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Objective: To assess the contribution of wild-type, mutant and loss of leucine-rich repeat kinase-2 (LRRK2; Lrrk2) on dendritic neuronal arborization., Background: LRRK2 mutations are recognized as the major genetic determinant of susceptibility to Parkinson's disease for which a cellular assay of Lrrk2 mutant function would facilitate the development of targeted molecular therapeutics., Methods: Dendritic neuronal arborization (neurite length, branching and the number of processes per cell) was quantified in primary hippocampal and midbrain cultures derived from five lines of recombinant LRRK2 mice, including human BAC wild-type and mutant overexpressors (Y1699C and G2019S), murine knock-out and G2019S knock-in animals., Results: Neuronal arborization in cultures from BAC Lrrk2 wild-type animals is comparable to non-transgenic littermate controls, despite high levels of human transgene expression. In contrast, primary neurons from both BAC mutant overexpressors presented with significantly reduced neuritic outgrowth and branching, although the total number of processes per cell remained comparable. The mutant-specific toxic gain-of-function observed in cultures from BAC mutant mice may be partially rescued by staurosporine treatment, a non-specific kinase inhibitor. In contrast, neuronal arborization is far more extensive in neuronal cultures derived from murine knock-out mice that lack endogenous Lrrk2 expression. In Lrrk2 G2019S knock-in mice, arguably the most physiologically relevant system, neuritic arborization is not impaired., Conclusions: Impairment of neuritic arborization is an exaggerated, albeit mutant specific, consequence of Lrrk2 over-expression in primary cultures. The phenotype and assay described provides a means to develop therapeutic agents that modulate the toxic gain-of-function conferred by mutant Lrrk2., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

Mutations in the gene encoding the E3 ubiquitin-protein ligase parkin have been shown to be a common genetic cause of familial early-onset Parkinson's disease (PD). In addition to its function in the ubiquitin-proteasome system (UPS), parkin has been ascribed general neuroprotective properties. Stress and mutation induced decreases in parkin solubility leading to compromised cytoprotection have recently been reported. We systematically investigated whether PD-related stresses including MG132 and epoxomicin (proteasomal impairment), tunicamycin (unfolded protein stress), and rotenone (mitochondrial dysfunction) resulted in expressional changes of parkin and other E3 ubiquitin ligases (dorfin, SIAH-1). Rotenone and tunicamycin did not change parkin mRNA levels, whereas proteasomal inhibition resulted in a reduction of parkin mRNA in PC12 cells as well as in SH-SY5Y cells. Therefore, surprisingly, cells did not react with a compensatory parkin upregulation under proteasomal inhibition, although, in parallel, parkin protein shifted to the insoluble fraction, reducing soluble parkin levels in the cytosol. Since the mRNA of the parkin-coregulated gene PACRG paralleled the parkin mRNA at least partly, we suspect a promoter-driven mechanism. Our study, therefore, shows a link between proteasomal impairment and parkin expression levels in cell culture, which is intriguing in the context of the described and debated proteasomal dysfunction in the substantia nigra of PD patients.

Pathogenic substitutions in the Lrrk2 protein have been shown to be an important cause of both familial and sporadic parkinsonism. The molecular pathway involved in Lrrk2 dopaminergic neuron degeneration remains elusive. Employing a combination of Lrrk2-mediated protein precipitation and tandem mass spectrometry, we identified 14 potential Lrrk2 binding partners. The majority of these interactions may be subgrouped into three functional cellular pathways: (i) chaperone-mediated response, (ii) proteins associated with the cytoskeleton and trafficking and (iii) phosphorylation and kinase activity. Future investigation of these candidates is now warranted and may help resolve the pathomechanism behind Lrrk2 neurodegeneration.

The goal of genetic association studies is to identify common (>5%) risk factors in complex disease traits. Herein we describe the first replicable 'functional' risk allele for Parkinson's disease. The leucine-rich repeat kinase 2 (Lrrk2) G2385R substitution is associated with familial parkinsonism, late-, and early-onset Parkinson's disease in ethnic Chinese Taiwanese. Crucially, we provide evidence of identity-by-descent and suggest that Lrrk2 G2385R carriers originate from one ancestor some 4800 years ago, at the start of Chinese civilization. Moreover, our findings demonstrate that common genetic coding variants contribute to Parkinson's disease in a population specific manner which may have important implications for future genome-wide association studies.