The δ _C Isoform of CaMKII Is Activated in Cardiac Hypertrophy and Induces Dilated Cardiomyopathy and Heart Failure

TLDR

Cardiac hypertrophy has been linked to nuclear CaMKIV or CaMKIIδB activity, yet CaMKIV is absent in heart and cardiomyocytes also express a cytoplasmic CaMKIIδC isoform. This study investigates whether the cytoplasmic CaMKIIδC isoform is upregulated after pressure overload and whether its enhanced activity can phosphorylate Ca²⁺‑regulatory proteins to drive hypertrophy. Transgenic mice were engineered to express the CaMKIIδC isoform in cardiomyocytes, enabling in vivo assessment of its phosphorylation targets. The transgene localized to the cytoplasm, and the mice developed dilated cardiomyopathy with markedly reduced fractional shortening, premature death, enlarged myocytes with impaired contractility and altered Ca²⁺ handling, and increased phosphorylation of RyR and phospholamban at CaMKII sites before heart failure, demonstrating that CaMKIIδC activation mediates Ca²⁺‑regulatory protein phosphorylation and contributes to dilated cardiomyopathy and heart failure.

Abstract

Recent studies have demonstrated that transgenic (TG) expression of either Ca 2+ /calmodulin-dependent protein kinase IV (CaMKIV) or CaMKIIδ B , both of which localize to the nucleus, induces cardiac hypertrophy. However, CaMKIV is not present in heart, and cardiomyocytes express not only the nuclear CaMKIIδ B but also a cytoplasmic isoform, CaMKIIδ C . In the present study, we demonstrate that expression of the δ C isoform of CaMKII is selectively increased and its phosphorylation elevated as early as 2 days and continuously for up to 7 days after pressure overload. To determine whether enhanced activity of this cytoplasmic δ C isoform of CaMKII can lead to phosphorylation of Ca 2+ regulatory proteins and induce hypertrophy, we generated TG mice that expressed the δ C isoform of CaMKII. Immunocytochemical staining demonstrated that the expressed transgene is confined to the cytoplasm of cardiomyocytes isolated from these mice. These mice develop a dilated cardiomyopathy with up to a 65% decrease in fractional shortening and die prematurely. Isolated myocytes are enlarged and exhibit reduced contractility and altered Ca 2+ handling. Phosphorylation of the ryanodine receptor (RyR) at a CaMKII site is increased even before development of heart failure, and CaMKII is found associated with the RyR in immunoprecipitates from the CaMKII TG mice. Phosphorylation of phospholamban is also increased specifically at the CaMKII but not at the PKA phosphorylation site. These findings are the first to demonstrate that CaMKIIδ C can mediate phosphorylation of Ca 2+ regulatory proteins in vivo and provide evidence for the involvement of CaMKIIδ C activation in the pathogenesis of dilated cardiomyopathy and heart failure.