The cytoplasmic domain of adherens-type junctions

Alterations in the distribution of actin and its binding proteins in the porcine endometrium during early pregnancy: Possible role in epithelial remodeling and embryo adhesion

During early pregnancy, uterine epithelial cells undergo major transformations in their cytoskeleton that make the endometrium receptive for conceptus attachment. Actin binding proteins (ABPs) such as cofilin, gelsolin, and vinculin are involved in regulating actin polymerization, severing or crosslinking actin to integrins. However, whether ABPs are involved in epithelial remodeling or embryo adhesion in pigs is unknown. Therefore, the expression and distribution of these proteins were investigated in porcine endometrium on Days 10 and 13 (pre-implantation period), and 16 (attachment phase) of the estrous cycle or pregnancy. While day and pregnancy status had no effect on ABP gene expression, the protein abundance of vinculin was significantly higher on Day 13 than on Day 10 (p < 0.05) of the estrous cycle, and its abundance was highest on Day 16 in the pregnant endometrium. Immunofluorescent staining showed alterations in the distribution of these proteins depending on the day of the estrous cycle or early pregnancy examined. Double immunofluorescent staining for the ABPs and actin revealed that while cofilin co-localized with actin in the apical epithelium on Days 13 and 16 of the estrous cycle, in pregnant animals, it was strongly associated with actin in the sub-epithelial stroma of the endometrium. Gelsolin was also co-localized with actin in the apical epithelium on Days 13 and 16 of the estrous cycle, but this association was absent in the pregnant endometrium. Vinculin co-localized with actin in the sub-epithelial stroma on Days 13 and 16 irrespective of the reproductive status, but was additionally associated with actin in the apical epithelium on Day 16 of pregnancy. Vinculin interacted with phosphorylated focal adhesion kinase in the endometrial epithelium, and the interaction was dependent on estradiol-17β, a conceptus-secreted pregnancy-recognition factor in pigs. Furthermore, silencing vinculin in the endometrial epithelial cells negatively affected trophoblast adhesion to them. In conclusion, the influence of stage and reproductive status on the specific localization of actin and its binding proteins in the porcine endometrium suggests that they play a role in regulating the endometrial cytoskeleton. Moreover, vinculin may facilitate conceptus attachment to the epithelium by interacting with focal adhesion kinase.

Identification of genes associated with primary open-angle glaucoma by bioinformatics approach

PURPOSE

This study aimed to identify associated genes with primary open-angle glaucoma (POAG) and explore the potentially modular mechanism underlying POAG.

METHODS

We downloaded gene expression profiles data GSE27276 from gene expression omnibus and identified differentially expressed genes between POAG patients and normal controls. Then, gene ontology analysis and kyoto encyclopedia of genes and genomes pathway enrichment were performed to predict the DEGs functions, followed with the construction, centrality analysis, and module mining of protein-protein interaction network.

RESULTS

A total of 552 DEGs including 249 up-regulated and 303 down-regulated genes were identified. The up-regulated DEGs were significantly involved in cell adhesion molecule, while the down-regulated DEGs were significantly involved in complement and coagulation cascades. Centrality analysis screened out 20 genes, among which COL4A4, COL3A1, COL1A2, ITGB5, COL5A2, and COL5A1 were shared in ECM-receptor interaction and focal adhesion pathways. In the sub-network, COL5A2, COL8A2, and COL5A1 were significantly enriched in biological function of eye morphogenesis and eye development, while LAMA5, COL3A1, COL1A2, and COL5A1 were significantly enriched in vasculature development and blood vessel development.

CONCLUSIONS

Six genes, including COL4A4, COL3A1, COL1A2, ITGB5, COL5A2, and COL5A1, ECM-receptor interaction and focal adhesion pathway, are potentially involved in the pathogenesis of POAG via participating in pathways of ECM-receptor interaction and focal adhesion.

Actin filament association at adherens junctions

The adherens junction (AJ) is a cadherin-based and actin filament associated cell-to-cell junction. AJs can contribute to tissue morphogenesis and homeostasis and their association with actin filaments is crucial for the functions. There are three types of AJs in terms of the mode of actin filament/AJ association. Among many actin-binding proteins associated with AJs, α-catenin is one of the most important actin filament/AJ linkers that functions in all types of AJs. Although α-catenin in cadherin-catenin complex appears to bind to actin filaments within cells, it fails to bind to actin filaments in vitro mysteriously. Recent report revealed that α-catenin in the complex can bind to actin filaments in vitro when forces are applied to the filament. In addition to force-sensitive vinculin binding, α-catenin has another force-sensitive property of actin filament-binding. Elucidation of its significance and the molecular mechanism is indispensable for understanding AJ formation and maintenance during tissue morphogenesis, function and repair. J. Med. Invest. 64: 14-19, February, 2017.

Fluctuation of zonulin levels in blood vs stability of antibodies

AIM

To evaluate the measurement of zonulin level and antibodies of zonulin and other tight junction proteins in the blood of controls and celiac disease patients.

METHODS

This study was conducted to assess the variability or stability of zonulin levels vs IgA and IgG antibodies against zonulin in blood samples from 18 controls at 0, 6, 24 and 30 h after blood draw. We also measured zonulin level as well as zonulin, occludin, vinculin, aquaporin 4 and glial fibrillary acidic protein antibodies in the sera of 30 patients with celiac disease and 30 controls using enzyme-linked immunosorbent assay methodology.

RESULTS

The serum zonulin level in 6 out of 18 subjects was low or < 2.8 ng/mL and was very close to the detection limit of the assay. The other 12 subjects had zonulin levels of > 2.8 ng/mL and showed significant fluctuation from sample to sample. Comparatively, zonulin antibody measured in all samples was highly stable and reproducible from sample to sample. Celiac disease patients showed zonulin levels with a mean of 8.5 ng/mL compared to 3.7 ng/mL in controls (P < 0.0001). Elevation of zonulin level at 2SD above the mean was demonstrated in 37% of celiac disease patients, while antibodies against zonulin, occludin and other tight junction proteins was detected in up to 86% of patients with celiac disease.

CONCLUSION

Due to its fluctuation, a single measurement of zonulin level is not recommended for assessment of intestinal barrier integrity. Measurement of IgG and IgA antibodies against zonulin, occludin, and other tight junction proteins is proposed for the evaluation of the loss of intestinal barrier integrity.

Actin-Based Adhesion Modules Mediate Cell Interactions with the Extracellular Matrix and Neighboring Cells

Cell adhesions link cells to the extracellular matrix (ECM) and to each other and depend on interactions with the actin cytoskeleton. Both cell-ECM and cell-cell adhesion sites contain discrete, yet overlapping, functional modules. These modules establish physical associations with the actin cytoskeleton, locally modulate actin organization and dynamics, and trigger intracellular signaling pathways. Interplay between these modules generates distinct actin architectures that underlie different stages, types, and functions of cell-ECM and cell-cell adhesions. Actomyosin contractility is required to generate mature, stable adhesions, as well as to sense and translate the mechanical properties of the cellular environment into changes in cell organization and behavior. Here, we review the organization and function of different adhesion modules and how they interact with the actin cytoskeleton. We highlight the molecular mechanisms of mechanotransduction in adhesions and how adhesion molecules mediate cross talk between cell-ECM and cell-cell adhesion sites.

Identification of morphogenetic capability limitations via a single starfish embryo/larva reconstruction method

Reconstruction of a starfish embryo provides unique morphogenesis during the developmental process that is not observed in normal development. Here, we established a novel method for reconstruction from single embryos/larvae. By using this method, we investigated the morphogenetic capabilities in critical steps during the reconstruction process as showed by the reconstructed embryos generated from embryos/larvae at the six developmental stages, or from segregated ectodermal and/or endomesodermal cells. Additionally, the novel method addressed several problems found in prior methods related to reproducibly generating reconstructed embryos. In the reconstructions from the various stage embryos/larvae, the morphogenetic capabilities were substantively reduced in the reconstructed embryos generated from 3-day bipinnaria (3dBp). The combination experiments using ectodermal or endomesodermal cells segregated from 2dBp or 3dBp showed a reduction of the morphogenetic capabilities in both cells types in 3dBp. The reconstructed embryos generated from ectodermal or endomesodermal cells segregated from 2dBp possessed partial morphological features, such as formation of the epithelium or blastopore, but all failed to develop into bipinnariae. These results indicate two limitations of the morphogenetic capabilities during the reconstruction process. Firstly, the morphogenetic capabilities to reconstruct an embryo are considerably reduced between 2dBp and 3dBp. Secondly, cells specified as ectoderm or endomesoderm possess limited morphogenetic capabilities to reconstruct bipinnaria. Furthermore, our results demonstrate that the interaction between these specified cell types is required for reconstruction.

