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

Abstract

Aims

Mitochondrial cardiomyopathy is associated with deleterious remodelling of cardiomyocyte Ca²⁺ signalling that is partly due to the suppressed expression of the sarcoplasmic reticulum (SR) Ca²⁺ 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). Ca²⁺ 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 Ca²⁺ signals of the cardiomyocytes that were accompanied by reduction in SR Ca²⁺ content and amplitude and duration of Ca²⁺ 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 Ca²⁺ signals, SR Ca²⁺ content, and Ca²⁺ spark properties to control levels.

Conclusion

Mitochondrial uncoupling results in fast transcriptional changes in CASQ2 expression that manifest as compromised Ca²⁺ 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.

E-C-coupling, Calcium release, ROS, Antioxidant therapy, Mitochondrial cardiomyopathy