Differential expression of AP-2alpha and AP-2beta in the developing chick retina: repression of R-FABP promoter activity by AP-2.

Retinal fatty acid binding protein (R-FABP) is the avian counterpart of murine brain FABP implicated in glial cell differentiation and neuronal cell migration. R-FABP is highly expressed in the undifferentiated retina and brain of chick embryos. We have previously shown by in vitro studies that the AP-2 transcription factor binds to a consensus AP-2 binding site in the R-FABP promoter region. Based on the expression pattern of AP-2 in the developing retina and on mutational analysis of the AP-2 binding site in DNA transfection experiments, we proposed that AP-2 could be involved in the down-regulation of R-FABP transcription. Here, we describe the cDNA isolation of two members of the AP-2 family expressed in the chick retina, AP-2alpha and AP-2beta. We show that R-FABP mRNA and the AP-2 factors are expressed in mutually exclusive patterns in the differentiating retina: whereas AP-2alpha and AP-2beta are selectively expressed either in amacrine, or in amacrine and horizontal cells, respectively, R-FABP mRNAis found in Muller glial cells and/or bipolar cells. Furthermore, a decrease in R-FABP-dependent expression is obtained upon cotransfection of primary retinal cultures with AP-2 expression vectors and a CAT reporter construct. The early and cell-specific expression of AP-2alpha and AP-2beta in the developing retina suggest a role for this transcription factor family in the early steps of amacrine and horizontal cell differentiation. Repression of the R-FABP gene in these cells may be an important component of their developmental program.