Ryanodine receptor regulation by intramolecular interaction between cytoplasmic and transmembrane domains

The Pentameric Ligand-Gated Ion Channel Family: A New Member of the Voltage Gated Ion Channel Superfamily?

In this report we present seven lines of bioinformatic evidence supporting the conclusion that the Pentameric Ligand-gated Ion Channel (pLIC) Family is a member of the Voltage-gated Ion Channel (VIC) Superfamily. In our approach, we used the Transporter Classification Database (TCDB) as a reference and applied a series of bioinformatic methods to search for similarities between the pLIC family and members of the VIC superfamily. These include: (1) sequence similarity, (2) compatibility of topology and hydropathy profiles, (3) shared domains, (4) conserved motifs, (5) similarity of Hidden Markov Model profiles between families, (6) common 3D structural folds, and (7) clustering analysis of all families. Furthermore, sequence and structural comparisons as well as the identification of a 3-TMS repeat unit in the VIC superfamily suggests that the sixth transmembrane segment evolved into a re-entrant loop. This evidence suggests that the voltage-sensor domain and the channel domain have a common origin. The classification of the pLIC family within the VIC superfamily sheds light onto the topological origins of this family and its evolution, which will facilitate experimental verification and further research into this superfamily by the scientific community.

A novel RyR2 mutation associated with co-morbid catecholaminergic polymorphic ventricular tachycardia (CPVT) and benign epilepsy with centrotemporal spikes (BECTS)

In this case report, we describe a 14-year-old patient with a novel RyR2 gene mutation (c.6577G > T/p.Val2193Leu), identified through a comprehensive review of medical history, examination findings, and follow-up data. The pathogenic potential of this mutation, which results in the loss of some interatomic forces and compromises the closure of the RyR2 protein pore leading to calcium leakage, was analyzed using the I-TASSER Suite to predict the structural changes in the protein. This mutation manifested clinically as co-morbid catecholaminergic polymorphic ventricular tachycardia (CPVT) and benign epilepsy with centrotemporal spikes (BECTS), a combination not previously documented in the same patient. While seizures were successfully managed with levetiracetam, the patient's exercise-induced syncope episodes could not be controlled with metoprolol, highlighting the complexity and challenge in managing CPVT associated with this novel RyR2 variation.

Molecular, Subcellular, and Arrhythmogenic Mechanisms in Genetic RyR2 Disease

The ryanodine receptor (RyR2) has a critical role in controlling Ca²⁺ release from the sarcoplasmic reticulum (SR) throughout the cardiac cycle. RyR2 protein has multiple functional domains with specific roles, and four of these RyR2 protomers are required to form the quaternary structure that comprises the functional channel. Numerous mutations in the gene encoding RyR2 protein have been identified and many are linked to a wide spectrum of arrhythmic heart disease. Gain of function mutations (GoF) result in a hyperactive channel that causes excessive spontaneous SR Ca²⁺ release. This is the predominant cause of the inherited syndrome catecholaminergic polymorphic ventricular tachycardia (CPVT). Recently, rare hypoactive loss of function (LoF) mutations have been identified that produce atypical effects on cardiac Ca²⁺ handling that has been termed calcium release deficiency syndrome (CRDS). Aberrant Ca²⁺ release resulting from both GoF and LoF mutations can result in arrhythmias through the Na⁺/Ca²⁺ exchange mechanism. This mini-review discusses recent findings regarding the role of RyR2 domains and endogenous regulators that influence RyR2 gating normally and with GoF/LoF mutations. The arrhythmogenic consequences of GoF/LoF mutations will then be discussed at the macromolecular and cellular level.

Classification and correlation of RYR2 missense variants in individuals with catecholaminergic polymorphic ventricular tachycardia reveals phenotypic relationships

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is predominantly caused by heterozygous missense variants in the cardiac ryanodine receptor, RYR2. However, many RYR2 missense variants are classified as variants of uncertain significance (VUS). We systematically re-evaluated all RYR2 variants in healthy individuals and those with CPVT or arrhythmia using the 2015 American College of Medical Genomics guidelines. RYR2 variants were identified by the NW Genomic Laboratory Hub, from the published literature and databases of sequence variants. Each variant was assessed based on minor allele frequencies, in silico prediction tools and appraisal of functional studies and classified according to the ACMG-AMP guidelines. Phenotype data was collated where available. Of the 326 identified RYR2 missense variants, 55 (16.9%), previously disease-associated variants were reclassified as benign. Application of the gnomAD database of >140,000 controls allowed reclassification of 11 variants more than the ExAC database. CPVT-associated RYR2 variants clustered predominantly between amino acid positions 3949-4332 and 4867-4967 as well as the RyR and IP3R homology-associated and ion transport domains (p < 0.005). CPVT-associated RYR2 variants occurred at more conserved amino acid positions compared with controls, and variants associated with sudden death had higher conservation scores (p < 0.005). There were five potentially pathogenic RYR2 variants associated with sudden death during sleep which were located almost exclusively in the C-terminus of the protein. In conclusion, control sequence databases facilitate reclassification of RYR2 variants but the majority remain as VUS. Notably, pathogenic variants in RYR2 are associated with death in sleep.

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.

Genetic Landscape of Sporadic Unilateral Adrenocortical Adenomas Without PRKACA p.Leu206Arg Mutation

CONTEXT

Adrenocortical adenomas (ACAs) are among the most frequent human neoplasias. Genetic alterations affecting the cAMP/protein kinase A signaling pathway are common in cortisol-producing ACAs, whereas activating mutations in the gene encoding β-catenin (CTNNB1) have been reported in a subset of both benign and malignant adrenocortical tumors. However, the molecular pathogenesis of most ACAs is still largely unclear.

OBJECTIVE

The aim of the study was to define the genetic landscape of sporadic unilateral ACAs.

DESIGN AND SETTING

Next-generation whole-exome sequencing was performed on fresh-frozen tumor samples and corresponding normal tissue samples.

PATIENTS

Ninety-nine patients with ACAs (74 cortisol-producing and 25 endocrine inactive) negative for p.Leu206Arg PRKACA mutation.

MAIN OUTCOME MEASURES

Identification of known and/or new genetic alterations potentially involved in adrenocortical tumorigenesis and autonomous hormone secretion, genotype-phenotype correlation.

RESULTS

A total of 706 somatic protein-altering mutations were detected in 88 of 99 tumors (median, six per tumor). We identified several mutations in genes of the cAMP/protein kinase A pathway, including three novel mutations in PRKACA, associated with female sex and Cushing's syndrome. We also found genetic alterations in different genes involved in the Wnt/β-catenin pathway, associated with larger tumors and endocrine inactivity, and notably, in many genes of the Ca(2+)-signaling pathway. Finally, by comparison of our genetic data with those available in the literature, we describe a comprehensive genetic landscape of unilateral ACAs.

CONCLUSIONS

This study provides the largest sequencing effort on ACAs to date. We thereby identified somatic alterations affecting known and novel pathways potentially involved in adrenal tumorigenesis.

