Polarized distribution of integrin and fibronectin in retinal pigment epithelium

We have examined the presence and distribution of integrin and fibronectin in the retinas of 21-day chick embryos and adult rats, with particular emphasis on the question of localization in the retinal pigment epithelium (RPE). Isolated sheets of RPE solubilized and separated by gel electrophoresis contain integrin, as indicated by immunoblotting with polyclonal and monoclonal antibodies to the complex. By the same technique, antibodies to fibronectin reacted with a single protein in the isolated RPE. In both chick and rat, integrin and fibronectin were localized by indirect immunofluorescence exclusively to the basement membrane of the RPE, the choriocapillaris and the retinal-vitreal border. When isolated RPE cells from chick retinas were examined, integrin was seen to be present along the basolateral surfaces of the cells as well. Similarly, in the intact rat retina, staining for integrin could be seen along the lateral surfaces of some of the RPE cells. Neither integrin nor fibronectin were present along the apical surfaces of the RPE in either rat or chick. The close similarity between the location of integrin and fibronectin supports the idea that the RPE adheres to the basal lamina at least in part via integrin-fibronectin linkages. A clear implication of our results is that the adhesion between RPE and retina requires a different set of linkage proteins.