Characterization and comparative analyses of transcriptomes for in vivo and in vitro produced peri-implantation conceptuses and endometria from sheep

An increasing number of reports indicate that in vitro fertilization (IVF) is highly associated with long‑term side effects on embryonic and postnatal development, and can sometimes result in embryonic implant failure. While high‑throughput gene expression analysis has been used to explore the mechanisms underlying IVF-induced side effects on embryonic development, little is known about the effects of IVF on conceptus–endometrial interactions during the peri-implantation period. Using sheep as a model, we performed a comparative transcriptome analysis between in vivo (IVO; in vivo fertilized followed by further development in the uterus) and in vitro produced (IVP; IVF with further culture in the incubator) conceptuses, and the caruncular and intercaruncular areas of the ovine endometrium. We identified several genes that were differentially expressed between the IVO and IVP groups on day 17, when adhesion between the trophoblast and the uterine luminal epithelium begins in sheep. By performing Gene Ontology enrichment analysis and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis, we found that, in the conceptus, differentially expressed genes (DEGs) were associated mainly with functions relating to cell binding and the cell cycle. In the endometrial caruncular area, DEGs were involved in cell adhesion/migration and apoptosis, and in the intercaruncular area, they were significantly enriched in pathways of signal transduction and transport. Thus, these DEGs are potential candidates for further exploring the mechanism underlying IVF/IVP-induced embryonic implant failure that occurs due to a loss of interaction between the conceptus and endometrium during the peri-implantation period.

Vinculin, cell mechanics and tumour cell invasion

The focal adhesion protein, vinculin, is important for transmitting mechanical forces and orchestrating mechanical signalling events. Deregulation of vinculin results in altered cell adhesion, contractility, motility and growth, all of which are important processes in cancer metastasis. This review summarises recent reports on the role of vinculin in cellular force generation and signalling, and discusses implications for a role of vinculin in promoting cancer cell migration in 3D environments.

Cadherins in brain morphogenesis and wiring

Cadherins are Ca(2+)-dependent cell-cell adhesion molecules that play critical roles in animal morphogenesis. Various cadherin-related molecules have also been identified, which show diverse functions, not only for the regulation of cell adhesion but also for that of cell proliferation and planar cell polarity. During the past decade, understanding of the roles of these molecules in the nervous system has significantly progressed. They are important not only for the development of the nervous system but also for its functions and, in turn, for neural disorders. In this review, we discuss the roles of cadherins and related molecules in neural development and function in the vertebrate brain.

Cadherin-actin interactions at adherens junctions

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

Adiponectin attenuates endothelial dysfunction induced by oxidised low-density lipoproteins

We investigated whether the adipocytokine, adiponectin, protected the endothelium against damage induced by oxidised low-density lipoprotein cholesterol (oxLDL). Human umbilical vein endothelial cells were cultured with either 200 or 350 microg/ml oxLDL, with or without adiponectin purified from human serum (12 microg/ml). Cellular oxidative status was assessed by measuring reactive oxygen species (ROS), peroxynitrite and glutathione (GSH) levels, while cell function was evaluated by measuring nitric oxide (NO) levels and immunohistochemical examination of proteins in the adherens cell junction. At a concentration of 200 microg/ml, oxLDL induced a small increase in ROS and peroxynitrite levels, a two-fold increase in GSH levels and no changes in NO levels or localisation of proteins in the adherens junction. However, 350 microg/ml of oxLDL induced a marked increase in ROS and peroxynitrite levels, a four-fold reduction in GSH levels and a significant decrease in NO levels and disruption of the adherens junctions. Addition of adiponectin to the cultures resulted in maintenance of normal ROS, peroxynitrite and GSH levels, with no change in either NO levels or protein localisation in the adherens junction. This study demonstrates that adiponectin protects against endothelial dysfunction and cellular disruption induced by oxLDL, with this effect being due, in part, to maintenance of intracellular GSH levels.

Trypanosoma cruzi infection disrupts vinculin costameres in cardiomyocytes

Chagas' disease cardiomyopathy is an important manifestation of Trypanosoma cruzi infection, leading to cardiac dysfunction and serious arrhythmias. We have here investigated by indirect immunofluorescence assay the distribution of vinculin, a focal adhesion protein with a major role in the transmission of contraction force, during the T. cruzi-cardiomyocyte infection in vitro and in vivo. No change in vinculin distribution was observed after 24 h of infection, where control and T. cruzi-infected cardiomyocytes displayed vinculin localized at costameres and intercalated discs. On the other hand, a clear disruption of vinculin costameric distribution was noted after 72 h of infection. A significant reduction in the levels of vinculin expression was observed at all times of infection. In murine experimental Chagas' disease, alteration in the vinculin distribution was also detected in the infected myocardium, with no costameric staining in infected myocytes and irregular alignment of intercalated discs in cardiac fibers. These data suggest that the disruption of costameric vinculin distribution and the enlargement of interstitial space due to inflammatory infiltration may contribute to the reduction of transmission of cardiac contraction force, leading to alterations in the heart function in Chagas' disease.

Transient myofibroblast differentiation of interstitial fibroblastic cells relevant to tubular dilatation in uranyl acetate-induced acute renal failure in rats

To investigate the mechanisms of myofibroblast differentiation of interstitial fibroblastic cells (FCs) in rats with uranyl acetate-induced acute renal failure (ARF), we examined the relationship between the expression of alpha-smooth muscle actin (alpha-SMA), myofibroblast phenotype and tubular dilatation as well as cell shape and adhesion of FCs. Peritubular alpha-SMA-positive myofibroblasts appeared after induction of ARF and extended along the damaged, dilated proximal tubules and then almost disappeared after proximal tubular recovery. The perimeter of proximal tubules correlated with fractional areas stained for alpha-SMA (P<0.001). Most alpha-SMA-positive cells did not incorporate [3H]-thymidine, indicating a low proliferative activity. Transmission electron microscopy showed that FCs increasingly attached to the tubular basement membrane by elongated cytoplasm-containing microfilament bundles, which formed abundant adherens and gap junctions from day 4 to day 7. Scanning electron microscopy showed hypertrophic FCs covering large areas of tubules after induction of ARF. Administration of chlorpromazine, which can inhibit cytoskeletal movement, after induction of ARF partially inhibited myofibroblast differentiation of FCs immunohistochemically and morphologically and resulted in more dilated proximal tubules in concert with aggravation of renal dysfunction and inhibition of regenerative repair at day 4 than vehicle-administered rats. Our results indicate that mechanical tension, judged by tubular dilatation, may contribute to the induction of alpha-SMA phenotype with increased stress fiber formation and intercellular junctions in FCs to support damaged nephron structures by adjusting tensional homeostasis in rats with uranyl acetate-induced ARF.

Urinary bladder contraction and relaxation: physiology and pathophysiology

The detrusor smooth muscle is the main muscle component of the urinary bladder wall. Its ability to contract over a large length interval and to relax determines the bladder function during filling and micturition. These processes are regulated by several external nervous and hormonal control systems, and the detrusor contains multiple receptors and signaling pathways. Functional changes of the detrusor can be found in several clinically important conditions, e.g., lower urinary tract symptoms (LUTS) and bladder outlet obstruction. The aim of this review is to summarize and synthesize basic information and recent advances in the understanding of the properties of the detrusor smooth muscle, its contractile system, cellular signaling, membrane properties, and cellular receptors. Alterations in these systems in pathological conditions of the bladder wall are described, and some areas for future research are suggested.

