Organizing Junctions at the Cell-Cell Interface
2008; Cell Press; Volume: 135; Issue: 5 Linguagem: Inglês
10.1016/j.cell.2008.11.002
ISSN1097-4172
Autores Tópico(s)Cellular Mechanics and Interactions
ResumoThe zonula adherens (ZA) is a specialized cadherin-based structure found at the contacts between epithelial cells. Meng et al., 2008Meng W. Mushika Y. Ichii T. Takeichi M. Cell. 2008; (this issue)Google Scholar now identify a protein complex containing the microtubule minus-end-binding protein Nezha, which provides a critical link between microtubules and cadherins in ZA biogenesis and maintenance. The zonula adherens (ZA) is a specialized cadherin-based structure found at the contacts between epithelial cells. Meng et al., 2008Meng W. Mushika Y. Ichii T. Takeichi M. Cell. 2008; (this issue)Google Scholar now identify a protein complex containing the microtubule minus-end-binding protein Nezha, which provides a critical link between microtubules and cadherins in ZA biogenesis and maintenance. Classical cadherin adhesion receptors play critical roles in tissue organization. They mediate cell-cell recognition and morphogenesis during development and support cohesion in established tissues. Cadherin dysfunction can have detrimental effects such as promoting the metastasis of epithelial cancers. The diverse morphogenetic roles of cadherin receptors are not due solely to their intrinsic adhesive properties but also depend on the crosstalk between cadherins and key elements of the cytoplasmic machinery involved in signal transduction, membrane trafficking, and cytoskeletal organization. Although cadherins have long been thought to cooperate with the actin cytoskeleton, less attention has been paid to their capacity to interact with microtubules. Reporting in this issue, Meng et al., 2008Meng W. Mushika Y. Ichii T. Takeichi M. Cell. 2008; (this issue)Google Scholar now identify a new cadherin-based protein complex in mammalian epithelial cells that can anchor microtubule minus ends to the zonula adherens (ZA) and is necessary for the biogenesis of this specialized junction. Cadherins localize at epithelial cell-cell contacts in two patterns: the first comprises clusters and strands distributed throughout the lateral interface, and the second consists of an apically located linear band that is thought to mark the ZA, a specialized cell-cell junction (Kametani and Takeichi, 2007Kametani Y. Takeichi M. Nat. Cell Biol. 2007; 9: 92-98Crossref PubMed Scopus (165) Google Scholar). As little is known about how the ZA is generated and maintained, Meng and colleagues set out to identify new regulators of the ZA in mammalian cells by searching for proteins that bind to the cadherin-associated protein p120-catenin. They uncover a new interacting partner, a pleckstrin homology (PH) domain protein of unknown function called PLEKHA7 (PH domain-containing family A, member 7). Immunoprecipitation analysis reveals that PLEKHA7 interacts with the E-cadherin-catenin complex. PLEKHA7 specifically colocalizes with the pool of E-cadherin at the ZA in a p120-dependent manner. When p120 was depleted from the cells, or when binding of E-cadherin to p120 was disrupted by mutation, PLEKHA7 was no longer found at the cell junctions. Importantly, Meng et al. report that PLEKHA7 expression is needed for the maintenance of ZA integrity. Specifically, PLEKHA7 appears to promote the incorporation of cadherin clusters into the higher-order structure of the ZA. The authors further identify another poorly characterized protein, which they call Nezha (formerly KIAA1543), as a binding partner of PLEKHA7. Depletion of Nezha from cultured human cells disrupted the ZA, implying that Nezha might mediate the impact of PLEKHA7 on the biogenesis of the ZA. Indeed, localization of Nezha to the ZA is perturbed when PLEKHA7 is absent. Interestingly, Nezha is only found at the ZA at late stages of junction maturation, after PLEKHA7 is already in place. Although PLEKHA7 appears to be necessary to recruit Nezha into the ZA, the association between these two proteins is likely to be highly regulated. During their characterization of Nezha, the authors make the surprising observation that it is a microtubule minus-end-binding protein. Microtubules are polarized polymers whose ends possess distinct dynamic properties. Microtubule plus ends serve as sites of polymerization; they are highly dynamic and associate with a large number of cellular factors (Akhmanova and Steinmetz, 2008Akhmanova A. Steinmetz M.O. Nat. Rev. Mol. Cell Biol. 2008; 9: 309-322Crossref PubMed Scopus (719) Google Scholar). The minus ends, on the other hand, serve as sites of microtubule anchoring or disassembly, and only a few proteins are known to specifically interact with them (Dammerman et al., 2003Dammerman A. Desai A. Oegema K. Curr. Biol. 2003; 13: R614-R624Abstract Full Text Full Text PDF PubMed Scopus (116) Google Scholar). The authors find that endogenous Nezha localizes to the minus ends of noncentrosomal microtubules in vivo and confirm that this protein binds to the minus ends of microtubules in vitro. Time-lapse microscopy of live cultured human cells reveals that Nezha-containing foci at cell-cell contacts appear to serve as sites of microtubule outgrowth and anchoring. Further, depolymerization of microtubules with nocodazole ultimately causes the ZA to fragment. This suggests an attractive model whereby the cadherin-based Nezha/PLEKHA7 complex supports ZA biogenesis by anchoring microtubules at their minus ends (Figure 1). Obtaining definitive proof for this model will require evidence that a significant population of microtubules is anchored by their minus ends at the ZA, which remains a challenge due to the high density of three-dimensional microtubule networks