Parkinson's disease (PD) is a neurodegenerative movement disorder which affects 5% of the world population over the age of 85 and may affect some individuals as young as 40. More than 15,000 U.S. Veteran's suffer from this disease which is characterized by progressive loss of neurons that control movement via the action of the neurotransmitter dopamine. The neurons are lost through deterioration of their antioxidant defense mechanisms and the adverse effects of inflammation. L-dihydroxyphenylalanine (levodopa, L-DOPA) is the drug of choice for treatment of the symptoms of this disease, serving to replenish dopamine. However, there has been concern that chronic administration of L-DOPA could contribute to progressive neurodegeneration. From a clinical assessment standpoint the ELLDOPA (Early versus Late Levodopa) multicenter clinical trial did not support the hypothesis that L-DOPA accelerates the progression of PD;but contradictory results from the neuroimaging component of the same trial left the issue unresolved. Thus, the inherent chemical nature of L-DOPA as an initiator of oxidative stress and the neuroimaging results of the clinical trial have left the question unanswered whether L-DOPA treatment may contribute to neurodegeneration either directly or in combination with the neuroinflammatory aspect of Parkinson's disease. This project is designed from a "bedside to bench" perspective of translational pharmacology, whereby a drug in common use is re-examined from a new vantage point to seek therapeutic improvement. The concepts and pathways of redox signal transduction, which are the focal point of research in the principal investigator's laboratory, were not well known when therapy with L-DOPA was initiated in the 1960s. Also a more recent development is the understanding that oxidative stress, a key factor in PD, is likely to perturb natural redox signaling and promote inflammatory responses and apoptotic cell death. Recent studies in the laboratory of the Principal Investigator have revealed that treatment of human dopaminergic neurons in culture with L-DOPA leads to decreases in the activities of two key enzyme systems glutaredoxin and thioredoxin. These enzymes maintain the thiol status of cysteine residues and the corresponding functions of protein intermediates in signaling pathways that govern whether a cell survives chemical insults or commits to programmed cell death (apoptosis). Concomitant with the L-DOPA-induced deficiency in these enzymes, there is an increase in neuron cell death.
Three aims are proposed to address the following key questions: 1. What molecular control mechanisms regulated by the glutaredoxin or thioredoxin enzymes are altered by L-DOPA treatment of neurons that cause them to switch from survival mode to programmed cell death? 2. How does L-DOPA treatment alter the pro-inflammatory response system in microglial cells and/or the antioxidant response system in astroglial cells controlled by the glutaredoxin or thioredoxin enzymes? 3. How do co-administered dopaminergic agonists or inducers of the antioxidant response element (ARE) alter pro-inflammatory signaling and induction of antioxidant enzymes to provide a net neuro-protective effect in the context of L-DOPA treatment? These studies will be pursued with neuronal, microglial, and astroglial cell lines in culture, separately and in co-culture. Experimentation will progress to studies of mouse midbrain slices containing the substantia nigra pars compacta, the focal region of Parkinson's disease where all three cell types reside. By learning how L-Dopa initiates neuronal cell death and how it and co-administered agents may inhibit pro-inflammatory responses from neighboring microglia cells and stimulate anti-oxidant responses in astroglial cells new insights regarding therapy will be realized. Accordingly, improved therapy of the second most prevalent neurodegenerative disease of the elderly can be anticipated.
Parkinson's disease (PD) is the second most common neurodegenerative disease affecting 5% of the population over the age of 85. Nearly a million Americans suffer from PD. The average age of onset is 60 years old, but some cases involve people as young as 30. In the VHA 1999 Large National Health Survey of Veteran Enrollees, of nearly 900,000 respondents >1.6% reported a diagnosis of PD. Studies of the cellular and biochemical mechanisms for onset and progression of PD and the effects of treatment with L-DOPA, as addressed by this proposal, are important for advancing alternatives in therapy that would be beneficial to veterans. Thiol-Disulfide Oxidoreductase enzymes, especially glutaredoxin and thioredoxin, are critical to mitigating the effects of oxidative stress in neurodegenerative diseases, as well as in cardiovascular disease, diabetes and aging, all of which are prevalent among veterans. Increased knowledge of how these enzymes regulate the balance between cell survival and cell death will promote innovative therapeutic development.