Investigation Into The Role Of SNCA And LRRK2 Mutations In The Pathogenesis Of Parkinson’s Disease

Parkinson’s disease(PD) is one of the most popular neurodegenerative diseases with approximately 1-2% of the population over 65 years suffering from this disease. It is clinically manifested by motor symptoms and non-motor symptoms, with progressive loss of dopaminergic(DAergic) neurons in substantia nigra(SN) and formation of Lewy bodies(LBs) in surviving cells as its hallmarks. Most of the PD cases are sporadic and the culprit behind it remains unclear. Familial PD could be caused by various mutations of genes. Although familial PD only accounts for a small percentage of PD cases, the investigations into familial PD shed a new light on the genetic causes of PD.SNCA and LRRK2 are two main pathogenic genes in PD, which both contribute to the autosomal-dominant PD. However there are significant differences between SNCA-linked PD and LRRK2-linked PD. SNCA mutations are quite rare, but highly penetrant and show a clinical phenotype of early-onset, fast progress and atypical symptoms. On the contrary, LRRK2 mutations are more common, but have a quite low penetrance and exhibit characteristics of late-onset, slow progress and Levodopa-responsive tremor-dominant symptoms, which are clinically indistinguishable from idiopathic PD. Our researches focus on the two main pathogenic genes and use the αsynA53T and LRRK2G2019 S mouse models to investigate the roles of αsynA53T and LRRK2G2019 S mutations in the pathogenesis of PD. Part I: Olfactory Dysfunction and Neurotransmitter Disturbance in Olfactory Bulb of Transgenic Mice Expressing Human A53 T Mutant α-synucleinParkinson disease is a multi-system neurodegenerative disease characterized by both motor and non-motor symptoms. Hyposmia is one of the early non-motor symptoms occurring in more than 90% of Parkinson disease cases, which can precede motor symptoms even several years. Up to now, the relationship between hyposmia and Parkinson disease remains elusive. Lack of proper animal models of hyposmia restricts the investigation. In this study we assessed olfactory function in Prp-A53T-α-synuclein transgenic(αSynA53T) mice which had been reported to show age-dependent motor impairments and intracytoplasmic inclusions. We also examined cholinergic and dopaminergic systems in olfactory bulb of αSynA53T mice by immunofluorescent staining, enzyme linked immunosorbent assay and western blot. We found that compared to wild type littermates, αSynA53T mice at 6 months or older displayed a deficit of odor discrimination and odor detection. No significant changes were found in olfactory memory and odor habituation. Furthermore compared to wildtype littermates, in olfactory bulb of αSynA53T mice at 10 months old we detected a marked decrease of cholinergic neurons in mitral cell layer and a decrease of acetylcholinesterase activity, while dopaminergic neurons were found increased in glomerular layer, accompanied with an increase of tyrosine hydroxylase protein. Our studies indicate that αSynA53T mice have olfactory dysfunction before motor deficits occur, and the cholinergic and dopaminergic disturbance might be responsible for the PD-related olfactory dysfunction. Part II: LRRK2G2019 S mutation and ageing confer susceptibility to proteasome inhibitor induced neurotoxicity in nigrostriatal dopaminergic systemMutations in LRRK2 are the common genetic causes in PD. Among the various mutations of the gene, LRRK2G2019 S is the most common one. The penetrance of LRRK2G2019 S is incomplete and is age-dependent. It is speculated that environmental factors or ageing could contribute to the pathogenesis of LRRK2G2019S-related PD. In our study we used the BAC LRRK2G2019 S mice to determine whether the LRRK2G2019 S mice were more susceptible to proteasome inhibitor-induced nigral DAergic neuron injury and whether ageing played a role. We found that lactacystin treatment in tg mice caused greater decline of striatal DA content than wild-type littermates at both 5 and 12 months old. Tg mice at 12 months old showed a tendency to have fewer TH-positive neurons in the substantia nigra than wt littermates. Both tg and wt mice at 12 months old lost more TH-positive neurons after lactacystin treatment than 5-m-old mice, indicating that age played an important role in DAergic system degeneration. Compared with the 5-m-old mice, the number of activated microglia in the injection side of SN in the 12-m-old mice was significantly increased by lactacystin treatment. Our study suggests that both the LRRK2G2019 S predisposition and ageing confer more vulnerability to proteasome inhibition-induced nigrostriatal DAergic degeneration.