Ryanodine receptor modification and regulation by intracellular Ca2+ and Mg2+ in healthy and failing human hearts

Abnormal phosphorylation / dephosphorylation and Ca2+ dysfunction in heart failure

Heart failure (HF) can be caused by a variety of causes characterized by abnormal myocardial systole and diastole. Ca2+ current through the L-type calcium channel (LTCC) on the membrane is the initial trigger signal for a cardiac cycle. Declined systole and diastole in HF are associated with dysfunction of myocardial Ca2+ function. This disorder can be correlated with unbalanced levels of phosphorylation / dephosphorylation of LTCC, endoplasmic reticulum (ER), and myofilament. Kinase and phosphatase activity changes along with HF progress, resulting in phased changes in the degree of phosphorylation / dephosphorylation. It is important to realize the phosphorylation / dephosphorylation differences between a normal and a failing heart. This review focuses on phosphorylation / dephosphorylation changes in the progression of HF and summarizes the effects of phosphorylation / dephosphorylation of LTCC, ER function, and myofilament function in normal conditions and HF based on previous experiments and clinical research. Also, we summarize current therapeutic methods based on abnormal phosphorylation / dephosphorylation and clarify potential therapeutic directions.

Inhibition of adenylyl cyclase 8 prevents the upregulation of Orai1 channel, which improves cardiac function after myocardial infarction

The upregulation of Orai1 and subsequent store-operated Ca2+ entry (SOCE) has been associated with adverse cardiac remodeling and heart failure (HF). However, the mechanism underlying Orai1 upregulation and its role in myocardial infarction remains unclear. Our study investigated the role of Orai1 in activating adenylyl cyclase 8 (AC8) and cyclic AMP (cAMP) response element-binding protein (CREB), as well as its contribution to cardiac dysfunction induced by ischemia and reperfusion (I/R). We found that I/R evoked an increase in the expression of Orai1 and AC8 in rats' hearts, resulting in a substantial rise in diastolic Ca2+ concentration ([Ca2+]i), and reduced ventricular contractions. The expression of Orai1 and AC8 was also increased in ventricular biopsies of post-ischemic HF patients. Mechanistically, we demonstrate that I/R activation of Orai1 stimulated AC8, which produced cAMP and phosphorylated CREB. Subsequently, p-CREB activated the ORAI1 promoter, resulting in Orai1 upregulation and SOCE exacerbation. Intramyocardial administration of AAV9 carrying AC8 short hairpin RNA decreased the expression of AC8, Orai1 and CREB, which restored diastolic [Ca2+]i and improved cardiac contraction. Therefore, our data suggests that the axis composed by Orai1/AC8/CREB plays a critical role in I/R-induced cardiac dysfunction, representing a potential new therapeutic target to limit the progression of the disease toward HF.

Roles of β-adrenoceptor Subtypes and Therapeutics in Human Cardiovascular Disease: Heart Failure, Tachyarrhythmias and Other Cardiovascular Disorders

β-Adrenoceptors (β-ARs) provide an important therapeutic target for the treatment of cardiovascular disease. Three β-ARs, β1-AR, β2-AR, β3-AR are localized to the human heart. Activation of β1-AR and β2-ARs increases heart rate, force of contraction (inotropy) and consequently cardiac output to meet physiological demand. However, in disease, chronic over-activation of β1-AR is responsible for the progression of disease (e.g. heart failure) mediated by pathological hypertrophy, adverse remodelling and premature cell death. Furthermore, activation of β1-AR is critical in the pathogenesis of cardiac arrhythmias while activation of β2-AR directly influences blood pressure haemostasis. There is an increasing awareness of the contribution of β2-AR in cardiovascular disease, particularly arrhythmia generation. All β-blockers used therapeutically to treat cardiovascular disease block β1-AR with variable blockade of β2-AR depending on relative affinity for β1-AR vs β2-AR. Since the introduction of β-blockers into clinical practice in 1965, β-blockers with different properties have been trialled, used and evaluated, leading to better understanding of their therapeutic effects and tolerability in various cardiovascular conditions. β-Blockers with the property of intrinsic sympathomimetic activity (ISA), i.e. β-blockers that also activate the receptor, were used in the past for post-treatment of myocardial infarction and had limited use in heart failure. The β-blocker carvedilol continues to intrigue due to numerous properties that differentiate it from other β-blockers and is used successfully in the treatment of heart failure. The discovery of β3-AR in human heart created interest in the role of β3-AR in heart failure but has not resulted in therapeutics at this stage.

FKBP12 binds to the cardiac ryanodine receptor with negative cooperativity: implications for heart muscle physiology in health and disease

Cardiac ryanodine receptors (RyR2) release the Ca²⁺ from intracellular stores that is essential for cardiac myocyte contraction. The ion channel opening is tightly regulated by intracellular factors, including the FK506 binding proteins, FKBP12 and FKBP12.6. The impact of these proteins on RyR2 activity and cardiac contraction is debated, with often apparently contradictory experimental results, particularly for FKBP12. The isoform that regulates RyR2 has generally been considered to be FKBP12.6, despite the fact that FKBP12 is the major isoform associated with RyR2 in some species and is bound in similar proportions to FKBP12.6 in others, including sheep and humans. Here, we show time- and concentration-dependent effects of adding FKBP12 to RyR2 channels that were partly depleted of FKBP12/12.6 during isolation. The added FKBP12 displaced most remaining endogenous FKBP12/12.6. The results suggest that FKBP12 activates RyR2 with high affinity and inhibits RyR2 with lower affinity, consistent with a model of negative cooperativity in FKBP12 binding to each of the four subunits in the RyR tetramer. The easy dissociation of some FKBP12/12.6 could dynamically alter RyR2 activity in response to changes in in vivo regulatory factors, indicating a significant role for FKBP12/12.6 in Ca²⁺ signalling and cardiac function in healthy and diseased hearts. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

How does flecainide impact RyR2 channel function?

