Mitochondrial uncoupling downregulates calsequestrin expression and reduces SR Ca2+ stores in cardiomyocytes.

AIMS Mitochondrial cardiomyopathy is associated with deleterious remodelling of cardiomyocyte Ca(2+) signalling that is partly due to the suppressed expression of the sarcoplasmic reticulum (SR) Ca(2+) buffer calsequestrin (CASQ2). This study was aimed at determining whether CASQ2 downregulation is directly caused by impaired mitochondrial function. METHODS AND RESULTS Mitochondrial stress was induced in cultured neonatal rat cardiomyocytes by means of the mitochondrial uncoupler carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone (FCCP). Ca(2+) transients and reactive oxygen species (ROS) were measured by confocal microscopy using the indicators fluo-4 and MitoSOX red, respectively. Mitochondrial stress led to concentration-dependent downregulation of calsequestrin (CASQ2) and changes in the Ca(2+) signals of the cardiomyocytes that were accompanied by reduction in SR Ca(2+) content and amplitude and duration of Ca(2+) sparks. Caspase 3, p38, and p53 inhibitors had no effect on FCCP-induced CASQ2 downregulation; however, it was attenuated by the ROS scavenger N-acetylcysteine (NAC). Importantly, NAC not only decreased FCCP-induced ROS production, but it also restored the Ca(2+) signals, SR Ca(2+) content, and Ca(2+) spark properties to control levels. CONCLUSION Mitochondrial uncoupling results in fast transcriptional changes in CASQ2 expression that manifest as compromised Ca(2+) signalling, and these changes can be prevented by ROS scavengers. As impaired mitochondrial function has been implicated in several cardiac pathologies as well as in normal ageing, the mechanisms described here might be involved in a wide spectrum of cardiac conditions.