Twenty years since the discovery of the parkin gene

Parkinson's disease (PD) is a common neurodegenerative disorder characterized by loss of dopaminergic neurons in the substantia nigra. Several lines of evidence strongly implicate mitochondrial dysfunction as a major causative factor in PD, although the molecular mechanisms responsible for mitochondrial dysfunction are poorly understood. Recently, loss-of-function mutations in the parkin gene, which encodes a ubiquitin-protein ligase, were found to underlie a familial form of PD known as autosomal recessive juvenile parkinsonism (AR-JP). To gain insight into the molecular mechanism responsible for selective cell death in AR-JP, we have created a Drosophila model of this disorder. Drosophila parkin null mutants exhibit reduced lifespan, locomotor defects, and male sterility. The locomotor defects derive from apoptotic cell death of muscle subsets, whereas the male sterile phenotype derives from a spermatid individualization defect at a late stage of spermatogenesis. Mitochondrial pathology is the earliest manifestation of muscle degeneration and a prominent characteristic of individualizing spermatids in parkin mutants. These results indicate that the tissue-specific phenotypes observed in Drosophila parkin mutants result from mitochondrial dysfunction and raise the possibility that similar mitochondrial impairment triggers the selective cell loss observed in AR-JP.

It is widely accepted that the familial Parkinson's disease (PD)-linked gene product, parkin, functions as a ubiquitin ligase involved in protein turnover via the ubiquitin-proteasome system. Substrates ubiquitinated by parkin are hence thought to be destined for proteasomal degradation. Because we demonstrated previously that parkin interacts with and ubiquitinates synphilin-1, we initially expected synphilin-1 degradation to be enhanced in the presence of parkin. Contrary to our expectation, we found that synphilin-1 is normally ubiquitinated by parkin in a nonclassical, proteasomal-independent manner that involves lysine 63 (K63)-linked polyubiquitin chain formation. Parkin-mediated degradation of synphilin-1 occurs appreciably only at an unusually high parkin to synphilin-1 expression ratio or when primed for lysine 48 (K48)-linked ubiquitination. In addition we found that parkin-mediated ubiquitination of proteins within Lewy-body-like inclusions formed by the coexpression of synphilin-1, alpha-synuclein, and parkin occurs predominantly via K63 linkages and that the formation of these inclusions is enhanced by K63-linked ubiquitination. Our results suggest that parkin is a dual-function ubiquitin ligase and that K63-linked ubiquitination of synphilin-1 by parkin may be involved in the formation of Lewy body inclusions associated with PD.

A putative G protein-coupled transmembrane polypeptide, named Pael receptor, was identified as an interacting protein with Parkin, a gene product responsible for autosomal recessive juvenile Parkinsonism (AR-JP). When overexpressed in cells, this receptor tends to become unfolded, insoluble, and ubiquitinated in vivo. The insoluble Pael receptor leads to unfolded protein-induced cell death. Parkin specifically ubiquitinates this receptor in the presence of ubiquitin-conjugating enzymes resident in the endoplasmic reticulum and promotes the degradation of insoluble Pael receptor, resulting in suppression of the cell death induced by Pael receptor overexpression. Moreover, the insoluble form of Pael receptor accumulates in the brains of AR-JP patients. Here, we show that the unfolded Pael receptor is a substrate of Parkin, the accumulation of which may cause selective neuronal death in AR-JP.