Abstract Fri136: Heart-specific Letm1 null mice have altered mitochondrial Ca ²⁺ handling leading to cardiomyopathy and death

Mitochondrial calcium (mCa 2+ ) overload contributes to several diseases, including myocardial infarction, a leading cause of death worldwide. Identifying the key proteins that mediate mCa 2+ transport is essential for understanding Ca 2+ handling mechanisms and developing effective treatments. Acute mCa 2+ uptake is known to be mediated by the MCU complex and Na + -dependent Ca 2+ efflux is understood to be mediated by NCLX, however the molecular identity and physiological role of H + -dependent Ca 2+ efflux remains debated. LETM1 (leucine–zipper–EF hand-containing transmembrane region) is thought to be a mitochondrial Ca 2+ /H + antiporter, but its physiological role in vivo remains uncertain. We assessed the hypothesis that LETM1 is a key effector of mCa 2+ homeostasis in the heart. Here we used a conditional cardiomyocyte-specific Letm1 deletion strategy in mice ( Letm1 -Myh6-MCM). Letm1 fl/fl-Myh6-MCM mice, as well as control mice ( Letm1fl/fl littermate or Myh6-MCM mice) were treated at 4 weeks of age with intraperitoneally administered tamoxifen for three consecutive days. Interestingly, deletion of Letm1 using this strategy results in lethality at ~12 weeks of age (8 weeks post tamoxifen). We observed that Letm1 fl/fl-Mhy6-MCM mice had normal cardiac function at 8 weeks of age but progressed to heart failure by 12 weeks of age as measured by echocardiography. Isolated cardiac mitochondria showed mCa 2+ overload and decreased oxygen consumption rate. Cardiac mitochondria were also swollen and showed aberrant cristae and overall architecture assessed by EM microscopy. Moreover, Letm1 fl/fl-Mhy6-MCM hearts at 12 weeks of age exhibited increased levels of cardiomyocyte cell, evaluated by in vivo cardiomyocyte uptake of Evan’s blue dye. Cardiac mitochondria from these mice showed reduced total mCa 2+ efflux as well as decreased Na + -independent Ca 2+ efflux, suggesting a reduction in H + /Ca 2+ exchange in Letm1 fl/fl-Mhy6-MCM mitochondria. In conclusion, LETM1 appears to be a mCa 2+ efflux pathway under physiologic conditions, which is critical for mitochondrial function and cardiac survival.