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.

Proteolytic fragmentation of inositol 1,4,5-trisphosphate receptors: a novel mechanism regulating channel activity?

Inositol 1,4,5-trisphosphate receptors (IP3 Rs) are a family of ubiquitously expressed intracellular Ca(2+) release channels. Regulation of channel activity by Ca(2+) , nucleotides, phosphorylation, protein binding partners and other cellular factors is thought to play a major role in defining the specific spatiotemporal characteristics of intracellular Ca(2+) signals. These properties are, in turn, believed pivotal for the selective and specific physiological activation of Ca(2+) -dependent effectors. IP3 Rs are also substrates for the intracellular cysteine proteases, calpain and caspase. Cleavage of the IP3 R has been proposed to play a role in apoptotic cell death by uncoupling regions important for IP3 binding from the channel domain, leaving an unregulated leaky Ca(2+) pore. Contrary to this hypothesis, we demonstrate following proteolysis that N- and C-termini of IP3 R1 remain associated, presumably through non-covalent interactions. Further, we show that complementary fragments of IP3 R1 assemble into tetrameric structures and retain their ability to be regulated robustly by IP3 . While peptide continuity is clearly not necessary for IP3 -gating of the channel, we propose that cleavage of the IP3 R peptide chain may alter other important regulatory events to modulate channel activity. In this scenario, stimulation of the cleaved IP3 R may support distinct spatiotemporal Ca(2+) signals and activation of specific effectors. Notably, in many adaptive physiological events, the non-apoptotic activities of caspase and calpain are demonstrated to be important, but the substrates of the proteases are poorly defined. We speculate that proteolytic fragmentation may represent a novel form of IP3 R regulation, which plays a role in varied adaptive physiological processes.

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.

Molecular characteristics, mRNA expression, and alternative splicing of a ryanodine receptor gene in the oriental fruit fly, Bactrocera dorsalis (Hendel)

Ryanodine receptors (RyRs) are a distinct class of ligand-gated channels controlling the release of calcium from intracellular stores. The emergence of diamide insecticides, which selectively target insect RyRs, has promoted the study of insect RyRs. In the present study, the full-length RyR cDNA (BdRyR) was cloned and characterized from the oriental fruit fly, Bactrocera dorsalis (Hendel), a serious pest of fruits and vegetables throughout East Asia and the Pacific Rim. The cDNA of BdRyR contains a 15,420-bp open reading frame encoding 5,140 amino acids with a predicted molecular weight of 582.4 kDa and an isoelectric point of 5.38. BdRyR shows a high level of amino acid sequence identity (78 to 97%) to other insect RyR isoforms. All common structural features of the RyRs are present in the BdRyR, including a well-conserved C-terminal domain containing consensus calcium-binding EF-hands and six transmembrane domains, and a large N-terminal domain. Quantitative real-time PCR analyses revealed that BdRyR was expressed at the lowest and highest levels in egg and adult, respectively, and that the BdRyR expression levels in the third instar larva, pupa and adult were 166.99-, 157.56- and 808.56-fold higher, respectively, than that in the egg. Among different adult body parts, the highest expression level was observed in the thorax compared with the head and abdomen. In addition, four alternative splice sites were identified in the BdRyR gene, with the first, ASI, being located in the central part of the predicted second spore lysis A/RyR domain. Diagnostic PCR analyses revealed that alternative splice variants were generated not only in a tissue-specific manner but also in a developmentally regulated manner. These results lay the foundation for further understanding the structural and functional properties of BdRyR, and the molecular mechanisms for target site resistance in B. dorsalis.

Overexpression of ryanodine receptor type 1 enhances mitochondrial fragmentation and Ca2+-induced ATP production in cardiac H9c2 myoblasts

Ca(+) influx to mitochondria is an important trigger for both mitochondrial dynamics and ATP generation in various cell types, including cardiac cells. Mitochondrial Ca(2+) influx is mainly mediated by the mitochondrial Ca(2+) uniporter (MCU). Growing evidence also indicates that mitochondrial Ca(2+) influx mechanisms are regulated not solely by MCU but also by multiple channels/transporters. We have previously reported that skeletal muscle-type ryanodine receptor (RyR) type 1 (RyR1), which expressed at the mitochondrial inner membrane, serves as an additional Ca(2+) uptake pathway in cardiomyocytes. However, it is still unclear which mitochondrial Ca(2+) influx mechanism is the dominant regulator of mitochondrial morphology/dynamics and energetics in cardiomyocytes. To investigate the role of mitochondrial RyR1 in the regulation of mitochondrial morphology/function in cardiac cells, RyR1 was transiently or stably overexpressed in cardiac H9c2 myoblasts. We found that overexpressed RyR1 was partially localized in mitochondria as observed using both immunoblots of mitochondrial fractionation and confocal microscopy, whereas RyR2, the main RyR isoform in the cardiac sarcoplasmic reticulum, did not show any expression at mitochondria. Interestingly, overexpression of RyR1 but not MCU or RyR2 resulted in mitochondrial fragmentation. These fragmented mitochondria showed bigger and sustained mitochondrial Ca(2+) transients compared with basal tubular mitochondria. In addition, RyR1-overexpressing cells had a higher mitochondrial ATP concentration under basal conditions and showed more ATP production in response to cytosolic Ca(2+) elevation compared with nontransfected cells as observed by a matrix-targeted ATP biosensor. These results indicate that RyR1 possesses a mitochondrial targeting/retention signal and modulates mitochondrial morphology and Ca(2+)-induced ATP production in cardiac H9c2 myoblasts.

Molecular cloning, characterization and expression profiling of a ryanodine receptor gene in Asian corn borer, Ostrinia furnacalis (Guenée)

Ryanodine receptor (RyR) Ca(2+) release channel is the target of diamide insecticides, which show selective insecticidal activity against lepidopterous insects. To study the molecular mechanisms underlying the species-specific action of diamide insecticides, we have cloned and characterized the entire cDNA sequence of RyR from Ostrinia furnacalis (named as OfRyR). The OfRyR mRNA has an Open Reading Frame of 15324 bp nucleotides and encodes a 5108 amino acid polypeptide that displays 79-97% identity with other insects RyR proteins and shows the greatest identity with Cnaphalocrocis medinalis RyR (97%). Quantitative real-time PCR showed that the OfRyR was expressed at the lowest level in egg and the highest level in adult. The relative expression level of OfRyR in first, third and fifth-instar larva were 1.28, 1.19 and 1.99 times of that in egg. Moreover, two alternative splicing sites were identified in the OfRyR gene. One pair of mutually exclusive exons (a/b) were present in the central part of the predicted SPRY domain, and an optional exon (c) was located between the third and fourth RyR domains. Diagnostic PCR demonstrated that exons a and b existed in all developmental stages of OfRyR cDNA, but exon c was not detected in the egg cDNA. And the usage frequencies of these exons showed a significant difference between different developmental stages. These results provided the crucial basis for the functional expression of OfRyR and for the discovery of compound with potentially selective insect activtity.

