Using cGMP analogues to modulate photoreceptor light sensitivity: Perspectives for the treatment of retinal degeneration

Abstract Cyclic nucleotide-gated (CNG) channels play an essential role within the phototransduction cascade in vertebrates. Although rod and cone light responses are mediated through similar pathways, each photoreceptor type relies on a specific CNG-channel isoform. In many forms of retinal degeneration, increased cGMP levels initiate a pathophysiological rollercoaster, which starts with an enhanced CNG-channel activation, often in rod photoreceptors. This causes cell death of both rods and cones, and eventually leads to complete blindness. Unfortunately, the targeting of the desired channel isoform still constitutes the bottleneck for many therapeutic schemes. Here, we present a novel strategy, based on cGMP analogues with opposite types of action, which allows for the selective modulation of either rod or cone photoreceptors. A combined treatment with a weak rod-selective CNG-channel inhibitor (Rp-8-Br-PET-cGMPS) and a cone-selective CNG-channel activator (8-pCPT-cGMP) preserved the normal CNG-channel function at physiological and pathological cGMP levels. The effectiveness of this approach was tested and confirmed in explanted mouse retina. Under physiological conditions, the inhibitor silenced the rods selectively and decreased the dependency of cone responses on light intensity. Remarkably, the activator, when applied together with the inhibitor, reinstated only the light responsiveness of cones. Yet, when applied alone, the activator dampened rod responses more strongly than those of cones. Hence, combinations of cGMP analogues with desired properties may elegantly address the isoform-specificity problem in future pharmacological therapies. Beyond therapies for retinal degeneration diseases, treatments based on this strategy may allow modulation of visual performance in certain light environments or disease conditions. One Sentence Summary A combination of cGMP analogues with opposite types of action can selectively modulate either rod or cone photoreceptor function.