Requirement of nectin, but not cadherin, for formation of claudin-based tight junctions in annexin II-knockdown MDCK cells

Diagnostic and Prognostic Roles of Urine Nectin-2 and Nectin-4 in Human Bladder Cancer

The clinical utility of urine nectins in bladder cancer (BCa) is unclear. We investigated the potential diagnostic and prognostic values of urine Nectin-2 and Nectin-4. Levels of urine Nectin-2, Nectin-4, and NMP-22 were quantified using an enzyme-linked immunosorbent assay in 122 patients with BCa, consisting of 78 with non-muscle-invasive BCa (NMIBC) and 44 with muscle-invasive BCa (MIBC), and ten healthy controls. Tumor nectin expression in MIBC was evaluated with immunohistochemical staining of transurethral resection specimens. The level of urine Nectin-4 (mean: 18.3 ng/mL) was much higher than that of urine Nectin-2 (mean: 0.40 ng/mL). The sensitivities of Nectin-2, Nectin-4, NMP-22, and cytology assays were 84%, 98%, 52%, and 47%, respectively; their specificities were 40%, 80%, 100%, and 100%, respectively. Both urine Nectin-2 and Nectin-4, though not NMP-22, were found to be significantly more sensitive than cytology. A four-titer grouping based on levels of urine Nectin-2/Nectin-4 (low/high, high/high, low/low, and high/low) showed a high capability for discriminating between NMIBC and MIBC. Neither urine Nectin-2 nor Nectin-4 levels had a significant prognostic value in NMIBC or MIBC. Urine levels correlated with tumor expression and serum levels in the Nectin-4 analysis, but not in the Nectin-2 analysis. Urine nectins are potential diagnostic biomarkers for BCa.

Identification of lysophosphatidic acid in serum as a factor that promotes epithelial apical junctional complex organization

The apical junctional complex (AJC) consists of adherens junctions (AJs) and tight junctions and regulates epithelial integrity and remodeling. However, it is unclear how AJC organization is regulated based on environmental cues. We found here using cultured EpH4 mouse mammary epithelial cells that fetal bovine serum (FBS) in a culture medium showed an activity to promote AJC organization and that FBS showed an activity to promote tight junction formation even in the absence of AJ proteins, such as E-cadherin, αE-catenin, and afadin. Furthermore, we purified the individual factor responsible for these functions from FBS and identified this molecule as lysophosphatidic acid (LPA). In validation experiments, purified LPA elicited the same activity as FBS. In addition, we found that the AJC organization–promoting activity of LPA was mediated through the LPA receptor 1/5 via diacylglycerol–novel PKC and Rho–ROCK pathway activation in a mutually independent, but complementary, manner. We demonstrated that the Rho–ROCK pathway activation–mediated AJC organization was independent of myosin II-induced actomyosin contraction, although this signaling pathway was previously shown to induce myosin II activation. These findings are in contrast to the literature, as previous results suggested an AJC organization–disrupting activity of LPA. The present results indicate that LPA in serum has an AJC organization–promoting activity in a manner dependent on or independent of AJ proteins.

Role of Nectin‑4 protein in cancer (Review)

The Nectin cell adhesion molecule (Nectin) family members are Ca2+‑independent immunoglobulin‑like cellular adhesion molecules (including Nectins 1‑4), involved in cell adhesion via homophilic/heterophilic interplay. In addition, the Nectin family plays a significant role in enhancing cellular viability and movement ability. In contrast to enrichment of Nectins 1‑3 in normal tissues, Nectin‑4 is particularly overexpressed in a number of tumor types, including breast, lung, urothelial, colorectal, pancreatic and ovarian cancer. Moreover, the upregulation of Nectin‑4 is an independent biomarker for overall survival in numerous cancer types. A large number of studies have revealed that high expression of Nectin‑4 is closely related to tumor occurrence and development in various cancer types, but the manner in which Nectin‑4 protein contributes to the onset and development of these malignancies is yet unknown. The present review summarizes the molecular mechanisms and functions of Nectin‑4 protein in the biological processes and current advances with regard to its expression and regulation in various cancer types.

Effects of ozone and particulate matter on airway epithelial barrier structure and function: a review of in vitro and in vivo studies

The airway epithelium represents a crucial line of defense against the spread of inhaled pathogens. As the epithelium is the first part of the body to be exposed to the inhaled environment, it must act as both a barrier to and sentinel against any inhaled agents. Despite its vital role in limiting the spread of inhaled pathogens, the airway epithelium is also regularly exposed to air pollutants which disrupt its normal function. Here we review the current understanding of the structure and composition of the airway epithelial barrier, as well as the impact of inhaled pollutants, including the reactive gas ozone and particulate matter, on epithelial function. We discuss the current in vitro, rodent model, and human exposure findings surrounding the impact of various inhaled pollutants on epithelial barrier function, mucus production, and mucociliary clearance. Detailed information on how inhaled pollutants impact epithelial structure and function will further our understanding of the adverse health effects of air pollution exposure.

