A postulated role of the near amino-terminal domain of the ryanodine receptor in the regulation of the sarcoplasmic reticulum Ca(2+) channel

Structure of RyR1 in native membranes

Ryanodine receptor 1 (RyR1) mediates excitation–contraction coupling by releasing Ca²⁺ from sarcoplasmic reticulum (SR) to the cytoplasm of skeletal muscle cells. RyR1 activation is regulated by several proteins from both the cytoplasm and lumen of the SR. Here, we report the structure of RyR1 from native SR membranes in closed and open states. Compared to the previously reported structures of purified RyR1, our structure reveals helix‐like densities traversing the bilayer approximately 5 nm from the RyR1 transmembrane domain and sarcoplasmic extensions linking RyR1 to a putative calsequestrin network. We document the primary conformation of RyR1 in situ and its structural variations. The activation of RyR1 is associated with changes in membrane curvature and movement in the sarcoplasmic extensions. Our results provide structural insight into the mechanism of RyR1 in its native environment.

The N-Terminal Region of the Ryanodine Receptor Affects Channel Activation

Mutations in the cardiac ryanodine receptor (RyR2), the ion channel responsible for release of calcium ions from intracellular stores into cytoplasm, are the cause of several inherited cardiac arrhythmias. At the molecular level, disease symptoms can be mimicked by domain peptides from mutation-prone regions of RyR2 that bind to RyR2 and activate it. Here we show that the domain peptide DP_cpvtN2, corresponding to the central helix of the N-terminal region of RyR2, activates the RyR2 channel. Structural modeling of interaction between DP_cpvtN2 and the N-terminal region of RyR2 in the closed and open conformation provided three plausible structures of the complex. Only one of them could explain the dependence of RyR2 activity on concentration of DP_cpvtN2. The structure of the complex was at odds with the previously proposed “domain switch” mechanism of competition between domain peptides and ryanodine receptor domains. Likewise, in structural models of the N-terminal region, the conformational changes induced by DP_cpvtN2 binding were different from those induced by mutation of central helix amino acids. The activating effect of DP_cpvtN2 binding and of mutations in the central helix could be explained by their similar effect on the transition energy between the closed and open conformation of RyR2.

Structural and functional interactions within ryanodine receptor

The ryanodine receptor/Ca2+ release channel plays a pivotal role in skeletal and cardiac muscle excitation-contraction coupling. Defective regulation leads to neuromuscular disorders and arrhythmogenic cardiac disease. This mini-review focuses on channel regulation through structural intra- and inter-subunit interactions and their implications in ryanodine receptor pathophysiology.

Bioinformatic mapping and production of recombinant N-terminal domains of human cardiac ryanodine receptor 2

We report the domain analysis of the N-terminal region (residues 1-759) of the human cardiac ryanodine receptor (RyR2) that encompasses one of the discrete RyR2 mutation clusters associated with catecholaminergic polymorphic ventricular tachycardia (CPVT1) and arrhythmogenic right ventricular dysplasia (ARVD2). Our strategy utilizes a bioinformatics approach complemented by protein expression, solubility analysis and limited proteolytic digestion. Based on the bioinformatics analysis, we designed a series of specific RyR2 N-terminal fragments for cloning and overexpression in Escherichia coli. High yields of soluble proteins were achieved for fragments RyR2(1-606)xHis(6), RyR2(391-606)xHis(6), RyR2(409-606)xHis(6), Trx.RyR2(384-606)xHis(6), TrxxRyR2(391-606)xHis(6) and Trx.RyR2(409-606)xHis(6). The folding of RyR2(1-606)xHis(6) was analyzed by circular dichroism spectroscopy resulting in alpha-helix and beta-sheet content of approximately 23% and approximately 29%, respectively, at temperatures up to 35 degrees C, which is in agreement with sequence based secondary structure predictions. Tryptic digestion of the largest recombinant protein, RyR2(1-606)xHis(6), resulted in the appearance of two specific subfragments of approximately 40 and 25 kDa. The 25 kDa fragment exhibited greater stability. Hybridization with anti-His(6).Tag antibody indicated that RyR2(1-606)xHis(6) is cleaved from the N-terminus and amino acid sequencing of the proteolytic fragments revealed that digestion occurred after residues 259 and 384, respectively.

