Abstract 11016: Bag3 Regulates Contractility and Ca Homeostasis in Adult Mouse Ventricular Myocytes

Abstract

Bcl2-associated athanogene 3 (BAG3) is a 575 amino acid anti-apoptotic protein that is constitutively expressed in the heart. BAG3 mutations are associated with familial dilated cardiomyopathy and reduced BAG3 expression was observed in murine transverse aortic constriction and porcine myocardial infarction models. BAG3 is expressed in the sarcolemma and t-tubules in adult myocytes. To simulate reduced BAG3 expression in disease states, we downregulated BAG3 in adult myocytes by shRNA (shBAG3). Despite ~50% reduction in BAG3 by shRNA, expression of α-subunit of L-type Ca channel, SR Ca-ATPase, Na/Ca exchanger, α1- and α2-subunits of Na-K-ATPase, and ryanodine receptor was unchanged. At baseline, BAG3 downregulation had no effect on myocyte contraction and [Ca]i dynamics. After isoproterenol, BAG3 downregulation resulted in reduced myocyte contraction amplitudes, shortening and re-lengthening velocities, lower systolic [Ca]i and [Ca]i transient amplitudes, and prolongation in both APD50 and APD90. L-type Ca current (ICa) and sarcoplasmic reticulum (SR) Ca content but not Na/Ca exchange current or SR Ca uptake were reduced in isoproterenol-treated shBAG3 myocytes. Forskolin or dibutyrl cAMP restored ICa amplitude in shBAG3 myocytes to that observed in WT myocytes. On the other hand, BAG3 overexpression resulted in enhanced myocyte contractility after isoproterenol. In addition, BAG3 overexpression rescued the depressed contraction amplitudes in myocytes overexpressing the HIV protein Tat, and improved myocardial contractility in Tg26 (HIV-1 transgenic) mice. We conclude: (1) BAG3 modulates myocyte contraction, Ca dynamics and action potential duration by regulating ion channels and provides a novel paradigm for contractile dysfunction and increased arrhythmia risks in familial dilated cardiomyopathy; (2) BAG3 is a therapeutic target for cardiomyopathy including HIV-associated cardiomyopathy.