CALCIUM HOMEOSTASIS IN MAMMALIAN ROD AND CONE PHOTORECEPTORS

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ABSTRACT Calcium (Ca2+) is a ubiquitous signaling molecule that controls the function and survival of neurons. The disrupted Ca2+ homeostasis in a wide range of photoreceptor mutations is believed to cause cell death, retinal degeneration and blindness. In vertebrate photoreceptors, Ca2+ changes also modulate the shutoff of the phototransduction cascade to accelerate light response recovery and background adaptation. It is thought that the concentration of Ca2+ in the outer segments of vertebrate photoreceptors is controlled by a dynamic balance between influx via the cGMP-gated (CNG) channels and extrusion via cell-specific Na+/Ca2+, K+ exchangers (NCKX), NCKX1 in rods and NCKX2 in cones. However, the extent to which these exchangers control the Ca2+ homeostasis in mammalian photoreceptors and modulate phototransduction and cell survival has not been determined. In addition, it is not known whether other active or passive mechanisms for extruding Ca2+ are at play in the outer segments of mammalian rods and cones. We will perform experiments to establish the role of CNG and NCKX1 in regulating the Ca2+ homeostasis in mammalian rods and their effect on long-term rod survival and degeneration. We will also test the hypothesis that abnormal photoreceptor Ca2+ homeostasis mediates photoreceptor degeneration in a variety of blinding diseases and will determine the therapeutic potential of restoring the Ca2+ flux balance in photoreceptor channelopathies. We have identified NCKX4 as a second Na+/Ca2+, K+ exchanger expressed in mammalian cones. We will perform experiments to analyze the expression profile, morphology, and functional properties of NCKX2- and NCKX4- deficient mouse cones. These experiments will establish the molecular mechanisms for the efficient extrusion of Ca2+ from mammalian cone photoreceptors critical for the fast response kinetics and background adaptation of cones as our daytime photoreceptors as well as their effect on cone long-term survival and degeneration. Collectively, our experiments will establish the molecular mechanisms that mediate the extrusion of Ca2+ from mammalian photoreceptors. They will also help us understand the link between abnormal Ca2+ homeostasis and photoreceptor degeneration and might potentially lead to the development of treatments for channelopaties.