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

Qiang Liu,Jian-Xin Shen,Michael R Sierks,Sharareh Emadi,Jie Wu,Ashley I Bush

doi:10.1371/journal.pone.0055886

Abstract

Cigarette smoking is associated with a decreased incidence of Parkinson disease (PD) through unknown mechanisms. Interestingly, a decrease in the numbers of α4β2 nicotinic acetylcholine receptors (α4β2-nAChRs) in PD patients suggests an α4β2-nAChR-mediated cholinergic deficit in PD. Although oligomeric forms of α-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 α-synuclein selectively inhibits human α4β2-nAChR-mediated currents in a dose-dependent, non-competitive and use-independent manner. We show that pre-loading cells with guanyl-5′-yl thiophosphate fails to prevent this inhibition, suggesting that the α-synuclein-induced inhibition of α4β2-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 α-synuclein on α4β2-nAChRs, but not on α4β4- or α7-nAChRs, suggesting nAChR subunit selectivity of oligomeric α-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 α-synuclein aggregates (but not monomeric, small oligomeric or fibrillar α-synuclein aggregates) exhibit the inhibitory effect on human α4β2-nAChRs. Collectively, we have provided direct evidence that α4β2-nAChR is a sensitive target to mediate oligomeric α-synuclein-induced modulation of cholinergic signaling, and our data imply that therapeutic strategies targeted toward α4β2-nAChRs may have potential for developing new treatments for PD.

Highlights

Parkinson disease (PD) is one of the most common neurodegenerative disorders affecting more than half a million people in the United States, with annual costs estimated at 10 billion dollars [1]
The neuropathological hallmarks of PD are progressive loss of dopaminergic neurons in the substantia nigra pars compacta (SNc) and microscopic proteinaceous inclusions, composed mainly of aggregated fibrillar a-synuclein in neurons and glia [2,3]. aSynuclein, an abundant presynaptic protein in the central nervous system (CNS), consists of a 140 amino-acid sequence that is highly homologous across human, rat and mouse [3]
Distinct oligomeric and fibrillar forms of asynuclein were generated by incubating monomeric a-synuclein for different lengths of time and aggregate morphologies were analyzed by atomic force microscopy (AFM) (Fig. 1A and B)

Summary

Introduction

Parkinson disease (PD) is one of the most common neurodegenerative disorders affecting more than half a million people in the United States, with annual costs estimated at 10 billion dollars [1]. A-Synuclein has been tightly linked to PD [4,5] and other related neurodegenerative disorders such as multiple systems atrophy (MSA), Hallervorden-Spatz disease, neurodegeneration with brain iron accumulation type-1, and Niemann-Pick Type C Disease [6,7]. A-Synuclein is considered a cytosolic protein, and its pathogenic effect was assumed limited to the cytoplasm of single cells [10]. Recent studies have suggested that a-synuclein has extracellular pathogenic effects [11,12,13,14]. Various studies have shown that the extracellular addition of aggregated a-synuclein to culture medium is cytotoxic [15,16,17,18,19,20,21]

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Discussion

Conclusion