Role of the prolyl isomerase Pin1 in the pathogenesis of Parkinson's disease and neuroprotection by novel targeted compounds in pre-clinical animal models of the disease
Parkinson's disease (PD) is a chronic and progressive neurodegenerative disorder named by the French neurologist Jean-Martin Charot, after the British physician James Parkinson who first described the disease as "Shaking Palsy". Pathologically, PD is characterized by a progressive loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc), a marked reduction of dopamine in the striatum, the presence of ubiquitin and α-synuclein positive cytoplasmic inclusions known as Lewy bodies, and depigmentation of the locus cereleus. The prevailing theory regarding processes that are likely to account for progressive degeneration of the dopaminergic neurons in the nigrostriatal axis involves on mitochondrial dysfunction, oxidative stress, excitotoxicity and neuroinflammation. Of those, neuroinflammation and oxidative stress have gained the most focus recently. Research over the last decade has provided extensive evidence that the sustained microglial and astroglial neuroinflammatory responses cause progressive and delayed dopaminergic neurodegeneration. However, the mechanisms by which glial cells activation and subsequent inflammation lead to dopaminergic neurodegeneration remain poorly understood. Development of effective therapeutic approaches to halt the disease progression of PD is of paramount importance. My objective in this Ph.D. thesis work was to characterize important signaling molecules activated in neuroinflammation mediated neurodegenerative pathways as well as characterze of novel compounds in a pre-clinical mouse model of PD. Protein interacting with never in mitosis - A (Pin1) is a peptidyl-prolyl isomerase, that specifically recognizes phosphorylated serine or threonine residues immediately preceding proline (pSer/Thr-Pro) in a subset of proteins in which it isomerizes the cis/trans conformation of the peptide bond. High expression levels of Pin1 in terminally differentiated and post mitotic neurons suggest that it plays an important role in neurons, except in cell cycle regulation and proliferation. Recently, Pin1 overexpression was shown to facilitate formation of &alpha-synuclein inclusions in a cellular model of &alpha-synuclein aggregation. Pin1 was also localized in Lewy bodies in PD patients. But the level, activity, and role of Pin1 in the pathogenesis of PD are incompletely known. We hypothesized that Pin1 is differentially activated in neuronal and glial cells of the nigral dopaminergic system, and that it regulates NF-κB-mediated sustained neuroinflammatory processes in cell culture and in an animal model of Parkinson's disease. Herein, we demonstrate for the first time that there is a dopaminergic neuron specific upregulation of Pin1 in human postmortem PD brain sections as well as in cell culture and animal models. We observed a rapid increase in Pin1 expression in both the 1-methyl-4phenyl pyridinium (MPP+)-treated cell culture and in 1-methyl-4-phenyl-1-2-3-6-tetrahydroptridine MPTP treated mice. Also, Pin1 acts as an important pro-apoptotic factor in the selective degeneration of dopaminergic neurons. Importantly, pharmacological inhibition of Pin1 attenuates MPTP-induced Pin1 expression in vitro and in vivo and protects against MPTP-induced neurodegeneration in the nigrostriatal axis. We also demonstrate for the first time that, microglia and astrocytes express Pin1 and that there is a strong association between Pin1 and NF-κB p65 in BV2 microglial cells. Recent studies demonstrated that the promoter regions of proinflammatory molecules contain the DNA binding site for NF-κB. We have shown that Pin1 and NF-κB p65 inhibition by Juglone leads to attenuation of glial cells activation and subsequent reduction of proinflammatory reactions in cell culture and in an animal model of PD. While characterizing the role of Pin1 in the pathogenesis of PD, we also tested the efficacy of novel compounds for protection of dopaminergic neurons in a mouse model of PD. We demonstrate that the novel compound diapocynin, a metabolite of apocynin, blocks MPTP-induced activation of microglial and astroglial cells, thus inhibiting the inflammatory and oxidative stress processes in MPTP-treated mice. Diapocynin also protects the nigrostriatum against MPTP toxicity. The final chapter of this work characterizes the anti-inflammatory and neuroprotective properties of mito-apocynin, a mitochondria targeted compound in the MPTP mouse model of PD. Mitoapocynin prevents the behavioral imapariments and dopamine loss caused by MPTP-induced toxicity. We have shown that mito-apocynin protects the nigrostriatum by attenuating glial cell mediated neuroinflammation and oxidative stress. Collectively, the research described herein characterizes the novel and important roles of Pin1 in the neuroinflammation and pathophysiology of Parkinson's disease, as well as establishes the efficacy of novel compounds in protection of the nigrostriatum in a pre-clinical mouse model of Parkinson's disease.