Ubiquitination of a New Form of α-Synuclein by Parkin from Human Brain: Implications for Parkinson's Disease

TLDR

Parkinson’s disease is a neurodegenerative disorder marked by α‑synuclein and ubiquitin inclusions, with rare inherited forms caused by mutations in α‑synuclein or the E3 ligase parkin. The study tested whether parkin normally ubiquitinates α‑synuclein and whether this process is disrupted in autosomal recessive PD. Normal human brain contains a parkin–UbcH7–glycosylated α‑synuclein (αSp22) complex, and mutant parkin fails to bind or ubiquitinate αSp22, leading to its accumulation in parkin‑deficient PD brains, indicating that parkin’s ubiquitination of αSp22 is critical and its loss underlies α‑synuclein aggregation.

Abstract

Parkinson's disease (PD) is a common neurodegenerative disorder characterized by the progressive accumulation in selected neurons of protein inclusions containing α-synuclein and ubiquitin. Rare inherited forms of PD are caused by autosomal dominant mutations in α-synuclein or by autosomal recessive mutations in parkin, an E3 ubiquitin ligase. We hypothesized that these two gene products interact functionally, namely, that parkin ubiquitinates α-synuclein normally and that this process is altered in autosomal recessive PD. We have now identified a protein complex in normal human brain that includes parkin as the E3 ubiquitin ligase, UbcH7 as its associated E2 ubiquitin conjugating enzyme, and a new 22-kilodalton glycosylated form of α-synuclein (αSp22) as its substrate. In contrast to normal parkin, mutant parkin associated with autosomal recessive PD failed to bind αSp22. In an in vitro ubiquitination assay, αSp22 was modified by normal but not mutant parkin into polyubiquitinated, high molecular weight species. Accordingly, αSp22 accumulated in a non-ubiquitinated form in parkin-deficient PD brains. We conclude that αSp22 is a substrate for parkin's ubiquitin ligase activity in normal human brain and that loss of parkin function causes pathological αSp22 accumulation. These findings demonstrate a critical biochemical reaction between the two PD-linked gene products and suggest that this reaction underlies the accumulation of ubiquitinated α-synuclein in conventional PD.