Cytoskeletal remodeling of the airway smooth muscle cell: a mechanism for adaptation to mechanical forces in the lung

Airway smooth muscle is continuously subjected to mechanical forces caused by changes in lung volume during breathing. These mechanical oscillations have profound effects on airway smooth muscle contractility both in vivo and in vitro. Alterations in airway smooth muscle properties in response to mechanical forces may result from adaptive changes in the organization of the actin cytoskeleton. Recent advances suggest that in airway smooth muscle, two cytosolic signaling proteins that associate with focal adhesion complexes, focal adhesion kinase (FAK) and paxillin, are involved in transducing external mechanical signals. FAK and paxillin regulate changes in the organization of the actin cytoskeleton and the activation of contractile proteins. Actin is in a dynamic state in airway smooth muscle and undergoes polymerization and depolymerization during the contraction-relaxation cycle. The organization of the cytoskeletal proteins, vinculin, talin, and alpha-actinin, which mediate linkages between actin filaments and transmembrane integrins, is also regulated by contractile stimulation in airway smooth muscle. The fluidity of the cytoskeletal structure of the airway smooth muscle cell may be fundamental to its ability to adapt and respond to the mechanical forces imposed on it in the lung during breathing.

Involvement of nectin in the localization of IQGAP1 at the cell-cell adhesion sites through the actin cytoskeleton in Madin-Darby canine kidney cells

IQGAP1, a putative downstream target of the Rho family small G proteins, Cdc42 and Rac, localizes at adherens junctions (AJs) in epithelial cells. It has been suggested that IQGAP1 localizes at AJs through its binding to beta-catenin, and negatively regulates the E-cadherin-mediated cell-cell adhesion. Nectin is a Ca(2+)-independent, immunoglobulin-like cell-cell adhesion molecule that localizes at AJs. Nectin is associated with E-cadherin through their respective cytoplasmic tail-binding proteins, afadin and catenins, and involved in the formation of AJs cooperatively with E-cadherin. Here we investigated a role of nectin in the localization of IQGAP1 at AJs. Ca(2+) chelation from the medium causes disruption of the E-cadherin-mediated cell-cell adhesion, but not the nectin-based cell-cell adhesion, in Madin-Darby canine kidney (MDCK) cells. IQGAP1 remained at the residual nectin-based cell-cell adhesion sites where the E-cadherin immunofluorescence signal disappeared. Restoration of Ca(2+) in the medium causes re-accumulation of E-cadherin to the residual nectin-based cell-cell adhesion sites to re-form AJs. Nectin inhibitors inhibit this re-accumulation of E-cadherin to re-form AJs by impairing the nectin-based cell-cell adhesion. The nectin inhibitors also reduced the localization of IQGAP1 at the cell-cell adhesion sites. When MDCK cells were incubated with microbeads coated with the extracellular fragment of nectin that interacts with cellular nectin, IQGAP1 also accumulated at the bead-MDCK cell contact sites. The accumulation of IQGAP1 at the cell-cell adhesion sites was inhibited by actin filament-disrupting agents, latrunculin A and cytochalasin D. These results indicate that nectin is involved in the localization of IQGAP1 at AJs through the actin cytoskeleton.

Antagonistic and agonistic effects of an extracellular fragment of nectin on formation of E-cadherin-based cell-cell adhesion

BACKGROUND

Nectin is a Ca2+-independent immunoglobulin-like cell-cell adhesion molecule at the E-cadherin-based cell-cell adherens junctions (AJs), and comprises a family consisting of four members, nectin-1, -2, -3, and -4. Nectin and E-cadherin are associated with afadin and alpha-catenin, actin filament (F-actin)-binding proteins connecting respective adhesion molecules to the actin cytoskeleton, but the role of nectin in the formation of the E-cadherin-based cell-cell AJs has not yet been fully understood. To obtain evidence for this role of nectin, we attempted to develop an antagonist and/or agonist of nectin.

RESULTS

We made a recombinant extracellular fragment of nectin-3 (Nef-3). Nef-3 trans-interacted with cellular nectin-1 and thereby diminished the formation of the nectin-1-based cell-cell adhesion. This resulted in a reduction of the formation of the E-cadherin-based cell-cell adhesion in L fibroblasts stably expressing both exogenous nectin-1alpha and E-cadherin (nectin-1-EL cells) and MDCK cells stably expressing exogenous nectin-1alpha (nectin-1-MDCK cells). This antagonistic effect of Nef-3 was also observed in L cells stably expressing exogenous E-cadherin alone (EL cells) and wild-type MDCK cells. Conversely, Nef-3 coated on microbeads first recruited the nectin-afadin complex and then the E-cadherin-catenin complex to the bead-cell contact sites in nectin-1-EL and nectin-1-MDCK cells.

CONCLUSION

These results suggest that nectin is necessary and sufficient for the recruitment of E-cadherin to the nectin-based cell-cell adhesion sites and involved in the formation of E-cadherin-based cell-cell AJs.

Involvement of nectin in the localization of junctional adhesion molecule at tight junctions

Junctional adhesion molecule (JAM) is a Ca2+-independent immunoglobulin-like cell-cell adhesion molecule which localizes at tight junctions (TJs). Claudin is a key cell-cell adhesion molecule that forms TJ strands at TJs. JAM is associated with claudin through their cytoplasmic tail-binding protein, ZO-1. JAM is furthermore associated with Par-3, a cell polarity protein which forms a ternary complex with Par-6 and atypical protein kinase C. Nectin is another Ca2+-independent immunoglobulin-like cell-cell adhesion molecule which localizes at adherens junctions (AJs). Nectin is associated with E-cadherin through their respective cytoplasmic tail-binding proteins, afadin and catenins, and involved in the formation of AJs cooperatively with E-cadherin. We show here that nectin is furthermore involved in the localization of JAM at TJs. During the formation of the junctional complex consisting of AJs and TJs in Madin-Darby canine kidney (MDCK) cells, JAM was recruited to the nectin-based cell-cell adhesion sites. This recruitment of JAM was inhibited by nectin inhibitors, which inhibited the trans-interaction of nectin. Microbeads coated with the extracellular fragment of nectin, that interacted with cellular nectin, also recruited JAM to the bead-MDCK cell contact sites. Furthermore, when cadherin-deficient L fibroblasts stably expressing both exogenous JAM and nectin (nectin-JAM-L cells) were co-cultured with L fibroblasts expressing only nectin (nectin-L cells), JAM was concentrated at the cell-cell adhesion sites between nectin-JAM-L and nectin-L cells without the trans-interaction of JAM. Analyses of the localization and immunoprecipitation of JAM revealed that it was associated with nectin through afadin and ZO-1. These results suggest that nectin has a role in the localization of JAM at TJs in the process of the formation of the junctional complex in epithelial cells.

Decreased distribution of lung epithelial junction proteins after intratracheal antigen or lipopolysaccharide challenge: correlation with neutrophil influx and levels of BALF sE-cadherin

Distribution of airway junctional complex proteins after antigen or lipopolysaccharide challenge in sensitized or naive mice, respectively, was investigated. E-cadherin immunoreactivity was detected continuously along neighboring epithelial cell borders and between adjacent alveolar epithelial cells in naive and saline-challenged mice. Occludin and ZO-1 immunoreactivity were observed in the tight junction areas. Both challenges induced changes in epithelial morphology and phenotype, accompanied initially by focal loss of epithelial E-cadherin that increased in size with time and number of allergen challenges. Allergen challenge also led to focal loss of occludin and ZO-1. Western blot analysis revealed increased levels of sE-cadherin in lavage fluid after either challenge, and this increase correlated with lavage neutrophil numbers (P = 0.002). Immunocytochemistry of lavage cells 6 h after either challenge revealed E-cadherin epitopes within cytoplasmic vacuoles of neutrophils, the major cell type. In contrast, peripheral blood neutrophils or tissue neutrophils before epithelial transmigration were negative, suggesting that in airway inflammation, E-cadherin extracellular domain is cleaved by neutrophils during epithelial penetration, instigating the destabilization of adherens and tight junctions. This junctional deterioration could lead to a progressive decrease in epithelial integrity and induce alterations in epithelial morphology, with consequent enhanced paracellular transit of antigens and pathogens.

Role of nectin in organization of tight junctions in epithelial cells

BACKGROUND

In polarized epithelial cells, cell-cell adhesion forms specialized membrane structures comprised of claudin-based tight junctions (TJs) and of E-cadherin-based adherens junctions (AJs). These structures are aligned from the apical to the basal side of the lateral membrane, but the mechanism of this organization remains unknown. Nectin is a Ca2+ independent immunoglobulin-like cell-cell adhesion molecule which localizes at AJs. Nectin is associated with E-cadherin through their respective cytoplasmic tail-binding proteins, afadin and catenins, and involved in the formation of AJs in cooperation with E-cadherin. We show here that nectin is also involved in the formation of TJs.

