Submembranous junctional plaque proteins include potential tumor suppressor molecules

Claudin 18.2 as a novel therapeutic target

Claudin 18.2, a tight-junction molecule predominantly found in the nonmalignant gastric epithelium, becomes accessible on the tumour cell surface during malignant transformation, thereby providing an appealing target for cancer therapy. Data from two phase III trials testing the anti-claudin 18.2 antibody zolbetuximab have established claudin 18.2-positive advanced-stage gastric cancers as an independent therapeutic subset that derives benefit from the addition of this agent to chemotherapy. This development has substantially increased the percentage of patients eligible for targeted therapy. Furthermore, newer treatments, such as high-affinity monoclonal antibodies, bispecific antibodies, chimeric antigen receptor T cells and antibody-drug conjugates capable of bystander killing effects, have shown considerable promise in patients with claudin 18.2-expressing gastric cancers. This new development has resulted from drug developers moving beyond traditional targets, such as driver gene alterations or growth factors. In this Review, we highlight the biological rationale and explore the clinical activity of therapies that target claudin 18.2 in patients with advanced-stage gastric cancer and explore the potential for expansion of claudin 18.2-targeted therapies to patients with other claudin 18.2-positive solid tumours.

Arginine affects growth and integrity of grass carp enterocytes by regulating TOR signaling pathway and tight junction proteins

Dietary arginine (Arg) could improve the intestinal structure and absorption of grass carp (Ctenopharyngodon idellus); however, the mechanism of Arg on intestinal morphology improvement was unclear. The present study aimed to explain the possible mechanism of the positive effect of Arg on intestinal epithelial cells of grass carp. An in vitro study was conducted through a primary culture model to assess the growth, cell viability, mRNA expressions of TOR signal pathway, and tight junction proteins of enterocytes after culture in the medium with 6 levels of Arg (0, 0.1, 0.2, 0.5, 1.0, and 2.0 mmol/L). The results showed that 0.5 mmol/L Arg improved the cell number and decreased the lactate dehydrogenase and creatine kinase activities in culture medium (P < 0.05). The alkaline phosphatase activity in cell lysis buffer was depressed by 1 and 2 mmol/L Arg (P < 0.05). The nitric oxide (NO) content showed an increasing trend with the Arg content (P < 0.05), whereas the NO synthase activity showed an opposite trend to NO. TOR expression was higher in 0.2 and 0.5 mmol/L groups, whereas S6K1 expression in 1.0 mmol/L and 2.0 mmol/L groups were lower (P < 0.05). The mRNA expressions of occludin, claudin 3, and claudin c in 0.5 mmol/L group were the highest, while ZO-1 and claudin b expressions were higher in 0.2 and 0.5 mmol/L groups (P < 0.05). This study indicated that Arg enhanced the growth and integrity of intestinal epithelial cells of grass carp through upregulation of mRNA expression of TOR signal pathway and tight junction proteins at an optimal Arg content of 0.2-0.5 mmol/L.

X-RAY IRRADIATION AFFECTS MORPHOLOGY, PROLIFERATION AND MIGRATION RATE OF HEALTHY AND CANCER CELLS

Cytoskeleton plays a central role in many cellular processes, such as migration, adhesion and proliferation. Alterations of its structural properties are commonly associated with different diseases (malignancy, cardiac hypertrophy, etc.). In this work, we studied the effects of X-radiations on cytoskeleton architecture of two cell lines: BALBc/3T3 and Simian virus 40-transformed BALBc/3T3 (SVT2) cells. In agreement with the current literature, we observed reduced adhesion and increased motility of SVT2 cells respect to non-transformed BALBc/3T3. In addition, we showed that two different doses of X-rays (1 and 2 Gy) increased cell-dish adhesiveness and reduced cell proliferation and cell motility of transformed cells, whereas minor effects were measured on the normal counterpart. These results suggested that low doses or fractioning of X-rays may have a normalization effect on the investigated parameters for the transformed cell phenotype.

Global dissociation of HuR-mRNA complexes promotes cell survival after ionizing radiation

Ionizing radiation (IR) triggers adaptive changes in gene expression. Here, we show that survival after IR strongly depends on the checkpoint kinase Chk2 acting upon its substrate HuR, an RNA-binding protein that stabilizes and/or modulates the translation of target mRNAs. Microarray analysis showed that in human HCT116 colorectal carcinoma cells (WT), IR-activated Chk2 triggered the dissociation of virtually all of HuR-bound mRNAs, since IR did not dissociate HuR target mRNAs in Chk2-null (CHK2-/-) HCT116 cells. Accordingly, several HuR-interacting mRNAs encoding apoptosis- and proliferation-related proteins (TJP1, Mdm2, TP53BP2, Bax, K-Ras) dissociated from HuR in WT cells, but remained bound and showed altered post-transcriptional regulation in CHK2-/- cells. Use of HuR mutants that were not phosphorylatable by Chk2 (HuR(3A)) and HuR mutants mimicking constitutive phosphorylation by Chk2 (HuR(3D)) revealed that dissociation of HuR target transcripts enhanced cell survival. We propose that the release of HuR-bound mRNAs via an IR-Chk2-HuR regulatory axis improves cell outcome following IR.

Junctional music that the nucleus hears: cell-cell contact signaling and the modulation of gene activity

Cell-cell junctions continue to capture the interest of cell and developmental biologists, with an emerging area being the molecular means by which junctional signals relate to gene activity in the nucleus. Although complexities often arise in determining the direct versus indirect nature of such signal transduction, it is clear that such pathways are essential for the function of tissues and that alterations may contribute to many pathological outcomes. This review assesses a variety of cell-cell junction-to-nuclear signaling pathways, and outlines interesting areas for further study.

Up-regulation of the tight-junction protein ZO-1 by substance P and IGF-1 in A431 cells

The formation of a barrier by tight junctions is important in epithelia of various tissues. Substance P (SP) and insulin-like growth factor (IGF)-1 synergistically promote barrier function in the corneal epithelium. We have now examined the effects of SP and IGF-1 on expression of the tight-junction protein zonula occludens (ZO)-1 in A431 human epidermoid carcinoma cells. Reverse transcription-polymerase chain reaction (RT-PCR) and immunoblot analyses revealed that SP and IGF-1 increased the amounts of ZO-1 mRNA and protein in these cells in a concentration-dependent manner, with neither SP nor IGF-1 alone having such an effect. The SP- and IGF-1-induced up-regulation of ZO-1 was accompanied by phosphorylation of extracellular signal-regulated kinase (ERK), and both of these effects were blocked by PD98059, an inhibitor of ERK activation. SP and IGF-1 also increased the transepithelial electrical resistance (TER) (an indicator of barrier function) of an A431 cell monolayer in a manner sensitive to PD98059. Our results thus suggest that the synergistic induction of ZO-1 expression by SP and IGF-1 may promote barrier function in skin epithelial cells.

