Regulation of epithelial wound closure and intercellular adhesion by interaction of AF6 with actin cytoskeleton

Afadin mediates cadherin-catenin complex clustering on F-actin linked to cooperative binding and filament curvature

The E-cadherin-β-catenin-αE-catenin (cadherin-catenin) complex couples the cytoskeletons of neighboring cells at adherens junctions (AJs) to mediate force transmission across epithelia. Mechanical force and auxiliary binding partners converge to stabilize the cadherin-catenin complex's inherently weak binding to actin filaments (F-actin) through unclear mechanisms. Here, we show that afadin's coiled-coil (CC) domain and vinculin synergistically enhance the cadherin-catenin complex's F-actin engagement. The cryo-electron microscopy (cryo-EM) structure of an E-cadherin-β-catenin-αE-catenin-vinculin-afadin-CC supra-complex bound to F-actin reveals that afadin-CC bridges adjacent αE-catenin actin-binding domains along the filament, stabilizing flexible αE-catenin segments implicated in mechanical regulation. These cooperative binding contacts promote the formation of supra-complex clusters along F-actin. Additionally, cryo-EM variability analysis links supra-complex binding along individual F-actin strands to nanoscale filament curvature, a deformation mode associated with cytoskeletal forces. Collectively, this work elucidates a mechanistic framework by which vinculin and afadin tune cadherin-catenin complex-cytoskeleton coupling to support AJ function across varying mechanical regimes.

Afadin mediates cadherin-catenin complex clustering on F-actin linked to cooperative binding and filament curvature

The E-cadherin-β-catenin-αE-catenin (cadherin-catenin) complex couples the cytoskeletons of neighboring cells at adherens junctions (AJs) to mediate force transmission across epithelia. Mechanical force and auxiliary binding partners converge to stabilize the cadherin-catenin complex's inherently weak binding to actin filaments (F-actin) through unclear mechanisms. Here we show that afadin's coiled-coil (CC) domain and vinculin synergistically enhance the cadherin-catenin complex's F-actin engagement. The cryo-EM structure of an E-cadherin-β-catenin-αE-catenin-vinculin-afadin-CC supra-complex bound to F-actin reveals that afadin-CC bridges adjacent αE-catenin actin-binding domains along the filament, stabilizing flexible αE-catenin segments implicated in mechanical regulation. These cooperative binding contacts promote the formation of supra-complex clusters along F-actin. Additionally, cryo-EM variability analysis links supra-complex binding along individual F-actin strands to nanoscale filament curvature, a deformation mode associated with cytoskeletal forces. Collectively, this work elucidates a mechanistic framework by which vinculin and afadin tune cadherin-catenin complex-cytoskeleton coupling to support AJ function across varying mechanical regimes.

Adherens junction: the ensemble of specialized cadherin clusters

The cell-cell connections in adherens junctions (AJs) are mediated by transmembrane receptors, type I cadherins (referred to here as cadherins). These cadherin-based connections (or trans bonds) are weak. To upregulate their strength, cadherins exploit avidity, the increased affinity of binding between cadherin clusters compared with isolated monomers. Formation of such clusters is a unique molecular process that is driven by a synergy of direct and indirect cis interactions between cadherins located at the same cell. In addition to their role in adhesion, cadherin clusters provide structural scaffolds for cytosolic proteins, which implicate cadherin into different cellular activities and signaling pathways. The cluster lifetime, which depends on the actin cytoskeleton, and on the mechanical forces it generates, determines the strength of AJs and their plasticity. The key aspects of cadherin adhesion, therefore, cannot be understood at the level of isolated cadherin molecules, but should be discussed in the context of cadherin clusters.

Computational modeling and simulation of epithelial wound closure

Wounds in the epithelium may lead to serious injurious events or chronic inflammatory diseases, however, multicellular organisms have the ability to self-repair wounds through the movement of epithelial cell toward the wound area. Despite intensive studies exploring the mechanism of wound closure, the role of mechanics in epithelial wound closure is still not well explained. In order to investigate the role of mechanical properties on wound closure process, a three-dimensional continuum physics-based computational model is presented in this study. The model takes into account the material property of the epithelial cell, intercellular interactions between neighboring cells at cell-cell junctions, and cell-substrate adhesion between epithelial cells and ECM. Through finite element simulation, it is found that the closure efficiency is related to the initial gap size and the intensity of lamellipodial protrusion. It is also shown that cells at the wound edge undergo higher stress compared with other cells in the epithelial monolayer, and the cellular normal stress dominates over the cellular shear stress. The model presented in this study can be employed as a numerical tool to unravel the mechanical principles behind the complex wound closure process. These results might have the potential to improve effective wound management and optimize the treatment.

Systems biology and OMIC data integration to understand gastrointestinal cancers

Gastrointestinal (GI) cancers are a set of diverse diseases affecting many parts/ organs. The five most frequent GI cancer types are esophageal, gastric cancer (GC), liver cancer, pancreatic cancer, and colorectal cancer (CRC); together, they give rise to 5 million new cases and cause the death of 3.5 million people annually. We provide information about molecular changes crucial to tumorigenesis and the behavior and prognosis. During the formation of cancer cells, the genomic changes are microsatellite instability with multiple chromosomal arrangements in GC and CRC. The genomically stable subtype is observed in GC and pancreatic cancer. Besides these genomic subtypes, CRC has epigenetic modification (hypermethylation) associated with a poor prognosis. The pathway information highlights the functions shared by GI cancers such as apoptosis; focal adhesion; and the p21-activated kinase, phosphoinositide 3-kinase/Akt, transforming growth factor beta, and Toll-like receptor signaling pathways. These pathways show survival, cell proliferation, and cell motility. In addition, the immune response and inflammation are also essential elements in the shared functions. We also retrieved information on protein-protein interaction from the STRING database, and found that proteins Akt1, catenin beta 1 (CTNNB1), E1A binding protein P300, tumor protein p53 (TP53), and TP53 binding protein 1 (TP53BP1) are central nodes in the network. The protein expression of these genes is associated with overall survival in some GI cancers. The low TP53BP1 expression in CRC, high EP300 expression in esophageal cancer, and increased expression of Akt1/TP53 or low CTNNB1 expression in GC are associated with a poor prognosis. The Kaplan Meier plotter database also confirmed the association between expression of the five central genes and GC survival rates. In conclusion, GI cancers are very diverse at the molecular level. However, the shared mutations and protein pathways might be used to understand better and reveal diagnostic/prognostic or drug targets.

