Differential Ca2+ sensitivity of RyR2 mutations reveals distinct mechanisms of channel dysfunction in sudden cardiac death

Cardiac side effects of RNA-based SARS-CoV-2 vaccines: Hidden cardiotoxic effects of mRNA-1273 and BNT162b2 on ventricular myocyte function and structure

BACKGROUND AND PURPOSE

To protect against SARS-CoV-2 infection, the first mRNA-based vaccines, Spikevax (mRNA-1273, Moderna) and Comirnaty (BNT162b2, Pfizer/Biontech), were approved in 2020. The structure and assembly of the immunogen-in both cases, the SARS-CoV-2 spike (S) glycoprotein-are determined by a messenger RNA sequence that is translated by endogenous ribosomes. Cardiac side-effects, which for the most part can be classified by their clinical symptoms as myo- and/or pericarditis, can be caused by both mRNA-1273 and BNT162b2.

EXPERIMENTAL APPROACH

As persuasive theories for the underlying pathomechanisms have yet to be developed, this study investigated the effect of mRNA-1273 and BNT162b2 on the function, structure, and viability of isolated adult rat cardiomyocytes over a 72 h period.

KEY RESULTS

In the first 24 h after application, both mRNA-1273 and BNT162b2 caused neither functional disturbances nor morphological abnormalities. After 48 h, expression of the encoded spike protein was detected in ventricular cardiomyocytes for both mRNAs. At this point in time, mRNA-1273 induced arrhythmic as well as completely irregular contractions associated with irregular as well as localized calcium transients, which provide indications of significant dysfunction of the cardiac ryanodine receptor (RyR2). In contrast, BNT162b2 increased cardiomyocyte contraction via significantly increased protein kinase A (PKA) activity at the cellular level.

CONCLUSION AND IMPLICATIONS

Here, we demonstrated for the first time, that in isolated cardiomyocytes, both mRNA-1273 and BNT162b2 induce specific dysfunctions that correlate pathophysiologically to cardiomyopathy. Both RyR2 impairment and sustained PKA activation may significantly increase the risk of acute cardiac events.

TRIC-A Channel Maintains Store Calcium Handling by Interacting With Type 2 Ryanodine Receptor in Cardiac Muscle

RATIONALE

Trimeric intracellular cation (TRIC)-A and B are distributed to endoplasmic reticulum/sarcoplasmic reticulum intracellular Ca²⁺ stores. The crystal structure of TRIC has been determined, confirming the homotrimeric structure of a potassium channel. While the pore architectures of TRIC-A and TRIC-B are conserved, the carboxyl-terminal tail (CTT) domains of TRIC-A and TRIC-B are different from each other. Aside from its recognized role as a counterion channel that participates in excitation-contraction coupling of striated muscles, the physiological function of TRIC-A in heart physiology and disease has remained largely unexplored.

OBJECTIVE

In cardiomyocytes, spontaneous Ca²⁺ waves, triggered by store overload-induced Ca²⁺ release mediated by the RyR₂ (type 2 ryanodine receptor), develop extrasystolic contractions often associated with arrhythmic events. Here, we test the hypothesis that TRIC-A is a physiological component of RyR₂-mediated Ca²⁺ release machinery that directly modulates store overload-induced Ca²⁺ release activity via CTT.

METHODS AND RESULTS

We show that cardiomyocytes derived from the TRIC-A^-/- (TRIC-A knockout) mice display dysregulated Ca²⁺ movement across sarcoplasmic reticulum. Biochemical studies demonstrate a direct interaction between CTT-A and RyR₂. Modeling and docking studies reveal potential sites on RyR₂ that show differential interactions with CTT-A and CTT-B. In HEK293 (human embryonic kidney) cells with stable expression of RyR₂, transient expression of TRIC-A, but not TRIC-B, leads to apparent suppression of spontaneous Ca²⁺ oscillations. Ca²⁺ measurements using the cytosolic indicator Fura-2 and the endoplasmic reticulum luminal store indicator D1ER suggest that TRIC-A enhances Ca²⁺ leak across the endoplasmic reticulum by directly targeting RyR₂ to modulate store overload-induced Ca²⁺ release. Moreover, synthetic CTT-A peptide facilitates RyR₂ activity in lipid bilayer reconstitution system, enhances Ca²⁺ sparks in permeabilized TRIC-A^-/- cardiomyocytes, and induces intracellular Ca²⁺ release after microinjection into isolated cardiomyocytes, whereas such effects were not observed with the CTT-B peptide. In response to isoproterenol stimulation, the TRIC-A^-/- mice display irregular ECG and develop more fibrosis than the WT (wild type) littermates.

