Abstract 3442: Normal Circadian Rhythms In Calcineurin-dependent Signaling And Protein Phosphorylation Are Disrupted In Heart Failure

Abstract

Many important cardiovascular factors, including metabolism, heart rate, blood pressure, and hormone release, oscillate over a 24-hour period. In humans, the incidents of adverse cardiac events, such as myocardial infarction, and ventricular tachycardia vary according to the time of the day. Despite overwhelming evidence of the importance of circadian rhythms in cardiovascular health and disease, little is known regarding the circadian regulation of intracellular signaling pathways involved in normal cardiac function and heart failure. Here we show that there are large circadian oscillations in calcineurin-dependent activities in normal, healthy mouse hearts. This finding is remarkable because activation of the protein phosphatase calcineurin has primarily been thought of as a pathological process driving cardiac hypertrophy and failure. In the hearts of wild type C57BL6 mice there was a 17-fold change in mRNA levels for the calcineurin/NFAT target gene RCAN1.4 that peaked in the early morning at the beginning of the animal’s sedentary phase. Nuclear occupancy of calcineurin-regulated transcription factor NFAT coincided with the changes in mRNA suggesting that circadian changes in calcineurin activity underlie these oscillations. Phosphorylation of the protein phosphatase 1 inhibitor 1 (I-1), a direct calcineurin substrate, and phospholamban (PLB), an indirect target, oscillated directly out of phase with calcineurin-dependent signaling. We propose that temporal separation of peak kinase and phosphatase activities form interdependent feedback loops helping coordinate cardiac function. In pressure overload-induced failing hearts, oscillations in calcineurin-dependent transcription persisted, however both the peak and trough activities were markedly elevated compared to control hearts. In contrast, oscillations in PLB and I-1 phosphorylation were largely lost in failing hearts where phosphorylation of I-1 could no longer be detected and PLB phosphorylation remained elevated throughout. Together, our results suggest that the normal relationships between circadian oscillations in protein phosphorylation and dephosphorylation are disrupted in heart failure. This research has received full or partial funding support from the American Heart Association, AHA South Central Affiliate (Arkansas, New Mexico, Oklahoma & Texas).