Central core disease mutations R4892W, I4897T and G4898E in the ryanodine receptor isoform 1 reduce the Ca2+ sensitivity and amplitude of Ca2+-dependent Ca2+ release

Preclinical model systems of ryanodine receptor 1-related myopathies and malignant hyperthermia: a comprehensive scoping review of works published 1990-2019

BACKGROUND

Pathogenic variations in the gene encoding the skeletal muscle ryanodine receptor (RyR1) are associated with malignant hyperthermia (MH) susceptibility, a life-threatening hypermetabolic condition and RYR1-related myopathies (RYR1-RM), a spectrum of rare neuromuscular disorders. In RYR1-RM, intracellular calcium dysregulation, post-translational modifications, and decreased protein expression lead to a heterogenous clinical presentation including proximal muscle weakness, contractures, scoliosis, respiratory insufficiency, and ophthalmoplegia. Preclinical model systems of RYR1-RM and MH have been developed to better understand underlying pathomechanisms and test potential therapeutics.

METHODS

We conducted a comprehensive scoping review of scientific literature pertaining to RYR1-RM and MH preclinical model systems in accordance with the PRISMA Scoping Reviews Checklist and the framework proposed by Arksey and O'Malley. Two major electronic databases (PubMed and EMBASE) were searched without language restriction for articles and abstracts published between January 1, 1990 and July 3, 2019.

RESULTS

Our search yielded 5049 publications from which 262 were included in this review. A majority of variants tested in RYR1 preclinical models were localized to established MH/central core disease (MH/CCD) hot spots. A total of 250 unique RYR1 variations were reported in human/rodent/porcine models with 95% being missense substitutions. The most frequently reported RYR1 variant was R614C/R615C (human/porcine total n = 39), followed by Y523S/Y524S (rabbit/mouse total n = 30), I4898T/I4897T/I4895T (human/rabbit/mouse total n = 20), and R163C/R165C (human/mouse total n = 18). The dyspedic mouse was utilized by 47% of publications in the rodent category and its RyR1-null (1B5) myotubes were transfected in 23% of publications in the cellular model category. In studies of transfected HEK-293 cells, 57% of RYR1 variations affected the RyR1 channel and activation core domain. A total of 15 RYR1 mutant mouse strains were identified of which ten were heterozygous, three were compound heterozygous, and a further two were knockout. Porcine, avian, zebrafish, C. elegans, canine, equine, and drosophila model systems were also reported.

CONCLUSIONS

Over the past 30 years, there were 262 publications on MH and RYR1-RM preclinical model systems featuring more than 200 unique RYR1 variations tested in a broad range of species. Findings from these studies have set the foundation for therapeutic development for MH and RYR1-RM.

Dantrolene rescues aberrant N-terminus intersubunit interactions in mutant pro-arrhythmic cardiac ryanodine receptors

AIMS

The ryanodine receptor (RyR2) is an intracellular Ca(2+) release channel essential for cardiac excitation-contraction coupling. Abnormal RyR2 channel function results in the generation of arrhythmias and sudden cardiac death. The present study was undertaken to investigate the mechanistic basis of RyR2 dysfunction in inherited arrhythmogenic cardiac disease.

METHODS AND RESULTS

We present several lines of complementary evidence, indicating that the arrhythmia-associated L433P mutation disrupts RyR2 N-terminus self-association. A combination of yeast two-hybrid, co-immunoprecipitation, and chemical cross-linking assays collectively demonstrate that a RyR2 N-terminal fragment carrying the L433P mutation displays substantially reduced self-interaction compared with wild type. Moreover, sucrose density gradient centrifugation reveals that the L433P mutation impairs tetramerization of the full-length channel. [(3)H]Ryanodine-binding assays demonstrate that disrupted N-terminal intersubunit interactions within RyR2(L433P) confer an altered sensitivity to Ca(2+) activation. Calcium imaging of RyR2(L433P)-expressing cells reveals substantially prolonged Ca(2+) transients and reduced Ca(2+) store content indicating defective channel closure. Importantly, dantrolene treatment reverses the L433P mutation-induced impairment and restores channel function.