CONCLUSIONS

In addition to the ion-conducting function, TRIC-A functions as an accessory protein of RyR₂ to modulate sarcoplasmic reticulum Ca²⁺ handling in cardiac muscle.

Non-ventricular, Clinical, and Functional Features of the RyR2(R420Q) Mutation Causing Catecholaminergic Polymorphic Ventricular Tachycardia

INTRODUCTION AND OBJECTIVES

Catecholaminergic polymorphic ventricular tachycardia is a malignant disease, due to mutations in proteins controlling Ca(2+) homeostasis. While the phenotype is characterized by polymorphic ventricular arrhythmias under stress, supraventricular arrhythmias may occur and are not fully characterized.

METHODS

Twenty-five relatives from a Spanish family with several sudden deaths were evaluated with electrocardiogram, exercise testing, and optional epinephrine challenge. Selective RyR2 sequencing in an affected individual and cascade screening in the rest of the family was offered. The RyR2(R420Q) mutation was generated in HEK-293 cells using site-directed mutagenesis to conduct in vitro functional studies.

RESULTS

The exercise testing unmasked catecholaminergic polymorphic ventricular tachycardia in 8 relatives (sensitivity = 89%; positive predictive value = 100%; negative predictive value = 93%), all of them carrying the heterozygous RyR2(R420Q) mutation, which was also present in the proband and a young girl without exercise testing, a 91% penetrance at the end of the follow-up. Remarkably, sinus bradycardia, atrial and junctional arrhythmias, and/or giant post-effort U-waves were identified in patients. Upon permeabilization and in intact cells, the RyR2(R420Q) expressing cells showed a smaller peak of Ca(2+) release than RyR2 wild-type cells. However, at physiologic intracellular Ca(2+) concentration, equivalent to the diastolic cytosolic concentration, the RyR2(R420Q) released more Ca(2+) and oscillated faster than RyR2 wild-type cells.

CONCLUSIONS

The missense RyR2(R420Q) mutation was identified in the N-terminus of the RyR2 gene in this highly symptomatic family. Remarkably, this mutation is associated with sinus bradycardia, atrial and junctional arrhythmias, and giant U-waves. Collectively, functional heterologous expression studies suggest that the RyR2(R420Q) behaves as an aberrant channel, as a loss- or gain-of-function mutation depending on cytosolic intracellular Ca(2+) concentration.

Study of molecular mechanism of Prostaglandin E1 in inhibiting coronary heart disease

Prostaglandin E1 has been used clinically for improving heart diseases. In this study, we examined the effect of Prostaglandin E1 on blood lipid levels, heart protein and genes expression in coronary heart disease (CHD) rats. Female rats were fed either a control diet or hypercholesterolemic diet for 14 weeks. The feeding of a hypercholesterolemic diet (HCD) increased the serum TC, TG, and LDL-c levels, decreased the serum HDL-c, E2, P, FSH, LH and PRL levels in CHD rats. In addition, The feeding of a HCD diet markedly increased the content of serum TXA2, TXB2, and decreased the content of serum PGI2, and PGI2/TXA2, 6-Keto PGF1a. Furthermore, the feeding of a hypercholesterolemic diet markedly increased expression levels of myocardium Fas and Caspase-3 protein and mRNA levels, vascular endothelial growth factor and basic fibroblast growth factor mRNA, and decreased RyR2 mRNA in CHD rats. The feeding of Prostaglandin E1 for 14 weeks significantly reversed these abnormal biochemical indexes in rats. These findings suggest that Prostaglandin E1 play a obvious heart protective effect. The mechanisms may be related to restraining the excessive activation of Fas and Caspase-3 protein and modulating some gene expressions associated with CHD.