The tight junction protein ZO-2 mediates proliferation of vascular smooth muscle cells via regulation of Stat1

AIMS

Recent evidence suggests that the zonula occludens protein 2 (ZO-2) might have additional cellular functions, beyond regulation of paracellular permeability of epithelial and endothelial cells. Deregulation of ZO-2 in response to ischaemia, hypertensive stress, and vascular injury implies its involvement in cardiovascular disorders, most likely via regulating the functional behaviour of vascular smooth muscle cells (VSMC). However, a role of ZO-2 in VSMC biology has yet to be established. Our study was designed to understand the specific functions of ZO-2 in human VSMC.

METHODS AND RESULTS

The expression of ZO-2 and Stat1 upon vascular injury was studied using ex vivo organ culture of coronary arteries combined with immunohistochemistry. ZO-2 silencing in human primary VSMC was achieved by means of lentiviral gene transfer. Cell proliferation was assessed by analysing DNA synthesis and by cell counting. Stat1 expression was examined using immunoblotting, immunocytochemistry, TaqMan, and fluorescence activated cell sorting (FACS) analysis. Functional relevance of Stat1 up-regulation was studied using a Stat1 promoter-luciferase reporter assay and intracellular microinjections of a Stat1 specific antibody. ZO-2 was highly expressed in the media and neointima of dilated but not of control arteries, whereas expression of the transcription factor Stat1 was inversely regulated upon injury. Analysis of VSMC with down-regulated ZO-2 revealed increased expression of Stat1 in these cells, whereas Stat1 phosphorylation was not affected. Stat1 up-regulation in VSMC with ZO-2 silencing resulted in a coordinate activation of Stat1-specific genes and consequently led to inhibition of cell proliferation. This effect was restored by microinjection of a Stat1 neutralising antibody.

CONCLUSION

Our data suggest that the tight junction protein ZO-2 is involved in regulation of VSMC growth control upon vascular injury that is mediated by the transcription factor Stat1. Our findings point to a novel function of ZO-2 in VSMC and implicate ZO-2 as a novel important molecular target in pathological states of vascular remodelling in cardiovascular diseases.

IQGAP3 regulates cell proliferation through the Ras/ERK signalling cascade

Proliferation of epithelial cells must be spatiotemporally regulated to maintain the organization of epithelial sheets. Here we show that the IQGAP family, comprising IQGAP1, 2 and 3, underlies lateral cell-cell contacts of epithelial cells. Of the three proteins, IQGAP3 is unique in that its expression is specifically confined to proliferating cells. Knockdown of IQGAP3 in cultured epithelial cells caused inhibition of proliferation and ERK activity. When exogenously expressed in quiescent cells, IQGAP3 was capable of inducing cell-cycle re-entry, which was completely inhibited by the MEK inhibitor U0126. Thus, IQGAP3 is necessary and sufficient for driving cell proliferation and ERK acts downstream of IQGAP3. Furthermore, IQGAP3 specifically interacted with the active, GTP-bound form of Ras, and in IQGAP3 knockdown cells, the activity of Ras, but not of other small GTPases, was inhibited. Thus, IQGAP3 regulates the promotion of cell proliferation through Ras-dependent ERK activation.

Optimized proteomic analysis on gels of cell-cell adhering junctional membrane proteins

A high level of structural organization of functional membrane domains in very narrow regions of a plasma membrane is crucial for the functions of plasma membranes and various other cellular functions. Conventional proteomic analyses are based on total soluble cellular proteins. Thus, because of insolubility problems, they have major drawbacks for use in analyses of low-abundance proteins enriched in very limited and specific areas of cells, as well as in analyses of the membrane proteins in two-dimensional gels. We optimized proteomic analyses of cell-cell adhering junctional membrane proteins on gels. First, we increased the purity of cell-cell junctions, which are very limited and specific areas for cell-cell adhesion, from hepatic bile canaliculi. We then enriched junctional membrane proteins via a guanidine treatment; these became selectively detectable on two- dimensionally electrophoresed gels after treatment with an extremely high concentration of NP-40. The framework of major junctional integral membrane proteins was shown on gels. These included six novel junctional membrane proteins of type I, type II, and tetraspanin, which were identified by mass spectrometry and by a database sequence homology search, as well as 12 previously identified junctional membrane proteins, such as cadherins and claudins.

Evidence for a potential tumor suppressor role for the Na,K-ATPase beta1-subunit

The Na,K-ATPase, consisting of two essential subunits (alpha, beta), plays a critical role in the regulation of ion homeostasis in mammalian cells. Recent studies indicate that reduced expression of the beta1 isoform (NaK-beta1) is commonly observed in carcinoma and is associated with events involved in cancer progression. In this study, we present evidence that repletion of NaK-beta1 in Moloney sarcoma virus-transformed Madin-Darby canine kidney cells (MSV-MDCK), a highly tumorigenic cell line, inhibits anchorage independent growth and suppresses tumor formation in immunocompromised mice. Additionally, using an in vitro cell-cell aggregation assay, we showed that cell aggregates of NaK-beta1 subunit expressing MSV-MDCK cells have reduced extracellular regulated kinase (ERK) 1/2 activity compared with parental MSV-MDCK cells. Finally, using immunohistochemistry and fully quantitative image analysis approaches, we showed that the levels of phosphorylated ERK 1/2 are inversely correlated to the NaK-beta1 levels in the tumors. These findings reveal for the first time that NaK-beta1 has a potential tumor-suppressor function in epithelial cells.

E-cadherin as an invasion suppressor

The loss of epithelial differentiation in carcinomas, which is accompanied by increased mobility and invasiveness of the tumour cells, is often a consequence of reduced intercellular adhesion. Recent reports have indicated that the primary cause for the 'scattering' of the cells in invasive carcinomas is a disturbance of the integrity of intercellular junctions often involving the cell adhesion molecule E-cadherin. It has also been suggested that during invasion, carcinoma cells convert to a sort of mesenchymal stage, as do normal epithelial cells during development. Permanent and transient molecular mechanisms lead to the impairment of junction integrity of epithelial cells and thus to the progression of carcinomas towards a more invasive state.

Epithelial tight junctions, gene expression and nucleo-junctional interplay

Tight junctions are components of the junctional complex linking neighbouring epithelial cells and are important for barrier formation. Recent evidence suggests that tight junctions also participate in signal transduction mechanisms that regulate epithelial cell proliferation, gene expression, differentiation and morphogenesis. One important class of tight-junction-associated signal transduction mechanism is based on dual localisation of certain proteins both at junctions and in the nucleus. These proteins and their partners participate in various steps of gene expression, ranging from regulation of transcription and chromatin structure to mRNA processing and translation. In cancer tissues, their expression is often deregulated in a manner that suggests that tight junctions function as suppressors of proliferation and transformation.

