Increased Neurotrophic Factor Levels in Ventral Mesencephalic Cultures Do Not Explain the Protective Effect of Osteopontin and the Synthetic 15-mer RGD Domain Against MPP+ Toxicity

The synthetic 15-mer arginine-glycine-aspartic acid (RGD) domain of osteopontin (OPN) is protective in vitro and in vivo against dopaminergic cell death and this protective effect may be mediated through interaction with integrin receptors to regulate neurotrophic factor levels. We now examine this concept in rat primary ventral mesencephalic (VM) cultures. 1-Methyl-4-phenylpyridinium (MPP+) exposure reduced tyrosine hydroxylase (TH)-positive cell number and activated glial cells as shown by increased glial fibrillary acidic protein (GFAP), oxycocin-42 (OX-42) and ectodermal dysplasia 1 (ED-1) immunoreactivity. Both OPN and the RGD domain of OPN were equally protective against MPP+ toxicity in VM cultures and both increased glial-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor (BDNF) levels. The effects of OPN and the RGD domain were accompanied by a decrease in numbers of activated microglia but with no change in astrocyte number. However, full-length OPN and the RGD domain of OPN remained protective against MPP+ toxicity in the presence of a GDNF neutralising antibody. This suggests that increased GDNF levels do not underlie the protective effect observed with OPN. Rather, OPN's protective effect may be mediated through decreased glial cell activation.