On the mechanism of internalization of α-synuclein into microglia: Roles of ganglioside GM1 and lipid raft

α-Synuclein (α-syn) has been known to be a key player of the pathogenesis of Parkinson's disease and has recently been detected in extracellular biological fluids and shown to be rapidly secreted from cells. The penetration of α-syn into cells has also been observed. In this study, we observed that dl-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol, a glucosyltransferase inhibitor, and proteinase K inhibited the internalization of extracellular monomeric α-syn into BV-2 cells, and the addition of monosialoganglioside GM1 ameliorated the inhibition of α-syn internalization in dl-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol- treated BV-2 cells. Furthermore, inhibition of clathrin-, caveolae-, and dynamin-dependent endocytosis did not prevent the internalization of α-syn, but disruption of lipid raft inhibited it. Inhibition of macropinocytosis and disruption of actin and microtubule structures also did not inhibit the internalization of α-syn. In addition, we further confirmed these observations by co-culture system of BV-2 cells and α-syn-over- expressing SH-SY5Y cells. These findings suggest that extracellular α-syn is internalized into microglia via GM1 as well as hitherto-unknown protein receptors in clathrin-, caveolae-, and dynamin-independent, but lipid raft-dependent manner. Elucidation of the mechanism involved in internalization of α-syn should be greatly helpful in the development of new treatments of α-syn-related neurodegenerative diseases.