Regulatory mechanisms required for DE-cadherin function in cell migration and other types of adhesion

Peripodial adherens junctions regulate Ajuba-Yorkie signaling to preserve fly eye morphology

The Drosophila eye develops from the larval eye disc, a flattened vesicle comprised of continuous retinal and peripodial epithelia (PE). The PE is an epithelium that plays a supporting role in retinal neurogenesis, but gives rise to cuticle in the adult. We report here that the PE is also necessary to preserve the morphology of the retinal epithelium. Depletion of the adherens junction (AJ) components β-Catenin (β-Cat), DE-Cadherin or α-Catenin from the PE leads to altered disc morphology, characterized by retinal displacement (RDis); so too does loss of the Ajuba protein Jub, an AJ-associated regulator of the transcriptional coactivator Yorkie (Yki). Restoring AJs or overexpressing Yki in β-Cat deficient PE results in suppression of RDis. Additional suppressors of AJ-dependent RDis include knockdown of Rho kinase (Rok) and Dystrophin (Dys). Furthermore, knockdown of βPS integrin (Mys) from the PE results in RDis, while overexpression of Mys can suppress RDis induced by the loss of β-Cat. We thus propose that AJ-Jub-Yki signaling in PE cells regulates PE cell contractile properties and/or attachment to the extracellular matrix to promote normal eye disc morphology.

Mind bomb 2 promotes cell migration and epithelial structure by regulating adhesion complexes and the actin cytoskeleton

Cell migration is essential in animal development and co-opted during metastasis and inflammatory diseases. Some cells migrate collectively, which requires them to balance epithelial characteristics such as stable cell-cell adhesions with features of motility like rapid turnover of adhesions and dynamic cytoskeletal structures. How this is regulated is not entirely clear but important to understand. While investigating Drosophila oogenesis, we found that the putative E3 ubiquitin ligase, Mind bomb 2 (Mib2), is required to promote epithelial stability and the collective cell migration of border cells. Through biochemical analysis, we identified components of Mib2 complexes, which include E-cadherin and α- and β-catenins, as well as actin regulators. We also found that three Mib2 interacting proteins, RhoGAP19D, Supervillin, and Myosin heavy chain-like, affect border cell migration. mib2 mutant main body follicle cells have drastically reduced E-cadherin-based adhesion complexes and diminished actin filaments. We conclude that Mib2 acts to stabilize E-cadherin-based adhesion complexes and promote a robust actin cytoskeletal network, which is important for maintenance of epithelial integrity. The interaction with cadherin adhesion complexes and other cytoskeletal regulators contribute to its role in collective cell migration. Since Mib2 is well conserved, it may have similar functional significance in other organisms.

E-cadherin acts as a positive regulator of the JAK-STAT signaling pathway during Drosophila oogenesis

The JAK-STAT pathway is evolutionary conserved. The simplicity of this signaling in Drosophila, due to the limited redundancy between pathway components, makes it an ideal model for investigation. In the Drosophila follicular epithelium, highly stereotyped functions of JAK-STAT signaling have been well characterized, but how signaling activity is regulated precisely to allow the different outcomes is not well understood. In this tissue, the ligand is secreted by the polar cells positioned at each follicle extremity, thus generating a gradient of JAK-STAT activity in adjacent cells. One way to control the delivered quantity of ligand is by regulating the number of polar cells, which is reduced by apoptosis to exactly two at each pole by mid-oogenesis. Hence, JAK-STAT activity is described as symmetrical between follicle anterior and posterior regions. Here, we show that JAK-STAT signaling activity is actually highly dynamic, resulting in asymmetry between poles by mid-oogenesis. Interestingly, we found similar temporal dynamics at follicle poles in the accumulation of the adherens junction E-cadherin protein. Remarkably, E-cadherin and JAK-STAT signaling not only display patterning overlaps but also share functions during oogenesis. In particular, we show that E-cadherin, like JAK-STAT signaling, regulates polar cell apoptosis non-cell-autonomously from follicle cells. Finally, our work reveals that E-cadherin is required for optimal JAK-STAT activity throughout oogenesis and that E-cadherin and Stat92E, the transcription factor of the pathway, form part of a physical complex in follicle cells. Taken together, our study establishes E-cadherin as a new positive regulator of JAK-STAT signaling during oogenesis.

Tricellulin secures the epithelial barrier at tricellular junctions by interacting with actomyosin

The epithelial cell sheet functions as a barrier to prevent invasion of pathogens. It is necessary to eliminate intercellular gaps not only at bicellular junctions, but also at tricellular contacts, where three cells meet, to maintain epithelial barrier function. To that end, tight junctions between adjacent cells must associate as closely as possible, particularly at tricellular contacts. Tricellulin is an integral component of tricellular tight junctions (tTJs), but the molecular mechanism of its contribution to the epithelial barrier function remains unclear. In this study, we revealed that tricellulin contributes to barrier formation by regulating actomyosin organization at tricellular junctions. Furthermore, we identified α-catenin, which is thought to function only at adherens junctions, as a novel binding partner of tricellulin. α-catenin bridges tricellulin attachment to the bicellular actin cables that are anchored end-on at tricellular junctions. Thus, tricellulin mobilizes actomyosin contractility to close the lateral gap between the TJ strands of the three proximate cells that converge on tricellular junctions.

Force-induced changes of α-catenin conformation stabilize vascular junctions independently of vinculin

Cadherin-mediated cell adhesion requires anchoring via the β-catenin–α-catenin complex to the actin cytoskeleton, yet, α-catenin only binds F-actin weakly. A covalent fusion of VE-cadherin to α-catenin enhances actin anchorage in endothelial cells and strongly stabilizes endothelial junctions in vivo, blocking inflammatory responses. Here, we have analyzed the underlying mechanism. We found that VE-cadherin–α-catenin constitutively recruits the actin adaptor vinculin. However, removal of the vinculin-binding region of α-catenin did not impair the ability of VE-cadherin–α-catenin to enhance junction integrity. Searching for an alternative explanation for the junction-stabilizing mechanism, we found that an antibody-defined epitope, normally buried in a short α1-helix of the actin-binding domain (ABD) of α-catenin, is openly displayed in junctional VE-cadherin–α-catenin chimera. We found that this epitope became exposed in normal α-catenin upon triggering thrombin-induced tension across the VE-cadherin complex. These results suggest that the VE-cadherin–α-catenin chimera stabilizes endothelial junctions due to conformational changes in the ABD of α-catenin that support constitutive strong binding to actin.