CONCLUSION

The N-terminus domain constitutes an important structural determinant for the functional oligomerization of RyR2. Our findings are consistent with defective N-terminus self-association as a molecular mechanism underlying RyR2 channel deregulation in inherited arrhythmogenic cardiac disease. Significantly, the therapeutic action of dantrolene may occur via the restoration of normal RyR2 N-terminal intersubunit interactions.

Pore dynamics and conductance of RyR1 transmembrane domain

Ryanodine receptors (RyR) are calcium release channels, playing a major role in the regulation of muscular contraction. Mutations in skeletal muscle RyR (RyR1) are associated with congenital diseases such as malignant hyperthermia and central core disease (CCD). The absence of high-resolution structures of RyR1 has limited our understanding of channel function and disease mechanisms at the molecular level. Previously, we have reported a hypothetical structure of the RyR1 pore-forming region, obtained by homology modeling and supported by mutational scans, electrophysiological measurements, and cryo-electron microscopy. Here, we utilize the expanded model encompassing six transmembrane helices to calculate the RyR1 pore region conductance, to analyze its structural stability, and to hypothesize the mechanism of the Ile4897 CCD-associated mutation. The calculated conductance of the wild-type RyR1 suggests that the proposed pore structure can sustain ion currents measured in single-channel experiments. We observe a stable pore structure on timescales of 0.2 μs, with multiple cations occupying the selectivity filter and cytosolic vestibule, but not the inner chamber. We further suggest that stability of the selectivity filter critically depends on the interactions between the I4897 residue and several hydrophobic residues of the neighboring subunit. Loss of these interactions in the case of polar substitution I4897T results in destabilization of the selectivity filter, a possible cause of the CCD-specific reduced Ca(2+) conductance.

Novel RYR1 missense mutations in six Chinese patients with central core disease

Central core disease (CCD) is a genetically heterogeneous congenital myopathy, and ryanodine receptor 1 (RYR1, gene ID6261) is the only pathogenicity gene until now. Data on mutation characteristics of RYR1 in the Chinese CCD population are scarce. This study searched for mutations in the C-terminal-encoding domain of RYR1 in six Chinese patients with CCD, and identified five missense mutations (N4807F, R4861H, R4893P, G4897D, and I4898T). Among them, N4807F, G4897D were novel while R4861H, R4893P, and I4898T were previously reported. All missense mutations were highly conserved across the species of human, mouse, rabbit, fish, and pig. This study found that mutations could be identified in about 85% CCD patients, even if only the C-terminal-encoding region of RYR1 was screened. Many mutations clustered in exons 100-102.

Clinical features and ryanodine receptor type 1 gene mutation analysis in a Chinese family with central core disease

Central core disease is a rare inherited neuromuscular disorder caused by mutations in ryanodine receptor type 1 gene. The clinical phenotype of the disease is highly variable. We report a Chinese pedigree with central core disease confirmed by the gene sequencing. All 3 patients in the family presented with mild proximal limb weakness. The serum level of creatine kinase was normal, and electromyography suggested myogenic changes. The histologic analysis of muscle biopsy showed identical central core lesions in almost all of the muscle fibers in the index case. Exon 90-106 in the C-terminal domain of the ryanodine receptor type 1 gene was amplified using polymerase chain reaction. One heterozygous missense mutation G14678A (Arg4893Gln) in exon 102 was identified in all 3 patients. This is the first report of a familial case of central core disease confirmed by molecular study in mainland China.