Afadin (AF6) in cancer progression: A multidomain scaffold protein with complex and contradictory roles

Adherens (AJ) and tight junctions (TJ) maintain cell-cell adhesions and cellular polarity in normal tissues. Afadin, a multi-domain scaffold protein, is commonly found in both adherens and tight junctions, where it plays both structural and signal-modulating roles. Afadin is a complex modulator of cellular processes implicated in cancer progression, including signal transduction, migration, invasion, and apoptosis. In keeping with the complexities associated with the roles of adherens and tight junctions in cancer, afadin exhibits both tumor suppressive and pro-metastatic functions. In this review, we will explore the dichotomous roles that afadin plays during cancer progression.

mTOR and S6K1 drive polycystic kidney by the control of Afadin-dependent oriented cell division

mTOR activation is essential and sufficient to cause polycystic kidneys in Tuberous Sclerosis Complex (TSC) and other genetic disorders. In disease models, a sharp increase of proliferation and cyst formation correlates with a dramatic loss of oriented cell division (OCD). We find that OCD distortion is intrinsically due to S6 kinase 1 (S6K1) activation. The concomitant loss of S6K1 in Tsc1-mutant mice restores OCD but does not decrease hyperproliferation, leading to non-cystic harmonious hyper growth of kidneys. Mass spectrometry-based phosphoproteomics for S6K1 substrates revealed Afadin, a known component of cell-cell junctions required to couple intercellular adhesions and cortical cues to spindle orientation. Afadin is directly phosphorylated by S6K1 and abnormally decorates the apical surface of Tsc1-mutant cells with E-cadherin and α-catenin. Our data reveal that S6K1 hyperactivity alters centrosome positioning in mitotic cells, affecting oriented cell division and promoting kidney cysts in conditions of mTOR hyperactivity.

Keeping RNA polymerase II on the run: Functions of MLL fusion partners in transcriptional regulation

Since the identification of key MLL fusion partners as transcription elongation factors regulating expression of HOX cluster genes during hematopoiesis, extensive work from the last decade has resulted in significant progress in our overall mechanistic understanding of role of MLL fusion partner proteins in transcriptional regulation of diverse set of genes beyond just the HOX cluster. In this review, we are going to detail overall understanding of role of MLL fusion partner proteins in transcriptional regulation and thus provide mechanistic insights into possible MLL fusion protein-mediated transcriptional misregulation leading to aberrant hematopoiesis and leukemogenesis.

Afadin Downregulation by Helicobacter pylori Induces Epithelial to Mesenchymal Transition in Gastric Cells

Afadin is a cytoplasmic protein of the adherens junctions, which regulates the formation and stabilization of both the adherens and the tight junctions. Aberrant expression of Afadin has been shown in cancer and its loss has been associated with epithelial-to-mesenchymal transition (EMT). EMT is characterized by the change from an epithelial to a mesenchymal phenotype, with modifications on the expression of adhesion molecules and acquisition of a migratory and invasive cell behavior. While it is known that Helicobacter pylori disrupts the tight and the adherens junctions and induces EMT, the effect of the bacteria on Afadin is still unknown. The aim of this study was to disclose the effect of H. pylori on Afadin and its impact in the induction of an EMT phenotype in gastric cells. Using two different cell lines, we observed that H. pylori infection decreased Afadin protein levels, independently of CagA, T4SS, and VacA virulence factors. H. pylori infection of cell lines recapitulated several EMT features, displacing and downregulating multiple proteins from cell–cell junctions, and increasing the expression of ZEB1, Vimentin, Slug, N-cadherin, and Snail. Silencing of Afadin by RNAi promoted delocalization of junctional proteins from the cell–cell contacts, increased paracellular permeability, and decreased transepithelial electrical resistance, all compatible with impaired junctional integrity. Afadin silencing also led to increased expression of the EMT marker Snail, and to the formation of actin stress fibers, together with increased cell motility and invasion. Finally, and in line with our in vitro data, the gastric mucosa of individuals infected with H. pylori showed decrease/loss of Afadin membrane staining at cell–cell contacts significantly more frequently than uninfected individuals. In conclusion, Afadin is downregulated by H. pylori infection in vitro and in vivo, and its downregulation leads to the emergence of EMT and to the acquisition of an aggressive phenotype in gastric cells, which can contribute to gastric carcinogenesis.

Cadherin controls nectin recruitment into adherens junctions by remodeling the actin cytoskeleton

The mechanism that coordinates activities of different adhesion receptors is poorly understood. We investigated this mechanism by focusing on the nectin-2 and E-cadherin adherens junction receptors. We found that, cadherin was not required for the basic process of nectin junction formation because nectin-2 formed junctions in cadherin-deficient A431D cells. Formation of nectin-2 junctions in these cells, however, became regulated by cadherin as soon as E-cadherin was re-expressed. E-cadherin recruited nectin-2 into adherens junctions, where both proteins formed distinct but tightly associated clusters. Live-cell imaging showed that the appearance of E-cadherin clusters often preceded that of nectin-2 clusters at sites of junction assembly. Inactivation of E-cadherin clustering by different strategies concomitantly suppressed the formation of nectin clusters. Furthermore, cadherin significantly increased the stability of nectin clusters, thereby making them resistant to the BC-12 antibody, which targets the nectin-2 adhesion interface. By testing different E-cadherin-α-catenin chimeras, we showed that the recruitment of nectin into chimera junctions is mediated by the actin-binding domain of α-catenin. Our data suggests that E-cadherin regulates assembly of nectin junctions through α-catenin-induced remodeling of the actin cytoskeleton around the cadherin clusters.

