Rod Genes Are Only Modestly and Temporarily Affected in rd7
In sharp contrast to changes in cone gene expression, rod-specific genes were much less severely affected in the rd7 mutant. Microarray and in situ hybridization analysis of numerous rod genes failed to reveal marked changes in expression levels at P14 and P21 (see Figure 2, lower left photomicrographs; Table S2). In addition to the three rod genes depicted in Figure 2, in situ hybridization analysis on an additional 19 rod-specific and pan-photoreceptor genes demonstrated only a very mild diminution of expression in two of these genes, gucy2e and Rgs9, at P14, and an increase in expression in two, Nr2e3, and Cnga1 (Table S2).
Despite the minimal changes in rod gene expression at later postnatal timepoints, there was evidence of a significant delay in the onset of rhodopsin (Rho) expression in rd7 mutants relative to wild-type. Microarray analysis at P6 demonstrated five cDNA spots that were down-regulated in three out of three experiments. Of these spots, three corresponded to rhodopsin (Table S3). In situ hybridization analysis of several rod-specific genes at P6 revealed that rhodopsin alone showed a markedly lower level of expression compared to wild type (Figure 6; unpublished data). Despite this modest delay in the onset of rhodopsin expression, by P14 the gene had attained nearly normal levels in the rd7 mutant (see Figure 2, lower left photomicrographs). This latter finding suggests that all the rod- and many cone-specific genes are coexpressed in the majority of photoreceptors in the rd7 mutant.
Figure 6  The Onset of rhodopsin Expression Is Delayed in the rd7 Mutant Retina
Note the nearly undetectable staining for rhodopsin in this P6 mutant retina (top right). The majority of rod-specific genes did not show this delay in expression onset, as indicated by the normal amount of staining for Pde6a in the mutant at P6 (bottom images).