α-Synuclein fate is determined by USP9X-regulated monoubiquitination

Abstract

α-Synuclein is central to the pathogenesis of Parkinson disease (PD). Mutations as well as accumulation of α-synuclein promote the death of dopaminergic neurons and the formation of Lewy bodies. α-Synuclein is monoubiquitinated by SIAH, but the regulation and roles of monoubiquitination in α-synuclein biology are poorly understood. We now report that the deubiquitinase USP9X interacts in vivo with and deubiquitinates α-synuclein. USP9X levels are significantly lower in cytosolic fractions of PD substantia nigra and Diffuse Lewy Body disease (DLBD) cortices compared to controls. This was associated to lower deubiquitinase activity toward monoubiquitinated α-synuclein in DLBD cortical extracts. A fraction of USP9X seems to be aggregated in PD and DLBD, as USP9X immunoreactivity is detected in Lewy bodies. Knockdown of USP9X expression promotes accumulation of monoubiquitinated α-synuclein species and enhances the formation of toxic α-synuclein inclusions upon proteolytic inhibition. On the other hand, by manipulating USP9X expression levels in the absence of proteolytic impairment, we demonstrate that monoubiquitination controls the partition of α-synuclein between different protein degradation systems. Deubiquitinated α-synuclein is mostly degraded by autophagy, while monoubiquitinated α-synuclein is preferentially degraded by the proteasome. Moreover, monoubiquitination promotes the degradation of α-synuclein, whereas deubiquitination leads to its accumulation, suggesting that the degradation of deubiquitinated α-synuclein by the autophagy pathway is less efficient than the proteasomal one. Lower levels of cytosolic USP9X and deubiquitinase activity in α-synucleinopathies may contribute to the accumulation and aggregation of monoubiquitinated α-synuclein in Lewy bodies. Our data indicate that monoubiquitination is a key determinant of α-synuclein fate. P arkinson disease (PD) is mainly characterized by the death of dopaminergic neurons in the substantia nigra and presence of inclusions called Lewy bodies. Even though Lewy bodies are composed predominantly of α-synuclein, the mechanisms by which Lewy bodies are formed and their role in the death of dopaminergic neurons remain elusive (1, 2).α-Synuclein purified from Lewy bodies is monoubiquitinated (3-5). Although some ubiquitin-ligases can polyubiquitinate α-synuclein (6-8), the E3 ubiquitin-ligase responsible for α-synuclein monoubiquitination has remained obscure. We and others have recently shown that α-synuclein is monoubiquitinated by the E3 ubiquitin-ligase SIAH both in vitro and in vivo (9-11). SIAH is present in Lewy bodies (9) and monoubiquitinates α-synuclein at the same lysine residues that are monoubiquitinated in α-synuclein immunopurified from Lewy bodies (5, 10). Most importantly, we found that in the presence of proteolytic inhibitors, monoubiquitination of α-synuclein promoted by SIAH leads to a marked increase in the aggregation of α-synuclein and formation of inclusions, which are toxic to cells (10-12). Therefore, our data indicate that monou...

Synthetic polyubiquitinated α-Synuclein reveals important insights into the roles of the ubiquitin chain in regulating its pathophysiology

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Identification of Ubiquitin-specific Protease 9X (USP9X) as a Deubiquitinase Acting on Ubiquitin-Peroxin 5 (PEX5) Thioester Conjugate

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USP9X deletion elevates the density of oligodendrocytes within the postnatal dentate gyrus

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