Dependency of calcium alternans on ryanodine receptor refractoriness

Background

Rapid pacing rates induce alternations in the cytosolic calcium concentration caused by fluctuations in calcium released from the sarcoplasmic reticulum (SR). However, the relationship between calcium alternans and refractoriness of the SR calcium release channel (RyR2) remains elusive.

Methodology/Principal Findings

To investigate how ryanodine receptor (RyR2) refractoriness modulates calcium handling on a beat-to-beat basis using a numerical rabbit cardiomyocyte model. We used a mathematical rabbit cardiomyocyte model to study the beat-to-beat calcium response as a function of RyR2 activation and inactivation. Bi-dimensional maps were constructed depicting the beat-to-beat response. When alternans was observed, a novel numerical clamping protocol was used to determine whether alternans was caused by oscillations in SR calcium loading or by RyR2 refractoriness. Using this protocol, we identified regions of RyR2 gating parameters where SR calcium loading or RyR2 refractoriness underlie the induction of calcium alternans, and we found that at the onset of alternans both mechanisms contribute. At low inactivation rates of the RyR2, calcium alternans was caused by alternation in SR calcium loading, while at low activation rates it was caused by alternation in the level of available RyR2s.

Conclusions/Significance

We have mapped cardiomyocyte beat-to-beat responses as a function of RyR2 activation and inactivation, identifying domains where SR calcium load or RyR2 refractoriness underlie the induction of calcium alternans. A corollary of this work is that RyR2 refractoriness due to slow recovery from inactivation can be the cause of calcium alternans even when alternation in SR calcium load is present.

Heterogeneity of ryanodine receptor dysfunction in a mouse model of catecholaminergic polymorphic ventricular tachycardia

RATIONALE

Most cardiac ryanodine receptor (RyR2) mutations associated with catecholaminergic polymorphic ventricular tachycardia (CPVT) are postulated to cause a distinctive form of Ca(2+) release dysfunction. Considering the spread distribution of CPVT mutations, we hypothesized that dysfunctional heterogeneity also was feasible.

OBJECTIVE

To determine the molecular and cellular mechanisms by which a novel RyR2-V2475F mutation associated with CPVT in humans triggers Ca(2+)-dependent arrhythmias in whole hearts and intact mice.

METHODS AND RESULTS

Recombinant channels harboring CPVT-linked RyR2 mutations were functionally characterized using tritiated ryanodine binding and single-channel recordings. Homologous recombination was used to generate a knock-in mouse bearing the RyR2-V2475F mutation. Ventricular myocytes from mice heterozygous for the mutation (RyR2-V2475F(+/-)) and their wild-type littermates were Ca(2+)-imaged by confocal microscopy under conditions that mimic stress. The propensity of wild-type and RyR2-V2475F(+/-) mice to have development of arrhythmias was tested at the whole heart level and in intact animals. Recombinant RyR2-V2475F channels displayed increased cytosolic Ca(2+) activation, abnormal protein kinase A phosphorylation, and increased activation by luminal Ca(2+). The RyR2-V2475F mutation appears embryonic-lethal in homozygous mice, but heterozygous mice have no alterations at baseline. Spontaneous Ca(2+) release events were more frequent and had shorter latency in isoproterenol-stimulated cardiomyocytes from RyR2-V2475F(+/-) hearts, but their threshold was unchanged with respect to wild-type. Adrenergically triggered tachyarrhythmias were more frequent in RyR2-V2475F(+/-) mice.

CONCLUSIONS

The mutation RyR2-V2475F is phenotypically strong among other CPVT mutations and produces heterogeneous mechanisms of RyR2 dysfunction. In living mice, this mutation appears too severe to be harbored in all RyR2 channels but remains undetected under basal conditions if expressed at relatively low levels. β-adrenergic stimulation breaks the delicate Ca(2+) equilibrium of RyR2-V2475F(+/-) hearts and triggers life-threatening arrhythmias.

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.

A novel ryanodine receptor mutation linked to sudden death increases sensitivity to cytosolic calcium

RATIONALE

Mutations in the cardiac type 2 ryanodine receptor (RyR2) have been linked to catecholaminergic polymorphic ventricular tachycardia (CPVT). CPVT-associated RyR2 mutations cause fatal ventricular arrhythmias in young individuals during β-adrenergic stimulation.

