Characterisation of Sirtuins in Parkinson's disease

Abstract

Parkinson’s disease (PD) is a progressive, age-related, neurodegenerative disorder characterised by loss of dopaminergic neurones in substantia nigra pars compacta (SNpc) with the formation of α-synuclein rich Lewy bodies. The exact mechanism behind SNpc cell death is still unclear but at the molecular level, oxidative stress and mitochondrial dysfunction are thought to be involved. Sirtuins (SIRTs) are NAD+ dependent protein deacetylases and/or ADP-ribosyltransferases, that modulate apoptosis, gene expression, stress resistance and anti-oxidant defence mechanisms by targeting histone and non-histone proteins. Recent evidence has suggested that SIRT1 and SIRT3 are neuroprotective and SIRT2 promotes neuronal death. This study investigated the role of SIRTs in oxidative stress mediated cell death and PD. The toxicity of diquat and rotenone, which produce oxidative and mitochondrial stress, were measured in dopaminergic SH-SY5Y cells and the effect of over-expression and inhibition of deacetylase activity of SIRTs on cell viability after toxin treatment was determined. Over-expression of SIRT1, SIRT2 and SIRT3 protected the cells from toxin induced cell death. The protection conferred by SIRT1 was partially independent of its deacetylase activity, which was mediated through the repression of NF-кB expression. On the other hand, protection exerted by SIRT2 and SIRT3 was entirely dependent on their enzymatic activity and was induced through higher expression of SOD2. SIRTs also reduced the formation of α-synuclein aggregates although only SIRT3 was co-localised with α-synuclein. In post-mortem brain tissue obtained from patients with Parkinson’s disease, Parkinson’s disease with dementia, dementia with Lewy bodies and Alzheimer’s disease, the activity of SIRT1 was observed to be down-regulated whereas, SIRT2 showed increased activity compared to controls. The increased activity of SIRT2 is possibly a compensatory effect to combat oxidative stress. SIRT3 was observed to be active in microglial cells in disease, implying an activation of anti-oxidant defence mechanism towards neuronal stress in neurodegenerative disorders. In conclusion, the main results of this thesis suggest that SIRTs rescue cells from oxidative stress and reduce the formation of α-synuclein aggregates. The mechanism through which they confer the protection is through enhancement of anti-oxidant pathways and repression of inflammatory responses.