Afadin controls cadherin cluster stability using clathrin-independent mechanism

Afadin is an actin-binding protein that interacts with the intracellular region of the transmembrane proteins, nectins. In collaboration with other transmembrane proteins, cadherins, nectins form adherens junctions, a major type of cell-cell adhesive structures in the multicellular organisms. To elucidate the afadin function, we studied adherens junction defects induced by afadin depletion in epithelial A431 cells. We have found that the cells lacking afadin exhibit no abnormalities in morphology or in general dynamics of adherens junctions in the confluent cell cultures. The only observed difference is a slight increase in the rate of cadherin turnover in these junctions. However, afadin depletion strongly affects the assembly of new adherens junctions immediately after two cells touch one another: initiation of new junctions is significantly delayed, the growth of the nascent junctions stagnates, and their lifetime shortens. As a result, the afadin-depleted cells need much more time to establish the mature junctional structures. This defect is not caused by the clathrin-dependent endocytosis of cadherin clusters that was monitored using live-cell imaging of A431 cells co-expressing GFP-tagged E-cadherin and mCherry-tagged clathrin light chain. Taken together our data show that afadin reinforces adherens junctions and that this process is crucial for the fast formation of adherens junctions at the sites of new cell-cell contacts.

Small-molecule inhibitors of AF6 PDZ-mediated protein-protein interactions

PDZ (PSD-95, Dlg, ZO-1) domains are ubiquitous interaction modules that are involved in many cellular signal transduction pathways. Interference with PDZ-mediated protein-protein interactions has important implications in disease-related signaling processes. For this reason, PDZ domains have gained attention as potential targets for inhibitor design and, in the long run, drug development. Herein we report the development of small molecules to probe the function of the PDZ domain from human AF6 (ALL1-fused gene from chromosome 6), which is an essential component of cell-cell junctions. These compounds bind to AF6 PDZ with substantially higher affinity than the peptide (Ile-Gln-Ser-Val-Glu-Val) derived from its natural ligand, EphB2. In intact cells, the compounds inhibit the AF6-Bcr interaction and interfere with epidermal growth factor (EGF)-dependent signaling.

M3 muscarinic acetylcholine receptor dysfunction inhibits Rac1 activity and disrupts VE-cadherin/β-catenin and actin cytoskeleton interaction

The objective was to investigate whether M3 muscarinic acetylcholine receptor (mAChR) dysfunction disrupts the linkage between the vascular endothelial (VE)-cadherin in the adherens junctional complex and the actin-based cytoskeleton, increasing vascular permeability in atherosclerosis. Western blotting revealed that a selective M3 receptor antagonist, 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP), and M3 receptor siRNA decrease VE-cadherin and β-catenin in Triton X-100-insoluble fractions, indicating that M3 receptor inhibition weakens the linkage between the VE-cadherin/β-catenin complex and the actin cytoskeleton. Co-immunoprecipitation assays showed that M3 receptor inhibition reduces Rac1 activity and the association of IQ motif-containing GTPase-activating protein 1 (IQGAP1) with Ras-related C3 botulinum toxin substrate 1 (Rac1), while increasing the interaction between IQGAP1 and β-catenin. Using IQGAP1 siRNA, we found that IQGAP1 is required for stable interaction between VE-cadherin/β-catenin and the actin cytoskeleton in quiescent endothelial cells; IQGAP1 siRNA augments the M3 receptor inhibition-induced dissociation between them. Moreover, S-nitroso-N-acetylpenicillamine (SNAP), a nitric oxide (NO) donor, attenuates this disassociation and Rac1 activity inhibition. The M3 receptor facilitates interaction of the VE-cadherin-based adherens junctional complex and the actin-based cytoskeleton by maintaining Rac1 activity, which regulates the interaction between IQGAP1/Rac1 and IQGAP1/β-catenin, and may contribute to endothelial barrier function under physiological conditions.

The adherens junction protein afadin is an AKT substrate that regulates breast cancer cell migration

The PI3K-AKT signaling pathway regulates all phenotypes that contribute to progression of human cancers, including breast cancer. AKT mediates signal relay by phosphorylating numerous substrates, which are causally implicated in biologic responses such as cell growth, survival, metabolic reprogramming, migration, and invasion. Here a new AKT substrate is identified, the adherens junction protein Afadin, which is phosphorylated by AKT at Ser1718. Importantly, under conditions of physiologic IGF-1 signaling and oncogenic PI3K and AKT, Afadin is phosphorylated by all AKT isoforms, and this phosphorylation elicits a relocalization of Afadin from adherens junctions to the nucleus. Also, phosphorylation of Afadin increased breast cancer cell migration that was dependent on Ser1718 phosphorylation. Finally, nuclear localization of Afadin was observed in clinical breast cancer specimens, indicating that regulation of Afadin by the PI3K-AKT pathway has pathophysiologic significance.

IMPLICATIONS

Phosphorylation of the adhesion protein Afadin by AKT downstream of the PI3K pathway, leads to redistribution of Afadin and controls cancer cell migration.

