A ß-catenin chromobody-based probe highlights endothelial maturation during vascular morphogenesis in vivo.

Visualization of protein dynamics is a crucial step in understanding cellular processes. Chromobodies, fluorescently labelled single-domain antibodies, have emerged as versatile probes for live cell imaging of endogenous proteins. However, how these chromobodies behave in vivo and how accurately they monitor tissue changes remain poorly explored. Here, we generated an endothelial-specific ß-catenin chromobody-derived probe and analyzed its expression pattern during cardiovascular development in zebrafish. Using high-resolution confocal imaging, we show that the chromobody signal correlates with the localization of ß-catenin in the nucleus and at cell-cell junctions, and thereby can be used to assess endothelial maturation. Loss of Cadherin 5 strongly affects the localization of the chromobody at the cell membrane, confirming the cadherin-based adherens junction role of ß-catenin. Furthermore, using a genetic model to block blood flow, we observed that cell junctions are compromised in most endothelial cells but not in the endocardium, highlighting the heterogeneous response of the endothelium to the lack of blood flow. Overall, our data further expand the use of chromobodies for in vivo applications and illustrate their potential to monitor tissue morphogenesis at high resolution.

Actin Cytoskeleton Remodeling Accompanied by Redistribution of Adhesion Proteins Drives Migration of Cells in Different EMT States.

Epithelial-mesenchymal transition (EMT) is a process during which epithelial cells lose epithelial characteristics and gain mesenchymal features. Here, we used several cell models to study migratory activity and redistribution of cell-cell adhesion proteins in cells in different EMT states: EGF-induced EMT of epithelial IAR-20 cells; IAR-6-1 cells with a hybrid epithelial-mesenchymal phenotype; and their more mesenchymal derivatives, IAR-6-1-DNE cells lacking adherens junctions. In migrating cells, the cell-cell adhesion protein α-catenin accumulated at the leading edges along with ArpC2/p34 and α-actinin. Suppression of α-catenin shifted cell morphology from fibroblast-like to discoid and attenuated cell migration. Expression of exogenous α-catenin in MDA-MB-468 cells devoid of α-catenin drastically increased their migratory capabilities. The Y654 phosphorylated form of ß-catenin was detected at integrin adhesion complexes (IACs). Co-immunoprecipitation studies indicated that α-catenin and pY654-ß-catenin were associated with IAC proteins: vinculin, zyxin, and α-actinin. Taken together, these data suggest that in cells undergoing EMT, catenins not participating in assembly of adherens junctions may affect cell migration.

MARCH family E3 ubiquitin ligases selectively target and degrade cadherin family proteins.

Cadherin family proteins play a central role in epithelial and endothelial cell-cell adhesion. The dynamic regulation of cell adhesion is achieved in part through endocytic membrane trafficking pathways that modulate cadherin cell surface levels. Here, we define the role for various MARCH family ubiquitin ligases in the regulation of cadherin degradation. We find that MARCH2 selectively downregulates VE-cadherin, resulting in loss of adherens junction proteins at cell borders and a loss of endothelial barrier function. Interestingly, N-cadherin is refractory to MARCH ligase expression, demonstrating that different classical cadherin family proteins are differentially regulated by MARCH family ligases. Using chimeric cadherins, we find that the specificity of different MARCH family ligases for different cadherins is conferred by the cadherin transmembrane domain. Further, juxta-membrane lysine residues are required for cadherin degradation by MARCH proteins. These findings expand our understanding of cadherin regulation and highlight a new role for mammalian MARCH family ubiquitin ligases in differentially regulating cadherin turnover.

HIF2α-dependent Dock4/Rac1-signaling regulates formation of adherens junctions and cell polarity in normoxia.

