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C. Rahner, M. Fukuhara, S. Peng, L.J. Rizzolo; Retinal secretions modulate the structure of tight junctions in the retinal pigment epithelium . Invest. Ophthalmol. Vis. Sci. 2004;45(13):3683.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: In principal, the permeability of tight junctions in the RPE can be changed by modulating the composition of the junction or by altering its structure. This study examines whether retinal secretions, alone or in collaboration with basal factors, alter the fine–structure of tight junctions. Methods: RPE, in vivo and in culture, was examined by transmission electron microscopy of freeze–fracture replicas. RPE was isolated from chick embryos at various embryonic stages. The RPE was cultured on filters that separated the apical and basal medium chambers. The basal chamber contained a base medium (SF3), SF3+bovine pituitary extract or SF3+medium conditioned by chick embryonic fibroblasts (CEF). The apical chamber contained SF3 or SF3 conditioned by embryonic day 14 (E14) neural retinas (rcSF3). We determined the number of strands parallel to the plane of the monolayer, the depth (distance between the apical and basal–most strands) and complexity (reciprocal of the distance between branch points). The transepithelial electrical resistance (TER) was used to monitor function. Results: During development in vivo, isolated patches of junctional strands gradually expanded and coalesced to form a continuous junctional network by E14. In culture, very few junctions formed when SF3 was in both medium chambers. Pituitary extract induced the formation of an incomplete tight junctional network. By contrast, CEF medium induced a continuous, but heterogenous network. Surprisingly, rcSF3 also promoted an epitheloid phenotype of the cultures and a continuous network of junctions. The combination of rcSF3 with either basal medium supplement synergistically increased the TER, but by different mechanisms. With pituitary extract, discontinuities of the junction were eliminated, whereas with CEF the junctions were remodeled to form a more homogeneous network. The TER and structure of the junctions were more in vivo–like with the combination of pituitary extract and rcSF3. Notably, the E7 and E14 cultures exhibited the same structure even though the E14 cultures had a higher TER. Conclusions: Retinal factors induce the formation of tight junctional strands and their assembly into a continuous network that can form a permeability barrier. This retinal–stimulated pathway intersects and modulates pathways stimulated by interactions at the basolateral membrane. Although retinal factors induced E7 and E14 cultures to form junctions of similar structure, the difference in TER indicates that the properties of individual junctional strands must differ in the two cultures.
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