Local Synthesis of Sarcolemma and Sarcoplasmic Reticulum Membrane Proteins in Cardiac Myocytes

Abstract

Classically, cardiac sarcolemma (SL) and sarcoplasmic reticulum (SR) membrane proteins are thought to be synthesized and processed on the rough ER, in the perinuclear region and then transported to locations of employment in the SR and SL. However, this view is largely based on studies in non-myocyte cell types. Therefore, we investigated the localization and regulation of synthesis of several key sarcolemmal SL and SR membrane proteins in adult cardiac myocytes. in contrast to the canonical view, in mouse ventricular myocytes we identified synthesis machinery and translation for such proteins, namely SERCA2a, RyR2, Cav1.2 and Nav1.5 localized not only at the perinuclear area, but spread throughout the entire cell volume, particularly in the vicinity of surface/T-tubule and SR membranes. inhibition of microtubular trafficking by colchicine resulted in redistribution of the mRNA signal for these proteins towards the nuclei, consistent with microtubule trafficking providing a dedicated pool of mRNA for local protein translation and synthesis. Active translation of SERCA2a mRNA throughout the cardiac myocyte space was confirmed through visualization of active translation sites using a novel in-situ hybridization assay, which we developed. Collectively, these results suggest that synthesis of SR and SL proteins in cardiomyocytes occurs locally at sites of utilization from dedicated, local mRNA pools.