The Regulation of Intestinal Mucosal Barrier by Myosin Light Chain Kinase/Rho Kinases

The intestinal epithelial apical junctional complex, which includes tight and adherens junctions, contributes to the intestinal barrier function via their role in regulating paracellular permeability. Myosin light chain II (MLC-2), has been shown to be a critical regulatory protein in altering paracellular permeability during gastrointestinal disorders. Previous studies have demonstrated that phosphorylation of MLC-2 is a biochemical marker for perijunctional actomyosin ring contraction, which increases paracellular permeability by regulating the apical junctional complex. The phosphorylation of MLC-2 is dominantly regulated by myosin light chain kinase- (MLCK-) and Rho-associated coiled-coil containing protein kinase- (ROCK-) mediated pathways. In this review, we aim to summarize the current state of knowledge regarding the role of MLCK- and ROCK-mediated pathways in the regulation of the intestinal barrier during normal homeostasis and digestive diseases. Additionally, we will also suggest potential therapeutic targeting of MLCK- and ROCK-associated pathways in gastrointestinal disorders that compromise the intestinal barrier.

Understanding the Molecular and Cell Biological Mechanisms of Electrical Synapse Formation

In this review article, we will describe the recent advances made towards understanding the molecular and cell biological mechanisms of electrical synapse formation. New evidence indicates that electrical synapses, which are gap junctions between neurons, can have complex molecular compositions including protein asymmetries across joined cells, diverse morphological arrangements, and overlooked similarities with other junctions, all of which indicate new potential roles in neurodevelopmental disease. Aquatic organisms, and in particular the vertebrate zebrafish, have proven to be excellent models for elucidating the molecular mechanisms of electrical synapse formation. Zebrafish will serve as our main exemplar throughout this review and will be compared with other model organisms. We highlight the known cell biological processes that build neuronal gap junctions and compare these with the assemblies of adherens junctions, tight junctions, non-neuronal gap junctions, and chemical synapses to explore the unknown frontiers remaining in our understanding of the critical and ubiquitous electrical synapse.

Reciprocal Association between the Apical Junctional Complex and AMPK: A Promising Therapeutic Target for Epithelial/Endothelial Barrier Function?

Epithelial/endothelial cells adhere to each other via cell–cell junctions including tight junctions (TJs) and adherens junctions (AJs). TJs and AJs are spatiotemporally and functionally integrated, and are thus often collectively defined as apical junctional complexes (AJCs), regulating a number of spatiotemporal events including paracellular barrier, selective permeability, apicobasal cell polarity, mechano-sensing, intracellular signaling cascades, and epithelial morphogenesis. Over the past 15 years, it has been acknowledged that adenosine monophosphate (AMP)-activated protein kinase (AMPK), a well-known central regulator of energy metabolism, has a reciprocal association with AJCs. Here, we review the current knowledge of this association and show the following evidences: (1) as an upstream regulator, AJs activate the liver kinase B1 (LKB1)–AMPK axis particularly in response to applied junctional tension, and (2) TJ function and apicobasal cell polarization are downstream targets of AMPK and are promoted by AMPK activation. Although molecular mechanisms underlying these phenomena have not yet been completely elucidated, identifications of novel AMPK effectors in AJCs and AMPK-driven epithelial transcription factors have enhanced our knowledge. More intensive studies along this line would eventually lead to the development of AMPK-based therapies, enabling us to manipulate epithelial/endothelial barrier function.

Implications of AMPK in the Formation of Epithelial Tight Junctions

Tight junctions (TJ) play an essential role in the epithelial barrier. By definition, TJ are located at the demarcation between the apical and baso-lateral domains of the plasma membrane in epithelial cells. TJ fulfill two major roles: (i) TJ prevent the mixing of membrane components; and (ii) TJ regulate the selective paracellular permeability. Disruption of TJ is regarded as one of the earliest hallmarks of epithelial injury, leading to the loss of cell polarity and tissue disorganization. Many factors have been identified as modulators of TJ assembly/disassembly. More specifically, in addition to its role as an energy sensor, adenosine monophosphate-activated protein kinase (AMPK) participates in TJ regulation. AMPK is a ubiquitous serine/threonine kinase composed of a catalytic α-subunit complexed with regulatory β-and γ-subunits. AMPK activation promotes the early stages of epithelial TJ assembly. AMPK phosphorylates the adherens junction protein afadin and regulates its interaction with the TJ-associated protein zonula occludens (ZO)-1, thereby facilitating ZO-1 distribution to the plasma membrane. In the present review, we detail the signaling pathways up-and down-stream of AMPK activation at the time of Ca2+-induced TJ assembly.

Interplay between tight junctions & adherens junctions

Cell-cell adhesions are critical for the development and maintenance of tissues. Present at sites of cell-cell contact are the adherens junctions and tight junctions. The adherens junctions mediate cell-cell adhesion via the actions of nectins and cadherins. The tight junctions regulate passage of ions and small molecules between cells and establish cell polarity. Historically, the adherens and tight junctions have been thought of as discrete complexes. However, it is now clear that a high level of interdependency exists between the two junctional complexes. The adherens junctions and tight junctions are physically linked, by the zonula occludens proteins, and linked via signaling molecules including several polarity complexes and actin cytoskeletal modifiers. This review will first describe the individual components of both the adherens and tight junctions and then discuss the coupling of the two complexes with an emphasis on the signaling links and physical interactions between the two junctional complexes.