Flecainide, a cardiac class 1C blocker of the surface membrane sodium channel (NaV1.5), has also been reported to reduce cardiac ryanodine receptor (RyR2)-mediated sarcoplasmic reticulum (SR) Ca2+ release. It has been introduced as a clinical antiarrhythmic agent for catecholaminergic polymorphic ventricular tachycardia (CPVT), a condition most commonly associated with gain-of-function RyR2 mutations. Current debate concerns both cellular mechanisms of its antiarrhythmic action and molecular mechanisms of its RyR2 actions. At the cellular level, it targets NaV1.5, RyR2, Na+/Ca2+ exchange (NCX), and additional proteins involved in excitation-contraction (EC) coupling and potentially contribute to the CPVT phenotype. This Viewpoint primarily addresses the various direct molecular actions of flecainide on isolated RyR2 channels in artificial lipid bilayers. Such studies demonstrate different, multifarious, flecainide binding sites on RyR2, with voltage-dependent binding in the channel pore or voltage-independent binding at distant peripheral sites. In contrast to its single NaV1.5 pore binding site, flecainide may bind to at least four separate inhibitory sites on RyR2 and one activation site. None of these binding sites have been specifically located in the linear RyR2 sequence or high-resolution structure. Furthermore, it is not clear which of the inhibitory sites contribute to flecainide's reduction of spontaneous Ca2+ release in cellular studies. A confounding observation is that flecainide binding to voltage-dependent inhibition sites reduces cation fluxes in a direction opposite to physiological Ca2+ flow from SR lumen to cytosol. This may suggest that, rather than directly blocking Ca2+ efflux, flecainide can reduce Ca2+ efflux by blocking counter currents through the pore which otherwise limit SR membrane potential change during systolic Ca2+ efflux. In summary, the antiarrhythmic effects of flecainide in CPVT seem to involve multiple components of EC coupling and multiple actions on RyR2. Their clarification may identify novel specific drug targets and facilitate flecainide's clinical utilization in CPVT.

RyR2 and Calcium Release in Heart Failure

Heart Failure (HF) is defined as the inability of the heart to efficiently pump out enough blood to maintain the body's needs, first at exercise and then also at rest. Alterations in Ca2+ handling contributes to the diminished contraction and relaxation of the failing heart. While most Ca2+ handling protein expression and/or function has been shown to be altered in many models of experimental HF, in this review, we focus in the sarcoplasmic reticulum (SR) Ca2+ release channel, the type 2 ryanodine receptor (RyR2). Various modifications of this channel inducing alterations in its function have been reported. The first was the fact that RyR2 is less responsive to activation by Ca2+ entry through the L-Type calcium channel, which is the functional result of an ultrastructural remodeling of the ventricular cardiomyocyte, with fewer and disorganized transverse (T) tubules. HF is associated with an elevated sympathetic tone and in an oxidant environment. In this line, enhanced RyR2 phosphorylation and oxidation have been shown in human and experimental HF. After several controversies, it is now generally accepted that phosphorylation of RyR2 at the Calmodulin Kinase II site (S2814) is involved in both the depressed contractile function and the enhanced arrhythmic susceptibility of the failing heart. Diminished expression of the FK506 binding protein, FKBP12.6, may also contribute. While these alterations have been mostly studied in the left ventricle of HF with reduced ejection fraction, recent studies are looking at HF with preserved ejection fraction. Moreover, alterations in the RyR2 in HF may also contribute to supraventricular defects associated with HF such as sinus node dysfunction and atrial fibrillation.

Flecainide Paradoxically Activates Cardiac Ryanodine Receptor Channels under Low Activity Conditions: A Potential Pro-Arrhythmic Action

Cardiac ryanodine receptor (RyR2) mutations are implicated in the potentially fatal catecholaminergic polymorphic ventricular tachycardia (CPVT) and in atrial fibrillation. CPVT has been successfully treated with flecainide monotherapy, with occasional notable exceptions. Reported actions of flecainide on cardiac sodium currents from mice carrying the pro-arrhythmic homozygotic RyR2-P2328S mutation prompted our explorations of the effects of flecainide on their RyR2 channels. Lipid bilayer electrophysiology techniques demonstrated a novel, paradoxical increase in RyR2 activity. Preceding flecainide exposure, channels were mildly activated by 1 mM luminal Ca²⁺ and 1 µM cytoplasmic Ca²⁺, with open probabilities (P_o) of 0.03 ± 0.01 (wild type, WT) or 0.096 ± 0.024 (P2328S). Open probability (P_o) increased within 0.5 to 3 min of exposure to 0.5 to 5.0 µM cytoplasmic flecainide, then declined with higher concentrations of flecainide. There were no such increases in a subset of high P_o channels with P_o ≥ 0.08, although P_o then declined with ≥5 µM (WT) or ≥50 µM flecainide (P2328S). On average, channels with P_o < 0.08 were significantly activated by 0.5 to 10 µM of flecainide (WT) or 0.5 to 50 µM of flecainide (P2328S). These results suggest that flecainide can bind to separate activation and inhibition sites on RyR2, with activation dominating in lower activity channels and inhibition dominating in more active channels.

