Laminin-5-integrin interaction signals through PI 3-kinase and Rac1b to promote assembly of adherens junctions in HT-29 cells

Junctional epithelium and hemidesmosomes: Tape and rivets for solving the "percutaneous device dilemma" in dental and other permanent implants

The percutaneous device dilemma describes etiological factors, centered around the disrupted epithelial tissue surrounding non-remodelable devices, that contribute to rampant percutaneous device infection. Natural percutaneous organs, in particular their extracellular matrix mediating the "device"/epithelium interface, serve as exquisite examples to inspire longer lasting long-term percutaneous device design. For example, the tooth's imperviousness to infection is mediated by the epithelium directly surrounding it, the junctional epithelium (JE). The hallmark feature of JE is formation of hemidesmosomes, cell/matrix adhesive structures that attach surrounding oral gingiva to the tooth's enamel through a basement membrane. Here, the authors survey the multifaceted functions of the JE, emphasizing the role of the matrix, with a particular focus on hemidesmosomes and their five main components. The authors highlight the known (and unknown) effects dental implant - as a model percutaneous device - placement has on JE regeneration and synthesize this information for application to other percutaneous devices. The authors conclude with a summary of bioengineering strategies aimed at solving the percutaneous device dilemma and invigorating greater collaboration between clinicians, bioengineers, and matrix biologists.

Sci-Seq of Human Fetal Salivary Tissue Introduces Human Transcriptional Paradigms and a Novel Cell Population

Multiple pathologies and non-pathological factors can disrupt the function of the non-regenerative human salivary gland including cancer and cancer therapeutics, autoimmune diseases, infections, pharmaceutical side effects, and traumatic injury. Despite the wide range of pathologies, no therapeutic or regenerative approaches exist to address salivary gland loss, likely due to significant gaps in our understanding of salivary gland development. Moreover, identifying the tissue of origin when diagnosing salivary carcinomas requires an understanding of human fetal development. Using computational tools, we identify developmental branchpoints, a novel stem cell-like population, and key signaling pathways in the human developing salivary glands by analyzing our human fetal single-cell sequencing data. Trajectory and transcriptional analysis suggest that the earliest progenitors yield excretory duct and myoepithelial cells and a transitional population that will yield later ductal cell types. Importantly, this single-cell analysis revealed a previously undescribed population of stem cell-like cells that are derived from SD and expresses high levels of genes associated with stem cell-like function. We have observed these rare cells, not in a single niche location but dispersed within the developing duct at later developmental stages. Our studies introduce new human-specific developmental paradigms for the salivary gland and lay the groundwork for the development of translational human therapeutics.

Electrochemical Impedance Spectroscopy as a Tool for Monitoring Cell Differentiation from Floor Plate Progenitors to Midbrain Neurons in Real Time

Here shows that electrical impedance spectroscopy can be used as a non-invasive and real time tool to probe cell adhesion and differentiation from midbrain floor plate progenitors into midbrain neurons on Au electrodes coated with human laminin. The electrical data and equivalent circuit modeling are consistent with standard microscopy analysis and reveal that within the first 6 hours progenitor cells sediment and attach to the electrode within 40 hours. Between 40 and 120 hours, midbrain progenitor cells differentiate into midbrain neurons, followed by an electrochemically stable maturation phase. The ability to sense and characterize non-invasively and in real time cell differentiation opens up unprecedented avenues for implantable therapies and differentiation strategies.

Laminin 332 in cancer: When the extracellular matrix turns signals from cell anchorage to cell movement

Laminin 332 is crucial in the biology of epithelia. This large extracellular matrix protein consists of the heterotrimeric assembly of three subunits - α3, β3, and γ2 - and its multifunctionality relies on a number of extracellular proteolytic processing events. Laminin 332 is central to normal epithelium homeostasis by sustaining cell adhesion, polarity, proliferation, and differentiation. It also supports a major function in epithelial tissue formation, repair, and regeneration by buttressing cell migration and survival and basement membrane assembly. Interest in this protein increased after the discovery that its expression is perturbed in tumor cells, cancer-associated fibroblasts, and the tumor microenvironment. This review summarizes current knowledge regarding the established involvement of the laminin 332 γ2 chain in tumor invasiveness and discusses the role of its α3 and β3 subunits.

