1 Ca 2+ release during EC coupling that results from the mutation producing a dominant-negative suppression of RYR1 channel Ca 2+ ion permeation

Type 1 ryanodine receptor knock-in mutation causing central core disease of skeletal muscle also displays a neuronal phenotype

The type 1 ryanodine receptor (RyR1) is expressed widely in the brain, with high levels in the cerebellum, hippocampus, and hypothalamus. We have shown that L-type Ca(2+) channels in terminals of hypothalamic magnocellular neurons are coupled to RyRs, as they are in skeletal muscle, allowing voltage-induced Ca(2+) release (VICaR) from internal Ca(2+) stores without Ca(2+) influx. Here we demonstrate that RyR1 plays a role in VICaR in nerve terminals. Furthermore, in heterozygotes from the Ryr1(I4895T/WT) (IT/+) mouse line, carrying a knock-in mutation corresponding to one that causes a severe form of human central core disease, VICaR is absent, demonstrating that type 1 RyR mediates VICaR and that these mice have a neuronal phenotype. The absence of VICaR was shown in two ways: first, depolarization in the absence of Ca(2+) influx elicited Ca(2+)syntillas (scintilla, spark, in a nerve terminal, a SYNaptic structure) in WT, but not in mutant terminals; second, in the presence of extracellular Ca(2+), IT/+ terminals showed a twofold decrease in global Ca(2+) transients, with no change in plasmalemmal Ca(2+) current. From these studies we draw two conclusions: (i) RyR1 plays a role in VICaR in hypothalamic nerve terminals; and (ii) a neuronal alteration accompanies the myopathy in IT/+ mice, and, possibly in humans carrying the corresponding RyR1 mutation

Muscle weakness in Ryr1I4895T/WT knock-in mice as a result of reduced ryanodine receptor Ca2+ ion permeation and release from the sarcoplasmic reticulum

The type 1 isoform of the ryanodine receptor (RYR1) is the Ca2+ release channel of the sarcoplasmic reticulum (SR) that is activated during skeletal muscle excitation–contraction (EC) coupling. Mutations in the RYR1 gene cause several rare inherited skeletal muscle disorders, including malignant hyperthermia and central core disease (CCD). The human RYR1I4898T mutation is one of the most common CCD mutations. To elucidate the mechanism by which RYR1 function is altered by this mutation, we characterized in vivo muscle strength, EC coupling, SR Ca2+ content, and RYR1 Ca2+ release channel function using adult heterozygous Ryr1I4895T/+ knock-in mice (IT/+). Compared with age-matched wild-type (WT) mice, IT/+ mice exhibited significantly reduced upper body and grip strength. In spite of normal total SR Ca2+ content, both electrically evoked and 4-chloro-m-cresol–induced Ca2+ release were significantly reduced and slowed in single intact flexor digitorum brevis fibers isolated from 4–6-mo-old IT/+ mice. The sensitivity of the SR Ca2+ release mechanism to activation was not enhanced in fibers of IT/+ mice. Single-channel measurements of purified recombinant channels incorporated in planar lipid bilayers revealed that Ca2+ permeation was abolished for homotetrameric IT channels and significantly reduced for heterotetrameric WT:IT channels. Collectively, these findings indicate that in vivo muscle weakness observed in IT/+ knock-in mice arises from a reduction in the magnitude and rate of RYR1 Ca2+ release during EC coupling that results from the mutation producing a dominant-negative suppression of RYR1 channel Ca2+ ion permeation