Katsumi Fumoto

GSK-3β-regulated interaction of BICD with dynein is involved in microtubule anchorage at centrosome

Microtubule arrays direct intracellular organization and define cellular polarity. Here, we show a novel function of glycogen synthase kinase‐3β (GSK‐3β) in the organization of microtubule arrays through the interaction with Bicaudal‐D (BICD). BICD is known to form a complex with dynein–dynactin and to function in the intracellular vesicle trafficking. Our data revealed that GSK‐3β is required for the binding of BICD to dynein but not to dynactin. Knockdown of GSK‐3β or BICD reduced centrosomally focused microtubules and induced the mislocalization of centrosomal proteins. The unfocused microtubules in GSK‐3β knockdown cells were rescued by the expression of the dynein intermediate chain‐BICD fusion protein. Microtubule regrowth assays showed that GSK‐3β and BICD are required for the anchoring of microtubules to the centrosome. These results imply that GSK‐3β may function in transporting centrosomal proteins to the centrosome by stabilizing the BICD1 and dynein complex, resulting in the regulation of a focused microtubule organization.

Axin localizes to the centrosome and is involved in microtubule nucleation

Axin is known to have an important role in the degradation of β‐catenin in the Wnt pathway. Here, we reveal a new function of Axin at the centrosome. Axin was localized to the centrosome in various cell lines and formed a complex with γ‐tubulin. Knockdown of Axin reduced the localization of γ‐tubulin and γ‐tubulin complex protein 2—components of the γ‐tubulin ring complex—to the centrosome and the centrosomal microtubule nucleation activity after treatment with nocodazole. These phenotypes could not be rescued by the reduction in the levels of β‐catenin. Although the expression of Axin rescued these phenotypes in Axin‐knockdown cells, overexpression of Axin2, which is highly homologous to Axin, could not. Axin2 was also localized to the centrosome, but it did not form a complex with γ‐tubulin. These results suggest that Axin, but not Axin2, is involved in microtubule nucleation by forming a complex with γ‐tubulin at the centrosome.

GSK-3β regulates proper mitotic spindle formation in cooperation with a component of the γ-tubulin ring complex, GCP5

Glycogen synthase kinase-3β (GSK-3β) is known to play a role in the regulation of the dynamics of microtubule networks in cells. Here we show the role of GSK-3β in the proper formation of the mitotic spindles through an interaction with GCP5, a component of the γ-tubulin ring complex (γTuRC). GCP5 bound directly to GSK-3β in vitro, and their interaction was also observed in intact cells at endogenous levels. Depletion of GCP5 dramatically reduced the GCP2 and γ-tubulin in the γTuRC fraction of sucrose density gradients and disrupted γ-tubulin localization to the spindle poles in mitotic cells. GCP5 appears to be required for the formation or stability of γTuRC and the recruitment of γ-tubulin to the spindle poles. A GSK-3 inhibitor not only led to the accumulation of γ-tubulin and GCP5 at the spindle poles but also enhanced microtubule nucleation activity at the spindle poles. Depletion of GCP5 rescued this disrupted organization of spindle poles observed in cells treated with the GSK-3 inhibitor. Furthermore, the inhibition of GSK-3 enhanced the binding of γTuRC to the centrosome isolated from mitotic cells in vitro. Our findings suggest that GSK-3β regulates the localization of γTuRC, including GCP5, to the spindle poles, thereby controlling the formation of proper mitotic spindles.

Phosphorylation of myosin II regulatory light chain is necessary for migration of HeLa cells but not for localization of myosin II at the leading edge

