Rafael Mejia-Alvarez, M.D., Ph.D.

IP₃R Type 2 Dysfunction and Its Pro-arrhythmic Actions

We recently reported that a single-point mutation in the type 1 inositol 1,4,5-trisphosphate receptor (IP₃R1), which is predominantly expressed in cardiac Purkinje fibers, produces a gain-of-function phenotype through an abnormally increased sensitivity to luminal Ca²⁺ regulation (Tambeaux et al., 2023). Furthermore, a transgenic mouse model harboring this gain-of-function mutation exhibits arrhythmogenic activity characterized by spontaneous Ca²⁺ release events associated with delayed afterdepolarizations.

Interestingly, our group has recently identified a similar mutation in the type 2 IP₃ receptor (IP₃R2), which is predominantly expressed in ventricular myocytes, in patients who survived idiopathic ventricular fibrillation (IVF) and have a family history of arrhythmias and sudden cardiac death (SCD). Our preliminary studies indicate that this mutation significantly increases both the basal activity of IP₃R2 and the magnitude of evoked Ca²⁺ release.

These observations have led us to hypothesize that gain-of-function mutations in either IP₃R1 or IP₃R2 increase the risk of IVF, and consequently SCD, by disrupting local diastolic Ca²⁺ homeostasis near sarcoplasmic reticulum (SR) Ca²⁺ release sites in Purkinje cells and ventricular myocytes, respectively.

Impact of Sarcoplasmic Reticulum Ca²⁺ Release on Action Potential Morphology in the Mouse Heart

The objective of this project is to investigate, in intact beating hearts, the cardioprotective effects of imperacalcin (ImpCa), a peptide isolated from scorpion venom that has been proposed to prevent or suppress episodes of catecholaminergic polymorphic ventricular tachycardia (CPVT).

Recent studies have shown that, in a CPVT-prone transgenic mouse expressing a loss-of-function RyR2 mutation (RyR2-A4860G⁺/⁻), ImpCa specifically binds to ryanodine receptors and effectively terminates CPVT episodes. We hypothesize that ImpCa increases Ca²⁺ leak through RyR channels, thereby preventing or reversing SR Ca²⁺ overload. This reduction in SR Ca²⁺ load is expected to limit Na⁺/Ca²⁺ exchanger (NCX) overactivation and consequently reduce the occurrence of delayed afterdepolarizations that trigger CPVT episodes.

To test this hypothesis, intracellular Ca²⁺ transients are measured in intact beating hearts using pulsed local-field fluorescence microscopy (PLFFM), while action potentials are simultaneously recorded using intracellular microelectrode techniques.