Precise regulation of the photoreceptor phosphodiesterase (PDE6) is critical in controlling cGMP levels during all phases of the visual process in retinal rod and cone photoreceptors. The rod PDE core enzyme is an αβ heterodimer, and each subunit contains both a catalytic domain (where cGMP is hydrolyzed) and two regulatory GAF domains (which bind cGMP). The inhibitory γ subunit of PDE interacts with the catalytic domain to block cGMP hydrolysis, and with the GAF domain to allosterically stabilize cGMP binding.

While the initial activation mechanism of the rod PDE holoenzyme (αβ_γ2) by the photoreceptor G-protein, transducin, is well understood at the molecular level, we know much less about PDE regulation during light adaptation or rod photoresponse saturation. This information is needed because the human visual system primarily operates in photopic environments where severe desensitization or complete shutdown of the rod phototransduction pathway is the norm.

We propose that intrinsic, allosteric changes within the rod PDE holoenzyme, as well as extrinsic factors that bind to PDE, both contribute to regulating PDE structure and function in rod photoreceptors. We hypothesize that the cGMP-binding GAF domains of rod PDE have both common and divergent regulatory mechanisms compared to other GAF-containing PDE family members. We also hypothesize that other signaling proteins in the rod outer segment interact with PDE to modulate its function during persistent activation of the visual transduction pathway.

Cone photoreceptors have been assumed to follow the same phototransduction pathway as has been described for rod photoreceptors, because they contain proteins homologous to most rod phototransduction proteins. However, the lack of quantitative biochemical information about the cone visual transduction pathway has limited our ability to explain at the molecular level why cone photoreceptors are ~100-fold less sensitive than rods, or why their photoresponse kinetics are faster and never saturate. We hypothesize that differences in cGMP metabolism-and, specifically, cone PDE6 regulation-account for some of the physiological differences between rods and cones.

The recent successes of phosphodiesterase inhibitors as therapeutic tools for several diseases has resulted, in large measure, from the development of potent and selective drugs that target only a single PDE family. The most remarkable is the use of PDE5 inhibitors for the treatment of erectile dysfunction. However, of all 11 PDE families, PDE5 is the most similar to photoreceptor PDE6, based on structural similarities, enzymological and pharmacological criteria. Relatively little is known about differences in the drug binding sites of PDE5, rod PDE6, and cone PDE6, and we hypothesize that subtle differences in the active sites of these three enzymes can be distinguished for the purpose of rational drug design of compounds that show greater selectivity.

Mutations in the catalytic or inhibitory subunits of PDE are responsible for several forms of retinal disease in humans and in animals. A better understanding of PDE structure, function, regulation and pharmacology will help explain how alterations in this central enzyme of visual transduction lead to visual disorders, retinal degeneration, and blindness. Our work will lead to better therapeutic strategies to regulate cGMP metabolism in retinal rods and cones as well as in other tissues where PDE5 is involved in cell signaling pathways.