RESULTS

During the formation of the junctional complex consisting of AJs and TJs in Madin-Darby canine kidney (MDCK) cells, claudin and occludin accumulated at the apical sites of the nectin-based cell-cell adhesion sites. This accumulation of claudin and occludin was inhibited by inhibitors acting on the trans interaction of nectin. The barrier function of TJs was also impaired by the nectin inhibitors. It has been shown that a phorbol ester promotes the formation of a TJ-like structure in an E-cadherin-independent manner. This phorbol ester-induced formation of the TJ-like structure was also inhibited by the nectin inhibitors.

CONCLUSIONS

These results suggest a role of the nectin-afadin system in the organization of TJs as well as AJs in epithelial cells.

Nectin couples cell-cell adhesion and the actin scaffold at heterotypic testicular junctions

Actin-based cell-cell adherens junctions (AJs) are crucial not only for mechanical adhesion but also for cell morphogenesis and differentiation. While organization of homotypic AJs is attributed mostly to classic cadherins, the adhesive mechanism of heterotypic AJs in more complex tissues remains to be clarified. Nectin, a member of a family of immunoglobulin-like adhesion molecules at various AJs, is a possible organizer of heterotypic AJs because of its unique heterophilic trans-interaction property. Recently, nectin-2 (-/-) mice have been shown to exhibit the defective sperm morphogenesis and the male-specific infertility, but the role of nectin in testicular AJs has not been investigated. We show here the heterotypic trans-interaction between nectin-2 in Sertoli cells and nectin-3 in spermatids at Sertoli-spermatid junctions (SspJs), heterotypic AJs in testes. Moreover, each nectin-based adhesive membrane domain exhibits one-to-one colocalization with each actin bundle underlying SspJs. Inactivation of the mouse nectin-2 gene causes not only impaired adhesion but also loss of the junctional actin scaffold at SspJs, resulting in aberrant morphogenesis and positioning of spermatids. Localization of afadin, an adaptor protein of nectin with the actin cytoskeleton, is also nectin-2 dependent at SspJs. These results indicate that the nectin-afadin system plays essential roles in coupling cell-cell adhesion and the cortical actin scaffold at SspJs and in subsequent sperm morphogenesis.

Nitric oxide increases albumin permeability of isolated rat glomeruli via a phosphorylation-dependent mechanism

Nitric oxide (NO) has been implicated in the induction of proteinuria in acute inflammatory glomerulonephritis and in the increased vascular permeability seen in various other disease conditions. The complicated interactions of NO with other factors in vivo hinder analysis of the mechanisms involved. By use of a recently introduced method for measuring albumin permeability (P(a)) in isolated glomeruli, the question of whether NO has a direct effect on the permeability barrier of glomerular tufts was examined and the potential mechanisms were explored. Exposure of isolated glomeruli to three NO donors, s-nitroso-N-acetyl-penicillamine (SNAP), (Z)-1-[-2-(aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate (DETA-NONOate), and sodium nitroprusside, all increased the P(a). This action of NO was time- and concentration-dependent and could be mimicked by 8-bromoguanosine 3', 5'-cyclic monophosphate. Western blot analysis of the proteins from NO donor-treated glomeruli revealed an increase of phosphotyrosine levels of proteins of molecular mass about 120 and 70 kD. The demonstration that pretreatment of glomeruli with the tyrosine kinase inhibitor, genistein, could largely prevent the effect of SNAP and DETA-NONOate confirmed the crucial role of tyrosine phosphorylation in the NO-induced increase of P(a). Furthermore, the tyrosine phosphatase inhibitor, phenylarsine oxide (PAO), could mimic the action of NO on P(a). NO-enhanced tyrosine phosphorylation was further confirmed by immunofluorescence staining, where positive cells in SNAP- and PAO-treated glomeruli were much more frequent than that in controls. By use of dual-label staining in combination with podocyte specific marker, nephrin, it was observed that most of the phosphorylated positive cells corresponded to podocytes. These results suggest that NO impairs the glomerular permeability barrier through a tyrosine phosphorylation-dependent mechanism.

Human hepatocyte culture systems for the in vitro evaluation of cytochrome P450 expression and regulation

Primary cultures of human hepatocytes have been used extensively by both academic and industrial laboratories for evaluating the hepatic disposition of drugs and other xenobiotics. Their primary utility has been for assessing the induction potential of new chemical entities (NCEs) and they continue to serve as the gold standard. Primary considerations for conducting in vitro drug testing utilizing cultures of human hepatocytes, such as the effects of culture and study conditions, are discussed. The maintenance of normal cellular physiology and intercellular contacts in vitro is of particular importance for optimal phenotypic gene expression and response to drugs and other xenobiotics. Significant advances in our understanding of cytochrome P450 (CYP450) enzyme regulation have been made with the recent identification of the nuclear receptors mediating the induction of CYP2B and CYP3A enzymes. In particular, the activation of pregnane X receptor (PXR) by prototypical inducers of CYP3A has been found to correlate well with the species-specific modulation of CYP3A by various drugs and other xenobiotics. Concomitant with the discovery of PXR has been the identification of compounds that may act synergistically or antagonistically on multiple receptors (e.g., co-repressors and/or co-activators of the receptor) introducing novel mechanisms of drug-drug interactions. Differential expression of the individual isoforms of the major CYP450 enzymes over time in culture suggest that this model system is not reflective of in vivo profiles and, therefore, may be limited in its application for drug metabolism studies. Overall, primary cultures of human hepatocytes can serve as a sensitive and selective model for predicting the regulation of CYP450 modulation by drugs and other xenobiotics. Considerations and recommendations for standardizing testing conditions and choosing relevant endpoint(s) are presented.

Localization of plectin and other related proteins along the sarcolemma in smooth muscle cells of rat colon

Plectin is a versatile linker protein which is associated with various types of cytoskeletal components and/or filaments including intermediate filaments. To better understand the functional roles of plectin in smooth muscle cells, we examined the distribution of plectin and other related proteins in rat colon smooth muscles by confocal laser and electron microscopy. The sarcolemma of smooth muscle cells exhibits two ultrastructurally distinct domains, domains associated with dense plaques and caveola-rich domains. Staining with anti-plectin and anti-desmin antibodies showed that plectin was localized along the sarcolemma in an intermittent manner and desmin was distributed in the sarcoplasm and intermittently at the cell periphery where it was codistributed with desmin. Plectin exhibited complementary and non-overlapping distribution to caveolin-1 and dystrophin, components of caveola domains, whereas plectin was codistributed with vinculin, talin and integrin beta1, components of dense plaques. Plectin was also codistributed with beta2-chain laminin but not with beta1-chain laminin. Electron microscopic observations on the sarcolemma revealed close association of intermediate filaments with dense plaques. Correlated confocal and electron microscopy clearly demonstrated that anti-plectin fluorescence corresponded to dense plaques but not to caveola domains in electron microscopic images. These findings indicate that plectin is confined to dense plaques to which desmin intermediate filaments may be anchored in rat colon smooth muscle cells.

Role of NCAM, cadherins, and microfilaments in cell-cell contact formation in TM4 immature mouse sertoli cells

To determine events that lead to the formation of intercellular contacts, we examined the spatial and temporal distribution of NCAM, cadherins, and F-actin in TM4 cells by immunofluorescence and laser scanning confocal microscopy. TM4 cells exhibited epithelioid characteristics and formed large overlapping lamella-like cell-cell contacts that contained a high concentration of NCAM. NCAM-rich lamellae formed from smaller NCAM patches at the ends of filopodia-like contacts between adjacent cells. Cadherins, as visualized by a pan-cadherin antibody, were present in a pattern distinctly different from that of NCAM. Although in filopodia-like contacts, both cadherins and NCAM were often concentrated at filopodial tips, in the larger lamella-like contacts that developed later, cadherins were located in an irregular punctate pattern only at the distal and more apical margins of the slanted NCAM-rich contact regions. Patterns of NCAM and microfilament (MF) bundle distribution were distinctly different, suggesting that the ends of these MF bundles were not physically linked to NCAM. By contrast, cadherins were concentrated at the ends of MF bundles at all stages of contact formation examined. Interestingly, this association of cadherins with MF bundles was mostly seen at the edge of the overlapping processes. In the lower cell process, MF bundles at the contact site were often arranged in random fashion, indicating an asymmetric distribution of MF in the junctional region. However, N-cadherin was enriched only at sites where MF bundles from both the upper and lower cell processes were aligned and terminated at the junctional membrane. Thus the organization of the actin cytoskeleton at cell-cell contact sites is influenced by the differential localization of different cadherins. These data also suggest that different mechanisms are involved in the accumulation of NCAM and cadherins in cell-cell contact regions.