RAC1B: A Rho GTPase with Versatile Functions in Malignant Transformation and Tumor Progression

RAC1B is an alternatively spliced isoform of the monomeric GTPase RAC1. It differs from RAC1 by a 19 amino acid in frame insertion, termed exon 3b, resulting in an accelerated GDP/GTP-exchange and an impaired GTP-hydrolysis. Although RAC1B has been ascribed several protumorigenic functions such as cell cycle progression and apoptosis resistance, its role in malignant transformation, and other functions driving tumor progression like epithelial-mesenchymal transition, migration/invasion and metastasis are less clear. Insertion of exon 3b endows RAC1B with specific biochemical properties that, when compared to RAC1, encompass both loss-of-functions and gain-of-functions with respect to the type of upstream activators, downstream targets, and binding partners. In its extreme, this may result in RAC1B and RAC1 acting in an antagonistic fashion in regulating a specific cellular response with RAC1B behaving as an endogenous inhibitor of RAC1. In this review, we strive to provide the reader with a comprehensive overview, rather than critical discussions, on various aspects of RAC1B biology in eukaryotic cells.

A Review of Integrin-Mediated Endothelial Cell Phenotype in the Design of Cardiovascular Devices

Sustained biomaterial thromboresistance has long been a goal and challenge in blood-contacting device design. Endothelialization is one of the most successful strategies to achieve long-term thromboresistance of blood-contacting devices, with the endothelial cell layer providing dynamic hemostatic regulation. It is well established that endothelial cell behavior is influenced by interactions with the underlying extracellular matrix (ECM). Numerous researchers have sought to exploit these interactions to generate improved blood-contacting devices by investigating the expression of hemostatic regulators in endothelial cells on various ECM coatings. The ability to select substrates that promote endothelial cell-mediated thromboresistance is crucial to advancing material design strategies to improve cardiovascular device outcomes. This review provides an overview of endothelial cell regulation of hemostasis, the major components found within the cardiovascular basal lamina, and the interactions of endothelial cells with prominent ECM components of the basement membrane. A summary of ECM-mimetic strategies used in cardiovascular devices is provided with a focus on the effects of key adhesion modalities on endothelial cell regulators of hemostasis.

Human salivary peptide histatin-1 stimulates epithelial and endothelial cell adhesion and barrier function

Histatins are multifunctional histidine-rich peptides secreted by the salivary glands and exclusively present in the saliva of higher primates, where they play a fundamental role in the protection of the oral cavity. Our previously published results demonstrated that histatin-1 (Hst1) promotes cell-substrate adhesion in various cell types and hinted that it could also be involved in cell-cell adhesion, a process of fundamental importance to epithelial and endothelial barriers. Here we explore the effects of Hst1 on cellular barrier function. We show that Hst1 improved endothelial barrier integrity, decreased its permeability for large molecules, and prevented translocation of bacteria across epithelial cell layers. These effects are mediated by the adherens junction protein E-cadherin (E-cad) and by the tight junction protein zonula occludens 1, as Hst1 increases the levels of zonula occludens 1 and of active E-cad. Hst1 may also promote epithelial differentiation as Hst1 induced transcription of the epithelial cell differentiation marker apolipoprotein A-IV (a downstream E-cad target). In addition, Hst1 counteracted the effects of epithelial-mesenchymal transition inducers on the outgrowth of oral cancer cell spheroids, suggesting that Hst1 affects processes that are implicated in cancer progression.-Van Dijk, I. A., Ferrando, M. L., van der Wijk, A.-E., Hoebe, R. A., Nazmi, K., de Jonge, W. J., Krawczyk, P. M., Bolscher, J. G. M., Veerman, E. C. I., Stap, J. Human salivary peptide histatin-1 stimulates epithelial and endothelial cell adhesion and barrier function.

A crosstalk between muscarinic and CRF2 receptors regulates cellular adhesion properties of human colon cancer cells

Patients with inflammatory bowel disease often suffer from chronic and relapsing intestinal inflammation that favor the development of colitis associated cancer. An alteration of the epithelial intestinal barrier function observed in IBD is supposed to be a consequence of stress. It has been proposed that corticotrophin-releasing factor receptor (CRF2), one of the two receptors of CRF, the principal neuromediator of stress, acts on cholinergic nerves to induce stress-mediated epithelial barrier dysfunction. Non-neuronal acetylcholine (Ach) and muscarinic receptors (mAchR) also contribute to alterations of epithelial cell functions. In this study, we investigated the mechanisms through which stress and Ach modulate epithelial cell adhesive properties. We show that Ach-induced activation of mAchR in HT-29 cells results in cell dissociation together with changes in cell-matrix contacts, which correlates with the acquisition of invasive potential consistent with a matrix metalloproteinase (MMP) mode of invasion. These processes result from mAchR subsequent stimulation of the cascade of src/Erk and FAK activation. Ach-induced secretion of laminin 332 leads to α3β1 integrin activation and RhoA-dependent reorganization of the actin cytoskeleton. We show that Ach-mediated effects on cell adhesion are blocked by astressin 2b, a CRF2 antagonist, suggesting that Ach action depends partly on CRF2 signaling. This is reinforced by the fact that Ach-mediated activation of mAchR stimulates both the synthesis and the release of CRF2 ligands in HT-29 cells (effects blocked by atropine). In summary, our data provides evidence for a novel intracellular circuit involving mAchR acting on CRF2-signaling that could mediate colonic mucosal barrier dysfunction and exacerbate mucosal inflammation.