Ca2+ release in muscle fibers expressing R4892W and G4896V type 1 ryanodine receptor disease mutants

The large and rapidly increasing number of potentially pathological mutants in the type 1 ryanodine receptor (RyR1) prompts the need to characterize their effects on voltage-activated sarcoplasmic reticulum (SR) Ca²⁺ release in skeletal muscle. Here we evaluated the function of the R4892W and G4896V RyR1 mutants, both associated with central core disease (CCD) in humans, in myotubes and in adult muscle fibers. For both mutants expressed in RyR1-null (dyspedic) myotubes, voltage-gated Ca²⁺ release was absent following homotypic expression and only partially restored following heterotypic expression with wild-type (WT) RyR1. In muscle fibers from adult WT mice, both mutants were expressed in restricted regions of the fibers with a pattern consistent with triadic localization. Voltage-clamp-activated confocal Ca²⁺ signals showed that fiber regions endowed with G4896V-RyR1s exhibited an ∼30% reduction in the peak rate of SR Ca²⁺ release, with no significant change in SR Ca²⁺ content. Immunostaining revealed no associated change in the expression of either α1S subunit (Cav1.1) of the dihydropyridine receptor (DHPR) or type 1 sarco(endo)plasmic reticulum Ca²⁺ ATPase (SERCA1), indicating that the reduced Ca²⁺ release resulted from defective RyR1 function. Interestingly, in spite of robust localized junctional expression, the R4892W mutant did not affect SR Ca²⁺ release in adult muscle fibers, consistent with a low functional penetrance of this particular CCD-associated mutant.

Novel excitation-contraction uncoupled RYR1 mutations in patients with central core disease

Central core disease, one of the most common congenital myopathies in humans, has been linked to mutations in the RYR1 gene encoding the Ca(2+) release channel of the sarcoplasmic reticulum (RyR1). Functional analyses showed that disease-associated RYR1 mutations led to impairment of skeletal muscle Ca(2+) homeostasis; however, thorough understanding of the molecular mechanisms underlying central core disease and other RyR1-related conditions is still lacking. We screened by sequencing the complete RYR1 transcripts in ten unrelated patients with central core disease and identified five novel, p.M4640R, p.L4647P, p.F4808L, p.D4918N and p.F4941C, and four recurrent mutations. Four of the novel mutations involved amino acid residues that were positioned within putative transmembrane segments of the RyR1. The pathogenic character of the identified mutations was demonstrated by bioinformatic analyses and by the in vitro functional studies in HEK293 cells and RYR1-null (dyspedic) myotubes. Characterization of Ca(2+) channel properties of RyR1s carrying one recurrent and two novel mutations upholds the view that diminished intracellular Ca(2+) release caused by impaired Ca(2+) channel gating and/or Ca(2+) permeability is an important component of central core disease etiology. This study expands the list of functionally characterized disease-associated RyR1 mutations, increasing the value of genetic diagnosis for RyR1-related disorders.

Core myopathies

The core myopathies, Central Core Disease and Multiminicore Disease, are heterogeneous congenital myopathies with the common defining histopathological feature of focally reduced oxidative enzyme activity (central cores, multiminicores). Mutations in the gene encoding for the skeletal muscle ryanodine (RyR1) receptor are the most common cause. Mutations in the selenoprotein N (SEPN1) gene cause a less common variant. Pathogenic mechanisms underlying dominant RYR1 mutations have been extensively characterized, whereas those associated with recessive RYR1 and SEPN1 mutations are emerging. Identifying a specific genetic defect from the histopathological diagnosis of a core myopathy is complex and ought to be informed by a combined appraisal of histopathological, clinical, and, increasingly, muscle magnetic resonance imaging data. The present review aims at giving an overview of the main genetic and clinicopathological findings, with a major emphasis on features likely to inform the diagnostic process, as well as current treatments and perspectives for future research.

Calcitonin gene-related peptide restores disrupted excitation-contraction coupling in myotubes expressing central core disease mutations in RyR1