Using fly genetics to dissect the cytoskeletal machinery of neurons during axonal growth and maintenance

The extension of long slender axons is a key process of neuronal circuit formation, both during brain development and regeneration. For this, growth cones at the tips of axons are guided towards their correct target cells by signals. Growth cone behaviour downstream of these signals is implemented by their actin and microtubule cytoskeleton. In the first part of this Commentary, we discuss the fundamental roles of the cytoskeleton during axon growth. We present the various classes of actin- and microtubule-binding proteins that regulate the cytoskeleton, and highlight the important gaps in our understanding of how these proteins functionally integrate into the complex machinery that implements growth cone behaviour. Deciphering such machinery requires multidisciplinary approaches, including genetics and the use of simple model organisms. In the second part of this Commentary, we discuss how the application of combinatorial genetics in the versatile genetic model organism Drosophila melanogaster has started to contribute to the understanding of actin and microtubule regulation during axon growth. Using the example of dystonin-linked neuron degeneration, we explain how knowledge acquired by studying axonal growth in flies can also deliver new understanding in other aspects of neuron biology, such as axon maintenance in higher animals and humans.

Epithelial barrier assembly requires coordinated activity of multiple domains of the tight junction protein ZO-1

Tight junctions (TJs) regulate the paracellular movement of ions, macromolecules and immune cells across epithelia. Zonula occludens (ZO)-1 is a multi-domain polypeptide required for the assembly of TJs. MDCK II cells lacking ZO-1, and its homolog ZO-2, have three distinct phenotypes: reduced localization of occludin and some claudins to the TJs, increased epithelial permeability, and expansion of the apical actomyosin contractile array found at the apical junction complex (AJC). However, it is unclear exactly which ZO-1 binding domains are required to coordinate these activities. We addressed this question by examining the ability of ZO-1 domain-deletion transgenes to reverse the effects of ZO depletion. We found that the SH3 domain and the U5 motif are required to recruit ZO-1 to the AJC and that localization is a prerequisite for normal TJ and cytoskeletal organization. The PDZ2 domain is not required for localization of ZO-1 to the AJC, but is necessary to establish the characteristic continuous circumferential band of ZO-1, occludin and claudin-2. PDZ2 is also required to establish normal permeability, but is not required for normal cytoskeletal organization. Finally, our results demonstrate that PDZ1 is crucial for the normal organization of both the TJ and the AJC cytoskeleton. Our results establish that ZO-1 acts as a true scaffolding protein and that the coordinated activity of multiple domains is required for normal TJ structure and function.

Rap1 and Canoe/afadin are essential for establishment of apical-basal polarity in the Drosophila embryo

The small GTPase Rap1 and the actin-junctional linker protein Canoe/afadin are essential for the initial establishment of polarity in Drosophila, acting upstream of Bazooka/Par3 and the adherens junctions. However, feedback and cross-regulation occur, so polarity establishment is regulated by a network of proteins rather than a linear pathway.

The establishment and maintenance of apical–basal cell polarity is critical for assembling epithelia and maintaining organ architecture. Drosophila embryos provide a superb model. In the current view, apically positioned Bazooka/Par3 is the initial polarity cue as cells form during cellularization. Bazooka then helps to position both adherens junctions and atypical protein kinase C (aPKC). Although a polarized cytoskeleton is critical for Bazooka positioning, proteins mediating this remained unknown. We found that the small GTPase Rap1 and the actin-junctional linker Canoe/afadin are essential for polarity establishment, as both adherens junctions and Bazooka are mispositioned in their absence. Rap1 and Canoe do not simply organize the cytoskeleton, as actin and microtubules become properly polarized in their absence. Canoe can recruit Bazooka when ectopically expressed, but they do not obligatorily colocalize. Rap1 and Canoe play continuing roles in Bazooka localization during gastrulation, but other polarity cues partially restore apical Bazooka in the absence of Rap1 or Canoe. We next tested the current linear model for polarity establishment. Both Bazooka and aPKC regulate Canoe localization despite being “downstream” of Canoe. Further, Rap1, Bazooka, and aPKC, but not Canoe, regulate columnar cell shape. These data reshape our view, suggesting that polarity establishment is regulated by a protein network rather than a linear pathway.

A Serrate-Notch-Canoe complex mediates glial-neuroepithelial cell interactions essential during Drosophila optic lobe development

It is firmly established that neuron-glia interactions are fundamental across species for the correct establishment of a functional brain. Here, we found that the glia of the Drosophila larval brain display an essential non-autonomous role during the development of the optic lobe. The optic lobe develops from neuroepithelial cells that proliferate by dividing symmetrically until they switch to asymmetric/differentiative divisions generating neuroblasts. The proneural gene lethal of scute (l'sc) is transiently activated by the Epidermal Growth Factor Receptor (EGFR)/Ras signal transduction pathway at the leading edge of a proneural wave that sweeps from medial to lateral neuroepithelium promoting this switch. This process is tightly regulated by the tissue-autonomous function within the neuroepithelium of multiple signaling pathways, including EGFR/Ras and Notch. This study shows that the Notch ligand Serrate (Ser) is expressed in the glia and it forms a complex in vivo with Notch and Canoe, which colocalize at the adherens junctions of neuroepithelial cells. This complex is crucial for glial-neuroepithelial cell interactions during optic lobe development. Ser is tissue-autonomously required in the glia where it activates Notch to regulate its proliferation, and non-autonomously in the neuroepithelium where Ser induces Notch signaling to avoid the premature activation of the EGFR/Ras pathway and hence of L'sc. Interestingly, different Notch activity reporters showed very different expression patterns in the glia and in the neuroepithelium, suggesting the existence of tissue-specific factors that promote the expression of particular Notch target genes or/and a reporter response dependent on different thresholds of Notch signaling.

A genome-wide functional screen shows MAGI-1 is an L1CAM-dependent stabilizer of apical junctions in C. elegans

BACKGROUND

In multicellular organisms, cell-cell junctions are involved in many aspects of tissue morphogenesis. α-catenin links the cadherin-catenin complex (CCC) to the actin cytoskeleton, stabilizing cadherin-dependent adhesions.