Kinetic determination of focal adhesion protein formation

I examined the binding kinetics between integrin (alpha(IIb)beta(3)) and purified focal adhesion proteins, including alpha-actinin, filamin, vinculin, talin, and F-actin. Using static light-scatter technique, I observed affinities of the order talin > filamin > F-actin > alpha-actinin > (talin when bound to vinculin) which were lower when integrin was complexed with fibronectin. No binding between integrin and vinculin was detected. The calculated dissociation constants (K(d)) ranged between 0.4 microM and 5 microM. These results in part confirm previously published data using different methods. The modest affinity with which the focal adhesion proteins interact in vitro might be indicative of how cells, e.g., thrombocytes, gain a high degree of versatility and velocity.

Low concentrations of fibrinogen increase cell migration speed on fibronectin/fibrinogen composite cables

Optimal cell migration rate in a given direction (velocity) is a function of speed and directional persistence. Migration speed has been reported to be a function of adhesion strength such that optimal cell migration occurs where the cell is able to form enough stable attachments for good traction while allowing attachments at the trailing end to be broken during locomotion. This is particularly important in peripheral nerve regeneration where rapid Schwann cell recruitment across the injury site will lead to better functional recovery and reduced end organ atrophy. The aim here was to investigate the effects of changing adhesion properties of Fn materials by adding fibrinogen in order to design an optimal material for repair processes. Cell migration on Fn/Fg-cables increased with increasing content of %Fg to a peak cell migration velocity (Schwann cells) of 49 microm/h, at 50% Fg. Further increases in Fg content hindered cell migration. Vinculin-rich attachment plaques were reduced in a dose-dependent manner as the content of %Fg was increased whilst cells at the optimum Fg proportion for cell migration were moderately well spread. These results support the idea that optimum cell migration rates occur at intermediate attachment conditions, in this case at 50% Fg. These results show that incorporation of Fg into Fn-based materials will enhance the speed of Schwann cell migration and this would be likely to improve peripheral nerve regeneration. Indeed, directionally aligned Fn-based materials can now be engineered to give optimal cell velocity during repair cell recruitment in a range of tissue repair or tissue engineering applications.

Monocytes induce reversible focal changes in vascular endothelial cadherin complex during transendothelial migration under flow

The vascular endothelial cell cadherin complex (VE-cadherin, alpha-, beta-, and gamma-catenin, and p120/p100) localizes to adherens junctions surrounding vascular endothelial cells and may play a critical role in the transendothelial migration of circulating blood leukocytes. Previously, we have reported that neutrophil adhesion to human umbilical vein endothelial cell (HUVEC) monolayers, under static conditions, results in a dramatic loss of the VE-cadherin complex. Subsequent studies by us and others (Moll, T., E. Dejana, and D. Vestweber. 1998. J. Cell Biol. 140:403-407) suggested that this phenomenon might reflect degradation by neutrophil proteases released during specimen preparation. We postulated that some form of disruption of the VE-cadherin complex might, nonetheless, be a physiological process during leukocyte transmigration. In the present study, the findings demonstrate a specific, localized effect of migrating leukocytes on the VE-cadherin complex in cytokine-activated HUVEC monolayers. Monocytes and in vitro differentiated U937 cells induce focal loss in the staining of VE-cadherin, alpha-catenin, beta-catenin, and plakoglobin during transendothelial migration under physiological flow conditions. These events are inhibited by antibodies that prevent transendothelial migration and are reversed following transmigration. Together, these data suggest that an endothelial-dependent step of transient and focal disruption of the VE-cadherin complex occurs during leukocyte transmigration.

Interaction of nectin with afadin is necessary for its clustering at cell-cell contact sites but not for its cis dimerization or trans interaction

We have recently found a novel functional unit of cell-cell adhesion at cadherin-based adherens junctions, consisting of at least nectin, a homophilic cell adhesion molecule, and afadin, an actin filament-binding protein, which connects nectin to the actin cytoskeleton. Here we studied a mechanism of cell-cell adhesion of the nectin-afadin system by use of a cadherin-deficient L cell line stably expressing the intact form of mouse nectin-2alpha, a truncated form of nectin-2alpha incapable of interacting with afadin (nectin-2alpha-DeltaC), or a point-mutated form of nectin-2alpha capable of interacting with afadin and a cadherin-expressing EL cell line, which transiently expressed the point-mutated form of nectin-2alpha. We found that the interaction of nectin-2alpha with afadin was necessary for their clustering at cell-cell contact sites. However, nectin-2alpha-DeltaC showed cis dimerization and trans interaction, both of which did not require the interaction of nectin-2alpha with afadin. We have previously shown in EL cells that the interaction of nectin-1 with afadin is necessary for its recruitment to adherens junctions. We found that the trans interaction of nectin-2alpha was furthermore necessary for this recruitment. On the basis of these observations, we propose a model for the mechanism of cell-cell adhesion of nectin and roles of afadin in this mechanism.

Transglutaminase type 1 and its cross-linking activity are concentrated at adherens junctions in simple epithelial cells

Transglutaminase type 1 was identified as a tyrosine-phosphorylated protein from the isolated junctional fraction of the mouse liver. This enzyme was reported to be involved in the covalent cross-linking of proteins in keratinocytes, but its expression and activity in other cell types have not been examined. Northern blotting revealed that transglutaminase type 1 was expressed in large amounts in epithelial tissues (lung, liver, and kidney), which was also confirmed by immunoblotting with antibodies raised against mouse recombinant protein. Immunoblotting of the isolated junctional fraction revealed that transglutaminase type 1 was concentrated in the fraction not only as a 97-kDa form but also as forms of various molecular masses cross-linked to other proteins. In agreement with this finding, endogenous transglutaminase type 1 was immunofluorescently colocalized with E-cadherin in cultured simple epithelial cells. In the liver and kidney, immunoelectron microscopy revealed that transglutaminase type 1 was concentrated, albeit not exclusively, at cadherin-based adherens junctions. Furthermore, by in vitro and in vivo labeling, transglutaminase cross-linking activity was also shown to be concentrated at intercellular junctions of simple epithelial cells. These findings suggested that the formation of covalently cross-linked multimolecular complexes by transglutaminase type 1 is an important mechanism for maintenance of the structural integrity of simple epithelial cells, especially at cadherin-based adherens junctions.

A novel component of epidermal cell-matrix and cell-cell contacts: transmembrane protein type XIII collagen

Type XIII collagen is a short chain collagen which has recently been shown to be a transmembrane protein. The purpose of this study was to elucidate the presence and localization of type XIII collagen in normal human skin and cultured keratinocytes. Expression of type XIII collagen was demonstrated in normal human skin and epidermis at the RNA level using reverse transcription followed by polymerase chain reaction and at the protein level using western blotting and indirect immunofluorescence labeling. Immunolabeling of epidermis revealed type XIII collagen both in the cell-cell contact sites and in the dermal-epidermal junction. In cultured keratinocytes type XIII collagen epitopes were detected in focal contacts and in intercellular contacts. The results of this study show very little colocalization of type XIII collagen and desmosomal components at the light microscopic level. Thus, these results suggest that type XIII collagen is unlikely to be a component of desmosomes. Instead, the punctate labeling pattern of type XIII collagen at the cell-cell contact sites and high degree of colocalization with E-cadherin suggests that type XIII collagen is very likely to be closely associated with adherens type junctions, and may, in fact, be a component of these junctions.

Similar and differential behaviour between the nectin-afadin-ponsin and cadherin-catenin systems during the formation and disruption of the polarized junctional alignment in epithelial cells

BACKGROUND

We have recently identified a novel cell-cell adhesion system, named NAP system, which is localized at cadherin-based cell-cell adherens junctions (AJs). The NAP system is composed of at least nectin, afadin and ponsin. Nectin is an immunoglobulin-like cell adhesion molecule. Afadin is an actin filament-binding protein which associates nectin with the actin cytoskeleton. Ponsin is an afadin-binding protein which furthermore binds to vinculin and provides a possible linkage of nectin-afadin to cadherin-catenin through vinculin. We compared here the behaviour of the NAP and cadherin-catenin systems during the formation and disruption of the polarized junctional alignment in epithelial cells.