Summary: There are novel antibody epitopes at the actin-binding domain of α-catenin that correlate with high affinity binding and are exposed in junction-stabilizing VE-cadherin–α-catenin fusion proteins.

Tissue fluidity mediated by adherens junction dynamics promotes planar cell polarity-driven ommatidial rotation

The phenomenon of tissue fluidity-cells' ability to rearrange relative to each other in confluent tissues-has been linked to several morphogenetic processes and diseases, yet few molecular regulators of tissue fluidity are known. Ommatidial rotation (OR), directed by planar cell polarity signaling, occurs during Drosophila eye morphogenesis and shares many features with polarized cellular migration in vertebrates. We utilize in vivo live imaging analysis tools to quantify dynamic cellular morphologies during OR, revealing that OR is driven autonomously by ommatidial cell clusters rotating in successive pulses within a permissive substrate. Through analysis of a rotation-specific nemo mutant, we demonstrate that precise regulation of junctional E-cadherin levels is critical for modulating the mechanical properties of the tissue to allow rotation to progress. Our study defines Nemo as a molecular tool to induce a transition from solid-like tissues to more viscoelastic tissues broadening our molecular understanding of tissue fluidity.

Germline CDH1 G212E Missense Variant: Combining Clinical, In Vitro and In Vivo Strategies to Unravel Disease Burden

Simple Summary

Hereditary diffuse gastric cancer (HDGC) is an inherited cancer syndrome associated with CDH1 germline mutations. The increasing detection of CDH1 genetic variants due to multigene panel testing poses a serious clinical challenge and urges the development of effective classification strategies. In this study, we describe the identification of the novel CDH1 G212E variant in a large family strongly affected by diffuse gastric cancer. Through a comprehensive characterization pipeline, we provide evidence of the damaging nature of this genetic alteration, thus impacting patient management and family screening.

Abstract

E-cadherin, encoded by CDH1, is an essential molecule for epithelial homeostasis, whose loss or aberrant expression results in disturbed cell–cell adhesion, increased cell invasion and metastasis. Carriers of CDH1 germline mutations have a high risk of developing diffuse gastric cancer and lobular breast cancer, associated with the cancer syndrome Hereditary Diffuse Gastric Cancer (HDGC). The ubiquitous availability of cancer panels has led to the identification of an increasing amount of “incidental” CDH1 genetic variants that pose a serious clinical challenge. This has sparked intensive research aiming at an accurate classification of the variants and consequent validation of their clinical relevance. The present study addressed the significance of a novel CDH1 variant, G212E, identified in an unusually large pedigree displaying strong aggregation of diffuse gastric cancer. We undertook a comprehensive pipeline encompassing family data, in silico predictions, in vitro assays and in vivo strategies, which validated the deleterious phenotype induced by this genetic alteration. In particular, we demonstrated that the G212E variant affects the stability and localization, as well as the adhesive and anti-invasive functions of E-cadherin, triggering epithelial disruption and disorganization. Our findings illustrate the clinical implication of a complementary approach for effective variant categorization and patient management.

Transient opening of tricellular vertices controls paracellular transport through the follicle epithelium during Drosophila oogenesis

Paracellular permeability is regulated to allow solute transport or cell migration across epithelial or endothelial barriers. However, how cell-cell junction dynamics controls paracellular permeability is poorly understood. Here, we describe patency, a developmentally regulated process in Drosophila oogenesis, during which cell vertices in the follicular epithelium open transiently to allow paracellular transport of yolk proteins for uptake by the oocyte. We show that the sequential removal of E-cadherin, N-cadherin, NCAM/Fasciclin 2, and Sidekick from vertices precedes their basal-to-apical opening, while the subsequent assembly of tricellular occluding junctions marks the termination of patency and seals the paracellular barrier. E-cadherin-based adhesion is required to limit paracellular channel size, whereas stabilized adherens junctions, prolonged NCAM/Fasciclin 2 expression, blocked endocytosis, or increased actomyosin contractility prevent patency. Our findings reveal a key role of cell vertices as gateways controlling paracellular transport and demonstrate that dynamic regulation of adhesion and actomyosin contractility at vertices governs epithelial barrier properties.

Distinct actin-dependent nanoscale assemblies underlie the dynamic and hierarchical organization of E-cadherin

Cadherins are transmembrane adhesion proteins required for the formation of cohesive tissues.^1-4 Intracellular interactions of E-cadherin with the Catenin family proteins, α- and β-catenin, facilitate connections with the cortical actomyosin network. This is necessary for maintaining the integrity of cell-cell adhesion in epithelial tissues.^5-11 The supra-molecular architecture of E-cadherin is an important feature of its adhesion function; cis and trans interactions of E-cadherin are deployed^12-15 to form clusters, both in cis and trans.^11,16-21 Studies in Drosophila embryo have also shown that Drosophila E-cadherin (dE-cad) is organized as finite-sized dynamic clusters that localize with actin patches at cell-cell junctions, in continuous exchange with the extra-junctional pool of dE-cad surrounding the clusters.^11,19 Here, we use the ectopic expression of dE-cad in larval hemocytes, which lack endogenous dE-cad to recapitulate functional cell-cell junctions in a convenient model system. We find that, while dE-cad at cell-cell junctions in hemocytes exhibits a clustered trans-paired organization similar to that reported previously in embryonic epithelial tissue, extra-junctional dE-cad is also organized as relatively immobile nanoclusters as well as more loosely packed diffusive oligomers. Oligomers are promoted by cis interactions of the ectodomain, and their growth is counteracted by the activity of cortical actomyosin. Oligomers in turn promote assembly of dense nanoclusters that require cortical actomyosin activity. Thus, cortical actin activity remodels oligomers and generates nanoclusters. The requirement for dynamic actin in the organization of dE-cad at the nanoscale may provide a mechanism to dynamically tune junctional strength.

Protein phosphatase 1 activity controls a balance between collective and single cell modes of migration

Collective cell migration is central to many developmental and pathological processes. However, the mechanisms that keep cell collectives together and coordinate movement of multiple cells are poorly understood. Using the Drosophila border cell migration model, we find that Protein phosphatase 1 (Pp1) activity controls collective cell cohesion and migration. Inhibition of Pp1 causes border cells to round up, dissociate, and move as single cells with altered motility. We present evidence that Pp1 promotes proper levels of cadherin-catenin complex proteins at cell-cell junctions within the cluster to keep border cells together. Pp1 further restricts actomyosin contractility to the cluster periphery rather than at individual internal border cell contacts. We show that the myosin phosphatase Pp1 complex, which inhibits non-muscle myosin-II (Myo-II) activity, coordinates border cell shape and cluster cohesion. Given the high conservation of Pp1 complexes, this study identifies Pp1 as a major regulator of collective versus single cell migration.