The expression of Nectin-4 on the surface of ovarian cancer cells alters their ability to adhere, migrate, aggregate, and proliferate

The cell adhesion molecule Nectin-4 is overexpressed in epithelial cancers, including ovarian cancer. The objective of this study was to determine the biological significance of Nectin-4 in the adhesion, aggregation, migration, and proliferation of ovarian cancer cells. Nectin-4 and its binding partner Nectin-1 were detected in patients' primary tumors, omental metastases, and ascites cells. The human cell lines NIH:OVCAR5 and CAOV3 were genetically modified to alter Nectin-4 expression. Cells that overexpressed Nectin-4 adhered to Nectin-1 in a concentration and time-dependent manner, and adhesion was inhibited by antibodies to Nectin-4 and Nectin-1, as well as synthetic Nectin peptides. In functional assays, CAOV3 cells with Nectin-4 knock-down were unable to form spheroids and migrated more slowly than CAOV3 parental cells expressing Nectin-4. NIH:OVCAR5 parental cells proliferated more rapidly, migrated faster, and formed larger spheroids than either the Nectin-4 knock-down or over-expressing cells. Parental cell lines expressed higher levels of epithelial markers and lower levels of mesenchymal markers compared to Nectin-4 knock-down cells, suggesting a role for Nectin-4 in epithelial-mesenchymal transition. Our results demonstrate that Nectin-4 promotes cell-cell adhesion, migration, and proliferation. Understanding the biology of Nectin-4 in ovarian cancer progression is critical to facilitate its development as a novel therapeutic target.

The expression of the Nectin complex in human breast cancer and the role of Nectin-3 in the control of tight junctions during metastasis

Introduction

Nectins are a family of integral protein molecules involved in the formation of functioning Adherens and Tight Junctions (TJ). Aberrant expression is associated with cancer progression but little is known how this effects changes in cell behaviour. This study aimed to ascertain the distribution of Nectins-1 to -4 in human breast cancer and the effect on junctional integrity of Nectin-3 modulation in human endothelial and breast cancer cells.

Methods

A human breast tissue cohort was processed for Q-PCR and immunohistochemistry for analysis of Nectin-1/-2/-3/-4. Nectin-3 over-expression was induced in the human breast cancer cell line MDA-MB-231 and the human endothelial cell line HECV. Functional testing was carried out to ascertain changes in cell behaviour.

Results

Q-PCR revealed a distinct reduction in node positive tumours and in patients with poor outcome. There was increased expression of Nectin-1/-2 in patients with metastatic disease, Nectin-3/-4 was reduced. IHC revealed that Nectin-3 expression showed clear changes in distribution between normal and cancerous cells. Nectin-3 over-expression in MDA-MB-231 cells showed reduced invasion and migration even when treated with HGF. Changes in barrier function resulted in MDAN3 cells showing less change in resistance after 2h treatment with HGF (p<0.001). Nectin-3 transformed endothelial cells were significantly more adhesive, irrespective of treatment with HGF (p<0.05) and had reduced growth. Barrier function revealed that transformed HECV cells had significantly tighter junctions that wildtype cells when treated with HGF (p<0.0001). HGF-induced changes in permeability were also reduced. Overexpression of Nectin-3 produced endothelial cells with significantly reduced ability to form tubules (p<0.0001). Immunoprecipitation studies discovered hitherto novel associations for Nectin-3. Moreover, HGF appeared to exert an effect on Nectin-3 via tyrosine and threonine phosphorylation.

Conclusions

Nectin-3 may be a key component in the formation of cell junctions and be a putative suppressor molecule to the invasion of breast cancer cells.

Nectin and junctional adhesion molecule are critical cell adhesion molecules for the apico-basal alignment of adherens and tight junctions in epithelial cells

Tight junctions (TJs) and adherens junctions (AJs) form an apical junctional complex at the apical side of the lateral membranes of epithelial cells, in which TJs are aligned at the apical side of AJs. Many cell adhesion molecules (CAMs) and cell polarity molecules (CPMs) cooperatively regulate the formation of the apical junctional complex, but the mechanism for the alignment of TJs at the apical side of AJs is not fully understood. We developed a cellular system with which epithelial-like TJs and AJs were reconstituted in fibroblasts and analyzed the cooperative roles of CAMs and CPMs. We exogenously expressed various combinations of CAMs and CPMs in fibroblasts that express negligible amounts of these molecules endogenously. In these cells, the nectin-based cell-cell adhesion was formed at the apical side of the junctional adhesion molecule (JAM)-based cell-cell adhesion, and cadherin and claudin were recruited to the nectin-3- and JAM-based cell-cell adhesion sites to form AJ-like and TJ-like domains, respectively. This inversed alignment of the AJ-like and TJ-like domains was reversed by complementary expression of CPMs Par-3, atypical protein kinase C, Par-6, Crb3, Pals1 and Patj. We describe the cooperative roles of these CAMs and CPMs in the apico-basal alignment of TJs and AJs in epithelial cells.

Binding between the junctional proteins afadin and PLEKHA7 and implication in the formation of adherens junction in epithelial cells

Adherens junction (AJ) is a specialized cell-cell junction structure that plays a role in mechanically connecting adjacent cells to resist strong contractile forces and to maintain tissue structure, particularly in the epithelium. AJ is mainly comprised of cell adhesion molecules cadherin and nectin and their associating cytoplasmic proteins including β-catenin, α-catenin, p120(ctn), and afadin. Our series of studies have revealed that nectin first forms cell-cell adhesion and then recruits cadherin to form AJ. The recruitment of cadherin by nectin is mediated by the binding of α-catenin and p120(ctn) to afadin. Recent studies showed that PLEKHA7 binds to p120(ctn), which is associated with E-cadherin, and maintains the integrity of AJ in epithelial cells. In this study, we showed that PLEKHA7 bound to afadin in addition to p120(ctn) and was recruited to the nectin-3α-based cell-cell adhesion site in a manner dependent on afadin, but not on p120(ctn). The binding of PLEKHA7 to afadin was required for the proper formation of AJ, but not for the formation of tight junction, in EpH4 mouse mammary gland epithelial cells. These results indicate that PLEKHA7 plays a cooperative role with nectin and afadin in the proper formation of AJ in epithelial cells.

