Calcineurin and its regulator, RCAN1, confer time-of-day changes in susceptibility of the heart to ischemia/reperfusion.

Many important components of the cardiovascular system display circadian rhythmicity. In both humans and mice, cardiac damage from ischemia/reperfusion (I/R) is greatest at the transition from sleep to activity. The causes of this window of susceptibility are not fully understood. In the murine heart we have reported high amplitude circadian oscillations in the expression of the cardioprotective protein regulator of calcineurin 1 (Rcan1). This study was designed to test whether Rcan1 contributes to the circadian rhythm in cardiac protection from I/R damage. Wild type (WT), Rcan1 KO, and Rcan1-Tg mice, with cardiomyocyte-specific overexpression of Rcan1, were subjected to 45min of myocardial ischemia followed by 24h of reperfusion. Surgeries were performed either during the first 2h (AM) or during the last 2h (PM) of the animal's light phase. The area at risk was the same for all genotypes at either time point; however, in WT mice, PM-generated infarcts were 78% larger than AM-generated infarcts. Plasma cardiac troponin I levels were likewise greater in PM-operated animals. In Rcan1 KO mice there was no significant difference between the AM- and PM-operated hearts, which displayed greater indices of damage similar to that of PM-operated WT animals. Mice with cardiomyocyte-specific overexpression of human RCAN1, likewise, showed no time-of-day difference, but had smaller infarcts comparable to those of AM-operated WT mice. In vitro, cardiomyocytes depleted of RCAN1 were more sensitive to simulated I/R and the calcineurin inhibitor, FK506, restored protection. FK506 also conferred protection to PM-infarcted WT animals. Importantly, transcription of core circadian clock genes was not altered in Rcan1 KO hearts. These studies identify the calcineurin/Rcan1-signaling cascade as a potential therapeutic target through which to benefit from innate circadian changes in cardiac protection without disrupting core circadian oscillations that are essential to cardiovascular, metabolic, and mental health.