The Parkinson ′s Disease Associated BAP1/ASXL3 Complex Regulates the Internalization of α-Synuclein Fibrils by Reprogramming the Cell Surface Glycoproteome

A central hallmark of Parkinson′s disease (PD) is the spread of α-Synuclein (α-Syn) aggregates, which is thought to contribute to its progressive nature. To better understand the mechanism of cellular internalization of α-Syn fibrils, we conducted a genome wide CRISPR activation (CRISPRa) screen to identify genetic modifiers of fibril uptake. We report here BRCA associated protein 1 (BAP1) as a regulator of fibril uptake into cells and validate its association with the genetic risk of PD. BAP1 regulates fibril entry into cells by acting as a master transcriptional regulator of the cell surface glycoproteome. BAP1 downregulates Heparan Sulfate Proteoglycan (HSPG) and upregulates O-linked glycoprotein expression, the net effect of which is to reduce fibril internalization. The effect of enhancing BAP1 expression is cell-type specific, as it reduces αSyn uptake in iPSC-derived dopaminergic neurons, but not in microglia. Using human midbrain organoids with integrated microglia, we find this effect is mediated through ASXL3 another PD risk gene and a non-obligatory component of the BAP1 Polycomb Repressive-Deubiquitinase Complex (PR-DUB). These findings are consistent with higher BAP1/ASXL3 co-expression in regions of the brain less vulnerable to PD pathology. Together, the work uncovers a novel role for BAP1/ASXL3 in regulating αSyn fibril internalization by remodelling the cell surface glycoproteome.