ZO-1 recruitment to α-catenin--a novel mechanism for coupling the assembly of tight junctions to adherens junctions

The formation of a barrier between epithelial cells is a fundamental determinant of cellular homeostasis, protecting underlying cells against pathogens, dehydration and damage. Assembly of the tight junction barrier is dependent upon neighboring epithelial cells binding to one another and forming adherens junctions, but the mechanism for how these processes are linked is poorly understood. Using a knockdown and substitution system, we studied whether ZO-1 binding to α-catenin is required for coupling tight junction assembly to the formation of adherens junctions. We found that preventing ZO-1 binding to α-catenin did not appear to affect adherens junctions. Rather the assembly and maintenance of the epithelial barrier were disrupted. This disruption was accompanied by alterations in the mobility of ZO-1 and the organization of the actin cytoskeleton. Thus, our study identifies α-catenin binding to ZO-1 as a new mechanism for coupling the assembly of the epithelial barrier to cell-to-cell adhesion.

Role of the cytoskeleton in formation and maintenance of angiogenic sprouts

Angiogenesis is the formation of new blood vessels from pre-existing structures, and is a key step in tissue and organ development, wound healing and pathological events. Changes in cell shape orchestrated by the cytoskeleton are integral to accomplishing the various steps of angiogenesis, and an intact cytoskeleton is also critical for maintaining newly formed structures. This review focuses on how the 3 main cytoskeletal elements--microfilaments, microtubules, and intermediate filaments--regulate the formation and maintenance of angiogenic sprouts. Multiple classes of compounds target microtubules and microfilaments, revealing much about the role of actin and tubulin and their associated molecules in angiogenic sprout formation and maintenance. In contrast, intermediate filaments are much less studied, yet intriguing evidence suggests a vital, but unresolved, role in angiogenic sprouting. This review discusses evidence for regulatory molecules and pharmacological compounds that affect actin, microtubule and intermediate filament dynamics to alter various steps of angiogenesis, including endothelial sprout formation and maintenance.

AMP-activated protein kinase (AMPK) activation and glycogen synthase kinase-3β (GSK-3β) inhibition induce Ca2+-independent deposition of tight junction components at the plasma membrane

Extracellular Ca(2+) is essential for the development of stable epithelial tight junctions. We find that in the absence of extracellular Ca(2+), AMP-activated protein kinase (AMPK) activation and glycogen synthase kinase (GSK)-3β inhibition independently induce the localization of epithelial tight junction components to the plasma membrane. The Ca(2+)-independent deposition of junctional proteins induced by AMPK activation and GSK-3β inhibition is independent of E-cadherin. Furthermore, the nectin-afadin system is required for the deposition of tight junction components induced by AMPK activation, but it is not required for that induced by GSK-3β inhibition. Phosphorylation studies demonstrate that afadin is a substrate for AMPK. These data demonstrate that two kinases involved in regulating cell growth and metabolism act through distinct pathways to influence the deposition of the components of epithelial tight junctions.

Nectin 4 overexpression in ovarian cancer tissues and serum: potential role as a serum biomarker

Early detection of ovarian cancer is difficult owing to the lack of specific and sensitive tests available. Previously, we found expression of nectin 4 to be increased in ovarian cancer compared with normal ovaries. Reverse transcriptase-polymerase chain reaction (RT-PCR) and quantitative RT-PCR validated the overexpression of nectin 4 messenger RNA in ovarian cancer compared with normal ovarian cell lines and tissues. Protein levels of nectin 4 were elevated in ovarian cancer cell lines and tissue compared with normal ovarian cell lines as demonstrated by Western immunoblotting, flow cytometry, and immunohistochemical staining of tissue microarray slides. Cleaved nectin 4 was detectable in a number of patient serum samples by enzyme-linked immunosorbent assay. In patients with benign gynecologic diseases with high serum CA125 levels, nectin 4 was not detected in the majority of cases, suggesting that nectin 4 may serve as a potential biomarker that helps discriminate benign gynecologic diseases from ovarian cancer in a panel with CA125.

Annexin 2 regulates endothelial morphogenesis by controlling AKT activation and junctional integrity

Sprouting angiogenesis is a multistep process that involves endothelial cell activation, basement membrane degradation, proliferation, lumen formation, and stabilization. In this study, we identified annexin 2 as a regulator of endothelial morphogenesis using a three-dimensional in vitro model where sprouting angiogenesis was driven by sphingosine 1-phosphate and angiogenic growth factors. We observed that sphingosine 1-phosphate triggered annexin 2 translocation from the cytosol to the plasma membrane and its association with vascular endothelial (VE)-cadherin. In addition, annexin 2 depletion attenuated Akt activation, which was associated with increased phosphorylation of VE-cadherin and endothelial barrier leakage. Disrupting homotypic VE-cadherin interactions with EGTA, antibodies to the extracellular domain of VE-cadherin, or gene silencing all resulted in decreased Akt (but not Erk1/2) activation. Furthermore, expression of constitutively active Akt restored reduced endothelial sprouting responses observed with annexin 2 and VE-cadherin knockdown. Collectively, we report that annexin 2 regulates endothelial morphogenesis through an adherens junction-mediated pathway upstream of Akt.

