The Transgenic Expression of Highly Inhibitory Monomeric Forms of Phospholamban in Mouse Heart Impairs Cardiac Contractility

Transgenic mice were generated with cardiac-specific overexpression of the monomeric, dominant-acting, superinhibitory L37A and I40A mutant forms of phospholamban (PLN), and their phenotypes were compared with wild-type (wt) mice or 2-fold overexpressors of wt PLN (wtOE). The level of PLN monomer in cardiac microsomes was increased 11-13-fold, and the apparent affinity of the sarco(endo)plasmic reticulum Ca(2+)-ATPase for Ca(2+) was decreased from pCa 6.22 in wt or 6.12 in wtOE to 5.81 in L37A and 5.72 in I40A. Basal physiological parameters, measured in isolated myocytes, indicated a significant reduction in the rates of shortening (+dL/dt) and relengthening (-dL/dt). Hemodynamic measurements indicated that peak systolic pressure was unaffected but that pressure changes (+dP/dt and -dP/dt) were lowered significantly in both mutant lines, and relaxation time (tau) was also lengthened significantly. Echocardiography for both mutants showed depressed systolic function and an increase in left ventricular mass of over 1.4-fold. Significant decreases in left ventricular shortening fraction and velocity of circumferential shortening and increases in ejection time were corrected by isoproterenol. The use of antibodies specific against Ser(16)- and Thr(17)-PLN peptides showed that phosphorylation of both pentameric and monomeric PLN were increased between 1.2- and 2.4-fold in both the L37A and I40A lines but not in the wtOE line. These observations show that overexpression of superinhibitory mutant forms of PLN causes depression of contractile parameters with induction of cardiac hypertrophy, as assessed with echocardiography.