RESULTS

To identify modulators of cadherin-based cell adhesion, we conducted a genome-wide RNAi screen in C. elegans and uncovered MAGI-1, a highly conserved protein scaffold. Loss of magi-1 function in wild-type embryos results in disorganized epithelial migration and occasional morphogenetic failure. MAGI-1 physically interacts with the putative actin regulator AFD-1/afadin; knocking down magi-1 or afd-1 function in a hypomorphic α-catenin background leads to complete morphogenetic failure and actin disorganization in the embryonic epidermis. MAGI-1 and AFD-1 localize to a unique domain in the apical junction and normal accumulation of MAGI-1 at junctions requires SAX-7/L1CAM, which can bind MAGI-1 via its C terminus. Depletion of MAGI-1 leads to loss of spatial segregation and expansion of apical junctional domains and greater mobility of junctional proteins.

CONCLUSIONS

Our screen is the first genome-wide approach to identify proteins that function synergistically with the CCC during epidermal morphogenesis in a living embryo. We demonstrate novel physical interactions between MAGI-1, AFD-1/afadin, and SAX-7/L1CAM, which are part of a functional interactome that includes components of the core CCC. Our results further suggest that MAGI-1 helps to partition and maintain a stable, spatially ordered apical junction during morphogenesis.

The actin-binding protein Canoe/AF-6 forms a complex with Robo and is required for Slit-Robo signaling during axon pathfinding at the CNS midline

Axon guidance is a key process during nervous system development and regeneration. One of the best established paradigms to study the mechanisms underlying this process is the axon decision of whether or not to cross the midline in the Drosophila CNS. An essential regulator of that decision is the well conserved Slit-Robo signaling pathway. Slit guidance cues act through Robo receptors to repel axons from the midline. Despite good progress in our knowledge about these proteins, the intracellular mechanisms associated with Robo function remain poorly defined. In this work, we found that the scaffolding protein Canoe (Cno), the Drosophila orthologue of AF-6/Afadin, is essential for Slit-Robo signaling. Cno is expressed along longitudinal axonal pioneer tracts, and longitudinal Robo/Fasciclin2-positive axons aberrantly cross the midline in cno mutant embryos. cno mutant primary neurons show a significant reduction of Robo localized in growth cone filopodia and Cno forms a complex with Robo in vivo. Moreover, the commissureless (comm) phenotype (i.e., lack of commissures due to constitutive surface presentation of Robo in all neurons) is suppressed in comm, cno double-mutant embryos. Specific genetic interactions between cno, slit, robo, and genes encoding other components of the Robo pathway, such as Neurexin-IV, Syndecan, and Rac GTPases, further confirm that Cno functionally interacts with the Slit-Robo pathway. Our data argue that Cno is a novel regulator of the Slit-Robo signaling pathway, crucial for regulating the subcellular localization of Robo and for transducing its signaling to the actin cytoskeleton during axon guidance at the midline.

Under construction: building the macromolecular superstructure and signaling components of an electrical synapse

A great deal is now known about the protein components of tight junctions and adherens junctions, as well as how these are assembled. Less is known about the molecular framework of gap junctions, but these also have membrane specializations and are subject to regulation of their assembly and turnover. Thus, it is reasonable to consider that these three types of junctions may share macromolecular commonalities. Indeed, the tight junction scaffolding protein zonula occluden-1 (ZO-1) is also present at adherens and gap junctions, including neuronal gap junctions. On the basis of these earlier observations, we more recently found that two additional proteins, AF6 and MUPP1, known to be associated with ZO-1 at tight and adherens junctions, are also components of neuronal gap junctions in rodent brain and directly interact with connexin36 (Cx36) that forms these junctions. Here, we show by immunofluorescence labeling that the cytoskeletal-associated protein cingulin, commonly found at tight junctions, is also localized at neuronal gap junctions throughout the central nervous system. In consideration of known functions related to ZO-1, AF6, MUPP1, and cingulin, our results provide a context in which to examine functional relationships between these proteins at Cx36-containing electrical synapses in brain--specifically, how they may contribute to regulation of transmission at these synapses, and how they may govern gap junction channel assembly and/or disassembly.

Immunoglobulin superfamily receptors and adherens junctions

The immunogroblin (Ig) superfamily proteins characterized by the presence of Ig-like domains are involved in various cellular functions. The properties of the Ig-like domains to form rod-like structures and to bind specifically to other proteins make them ideal for cell surface receptors and cell adhesion molecules (CAMs). Ig-CAMs, nectins in mammals and Echinoid in Drosophila, are crucial components of cadherin-based adherens junctions in the epithelium. Nectins form cell-cell adhesion by their trans-interactions and recruit cadherins to the nectin-initiated cell-cell adhesion site to establish adherens junctions. Thereafter junction adhesion molecules, occludin, and claudins, are recruited to the apical side of adherens junctions to establish tight junctions. The recruitment of these molecules by nectins is mediated both by the direct and indirect interactions of afadin with many proteins, such as catenins, and zonula occludens proteins, and by the nectin-induced reorganization of the actin cytoskeleton. Nectins contribute to the formation of both homotypic and heterotypic types of cell-cell junctions, such as synapses in the brain, contacts between pigment and non-pigment cell layers of the ciliary epithelium in the eye, Sertoli cell-spermatid junctions in the testis, and sensory cells and supporting cells in the sensory organs. In addition, cis- and trans-interactions of nectins with various cell surface proteins, such as integrins, growth factor receptors, and nectin-like molecules (Necls) play important roles in the regulation of many cellular functions, such as cell polarization, movement, proliferation, differentiation, survival, and cell sorting. Furthermore, the Ig-CAMs are implicated in many human diseases including viral infections, ectodermal dysplasia, cancers, and Alzheimer's disease.

