Role of the low-affinity NGF receptor (p75) in survival of retinal bipolar cells

We have examined the role of neurotrophins in promoting survival of mammalian rod bipolar cells (RBC) in culture. Retinas taken from 8- to 10-day-old Long-Evans rats were dissociated and cultured in media supplemented with either nerve growth factor (NGF), neurotrophin-3 (NT-3), brain-derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF), or basic fibroblast growth factor (FGF-2). Survival was measured by the number of cells that were immunoreactive for α-, β-, γ-PKC, a bipolar cell-specific marker. Compared to untreated cultures, CNTF had no effect on RBC survival, while NGF and NT-3 increased survival only slightly. BDNF, however, increased survival by approximately 300%. Similar results were obtained with FGF-2. Both nerve growth factor (NGF) and an antibody (anti-REX) which interferes with binding to the 75-kD low-affinity neurotrophin receptor (p75(NTR)) eliminated BDNF-promoted survival, but had no effect on FGF-2-mediated survival. Interestingly, p75(NTR) was expressed by retinal glia (Muller cells), but not by the bipolar cells themselves, providing for the possibility that BDNF might induce Muller cells to produce a secondary factor, perhaps FGF-2, which directly rescues RBCs. In support of this hypothesis, an antibody that neutralizes FGF-2 attenuated the trophic effects of BDNF, and dramatically reduced survival in cultures with no added growth factors, indicating that there may be an endogenous source of FGF-2 that promotes survival of RBCs in culture. We suggest that BDNF increases production or release of FGF-2 by binding to p75(NTR) on Muller cells.