The role of transbilayer asymmetry of phospholipids in exocytosis

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In this study, we investigated the role of membrane components other than membrane protein such as phospholipids and cholesterol in exocytotic release.1.The role of transbilayer asymmetric distribution of phospholipids in exocytosisIn order to disrupt the transbilayer asymmetry of phospholipids, lipid scramblase, which catalyze hi-directional movement of phospholipids between two membrane layers, was expressed in mast cells or PC12 cells. We found that exocytosis was significantly inhibited in these cells. This result suggests that transbilayer asymmetry of phospholipids plays an important role in exocytotic release.2.The role of cholesterol in the plasma membrane in exocytotic processThe effects of cholesterol depletion from the plasma membrane with methyl-β-cyclodextrin (MβCD) on exocytotic processes were investigated in mast cells. Pretreatment with MβCD inhibited antigen-evoked exocytosis. The inhibition of exocytosis by MβCD was observed even under stimulation with phorbol ester a … More nd calcium ionophore. Therefore, MβCD affected membrane fusion between granule and the plasma membrane. Intracellular Ca^<2+> measurements revealed that MβCD inhibited Ca^<2+> influx from the extracellular medium through the store-operated calcium channel (SOC) without affecting Ca^<2+> from Ca^<2+> store. These results suggest that cholesterol depletion inhibits degranulation mainly by impairing SOC and exocytotic membrane fusion between granules and the plasma membrane.3.Raft protein flotillin regulates IgE receptor mediated signalingWe investigated the role of flotillin, which is localized in lipid rafts, in the early exocytotic process. We obtained flotillin-1 knockdown RBL-2H3 cells. In these cells, we observed a significant decrease in Ca^<2+> tyrosine phosphorylation of IgE-R, and exocytosis. We also found that flotillin is associated with Lyn in lipid rafts. Antigen stimulation causes augmentation of flotillin binding to Lyn, resulting in increased activity of Lyn. These results suggest that flotillin-1 is an essential molecule in IgE-R mediated activation, regulating the kinase activity of Lyn. Less