Evaluation of neurochemical and functional effects of glial cell-derived neurotrophic factor gene delivery using a tetracycline-regulatable adeno-associated viral vector (2011)

Gene transfer to the brain is a promising therapeutic strategy for a variety of neurodegenerative disorders including Parkinson‟s disease (PD). PD is the second most common neurodegenerative disease. Although many drugs have been developed and introduced into the market to provide symptomatic treatment, there is still no cure for PD. Glial cell line-derived neurotrophic factor (GDNF) is a potent survival factor for injured nigrostriatal dopamine neurons and is currently being evaluated as a potential treatment for PD. Gene therapy allows localized, long-term and stable transgene expression after a single intervention to obtain a therapeutic effect. Regulatable promoters for transgene expression furthermore allow optimizing GDNF concentration to avoid undesirable biological activity and clinical side effects. In the first part of the study, an autoregulatory tetracycline-inducible recombinant adeno-associated viral vector (rAAV-pTetbidiON) utilizing the rtTAM2 reverse tetracycline transactivator (rAAV-rtTAM2) was used to conditionally express the human GDNF cDNA. Eight weeks after a single intrastriatal injection of the rAAV-rtTAM2-GDNF vector encapsidated into AAV serotype 1 capsids (rAAV2/1), the GDNF protein level was respectively 15 fold higherand undistinguishable from the endogenous level in doxycycline(Dox) treated and untreated animals. However, a residual GDNF expression in the uninduced animals was evidenced by a sensitive immunohistochemical staining. As compared to rAAV2/1-rtTAM2-GDNF, the rAAV2/1-rtTAM2-WPRE-GDNF vector harboring a woodchuck hepatitis post-transcriptional regulatory element, which increases and stabilizes the transgene transcript, expressed a similar concentration of GDNF in the induced state but a basal level ~2.5-fold higher than the endogenous striatal level. However, the distribution of GDNF in the striatum in induced state was more widespread using the rAAV2/1-rtTAM2-WPRE-GDNF vector as compared to rAAV2/1-rtTAM2- GDNF. As a proof for biological activity, for both vectors, downregulation of tyrosine hydroxylase (TH) was evidenced in dopaminergic terminals of Dox-treated but not untreated animals. In the second part of my study, functional (behavioural) and neurochemical changes mediated by delayed intrastriatal GDNF gene delivery in the partial Parkinson‟s disease rat model were investigated. The rAAV2/1-rtTAM2-WPRE-GDNF vector (3.5 108 viral genomes) was administered unilaterally in the rat striatum 5 weeks after intrastriatal injection of 6-hydroxydopamine (6-OHDA) which produces a partial and progressive lesion of the nigro-striatal dopaminergic pathway. Rats were treated with Dox or untreated from the day of vector injection until sacrifice at 4 or 14 weeks (continuous treatment). A sub-group was Dox-treated for 7 weeks (temporary treatment) then untreated until 14 weeks. In the absence of Dox, the GDNF tissue concentration was found to be equivalent to the endogenous level in 6-OHDA-lesioned rats. In the presence of Dox, it was ~10-fold higher. Dox-dependent behavioral improvements were demonstrated 4 weeks post-vector injection. At later time points, spontaneous partial recovery was observed in all rats, but no further improvement was found in Dox-treated animals. Moreover GDNF gene delivery only transiently improved dopaminergic function. Over the long term, TH was more abundant, but not functional, and the increase was lost when GDNF gene expression was switched off. The third part of my study consisted in the evaluation of the respective dose-range of therapeutical and undesirable effects of GDNF. Functional effects appeared after delivery of 3.5 108 viral particles which produced 200-300 pg/mg protein of GDNF in the lesioned rat striatum (see above). In order to evaluate the viral dose producing undesirable effects, we compared two different doses of vector: 3.5x108 and 4.4x109 viral genome. In the low dose group, the GDNF concentration in the striatum was ~300 pg/mg protein in the Dox-treated animals and equivalent to the endogenous level in untreated animals (~20 pg/mg protein). In contrast, in the high dose group, GDNF levels reached ~1200 pg/mg protein in induced animals but up to ~300 pg/mg protein in uniduced animals. In the low dose group, Dox-dependent downregulation of TH but no asymetrical behaviour was evidenced. In the high dose group, TH downregulation was observed in both Dox+ and Dox-rats. In addition, amphetamine-induced rotational behaviour was evidenced in Dox+ but not in Dox-rats. These data suggest that low doses of virus are sufficient to induce therapeutically-relevant but not undesirable functional effects of GDNF. Nevertheless,a neurochemical effect of GDNF (TH down-regulation) did appear at low dose. In order to understand the GDNF-induced motor asymmetry, we investigated the anatomical pattern of TH down regulation in striatum. Strikingly, there was a greater loss of TH labeling in striosomes than in the surrounding matrix. Receptors which are known to be differentially expressed in the striosomes i.e. µ-opioid receptor(MOR-1) and N-methyl-D-aspartic acid (NMDA) receptor 1 (NR1) as compared to the matrix were analyzed in the high-dose group of animals. MOR-1 was not affected by GDNF gene delivery. In contrast, NR1 was down regulated. The potential relationship between TH and NR1 down-regulation as well as other previously described neurochemical effects of GDNF (as enhancement of DA release and metabolism, of DA neurons excitability or of TH phosphorylation) and behavioural asymmetry remains to be clarified. As summary, our data suggest that behavioural and neurochemical effects of striatal delivery of GDNF can be controlled by Dox by using the autoregulatory rAAV2/1-TetON- GDNF vector, provided the dose range of gene delivery is carefully adjusted.