INTERPHOTORECEPTOR RETINOID-BINDING PROTEIN (IRBP) - EXPRESSION IN THE ADULT AND DEVELOPING XENOPUS RETINA

Apposition of the neural retina and pigment epithelium is critical to photoreceptor development and function. Interphotoreceptor retinoid-bi nding protein (IRBP) is a major component of the extracellular matrix separating these epithelia in the African clawed frog Xenopus laevis ( Gonzalez-Fernandez et al., [1993], J. Cell Sci. 105:7-21). In the adul t retina, IRBP appears to mediate the transport of hydrophobic molecul es, particularly retinoids and fatty acids, within the hydrophilic ext racellular domain. In this paper, we compare the distribution of IRBP and its mRNA in adult and embryonic Xenopus retina. Xenopus IRBP antis ense RNA, labeled with tritium or digoxigenin, was used for in situ hy bridizaton studies. For immunohistochemistry, we used an antiserum aga inst Xenopus IRBP expressed in Escherichia coli. In the adult, we foun d that IRBP is synthesized at similar levels by both rods and cones. T he protein is restricted to the interphotoreceptor matrix, with lesser amounts in the pigment epithelial cytoplasm. In the embryo, expressio n of the mRNA for IRBP is restricted to the central retina, where phot oreceptor differentiation has taken place. By contrast, the protein is distributed throughout the embryonic subretinal space. Therefore, the presence of IRBP precedes photoreceptor differentiation. In summary, IRBP is synthesized by both rods and cones and may be internalized by the pigment epithelium. In the embryo, IRBP is synthesized by the cent ral retina and diffuses through the matrix, reaching the undifferentia ted peripheral retina. In view of its ligand-binding properties, diffu sion of IRBP may provide the peripheral neural retina with a vehicle t o transport retinoids and docosahexaenoic acid (molecules critical to normal retinal development) from the pigment epithelium. (C) 1996 Wile y-Liss, Inc.