OBJECTIVE

This study sought to determine the effects of a novel RyR2-G230C mutation and whether this mutation and RyR2-P2328S alter the sensitivity of the channel to luminal calcium (Ca(2+)).

METHODS AND RESULTS

Functional characterizations of recombinant human RyR2-G230C channels were performed under conditions mimicking stress. Human RyR2 mutant channels were generated by site-directed mutagenesis and heterologously expressed in HEK293 cells together with calstabin2. RyR2 channels were measured to examine the regulation of the channels by cytosolic versus luminal sarcoplasmic reticulum Ca(2+). A 50-year-old white man with repeated syncopal episodes after exercise had a cardiac arrest and harbored the mutation RyR2-G230C. cAMP-dependent protein kinase-phosphorylated RyR2-G230C channels exhibited a significantly higher open probability at diastolic Ca(2+) concentrations, associated with a depletion of calstabin2. The luminal Ca(2+) sensitivities of RyR2-G230C and RyR2-P2328S channels were WT-like.

CONCLUSIONS

The RyR2-G230C mutant exhibits similar biophysical defects compared with previously characterized CPVT mutations: decreased binding of the stabilizing subunit calstabin2 and a leftward shift in the Ca(2+) dependence for activation under conditions that simulate exercise, consistent with a "leaky" channel. Both RyR2-G230C and RyR2-P2328S channels exhibit normal luminal Ca(2+) activation. Thus, diastolic sarcoplasmic reticulum Ca(2+) leak caused by reduced calstabin2 binding and a leftward shift in the Ca(2+) dependence for activation by diastolic levels of cytosolic Ca(2+) is a common mechanism underlying CPVT.

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.

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.

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.

Mechanisms and potential therapeutic targets for ventricular arrhythmias associated with impaired cardiac calcium cycling

The close relationship between life-threatening ventricular arrhythmias and contractile dysfunction in the heart implicates intracellular calcium cycling as an important underlying mechanism of arrhythmogenesis. Despite this close association, however, the mechanisms of arrhythmogenesis attributable to impaired calcium cycling are not fully appreciated or understood. In this report we review some of the current thinking regarding arrhythmia mechanisms associated with either abnormal impulse initiation (i.e. arrhythmia triggers) or impulse propagation (i.e. arrhythmia substrates). In all cases, the mechanisms are primarily related to dysfunction of calcium regulatory proteins associated with the sarcomere. These findings highlight the broad scope of arrhythmias associated with abnormal calcium cycling, and provide a basis for a causal relationship between cardiac electrical instability and contractile dysfunction. Moreover, calcium cycling proteins may provide much needed targets for novel antiarrhythmic therapies.

Knockin Animal Models of Inherited Arrhythmogenic Diseases: What Have We Learned From Them?

Mouse models are becoming an increasingly accepted method of studying human diseases. Knockin and knockout techniques have several advantages over traditional transgenic overexpression, and the versatility of the knockin mouse allows the study of both gain of function mutations through targeted mutagenesis, as well as the replacement of one gene by another functional gene. Here, we will review the methods available to generate knockin mice; provide an overview of the techniques used to study electrophysiology in the mice at the cellular, organ, and whole animal level; and highlight knockin mice that have implications for inherited arrhythmias. Specifically, we will focus on models that used knockin mice to clarify gene expression, identify similarities and differences between related genes, and model human arrhythmia syndromes. Our goal is to provide the reader with a general understanding of studies done on knockin mouse models of inherited arrhythmias as well as ideas for future directions.

