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OBJECTIVE Changes in retinal photopigments represent a fundamental step in the evolution of visual systems, in that addition of new pigment types or alterations in the spectral absorption properties of existing pigments modify visual capacities and thus open new visual worlds. To provide a tool that would allow direct examination of the changes caused by the presence of novel photopigments, this study was designed to determine whether a gene encoding a human cone photopigment introduced into the mouse genome would be expressed in a cone-specific manner and would support phototransduction. METHODS Mice transgenic for the human long wavelength-sensitive (L) photopigment were generated by microinjection of fertilized mouse eggs. RNA expression in different tissues was monitored by reverse transcription-polymerase chain reaction analysis. Photopigment protein was localized in retinal cross sections and wholemounts by antibody staining. Light transduction of the cone photopigments was assessed by flicker photometric electroretinography (ERG). RESULTS The human transgene was expressed specifically in the mouse cones in quantities comparable to those of the mouse middle wavelength-sensitive (M) pigment gene. Immunocytochemical analysis showed that the human L pigment was abundantly synthesized in most mouse cones, was translocated to the outer segments, and caused no detectable cone degeneration. Electroretinographic spectral sensitivity analysis showed that the human L pigment was efficient in eliciting an electrical response. The degree of expression of the transgene in the two founders correlated well with the spectral responsivity of the ERG. CONCLUSIONS The human L photopigment transduces light efficiently in mouse cones, implying that all protein domains necessary for efficient interaction with intracellular transport and signal transduction machineries in mouse cones have been conserved through evolution. The expression of the human L photopigment gene in both classes of cone of the mouse retina indicates that the transgene did not have the regulatory elements necessary for restricting its expression to mouse M cones or that such elements are not recognized in mouse UV-sensitive cones.
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