Overexpression of Myosin Phosphatase Reduces Ca²⁺ Sensitivity of Contraction and Impairs Cardiac Function

Abstract

Background: Phosphorylation of the regulatory light chain of myosin (MLC) has roles in cardiac function. In vitro, myosin phosphatase target subunit 2 (MYPT2) is a strongly suspected regulatory subunit of cardiac myosin phosphatase (MP), but there is no in-vivo evidence regarding the functions of MYPT2 in the heart. Methods and Results: Transgenic mice (Tg) overexpressing MYPT2 were generated using the α-MHC promoter. Tg hearts showed an increased expression of MYPT2 and concomitant increase of the endogenous catalytic subunit of type 1 phosphatase (PP1cδ), resulting in an increase of the MP holoenzyme. The level of phosphorylation of ventricular MLC was reduced. The pCa-tension relationship, using β-escin permeabilized fibers, revealed decreased Ca²⁺ sensitization of contraction in the Tg heart. LV enlargement with associated impairment of function was observed in the Tg heart and ultrastructural examination showed cardiomyocyte degeneration. Conclusions: Overexpression of MYPT2 and the increase in PP1cδ resulted in an increase of the MP holoenzyme and a decrease in the level of MLC phosphorylation. The latter induced Ca²⁺ desensitization of contraction and decreased LV contractility, resulting in LV enlargement. Thus, MYPT2 is truly the regulatory subunit of cardiac MP in-vivo and plays a significant role in modulating cardiac function.