Transmembrane Signaling Regulates the Remodeling of the Actin Cytoskeleton: Roles of PKC and Adducin

Abstract

Members of the conventional protein kinase C (cPKC) family are activated by both DAG and Ca2+ and have been implicated in the regulation of the actin cytoskeleton. Drosophila contains two cPKCs, Pkc53E (Pkc1) and eye-PKC (Pkc2); mutants missing each PKC lead to retinal degeneration. While eye-PKC is critical for the visual signaling, the role of Pkc53E is not known. We identified a photoreceptor-specific isoform of Pkc53E and show Pkc53E-RNAi negatively impacts the actin cytoskeleton of rhabdomeres. Interestingly, Pkc53E-RNAi enhances the degeneration of norpAP24 photoreceptors, suggesting Pkc53E could be activated independently of NorpA/PLCβ4. We further demonstrate that in norpAP24 photoreceptors Plc21C can be activated by Gq, which is responsible for the activation of Pkc53E. We explored whether Pkc53E regulates adducin in Drosophila photoreceptors. Adducin cross-links the actin cytoskeleton to the spectrin network, which is blunted by PKC phosphorylation. Importantly, we observed that phosphorylation of adducin was greatly reduced in a null allele of pkc53E. Downregulation of hts that encodes Drosophila adducin, exerts a more severe effect than Pkc53E-RNAi to impact the actin cytoskeleton. In contrast, overexpression of a mCherry tagged Add2, one of the three Drosophila adducin isoforms, led to the apical expansion of rhabdomeres with overgrowth of the actin cytoskeleton. This phenotype is likely caused by the dominant-negative activity of the tagged Add2 as it also was observed in α-spectrin-RNAi or β-spectrin-RNAi. Interestingly, downregulation of Pkc53E does not suppress the expansion of rhabdomeres during development, but negatively affects the appearance of rhabdomeres in adult photoreceptors. We conclude that Drosophila adducin has two distinct functions: in pupal photoreceptors, it regulates rhabdomere morphogenesis, which is independent of Pkc53E. In adult photoreceptors, it promotes the maintenance of the actin cytoskeleton, which is regulated by Pkc53E in response to the light-induced activation of the PLCβ activity.