Is the Debate on the Flecainide Action on the RYR2 in CPVT Closed?

Flecainide Specifically Targets the Monovalent Countercurrent Through the Cardiac Ryanodine Receptor, While a Dominant Opposing Ca2+/Ba2+ Current Is Present

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a highly arrhythmogenic syndrome triggered by stress, primarily linked to gain-of-function point mutations in the cardiac ryanodine receptor (RyR2). Flecainide, as an effective therapy for CPVT, is a known blocker of the surface-membrane Na+ channel, also affecting the intracellular RyR2 channel. The therapeutic relevance of the flecainide-RyR2 interaction remains controversial, as flecainide blocks only the RyR2 current flowing in the opposite direction to the physiological Ca2+ release from the sarcoplasmic reticulum (SR). However, it has been proposed that charge-compensating countercurrent from the cytosol to SR lumen plays a critical role, and its reduction may indeed suppress excessive diastolic SR Ca2+ release through RyR2 channels in CPVT. Monitoring single-channel properties, we examined whether flecainide can target intracellular pathways for charge-balancing currents carried by RyR2 and SR Cl- channels under cell-like conditions. Particularly, the Tris+ countercurrent flowed through the RyR2 channel simultaneously with a dominant reverse Ca2+/Ba2+ current. We demonstrate that flecainide blocked the RyR2-mediated countercurrent without affecting channel activity. In contrast, the SR Cl- channel was completely resistant to flecainide. Based on these findings, it is reasonable to propose that the primary intracellular target of flecainide in vivo is the RyR2-mediated countercurrent.

State of the art in Purkinje bioengineering

This review article will cover the recent developments in the new evolving field of Purkinje bioengineering and the development of human Purkinje networks. Recent work has progressed to the point of a methodological and systematic process to bioengineer Purkinje networks. This involves the development of 3D models based on human anatomy, followed by the development of tunable biomaterials, and strategies to reprogram stem cells to Purkinje cells. Subsequently, the reprogrammed cells and the biomaterials are coupled to bioengineer Purkinje networks, which are then tested using a small animal injury model. In this article, we discuss this process as a whole and then each step separately. We then describe potential applications of bioengineered Purkinje networks and challenges in the field that need to be overcome to move this field forward. Although the field of Purkinje bioengineering is new and in a state of infancy, it holds tremendous potential, both for therapeutic applications and to develop tools that can be used for disease modeling.

Store-Operated Ca2+ Entry as a Putative Target of Flecainide for the Treatment of Arrhythmogenic Cardiomyopathy

Arrhythmogenic cardiomyopathy (ACM) is a genetic disorder that may lead patients to sudden cell death through the occurrence of ventricular arrhythmias. ACM is characterised by the progressive substitution of cardiomyocytes with fibrofatty scar tissue that predisposes the heart to life-threatening arrhythmic events. Cardiac mesenchymal stromal cells (C-MSCs) contribute to the ACM by differentiating into fibroblasts and adipocytes, thereby supporting aberrant remodelling of the cardiac structure. Flecainide is an Ic antiarrhythmic drug that can be administered in combination with β-adrenergic blockers to treat ACM due to its ability to target both Nav1.5 and type 2 ryanodine receptors (RyR2). However, a recent study showed that flecainide may also prevent fibro-adipogenic differentiation by inhibiting store-operated Ca2+ entry (SOCE) and thereby suppressing spontaneous Ca2+ oscillations in C-MSCs isolated from human ACM patients (ACM C-hMSCs). Herein, we briefly survey ACM pathogenesis and therapies and then recapitulate the main molecular mechanisms targeted by flecainide to mitigate arrhythmic events, including Nav1.5 and RyR2. Subsequently, we describe the role of spontaneous Ca2+ oscillations in determining MSC fate. Next, we discuss recent work showing that spontaneous Ca2+ oscillations in ACM C-hMSCs are accelerated to stimulate their fibro-adipogenic differentiation. Finally, we describe the evidence that flecainide suppresses spontaneous Ca2+ oscillations and fibro-adipogenic differentiation in ACM C-hMSCs by inhibiting constitutive SOCE.

Molecular, Subcellular, and Arrhythmogenic Mechanisms in Genetic RyR2 Disease

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

Moving in the right direction: elucidating the mechanisms of interaction between flecainide and the cardiac ryanodine receptor

Flecainide is used to treat catecholaminergic polymorphic ventricular tachycardia (CPVT), an arrhythmia caused by disrupted cellular Ca²⁺ handling following β-adrenergic stimulation. The clinical efficacy of flecainide in this context involves complex effects on multiple ion channels that may be influenced by the disease state. A compelling narrative has been constructed around flecainide's nonselective block of sarcoplasmic reticulum (SR) lumen-to-cytoplasm Ca²⁺ release through intracellular calcium release channels (RyR2). However, ion fluxes across the SR membrane during heart contraction are bidirectional, and here, we review experimental evidence that flecainide's principal action on RyR2 involves the partial block of ion flow in the cytoplasm-to-lumen direction (i.e., flecainide inhibits RyR2-mediated SR 'countercurrent'). Experimental approaches that could advance new knowledge on the mechanism of RyR2 block by flecainide are proposed. Some impediments to progress in this area, that must be overcome to enable the development of superior drugs to treat CPVT, are also considered.