Vision is essential for vertebrates including humans. Sustained vision is accomplished by the retinoid metabolism, ‘visual cycle’, where all-trans retinol (atROL) is released from photoreceptors upon light absorption and is incorporated into the adjacent retinal pigment epithelium (RPE). In the process, a receptor of atROL is assumed; however, the entity has been missing for a decade. In my talk, I would like to give a presentation about our recent finding showing that low-density lipoprotein (LDL)-related receptor-5 (Lrp5) protein is a strong candidate for the receptor of atROL in the visual cycle (Takita and Seko, iScience 2020; 23(12): 101762). We generated and analyzed the digenic eyes shut homolog (eys)+/-; lrp5+/- zebrafish, the same form of gene defects emerged from a human case report of a candidate retinitis pigmentosa, the most common genetic disorder in inherited retinal dystrophies. The mutant photoreceptor cell layer (PRCL) was mildly thinner than the wild-type PRCL and global gene expression analysis revealed that the hallmark was remarkable decrease in the expression level of the retinol binding protein 1a (rbp1a) gene. The genetic interaction study using rbp1a-/- and lrp5-/- zebrafish eyes clarified that rbp1a gene played a role downstream of lrp5 gene. Immunohistochemical analysis revealed that Lrp5 protein was colocalized with Rbp1a protein at the microvilli of RPE cells in the outer retina. Furthermore, in vitro binding assay using full-length Rbp1a protein and C-terminal intracellular region of Lrp5 protein revealed that they directly bound each other. Collectively, these results strongly suggest that Lrp5 protein is a potent candidate of the receptor of atROL in the visual cycle. This work also indicates that combination of detection and comprehensive analysis of polygenic inheritance could be a powerful tool to dissect molecular networks such as protein-protein interactions and protein complexes not only in vision but also in other fields of biology and medicine.
Dr.Shimpei Takita has primarily worked on vision having interests in identification of key genes important for specialised sensory neurons and their supporting cells (i) to be specialised for detecting external stimuli and (ii) to be finely-organised and well-maintained.