Disruption of EGF Feedback by Intestinal Tumors and Neighboring Cells in Drosophila

In healthy adult organs, robust feedback mechanisms control cell turnover to enforce homeostatic equilibrium between cell division and death [1, 2]. Nascent tumors must subvert these mechanisms to achieve cancerous overgrowth [3-7]. Elucidating the nature of this subversion can reveal how cancers become established and may suggest strategies to prevent tumor progression. In adult Drosophila intestine, a well-studied model of homeostatic cell turnover, the linchpin of cell equilibrium is feedback control of the epidermal growth factor (EGF) protease Rhomboid (Rho). Expression of Rho in apoptotic cells enables them to secrete EGFs, which stimulate nearby stem cells to undergo replacement divisions [8]. As in mammals, loss of adenomatous polyposis coli (APC) causes Drosophila intestinal stem cells to form adenomas [9]. Here, we demonstrate that Drosophila APC-/- tumors trigger widespread Rho expression in non-apoptotic cells, resulting in chronic EGF signaling. Initially, nascent APC-/- tumors induce rho in neighboring wild-type cells via acute, non-autonomous activation of Jun N-terminal kinase (JNK). During later growth and multilayering, APC-/- tumors induce rho in tumor cells by autonomous downregulation of E-cadherin (E-cad) and consequent activity of p120-catenin. This sequential dysregulation of tumor non-autonomous and -autonomous EGF signaling converts tissue-level feedback into feed-forward activation that drives cancerous overgrowth. Because Rho, EGF receptor (EGFR), and E-cad are associated with colorectal cancer in humans [10-17], our findings may shed light on how human colorectal tumors progress.

The multifarious regulation of the apical junctional complex

Epithelial cells form highly organized polarized sheets with characteristic cell morphologies and tissue architecture. Cell–cell adhesion and intercellular communication are prerequisites of such cohesive sheets of cells, and cell connectivity is mediated through several junctional assemblies, namely desmosomes, adherens, tight and gap junctions. These cell–cell junctions form signalling hubs that not only mediate cell–cell adhesion but impact on multiple aspects of cell behaviour, helping to coordinate epithelial cell shape, polarity and function. This review will focus on the tight and adherens junctions, constituents of the apical junctional complex, and aims to provide a comprehensive overview of the complex signalling that underlies junction assembly, integrity and plasticity.

Cell-Cell Adhesion and Myosin Activity Regulate Cortical Actin Assembly in Mammary Gland Epithelium on Concaved Surface

It has been demonstrated that geometry can affect cell behaviors. Though curvature-sensitive proteins at the nanoscale are studied, it is unclear how cells sense curvature at the cellular and multicellular levels. To characterize and determine the mechanisms of curvature-dependent cell behaviors, we grow cells on open channels of the 60-µm radius. We found that cortical F-actin is 1.2-fold more enriched in epithelial cells grown on the curved surface compared to the flat control. We observed that myosin activity is required to promote cortical F-actin formation. Furthermore, cell–cell contact was shown to be indispensable for curvature-dependent cortical actin assembly. Our results indicate that the actomyosin network coupled with adherens junctions is involved in curvature-sensing at the multi-cellular level.

A conserved major facilitator superfamily member orchestrates a subset of O-glycosylation to aid macrophage tissue invasion

Aberrant display of the truncated core1 O-glycan T-antigen is a common feature of human cancer cells that correlates with metastasis. Here we show that T-antigen in Drosophila melanogaster macrophages is involved in their developmentally programmed tissue invasion. Higher macrophage T-antigen levels require an atypical major facilitator superfamily (MFS) member that we named Minerva which enables macrophage dissemination and invasion. We characterize for the first time the T and Tn glycoform O-glycoproteome of the Drosophila melanogaster embryo, and determine that Minerva increases the presence of T-antigen on proteins in pathways previously linked to cancer, most strongly on the sulfhydryl oxidase Qsox1 which we show is required for macrophage tissue entry. Minerva's vertebrate ortholog, MFSD1, rescues the minerva mutant's migration and T-antigen glycosylation defects. We thus identify a key conserved regulator that orchestrates O-glycosylation on a protein subset to activate a program governing migration steps important for both development and cancer metastasis.

In locus analysis of patterning evolution of the BMP type II receptor Wishful thinking

Proper tissue patterning is an essential step during organ formation. During this process, genes are expressed in distinct patterns, defining boundaries for future functional domains. The bone morphogenetic protein (BMP) signaling pathway sets the anterior domain during eggshell patterning. Previously, the Drosophila melanogaster homolog of BMPR2, Wishful thinking (WIT), was shown to be required for BMP signaling and patterning during eggshell formation. Expressed in a conserved anterior pattern, the width of wit patterning in the follicular epithelium is evolutionarily divergent between Drosophila species. We used genome editing to demonstrate how the gene pattern divergence is controlled in cis within the wit locus of D. virilis Furthermore, unlike direct targets of BMP signaling, we demonstrate how one transcription factor binding site shapes the pattern of WIT in D. melanogaster by negative regulation. However, changes in this site are not sufficient to explain the evolution of wit patterning, suggesting that a positive regulatory element that controls pattern divergence remains to be discovered.

Ovarian polarity and cell shape determination by Btk29A in Drosophila

Drosophila Btk29A is a Tec family nonreceptor tyrosine kinase, the ortholog of which causes X-linked agammaglobulinemia in humans when mutant. In Btk29A^ficP mutant ovaries, multiple defects are observed: extrapolar cells form ectopically; osk mRNA fails to accumulate posteriorly in mature oocytes; the shape and alignment of follicle cells are grossly distorted. All these phenotypes are rescued by selectively overexpressing the type 2 isoform of wild-type Btk29A in follicle cells. Expression of certain proteins enriched in adherens junctions is markedly affected in Btk29A^ficP mutants; the anterior-posterior gradient normally observed in the expression of DE-Cadherin and Armadillo are lost and Canoe is sequestered from adherens junctions. Intriguingly, tyrosine phosphorylation of Canoe is reduced in Btk29A^ficP mutants. It is proposed that Btk29A is required for the establishment of egg chamber polarity presumably through the regulation of subcellular localization of its downstream proteins, including Cno.