Membrane-Type 1 Matrix Metalloproteinase Expression Is Regulated by Zonula Occludens-1 in Human Breast Cancer Cells

The acquisition of a migratory/invasive phenotype by tumor cells is characterized by the loss of cell-cell adhesion contacts and the expression of degradative properties. In this study, we examined the effect of the disorganization of occludin/zonula occludens (ZO)-1 tight junction (TJ) complexes on the expression of membrane-type 1 matrix metalloproteinase (MT1-MMP). We first compared the expression of MT1-MMP and the localization of occludin/ZO-1 complexes in breast tumor cell lines displaying various degrees of invasiveness. We showed that the expression of MT1-MMP in invasive breast tumor cell lines correlates with the absence of occludin and with a cytoplasmic localization of ZO-1. In contrast, noninvasive cell lines displayed a membrane staining for both ZO-1 and occludin and did not express MT1-MMP. In vivo, cytoplasmic ZO-1 and MT1-MMP could be detected in invasive tumor clusters of human breast carcinomas. We then used RNA interference strategy to inhibit ZO-1 expression in invasive BT549 cells and to evaluate the effect of ZO-1 down-regulation on MT1-MMP expression. We observed that ZO-1 small interfering RNA transfection down-regulates MT1-MMP mRNAs and proteins and subsequently decreases the ability of tumor cells to invade a reconstituted basement membrane in a Boyden chamber assay. Inversely, transfection of expression vectors encoding wild-type ZO-1 or the NH2-terminal fragment of ZO-1 comprising the PSD95/DLG/ZO-1 domains in BT549 activated a human MT1-MMP promoter luciferase reporter construct and increased cell invasiveness. Such transfections concomitantly activated the beta-catenin/TCF/LEF pathway. Our results therefore show that ZO-1, besides its structural role in TJ assembly, can intervene in signaling events promoting tumor cell invasion.

Synthesis of junctional proteins in metastasizing colon cancer cells

Various authors have reported reduced synthesis of epithelial junctional proteins during dedifferentiation, tumorigenesis and metastasis in a great variety of tumors. Consequently, it is generally accepted that loss of adhesive molecules and adhesion structures is implicated in the development of an invasive phenotype and poor patient prognosis. Colon carcinomas, on the other hand, were shown to behave differently as synthesis of main adhesive proteins continues despite the development of an invasive phenotype. In this study we used cultured cells grown under conditions that inhibited intercellular adhesion (low Ca2+ concentration) and compared these results with data obtained from metastasizing colon cancer cells (signet ring cell carcinoma). Characterization of these proteins and their structures were performed by immunoprecipitations, Western blot analysis, immunohistochemistry, pre-embedding immuno-electron microscopy, and a new method to perform immuno-electron microscopy on paraffin-embedded material, which we present in this paper. We demonstrate that synthesis carries on for both, the desmosomal and the proteins of the zonula adhaerens. While, however, the assembly of desmosomal structures in the form of half-desmosomes at the cell surface continues, those of the zonula adhaerens did not. Instead E-cadherin was found, although associated with alpha-catenin, beta-catenin, and plakoglobin, evenly distributed at the plasma membrane of the cultured cells and also at the surface of the dissociated tumor cells. We conclude from our observations that continued expression and synthesis of junctional proteins do not necessarily contribute to the suppression of tumor invasion and metastasis of colon cancer.

Reduced E-cadherin expression contributes to the loss of p27kip1-mediated mechanism of contact inhibition in thyroid anaplastic carcinomas

In the present study, we have characterized several human thyroid cancer cell lines of different histotypes for their responsiveness to contact inhibition. We found that cells derived from differentiated carcinoma (TPC-1, WRO) arrest in G(1) phase at confluence, whereas cells derived from anaplastic carcinoma (ARO, FRO and FB1) continue to grow after reaching confluence. Furthermore, we provide experimental evidence that the axis, E-cadherin/beta-catenin/p27(Kip1), represents an integral part of the regulatory mechanism that controls proliferation at a high cell density, whose disruption may play a key role in determining the clinical behaviour of thyroid cancer. This conclusion derives from the finding that: (i) the expression of p27(Kip1) is enhanced at high cell density only in cells responsive to contact inhibition (TPC-1, WRO), but not in contact-inhibition resistant cells (ARO, FRO or FB1 cells); (ii) the increase in p27(Kip1) also resulted in increased levels of p27(Kip1) bound to cyclin E-Cdk2 complex, a reduction in cyclin E-Cdk2 activity and dephosphorylation of the retinoblastoma protein; (iii) antisense inhibition of p27(Kip1) upregulation at high cell density in confluent-sensitive cells completely prevents the confluence-induced growth arrest; (iv) proper expression and/or membrane localization of E-cadherin is observed only in cells responsive to contact inhibition (TPC-1, NPA, WRO) but not in unresponsive cells (ARO, FRO or FB1); (v) disruption of E-cadherin-mediated cell-cell contacts at high cell density induced by an anti-E-cadherin neutralizing antibody, inhibits the induction of p27(kip1) and restores proliferation in contact-inhibited cells; (vi) re-expression of E-cadherin into cells unresponsive to contact inhibition (ARO, FB1) induces a p27(kip1) expression and growth arrest. In summary, our data indicate that the altered response to contact inhibition exhibited by thyroid anaplastic cancer cells is due to the failure to upregulate p27(Kip1) in response to cell-cell interactions.

Cell Adhesion, Polarity, and Epithelia in the Dawn of Metazoans

Transporting epithelia posed formidable conundrums right from the moment that Du Bois Raymond discovered their asymmetric behavior, a century and a half ago. It took a century and a half to start unraveling the mechanisms of occluding junctions and polarity, but we now face another puzzle: lest its cells died in minutes, the first high metazoa (i.e., higher than a sponge) needed a transporting epithelium, but a transporting epithelium is an incredibly improbable combination of occluding junctions and cell polarity. How could these coincide in the same individual organism and within minutes? We review occluding junctions (tight and septate) as well as the polarized distribution of Na+-K+-ATPase both at the molecular and the cell level. Junctions and polarity depend on hosts of molecular species and cellular processes, which are briefly reviewed whenever they are suspected to have played a role in the dawn of epithelia and metazoan. We come to the conclusion that most of the molecules needed were already present in early protozoan and discuss a few plausible alternatives to solve the riddle described above.

Ezrin gene, coding for a membrane-cytoskeleton linker protein, is regionally expressed in the developing mouse neuroepithelium

Ezrin is a member of the Ezrin, Radixin, Moesin (ERM) proteins family that are proposed to act as linkers between the cytoskeleton and plasma membrane. Ezrin regulates cell-cell and cell-matrix interactions playing a role in the regulation of cellular adhesion, movement and morphology in epithelia. Alterations in the expression of Ezrin and other members of ERM family have also been observed in brain tumours. Here we report the expression pattern of Ezrin during mouse neural development, from early stages to postnatal stages. In young and middle gestation embryos, Ezrin is expressed in the roof plate of the neural tube, in the presumptive domain of the choroidal plexus, and in some precise domains of ventricular epithelium. These domains are distributed in basal and alar neuroepithelial regions, some of them in relation to the expression of cadherins. At later gestation and postnatal stages, Ezrin expression is maintained on the mature choroidal plexus and is weakly detected in the proliferative regions of the mature brain.

