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

J. Neurochem. (2011) 119, 815–825. Abnormalities of α‐synuclein (α‐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 α‐syn on NMDARs was investigated by examining the alterations of surface NMDAR NR1 subunits in MES23.5 dopaminergic cells transfected with the human α‐syn gene as well as in cells treated with extracellularly added human α‐syn. As demonstrated previously that α‐syn can enter cells in a non‐endocytic manner without being degraded by the cellular proteolytic systems, the extracellularly added α‐syn entered the cytoplasm of MES23.5 cells in a concentration‐dependent manner. Both the α‐syn‐transfected cells and α‐syn‐treated cells exhibited increased intracellular α‐syn levels and reduced surface NR1 without altering the total NR1. The α‐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 α‐syn may promote clathrin‐mediated NMDAR endocytosis.