Abstract 33: Contractile Dysfunction In The Mouse Heart Caused By Phospholipase C beta1b Mediated Activation Of Protein Kinase Calpha

Abstract

The activity of the early signaling enzyme, phospholipase Cβ1b (PLCβ1b), is elevated in diseased myocardium and activity increases with disease progression. PLCβ1b and the alternative splice variant, PLCβ1a, were expressed in mouse hearts using adeno-associated viral constructs (rAAV6-FLAG-PLCβ1b, rAAV6-FLAG- PLCβ1a) delivered intravenously. Functional responses were assessed in vivo and confirmatory mechanistic studies were conducted in neonatal rat ventricular myocytes (NRVM). FLAG-PLCβ1b was expressed in all of the chambers of the mouse heart, but was highest in left ventricle, where expression was observed in >90% of the cells and was localized to the sarcolemma and T-tubules. Heightened PLCβ1b expression caused a rapid loss of contractility and down-regulation of Phospholamban expression. The loss of contractility induced by PLCβ1b was reversed by inhibition of protein kinase Cα (PKCα). PLCβ1a did not affect contractile function or phospholamban expression. Mechanistic analysis performed in neonatal rat cardiomyocytes confirmed PLCβ1b increased the membrane association of PKCα as well as downstream dephosphorylation of phospholamban and depletion of the Ca2+ stores of the sarcoplasmic reticulum, both of which were mediated by PKCα. Trans-aortic constriction (TAC) resulted in progressive hypertrophy together with reduced contractility in PLCβ1a expressing mice. In PLCβ1b-expressing mice, TAC induced a similar hypertrophic response, but did not cause further contractile depression above that due to PLCβ1b expression alone, suggesting that PLCβ1b is responsible for lowering contractility in response to pressure overload. We conclude that heightened PLCβ1b activity observed in diseased myocardium contributes to pathology by PKCα-mediated contractile dysfunction. PLCβ1b is a cardiac-specific signaling system, and thus provides an ideal therapeutic target for the development of well-tolerated inotropic agents for use in failing myocardium.