RESULTS

At the early stage of the formation of the polarized junctional alignment in MTD-1 A cells, primordial spot-like junctions were formed at the tips of thin cellular protrusions radiating from adjacent cells. Nectin, afadin, ponsin, cadherin and catenin were simultaneously recruited to these junctions. As the cell polarization proceeded, the spot-like junctions were gradually fused to form belt-like AJs where all these proteins were concentrated. The disruption of cell-cell AJs in MDCK cells by culturing at a low Ca2+ concentration caused rapid endocytosis of cadherin, but not that of nectin or afadin. Addition of 12-O-tetradecanoylphorbol-13-acetate to the cells formed a tight junction-like structure where nectin and afadin, but not cadherin, accumulated.

CONCLUSION

These results indicate that the NAP and cadherin-catenin systems show similar and differential behaviour during the formation and disruption of the polarized junctional alignment in epithelial cells.

Afadin: A key molecule essential for structural organization of cell-cell junctions of polarized epithelia during embryogenesis

Afadin is an actin filament-binding protein that binds to nectin, an immunoglobulin-like cell adhesion molecule, and is colocalized with nectin at cadherin-based cell-cell adherens junctions (AJs). To explore the function of afadin in cell-cell adhesion during embryogenesis, we generated afadin(-/-) mice and embryonic stem cells. In wild-type mice at embryonic days 6.5-8.5, afadin was highly expressed in the embryonic ectoderm and the mesoderm, but hardly detected in the extraembryonic regions such as the visceral endoderm. Afadin(-/-) mice showed developmental defects at stages during and after gastrulation, including disorganization of the ectoderm, impaired migration of the mesoderm, and loss of somites and other structures derived from both the ectoderm and the mesoderm. Cystic embryoid bodies derived from afadin(-/-) embryonic stem cells showed normal organization of the endoderm but disorganization of the ectoderm. Cell-cell AJs and tight junctions were improperly organized in the ectoderm of afadin(-/-) mice and embryoid bodies. These results indicate that afadin is highly expressed in the ectoderm- derived cells during embryogenesis and plays a key role in proper organization of AJs and tight junctions of the highly expressing cells, which is essential for proper tissue morphogenesis.

Involvement of Cdc42 small G protein in cell-cell adhesion, migration and morphology of MDCK cells

The Rho small G protein family consists of the Rho, Rac, and Cdc42 subfamilies and regulates various cell functions through reorganization of the actin cytoskeleton. We previously showed that the Rho subfamily regulates the formation of stress fibers and focal adhesions whereas the Rac subfamily regulates the E-cadherin-based cell-cell adhesion in MDCK cells. We studied here the function of the Cdc42 subfamily, consisting of two members, Cdc42Hs and G25k, in cell adhesion, migration, and morphology of MDCK cells. For this purpose, we made and used MDCK cell lines stably expressing each of dominant active mutants of Cdc42Hs (sMDCK-Cdc42HsDA) and G25K (sMDCK-G25KDA). Actin filaments at the cell-cell adhesion sites increased in both sMDCK-Cdc42HsDA and -G25KDA cells. Both E-cadherin and beta-catenin, adherens junctional proteins, at the cell-cell adhesion sites also increased in both sMDCK-Cdc42HsDA and -G25KDA cells. Electron microscopic analysis revealed that sMDCK-Cdc42HsDA cells tightly contacted with each other throughout the lateral membranes. Moreover, both the HGF- and TPA-induced disruption of the cadherin-based cell-cell adhesion and the subsequent cell migration were inhibited in both sMDCK-Cdc42HsDA and -G25KDA cells. Co-expression of the dominant negative mutant of Rac1, a member of the Rac subfamily, with the dominant active mutant of Cdc42Hs did not inhibit the increased accumulation of actin filaments at the cell-cell adhesion sites. These results suggest that the Cdc42 subfamily is involved in the cadherin-based cell-cell adhesion in a manner independent of the Rac subfamily. Furthermore, the cells were frequently enveloped by the large multinuclear cells in both sMDCK-Cdc42HsDA and -G25KDA cells. Video microscopic analysis revealed that the cells were engulfed by the large cells during cytokinesis.

Nectin/PRR: an immunoglobulin-like cell adhesion molecule recruited to cadherin-based adherens junctions through interaction with Afadin, a PDZ domain-containing protein

We have isolated a novel actin filament-binding protein, named afadin, localized at cadherin-based cell-cell adherens junctions (AJs) in various tissues and cell lines. Afadin has one PDZ domain, three proline-rich regions, and one actin filament-binding domain. We found here that afadin directly interacted with a family of the immunoglobulin superfamily, which was isolated originally as the poliovirus receptor-related protein (PRR) family consisting of PRR1 and -2, and has been identified recently to be the alphaherpes virus receptor. PRR has a COOH-terminal consensus motif to which the PDZ domain of afadin binds. PRR and afadin were colocalized at cadherin-based cell-cell AJs in various tissues and cell lines. In E-cadherin-expressing EL cells, PRR was recruited to cadherin-based cell-cell AJs through interaction with afadin. PRR showed Ca2+-independent cell-cell adhesion activity. These results indicate that PRR is a cell-cell adhesion molecule of the immunoglobulin superfamily which is recruited to cadherin-based cell-cell AJs through interaction with afadin. We rename PRR as nectin (taken from the Latin word "necto" meaning "to connect").

Differential behavior of E-cadherin and occludin in their colocalization with ZO-1 during the establishment of epithelial cell polarity

At the initial stage of cell-cell contact of epithelial cells, primordial spot-like junctions are formed at the tips of thin cellular protrusions radiating from adjacent cells, where E-cadherin and ZO-1 are precisely coconcentrated (Yonemura et al., 1995, J. Cell Sci. 108:127-142). In fully polarized epithelial cells, E-cadherin and ZO-1 are completely sorted into belt-like adherens junctions (AJ) and tight junctions (TJ), respectively. Here we examined the behavior of occludin, an integral membrane protein consisting of TJ, during the establishment of epithelial cell polarity. Using confocal immunofluorescence microscopy, we quantitatively compared the spatial relationship of occludin/ZO-1 with that of E-cadherin/ZO-1 during epithelial cellular polarization by replating or wounding cultured mouse epithelial cells (MTD1-A). At the initial stage of cell-cell contact, E-cadherin and ZO-1 appeared to be simultaneously recruited to the primordial form of spot-like junctions at the tips of cellular processes which showed no concentration of occludin. Then, as cellular polarization proceeded, occludin was gradually accumulated at the ZO-1-positive spot-like junctions to form belt-like TJ, and in a complementary manner E-cadherin was sorted out from the ZO-1-positive spot-like junctions to form belt-like AJ. The molecular mechanism of TJ/AJ formation during epithelial cellular polarization is discussed with special reference to the roles of ZO-1.

Ponsin/SH3P12: an l-afadin- and vinculin-binding protein localized at cell-cell and cell-matrix adherens junctions

We recently isolated a novel actin filament (F-actin)-binding protein, afadin, that has two isoforms, l- and s-afadins. l-Afadin is ubiquitously expressed and specifically localized at zonula adherens (ZA) in epithelial cells and at cell-cell adherens junction (AJ) in nonepithelial cells, whereas s-afadin is abundantly expressed in neural tissue. l-Afadin has one PDZ domain, three proline-rich regions, and one F-actin-binding domain, whereas s-afadin lacks the third proline-rich region and the F-actin-binding domain. To understand the molecular mechanism of the specific localization of l-afadin at ZA in epithelial cells and at cell-cell AJ in nonepithelial cells, we attempted here to identify an l-afadin-binding protein(s) and isolated a protein, named ponsin. Ponsin had many splicing variants and the primary structures of two of them were determined. Both the two variants had three Src homology 3 (SH3) domains and turned out to be splicing variants of SH3P12. The third proline-rich region of l-afadin bound to the region of ponsin containing the second and third SH3 domains. Ponsin was ubiquitously expressed and localized at ZA in epithelial cells, at cell-cell AJ in nonepithelial cells, and at cell-matrix AJ in both types of cells. Ponsin furthermore directly bound vinculin, an F-actin-binding protein localized at ZA in epithelial cells, at cell-cell AJ in nonepithelial cells, and at cell-matrix AJ in both types of cells. Vinculin has one proline-rich region where two proline-rich sequences are located. The proline-rich region bound to the region of ponsin containing the first and second SH3 domains. l-Afadin and vinculin bound to ponsin in a competitive manner and these three proteins hardly formed a ternary complex. These results indicate that ponsin is an l-afadin- and vinculin-binding protein localized at ZA in epithelial cells, at cell-cell AJ in nonepithelial cells, and at cell-matrix AJ in both types of cells.