Mutant α-actinin-4 promotes tumorigenicity and regulates cell motility of a human lung carcinoma

The precise role of alpha-actinin-4 encoding gene (ACTN4) is not very well understood. It has been reported to elicit tumor suppressor activity and to regulate cellular motility. To further assess the function of human ACTN4, we studied a lung carcinoma cell line expressing a mutated alpha-actinin-4, which is recognized as a tumor antigen by autologous CD8(+) cytotoxic T lymphocytes (CTL). Confocal immunofluorescence microscopy indicated that, while wild-type (WT) alpha-actinin-4 stains into actin cytoskeleton and cell surface ruffles, the mutated protein is only dispersed in the cytoplasm of the lung carcinoma cells. This loss of association with the cell surface did not appear to correlate with a decrease in in vitro alpha-actinin-4 crosslinking to filamentous (F)-actin. Interestingly, experiments using cell lines stably expressing ACTN4 demonstrated that as opposed to WT gene, mutant ACTN4 was unable to inhibit tumor cell growth in vitro and in vivo. Moreover, the expression of mutant alpha-actinin-4 resulted in the loss of tumor cell capacity to migrate. The identification of an inactivating mutation in ACTN4 emphasizes its role as a tumor suppressor gene and underlines the involvement of cytoskeleton alteration in tumor development and metastasis.

Junctional complex revisited by high-resolution scanning electron microscopy

The present study correlates the ultrastructural morphology of junctional complexes as revealed by transmission electron microscopy (TEM) with that observed by high-resolution scanning electron microscopy (HRSEM), thanks to a new modification of the osmium tetroxide maceration technique. The removal of all cytoplasmic organelles by this technique allows the inspection of the inner side of the plasmalemma. With this treatment, a continuous band of tightly packed particles is observed at the most apical portion of lateral membranes. Just below this band, irregular clusters of apparently identical particles are placed all around the cellular contour. The topographical correspondence among these clusters and spot desmosomes seen by TEM identifies them as desmosomes. The continuous band seems to represent the combination of both zonulae, occludens and adherens. Regarding the nature of the particles, we suppose that they probably consist of peripheral membrane proteins clustered at the cytoplasmic surface of intercellular junctions and involved in the linkage between cytoskeleton and plasmalemma.

Signalling to and from tight junctions

Tight junctions have long been regarded as simple barriers that separate compartments of different compositions, but recent research indicates that different types of signalling proteins and transduction pathways are associated with these junctions. They receive and convert signals from the cell interior to regulate junction assembly and function, and transmit signals to the cell interior to modulate gene expression and cell behaviour.

The structure and function of myelin: from inert membrane to perfusion pump

The goal of this overview is to propose a novel structure/function model of central nervous system myelin. Although myelin is known to be a compact multilamellar structure that wraps around axons, the biologic role this structure plays in the nervous system remains an enigma. One means of ascertaining myelin's biologic role is by analyzing its structure. The recent discovery of tight junctions in myelin may be the key that unlocks the mysterious black box of myelin structure/function. Tight junctions in other cell types are invariably adjacent to adherens junctions, with both of these junctional plaques playing critical roles in paracellular barrier function, i.e., adhesion of cell membranes, signal transduction, and fluid movement between cells via aqueous pores and channels. The application of current knowledge about junctional plaques to myelin is an original concept. This knowledge, taken together with evidence from studies of normal and pathologic myelin, supports the possibility that a primary function of junctional plaques in myelin is to perfuse the periaxonal space.

Regulation of intercellular tight junctions by zonula occludens toxin and its eukaryotic analogue zonulin

The intestinal epithelium represents the largest interface between the external environment and the internal host milieu and constitutes the major barrier through which molecules can either be absorbed or secreted. There is now substantial evidence that tight junctions (tj) play a major role in regulating epithelial permeability by influencing paracellular flow of fluid and solutes. Tj are one of the hallmarks of absorptive and secretory epithelia. Evidence now exists that tj are dynamic rather than static structures and readily adapt to a variety of developmental, physiological, and pathological circumstances. These adaptive mechanisms are still incompletely understood. Activation of PKC either by Zonula occludens toxin (Zot) or by phorbol esters increases paracellular permeability. Alteration of epithelial tj is a recently described property for infectious agents. Clostridium difficile toxin A and B and influenza and vesicular stomatitis viruses have been shown to loosen tj in tissue culture monolayers. Unlike what occurs after the Zot stimulus, these changes appear to be irreversible and are associated with destruction of the tj complex. On the basis of this observation, we postulated that Zot may mimic the effect of a functionally and immunologically related endogenous modulator of epithelial tj. We were able to identify an intestinal Zot analogue, which we named zonulin. It is conceivable that the zonulins participate in the physiological regulation of intercellular tj not only in the small intestine, but also throughout a wide range of extraintestinal epithelia as well as the ubiquitous vascular endothelium, including the blood-brain barrier. Disregulation of this hypothetical zonulin model may contribute to disease states that involve disordered intercellular communication, including developmental and intestinal disorders, tissue inflammation, malignant transformation, and metastasis.

Zonula adherens formation in Caenorhabditis elegans requires dlg-1, the homologue of the Drosophila gene discs large

The correct assembly of junction components, such as E-cadherin and beta-catenin, into the zonula adherens is fundamental for the function of epithelia, both in flies and in vertebrates. In C. elegans, however, the cadherin-catenin system is not essential for general adhesion, raising the question as to the genetic basis controlling junction morphogenesis in nematodes. Here we show that dlg-1, the C. elegans homologue of the Drosophila tumour-suppressor gene discs-large, plays a crucial role in epithelial development. DLG-1 is restricted to adherens junctions of all embryonic epithelia, which contrasts with the localisation of the Drosophila and vertebrate homologues in septate and tight junctions, respectively. Proper localisation of DLG-1 requires the basolateral LET-413 protein, but is independent of the cadherin-catenin system. Embryos in which dlg-1 activity was eliminated by RNA-mediated interference fail to form a continuous belt of junction-associated antigens and arrest development. Loss of dlg-1 activity differentially affects localisation of proteins normally enriched apically to the zonula adherens. While the distribution of an atypical protein kinase C (PKC-3) and other cytoplasmic proteins (PAR-3, PAR-6) is not affected in dlg-1 (RNAi) embryos, the transmembrane protein encoded by crb-1, the C. elegans homologue of Drosophila crumbs, is no longer concentrated in this domain. In contrast to Drosophila, however, crb-1 and a second crb-like gene are not essential for epithelial development in C. elegans. Together the data indicate that several aspects of the spatial organisation of epithelial cells and its genetic control differ between flies, worms, and vertebrates, while others are conserved. The molecular nature of DLG-1 makes it a likely candidate to participate in the organisation of a protein scaffold that controls the assembly of junction components into the zonula adherens.

Localization of the Drosophila MAGUK protein Polychaetoid is controlled by alternative splicing

The Drosophila membrane-associated guanylate kinase (MAGUK) protein Polychaetoid (Pyd) is required for dorsal closure of the embryo, sensory organ patterning, and cell fate specification in the developing eye. We demonstrate that pyd is alternatively spliced resulting in two isoforms that differ by the presence or absence of exon 6. To determine the role of alternative splicing in Pyd function, we generated antibodies specific for each isoform. We find that the exon 6(+) form of Pyd is localized at adherens junctions of embryonic and imaginal epithelia, while the exon 6(-) form is distributed broadly along the lateral membrane. These results suggest that localization of Pyd is controlled by alternative splicing and raise the possibility that exon 6 represents a distinct protein-protein interaction domain.