Mechanosensing by the α6-integrin confers an invasive fibroblast phenotype and mediates lung fibrosis

Matrix stiffening is a prominent feature of pulmonary fibrosis. In this study, we demonstrate that matrix stiffness regulates the ability of fibrotic lung myofibroblasts to invade the basement membrane (BM). We identify α₆-integrin as a mechanosensing integrin subunit that mediates matrix stiffness-regulated myofibroblast invasion. Increasing α₆-expression, specifically the B isoform (α₆B), couples β₁-integrin to mediate MMP-2-dependent pericellular proteolysis of BM collagen IV, leading to myofibroblast invasion. Human idiopathic pulmonary fibrosis lung myofibroblasts express high levels of α₆-integrin in vitro and in vivo. Genetic ablation of α₆ in collagen-expressing mesenchymal cells or pharmacological blockade of matrix stiffness-regulated α₆-expression protects mice against bleomycin injury-induced experimental lung fibrosis. These findings suggest that α₆-integrin is a matrix stiffness-regulated mechanosensitive molecule which confers an invasive fibroblast phenotype and mediates experimental lung fibrosis. Targeting this mechanosensing α₆(β₁)-integrin offers a novel anti-fibrotic strategy against lung fibrosis.

Matrix stiffening is a feature of pulmonary fibrosis, and is amplified by lung myofibroblasts. Here the authors find that a6 integrin expression is upregulated on lung myofibroblasts in response to matrix stiffness, and this integrin is required for myofibroblast invasion, and fibrosis in an experimental disease model.

α3 Integrin of Cell-Cell Contact Mediates Kidney Fibrosis by Integrin-Linked Kinase in Proximal Tubular E-Cadherin Deficient Mice

Loss of E-cadherin marks a defect in epithelial integrity and polarity during tissue injury and fibrosis. Whether loss of E-cadherin plays a causal role in fibrosis is uncertain. α3β1 Integrin has been identified to complex with E-cadherin in cell-cell adhesion, but little is known about the details of their cross talk. Herein, E-cadherin gene (Cdh1) was selectively deleted from proximal tubules of murine kidney by Sglt2Cre. Ablation of E-cadherin up-regulated α3β1 integrin at cell-cell adhesion. E-cadherin-deficient proximal tubular epithelial cell displayed enhanced transforming growth factor-β1-induced α-smooth muscle actin (α-SMA) and vimentin expression, which was suppressed by siRNA silencing of α3 integrin, but not β1 integrin. Up-regulation of transforming growth factor-β1-induced α-SMA was mediated by an α3 integrin-dependent increase in integrin-linked kinase (ILK). Src phosphorylation of β-catenin and consequent p-β-catenin-Y654/p-Smad2 transcriptional complex underlies the transcriptional up-regulation of ILK. Kidney fibrosis after unilateral ureteric obstruction or ischemia reperfusion was increased in proximal tubule E-cadherin-deficient mice in comparison to that of E-cadherin intact control mice. The exacerbation of fibrosis was explained by the α3 integrin-dependent increase of ILK, β-catenin nuclear translocation, and α-SMA/proximal tubular-specific Cre double positive staining in proximal tubular epithelial cell. These studies delineate a nonconventional integrin/ILK signaling by α3 integrin-dependent Src/p-β-catenin-Y654/p-Smad2-mediated up-regulation of ILK through which loss of E-cadherin leads to kidney fibrosis.

The FHL2 regulation in the transcriptional circuitry of human cancers

The Four-and-a-half LIM (FHL)-only protein is a subfamily of protein members under the LIM-only protein family. These proteins are identified by their characteristic four and a half cysteinerich LIM homeodomain. Five members have been categorized into the FHL subfamily, which are FHL1, FHL2, FHL3, FHL4 and activator of CREM in testis (ACT) in human. FHL2 is amongst the most examined members within the family. Fhl2, the gene that code for the protein, is transcriptionally regulated by diverse types of transcription factors, for example, p53, serum response factor (SRF), and specificity protein 1 (Sp1). The expression of FHL2 is found in different tissues and organs and has been reported as a critical participant influencing the wide types of cancer such as breast cancer, gastrointestinal (GI) cancers, liver cancer and prostate cancer. The expression profile of FHL2 appeared to have a significant functional role in the carcinogenesis of these cancers which are mediated by different types of transcription factor including both tumor suppressors and inducers. In this review, we will first describe the molecular network governing FHL2 expression, which focus on the transcription factors conveying FHL2-initiated responses. In the second part, FHL2-linked cancers and the underlying molecular machinery will be discussed. Factors other than transcriptional regulation which may involve the cancer progression such as mutations of fhl2 and posttranslational modifications of the protein will also be mentioned.