Hypoxia-inducible factors (HIF) 1 and 2 regulate similar but distinct sets of target genes. Although HIFs are best known for their roles in mediating the hypoxia response accumulating evidence suggests that under certain conditions HIFs, particularly HIF2, may function also under normoxic conditions. Here we report that HIF2α functions under normoxic conditions in kidney epithelial cells to regulate formation of adherens junctions. HIF2α expression was required to induce Dock4/Rac1/Pak1-signaling mediating stability and compaction of E-cadherin at nascent adherens junctions. Impaired adherens junction formation in HIF2α- or Dock4-deficient cells led to aberrant cyst morphogenesis in 3D kidney epithelial cell cultures. Taken together, we show that HIF2α functions in normoxia to regulate epithelial morphogenesis.

Neurotensin modulates ovarian vascular permeability via adherens junctions.

Neurotensin (NTS) is a 13-amino acid peptide which is highly expressed in the mammalian ovary in response to the luteinizing hormone surge. Antibody neutralization of NTS in the ovulatory follicle of the cynomolgus macaque impairs ovulation and induces follicular vascular dysregulation, with excessive pooling of red blood cells in the follicle antrum. We hypothesize that NTS is an essential intrafollicular regulator of vascular permeability. In the present study, follicle injection of the NTS receptor antagonist SR142948 also resulted in vascular dysregulation. To measure vascular permeability changes in vitro, primary macaque ovarian microvascular endothelial cells (mOMECs) were enriched from follicle aspirates and studied in vitro. When treated with NTS, permeability of mOMECs decreased. RNA sequencing (RNA-Seq) of mOMECs revealed high mRNA expression of the permeability-regulating adherens junction proteins N-cadherin (CDH2) and K-cadherin (CDH6). Immunofluorescent detection of CDH2 and CDH6 confirmed expression and localized these cadherins to the cell-cell boundaries, consistent with function as components of adherens junctions. mOMECs did not express detectable levels of the typical vascular endothelial cadherin, VE-cadherin (CDH5) as determined by RNA-Seq, qPCR, western blot, and immunofluorescence. Knockdown of CDH2 or CDH6 via siRNA abrogated the NTS effect on mOMEC permeability. Collectively, these data suggest that NTS plays an ovulation-critical role in vascular permeability maintenance, and that CDH2 and CDH6 are involved in the permeability modulating effect of NTS on the ovarian microvasculature. NTS can be added to a growing number of angiogenic regulators which are critical for successful ovulation.

α-catenin middle- and actin-binding domain unfolding mutants differentially impact epithelial strength and sheet migration.

α-catenin (α-cat) displays force-dependent unfolding and binding to actin filaments through direct and indirect means, but features of adherens junction structure and function most vulnerable to loss of these allosteric mechanisms have not been directly compared. By reconstituting an α-cat F-actin-binding domain unfolding mutant known to exhibit enhanced binding to actin (α-cat-H0-FABD+) into α-cat knockout Madin Darby Canine Kidney (MDCK) cells, we show that partial loss of the α-cat catch bond mechanism (via an altered H0 α-helix) leads to stronger epithelial sheet integrity with greater colocalization between the α-cat-H0-FABD+ mutant and actin. α-cat-H0-FABD+ -expressing cells are less efficient at closing scratch-wounds, suggesting reduced capacity for more dynamic cell-cell coordination. Evidence that α-cat-H0-FABD+ is equally accessible to the conformationally sensitive α18 antibody epitope as WT α-cat and shows similar vinculin recruitment suggests this mutant engages lower tension cortical actin networks, as its M-domain is not persistently open. Conversely, α-cat-M-domain salt-bridge mutants with persistent recruitment of vinculin and phosphorylated myosin light chain show only intermediate monolayer adhesive strengths, but display less directionally coordinated and thereby slower migration speeds during wound-repair. These data show α-cat M- and FABD-unfolding mutants differentially impact cell-cell cohesion and migration properties, and suggest signals favoring α-cat-cortical actin interaction without persistent M-domain opening may improve epithelial monolayer strength through enhanced coupling to lower tension actin networks.

