Phospholamban Ablation Rescues SR Ca2+ Loading But Not Cardiac Function In CaMKIIλC Transgenic Mice

Abstract

Ca2+/calmodulin dependent kinase II (CaMKII) is a key candidate for controlling phosphorylation and function of Ca2+ regulatory proteins of the sarcoplasmic reticulum (SR), including the ryanodine receptors (RyR2) and phospholamban (PLB). The predominant form of CaMKII in the heart is the λ isoform. Previous work from our laboratory has shown that transgenic (TG) mice overexpressing CaMKIIλC in the heart develop dilated cardiomyopathy due to increased phosphorylation of the RyR2 and enhanced SR Ca2+ leak. The associated lowering of SR Ca2+ load likely contributes to decreased ventricular contractility. Since PLB limits SR Ca2+ uptake, we hypothesized that PLB ablation would recover SR Ca2+ load and rescue the heart failure phenotype. Accordingly, we crossed PLB −/ − mice with CaMKIIλ TG and found surprisingly, that the phenotype was exaggerated in the double mutant (DM) mice. DM showed severe dilated cardiomyopathy and contractile failure despite Ca2+ load repletion, and normalized twitch Ca2+ transients relative to TG. There was no significant change in expression levels of calsequestrin, SERCA, and Na+/Ca2+exchanger, major Ca2+ handling proteins. RyR2 phosphorylation at known CaMKII sites (Ser2815 and 2809) was not greater in DM vs TG mice. However, SR Ca2+ sparks were increased, presumably due to the normalized Ca2+ load. Elevated Ca2+ and CaMKII have been implicated in cardiomyocyte apoptosis and preliminary studies suggest an increase in TUNEL positive cells in DM. We are currently testing the hypothesis that increased apoptosis contributes to the more severe phenotype in DM vs TG.