Phosphorylation potential of Drosophila E-Cadherin intracellular domain is essential for development and adherens junction biosynthetic dynamics regulation

Phosphorylation of a highly conserved serine cluster in the intracellular domain of E-Cadherin is essential for binding to β-Catenin in vitro In cultured cells, phosphorylation of specific serine residues within the cluster is also required for regulation of adherens junction (AJ) stability and dynamics. However, much less is known about how such phosphorylation of E-Cadherin regulates AJ formation and dynamics in vivo In this report, we generated an extensive array of Drosophila E-Cadherin (DE-Cad) endogenous knock-in alleles that carry mutations targeting this highly conserved serine cluster. Analyses of these mutations suggest that the overall phosphorylation potential, rather than the potential site-specific phosphorylation, of the serine cluster enhances the recruitment of β-Catenin by DE-Cad in vivo Moreover, phosphorylation potential of the serine cluster only moderately increases the level of β-Catenin in AJs and is in fact dispensable for AJ formation in vivo Nonetheless, phosphorylation-dependent recruitment of β-Catenin is essential for development, probably by enhancing the interactions between DE-Cad and α-Catenin. In addition, several phospho-mutations dramatically reduced the biosynthetic turnover rate of DE-Cad during apical-basal polarization, and such biosynthetically stable DE-Cad mutants specifically rescued the polarity defects in embryonic epithelia lacking the polarity proteins Stardust and Crumbs.

α-Catenin stabilises Cadherin-Catenin complexes and modulates actomyosin dynamics to allow pulsatile apical contraction

We have investigated how cell contractility and adhesion are functionally integrated during epithelial morphogenesis. To this end, we have analysed the role of α-Catenin, a key molecule linking E-Cadherin-based adhesion and the actomyosin cytoskeleton, during Drosophila embryonic dorsal closure, by studying a newly developed allelic series. We find that α-Catenin regulates pulsatile apical contraction in the amnioserosa, the main force-generating tissue driving closure of the embryonic epidermis. α-Catenin controls actomyosin dynamics by stabilising and promoting the formation of actomyosin foci, and also stabilises DE-Cadherin (Drosophila E-Cadherin, also known as Shotgun) at the cell membrane, suggesting that medioapical actomyosin contractility regulates junction stability. Furthermore, we uncover a genetic interaction between α-Catenin and Vinculin, and a tension-dependent recruitment of Vinculin to amniosersoa apical cell membranes, suggesting the existence of a mechano-sensitive module operating in this tissue.

Reevaluating αE-catenin monomer and homodimer functions by characterizing E-cadherin/αE-catenin chimeras

Our results demonstrate that E-cadherin/αE-catenin chimeras homodimerize and do not mimic αE-catenin in the native CCC, and imply that both CCC-bound monomer and cytosolic homodimer αE-catenin are required for strong cell adhesion.

As part of the E-cadherin–β-catenin–αE-catenin complex (CCC), mammalian αE-catenin binds F-actin weakly in the absence of force, whereas cytosolic αE-catenin forms a homodimer that interacts more strongly with F-actin. It has been concluded that cytosolic αE-catenin homodimer is not important for intercellular adhesion because E-cadherin/αE-catenin chimeras thought to mimic the CCC are sufficient to induce cell–cell adhesion. We show that, unlike αE-catenin in the CCC, these chimeras homodimerize, bind F-actin strongly, and inhibit the Arp2/3 complex, all of which are properties of the αE-catenin homodimer. To more accurately mimic the junctional CCC, we designed a constitutively monomeric chimera, and show that E-cadherin–dependent cell adhesion is weaker in cells expressing this chimera compared with cells in which αE-catenin homodimers are present. Our results demonstrate that E-cadherin/αE-catenin chimeras used previously do not mimic αE-catenin in the native CCC, and imply that both CCC-bound monomer and cytosolic homodimer αE-catenin are required for strong cell–cell adhesion.

N-cadherin negatively regulates collective Drosophila glial migration via actin cytoskeleton remodeling

Cell migration is an essential and highly regulated process. During development, glia and neurons migrate over long distances, in most cases collectively, to reach their final destination and build the sophisticated architecture of the nervous system, the most complex tissue of the body. Collective migration is highly stereotyped and efficient, defects in the process leading to severe human diseases that include mental retardation. This dynamic process entails extensive cell communication and coordination, hence the real challenge is to analyze it in the whole organism and at cellular resolution. We here investigate the impact of the N-cadherin adhesion molecule on collective glial migration using the Drosophila developing wing and cell-type specific manipulation of gene expression. We show that N-cadherin timely accumulates in glial cells and that its levels affect migration efficiency. N-cadherin works as a molecular brake in a dosage dependent manner by negatively controlling actin nucleation and cytoskeleton remodeling through α/β catenins. This is the first in vivo evidence for N-cadherin negatively and cell autonomously controlling collective migration.

Evolutionary conservation of early mesoderm specification by mechanotransduction in Bilateria

The modulation of developmental biochemical pathways by mechanical cues is an emerging feature of animal development, but its evolutionary origins have not been explored. Here we show that a common mechanosensitive pathway involving β-catenin specifies early mesodermal identity at gastrulation in zebrafish and Drosophila. Mechanical strains developed by zebrafish epiboly and Drosophila mesoderm invagination trigger the phosphorylation of β-catenin–tyrosine-667. This leads to the release of β-catenin into the cytoplasm and nucleus, where it triggers and maintains, respectively, the expression of zebrafish brachyury orthologue notail and of Drosophila Twist, both crucial transcription factors for early mesoderm identity. The role of the β-catenin mechanosensitive pathway in mesoderm identity has been conserved over the large evolutionary distance separating zebrafish and Drosophila. This suggests mesoderm mechanical induction dating back to at least the last bilaterian common ancestor more than 570 million years ago, the period during which mesoderm is thought to have emerged.

Mechanical cues can induce morphogenetic processes during development. Here the authors show that mechanical changes during embryonic development in both zebrafish and Drosophila lead to nuclear localization of β-catenin, which regulates genes required for early mesoderm development in both species.