Suppression of the tumorigenicity of mutant p53-transformed rat embryo fibroblasts through expression of a newly cloned rat nonmuscle myosin heavy chain-B

In our previous study, a rat homolog of human nonmuscle myosin heavy chain-B (nmMHC-B) was identified by mRNA differential display comparing of transformed against nontransformed Rat 6 cells overexpressing mutant p53val135 gene. The nmMHC-B was found to be expressed in normal Rat 6 embryo fibroblast cell line, but markedly suppressed in the mutant p53val135-transformed Rat 6 cells. To examine the possible involvement of nmMHC-B in cell transformation, we first cloned and sequenced the full length cDNA of rat nmMHC-B, which was then cloned into an ecdysone-expression vector. The resulting construct was introduced into the T2 cell line, a mutant p53val135-transformed Rat 6 cells lacking the expression of the endogenous nmMHC-B. The clonal transfectants, expressing muristerone A-induced nmMHC-B, displayed a slightly flatter morphology and reached to a lower saturation density compared to the parental transformed cells. Reconstitution of actin filamental bundles was also clearly seen in cells overexpressing the nmMHC-B. In soft agar assays, nmMHC-B transfectants formed fewer and substantially smaller colonies than the parental cells in response to muristerone A induction. Moreover, it was strikingly effective in suppressing the tumorigenicity of the T2 cells when tested in nude mice. Thus, the nmMHC-B, known as a component of the cytoskeletal network, may act as a tumor suppressor gene. Our current finding may reveal a novel role of nmMHC-B in regulating cell growth and cell signaling in nonmuscle cells. Oncogene (2001) 20, 58 - 68.

Altered expression of the ERM proteins in lung adenocarcinoma

Radixin is a member of the ERM (ezrin/radixin/moesin) protein family that is proposed to function as a membrane-cytoskeletal linker. Using differential display analysis, we have identified radixin as a gene down-regulated in primary lung adenocarcinoma. Real-time quantitative reverse transcription polymerase chain reaction confirmed that radixin mRNA was decreased, both in 10 early-stage bronchioloalveolar carcinomas and in 16 invasive lung adenocarcinomas, by 69% (p = 0.0002) and 82% (p < 0.0001), respectively, compared with 9 nontumor lung tissues. Similarly, moesin and ezrin mRNA levels were reduced in lung adenocarcinoma. Immunohistochemistry confirmed that cancer cells expressed very little radixin and moesin, whereas non-neoplastic alveolar and bronchiolar epithelial cells, and endothelial cells, including those within the tumor stroma, were consistently positive for these two proteins. Ezrin was localized in the apical surface of non-neoplastic bronchiolar and alveolar epithelial cells and, in contrast to radixin and moesin, the majority of tumor cells retained expression of ezrin. Localization of ezrin was altered in a significant proportion of tumor cells: whereas tumor cells forming lumina displayed membranous staining on the apical side, tumor cells with disorganized structures were either negative or diffusely positive for ezrin in the cytoplasm. Furthermore, a fraction of tumor cells invading the stroma in a scattered manner were strongly positive for ezrin. In conclusion, expression of radixin and moesin is down-regulated in lung adenocarcinoma, including early-stage bronchioloalveolar carcinoma. An intriguing implication of this finding is that these two genes may function as tumor suppressors in lung adenocarcinoma oncogenesis. Although structurally related to radixin and moesin, ezrin may have a distinct function in tumor-cell invasion.

Organization and expression of the human zo-2 gene (tjp-2) in normal and neoplastic tissues

One of the tight junction components, zonula occludens protein 2 (ZO-2), is expressed as two isoforms, ZO-2A and ZO-2C, in normal epithelia. In pancreatic adenocarcinoma of the ductal type ZO-2A is absent, but none of the common mechanisms of gene inactivation is responsible for lack of ZO-2A expression. In the current study, we report the complete organization of the human zo-2 gene (tjp-2), its alternative splicing, and its expression in normal and neoplastic tissues of several organ sites. In addition to pancreatic adenocarcinoma, ZO-2 was found to be de-regulated in breast adenocarcinoma, but not in colon or prostate adenocarcinoma. The latter are considered to be of acinar rather than ductal type. Thus, our data indicate the importance of zo-2 (tjp-2) gene regulation in ductal cancer development and should help to understand the defects of intercellular interactions, critical for suppressing the malignant phenotype.

Protein kinase C activation leads to dephosphorylation of occludin and tight junction permeability increase in LLC-PK1 epithelial cell sheets

Activation of protein kinase C by exposure of LLC-PK1 renal epithelial cells to 10(−7) M TPA, a tumor promoting phorbol ester, results in a rapid and sustained increase in paracellular permeability as evidenced by a decrease in transepithelial electrical resistance. Occludin, the first identified transmembrane protein to be localized to the tight junction of both epithelial and endothelial cells is thought play an important role in tight junction barriers. Although transepithelial electrical resistance fell to less than 20% of initial values within 1 hour of TPA exposure, transmission electron microscopy showed no change in the gross morphology of the tight junction of cells treated with 10(−7) M TPA for up to 2 hours. Immunofluorescence microscopy revealed a more rapid change in the membrane distribution of ZO-1 compared to occludin in the TPA-treated cells. Immunoblot analysis indicated that occludin levels in total cell lysates as well as cytosolic, membrane (Triton-X soluble) and cytoskeletal (Triton-X insoluble) fractions remained unchanged for at least 2 hours in cells treated with 10(−7) M TPA compared to their corresponding control cells. As the phosphorylation state of occludin is thought to be important in both tight junction assembly and regulation, the effect of phorbol ester treatment on the phosphorylation of occludin was investigated. Surprisingly, activation of protein kinase C with 10(−7) M TPA resulted in a time-dependent decrease in threonine phosphorylation of occludin which correlated closely with the rapid decrease in transepithelial electrical resistance. This dephosphorylation of occludin, occurring after activation of a serine/threonine kinase by TPA, suggested that protein kinase C was not acting directly on this tight junction target protein. If occludin dephosphorylation is involved in increasing tight junction permeability, then protein kinase C is apparently further upstream in the signaling pathway regulating epithelial barrier function, with a downstream serine/threonine phosphatase acting upon occludin.