N-terminus oligomerization regulates the function of cardiac ryanodine receptors

The ryanodine receptor (RyR) is an ion channel composed of four identical subunits mediating calcium efflux from the endo/sarcoplasmic reticulum of excitable and non-excitable cells. We present several lines of evidence indicating that the RyR2 N-terminus is capable of self-association. A combination of yeast two-hybrid screens, co-immunoprecipitation analysis, chemical crosslinking and gel filtration assays collectively demonstrate that a RyR2 N-terminal fragment possesses the intrinsic ability to oligomerize, enabling apparent tetramer formation. Interestingly, N-terminus tetramerization mediated by endogenous disulfide bond formation occurs in native RyR2, but notably not in RyR1. Disruption of N-terminal inter-subunit interactions within RyR2 results in dysregulation of channel activation at diastolic Ca(2+) concentrations from ryanodine binding and single channel measurements. Our findings suggest that the N-terminus interactions mediating tetramer assembly are involved in RyR channel closure, identifying a crucial role for this structural association in the dynamic regulation of intracellular Ca(2+) release.

Cardiac phenotype of Duchenne Muscular Dystrophy: insights from cellular studies

Dilated cardiomyopathy is a serious and almost inevitable complication of Duchenne Muscular Dystrophy, a devastating and fatal disease of skeletal muscle resulting from the lack of functional dystrophin, a protein linking the cytoskeleton to the extracellular matrix. Ultimately, it leads to congestive heart failure and arrhythmias resulting from both cardiac muscle fibrosis and impaired function of the remaining cardiomyocytes. Here we summarize findings obtained in several laboratories, focusing on cellular mechanisms that result in degradation of cardiac functions in dystrophy.

Enhanced binding of calmodulin to the ryanodine receptor corrects contractile dysfunction in failing hearts

AIMS

The channel function of the cardiac ryanodine receptor (RyR2) is modulated by calmodulin (CaM). However, the involvement of CaM in aberrant Ca(2+) release in diseased hearts remains unclear. Here, we investigated the pathogenic role of defective CaM binding to the RyR2 in the channel dysfunction associated with heart failure.

METHODS AND RESULTS

The involvement of CaM in aberrant Ca(2+) release was assessed in normal and pacing-induced failing canine hearts. The apparent affinity of CaM for RyR2 was considerably lower in failing sarcoplasmic reticulum (SR) compared with normal SR. Thus, the amount of CaM bound to RyR2 was markedly decreased in failing myocytes. Expression of the CaM isoform Gly-Ser-His-CaM (GSH-CaM), which has much higher binding affinity than wild-type CaM for RyR1, restored normal CaM binding to RyR2 in both SR and myocytes of failing hearts. The Ca(2+) spark frequency (SpF) was markedly higher and the SR Ca(2+) content was lower in failing myocytes compared with normal myocytes. The incorporation of GSH-CaM into the failing myocytes corrected the aberrant SpF and SR Ca(2+) content to normal levels.

CONCLUSION

Reduced CaM binding to RyR2 seems to play a critical role in the pathogenesis of aberrant Ca(2+) release in failing hearts. Correction of the reduced CaM binding to RyR2 stabilizes the RyR2 channel function and thereby restores normal Ca(2+) handling and contractile function to failing hearts.

A mechanistic description of gating of the human cardiac ryanodine receptor in a regulated minimal environment

Cardiac muscle contraction, triggered by the action potential, is mediated by the release of Ca²⁺ from the sarcoplasmic reticulum through ryanodine receptor (RyR)2 channels. In situ, RyR2 gating is modulated by numerous physiological and pharmacological agents, and altered RyR2 function underlies the occurrence of arrhythmias in both inherited and acquired diseases. To understand fully the mechanisms underpinning the regulation of RyR2 in the normal heart and how these systems are altered in pathological conditions, we must first gain a detailed knowledge of the fundamental processes of RyR2 gating. In this investigation, we provide key novel mechanistic insights into the physical reality of RyR2 gating revealed by new experimental and analytical approaches. We have examined in detail the single-channel gating kinetics of the purified human RyR2 when activated by cytosolic Ca²⁺ in a stringently regulated environment where the modulatory influence of factors external to the channel were minimized. The resulting gating schemes are based on an accurate description of single-channel kinetics using hidden Markov model analysis and reveal several novel aspects of RyR2 gating behavior: (a) constitutive gating is observed as unliganded opening events; (b) binding of Ca²⁺ to the channel stabilizes it in different open states; (c) RyR2 exists in two preopening closed conformations in equilibrium, one of which binds Ca²⁺ more readily than the other; (d) the gating of RyR2 when bound to Ca²⁺ can be described by a kinetic scheme incorporating bursts; and (e) analysis of flicker closing events within bursts reveals gating activity that is not influenced by ligand binding. The gating schemes generated in this investigation provide a framework for future studies in which the mechanisms of action of key physiological regulatory factors, disease-linked mutations, and potential therapeutic compounds can be described precisely.

Structural and functional conservation of key domains in InsP3 and ryanodine receptors

Inositol 1,4,5-trisphosphate receptors (InsP₃R) and ryanodine receptors (RyR) are tetrameric intracellular Ca²⁺ channels¹. For each, the pore is formed by C-terminal transmembrane domains and regulated by signals detected by the large cytosolic structures. InsP₃R gating is initiated by InsP₃ binding to the InsP₃-binding core (IBC, residues 224-604 of InsP₃R1)² and it requires the suppressor domain (SD, residues 1-223)^2-8. We present structures of the N-terminal region (NT) of InsP₃R1 with (3.6 Å) and without (3.0 Å) InsP₃ bound. The arrangement of the three NT domains, the SD, IBC-β and IBC-α, identifies two discrete interfaces (α and β) between the IBC and SD. Similar interfaces occur between equivalent domains (A, B and C) in RyR1⁹. The orientations of the three domains docked into a tetrameric structure of InsP₃R¹⁰ and of the ABC domains in RyR⁹ are remarkably similar. The importance of the α-interface for activation of InsP₃R and RyR is confirmed by mutagenesis and, for RyR, by disease-causing mutations^9,11,12. InsP₃ causes partial closure of the clam-like IBC, disrupting the β-interface and pulling the SD towards the IBC. This reorients an exposed SD loop (HS-loop) that is essential for InsP₃R activation⁷. The loop is conserved in RyR and includes mutations associated with malignant hyperthermia and central core disease^9,11,12. The HS-loop interacts with an adjacent NT, suggesting that activation re-arranges inter-subunit interactions. The A-domain of RyR functionally replaced the SD in a full-length InsP₃R, and an InsP₃R in which its C-terminal transmembrane region was replaced by that from RyR1 was gated by InsP₃ and blocked by ryanodine. Activation mechanisms are conserved between InsP₃R and RyR. Allosteric modulation of two similar domain interfaces within an N-terminal subunit re-orients the first domain (SD or A-domain), allowing it, via interactions of the second domain of an adjacent subunit (IBC-β or B-domain), to gate the pore.