Nicotine-induced cellular stresses and autophagy in human cancer colon cells: A supportive effect on cell homeostasis via up-regulation of Cox-2 and PGE(2) production

Nicotine, one of the active components in cigarette smoke, has been described to contribute to the protective effect of smoking in ulcerative colitis (UC) patients. Furthermore, the nicotinic acetylcholine receptor α7 subunit (α7nAChR) expressed on immune cells, is an essential regulator of inflammation. As intestinal epithelial cells also express α7nAChR, we investigated how nicotine could participate in the homeostasis of intestinal epithelial cells. First, using the human adenocarcinoma cell line HT-29, we revealed that nicotine, which triggers an influx of extracellular Ca(2+) following α7nAChR stimulation, induces mitochondrial reactive oxygen species (ROS) production associated with a disruption of the mitochondrial membrane potential and endoplasmic reticulum stress. This results in caspase-3 activation, which in turn induces apoptosis. Additionally, we have shown that nicotine induces a PI3-K dependent up-regulation of cyclooxygenase-2 (Cox-2) expression and prostaglandin E2 (PGE2) production. In this context, we suggest that this key mediator participates in the cytoprotective effects of nicotine against apoptosis by stimulating autophagy in colon cancer cells. Our results provide new insight into one potential mechanism by which nicotine could protect from UC and suggest an anti-inflammatory role for the cholinergic pathway at the epithelial cell level.

P-cadherin signals through the laminin receptor α6β4 integrin to induce stem cell and invasive properties in basal-like breast cancer cells

P-cadherin is a classical cell-cell adhesion molecule that, in contrast to E-cadherin, has a positive role in breast cancer progression, being considered a poor prognostic factor in this disease. In previous reports, we have shown that this protein induces cancer stem cell and invasive properties to basal-like breast cancer cells. Here, we clarify the downstream signaling pathways that are triggered by P-cadherin to mediate these effects. We demonstrated that P-cadherin inhibition led to a significant decreased adhesion of cancer cells to the basement membrane substrate laminin, as well as to a major reduction in the expression of the laminin receptor α6β4 integrin. Remarkably, the expression of this heterodimer was required for the invasive capacity and increased mammosphere forming efficiency induced by P-cadherin expression. Moreover, we showed that P-cadherin transcriptionally up-regulates the α6 integrin subunit expression and directly interacts with the β4 integrin subunit. We still showed that P-cadherin downstream signaling, in response to laminin, involves the activation of focal adhesion (FAK), Src and AKT kinases. The association between the expression of P-cadherin, α6β4 heterodimer and the active FAK and Src phosphorylated forms was validated in vivo. Our data establish that there is a crosstalk between P-cadherin and the laminin receptor α6β4 integrin signaling pathway, which link has never been previously described. The activation of this heterodimer explains the stem cell and invasive properties induced by P-cadherin to breast cancer cells, pointing to a new molecular mechanism that may be targeted to counteract the effects induced by this adhesion molecule.

Signaling pathways in anchoring junctions of epithelial cells: cell-to-cell and cell-to-extracellular matrix interactions

Epithelial cells form the epithelium, one of the basic tissues of the human body. These cells present specializations from tissue to tissue, determining different structures and functions. Tissues formed by epithelial cells are characterized by the few extracellular matrix found between adjacent cells. In this way, to preserve tissue integrity, cells have to stick to each other and have to maintain a strict communication with the environment via cell junctions. Signal transduction is the main way of cell communication, being vital for the regulation of cell survival and proliferation. In cell junctions, this communication occurs through cell adhesion molecules that promote cell-to-cell and cell-to-extracellular matrix adhesion, as well as, enable the flow of information to the inside and to the outside of the cell. These molecules include integrins and cadherins, among others. The impairment of cell signaling in epithelial junctions has been involved in several pathological processes that underlie the development of, for example, colorectal cancer. Thus, epithelial cell signaling mediators have been explored as potential therapeutic targets and efforts have been made to achieve a deeper understanding of molecular events that occur at cell junctions. In this review, we address the current knowledge on the main signaling events that take place in anchoring junctions of epithelial cells, focusing both on cell-to-cell and cell-to-matrix interactions. To conclude, we explore some relevant consequences from epithelial cell signaling impairment and demonstrate that the molecular mediators of the pathways analyzed may be putative therapeutic targets.

