The study of the deubiquitinase USP8 in Parkinson's disease pathogenesis

Parkinson's disease is the second commonest neurodegenerative disease currently treated symptomatically. It is a multifactorial disease involving mechanisms ranging from protein aggregation to mitochondrial dysfunction, oxidative stress and dopamine dysregulation. The levels of α-synuclein have been causatively linked to the development and progression of Parkinson's disease. Therefore α-synuclein lowering strategies are valid approaches in Parkinson's disease. Neuropathologically, Lewy Bodies in the vulnerable substantia nigra of Parkinson's disease patients are less ubiquitinated and specifically less K-63 ubiquitinated than Lewy bodies in the cortex, suggesting differential activation or regulation of ubiquitin interactors. A targeted screen for such interactors revealed that the Deubiquitinating enzyme Usp8 is upregulated in the substantia nigra of Parkinson's disease brains and is inversely correlated with the degree of total and K-63 ubiquitination. Using genetic knockdown and overexpression techniques, Usp8 was found to colocalize and directly interact with α-synuclein. It was found to de-ubiquitinate α-synuclein and increase its half-life. Its knockdown increased the total and K-63 α-synuclein ubiquitination and decreased its levels by 35% at least partly by increasing its degradation via the lysosome. In vivo in the Drosophila melanogaster, Usp8 knockdown demonstrated protection against α-synuclein toxicity. It rescued in a specific manner the rough eye phenotype, the age-dependent locomotive defect and the loss of dopaminergic neurons caused by the expression of α-synuclein. Specific and effective pharmacological Usp8 inhibition also has the potential to lower α-synuclein levels. Collectively, the evidence produced in my thesis suggests that Usp8 could be a potential target for the future disease-modifying therapies in Parkinson's disease.