Ca2+ mishandling in heart failure: Potential targets

Ca²⁺ mishandling is a common feature in several cardiovascular diseases such as heart failure (HF). In many cases, impairment of key players in intracellular Ca²⁺ homeostasis has been identified as the underlying mechanism of cardiac dysfunction and cardiac arrhythmias associated with HF. In this review, we summarize primary novel findings related to Ca²⁺ mishandling in HF progression. HF research has increasingly focused on the identification of new targets and the contribution of their role in Ca²⁺ handling to the progression of the disease. Recent research studies have identified potential targets in three major emerging areas implicated in regulation of Ca²⁺ handling: the innate immune system, bone metabolism factors and post-translational modification of key proteins involved in regulation of Ca²⁺ handling. Here, we describe their possible contributions to the progression of HF.

Cardiac function dependence on carbon monoxide

Nitric oxide, studied to evaluate its role in cardiovascular physiology, has cardioprotective and therapeutic effects in cellular signaling, mitochondrial function, and in regulating inflammatory processes. Heme oxygenase (major role in catabolism of heme into biliverdin, carbon monoxide (CO), and iron) has similar effects as well. CO has been suggested as the molecule that is responsible for many of the above mentioned cytoprotective and therapeutic pathways as CO is a signaling molecule in the control of physiological functions. This is counterintuitive as toxic effects are related to its binding to hemoglobin. However, CO is normally produced in the body. Experimental evidence indicates that this toxic gas, CO, exerts cytoprotective properties related to cellular stress including the heart and is being assessed for its cytoprotective and cytotherapeutic properties. While survival of adult cardiomyocytes depends on oxidative phosphorylation (survival and resulting cardiac function is impaired by mitochondrial damage), mitochondrial biogenesis is modified by the heme oxygenase-1/CO system and can result in promotion of mitochondrial biogenesis by associating mitochondrial redox status to the redox-active transcription factors. It has been suggested that the heme oxygenase-1/CO system is important in differentiation of embryonic stem cells and maturation of cardiomyocytes which is thought to mitigate progression of degenerative cardiovascular diseases. Effects on other cardiac cells are being studied. Acute exposure to air pollution (and, therefore, CO) is associated with cardiovascular mortality, myocardial infarction, and heart failure, but changes in the endogenous heme oxygenase-1 system (and, thereby, CO) positively affect cardiovascular health. We will review the effect of CO on heart health and function in this article.

THE ROLE OF PHOSPHORYLATION IN ATRIAL FIBRILLATION: A FOCUS ON MASS SPECTROMETRY APPROACHES

Atrial fibrillation (AF) is the most common arrhythmia worldwide. It is associated with significant increases in morbidity in the form of stroke and heart failure, and a doubling in all-cause mortality. The pathophysiology of AF is incompletely understood, and this has contributed to a lack of effective treatments and disease-modifying therapies. An important cellular process that may explain how risk factors give rise to AF includes post-translational modification (PTM) of proteins. As the most commonly occurring PTM, protein phosphorylation is especially relevant. Although many methods exist for studying protein phosphorylation, a common and highly resolute technique is mass spectrometry (MS). This review will discuss recent evidence surrounding the role of protein phosphorylation in the pathogenesis of AF. MS-based technology to study phosphorylation and uses of MS in other areas of medicine such as oncology will also be presented. Based on these data, future goals and experiments will be outlined that utilize MS technology to better understand the role of phosphorylation in AF and elucidate its role in AF pathophysiology. This may ultimately allow for the development of more effective AF therapies.

Stress-driven cardiac calcium mishandling via a kinase-to-kinase crosstalk

Calcium homeostasis in the cardiomyocyte is critical to the regulation of normal cardiac function. Abnormal calcium dynamics such as altered uptake by the sarcoplasmic reticulum (SR) Ca²⁺-ATPase and increased diastolic SR calcium leak are involved in the development of maladaptive cardiac remodeling under pathological conditions. Ca²⁺/calmodulin-dependent protein kinase II-δ (CaMKIIδ) is a well-recognized key molecule in calcium dysregulation in cardiomyocytes. Elevated cellular stress is known as a common feature during pathological remodeling, and c-jun N-terminal kinase (JNK) is an important stress kinase that is activated in response to intrinsic and extrinsic stress stimuli. Our lab recently identified specific actions of JNK isoform 2 (JNK2) in CaMKIIδ expression, activation, and CaMKIIδ-dependent SR Ca²⁺ mishandling in the stressed heart. This review focuses on the current understanding of cardiac SR calcium handling under physiological and pathological conditions as well as the newly identified contribution of the stress kinase JNK2 in CaMKIIδ-dependent SR Ca²⁺ abnormal mishandling. The new findings identifying dual roles of JNK2 in CaMKIIδ expression and activation are also discussed in this review.