Cooperation of nectin-1 and nectin-3 is required for normal ameloblast function and crown shape development in mouse teeth

Nectins are immunoglobulin-like cell adhesion proteins and their interactions recruit various cell-cell junctions. Mutations in human NECTIN-1 cause an ectodermal dysplasia syndrome, but Nectin-1 null mice have only slight defects in teeth, suggesting compensation by other nectin(s). We observed overlapping expression of nectin-3 with nectin-1 and enamel abnormality in the nectin-3 mutant. We, therefore, generated nectin-1;nectin-3 compound mutants. However, all teeth developed and no significant dental abnormalities were observed before birth. At postnatal day 10, the upper molars of compound mutants exhibited conical crown shape and retarded enamel maturation. Nectin-1 was expressed in ameloblasts whereas nectin-3 was expressed in neighboring stratum intermedium cells at this stage. The immunohistochemical localization and electron microscopical observations indicated that the desmosomal junctions between stratum intermedium and ameloblasts were significantly reduced. These results suggest that heterophilic interaction between nectin-1 and nectin-3 recruits desmosomal junctions, and that these are required for proper enamel formation.

Involvement of the interaction of afadin with ZO-1 in the formation of tight junctions in Madin-Darby canine kidney cells

Tight junctions (TJs) and adherens junctions (AJs) are major junctional apparatuses in epithelial cells. Claudins and junctional adhesion molecules (JAMs) are major cell adhesion molecules (CAMs) at TJs, whereas cadherins and nectins are major CAMs at AJs. Claudins and JAMs are associated with ZO proteins, whereas cadherins are associated with beta- and alpha-catenins, and nectins are associated with afadin. We previously showed that nectins first form cell-cell adhesions where the cadherin-catenin complex is recruited to form AJs, followed by the recruitment of the JAM-ZO and claudin-ZO complexes to the apical side of AJs to form TJs. It is not fully understood how TJ components are recruited to the apical side of AJs. We studied the roles of afadin and ZO-1 in the formation of TJs in Madin-Darby canine kidney (MDCK) cells. Before the formation of TJs, ZO-1 interacted with afadin through the two proline-rich regions of afadin and the SH3 domain of ZO-1. During and after the formation of TJs, ZO-1 dissociated from afadin and associated with JAM-A. Knockdown of afadin impaired the formation of both AJs and TJs in MDCK cells, whereas knockdown of ZO-1 impaired the formation of TJs, but not AJs. Re-expression of full-length afadin restored the formation of both AJs and TJs in afadin-knockdown MDCK cells, whereas re-expression of afadin-DeltaPR1-2, which is incapable of binding to ZO-1, restored the formation of AJs, but not TJs. These results indicate that the transient interaction of afadin with ZO-1 is necessary for the formation of TJs in MDCK cells.

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

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

Involvement of IQGAP3, a regulator of Ras/ERK-related cascade, in hepatocyte proliferation in mouse liver regeneration and development

The spatio-temporal regulation of hepatocyte proliferation is a critical issue in liver regeneration. Here, in normal and regenerating liver as well as in developing liver, we examined its expression/localization of IQGAP3, which was most recently reported as a Ras/Rac/Cdc42-binding proliferation factor associated with cell-cell contacts in epithelial-type cells. In parallel, the expression/localization of Rac/Cdc42-binding IQGAP1/2 was examined. IQGAP3 showed a specific expression in proliferating hepatocytes positive for the proliferating marker Ki-67, the levels of expressions of mRNAs and proteins were significantly increased in hepatocytes in liver regeneration and development. In immunofluorescence, IQGAP3 was highly enriched at cell-cell contacts of hepatocytes. IQGAP1 and IQGAP2 were exclusively expressed in Kupffer and sinusoidal endothelial cells, respectively, in normal, regenerating, and developing liver. The expression of IQGAP1, but not of IQGAP2, was increased in CCl4-induced (but not in partial hepatectomy-induced) liver regeneration. Exclusive expression/localization of IQGAP3 to hepatocytes in the liver likely reflects the specific involvement of the IQGAP3/Ras/ERK signaling cascade in hepatocyte proliferation in addition to the previously identified signaling pathways, possibly by integrating cell-cell contact-related proliferating signaling events. On the other hand, the Rac/Cdc42-binding properties of IQGAP1/2/3 may be related to the distinct modes of remodeling due to the different strategies which induced proliferation of liver cells; partial hepatectomy, CCl4 injury, or embryonic development. Thus, the functional orchestration of Ras and the Ras homologous (Rho) family proteins Rac/Cdc42 likely plays a critical role in liver regeneration and development.

Annexin 2 has a dual role as regulator and effector of v-Src in cell transformation

Cell transformation by v-Src involves rearrangement of the actin cytoskeleton, disassembly of focal adhesions, and the development of anchorage-independent growth. Here, we report that this is dependent on annexin 2, a v-Src substrate and calcium-dependent regulator of actin dynamics. Using a thermoactivatable mutant of v-Src, we show that at the permissive temperature, annexin 2 becomes phosphorylated and colocalizes with activated v-Src and focal adhesion kinase both at the plasma membrane and in a Rab11-positive compartment of the endosomal pathway. In cells depleted of annexin 2 by small interfering RNA, v-Src becomes activated at the permissive temperature but does not target to the plasma membrane or to perinuclear vesicles, and cell transformation does not occur. Our findings reveal a dual role for annexin 2, first as a regulator of v-Src trafficking and targeting and second as a v-Src effector in the reorganization of actin.