Cell migration: from tissue culture to embryos

Cell migration is a fundamental process that occurs during embryo development. Classic studies using in vitro culture systems have been instrumental in dissecting the principles of cell motility and highlighting how cells make use of topographical features of the substrate, cell-cell contacts, and chemical and physical environmental signals to direct their locomotion. Here, we review the guidance principles of in vitro cell locomotion and examine how they control directed cell migration in vivo during development. We focus on developmental examples in which individual guidance mechanisms have been clearly dissected, and for which the interactions among guidance cues have been explored. We also discuss how the migratory behaviours elicited by guidance mechanisms generate the stereotypical patterns of migration that shape tissues in the developing embryo.

Three functions of cadherins in cell adhesion

Cadherins are transmembrane proteins that mediate cell-cell adhesion in animals. By regulating contact formation and stability, cadherins play a crucial role in tissue morphogenesis and homeostasis. Here, we review the three major functions of cadherins in cell-cell contact formation and stability. Two of those functions lead to a decrease in interfacial tension at the forming cell-cell contact, thereby promoting contact expansion--first, by providing adhesion tension that lowers interfacial tension at the cell-cell contact, and second, by signaling to the actomyosin cytoskeleton in order to reduce cortex tension and thus interfacial tension at the contact. The third function of cadherins in cell-cell contact formation is to stabilize the contact by resisting mechanical forces that pull on the contact.

Fusing VE-cadherin to α-catenin impairs fetal liver hematopoiesis and lymph but not blood vessel formation

We have recently shown that genetic replacement of VE-cadherin by a VE-cadherin-α-catenin fusion construct strongly impairs opening of endothelial cell contacts during leukocyte extravasation and induction of vascular permeability in adult mice. Here we show that this mutation leads to lethality at midgestation on a clean C57BL/6 background. Investigating the reasons for embryonic lethality, we observed a lack of fetal liver hematopoiesis and severe lymphedema but no detectable defects in blood vessel formation and remodeling. As for the hematopoiesis defect, VE-cadherin-α-catenin affected neither the generation of hematopoietic stem and progenitor cells (HSPCs) from hemogenic endothelium nor their differentiation into multiple hematopoietic lineages. Instead, HSPCs accumulated in the fetal circulation, suggesting that their entry into the fetal liver was blocked. Edema formation was caused by disturbed lymphatic vessel development. Lymphatic progenitor cells of VE-cadherin-α-catenin-expressing embryos were able to leave the cardinal vein and migrate to the site of the first lymphatic vessel formation, yet subsequently, these cells failed to form large lumenized lymphatic vessels. Thus, stabilizing endothelial cell contacts by a covalent link between VE-cadherin and α-catenin affects recruitment of hematopoietic progenitors into the fetal liver and the development of lymph but not blood vessels.

Adherens junction turnover: regulating adhesion through cadherin endocytosis, degradation, and recycling

Adherens junctions are important mediators of intercellular adhesion, but they are not static structures. They are regularly formed, broken, and rearranged in a variety of situations, requiring changes in the amount of cadherins, the main adhesion molecule in adherens junctions, present at the cell surface. Thus, endocytosis, degradation, and recycling of cadherins are crucial for dynamic regulation of adherens junctions and control of intercellular adhesion. In this chapter, we review the involvement of cadherin endocytosis in development and disease. We discuss the various endocytic pathways available to cadherins, the adaptors involved, and the sorting of internalized cadherin for recycling or lysosomal degradation. In addition, we review the regulatory pathways controlling cadherin endocytosis and degradation, including regulation of cadherin endocytosis by catenins, cadherin ubiquitination, and growth factor receptor signaling pathways. Lastly, we discuss the proteolytic cleavage of cadherins at the plasma membrane.

Cadherin switching during the formation and differentiation of the Drosophila mesoderm - implications for epithelial-to-mesenchymal transitions

Epithelial-to-mesenchymal transition (EMT) is typically accompanied by downregulation of epithelial (E-) cadherin, and is often additionally accompanied by upregulation of a mesenchymal or neuronal (N-) cadherin. Snail represses transcription of the E-cadherin gene both during normal development and during tumour spreading. The formation of the mesodermal germ layer in Drosophila, considered a paradigm of a developmental EMT, is associated with Snail-mediated repression of E-cadherin and the upregulation of N-cadherin. By using genetic manipulation to remove or overexpress the cadherins, we show here that the complementarity of cadherin expression is not necessary for the segregation or the dispersal of the mesodermal germ layer in Drosophila. However, we discover different effects of E- and N-cadherin on the differentiation of subsets of mesodermal derivatives, which depend on Wingless signalling from the ectoderm, indicating differing abilities of E- and N-cadherin to bind to and sequester the common junctional and signalling effector β-catenin. These results suggest that the downregulation of E-cadherin in the mesoderm might be required to facilitate optimal levels of Wingless signalling.

Patterns in space: coordinating adhesion and actomyosin contractility at E-cadherin junctions

Cadherin adhesion receptors are fundamental determinants of tissue organization in health and disease. Increasingly, we have come to appreciate that classical cadherins exert their biological actions through active cooperation with the contractile actin cytoskeleton. Rather than being passive resistors of detachment forces, cadherins can regulate the assembly and mechanics of the contractile apparatus itself. Moreover, coordinate spatial patterning of adhesion and contractility is emerging as a determinant of morphogenesis. Here we review recent developments in cadherins and actin cytoskeleton cooperativity, by focusing on E-cadherin adhesive patterning in the epithelia. Next, we discuss the underlying principles of cellular rearrangement during Drosophila germband extension and epithelial cell extrusion, as models of how planar and apical-lateral patterns of contractility organize tissue architecture.

Collective cell migration of epithelial and mesenchymal cells

Directional cell migration is required for proper embryogenesis, immunity, and healing, and its underpinning regulatory mechanisms are often hijacked during diseases such as chronic inflammations and cancer metastasis. Studies on migratory epithelial tissues have revealed that cells can move as a collective group with shared responsibilities. First thought to be restricted to proper epithelial cell types able to maintain stable cell-cell junctions, the field of collective cell migration is now widening to include cooperative behavior of mesenchymal cells. In this review, we give an overview of the mechanisms driving collective cell migration in epithelial tissues and discuss how mesenchymal cells can cooperate to behave as a collective in the absence of bona fide cell-cell adhesions.

