Alterations of Phospholamban Function Can Exhibit Cardiotoxic Effects Independent of Excessive Sarcoplasmic Reticulum Ca2+-ATPase Inhibition

Background-Low activity of the sarco(endo) plasmic reticulum Ca2+-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, PLNR9C, that triggers cardiac failure in humans and mice. Methods and Results-Because PLNR9C inhibits SERCA2a mainly by preventing deactivation of wild-type PLN, SERCA2a activity could be increased stepwise by generating mice that carry a PLNR9C 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 Ca2+ uptake rates and improved hemodynamics compared with PLN+/+ + TgPLN(R9C) mice but still responded poorly to beta-adrenergic stimulation because PLNR9C impairs protein kinase A-mediated phosphorylation of both wild-type and mutant PLN. PLN+/+ + TgPLNR9C 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 Ca2+ 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 PLNwt and the time of onset of heart failure and show that a weak inhibitor of SERCA2a, PLNR9C, which is diminished in its ability to modify the level of SERCA2a activity, leads to heart failure despite fast sarcoplasmic reticulum Ca2+ reuptake. (Circulation. 2009; 119: 436-444.)