Abstract 281: The Calcineurin/Rcan1 Axis Influences Mitochondrial Dynamics, Metabolism, and Biogenesis

Abstract

Rationale and Objective: The Regulator of Calcineurin 1 (RCAN1) inhibits calcineurin (CN), a Ca 2+ -activated protein phosphatase important in cardiac remodeling. The hearts and brains of Rcan1 KO mice are more susceptible to damage from ischemia/reperfusion (I/R). As mitochondria are known to be key mediators of I/R damage, the goal of our studies was to determine the impact of the CN/RCAN1 signaling axis on the mitochondrial network and function. Methods and Results: Using both neonatal and isolated adult cardiomyocytes, we found that, the mitochondrial network was more fragmented in the absence of RCAN1, due to increased CN-dependent activation of the fission protein DRP1. RCAN1-deficient cardiomyocytes had reduced mitochondrial membrane potential, consumed less O 2 , and generated lower levels of ROS. Importantly, they also displayed a reduced capacity for mitochondrial Ca 2+ uptake. RCAN1-depleted cardiomyocytes were more sensitive to I/R, however, pharmacological inhibition of CN, DRP1, or calpains (Ca 2+ -activated proteases) restored protection, suggesting that, in the absence of RCAN1, calpain-mediated damage following I/R is greater because of a decrease in the ability of mitochondria to buffer cytoplasmic Ca 2+ . Over-expression of RCAN1 was sufficient to enhance fusion. In addition to its impact on mitochondrial dynamics, CN activation also promoted mitochondrial biogenesis. We demonstrate that expression of the 1b isoform of PGC1α (a master regulator of mitochondrial biogenesis) is directly under CN control . In vivo, a cardiomyocyte-specific CN transgene caused a more rapid increase in: transcript levels for P gc 1α-1b and PolG (the mitochondrial DNA polymerase), mtDNA content, and the abundance of electron transport complex proteins. Despite the increase in mitochondrial content, changes in metabolic enzymes were consistent with a shift toward glycolysis rather than fatty acid oxidation. Conclusions: The CN/RCAN1 signaling axis helps determine both the number of mitochondria (DRP1-dependent fission) and the total mitochondrial mass ( Pgc1α -dependent biogenesis). Maintaining the proper level of these two processes is essential to cardiovascular health and metabolic flexibility.