Cadherin-actin interactions at adherens junctions

The adherens junction (AJ) is a major cell-cell junction that mediates cell recognition, adhesion, morphogenesis, and tissue integrity. Although AJs transmit forces generated by actomyosin from one cell to another, AJs have long been considered as an area where signal transduction from cadherin ligation takes place through cell adhesion. Through the efforts to understand embryonic or cellular morphogenesis, dynamic interactions between the AJ and actin filaments have become crucial issues to be addressed since actin association is essential for AJ development, remodeling and function. Here, I provide an overview of cadherin-actin interaction from morphological aspects and of possible molecular mechanisms revealed by recent studies.     

Molecular heterogeneity of adherens junctions

We describe here the subcellular distributions of three junctional proteins in different adherens-type contacts. The proteins examined include vinculin, talin, and a recently described 135-kD protein (Volk, T., and B. Geiger, 1984, EMBO (Eur. Mol. Biol. Organ.) J., 10:2249-2260). Immunofluorescent localization of the three proteins indicated that while vinculin was ubiquitously present in all adherens junctions, the other two showed selective and mutually exclusive association with either cell-substrate or cell-cell adhesions. Talin was abundant in focal contacts and in dense plaques of smooth muscle, but was essentially absent from intercellular junctions such as intercalated disks or adherens junctions of lens fibers. The 135-kD protein, on the other hand, was present in the latter two loci and was apparently absent from membrane-bound plaques of gizzard or from focal contacts. Radioimmunoassay of tissue extracts and immunolabeling of cultured chick lens cells indicated that the selective presence of talin and of the 135-kD protein in different cell contacts is spatially regulated within individual cells. On the basis of these findings it was concluded that adherens junctions are molecularly heterogeneous and consist of at least two major subgroups. Contacts with noncellular substrates contain talin and vinculin but not the 135-kD protein, whereas their intercellular counterparts contain the latter two proteins and are devoid of talin. The significance of these results and their possible relationships to contact-induced regulation of cell behavior are discussed.     

Molecular architecture of adherens junctions.

Adherens junctions are composed of a cadherin-catenin complex and its associated proteins. Recently, an increasing number of novel members of adherens junctions, including membrane and PDZ proteins, have been reported. Interactions among these components in adherens junctions seem to be dynamically regulated during the formation of adherens junction complexes in epithelial cells.     

Myosin-dependent remodeling of adherens junctions protects junctions from Snail-dependent disassembly

Although Snail is essential for disassembly of adherens junctions during epithelial–mesenchymal transitions (EMTs), loss of adherens junctions in Drosophila melanogaster gastrula is delayed until mesoderm is internalized, despite the early expression of Snail in that primordium. By combining live imaging and quantitative image analysis, we track the behavior of E-cadherin–rich junction clusters, demonstrating that in the early stages of gastrulation most subapical clusters in mesoderm not only persist, but move apically and enhance in density and total intensity. All three phenomena depend on myosin II and are temporally correlated with the pulses of actomyosin accumulation that drive initial cell shape changes during gastrulation. When contractile myosin is absent, the normal Snail expression in mesoderm, or ectopic Snail expression in ectoderm, is sufficient to drive early disassembly of junctions. In both cases, junctional disassembly can be blocked by simultaneous induction of myosin contractility. Our findings provide in vivo evidence for mechanosensitivity of cell–cell junctions and imply that myosin-mediated tension can prevent Snail-driven EMT.     

