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

Regulation by afadin of cyclical activation and inactivation of Rap1, Rac1, and RhoA small G proteins at leading edges of moving NIH3T3 cells

Cyclical activation and inactivation of Rho family small G proteins, such as Rho, Rac, and Cdc42, are needed for moving cells to form leading edge structures in response to chemoattractants. However, the mechanisms underlying the dynamic regulation of their activities are not fully understood. We recently showed that another small G protein, Rap1, plays a crucial role in the platelet-derived growth factor (PDGF)-induced formation of leading edge structures and activation of Rac1 in NIH3T3 cells. We showed here that knockdown of afadin, an actin-binding protein, in NIH3T3 cells resulted in a failure to develop leading edge structures in association with an impairment of the activation of Rap1 and Rac1 and inactivation of RhoA in response to PDGF. Overexpression of a constitutively active mutant of Rap1 (Rap1-CA) and knockdown of SPA-1, a Rap1 GTPase-activating protein that was negatively regulated by afadin by virtue of binding to it, in afadin-knockdown NIH3T3 cells restored the formation of leading edge structures and the reduction of the PDGF-induced activation of Rac1 and inactivation of RhoA, suggesting that the inactivation of Rap1 by SPA-1 is responsible for inhibition of the formation of leading edge structures. The effect of Rap1-CA on the restoration of the formation of leading edge structures and RhoA inactivation was diminished by additional knockdown of ARAP1, a Rap-activated Rho GAP, which localized at the leading edges of moving NIH3T3 cells. These results indicate that afadin regulates the cyclical activation and inactivation of Rap1, Rac1, and RhoA through SPA-1 and ARAP1.

Involvement of afadin in the formation and remodeling of synapses in the hippocampus

In the hippocampus, synapses are formed between mossy fiber terminals and CA3 pyramidal cell dendrites and comprise highly developed synaptic junctions (SJs) and puncta adherentia junctions (PAJs). Dynamic remodeling of synapses in the hippocampus is implicated in learning and memory. Components of both the nectin-afadin and cadherin-catenin cell adhesion systems exclusively accumulate at PAJs. We investigated the role of afadin at synapses in mice in which the afadin gene was conditionally inactivated in hippocampal neurons. In these mutant mice, the signals for not only nectins, but also N-cadherin and beta-catenin, were hardly detected in the CA3 area, in addition to loss of the signal for afadin, resulting in disruption of PAJs. Ultrastructural analysis revealed an increase in the number of perforated synapses, suggesting the instability of SJs. These results indicate that afadin is involved not only in the assembly of nectins and cadherins at synapses, but also in synaptic remodeling.

Drosophila big brain does not act as a water channel, but mediates cell adhesion

The neurogenic gene Drosophila big brain (bib) has a high sequence homology to aquaporin-4. However, its cellular functions in Drosophila neurogenesis have remained elusive. Here we investigated cell adhesion, and the ion and water permeability of Bib. The adhesive function was examined by a cell aggregation assay using L cells. Bib-transfected L cells formed aggregated clusters, while control-L cells remained as a single cell suspension. Ion permeation was not confirmed in L cells stably expressing Bib. When expressed in COS7 cells, Bib exhibited limited water permeability. This newly found cell adhesive function of Bib may be important for Drosophila neurogenesis.

Endothelial signaling in paracellular and transcellular leukocyte transmigration

As the primary physical barrier between blood and tissue compartments within the body, blood vessel endothelial cells and integrity of the cell junctions connecting them must be carefully regulated to support leukocyte transendothelial migration only when necessary. Leukocytes utilize two independent routes across the endothelium: the paracellular route involves migration in-between adjacent endothelial cells and requires the transient disassembly of endothelial cell junctions, while the transcellular route occurs directly through an individual endothelial cell, likely requiring the formation of a channel or pore. In this review, I will first summarize the signaling events that are transduced by leukocyte engagement of endothelial cell-surface receptors like ICAM-1 and VCAM-1. Some of these signals include activation of GTPases, production of reactive oxygen species, and phosphorylation of target proteins. These signaling pathways converge to cause junctional disruption, cytoskeletal remodeling, and/or the membrane fusion events that are associated with leukocyte transendothelial migration. The review will conclude with a detailed discussion of the newly characterized transmigratory cup structure, and the recent advances made towards understanding the mechanisms of transcellular transendothelial migration.

