Targeting ryanodine receptors for anti-arrhythmic therapy

Abstract

Antiarrhythmic drugs are a group of pharmaceuticals that suppress or prevent abnormal heart rhythms, which are often associated with substantial morbidity and mortality. Current antiarrhythmic drugs that typically target plasma membrane ion channels have limited clinical success and in some cases have been described as being pro-arrhythmic. However, recent studies suggest that pathological release of calcium (Ca(2+)) from the sarcoplasmic reticulum via cardiac ryanodine receptors (RyR2) could represent a promising target for antiarrhythmic therapy. Diastolic SR Ca(2+) release has been linked to arrhythmogenesis in both the inherited arrhythmia syndrome 'catecholaminergic polymorphic ventricular tachycardia' and acquired forms of heart disease (eg, atrial fibrillation, heart failure). Several classes of pharmaceuticals have been shown to reduce abnormal RyR2 activity and may confer protection against triggered arrhythmias through reduction of SR Ca(2+) leak. In this review, we will evaluate the current pharmacological methods for stabilizing RyR2 and suggest treatment modalities based on current evidence of molecular mechanisms.

Figure 1

Dantrolene inhibits catecholaminergic polymorphic ventricular tachycardia in mice. (A) Representative images of Ca²⁺ sparks in cardiomyocytes isolated from heterozygous RyR2-R2474S/+ knock-in mice, showing that dantrolene reduces Ca²⁺ spark frequency following isoproterenol exposure. (B) Bar graph showing that dantrolene suppresses abnormal Ca²⁺ spark frequency in RyR2-R2474S/+ mutant mice after isoproterenol exposure. (C) Telemetric ECG recordings reveal exercise-induced ventricular tachycardia in a RyR2-R2474S/+ mouse, which was suppressed by dantrolene. (D) Bar graphs revealing that dantrolene suppresses the incidence of exercise or epinephrine induced ventricular tachycardia (VT) in RyR2-R2474S/+ mice. Adapted from Kobayashi et al.

Figure 2

Anti-arrhythmic effects of 1,4-benzothiazepine JTV-519 in FKBP12.6^+/− mice. (A–B) Representative immunoblots and quantifications for RyR2, RyR2-pSer²⁸⁰⁹ (PKA phosphorylation site on RyR2), and calstabin2 (FKBP12.6) from wildtype (WT), calstabin2^+/− heterozygous, and calstabin2^−/− (FKBP12.6^−/−) knockout mice. Whereas exercise increased PKA phosphorylation of RyR2 and decreased calstabin2 (FKBP12.6) binding to RyR2, JTV-519 prevented calstabin2 dissociation. (C–D) Representative single channel recordings in planar lipid bilayers showing that JVT519 reduced the open probability of RyR2 isolated from calstabin2^+/− but not calstabin2^−/− mice, consistent with calstabin2 (FKBP12.6) being required for the therapeutic effects of JTV519. (E–F) ECG tracings showing that JTV-519 reduces ventricular arrhythmias in calstabin2^+/− but not calstabin2^−/− mice. ^*P<0.05. Adapted from Wehrens et al.

Figure 3

Prevention of triggered arrhythmias by flecainide. (A–B) Concentration-dependent effects of flecainide on single sheep RyR2 channels in lipid bilayers. Flecainide decreases open probability (Po) and mean open time (To), and does not significantly alter the mean closed time (Tc) of RyR2. ^*P<0.02, ^**P<0.01 and ^***P<0.001. (C–D) Effects of flecainide on isoproterenol (ISO) stimulated calsequestrin-deficient cardiomyocytes. Whereas ISO evoked spontaneous SR Ca²⁺ release events (^*), flecainide reduced the number of Ca²⁺ releases and triggered beats (†). ^**P=0.0078 and ^***P <0.001. (E) Cartoon illustrating dual effects of flecainide action on SR Ca²⁺ release (red tracing) and inhibition of premature beats triggered by delayed after depolarization (black tracing). Adapted from Watanabe et al.