Retinal pigment epithelial cells from dystrophic rats form normal tight junctions in vitro

In the genetically defective Royal College of Surgeons (RCS) rat model for retinal degeneration, a breakdown occurs in the retinal pigment epithelial (RPE) cell tight junctions just as the photoreceptors begin to degenerate. These experiments sought to determine the impact of the RPE genetic defect on this alteration in the RPE cell tight junctions. Retinal pigment epithelial cell cultures prepared from RCS and control rats were treated with hormonally defined medium (HDM), base medium conditioned by RCS or control retinas, or unconditioned base medium. The tight junctions formed by these cultures were assayed functionally by measuring transepithelial electrical resistance and permeability. Junction structure was evaluated by immunolocalization of the tight junction protein zonula occludens 1 and of the junction-associated actin microfilaments. Retinal pigment epithelial cultures from dystrophic rats formed structurally and functionally normal tight junctions when maintained in hormonally defined medium. The junctions remained stable when the medium bathing the apical surface was switched to base medium preconditioned by normal retinas. In contrast, cultures treated with medium preconditioned by degenerating dystrophic retinas or with unconditioned medium exhibited a breakdown in their tight junctions. Retinal pigment epithelial cells isolated from dystrophic RCS rats can form tight junctions normally in vitro. Normal, but not dystrophic, retinas release factors that support RPE tight junctions. Therefore, the junctional abnormality seen in dystrophic rat RPE cells in vivo is probably caused by the loss of trophic factors normally provided by the healthy neural retina rather than by a direct effect of the genetic defect on the tight junctions.