Human α4β2 Nicotinic Acetylcholine Receptor as a Novel Target of Oligomeric α-Synuclein

Cigarette smoking is associated with a decreased incidence of Parkinson disease (PD) through unknown mechanisms. Interestingly, a decrease in the numbers of alpha 4 beta 2 nicotinic acetylcholine receptors (alpha 4 beta 2-nAChRs) in PD patients suggests an alpha 4 beta 2-nAChR-mediated cholinergic deficit in PD. Although oligomeric forms of alpha-synuclein have been recognized to be toxic and involved in the pathogenesis of PD, their direct effects on nAChR-mediated cholinergic signaling remains undefined. Here, we report for the first time that oligomeric alpha-synuclein selectively inhibits human alpha 4 beta 2-nAChR-mediated currents in a dose-dependent, non-competitive and use-independent manner. We show that pre-loading cells with guanyl-59-yl thiophosphate fails to prevent this inhibition, suggesting that the alpha-synuclein-induced inhibition of alpha 4 beta 2-nAChR function is not mediated by nAChR internalization. By using a pharmacological approach and cultures expressing transfected human nAChRs, we have shown a clear effect of oligomeric alpha-synuclein on alpha 4 beta 2-nAChRs, but not on alpha 4 beta 4- or alpha 7-nAChRs, suggesting nAChR subunit selectivity of oligomeric alpha-synuclein-induced inhibition. In addition, by combining the size exclusion chromatography and atomic force microscopy (AFM) analyses, we find that only large (>4 nm) oligomeric alpha-synuclein aggregates (but not monomeric, small oligomeric or fibrillar alpha-synuclein aggregates) exhibit the inhibitory effect on human alpha 4 beta 2-nAChRs. Collectively, we have provided direct evidence that alpha 4 beta 2-nAChR is a sensitive target to mediate oligomeric alpha-synuclein-induced modulation of cholinergic signaling, and our data imply that therapeutic strategies targeted toward alpha 4 beta 2-nAChRs may have potential for developing new treatments for PD.