Repeat DNA methylation is modulated by adherens junction signaling.

Through its involvement in gene transcription and heterochromatin formation, DNA methylation regulates how cells interact with their environment. Nevertheless, the extracellular signaling cues that modulate the distribution of this central chromatin modification are largely unclear. DNA methylation is highly abundant at repetitive elements, but its investigation in live cells has been complicated by methodological challenges. Utilizing a CRISPR/dCas9 biosensor that reads DNA methylation of human α-satellite repeats in live cells, we here uncover a signaling pathway linking the chromatin and transcriptional state of repetitive elements to epithelial adherens junction integrity. Specifically, we find that in confluent breast epithelial cell monolayers, α-satellite repeat methylation is reduced by comparison to low density cultures. This is coupled with increased transcriptional activity at repeats. Through comprehensive perturbation experiments, we identify the junctional protein E-cadherin, which links to the actin cytoskeleton, as a central molecular player for signal relay into the nucleus. Furthermore, we find that this pathway is impaired in cancer cells that lack E-cadherin and are not contact-inhibited. This suggests that the molecular connection between cell density and repetitive element methylation could play a role in the maintenance of epithelial tissue homeostasis.

The Dilute domain in Canoe is not essential for linking cell junctions to the cytoskeleton but supports morphogenesis robustness.

Robust linkage between adherens junctions and the actomyosin cytoskeleton allows cells to change shape and move during morphogenesis without tearing tissues apart. The Drosophila multidomain protein Canoe and its mammalian homolog afadin are crucial for this, as in their absence many events of morphogenesis fail. To define the mechanism of action for Canoe, we are taking it apart. Canoe has five folded protein domains and a long intrinsically disordered region. The largest is the Dilute domain, which is shared by Canoe and myosin V. To define the roles of this domain in Canoe, we combined biochemical, genetic and cell biological assays. AlphaFold was used to predict its structure, providing similarities and contrasts with Myosin V. Biochemical data suggested one potential shared function - the ability to dimerize. We generated Canoe mutants with the Dilute domain deleted (CnoΔDIL). Surprisingly, they were viable and fertile. CnoΔDIL localized to adherens junctions and was enriched at junctions under tension. However, when its dose was reduced, CnoΔDIL did not provide fully wild-type function. Furthermore, canoeΔDIL mutants had defects in the orchestrated cell rearrangements of eye development. This reveals the robustness of junction-cytoskeletal connections during morphogenesis and highlights the power of natural selection to maintain protein structure.

ESCRT-I protein UBAP1 controls ventricular expansion and cortical neurogenesis via modulating adherens junctions of radial glial cells.

Intricate cerebral cortex formation is orchestrated by the precise behavior and division dynamics of radial glial cells (RGCs). Endocytosis functions in the recycling and remodeling of adherens junctions (AJs) in response to changes in RGC activity and function. Here, we show that conditional disruption of ubiquitin-associated protein 1 (UBAP1), a component of endosomal sorting complex required for transport (ESCRT), causes severe brain dysplasia and prenatal ventriculomegaly. UBAP1 depletion disrupts the AJs and polarity of RGCs, leading to failure of apically directed interkinetic nuclear migration. Accordingly, UBAP1 knockout or knockdown results in reduced proliferation and precocious differentiation of neural progenitor cells. Mechanistically, UBAP1 regulates the expression and surface localization of cell adhesion molecules, and ß-catenin over-expression significantly rescues the phenotypes of Ubap1 knockdown in vivo. Our study reveals a critical physiological role of the ESCRT machinery in cortical neurogenesis by regulating AJs of RGCs.

The zonula adherens matura redefines the apical junction of intestinal epithelia.

