Characterization of the Fidelity to PD of a Unique Rat Model

Parkinson disease (PD), the second most prevalent neurodegenerative disorder of the elderly, is associated with movement and cognitive symptoms, and current pharmacological treatments result in transient symptomatic relief but are not disease-modifying. PD results from degeneration of dopaminergic neurons in the substantia nigra (SNPC), and loss of striatal dopaminergic function leads to resting tremor, bradykinesia, rigidity, and postural instability. Current treatments for PD have been focused on augmenting dopamine levels and include levodopa, dopamine agonists, and MAO-B inhibitors. Arguably the first biochemical deficits in PD, which arise before symptoms appear, are loss of parenchymal glutathione (GSH), the brain¡¦s major small molecule antioxidant, and elevation of its oxidized form (GSSG) (Chinta et al., 2006). This has led researchers to show that PD is associated with oxidative stress, though the cause of PD remains unknown. Familial PD cases have been linked to altered genes, among which is PTEN-induced putative kinase-1 (Pink-1). Environmental factors, such as heavy metals or metalloids, herbicides, and certain industrial solvents, have been implicated in PD. We recently published oxidative stress and redox proteomics studies of a C.elegans model of PD (Di Domenico et al., 2012). We pioneered the techniques of redox proteomics in neurodegenerative disorders (review: Butterfield et al., 2012). Genetic models of PD, mostly mice bearing mutations in genes such as alpha-synuclein (ƒÑSN), leucine rich repeat kinase 2 (Lrrk2), parkin (Park2), DJ-1 (Park7), and Pink-1(Park6), have some construct validity, but for unknown reasons do not have pathology or dopaminergic neuronal loss in the SNPC. An ideal animal model of PD would display progressive loss of dopaminergic neurons, and formation of Lewy bodies, exhibit motor deficits, and also mimic non-dopaminergic characteristics of PD, i.e., loss of cognition and sense of smell (anosmia). According to Dawson (2010), no mouse model fulfills these criteria. Relative to the mouse, the rat, with a larger brain size and closer resemblance to human physiology, is capable of higher components of learning and behavior. Sage Labs (Horizon) has produced a Pink-1 knockout rat [PINK-1 KO] that exhibits progressive dopaminergic loss in the SNPC, moderate to severe motor deficits, and non-motor characteristics of PD, i.e., anosmia. At 8 months of age, abundant pathology in the SNPC and PD-relevant symptomology are present. It would be helpful to utilize this exciting novel biological tool for environmental studies relevant to environmental-gene interaction in the neurodegeneration associated with PD. However, this laudable goal is presently premature since this rat has not been fully characterized for goodness of fit to PD. Hence, to fill this critical need this R21 proposal will achieve the goal of characterizing this unique rat model of PD to determine with what fidelity PD characteristics are manifested in this rat. The following interrelated Specific Aims will be performed: SA 1. Determine in the SNPC the age dependence of: (A) Pathology [DT imaging; MRS-determined neurochemical and metabolic signatures; ƒÑ-SN levels and histopathology, i.e., loss of dopaminergic neurons; presence of Lewy bodies; nature of mitochondrial dysfunction (Complex I? and/or III?)]; (B) Behavior [bradykinesia; tremor; motor deficits; anosmia; cognitive dysfunction]. (C) Response of endpoints of (A) and (B) above to L-DOPA treatment. SA2. (A) Assess oxidative stress (protein oxidation, lipid peroxidation, ratio of GSH/GSSG) in SNPC and cortex of the rats used in SA1 to test the hypothesis that alterations found in SA1 are associated with oxidative stress and that L-DOPA treatment modulates these alterations. (B) Perform redox proteomics of the SNPC from rats at the age when divergence of characteristics of SA1 and SA2 from young PINK1 KO rats appear in order to identify the oxidatively modified proteins and associated pathways altered. Successful characterization of this unique rat model of PD will provide: (a) Necessary knowledge of the fidelity of this rat missing a PD-associated gene (the ultimate mutation) to PD; (b) The role of mitochondria in these changes; (c) The basis for future studies to interrogate specific pathways uncovered by redox proteomics to investigate environmental-gene interactions of relevance to PD; and (d) insights into molecular processes by which L-DOPA may modulate alterations uncovered in familial PD associated with mutations in Pink-1.