Central core disease (CCD) is a congenital human myopathy associated with mutations in the gene encoding the skeletal muscle ryanodine receptor (RyR1), resulting in skeletal muscle weakness and lower limb deformities. The muscle weakness can be at least partially explained by a reduced magnitude of voltage-gated Ca(2+) release (VGCR). To date, only a few studies have focused on identifying potential therapeutic agents for CCD. Therefore, in this work we investigated the potential use of the calcitonin gene related peptide (CGRP) to restore VGCR in myotubes expressing CCD RyR1 mutants. We also examined the influence of CCD mutants on Ca(2+)-dependent processes involved in myogenesis (myoblast fusion and sarcoendoplasmic reticulum Ca(2+)-ATPase isoform 2 (SERCA2) gene expression). C2C12 cells were transfected with cDNAs encoding either wild-type RyR1 or CCD mutants, and then exposed to CGRP (100 nm, 1-4 h). Expression of the I4897T mutant significantly inhibited SERCA2 gene expression and myoblast fusion, whereas the Y523S mutant exerted the opposite effect. Interestingly, both mutants clearly inhibited VGCR (50%), due to a reduction in SR Ca(2+) content. However, no major changes due to CGRP or CCD mutants were observed in I(CaL). Our data suggest that the Y523S mutant results in store depletion via decompensated SR Ca(2+) leak, while the I4897T mutant inhibits SERCA2 gene expression. Remarkably, in both cases CGRP restored VGCR, likely to have been by enhancing phospholamban (PLB) phosphorylation, SERCA activity and SR Ca(2+) content. Taken together, our data show that in the C2C12 model system, changes in excitation-contraction coupling induced by the expression of RyR1 channels bearing CCD mutations Y523S or I4897T can be reversed by CGRP.

Malignant hyperthermia: a pharmacogenetic disorder

Malignant hyperthermia (MH) is a pharmacogenetic disorder triggered by volatile anesthetics or depolarizing muscle relaxants in predisposed individuals. Exercise or stress-induced MH episodes, in the absence of any obvious pharmacological trigger, have been reported, but these are rare. A considerable effort has taken place over the last two decades to identify mutations associated with MH and characterize their functional effects. A number of different, but complementary systems, have been developed and implemented to this end. The results of such studies have identified commonalities in functional affects of mutations, and also uncovered unexpected complexities in both the structure and function of the skeletal muscle calcium-release channel. The following review is an attempt to provide a summary of the background to current MH research, and highlight some recent advances in our knowledge of the molecular basis of the phenotypic expression of this disorder.

Diagnostics and therapy of muscle channelopathies--Guidelines of the Ulm Muscle Centre

This article is dedicated to our teacher, Prof. Erich Kuhn, Heidelberg, on the occasion of his 88th birthday on 23rd November 2008. In contrast to muscular dystrophies, the muscle channelopathies, a group of diseases characterised by impaired muscle excitation or excitation-contraction coupling, can fairly well be treated with a whole series of pharmacological drugs. However, for a proper treatment proper diagnostics are essential. This article lists state-of-the-art diagnostics and therapies for the two types of myotonic dystrophies, for recessive and dominant myotonia congenita, for the sodium channel myotonias, for the primary dyskalemic periodic paralyses, for central core disease and for malignant hyperthermia susceptibility in detail. In addition, for each disorder a short summary of aetiology, symptomatology, and pathogenesis is provided.

Single channel properties of heterotetrameric mutant RyR1 ion channels linked to core myopathies

Skeletal muscle excitation-contraction coupling involves activation of homotetrameric ryanodine receptor ion channels (RyR1s), resulting in the rapid release of Ca(2+) from the sarcoplasmic reticulum. Previous work has shown that Ca(2+) release is impaired by mutations in RyR1 linked to Central Core Disease and Multiple Minicore Disease. We studied the consequences of these mutations on RyR1 function, following their expression in human embryonic kidney 293 cells and incorporation in lipid bilayers. RyR1-G4898E, -G4898R, and -DeltaV4926/I4927 mutants in the C-terminal pore region of RyR1 and N-terminal RyR1-R110W/L486V mutant all showed negligible Ca(2+) permeation and loss of Ca(2+)-dependent channel activity but maintained reduced K(+) conductances. Co-expression of wild type and mutant RyR1s resulted in Ca(2+)-dependent channel activities that exhibited intermediate Ca(2+) selectivities compared with K(+), which suggested the presence of tetrameric RyR1 complexes composed of wild type and mutant subunits. The number of wild-type subunits to maintain a functional heterotetrameric channel differed among the four RyR1 mutants. The results indicate that homozygous RyR1 mutations associated with core myopathies abolish or greatly reduce sarcoplasmic reticulum Ca(2+) release during excitation-contraction coupling. They further suggest that in individuals, expressing wild type and mutant alleles, a substantial portion of RyR1 channels is able to release Ca(2+) from sarcoplasmic reticulum.

