Functional effects of AAV2-GDNF on the dopaminergic nigrostriatal pathway in parkinsonian rhesus monkeys

Abstract

We investigated the safety and neuroregenerative potential of an adeno-associated virus (AAV2) containing human glial cell line-derived neurotrophic factor (GDNF) in an MPTP primate model of Parkinson's disease. Dopaminergic function was evaluated by positron emission tomography with 6-[(18)F]fluoro-l-m-tyrosine (FMT) before and after AAV2-GDNF or phosphate-buffered saline infusion bilaterally into the putamen. FMT uptake was significantly increased bilaterally in the putamen of AAV2-GDNF but not phosphate-buffered saline-treated animals 6 months after infusion, indicating increased dopaminergic activity in the nigrostriatal pathways. AAV2-GDNF-treated animals also showed clinical improvement without adverse effects. These findings are consistent with our previous report in aged nonhuman primates that showed evidence of enhanced use of striatal dopamine and dopaminergic nigrostriatal innervation. Clinical improvement and evidence of functional recovery in the nigrostriatal pathway, and the absence of adverse effects, support the safety of this approach for the delivery of GDNF over a 6-month period.

FIG. 1.

FMT-PET images before and after treatment. Representative parametric PET images show increased FMT uptake bilaterally for a GDNF-treated monkey but minimal change in FMT uptake for a control monkey.

FIG. 2.

FMT-PET uptake (K_i) in the putamen. (A) Mean FMT uptake in fully lesioned (right) and partially lesioned (left) putamen at baseline and post-treatment for GDNF-treated (solid columns) and PBS control monkeys (shaded columns). GDNF-treated monkeys showed significant bilateral increases in K_i values at 6 months compared with baseline (paired t test, *p < 0.05), whereas control monkeys did not show a significant change. FMT-PET uptake in both hemispheres was also significantly greater at 6 months in the AAV2-GDNF monkeys compared with the controls (unpaired t test, **p < 0.01). (B) Percentage increase in FMT-PET uptake 6 months after treatment compared with the baseline PET scans was greater in the partially lesioned right putamen. Error bars indicate the SEM.

FIG. 3.

Clinical rating scale (CRS) scores. AAV2-GDNF-treated monkeys (triangles) showed rapid improvement in CRS score after treatment relative to controls (circles). A significant reduction in CRS score was observed to occur during the 9 months after treatment and was maintained at the longest time point of 12 months (two-way ANOVA, AAV2-GDNF vs. controls, p < 0.001). Error bars indicate the SEM.

FIG. 4.

Bilateral increases in FMT uptake correlate with reductions in clinical rating scale (CRS) scores. Shown are independent correlations between the percentage change in CRS score 6 months after treatment and the percentage increase in FMT-PET signal for the left and right putamen of AAV2-GDNF (A and B) and control (C and D) monkeys. AAV2-GDNF monkeys displayed a significant linear correlation (solid lines) between changes in CRS score and PET signal for both the partially lesioned left putamen (A) and fully lesioned right putamen (B). Correlations for the control monkeys (dotted lines) were not statistically significant for either the partial (C) or the fully (D) lesioned hemisphere.

FIG. 5.

Enhanced expression of tyrosine hydroxylase (TH) after AAV2-GDNF delivery. Immunostaining for TH demonstrated almost complete destruction of TH-positive fibers in the fully MPTP lesioned (right) caudate–putamen compared with the partially lesioned (left) hemisphere. Twelve months after bilateral AAV2-GDNF delivery (A), and the subsequent upregulation of GDNF in the putamen (A, middle), TH expression in both hemispheres was enhanced relative to PBS-treated controls (B). Very intense TH staining was observed in the left putamen of AAV2-GDNF-treated monkeys (left) with no evident sign of MPTP-induced lesioning. Within the right putamen of AAV2-GDNF-treated monkeys, we observed upregulation of TH, particularly in the medial–ventral region (right). TH staining in the right putamen of PBS controls was restricted to a limited number of TH-positive fibers with none of the diffuse TH staining seen in either the left putamen or AAV2-GDNF-treated monkeys.