The immunoglobulin-like cell adhesion molecule nectin and its associated protein afadin

Nectins are immunoglobulin-like cell adhesion molecules (CAMs) that compose a family of four members. Nectins homophilically and heterophilically interact in trans with each other to form cell-cell adhesions. In addition, they heterophilically interact in trans with other immunoglobulin-like CAMs. Nectins bind afadin, an actin filament (F-actin)-binding protein, at its cytoplasmic tail and associate with the actin cytoskeleton. Afadin additionally serves as an adaptor protein by further binding many scaffolding proteins and F-actin-binding proteins and contributes to the association of nectins with other cell-cell adhesion and intracellular signaling systems. Nectins and afadin play roles in the formation of a variety of cell-cell junctions cooperatively with, or independently of, cadherins. Cooperation between nectins and cadherins is required for the formation of cell-cell junctions; cadherins alone are not sufficient. Additionally, nectins regulate many other cellular activities (such as movement, proliferation, survival, differentiation, polarization, and the entry of viruses) in cooperation with other CAMs and cell surface membrane receptors.

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.

Coordination of synaptic adhesion with dendritic spine remodeling by AF-6 and kalirin-7

Remodeling of central excitatory synapses is crucial for synapse maturation and plasticity, and contributes to neurodevelopmental and psychiatric disorders. Remodeling of dendritic spines and the associated synapses has been postulated to require the coordination of adhesion with spine morphology and stability; however, the molecular mechanisms that functionally link adhesion molecules with regulators of dendritic spine morphology are mostly unknown. Here, we report that spine size and N-cadherin content are tightly coordinated. In rat mature cortical pyramidal neurons, N-cadherin-dependent adhesion modulates the morphology of existing spines by recruiting the Rac1 guanine-nucleotide exchange factor kalirin-7 to synapses through the scaffolding protein AF-6/afadin. In pyramidal neurons, N-cadherin, AF-6, and kalirin-7 colocalize at synapses and participate in the same multiprotein complexes. N-cadherin clustering promotes the reciprocal interaction and recruitment of N-cadherin, AF-6, and kalirin-7, increasing the content of Rac1 and in spines and PAK (p21-activated kinase) phosphorylation. N-cadherin-dependent spine enlargement requires AF-6 and kalirin-7 function. Conversely, disruption of N-cadherin leads to thin, long spines, with reduced Rac1 contact, caused by uncoupling of N-cadherin, AF-6, and kalirin-7 from each other. By dynamically linking N-cadherin with a regulator of spine plasticity, this pathway allows synaptic adhesion molecules to rapidly coordinate spine remodeling associated with synapse maturation and plasticity. This study hence identifies a novel mechanism whereby cadherins, a major class of synaptic adhesion molecules, signal to the actin cytoskeleton to control the morphology of dendritic spines, and outlines a mechanism that underlies the coordination of synaptic adhesion with spine morphology.

Ras oncogenes: split personalities

Extensive research on the Ras proteins and their functions in cell physiology over the past 30 years has led to numerous insights that have revealed the involvement of Ras not only in tumorigenesis but also in many developmental disorders. Despite great strides in our understanding of the molecular and cellular mechanisms of action of the Ras proteins, the expanding roster of their downstream effectors and the complexity of the signalling cascades that they regulate indicate that much remains to be learnt.

Establishment of cell polarity by afadin during the formation of embryoid bodies

Afadin directly links nectin, an immunoglobulin-like cell-cell adhesion molecule, to actin filaments (F-actin) at adherens junctions (AJs). The nectin-afadin complex is important for the formation of not only AJs but also tight junctions (TJs) in epithelial cells. Studies using afadin-knockout mice have revealed that afadin is indispensable for embryonic development by organizing the formation of cell-cell junctions. However, the molecular mechanism of cell-cell junction disorganization during embryonic development in afadin-knockout mice is poorly understood. To address this, we took advantage of embryoid bodies (EBs) as a model system. The formation of cell-cell junctions including AJs and TJs was impaired in afadin-null EBs. The proper accumulation of the Par complex and the activation of Cdc42 and atypical PKC (aPKC), which are crucial for the formation of cell polarity, were also inhibited by knockout of afadin. In addition, the disruption of afadin caused the abnormal deposition of laminin and the dislocalization of its receptors integrin alpha(6) and integrin beta(1). These results indicate that afadin organizes the formation of cell-cell junctions by regulating cell polarization in early embryonic development.

