Loss of Pde6 reduces cell body Ca2+ transients within photoreceptors

Modulation of Ca²⁺ within cells is tightly regulated through complex and dynamic interactions between the plasma membrane and internal compartments. In this study, we exploit in vivo imaging strategies based on genetically encoded Ca²⁺ indicators to define changes in perikaryal Ca²⁺ concentration of intact photoreceptors. We developed double-transgenic zebrafish larvae expressing GCaMP3 in all cones and tdTomato in long-wavelength cones to test the hypothesis that photoreceptor degeneration induced by mutations in the phosphodiesterase-6 (Pde6) gene is driven by excessive Ca²⁺]_i levels within the cell body. Arguing against Ca²⁺ overload in Pde6 mutant photoreceptors, simultaneous analysis of cone photoreceptor morphology and Ca²⁺ fluxes revealed that degeneration of pde6c^w59 mutant cones, which lack the cone-specific cGMP phosphodiesterase, is not associated with sustained increases in perikaryal Ca²⁺]_i. Analysis of Ca²⁺]_i in dissociated Pde6β^rd1mouse rods shows conservation of this finding across vertebrates. In vivo, transient and Pde6-independent Ca²⁺ elevations (‘flashes'') were detected throughout the inner segment and the synapse. As the mutant cells proceeded to degenerate, these Ca²⁺ fluxes diminished. This study thus provides insight into Ca²⁺ dynamics in a common form of inherited blindness and uncovers a dramatic, light-independent modulation of Ca²⁺]_i that occurs in normal cones.