Alterations of Phospholamban Function Can Exhibit Cardiotoxic Effects Independent of Excessive Sarcoplasmic Reticulum Ca ²⁺ -ATPase Inhibition

Background— Low activity of the sarco(endo)plasmic reticulum Ca 2+ -ATPase (SERCA2a) resulting from strong inhibition by phospholamban (PLN) can depress cardiac contractility and lead to dilated cardiomyopathy and heart failure. Here, we investigated whether PLN exhibits cardiotoxic effects via mechanisms other than chronic inhibition of SERCA2a by studying a PLN mutant, PLN R9C , that triggers cardiac failure in humans and mice. Methods and Results— Because PLN R9C inhibits SERCA2a mainly by preventing deactivation of wild-type PLN, SERCA2a activity could be increased stepwise by generating mice that carry a PLN R9C transgene and 2, 1, or 0 endogenous PLN alleles (PLN +/+ +TgPLN R9C , PLN +/− +TgPLN R9C , and PLN −/− +TgPLN R9C , respectively). PLN −/− +TgPLN R9C hearts demonstrated accelerated sarcoplasmic reticulum Ca 2+ uptake rates and improved hemodynamics compared with PLN +/+ +TgPLN R9C mice but still responded poorly to β-adrenergic stimulation because PLN R9C impairs protein kinase A–mediated phosphorylation of both wild-type and mutant PLN. PLN +/+ +TgPLN R9C mice died of heart failure at 21±6 weeks, whereas heterozygous PLN +/− +TgPLN R9C mice survived to 48±11 weeks, PLN −/− +TgPLN R9C mice to 66±19 weeks, and wild-type mice to 94±27 weeks ( P <0.001). Although Ca 2+ reuptake kinetics in young PLN −/− +TgPLN R9C mice exceeded those measured in wild-type control animals, this parameter alone was not sufficient to prevent the eventual development of dilated cardiomyopathy. Conclusions— The data demonstrate an association between the dose-dependent inhibition of SERCA2a activity by PLN wt and the time of onset of heart failure and show that a weak inhibitor of SERCA2a, PLN R9C , which is diminished in its ability to modify the level of SERCA2a activity, leads to heart failure despite fast sarcoplasmic reticulum Ca 2+ reuptake.