A SPLICE VARIANT OF TRKB AND BRAIN-DERIVED NEUROTROPHIC FACTOR ARE COEXPRESSED IN RETINAL PIGMENTED EPITHELIAL-CELLS AND PROMOTE DIFFERENTIATED CHARACTERISTICS

There is evidence suggesting reciprocal trophic interactions between p hotoreceptors and the retinal pigmented epithelium (RPE), but the fact ors involved have not been identified. In this study, we investigated the hypothesis that one or more known neurotrophic factors act upon th e RPE. Cultured human and freshly isolated bovine RPE cells demonstrat ed saturable specific binding for [I-125]labeled BDNF, NT-4/5 and NT-3 with little specific binding for CNTF and none for NGF. Cross-competi tion experiments showed that BDNF is the preferred ligand and cross-li nking of [I-125]BDNF resulted in a doublet at 160 kd that was increase d in RPE cells incubated in all-trans retinoic acid. There was basal p hosphorylation of a 145 kd protein recognized by an anti-trk antibody that was increased in RPE cells pulsed with BDNF. RT-PCR with primers spanning the transmembrane domain demonstrated that RPE cells express trkB mRNA lacking a region homologous to exon 9 of chicken trkB, a spl ice variant that has been demonstrated to preferentially interact with BDNF. Northern blots demonstrated that cultured RPE cells also expres s mRNA for BDNF. BDNF did not stimulate proliferation or increase surv ival of RPE cells in serum-free medium, but promoted a differentiated morphology and increased the expression of cellular retinaldehyde bind ing protein, a marker of the differentiated state in RPE cells. An RPE cell line that spontaneously shows differentiated features showed a h igh level of BDNF mRNA. These data demonstrate that RPE cells express a shea splice variant of trkB whose activation correlates with express ion of differentiated characteristics and the cells themselves are cap able of producing a ligand for the receptors. Signaling through trkB c ould play a role in differentiation of RPE cells during development an d maintenance of the differentiated state in adult RPE. (C) 1998 Elsev ier Science B.V.