Structural Basis of Alpha Synuclein Assembly Toxicity Inhibition by Human Serum Albumin

Abstract

The generation of oligomeric intermediates arising during the aggregation of alpha synuclein (αS) is a central feature of Lewy body diseases such as Parkinson's disease (PD). Recent findings suggest that trace amounts of αS are released into the extracellular environment by unconventional exocytosis from neurons, and that these extracellular species contribute to the pathology observed in PD. Extracellular chaperones, such as Human Serum Albumin (HSA), have been shown to inhibit the self-association of αS. However, the mechanism underlying αS aggregation inhibition by the endogenous fatty acid (FA)-transporter HSA remains poorly understood. By combining solution NMR with biolayer interferometry, dynamic light scattering, electron microscopy, wide-angle X-ray diffraction, size exclusion chromatography coupled with multiple angle light scattering, fluorescence spectroscopy and cell viability assays, we provide an unprecedented mechanism of αS assembly (αSn) toxicity inhibition by HSA. Here we show that HSA binds αSn­ with sub µM affinity through hydrophobic interactions that are independent of FA binding to the two high-affinity sites in HSA. HSA-binding of αSn shifts the populations of low molecular weight (MW) oligomers and high MW fibrils into “worm-like” intermediates with reduced toxicity and, concurrently, perturbs the conversion of NMR-visible monomers into NMR-invisible αSn. Notably, we show that HSA inhibits the interaction of αSn with membranes and that the effects of HSA and membrane addition to αSn are non-additive, arising possibly from the direct interactions of HSA with membranes. These results reveal that extracellular chaperoneshelp maintain protein homeostasisnot only by assisting the folding and assembly of proteins,but unexpectedly, also suppressing aberrant interactions with membranes, which promote the formation of toxic intermediates and enhance neuronal dysfunction.