Cell-cell apical junctions of epithelia consist of multiprotein complexes that organize as belts regulating cell-cell adhesion, permeability, and mechanical tension: the tight junction (zonula occludens), the zonula adherens (ZA), and the macula adherens. The prevailing dogma is that at the ZA, E-cadherin and catenins are lined with F-actin bundles that support and transmit mechanical tension between cells. Using super-resolution microscopy on human intestinal biopsies and Caco-2 cells, we show that two distinct multiprotein belts are basal of the tight junctions as the intestinal epithelia mature. The most apical is populated with nectins/afadin and lined with F-actin; the second is populated with E-cad/catenins. We name this dual-belt architecture the zonula adherens matura. We find that the apical contraction apparatus and the dual-belt organization rely on afadin expression. Our study provides a revised description of epithelial cell-cell junctions and identifies a module regulating the mechanics of epithelia.

Cortactin is in a complex with VE-cadherin and is required for endothelial adherens junction stability through Rap1/Rac1 activation.

Vascular permeability is mediated by Cortactin (Cttn) and regulated by several molecules including cyclic-adenosine-monophosphate, small Rho family GTPases and the actin cytoskeleton. However, it is unclear whether Cttn directly interacts with any of the junctional components or if Cttn intervenes with signaling pathways affecting the intercellular contacts and the cytoskeleton. To address these questions, we employed immortalized microvascular myocardial endothelial cells derived from wild-type and Cttn-knock-out mice. We found that lack of Cttn compromised barrier integrity due to fragmented membrane distribution of different junctional proteins. Moreover, immunoprecipitations revealed that Cttn is within the VE-cadherin-based adherens junction complex. In addition, lack of Cttn slowed-down barrier recovery after Ca2+ repletion. The role of Cttn for cAMP-mediated endothelial barrier regulation was analyzed using Forskolin/Rolipram. In contrast to Cttn-KO, WT cells reacted with increased transendothelial electrical resistance. Absence of Cttn disturbed Rap1 and Rac1 activation in Cttn-depleted cells. Surprisingly, despite the absence of Cttn, direct activation of Rac1/Cdc42/RhoA by CN04 increased barrier resistance and induced well-defined cortical actin and intracellular actin bundles. In summary, our data show that Cttn is required for basal barrier integrity by allowing proper membrane distribution of junctional proteins and for cAMP-mediated activation of the Rap1/Rac1 signaling pathway.

Adherens junctions as molecular regulators of emergent tissue mechanics.

Tissue and organ development during embryogenesis relies on the collective and coordinated action of many cells. Recent studies have revealed that tissue material properties, including transitions between fluid and solid tissue states, are controlled in space and time to shape embryonic structures and regulate cell behaviours. Although the collective cellular flows that sculpt tissues are guided by tissue-level physical changes, these ultimately emerge from cellular-level and subcellular-level molecular mechanisms. Adherens junctions are key subcellular structures, built from clusters of classical cadherin receptors. They mediate physical interactions between cells and connect biochemical signalling to the physical characteristics of cell contacts, hence playing a fundamental role in tissue morphogenesis. In this Review, we take advantage of the results of recent, quantitative measurements of tissue mechanics to relate the molecular and cellular characteristics of adherens junctions, including adhesion strength, tension and dynamics, to the emergent physical state of embryonic tissues. We focus on systems in which cell-cell interactions are the primary contributor to morphogenesis, without significant contribution from cell-matrix interactions. We suggest that emergent tissue mechanics is an important direction for future research, bridging cell biology, developmental biology and mechanobiology to provide a holistic understanding of morphogenesis in health and disease.

PLK1 and its substrate MISP facilitate intrahepatic cholangiocarcinoma progression by promoting lymphatic invasion and impairing E-cadherin adherens junctions.

