Language：English   Publishing type：Research paper (scientific journal)   Publisher：Journal of Cell Biology  

The epithelial cell sheet functions as a barrier to prevent invasion of pathogens. It is necessary to eliminate intercellular gaps not only at bicellular junctions, but also at tricellular contacts, where three cells meet, to maintain epithelial barrier function. To that end, tight junctions between adjacent cells must associate as closely as possible, particularly at tricellular contacts. Tricellulin is an integral component of tricellular tight junctions (tTJs), but the molecular mechanism of its contribution to the epithelial barrier function remains unclear. In this study, we revealed that tricellulin contributes to barrier formation by regulating actomyosin organization at tricellular junctions. Furthermore, we identified α-catenin, which is thought to function only at adherens junctions, as a novel binding partner of tricellulin. α-catenin bridges tricellulin attachment to the bicellular actin cables that are anchored end-on at tricellular junctions. Thus, tricellulin mobilizes actomyosin contractility to close the lateral gap between the TJ strands of the three proximate cells that converge on tricellular junctions.

DOI： 10.1083/jcb.202009037

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Journal of Cell Biology  

Epithelial cells are constantly exposed to osmotic stress. The influx of water molecules into the cell in a hypo-osmotic environment increases plasma membrane tension as it rapidly expands. Therefore, the plasma membrane must be supplied with membrane lipids since expansion beyond its elastic limit will cause the cell to rupture. However, the molecular mechanism to maintain a constant plasma membrane tension is not known. In this study, we found that the apical membrane selectively expands when epithelial cells are exposed to hypo-osmotic stress. This requires the activation of mTORC2, which enhances the transport of secretory vesicles containing sphingomyelin, the major lipid of the apical membrane. We further show that the mTORC2-Rab35 axis plays an essential role in the defense against hypotonic stress by promoting the degradation of the actin cortex through the up-regulation of PI(4,5)P2 metabolism, which facilitates the apical tethering of sphingomyelin-loaded vesicles to relieve plasma membrane tension.

DOI： 10.1083/jcb.202106160

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Language：English   Publishing type：Research paper (scientific journal)  

The cytoplasm in mammalian cells is considered homogeneous. In this study, we report that the cytoplasmic fluidity is regulated in the blebbing cells; the cytoplasm of rapidly expanding membrane blebs is more disordered than the cytoplasm of retracting blebs. The increase of cytoplasmic fluidity in the expanding bleb is caused by a sharp rise in the calcium concentration. The STIM-Orai1 pathway regulates this rapid and restricted increase of calcium in the expanding blebs. Conversely, activated ERM protein binds to Orai1 to inhibit the store-operated calcium entry in retracting blebs, which results in decreased in cytoplasmic calcium, rapid reassembly of the actin cortex.

DOI： 10.1038/s41467-020-20826-5

Language：English   Publishing type：Research paper (scientific journal)  

Tiam1 is one of the most extensively analyzed activators of the small GTPase Rac. However, fundamental aspects of its regulation are poorly understood. Here we demonstrate that Tiam1 is functionally suppressed by internal interactions and that the PAR complex participates in its full activation. The N-terminal region of Tiam1 binds to the protein-binding and catalytic domains to inhibit its localization and activation. Atypical PKCs phosphorylate Tiam1 to relieve its intramolecular interactions, and the subsequent stabilization of its interaction with PAR3 allows it to exert localized activity. By analyzing Tiam1 regulation by PAR3-aPKC within the context of PDGF signaling, we also show that PAR3 directly binds PDGF receptor β. Thus we provide the first evidence for the negative regulation of Tiam1 by internal interactions, elucidate the nature of Tiam1 regulation by the PAR complex, and reveal a novel role for the PAR complex in PDGF signaling.