Intracellular calcium leak in heart failure and atrial fibrillation: a unifying mechanism and therapeutic target

Ca2+ is a fundamental second messenger in all cell types and is required for numerous essential cellular functions, including cardiac and skeletal muscle contraction. The intracellular concentration of free Ca2+ ([Ca2+]) is regulated primarily by ion channels, pumps (ATPases), exchangers and Ca2+-binding proteins. Defective regulation of [Ca2+] is found in a diverse spectrum of pathological states that affect all the major organs. In the heart, abnormalities in the regulation of cytosolic and mitochondrial [Ca2+] occur in heart failure (HF) and atrial fibrillation (AF), two common forms of heart disease and leading contributors to morbidity and mortality. In this Review, we focus on the mechanisms that regulate ryanodine receptor 2 (RYR2), the major sarcoplasmic reticulum (SR) Ca2+-release channel in the heart, how RYR2 becomes dysfunctional in HF and AF, and its potential as a therapeutic target. Inherited RYR2 mutations and/or stress-induced phosphorylation and oxidation of the protein destabilize the closed state of the channel, resulting in a pathological diastolic Ca2+ leak from the SR that both triggers arrhythmias and impairs contractility. On the basis of our increased understanding of SR Ca2+ leak as a shared Ca2+-dependent pathological mechanism in HF and AF, a new class of drugs developed in our laboratory, known as rycals, which stabilize RYR2 channels and prevent Ca2+ leak from the SR, are undergoing investigation in clinical trials.

Multisite phosphorylation of the cardiac ryanodine receptor: a random or coordinated event?

Many proteins are phosphorylated at more than one phosphorylation site to achieve precise tuning of protein function and/or integrate a multitude of signals into the activity of one protein. Increasing the number of phosphorylation sites significantly broadens the complexity of molecular mechanisms involved in processing multiple phosphorylation sites by one or more distinct kinases. The cardiac ryanodine receptor (RYR2) is a well-established multiple phospho-target of kinases activated in response to β-adrenergic stimulation because this Ca²⁺ channel is a critical component of Ca²⁺ handling machinery which is responsible for β-adrenergic enhancement of cardiac contractility. Our review presents a selective overview of the extensive, often conflicting, literature which focuses on identifying reliable lines of evidence to establish if multiple RYR2 phosphorylation is achieved randomly or in a specific sequence, and whether phosphorylation at individual sites is functionally specific and additive or similar and can therefore be substituted.

Ryanodine Receptor Type 2: A Molecular Target for Dichlorodiphenyltrichloroethane- and Dichlorodiphenyldichloroethylene-Mediated Cardiotoxicity

Dichlorodiphenyltrichloroethane (DDT) and its metabolite dichlorodiphenyl-dichloroethylene (DDE) are ubiquitously found in the environment and linked to cardiovascular diseases-with a majority of the work focused on hypertension. Studies investigating whether DDx can interact with molecular targets on cardiac tissue to directly affect cardiac function are lacking. Therefore, we investigated whether o,p'-DDT, p,p'-DDT, o,p'-DDE, or p,p'-DDE (DDx, collectively) can directly alter the function of human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) by assessing their effect(s) on hiPSC-CMs Ca2+ dynamics. DDx (0.1-10 µM) affected hiPSC-CMs synchronous Ca2+ oscillation frequency in a concentration-dependent manner, with p,p'-DDT and p,p'-DDE also decreasing Ca2+ stores. HEK-RyR2 cells cultured under antibiotic selection to induce expression of wild-type mouse ryanodine receptor type 2 (RyR2) are used to further investigate whether DDx alters hiPSC-CMs Ca2+ dynamics through engagement with RyR2, a protein critical for cardiac muscle excitation-contraction coupling (ECC). Acute treatment with 10 µM DDx failed to induce Ca2+ release in HEK293-RyR2, whereas pretreatment with DDx (0.1-10 µM) for 12- or 24-h significantly decreased sarcoplasmic reticulum Ca2+ stores in HEK-RyR2 cells challenged with caffeine (1 mM), an RyR agonist. [3H]ryanodine-binding analysis using murine cardiac RyR2 homogenates further confirmed that all DDx isomers (10 µM) can directly engage with RyR2 to favor an open (leaky) confirmation, whereas only the DDT isomers (10 µM) modestly (≤10%) inhibited SERCA2a activity. The data demonstrate that DDx increases heart rate and depletes Ca2+ stores in human cardiomyocytes through a mechanism that impairs RyR2 function and Ca2+ dynamics.

IMPACT STATEMENT

DDT/DDE interactions with RyR2 alter cardiomyocyte Ca2+ dynamics that may contribute to adverse cardiovascular outcomes associated with exposures.

