【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 transduction 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 have generated NCKX1-deficient mice that have allowed us to establish the role of NCKX1 in regulating the Ca2+ homeostasis in mammalian rods and its effect on phototransduction and long-term rod survival and degeneration. Surprisingly, our results indicate the existence of additional, NCKX1-independent mechanism(s) for extruding Ca2+ from mammalian rods. We have also identified NCKX4 as a second Na+/Ca2+, K+ exchanger expressed in mammalian cones and have performed electrophysiological recordings from NCKX2- and NCKX4-deficient mouse cones. Our results demonstrate that the combined action of both NCKX2 and NCKX4 is required 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. Collectively, our results establish the molecular mechanisms that mediate the extrusion of Ca2+ from mammalian photoreceptors.
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 transduction 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 have generated NCKX1-deficient mice that have allowed us to establish the role of NCKX1 in regulating the Ca2+ homeostasis in mammalian rods and its effect on phototransduction and long-term rod survival and degeneration. Surprisingly, our results indicate the existence of additional, NCKX1-independent mechanism(s) for extruding Ca2+ from mammalian rods. We have also identified NCKX4 as a second Na+/Ca2+, K+ exchanger expressed in mammalian cones and have performed electrophysiological recordings from NCKX2- and NCKX4-deficient mouse cones. Our results demonstrate that the combined action of both NCKX2 and NCKX4 is required 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. Collectively, our results establish the molecular mechanisms that mediate the extrusion of Ca2+ from mammalian photoreceptors.