Calcium-induced assembly of adherens junctions in keratinocytes

Extracellular calcium concentration has been shown to control the stratification of cultured keratinocytes, presumably by regulation of formation of desmosomes. Previous studies have shown that keratinocytes cultured in medium containing 0.1 mM Ca++ form loose colonies without desmosomes. If the Ca++ is raised to 1 mM, desmosomes are assembled and the distribution of keratin filaments is altered. We have examined the disposition of vinculin and actin in keratinocytes under similar conditions. Using immunofluorescence microscopy we show that raising [Ca++] in the medium dramatically alters the distribution of vinculin and actin and results in the formation of adherens-type junctions within 15 min after switching to high calcium medium. Borders of cells at the edge of colonies, which are not proximal to other cells, are not affected, while cells in the interior of the colony form junctions around their periphery. Attachment plaques in keratinocytes grown in low calcium medium are located at the ventral plane of the cell, but junctions formed after switching to high calcium are not, as demonstrated by interference reflection microscopy. In cells colabeled with antibodies against vinculin and desmoplakin, vinculin-containing adherens junctions were visible before desmosomal junctions when cells were switched to high calcium. Although newly formed vinculin-containing structures in high calcium cells, like desmosomes, colocalize with phase-dense structures, superimposition of video fluorescence images using digitized fluorescence microscopy indicates that adherens junctions and desmosomes are discrete structures. Adherens junctions, like desmosomes, may play an essential role in controlling stratification of keratinocytes.     

Trypanosoma cruzi alters adherens junctions in cardiomyocytes

We analyzed the distribution and expression of cadherin and beta-catenin during Trypanosoma cruzi-cardiomyocyte interaction. Confocal microscopy revealed cadherin associated with beta-catenin at the cell-cell contacts. After 24h of infection, the spatial distribution and expression of both adherens junction (AJ) proteins remained unaltered. In contrast, loss of N-cadherin-catenin complex was visualized in highly infected cardiomyocytes. Immunoblotting assays corroborated the spatial disorder, showing a 46% reduction in both N-cadherin and beta-catenin expression at later infection (72h of infection). Our data demonstrate that T. cruzi infection disturbs AJs, which can result in loss of cardiac tension and may contribute to the cardiac dysfunctions present in T. cruzi infection.     

Isolation of cell-to-cell adherens junctions from rat liver

A new isolation procedure for cell-to-cell adherens junctions has been developed using rat liver. From the bile canaliculi-enriched fraction obtained by homogenization of the liver and sucrose gradient centrifugation, the fraction rich in adherens junction was recovered by detergent treatment followed by sucrose gradient centrifugation. Light and electron microscopy revealed that this final fraction was mainly composed of the belt-like adherens junctions with their associated short actin filaments. Biochemical and immunological analyses have shown that vinculin is highly enriched in this fraction. Considering that vinculin is known to be localized in the cell-to-cell adherens junctions, we can conclude that we have succeeded in isolating the cell-to-cell adherens junctions. Furthermore, the constituents of the undercoat (dense layer underlying the membrane) of adherens junctions were selectively extracted from the fraction rich in junctions. Upon SDS electrophoresis of this extract, 10 polypeptides including vinculin, alpha-actinin, and actin were dominant. The results obtained are discussed with special reference to the molecular organization of the undercoats of cell-to-cell adherens junctions.     

E-N-cadherin heterodimers define novel adherens junctions connecting endoderm-derived cells

Intercellular junctions play a pivotal role in tissue development and function and also in tumorigenesis. In epithelial cells, decrease or loss of E-cadherin, the hallmark molecule of adherens junctions (AJs), and increase of N-cadherin are widely thought to promote carcinoma progression and metastasis. In this paper, we show that this "cadherin switch" hypothesis does not hold for diverse endoderm-derived cells and cells of tumors derived from them. We show that the cadherins in a major portion of AJs in these cells can be chemically cross-linked in E-N heterodimers. We also show that cells possessing E-N heterodimer AJs can form semistable hemihomotypic AJs with purely N-cadherin-based AJs of mesenchymally derived cells, including stroma cells. We conclude that these heterodimers are the major AJ constituents of several endoderm-derived tissues and tumors and that the prevailing concept of antagonistic roles of these two cadherins in developmental and tumor biology has to be reconsidered.     