Uncoupled IP3 receptor can function as a Ca2+-leak channel: cell biological and pathological consequences

Ca(2+) release via intracellular release channels, IP(3)Rs (inositol 1,4,5-trisphosphate receptors) and RyRs (ryanodine receptors), is perhaps the most ubiquitous and versatile cellular signalling mechanism, and is involved in a vast number of cellular processes. In addition to this classical release pathway there is limited, but yet persistent, information about less well-defined Ca(2+)-leak pathways that may play an important role in the control of the Ca(2+) load of the endo(sarco)plasmic reticulum. The mechanisms responsible for this 'basal' leak are not known, but recent data suggest that both IP(3)Rs and RyRs may also operate as Ca(2+)-leak channels, particularly in pathological conditions. Proteolytic cleavage or biochemical modification (such as hyperphosphorylation or nitrosylation), for example, occurring during conditions of cell stress or apoptosis, can functionally uncouple the cytoplasmic control domains from the channel domain of the receptor. Highly significant information has been obtained from studies of malfunctioning channels in various disorders; for example, RyRs in cardiac malfunction or genetic muscle diseases and IP(3)Rs in neurodegenerative diseases. In this review we aim to summarize the existing information about functionally uncoupled IP(3)R and RyR channels, and to discuss the concept that those channels can participate in Ca(2+)-leak pathways.

Mutation-linked defective interdomain interactions within ryanodine receptor cause aberrant Ca²⁺release leading to catecholaminergic polymorphic ventricular tachycardia

BACKGROUND

The molecular mechanism by which catecholaminergic polymorphic ventricular tachycardia is induced by single amino acid mutations within the cardiac ryanodine receptor (RyR2) remains elusive. In the present study, we investigated mutation-induced conformational defects of RyR2 using a knockin mouse model expressing the human catecholaminergic polymorphic ventricular tachycardia-associated RyR2 mutant (S2246L; serine to leucine mutation at the residue 2246).

METHODS AND RESULTS

All knockin mice we examined produced ventricular tachycardia after exercise on a treadmill. cAMP-dependent increase in the frequency of Ca²⁺ sparks was more pronounced in saponin-permeabilized knockin cardiomyocytes than in wild-type cardiomyocytes. Site-directed fluorescent labeling and quartz microbalance assays of the specific binding of DP2246 (a peptide corresponding to the 2232 to 2266 region: the 2246 domain) showed that DP2246 binds with the K201-binding sequence of RyR2 (1741 to 2270). Introduction of S2246L mutation into the DP2246 increased the affinity of peptide binding. Fluorescence quench assays of interdomain interactions within RyR2 showed that tight interaction of the 2246 domain/K201-binding domain is coupled with domain unzipping of the N-terminal (1 to 600)/central (2000 to 2500) domain pair in an allosteric manner. Dantrolene corrected the mutation-caused domain unzipping of the domain switch and stopped the exercise-induced ventricular tachycardia.

CONCLUSIONS

The catecholaminergic polymorphic ventricular tachycardia-linked mutation of RyR2, S2246L, causes an abnormally tight local subdomain-subdomain interaction within the central domain involving the mutation site, which induces defective interaction between the N-terminal and central domains. This results in an erroneous activation of Ca²⁺ channel in a diastolic state reflecting on the increased Ca²⁺ spark frequency, which then leads to lethal arrhythmia.

Characterization of a novel mutation in the cardiac ryanodine receptor that results in catecholaminergic polymorphic ventricular tachycardia

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an arrhythmogenic disease that manifests as syncope or sudden death during high adrenergic tone in the absence of structural heart defects. It is primarily caused by mutations in the cardiac ryanodine receptor (RyR2). The mechanism by which these mutations cause arrhythmia remains controversial, with discrepant findings related to the role of the RyR2 binding protein FKBP12.6. The purpose of this study was to characterize a novel RyR2 mutation identified in a kindred with clinically diagnosed CPVT. Single-strand conformational polymorphism analysis and direct DNA sequencing were used to screen the RyR2 gene for mutations. Site-directed mutagenesis was employed to introduce the mutation into the mouse RyR2 cDNA. The impact of the mutation on the interaction between RyR2 and a 12.6 kDa FK506 binding protein (FKBP12.6) was determined by immunoprecipitation and immunoblotting and its effect on RyR2 function was characterized by single cell Ca(2+) imaging and [(3)H]ryanodine binding. A novel CPVT mutation, E189D, was identified. The E189D mutation does not alter the affinity of the channel for FKBP12.6, but it increases the propensity for store-overload-induced Ca(2+) release (SOICR). Furthermore, the E189D mutation enhances the basal channel activity of RyR2 and its sensitivity to activation by caffeine. The E189D RyR2 mutation is causative for CPVT and functionally increases the propensity for SOICR without altering the affinity for FKBP12.6. These observations strengthen the notion that enhanced SOICR, but not altered FKBP12.6 binding, is a common mechanism by which RyR2 mutations cause arrhythmias.

Intracellular Ca2+ storage in health and disease: a dynamic equilibrium

Homeostatic control of the endoplasmic reticulum (ER) both as the site for protein handling (synthesis, folding, trafficking, disaggregation and degradation) and as a Ca2+ store is of crucial importance for correct functioning of the cell. Disturbance of the homeostatic control mechanisms leads to a vast array of severe pathologies. The Ca2+ content of the ER is a dynamic equilibrium between active uptake via Ca2+ pumps and Ca2+ release by a number of highly regulated Ca2+-release channels. Regulation of the Ca2+-release channels is very complex and several mechanisms are still poorly understood or controversial. There is increasing evidence that a number of unrelated proteins, either by themselves or in association with other Ca2+ channels, can provide additional Ca2+-leak pathways. The ER is a dynamic organelle and changes in its size and components have been described, either as a result of (de)differentiation processes affecting the secretory capacity of cells, or as a result of adaptation mechanisms to diverse stress conditions such as the unfolded protein response and autophagy. In this review we want to give an overview of the current knowledge of the (short-term) regulatory mechanisms that affect Ca2+-release and Ca2+-leak pathways and of the (long-term) adaptations in ER size and capacity. Understanding of the consequences of these mechanisms for cellular Ca2+ signaling could provide a huge therapeutic potential.

Ryanodine receptor mutations in arrhythmia: The continuing mystery of channel dysfunction

Mutations in RyR2 are causative of an inherited disorder which often results in sudden cardiac death. Dysfunctional channel behaviour has been the subject of many investigations varying from single channel analysis through to complex animal models. This review discusses recent advances in the field, describes the controversy surrounding the exact consequences of RyR2 mutation and how the disparate data may be reconciled. This heterogeneity of function with respect to the effects of polymorphisms, phosphorylation, cytosolic and luminal Ca(2+) as well as inter-domain interactions may have important implications for the recent pharmaceutical therapies which have been put forward. We surmise that a comprehensive characterisation of mutations on a case-by-case basis may be beneficial for the development of specifically targeted therapies.