CRF2 signaling is a novel regulator of cellular adhesion and migration in colorectal cancer cells

Stress has been proposed to be a tumor promoting factor through the secretion of specific neuromediators, such as Urocortin2 and 3 (Ucn2/3), however its role in colorectal cancer (CRC) remains elusive. We observed that Ucn2/3 and their receptor the Corticotropin Releasing Factor receptor 2 (CRF2) were up-regulated in high grade and poorly differentiated CRC. This suggests a role for CRF2 in the loss of cellular organization and tumor progression. Using HT-29 and SW620 cells, two CRC cell lines differing in their abilities to perform cell-cell contacts, we found that CRF2 signals through Src/ERK pathway to induce the alteration of cell-cell junctions and the shuttle of p120ctn and Kaiso in the nucleus. In HT-29 cells, this signaling pathway also leads to the remodeling of cell adhesion by i) the phosphorylation of Focal Adhesion Kinase and ii) a modification of actin cytoskeleton and focal adhesion complexes. These events stimulate cell migration and invasion. In conclusion, our findings indicate that CRF2 signaling controls cellular organization and may promote metastatic potential of human CRC cells through an epithelial-mesenchymal transition like process. This contributes to the comprehension of the tumor-promoting effects of stress molecules and designates Ucn2/3-CRF2 tandem as a target to prevent CRC progression and aggressiveness.

Integrins and cadherins join forces to form adhesive networks

Cell-cell and cell-extracellular-matrix (cell-ECM) adhesions have much in common, including shared cytoskeletal linkages, signaling molecules and adaptor proteins that serve to regulate multiple cellular functions. The term 'adhesive crosstalk' is widely used to indicate the presumed functional communication between distinct adhesive specializations in the cell. However, this distinction is largely a simplification on the basis of the non-overlapping subcellular distribution of molecules that are involved in adhesion and adhesion-dependent signaling at points of cell-cell and cell-substrate contact. The purpose of this Commentary is to highlight data that demonstrate the coordination and interdependence of cadherin and integrin adhesions. We describe the convergence of adhesive inputs on cell signaling pathways and cytoskeletal assemblies involved in regulating cell polarity, migration, proliferation and survival, differentiation and morphogenesis. Cell-cell and cell-ECM adhesions represent highly integrated networks of protein interactions that are crucial for tissue homeostasis and the responses of individual cells to their adhesive environments. We argue that the machinery of adhesion in multicellular tissues comprises an interdependent network of cell-cell and cell-ECM interactions and signaling responses, and not merely crosstalk between spatially and functionally distinct adhesive specializations within cells.

Enterocytic differentiation is modulated by lipid rafts-dependent assembly of adherens junctions

Integrity of the epithelial barrier is determined by apical junctional complexes which also participate in the signalling pathways inducing intestinal cell differentiation. Lipid rafts (LR) have been proposed to play a role in the organization and the function of these intercellular complexes. This study investigated potential mechanisms by which LR could participate in the establishment of adherens junctions (AJ) and the initiation of enterocytic differentiation. We showed that the differentiation of epithelial cells in rat colons correlates with the emergence of LR. Using HT-29 cells we demonstrated that during the differentiation process, LR are required for the recruitment and the association of p120ctn to E-cadherin. Silencing of flotillin-1, a LR component, alters the recruitment of AJ proteins in LR and delays the expression of differentiation markers. Furthermore, the ability of p120ctn/E-cadherin complexes to support cell differentiation is altered in HT-29 Rac1N17 cells. These results show a contributory role of LR in the enterocytic differentiation process, which serve as signalling platforms for Rac1-mediated organization of AJ. A better understanding of the mechanism involved in the establishment of junctional complex and their role in enterocytic differentiation provides new insights into the regulation of intestinal homeostasis.

Basement membrane diseases in zebrafish

Basement membranes (BMs) are a complex, sheet-like network of specialized extracellular matrix that underlies epithelial cells and surrounds muscle cells. They provide adherence between neighboring tissues, permit some flexibility of these adherent structures, and can act as a store for growth factors and as a guide for cell migration. The BM is not just a static structure; its deposition and remodeling are important for many processes including embryonic development, immune response, and wound healing. To date, dysfunction in BM deposition or remodeling has been linked to many human congenital disorders and diseases, affecting many different tissues in the body, including malformations, dystrophies, and cancer. However, many questions remain to be answered on the role BM proteins, and their mutations, play in the pathogenesis of human disease. In recent years, the zebrafish (Danio rerio) has emerged as a powerful animal model for human development and disease. In the first part of this chapter, we provide an overview of described defects caused by BM dysfunction in zebrafish, including development and function of notochord, muscle, central nervous system, skin, cardiovascular system, and kidney. In the second part, we will describe details of methods used to visualize and assess the structure of the BM in zebrafish, and to functionally analyze its different components.

