Possible role of cortactin phosphorylation by protein kinase Cα in actin-bundle formation at growth cone

Cortactin: A Major Cellular Target of the Gastric Carcinogen Helicobacter pylori

Cortactin is an actin binding protein and actin nucleation promoting factor regulating cytoskeletal rearrangements in nearly all eukaryotic cell types. From this perspective, cortactin poses an attractive target for pathogens to manipulate a given host cell to their own benefit. One of the pathogens following this strategy is Helicobacter pylori, which can cause a variety of gastric diseases and has been shown to be the major risk factor for the onset of gastric cancer. During infection of gastric epithelial cells, H. pylori hijacks the cellular kinase signaling pathways, leading to the disruption of key cell functions. Specifically, by overruling the phosphorylation status of cortactin, H. pylori alternates the activity of molecular interaction partners of this important protein, thereby manipulating the performance of actin-cytoskeletal rearrangements and cell movement. In addition, H. pylori utilizes a unique mechanism to activate focal adhesion kinase, which subsequently prevents host epithelial cells from extensive lifting from the extracellular matrix in order to achieve chronic infection in the human stomach.

Bmal1-deficiency affects glial synaptic coverage of the hippocampal mossy fiber synapse and the actin cytoskeleton in astrocytes

Bmal1 is an essential component of the molecular clockwork, which drives circadian rhythms in cell function. In Bmal1-deficient (Bmal1-/-) mice, chronodisruption is associated with cognitive deficits and progressive brain pathology including astrocytosis indicated by increased expression of glial fibrillary acidic protein (GFAP). However, relatively little is known about the impact of Bmal1-deficiency on astrocyte morphology prior to astrocytosis. Therefore, in this study we analysed astrocyte morphology in young (6-8 weeks old) adult Bmal1-/- mice. At this age, overall GFAP immunoreactivity was not increased in Bmal1-deficient mice. At the ultrastructural level, we found a decrease in the volume fraction of the fine astrocytic processes that cover the hippocampal mossy fiber synapse, suggesting an impairment of perisynaptic processes and their contribution to neurotransmission. For further analyses of actin cytoskeleton, which is essential for distal process formation, we used cultured Bmal1-/- astrocytes. Bmal1-/- astrocytes showed an impaired formation of actin stress fibers. Moreover, Bmal1-/- astrocytes showed reduced levels of the actin-binding protein cortactin (CTTN). Cttn promoter region contains an E-Box like element and chromatin immunoprecipitation revealed that Cttn is a potential Bmal1 target gene. In addition, the level of GTP-bound (active) Rho-GTPase (Rho-GTP) was reduced in Bmal1-/- astrocytes. In summary, our data demonstrate that Bmal1-deficiency affects morphology of the fine astrocyte processes prior to strong upregulation of GFAP, presumably because of impaired Cttn expression and reduced Rho-GTP activation. These morphological changes might result in altered synaptic function and, thereby, relate to cognitive deficits in chronodisruption.

Phosphorylation of cortactin by cyclin-dependent kinase 5 modulates actin bundling by the dynamin 1-cortactin ring-like complex and formation of filopodia and lamellipodia in NG108-15 glioma-derived cells

Dynamin copolymerizes with cortactin to form a ring-like complex that bundles and stabilizes actin filaments. Actin bundle formation is crucial for generation of filopodia and lamellipodia, which guide migration, invasion, and metastasis of cancer cells. However, it is unknown how the dynamin-cortactin complex regulates actin bundle formation. The present study investigated phosphorylation of cortactin by cyclin-dependent kinase 5 (CDK5) and its effect on actin bundle formation by the dynamin-cortactin complex. CDK5 directly phosphorylated cortactin at T145/T219 in vitro. Phosphomimetic mutants in which one or both of these threonine residues was substituted by aspartate were used. The three phosphomimetic mutants (T145D, T219D and T145DT219D) had a decreased affinity for F-actin. Furthermore, electron microscopy demonstrated that these phosphomimetic mutants could not form a ring-like complex with dynamin 1. Consistently, the dynamin 1-phosphomimetic cortactin complexes exhibited decreased actin-bundling activity. Expression of the phosphomimetic mutants resulted in not only aberrant lamellipodia and short filopodia but also cell migration in NG108-15 glioma-derived cells. These results indicate that phosphorylation of cortactin by CDK5 regulates formation of lamellipodia and filopodia by modulating dynamin 1/cortactin-dependent actin bundling. Taken together, these findings suggest that CDK5 is a potential molecular target for anticancer therapy.

EXPRESS: F-actin links Epac-PKC signaling to purinergic P2X3 receptors sensitization in dorsal root ganglia following inflammation

Sensitization of purinergic P2X3 receptors (P2X3Rs) contributes to the production of exaggerated nociceptive responses following inflammatory injury. We showed previously that prostaglandin E2 (PGE2) potentiates P2X3R-mediated ATP currents in dorsal root ganglion neurons isolated from both control and complete Freund’s adjuvant-induced inflamed rats. PGE2 potentiation of ATP currents depends only on PKA signaling in control neurons, but it depends on both PKA and PKC signaling in inflamed neurons. We further found that inflammation evokes an increase in exchange proteins directly activated by cAMP (Epacs) in dorsal root ganglions. This increase promotes the activation of PKC to produce a much enhanced PGE2 effect on ATP currents and to elicit Epac-dependent flinch nocifensive behavioral responses in complete Freund’s adjuvant rats. The link between Epac-PKC signaling and P2X3R sensitization remains unexplored. Here, we show that the activation of Epacs promotes the expression of phosphorylated PKC and leads to an increase in the cytoskeleton, F-actin, expression at the cell perimeter. Depolymerization of F-actin blocks PGE2-enhanced ATP currents and inhibits P2X3R-mediated nocifensive responses after inflammation. Thus, F-actin is dynamically involved in the Epac-PKC-dependent P2X3R sensitization. Furthermore, Epacs induce a PKC-dependent increase in the membrane expression of P2X3Rs. This increase is abolished by F-actin depolymerization, suggesting that F-actin mediates Epac-PKC signaling of P2X3R membrane expression. Thus, after inflammation, an Epac-PKC dependent increase in F-actin in dorsal root ganglion neurons enhances the membrane expression of P2X3Rs to bring about sensitization of P2X3Rs and abnormal pain behaviors.