Na+-dependent SR Ca2+ overload induces arrhythmogenic events in mouse cardiomyocytes with a human CPVT mutation

AIMS

Mutations in the cardiac ryanodine receptor Ca(2+) release channel, RyR2, underlie catecholaminergic polymorphic ventricular tachycardia (CPVT), an inherited life-threatening arrhythmia. CPVT is triggered by spontaneous RyR2-mediated sarcoplasmic reticulum (SR) Ca(2+) release in response to SR Ca(2+) overload during beta-adrenergic stimulation. However, whether elevated SR Ca(2+) content--in the absence of protein kinase A activation--affects RyR2 function and arrhythmogenesis in CPVT remains elusive.

METHODS AND RESULTS

Isolated murine ventricular myocytes harbouring a human RyR2 mutation (RyR2(R4496C+/-)) associated with CPVT were investigated in the absence and presence of 1 micromol/L JTV-519 (RyR2 stabilizer) followed by 100 micromol/L ouabain intervention to increase cytosolic [Na(+)] and SR Ca(2+) load. Changes in membrane potential and intracellular [Ca(2+)] were monitored with whole-cell patch-clamping and confocal Ca(2+) imaging, respectively. At baseline, action potentials (APs), Ca(2+) transients, fractional SR Ca(2+) release, and SR Ca(2+) load were comparable in wild-type (WT) and RyR2(R4496C+/-) myocytes. Ouabain evoked significant increases in diastolic [Ca(2+)], peak systolic [Ca(2+)], fractional SR Ca(2+) release, and SR Ca(2+) content that were quantitatively similar in WT and RyR2(R4496C+/-) myocytes. Ouabain also induced arrhythmogenic events, i.e. spontaneous Ca(2+) waves, delayed afterdepolarizations and spontaneous APs, in both groups. However, the ouabain-induced increase in the frequency of arrhythmogenic events was dramatically larger in RyR2(R4496C+/-) when compared with WT myocytes. JTV-519 greatly reduced the frequency of ouabain-induced arrhythmogenic events.

CONCLUSION

The elevation of SR Ca(2+) load--in the absence of beta-adrenergic stimulation--is sufficient to increase the propensity for triggered arrhythmias in RyR2(R4496C+/-) cardiomyocytes. Stabilization of RyR2 by JTV-519 effectively reduces these triggered arrhythmias.

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.

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.

[Molecular architecture of the sarcoplasmic reticulum and its role in the ECC]

The sarcoplasmic reticulum (SR) plays a fundamental role in excitation-contraction coupling, which propagates the electric signal conversion along the muscle fiber's plasmic membrane to a mechanical event manifested as a muscle contraction. It plays a crucial role in calcium homeostasis and intracellular calcium storage control (storage, liberation and uptake) necessary for fiber muscle contraction and then relaxation. These functions take place at the triad, made up of individualized SR subdomains where the protein-specific organization provides efficient and fast coupling. Ryanodine receptors (RyR) and dihydropyridine receptors (DHPR) mainly act in calcium exchanges in the SR. This particular structural and molecular architecture must be correlated to its functional specificity.

Increased Ca2+ sensitivity of the ryanodine receptor mutant RyR2R4496C underlies catecholaminergic polymorphic ventricular tachycardia

Cardiac ryanodine receptor (RyR2) mutations are associated with autosomal dominant catecholaminergic polymorphic ventricular tachycardia, suggesting that alterations in Ca(2+) handling underlie this disease. Here we analyze the underlying Ca(2+) release defect that leads to arrhythmia in cardiomyocytes isolated from heterozygous knock-in mice carrying the RyR2(R4496C) mutation. RyR2(R4496C-/-) littermates (wild type) were used as controls. [Ca(2+)](i) transients were obtained by field stimulation in fluo-3-loaded cardiomyocytes and viewed using confocal microscopy. In our basal recording conditions (2-Hz stimulation rate), [Ca(2+)](i) transients and sarcoplasmic reticulum Ca(2+) load were similar in wild-type and RyR2(R4496C) cells. However, paced RyR2(R4496C) ventricular myocytes presented abnormal Ca(2+) release during the diastolic period, viewed as Ca(2+) waves, consistent with the occurrence of delayed afterdepolarizations. The occurrence of this abnormal Ca(2+) release was enhanced at faster stimulation rates and by beta-adrenergic stimulation, which also induced triggered activity. Spontaneous Ca(2+) sparks were more frequent in RyR2(R4496C) myocytes, indicating increased RyR2(R4496C) activity. When permeabilized cells were exposed to different cytosolic [Ca(2+)](i), RyR2(R4496C) showed a dramatic increase in Ca(2+) sensitivity. Isoproterenol increased [Ca(2+)](i) transient amplitude and Ca(2+) spark frequency to the same extent in wild-type and RyR2(R4496C) cells, indicating that the beta-adrenergic sensitivity of RyR2(R4496C) cells remained unaltered. This effect was independent of protein expression variations because no difference was found in the total or phosphorylated RyR2 expression levels. In conclusion, the arrhythmogenic potential of the RyR2(R4496C) mutation is attributable to the increased Ca(2+) sensitivity of RyR2(R4496C), which induces diastolic Ca(2+) release and lowers the threshold for triggered activity.

Defective domain-domain interactions within the ryanodine receptor as a critical cause of diastolic Ca2+ leak in failing hearts

AIMS

A domain peptide (DP) matching the Gly(2460)-Pro(2495) region of the cardiac type-2 ryanodine receptor (RyR2), DPc10, is known to mimic channel dysfunction associated with catecholaminergic polymorphic ventricular tachycardia (CPVT), owing to its interference in a normal interaction of the N-terminal (1-600) and central (2000-2500) domains (viz. domain unzipping). Using DPc10 and two other DPs harboring different mutation sites, we investigated the underlying mechanism of abnormal Ca(2+) cycling in failing hearts.

METHODS AND RESULTS

Sarcoplasmic reticulum (SR) vesicles and cardiomyocytes were isolated from dog left ventricular muscles for Ca(2+) leak and spark assays. The RyR2 moiety of the SR was fluorescently labelled with methylcoumarin acetate (MCA) using DPs corresponding to the 163-195 and 4090-4123 regions of RyR2 (DP163-195 and DP4090-4123, respectively) as site-directed carriers. Both DPs mediated a specific MCA fluorescence labelling of RyR2. Addition of either DP to the MCA-labelled SR induced domain unzipping, as evidenced by an increased accessibility of the bound MCA to a large-size fluorescence quencher. Both SR Ca(2+) leak and Ca(2+) spark frequency (SpF) were markedly increased in failing cardiomyocytes. Upon introduction of DP163-195 or DP4090-4123 into normal SR or cardiomyocytes, both Ca(2+) leak and SpF increased to the levels comparable with those of failing myocytes. K201 (JTV519) suppressed all of the effects induced by DP163-195 (domain unzipping and increased Ca(2+) leak and SpF) or those in failing cardiomyocytes, but did not suppress the effects induced by DP4090-4123.

