Abstract 356: CaMKII Cytoplasmic and Nuclear Isoforms Differentially Affect Calcium Handling but Similarly Regulate HDAC/MEF2 Transcriptional Responses
The δ isoforms of Ca/calmodulin-dependent protein kinase II (CaMKII) predominate in the heart and both nuclear δB and cytoplasmic δC splice variants of CaMKII are present in cardiomyocytes. We hypothesized that nuclear and cytoplasmic CaMKIIδ isoforms regulate separate signaling pathways, with δB involved in nuclear transcriptional regulation and δC in cytosolic Ca handling. Surprisingly we found that either CaMKIIδB or δC could activate MEF2-luciferase gene expression in neonatal rat ventricular myocytes (NRVMs). To determine whether this occurred in vivo, CaMKIIδB or δC transgenic (TG) mice were crossed with MEF2/3-galactosidase indicator mice. Increased MEF2 activation, assessed by β-gal activity, was elicited by both CaMKII isoforms. Furthermore either CaMKIIδB or δC, expressed in NRVMs, increased histone deacetylase 4 (HDAC4) association with 14 −3−3 and induced HDAC4 translocation from nucleus to cytoplasm, suggesting that both CaMKIIδ isoforms regulate HDAC4 phosphorylation. Finally, HDAC4 kinase activity was markedly increased in cardiac homogenates from either CaMKIIδB or δC TG mice. In contrast to the equivalence of CaMKIIδB and CaMKIIδC in HDAC and MEF2 regulation, only CaMKIIδC directly altered Ca handling. In hearts from CaMKIIδC (but not CaMKIIδB) TG mice there was increased phosphorylation of ryanodine receptor (RyR) and phospholamban (PLB) at known CaMKII sites, enhanced sarcoplasmic reticulum (SR) Ca spark frequency and decreased SR Ca content. The finding that the cytoplasmic and nuclear CaMKIIδ isoforms have distinct effects on Ca handling proteins and Ca regulation, but similar effects on HDAC mediated MEF2 gene expression, is consistent with our observation that TG expression of the nuclear CaMKIIδB induces cardiac hypertrophy while TG expression of the cytoplasmic CaMKIIδC leads to decompensated hypertrophy, Ca dysregulation and severe heart failure. Differential patterns of isoform activation in vivo may thus have distinct roles in the pathogenesis of cardiac hypertrophy and heart failure.