Exposure to external environment of low ion concentrations is the trigger for rapid wound closure in Xenopus laevis embryos

Wounds in Xenopus laevis embryos close rapidly, as previously described. In this study, we examined the dependence on extracellular Na(+) and/or Cl(-) ion concentrations of the closure of wounds in Xenopus embryos inflicted by thermal injury. Wound closure did not occur in normal amphibian medium (100% NAM), while wound areas remarkably decreased either in 10-50% NAM or in 100% NAM lacking Na(+) or Cl(-). Similarly, wound areas did not change in a set of Na(+) and Cl(-) ion concentrations equivalent to those of the humoral fluids of intact Xenopus embryos, but rapid wound closure was induced by decreasing the concentration of either of the two ions. A tangential accumulation of actin cytoskeleton along the wound edge was associated with wound closure. However, a similar actin alignment formed even under the 100% NAM condition, in which wounds did not close, as stated above. The epidermis around the wound edge exhibited ellipse-shaped hypertrophy, and the marginal cells centripetally elongated during wound closure. On the other hand, no distinct morphological changes occurred in 100% NAM, although the epidermis was somewhat thickened. Thus, the morphological changes in the epidermis specific to the low ionic environment most likely play active roles in the wound closure of Xenopus laevis embryos, whereas the tangential actin alignment alone may be insufficient. Taken together, we propose that the wound closure in Xenopus embryos is triggered by a decline in either the extracellular Na(+) or Cl(-) ion concentration, and that this process is required for the abovementioned changes in the shape of the marginal cells.

Computer simulations of cell sorting due to differential adhesion

The actions of cell adhesion molecules, in particular, cadherins during embryonic development and morphogenesis more generally, regulate many aspects of cellular interactions, regulation and signaling. Often, a gradient of cadherin expression levels drives collective and relative cell motions generating macroscopic cell sorting. Computer simulations of cell sorting have focused on the interactions of cells with only a few discrete adhesion levels between cells, ignoring biologically observed continuous variations in expression levels and possible nonlinearities in molecular binding. In this paper, we present three models relating the surface density of cadherins to the net intercellular adhesion and interfacial tension for both discrete and continuous levels of cadherin expression. We then use then the Glazier-Graner-Hogeweg (GGH) model to investigate how variations in the distribution of the number of cadherins per cell and in the choice of binding model affect cell sorting. We find that an aggregate with a continuous variation in the level of a single type of cadherin molecule sorts more slowly than one with two levels. The rate of sorting increases strongly with the interfacial tension, which depends both on the maximum difference in number of cadherins per cell and on the binding model. Our approach helps connect signaling at the molecular level to tissue-level morphogenesis.

Cooperative role of nectin-nectin and nectin-afadin interactions in formation of nectin-based cell-cell adhesion

The nectin cell adhesion molecules interact in trans with each other through their extracellular regions and with afadin through their cytoplasmic tails, forming adherens junctions in cooperation with cadherins. In a single cell, Necl-5 (nectin-like molecule-5) localizes at the leading edge and regulates directional cell movement in response to a chemoattractant. In such a single cell, afadin also localizes at the leading edge without interacting with nectins or Necl-5. It remains unknown how the nectin-nectin and nectin-afadin interactions are initiated when moving cells contact each other to initiate the formation of adherens junctions. We show here that the Necl-5-nectin interaction induced by cell-cell contact enhances the nectin-afadin interaction. This interaction then enhances the nectin-nectin interaction, which further enhances the nectin-afadin interaction in a positive feedback manner. Thus, the Necl-5-nectin, nectin-nectin, and nectin-afadin interactions cooperatively increase the clustering of the nectin-afadin complex at the cell-cell contact sites, promoting the formation of the nectin-based cell-cell adhesion.

Canoe functions at the CNS midline glia in a complex with Shotgun and Wrapper-Nrx-IV during neuron-glia interactions

Vertebrates and insects alike use glial cells as intermediate targets to guide growing axons. Similar to vertebrate oligodendrocytes, Drosophila midline glia ensheath and separate axonal commissures. Neuron-glia interactions are crucial during these events, although the proteins involved remain largely unknown. Here, we show that Canoe (Cno), the Drosophila ortholog of AF-6, and the DE-cadherin Shotgun (Shg) are highly restricted to the interface between midline glia and commissural axons. cno mutant analysis, genetic interactions and co-immunoprecipitation assays unveil Cno function as a novel regulator of neuron-glia interactions, forming a complex with Shg, Wrapper and Neurexin IV, the homolog of vertebrate Caspr/paranodin. Our results also support additional functions of Cno, independent of adherens junctions, as a regulator of adhesion and signaling events in non-epithelial tissues.

Downregulation of thrombomodulin, a novel target of Snail, induces tumorigenesis through epithelial-mesenchymal transition

The expression of thrombomodulin (TM), a calcium-dependent adhesion molecule, is frequently downregulated in various cancer types. However, the mechanism responsible for the low expression level of TM in tumorigenesis is unknown. Here, an inverse expression of TM and Snail was detected in different cancer cell lines. We further confirmed this inverse relation using the epithelial-mesenchymal transition cell model in HaCaT and A431 cells. We demonstrated that Snail suppressed TM expression by binding to E-box (CACCTG) in TM promoter. Moreover, TM knockdown by short hairpin RNA disrupted E-cadherin-mediated cell junctions and contributed to tumorigenesis. In the calcium switch assay, E-cadherin lost the ability to associate with β-catenin and accumulated in cytoplasm in TM knockdown cells. Meanwhile, wound healing and invasive assays showed that TM knockdown promoted cell motility. A subcutaneous injection of TM knockdown transfectants into immunocompromised mice induced squamous cell carcinoma-like tumors. Besides, forced expression of murine TM in TM knockdown cells made the cells reassume epithelium-like morphology and increased calcium-dependent association of E-cadherin and β-catenin. In conclusion, TM, a novel downstream target of Snail in epithelial-mesenchymal transition, is required for maintaining epithelial morphology and functions as a tumor suppressor.

