Embryonic markers of cone differentiation

PURPOSE: Photoreceptor cells are born in two distinct phases of vertebrate retinogenesis. In the mouse retina, cones are born primarily during embryogenesis, while rod formation occurs later in embryogenesis and early postnatal ages. Despite this dichotomy in photoreceptor birthdates, the visual pigments and phototransduction machinery are not reactive to visual stimulus in either type of photoreceptor cell until the second postnatal week. Several markers of early cone formation have been identified, including Otx2, Crx, Blimp1, NeuroD, Trβ2, Rorβ, and Rxrγ, and all are thought to be involved in cellular determination. However, little is known about the expression of proteins involved in cone visual transduction during early retinogenesis. Therefore, we sought to characterize visual transduction proteins that are expressed specifically in photoreceptors during mouse embryogenesis. METHODS: Eye tissue was collected from control and phosducin-null mice at embryonic and early postnatal ages. Immunohistochemistry and quantitative reverse transcriptase-PCR (qPCR) were used to measure the spatial and temporal expression patterns of phosducin (Pdc) and cone transducin γ (Gngt2) proteins and transcripts in the embryonic and early postnatal mouse retina. RESULTS: We identified the embryonic expression of phosducin (Pdc) and cone transducin γ (Gngt2) that coincides temporally and spatially with the earliest stages of cone histogenesis. Using immunohistochemistry, the phosducin protein was first detected in the retina at embryonic day (E)12.5, and cone transducin γ was observed at E13.5. The phosducin and cone transducin γ proteins were seen only in the outer neuroblastic layer, consistent with their expression in photoreceptors. At the embryonic ages, phosducin was coexpressed with Rxrγ, a known cone marker, and with Otx2, a marker of photoreceptors. Pdc and Gngt2 mRNAs were detected as early as E10.5 with qPCR, although at low levels. CONCLUSIONS: Visual transduction proteins are expressed at the earliest stages in developing cones, well before the onset of opsin gene expression. Given the delay in opsin expression in rods and cones, we speculate on the embryonic function of these G-protein signaling components beyond their roles in the visual transduction cascade.