Proteins within the intracellular calcium store determine cardiac RyR channel activity and cardiac output

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.

The function and regulation of calsequestrin-2: implications in calcium-mediated arrhythmias

Cardiac arrhythmias are life-threatening events in which the heart develops an irregular rhythm. Mishandling of Ca²⁺ within the myocytes of the heart has been widely demonstrated to be an underlying mechanism of arrhythmogenesis. This includes altered function of the ryanodine receptor (RyR2)-the primary Ca²⁺ release channel located to the sarcoplasmic reticulum (SR). The spontaneous leak of SR Ca²⁺ via RyR2 is a well-established contributor in the development of arrhythmic contractions. This leak is associated with increased channel activity in response to changes in SR Ca²⁺ load. RyR2 activity can be regulated through several avenues, including interactions with numerous accessory proteins. One such protein is calsequestrin-2 (CSQ2), which is the primary Ca²⁺-buffering protein within the SR. The capacity of CSQ2 to buffer Ca²⁺ is tightly associated with the ability of the protein to polymerise in response to changing Ca²⁺ levels. CSQ2 can itself be regulated through phosphorylation and glycosylation modifications, which impact protein polymerisation and trafficking. Changes in CSQ2 modifications are implicated in cardiac pathologies, while mutations in CSQ2 have been identified in arrhythmic patients. Here, we review the role of CSQ2 in arrhythmogenesis including evidence for the indirect and direct regulation of RyR2 by CSQ2, and the consequences of a loss of functional CSQ2 in Ca²⁺ homeostasis and Ca²⁺-mediated arrhythmias.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12551-021-00914-6.

Luminal Ca2+ regulation of RyR1 Ca2+ channel leak activation and inactivation in sarcoplasmic reticulum membrane vesicles

In this study, we tested the hypothesis that the RyR1 Ca²⁺ channel closure is sensitive to outward trans-SR membrane Ca²⁺ gradients established by SERCA1 pumping. To perform these studies, we employed stopped-flow rapid-kinetic fluorescence methods to measure and assess how variation in trans-SR membrane Ca²⁺ distribution affects evolution of RyR1 Ca²⁺ leaks in RyR1/ CASQ1/SERCA1-rich membrane vesicles. Our studies showed that rapid filling of a Mag-Fura-2-sensitive free Ca²⁺ pool during SERCA1-mediated Ca²⁺ sequestration appears to be a crucial condition allowing RyR1 Ca²⁺ channels to close once reloading of luminal Ca²⁺ stores is complete. Disruption in the filling of this pool caused activation of Ruthenium Red inhibitable RyR1 Ca²⁺ leaks, suggesting that SERCA1 pump formation of outward Ca²⁺ gradients is an important aspect of Ca²⁺ flux control channel opening and closing. In addition, our observed ryanodine-induced shift in luminal Ca²⁺ from free to a CTC-Ca⁺-sensitive, CASQ1-associated bound compartment underscores the complex organization and regulation of SR luminal Ca²⁺. Our study provides strong evidence that RyR1 functional states directly and indirectly influence the compartmentation of luminal Ca²⁺. This, in turn, is influenced by the activity of SERCA1 pumps to fill luminal pools while synchronously reducing Ca²⁺ levels on the cytosolic face of RyR1 channels.

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.

Targeting Calcium Homeostasis in Myocardial Ischemia/Reperfusion Injury: An Overview of Regulatory Mechanisms and Therapeutic Reagents

Calcium homeostasis plays an essential role in maintaining excitation–contraction coupling (ECC) in cardiomyocytes, including calcium release, recapture, and storage. Disruption of calcium homeostasis may affect heart function, leading to the development of various heart diseases. Myocardial ischemia/reperfusion (MI/R) injury may occur after revascularization, which is a treatment used in coronary heart disease. MI/R injury is a complex pathological process, and the main cause of increased mortality and disability after treatment of coronary heart disease. However, current methods and drugs for treating MI/R injury are very scarce, not ideal, and have limitations. Studies have shown that MI/R injury can cause calcium overload that can further aggravate MI/R injury. Therefore, we reviewed the effects of critical calcium pathway regulators on MI/R injury and drew an intuitive diagram of the calcium homeostasis pathway. We also summarized and analyzed calcium pathway-related or MI/R drugs under research or marketing by searching Therapeutic Target and PubMed Databases. The data analysis showed that six drugs and corresponding targets are used to treat MI/R injury and involved in calcium signaling pathways. We emphasize the relevance of further detailed investigation of MI/R injury and calcium homeostasis and the therapeutic role of calcium homeostasis in MI/R injury, which bridges basic research and clinical applications of MI/R injury.

