| This thesis describes a molecular genetic approach to characterize Drosophila phototransduction, a signaling cascade that has served as model for a variety of sensory modalities. The Drosophila cascade is initiated by activation of the photo-receptor, rhodopsin, which consists of an opsin and a chromophore, 11-cis-retinal or similar derivatives. In the first part of my thesis, we establish the Drosophila visual system as a genetic model for characterizing retinoid binding proteins and retinal metabolism. We identified a novel CRAL-TRIO domain protein, PINTA, which binds to all-trans retinol. We demonstrate that PINTA functions subsequent to the production of vitamin A and is expressed and required in the retinal pigment cells. Our results represent the first genetic evidence for a role for the retinal pigment cells in the visual response and suggest that these cells are the closest invertebrate equivalent to the RPE.; In the second part of my thesis, I focused on the mechanism of Ca 2+ extrusion in the photoreceptor cells. In sensory neurons, such as Drosophila photoreceptors, Ca2+ entry is crucial for activation and subsequent attenuation of signaling. In this thesis we establish that Na+/Ca2+ exchanger, CalX, is the primary Ca2+ extrusion mechanism in the photoresponse. Loss of CalX activity results in a transient response to light, a dramatic decrease in signal amplification and unusually rapid adaptation. Overexpression of CalX had reciprocal effects and greatly suppressed the retinal degeneration caused by constitutive activity of the TRP channel. These latter results provide genetic evidence that Ca2+ overload is responsible for a form of retinal degeneration resulting from defects in the TRP channel. Moreover, retinal degeneration in the trp loss-of-function mutant was due primarily to deficient Ca2+ entry rather than disruption of the TRP anchoring function. |