Interrelation between ROS and Ca2+ in aging and age-related diseases

Calcium (Ca2+) and reactive oxygen species (ROS) are versatile signaling molecules coordinating physiological and pathophysiological processes. While channels and pumps shuttle Ca2+ ions between extracellular space, cytosol and cellular compartments, short-lived and highly reactive ROS are constantly generated by various production sites within the cell. Ca2+ controls membrane potential, modulates mitochondrial adenosine triphosphate (ATP) production and affects proteins like calcineurin (CaN) or calmodulin (CaM), which, in turn, have a wide area of action. Overwhelming Ca2+ levels within mitochondria efficiently induce and trigger cell death. In contrast, ROS comprise a diverse group of relatively unstable molecules with an odd number of electrons that abstract electrons from other molecules to gain stability. Depending on the type and produced amount, ROS act either as signaling molecules by affecting target proteins or as harmful oxidative stressors by damaging cellular components. Due to their wide range of actions, it is little wonder that Ca2+ and ROS signaling pathways overlap and impact one another. Growing evidence suggests a crucial implication of this mutual interplay on the development and enhancement of age-related disorders, including cardiovascular and neurodegenerative diseases as well as cancer.

Understanding How Phosphorylation and Redox Modifications Regulate Cardiac Ryanodine Receptor Type 2 Activity to Produce an Arrhythmogenic Phenotype in Advanced Heart Failure

Heart failure (HF) is a global pandemic with significant mortality and morbidity. Despite current medications, 50% of individuals die within 5 years of diagnosis. Of these deaths, 30-50% will be a result of sudden cardiac death from ventricular arrhythmias. This review discusses two stress-induced mechanisms, phosphorylation from chronic β-adrenoceptor (β-AR) stimulation and thiol modifications from oxidative stress, and how they modulate the cardiac ryanodine receptor type 2 (RyR2) and foster an arrhythmogenic phenotype. Calcium (Ca²⁺) is the ubiquitous secondary messenger of excitation-contraction coupling and provides a common pathway for contractile dysfunction and arrhythmia genesis. In a healthy heart, Ca²⁺ is released from the sarcoplasmic reticulum (SR) by RyR2. The open probability of RyR2 is under the dynamic influence of co-proteins, ions, and kinases that are in strict balance to ensure normal physiological functioning. In HF, chronic β-AR activity and production of reactive oxygen species and reactive nitrogen species provide two stress-induced mechanisms uncoupling RyR2 control, resulting in pathological diastolic SR Ca²⁺ leak. This increased cytosolic [Ca²⁺] promotes Ca²⁺ extrusion via the local Na⁺/Ca²⁺ exchanger, resulting in net sarcolemmal depolarization, delayed after depolarization and ventricular arrhythmia. Experimental models researching oxidative stress and phosphorylation have aimed to identify how post-translational modifications to the RyR2 macromolecular complex, and the associated Na⁺/Ca²⁺ cycling proteins, result in pathological Ca²⁺ handling and diastolic leak. However, the causative molecular changes remain controversial and undefined. Through understanding the molecular mechanisms that produce an arrhythmic phenotype, novel therapeutic targets to treat HF and prevent its malignant course can be identified.

Sarcoplasmic reticulum calcium mishandling: central tenet in heart failure?

Excitation-contraction coupling links excitation of the sarcolemmal surface membrane to mechanical contraction. In the heart this link is established via a Ca²⁺-induced Ca²⁺ release process, which, following sarcolemmal depolarisation, prompts Ca²⁺ release from the sarcoplasmic reticulum (SR) though the ryanodine receptor (RyR2). This substantially raises the cytoplasmic Ca²⁺ concentration to trigger systole. In diastole, Ca²⁺ is removed from the cytoplasm, primarily via the sarcoplasmic-endoplasmic reticulum Ca²⁺-dependent ATPase (SERCA) pump on the SR membrane, returning Ca²⁺ to the SR store. Ca²⁺ movement across the SR is thus fundamental to the systole/diastole cycle and plays an essential role in maintaining cardiac contractile function. Altered SR Ca²⁺ homeostasis (due to disrupted Ca²⁺ release, storage, and reuptake pathways) is a central tenet of heart failure and contributes to depressed contractility, impaired relaxation, and propensity to arrhythmia. This review will focus on the molecular mechanisms that underlie asynchronous Ca²⁺ cycling around the SR in the failing heart. Further, this review will illustrate that the combined effects of expression changes and disruptions to RyR2 and SERCA2a regulatory pathways are critical to the pathogenesis of heart failure.

Calcium Signaling in Cardiomyocyte Function

Rhythmic increases in intracellular Ca²⁺ concentration underlie the contractile function of the heart. These heart muscle-wide changes in intracellular Ca²⁺ are induced and coordinated by electrical depolarization of the cardiomyocyte sarcolemma by the action potential. Originating at the sinoatrial node, conduction of this electrical signal throughout the heart ensures synchronization of individual myocytes into an effective cardiac pump. Ca²⁺ signaling pathways also regulate gene expression and cardiomyocyte growth during development and in pathology. These fundamental roles of Ca²⁺ in the heart are illustrated by the prevalence of altered Ca²⁺ homeostasis in cardiovascular diseases. Indeed, heart failure (an inability of the heart to support hemodynamic needs), rhythmic disturbances, and inappropriate cardiac growth all share an involvement of altered Ca²⁺ handling. The prevalence of these pathologies, contributing to a third of all deaths in the developed world as well as to substantial morbidity makes understanding the mechanisms of Ca²⁺ handling and dysregulation in cardiomyocytes of great importance.