Fibroblast transgelin and smooth muscle SM22alpha are the same protein, the expression of which is down-regulated in many cell lines

In this report we investigate the expression and relationship of transgelin (Tg), a transformation and shape-change sensitive actin gelling protein found in fibroblasts and smooth muscle [Shapland et al., 1988: J. Cell. Biol. 107:153-161; Shapland et al., 1993: J. Cell. Biol. 121:1065-1073], to SM22alpha, a smooth muscle protein of unknown function [Lees-Millar et al., 1987: J. Biol. Chem. 262:2988-2993; Solway et al., 1995: J. Biol. Chem. 270:13460-13469]. To clarify their relationship we have cloned and sequenced the cDNA encoding Tg from cultures of rat embryo fibroblasts. The sequences of fibroblast Tg and the smooth muscle isoform SM22 are identical [Prinjha et al., 1994: Cell Motil. Cytoskeleton 28:243-255; Shanahan et al., 1993: Circ. Res. 73:193-204; Solway et al., 1995]. These data, coupled with our immunoblot analysis and previous observations [Shapland et al., 1988; Shapland et al., 1993], demonstrate that Tg expression is not restricted to smooth muscle since this protein is also present in normal mesenchymal cells. However, we also show that Tg, although present in secondary cultures of mouse and rat embryo fibroblasts, is absent in many apparently normal fibroblast cell lines. Tg down-regulation may therefore be an early and sensitive marker for the onset of transformation. A functional role for Tg is unlikely to directly involve Ca2+ since it neither contains a functional EF hand nor binds 45Ca2+.

The tight junction protein ZO-1 and an interacting transcription factor regulate ErbB-2 expression

Epithelial tight junctions regulate paracellular diffusion and restrict the intermixing of apical and basolateral plasma membrane components. We now identify a Y-box transcription factor, ZONAB (ZO-1-associated nucleic acid-binding protein), that binds to the SH3 domain of ZO-1, a submembrane protein of tight junctions. ZONAB localizes to the nucleus and at tight junctions, and binds to sequences of specific promoters containing an inverted CCAAT box. In reporter assays, ZONAB and ZO-1 functionally interact in the regulation of the ErbB-2 promoter in a cell density-dependent manner. In stably transfected overexpressing cells, ZO-1 and ZONAB control expression of endogenous ErbB-2 and function in the regulation of paracellular permeability. These data indicate that tight junctions directly participate in the control of gene expression and suggest that they function in the regulation of epithelial cell differentiation.

Dissociation of intra- and extracellular domains of desmosomal cadherins and E-cadherin in Hailey-Hailey disease and Darier's disease

In order to clarify the pathomechanism of acantholysis in Hailey-Hailey disease (HHD) and Darier's disease (DD), the distribution of desmosomal and adherens junction-associated proteins was studied in the skin of patients with HHD (n = 4) and DD (n = 3). Domain-specific antibodies were used to determine the cellular localization of the desmosomal transmembrane glycoproteins (desmogleins 1 and 3 and desmocollin), desmosomal plaque proteins (desmoplakin, plakophilin and plakoglobin) and adherens junction-associated proteins (E-cadherin, alpha-catenin, beta-catenin and actin). A significant difference in staining patterns between intra- and extracellular domains of desmosomal cadherins and E-cadherin was demonstrated in acantholytic cells in both HHD and DD, but not in those in pemphigus vulgaris and pemphigus foliaceus samples used as controls. In acantholytic cells in HHD and DD, antibodies against attachment plaque proteins and intracellular epitopes of desmosomal cadherins exhibited diffuse cytoplasmic staining, whereas markedly reduced staining was observed with antibodies against extracellular epitopes of the desmogleins. Similarly, membrane staining of an intracellular epitope of E-cadherin was preserved, while immunoreactivity of an extracellular epitope of E-cadherin was destroyed. While the DD gene has been identified as ATP2A2, the gene for HHD has not been clarified. The dissociation of intra- and extracellular domains of desmosomal cadherin and E-cadherin is characteristic of the acantholytic cells in HHD and DD, and not of pemphigus. This common phenomenon in HHD and DD might be closely related to the pathophysiological mechanisms in both conditions.

zo-2 gene alternative promoters in normal and neoplastic human pancreatic duct cells

We have observed that 2 forms of zonula occludens 2 (ZO-2) protein, ZO-2A and ZO-2C, are expressed in normal human pancreatic duct cells, but only ZO-2C in pancreatic duct adenocarcinoma. We report here partial organization of the zo-2 gene. Transcription of 2 forms of ZO-2 mRNA is driven by alternative promoters P(A) and P(C). Lack of expression of ZO-2A in neoplastic cells is caused by inactivation of the downstream promoter P(A). Analysis of the promoter P(A) sequence and function in normal and neoplastic cells demonstrated that neither structural changes (mutations) nor a change in the pool of transcription factors is responsible for its inactivation. Although hypermethylation was found in a large number of cancer clones, treatment with 5-aza-2'-deoxycytidine did not fully cause the promoter function to recover. We conclude that the initial down-regulation of zo-2 promoter P(A) activity in pancreatic duct carcinomas is due to the structural or functional alteration(s) in the regulatory elements, localized outside the analyzed promoter region. Methylation of P(A) is responsible for the inactivation of the suppressed promoter at the late stages of tumor development.

Vascular endothelial (VE)-cadherin: only an intercellular glue?

Data collected during the past years indicate that AJ- and more specifically VE-cadherin play an important role in endothelial cell biology. VE-cadherin may transfer information intracellularly through interaction with a complex network of cytoskeletal and signaling molecules. Expression of VE-cadherin is required for the control of vascular permeability and vascular integrity. In addition, the molecule may exert a morphogenetic role modulating the capacity of endothelial cells to organize into tubular-like structures. VE-cadherin presents many structural and sequence homologies to the other members of the family and apparently binds to the same intracellular molecules. However, remarkably, VE-cadherin may transfer specific signals to endothelial cells to modulate their functional reactivity.

A colorectal cell line with alterations in E-cadherin and epithelial biology may be an in vitro model of colitis

BACKGROUND

It has been shown previously in ulcerative colitis tissue that E-cadherin can occasionally be mutated in the extracellular domain early in neoplastic progression. E-cadherin is known to maintain differentiation and inhibits invasion in vivo.

AIMS

To assess the mechanisms by which such dysfunction occurs.

METHODS

Four human colorectal cancer cell lines, HCA-7 colonies 1, 3, 6, and 30, derived from a single heterogeneous colorectal cancer were studied. The HCA-7 cell line has p53 mutations and a random errors of replication "positive" phenotype, as is seen in early colitis associated cancers or hereditary nonpolyposis coli cancer (HNPCC).

RESULTS

Cell lines 6 and 30 expressed E-cadherin abundantly and this correlated positively with their degree of differentiation and organisation; however, both cell lines had loss of heterozygosity of E-cadherin. Interestingly, E-cadherin production was downregulated in the poorly differentiated cell line 1, and this was associated with major chromosomal rearrangements of 16q. This cell line also had a mutation in the homophilic binding domain of exon 4, which was associated with disaggregation by low titres of a function blocking antibody, and an invasive phenotype.

CONCLUSIONS

These multiple biological alterations further characterise the complex association that E-cadherin has with tumour heterogeneity and suggest that this series of cell lines may be a useful model of colitis associated or HNPCC associated tumorigenesis.

