Alpha S. Yap

Cadherins and cancer: how does cadherin dysfunction promote tumor progression?

It has long been recognized that the cell–cell adhesion receptor, E-cadherin, is an important determinant of tumor progression, serving as a suppressor of invasion and metastasis in many contexts. Yet how the loss of E-cadherin function promotes tumor progression is poorly understood. In this review, we focus on three potential underlying mechanisms: the capacity of E-cadherin to regulate β-catenin signaling in the canonical Wnt pathway; its potential to inhibit mitogenic signaling through growth factor receptors and the possible links between cadherins and the molecular determinants of epithelial polarity. Each of these potential mechanisms provides insights into the complexity that is likely responsible for the tumor-suppressive action of E-cadherin.

Lateral clustering of the adhesive ectodomain: a fundamental determinant of cadherin function.

BACKGROUND Classical cadherin-based cellular adhesion is mediated by a multicomponent protein complex that links the adhesive binding activity of the cadherin ectodomain to the actin cytoskeleton. Despite the importance of cadherins in morphogenesis and development, we know very little about how cells determine and alter cadherin adhesive strength. In this study, we sought to identify specific cellular mechanisms that modulate cadherin function by studying adhesion between cells transfected with Xenopus C-cadherin mutant molecules and substrata coated with the purified ectodomain of C-cadherin. RESULTS Using the FKBP-FK1012 protein oligomerization system, we found that forced clustering, in cells, of cadherin mutants lacking the cytoplasmic tail significantly increased cellular adhesive strength. Therefore, redistribution of the adhesive binding sites of cells into clusters can influence adhesion independently of other protein interactions mediated by the cadherin cytoplasmic tail. Furthermore, cells transfected with full-length C-cadherin demonstrated dynamic changes in adhesion over time that correlated with clustering but not with changes in the surface expression of C-cadherin or in the composition of the cadherin-catenin complex. The cytoplasmic tail was, however, necessary for clustering of wild-type cadherin. CONCLUSIONS These studies directly demonstrate a fundamental role for lateral clustering in cadherin function. The distribution of cadherin binding sites presented at the cell surface, a cellular property which is regulated by the cadherin cytoplasmic tail, is an important mechanism which modulates cellular adhesion independently of cytoskeletal activity or signalling.

Cadherin-Directed Actin Assembly: E-Cadherin Physically Associates with the Arp2/3 Complex to Direct Actin Assembly in Nascent Adhesive Contacts

Cadherin cell adhesion molecules are major determinants of tissue patterning which function in cooperation with the actin cytoskeleton. In the context of stable adhesion, cadherin/catenin complexes are often envisaged to passively scaffold onto cortical actin filaments. However, cadherins also form dynamic adhesive contacts during wound healing and morphogenesis. Here actin polymerization has been proposed to drive cell surfaces together, although F-actin reorganization also occurs as cell contacts mature. The interaction between cadherins and actin is therefore likely to depend on the functional state of adhesion. We sought to analyze the relationship between cadherin homophilic binding and cytoskeletal activity during early cadherin adhesive contacts. Dissecting the specific effect of cadherin ligation alone on actin regulation is difficult in native cell-cell contacts, due to the range of juxtacrine signals that can arise when two cell surfaces adhere. We therefore activated homophilic ligation using a specific functional recombinant protein. We report the first evidence that E-cadherin associates with the Arp2/3 complex actin nucleator and demonstrate that cadherin binding can exert an active, instructive influence on cells to mark sites for actin assembly at the cell surface.

Direct cadherin-activated cell signaling: a view from the plasma membrane

Classical cadherin adhesion molecules are key determinants of cell recognition and tissue morphogenesis, with diverse effects on cell behavior. Recent developments indicate that classical cadherins are adhesion-activated signaling receptors. In particular, early–immediate Rac signaling is emerging as a mechanism to coordinate cadherin–actin integration at the plasma membrane.

Tissue organization by cadherin adhesion molecules: dynamic molecular and cellular mechanisms of morphogenetic regulation

This review addresses the cellular and molecular mechanisms of cadherin-based tissue morphogenesis. Tissue physiology is profoundly influenced by the distinctive organizations of cells in organs and tissues. In metazoa, adhesion receptors of the classical cadherin family play important roles in establishing and maintaining such tissue organization. Indeed, it is apparent that cadherins participate in a range of morphogenetic events that range from support of tissue integrity to dynamic cellular rearrangements. A comprehensive understanding of cadherin-based morphogenesis must then define the molecular and cellular mechanisms that support these distinct cadherin biologies. Here we focus on four key mechanistic elements: the molecular basis for adhesion through cadherin ectodomains, the regulation of cadherin expression at the cell surface, cooperation between cadherins and the actin cytoskeleton, and regulation by cell signaling. We discuss current progress and outline issues for further research in these fields.

