GDNF is currently the most effective, and best studied, survival factor acting on dopamine neurons (the neurons that die in Parkinson´s disease, PD). Infusion of GDNF into the brain has been explored as a potential protective treatment in patients, but with uncertain and variable outcomes. This project is based on preliminary observations indicating that dopamine neurons expressing increased levels of alpha-synuclein are unresponsive to GDNF. The purpose of the project is to explore further the cellular changes that prevent GDNF from exerting its neuroprotective action.
For these studies we will use a novel pre-clinical model of PD where human alpha-synuclein is expressed selectively in the dopamine neurons by injection of an AAV-a-synuclein vector into the substantia nigra. The cellular changes induced by GDNF will be studied at 3 weeks after vector injection, and compared with the response seen in the substatia nigra injected with a control vector. The experiments are designed to identify which step in GDNF signaling is affected by alpha-synuclein (i.e, at the level of the Ret or GRFa1 receptor, or at the level of one of the steps in the intracellular signaling cascade). Of particular interest is the possible involvement of the transcription factor Nurr1, which is an important regulator of the GDNF Ret receptor. This transcription factor has previously been observed to be downregulated in patients with PD. In the experiments planned here we will explore the possibility to restore GDNF signaling in alpha-synuclein expressing cells by over-expression of Nurr1 in the affected neurons.
Relevance to Diagnosis/Treatment of Parkinson’s Disease:
The potency of GDNF as a neuroprotective agent is well established in 6-OHDA and MPTP pre-clinical models. The current efforts to develop GDNF, or its close relative neurturin, as a neuroprotective therapy are based on promising results obtained in these pre-clinical studies. The results of the proposed experiments, in a pre-clinical model of PD that replicates more faithfully the neuropathological changes seen in the human disease, will have implications for the ongoing efforts to use GDNF as a neuroprotective agent in PD patients, and may also suggest new interesting targets, Nurr1 and two recently discovered micro-RNAs (miR-21 and miR22) for the development of alternative neuroprotective strategies.
The results will tell us, first, whether the ability of the affected dopamine neurons to respond to GDNF is impaired in cells that over-express alpha-synuclein, and, secondly, whether this impairment is caused by a downregulation of its receptor, or due to a block in the intracellular signaling cascade. We will also know whether expression of Nurr1 will be able to reverse this effect.
We have in a previous study shown that GDNF, which is known to be a potent neuroprotective factor in standard neurotoxin models, is unable to protect the midbrain dopamine neurons against α-synuclein-induced cell death. In the experiments carried out within this project we show that this failure is caused by a blockade of the intracellular signaling response to both exogenous and endogenous GDNF, and that this down-regulation is induced by increased cellular levels of α-synuclein. The results, moreover, provide evidence that the blockade of the GDNF trophic effect is caused by a down-regulation of the transcription factor Nurr1, which in turn results in a reduced cellular expression of the GDNF receptor Ret. The results add support for the idea that the adverse/toxic effects of elevated levels of α-synuclein, and its ability to impair dopaminergic neurotransmission, is caused - at least in part - by its interaction with Nurr1, pointing to Nurr1 as an interesting novel therapeutic taget for neuroprotection in PD.
A publication of these findings is now in preparation.