DOI： 10.1091/mbc.E15-09-0670

Language：English   Publishing type：Research paper (scientific journal)  

Microtubules (MTs) play critical roles in various cellular events, including cell migration. End-binding proteins (EBs) accumulate at the ends of growing MTs and regulate MT end dynamics by recruiting other plus end-tracking proteins (+TIPs). However, how EBs contribute to MT dynamics through +TIPs remains elusive. We focused on tau-tubulin kinase 2 (TTBK2) as an EB1/3-binding kinase and confirmed that TTBK2 acted as a +TIP. We identified MT-depolymerizing kinesin KIF2A as a novel substrate of TTBK2. TTBK2 phosphorylated KIF2A at S135 in intact cells in an EB1/3-dependent fashion and inactivated its MT-depolymerizing activity in vitro. TTBK2 depletion reduced MT lifetime (facilitated shrinkage and suppressed rescue) and impaired HeLa cell migration, and these phenotypes were partially restored by KIF2A co-depletion. Expression of nonphosphorylatable KIF2A, but not wild-type KIF2A, reduced MT lifetime and slowed down the cell migration. These findings indicate that TTBK2 with EB1/3 phosphorylates KIF2A and antagonizes KIF2A-induced depolymerization at MT plus ends for cell migration.

DOI： 10.1083/jcb.201412075

Language：English   Publishing type：Research paper (scientific journal)  

The organization of the Golgi apparatus is essential for cell polarization and its maintenance. The polarity regulator PAR complex (PAR3, PAR6, and aPKC) plays critical roles in several processes of cell polarization. However, how the PAR complex participates in regulating the organization of the Golgi remains largely unknown. Here we demonstrate the functional cross-talk of the PAR complex with CLASP2, which is a microtubule plus-end-tracking protein and is involved in organizing the Golgi ribbon. CLASP2 directly interacted with PAR3 and was phosphorylated by aPKC. In epithelial cells, knockdown of either PAR3 or aPKC induced the aberrant accumulation of CLASP2 at the trans-Golgi network (TGN) concomitantly with disruption of the Golgi ribbon organization. The expression of a CLASP2 mutant that inhibited the PAR3-CLASP2 interaction disrupted the organization of the Golgi ribbon. CLASP2 is known to localize to the TGN through its interaction with the TGN protein GCC185. This interaction was inhibited by the aPKC-mediated phosphorylation of CLASP2. Furthermore, the nonphosphorylatable mutant enhanced the colocalization of CLASP2 with GCC185, thereby perturbing the Golgi organization. On the basis of these observations, we propose that PAR3 and aPKC control the organization of the Golgi through CLASP2 phosphorylation.

DOI： 10.1091/mbc.E14-09-1382

Language：English   Publishing type：Research paper (scientific journal)  

Migrating cells acquire front-rear polarity with a leading edge and a trailing tail for directional movement. The Rac exchange factor Tiam1 participates in polarized cell migration with the PAR complex of PAR3, PAR6, and atypical protein kinase C. However, it remains largely unknown how Tiam1 is regulated and contributes to the establishment of polarity in migrating cells. We show here that Tiam1 interacts directly with talin, which binds and activates integrins to mediate their signaling. Tiam1 accumulated at adhesions in a manner dependent on talin and the PAR complex. The interactions of talin with Tiam1 and the PAR complex were required for adhesion-induced Rac1 activation, cell spreading, and migration toward integrin substrates. Furthermore, Tiam1 acted with talin to regulate adhesion turnover. Thus, we propose that Tiam1, with the PAR complex, binds to integrins through talin and, together with the PAR complex, thereby regulates Rac1 activity and adhesion turnover for polarized migration.

DOI： 10.1083/jcb.201202041

Language：English   Publishing type：Research paper (scientific journal)  

Cadherin trafficking controls tissue morphogenesis and cell polarity. The endocytic adaptor Numb participates in apicobasal polarity by acting on intercellular adhesions in epithelial cells. However, it remains largely unknown how Numb controls cadherin-based adhesion. Here, we found that Numb directly interacted with p120 catenin (p120), which is known to interact with E-cadherin and prevent its internalization. Numb accumulated at intercellular adhesion sites and the apical membrane in epithelial cells. Depletion of Numb impaired E-cadherin internalization, whereas depletion of p120 accelerated internalization. Expression of the Numb-binding fragment of p120 inhibited E-cadherin internalization in a dominant-negative fashion, indicating that Numb interacts with the E-cadherin/p120 complex and promotes E-cadherin endocytosis. Impairment of Numb induced mislocalization of E-cadherin from the lateral membrane to the apical membrane. Atypical protein kinase C (aPKC), a member of the PAR complex, phosphorylated Numb and inhibited its association with p120 and α-adaptin. Depletion or inhibition of aPKC accelerated E-cadherin internalization. Wild-type Numb restored E-cadherin internalization in the Numb-depleted cells, whereas a phosphomimetic mutant or a mutant with defective α-adaptin-binding ability did not restore the internalization. Thus, we propose that aPKC phosphorylates Numb to prevent its binding to p120 and α-adaptin, thereby attenuating E-cadherin endocytosis to maintain apicobasal polarity.

