Interphotoreceptor matrix retinoid binding protein

Fig. 3. Visual cycle retinoids and localion of retinoid-binding proteins in the pigment epithelium, interphotoreceptor matrix and distal retina. Key AT, ali-trans configuration M. Milller cell—a glial cell that spans the retina from the vitreous surface to the external limiting membrane. This membrane is formed by the Muller cell ciliary processes that project into the space filled with...

Interphotoreceptor retinoid binding protein (IRBP) occupies the space between the RPE and the rod outer segment (ROS), the interphotoreceptor matrix, and binds both llcROH and llcRCHO. Retinoids do not require IRBP for transfer between the two membranes, however, because IRBP null mice show no gross abnormalities of the visual cycle, even though they have severe retinal abnormalities. Although retinoids have very limited solubility in the aqueous phase, membranes of the RPE and ROS transfer retinoids between them in the absence of IRBP. 

The cause or causes of retinitis pigmentosa are yet to be found and at present a possible role for vitamin A in this condition has not been established. However, the above studies have enabled us to eliminate some possibilities and to identify others. The retinitis pigmentosa eye was normal in its ability to store retinoid, to esterify retinol, and to convert the all-trans isomer to 11-cis. A major abnormality was the absence of interstitial retinol-binding protein. It has not been established whether this is a selective loss, or whether other components of the interphotoreceptor matrix are also involved. Clearly, much more woric is needed before we arrive at an insight into the causes of chorioretinal binding diseases. 

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