Three-dimensional localization of serine 2808, a phosphorylation site in cardiac ryanodine receptor

Type 2 ryanodine receptor (RyR2) is the major calcium release channel in cardiac muscle. Phosphorylation of RyR2 by cAMP-dependent protein kinase A and by calmodulin-dependent protein kinase II modulates channel activity. Hyperphosphorylation at a single amino acid residue, Ser-2808, has been proposed to directly disrupt the binding of a 12.6-kDa FK506-binding protein (FKBP12.6) to RyR2, causing a RyR2 malfunction that triggers cardiac arrhythmias in human heart failure. To determine the structural basis of the interaction between Ser-2808 and FKBP12.6, we have employed two independent approaches to map this phosphorylation site in RyR2 by three-dimensional cryo-electron microscopy. In one approach, we inserted a green fluorescent protein (GFP) after amino acid Tyr-2801, and mapped the GFP three-dimensional location in the RyR2 structure. In another approach, the binding site of monoclonal antibody 34C was mapped in the three-dimensional structure of skeletal muscle RyR1. The epitope of antibody 34C has been mapped to amino acid residues 2,756 through 2,803 of the RyR1 sequence, corresponding to residues 2,722 through 2,769 of the RyR2 sequence. These locations of GFP insertion and antibody binding are adjacent to one another in domain 6 of the cytoplasmic clamp region. Importantly, the three-dimensional location of the Ser-2808 phosphorylation site is 105-120 A distance from the FKBP12.6 binding site mapped previously, indicating that Ser-2808 is unlikely to be directly involved in the binding of FKBP12.6 to RyR2, as had been proposed previously.

Multiscale modeling of calcium signaling in the cardiac dyad

Calcium (Ca(2+))-induced Ca(2+)-release (CICR) takes place in spatially restricted microdomains known as dyads. The length scale over which CICR occurs is on the order of nanometers and relevant time scales range from micro- to milliseconds. Quantitative understanding of CICR therefore requires development of models that are applicable over a range of spatio-temporal scales. We will present several new approaches for multiscale modeling of CICR. First, we present a model of dyad Ca(2+) dynamics in which the Fokker-Planck equation (FPE) is solved for the probability P(x, t) that a Ca(2+) ion is located at dyad position x at time t. Using this model, we demonstrate that (a) Ca(2+) signaling in the dyad is mediated by approximately tens of Ca(2+) ions; (b) these signaling events are noisy due to the small number of ions involved; and (c) the geometry of the RyR (ryanodine receptors) protein may function to restrict the diffusion of and to "funnel" Ca(2+) ions to activation-binding sites on the RyR, thus increasing RyR open probability and excitation-contraction (EC) coupling gain. Simplification of this model to one in which the dyadic space is represented using a single compartment yields the stochastic local-control model of CICR developed previously. We have shown that this model captures fundamental properties of CICR, such as graded release and voltage-dependent gain, may be integrated within a model of the myocyte and may be simulated in reasonable times using a combination of efficient numerical methods and parallel computing, but remains too complex for general use in cell simulations. To address this problem, we show how separation of time scales may be used to formulate a model in which nearby L-type Ca(2+) channels (LCCs) and RyRs gate as a coupled system that may be described using low-dimensional systems of ordinary differential equations, thus reducing computational complexity while capturing fundamentally important properties of CICR. The simplified model may be solved many orders of magnitude faster than can either of the more detailed models, thus enabling incorporation into tissue-level simulations.

Protein geometry and placement in the cardiac dyad influence macroscopic properties of calcium-induced calcium release

In cardiac ventricular myocytes, events crucial to excitation-contraction coupling take place in spatially restricted microdomains known as dyads. The movement and dynamics of calcium (Ca2+) ions in the dyad have often been described by assigning continuously valued Ca2+ concentrations to one or more dyadic compartments. However, even at its peak, the estimated number of free Ca2+ ions present in a single dyad is small (approximately 10-100 ions). This in turn suggests that modeling dyadic calcium dynamics using laws of mass action may be inappropriate. In this study, we develop a model of stochastic molecular signaling between L-type Ca2+ channels (LCCs) and ryanodine receptors (RyR2s) that describes: a), known features of dyad geometry, including the space-filling properties of key dyadic proteins; and b), movement of individual Ca2+ ions within the dyad, as driven by electrodiffusion. The model enables investigation of how local Ca2+ signaling is influenced by dyad structure, including the configuration of key proteins within the dyad, the location of Ca2+ binding sites, and membrane surface charges. Using this model, we demonstrate that LCC-RyR2 signaling is influenced by both the stochastic dynamics of Ca2+ ions in the dyad as well as the shape and relative positioning of dyad proteins. Results suggest the hypothesis that the relative placement and shape of the RyR2 proteins helps to "funnel" Ca2+ ions to RyR2 binding sites, thus increasing excitation-contraction coupling gain.

