Alterations of Nuclear Ca2+-Dependent Signalling in Heart Failure

Abstract

A hallmark of heart failure is impaired Ca2+ handling of cardiomyocytes. We previously showed that specific alterations in nuclear Ca2+ handling precede changes in cytoplasmic Ca2+ handling as heart failure progresses. However, a direct link between changes of nucleoplasmic Ca2+ handling and altered excitation-transcription coupling during the heart failure progression was not previously established. We thus characterized changes of nuclear Ca2+ handling and the activation of nuclear Ca2+-dependent transcription factors under low and high pacing frequencies at the early and late stage of hypertrophy in mouse model of pressure overload.Ventricular cardiomyocytes were isolated 1 and 7 weeks after transverse aortic constriction (TAC) in adult wild-type mice. Subcellular [Ca2+] transients were recorded in electrically stimulated CMs loaded with Fluo-4/AM. Phosphorylation levels of CaMKII and nuclear accumulation of HDAC4 were quantified by immunocytostaining.During the early remodelling (i.e. 1 week after TAC intervention) - in contrast to diastolic [Ca2+] in the cytoplasm - diastolic [Ca2+] in the nucleus was already elevated at very low stimulation rate (0.5 Hz) as compared to the non-failing group, and than overproportionally increased with faster stimulation rates. In failing cardiomyocytes (7 weeks after TAC intervention), the changes in nucleoplasmic and cytoplasmic diastolic [Ca2+] were qualitatively comparable, though the increase was more pronounced in the nuclear compartment. High pacing frequency caused significantly higher phosphorylation of CaMKII and corresponding HDAC4 translocation in cardiomyocytes from hypertrophic hearts compared to healthy controls, with the significantly higher increase in CaMKII phosphorylation in the nucleoplasmic compartment as compared to cytoplasm.In conclusion, we found that the increased stimulation frequency led to a higher build-up of diastolic [Ca2+] in cardiomyocytes from hypertrophied hearts, especially in the nucleoplasmic compartment, which may be involved in the dysregulation of Ca2+-dependent gene transcription and progression of adverse cardiac remodeling.