Differential regulation of distinct phenotypic features of serotonergic neurons by bone morphogenetic proteins

Bone morphogenetic proteins (BMPs), growth and differentiation factor 5 (GD F5) and glial cell line-derived neurotrophic factor (GDNF) are members of t he transforming growth factor-beta superfamily that have been implicated in tissue growth and differentiation. Several BMPs are expressed in embryonic and adult brain. We show now that BMP-2, -6 and -7 and GDF5 are expressed in the embryonic rat hindbrain raphe. To start to define roles for BMPs in the regulation of serotonergic (5-HT) neuron development, we have generated serum-free cultures of 5-HT neurons isolated from the embryonic (E14) rat raphe. Addition of saturating concentrations (10 ng/mL) of BMP-6 and GDF5 a ugmented numbers of tryptophan hydroxylase (TpOH) -immunoreactive neurons a nd cells specifically taking up 5,7-dihydroxytryptamine (5,7-DHT) by about two-fold. Alterations in 5-HT neuron numbers were due to the induction of s erotonergic markers rather than increased survival, as shown by the efficac y of short-term treatments. Importantly, BMP-7 selectively induced 5,7-DHT uptake without affecting TpOH immunoreactivity. BMP-6 and -7 also promoted DNA synthesis and increased numbers of cells immunoreactive for vimentin an d glial fibrillary acidic protein (GFAP). Pharmacological suppression of ce ll proliferation or glial development abolished the induction of serotonerg ic markers by BMP-6 and -7, suggesting that BMPs act indirectly by stimulat ing synthesis or release of glial-derived serotonergic differentiation fact ors. Receptor bodies for the neurotrophin receptor trkB, but not trkC, abol ished the BMP-mediated effects on serotonergic development, suggesting that the glia-derived factor is probably brain-derived neurotrophic factor (BDN F) or neurotrophin-4. In support of this notion, we detected increased leve ls of BDNF mRNA in BMP-treated cultures. Together, these data suggest both distinct and overlapping roles of several BMPs in regulating 5-HT neuron de velopment.