Two secreted retinal factors regulate different stages of development of the outer blood-retinal barrier.

The retinal pigment epithelium (RPE) lies at the interface between the neural retina and the choriocapillaries where it forms a blood-retinal barrier. Like endothelial regions of the blood-brain barrier, the development of the RPE barrier is a gradual, multistep process. A culture model of chick RPE was used to study this development. The permeability of the tight junctions that limit diffusion between neighboring RPE cells was measured as the transepithelial electrical resistance (TER). Embryonic day 14 (E14) retinas were used to make a conditioned medium that lowered the permeability of cultured RPE. The TER of cultures prepared from E14 RPE was twice that of E7 RPE. In each culture, retinal conditioned medium increases the TER 2-2.5 fold. The active factors of conditioned medium that affected each culture had different physical properties. The factor that affected E7 was protease-resistant with a Mr<10 kDa, but the factor that affected E14 appeared to be a protein of approximately 49 kDa. Unlike the effect of astrocyte conditioned medium on endothelia, retinal conditioned medium did not act synergistically with cAMP. These data indicate that the chick retina, which lacks astrocytes, uses different diffusible factors to regulate different stages of tight junction development.

Mannose receptor is expressed in normal and dystrophic retinal pigment epithelium.

In normal retinas, the phagocytosis of shed photoreceptor outer segments is mediated in part through a mannose receptor protein located in the apical retinal pigment epithelium membrane. As dystrophic rats of the Royal College of Surgeons have a defect in which the retinal pigment epithelium (RPE) is unable to phagocytize the shed outer segments, it is hypothesized that mannose receptor expression will be lost with the progression of photoreceptor degeneration. Immunohistochemical and molecular techniques have been used to study the developmental expression of the mannose receptor in normal and dystrophic retinal pigment epithelium. By immunofluorescence, the mannose receptor is localized to the retinal pigment epithelium, apical membrane region, beginning around 5 days postnatally in both normal and dystrophic retinas. In immunoblots, bands at 175 kDa are labelled by an anti-mannose receptor antibody in apical membrane samples from both normal and dystrophic RPE at all developmental times sampled. RT-PCR analysis reveals that mannose receptor message is present in normal and dystrophic RPE samples at all developmental time points examined. The present study demonstrates that the expression of the mannose receptor begins prior to outer segment differentiation and the initiation of phagocytosis in both normal and dystrophic RPE. Expression of the mannose receptor continues to be unchanged during the progression of photoreceptor degeneration in the dystrophic retina.

Is another mannose receptor-like lectin present in retinal pigment epithelium?

PURPOSE: This study was designed to investigate the presence of the secretory phospholipase A2 receptor in RPE, a protein closely related to the phagocytic mannose receptor. METHODS: Proteins from cultured rat, pig, and human RPE were separated by SDS-PAGE and immunoblotted with a polyclonal guinea pig sPLA2 receptor antibody. RT-PCR was performed on rat and pig RPE samples using primers designed from published rat pancreatic sPLA2 receptor sequences. RESULTS: The sPLA2 receptor protein was not detected in rat, pig, or human RPE by immunoblots. Additionally, message for this receptor was not detected in rat or pig RPE. CONCLUSIONS: With these techniques, these data demonstrate that the sPLA2 receptor is undetectable in the RPE.