Enzymatic activity and gene expression of diacylglycerol kinase isozymes in developing retina of rats

Photoreceptors contain highly specialized structures for phototransduction, which is mediated by rhodopsins and heterotrimeric G-proteins. The signal is transmitted through the cGMP cascade, which controls cGMP-gated cation channels in mammals, while in flies it is operated by phosphoinositide (PI) cascade through a second messenger diacylglycerol (DG), which engenders the opening of Ca2+ channels. Recent studies suggest that PI-related signaling cascade is also involved in the phototransduction in mammalian retina. This study examined whether one PI-related enzyme, diacylglycerol kinase (DGK), which is regarded as a regulator of the DG signal through its metabolism, is expressed in mammalian retina. Enzymatic assay, Northern blot and RT-PCR analyses, and in situ hybridization histochemistry were performed to assess the expression profile of DGK isozymes and their cellular localization. In rat retina DGKε, DGKζ, and DGKι are the dominant species with distinct patterns of expression. At the cellular level, DGKε is the only one detected intensely in the photoreceptor layer, although DGKι and DGKζ are observed in bipolar and ganglion cell layers. These results suggest that each DGK isozyme plays a different role in the signal transduction in distinct cell types and that DGKε is a candidate involved in the photoreceptor PI signaling machinery.