Cone cell survival and downregulation of GCAP1 protein in the retinas of GC1 knockout mice

Purpose. To examine the spatial and temporal characteristics of cone cell survival and the expression of guanylate cyclase activating proteins (GCAPs) in the guanylate cyclase (GC)-1 knockout (KO) mouse retina. Methods. Immunohistochemical analyses with peanut agglutinin and an antibody specific for cone transducin were used to examine cone cell survival in the GC1 KO retina at 4, 5, 9, 16, and 24 weeks of age. Immunohistochemical and Northern and Western blot analyses were used to examine the expression of GCAP1 and GCAP2 in 4- to 5-week-old mice. Results. The number of cone cells appeared normal throughout the superior and inferior retinal regions in 4- and 5-week-old GC1 KO mice but gradually decreased by 6 months. Cone cell loss was exacerbated in the inferior retinal region, with only 2% to 8% remaining by 6 months of age; however, 40% to 70% of the cone cells survived in the superior region at this age. GCAP1 and GCAP2 protein levels were downregulated in GC1 KO retinas at 4 weeks of age and GCAP1 immunostaining was absent from the photoreceptor outer segments. Conclusions. The results of this study show that the rate of cone cell loss in the GC1 KO mouse is comparable to that previously described in the GUCY1*B chicken and in humans with Leber congenital amaurosis (LCA)-1. The GCAP expression data, when combined with those of previous electrophysiological studies of the GC1 KO mouse retina, provide evidence that GC1-GCAP1 interactions are essential for cone cell function in mice and that GC2 and GCAP2 activities contribute to the rod cell response in the absence of GC1.