Intrahepatic cholangiocarcinoma (iCCA) is a subtype of CCA and has a high mortality rate and a relatively poor prognosis. However, studies focusing on increased cell motility and loss of epithelial integrity during iCCA progression remain relatively scarce. We collected seven fresh tumor samples from four patients to perform RNA sequencing (RNA-seq) and assay for transposase-accessible chromatin using sequencing (ATAC-seq) to determine the transcriptome profile and chromatin accessibility of iCCA. The increased expression of cell cycle regulators, including PLK1 and its substrate MISP, was identified. Ninety-one iCCA patients were used to validate the clinical significance of PLK1 and MISP. The upregulation of PLK1 and MISP was determined in iCCA tissues. Increased expression of PLK1 and MISP was significantly correlated with tumor number, N stage, and lymphatic invasion in an iCCA cohort. Knockdown of PLK1 or MISP reduced trans-lymphatic endothelial migration and wound healing and affected focal adhesions in vitro. In cell‒cell junctions, MISP localized to adherens junctions and suppressed E-cadherin dimerization. PLK1 disrupted adherens junctions in a myosin-dependent manner. Furthermore, PLK1 and MISP promoted cell proliferation in vitro and tumorigenesis in vivo. In iCCA, PLK1 and MISP promote aggressiveness by increasing lymphatic invasion, tumor growth, and motility through the repression of E-cadherin adherens junctions.

Basal spot junctions of Drosophila epithelial tissues respond to morphogenetic forces and regulate Hippo signaling.

Organ size is controlled by numerous factors including mechanical forces, which are mediated in part by the Hippo pathway. In growing Drosophila epithelial tissues, cytoskeletal tension influences Hippo signaling by modulating the localization of key pathway proteins to different apical domains. Here, we discovered a Hippo signaling hub at basal spot junctions, which form at the basal-most point of the lateral membranes and resemble adherens junctions in protein composition. Basal spot junctions recruit the central kinase Warts via Ajuba and E-cadherin, which prevent Warts activation by segregating it from upstream Hippo pathway proteins. Basal spot junctions are prominent when tissues undergo morphogenesis and are highly sensitive to fluctuations in cytoskeletal tension. They are distinct from focal adhesions, but the latter profoundly influences basal spot junction abundance by modulating the basal-medial actomyosin network and tension experienced by spot junctions. Thus, basal spot junctions couple morphogenetic forces to Hippo pathway activity and organ growth.

Preexisting tissue mechanical hypertension at adherens junctions disrupts apoptotic extrusion in epithelia.

Apical extrusion is a tissue-intrinsic process that allows epithelia to eliminate unfit or surplus cells. This is exemplified by the early extrusion of apoptotic cells, which is critical to maintain the epithelial barrier and prevent inflammation. Apoptotic extrusion is an active mechanical process, which involves mechanotransduction between apoptotic cells and their neighbors, as well as local changes in tissue mechanics. Here we report that the preexisting mechanical tension at adherens junctions (AJs) conditions the efficacy of apoptotic extrusion. Specifically, increasing baseline mechanical tension by overexpression of a phosphomimetic Myosin II regulatory light chain (MRLC) compromises apoptotic extrusion. This occurs when tension is increased in either the apoptotic cell or its surrounding epithelium. Further, we find that the proinflammatory cytokine, TNFα, stimulates Myosin II and increases baseline AJ tension to disrupt apical extrusion, causing apoptotic cells to be retained in monolayers. Importantly, reversal of mechanical tension with an inhibitory MRLC mutant or tropomyosin inhibitors is sufficient to restore apoptotic extrusion in TNFα-treated monolayers. Together, these findings demonstrate that baseline levels of tissue tension are important determinants of apoptotic extrusion, which can potentially be coopted by pathogenetic factors to disrupt the homeostatic response of epithelia to apoptosis.

Tight junctions.