Adherens junctions determine the apical position of the midbody during follicular epithelial cell division

Cytokinesis is asymmetric along the apical-basal axis of epithelial cells, positioning the midbody near the apical domain. However, little is known about the mechanism and purpose of this asymmetry. We use live imaging of Drosophila follicle cell division to show that asymmetric cytokinesis does not result from intrinsic polarization of the main contractile ring components. We show that adherens junctions (AJs) maintain close contact with the apical side of the contractile ring during cytokinesis. Asymmetric distribution of AJ components within follicle cells and in the otherwise unpolarized S2 cells is sufficient to recruit the midbody, revealing that asymmetric cytokinesis is determined by apical AJs in the epithelia. We further show that ectopic midbody localization induces epithelial invaginations, shifting the position of the apical interface between daughter cells relative to the AB axis of the tissue. Thus, apical midbody localization is essential to maintain epithelial tissue architecture during proliferation.

The Hippo pathway polarizes the actin cytoskeleton during collective migration of Drosophila border cells

Collective migration of Drosophila border cells depends on a dynamic actin cytoskeleton that is highly polarized such that it concentrates around the outer rim of the migrating cluster of cells. How the actin cytoskeleton becomes polarized in these cells to enable collective movement remains unknown. Here we show that the Hippo signaling pathway links determinants of cell polarity to polarization of the actin cytoskeleton in border cells. Upstream Hippo pathway components localize to contacts between border cells inside the cluster and signal through the Hippo and Warts kinases to polarize actin and promote border cell migration. Phosphorylation of the transcriptional coactivator Yorkie (Yki)/YAP by Warts does not mediate the function of this pathway in promoting border cell migration, but rather provides negative feedback to limit the speed of migration. Instead, Warts phosphorylates and inhibits the actin regulator Ena to activate F-actin Capping protein activity on inner membranes and thereby restricts F-actin polymerization mainly to the outer rim of the migrating cluster.

Monomeric α-catenin links cadherin to the actin cytoskeleton

The linkage of adherens junctions to the actin cytoskeleton is essential for cell adhesion. The contribution of the cadherin-catenin complex to the interaction between actin and the adherens junction remains an intensely investigated subject that centres on the function of α-catenin, which binds to cadherin through β-catenin and can bind F-actin directly or indirectly. Here, we delineate regions within Drosophila α-Catenin (α-Cat) that are important for adherens junction performance in static epithelia and dynamic morphogenetic processes. Moreover, we address whether persistent α-catenin-mediated physical linkage between cadherin and F-actin is crucial for cell adhesion and characterize the functions of α-catenin monomers and dimers at adherens junctions. Our data support the view that monomeric α-catenin acts as an essential physical linker between the cadherin-β-catenin complex and the actin cytoskeleton, whereas α-catenin dimers are cytoplasmic and form an equilibrium with monomeric junctional α-catenin.

Dimer asymmetry defines α-catenin interactions

The F-actin binding cytoskeletal protein α-catenin interacts with β-catenin-cadherin complexes and stabilizes cell-cell junctions. The β-catenin–α-catenin complex cannot bind to F-actin, whereas interactions of α-catenin with the cytoskeletal protein vinculin appear necessary to stabilize adherens junctions. Here we report the crystal structure of nearly full-length human α-catenin at 3.7 Å resolution. α-Catenin forms an asymmetric dimer, where the four-helix bundle domains of each subunit engage in distinct intermolecular interactions. This results in a left handshake-like dimer, where the two subunits have remarkably different conformations. The crystal structure explains why dimeric α-catenin has a higher affinity for F-actin than monomeric α-catenin, why the β-catenin–α-catenin complex does not bind to F-actin, how activated vinculin links the cadherin-catenin complex to the cytoskeleton, and why α-catenin but not inactive vinculin can bind to F-actin.

The Drosophila BMPRII, wishful thinking, is required for eggshell patterning

The Drosophila eggshell is an elaborate structure that is derived from a monolayer of follicular epithelium surrounding the developing oocyte within the female ovary. The bone morphogenetic protein (BMP) signaling pathway is essential for controlling the patterning and morphogenesis of the eggshell. During oogenesis, the roles of patterning and morphogenesis by the BMP type I receptor thickveins (tkv) have been studied extensively. However, signaling through this pathway requires both type I and II receptors, and the latter has yet to be established in oogenesis. We focus on wishful thinking (wit), the Drosophila homolog to the mammalian BMP type II receptor, BMPRII. We found that wit is expressed dynamically in the FCs of D. melanogaster in an evolutionary conserved pattern. The expression patterns are highly correlated with the dynamics of the BMP signaling, which is consistent with our finding that wit is a target of BMP signaling. Furthermore, we established that WIT is necessary for BMP signaling, and loss of WIT is associated with cell autonomous loss of BMP responses. Of importance, we demonstrated that perturbations in WIT led to changes in eggshell morphologies in domains that are patterned by BMP signaling. Previous studies have shown a role for WIT in BMP signaling during neurogenesis; however, our results reveal a role for WIT in epithelial cells' development.

Group choreography: mechanisms orchestrating the collective movement of border cells

Cell movements are essential for animal development and homeostasis but also contribute to disease. Moving cells typically extend protrusions towards a chemoattractant, adhere to the substrate, contract and detach at the rear. It is less clear how cells that migrate in interconnected groups in vivo coordinate their behaviour and navigate through natural environments. The border cells of the Drosophila melanogaster ovary have emerged as an excellent model for the study of collective cell movement, aided by innovative genetic, live imaging, and photomanipulation techniques. Here we provide an overview of the molecular choreography of border cells and its more general implications.

Aneuploidy-induced delaminating cells drive tumorigenesis in Drosophila epithelia

Genomic instability has been observed in essentially all sporadic carcinomas. Here we use Drosophila epithelial cells to address the role of chromosomal instability in cancer development as they have proved useful for elucidating the molecular mechanisms underlying tumorigenic growth. We first show that chromosomal instability leads to an apoptotic response. Interestingly, this response is p53 independent, as opposed to mammalian cells, and depends on the activation of the c-Jun N-terminal kinase (JNK) signaling cascade. When prevented from undergoing programmed cell death (PCD), chromosomal instability induces neoplasic overgrowth. These tumor-like tissues are able to grow extensively and metastasize when transplanted into the abdomen of adult hosts. Detailed analysis of the tumors allows us to identify a delaminating cell population as the critical one in driving tumorigenesis. Cells loose their apical-basal polarity, mislocalize DE-cadherin, and delaminate from the main epithelium. A JNK-dependent transcriptional program is activated specifically in delaminating cells and drives nonautonomous tissue overgrowth, basement membrane degradation, and invasiveness. These findings unravel a general and rapid tumorigenic potential of genomic instability, as opposed to its proposed role as a source of mutability to select specific tumor-prone aneuploid cells, and open unique avenues toward the understanding of the role of genomic instability in human cancer.

