Effect of Rds abundance on cone outer segment morphogenesis, photoreceptor gene expression, and outer limiting membrane integrity

We examined the molecular, structural, and functional consequences on cone photoreceptors of the neural retinal leucine zipper knockout (Nrl^-/-) mice when only one allele of retinal degeneration slow (Rds) is present (Rds^+/-/Nrl^-/-). Quantitative RT-PCR and immunoblot analysis were used to assess the expression levels of several phototransduction genes; electroretinography was used to assess quantitatively the retinal responsiveness to light; and immunohistochemistry and ultrastructural analysis were used to examine retinal protein distribution and morphology, respectively. In Rds/Nrl double-null mice, S-cones form dysmorphic outer segments that lack lamellae and fail to associate properly with the cone matrix sheath and the outer limiting membrane. In Rds^+/-/Nrl^-/- mice, cones form oversized and disorganized outer segment lamellae; although outer limiting membrane associations are maintained, normal interactions with cone matrix sheaths are not, and photoreceptor rosette formation is observed. These retinas produce significantly higher photopic a-wave and b-wave amplitudes than do those of Rds^-/-/Nrl^-/- mice, and the levels of several cone phototransduction genes are significantly increased coincidently with the presence of Rds and partial lamellae formation. Thus, as in rod photoreceptors, expression of only one Rds allele is unable to support normal outer segment morphogenesis in cones. However, cone lamellae assembly, albeit disorganized, concomitantly permits outer limiting membrane association, and this appears to be linked to photoreceptor rosette formation in the rodless (cone-only) Nrl ^-/- retina. In addition, photoreceptor gene expression alterations occur in parallel with changes in Rds levels.