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.

Temporal and spatial localization of nectin-1 and l-afadin during synaptogenesis in hippocampal neurons

Nectins are cell adhesion molecules that, together with the intracellular binding partner afadin, mediate adhesion and signaling at a variety of intercellular junctions. In this work we studied the distribution of nectin-1 and afadin during hippocampal synapse formation using cultured primary hippocampal neurons. Nectin-1 and afadin cluster at developing synapses between hippocampal neurons. These nectin-afadin clusters uniformly colocalize with N-cadherin-catenin pairs, suggesting that formation of developing synapses involves participation of both bimolecular systems. Nectin-1 is initially expressed at excitatory and inhibitory synapses but is progressively lost at inhibitory synapses during their maturation. Treatment of neurons with actin depolymerizing agents disrupts the synaptically localized nectin-1 and afadin cluster at an early stage and elicits nectin-1 ectodomain shedding. These data indicate that the synaptic localization of nectin-1 and l-afadin are F-actin-dependent and that the shedding of nectin-1 is a mechanism contributing to synaptic plasticity.

Organization of multiprotein complexes at cell-cell junctions

The formation of stable cell–cell contacts is required for the generation of barrier-forming sheets of epithelial and endothelial cells. During various physiological processes like tissue development, wound healing or tumorigenesis, cellular junctions are reorganized to allow the release or the incorporation of individual cells. Cell–cell contact formation is regulated by multiprotein complexes which are localized at specific structures along the lateral cell junctions like the tight junctions and adherens junctions and which are targeted to these site through their association with cell adhesion molecules. Recent evidence indicates that several major protein complexes exist which have distinct functions during junction formation. However, this evidence also indicates that their composition is dynamic and subject to changes depending on the state of junction maturation. Thus, cell–cell contact formation and integrity is regulated by a complex network of protein complexes. Imbalancing this network by oncogenic proteins or pathogens results in barrier breakdown and eventually in cancer. Here, I will review the molecular organization of the major multiprotein complexes at junctions of epithelial cells and discuss their function in cell–cell contact formation and maintenance.

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.

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

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

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.

The novel GTPase Rit differentially regulates axonal and dendritic growth

The Rit GTPase is widely expressed in developing and adult nervous systems, and our previous data with pheochromocytoma cells implicate Rit signaling in NGF-induced neurite outgrowth. In this study, we investigated a role for Rit in neuronal morphogenesis. Expression of a dominant-negative (dn) Rit mutant in hippocampal neurons inhibited axonal growth but potentiated dendritic growth. Conversely, a constitutively active (ca) Rit mutant promoted axonal growth but inhibited dendritic growth. Dendritogenesis is regulated differently in sympathetic neurons versus hippocampal neurons in that sympathetic neurons require NGF and bone morphogenetic proteins (BMPs) to trigger dendritic growth. Despite these differences, dnRit potentiated and caRit blocked BMP7-induced dendritic growth in sympathetic neurons. Biochemical studies indicated that BMP7 treatments that caused dendritic growth also decreased Rit GTP loading. Additional studies demonstrate that caRit increased extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation and pharmacological inhibition of MEK1 (mitogen-activated protein kinase/ERK 1) blocked the axon-promoting and dendrite-inhibiting effects of caRit. These observations suggest that Rit is a convergence point for multiple signaling pathways and it functions to promote axonal growth but inhibit dendritic growth via activation of ERK1/2. Modulation of the activational status of Rit may therefore represent a generalized mechanism across divergent neuronal cell types for regulating axonal versus dendritic growth modes.

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.