Phenytoin Reduces Activity of Cardiac Ryanodine Receptor 2; A Potential Mechanism for Its Cardioprotective Action

Phenytoin is a hydantoin derivative that is used clinically for the treatment of epilepsy and has been reported to have antiarrhythmic actions on the heart. In a failing heart, the elevated diastolic Ca²⁺ leak from the sarcoplasmic reticulum can be normalized by the cardiac ryanodine receptor 2 (RyR2) inhibitor, dantrolene, without inhibiting Ca²⁺ release during systole or affecting Ca²⁺ release in normal healthy hearts. Unfortunately, dantrolene is hepatotoxic and unsuitable for chronic long-term administration. Because phenytoin and dantrolene belong to the hydantoin class of compounds, we test the hypothesis that dantrolene and phenytoin have similar inhibitory effects on RyR2 using a single-channel recording of RyR2 activity in artificial lipid bilayers. Phenytoin produced a reversible inhibition of RyR2 channels from sheep and human failing hearts. It followed a hyperbolic dose response with maximal inhibition of ∼50%, Hill coefficient ∼1, and IC₅₀ ranging from 10 to 20 µM. It caused inhibition at diastolic cytoplasmic [Ca²⁺] but not at Ca²⁺ levels in the dyadic cleft during systole. Notably, phenytoin inhibits RyR2 from failing human heart but not from healthy heart, indicating that phenytoin may selectively target defective RyR2 channels in humans. We conclude that phenytoin could effectively inhibit RyR2-mediated release of Ca²⁺ in a manner paralleling that of dantrolene. Moreover, the IC₅₀ of phenytoin in RyR2 is at least threefold lower than for other ion channels and clinically used serum levels, pointing to phenytoin as a more human-safe alternative to dantrolene for therapies against heart failure and cardiac arrythmias. SIGNIFICANCE STATEMENT: We show that phenytoin, a Na channel blocker used clinically for treatment of epilepsy, is a diastolic inhibitor of cardiac calcium release channels [cardiac ryanodine receptor 2 (RyR2)] at doses threefold lower than its current therapeutic levels. Phenytoin inhibits RyR2 from failing human heart and not from healthy heart, indicating that phenytoin may selectively target defective RyR2 channels in humans and pointing to phenytoin as a more human-safe alternative to dantrolene for therapies against heart failure and cardiac arrhythmias.

B-type natriuretic peptide and its role in altering Ca2+-regulatory proteins in heart failure-mechanistic insights

Heart failure (HF) is a worldwide disease with high levels of morbidity and mortality. The pathogenesis of HF is complicated and involves imbalances in hormone and electrolyte. B-type natriuretic peptide (BNP) has served as a biomarker of HF severity, and in recent years, it has been used to treat the disease, thanks to its cardio-protective effects, such as diuresis, natriuresis, and vasodilatation. In stage C/D HF, symptoms are severe despite elevated BNP. Disturbances in Ca²⁺ homeostasis are often a dominating feature of the disease, causing Ca²⁺-regulatory protein dysfunction, including reduced expression and activity of sarcoplasmic reticulum Ca²⁺-ATPase2a (SERCA2a), impaired ryanodine receptors (RYRs) function, intensive Na⁺-Ca²⁺ exchanger (NCX), and downregulation of S100A1. The relationship between natriuretic peptides (NPs) and Ca²⁺-regulatory proteins has been widely studied and represents important mechanisms in the etiology of HF. In this review, we present evidence that BNP may regulate Ca²⁺-regulatory proteins, in particular, suppressing SERCA2a and S100A1 expression. However, relationships between BNP and other Ca²⁺-regulatory proteins remain vague.

Hyperactive ryanodine receptors in human heart failure and ischaemic cardiomyopathy reside outside of couplons

Aims

In ventricular myocytes from humans and large mammals, the transverse and axial tubular system (TATS) network is less extensive than in rodents with consequently a greater proportion of ryanodine receptors (RyRs) not coupled to this membrane system. TATS remodelling in heart failure (HF) and after myocardial infarction (MI) increases the fraction of non-coupled RyRs. Here we investigate whether this remodelling alters the activity of coupled and non-coupled RyR sub-populations through changes in local signalling. We study myocytes from patients with end-stage HF, compared with non-failing (non-HF), and myocytes from pigs with MI and reduced left ventricular (LV) function, compared with sham intervention (SHAM).

Methods and results

Single LV myocytes for functional studies were isolated according to standard protocols. Immunofluorescent staining visualized organization of TATS and RyRs. Ca²⁺ was measured by confocal imaging (fluo-4 as indicator) and using whole-cell patch-clamp (37°C). Spontaneous Ca²⁺ release events, Ca²⁺ sparks, as a readout for RyR activity were recorded during a 15 s period following conditioning stimulation at 2 Hz. Sparks were assigned to cell regions categorized as coupled or non-coupled sites according to a previously developed method. Human HF myocytes had more non-coupled sites and these had more spontaneous activity than in non-HF. Hyperactivity of these non-coupled RyRs was reduced by Ca²⁺/calmodulin-dependent kinase II (CaMKII) inhibition. Myocytes from MI pigs had similar changes compared with SHAM controls as seen in human HF myocytes. As well as by CaMKII inhibition, in MI, the increased activity of non-coupled sites was inhibited by mitochondrial reactive oxygen species (mito-ROS) scavenging. Under adrenergic stimulation, Ca²⁺ waves were more frequent and originated at non-coupled sites, generating larger Na⁺/Ca²⁺ exchange currents in MI than in SHAM. Inhibition of CaMKII or mito-ROS scavenging reduced spontaneous Ca²⁺ waves, and improved excitation–contraction coupling.