Roles of cell adhesion molecules nectin and nectin-like molecule-5 in the regulation of cell movement and proliferation

In response to chemoattractants, migrating cells form protrusions, such as lamellipodia and filopodia, and structures, such as ruffles over lamellipodia, focal complexes and focal adhesions at leading edges. The formation of these leading edge structures is essential for directional cell movement. Nectin-like molecule-5 (Necl-5) interacts in cis with PDGF receptor and integrin alpha(v)beta(3), and enhances the activation of signalling molecules associated with these transmembrane proteins, which results in the formation of leading edge structures and enhancement of directional cell movement. When migrating cells come into contact with each other, cell-cell adhesion is initiated, resulting in reduced cell velocity. Necl-5 first interacts in trans with nectin-3. This interaction is transient and induces down-regulation of Necl-5 expression at the cell surface, resulting in reduced cell movement. Cell proliferation is also suppressed by the down-regulation of Necl-5, because the inhibitory effect of Necl-5 on Sprouty2, a negative regulator of the Ras signalling, is diminished. PDGF receptor and integrin alpha(v)beta(3), which have interacted with Necl-5, then form a complex with nectin, which initiates cell-cell adhesion and recruits cadherin to the nectin-based cell-cell adhesion sites to form stable adherens junctions. The formation of adherens junctions stops cell movement, in part through inactivation of integrin alpha(v)beta(3) caused by the trans-interaction of nectin. Thus, nectin and Necl-5 play key roles in the regulation of cell movement and proliferation.

The cell adhesion molecule nectin-1 is critical for normal enamel formation in mice

Nectin-1 is a member of a sub-family of immunoglobulin-like adhesion molecules and a component of adherens junctions. In the current study, we have shown that mice lacking nectin-1 exhibit defective enamel formation in their incisor teeth. Although the incisors of nectin-1-null mice were hypomineralized, the protein composition of the enamel matrix was unaltered. While strong immunostaining for nectin-1 was observed at the interface between the maturation-stage ameloblasts and the underlying cells of the stratum intermedium (SI), its absence in nectin-1-null mice correlated with separation of the cell layers at this interface. Numerous, large desmosomes were present at this interface in wild-type mice; however, where adhesion persisted in the mutant mice, the desmosomes were smaller and less numerous. Nectins have been shown to regulate tight junction formation; however, this is the first report showing that they may also participate in the regulation of desmosome assembly. Importantly, our results show that integrity of the SI–ameloblast interface is essential for normal enamel mineralization.

Nectins and nectin-like molecules: roles in contact inhibition of cell movement and proliferation

Nectins and nectin-like molecules (Necls) are immunoglobulin-like transmembrane cell adhesion molecules that are expressed in various cell types. Homophilic and heterophilic engagements between family members provide cells with molecular tools for intercellular communications. Nectins primarily regulate cell-cell adhesions, whereas Necls are involved in a greater variety of cellular functions. Recent studies have revealed that nectins and NECL-5, in cooperation with integrin alphavbeta3 and platelet-derived growth factor receptor, are crucial for the mechanisms that underlie contact inhibition of cell movement and proliferation; this has important implications for the development and tissue regeneration of multicellular organisms and the phenotypes of cancer cells.

Downregulation of integrins by von Hippel-Lindau (VHL) tumor suppressor protein is independent of VHL-directed hypoxia-inducible factor alpha degradation

Inactivation of the von Hippel-Lindau (VHL) tumor suppressor gene occurs in the majority of clear-cell renal cell carcinomas (RCCs). It was previously shown that VHL decreased the abundance of integrins alpha2, alpha5, and beta1, which is consistent with VHL-associated changes in cell-cell and cell - extracellular matrix adhesions. We investigated the mechanism by which VHL downregulates integrins. Although VHL can target hypoxia-inducible factor alpha (HIFalpha) subunits for degradation, VHL-dependent reduction of integrins was independent of O2 concentration and HIFalpha levels. VHL reduced the half-lives of integrins, and this activity was blocked by proteasomal inhibition. Although ectopically expressed FLAG-VHL retained HIFalpha degradation activity, it neither downregulated integrins nor promoted adherens and tight intercellular junctions, in contrast to expressed wild-type VHL. Moreover, integrins co-immunoprecipitated with wild-type VHL, but not FLAG-VHL. These data indicate that the downregulation of integrins by VHL is distinct from the regulation of HIFalpha subunits by VHL, and suggests that the loss of this activity contributes to VHL-associated RCC development through disruption of adherens and tight junctions.

Novel role of nectin: implication in the co-localization of JAM-A and claudin-1 at the same cell-cell adhesion membrane domain

Tight junctions (TJs) are formed at the apical side of adherens junctions (AJs) in epithelial cells. Major cell adhesion molecules (CAMs) at TJs are JAM and claudin, whereas major CAMs at AJs are nectin and cadherin. We previously showed that nectin initially forms cell-cell adhesion and then recruits cadherin to the nectin-based cell-cell adhesion sites to form AJs, followed by the recruitment of JAM and claudin to the apical side of AJs to form TJs. We investigated the roles of nectin in the formation of TJs by expressing various combinations of CAMs in L fibroblasts with no TJs or AJs. Co-expression of one of the AJ CAMs and one of the TJ CAMs formed two separate cell-cell adhesion membrane domains (CAMDs). Co-expression of nectin-3 and E-cadherin formed the same CAMD, but co-expression of JAM-A and claudin-1 did not form the same CAMD. Co-expression of JAM-A and claudin-1 with nectin-3, but not E-cadherin, made them form the same CAMD, which was separated from the nectin-based CAMD. Nectin-3 required afadin, a nectin- and F-actin-binding protein, for this ability. In conclusion, nectin plays a novel role in the co-localization of JAM and claudin at the same CAMD.

