α-Synuclein promotes clathrin-mediated NMDA receptor endocytosis and attenuates NMDA-induced dopaminergic cell death

Abnormalities of alpha-synuclein (alpha-syn) and NMDA receptors (NMDARs) are implicated in the pathogenesis of Parkinson's disease. However, how these proteins interact with each other has not been elucidated. Here, the effect of alpha-syn on NMDARs was investigated by examining the alterations of surface NMDAR NR1 subunits in MES23.5 dopaminergic cells transfected with the human alpha-syn gene as well as in cells treated with extracellularly added human alpha-syn. As demonstrated previously that alpha-syn can enter cells in a non-endocytic manner without being degraded by the cellular proteolytic systems, the extracellularly added alpha-syn entered the cytoplasm of MES23.5 cells in a concentration-dependent manner. Both the alpha-syn-transfected cells and alpha-syn-treated cells exhibited increased intracellular alpha-syn levels and reduced surface NR1 without altering the total NR1. The alpha-syn-induced surface NR1 reduction was accompanied by suppression of NMDA-elicited intracellular Ca2+ elevation and reductions of NMDA-induced caspase 3 activation and cell death, which was abolished by hypotonic shock and K+ depletion, a procedure that blocks clathrin-mediated endocytosis, and by suppression of RAB5B expression with anti-RAB5B oligonucleotides. The data obtained provide evidence for the first time that alpha-syn may promote clathrin-mediated NMDAR endocytosis.