Cerebrospinal fluid lipoproteins inhibit α-synuclein aggregation by interacting with oligomeric species in seed amplification assays

Abstract

Abstract Background: Aggregation of α-synuclein (α-syn) is a prominent feature of Parkinson’s disease (PD) and other synucleinopathies. In these diseases, the extracellular spreading of misfolded α-syn significantly contributes to the cell-to-cell propagation of the α-syn misfolding pathology in a prion-like fashion. Therefore, extracellular α-syn aggregates are considered primary targets both for diagnostics and for novel disease modifying therapies. Currently, α-syn seed amplification assays (SAAs) using cerebrospinal fluid (CSF) represent the most promising diagnostic tools for synucleinopathies. However, CSF itself contains several compounds that can modulate the aggregation of α-syn in a patient-dependent manner, potentially sabotaging unoptimized α-syn SAAs and preventing seed quantification. Methods: In this study, we characterized the inhibitory effect of CSF on in vitro α-syn aggregation by means of CSF fractionation, mass spectrometry, dot-blot, Western blot, transmission electron microscopy, solution nuclear magnetic resonance spectroscopy, a highly accurate and standardized diagnostic SAA, and different in vitro aggregation conditions to evaluate spontaneous aggregation of α-syn. Results: We found the high-molecular weight fraction of CSF (>100,000 Da) to be highly inhibitory and identified lipoproteins to be the main drivers of this effect. We evaluated direct interaction between lipoprotein and α-syn and observed lipoprotein-α-syn complexes by transmission electron microscopy. Direct interaction between lipoproteins and monomeric α-syn was not detected by solution nuclear magnetic resonance spectroscopy, suggesting interaction between lipoproteins and oligomeric/proto-fibrillary α-syn intermediates instead. Lastly, we observed significantly slower amplification of α-syn seeds in PD CSF when lipoproteins were added to the reaction mix of a highly accurate diagnostic SAA. Conclusions: Our results describe a novel interaction between lipoproteins and α-syn aggregates that inhibits the formation of α-syn fibrils and could have relevant biological and translational implications. Indeed, the donor-specific inhibition of CSF on α-syn aggregation explains the lack of quantitative results so far obtained by the analysis of SAA-derived kinetic parameters. Furthermore, our data show that apolipoproteins are the main inhibitory components of CSF, suggesting that lipoprotein concentration measurements could be incorporated into data analysis models to eliminate the confounding effects of CSF milieu on α-syn quantification efforts.