Molecular characterization of flubendiamide sensitivity in the lepidopterous ryanodine receptor Ca(2+) release channel

Flubendiamide is a benzenedicarboxamide derivative that shows selective insecticidal activity against lepidopterous insects. The specific modulatory effects of flubendiamide on ryanodine binding in insect muscle microsomal membranes suggest that the ryanodine receptor (RyR) Ca(2+) release channel is a primary target of flubendiamide. However, the molecular mechanisms underlying the species-specific action of flubendiamide are unclear. We have cloned cDNA encoding a novel RyR from the lepidopterous silkworm RyR (sRyR) and tested the sensitivity to flubendiamide of the recombinant sRyR in HEK293 cells. Confocal localization studies and Ca(2+) imaging techniques revealed that sRyRs form Ca(2+) release channels in the endoplasmic reticulum. Importantly, flubendiamide induced release of Ca(2+) through the sRyR, but not through the rabbit RyR isoforms. Photoaffinity labeling of sRyR deletion mutants using a photoreactive derivative revealed that flubendiamide is mainly incorporated into the transmembrane domain (amino acids 4111-5084) of the sRyR. The rabbit cardiac muscle isoform RyR2 (rRyR2) and the RyR mutant carrying a replacement of the transmembrane domain (residues 4084-5084) with its counterpart sequence from rRyR2 (residues 3936-4968) were not labeled by the photoreactive compound. This replacement in the sRyR significantly impaired the responses to flubendiamide but only marginally reduced the sensitivity to caffeine, a general RyR activator. Furthermore, deletion of the N-terminal sequence (residues 183-290) abolished the responses of the sRyR to flubendiamide but not the sensitivity to caffeine. Our results suggest that the transmembrane domain plays an important role in the formation of an action site for flubendiamide, while the N-terminus is a structural requirement for flubendiamide-induced activation of the sRyR.

Genetic variation in RYR1 and malignant hyperthermia phenotypes

BACKGROUND

Malignant hyperthermia (MH) is associated, in the majority of cases, with mutations in RYR1, the gene encoding the skeletal muscle ryanodine receptor. Our primary aim was to assess whether different RYR1 variants are associated with quantitative differences in MH phenotype.

METHODS

The degree of in vitro pharmacological muscle contracture response and the baseline serum creatine kinase (CK) concentration were used to generate a series of quantitative phenotypes for MH. We then undertook the most extensive RYR1 genotype-phenotype correlation in MH to date using 504 individuals from 204 MH families and 23 RYR1 variants. We also determined the association between a clinical phenotype and both the laboratory phenotype and RYR1 genotype.

RESULTS

We report a novel correlation between the degree of in vitro pharmacological muscle contracture responses and the onset time of the clinical MH response in index cases (P<0.05). There was also a significant correlation between baseline CK concentration and clinical onset time (P=0.039). The specific RYR1 variant was a significant determinant of the severity of each laboratory phenotype (P<0.0001).

CONCLUSIONS

The MH phenotype differs significantly with different RYR1 variants. Variants leading to more severe MH phenotype are distributed throughout the gene and tend to lie at relatively conserved sites in the protein. Differences in phenotype severity between RYR1 variants may explain the variability in clinical penetrance of MH during anaesthesia and why some variants have been associated with exercise-induced rhabdomyolysis and heat stroke. They may also inform a mutation screening strategy in cases of idiopathic hyperCKaemia.

Ryanodine receptor-mediated arrhythmias and sudden cardiac death

The cardiac ryanodine receptor-Ca²⁺ release channel (RyR2) is an essential sarcoplasmic reticulum (SR) transmembrane protein that plays a central role in excitation–contraction coupling (ECC) in cardiomyocytes. Aberrant spontaneous, diastolic Ca²⁺ leak from the SR due to dysfunctional RyR2 contributes to the formation of delayed after-depolarisations, which are thought to underlie the fatal arrhythmia that occurs in both heart failure (HF) and in catecholaminergic polymorphic ventricular tachycardia (CPVT). CPVT is an inherited disorder associated with mutations in either the RyR2 or a SR luminal protein, calsequestrin. RyR2 shows normal function at rest in CPVT but the RyR2 dysfunction is unmasked by physical exercise or emotional stress, suggesting abnormal RyR2 activation as an underlying mechanism. Several potential mechanisms have been advanced to explain the dysfunctional RyR2 observed in HF and CPVT, including enhanced RyR2 phosphorylation status, altered RyR2 regulation at luminal/cytoplasmic sites and perturbed RyR2 intra/inter-molecular interactions. This review considers RyR2 dysfunction in the context of the structural and functional modulation of the channel, and potential therapeutic strategies to stabilise RyR2 function in cardiac pathology.

