How to Mend a Broken Heart: Massive Endocytosis and the Role of Lipidic Forces in Membrane Trafficking
Fine, Michael Jon
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Novel forms of membrane internalization defined as massive endocytosis (MEND) were characterized for mechanism and physiological significance in isolated cells and intact cardiac tissue. These non-canonical forms of membrane trafficking are related to perturbations within the outer lipid monolayer of the plasmalemma or through coupled calcium-mediated fusion and phosphatidylinositol 4,5-bisphosphate (PIP2) signaling. Both forms of MEND do not rely on classical endocytic proteins such as clathrin or dynamin and appear independent of the cytoskeletal or extracellular matrix. A hypothesis emerged implicating lipidic driven processes as the mechanistic basis of MEND. One explanation involves lipid domain formation with excessive inward curvature from rapid accumulation of PIP2 within the inner monolayer subsequent to large calcium transients. Alternatively, perturbation of the outer monolayer through amphipathic detergents or modulation of lipid content also promotes inward curvature of the membrane leading to massive endocytosis. Electrophysiological and optical methods support MEND preferentially internalizing membrane of liquid-ordered (Lo) domains leading to the potential selection of certain proteins and markers due to physiological segregation into their respective energetically favorable domains. MEND demonstrates that lipidic reorganization of the membrane may be sufficient for rapid and selective internalization of the plasma membrane. The implications of such a novel form of endocytosis could dramatically impact numerous physiological and cellular activities. In cardiac pathologies, short-term changes in the surface membrane expression of vital ionic transporters have been implicated in hypertrophy, atrial defibrillation, and damage post myocardial ischemia. It is possible that the properties underlying MEND mechanics may be responsible for some of these acute cardiac changes. Protocols that induce massive endocytosis were performed on isolated cardiac myocytes with similar results described in the fibroblast cell lines. MEND occurs in cardiomyocytes with calcium transients. MEND also occurs upon isolation of myocytes without stimulation. While this indicates that MEND does occur in primary cells, protocols to mimic MEND in intact cardiac tissue have remained inconclusive. Cellular processes during the isolation procedures cause dramatic changes in the membrane of cardiac myocytes. Subsequently, it is likely isolated cardiomyocytes have distinct differences from unstressed intact tissue. MEND may still occur in cardiac tissue as well as other physiological systems and newer protocols to monitor membrane movements in intact tissues are currently being investigated.