Tight junction protein ZO-2 is differentially expressed in normal pancreatic ducts compared to human pancreatic adenocarcinoma

Differential display of hamster mRNA identified a fragment present in normal pancreatic duct cells that is not expressed in pancreatic duct carcinoma cells. Sequence analysis showed an 88% and 82% identity, respectively, to the cDNA of the canine and human tight junction zo-2 gene. Semi-quantitative RT-PCR analysis of human ZO-2 revealed a striking difference in the expression of various regions of the ZO-2 transcript in normal and neoplastic cells and the presence of an abnormality at the 5'-end of mRNA. RACE analysis identified 2 human ZO-2 mRNAs that encode proteins of different lengths, designated as ZO-2A and ZO-2C. The difference between the 2 forms of ZO-2 is the absence of 23 amino acid residues at the N terminus of ZO-2C compared with ZO-2A. Although ZO-2C was expressed in normal pancreatic cells and a majority of neoplastic tissues analyzed, ZO-2A was undetectable except in one case in all of the pancreatic adenocarcinomas analyzed. This suggests the presence of a yet to be identified motif important for cell-growth regulation within the 23-amino acid residue N-terminal peptide of ZO-2A, MPVRGDRGFPPRRELSGWLRAPG.

Developmental expression of ZO-1 antigen in the mouse blood-brain barrier

Tight junction biogenesis during blood-brain barrier development (BBB) in mesencephalon microvessels of mouse embryos of day 9, foetuses of day 15 and 19 and new-born (2-day-old) mice was examined by light and electron microscopy, using monoclonal antibodies recognizing the tight junction peripheral membrane protein ZO-1. A faint spot-like staining began to be recognizable under the light microscope in day 15 vessels in which the endothelial cells showed isolated fusion points between the external plasma membrane leaflets under the electron microscope. A stronger labelling was present in microvessels of day 19 foetuses and new-born animals when the endothelial tight junction appeared completely differentiated. In the immunogold study, gold particles were seen scattered throughout the cytoplasm of endothelial cells of day 15 foetuses. In day 19 foetuses and in the new-born mice, gold particles were located only at the cytoplasmic surfaces of the tight junctions. The results indicate that the ZO-1 protein is a specific molecular marker in the developing brain endothelial tight junctions and that its expression takes place parallel to BBB morphofunctional maturation.

A unique talin homologue with a villin headpiece-like domain is required for multicellular morphogenesis in Dictyostelium

Molecules involved in the interaction between the extracellular matrix, cell membrane and cytoskeleton are of central importance in morphogenesis. Talin is a large cytoskeletal protein with a modular structure consisting of an amino-terminal membrane-interacting domain, with sequence similarities to members of the band 4.1 family, and a carboxy-terminal region containing F-actin-binding and vinculin-binding domains [1] [2]. It also interacts with the cytoplasmic tail of beta integrins which, on the external face of the membrane, bind to extracellular matrix proteins [3]. The possible roles of talin in multicellular morphogenesis in development remain largely unexplored. In Dictyostelium, a eukaryotic microorganism capable of multicellular morphogenesis, a talin homologue (TALA) has previously been identified and shown to play an important role in cell-to-substrate adhesion and maintenance of normal elastic properties of the cell [4] [5] [6]. Here, we describe a second talin homologue (TALB) that is required for multicellular morphogenesis in the development of Dictyostelium. Unlike any other talin characterised to date, it contains an additional carboxy-terminal domain homologous to the villin headpiece.

Functional domains of alpha-catenin required for the strong state of cadherin-based cell adhesion

The interaction of cadherin-catenin complex with the actin-based cytoskeleton through alpha-catenin is indispensable for cadherin-based cell adhesion activity. We reported previously that E-cadherin-alpha-catenin fusion molecules showed cell adhesion and cytoskeleton binding activities when expressed in nonepithelial L cells. Here, we constructed deletion mutants of E-cadherin-alpha-catenin fusion molecules lacking various domains of alpha-catenin and introduced them into L cells. Detailed analysis identified three distinct functional domains of alpha-catenin: a vinculin/alpha-actinin-binding domain, a ZO-1-binding domain, and an adhesion-modulation domain. Furthermore, cell dissociation assay revealed that the fusion molecules containing the ZO-1-binding domain in addition to the adhesion-modulation domain conferred the strong state of cell adhesion activity on transfectants, although those lacking the ZO-1-binding domain conferred only the weak state. The disorganization of actin-based cytoskeleton by cytochalasin D treatment shifted the cadherin-based cell adhesion from the strong to the weak state. In the epithelial cells, where alpha-catenin was not precisely colocalized with ZO-1, the ZO-1-binding domain did not completely support the strong state of cell adhesion activity. Our studies showed that the interaction of alpha-catenin with the actin-based cytoskeleton through the ZO-1-binding domain is required for the strong state of E-cadherin-based cell adhesion activity.

Characterization of ZO-2 as a MAGUK family member associated with tight as well as adherens junctions with a binding affinity to occludin and alpha catenin

ZO-2, a member of the MAGUK family, was thought to be specific for tight junctions (TJs) in contrast to ZO-1, another MAGUK family member, which is localized at TJs and adherens junctions (AJs) in epithelial and nonepithelial cells, respectively. Mouse ZO-2 cDNA was isolated, and a specific polyclonal antibody was generated using corresponding synthetic peptides as antigens. Immunofluorescence microscopy with this polyclonal antibody revealed that, similarly to ZO-1, in addition to TJs in epithelial cells, ZO-2 was also concentrated at AJs in nonepithelial cells such as fibroblasts and cardiac muscle cells lacking TJs. When NH2-terminal dlg-like and COOH-terminal non-dlg-like domains of ZO-2 (N-ZO-2 and C-ZO-2, respectively) were separately introduced into cultured cells, N-ZO-2 was colocalized with endogenous ZO-1/ZO-2, i.e. at TJs in epithelial cells and at AJs in non-epithelial cells, whereas C-ZO-2 was distributed along actin filaments. Consistently, occludin as well as alpha catenin directly bound to N-ZO-2 as well as the NH2-terminal dlg-like portion of ZO-1 (N-ZO-1) in vitro. Furthermore, immunoprecipitation experiments revealed that the second PDZ domain of ZO-2 was directly associated with N-ZO-1. These findings indicated that ZO-2 forms a complex with ZO-1/occludin or ZO-1/alpha catenin to establish TJ or AJ domains, respectively.