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.

Diseases associated with altered ryanodine receptor activity

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

Ryanodine 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.

Mice with the R176Q cardiac ryanodine receptor mutation exhibit catecholamine-induced ventricular tachycardia and cardiomyopathy

Mutations in the cardiac ryanodine receptor 2 (RyR2) have been associated with catecholaminergic polymorphic ventricular tachycardia and a form of arrhythmogenic right ventricular dysplasia. To study the relationship between RyR2 function and these phenotypes, we developed knockin mice with the human disease-associated RyR2 mutation R176Q. Histologic analysis of hearts from RyR2(R176Q/+) mice revealed no evidence of fibrofatty infiltration or structural abnormalities characteristic of arrhythmogenic right ventricular dysplasia, but right ventricular end-diastolic volume was decreased in RyR2(R176Q/+) mice compared with controls, indicating subtle functional impairment due to the presence of a single mutant allele. Ventricular tachycardia (VT) was observed after caffeine and epinephrine injection in RyR2(R176Q/+), but not in WT, mice. Intracardiac electrophysiology studies with programmed stimulation also elicited VT in RyR2(R176Q/+) mice. Isoproterenol administration during programmed stimulation increased both the number and duration of VT episodes in RyR2(R176Q/+) mice, but not in controls. Isolated cardiomyocytes from RyR2(R176Q/+) mice exhibited a higher incidence of spontaneous Ca(2+) oscillations in the absence and presence of isoproterenol compared with controls. Our results suggest that the R176Q mutation in RyR2 predisposes the heart to catecholamine-induced oscillatory calcium-release events that trigger a calcium-dependent ventricular arrhythmia.

Arrhythmogenesis in catecholaminergic polymorphic ventricular tachycardia: insights from a RyR2 R4496C knock-in mouse model

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited disease characterized by life threatening arrhythmias and mutations in the gene encoding the ryanodine receptor (RyR2). Disagreement exists on whether (1) RyR2 mutations induce abnormal calcium transients in the absence of adrenergic stimulation; (2) decreased affinity of mutant RyR2 for FKBP12.6 causes CPVT; (3) K201 prevent arrhythmias by normalizing the FKBP12.6-RyR2 binding. We studied ventricular myocytes isolated from wild-type (WT) and knock-in mice harboring the R4496C mutation (RyR2(R4496C+/-)). Pacing protocols did not elicit delayed afterdepolarizations (DADs) (n=20) in WT but induced DADs in 21 of 33 (63%) RyR2(R4496C+/-) myocytes (P=0.001). Superfusion with isoproterenol (30 nmol/L) induced small DADs (45%) and no triggered activity in WT myocytes, whereas it elicited DADs in 87% and triggered activity in 60% of RyR2(R4496C+/-) myocytes (P=0.001). DADs and triggered activity were abolished by ryanodine (10 micromol/L) but not by K201 (1 micromol/L or 10 micromol/L). In vivo administration of K201 failed to prevent induction of polymorphic ventricular tachycardia (VT) in RyR2(R4496C+/-) mice. Measurement of the FKBP12.6/RyR2 ratio in the heavy sarcoplasmic reticulum membrane showed normal RyR2-FKBP12.6 interaction both in WT and RyR2(R4496C+/-) either before and after treatment with caffeine and epinephrine. We suggest that (1) triggered activity is the likely arrhythmogenic mechanism of CPVT; (2) K201 fails to prevent DADs in RyR2(R4496C+/-) myocytes and ventricular arrhythmias in RyR2(R4496C+/-) mice; and (3) RyR2-FKBP12.6 interaction in RyR2(R4496C+/-) is identical to that of WT both before and after epinephrine and caffeine, thus suggesting that it is unlikely that the R4496C mutation interferes with the RyR2/FKBP12.6 complex.

