Both rod and cone photoreceptors absorb light, triggering an amplification cascade, which produces membrane hyperpolarization through closure of selective ion channels. Radiating from the light-initiated event is a G-protein-coupled response of secondary activities, including rhodopsin or cone opsin receptor shut-off through a GRK1 phosphorylation and subsequent binding of either rod or cone arrestin. Cone photoreceptors are distinct from rods in morphology, light sensitivity, recovery rate, thermal stability, timing of outer segment shedding and resistance to programmed cell death by apoptosis. Characterization of the dynamic interactions and functions of these cone gene and their gene products may provide a basis for diagnosis, treatment or prevention of age related macular degeneration and other retinal rod and cone degenerations, thus preserving vision for currently untreatable forms of blindness. To address the distinct aspects inherent to the cone photo-transduction pathway and to accomplish our goals, experiments are designed to explore the function(s) of cone arrestin (CAR), its targeted G protein-coupled receptors (S and M opsin pigments) and other potential relevant partners in the cone synapse. Our working hypothesis, based in part on our ongoing biochemical and electrophysiological studies, support a role for CAR in regulating cone photo-transduction through binding to light-activated, GRK1 phosphorylated S and M opsins. We propose that when this X-chromosomal gene encoding CAR is genetically deleted with mouse knockout (KO) technology, a defective receptor shut-off will lead to a delayed recovery of cone photoresponses. To test this hypothesis, the specific aims and experimental design include 1) characterization of the morphological, biochemical and electrophysiological retinal phenotypes of the newly generated Car KO. Further experiments will explore these parameters in Grk1/Car double KO mice on two genetic backgrounds (transducin alpha -/- with normal rod morphology but no rod response and Nrl -/- with pure cone retina) compared to wildtype;2) examine the effects of Grk1 S and M opsin phosphorylation and CAR binding on the cone visual retinoid cycle pathway;and 3) identification of other potential interacting cone synaptic partners for CAR and its alternatively spliced isoforms. Studies of the photo-transduction cascade and the molecular triggers for initiation and termination of high acuity vision are vital for sustaining lifelong vision.
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