Abnormal Ca2+ handling in cardiac muscle cells is associated with a wide range of human cardiac diseases, including heart failure and cardiac arrhythmias. Zebrafish tremblor (tre) mutant embryos manifest unsynchronized cardiac contractions due to a Ca2+ extrusion defect in cardiomyocytes and thus are used as an animal model for aberrant Ca2+ homeostasis-induced cardiac arrhythmia. To further dissect molecular mechanisms regulating cardiac Ca2+ homeostasis, we conducted a chemical suppressor screen on tre and found that efsevin, a synthetic compound, potently suppresses cardiac fibrillation and restores rhythmic cardiac contractions in tre embryos. In addition, the treatment with efsevin blocks the propagation of arrhythmogenic Ca2+ waves and accelerates the decay phase of Ca2+ sparks in adult murine cardiomyocytes under Ca2+ overload conditions, demonstrating that efsevin modulates Ca2+ handling in both embryonic and adult cardiac tissues. Through a biochemical pulldown assay, we identified a direct interaction between efsevin and VDAC2, a mitochondrial outer membrane voltage dependent anion channel. Overexpression of VDAC2 restores synchronized cardiac contraction in tre and knocking down VDAC2 activity abolishes the rescue effect of efsevin on tre, suggesting that efsevin modulates cardiac Ca2+ homeostasis by potentiating VDAC2 activity. We further showed that enhancing mitochondria Ca2+ uptake by overexpressing MICU or MCU suppresses cardiac fibrillation in tre just like VDAC2 does. Interestingly, this suppressive effect is absent in tre/vdac2 double deficient embryos and co-expression of VDAC2 and MICU or MCU results in synergistic rescue effect on tre, indicating a critical role for mitochondria in regulating cardiac Ca2+ handling and rhythmicity and suggesting that VDAC2 functions as a gate for transporting Ca2+ across the outer membrane. Taken together, our findings identify efsevin as a potent pharmacological tool to modulate cardiac Ca2+ handling, suggest a critical role of mitochondria in the control of cardiac rhythmicity and establish VDAC2 as a modulator of cardiac Ca2+ handling and a potential therapeutic target for the treatment of arrhythmias.
In search of the sources of cardiac fibrillation