Plumbing for the heart

Fixing a leaky calcium channel could be a promising approach for protecting against life-threatening cardiac arrhythmias, according to a recent paper in Science. The research by Marks and colleagues shows that an experimental drug known as JTV519 enhances the binding of the protein calstabin-2 to a calcium channel present in heart cells, thereby stabilizing the channel in its closed state and preventing potentially dangerous calcium leakage.

Calstabin-2 forms part of the ryanodine receptor–calcium-release channel (RyR2) complex that regulates calcium release from the sarcoplasmic reticulum (SR). Calstabin-2, which is bound to RyR2, helps maintain the closed state of the channel during the diastolic (resting) phase of the cardiac cycle. But in patients with heart failure, and in some inherited forms of exercise-induced sudden cardiac death (SCD), depletion of calstabin-2 from the RyR2 complex has been shown to cause calcium leakage that can trigger fatal ventricular tachyarrhythmia (VT).

Successful prevention or treatment of SCD has been hindered by an incomplete understanding of the molecular mechanisms that cause VT. However, JTV519 was recently shown to reduce diastolic SR calcium leakage in an animal model of heart failure, and now Marks and colleagues have clarified the underlying mechanism in a series of experiments using mice that are either partially or completely deficient in calstabin-2.

When calstabin-2^+/− mice were subjected to an exercise protocol designed to induce stress, 100% developed VT and 89% subsequently died. By contrast, no arrhythmias were observed in calstabin-2^+/− mice that were treated with JTV519 before exercise. However, in calstabin-2-null mice, the protective effect of JTV519 was negated: all treated and untreated null mice in the study developed VT and died. This indicates that JTV519 exerts its protective effect via calstabin-2, but to characterize this further the authors turned their attention to the relationship between exercise, phosphorylation of RyR2 and calstabin-2 depletion.

Exercise is known to induce protein kinase A (PKA) phosphorylation of RyR2, which in turn triggers the dissociation of calstabin-2 from RyR2. Correspondingly, the value of P_O (probability that channels will be open) for RyR2 significantly increases in calstabin-2^+/− mice during exercise, compared with calstabin-2^+/+ mice, when the channels are examined under conditions that simulate diastole. The authors showed that JTV519 prevents the depletion of calstabin-2 that is observed in mice during exercise and significantly reduced P_O in calstabin-2^+/− mice but not in calstabin-2-null mice. However, JTV519 had no direct effect on this phosphorylation, indicating that it increases the affinity of calstabin-2 for RyR2 even when the channel is phosphorylated by PKA. Indeed, further experiments showed that the affinity of calstabin-2 for both wild-type and a mutant, constitutively PKA-phosphorylated RyR2 was increased in the presence of JTV519, regardless of phosphorylation status of the channel.

The authors speculate that calcium leakage resulting from calstabin-2-deficient RyR2 alone might not be sufficient to cause SCD, but that when combined with exercise, the resulting phosphorylation of RyR2 and subsequent dissociation of already depleted levels of calstabin-2 is enough to cause a fatal arrhythmia. JTV519 has therefore provided both an insight into this mechanism and a potential therapeutic strategy to prevent SCD.