The effect of hypothalamic peptides, neurohormone C and proline-rich peptide-1on the Ca2+-handling system in heartin pathophysiological conditions

Atthe Institute of Biochemistry named after H. Buniatyan we discovered and studied hypothalamic peptides with coronary dilatory and antioxidant activities:neurohormone C (NC) and proline-rich peptide-1 (PRP-1). Both NC and PRP-1 exhibit cardioprotective effects, in part by restoring the calcium affinity for calcium-binding membrane proteins in cardiomyocytes. This affinity is diminished in the sarcoplasmic reticulum and mitochondriawith myocardial damage, heart failure, pancreatic necrosis and crush syndrome caused by isoproterenol. The peptides can also destroy the four detected toxic peptides and myocardial depressant factor, and protect against ischemia-reperfusion injury. Further studies of these peptides may be promising for the treatment of patients at high risk of cardiovascular disease, regardless of pathology.

Core skeletal muscle ryanodine receptor calcium release complex

The core skeletal muscle ryanodine receptor (RyR1) calcium release complex extends through three compartments of the muscle fibre, linking the extracellular environment through the cytoplasmic junctional gap to the lumen of the internal sarcoplasmic reticulum (SR) calcium store. The protein complex is essential for skeletal excitation-contraction (EC)-coupling and skeletal muscle function. Its importance is highlighted by perinatal death if any one of the EC-coupling components are missing and by myopathies associated with mutation of any of the proteins. The proteins essential for EC-coupling include the DHPR α_1S subunit in the transverse tubule membrane, the DHPR β_1a subunit in the cytosol and the RyR1 ion channel in the SR membrane. The other core proteins are triadin and junctin and calsequestrin, associated mainly with SR. These SR proteins are not essential for survival but exert structural and functional influences that modify the gain of EC-coupling and maintain normal muscle function. This review summarises our current knowledge of the individual protein/protein interactions within the core complex and their overall contribution to EC-coupling. We highlight significant areas that provide a continuing challenge for the field. Additional important components of the Ca²⁺ release complex, such as FKBP12, calmodulin, S100A1 and Stac3 are identified and reviewed elsewhere.

Control of cardiac ryanodine receptor by sarcoplasmic reticulum luminal Ca2

Jones et al. propose that SR luminal Ca²⁺ regulates RyR2 activity via a luminal Ca²⁺ sensor distinct from the cytosolic Ca²⁺ sensor.

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.

Calsequestrin interacts directly with the cardiac ryanodine receptor luminal domain

Cardiac muscle contraction requires sarcoplasmic reticulum (SR) Ca²⁺ release mediated by the quaternary complex comprising the ryanodine receptor 2 (RyR2), calsequestrin 2 (CSQ2), junctin (encoded by ASPH) and triadin. Here, we demonstrate that a direct interaction exists between RyR2 and CSQ2. Topologically, CSQ2 binding occurs at the first luminal loop of RyR2. Co-expression of RyR2 and CSQ2 in a human cell line devoid of the other quaternary complex proteins results in altered Ca²⁺-release dynamics compared to cells expressing RyR2 only. These findings provide a new perspective for understanding the SR luminal Ca²⁺ sensor and its involvement in cardiac physiology and disease.

Regulation of RYR2 by sarcoplasmic reticulum Ca(2+)