Zyxin and vinculin distribution at the cell-extracellular matrix attachment complex (CMAX) in corneal epithelial tissue are actin dependent

Avian embryonic corneal epithelia are two cell layers thick. If isolated without (-) basal lamina, the basal cells have unorganized actin and project cytoplasmic protrusions termed blebs. The actin-based cytoskeleton at the cell-extracellular matrix junction (termed the actin cortical mat) is disrupted. These epithelia respond to soluble extracellular matrix molecules by reorganizing the actin cortical mat. Sheets of epithelia were isolated + or -basal lamina. Epithelia isolated -basal lamina were cultured +/- laminin-1 and/or +/- cytochalasin D (CD). The intracellular localization of zyxin, vinculin, paxillin, focal adhesion kinase, and tensin was determined using indirect immunohistochemistry. Protein levels were determined by Western blot analysis. Zyxin and vinculin were concentrated in two areas of the tissue. The interface between the upper cell layer (periderm) and the basal cells. The second area of concentration was at the inferior 1-4 microns of the basal cells in an area with multiple actin bundles termed the actin cortical mat. The actin bundles align toward zyxin and vinculin that were located near basal lateral membranes. Zyxin was displaced from the basal compartment of blebbing basal cells. In contrast tensin, vinculin and focal adhesion kinase were found diffusely throughout the blebs. Zyxin and vinculin redistributed to the basal-lateral membranes as actin bundles reorganized in laminin-stimulated epithelia. In contrast to the altered protein distribution, extractable protein levels were similar in blebbing and laminin-stimulated epithelia. Zyxin, vinculin, and other associated proteins were disrupted in the CD-treated tissues and do not colocalize with each other or CD-induced actin aggregates. The intracellular localization of zyxin and vinculin were concentrated in distinct areas along the inferior basolateral membranes of basal cells termed the cell-extracellular matrix attachment complex (CMAX). The distribution of CMAX proteins was dependent upon actin bundle organization.

Different behavior of l-afadin and neurabin-II during the formation and destruction of cell-cell adherens junction

We have recently isolated two novel actin filament-binding proteins, l-afadin and neurabin-II and shown that they are localized at cell-cell adherens junction (AJ) in epithelial cells. We found here that l-afadin, neurabin-II, ZO-1, and E-cadherin showed similar and different behavior during the formation and destruction of cell-cell AJ in MDCK cells. In MDCK cells, the accumulation of both l-afadin and E-cadherin, but not that of ZO-1, changed in parallel depending on Rac small G protein activity. Dissociation of MDCK cells by culturing the cells at 2 microM Ca2+ caused rapid endocytosis of E-cadherin, but not that of l-afadin or ZO-1. Addition of phorbol 12-myristate 13-acetate to these dissociated cells formed a tight junction-like structure where ZO-1 and l-afadin, but not neurabin-II or E-cadherin, accumulated. We furthermore found that, in non-epithelial EL cells, which expressed E-cadherin and attached to each other, l-afadin, neurabin-II, ZO-1 and E-cadherin were all localized at AJ. In cadherin-deficient L cells, I-afadin was mainly localized at cell-cell contact sites, but ZO-1 was mainly localized at the tip area of cell processes. Neurabin-II did not accumulate at the plasma membrane area. Neither l-afadin nor neurabin-II significantly interacted with alpha-, beta-catenin, E-cadherin, ZO-1 or occludin.

The M-cadherin catenin complex interacts with microtubules in skeletal muscle cells: implications for the fusion of myoblasts

M-cadherin, a calcium-dependent intercellular adhesion molecule, is expressed in skeletal muscle cells. Its pattern of expression, both in vivo and in cell culture as well as functional studies, have implied that M-cadherin is important for skeletal muscle development, in particular the fusion of myoblasts into myotubes. M-cadherin formed complexes with the catenins in skeletal muscle cells similar to E-cadherin in epithelial cells. This suggested that the muscle-specific function of the M-cadherin catenin complex might be mediated by additional interactions with yet unidentified cellular components, especially cytoskeletal elements. These include the microtubules which also have been implicated in the fusion process of myoblasts. Here we present evidence that the M-cadherin catenin complex interacts with microtubules in myogenic cells by using three independent experimental approaches. (1) Analysis by laser scan microscopy revealed that the destruction of microtubules by nocodazole leads to an altered cell surface distribution of M-cadherin in differentiating myogenic cells. In contrast, disruption of actin filaments had little effect on the surface distribution of M-cadherin. (2) M-cadherin antibodies coimmunoprecipitated tubulin from extracts of nocodazole-treated myogenic cells but not of nocodazole-treated epithelial cells ectopically expressing M-cadherin. Vice versa, tubulin antibodies coimmunoprecipitated M-cadherin from extracts of nocodazole-treated myogenic cells but not of nocodazole-treated M-cadherin-expressing epithelial cells. (3) M-cadherin and the catenins, but not a panel of control proteins, were copolymerized with tubulin from myogenic cell extracts even after repeated cycles of assembly and disassemly of tubulin. Moreover, neither M-cadherin nor E-cadherin could be found in a complex with microtubules in epithelial cells ectopically expressing M-cadherin. Our data are consistent with the idea that the interaction of M-cadherin with microtubules might be essential to keep the myoblasts aligned during fusion, a process in which both M-cadherin and microtubules have been implicated.

Nexilin: a novel actin filament-binding protein localized at cell-matrix adherens junction

We isolated two novel actin filament (F-actin)-binding proteins from rat brain and rat 3Y1 fibroblast. They were splicing variants, and we named brain big one b-nexilin and fibroblast small one s-nexilin. b-Nexilin purified from rat brain was a protein of 656 amino acids (aa) with a calculated molecular weight of 78,392, whereas s-nexilin, encoded by the cDNA isolated from rat 3Y1 cells by the reverse transcriptase-PCR method, was a protein of 606 aa with a calculated molecular weight of 71,942. b-Nexilin had two F-actin- binding domains (ABDs) at the NH2-terminal and middle regions, whereas s-nexilin had one ABD at the middle region because 64 aa residues were deleted and 14 aa residues were inserted in the first NH2-terminal ABD of b-nexilin, and thereby the first ABD lost its activity. b- and s-nexilins bound along the sides of F-actin, but only b-nexilin showed F-actin cross-linking activity. b-Nexilin was mainly expressed in brain and testis, whereas s-nexilin was mainly expressed in testis, spleen, and fibroblasts, such as rat 3Y1 and mouse Swiss 3T3 cells, but neither b- nor s-nexilin was detected in liver, kidney, or cultured epithelial cells. An immunofluorescence microscopic study revealed that s-nexilin was colocalized with vinculin, talin, and paxillin at cell- matrix adherens junction (AJ) and focal contacts, but not at cell-cell AJ, in 3Y1 cells. Overexpressed b- and s-nexilins were localized at focal contacts but not at cell-cell AJ. These results indicate that nexilin is a novel F-actin-binding protein localized at cell-matrix AJ.

Actin ‘purse string’ filaments are anchored by E-cadherin-mediated adherens junctions at the leading edge of the epithelial wound, providing coordinated cell movement

At the leading edge of healing embryonic epithelium, cables of actin filaments appear to extend from cell to cell, forming a ring around the wound circumference. It has been hypothesized that this actin filament cable functions as a contractile ‘purse string’ to facilitate wound closure. We have observed this cable in large, circular healing epithelial wounds in corneas of adult mice. To elucidate the role of the actin filament cable, we characterized the molecular components associated with the cell-cell junction where the actin filament cable inserts and with the actin filament cable itself, and we studied the effect of disruption of the cable using an E-cadherin function-blocking antibody, ECCD-1. Localization of E-cadherin and the direct association of catenins with actin filament cable at the cell-cell interface of the actin cable confirmed that the cell-cell junction associated with the actin filament cable is an adherens junction. The E-cadherin function-blocking antibody caused disruption of the actin filament cable and induction of prominent lamellipodial extensions on cells at the leading edge, leading to a ragged uneven epithelial wound margin. These data demonstrate that cell-to-cell associated E-cadherin molecules link the actin filament cable, forming a functional adherens junction, and that the actin filament cable plays a role in coordinating cell movement.