Molecular characterization of the inositol 1,4,5-trisphosphate receptor pore-forming segment

Specific residues in the putative pore helix, selectivity filter, and S6 transmembrane helix of the inositol 1,4,5-trisphosphate receptor were mutated in order to examine their effects on channel function. Mutation of 5 of 8 highly conserved residues in the pore helix/selectivity filter region inactivated the channel (C2533A, G2541A, G2545A, G2546A, and G2547A). Of the remaining three mutants, C2527A and R2543A were partially active and G2549A behaved like wild type receptor. Mutation of a putative glycine hinge residue in the S6 helix (G2586A) or a putative gating residue at the cytosolic end of S6 helix (F2592A) had minimal effects on function, although channel function was inactivated by G2586P and F2592D mutations. The mutagenesis data are interpreted in the context of a structural homology model of the inositol 1,4,5-trisphosphate receptor.

Central core disease

Central core disease (CCD) is an inherited neuromuscular disorder characterised by central cores on muscle biopsy and clinical features of a congenital myopathy. Prevalence is unknown but the condition is probably more common than other congenital myopathies. CCD typically presents in infancy with hypotonia and motor developmental delay and is characterized by predominantly proximal weakness pronounced in the hip girdle; orthopaedic complications are common and malignant hyperthermia susceptibility (MHS) is a frequent complication. CCD and MHS are allelic conditions both due to (predominantly dominant) mutations in the skeletal muscle ryanodine receptor (RYR1) gene, encoding the principal skeletal muscle sarcoplasmic reticulum calcium release channel (RyR1). Altered excitability and/or changes in calcium homeostasis within muscle cells due to mutation-induced conformational changes of the RyR protein are considered the main pathogenetic mechanism(s). The diagnosis of CCD is based on the presence of suggestive clinical features and central cores on muscle biopsy; muscle MRI may show a characteristic pattern of selective muscle involvement and aid the diagnosis in cases with equivocal histopathological findings. Mutational analysis of the RYR1 gene may provide genetic confirmation of the diagnosis. Management is mainly supportive and has to anticipate susceptibility to potentially life-threatening reactions to general anaesthesia. Further evaluation of the underlying molecular mechanisms may provide the basis for future rational pharmacological treatment. In the majority of patients, weakness is static or only slowly progressive, with a favourable long-term outcome.

Two central core disease (CCD) deletions in the C-terminal region of RYR1 alter muscle excitation-contraction (EC) coupling by distinct mechanisms

Central core disease (CCD) and malignant hyperthermia (MH) are skeletal muscle disorders that are linked to mutations in the gene that encodes the type 1 ryanodine receptor (RYR1). The RYR1 ion channel plays a central role in excitation-contraction (EC) coupling by releasing Ca(2+) from an internal store. Pathogenic CCD mutations in RYR1 result in changes in the magnitude of Ca(2+) release during EC coupling. CCD has recently been linked to two novel deletions (c.12640_12648delCGCCAGTTC [p.Arg4214_Phe4216del] and c.14779_14784delGTCATC [p.Val4927_Ile4928del]) in the C-terminal region of RYR1. To determine the phenotypic consequences of these mutations and extend our understanding of the pathogenic mechanisms that underlie CCD, we determined functional effects on Ca(2+) release channel activity of analogous deletions (p.Arg4215_Phe4217del and p.Val4926_Ile4927del) engineered into rabbit RYR1 following expression in RYR1-null (dyspedic) myotubes and HEK293 cells. In addition, we assessed effects of the p.Arg4214 Phe4216del mutation on RYR1 function in lymphoblastoid cells obtained from CCD patients heterozygous for the mutation. Here we report that both deletions significantly reduce Ca(2+) release following RYR1 activation, but by different mechanisms. While the p.Arg4214_Phe4216del deletion promotes Ca(2+) depletion from intracellular stores by exhibiting a classic "leaky channel" behavior, the p.Val4927_Ile4928del deletion reduces Ca(2+) release by disrupting Ca(2+) gating and eliminating Ca(2+) permeation through the open channel.

