Phototransduction and Signaling in Photoreceptors

Johns Hopkins University, Baltimore MD

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Abstract

PROJECT SUMMARY There have been great advances over the past 30-40 years in understanding how light initiates vision in the rod and cone photoreceptors of the eye. Not only is the sequence of events in this process known, but the genes coding for the key phototransduction proteins have also been cloned. Mutations of many of these genes are found to be associated with various vision-impairing diseases. The phototransduction mechanism in rods is particularly well understood - down to quantitative details - and serves as a blueprint for understanding other G-protein-coupled-receptor signaling pathways all over the body. Now, a frontier of phototransduction research lies in cones, which are more important than rods for human vision. Although our knowledge about rod transduction generally applies to cones, much still remains unknown about how the substantially lower photosensitivity and faster response kinetics of cones compared to rods come about, as well as their far greater ability to adapt to bright light. Most phototransduction proteins have distinct rod- and cone-isoforms, but how these distinct isoforms underlie the overall physiological differences between rods and cones, and among cone types, remain unclear. The long-term objective of the proposed research is to understand in detail the phototransduction mechanism in retinal cones, especially its differences from that in rods. Starting with the visual pigment, we hope to systematically work our way down the signaling pathway. One major advance we have made recently is the solving of a protracted puzzle/controversy about the spontaneous activity of rod and cone holopigments in darkness, by showing that it indeed arises from a canonical isomerization of the pigment, except being driven by thermal energy instead of light. We have also developed a theory that successfully predicts the 107-fold range in thermal activity across the rod and cone pigments in the visible spectrum. For this grant's Aim 1, we shall capitalize on these advances and further examine the thermal activity of native or mutant pigments. For Aim 2, we address how the different thermal activities of red, green and blue pigments may affect the response properties of the associated cone types. Moving down the signaling cascade, we shall examine, for Aim 3, subtle aspects of transducin function. We shall also ask whether the lifetime of transducin dictates the decline of the light response in cones as it does in rods. Finally, for short Aim 4, we shall examine the reported biphasic feature of the primate cone response, which is quite distinct from the primate rod response. Sorting out the above issues will help understand not only normal photoreceptor physiology, but also questions such as the different susceptibilities of rods and cones to certain disease states.