A paracrine effect for neuron-derived BDNF in development of dorsal root ganglia: Stimulation of Schwann cell myelin protein expression by glial cells

Addition of neurons to cultures of non-neuronal cells derived from quail embryonic dorsal root ganglia causes a 2.5-fold increase in the proportion of cells that express the glial marker Schwann cell myelin protein (SMP) when compared to cultures devoid of neurons. This effect is mediated by BDNF because incubation with a trkB immunoadhesin that sequesters BDNF, but not with trkA or trkC immunoadhesins, abolishes this stimulation. This neuronal activity can be mimicked by treatment with soluble BDNF that stimulates specifically the conversion of SMP-negative glial cells into cells that express this phenotype. That BDNF is the endogenous neuron-derived factor affecting glial development is further supported by the observation that BDNF is extensively expressed in developing sensory neurons of the avian ganglia both in vivo and in vitro, but not by the satellite cells. These results show for the first time a paracrine role for neuronal BDNF on differentiation of peripheral glial cells. This effect of BDNF is likely to be mediated by the p75 neurotrophin receptor because: (1) p75 immunoreactive protien is expressed by a subset of satellite cells; (2) neutralization of p75 abolishes the BDNF-induced stimulation; (3) a treatment of non-neuronal cell cultures with equimolar concentrations of either soluble NGF or NT-3 also affects the proportion of cells that become SMP-positive. Whereas NGF stimulates the acquisition of this glial antigen to a similar extent as BDNF, NT-3 inhibits its expression, suggesting that distinct neurotrophins signal differentially through p75. These findings also suggest that the definitive phenotype of peripheral glia is determined by a balance between positive and inhibitory signals arising in adjacent neurons.