To investigate the role of phosphorylated myosin II regulatory light chain (MRLC) in living cell migration, these mutant MRLCs were engineered and introduced into HeLa cells. The mutant MRLCs include an unphosphorylatable form, in which both Thr-18 and Ser-19 were substituted with Ala (AA-MRLC), and pseudophosphorylated forms, in which Thr-18 and Ser-19 were replaced with Ala and Asp, respectively (AD-MRLC), and both Thr-18 and Ser-19 were replaced with Asp (DD-MRLC). Mutant MRLC-expressing cell monolayers were mechanically stimulated by scratching, and the cells were forced to migrate in a given direction. In this wound-healing assay, the AA-MRLC-expressing cells migrated much more slowly than the wild-type MRLC-expressing cells. In the case of DD-MRLC- and AD-MRLC-expressing cells, no significant differences compared with wild-type MRLC-expressing cells were observed in their migration speed. Indirect immunofluorescence staining showed that the accumulation of endogenous diphosphorylated MRLC at the leading edge was not observed in AA-MRLC-expressing cells, although AA-MRLC was incorporated into myosin heavy chain and localized at the leading edge. In conclusion, we propose that the phosphorylation of MRLC is required to generate the driving force in the migration of the cells but not necessary for localization of myosin II at the leading edge.

AIP regulates stability of Aurora-A at early mitotic phase coordinately with GSK-3β

Glycogen synthase kinase-3 (GSK-3β) regulates microtubule dynamics and cellular polarity through phosphorylating various microtubule associating proteins and plus-end tracking proteins. Although it was also reported that GSK-3β is inactivated by protein kinase B at the spindle poles, functions and targets of GSK-3β in the mitotic phase are unknown. Here, we identified Aurora-A-interacting protein (AIP), a negative regulator of Aurora-A, as a binding partner of GSK-3β. AIP was colocalized with Aurora-A and GSK-3β to the spindle poles in metaphase, and its depletion in cells stabilized and activated Aurora-A in early mitotic phase and caused mitotic cell arrest. Treatment of the cells with a GSK-3β inhibitor reduced the protein level of Aurora-A and this reduction was suppressed by AIP knockdown. AIP was phosphorylated by GSK-3β, and an AIP mutant in which the GSK-3β phosphorylation site was mutated could bind and downregulate Aurora-A more efficiently. These results suggest that GSK-3β modulates the early mitotic Aurora-A level through binding and phosphorylating AIP.

CKAP4 is a Dickkopf1 receptor and is involved in tumor progression

Dickkopf1 (DKK1) is a secretory protein that antagonizes oncogenic Wnt signaling by binding to the Wnt coreceptor low-density lipoprotein receptor-related protein 6 (LRP6). DKK1 may also regulate its own signaling to promote cancer cell proliferation, but the mechanism is not understood. Here, we identified cytoskeleton-associated protein 4 (CKAP4) as a DKK1 receptor and evaluated CKAP4-mediated DKK1 signaling in cancer cell proliferation. We determined that DKK1 binds CKAP4 and LRP6 with similar affinity but interacts with these 2 receptors with different cysteine-rich domains. DKK1 induced internalization of CKAP4 in a clathrin-dependent manner, further supporting CKAP4 as a receptor for DKK1. DKK1/CKAP4 signaling activated AKT by forming a complex between the proline-rich domain of CKAP4 and the Src homology 3 domain of PI3K, resulting in proliferation of normal cells and cancer cells. Expression of DKK1 and CKAP4 was frequent in tumor lesions of human pancreatic and lung cancers, and simultaneous expression of both proteins in patient tumors was negatively correlated with prognosis and relapse-free survival. An anti-CKAP4 antibody blocked the binding of DKK1 to CKAP4, suppressed AKT activity in a human cancer cell line, and attenuated xenograft tumor formation in immunodeficient mice. Together, our results suggest that CKAP4 is a potential therapeutic target for cancers that express both DKK1 and CKAP4.

Wnt5a signaling promotes apical and basolateral polarization of single epithelial cells.

Wnt signal plays important roles in polarization. Here intestinal epithelial cells are shown to form apicobasal polarization at a single-cell level in an extracellular matrix adhesion–dependent manner. Wnt5a signaling promotes single-cell polarization through balanced control between Rac1 and RhoA activities spatially.