AF6/s-afadin is a dual residency protein and localizes to a novel subnuclear compartment

The AF6/afadin protein is a component of cell membranes at specialized sites of cell-cell contact. Two main splice variants exist, known as l- and s-afadin, respectively. L-afadin is widely expressed in cells of epithelial origin, whilst s-afadin expression is restricted to the brain. Here we demonstrate that the short form of AF6/s-afadin is a dual residency protein able to localize to the plasma membrane or nucleus whilst the long form of AF6, l-afadin is unable to localize to the nucleus. AF6/s-afadin clusters in a distinctive speckled pattern in the nucleus, but is unable to do so when cell cycle progression is inhibited at the G(1)/S or G(2)/M checkpoints. The formation of AF6/s-afadin nuclear bodies is also sensitive to the transcriptional activity of the cell with inhibition of RNA polymerase activity abolishing AF6/s-afadin nuclear clustering. AF6/s-afadin nuclear bodies localize to a novel subnuclear compartment, failing to colocalize with other known nuclear bodies. Formation of the AF6/s-afadin nuclear foci can be regulated by specific growth factor receptor mediated signaling events and by cytoplasmic tyrosine kinases, but does not correlate with tyrosine phosphorylation of AF6/s-afadin. AF6/s-afadin is a candidate for mediating control of cellular growth processes by regulated translocation to the nucleus.

The cadherin superfamily in neuronal connections and interactions

Neural development and the organization of complex neuronal circuits involve a number of processes that require cell-cell interaction. During these processes, axons choose specific partners for synapse formation and dendrites elaborate arborizations by interacting with other dendrites. The cadherin superfamily is a group of cell surface receptors that is comprised of more than 100 members. The molecular structures and diversity within this family suggest that these molecules regulate the contacts or signalling between neurons in a variety of ways. In this review I discuss the roles of three subfamilies - classic cadherins, Flamingo/CELSRs and protocadherins - in the regulation of neuronal recognition and connectivity.

The Cdi/TESK1 kinase is required for Sevenless signaling and epithelial organization in the Drosophila eye

How cellular behaviors such as cell-to-cell communication, epithelial organization and cell shape reorganization are coordinated during development is poorly understood. The developing Drosophila eye offers an ideal model system to study these processes. Localized actin polymerization is required to constrict the apical surface of epithelial cells of the eye imaginal disc to maintain the refined arrangement of the developing ommatidia. The identity of each photoreceptor cell within the epithelium is determined by cell-to-cell contacts involving signal transduction events. The R7 photoreceptor cell requires the activity of the Sevenless RTK to adopt a proper cell fate. We performed an EP screen for negative regulators of this inductive process, and we identified the serine/threonine kinase Center divider (cdi) as a suppressor of the phenotype caused by an activated Sevenless receptor. Cdi is homologous to the human testis-specific kinase 1 (TESK1), a member of the LIM kinases involved in cytoskeleton control through ADF/cofilin phosphorylation. We have analyzed the effects of gain- and loss-of-function of cdi and found alterations in actin organization and in the adherens junctions proteins DE-cadherin and beta-catenin, as well as in Sevenless apical localization. Interference with the function of the ADF/cofilin phosphatase Slingshot (ssh), which antagonizes Cdi, also results in a suppression of signaling triggered by the Sevenless RTK. Our results reveal a critical interplay between the localization of molecules involved in epithelial organization and signal transduction.

Cdc42 GEF Tuba regulates the junctional configuration of simple epithelial cells

Epithelial cells are typically arranged in a honeycomb-like pattern, minimizing their cell–cell contact areas, which suggests that some tension operates for shaping of the cell boundaries. However, the molecular mechanisms that generate such tension remain unknown. We found that Tuba, which is a Cdc42-specific GEF, was concentrated at the apical-most region of cell junctions in simple epithelia via its interaction with ZO-1. RNAi–mediated depletion of Tuba altered the geometrical configuration of cell junctions, resulting in a curved and slack appearance. At the subcellular level, Tuba inactivation modified the assembly pattern of junctional F-actin and E-cadherin. Tuba RNAi also retarded cell junction formation in calcium-switch experiments. Suppression of Cdc42 activity or depletion of N-WASP, which is an effector of Cdc42, mimicked the effects of Tuba depletion. Conversely, overexpression of dominant-active Cdc42 or N-WASP enhanced the junction formation of Tuba-depleted cells. These results suggest that Tuba controls the shaping of cell junctions through the local activation of Cdc42 and its effectors.

