Abstract
BACKGROUND: Mitochondrial ATP production, essential for cardiomyocyte function, is regulated by mitochondrial Ca(2+) (mtCa(2+)). The primary route for mtCa(2+) influx is the mitochondrial calcium uniporter complex. The mitochondrial calcium uniporter complex subunit MICU (mitochondrial calcium uptake) 1 limits mtCa(2+) uptake, preventing mtCa(2+) overload. Although elevated mtCa(2+) has been observed in multiple diseases including heart failure, its effects on heart function remain elusive. METHODS: To investigate the impact of elevated mtCa(2+) in adult hearts, we generated a mouse model with cardiomyocyte-specific tamoxifen-inducible Micu1 deletion (Micu1(cKO)). Cardiac function was assessed through echocardiography. Mitochondria, adult cardiomyocytes, and tissue extracts were isolated from the left ventricle (LV) and right ventricle (RV) for comprehensive analysis at multiple time points ranging from 1 to 9 weeks post-tamoxifen injection. RESULTS: Acute MICU1 deficiency resulted in increased mtCa(2+) accompanied by reduced mitochondrial respiration in both the RV and LV. Contractile function, which was diminished in both ventricles initially, remained reduced in the RV upon prolonged MICU1 deficiency. In contrast, the LV exhibited signs of recovery over time, including restored ejection fraction concurrent with normalization of mtCa(2+) levels. This pattern was mirrored in cardiomyocyte contractility. In Micu1(cKO) RV, mtCa(2+) remained elevated, likely contributing to oxidative stress. As a potential mechanism underlying LV-specific recovery, EMRE (essential MCU [mitochondrial calcium uniporter] regulator), an mitochondrial calcium uniporter complex subunit that promotes mtCa(2+) uptake, was found to be downregulated only in the LV. This suggested that the LV initiated a compensatory response to elevated mtCa(2+), while the RV remained impacted. Supporting this, proteomics analysis indicated a divergent proteomic signature in Micu1(cKO) RV. Follow-up experiments suggested enhanced EMRE degradation in Micu1(cKO) LV mediated by m-AAA proteases through a PKA (protein kinase A)-regulated mechanism. In MICU1-deficient neonatal cardiomyocytes, pharmacological PKA inhibition was sufficient to decrease EMRE levels. Analysis of LV tissues from patients with dilated cardiomyopathy suggested that this pathway may be relevant in human DCM. CONCLUSIONS: While elevated mtCa(2+) disrupted cardiac function in both ventricles, it induced an LV-specific adaptive response that suppressed mtCa(2+) intake, contributing to the recovery of mitochondrial and cardiac function. The absence of this pathway in the RV has implications for therapeutics targeting RV dysfunction, a key determinant of mortality in heart failure.