Parkinson's Disease (PD) is a neurodegenerative disorder that primarily affects the aging brain, and the symptoms of PD are principally due to degeneration of nigrostriatal neurons. Current treatments center around restoring striatal dopamine levels and are effective initially. But these treatments gradually lose efficacy, and they do not alter the progression of the Disease. Thus, improved treatments for PD should first, restore the function of the nigrostriatal system, and second, alter progression of the Disease by protecting the remaining nigrostriatal neurons. We have begun to develop a gene therapy treatment for PD. Gene transfer is performed using a helper virus-free Herpes Simplex Virus vector system we pioneered. Two complementary treatment strategies are being implemented. First, restore striatal dopamine levels by producing dopamine in striatal cells. We have shown that expression of tyrosine hydroxylase (TH) in striatal cells supports long-term (1 year) biochemical and behavioral correction of a rat model of PD. Second, protect remaining nigrostriatal neurons by delivering specific neurotrophic factors to these neurons, and enhance the function of the remaining nigrostriatal neurons. Many investigators have shown that glial cell line-derived neurotrophic factor (GDNF) can protect nigrostriatal neurons in rodent and primate models of PD and that brain derived neurotrophic factor (BDNF) has similar capabilities. We have expressed specific neurotrophic factors (NGF, BDNF, NGF receptor) from HSV-1 vectors and demonstrated neuroprotection and other changes in neuronal physiology in specific systems. The function of the remaining nigrostriatal neurons might be enhanced by activating specific signal transduction pathways in these neurons. We have shown that a constitutively active protein kinase C (PKC) increases release of catecholamines from cultured neurons and targeting this PKC to nigrostriatal neurons modulates rotational behavior. We now propose to systematically develop an improved gene therapy for PD by first determining the preferred mechanism to support each treatment strategy and then combining the preferred strategies. The first specific aim will improve biochemical and behavioral correction by systematically investigating the preferred combination of genes (TH, GTP cyclohyrolase, aromatic amino acid decarboxylase, a vesicular monoamine transporter) to support production of dopamine. The second specific aim will improve protection of nigrostriatal neurons by systematically investigating the preferred neurotrophic factor(s) (GDNF and/or BDNF) and the preferred site of production.
This aim will also examine the function of the remaining nigrostriatal neurons by expressing the constitutively active PKC. The third specific aim will both restore striatal dopamine levels and improve protection of nigrostriatal neurons by systematically combining the preferred strategies from aims 1 and 2. Vectors will be evaluated in cultured cells and then in the rat models of PD for both long-term biochemical and behavioral correction and protection of nigrostriatal neurons. Potential changes in the neurochemistry of nigrostriatal or striatal neurons will also be examined. The long-term goal is to develop human gene therapy for PD.
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