Rod and Cone Pigment Regeneration in RPE65 ^-/- Mice

Abstract

RPE65 is a major protein in the retinal pigment epithelium (RPE) (Hamel et al., 1993), where it is required for the regeneration of 11-cis retinal, the native ligand of rod and cone opsins, in the dark (Redmond et al., 1998). Therefore, the retina of the Rpe65 ^-/- mouse is almost completely depleted of 11-cis retinal, resulting in a minimal level of photosensitivity. This observation poses several questions, of which we will only address three: first, which cell type (rods and/or cones) is responsible for the remaining photosensitivity; second, if only one cell type remains photosensitive, what happens to the other one; and third, what is the chromophore that enables the formation of light-sensitive pigment? Early reports have disagreed whether the remaining photosensitivity can be attributed to rod (Seeliger et al., 2001) or cone function (Redmond et al., 1998; Ekesten et al., 2001). Double knockout experiments, crossing the Rpe65 ^-/- with either the rhodopsin or the cone cGMP-gated channel knockout, respectively, revealed that the remaining photosensitivity in the young adult and old Rpe65 ^-/- mouse retina (>6 weeks-of-age) can be attributed to rod sensitivity (Seeliger et al., 2001). This leaves open the question as to the possible fate of the cone photoreceptors in the absence of RPE65. Likewise, early reports demonstrated a slow degeneration in particular of the rod photoreceptors (Redmond et al., 1998), suggesting that the accumulation of the retinyl ester in the RPE might contribute to the demise of the photoreceptors. However, in the Rpe65 ^-/-::Gnat1 ^-/- mouse, in which similar elevated amounts of retinyl ester have been reported to accumulate in the RPE, no rod degeneration occurs (Woodruff et al., 2003). And finally, with respect to the available chromophore; several groups have tried to obtain a spectrum of the pigment from pooled tissue and have failed to do so (Ablonzcy et al., 2001; C. H. Remé, personal communication). Thus, here we would like to further address these three key issues and how our laboratories have investigated them over the past 5 years.