Enhanced excitation-coupled calcium entry in myotubes expressing malignant hyperthermia mutation R163C is attenuated by dantrolene

Dantrolene is the drug of choice for the treatment of malignant hyperthermia (MH) and is also useful for treatment of spasticity or muscle spasms associated with several clinical conditions. The current study examines the mechanisms of dantrolene's action on skeletal muscle and shows that one of dantrolene's mechanisms of action is to block excitation-coupled calcium entry (ECCE) in both adult mouse flexor digitorum brevis fibers and primary myotubes. A second important new finding is that myotubes isolated from mice heterozygous and homozygous for the ryanodine receptor type 1 R163C MH susceptibility mutation show significantly enhanced ECCE rates that could be restored to those measured in wild-type cells after exposure to clinical concentrations of dantrolene. We propose that this gain of ECCE function is an important etiological component of MH susceptibility and possibly contributes to the fulminant MH episode. The inhibitory potency of dantrolene on ECCE found in wild-type and MH-susceptible muscle is consistent with the drug's clinical potency for reversing the MH syndrome and is incomplete as predicted by its efficacy as a muscle relaxant.

Agonists and antagonists of the cardiac ryanodine receptor: Potential therapeutic agents?

This review addresses the potential use of the intracellular ryanodine receptor (RyR) Ca(2+) release channel as a therapeutic target in heart disease. Heart disease encompasses a wide range of conditions with the major contributors to mortality and morbidity being ischaemic heart disease and heart failure (HF). In addition there are many rare, but devastating conditions, some of which are either genetically linked to the RyR and its regulatory proteins or involve drug-induced modification of the proteins. The defects in Ca(2+) signalling vary with the nature of the heart disease and the stage in its progress and therefore specific corrections require different modifications of Ca(2+) signalling. Compounds that activate the RyR are potential inotropic agents to increase the Ca(2+) transient and strength of contraction. Compounds that reduce RyR activity are potentially useful in conditions where excess RyR activity initiates arrhythmias, or depletes the Ca(2+) store, as in end stage HF. It has recently been discovered that the cardio-protective action of the drug JTV519 can be attributed partly to its ability to stabilise the interaction between the RyR and the 12.6 kDa binding protein for the commonly used immunosuppressive drug FK506 (FKBP12.6, known as tacrolimus). This has established the credibility of the RyR as a therapeutic target. We explore the possibility that mutations causing the rare RyR-linked arrhythmias will open the door to identification of novel RyR-based therapeutic agents. The use of regulatory binding sites within the RyR complex or on its associated proteins as templates for drug design is discussed.

A variably spliced region in the type 1 ryanodine receptor may participate in an inter-domain interaction

The aim of the present study was to examine residues that are variably spliced in the juvenile and adult isoforms of the skeletal-muscle RyR1 (type 1 ryanodine receptor). The juvenile ASI(-) splice variant is less active than the adult ASI(+) variant and is overexpressed in patients with DM (myotonic dystrophy) [Kimura, Nakamori, Lueck, Pouliquin, Aoike, Fujimura, Dirksen, Takahashi, Dulhunty and Sakoda (2005) Hum. Mol. Genet. 14, 2189-2200]. In the present study, we explore the ASI region using synthetic peptides corresponding to rabbit RyR1 residues Thr3471-Gly3500 either containing [PASI(+)] or lacking [PASI(-)] the ASI residues. Both peptides increased [3H]ryanodine binding to rabbit RyR1s, increased Ca2+ release from sarcoplasmic reti-culum vesicles and increased single RyR1 channel activity. The peptide PASI(-) was more active in each case than PASI(+). [3H]Ryanodine binding to recombinant ASI(+)RyR1 or ASI(-)-RyR1 was enhanced more by PASI(-) than PASI(+), with the greatest increase seen when PASI(-) was added to ASI(-)RyR1. The activation of the RyR channels is consistent with the hypo-thesis that the peptides interrupt an inhibitory inter-domain inter-action and that PASI(-) is more effective at interrupting this interaction than PASI(+). We therefore suggest that the ASI(-) sequence interacts more tightly than the ASI(+) sequence with its binding partner, so that the ASI(-)RyR1 is more strongly inhibited (less active) than the ASI(+)RyR1. Thus the affinity of the binding partners in this inter-domain interaction may deter-mine the activities of the mature and juvenile isoforms of RyR1 and the stronger inhibition in the juvenile isoform may contribute to the myopathy in DM.

Novel regulators of RyR Ca2+ release channels: insight into molecular changes in genetically-linked myopathies