Nectins and nectin-like molecules: roles in cell adhesion, polarization, movement, and proliferation

Nectins and nectin-like molecules (Necls) are immunoglobulin-like cell adhesion molecules that constitute families containing four and five members, respectively. All members, except for Necl-5, trans-interact homophilically. Furthermore, all members, including Necl-5, trans-interact heterophilically with their respective specific partners among the members. Necl-5 regulates cell movement and proliferation cooperatively with integrin alphavbeta3 and growth factor receptors. Nectins function as cell-cell adhesion molecules at a variety of cell-cell junctions, including adherens junctions, and regulate the initial step of cell-cell junction formation. Nectins and integrin alphavbeta3 are further involved in the cross-talk between cell-matrix and cell-cell junctions. Thus, both nectin and Necl family members play important roles in fundamental cellular functions, including cell adhesion, polarization, movement, and proliferation.

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

AF6 is a human multi-domain protein involved in signaling and organization of cell junctions during embryogenesis. Its homologue in rat is called afadin. Three different AF6 transcripts are known, but only isoform 1 (AF6i1) has been characterized as protein. We focused on the AF6 isoform 3 (AF6i3), which differs from the AF6i1 by an additional C-terminal F-actin-binding site. Knockdown of AF6i3 in epithelial cells, which express only this isoform, resulted in impaired E-cadherin-dependent intercellular adhesion due to concomitantly reduced association of E-cadherin with F-actin and p120-catenin. Impaired intercellular adhesion also accelerated wound closure due to increased directionality of cell migration and delayed de novo formation of cell junctions. In contrast to AF6i3, the AF6i1 displayed a reduced association with the actin cytoskeleton and did not stabilize intercellular adhesion. Therefore, we propose that the AF6i3 protein stabilizes E-cadherin-dependent adhesion during dynamic processes, such as wound closure and formation of cell junctions, by linking the E-cadherin-catenin complex to the actin cytoskeleton via its F-actin-binding site.

Expression of JAM-A, AF-6, PAR-3 and PAR-6 during the assembly and remodeling of RPE tight junctions

The tight junctions of the endothelial and epithelial regions of the blood-brain barrier are regulated by interactions with the neighboring tissue. We examined how the neural retina regulates the assembly of tight junctions in the retinal pigment epithelium (RPE). The proteins JAM-A, AF-6, PAR-3 and PAR-6 have been implicated in the assembly of other epithelial tight junctions. Using chick embryos and primary cell culture, we examined gene expression of these proteins during embryonic development, and whether retinal secretions regulate their expression. Three highly conserved RNA splice sites of AF-6 were identified in chick ocular tissues, but only two were expressed in RPE. JAM-A and AF-6 were expressed at relatively high levels early in development when adherens junctions form, but before tight junctions form. Expression of JAM-A and the AF-6 isoforms actually decreased when tight junctions were forming and expanding. The expression of PAR-3 and PAR-6 was constant. Despite the expression of these proteins in vitro (along with claudins, occludin, ZO-1 and ZO-2), the tight junctional networks that form were discontinuous (Rahner, C., Fukuhara, M., Peng, S., Kojima, S., Rizzolo, L.J., 2004. The apical and basal environments of the retinal pigment epithelium regulate the maturation of tight junctions during development. J. Cell Sci. 117, 3307-3318). The expression of these assembly proteins was unaffected by a retinal conditioned medium that induced the completion of tight junction formation. These data indicate that the early expression of the assembly proteins corresponds to the initial establishment of the adherens and tight junctions, but secretory products of the neural retina must induce the expression of additional proteins to complete the maturation process.