DOI： 10.1091/mbc.E11-03-0274

Language：English   Publishing type：Research paper (scientific journal)  

Polarised cell migration is required for various cell behaviours and functions. Actin and microtubules are coupled structurally and distributed asymmetrically along the front-rear axis of migrating cells. CLIP-associating proteins (CLASPs) accumulate near the ends of microtubules at the front of migrating cells to control microtubule dynamics and cytoskeletal coupling. Regional inhibition of GSK-3beta is responsible for this asymmetric distribution of CLASPs. However, it is not known how GSK-3beta regulates the activity of CLASPs for linkage between actin and microtubules. Here we identified IQGAP1, an actin-binding protein, as a novel CLASP-binding protein. GSK-3beta directly phosphorylates CLASP2 at Ser533 and Ser537 within the region responsible for the IQGAP1 binding. Phosphorylation of CLASP2 results in the dissociation of CLASP2 from IQGAP1, EB1 and microtubules. At the leading edges of migrating fibroblasts, CLASP2 near microtubule ends partially colocalises with IQGAP1. Expression of active GSK-3beta abrogates the distribution of CLASP2 on microtubules, but not that of a nonphosphorylatable CLASP2 mutant. The phosphorylated CLASP2 does not accumulate near the ends of microtubules at the leading edges. Thus, phosphorylation of CLASP2 by GSK-3beta appears to control the regional linkage of microtubules to actin filaments through IQGAP1 for cell migration.

DOI： 10.1242/jcs.046649

Event date： 2021.12 - 2022.6

Language：English  

Venue：オンライン   Country：Japan  

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Event date： 2021.6 - 2021.7

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Multicellular tissues are covered by a continuous sheet of epithelial cells. The epithelial cells adhere to each other through cell adhesion structures, the apical junctional complexes (AJC), that are reinforced by a network of actomyosin filaments. Contractility of the actomyosin network drives the constriction of the apical plasma membrane, a hallmark of epithelial cells that underlies cell shape changes during developmental morphogenesis and maintenance of tissue integrity in homeostasis. However, it is unclear how the core components of the AJC modulate apical contractility in the broader context of the epithelial cell sheet.
In this study, we present evidence that the membrane-associated guanylate kinases (MAGUK) family proteins MAGI-1 and MAGI-3 are key negative regulators of apical membrane contractility. We find that MAGI knockout in a model epithelial cell line enriches non-muscle myosin IIB (NMIIB) as well as the myosin activator Rho-associated protein kinase 1 (ROCK1) at AJC. As a result of the heightened cellular contractility, the MAGI knockout cell sheet is populated by cells having irregularly sized apical domains. Members of MAGI proteins are differentially recruited to the AJC by the scaffolding proteins afadin and the zonula occludens (ZO) family proteins. MAGI then recruit ankyrin-repeat-, SH3-domain- and proline-rich-region-containing protein 2 (ASPP2), which in turn controls the localizations of the polarity proteins Partitioning defective-3 (Par-3) and atypical protein kinase C (aPKC) at AJC. We further clarify the functional interaction between ASPP2 and Par-3 by elucidating the requirement for ASPP2 recruitment of protein phosphatase 1 (PP1) to retain aPKC at AJC, which is necessary to antagonize ROCK. These results taken together indicate that homeostasis of the epithelial sheet morphology requires conformity of two factors within a certain range by the constituent cells: junctional ROCK activity and NMIIB recruitment. Finally, signaling from AJC to Par-3-aPKC through MAGI is a crucial means of normalizing this activity level, owing to MAGIs’ role in regulating local phosphorylation dynamics through ASPP2-PP1.

Event date： 2019.12

Language：Japanese  

Venue：福岡市   Country：Japan  

Event date： 2019.6

Language：English  

Venue：東京   Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Symposium, workshop panel (public)  

Country：Japan  

Language：Japanese