There are many mutations in the ryanodine receptor (RyR) Ca2+ release channel that are implicated in skeletal muscle disorders and cardiac arrhythmias. More than 80 mutations in the skeletal RyR1 have been identified and linked to malignant hyperthermia, central core disease or multi-minicore disease, while more than 40 mutations in the cardiac RyR2 lead to ventricular arrhythmias and sudden cardiac death in patients with structurally normal hearts. These RyR mutations cause diverse changes in RyR activity which either excessively activate or block the channel in a manner that disrupts Ca2+ signalling in the muscle fibres. In a different myopathy, myotonic dystrophy (DM), a juvenile isoform of the skeletal RyR is preferentially expressed in adults. There are two regions of RyR1 that are variably spiced and developmentally regulated (ASI and ASII). The juvenile isoform (ASI(-)) is less active than the adult isoform (ASI(+)) and its over-expression in adults with DM may contribute to functional changes. Finally, mutations in an important regulator of the RyR, the Ca2+ binding protein calsequestrin (CSQ), have been linked to a disruption of Ca2+ homeostasis in cardiac myocytes that results in arrhythmias. We discuss evidence supporting the hypothesis that mutations in each of these situations alter protein/protein interactions within the RyR complex or between the RyR and its associated proteins. The disruption of these protein-protein interactions can lead either to excess Ca2+ release or reduced Ca2+ release and thus to abnormal Ca2+ homeostasis. Much of the evidence for disruption of protein-protein interactions has been provided by the actions of a group of novel RyR regulators, domain peptides with sequences that correspond to sequences within the RyR and which compete with the endogenous residues for their interaction sites.

Probing a putative dantrolene-binding site on the cardiac ryanodine receptor

Dantrolene is an inhibitor of intracellular Ca2+ release from skeletal muscle SR (sarcoplasmic reticulum). Direct photoaffinity labelling experiments using [3H]azidodantrolene and synthetic domain peptides have demonstrated that this drug targets amino acids 590-609 [termed DP1 (domain peptide 1)] of RyR1 (ryanodine receptor 1), the skeletal muscle RyR isoform. Although the identical sequence exists in the cardiac isoform, RyR2 (residues 601-620), specific labelling of RyR2 by dantrolene has not been demonstrated, even though some functional studies show protective effects of dantrolene on heart function. Here we test whether dantrolene-active domains exist within RyR2 and if so, whether this domain can be modulated. We show that elongated DP1 sequences from RyR1 (DP1-2s; residues 590-628) and RyR2 (DP1-2c; residues 601-639) can be specifically photolabelled by [3H]azidodantrolene. Monoclonal anti-RyR1 antibody, whose epitope is the DP1 region, can recognize RyR1 but not RyR2 in Western blot and immunoprecipitation assays, yet it recognizes both DP1-2c and DP1-2s. This suggests that although the RyR2 sequence has an intrinsic capacity to bind dantrolene in vitro, this site may be poorly accessible in the native channel protein. To examine whether it is possible to modulate this site, we measured binding of [3H]dantrolene to cardiac SR as a function of free Ca2+. We found that > or =10 mM EGTA increased [3H]dantrolene binding to RyR2 by approximately 2-fold. The data suggest that the dantrolene-binding site on RyR2 is conformationally sensitive. This site may be a potential therapeutic target in cardiovascular diseases sensitive to dysfunctional intracellular Ca2+ release.

Effects of peptide C corresponding to the Glu724-Pro760 region of the II-III loop of the DHP (dihydropyridine) receptor alpha1 subunit on the domain- switch-mediated activation of RyR1 (ryanodine receptor 1) Ca2+ channels

The Leu720-Leu764 region of the II-III loop of the dihydropyridine receptor is believed to be important for both orthograde and retrograde communications with the RyR (ryanodine receptor), but its actual role has not yet been resolved. Our recent studies suggest that voltage-dependent activation of the RyR channel is mediated by a pair of interacting N-terminal and central domains, designated as the 'domain switch'. To investigate the effect of peptide C (a peptide corresponding to residues Glu724-Pro760) on domain- switch-mediated activation of the RyR, we measured Ca2+ release induced by DP (domain peptide) 1 or DP4 (which activates the RyR by mediation of the domain switch) and followed the Ca2+ release time course using a luminal Ca2+ probe (chlortetracycline) under Ca2+-clamped conditions. Peptide C produced a significant potentiation of the domain-switch-mediated Ca2+ release, provided that the Ca2+ concentration was sufficiently low (e.g. 0.1 microM) and the Ca2+ channel was only partially activated by the domain peptide. However, at micromolar Ca2+ concentrations, peptide C inhibits activation. Covalent cross-linking of fluorescently labelled peptide C to the RyR and screening of the fluorescently labelled tryptic fragments permitted us to localize the peptide-C-binding site to residues 450-1400, which may represent the primary region involved in physical coupling. Based on the above findings, we propose that the physiological role of residues Glu724-Pro760 is to facilitate depolarization-induced and domain-switch-mediated RyR activation at sub- or near-threshold concentrations of cytoplasmic Ca2+ and to suppress activation upon an increase of cytoplasmic Ca2+.