CONCLUSION

Defective inter-domain interaction between N-terminal and central domains induces diastolic Ca(2+) leak, leading to heart failure and lethal arrhythmia. Mutation at the C-terminal region seen in CPVT does not seem to communicate with the aforementioned N-terminal and central inter-domain interaction, although spontaneous Ca(2+) leak is similarly induced.

Physiological consequences of the P2328S mutation in the ryanodine receptor (RyR2) gene in genetically modified murine hearts

Aim

To explore the physiological consequences of the ryanodine receptor (RyR2)-P2328S mutation associated with catecholaminergic polymorphic ventricular tachycardia (CPVT).

Methods

We generated heterozygotic (RyR2^p/s) and homozygotic (RyR2^s/s) transgenic mice and studied Ca²⁺ signals from regularly stimulated, Fluo-3-loaded, cardiac myocytes. Results were compared with monophasic action potentials (MAPs) in Langendorff-perfused hearts under both regular and programmed electrical stimulation (PES).

Results

Evoked Ca²⁺ transients from wild-type (WT), heterozygote (RyR2^p/s) and homozygote (RyR2^s/s) myocytes had indistinguishable peak amplitudes with RyR2^s/s showing subsidiary events. Adding 100 nm isoproterenol produced both ectopic peaks and subsidiary events in WT but not RyR2^p/s and ectopic peaks and reduced amplitudes of evoked peaks in RyR2^s/s. Regularly stimulated WT, RyR2^p/s and RyR2^s/s hearts showed indistinguishable MAP durations and refractory periods. RyR2^p/s hearts showed non-sustained ventricular tachycardias (nsVTs) only with PES. Both nsVTs and sustained VTs (sVTs) occurred with regular stimuli and PES with isoproterenol treatment. RyR2^s/s hearts showed higher incidences of nsVTs before but mainly sVTs after introduction of isoproterenol with both regular stimuli and PES, particularly at higher pacing frequencies. Additionally, intrinsically beating RyR2^s/s showed extrasystolic events often followed by spontaneous sVT.

Conclusion

The RyR2-P2328S mutation results in marked alterations in cellular Ca²⁺ homeostasis and arrhythmogenic properties resembling CPVT with greater effects in the homozygote than the heterozygote demonstrating an important gene dosage effect.

Endoplasmic reticulum Ca2+ measurements reveal that the cardiac ryanodine receptor mutations linked to cardiac arrhythmia and sudden death alter the threshold for store-overload-induced Ca2+ release

A number of RyR2 (cardiac ryanodine receptor) mutations linked to ventricular arrhythmia and sudden death are located within the last C-terminal ∼500 amino acid residues, which is believed to constitute the ion-conducting pore and gating domain of the channel. We have previously shown that mutations located near the C-terminal end of the predicted TM (transmembrane) segment 10, the inner pore helix, can either increase or decrease the propensity for SOICR (store-overload-induced Ca2+ release), also known as spontaneous Ca2+ release. In the present study, we have characterized an RyR2 mutation, V4653F, located in the loop between the predicted TM 6 and TM 7a, using an ER (endoplasmic reticulum)-targeted Ca2+-indicator protein (D1ER). We directly demonstrated that SOICR occurs at a reduced luminal Ca2+ threshold in HEK-293 cells (human embryonic kidney cells) expressing the V4653F mutant as compared with cells expressing the RyR2 wild-type. Single-channel analyses revealed that the V4653F mutation increased the sensitivity of RyR2 to activation by luminal Ca2+. In contrast with previous reports, the V4653 mutation did not alter FKBP12.6 (FK506-binding protein 12.6 kDa; F506 is an immunosuppressant macrolide)–RyR2 interaction. Luminal Ca2+ measurements also showed that the mutations R176Q/T2504M, S2246L and Q4201R, located in different regions of the channel, reduced the threshold for SOICR, whereas the A4860G mutation, located within the inner pore helix, increased the SOICR threshold. We conclude that the cytosolic loop between TM 6 and TM 7a plays an important role in determining the SOICR threshold and that the alteration of the threshold for SOICR is a common mechanism for RyR2-associated ventricular arrhythmia.

Ryanodine receptor mutations in arrhythmias: advances in understanding the mechanisms of channel dysfunction

The cardiac ryanodine receptor (RyR2) mediates rapid Ca2+ efflux from intracellular stores to effect myocyte contraction during the process of EC (excitation–contraction) coupling. It is now known that mutations in this channel perturb Ca2+ release function, leading to triggered arrhythmias that may cause SCD (sudden cardiac death). Resolving the precise molecular mechanisms by which SCD-linked RyR2 dysfunction occurs currently constitutes a burgeoning area of cardiac research. So far, defective channel phosphorylation, accessory protein binding, luminal/cytosolic Ca2+ sensing, and the disruption of interdomain interactions represent the main candidate mechanisms for explaining aberrant SR (sarcoplasmic reticulum) Ca2+ release via mutants of RyR2. It appears increasingly unlikely that a single exclusive common mechanism underlies every case of mutant channel dysfunction, and that each of these potential mechanisms may contribute to the resultant phenotype. The present review will consider very recent mechanistic developments in this field, including new observations from mutant RyR2 transgenic mouse models, peptide-probe studies, and the implications of functional and phenotypic heterogeneity of RyR2 mutations and polymorphisms.

A truncation in the RYR1 gene associated with central core lesions in skeletal muscle fibres

A novel single-nucleotide deletion in exon 100 of the RYR1 gene, corresponding to deletion of nucleotide 14,510 in the human RyR1 mRNA (c14510delA), was identified in a man with malignant hyperthermia and in his two daughters who were normal for malignant hyperthermia. This deletion results in a RyR1 protein lacking the last 202 amino acid residues. All three subjects heterozygotic for the mutated allele presented with a prevalence of type 1 fibres with central cores, although none experienced clinical signs of myopathy. Expression of the truncated protein resulted in non-functional RYR1 calcium release channels. Expression of wild-type and RyR1(R4836fsX4838) proteins resulted in heterozygotic release channels with overall functional properties similar to those of wild-type RyR1 channels. Nevertheless, small differences in sensitivity to calcium and caffeine were observed in heterotetrameric channels, which also presented an altered assembly/stability in sucrose-gradient centrifugation analysis. Altogether, these data suggest that altered RYR1 tetramer assembly/stability coupled with subtle chronic changes in Ca2+ homoeostasis over the long term may contribute to the development of core lesions and incomplete malignant hyperthermia susceptibility penetrance in individuals carrying this novel RYR1 mutation.