Cadherins in collective cell migration of mesenchymal cells

Immunity, embryogenesis and tissue repair rely heavily on cell migration. Cells can be seen migrating as individuals or large groups. In the latter case, collectiveness emerges via cell-cell interactions. In migratory epithelial cell sheets, classic Cadherins are critical to maintain tissue integrity, to promote coordination and establish cell polarity. However, recent evidence indicates that mesenchymal cells, migrating in streams such as neural crest or cancer cells, also exhibit collective migration. Here we will explore the idea that Cadherins play an essential role during collective migration of mesenchymal cells.

Vinculin-dependent Cadherin mechanosensing regulates efficient epithelial barrier formation

Proper regulation of the formation and stabilization of epithelial cell–cell adhesion is crucial in embryonic morphogenesis and tissue repair processes. Defects in this process lead to organ malformation and defective epithelial barrier function. A combination of chemical and mechanical cues is used by cells to drive this process. We have investigated the role of the actomyosin cytoskeleton and its connection to cell–cell junction complexes in the formation of an epithelial barrier in MDCK cells. We find that the E-cadherin complex is sufficient to mediate a functional link between cell–cell contacts and the actomyosin cytoskeleton. This link involves the actin binding capacity of α-catenin and the recruitment of the mechanosensitive protein Vinculin to tensile, punctate cell–cell junctions that connect to radial F-actin bundles, which we name Focal Adherens Junctions (FAJ). When cell–cell adhesions mature, these FAJs disappear and linear junctions are formed that do not contain Vinculin. The rapid phase of barrier establishment (as measured by Trans Epithelial Electrical Resistance (TER)) correlates with the presence of FAJs. Moreover, the rate of barrier establishment is delayed when actomyosin contraction is blocked or when Vinculin recruitment to the Cadherin complex is prevented. Enhanced presence of Vinculin increases the rate of barrier formation. We conclude that E-cadherin-based FAJs connect forming cell–cell adhesions to the contractile actomyosin cytoskeleton. These specialized junctions are sites of Cadherin mechanosensing, which, through the recruitment of Vinculin, is a driving force in epithelial barrier formation.

Microtubules and Lis-1/NudE/dynein regulate invasive cell-on-cell migration in Drosophila

The environment through which cells migrate in vivo differs considerably from the in vitro environment where cell migration is often studied. In vivo many cells migrate in crowded and complex 3-dimensional tissues and may use other cells as the substratum on which they move. This includes neurons, glia and their progenitors in the brain. Here we use a Drosophila model of invasive, collective migration in a cellular environment to investigate the roles of microtubules and microtubule regulators in this type of cell movement. Border cells are of epithelial origin and have no visible microtubule organizing center (MTOC). Interestingly, microtubule plus-end growth was biased away from the leading edge. General perturbation of the microtubule cytoskeleton and analysis by live imaging showed that microtubules in both the migrating cells and the substrate cells affect movement. Also, whole-tissue and cell autonomous deletion of the microtubule regulator Stathmin had distinct effects. A screen of 67 genes encoding microtubule interacting proteins uncovered cell autonomous requirements for Lis-1, NudE and Dynein in border cell migration. Net cluster migration was decreased, with initiation of migration and formation of dominant front cell protrusion being most dramatically affected. Organization of cells within the cluster and localization of cell-cell adhesion molecules were also abnormal. Given the established role of Lis-1 in migrating neurons, this could indicate a general role of Lis-1/NudE, Dynein and microtubules, in cell-on-cell migration. Spatial regulation of cell-cell adhesion may be a common theme, consistent with observing both cell autonomous and non-autonomous requirements in both systems.

Drosophila apc regulates delamination of invasive epithelial clusters

Border Cells in the Drosophila ovaries are a useful genetic model for understanding the molecular events underlying epithelial cell motility. During stage 9 of egg chamber development they detach from neighboring stretched cells and migrate between the nurse cells to reach the oocyte. RNAi screening allowed us to identify the dapc1 gene as being critical in this process. Clonal and live analysis showed a requirement of dapc1 in both outer border cells and contacting stretched cells for delamination. This mutant phenotype was rescued by dapc1 or dapc2 expression. Loss of dapc1 function was associated with an abnormal lasting accumulation of β-catenin/Armadillo and E-cadherin at the boundary between migrating border and stretched cells. Moreover, β-catenin/armadillo or E-cadherin downregulation rescued the dapc1 loss of function phenotype. Altogether these results indicate that Drosophila Apc1 is required for dynamic remodeling of β-catenin/Armadillo and E-cadherin adhesive complexes between outer border cells and stretched cells regulating proper delamination and invasion of migrating epithelial clusters.

N-cadherin-mediated cell-cell adhesion promotes cell migration in a three-dimensional matrix

Cancer cells that originate from epithelial tissues typically lose epithelial specific cell-cell junctions, but these transformed cells are not devoid of cell-cell adhesion proteins. Using hepatocyte-growth-factor-treated MDCK cells that underwent a complete epithelial-to-mesenchymal transition, we analyzed cell-cell adhesion between these highly invasive transformed epithelial cells in a three-dimensional (3D) collagen matrix. In a 3D matrix, these transformed cells formed elongated multicellular chains, and migrated faster and more persistently than single cells in isolation. In addition, the cell clusters were enriched with stress-fiber-like actin bundles that provided contractile forces. N-cadherin-knockdown cells failed to form cell-cell junctions or migrate, and the expression of the N-cadherin cytoplasmic or extracellular domain partially rescued the knockdown phenotype. By contrast, the expression of N-cadherin-α-catenin chimera rescued the knockdown phenotype, but individual cells within the cell clusters were less mobile. Together, our findings suggest that a dynamic N-cadherin and actin linkage is required for efficient 3D collective migration.

Mtss1 promotes cell-cell junction assembly and stability through the small GTPase Rac1

Cell-cell junctions are an integral part of epithelia and are often disrupted in cancer cells during epithelial-to-mesenchymal transition (EMT), which is a main driver of metastatic spread. We show here that Metastasis suppressor-1 (Mtss1; Missing in Metastasis, MIM), a member of the IMD-family of proteins, inhibits cell-cell junction disassembly in wound healing or HGF-induced scatter assays by enhancing cell-cell junction strength. Mtss1 not only makes cells more resistant to cell-cell junction disassembly, but also accelerates the kinetics of adherens junction assembly. Mtss1 drives enhanced junction formation specifically by elevating Rac-GTP. Lastly, we show that Mtss1 depletion reduces recruitment of F-actin at cell-cell junctions. We thus propose that Mtss1 promotes Rac1 activation and actin recruitment driving junction maintenance. We suggest that the observed loss of Mtss1 in cancers may compromise junction stability and thus promote EMT and metastasis.