Effects of azumolene on Ca2+ sparks in skeletal muscle fibers

Azumolene is an analog of dantrolene, the only approved medicine for treatment of malignant hyperthermia (MH). The pharmacological mechanism of these drugs is to inhibit skeletal muscle sarcoplasmic reticulum (SR) Ca2+ release by modulating the activity of the SR ryanodine receptor (RyR) Ca2+ release channel. To investigate the effects of azumolene on SR Ca2+ channel gating within skeletal muscle fibers, we monitored Ca2+ sparks in permeabilized frog skeletal muscle fibers. Application of 0.0001 to 10 microM azumolene suppressed the frequency of spontaneous Ca2+ sparks in a dose-dependent manner (EC50 = 0.25 microM; Hill coefficient = 1.44), but it did not cause systematic dose-dependent effects on the properties of the Ca2+ sparks. These results suggest that azumolene decreases the likelihood of Ca2+ release channel openings that initiate Ca2+ sparks, thereby decreasing spark frequency, but it has little effect on aggregate Ca2+ channel open times during a spark. To assess azumolene inhibition of RyRs activated in a manner analogous to those activated during an MH episode, we applied DP4, a synthetic peptide corresponding to a central region of RyR1 (Leu2442 to Pro2477), which mimics an MH modification. Azumolene also decreased Ca2+ spark frequency in a dose-dependent manner without altering spark properties in the DP4 MH model. We conclude that azumolene suppresses the opening rate but not the open time of RyR Ca2+ release channels within skeletal fibers.

Antibody probe study of Ca2+ channel regulation by interdomain interaction within the ryanodine receptor

N-terminal and central domains of ryanodine receptor 1 (RyR1), where many reported malignant hyperthermia (MH) mutations are localized, represent putative channel regulatory domains. Recent domain peptide (DP) probe studies led us to the hypothesis that these domains interact to stabilize the closed state of channel (zipping), while weakening of domain-domain interactions (unzipping) by mutation de-stabilizes the channel, making it leaky to Ca2+ or sensitive to the agonists of RyR1. As shown previously, DP1 (N-terminal domain peptide) and DP4 (central domain peptide) produced MH-like channel activation/sensitization effects, presumably by peptide binding to sites critical to stabilizing domain-domain interactions and resultant loss of conformational constraints. Here we report that polyclonal anti-DP1 and anti-DP4 antibodies also produce MH-like channel activation and sensitization effects as evidenced by about 4-fold enhancement of high affinity [3H]ryanodine binding to RyR1 and by a significant left-shift of the concentration-dependence of activation of sarcoplasmic reticulum Ca2+ release by polylysine. Fluorescence quenching experiments demonstrate that the accessibility of a DP4-directed, conformationally sensitive fluorescence probe linked to the RyR1 N-terminal domain is increased in the presence of domain-specific antibodies, consistent with the view that these antibodies produce unzipping of interacting domains that are of hindered accessibility to the surrounding aqueous environment. Our results suggest that domain-specific antibody binding induces a conformational change resulting in channel activation, and are consistent with the hypothesis that interacting N-terminal and central domains are intimately involved in the regulation of RyR1 channel function.

Ryanodine receptor regulation by intramolecular interaction between cytoplasmic and transmembrane domains

Ryanodine receptors (RyR) function as Ca(2+) channels that regulate Ca(2+) release from intracellular stores to control a diverse array of cellular processes. The massive cytoplasmic domain of RyR is believed to be responsible for regulating channel function. We investigated interaction between the transmembrane Ca(2+)-releasing pore and a panel of cytoplasmic domains of the human cardiac RyR in living cells. Expression of eGFP-tagged RyR constructs encoding distinct transmembrane topological models profoundly altered intracellular Ca(2+) handling and was refractory to modulation by ryanodine, FKBP12.6 and caffeine. The impact of coexpressing dsRed-tagged cytoplasmic domains of RyR2 on intracellular Ca(2+) phenotype was assessed using confocal microscopy coupled with parallel determination of in situ protein: protein interaction using fluorescence resonance energy transfer (FRET). Dynamic interactions between RyR cytoplasmic and transmembrane domains were mediated by amino acids 3722-4610 (Interacting or "I"-domain) which critically modulated intracellular Ca(2+) handling and restored RyR sensitivity to caffeine activation. These results provide compelling evidence that specific interaction between cytoplasmic and transmembrane domains is an important mechanism in the intrinsic modulation of RyR Ca(2+) release channels.

