The Y99C Mutation in Guanylyl Cyclase-Activating Protein 1 Increases Intracellular Ca2+and Causes Photoreceptor Degeneration in Transgenic Mice

Abstract

Guanylyl cyclase-activating proteins (GCAPs) are Ca²⁺-binding proteins that activate guanylyl cyclase when free Ca²⁺concentrations in retinal rods and cones fall after illumination and inhibit the cyclase when free Ca²⁺reaches its resting level in the dark. Several forms of retinal dystrophy are caused by mutations inGUCA1A,the gene coding for GCAP1. To investigate the cellular mechanisms affected by the diseased state, we created transgenic mice that express GCAP1 with a Tyr99Cys substitution (Y99C GCAP1) found in human patients with a late-onset retinal dystrophy (Payne et al., 1998). Y99C GCAP1 shifted the Ca²⁺sensitivity of the guanylyl cyclase in photoreceptors, keeping it partially active at 250 nmfree Ca²⁺, the normal resting Ca²⁺concentration in darkness. The enhanced activity of the cyclase in the dark increased cyclic nucleotide-gated channel activity and elevated the rod outer segment Ca²⁺concentration in darkness, measured by using fluo-5F and laser spot microscopy. In different lines of transgenic mice the magnitude of this effect rose with the Y99C GCAP1 expression. Surprisingly, there was little change in the rod photoresponse, indicating that dynamic Ca²⁺-dependent regulation of cGMP synthesis was preserved. However, the photoreceptors in these mice degenerated, and the rate of the cell loss increased with the level of the transgene expression, unlike in transgenic mice that overexpressed normal GCAP1. These results provide the first direct evidence that a mutation linked to congenital blindness increases Ca²⁺in the outer segment, which may trigger the apoptotic process.