Myosin II isoforms identify distinct functional modules that support integrity of the epithelial zonula adherens

Classic cadherin receptors cooperate with regulators of the actin cytoskeleton to control tissue organization in health and disease. At the apical junctions of epithelial cells, the cadherin ring of the zonula adherens (ZA) couples with a contiguous ring of actin filaments to support morphogenetic processes such as tissue integration and cellular morphology. However, the molecular mechanisms that coordinate adhesion and cytoskeleton at these junctions are poorly understood. Previously we identified non-muscle myosin II as a target of Rho signalling that supports cadherin junctions in mammalian epithelial cells. Myosin II has various cellular functions, which are increasingly attributable to the specific biophysical properties and regulation of its different isoforms. Here we report that myosin II isoforms have distinct and necessary roles at cadherin junctions. Although two of the three mammalian myosin II isoforms are found at the ZA, their localization is regulated by different upstream signalling pathways. Junctional localization of myosin IIA required E-cadherin adhesion, Rho/ROCK and myosin light-chain kinase, whereas junctional myosin IIB depended on Rap1. Further, these myosin II isoforms support E-cadherin junction integrity by different mechanisms. Myosin IIA RNA-mediated interference (RNAi) selectively perturbed the accumulation of E-cadherin in the apical ZA, decreased cadherin homophilic adhesion and disrupted cadherin clustering. In contrast, myosin IIB RNAi decreased filament content, altered dynamics, and increased the lateral movement of the perijunctional actin ring. Myosin IIA and IIB therefore identify two distinct functional modules, with different upstream signals that control junctional localization, and distinct functional effects. We propose that these two isoform-based modules cooperate to coordinate adhesion receptor and F-actin organization to form apical cadherin junctions.

Cortactin is necessary for E-cadherin–mediated contact formation and actin reorganization

Classical cadherin adhesion molecules are key determinants of cell–cell recognition during development and in post-embryonic life. A decisive step in productive cadherin-based recognition is the conversion of nascent adhesions into stable zones of contact. It is increasingly clear that such contact zone extension entails active cooperation between cadherin adhesion and the force-generating capacity of the actin cytoskeleton. Cortactin has recently emerged as an important regulator of actin dynamics in several forms of cell motility. We now report that cortactin is recruited to cell–cell adhesive contacts in response to homophilic cadherin ligation. Notably, cortactin accumulates preferentially, with Arp2/3, at cell margins where adhesive contacts are being extended. Recruitment of cortactin is accompanied by a ligation-dependent biochemical interaction between cortactin and the cadherin adhesive complex. Inhibition of cortactin activity in cells blocked Arp2/3-dependent actin assembly at cadherin adhesive contacts, significantly reduced cadherin adhesive contact zone extension, and perturbed both cell morphology and junctional accumulation of cadherins in polarized epithelia. Together, our findings identify a necessary role for cortactin in the cadherin–actin cooperation that supports productive contact formation.

E-cadherin junctions as active mechanical integrators in tissue dynamics

During epithelial morphogenesis, E-cadherin adhesive junctions play an important part in mechanically coupling the contractile cortices of cells together, thereby distributing the stresses that drive cell rearrangements at both local and tissue levels. Here we discuss the concept that cellular contractility and E-cadherin-based adhesion are functionally integrated by biomechanical feedback pathways that operate on molecular, cellular and tissue scales.

Dynamic microtubules regulate the local concentration of E-cadherin at cell-cell contacts.

In contrast to the well-established relationship between cadherins and the actin cytoskeleton, the potential link between cadherins and microtubules (MTs) has been less extensively investigated. We now identify a pool of MTs that extend radially into cell-cell contacts and are inhibited by manoeuvres that block the dynamic activity of MT plus-ends (e.g. in the presence of low concentrations of nocodazole and following expression of a CLIP-170 mutant). Blocking dynamic MTs perturbed the ability of cells to concentrate and accumulate E-cadherin at cell-cell contacts, as assessed both by quantitative immunofluorescence microscopy and fluorescence recovery after photobleaching (FRAP) analysis, but did not affect either transport of E-cadherin to the plasma membrane or the amount of E-cadherin expressed at the cell surface. This indicated that dynamic MTs allow cells to concentrate E-cadherin at cell-cell contacts by regulating the regional distribution of E-cadherin once it reaches the cell surface. Importantly, dynamic MTs were necessary for myosin II to accumulate and be activated at cadherin adhesive contacts, a mechanism that supports the focal accumulation of E-cadherin. We propose that this population of MTs represents a novel form of cadherin-MT cooperation, where cadherin adhesions recruit dynamic MTs that, in turn, support the local concentration of cadherin molecules by regulating myosin II activity at cell-cell contacts.

Myosin VI and vinculin cooperate during the morphogenesis of cadherin cell–cell contacts in mammalian epithelial cells

Cooperation between cadherins and the actin cytoskeleton controls many aspects of epithelial biogenesis. We report here that myosin VI critically regulates the morphogenesis of epithelial cell–cell contacts. As epithelial monolayers mature in culture, discontinuous cell–cell contacts are initially replaced by continuous (cohesive) contacts. Myosin VI is recruited to cell contacts as they become linear and cohesive, where it forms a biochemical complex with epithelial cadherin (E-cadherin). Myosin VI is necessary for strong cadherin adhesion, for cells to form cohesive linear contacts, and for the integrity of the apical junctional complex. We find that vinculin mediates this effect of myosin VI. Myosin VI is necessary for vinculin and E-cadherin to interact. A combination of gain and loss of function approaches identifies vinculin as a downstream effector of myosin VI that is necessary for the integrity of intercellular contacts. We propose that myosin VI and vinculin form a molecular apparatus that generates cohesive cell–cell contacts in cultured mammalian epithelia.