The mammalian retinal degeneration B2 gene is not required for photoreceptor function and survival

The retinal degeneration B (rdgB) gene in Drosophila is essential for photoreceptor function and survival. The rdgB mutant fly exhibits an abnormal electroretinogram and a light-dependent photoreceptor degeneration. The function of RdgB is not fully understood, but the presence of a phosphatidylinositol transfer protein domain suggests a possible role in phosphatidylinositol metabolism and signaling. Two mammalian homologs, M-RdgB1 and M-RdgB2, are known. While M-RdgB1 is widely expressed, M-RdgB2 is found primarily in the retina and the dentate gyrus. Functional conservation between the Drosophila and mammalian RdgBs was demonstrated by the ability of both M-RdgBs to rescue the photoreceptor phenotype in rdgB mutant flies through transgenic expression. To investigate the role of M-RdgB2 in the mammalian retina, we disrupted the m-rdgB2 gene in mice by gene targeting. The homozygous knockout mice are fertile and apparently healthy. By light microscopy, immunocytochemistry and electroretinograms, mice up to 18 months of age showed normal photoreceptor function and survival. The inner retinal neurons were also examined by immunolabeling with a number of cell-specific markers and no apparent defects were found in the major cell populations. We conclude that M-rdgB2 is not essential for phototransduction and photoreceptor survival. Thus, m-rdgB2 is not a candidate gene for human retinal degenerations. Whether M-rdgB2 has a role in visual processing in the inner retina, or whether it is required for hippocampal function, remains to be determined.