The calmodulin binding region of the skeletal ryanodine receptor acts as a self-modulatory domain

A synthetic peptide (CaMBP) matching amino acids 3614-3643 of the skeletal ryanodine receptor (RyR1) binds to both Ca2+-free calmodulin (CaM) and Ca2+-bound CaM with nanomolar affinity [J. Biol. Chem. 276 (2001) 2069]. We report here that CaMBP increases [3H]ryanodine binding to RyR1 in a dose- and Ca2+-dependent manner; it also induces Ca2+ release from SR vesicles, and increases open probability (P(o)) of single RyR channels reconstituted in planar lipid bilayers. Further, CaMBP removes CaM associated with SR vesicles and increases [3H]ryanodine binding to purified RyR1, suggesting that its mechanism of action is two-fold: it removes endogenous inhibitors and also interacts directly with complementary regions in RyR1. Remarkably, the N-terminus of CaMBP activates RyRs while the C-terminus of CaMBP inhibits RyR activity, suggesting the presence of two discrete functional subdomains within this region. A ryr1 mutant lacking this region, RyR1-Delta3614-3643, was constructed and expressed in dyspedic myoblasts (RyR1-knockout). The depolarization-, caffeine- and 4-chloro-m-cresol (4-CmC)-induced Ca2+ transients in these cells were dramatically reduced compared with cells expressing wild type RyR1. Deletion of the 3614-3643 region also resulted in profound changes in unitary conductance and channel gating. We thus propose that the RyR1 3614-3643 region acts not only as the CaM binding site, but also as an important modulatory domain for RyR1 function.

A new cardioprotective agent, JTV519, improves defective channel gating of ryanodine receptor in heart failure

Defective interaction between FKBP12.6 and ryanodine receptors (RyR) is a possible cause of cardiac dysfunction in heart failure (HF). Here, we assess whether the new cardioprotective agent JTV519 can correct it in tachycardia-induced HF. HF was induced in dogs by 4-wk rapid ventricular pacing, and sarcoplasmic reticulum (SR) was isolated from left ventricular muscles. In failing SR, JTV519 increased the rate of Ca(2+) release and [(3)H]ryanodine binding. RyR were then labeled in a site-directed fashion with the fluorescent conformational probe methylcoumarin acetamide. In failing SR, the polylysine induced a rapid change in methylcoumarin acetamide fluorescence, presumably because the channel opening preceding the Ca(2+) release was smaller than in normal SR (consistent with a decreased rate of Ca(2+) release in failing SR), and JTV519 increased it. In conclusion, JTV519, a new 1,4-benzothiazepine derivative, corrected the defective channel gating in RyR (increase in both the rapid conformational change and the subsequent Ca(2+) release rate) in HF.

Molecular genetics of ryanodine receptors Ca2+-release channels

The family of ryanodine receptor (RyR) genes encodes three highly related Ca(2+)-release channels: RyR1, RyR2 and RyR3. RyRs are known as the Ca(2+)-release channels that participate to the mechanism of excitation-contraction coupling in striated muscles, but they are also expressed in many other cell types. Actually, in several cells two or three RyR isoforms can be co-expressed and interactive feedbacks among them may be important for generation of intracellular Ca(2+) signals and regulation of specific cellular functions. Important developments have been obtained in understanding the biochemical complexity underlying the process of Ca(2+) release through RyRs. The 3-D structure of these large molecules has been obtained and some regulatory regions have been mapped within these 3-D reconstructions. Recent studies have clarified the role of protein kinases and phosphatases that, by physically interacting with RyRs, appear to play a role in the regulation of these Ca(2+)-release channels. These and other recent advancements in understanding RyR biology will be the object of this review.

Identification of a dantrolene-binding sequence on the skeletal muscle ryanodine receptor

Dantrolene is a drug that suppresses intracellular Ca(2+) release from sarcoplasmic reticulum (SR) in skeletal muscle and is used as a therapeutic agent in individuals susceptible to malignant hyperthermia. Although its precise mechanism of action has not been elucidated, we have identified the N-terminal region (amino acids 1-1400) of the skeletal muscle isoform of the ryanodine receptor (RyR1), the primary Ca(2+) release channel in SR, as a molecular target for dantrolene using the photoaffinity analog [(3)H]azidodantrolene. Here, we demonstrate that heterologously expressed RyR1 retains its capacity to be specifically labeled with [(3)H]azidodantrolene, indicating that muscle specific factors are not required for this ligand-receptor interaction. Synthetic domain peptides of RyR1 previously shown to affect RyR1 function in vitro and in vivo were exploited as potential drug binding site mimics and used in photoaffinity labeling experiments. Only DP1 and DP1-2s, peptides containing the amino acid sequence corresponding to RyR1 residues 590-609, were specifically labeled by [(3)H]azidodantrolene. A monoclonal anti-RyR1 antibody that recognizes RyR1 and its 1400-amino acid N-terminal fragment recognizes DP1 and DP1-2s in both Western blots and immunoprecipitation assays and specifically inhibits [(3)H]azidodantrolene photolabeling of RyR1 and its N-terminal fragment in SR. Our results indicate that synthetic domain peptides can mimic a native, ligand-binding conformation in vitro and that the dantrolene-binding site and the epitope for the monoclonal antibody on RyR1 are equivalent and composed of amino acids 590-609.