Genetic analysis of fin development in zebrafish identifies furin and hemicentin1 as potential novel fraser syndrome disease genes

Using forward genetics, we have identified the genes mutated in two classes of zebrafish fin mutants. The mutants of the first class are characterized by defects in embryonic fin morphogenesis, which are due to mutations in a Laminin subunit or an Integrin alpha receptor, respectively. The mutants of the second class display characteristic blistering underneath the basement membrane of the fin epidermis. Three of them are due to mutations in zebrafish orthologues of FRAS1, FREM1, or FREM2, large basement membrane protein encoding genes that are mutated in mouse bleb mutants and in human patients suffering from Fraser Syndrome, a rare congenital condition characterized by syndactyly and cryptophthalmos. Fin blistering in a fourth group of zebrafish mutants is caused by mutations in Hemicentin1 (Hmcn1), another large extracellular matrix protein the function of which in vertebrates was hitherto unknown. Our mutant and dose-dependent interaction data suggest a potential involvement of Hmcn1 in Fraser complex-dependent basement membrane anchorage. Furthermore, we present biochemical and genetic data suggesting a role for the proprotein convertase FurinA in zebrafish fin development and cell surface shedding of Fras1 and Frem2, thereby allowing proper localization of the proteins within the basement membrane of forming fins. Finally, we identify the extracellular matrix protein Fibrillin2 as an indispensable interaction partner of Hmcn1. Thus we have defined a series of zebrafish mutants modelling Fraser Syndrome and have identified several implicated novel genes that might help to further elucidate the mechanisms of basement membrane anchorage and of the disease's aetiology. In addition, the novel genes might prove helpful to unravel the molecular nature of thus far unresolved cases of the human disease.

There are a large number of human genetic syndromes with limb and digit deformities. It has been shown that the genes underlying these syndromes are well conserved in evolution, and most perform the same role even in the fins of fish. One such human syndrome is Fraser Syndrome, characterized by a number of defects including fusion of the fingers (syndactyly). Data obtained with corresponding mouse mutants suggest that all of these defects are due to transient basement membrane disruptions and epithelial blistering during development. Whilst some of the Fraser Syndrome genes have been identified, others are unknown. We show that mutation of the known Fraser Syndrome genes in zebrafish generate comparable blistering defects in the fins. Importantly, we have also identified additional genes and mechanisms required for the same processes. Included in this are hemicentin1, a gene whose function had thus far only been studied in nematodes, and furinA, encoding a proprotein convertase, for which we reveal a novel role in ectodomain shedding of Fras/Frem proteins. This work thus expands our understanding, not only of Fraser Syndrome, but also of the common processes of basement membrane formation and function during fin and limb development.

Cyclic mechanical stretch decreases cell migration by inhibiting phosphatidylinositol 3-kinase- and focal adhesion kinase-mediated JNK1 activation

Epithelial cell migration during wound healing requires coordinated signaling pathways that direct polarization of the leading and trailing ends of the cells, cytoskeletal organization, and remodeling of focal adhesions. These inherently mechanical processes are disrupted by cyclic stretch (CS), but the specific signaling molecules involved in this disruption are not well understood. In this study, we demonstrate that inhibition of phosphatidylinositol 3-kinase (PI3K) or expression of a dominant-negative form of PI3K caused inhibition of airway epithelial cell wound closure. CS caused a sustained decrease in activation of PI3K and inhibited wound healing. Expression of constitutively active PI3K stimulated translocation of Tiam1 to the membrane, increased Rac1 activity, and increased wound healing of airway epithelial cells. Increased Rac1 activity resulted in increased phosphorylation of JNK1. PI3K activation was not regulated by association with focal adhesion kinase. Restoration of efficient cell migration during CS required coexpression of constitutively active PI3K, focal adhesion kinase, and JIP3.

Integrins stimulate E-cadherin-mediated intercellular adhesion by regulating Src-kinase activation and actomyosin contractility

Cadherins and integrins are major adhesion molecules regulating cell-cell and cell-matrix interactions. In vitro and in vivo studies have demonstrated the existence of crosstalk between integrins and cadherins in cell adhesion and motility. We used a dual pipette assay to measure the force required to separate E-cadherin-producing cell doublets and to investigate the role of integrin in regulating the strength of intercellular adhesion. A greater force was required to separate cell doublets bound to fibronectin or vitronectin-coated beads than for doublets bound to polylysine-coated beads. This effect depended on cell spreading and the duration of stimulation. Cells expressing type II cadherin-7 also responded to fibronectin stimulation to produce a higher intercellular adhesion. Establishment of cadherin-mediated adhesion needed ROCK, MLCK and myosin ATPase II activity. The regulation of intercellular adhesion strength by integrin stimulation required activation of Src family kinases, ROCK and actomyosin contractility. These findings highlight the importance and mechanisms of molecular crosstalk between cadherins and integrins in the control of cell plasticity during histogenesis and morphogenesis.