Targeting and retention of type 1 ryanodine receptors to the endoplasmic reticulum

Most ryanodine receptors and their relatives, inositol 1,4,5-trisphosphate receptors, are expressed in the sarcoplasmic or endoplasmic reticulum (ER), where they mediate Ca(2+) release. We expressed fragments of ryanodine receptor type 1 (RyR1) in COS cells alone or fused to intercellular adhesion molecule-1 (ICAM-1), each tagged with yellow fluorescent protein, and used confocal imaging and glycoprotein analysis to identify the determinants of ER targeting and retention. Single transmembrane domains (TMD) of RyR1 taken from the first (TMD1-TMD2) or last (TMD5-TMD6) pair were expressed in the ER membrane. TMD3-TMD4 was expressed in the outer mitochondrial membrane. The TMD outer pairs (TMD1-TMD2 and TMD5-TMD6) retained ICAM-1, a plasma membrane-targeted protein, within the ER membrane. TMD1 alone provided a strong ER retention signal and TMD6 a weaker signal, but the other single TMD were unable to retain ICAM-1 in the ER. We conclude that TMD1 provides the first and sufficient signal for ER targeting of RyR1. The TMD outer pairs include redundant ER retention signals, with TMD1 providing the strongest signal.

Alternative Splicing of Ryanodine Receptors Modulates Cardiomyocyte Ca 2+ Signaling and Susceptibility to Apoptosis

Ca 2+ release via type 2 ryanodine receptors (RyR2) regulates cardiac function. Molecular cloning of human RyR2 identified 2 alternatively spliced variants, comprising 30- and 24-bp sequence insertions; yet their role in shaping cardiomyocyte Ca 2+ signaling and cell phenotype is unknown. We profiled the developmental regulation and the tissue and species specificity of these variants and showed that their recombinant expression in HL-1 cardiomyocytes profoundly modulated nuclear and cytoplasmic Ca 2+ release. All splice variants localized to the sarcoplasmic reticulum, perinuclear Golgi apparatus, and to finger-like invaginations of the nuclear envelope (nucleoplasmic reticulum). Strikingly, the 24-bp splice insertion that was present at low levels in embryonic and adult hearts was essential for targeting RyR2 to an intranuclear Golgi apparatus and promoted the intracellular segregation of this variant. The amplitude variability of nuclear and cytoplasmic Ca 2+ fluxes were reduced in nonstimulated cardiomyocytes expressing both 30- and 24-bp splice variants and were associated with lower basal levels of apoptosis. Expression of RyR2 containing the 24-bp insertion also suppressed intracellular Ca 2+ fluxes following prolonged caffeine exposure (1 mmol/L, 16 hours) that protected cells from apoptosis. The antiapoptotic effects of this variant were linked to increased levels of Bcl-2 phosphorylation. In contrast, RyR2 containing the 30-bp insertion, which was abundant in human embryonic heart but was decreased during cardiac development, did not protect cardiomyocytes from caffeine-evoked apoptosis. Thus, we provide the first evidence that RyR2 splice variants exquisitely modulate intracellular Ca 2+ signaling and are key determinants of cardiomyocyte apoptotic susceptibility.

Role of ryanodine receptor mutations in cardiac pathology: more questions than answers?

The RyR (ryanodine receptor) mediates rapid Ca2+ efflux from the ER (endoplasmic reticulum) and is responsible for triggering numerous Ca2+-activated physiological processes. The most studied RyR-mediated process is excitation-contraction coupling in striated muscle, where plasma membrane excitation is transmitted to the cell interior and results in Ca2+ efflux that triggers myocyte contraction. Recently, single-residue mutations in the cardiac RyR (RyR2) have been identified in families that exhibit CPVT (catecholaminergic polymorphic ventricular tachycardia), a condition in which physical or emotional stress can trigger severe tachyarrhythmias that can lead to sudden cardiac death. The RyR2 mutations in CPVT are clustered in the N- and C-terminal domains, as well as in a central domain. Further, a critical signalling role for dysfunctional RyR2 has also been implicated in the generation of arrhythmias in the common condition of HF (heart failure). We have prepared cardiac RyR2 plasmids with various CPVT mutations to enable expression and analysis of Ca2+ release mediated by the wild-type and mutated RyR2. These studies suggest that the mutational locus may be important in the mechanism of Ca2+ channel dysfunction. Understanding the causes of aberrant Ca2+ release via RyR2 may assist in the development of effective treatments for the ventricular arrhythmias that often leads to sudden death in HF and in CPVT.

Skeletal and cardiac ryanodine receptors exhibit different responses to Ca2+ overload and luminal ca2+

Spontaneous Ca(2+) release occurs in cardiac cells during sarcoplasmic reticulum Ca(2+) overload, a process we refer to as store-overload-induced Ca(2+) release (SOICR). Unlike cardiac cells, skeletal muscle cells exhibit little SOICR activity. The molecular basis of this difference is not well defined. In this study, we investigated the SOICR properties of HEK293 cells expressing RyR1 or RyR2. We found that HEK293 cells expressing RyR2 exhibited robust SOICR activity, whereas no SOICR activity was observed in HEK293 cells expressing RyR1. However, in the presence of low concentrations of caffeine, SOICR could be triggered in these RyR1-expressing cells. At the single-channel level, we showed that RyR2 is much more sensitive to luminal Ca(2+) than RyR1. To identify the molecular determinants responsible for these differences, we constructed two chimeras between RyR1 and RyR2, N-RyR1(1-4006)/C-RyR2(3962-4968) and N-RyR2(1-3961)/C-RyR1(4007-5037). We found that replacing the C-terminal region of RyR1 with the corresponding region of RyR2 (N-RyR1/C-RyR2) dramatically enhanced the propensity for SOICR and the response to luminal Ca(2+), whereas replacing the C-terminal region of RyR2 with the corresponding region of RyR1 (N-RyR2/C-RyR1) reduced the propensity for SOICR and the luminal Ca(2+) response. These observations indicate that the C-terminal region of RyR is a critical determinant of both SOICR and the response to luminal Ca(2+). These chimeric studies also reveal that the N-terminal region of RyR plays an important role in regulating SOICR and luminal Ca(2+) response. Taken together, our results demonstrate that RyR1 differs markedly from RyR2 with respect to their responses to Ca(2+) overload and luminal Ca(2+), and suggest that the lack of spontaneous Ca(2+) release in skeletal muscle cells is, in part, attributable to the unique intrinsic properties of RyR1.

