GDNF gene delivery in an animal model of Parkinson's disease. Long-term effects on intact, injured and transplanted dopamine neurons using lentiviral gene transfer

Abstract: Parkinson's disease is characterized by a progressive degeneration of dopaminergic neurons in the substantia nigra, leading to a loss of dopamine in the target structure striatum and development of motor symptoms, such as bradykinesia, rigidity and tremor. New experimental treatment strategies for Parkinson's disease are aimed at either preventing the degeneration of the dopaminergic neurons, or at restoring dopamine in the striatum by fetal dopaminergic transplants. In this thesis work, we have evaluated the long-term effects of glial cell line-derived neurotrophic factor (GDNF) on intact, injured and transplanted dopaminergic neurons following GDNF gene delivery using a viral vector system based on lentiviruses. The results show that the lentiviral vectors provide an efficient transfer of the GDNF gene into the nigrostriatal dopamine system, resulting in a stable and long-lasting expression of the GDNF protein at high levels. The neuroprotective effects of lentiviral-mediated delivery of GDNF were evaluated in a rat model of Parkinson's disease and demonstrated that continuous overexpression of GDNF in the striatum provided an efficient protection of the nigral dopamine neurons, however, improvements in motor function were not observed. Instead, GDNF induced an aberrant sprouting of nigrostriatal fibers in areas outside of the striatum, and the phenotypic expression of tyrosine hydroxylase was reduced in the preserved dopaminergic terminals. We further evaluated the GDNF-induced downregulation of tyrosine hydroxylase in the intact nigrostriatal dopamine system and showed that this effect was both time- and dose-dependent, and did not seem to have a detrimental effect on normal dopamine neurotransmission. The lentiviral vector was also used to study the long-term effects of GDNF on the survival and function of transplanted fetal dopamine neurons. GDNF initially increased the survival of the grafted dopamine neurons, however, the protected cells failed to survive long-term and the presence of GDNF around the grafts appeared to be detrimental to the transplant-induced recovery in these animals. Based on the observation that long-term and continuous GDNF delivery may have compromising effects on the functional outcome in either a neuroprotective or restorative paradigm, we developed a regulatable lentiviral vector system for controlled expression of GDNF in the rat brain. Efficient induction of GDNF gene expression was obtained following injection of the regulated lentiviral vector into the striatum, however, a significant basal expression was also observed, demonstrating the need to further improve the vector system for tight regulation in vivo. The findings of this thesis will have implications for the development of a GDNF gene therapy approach in Parkinson's disease.