Laminin-binding integrins and their tetraspanin partners as potential antimetastatic targets

Within the integrin family of cell adhesion receptors, integrins alpha3beta1, alpha6beta1, alpha6beta4 and alpha7beta1 make up a laminin-binding subfamily. The literature is divided on the role of these laminin-binding integrins in metastasis, with different studies indicating either pro- or antimetastatic functions. The opposing roles of the laminin-binding integrins in different settings might derive in part from their unusually robust associations with tetraspanin proteins. Tetraspanins organise integrins into multiprotein complexes within discrete plasma membrane domains termed tetraspanin-enriched microdomains (TEMs). TEM association is crucial to the strikingly rapid cell migration mediated by some of the laminin-binding integrins. However, emerging data suggest that laminin-binding integrins also promote the stability of E-cadherin-based cell-cell junctions, and that tetraspanins are essential for this function as well. Thus, TEM association endows the laminin-binding integrins with both pro-invasive functions (rapid migration) and anti-invasive functions (stable cell junctions), and the composition of TEMs in different cell types might help determine the balance between these opposing activities. Unravelling the tetraspanin control mechanisms that regulate laminin-binding integrins will help to define the settings where inhibiting the function of these integrins would be helpful rather than harmful, and may create opportunities to modulate integrin activity in more sophisticated ways than simple functional blockade.

The role of the basement membrane as a modulator of intestinal epithelial-mesenchymal interactions

Intestinal development is a process of continuous dynamic bidirectional crosstalk between epithelial and underlying mesenchymal cells. This crosstalk is mediated by well-dissected signaling pathways. Another crucial actor in the epithelio-mesenchymal interactions is the stromal microenvironment, which is composed of extracellular matrix molecules. Among them, the basement membrane (BM) molecules are secreted by the epithelium and mesenchyme in a complementary manner. These molecules signal back to the cells via the integrins or other specific receptors. In this review, we mainly focus on the BM molecules, particularly laminins. The major BM molecules are organized in a complex molecular network, which is highly variable among organs. Cell culture, coculture, and grafting models have been of great interest in understanding the importance of these molecules. Mouse gene ablation of laminin chains are interesting models, which often lead to embryonic death and are frequently accompanied by compensatory processes. Overall, the BM molecules have a crucial role in the careful maintenance of intestinal homeostasis.

Biological function of laminin-5 and pathogenic impact of its deficiency

The basement membrane glycoprotein laminin-5 is a key component of the anchoring complex connecting keratinocytes to the underlying dermis. It is secreted by keratinocytes as a cross-shaped heterotrimer of alpha3, beta3 and gamma2 chains and serves as a ligand of various transmembrane receptors, thereby regulating keratinocyte adhesion, motility and proliferation. In intact skin, laminin-5 provides essential links to both the hemidesmosomal alpha6beta4 integrin and the collagen type VII molecules which form the anchoring fibrils inserting into the dermis. If the basement membrane is injured, laminin-5 production increases rapidly. It then serves as a scaffold for cell migration, initiates the formation of hemidesmosomes and accelerates basement membrane restoration at the dermal-epidermal junction. Mutations of the laminin-5 genes or auto-antibodies against one of the subunits of laminin-5 may lead to a significant lack of this molecule in the epidermal basement membrane zone. The major contributions of laminin-5 to the resistance of the epidermis against frictional stress but also for basement membrane regeneration and repair of damaged skin are reflected by the phenotype of Herlitz junctional epidermolysis bullosa, which is caused by an inherited absence of functional laminin-5. This lethal disease becomes manifest in widespread blistering of skin and mucous membranes, impaired wound healing and chronic erosions containing exuberant granulation tissue. Here, we discuss current understanding of the biological functions of laminin-5, the pathogenic impact of its deficiency and implications on molecular approaches towards a therapy of junctional epidermolysis bullosa.

