Heparin-binding determinants of GDNF reduce its tissue distribution but are beneficial for the protection of nigral dopaminergic neurons

Abstract

Glial cell line-derived neurotrophic factor (GDNF) protects and repairs dopamine neurons. It binds to GDNF family receptor α1 (GFRα1) and activates receptor tyrosine kinase. Heparan sulphate proteoglycans (HSPGs) also participate in the signalling of GDNF, though binding to HS may hinder the diffusion of infused GDNF. We assessed the importance of heparin-binding determinants in the neuroprotective effects of GDNF in the 6-OHDA rat model of Parkinson's disease. We utilized a truncated, non-heparin-binding Δ38N-GDNF or combined wtGDNF with heparin-binding growth-associated molecule (HB-GAM, pleiotrophin). Tissue diffusion of wtGDNF ± HB-GAM and Δ38N-GDNF was also compared. A protective effect against ipsilateral d-amphetamine-induced turning was seen with 10 μg wtGDNF, 17 μg HB-GAM + 10 μg wtGDNF or 10 μg Δ38N-GDNF at 8 weeks post lesion. This effect was most pronounced with wtGDNF alone. HB-GAM (17 or 50 μg) also reduced rotational behaviour, but did not protect dopaminergic cells. Otherwise, the survival of TH-positive cells in the substantia nigra correlated with the behavioural data. Although Δ38N-GDNF was more widely distributed than wtGDNF (irrespective of its origin), stable in a brain extract, and potent in mitogen-activated kinase assay, it was inferior in vivo. The results imply that GDNF binding to HSs is needed for the optimum neuroprotective effect.