Various functions within our bodies require the generation and maintenance of compartments with distinct compositions, which in turn necessitate the formation of semipermeable cellular diffusion barriers. For example, the blood-brain barrier protects the brain by allowing only specific molecules to pass through. Another instance is the intestinal barrier, which allows the uptake of essential nutrients, while restricting the passage of pathogenic molecules and bacteria. Breakdown of such barriers causes various pathologies, such as brain or retinal edema, or diarrhoea. Epithelia and endothelia are the most common barrier-forming cells. Individual cells in such barriers are held together by cell-cell adhesion structures - also known as intercellular junctions - that are essential for barrier formation and maintenance. Here, we will focus on the structure and assembly of tight junctions (TJs) and their functions as barriers, but will refer to other adhesive structures crucial for barrier regulation such as adherens junctions (AJs) and focal adhesions to the extracellular matrix (ECM) (Figure 1A,B). We will also discuss additional functions of TJs in cell surface polarity and the regulation of gene expression, cell function, and cell behaviour.

Epithelial cell spreading assay on E-cadherin-coated glass or PDMS substrates for microscopy-based analysis of cadherin adhesions.

Adherens junctions (AJs) are multi-protein adhesion structures that couple contractile actomyosin networks of epithelial cells within a tissue. Here, we present an epithelial cell spreading assay on E-cadherin-coated glass or polydimethylsiloxane (PDMS) substrates for detailed microscopy-based analysis of cadherin adhesions. We describe steps for preparation of glass coverslips and PDMS gels, E-cadherin coating, and epithelial cell spreading. Epithelial cells can be seeded on E-cadherin-coated surfaces, thereby mimicking AJ formation in X-Y dimension, making it suitable for microscopy analysis. For complete details on the use and execution of this protocol, please refer to Noordstra et al. (2023).1.

Actin-dependent recruitment of AGO2 to the zonula adherens.

Adherens junctions are cadherin-based structures critical for cellular architecture. E-cadherin junctions in mature epithelial cell monolayers tether to an apical actomyosin ring to form the zonula adherens (ZA). We have previously shown that the adherens junction protein PLEKHA7 associates with and regulates the function of the core RNA interference (RNAi) component AGO2 specifically at the ZA. However, the mechanism mediating AGO2 recruitment to the ZA remained unexplored. Here, we reveal that this ZA-specific recruitment of AGO2 depends on both the structural and tensile integrity of the actomyosin cytoskeleton. We found that depletion of not only PLEKHA7, but also either of the three PLEKHA7-interacting, LIM-domain family proteins, namely LMO7, LIMCH1, and PDLIM1, results in disruption of actomyosin organization and tension, as well as disruption of AGO2 junctional localization and of its miRNA-binding ability. We also show that AGO2 binds Myosin IIB and that PLEKHA7, LMO7, LIMCH1, and PDLIM1 all disrupt interaction of AGO2 with Myosin IIB at the ZA. These results demonstrate that recruitment of AGO2 to the ZA is sensitive to actomyosin perturbations, introducing the concept of mechanosensitive RNAi machinery, with potential implications in tissue remodeling and in disease.

Ubiquitin ligase CHFR mediated degradation of VE-cadherin through ubiquitylation disrupts endothelial adherens junctions.

Vascular endothelial cadherin (VE-cadherin) expressed at endothelial adherens junctions (AJs) is vital for vascular integrity and endothelial homeostasis. Here we identify the requirement of the ubiquitin E3-ligase CHFR as a key mechanism of ubiquitylation-dependent degradation of VE-cadherin. CHFR was essential for disrupting the endothelium through control of the VE-cadherin protein expression at AJs. We observe augmented expression of VE-cadherin in endothelial cell (EC)-restricted Chfr knockout (ChfrΔEC) mice. We also observe abrogation of LPS-induced degradation of VE-cadherin in ChfrΔEC mice, suggesting the pathophysiological relevance of CHFR in regulating the endothelial junctional barrier in inflammation. Lung endothelial barrier breakdown, inflammatory neutrophil extravasation, and mortality induced by LPS were all suppressed in ChfrΔEC mice. We find that the transcription factor FoxO1 is a key upstream regulator of CHFR expression. These findings demonstrate the requisite role of the endothelial cell-expressed E3-ligase CHFR in regulating the expression of VE-cadherin, and thereby endothelial junctional barrier integrity.