Involvement of the nectin-afadin complex in PDGF-induced cell survival

The nectin-afadin complex is involved in the formation of cell-cell junctions, such as adherens junctions (AJs) and tight junctions (TJs). Nectins are Ca2+-independent immunoglobulin-like cell-cell adhesion molecules, whereas afadin is an intracellular nectin-binding protein that connects nectins to the cadherin-catenin system at AJs and to the claudin–zona-occludens (ZO) protein system at TJs. Afadin–/– mice show embryonic lethality, resulting from impaired migration and improper differentiation of cells due to disorganization of cell-cell junctions during gastrulation. However, it remains to be elucidated whether disruption of afadin affects apoptosis. In the present study, we first found that embryoid bodies derived from afadin-knockout embryonic stem (ES) cells contained many more apoptotic cells than those derived from wild-type ES cells. We also revealed that apoptosis induced by serum starvation or Fas-ligand stimulation was increased in cultured NIH3T3 cells when afadin or nectin-3 was knocked down. The nectin-afadin complex was involved in the platelet-derived growth factor (PDGF)-induced activation of phosphatidylinositol 3-kinase (PI3K)-Akt signaling for cell survival. This complex was associated with PDGF receptor on the plasma membrane at cell-cell adhesion sites. Thus, the nectin-afadin complex is involved in PDGF-induced cell survival, at least through the PI3K-Akt signaling pathway.

Anchoring junctions as drug targets: role in contraceptive development

In multicellular organisms, cell-cell interactions are mediated in part by cell junctions, which underlie tissue architecture. Throughout spermatogenesis, for instance, preleptotene leptotene spermatocytes residing in the basal compartment of the seminiferous epithelium must traverse the blood-testis barrier to enter the adluminal compartment for continued development. At the same time, germ cells must also remain attached to Sertoli cells, and numerous studies have reported extensive restructuring at the Sertoli-Sertoli and Sertoli-germ cell interface during germ cell movement across the seminiferous epithelium. Furthermore, the proteins and signaling cascades that regulate adhesion between testicular cells have been largely delineated. These findings have unveiled a number of potential "druggable" targets that can be used to induce premature release of germ cells from the seminiferous epithelium, resulting in transient infertility. Herein, we discuss a novel approach with the aim of developing a nonhormonal male contraceptive for future human use, one that involves perturbing adhesion between Sertoli and germ cells in the testis.

Cross-talk among integrin, cadherin, and growth factor receptor: roles of nectin and nectin-like molecule

Integrin, cadherin, and growth factor receptor are key molecules for fundamental cellular functions including cell movement, proliferation, differentiation, adhesion, and survival. These cell surface molecules cross-talk with each other in the regulation of such cellular functions. Nectin and nectin-like molecule (Necl) have been identified as cell adhesion molecules that belong to the immunoglobulin superfamily. Nectin and Necl play important roles in the integration of integrin, cadherin, and growth factor receptor at the cell-cell adhesion sites of contacting cells and at the leading edges of moving cells, and thus are also involved in the fundamental cellular functions together with integrin, cadherin, and growth factor receptor. This chapter describes how newly identified cell adhesion molecules, nectin and Necl, modulate the cross-talk among integrin, cadherin, and growth factor receptor and how these integrated molecules act in the regulation of fundamental cellular functions.

Cooperative roles of Par-3 and afadin in the formation of adherens and tight junctions

Par-3 is a cell-polarity protein that regulates the formation of tight junctions (TJs) in epithelial cells, where claudin is a major cell-cell adhesion molecule (CAM). TJs are formed at the apical side of adherens junctions (AJs), where E-cadherin and nectin are major CAMs. We have revealed that nectin first forms cell-cell adhesions, and then recruits cadherin to nectin-based cell-cell adhesion sites to form AJs and subsequently recruits claudin to the apical side of AJs to form TJs. The cytoplasmic tail of nectin binds afadin and Par-3. Afadin regulates the formation of AJs and TJs cooperatively with nectin. Here, we studied the role of Par-3 in the formation of these junctions by using Par-3-knockdown MDCK cells. Par-3 was necessary for the formation of AJs and TJs but was not necessary for nectin-based cell-cell adhesion. Par-3 promoted the association of afadin with nectin, whereas afadin was not necessary for the association of Par-3 with nectin. However, the association of afadin with nectin alone was not sufficient for the formation of AJs or TJs, and Par-3 and afadin cooperatively regulated it. We describe here these novel roles of Par-3 in the formation of junctional complexes.

The roles of nectins in cell adhesions: cooperation with other cell adhesion molecules and growth factor receptors

Nectins are Ca(2+)-independent Ig-like cell adhesion molecules (CAMs) which homophilically and heterophilically interact in trans with nectins and form cell-cell adhesion. This cell-cell adhesion is involved in the formation of many types of cell-cell junctions such as adherens junctions, tight junctions, and synaptic junctions, cooperatively with other CAMs such as cadherins and claudins. Nectins transduce signals cooperatively with integrin alpha(v)beta(3), and regulate formation of cell-cell junctions. In addition, nectin interacts in cis with PDGF receptor and regulates its signaling for anti-apoptosis. Furthermore, nectin interacts in trans with nectin-like molecule-5 (Necl-5) and regulate cell movement and proliferation. We describe cooperative roles of nectins with other CAMs and growth factor receptors.

