Early events in cell adhesion and polarity during epithelial-mesenchymal transition

Abstract

During evolution from primitive species, a mechanism, termed epithelial-mesenchymal transition (EMT), was established to create mesenchymal cells from a pre-existing epithelial cell layer. However, the direct and full conversion from an epithelial to a mesenchymal state is not observed in all species (Yin et al., 2009). Intermediate-staged cells harbor the greatest plasticity because they are able to either reverse to an epithelial state or progress to a mesenchymal phenotype. Epithelial cells are characterized by well-developed, intercellular contacts, whereas mesenchymal cells seldom form intercellular contacts. Therefore, to better characterize the hallmarks of intermediate-stage cells, it is important to understand how intercellular contacts change during early EMT. This Cell Science at a Glance article focuses on adhesion and polarity, which are restructured during EMT in vitro. The molecular anatomy of junctional complexes and the dynamics of some of their components during EMT will be described in intermediate stages that lead to a fully converted mesenchymal state.A prototypic monolayered epithelium is characterized by apical, lateral and basal plasma membrane domains. The lateral domain contains morphologically distinct, well-demarcated intercellular adhesive structures, including tight junctions (TJs), adherens junctions (AJs) and desmosomes, that contribute to the establishment of apical-basal polarity (Nelson, 2009). Below, we describe the dynamics involved in the formation of these junctional complexes during the establishment of apical-basal polarity.Nectins comprise a family of cell adhesion proteins that contain three immunoglobulin (Ig) domains. Nectins are involved in nascent contact between epithelial cells (Takai and Nakanishi, 2003) through Ca2+-independent heterophilic binding (e.g. nectin-1 binds to nectin-3), which forms the scaffold upon which E-cadherin-based adhesive structures assemble and recruit other adherens components. Through their PDZ-binding domain, nectins associate with afadin, a large cytoplasmic protein that binds directly to actin microfilaments (Takai and Nakanishi, 2003). E-cadherin, a prototypic, Ca2+-dependent, adhesion molecule that contains five Ig domains, subsequently accumulates at these nectin scaffolds and forms trans-homophilic interactions through its N-terminus, and cis-interactions between the first and second Ig domains (Harrison et al., 2011). The armadillo repeat-containing proteins β-catenin and p120-catenin interact with the cytoplasmic domain of E-cadherin through their C-terminal and juxtamembrane segments, respectively. β-catenin binds to α-catenin, which, in turn, binds to other proteins ensuring the connection to actin microfilaments (Nishimura and Takeichi, 2009). In conjunction with afadin, proteins that connect to actin microfilaments enforce nascent junctional stability. However, such a stabilizing effect is still debated, because actin polymerization is inhibited by monomeric α-catenin (the form that binds cytoplasmic β-catenin), but not dimeric α-catenin (Drees et al., 2005; Yamada et al., 2005).These nascent cadherin-based adhesions, often called ‘punctates’, progressively mature to form AJs (Adams et al., 1996). This first requires the active assembly of actin filaments within cadherin-rich punctates, which involves nucleation of actin polymerization by the actin-like protein complex ARP2/3 and actinin-4 (Tang and Brieher, 2012). Mature AJs provide a scaffold for the assembly of a circumferential acto-myosin contractile belt at the apex of epithelial cells to ensure stable interactions with neighboring cells that reside as epithelial sheets (Shapiro and Weis, 2009). EPLIN (also known as LIMA1), an actin-filament-bundling protein, establishes such connections in fully developed adherens junctions, forming the zonula adherens at the apex of the lateral domain of polarized epithelial cells (Taguchi et al., 2011). Therefore, actin dynamics have a profound effect on AJ stability.AJ formation and stability are controlled by several protein complexes, in particular GTPases and their corresponding G-protein exchange factors (GEFs). For example, TARA (also known as TRIOBP), an actin-binding adaptor protein, activates the GEF triple functional domain (TRIO), which in turn promotes actin polymerization through Rac activation (Yano et al., 2011). The Rac, CDC42 GTPases and ARP2/3 also induce localized actin polymerization to initiate nascent junctions in the vicinity of lamellipodia by forming complexes with the Abi-WAVE complex [the Abelson interactor proteins 1 and 2 in complex with a member of the Wislott-Aldrich syndrome protein (WASP) family]. Abi also binds Diaphanous, a member of the formin family, which competes for the same binding site as WAVE. This Abi-Diaphanous complex polymerizes linear actin filaments and stabilizes junctions. Both the Abi-Diaphanous and Abi-WAVE complexes form in the vicinity of E-cadherin–β-catenin complexes (Ryu