Remodeling of endothelial adherens junctions by Kaposi's sarcoma-associated herpesvirus

Vascular endothelial cadherin (VE-cadherin) connects neighboring endothelial cells (ECs) via interendothelial junctions and regulates EC proliferation and adhesion during vasculogenesis and angiogenesis. The cytoplasmic domain of VE-cadherin recruits α- and β-catenins and γ-catenin, which interact with the actin cytoskeleton, thus modulating cell morphology. Dysregulation of the adherens junction/cytoskeletal axis is a hallmark of invasive tumors. We now demonstrate that the transmembrane ubiquitin ligase K5/MIR-2 of Kaposi's sarcoma-associated herpesvirus targets VE-cadherin for ubiquitin-mediated destruction, thus disturbing EC adhesion. In contrast, N-cadherin levels in K5-expressing cells were increased compared to those in control cells. Steady-state levels of α- and β-catenins and γ-catenin in K5-expressing ECs were drastically reduced due to proteasomal destruction. Moreover, the actin cytoskeleton was rearranged, resulting in the dysregulation of EC barrier function as measured by electric cell-substrate impedance sensing. Our data represent the first example of a viral protein targeting adherens junction proteins and suggest that K5 contributes to EC proliferation, vascular leakage, and the reprogramming of the EC proteome during Kaposi's sarcoma tumorigenesis.     

Sticky business: orchestrating cellular signals at adherens junctions

Cohesive sheets of epithelial cells are a fundamental feature of multicellular organisms and are largely a product of the varied functions of adherens junctions. These junctions and their cytoskeletal associations contribute heavily to the distinct shapes, polarity, spatially oriented mitotic spindle planes, and cellular movements of developing tissues. Deciphering the underlying mechanisms that govern these conserved cellular rearrangements is a prerequisite to understanding vertebrate morphogenesis.     

Plasma membrane components of adherens junctions (Review)

This review focuses on the three known plasma membrane components of adherens junctions: E-cadherin, nectin-2 and vezatin. The structures of these three components are discussed, with particular emphasis on the molecular mechanisms by which E-cadherin and nectin-2 promote cell adhesion.     

Myofibroblast development is characterized by specific cell-cell adherens junctions

Myofibroblasts of wound granulation tissue, in contrast to dermal fibroblasts, join stress fibers at sites of cadherin-type intercellular adherens junctions (AJs). However, the function of myofibroblast AJs, their molecular composition, and the mechanisms of their formation are largely unknown. We demonstrate that fibroblasts change cadherin expression from N-cadherin in early wounds to OB-cadherin in contractile wounds, populated with alpha-smooth muscle actin (alpha-SMA)-positive myofibroblasts. A similar shift occurs during myofibroblast differentiation in culture and seems to be responsible for the homotypic segregation of alpha-SMA-positive and -negative fibroblasts in suspension. AJs of plated myofibroblasts are reinforced by alpha-SMA-mediated contractile activity, resulting in high mechanical resistance as demonstrated by subjecting cell pairs to hydrodynamic forces in a flow chamber. A peptide that inhibits alpha-SMA-mediated contractile force causes the reorganization of large stripe-like AJs to belt-like contacts as shown for enhanced green fluorescent protein-alpha-catenin-transfected cells and is associated with a reduced mechanical resistance. Anti-OB-cadherin but not anti-N-cadherin peptides reduce the contraction of myofibroblast-populated collagen gels, suggesting that AJs are instrumental for myofibroblast contractile activity.     

The role of adherens junctions in the developing neocortex

The disproportional enlargement of the neocortex through evolution has been instrumental in the success of vertebrates, in particular mammals. The neocortex is a multilayered sheet of neurons generated from a simple proliferative neuroepithelium through a myriad of mechanisms with substantial evolutionary conservation. This developing neuroepithelium is populated by progenitors that can generate additional progenitors as well as post-mitotic neurons. Subtle alterations in the production of progenitors vs. differentiated cells during development can result in dramatic differences in neocortical size. This review article will examine how cadherin adhesion proteins, in particular α-catenin and N-cadherin, function in regulating the neural progenitor microenvironment, cell proliferation, and differentiation in cortical development.