Year of Publication

2012

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Medicine

Department

Physiology

First Advisor

Dr. George M. Smith

Abstract

Parkinson's disease is characterized by progressive degeneration of substantia nigra (SN) and subsequently loss of the nigrostriatal circuit. Many strategies have attempted to reconstruct this circuit but failed to satisfy clinical trials. The inhibitory environment of the adult CNS and the long distance between the SN and the striatum make true reconstruction difficult. To reconstruct this circuit, we used a transplant-pathway targeting model. Several putative pathway targeting molecules were examined for their ability to direct the growth of axons from a dopaminergic transplant. For a proof-of-principle study, adenoviral and lentiviral encoded glial cell line-derived neurotrophic factor (GDNF), GDNF-receptor alpha1 (GFRa1 ), or netrin-1 were injected along the corpus callosum individually or in combination. Treatment with individual factors leads to modest growth with few axons extending the entire length of the pathway. Combined treatment with either GDNF/GFRa1 or GDNF/netrin-1 induced the most robust growth towards the contralateral striatum. GDNF/netrin-1 showed the most consistent growth, with about 80% of the axons growing to the farthest injection site on the contralateral side. To determine if this combination of guidance molecules could be used to reconstruct the nigrostriatal pathway, we examined axon outgrowth from transplants placed within the SN in the 6-0HDA-Iesioned hemiparkinsonian animal model. A pathway from the SN to the striatum was made by injecting lentivirus encoding either GDNF and netrin-1 or GDNF and GFRa1, along the internal capsule, from the SN to the striatum. In another cohort of animals lentivirus encoding GFP was used as a control. A piece of embryonic VM tissue was transplanted into the SN two weeks after lentivirus injections. Compare to the GFP control group, a significantly greater number of dopaminergic axons grew from the transplants towards the striatum ten weeks after transplantation. Retrograde tract tracing showed the dopaminergic axons were from A9 cells in the transplant. Behavioral studies showed a significant reduction in number of amphetamine-induced rotations in GDNF/netrin-1 treated animals. Functional recovery strongly correlated with the number of dopaminergic fibers growing out from the transplant. This study shows that a functional nigrostriatal pathway can be reconstructed by guiding axonal growth from the dopaminergic neurons transplanted in the SN along a preformed growth-supportive pathway extending into the striatum. Refinement of this technique could be beneficial for PD patients in the future.

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