Glia induce dendritic growth in cultures sympathetic neurons by modulating the balance between bone morphogenetic proteins (BMPs) and BMP antagonists

Dendritic growth in cultured sympathetic neurons requires specific trophic interactions. Previous studies have demonstrated that either coculture with glia or exposure to recombinant bone morphogenetic proteins (BMPs) is both necessary and sufficient to induce dendrite formation. These observations led us to test the hypothesis that BMPs mediate glial-induced dendritic growth. In situ hybridization and immunocytochemical studies indicate that the spatiotemporal expression of BMP5, -6, and -7 in rat superior cervical ganglia (SCG) is consistent with their proposed role in dendritogenesis. In vitro, both SCG glia and neurons were found to express BMP mRNA and protein when grown in the presence or absence of the other cell type. However, addition of ganglionic glia to cultured sympathetic neurons causes a marked increase in BMP proteins coincident with a significant decrease in follistatin and noggin. Functional assays indicate that glial-induced dendritic growth is significantly reduced by BMP7 antibodies and completely inhibited by exogenous noggin and follistatin. These data suggest that glia influence the rapid perinatal expansion of the dendritic arbor in sympathetic neurons by increasing BMP activity via modulation of the balance between BMPs and their antagonists.