Junctional adhesion molecule-a participates in the formation of apico-basal polarity through different domains

Junctional adhesion molecule (JAM)-A is an integral membrane protein at tight junctions of epithelial cells which associates with the cell polarity protein PAR-3. Here, we demonstrate that downregulation of JAM-A impairs the ability of MDCK II cells to form cysts in a three-dimensional matrix indicating the requirement of JAM-A for the development of apico-basal polarity. To define the regions of JAM-A important for this function, we have generated MDCK II cell lines stably expressing inducible JAM-A mutants. Mutants of JAM-A which were designed to mislocalize strongly impaired the development of cysts and the formation of functional tight junctions. Surprisingly, similar mutants that lacked the PDZ domain-binding motif at the C-terminus were still impaired in apico-basal polarity formation suggesting that additional regions within the cytoplasmic tail of JAM-A are important for the function of JAM-A. A JAM-A mutant lacking the first Ig-like domain necessary for homophilic binding localized to cell-cell contacts similar to wild-type JAM-A. However, despite this same localization, this mutant interfered with cell polarity and tight junction formation. Together our findings suggest an important role for JAM-A in the development of apico-basal polarity in epithelial cells and identify regions in JAM-A which are critical for this role.

The resilient synapse: insights from genetic interference of synaptic cell adhesion molecules

Synaptic cell adhesion molecules (SCAMs) are mostly membrane-anchored molecules with extracellular domains that extend into the synaptic cleft. Prototypical SCAMs interact with homologous or heterologous molecules on the surface of adjacent cells, ensuring the precise apposition of pre- and postsynaptic elements. More recent definitions of SCAMs often include molecules involved in axon pathfinding, cell recognition and synaptic differentiation events, making SCAMs functionally and molecularly a highly diverse group. In this review, we summarize the proposed in vivo functions of a large variety of SCAMs. We mainly focus on results obtained from analyses of genetic model organisms, mostly mouse knockout mutants, lacking expression of the respective candidate genes. In contrast to the substantial effect yielded by some knockouts of molecules involved in synaptic vesicle release, no SCAM mutant has been reported thus far that shows a prominently altered structure of the majority of synapses or even lacks synapses altogether. This surprising resilience of synaptic structure might be explained by a high redundancy between different SCAMs, by the assumption that the crucial molecular players in synapse structure have yet to be discovered or by a grand variability in the mechanisms of synapse formation that underlies the diversity of synapses. Whatever the final answer turns out to be, the genetic dissection of the SCAM superfamilies has led to a much better understanding of the different steps required to form, differentiate and modify a synapse.

Concerted upregulation of CLP36 and smooth muscle actin protein expression in human endometrium during decidualization

The human endometrium prepares for implantation of the blastocyst by reorganization of its whole cellular network. Endometrial stroma cells change their phenotype starting around the 23rd day of the menstrual cycle. These predecidual stroma cells first appear next to spiral arteries, and after implantation these cells further differentiate into decidual stroma cells. The phenotypical changes in these cells during decidualization are characterized by distinct changes in the actin filaments and filament-related proteins such as alpha-actinin. The carboxy-terminal LIM domain protein with a molecular weight of 36 kDa (CLP36) is a cytoskeletal component that has been shown to associate with contractile actin filaments and to bind to alpha-actinin supporting a role for CLP36 in cytoskeletal reorganization and signal transduction by binding to signaling proteins. The expression patterns of CLP36, alpha-actinin and actin were studied in endometrial stroma cells from different stages of the menstrual cycle and in decidual stroma cells from the 6th week of gestation until the end of pregnancy. During the menstrual cycle, CLP36 is only expressed in the luminal and glandular epithelium but not in endometrial stroma cells. During decidualization and throughout pregnancy, a parallel upregulation of CLP36 and smooth muscle actin, an early marker of decidualization in the baboon, was observed in endometrial decidual cells. Since both proteins maintain a high expression level throughout pregnancy, a role of both proteins is suggested in the stabilization of the cytoskeleton of these cells that come into close contact with invading trophoblast cells.

Correlated break at PARK2/FRA6E and loss of AF-6/Afadin protein expression are associated with poor outcome in breast cancer

