Mouse opsin promoter-directed Cre recombinase expression in transgenic mice.

PURPOSE Gene inactivation with homologous recombination in mice is a widely used tool to study gene function. However, many proteins play essential roles in a number of tissues and germline gene inactivation often results in embryonic lethality. To overcome this limitation and to dissect the functions of essential genes beyond embryonic development, we generated mouse rod opsin promoter-controlled cre transgenic mice with a goal of obtaining transgenic lines with a range of Cre activity in rod photoreceptors. METHODS Transgenic mice expressing Cre recombinase directed by a long or short mouse opsin promoter were generated. Candidate Cre-expressing lines were identified with RT-PCR and Western blot analysis. Potentially useful Cre-expressing lines were characterized further with immunohistochemistry, PCR, and functional analysis using a Cre-activatable lacZ reporter mouse strain (R26R) to determine temporal and spatial patterns of Cre expression. Retinal function and morphology in these mouse lines were analyzed with electroretinography (ERG) and light microscopy of hematoxylin and eosin stained retinal sections. RESULTS Transgenic mice expressing Cre in rod photoreceptors were generated. Characterization of candidate photoreceptor-specific Cre mice using immunohistochemistry and functional assays demonstrated that an efficient Cre-mediated recombination occurred in rod photoreceptor cells in one mouse line and a mosaic Cre-mediated recombination occurred in rod photoreceptors and rod bipolar cells in another mouse line. Further analysis of these mice with ERG and morphological examination suggested that the retinas of eight-month-old adults were normal. CONCLUSIONS We have generated transgenic mice expressing Cre recombinase in rod photoreceptors. One transgenic mouse line was capable of carrying out efficient Cre-mediated recombination in rod photoreceptors. Another transgenic mouse line was capable of carrying out mosaic Cre-mediated recombination in rod photoreceptors and bipolar cells across the whole retina. These mice will be useful tools for Cre/lox-based gene activation and inactivation, as well as genetic mosaics, in rod photoreceptors and rod bipolar cells.