PHF10 is required for cell proliferation in normal and SV40-immortalized human fibroblast cells

Normal human diploid fibroblasts have limited life span in culture and undergo replicative senescence after 50-60 population doublings. On the contrary, cancer cells typically divide indefinitely and are immortal. Expression of SV40 large T and small t antigens in human fibroblasts transiently extends their life span by 20-30 population doublings and facilitates immortalization. We have identified a rearrangement in chromosome 6 shared by SV40-transformed human fibroblasts. Rearrangements involving chromosome 6 are among the most frequent in human carcinogenesis. In this paper, we extend analysis of the 6q26-q27 region, a putative site for a growth suppressor gene designated SEN6 involved in immortalization of SV40-transformed cells. Detailed molecular characterization of the rearranged chromosomes (6q*, normal appearing; and 6q(t), translocated) in the SV40-immortalized cell line HALneo by isolating each of these 2 chromosomes in mouse/HAL somatic cell hybrids is presented. Analysis of these mouse/HAL somatic cell hybrids with polymorphic and nonpolymorphic markers revealed that the 6q* has undergone a chromosomal break in the MLLT4 gene (alias AF6). This result in conjunction with previous published observations leads us to conclude that SEN6 lies between MLLT4 and TBP at chromosomal region 6q27. Examination of different genes (MLLT4, DLL1, FAM120B, PHF10) located within this interval that are expressed in HS74 normal fibroblast cells reveals that overexpression of epitope-tagged truncated PHF10 cDNAs resulted in reduced cell proliferation in multiple cell lines. Paradoxically, down-regulation of PHF10 by RNAi also resulted in loss of cell proliferation in normal fibroblast cells, indicating PHF10 function is required for cell growth. Taken together, these observations suggest that decreased cell proliferation with epitope-tagged truncated PHF10 proteins may be due to dominant negative effects or due to unregulated expression of these mutant proteins. Hence we conclude that PHF10 is not SEN6 but is required for cell growth.

Localization of nectin-free afadin at the leading edge and its involvement in directional cell movement induced by platelet-derived growth factor

Afadin is an actin-filament-binding protein that binds to nectin, an immunoglobulin-like cell-cell adhesion molecule, and plays an important role in the formation of adherens junctions. Here, we show that afadin, which did not bind to nectin and was localized at the leading edge of moving cells, has another role: enhancement of the directional, but not random, cell movement. When NIH3T3 cells were stimulated with platelet-derived growth factor (PDGF), afadin colocalized with PDGF receptor, αvβ3 integrin and nectin-like molecule-5 at the leading edge and facilitated the formation of leading-edge structures and directional cell movement in the direction of PDGF stimulation. However, these phenotypes were markedly perturbed by knockdown of afadin, and were dependent on the binding of afadin to active Rap1. Binding of Rap1 to afadin was necessary for the recruitment of afadin and the tyrosine phosphatase SHP-2 to the leading edge. SHP-2 was previously reported to tightly regulate the activation of PDGF receptor and its downstream signaling pathway for the formation of the leading edge. These results indicate that afadin has a novel role in PDGF-induced directional cell movement, presumably in cooperation with active Rap1 and SHP-2.

The Drosophila afadin homologue Canoe regulates linkage of the actin cytoskeleton to adherens junctions during apical constriction

Cadherin-based adherens junctions (AJs) mediate cell adhesion and regulate cell shape change. The nectin–afadin complex also localizes to AJs and links to the cytoskeleton. Mammalian afadin has been suggested to be essential for adhesion and polarity establishment, but its mechanism of action is unclear. In contrast, Drosophila melanogaster’s afadin homologue Canoe (Cno) has suggested roles in signal transduction during morphogenesis. We completely removed Cno from embryos, testing these hypotheses. Surprisingly, Cno is not essential for AJ assembly or for AJ maintenance in many tissues. However, morphogenesis is impaired from the start. Apical constriction of mesodermal cells initiates but is not completed. The actomyosin cytoskeleton disconnects from AJs, uncoupling actomyosin constriction and cell shape change. Cno has multiple direct interactions with AJ proteins, but is not a core part of the cadherin–catenin complex. Instead, Cno localizes to AJs by a Rap1- and actin-dependent mechanism. These data suggest that Cno regulates linkage between AJs and the actin cytoskeleton during morphogenesis.

Junctional adhesion molecule A interacts with Afadin and PDZ-GEF2 to activate Rap1A, regulate beta1 integrin levels, and enhance cell migration

Junctional adhesion molecule-A (JAM-A) is a transmembrane tight junction protein that has been shown to regulate barrier function and cell migration through incompletely understood mechanisms. We have previously demonstrated that JAM-A regulates cell migration by dimerization of the membrane-distal immunoglobulin-like loop and a C-terminal postsynaptic density 95/disc-large/zona occludens (PDZ) binding motif. Disruption of dimerization resulted in decreased epithelial cell migration secondary to diminished levels of beta1 integrin and active Rap1. Here, we report that JAM-A is physically and functionally associated with the PDZ domain-containing molecules Afadin and PDZ-guanine nucleotide exchange factor (GEF) 2, but not zonula occludens (ZO)-1, in epithelial cells, and these interactions mediate outside-in signaling events. Both Afadin and PDZ-GEF2 colocalized and coimmunoprecipitated with JAM-A. Furthermore, association of PDZ-GEF2 with Afadin was dependent on the expression of JAM-A. Loss of JAM-A, Afadin, or PDZ-GEF2, but not ZO-1 or PDZ-GEF1, similarly decreased cellular levels of activated Rap1, beta1 integrin protein, and epithelial cell migration. The functional effects observed were secondary to decreased levels of Rap1A because knockdown of Rap1A, but not Rap1B, resulted in decreased beta1 integrin levels and reduced cell migration. These findings suggest that JAM-A dimerization facilitates formation of a complex with Afadin and PDZ-GEF2 that activates Rap1A, which regulates beta1 integrin levels and cell migration.