Common fragile sites (CFSs) are regions of chromosomal break that may play a role in oncogenesis. The most frequent alteration occurs at FRA3B, within the FHIT gene, at chromosomal region 3p14. We studied a series of breast carcinomas for break of a CFS at 6q26, FRA6E, and its associated gene PARK2, using fluorescence in situ hybridization on tissue microarrays (TMA). We found break of PARK2 in 6% of cases. We studied the PARK2-encoded protein Parkin by using immunohistochemistry on the same TMA. Loss of Parkin was found in 13% of samples but was not correlated with PARK2 break. PARK2 break but not Parkin expression was correlated with prognosis. Alteration of PARK2/FRA6E may cause haplo-insufficiency of one or several telomeric potential tumor suppressor genes (TSG). The AF-6/MLLT4 gene, telomeric of PARK2, encodes the Afadin scaffold protein, which is essential for epithelial integrity. Loss of Afadin was found in 14.5% of cases, and 36% of these cases showed PARK2 break. Loss of Afadin had prognostic impact, suggesting that AF-6 may be a TSG. Loss of Afadin was correlated with loss of FHIT expression, suggesting fragility of FRA6E and FRA3B in a certain proportion of breast tumors.

Interaction of integrin alpha(v)beta3 with nectin. Implication in cross-talk between cell-matrix and cell-cell junctions

Cell-matrix and cell-cell junctions cross-talk together, and these two junctions cooperatively regulate cell movement, proliferation, adhesion, and polarization. However, the mechanism of this cross-talk remains unknown. An immunoglobulin-like cell-cell adhesion molecule nectin first trans-interacts with each other to form cell-cell adhesion and induces activation of Rap1, Cdc42, and Rac small G proteins through c-Src. Trans-interacting nectin then recruits another cell-cell adhesion molecule cadherin to the nectin-based cell-cell adhesion sites and forms adherens junctions (AJs). Here, we show that integrin alpha(v)beta3 functionally and physically associates with nectin. Integrin alpha(v)beta3 colocalized with nectin at the nectin-based cell-cell adhesion sites. The association of integrin alpha(v)beta3 with nectin was direct and was mediated through their extracellular regions. This interaction was necessary for the nectin-induced signaling. Focal adhesion kinase, which relays the integrin-initiated outside-in signals to the intracellular signaling molecules, was also involved in the nectin-induced signaling. During the formation of AJs, the high affinity form of integrin alpha(v)beta3 co-localized with nectin at the primordial cell-cell contact sites, and then after the establishment of AJs, this high affinity form of integrin alpha(v)beta3 was converted to the low affinity form, which continued to co-localize with nectin. Thus, integrin alpha(v)beta3 and nectin play pivotal roles in the cross-talk between cell-matrix and cell-cell junctions and the formation of cadherin-based AJs.

Herpes simplex virus infection, with particular reference to the progression and complications of primary herpetic gingivostomatitis

Primary herpetic gingivostomatitis (PHGS) represents the clinically apparent pattern of primary herpes simplex virus (HSV) infection, since the vast majority of other primary infections are symptomless. PHGS is caused predominantly by HSV-1 and affects mainly children. Prodromal symptoms, such as fever, anorexia, irritability, malaise and headache, may occur in advance of disease. The disease presents as numerous pin-head vesicles, which rupture rapidly to form painful irregular ulcerations covered by yellow-grey membranes. Sub-mandibular lymphadenitis, halitosis and refusal to drink are usual concomitant findings. Following resolution of the lesions, the virus travels through the nerve endings to the nerve cells serving the affected area, whereupon it enters a latent state. When the host becomes stressed, the virus replicates and migrates in skin, mucosae and, in rare instances, the central nervous system. A range of morbidities, or even mortality, may then occur, i.e., recurrent HSV infections, which are directly or indirectly associated with PHGS. These pathological entities range from the innocuous herpes labialis to life-threatening meningoencephalitis.

Expression of the actin stress fiber-associated protein CLP36 in the human placenta

Differentiation processes in the trophoblast comprise polarization, cell fusion and migration. All these processes involve dramatic reorganizations of cytoskeletal proteins such as intermediate filaments or actin. Due to very restricted knowledge on cytoskeletal changes in trophoblast, we analyzed the protein expression of an actin stress fiber-associated protein, the carboxy-terminal LIM domain protein (CLP36). CLP36 belongs to the enigma family of proteins, binds to alpha-actinin and is involved in the cytoskeletal reorganization and signal transduction of a variety of cells. CLP36 protein was found to be exclusively expressed in the cytotrophoblast layer. Colocalization of CLP36 with Mib-1 revealed that CLP36 protein expression is restricted to proliferative and early post-proliferative trophoblast cells. Blockage of syncytial fusion by culture of villous explants in the presence of caspase 8 inhibitors further supported this notion since CLP36 was only found in the basal and proliferative layer of the multilayered cytotrophoblast. We present evidence for the exclusive protein expression of CLP36 in proliferative and early post-proliferative trophoblast cells. Pathological pregnancy syndromes such as preeclampsia are driven by alterations of trophoblast differentiation and turnover, where it needs to be elucidated whether CLP36 is involved in these alterations.

