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 Ca 2+ 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 Ca 2+ 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. (Circ J 2010; 74: 120 - 128)

Biochemical and Physiological Regulation of Cardiac Myocyte Contraction by Cardiac-Specific Myosin Light Chain Kinase

“…41 In contrast, transgenic mice overexpressing MYPT2 specifically in cardiac myocytes demonstrated enhanced expression of MYPT2 with a concomitant increase in the level of endogenous PP1c-δ, which resulted in a reduction of the level of in vivo MLC2v phosphorylation in the heart associated with a decrease in the myofilament response to Ca 2+ and a decreased left ventricular contractility. 61 These findings are consistent with the evidence obtained from TG-MLC2v(P-) mice and cMLCK-knockout mice as mentioned earlier.…”

Cardiac Myosin Light Chain Kinase Is Necessary for Myosin Regulatory Light Chain Phosphorylation and Cardiac Performance in Vivo

“…Transcriptional levels of brain natriuretic peptide, but not other hypertrophy markers, were increased in hearts from cMLCK neo/neo mice. The extent of change was modest but consistent with re-activation of a fetal gene program associated with pathological remodeling (36,37), including changes associated with decreased MLC2v phosphorylation with overexpression of MYPT2 (38).…”

Signaling to Myosin Regulatory Light Chain in Sarcomeres

“…Adenovirus-mediated gene transfer of MYPT2 and PP1c␦ into neonatal cardiac myocytes reduced RLC phosphorylation following stimulation with A23187 and blocked the angiotensin II-induced sarcomere organization in cultured cardiomyocytes (47). Transgenic mice overexpressing MYPT2 showed an increase in PP1c␦ forming the holoenzyme and decreased basal RLC phosphorylation and cardiac contractility (48).…”

“…Transgenic mice overexpressing MYPT2 showed a concomitant increase in endogenous PP1c␦, resulting in an increase in the myosin phosphatase holoenzyme (48). Ventricular RLC phosphorylation was reduced, resulting in left ventricular enlargement with associated impairment of contractile performance.…”