Diseases associated with altered ryanodine receptor activity

Mutations in two intracellular Ca2+ release channels or ryanodine receptors (RyR1 and RyR2) are associated with a number of human skeletal and cardiac diseases. This chapter discusses these diseases in terms of known mechanisms, controversies, and unanswered questions. We also compare the cardiac and skeletal muscle diseases to explore common mechanisms.

Ryanodine receptor type 1 (RyR1) possessing malignant hyperthermia mutation R615C exhibits heightened sensitivity to dysregulation by non-coplanar 2,2',3,5',6-pentachlorobiphenyl (PCB 95)

Malignant hyperthermia (MH) susceptibility is conferred by inheriting one of >60 missense mutations within the highly regulated microsomal Ca(2+) channel known as ryanodine receptor type 1 (RyR1). Although MH susceptible patients lack overt clinical signs, a potentially lethal MH syndrome can be triggered by exposure to halogenated alkane anesthetics. This study compares how non-coplanar 2,2',3,5',6-pentachlorobiphenyl (PCB 95), a congener identified in environmental and human samples, alters the binding properties of [(3)H]ryanodine to RyR1 in vitro. Junctional sarcoplasmic reticulum (SR) was isolated from skeletal muscle dissected from wild type pigs ((Wt)RyR1) and pigs homozygous for MH mutation R615C ((MH)RyR1), a mutation also found in humans. Although the level of (Wt)RyR1 and (MH)RyR1 expression is the same, (MH)RyR1 shows heightened sensitivity to activation and altered regulation by physiological cations. We report here that (MH)RyR1 shows more pronounced activation by Ca(2+), and is less sensitive to channel inhibition by Ca(2+) and Mg(2+), compared to (Wt)RyR1. In a buffer containing 100nM free Ca(2+), conditions typically found in resting cells, PCB 95 (50-1000nM) enhances the activity of (MH)RyR1 whereas it has no detectable effect on (Wt)RyR1. PCB 95 (2microM) decreases channel inhibition by Mg(2+) to a greater extent in (MH)RyR1 (IC(50) increased nine-fold) compared to (Wt)RyR1 (IC(50) increased by 2.5-fold). PCB95 reduces inhibition by Ca(2+) two-fold more with (MH)RyR1 than (Wt)RyR1. Our data suggest that non-coplanar PCBs are more potent and efficacious toward (MH)RyR1 than (Wt)RyR1, and have more profound effects on its cation regulation. Considering the important roles of Ca(2+) and Mg(2+) in regulating Ca(2+) signals involving RyR channels, these data provide the first mechanistic evidence that a genetic mutation known to confer susceptibility to pharmacological agents also enhances sensitivity to an environmental contaminant.

Sarcoplasmic reticulum: The dynamic calcium governor of muscle

The sarcoplasmic reticulum (SR) provides feedback control required to balance the processes of calcium storage, release, and reuptake in skeletal muscle. This balance is achieved through the concerted action of three major classes of SR calcium-regulatory proteins: (1) luminal calcium-binding proteins (calsequestrin, histidine-rich calcium-binding protein, junctate, and sarcalumenin) for calcium storage; (2) SR calcium release channels (type 1 ryanodine receptor or RyR1 and IP3 receptors) for calcium release; and (3) sarco(endo)plasmic reticulum Ca2+ -ATPase (SERCA) pumps for calcium reuptake. Proper calcium storage, release, and reuptake are essential for normal skeletal muscle function. We review SR structure and function during normal skeletal muscle activity, the proteins that orchestrate calcium storage, release, and reuptake, and how phenotypically distinct muscle diseases (e.g., malignant hyperthermia, central core disease, and Brody disease) can result from subtle alterations in the activity of several key components of the SR calcium-regulatory machinery.