Peptide probe study of the critical regulatory domain of the cardiac ryanodine receptor

The recently devised domain peptide probe technique was used to identify and characterize critical domains of the cardiac ryanodine receptor (RyR2). A synthetic peptide corresponding to the Gly(2460)-Pro(2495) domain of the RyR2, designated DPc10, enhanced the ryanodine binding activity and increased the sensitivity of the RyR2 to activating Ca(2+): the effects that resemble the typical phenotypes of cardiac diseases. A single Arg-to-Ser mutation made in DPc10, mimicking the recently reported Arg(2474)-to-Ser(2474) human mutation, abolished all of these effects that would have been produced by DPc10. On the basis of the principle of the domain peptide probe approach (see Model 1), these results indicate that the in vivo RyR2 domain corresponding to DPc10 plays a key role in the cardiac channel regulation and in the pathogenic mechanism. This domain peptide approach opens the new possibility in the studies of the regulatory and pathogenic mechanisms of the cardiac Ca(2+) channel.

Spectroscopic monitoring of local conformational changes during the intramolecular domain-domain interaction of the ryanodine receptor

The amino (N)-terminal and central regions of the ryanodine receptor (RyR) containing most mutation sites of malignant hyperthermia (MH) and central core disease (CCD) seem to be involved in the Ca(2+) channel regulation. Our recent peptide probe study (Yamamoto, T., El-Hayek, R., and Ikemoto, N. (2000) J. Biol. Chem. 275, 11618-11625) suggested the hypothesis that a close contact between the N-terminal and central domains (zipping) stabilizes the closed-state of the channel, while removal of the contact (unzipping) deblocks the channel, causing channel-activation effects. We here report the results of our recent effort to monitor local conformational changes in the putative domain-domain interaction site to test this hypothesis. The conformation-sensitive fluorescence probe, methyl coumarin acetamide (MCA), was incorporated into RyR in a protein- and site-specific manner by using DP4 (the peptide corresponding to the Leu(2442)-Pro(2477) region of the central domain) as a site-directing carrier. The site of MCA labeling was localized in the 150 kDa N-terminal region of RyR, indicating that DP4 and its in vivo counterpart (a portion of the central domain) interact with the N-terminal region. RyR-activating domain peptides, DP4 and DP1 (corresponding to the Leu(590)-Cys(609) region of the N-terminal domain), and depolarization of the T-tubule moiety of the triad (physiologic stimulation) induced a rapid decrease in the fluorescence intensity of the protein-bound MCA and Ca(2+) release at a somewhat slower rate. The accessibility of the protein-bound MCA to the fluorescence quencher was increased in the presence of DP4. These results are all consistent with the above hypothesis.

Interdomain interactions within ryanodine receptors regulate Ca2+ spark frequency in skeletal muscle

DP4 is a 36-residue synthetic peptide that corresponds to the Leu(2442)-Pro(2477) region of RyR1 that contains the reported malignant hyperthermia (MH) mutation site. It has been proposed that DP4 disrupts the normal interdomain interactions that stabilize the closed state of the Ca(2)+ release channel (Yamamoto, T., R. El-Hayek, and N. Ikemoto. 2000. J. Biol. Chem. 275:11618-11625). We have investigated the effects of DP4 on local SR Ca(2)+ release events (Ca(2)+ sparks) in saponin-permeabilized frog skeletal muscle fibers using laser scanning confocal microscopy (line-scan mode, 2 ms/line), as well as the effects of DP4 on frog SR vesicles and frog single RyR Ca(2)+ release channels reconstituted in planar lipid bilayers. DP4 caused a significant increase in Ca(2)+ spark frequency in muscle fibers. However, the mean values of the amplitude, rise time, spatial half width, and temporal half duration of the Ca(2)+ sparks, as well as the distribution of these parameters, remained essentially unchanged in the presence of DP4. Thus, DP4 increased the opening rate, but not the open time of the RyR Ca(2)+ release channel(s) generating the sparks. DP4 also increased [(3)H]ryanodine binding to SR vesicles isolated from frog and mammalian skeletal muscle, and increased the open probability of frog RyR Ca(2)+ release channels reconstituted in bilayers, without changing the amplitude of the current through those channels. However, unlike in Ca(2)+ spark experiments, DP4 produced a pronounced increase in the open time of channels in bilayers. The same peptide with an Arg(17) to Cys(17) replacement (DP4mut), which corresponds to the Arg(2458)-to-Cys(2458) mutation in MH, did not produce a significant effect on RyR activation in muscle fibers, bilayers, or SR vesicles. Mg(2)+ dependence experiments conducted with permeabilized muscle fibers indicate that DP4 preferentially binds to partially Mg(2)+-free RyR(s), thus promoting channel opening and production of Ca(2)+ sparks.

