MG56, a Member of the MBOAT Family of Proteins, Regulates Intracellular Calcium Signaling in Striated Muscle

Abstract

Calcium-induced-calcium-release (CICR) from the sarcoplasmic reticulum (SR) plays an integral role in excitation-contraction coupling - the driving mechanism behind synchronous cardiac muscle contractions. Alterations in CICR are commonly found in individuals suffering from cardiac arrhythmias, and closely associated with ventricular fibrillation, tachycardia, and sudden cardiac death. The ryanodine receptor 2 (RyR2) is a key mediator of CICR, functioning as a calcium channel along the SR. Elucidating the role of proteins that modulate RyR2-mediated CICR represents a major interest in cardiovascular research. We recently discovered a novel SR-resident membrane protein named mitsugumin 56 (MG56), which belongs to the membrane-bound o-acyltransferase (MBOAT) family of proteins. Knockout of MG56 produced a postnatal lethal phenotype. We have observed elevated Ca spark activity in MG56 null muscle fibers when compared with the wild type littermates. Using HEK293 cells with inducible RYR2 expression, we found that over-expression of MG56 led to suppression of RyR2-mediated store-overload induced calcium release (SOICR) from the endoplasmic reticulum (ER). The findings suggest the possibility that MG56 functions as a stabilizer of RyR channel function. Since MBOAT family proteins contain acyltransferase activity, we hypothesize that the putative acyltransferase function for MG56 may be involved in the Ca regulatory function. In preliminary studies, we found that pretreatment of muscle fibers with 2-bromohexanoic acid, an inhibitor of palmitoyl acyltransferases, led to suppression of Ca spark activity. The effect of 2- bromohexanoic acid was also observed in HEK293 cells expressing RyR2, where SOICR activity was reduced upon treatment. Future studies are required to dissect the functional interaction between MG56 and RyR2, and to delineate the potential role for MG56-mediated lipid modification of RyR2 in the overall Ca signaling in cardiac muscle.