Gene expression analysis of embryonic photoreceptor precursor cells using BAC-Crx-EGFP transgenic mouse.

Crx is a transcription factor which is predominantly expressed in developing and mature photoreceptor cells in the retina, and plays a crucial role in the terminal differentiation of both rods and cones. Crx is one of the earliest-expressed genes specifically in photoreceptor precursors, allowing us to trace photoreceptor precursor cells from embryonic stages to adult stage by visualizing Crx-expressing cells. In the current study, we generated a transgenic mouse line which expresses enhanced green fluorescence protein (EGFP) in the retina driven by the Crx promoter using bacterial artificial chromosome (BAC) transgenesis. EGFP-positive cells were observed in the presumptive photoreceptor layer in the retina at embryonic day 15.5 (E15.5), and continued to be expressed in developing and mature photoreceptor cells up to adult stage. We sorted EGFP-positive photoreceptor precursors at E17.5 using fluorescence-activated cell sorter (FACS), and subsequently performed microarray analysis of the FACS-sorted cells. We observed various photoreceptor genes, especially cone genes, are enriched in the EGFP-positive cells, indicating that embryonic cone photoreceptor precursors are enriched. In addition, we found that most of the EGFP-positive cells were post-mitotic cells. Thus, the transgenic line we established can serve as a useful tool to study both developing and mature photoreceptor cells, including embryonic cone precursors whose analysis has been difficult.