in differentiated epithelial cells. How might a minus-end-anchoring apparatus contribute to the biogenesis of the ZA? An important property of microtubule polarity is to provide directional tracks for motor-dependent transport of macromolecules and membranes. Indeed, the contribution of microtubules to cell-cell junctions has often been framed in terms of membrane transport. Consistent with this, Meng et al. report that kinesin-14/KIFC3, the minus-end-directed motor previously implicated in apical membrane transport in epithelial cells (Noda et al., 2001Noda Y. Okada Y. Saito N. Setou M. Xu Y. Zhang Z. Hirokawa N. J. Cell Biol. 2001; 155: 77-88Crossref PubMed Scopus (124) Google Scholar), is required for ZA integrity. It is tempting to speculate that Nezha-mediated anchorage of microtubule minus ends orients tracks for KIFC3-based delivery of molecules necessary for the biogenesis and maintenance of the ZA. However, the exact nature of the potential cargo remains to be determined. Interestingly, KIFC3 itself localizes to the ZA in a Nezha- and PLEKHA7-dependent fashion, suggesting that it may play a structural or signaling role or even participate in microtubule organization at epithelial cell-cell junctions. Irrespective of the underlying mechanism, characterization of Nezha and PLEKHA7 function emphasizes the importance of protein cooperativity in ZA biogenesis. Although PLEKHA7 is necessary to recruit Nezha into junctions, depletion of Nezha also causes PLEKHA7 to be lost from junctions. Thus, the incorporation of Nezha appears to stabilize PLEKHA7, either directly or indirectly. Similarly, microtubule depolymerization leads to the eventual loss of KIFC3 and Nezha and disruption of the ZA itself. This implies that the ZA, rather than being a static structure, is dynamic. Such cooperativity is likely to extend to other signaling and cytoskeletal interactions at the ZA. The findings of Meng et al. add to a growing body of evidence linking cadherins and microtubules. Microtubule disruption by overall depolymerization or by freezing the dynamic activity of the plus ends can perturb cadherin-based cell-cell contacts (Stehbens et al., 2006Stehbens S.J. Paterson A.D. Crampton M.S. Shewan A.M. Ferguson C. Akhmanova A. Parton R.G. Yap A.S. J. Cell Sci. 2006; 119: 1801-1811Crossref PubMed Scopus (138) Google Scholar, Waterman-Storer et al., 2000Waterman-Storer C.M. Salmon W.C. Salmon E.D. Mol. Biol. Cell. 2000; 11: 2471-2483Crossref PubMed Scopus (124) Google Scholar). Furthermore, other studies provide evidence that the dynamic plus ends of microtubules can also extend into cadherin-based junctions. Microtubules in fibroblasts and isolated epithelial cells are often organized as arrays with their minus ends anchored at centrosomes and plus ends facing the cell periphery. However, relocation of microtubule minus ends from the centrosomes to either the apical surface or to cell-cell contacts often occurs during epithelial biogenesis and is a process that depends on the microtubule minus-end-anchoring factor ninein (Dammerman et al., 2003Dammerman A. Desai A. Oegema K. Curr. Biol. 2003; 13: R614-R624Abstract Full Text Full Text PDF PubMed Scopus (116) Google Scholar, Lechler and Fuchs, 2007Lechler T. Fuchs E. J. Cell Biol. 2007; 176: 147-154Crossref PubMed Scopus (133) Google Scholar). Interestingly, Meng and colleagues report that Nezha does not colocalize with ninein, and, unlike PLEKHA7 and ninein, it does not bind to centrosomes. This observation will require further investigation to elucidate its place in the regulation of microtubule minus ends. Most importantly, this new study sheds light on the enigmatic biology of the ZA. This distinctive structure, first characterized by electron microscopy (Farquhar and Palade, 1963Farquhar M.G. Palade G.E. J. Cell Biol. 1963; 17: 375-412Crossref PubMed Scopus (1994) Google Scholar), is thought to be a higher-order molecular apparatus built on a cadherin core that also includes many other membrane proteins such as noncadherin adhesion molecules (Gumbiner, 2005Gumbiner B.M. Nat. Rev. Mol. Cell Biol. 2005; 6: 622-634Crossref PubMed Scopus (1127) Google Scholar). The observation by Meng et al. that Nezha, PLEKHA7, and KIFC3 appear to promote the accumulation of cadherin in the ZA at the expense of the cadherin pool at the cell's lateral contacts further supports the notion that the ZA represents a higher-order organization of cadherins at cell-cell junctions. Many functions have been suggested for the ZA beyond basic cadherin adhesion, including cell signaling, apical cell morphogenesis, and adhesive stabilization. However, efforts to dissect these roles for the ZA have been hampered by a lack of strategies to specifically perturb ZA formation without affecting basic cadherin function. The identification of mechanisms that regulate epithelial cell junction organization by Meng et al. and others opens the way for future studies examining the specific cellular and tissue functions of the ZA. Anchorage of Microtubule Minus Ends to Adherens Junctions Regulates Epithelial Cell-Cell ContactsMeng et al.CellNovember 28, 2008In BriefEpithelial cells contain noncentrosomal microtubules (MTs), whose minus ends are oriented apically. In contrast with the well-known interactions of the minus ends with the centrosome, little is known about the termination site of the noncentrosomal minus ends. Here we show that a population of MT minus ends is anchored at the zonula adherens (ZA), the apical-most part of the cadherin-based adherens junction, via a protein that we have termed Nezha. We initially identified PLEKHA7 as a ZA component and subsequently detected Nezha as a partner for PLEKHA7. Full-Text PDF Open Archive
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