Ca(2+) is arguably the most important ion involved in the contraction of the heart. The cardiac ryanodine receptor (RyR2), the major Ca(2+) release channel located in the sarcoplasmic reticulum (SR) membrane, is responsible for releasing the bulk of Ca(2+) required for contraction. Moreover, RyR2 is also crucial for maintaining SR Ca(2+) homeostasis by releasing Ca(2+) from the SR when it becomes overloaded with Ca(2+) . During normal contraction, RyR2 is activated by cytosolic Ca(2+) , whereas during store overload conditions, the opening of RyR2 is governed by SR Ca(2+) . Although the process of the cytosolic control of RyR2 is well established, the molecular mechanism by which SR luminal Ca(2+) regulates RyR2 has only recently been elucidated and remains controversial. In addition to the activation of RyR2, SR luminal Ca(2+) also determines when the RyR2 channel closes. RyR2-mediated Ca(2+) release from the SR does not continue until the SR is completely depleted. Rather, it ceases when SR luminal Ca(2+) falls below a certain level. Given the importance of SR Ca(2+) , it is not surprising that the SR luminal Ca(2+) level is tightly controlled by SR Ca(2+) -buffering proteins. Consequently, the opening and closing of RyR2 is heavily influenced by the presence of such proteins, particularly those associated with RyR2, such as calsequestrin and the histidine-rich Ca(2+) -binding protein. These proteins appear to indirectly alter RyR2 activity by modifying the microdomain SR Ca(2+) level surrounding RyR2.

A new cytoplasmic interaction between junctin and ryanodine receptor Ca2+ release channels

Junctin, a non-catalytic splice variant encoded by the aspartate-β-hydroxylase (Asph) gene, is inserted into the membrane of the sarcoplasmic reticulum (SR) Ca²⁺ store where it modifies Ca²⁺ signalling in the heart and skeletal muscle through its regulation of ryanodine receptor (RyR) Ca²⁺ release channels. Junctin is required for normal muscle function as its knockout leads to abnormal Ca²⁺ signalling, muscle dysfunction and cardiac arrhythmia. However, the nature of the molecular interaction between junctin and RyRs is largely unknown and was assumed to occur only in the SR lumen. We find that there is substantial binding of RyRs to full junctin, and the junctin luminal and, unexpectedly, cytoplasmic domains. Binding of these different junctin domains had distinct effects on RyR1 and RyR2 activity: full junctin in the luminal solution increased RyR channel activity by ∼threefold, the C-terminal luminal interaction inhibited RyR channel activity by ∼50%, and the N-terminal cytoplasmic binding produced an ∼fivefold increase in RyR activity. The cytoplasmic interaction between junctin and RyR is required for luminal binding to replicate the influence of full junctin on RyR1 and RyR2 activity. The C-terminal domain of junctin binds to residues including the S1–S2 linker of RyR1 and N-terminal domain of junctin binds between RyR1 residues 1078 and 2156.

Cardiac ryanodine receptor activation by a high Ca²⁺ store load is reversed in a reducing cytoplasmic redox environment

Here, we report the impact of redox potential on isolated cardiac ryanodine receptor (RyR2) channel activity and its response to physiological changes in luminal [Ca²⁺]. Basal leak from the sarcoplasmic reticulum is required for normal Ca²⁺ handling, but excess diastolic Ca²⁺ leak attributed to oxidative stress is thought to lower the threshold of RyR2 for spontaneous sarcoplasmic reticulum Ca²⁺ release, thus inducing arrhythmia in pathological situations. Therefore, we examined the RyR2 response to luminal [Ca²⁺] under reducing or oxidising cytoplasmic redox conditions. Unexpectedly, as luminal [Ca²⁺] increased from 0.1 to 1.5 mM, RyR2 activity declined when pretreated with cytoplasmic 1 mM DTT or buffered with GSH∶GSSG to a normal reduced cytoplasmic redox potential (−220 mV). Conversely, with 20 µM cytoplasmic 4,4′-DTDP or buffering of the redox potential to an oxidising value (−180 mV), RyR2 activity increased with increasing luminal [Ca²⁺]. The luminal redox potential was constant at −180 mV in each case. These responses to luminal [Ca²⁺] were maintained with cytoplasmic 2 mM Na₂ATP or 5 mM MgATP (1 mM free Mg²⁺). Overall, the results suggest that the redox potential in the RyR2 junctional microdomain is normally more oxidised than that of the bulk cytoplasm.