The secret life of α-catenin: moonlighting in morphogenesis

Cadherin-based intercellular adhesions are important determinants of proper tissue architecture. These adhesions must be both stable and dynamic to maintain tissue integrity as cells undergo morphogenetic movements during development. The role of α-catenin in this process has been vigorously debated due to conflicting in vitro and in vivo evidence regarding its molecular mechanism of action. Recent data supports the classical view that α-catenin facilitates actin attachments at adherens junctions, but also suggests that α-catenin may act as a force transducer, and may have additional roles in the cytoplasm. These multiple functions for α-catenin converge on the regulation of adhesion and may help to explain its stable yet dynamic nature.

Mutational analysis supports a core role for Drosophila α-catenin in adherens junction function

α-catenin associates the cadherin-catenin complex with the actin cytoskeleton. α-catenin binds to β-catenin, which links it to the cadherin cytoplasmic tail, and F-actin, but also to a multitude of actin-associated proteins. These interactions suggest a highly complex cadherin-actin interface. Moreover, mammalian αE-catenin has been implicated in a cadherin-independent cytoplasmic function in Arp2/3-dependent actin regulation, and in cell signaling. The function and regulation of individual molecular interactions of α-catenin, in particular during development, are not well understood. We have generated mutations in Drosophila α-Catenin (α-Cat) to investigate α-Catenin function in this model, and to establish a setup for testing α-Catenin-related constructs in α-Cat-null mutant cells in vivo. Our analysis of α-Cat mutants in embryogenesis, imaginal discs and oogenesis reveals defects consistent with a loss of cadherin function. Compromising components of the Arp2/3 complex or its regulator SCAR ameliorate the α-Cat loss-of-function phenotype in embryos but not in ovaries, suggesting negative regulatory interactions between α-Catenin and the Arp2/3 complex in some tissues. We also show that the α-Cat mutant phenotype can be rescued by the expression of a DE-cadherin::α-Catenin fusion protein, which argues against an essential cytosolic, cadherin-independent role of Drosophila α-Catenin.

Adherens junctions and cadherins in Drosophila development

Drosophila represents a paradigm for the analysis of the cellular, molecular and genetic mechanisms of development and is an ideal model system to study the contribution of Adherens Junctions (AJs) and their major components, cadherins, to morphogenesis. The combination of different techniques and approaches has allowed researchers to identify the requirements of these epithelial junctions in vivo in the context of a whole organism. The functional analysis of mutants for AJ core components, particularly for Drosophila DE-cadherin, has shown that AJs play critical roles in virtually all stages of development. For instance, AJs maintain tissue integrity while allowing the remodelling and homeostasis of many tissues. They control cell shape, contribute to cell polarity, facilitate cell-cell recognition during cell sorting, orient cell divisions, or regulate cell rearrangements, among other activities. Remarkably, these activities require a very fine control of the organisation and turnover of AJs during development. In addition, AJs engage in diverse and complex interactions with the cytoskeleton, signalling networks, intracellular trafficking machinery or polarity cues to perform these functions. Here, by summarising the requirements of AJs and cadherins during Drosophila morphogenesis, we illustrate the capital contribution of this model system to our knowledge of the mechanisms and biology of AJs.

Differential regulation of adherens junction dynamics during apical-basal polarization

Adherens junctions (AJs) in epithelial cells are constantly turning over to modulate adhesion properties under various physiological and developmental contexts, but how such AJ dynamics are regulated during the apical-basal polarization of primary epithelia remains unclear. Here, we used new and genetically validated GFP markers of Drosophila E-cadherin (DE-cadherin, hereafter referred to as DE-Cad) and β-catenin (Armadillo, Arm) to quantitatively assay the in vivo dynamics of biosynthetic turnover and membrane redistribution by fluorescence recovery after photobleaching (FRAP) assays. Our data showed that membrane DE-Cad and Arm in AJs of polarizing epithelial cells had much faster biosynthetic turnover than in polarized cells. Fast biosynthetic turnover of membrane DE-Cad is independent of actin- and dynamin-based trafficking, but is microtubule-dependent. Furthermore, Arm in AJs of polarizing cells showed a faster and diffusion-based membrane redistribution that was both quantitatively and qualitatively different from the slower and exchange-based DE-Cad membrane distribution, indicating that the association of Arm with DE-Cad is more dynamic in polarizing cells, and only becomes stable in polarized epithelial cells. Consistently, biochemical assays showed that the binding of Arm to DE-Cad is weaker in polarizing cells than in polarized cells. Our data revealed that the molecular interaction between DE-Cad and Arm is modulated during apical-basal polarization, suggesting a new mechanism that might be crucial for establishing apical-basal polarity through regulating the AJ dynamics.

Defective neuroepithelial cell cohesion affects tangential branchiomotor neuron migration in the zebrafish neural tube

Facial branchiomotor neurons (FBMNs) in zebrafish and mouse embryonic hindbrain undergo a characteristic tangential migration from rhombomere (r) 4, where they are born, to r6/7. Cohesion among neuroepithelial cells (NCs) has been suggested to function in FBMN migration by inhibiting FBMNs positioned in the basal neuroepithelium such that they move apically between NCs towards the midline of the neuroepithelium instead of tangentially along the basal side of the neuroepithelium towards r6/7. However, direct experimental evaluation of this hypothesis is still lacking. Here, we have used a combination of biophysical cell adhesion measurements and high-resolution time-lapse microscopy to determine the role of NC cohesion in FBMN migration. We show that reducing NC cohesion by interfering with Cadherin 2 (Cdh2) activity results in FBMNs positioned at the basal side of the neuroepithelium moving apically towards the neural tube midline instead of tangentially towards r6/7. In embryos with strongly reduced NC cohesion, ectopic apical FBMN movement frequently results in fusion of the bilateral FBMN clusters over the apical midline of the neural tube. By contrast, reducing cohesion among FBMNs by interfering with Contactin 2 (Cntn2) expression in these cells has little effect on apical FBMN movement, but reduces the fusion of the bilateral FBMN clusters in embryos with strongly diminished NC cohesion. These data provide direct experimental evidence that NC cohesion functions in tangential FBMN migration by restricting their apical movement.