Conclusions

In HF and after MI, RyR microdomain re-organization enhances spontaneous Ca²⁺ release at non-coupled sites in a manner dependent on CaMKII activation and mito-ROS production. This specific modulation generates a substrate for arrhythmia that appears to be responsive to selective pharmacologic modulation.

Ion channel gating in cardiac ryanodine receptors from the arrhythmic RyR2-P2328S mouse

Mutations in the cardiac ryanodine receptor Ca²⁺ release channel (RyR2) can cause deadly ventricular arrhythmias and atrial fibrillation (AF). The RyR2-P2328S mutation produces catecholaminergic polymorphic ventricular tachycardia (CPVT) and AF in hearts from homozygous RyR2^{P2328S/P2328S} (denoted RyR2^S/S) mice. We have now examined P2328S RyR2 channels from RyR2^S/S hearts. The activity of wild-type (WT) and P2328S RyR2 channels was similar at a cytoplasmic [Ca²⁺] of 1 mM, but P2328S RyR2 was significantly more active than WT at a cytoplasmic [Ca²⁺] of 1 µM. This was associated with a >10-fold shift in the half maximal activation concentration (AC₅₀) for Ca²⁺ activation, from ∼3.5 µM Ca²⁺ in WT RyR2 to ∼320 nM in P2328S channels and an unexpected >1000-fold shift in the half maximal inhibitory concentration (IC₅₀) for inactivation from ∼50 mM in WT channels to ≤7 μM in P2328S channels, which is into systolic [Ca²⁺] levels. Unexpectedly, the shift in Ca²⁺ activation was not associated with changes in sub-conductance activity, S2806 or S2814 phosphorylation or the level of FKBP12 (also known as FKBP1A) bound to the channels. The changes in channel activity seen with the P2328S mutation correlate with altered Ca²⁺ homeostasis in myocytes from RyR2^S/S mice and the CPVT and AF phenotypes.This article has an associated First Person interview with the first author of the paper.

Calmodulin inhibition of human RyR2 channels requires phosphorylation of RyR2-S2808 or RyR2-S2814

Calmodulin (CaM) is a Ca-binding protein that binds to, and can directly inhibit cardiac ryanodine receptor calcium release channels (RyR2). Animal studies have shown that RyR2 hyperphosphorylation reduces CaM binding to RyR2 in failing hearts, but data are lacking on how CaM regulates human RyR2 and how this regulation is affected by RyR2 phosphorylation. Physiological concentrations of CaM (100 nM) inhibited the diastolic activity of RyR2 isolated from failing human hearts by ~50% but had no effect on RyR2 from healthy human hearts. Using FRET between donor-FKBP12.6 and acceptor-CaM bound to RyR2, we determined that CaM binds to RyR2 from healthy human heart with a K_d = 121 ± 14 nM. Ex-vivo phosphorylation/dephosphorylation experiments suggested that the divergent CaM regulation of healthy and failing human RyR2 was caused by differences in RyR2 phosphorylation by protein kinase A and Ca-CaM-dependent kinase II. Ca²⁺-spark measurements in murine cardiomyocytes harbouring RyR2 phosphomimetic or phosphoablated mutants at S2814 and S2808 suggest that phosphorylation of residues corresponding to either human RyR2-S2808 or S2814 is both necessary and sufficient for RyR2 regulation by CaM. Our results challenge the current concept that CaM universally functions as a canonical inhibitor of RyR2 across species. Rather, CaM's biological action on human RyR2 appears to be more nuanced, with inhibitory activity only on phosphorylated RyR2 channels, which occurs during exercise or in patients with heart failure.

Increased risk of post-stroke epilepsy in Chinese patients with a TRPM6 polymorphism

OBJECTIVES

We attempted to determine whether a functional polymorphism of TRPM6 (rs2274924) is associated with susceptibility to epilepsy following ischemic stroke, and to further explore the effect of this polymorphism on serum levels of Mg²⁺ in post-stroke patients.

METHODS

We carried out a case-control study on 378 post-stroke epilepsy patients and 420 controls (stroke patients without secondary epilepsy). We used DNA sequencing to determine the genotypes of the TRPM6 rs2274924 polymorphism, and used the ion selective electrode method to measure serum levels of Mg²⁺.

RESULTS

The distribution of the CC genotype and the frequency of the C allele were significantly higher in the post-stroke epilepsy patients than in the controls (P < 0.01). With regard to the post-stroke epilepsy patients, the serum levels of Mg²⁺ decreased significantly in the TRPM6 rs2274924 C allele carriers compared to the rs2274924 T allele carriers.

CONCLUSION

The TRPM6 rs2274924 polymorphism may be associated with susceptibility to epilepsy following stroke, and the C allele may be associated with increased risk of post-stroke epilepsy. The TRPM6 rs2274924 polymorphism may also influence serum levels of Mg²⁺ in post-stroke epilepsy patients.

