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

Abstract Bone morphogenetic proteins (BMPs) are members of the transforming growth factor β (TGF‐β) 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 EC 50 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‐β 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 [ 3 H]dopamine without affecting the uptake of [ 3 H]5‐hydroxytryptamine and [ 3 H]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‐β superfamily, but act indirectly, possibly through glial cells.