Rescue of the mutant phenotype by reexpression of full-length vinculin in null F9 cells; effects on cell locomotion by domain deleted vinculin

Vinculin plays a role in signaling between integrins and the actin cytoskeleton. We reported earlier that F9-derived cells lacking vinculin are less spread, less adhesive, and move two times faster than wild-type F9 cells. Expression of intact vinculin in null cells restored all wild-type characteristics. In contrast, expression of the head (90 kDa) fragment exaggerated mutant characteristics, especially locomotion, which was double that of vinculin null cells. Expression of the tail domain also had a marked effect on locomotion in the opposite direction, reducing it to very low levels. The expression of the head plus tail domains together (no covalent attachment) effected a partial rescue towards wild-type phenotype, thus indicating that reexpressed polypeptides may be in their correct location and are interacting normally. Therefore, we conclude that: (1) the head domain is part of the locomotory force of the cell, modulated by the tail, and driven by the integrin/matrix connection; (2) intact vinculin is required for normal regulation of cell behavior, suggesting that vinculin head-tail interactions control cell adhesion, spreading, lamellipodia formation and locomotion.

Contributions of extracellular and intracellular domains of full length and chimeric cadherin molecules to junction assembly in epithelial cells

The integrity of cell-cell junctions in epithelial cells depends on functional interactions of both extracellular and intracellular domains of cadherins with other junction proteins. To examine the roles of the different domains of E-cadherin and desmoglein in epithelial junctions, we stably expressed full length desmoglein 1 and chimeras of E-cadherin and desmoglein 1 in A431 epithelial cells. Full length desmoglein 1 was able to incorporate into or disrupt endogenous desmosomes depending on expression level. Each of the chimeric cadherin molecules exhibited distinct localization patterns at the cell surface. A chimera of the desmoglein 1 extracellular domain and the E-cadherin intracellular domain was distributed diffusely at the cell surface while the reverse chimera, comprising the E-cadherin extracellular domain and the desmoglein 1 intracellular domain, localized in large, sometimes contiguous patches at cell-cell interfaces. Nevertheless, both constructs disrupted desmosome assembly. Expression of constructs containing the desmoglein 1 cytoplasmic domain resulted in approximately a 3-fold decrease in E-cadherin bound to plakoglobin and a 5- to 10-fold reduction in the steady-state levels of the endogenous desmosomal cadherins, desmoglein 2 and desmocollin 2, possibly contributing to the dominant negative effect of the desmoglein 1 tail. In addition, biochemical analysis of protein complexes in the stable lines revealed novel in vivo protein interactions. Complexes containing beta-catenin and desmoglein 1 were identified in cells expressing constructs containing the desmoglein 1 tail. Furthermore, interactions were identified between endogenous E-cadherin and the chimera containing the E-cadherin extracellular domain and the desmoglein 1 intracellular domain providing in vivo evidence for previously predicted lateral interactions of E-cadherin extracellular domains.

Neurabin-II/spinophilin. An actin filament-binding protein with one pdz domain localized at cadherin-based cell-cell adhesion sites

In a preceding paper, we reported a novel actin filament (F-actin)-binding protein, named neurabin, which was specifically expressed in neural tissue and implicated in neurite formation. We purified from rat brain another F-actin-binding protein, which had a domain organization similar to that of neurabin but was ubiquitously expressed, and named it neurabin-II. The original neurabin, renamed neurabin-I, had 1095 amino acids and a calculated Mr of 122,729, whereas neurabin-II had 817 amino acids and a calculated Mr of 89, 642. Both neurabin-I and -II had one F-actin-binding domain at the N-terminal region, one PDZ domain at the middle region, a domain known to interact with transmembrane proteins, and domains predicted to form coiled-coil structures at the C-terminal region. Both neurabin-I and -II bound along the sides of F-actin and showed F-actin-cross-linking activity. The subcellular distribution analysis indicated that neurabin-II was enriched at the postsynaptic density fraction in rat brain and the adherens junction fraction in rat liver. Immunofluorescence microscopic analysis revealed that neurabin-II was highly concentrated at the synapse in primary cultured rat hippocampal neurons and at the cadherin-based cell-cell adhesion sites in Madin-Darby canine kidney cells. Neurabin-II turned out to be the same as a recently reported protein phosphatase 1-binding protein named spinophilin. These results suggest that neurabin-II/spinophilin plays an important role in linking the actin cytoskeleton to the plasma membrane.

Vinculin is associated with the E-cadherin adhesion complex

Cadherins mediate calcium-dependent cell-cell adhesion, and this activity is regulated by cytoplasmic interactions between cadherins, catenins, and the actin-based cytoskeleton. alpha-Catenin plays a critical role in the transmembrane anchorage of cadherins, and deletion of alpha-catenin has been shown to inactivate cadherin-mediated adhesion, resulting in a nonadhesive phenotype. Here we show that serum starvation increases E-cadherin expression and induces E-cadherin-dependent adhesion in the MDA-MB-468 breast cancer cell line. This adhesion occurred despite a lack of alpha-catenin expression, which was caused by mutations in the alpha-catenin gene. Coprecipitation analysis suggests that this adhesion may be mediated by cytoplasmic connections from cadherins to the cytoskeleton involving vinculin. A high level of vinculin associated with E-cadherin immunoprecipitates was observed in MDA-MB-468 cells. In contrast, vinculin was not detected in E-cadherin complexes in the A431 and MCF-7 epithelial carcinoma cell lines, which express alpha-catenin. However, in reciprocal immunoprecipitations using anti-vinculin antibodies, E-cadherin associated strongly with vinculin in MDA-MB-468 cells and, to a lesser extent, in A431 and MCF-7 cells. These results suggest that both alpha-catenin and vinculin may be present in the adhesion complex. To test the hypothesis that vinculin associates with E-cadherin complexes via beta-catenin, excess recombinant beta-catenin or alpha-catenin fusion protein was added to MDA-MB-468 cell lysates. Both specifically inhibited the coprecipitation of E-cadherin with vinculin, suggesting competition for the same binding site. These results suggest that vinculin plays a role in the establishment or regulation of the cadherin-based cell adhesion complex by direct interaction with beta-catenin.

Afadin: A novel actin filament-binding protein with one PDZ domain localized at cadherin-based cell-to-cell adherens junction

A novel actin filament (F-actin)-binding protein with a molecular mass of approximately 205 kD (p205), which was concentrated at cadherin-based cell-to-cell adherens junction (AJ), was isolated and characterized. p205 was purified from rat brain and its cDNA was cloned from a rat brain cDNA library. p205 was a protein of 1,829 amino acids (aa) with a calculated molecular mass of 207,667 kD. p205 had one F-actin-binding domain at 1,631-1,829 aa residues and one PDZ domain at 1,016- 1,100 aa residues, a domain known to interact with transmembrane proteins. p205 was copurified from rat brain with another protein with a molecular mass of 190 kD (p190). p190 was a protein of 1,663 aa with a calculated molecular mass of 188,971 kD. p190 was a splicing variant of p205 having one PDZ domain at 1,009-1,093 aa residues but lacking the F-actin-binding domain. Homology search analysis revealed that the aa sequence of p190 showed 90% identity over the entire sequence with the product of the AF-6 gene, which was found to be fused to the ALL-1 gene, known to be involved in acute leukemia. p190 is likely to be a rat counterpart of human AF-6 protein. p205 bound along the sides of F-actin but hardly showed the F-actin-cross-linking activity. Northern and Western blot analyses showed that p205 was ubiquitously expressed in all the rat tissues examined, whereas p190 was specifically expressed in brain. Immunofluorescence and immunoelectron microscopic studies revealed that p205 was concentrated at cadherin-based cell-to-cell AJ of various tissues. We named p205 l-afadin (a large splicing variant of AF-6 protein localized at adherens junction) and p190 s-afadin (a small splicing variant of l-afadin). These results suggest that l-afadin serves as a linker of the actin cytoskeleton to the plasma membrane at cell-to-cell AJ.