Redox Dependent Modifications of Ryanodine Receptor: Basic Mechanisms and Implications in Heart Diseases

Heart contraction vitally depends on tightly controlled intracellular Ca regulation. Because contraction is mainly driven by Ca released from the sarcoplasmic reticulum (SR), this organelle plays a particularly important role in Ca regulation. The type two ryanodine receptor (RyR2) is the major SR Ca release channel in ventricular myocytes. Several cardiac pathologies, including myocardial infarction and heart failure, are associated with increased RyR2 activity and diastolic SR Ca leak. It has been suggested that the increased RyR2 activity plays an important role in arrhythmias and contractile dysfunction. Several studies have linked increased SR Ca leak during myocardial infarction and heart failure to the activation of RyR2 in response to oxidative stress. This activation might include direct oxidation of RyR2 as well as indirect activation via phosphorylation or altered interactions with regulatory proteins. Out of ninety cysteine residues per RyR2 subunit, twenty one were reported to be in reduced state that could be potential targets for redox modifications that include S-nitrosylation, S-glutathionylation, and disulfide cross-linking. Despite its clinical significance, molecular mechanisms of RyR dysfunction during oxidative stress are not fully understood. Herein we review the most recent insights into redox-dependent modulation of RyR2 during oxidative stress and heart diseases.

Ryanodine receptor Ca2+ release channel post-translational modification: Central player in cardiac and skeletal muscle disease

Calcium release from internal stores is a quintessential event in excitation-contraction coupling in cardiac and skeletal muscle. The ryanodine receptor Ca²⁺ release channel is embedded in the internal sarcoplasmic reticulum Ca²⁺ store, which releases Ca²⁺ into the cytoplasm, enabling contraction. Ryanodine receptors form the hub of a macromolecular complex extending from the extracellular space to the sarcoplasmic reticulum lumen. Ryanodine receptor activity is influenced by the integrated effects of associated co-proteins, ions, and post-translational phosphor and redox modifications. In healthy muscle, ryanodine receptors are phosphorylated and redox modified to basal levels, to support cellular function. A pathological increase in the degree of both post-translational modifications disturbs intracellular Ca²⁺ signalling, and is implicated in various cardiac and skeletal disorders. This review summarises our current understanding of the mechanisms linking ryanodine receptor post-translational modification to heart failure and skeletal myopathy and highlights the challenges and controversies within the field.

Dysregulated Zn2+ homeostasis impairs cardiac type-2 ryanodine receptor and mitsugumin 23 functions, leading to sarcoplasmic reticulum Ca2+ leakage

Aberrant Zn²⁺ homeostasis is associated with dysregulated intracellular Ca²⁺ release, resulting in chronic heart failure. In the failing heart a small population of cardiac ryanodine receptors (RyR2) displays sub-conductance-state gating leading to Ca²⁺ leakage from sarcoplasmic reticulum (SR) stores, which impairs cardiac contractility. Previous evidence suggests contribution of RyR2-independent Ca²⁺ leakage through an uncharacterized mechanism. We sought to examine the role of Zn²⁺ in shaping intracellular Ca²⁺ release in cardiac muscle. Cardiac SR vesicles prepared from sheep or mouse ventricular tissue were incorporated into phospholipid bilayers under voltage-clamp conditions, and the direct action of Zn²⁺ on RyR2 channel function was examined. Under diastolic conditions, the addition of pathophysiological concentrations of Zn²⁺ (≥2 nm) caused dysregulated RyR2-channel openings. Our data also revealed that RyR2 channels are not the only SR Ca²⁺-permeable channels regulated by Zn²⁺. Elevating the cytosolic Zn²⁺ concentration to 1 nm increased the activity of the transmembrane protein mitsugumin 23 (MG23). The current amplitude of the MG23 full-open state was consistent with that previously reported for RyR2 sub-conductance gating, suggesting that in heart failure in which Zn²⁺ levels are elevated, RyR2 channels do not gate in a sub-conductance state, but rather MG23-gating becomes more apparent. We also show that in H9C2 cells exposed to ischemic conditions, intracellular Zn²⁺ levels are elevated, coinciding with increased MG23 expression. In conclusion, these data suggest that dysregulated Zn²⁺ homeostasis alters the function of both RyR2 and MG23 and that both ion channels play a key role in diastolic SR Ca²⁺ leakage.

Calcium Signaling and Cardiac Arrhythmias

There has been a significant progress in our understanding of the molecular mechanisms by which calcium (Ca2+) ions mediate various types of cardiac arrhythmias. A growing list of inherited gene defects can cause potentially lethal cardiac arrhythmia syndromes, including catecholaminergic polymorphic ventricular tachycardia, congenital long QT syndrome, and hypertrophic cardiomyopathy. In addition, acquired deficits of multiple Ca2+-handling proteins can contribute to the pathogenesis of arrhythmias in patients with various types of heart disease. In this review article, we will first review the key role of Ca2+ in normal cardiac function-in particular, excitation-contraction coupling and normal electric rhythms. The functional involvement of Ca2+ in distinct arrhythmia mechanisms will be discussed, followed by various inherited arrhythmia syndromes caused by mutations in Ca2+-handling proteins. Finally, we will discuss how changes in the expression of regulation of Ca2+ channels and transporters can cause acquired arrhythmias, and how these mechanisms might be targeted for therapeutic purposes.