The Cardiac-specific Nuclear δB Isoform of Ca2+/Calmodulin-dependent Protein Kinase II Induces Hypertrophy and Dilated Cardiomyopathy Associated with Increased Protein Phosphatase 2A Activity*

The δ isoform of Ca2+/calmodulin-dependent protein kinase II (CaMKII) predominates in the heart. To investigate the role of CaMKII in cardiac function, we made transgenic (TG) mice that express the nuclear δB isoform of CaMKII. The expressed CaMKIIδB transgene was restricted to the myocardium and highly concentrated in the nucleus. Cardiac hypertrophy was evidenced by an increased left ventricle to body weight ratio and up-regulation of embryonic and contractile protein genes including atrial natriuretic factor, β-myosin heavy chain, and α-skeletal actin. Echocardiography revealed ventricular dilation and decreased cardiac function, which was also observed in hemodynamic measurements from CaMKIIδB TG mice. Surprisingly, phosphorylation of phospholamban at both Thr17 and Ser16 was significantly decreased in the basal state as well as upon adrenergic stimulation. This was associated with diminished sarcoplasmic reticulum Ca2+ uptake in vitro and altered relaxation properties in vivo. The activity and expression of protein phosphatase 2A were both found to be increased in CaMKII TG mice, and immunoprecipitation studies indicated that protein phosphatase 2A directly associates with CaMKII. Our findings are the first to demonstrate that CaMKII can induce hypertrophy and dilation in vivo and indicate that compensatory increases in phosphatase activity contribute to the resultant phenotype.