Cyclin-dependent kinase 2/cyclin E complex is involved in p120 catenin (p120ctn)-dependent cell growth control: a new role for p120ctn in cancer

Depending on its cellular localization, p120 catenin (p120ctn) can participate in various processes, such as cadherin-dependent cell-cell adhesion, actin cytoskeleton remodeling, and intracellular trafficking. Recent studies also indicate that p120ctn could regulate cell proliferation and contact inhibition. This report describes a new function of p120ctn in the regulation of cell cycle progression. Overexpression of the p120ctn isoform 3A in human colon adenocarcinoma cells (HT-29) results in cytoplasmic accumulation of the protein, as observed in many tumors. This cytoplasmic increase is correlated with a reduction in proliferation and inhibition of DNA synthesis. Under these conditions, experiments on synchronized cells revealed a prolonged S phase associated with cyclin E stabilization. Both confocal microscopy and biochemical analysis showed that cyclin E and cyclin-dependent kinase 2 colocalized with p120ctn in centrosomes during mitosis. These proteins are associated in a functional complex evidenced by coimmunoprecipitation experiments and the emergence of Thr199-phosphorylated nucleophosmin/B23. Such post-translational modification of this centrosomal target has been shown to trigger the initiation of centrosome duplication. Therefore, p120ctn-mediated accumulation of cyclin E in centrosomes may participate in abnormal amplification of centrosomes and the inhibition of DNA replication, thus leading to aberrant mitosis and polyploidy. Because these modifications are often observed in cancer, p120ctn may represent a new therapeutic target for future therapy.

Laminin alpha5 is essential for the formation of the zebrafish fins

The vertebrate fin fold, the presumptive evolutionary antecedent of the paired fins, consists of two layers of epidermal cells extending dorsally and ventrally over the trunk and tail of the embryo, facilitating swimming during the embryonic and larval stages. Development of the fin fold requires dramatic changes in cell shape and adhesion during early development, but the proteins involved in this process are completely unknown. In a screen of mutants defective in fin fold morphogenesis, we identified a mutant with a severe fin fold defect, which also displays malformed pectoral fins. We find that the cause of the defect is a non-sense mutation in the zebrafish lama5 gene that truncates laminin alpha5 before the C-terminal laminin LG domains, thereby preventing laminin alpha5 from interacting with its cell surface receptors. Laminin is mislocalized in this mutant, as are the membrane-associated proteins, actin and beta-catenin, that normally form foci within the fin fold. Ultrastructural analysis revealed severe morphological abnormalities and defects in cell-cell adhesion within the epidermis of the developing fin fold at 36 hpf, resulting in an epidermal sheet that can not extend away from the body. Examining the pectoral fins, we find that the lama5 mutant is the first zebrafish mutant identified in which the pectoral fins fail to make the transition from an apical epidermal ridge to an apical fold, a transformation that is essential for pectoral fin morphogenesis. We propose that laminin alpha5, which is concentrated at the distal ends of the fins, organizes the distal cells of the fin fold and pectoral fins in order to promote the morphogenesis of the epidermis. The lama5 mutant provides novel insight into the role of laminins in the zebrafish epidermis, and the molecular mechanisms driving fin formation in vertebrates.

Evolution of the Rho family of ras-like GTPases in eukaryotes

GTPases of the Rho family are molecular switches that play important roles in converting and amplifying external signals into cellular effects. Originally demonstrated to control the dynamics of the F-actin cytoskeleton, Rho GTPases have been implicated in many basic cellular processes that influence cell proliferation, differentiation, motility, adhesion, survival, or secretion. To elucidate the evolutionary history of the Rho family, we have analyzed over 20 species covering major eukaryotic clades from unicellular organisms to mammals, including platypus and opossum, and have reconstructed the ontogeny and the chronology of emergence of the different subfamilies. Our data establish that the 20 mammalian Rho members are structured into 8 subfamilies, among which Rac is the founder of the whole family. Rho, Cdc42, RhoUV, and RhoBTB subfamilies appeared before Coelomates and RhoJQ, Cdc42 isoforms, RhoDF, and Rnd emerged in chordates. In vertebrates, gene duplications and retrotranspositions increased the size of each chordate Rho subfamily, whereas RhoH, the last subfamily, arose probably by horizontal gene transfer. Rac1b, a Rac1 isoform generated by alternative splicing, emerged in amniotes, and RhoD, only in therians. Analysis of Rho mRNA expression patterns in mouse tissues shows that recent subfamilies have tissue-specific and low-level expression that supports their implication only in narrow time windows or in differentiated metabolic functions. These findings give a comprehensive view of the evolutionary canvas of the Rho family and provide guides for future structure and evolution studies of other components of Rho signaling pathways, in particular regulators of the RhoGEF family.

Crosstalk between different adhesion molecules

Cell adhesion molecules mediate cell-cell and cell-extracellular matrix adhesions, and coordination between these molecules is essential for tissue formation and morphogenesis. Crosstalk between integrins and cadherins may result from a physical response to integrin-mediated adhesion, complex cell differentiation processes, or direct signaling pathways linking the two adhesion systems. Nectins have recently been shown to regulate the organization of cadherins into adherens junctions and the formation of tight junctions by several processes. Furthermore, protocadherins can interact with extracellular matrix proteins or function by regulating classical cadherins.