Sodium-calcium exchange in invertebrate photoreceptors

Light activation, adaptation, and cell survival functions of the Na+/Ca2+ exchanger CalX

In sensory neurons, Ca(2+) entry is crucial for both activation and subsequent attenuation of signaling. Influx of Ca(2+) is counterbalanced by Ca(2+) extrusion, and Na(+)/Ca(2+) exchange is the primary mode for rapid Ca(2+) removal during and after sensory stimulation. However, the consequences on sensory signaling resulting from mutations in Na(+)/Ca(2+) exchangers have not been described. Here, we report that mutations in the Drosophila Na(+)/Ca(2+) exchanger calx have a profound effect on activity-dependent survival of photoreceptor cells. Loss of CalX activity resulted in a transient response to light, a dramatic decrease in signal amplification, and unusually rapid adaptation. Conversely, overexpression of CalX had reciprocal effects and greatly suppressed the retinal degeneration caused by constitutive activity of the TRP channel. These results illustrate the critical role of Ca(2+) for proper signaling and provide genetic evidence that Ca(2+) overload is responsible for a form of retinal degeneration resulting from defects in the TRP channel.

Photoreceptor cells dissociated from the compound lateral eye of the horseshoe crab, Limulus polyphemus, II: Function

A combination of enzymatic digestions and mechanical disruption was used to isolate photoreceptor cells from the compound lateral eye of the horseshoe crab, Limulus polyphemus. The cells were maintained in a culture medium and tested for function using whole-cell and cell-attached patch configurations of the gigaseal technique. The cells dissociated from the eye generated spontaneous voltage and current bumps in the dark, and depolarized in a graded fashion to increasing intensities of light over several decades, producing responses similar to those of cells in vivo. Currents evoked during voltage clamp were similar to those in ventral photoreceptor cells of Limulus, although transient currents in the dark- and light-activated currents were smaller in isolated lateral eye cells, perhaps because of the slow speed and spatial nonuniformity of the clamp in these large cells. In addition to isolated cells, dissociation of the compound eye produced small clusters of cells and isolated ommatidia which were also tested for function. Comparison of the electrical characteristics of isolated cells with those of cells in small clusters and in their ommatidial matrix suggests that the electrical junctions normally connecting photoreceptor cells within an ommatidium are functional in the latter groups, but not in isolated cells. Cell-attached patches of rhabdomeral membrane of isolated cells contained light-activated channels, resembling those observed in ventral photoreceptor cells, but no voltage-activated channels. Similar patches of arhabdomeral membrane contained voltage-activated channels, but no light-activated channels. We conclude that this preparation is suitable for studies of processes involved in generating the light response in invertebrate photoreceptor cells.