Retinal photoreceptor cells can maintain a wide operating range by adjusting their sensitivity as a function of ambient illumination. In microvillar photoreceptors, the predominant type amongst invertebrate organisms, light is encoded by coupling the photoexcited rhodopsin to the light-sensitive ion channels via activation of the phosphoinositide pathway. Light-induced elevation of cytosolic Ca 2+ has long been known to be a key factor in light adaptation, but its downstream targets and molecular mechanisms of action remain to be determined. Protein kinase C (PKC) is a particularly promising candidate not only because of its sensitivity to Ca2+ and diacyiglycerol (DAG), the two messengers generated by the light-triggered signaling cascade, but also because in Drosophila an eye-specific PKC partakes of the supramolecular light-signaling complex (the “transducisome”). The aim of this study is to test the hypothesis that, in microvillar photoreceptors of the file clam Lima scabra, Ca2+ may mediate light adaptation by activating PKC. We used Western blot analysis to detect two “classical” (i.e. Ca2+-dependent) PKC isoforms in eye homogenates: PKC-Apl I and PKC-α. Immunofluorescence studies on isolated retinal cells demonstrated that both isoforms are found in the photoreceptors; however, whereas Apl I is distributed throughout the cell, PKC-α localizes exclusively on the photosensitive lobe, suggesting some specific role in light signaling. Evidence that photostimulation activates PKC was obtained by demonstrating light-dependent translocation of PKC-α immunofluorescence from the cytosol to the lightsensitive membrane in a manner similar to phorbol esters, well-known activators of PKC. The effect of pharmacological inhibition of PKC, either by bisindolylmalemide derivatives preferentially effective on the α-isoform or by synthetic peptides corresponding to the autoinhibitory domain of the protein, was studied electrophysiologically. PKC antagonists enhanced photoreceptors basal sensitivity; more important, they specifically counteracted the desensitization of the photoresponse induced either by conditioning flashes or by steady background light. We propose that activation of an α-like isoform of PKC is an important contributor to the process of light adaptation initiated by photo-induced elevation of intracellular Ca2+; the existence of several key enzymatic steps along the light-signaling pathway that are targets of PKC phosphorylation lends further support to this scheme.
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