Cell-cell junction formation: the role of Rap1 and Rap1 guanine nucleotide exchange factors

Rap proteins are Ras-like small GTP-binding proteins that amongst others are involved in the control of cell-cell and cell-matrix adhesion. Several Rap guanine nucleotide exchange factors (RapGEFs) function to activate Rap. These multi-domain proteins, which include C3G, Epacs, PDZ-GEFs, RapGRPs and DOCK4, are regulated by various different stimuli and may function at different levels in junction formation. Downstream of Rap, a number of effector proteins have been implicated in junctional control, most notably the adaptor proteins AF6 and KRIT/CCM1. In this review, we will highlight the latest findings on the Rap signaling network in the control of epithelial and endothelial cell-cell junctions.

Specificity in Ras and Rap signaling

Ras and Rap proteins are closely related small GTPases. Whereas Ras is known for its role in cell proliferation and survival, Rap1 is predominantly involved in cell adhesion and cell junction formation. Ras and Rap are regulated by different sets of guanine nucleotide exchange factors and GTPase-activating proteins, determining one level of specificity. In addition, although the effector domains are highly similar, Rap and Ras interact with largely different sets of effectors, providing a second level of specificity. In this review, we discuss the regulatory proteins and effectors of Ras and Rap, with a focus on those of Rap.

c-Src-mediated epithelial cell migration and invasion regulated by PDZ binding site

c-Src tyrosine kinase controls proliferation, cell adhesion, and cell migration and is highly regulated. A novel regulatory mechanism to control c-Src function that has recently been identified involves the C-terminal amino acid sequence Gly-Glu-Asn-Leu (GENL) of c-Src as ligand for PDZ domains. Herein, we determined the biological relevance of this c-Src regulation in human breast epithelial cells. The intact GENL sequence maintained c-Src in an inactive state in starved cells and restricted c-Src functions that might lead to metastatic transformation under normal growth conditions. c-Src with a C-terminal Leu/Ala mutation in GENL (Src-A) promoted the activation and translocation of cortactin and focal adhesion kinase and increased the motility and persistence of cell migration on the basement membrane. Src-A promoted increased extracellular proteolytic activity, and in acinar cultures, it led to the escape of cells through the basement membrane into the surrounding matrix. We ascribe the regulatory function of C-terminal Leu to the role of GENL in modulating c-Src activity downstream of cell matrix adhesion. We propose that the C terminus of c-Src via its GENL sequence presents a mechanism that restricts c-Src in epithelia and prevents progression toward an invasive phenotype.

Role of AF6 protein in cell-to-cell spread of Herpes simplex virus 1

AF6 and its rat homologue afadin are multidomain proteins localized at cell junctions and involved in intercellular adhesion. AF6 interacts via its PDZ domain with nectin-1 at epithelial adherens junctions. Nectin-1 serves as a mediator of cell-to-cell spread for Herpes simplex virus 1 (HSV-1). We analyzed the role of AF6 protein in the viral spread and nectin-1 clustering at cell-cell contacts by knockdown of AF6 in epithelial cells. AF6 knockdown reduced efficiency of HSV-1 spreading, however, the clustering of nectin-1 at cell-cell contacts was not affected. Thus, AF6 protein is important for spreading of HSV-1 in epithelial cells, independently of nectin clustering, possibly by stabilization of the E-cadherin-dependent cell adhesion.

Tight junctions/adherens junctions: basic structure and function

Adherens and tight junctions are intercellular junctions crucial for epithelial adhesion and barrier function in a wide variety of tissues and organisms. In stratifying epithelia, such as the epidermis, the role of adherens and tight junctions was considered less important owing to the abundance of desmosomes, mediating firm mechanical stability between the cells, and to the barrier function of the stratum corneum, respectively. This view has changed in recent years because of different studies that showed the importance of these structures for proper skin physiology and barrier function. The current review provides an overview of the crucial molecular constituents of these structures and highlights some recent results on their regulation. In particular, I will discuss their importance in skin biology.

Dicer is involved in protection against influenza A virus infection

In mammals the interferon (IFN) system is a central innate antiviral defence mechanism, while the involvement of RNA interference (RNAi) in antiviral response against RNA viruses is uncertain. Here, we tested whether RNAi is involved in the antiviral response in mammalian cells. To investigate the role of RNAi in influenza A virus-infected cells in the absence of IFN, we used Vero cells that lack IFN-alpha and IFN-beta genes. Our results demonstrate that knockdown of a key RNAi component, Dicer, led to a modest increase of virus production and accelerated apoptosis of influenza A virus-infected cells. These effects were much weaker in the presence of IFN. The results also show that in both Vero cells and the IFN-producing alveolar epithelial A549 cell line influenza A virus targets Dicer at mRNA and protein levels. Thus, RNAi is involved in antiviral response, and Dicer is important for protection against influenza A virus infection.

Regulation of c-Src by binding to the PDZ domain of AF-6

c-Src is a tightly regulated non-receptor tyrosine kinase. We describe the C-terminus of c-Src as a ligand for a PDZ (postsynaptic density 95, PSD-95; discs large, Dlg; zonula occludens-1, ZO-1) domain. The C-terminal residue Leu of c-Src is essential for binding to a PDZ domain. Mutation of this residue does not affect the intrinsic kinase activity in vitro, but interferes with c-Src regulation in cells. As a candidate PDZ protein, we analysed AF-6, a junctional adhesion protein. The AF-6 PDZ domain restricts the number of c-Src substrates, whereas knockdown of AF-6 has the opposite effect. Binding of c-Src to the AF-6 PDZ domain interferes with phosphorylation of c-Src at Tyr527 by the C-terminal kinase, and reduces c-Src autophosphorylation at Tyr416, resulting in a moderately activated c-Src kinase. Unphosphorylated Tyr527 allows binding of c-Src to AF-6. This can be overcome by overexpression of CSK or strong activation of c-Src. c-Src is recruited by AF-6 to cell-cell contact sites, suggesting that c-Src is regulated by a PDZ protein in special cellular locations. We identified a novel type of c-Src regulation by interaction with a PDZ protein.