Mechanism of calsequestrin regulation of single cardiac ryanodine receptor in normal and pathological conditions

Release of Ca²⁺ from the sarcoplasmic reticulum (SR) drives contractile function of cardiac myocytes. Luminal Ca²⁺ regulation of SR Ca²⁺ release is fundamental not only in physiology but also in physiopathology because abnormal luminal Ca²⁺ regulation is known to lead to arrhythmias, catecholaminergic polymorphic ventricular tachycardia (CPVT), and/or sudden cardiac arrest, as inferred from animal model studies. Luminal Ca²⁺ regulates ryanodine receptor (RyR)2-mediated SR Ca²⁺ release through mechanisms localized inside the SR; one of these involves luminal Ca²⁺ interacting with calsequestrin (CASQ), triadin, and/or junctin to regulate RyR2 function.

CASQ2-RyR2 regulation was examined at the single RyR2 channel level. Single RyR2s were incorporated into planar lipid bilayers by the fusion of native SR vesicles isolated from either wild-type (WT), CASQ2 knockout (KO), or R33Q-CASQ2 knock-in (KI) mice. KO and KI mice have CPVT-like phenotypes. We show that CASQ2(WT) action on RyR2 function (either activation or inhibition) was strongly influenced by the presence of cytosolic MgATP. Function of the reconstituted CASQ2(WT)–RyR2 complex was unaffected by changes in luminal free [Ca²⁺] (from 0.1 to 1 mM). The inhibition exerted by CASQ2(WT) association with the RyR2 determined a reduction in cytosolic Ca²⁺ activation sensitivity. RyR2s from KO mice were significantly more sensitive to cytosolic Ca²⁺ activation and had significantly longer mean open times than RyR2s from WT mice. Sensitivity of RyR2s from KI mice was in between that of RyR2 channels from KO and WT mice. Enhanced cytosolic RyR2 Ca²⁺ sensitivity and longer RyR2 open times likely explain the CPVT-like phenotype of both KO and KI mice.

Glutathione transferases, regulators of cellular metabolism and physiology

BACKGROUND

The cytosolic glutathione transferases (GSTs) comprise a super family of proteins that can be categorized into multiple classes with a mixture of highly specific and overlapping functions.

SCOPE OF REVIEW

The review covers the genetics, structure and function of the human cytosolic GSTs with particular attention to their emerging roles in cellular metabolism.

MAJOR CONCLUSIONS

All the catalytically active GSTs contribute to the glutathione conjugation or glutathione dependant-biotransformation of xenobiotics and many catalyze glutathione peroxidase or thiol transferase reactions. GSTs also catalyze glutathione dependent isomerization reactions required for the synthesis of several prostaglandins and steroid hormones and the catabolism of tyrosine. An increasing body of work has implicated several GSTs in the regulation of cell signaling pathways mediated by stress-activated kinases like Jun N-terminal kinase. In addition, some members of the cytosolic GST family have been shown to form ion channels in intracellular membranes and to modulate ryanodine receptor Ca(2+) channels in skeletal and cardiac muscle.

GENERAL SIGNIFICANCE

In addition to their well established roles in the conjugation and biotransformation of xenobiotics, GSTs have emerged as significant regulators of pathways determining cell proliferation and survival and as regulators of ryanodine receptors that are essential for muscle function. This article is part of a Special Issue entitled Cellular functions of glutathione.

Functional interaction between calsequestrin and ryanodine receptor in the heart

Evidence obtained in the last two decades indicates that calsequestrin (CSQ2), as the major Ca(2+)-binding protein in the sarcoplasmic reticulum of cardiac myocytes, communicates changes in the luminal Ca(2+) concentration to the cardiac ryanodine receptor (RYR2) channel. This review summarizes the major aspects in the interaction between CSQ2 and the RYR2 channel. The single channel properties of RYR2 channels, discussed here in the context of structural changes in CSQ2 after Ca(2+) binding, are particularly important. We focus on five important questions concerning: (1) the method for reliable detection of CSQ2 on the reconstituted RYR2 channel complex; (2) the power of the procedure to strip CSQ2 from the RYR2 channel complex; (3) structural changes in CSQ2 upon binding of Ca(2+) which cause CSQ2 dissociation; (4) the potential role of CSQ2-independent regulation of the RYR2 activity by luminal Ca(2+); and (5) the vizualization of CSQ2 dissociation from the RYR2 channel complex on the single channel level. We discuss the potential sources of the conflicting experimental results which may aid detailed understanding of the CSQ2 regulatory role. Although we mainly focus on the cardiac isoform of the proteins, some aspects of more extensive work carried out on the skeletal isoform are also discussed.