Target genes of beta-catenin-T cell-factor/lymphoid-enhancer-factor signaling in human colorectal carcinomas

Mutations in the adenomatous polyposis coli or beta-catenin gene lead to cytosolic accumulation of beta-catenin and, subsequently, to increased transcriptional activity of the beta-catenin-T cell-factor/lymphoid-enhancer-factor complex. This process seems to play an essential role in the development of most colorectal carcinomas. To identify genes activated by beta-catenin overexpression, we used colorectal cell lines for transfection with the beta-catenin gene and searched for genes differentially expressed in the transfectants. There are four genes affected by beta-catenin overexpression; three overexpressed genes code for two components of the AP-1 transcription complex, c-jun and fra-1, and for the urokinase-type plasminogen activator receptor (uPAR), whose transcription is activated by AP-1. The direct interaction of the beta-catenin-T cell-factor/lymphoid-enhancer-factor complex with the promoter region of c-jun and fra-1 was shown in a gel shift assay. The concomitant increase in beta-catenin expression and the amount of uPAR was confirmed in primary colon carcinomas and their liver metastases at both the mRNA and the protein levels. High expression of beta-catenin in transfectants, as well as in additionally analyzed colorectal cell lines, was associated with decreased expression of ZO-1, which is involved in epithelial polarization. Thus, accumulation of beta-catenin indirectly affects the expression of uPAR in vitro and in vivo. Together with the other alterations, beta-catenin accumulation may contribute to the development and progression of colon carcinoma both by dedifferentiation and through proteolytic activity.

p27 is involved in N-cadherin-mediated contact inhibition of cell growth and S-phase entry

In this study the direct involvement of cadherins in adhesion-mediated growth inhibition was investigated. It is shown here that overexpression of N-cadherin in CHO cells significantly suppresses their growth rate. Interaction of these cells and two additional fibroblastic lines with synthetic beads coated with N-cadherin ligands (recombinant N-cadherin ectodomain or specific antibodies) leads to growth arrest at the G1 phase of the cell cycle. The cadherin-reactive beads inhibit the entry into S phase and the reduction in the levels of cyclin-dependent kinase (cdk) inhibitors p21 and p27, following serum-stimulation of starved cells. In exponentially growing cells these beads induce G1 arrest accompanied by elevation in p27 only. We propose that cadherin-mediated signaling is involved in contact inhibition of growth by inducing cell cycle arrest at the G1 phase and elevation of p27 levels.

Claudin multigene family encoding four-transmembrane domain protein components of tight junction strands

Two related integral membrane proteins, claudin-1 and -2, recently were identified as novel components of tight junction (TJ) strands. Here, we report six more claudin-like proteins, indicating the existence of a claudin gene family. Three of these were reported previously as RVP1, Clostridium perfringens enterotoxin receptor, and TMVCF, but their physiological functions were not determined. Through similarity searches followed by PCR, we isolated full length cDNAs of mouse RVP1, Clostridium perfringens enterotoxin receptor, and TMVCF as well as three mouse claudin-like proteins and designated them as claudin-3 to -8, respectively. All of these claudin family members showed similar patterns on hydrophilicity plots, which predicted four transmembrane domains in each molecule. Northern blotting showed that the tissue distribution pattern varied significantly, depending on claudin species. Similarly to claudin-1 and -2, when these claudins were HA-tagged and introduced into cultured Madin-Darby canine kidney cells, all showed a tendency to concentrate at TJs. Immunofluorescence and immunoelectron microscopy with polyclonal antibodies specific for claudin-3, -4, or -8 revealed that these molecules were exclusively concentrated at TJs in the liver and/or kidney. These findings indicated that multiple claudin family members are involved in the formation of TJ strands in various tissues.

The tight junction: morphology to molecules

The tight junction forms a regulated barrier in the paracellular pathway between epithelial and endothelial cells. This intercellular junction also demarcates the compositionally distinct apical and basolateral membranes. While the existence of a paracellular barrier in epithelia was hypothesized by physiologists over a century ago, the molecular characterization of the tight junction is a relatively new and rapidly expanding area of research. It is now recognized that the tight junction is comprised of at least nine peripheral and one integral membrane proteins. This complex includes members of a protein family related to tumor suppression and signal transduction, a rab protein, and a Ras target protein. The characteristics of, interactions between, and potential physiological roles of these proteins at the tight junction are discussed.

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.

The spatial organization of apical junctional complex-associated proteins in feline and human corneal endothelium

PURPOSE

Previous studies suggest that proteins associated with the apical junctional complex (AJC) play essential roles in the development, maintenance and regulation of barrier function in transport epithelium and vascular endothelium. The goal of this study is to identify and determine the spatial organization of several major AJC-associated proteins in normal human and feline corneal endothelium.

METHODS

Fresh corneal tissue was obtained from 4 recipient buttons removed during penetrating keratoplasty (two from keratoconus patients, and two from patients with post-traumatic stromal scarring) as well as from 16 cat eyes. En bloc double- and triple-labeling of corneas was performed using phalloidin, and mouse, rat or rabbit antibodies to ZO-1, occludin, pan-cadherin, alpha-catenin, beta-catenin and plakoglobin (gamma-catenin). The 3-D localization of the proteins was then determined in situ using laser confocal microscopy.

RESULTS

Similar staining patterns were obtained for the corneal endothelium of normal cat corneas and fresh human buttons. Apically, f-actin was arranged into dense peripheral bands (DPB) in individual cells that were separated from those in adjacent cells. Diffuse phalloidin staining also extended from the DPB into the cytoplasm apically. Although weaker, phalloidin staining also appeared to be associated with the basolateral cell borders. The adherens junction protein, cadherin, formed a thin pericellular band at the apical cell junctions between the DPB. In addition, cadherin staining also appeared to extend along the basolateral cell borders in a convoluted pattern. Staining for alpha-catenin, beta-catenin and plakoglobin each showed a nearly identical organization as cadherin. ZO-1 formed a single apical band that was localized between the DPB; however, no basolateral ZO-1 staining was detected. Interestingly, the distribution of ZO-1 was discontinuous around the cell, with the largest gaps occurring at the Y-junctions between adjacent endothelial cells. Positive staining for occludin was not detected in either human or feline corneal endothelium.

CONCLUSIONS

The composition and organization of the AJC of corneal endothelium appears to be different from that of classical transport epithelia; these findings may be related to functional differences between these two cell types.

Novel approaches for oral delivery of macromolecules

Traditional forms of administrations of nonabsorbable drugs and peptides often rely on their parenteral injection, since the intestinal epithelium is poorly permeable to these therapeutical agents. A number of innovative drug delivery approaches have been recently developed, including the drug entrapment within small vesicles or their passage through the intestinal paracellular pathway. Zonula occludens toxin, a recently discovered protein elaborated by Vibrio cholerae, provided tools to gain more insights on the pathophysiology of the regulation of intestinal permeability through the paracellular pathway and to develop alternative approaches for the oral delivery of drugs and macromolecules normally not absorbed through the intestine.

E-cadherin and catenins: molecules with versatile roles in normal and neoplastic epithelial cell biology

E-cadherin and its associated cytoplasmic proteins alpha-, beta-, and gamma-catenin, play a crucial role in epithelial cell-cell adhesion and in the maintenance of tissue architecture. Perturbation in the expression or function of any of these molecules results in loss of intercellular adhesion, with possible consequent cell transformation and tumour progression. The catenins are connected to many structural and functional proteins, which in turn influence their functions. Among these molecules are type 1 growth factor receptors, which along with other molecules are believed to alter the function of catenins through tyrosine phosphorylation. A recent finding is the association between the catenins and the adenomatous polyposis coli gene product (APC). APC mutation is an early event in colorectal carcinogenesis. It may possibly do so through perturbation of the critical cadherin/catenin complex. Further studies of the cadherin/catenin complex and its connections may give insight into the early molecular interactions critical to the initiation and progression oftumours, which should aid in the development of novel therapeutic strategies for both prevention and treatment.