The Rx-like Homeobox Gene (Rx-L) Is Necessary for Normal Photoreceptor Development

PURPOSE: The retinal homeobox (Rx) gene plays an essential role in retinal development. An Rx-like (Rx-L) gene from Xenopus laevis has been identified. The purpose of this study was to analyze the function of Rx-L in the developing retina. METHODS: DNA-binding properties of Rx-L were analyzed by electrophoretic mobility shift assay (EMSA), with in vitro-translated proteins and radiolabeled oligonucleotide probe. The Rx-L expression pattern was analyzed by in situ hybridization using whole or sectioned embryos and digoxigenin-labeled antisense riboprobes. Rx-L loss of function was studied by using antisense morpholino oligonucleotides targeted to the Rx-L translation initiation site. Embryos injected with control or Rx-L morpholinos were analyzed at stage 41 or 45. RESULTS: Rx-L shares homology with Rx at the homeo-, OAR, and Rx domains, but lacks an octapeptide motif. Rx-L is expressed in the developing retina beginning in the early tailbud stage. In the maturing retina, Rx-L expression is restricted primarily to the developing photoreceptor layer and the ciliary marginal zone. Rx-L can bind a photoreceptor conserved element-1 (PCE-1) oligonucleotide, an element conserved among all known photoreceptor gene promoters. In a promoter activity assay, Rx-L functions as a stronger transcriptional activator than Rx. Antisense morpholino-mediated knockdown of Rx-L expression resulted in a decrease in rhodopsin and red cone opsin expression levels in Xenopus retinas. Injection of the Rx-L antisense morpholino oligonucleotide also resulted in a decrease in the length of both rod and cone outer segments. CONCLUSIONS: The results suggest that Rx-L functions to regulate rod and cone development by activating photoreceptor-specific gene expression.