Wnt5a signaling controls cytokinesis by correctly positioning ESCRT-III at the midbody

Summary Wnts activate at least two signaling pathways, the &bgr;-catenin-dependent and -independent pathways. Although the &bgr;-catenin-dependent pathway is known to contribute to G1–S transition, involvement of the &bgr;-catenin-independent pathway in cell cycle regulation remains unclear. Here, we show that Wnt5a signaling, which activates the &bgr;-catenin-independent pathway, is required for cytokinesis. Dishevelled 2 (Dvl2), a mediator of Wnt signaling pathways, was localized to the midbody during cytokinesis. Beside the localization of Dvl2, Fz2, a Wnt receptor, was detected in the midbody with the endosomal sorting complex required for transport III (ESCRT-III) subunit, CHMP4B. Depletion of Wnt5a, its receptors, and Dvl increased multinucleation. The phenotype observed in Wnt5a-depleted cells was rescued by the addition of purified Wnt5a but not Wnt3a, which is a ligand for the &bgr;-catenin-dependent pathway. Moreover, depletion of Wnt5a signaling caused loss of stabilized microtubules and mislocalization of CHMP4B at the midbody, which affected abscission. Inhibition of the stabilization of microtubules at the midbody led to the mislocalization of CHMP4B, while depletion of CHMP4B did not affect the stabilization of microtubules, suggesting that the correct localization of CHMP4B depends on microtubules. Fz2 was localized to the midbody in a Rab11-dependent manner, probably along stabilized microtubules. Fz2 formed a complex with CHMP4B upon Wnt5a stimulation and was required for proper localization of CHMP4B at the midbody, while CHMP4B was not necessary for the localization of Fz2. These results suggest that Wnt5a signaling positions ESCRT-III in the midbody properly for abscission by stabilizing midbody microtubules.

Modulation of apical constriction by Wnt signaling is required for lung epithelial shape transition

In lung development, the apically constricted columnar epithelium forms numerous buds during the pseudoglandular stage. Subsequently, these epithelial cells change shape into the flat or cuboidal pneumocytes that form the air sacs during the canalicular and saccular (canalicular-saccular) stages, yet the impact of cell shape on tissue morphogenesis remains unclear. Here, we show that the expression of Wnt components is decreased in the canalicular-saccular stages, and that genetically constitutive activation of Wnt signaling impairs air sac formation by inducing apical constriction in the epithelium as seen in the pseudoglandular stage. Organ culture models also demonstrate that Wnt signaling induces apical constriction through apical actomyosin cytoskeletal organization. Mathematical modeling reveals that apical constriction induces bud formation and that loss of apical constriction is required for the formation of an air sac-like structure. We identify MAP/microtubule affinity-regulating kinase 1 (Mark1) as a downstream molecule of Wnt signaling and show that it is required for apical cytoskeletal organization and bud formation. These results suggest that Wnt signaling is required for bud formation by inducing apical constriction during the pseudoglandular stage, whereas loss of Wnt signaling is necessary for air sac formation in the canalicular-saccular stages. Summary: Epithelial morphogenesis during the transition from the early to late stage of lung development is coordinated by the activity of Wnt signaling.

The Dickkopf1-CKAP4 axis creates a novel signaling pathway and may represent a molecular target for cancer therapy

Dickkopf 1 (DKK1) is a secreted protein and antagonizes oncogenic Wnt signalling by binding to the Wnt co‐receptor, low‐density lipoprotein receptor‐related protein 6. DKK1 has also been suggested to regulate its own signalling, associated with tumour aggressiveness. However, the underlying mechanism by which DKK1 promotes cancer cell proliferation has remained to be clarified for a long time. The cytoskeleton‐associated protein 4 (CKAP4), originally identified as an endoplasmic reticulum membrane protein, was recently found to act as a novel DKK1 receptor. DKK1 stimulates cancer cell proliferation when CKAP4 is expressed on the cell surface membrane. Although there are no tyrosine residues in the intracellular region of CKAP4, CKAP4 forms a complex with PI3K upon the binding of DKK1, leading to the activation of Akt. Both DKK1 and CKAP4 are frequently expressed in pancreatic and lung tumours, and their simultaneous expression is negatively correlated with prognosis. Knockdown of CKAP4 in cancer cells and treatment of mice with the anti‐CKAP4 antibody inhibit Akt activity in cancer cells and suppress xenograft tumour formation, suggesting that CKAP4 may represent a therapeutic target for cancers expressing both DKK1 and CKAP4. This review will provide details of the novel DKK1‐CKAP4 signalling axis that promotes cancer proliferation and discuss the possibility of targeting this pathway in future cancer drug development.