Arrhythmogenic Mutation-Linked Defects in Ryanodine Receptor Autoregulation Reveal a Novel Mechanism of Ca 2+ Release Channel Dysfunction

Arrhythmogenic cardiac ryanodine receptor (RyR2) mutations are associated with stress-induced malignant tachycardia, frequently leading to sudden cardiac death (SCD). The causative mechanisms of RyR2 Ca 2+ release dysregulation are complex and remain controversial. We investigated the functional impact of clinically-severe RyR2 mutations occurring in the central domain, and the C-terminal I domain, a key locus of RyR2 autoregulation, on interdomain interactions and Ca 2+ release in living cells. Using high-resolution confocal microscopy and fluorescence resonance energy transfer (FRET) analysis of interaction between fusion proteins corresponding to amino- (N-) and carboxyl- (C-) terminal RyR2 domains, we determined that in resting cells, RyR2 interdomain interaction remained unaltered after introduction of SCD-linked mutations and normal Ca 2+ regulation was maintained. In contrast, after channel activation, the abnormal Ca 2+ release via mutant RyR2 was intrinsically linked to altered interdomain interaction that was equivalent with all mutations and exhibited threshold characteristics (caffeine >2.5 mmol/L; Ca 2+ >150 nmol/L). Noise analysis revealed that I domain mutations introduced a distinct pattern of conformational instability in Ca 2+ handling and interdomain interaction after channel activation that was absent in signals obtained from the central domain mutation. I domain–linked channel instability also occurred in intact RyR2 expressed in CHO cells and in HL-1 cardiomyocytes. These new insights highlight a critical role for mutation-linked defects in channel autoregulation, and may contribute to a molecular explanation for the augmented Ca 2+ release following RyR2 channel activation. Our findings also suggest that the mutational locus may be an important mechanistic determinant of Ca 2+ release channel dysfunction in arrhythmia and SCD.

Enhanced store overload-induced Ca2+ release and channel sensitivity to luminal Ca2+ activation are common defects of RyR2 mutations linked to ventricular tachycardia and sudden death

Ventricular tachycardia (VT) is the leading cause of sudden death, and the cardiac ryanodine receptor (RyR2) is emerging as an important focus in its pathogenesis. RyR2 mutations have been linked to VT and sudden death, but their precise impacts on channel function remain largely undefined and controversial. We have previously shown that several disease-linked RyR2 mutations in the C-terminal region enhance the sensitivity of the channel to activation by luminal Ca2+. Cells expressing these RyR2 mutants display an increased propensity for spontaneous Ca2+ release under conditions of store Ca2+ overload, a process we referred to as store overload-induced Ca2+ release (SOICR). To determine whether common defects exist in disease-linked RyR2 mutations, we characterized 6 more RyR2 mutations from different regions of the channel. Stable inducible HEK293 cell lines expressing Q4201R and I4867M from the C-terminal region, S2246L and R2474S from the central region, and R176Q(T2504M) and L433P from the N-terminal region were generated. All of these cell lines display an enhanced propensity for SOICR. HL-1 cardiac cells transfected with disease-linked RyR2 mutations also exhibit increased SOICR activity. Single channel analyses reveal that disease-linked RyR2 mutations primarily increase the channel sensitivity to luminal, but not to cytosolic, Ca2+ activation. Moreover, the Ca2+ dependence of [3H]ryanodine binding to RyR2 wild type and mutants is similar. In contrast to previous reports, we found no evidence that disease-linked RyR2 mutations alter the FKBP12.6-RyR2 interaction. Our data indicate that enhanced SOICR activity and luminal Ca2+ activation represent common defects of RyR2 mutations associated with VT and sudden death. A mechanistic model for CPVT/ARVD2 is proposed.

Ryanodine receptor dysfunction in arrhythmia and sudden cardiac death

Mutations in ryanodine receptor calcium ion-release channels (RyR2) have emerged as important causative players in exercise/stress-induced ventricular arrhythmia leading to sudden cardiac death (SCD). Thus, RyR2 represents an attractive therapeutic target, and a detailed understanding of the mechanistic basis of RyR2 dysfunction at the molecular, cellular and organ level is essential for the development of novel, more effective therapeutic approaches to prevent arrhythmia and SCD. Such advances will translate into a tremendous improvement in the survival and quality of life of SCD-susceptible individuals. In this review, the authors consider how recent knowledge gained from mutation identification, phenotypic manifestation and functional evaluation of RyR2 mutants, are being used to develop novel therapeutic strategies in RyR2-dependent arrhythmia.