Phosphoinositide roles in muscle membrane compartmentalization and remodeling. Ines Ribeiro, Amy Kiger. Division of Biological Sciences, University of California, San Diego, La Jolla, CA.

   Muscle cells, called myofibers, must maintain a highly compartmentalized plasma membrane, or sarcolemma, with cell remodeling during development and with use. The sarcolemma contains spatially restricted integrin adhesions that maintain myofiber attachments and sites of union with the transverse tubule (T-tubule) membranes, an internal membrane network continuous with the sarcolemma critical for excitation-contraction coupling. Although membrane trafficking is an important mechanism for the maintenance and remodeling of plasma membrane organization in many cell types, little is known about the mechanisms that regulate sarcolemmal compartmentalization in muscle. Phosphoinositide lipids, under the control of dedicated phosphatases and kinases, convey transient identity to trafficking membrane compartments. We found that both integrin adhesions and t-tubules are remodeled upon myofibril turnover in abdominal persistent larval muscles during metamorphosis and that this remodeling requires myotubularin (mtm) phosphoinositide phosphatase. We determined that Class II PI3-kinase, Pi3K68D, and Mtm co-regulate PI(3)P involved in integrin redelivery from endosomal-related compartments to the sarcolemma necessary for myofiber attachments, but are independently required for T-tubule remodeling. Interestingly, Pi3K68D from pupal thorax lysates co-immunoprecipitated Discs large (Dlg), a membrane associated guanylate kinase scaffold protein localized at T-tubules and neuromuscular junctions, suggesting involvement in a common pathway. Moreover, the localization and function of Rab6 GTPase, implicated in unconventional secretion in yeast, reveals an intricate trafficking pathway underlying sarcolemma membrane organization. Our results indicate that regulation of distinct phosphoinositide pools plays a central role in maintaining cell compartmentalization during muscle remodeling, with likely roles for integrin, Dlg and Class II PI3-kinase in MTM cellular pathways.