Mitochondrial Ca uptake influences energy production, cell survival, and Ca signaling. The mitochondrial calcium uniporter, MCU, is the primary route for uptake of Ca into the mitochondrial matrix. We have generated a zebrafish mutant that survives to adulthood and exhibits dramatic cardiac phenotypes resembling cardiomyopathy and sinus arrest. hearts contract weakly and have a smaller ventricle with a thin compact layer and reduced trabecular density. Damaged myofibrils and swollen mitochondria were present in the ventricles of mutants, along with gene expression changes indicative of cell stress and altered cardiac structure and function. Using electrocardiography, we found that hearts display conduction system defects and abnormal rhythm, with extended pauses resembling episodes of sinus arrest. Together, our findings suggest that proper mitochondrial Ca homeostasis is crucial for maintaining a healthy adult heart, and establish the mutant as a useful model for understanding the role of mitochondrial Ca handling in adult cardiac biology.