Ryanodine receptors and ventricular arrhythmias: emerging trends in mutations, mechanisms and therapies

It has been six years since the first reported link between mutations in the cardiac ryanodine receptor Ca(2+) release channel (RyR2) and catecholaminergic polymorphic ventricular tachycardia (CPVT), a malignant stress-induced arrhythmia. In this time, rapid advances have been made in identifying new mutations, and in understanding how these mutations disrupt normal channel function to cause VT that frequently degenerates into ventricular fibrillation (VF) and sudden death. Functional characterisation of these RyR2 Ca(2+) channelopathies suggests that mutations alter the ability of RyR2 to sense its intracellular environment, and that channel modulation via covalent modification, Ca(2+)- and Mg(2+)-dependent regulation and structural feedback mechanisms are catastrophically disturbed. This review reconciles the current status of RyR2 mutation-linked etiopathology, the significance of mutational clustering within the RyR2 polypeptide and the mechanisms underlying channel dysfunction. We will also review new data that explores the link between abnormal Ca(2+) release and the resultant cardiac electrical instability in VT and VF, and how these recent developments impact on novel anti-arrhythmic therapies. Finally, we evaluate the concept that mechanistic differences between CPVT and other arrhythmogenic disorders may preclude a common therapeutic strategy to normalise RyR2 function in cardiac disease.

Modulation of calcium signalling by dominant negative splice variant of ryanodine receptor subtype 3 in native smooth muscle cells

The ryanodine receptor subtype 3 (RYR3) is expressed ubiquitously but its physiological function varies from cell to cell. Here, we investigated the role of a dominant negative RYR3 isoform in Ca2+ signalling in native smooth muscle cells. We used intranuclear injection of antisense oligonucleotides to specifically inhibit endogenous RYR3 isoform expression. In mouse duodenum myocytes expressing RYR2 subtype and both spliced and non-spliced RYR3 isoforms, RYR2 and non-spliced RYR3 were activated by caffeine whereas the spliced RYR3 was not. Only RYR2 was responsible for the Ca2+-induced Ca2+ release mechanism that amplified Ca2+ influx- or inositol 1,4,5-trisphosphate-induced Ca2+ signals. However, the spliced RYR3 negatively regulated RYR2 leading to the decrease of amplitude and upstroke velocity of Ca2+ signals. Immunostaining in injected cells showed that the spliced RYR3 was principally expressed near the plasma membrane whilst the non-spliced isoform was revealed around the nucleus. This study shows for the first time that the short isoform of RYR3 controls Ca2+ release through RYR2 in native smooth muscle cells.

Amino acid residues Gln4020 and Lys4021 of the ryanodine receptor type 1 are required for activation by 4-chloro-m-cresol

The ryanodine receptor type 1 (RyR1) and type 2 (RyR2), but not type 3 (RyR3), are efficiently activated by 4-chloro-m-cresol (4-CmC). We previously showed that a 173-amino acid segment of RyR1 (residues 4007-4180) is required for channel activation by 4-CmC (Fessenden, J. D., Perez, C. F., Goth, S., Pessah, I. N., and Allen, P. D. (2003) J. Biol. Chem. 278, 28727-28735). In the present study, we used site-directed mutagenesis to identify individual amino acid(s) within this region that mediate 4-CmC activation. In RyR1, substitution of 11 amino acids conserved between RyR1 and RyR2, but divergent in RyR3, with their RyR3 counterparts reduced 4-CmC sensitivity to the same degree as substitution of the entire 173-amino acid segment. Further analysis of various RyR1 mutants containing successively smaller numbers of these mutations identified 2 amino acid residues (Gln(4020) and Lys(4021)) that, when mutated to their RyR3 counterparts (Leu(3873) and Gln(3874)), abolished 4-CmC activation of RyR1. Mutation of either of these residues alone did not abolish 4-CmC sensitivity, although Q4020L partially reduced 4-CmC-induced Ca(2+) transients. In addition, mutation of the corresponding residues in RyR3 to their RyR1 counterparts (L3873Q/Q3874K) imparted 4-CmC sensitivity to RyR3. Recordings of single RyR1 channels indicated that 4-CmC applied to either the luminal or cytoplasmic side activated the channel with equal potency. Secondary structure modeling in the vicinity of the Gln(4020)-Lys(4021) dipeptide suggests that the region contains a surface-exposed region adjacent to a hydrophobic segment, indicating that both hydrophilic and hydrophobic regions of RyR1 are necessary for 4-CmC binding to the channel and/or to translate allosteric 4-CmC binding into channel activation.

Mechanisms of Disease: ryanodine receptor defects in heart failure and fatal arrhythmia

Abnormal regulation of intracellular Ca(2+) by sarcoplasmic reticulum plays a part in the mechanism underlying contractile and relaxation dysfunction in heart failure (HF). The protein-kinase-A-mediated hyperphosphorylation of ryanodine receptors in the sarcoplasmic reticulum has been shown to cause the dissociation of FKBP12.6 (also known as calstabin-2) from ryanodine receptors in HF. In addition, several disease-linked mutations in the ryanodine receptors have been reported in patients with catecholaminergic polymorphic ventricular tachycardia or arrhythmogenic right ventricular cardiomyopathy type 2. The unique distribution of these mutation sites has led to the concept that the interaction among the putative regulatory domains within the ryanodine receptors has a key role in regulating channel opening. The knowledge gained from various studies of ryanodine receptors under pathologic conditions might lead to the development of new pharmacological or genetic strategies for the treatment of HF or cardiac arrhythmia. In this review, we focus on the role of the Ca(2+)-release channel, the ryanodine receptor, in the pathogenesis of HF and fatal arrhythmia, and the possibility of developing new therapeutic strategies for targeting this receptor.

Characteristics and actions of NAD(P)H oxidase on the sarcoplasmic reticulum of coronary artery smooth muscle

It has been reported that nonmitochondrial NAD(P)H oxidases make an important contribution to intracellular O2−· in vascular tissues and, thereby, the regulation of vascular function. Topological analyses have suggested that a well-known membrane-associated NAD(P)H oxidase may not release O2−· into the cytosol. It is imperative to clarify the source of intracellular O2−· associated with this enzyme and its physiological significance in vascular cells. The present study hypothesized that an NAD(P)H oxidase on the sarcoplasmic reticulum (SR) in coronary artery smooth muscle (CASM) regulates SR ryanodine receptor (RyR) activity by producing O2−· locally. Western blot analysis was used to detect NAD(P)H oxidase subunits in purified SR from CASM. Fluorescent spectrometric analysis demonstrated that incubation of SR with NADH time dependently produced O2−·, which could be substantially blocked by the specific NAD(P)H oxidase inhibitors diphenylene iodonium and apocynin and by SOD or its mimetic tiron. This SR NAD(P)H oxidase activity was also confirmed by HPLC analysis of conversion of NADH to NAD+. In experiments of lipid bilayer channel reconstitution, addition of NADH to the cis solution significantly increased the activity of RyR/Ca2+release channels from these SR preparations from CASM, with a maximal increase in channel open probability from 0.0044 ± 0.0005 to 0.0213 ± 0.0018; this effect of NADH was markedly blocked in the presence of SOD or tiron or the NAD(P)H oxidase inhibitors diphenylene iodonium, N-vanillylnonanamide, and apocynin. These results suggest that a local NAD(P)H oxidase system on SR from CASM regulates RyR/Ca2+channel activity and Ca2+release from SR by producing O2−·.