The skeletal muscle ryanodine receptor identified as a molecular target of [3H]azidodantrolene by photoaffinity labeling

Dantrolene is a skeletal muscle relaxant which acts by inhibiting intracellular Ca(2+) release from sarcoplasmic reticulum (SR). It is used primarily in the treatment of malignant hyperthermia (MH), a pharmacogenetic sensitivity to volatile anesthetics resulting in massive intracellular Ca(2+) release. Determination of the site and mechanism of action of dantrolene should contribute to the understanding of the regulation of intracellular Ca(2+) release in skeletal muscle. Photoaffinity labeling of porcine SR with [(3)H]azidodantrolene, a photoactivatable analogue of dantrolene, has identified a 160 kDa SR protein with immunologic cross-reactivity to skeletal muscle ryanodine receptor (RyR) as a possible target [Palnitkar et al. (1999) J. Med. Chem. 42, 1872-1880]. Here we demonstrate specific, AMP-PCP-enhanced, [(3)H]azidodantrolene photolabeling of both the RyR monomer and a 160 or 172 kDa protein in porcine and rabbit SR, respectively. The 160/172 kDa protein is shown to be the NH(2)-terminus of the RyR cleaved from the monomer by an endogenous protease activity consistent with that of n-calpain. MALDI-mass spectrometric analysis of the porcine 160 kDa protein identifies it as the 1400 amino acid NH(2)-terminal fragment of the skeletal muscle RyR reportedly generated by n-calpain [Shevchenko et al. (1998) J. Membr. Biol. 161, 33-34]. Immunoprecipitation of solubilized, [(3)H]azidodantrolene-photolabeled SR protein reveals that the cleaved 160/172 kDa protein remains associated with the C-terminal, 410 kDa portion of the RyR. [(3)H]Dantrolene binding to both the intact and the n-calpain-cleaved channel RyR is similarly enhanced by AMP-PCP. n-Calpain cleavage of the RyR does not affect [(3)H]dantrolene binding in the presence of AMP-PCP, but depresses drug binding in the absence of nucleotide. These results demonstrate that the NH(2)-terminus of the RyR is a molecular target for dantrolene, and suggest a regulatory role for both n-calpain activity and ATP in the interaction of dantrolene with the RyR in vivo.

Direct association of ligand-binding and pore domains in homo- and heterotetrameric inositol 1,4,5-trisphosphate receptors

Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) are a family of intracellular Ca(2+) channels that exist as homo- or heterotetramers. In order to determine whether the N-terminal ligand-binding domain is in close physical proximity to the C-terminal pore domain, we prepared microsomal membranes from COS-7 cells expressing recombinant type I and type III IP(3)R isoforms. Trypsin digestion followed by cross-linking and co-immunoprecipitation of peptide fragments suggested an inter-subunit N- and C-terminal interaction in both homo- and heterotetramers. This observation was further supported by the ability of in vitro translated C-terminal peptides to interact specifically with an N-terminal fusion protein. Using a (45)Ca(2+) flux assay, we provide functional evidence that the ligand-binding domain of one subunit can gate the pore domain of an adjacent subunit. We conclude that common structural motifs are shared between the type I and type III IP(3)Rs and propose that the gating mechanism of IP(3)R Ca(2+) channels involves the association of the N-terminus of one subunit with the C-terminus of an adjacent subunit in both homo- and heterotetrameric complexes.

Postulated role of interdomain interaction within the ryanodine receptor in Ca(2+) channel regulation

Localized distribution of malignant hyperthermia (MH) and central core disease (CCD) mutations in N-terminal and central domains of the ryanodine receptor suggests that the interaction between these domains may be involved in Ca(2+) channel regulation. To test this hypothesis, we investigated the effects of a new synthetic domain peptide DP4 corresponding to the Leu(2442)-Pro(2477) region of the central domain. DP4 enhanced ryanodine binding and induced a rapid Ca(2+) release. The concentration for half-maximal activation by agonists was considerably reduced in the presence of DP4. These effects of DP4 are analogous to the functional modifications of the ryanodine receptor caused by MH/CCD mutations (viz. hyperactivation of the channel and hypersensitization of the channel to agonists). Replacement of Arg of DP4 with Cys, mimicking the in vivo Arg(2458)-to-Cys(2458) mutation, abolished the activating effects of DP4. An N-terminal domain peptide DP1 (El-Hayek, R., Saiki, Y., Yamamoto, T., and Ikemoto, N. (1999) J. Biol. Chem. 274, 33341-33347) shows similar activation/sensitization effects. The addition of both DP4 and DP1 produced mutual interference of their activating functions. We tentatively propose that contact between the two (N-terminal and central) domains closes the channel, whereas removal of the contact by these domain peptides or by MH/CCD mutations de-blocks the channel, resulting in hyperactivation/hyper-sensitization effects.