The cytoplasmic plaque of tight junctions: a scaffolding and signalling center

The region of cytoplasm underlying the tight junction (TJ) contains several multimolecular protein complexes, which are involved in scaffolding of membrane proteins, regulation of cytoskeletal organization, establishment of polarity, and signalling to and from the nucleus. In this review, we summarize some of the most recent advances in understanding the identity of these proteins, their domain organization, their protein interactions, and their functions in vertebrate organisms. Analysis of knockdown and knockout model systems shows that several TJ proteins are essential for the formation of epithelial tissues and early embryonic development, whereas others appear to have redundant functions.

Nectin and nectin-like molecules: biology and pathology

Nectins and nectin-like molecules (Necls) are structurally related transmembrane proteins primarily involved in cell adhesion. Nectins and afadin, the adaptor or anchoring protein, stabilize the epithelium and endothelium and establish apical-basal polarity of epithelial cells, independently or in cooperation with other cell adhesion molecules. Necls facilitate cell-cell communication implicated in cell movement and proliferation, immune responses, and cancer cell phenotypes. Necls interact with nectins and specific ligands at cell-cell contacts, whereas Necls associate with integrin alpha v beta 3 and growth factor receptors on the same cell surface. Besides their roles in cell adhesion, nectins regulate the activities of Rho family small G proteins which play critical roles in maintaining the apical junctions of epithelial cells through reorganization of the actin cytoskeleton. Since mice lacking the Rho GDP-dissociation inhibitor (GDI)alpha show massive proteinuria and degeneration of renal epithelial cells, nectins and other cell adhesion molecules may play roles in the structural and functional aspects of renal diseases. Here we summarize our knowledge of nectins and Necls and discuss cell adhesion biology in the kidney.

Up-regulation of loricrin expression by cell adhesion molecule nectin-1 through Rap1-ERK signaling in keratinocytes

Nectin is an immunoglobulin-like cell-cell adhesion molecule, which plays essential roles in the initial step of formation of adherens junctions and tight junctions. We demonstrate here the role of nectin-1 in the epidermis using nectin-1-/- mice. Newborn nectin-1-/- pups showed shiny and slightly reddish skin; the amount of loricrin, one of the differentiation markers and also a major component of cornified cell envelopes, was markedly reduced in the epidermis of nectin-1-/- mice. The amounts of repetin and SPRRP, other components of cornified cell envelopes, were markedly elevated probably due to a compensatory mechanism to overcome the impaired expression of loricrin. However, cornified cells from nectin-1-/- mice were sensitive to mechanical stress. Moreover, Ca2+-induced activation of ERK through Rap1 and expression of loricrin were reduced in primary cultured nectin-1-/- keratinocytes; in turn, the inhibition of ERK activation reduced the amount of loricrin in wild-type keratinocytes. These results indicate that nectin-1 plays a key role in the expression of loricrin in the epidermis.

Role of tight junctions in cell proliferation and cancer

The acquisition of a cancerous phenotype by epithelial cells involves the disruption of intercellular adhesions. The reorganization of the E-cadherin/beta-catenin complex in adherens junctions during cell transformation is widely recognized. Instead the implication of tight junctions (TJs) in this process is starting to be unraveled. The aim of this article is to review the role of TJ proteins in cell proliferation and cancer.

Proteomic and bioinformatic analysis of epithelial tight junction reveals an unexpected cluster of synaptic molecules

BACKGROUND

Zonula occludens, also known as the tight junction, is a specialized cell-cell interaction characterized by membrane "kisses" between epithelial cells. A cytoplasmic plaque of approximately 100 nm corresponding to a meshwork of densely packed proteins underlies the tight junction membrane domain. Due to its enormous size and difficulties in obtaining a biochemically pure fraction, the molecular composition of the tight junction remains largely unknown.

RESULTS

A novel biochemical purification protocol has been developed to isolate tight junction protein complexes from cultured human epithelial cells. After identification of proteins by mass spectroscopy and fingerprint analysis, candidate proteins are scored and assessed individually. A simple algorithm has been devised to incorporate transmembrane domains and protein modification sites for scoring membrane proteins. Using this new scoring system, a total of 912 proteins have been identified. These 912 hits are analyzed using a bioinformatics approach to bin the hits in 4 categories: configuration, molecular function, cellular function, and specialized process. Prominent clusters of proteins related to the cytoskeleton, cell adhesion, and vesicular traffic have been identified. Weaker clusters of proteins associated with cell growth, cell migration, translation, and transcription are also found. However, the strongest clusters belong to synaptic proteins and signaling molecules. Localization studies of key components of synaptic transmission have confirmed the presence of both presynaptic and postsynaptic proteins at the tight junction domain. To correlate proteomics data with structure, the tight junction has been examined using electron microscopy. This has revealed many novel structures including end-on cytoskeletal attachments, vesicles fusing/budding at the tight junction membrane domain, secreted substances encased between the tight junction kisses, endocytosis of tight junction double membranes, satellite Golgi apparatus and associated vesicular structures. A working model of the tight junction consisting of multiple functions and sub-domains has been generated using the proteomics and structural data.

CONCLUSION

This study provides an unbiased proteomics and bioinformatics approach to elucidate novel functions of the tight junction. The approach has revealed an unexpected cluster associating with synaptic function. This surprising finding suggests that the tight junction may be a novel epithelial synapse for cell-cell communication.

REVIEWERS

This article was reviewed by Gáspár Jékely, Etienne Joly and Neil Smalheiser.