Targeted expression of Cre recombinase to cone photoreceptors in transgenic mice.

Purpose: Disruption of widely expressed essential genes in mice often leads to embryonic or neonatal lethality. To circumvent this problem and dissect gene functions in the cone photoreceptors, we elected to generate cone photoreceptor specific cre transgenic mice.

Methods: Transgenic mice expressing Cre recombinase directed by the human red/green pigment (HRGP) gene promoter were generated. Candidate Cre-expressing lines were identified with RT-PCR. Cre-expressing mice were characterized with immunocytochemical assays and functional studies using a Cre activatable lacZ reporter mouse strain (R26R). Cone distribution was determined by immunohistochemistry and retinal function was measured by electroretinography (ERG) on six month old HRGP-cre mice.

Results: RT-PCR analysis suggested that several transgenic lines expressed cre mRNA in the retina. Beta-galactosidase staining on retinal flat mounts and sections from F1 mice derived from the HRGP-cre and R26R reporter mice suggested that two mouse strains were capable of carrying out efficient Cre mediated recombination in cone photoreceptors. Immunocytochemical staining of retinal sections demonstrated that Cre expression was localized to cone photoreceptors. Cone distribution and cone ERG analysis suggested that cone photoreceptors were normal in adult HRGP-cre mice.

Conclusions: We have generated transgenic mice that efficiently express Cre recombinase in cone photoreceptor cells. The adult transgenic mice have normal cone distribution and function. They can be used in conditional knockout experiments for gene function studies in cone photoreceptors.