Bone Morphogenetic Proteins: Neurotrophic Roles for Midbrain Dopaminergic Neurons and Implications of Astroglial Cells

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor beta (TGF-beta) superfamily that have been implicated in tissue growth and remodelling. Recent evidence suggests that several BMPs are expressed in the developing and adult brain. Specifically, we show that BMP 2 and BMP 6 are expressed in the developing midbrain floor of the rat. We studied potential neurotrophic effects of BMPs on the in vitro survival, transmitter uptake and protection against MPP+ toxicity of mesencephalic dopaminergic neurons cultured from the embryonic midbrain floor at embryonic day (E) 14. At 10 ng/ml and under serum-free conditions, most BMPs promoted the survival of dopaminergic neurons visualized by tyrosine hydroxylase immunocytochemistry during an 8-day culture period, but to varying extents (relative potencies: BMP 6 = 12 > 2, 4, 7). BMPs 6 and 12 were as effective as fibroblast growth factor-2 (FGF-2) and glial cell line-derived neurotrophic factor, promoting survival 1.7-fold compared with controls. BMPs 9 and 11 were not effective. Dose-response curves revealed an EC50 for BMPs 2, 6 and 12 of 2 ng/ml. BMPs 2, 4, 6, 7, 9 and 12 also promoted DNA synthesis and astroglial cell differentiation, visualized by 5-bromodeoxyuridine (BrdU) incorporation and glial fibrillary acidic protein (GFAP) immunocytochemistry respectively. Suppression of cell proliferation and subsequent maturation of GFAP-positive cells by 5-fluorodeoxyuridine or aminoadipic acid abolished the neuron survival-promoting effect of BMP 2. This suggests that BMPs, like other non-TGF-beta factors affecting dopaminergic neuron survival, act indirectly, probably by stimulating the synthesis and/or release of glial-derived trophic factors. BMP 6 and BMP 7 also increased the uptake of [3H]dopamine without affecting the uptake of [3H]5-hydroxytryptamine and [3H]GABA, underscoring the specificity of the trophic effect. We conclude that several BMPs share a neurotrophic capacity for dopaminergic midbrain neurons with other members of the TGF-beta superfamily, but act indirectly, possibly through glial cells.