Molecular Mechanism Of Neurodegenerative Diseases

Alzheimer's disease (AD) and Parkinson's disease (PD) are the most common aging dependent neurodegenerative diseases. AD and PD are both characterized by a selective and symmetric loss of neurons in specific brain regions, resulting disruption of motor, sensory or cognitive nervous systems, eventually various severe disabilities of the affected individuals. Most cases of AD and PD occur sporadically and late in life, rare familial forms of these diseases are also identified. Studies of the familial forms of the diseases revealed number of mutations in different causative genes. The causes of sporadic cases of the diseases remain elusive. Genetic polymorphisms and/or environmental stresses are indicated to increase the vuleraibility of affected individuals. Currently, there is no cure for both AD and PD. Understanding the molecular mechanism of these dieases will be essential to design effective treatment stratatgies.Hyperphosphorylation of microtubule-associated protein tau at specific sites is a key pathological furture of AD. Protein kinase A (PKA) is a crucial kinase to generate AD-like hyperphosphorylation of tua protein in the cell. In the study presented in Chapter 1, we found that injection of isoproterenol (ISO), a PKA specific activator, into hippocampus of rat brain induced PKA overactivation, activation of superoxide dismutase (SOD), elevation of malondialdehyde (MDA), and eventual tau hyperphosphorylation. The results suggest that ISO causes oxidative stress in rat brain. Pre-infusion of melatonin intraperitoneally partially reversed ISO-induced tau hyperphosphorylation. Furthermore, melatonin inhibits ISO-induced PKA activation, enhanced SOD activity, decreased the level of MDA in rat brain tissues. This study suggested that ISO likely induce abnormal hyperphosphorylation of tau via activation of PKA and increase of oxidative stress. Melatonin protects against ISO-induced tau hyperphosphorylation through suppression of both PKA overactivation and oxidative stress.Mutations in the PINK1 gene are linked to autosomal recessive early onset familial form of PD. The physiological function of PINK1 and pathological abnormality of PD-associated PINK1 mutants are largely unknown. In studies presendeted in Chapeter 2, I show that inactivation of Drosophila PINK1 (dPINK1) using RNAi results in progressive loss of dopaminergic (DA) neurons and in ommatidial degeneration of the compound eye, which is rescued by expression of human PINK1 (hPINK1). Expression of human SOD1 suppresses neurodegeneration induced by dPINK1 inactivation. Moreover, treatment of dPINK1 RNAi flies with the antioxidants SOD and vitamin E significantly inhibits ommatidial degeneration. Thus, dPINK1 plays an essential role in maintaining neuronal survival by preventing neurons from undergoing oxidative stress, thereby suggesting a potential mechanism by which a reduction in PINK1 function leads to PD-associated neurodegeneration.Mutations in parkin, the PTEN-induced kinase 1 (PINK1) and DJ-1 are individually linked to autosomal recessive early-onset familial forms of PD. Despite the fact that mutations in these genes cause the same disease, their functional relationships and how respective disease-associated mutations cause selective neuronal loss and Lewy body formation are largely unknown. In the study presented in Chapter 4, we demonstrate that parkin, PINK1 and DJ-1 form a complex to promote ubiquitination and degradation of parkin substrates, including parkin itself and synphilin-1. Genetic ablation of either PINK1 or DJ-1 resulted in decreased acute degradation and increased accumulation of parkin substrates. PD-pathogenic parkin and PINK1 mutations impair degradation activity of the complex. This study identifies a novel functional ubiquitin E3 ligase complex consisting of parkin, PINK1 and DJ-1 and suggests that PD-pathogenic parkin, PINK1, or DJ-1 mutations disrupt E3 ligase activity of the complex, which may constitute a mechanism underlying PD pathogenesis.