Ca2+ signaling in HEK-293 and skeletal muscle cells expressing recombinant ryanodine receptors harboring malignant hyperthermia and central core disease mutations

Malignant hyperthermia (MH) and central core disease (CCD) are caused by mutations in the RYR1 gene encoding the skeletal muscle isoform of the ryanodine receptor (RyR1), a homotetrameric Ca(2+) release channel. Rabbit RyR1 mutant cDNAs carrying mutations corresponding to those in human RyR1 that cause MH and CCD were expressed in HEK-293 cells, which do not have endogenous RyR, and in primary cultures of rat skeletal muscle, which express rat RyR1. Analysis of intracellular Ca(2+) pools was performed using aequorin probes targeted to the lumen of the endo/sarcoplasmic reticulum (ER/SR), to the mitochondrial matrix, or to the cytosol. Mutations associated with MH caused alterations in intracellular Ca(2+) homeostasis different from those associated with CCD. Measurements of luminal ER/SR Ca(2+) revealed that the mutations generated leaky channels in all cases, but the leak was particularly pronounced in CCD mutants. Cytosolic and mitochondrial Ca(2+) transients induced by caffeine stimulation were drastically augmented in the MH mutant, slightly reduced in one CCD mutant (Y523S) and completely abolished in another (I4898T). The results suggest that local Ca(2+) derangements of different degrees account for the specific cellular phenotypes of the two disorders.

Intracellular Ca2+ dynamics in malignant hyperthermia and central core disease: established concepts, new cellular mechanisms involved

Malignant hyperthermia (MH) and central core disease (CCD) are inherited human disorders of skeletal muscle Ca2+ homeostasis. Both MH and CCD are linked to mutations and/or deletions in the gene encoding the skeletal muscle ryanodine receptor (RyR1), the intracellular Ca2+ release channel, which is essential to excitation-contraction (EC) coupling. Our knowledge on how mutations in RyR1 disrupt intracellular Ca2+ homeostasis and EC coupling, eventually leading to MH and CCD has been recently improved, thanks to multidisciplinary studies ranging from clinical, single channel recordings, patch-clamp experiments, and molecular biology. This review presents a brief historical perspective, on how pioneer studies resulted in associating MH and CCD to RyR1. The review is also focused on discussing novel results in regard to pathophysiological consequences of specific MH/CCD RyR1 mutant proteins, which are representative of the different cellular mechanisms that are linked to either phenotype.

Dantrolene stabilizes domain interactions within the ryanodine receptor

Interdomain interactions between N-terminal and central domains serving as a "domain switch" are believed to be essential to the functional regulation of the skeletal muscle ryanodine receptor-1 Ca(2+) channel. Mutational destabilization of the domain switch in malignant hyperthermia (MH), a genetic sensitivity to volatile anesthetics, causes functional instability of the channel. Dantrolene, a drug used to treat MH, binds to a region within this proposed domain switch. To explore its mechanism of action, the effect of dantrolene on MH-like channel activation by the synthetic domain peptide DP4 or anti-DP4 antibody was examined. A fluorescence probe, methylcoumarin acetate, was covalently attached to the domain switch using DP4 as a delivery vehicle. The magnitude of domain unzipping was determined from the accessibility of methylcoumarin acetate to a macromolecular fluorescence quencher. The Stern-Volmer quenching constant (K(Q)) increased with the addition of DP4 or anti-DP4 antibody. This increase was reversed by dantrolene at both 37 and 22 degrees C and was unaffected by calmodulin. [(3)H]Ryanodine binding to the sarcoplasmic reticulum and activation of sarcoplasmic reticulum Ca(2+) release, both measures of channel activation, were enhanced by DP4. These activities were inhibited by dantrolene at 37 degrees C, yet required the presence of calmodulin at 22 degrees C. These results suggest that the mechanism of action of dantrolene involves stabilization of domain-domain interactions within the domain switch, preventing domain unzipping-induced channel dysfunction. We suggest that temperature and calmodulin primarily affect the coupling between the domain switch and the downstream mechanism of regulation of Ca(2+) channel opening rather than the domain switch itself.