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

Adherens junctions (AJs) and tight junctions (TJs) comprise a junctional complex which plays key roles not only in cell adhesion and polarization but also in regulation of cell movement and proliferation in epithelial cells. E-Cadherin and nectin are major cell-cell adhesion molecules (CAMs) at AJs, whereas claudin is a major CAM at TJs. We have shown that the cadherin-based cell-cell adhesion is not formed in MDCK cells in which annexin II, a Ca(2+)- and phospholipid-binding protein, is knocked down. Here, we found that TJs and the nectin-based cell-cell adhesions were formed in annexin II-knockdown cells. The formation of TJs in annexin II-knockdown MDCK cells required the nectin-based cell-cell adhesion and afadin, a nectin- and actin-filament-binding protein. In addition, it required the activation of Cdc42 and Rac small G proteins and subsequent reorganization of the IQGAP1-dependent actin cytoskeleton which were induced by the nectin-based cell-cell adhesion. These results indicate that the nectin-based cell-cell adhesion and afadin, but not the cadherin-based cell-cell adhesion, are necessary for the formation of TJs and that the signaling by nectin and the subsequent reorganization of the actin cytoskeleton are also necessary for the formation of TJs under certain conditions.

Regulation of the Assembly and Adhesion Activity of E-cadherin by Nectin and Afadin for the Formation of Adherens Junctions in Madin-Darby Canine Kidney Cells

The Ca2+-independent immunoglobulin-like molecule nectin first forms cell-cell adhesion and then assembles cadherin at nectin-based cell-cell adhesion sites, resulting in the formation of adherens junctions (AJs). Afadin is a nectin- and actin filament-binding protein that connects nectin to the actin cytoskeleton. Here, we studied the roles and modes of action of nectin and afadin in the formation of AJs in cultured MDCK cells. The trans-interaction of nectin assembled E-cadherin, which associated with p120(ctn), beta-catenin, and alpha-catenin, at the nectin-based cell-cell adhesion sites in an afadin-independent manner. However, the assembled E-cadherin showed weak cell-cell adhesion activity and might be the non-trans-interacting form. This assembly was mediated by the IQGAP1-dependent actin cytoskeleton, which was organized by Cdc42 and Rac small G proteins that were activated by the action of trans-interacting nectin through c-Src and Rap1 small G protein in an afadin-independent manner. However, Rap1 bound to afadin, and this Rap1-afadin complex then interacted with p120(ctn) associated with non-trans-interacting E-cadherin, thereby causing the trans-interaction of E-cadherin. Thus, nectin regulates the assembly and cell-cell adhesion activity of E-cadherin through afadin, nectin signaling, and p120(ctn) for the formation of AJs in Madin-Darby canine kidney cells.

Activity-dependent dendritic spine structural plasticity is regulated by small GTPase Rap1 and its target AF-6

Activity-dependent remodeling of dendritic spines is essential for neural circuit development and synaptic plasticity, but the mechanisms that coordinate synaptic structural and functional plasticity are not well understood. Here we investigate the signaling pathways that enable excitatory synapses to undergo activity-dependent structural modifications. We report that activation of NMDA receptors in cultured cortical neurons induces spine morphogenesis and activation of the small GTPase Rap1. Rap1 bimodally regulates spine morphology: activated Rap1 recruits the PDZ domain-containing protein AF-6 to the plasma membrane and induces spine neck elongation, while inactive Rap1 dissociates AF-6 from the membrane and induces spine enlargement. Rap1 also regulates spine content of AMPA receptors: thin spines induced by Rap1 activation have reduced GluR1-containing AMPA receptor content, while large spines induced by Rap1 inactivation are rich in AMPA receptors. These results identify a signaling pathway that regulates activity-dependent synaptic structural plasticity and coordinates it with functional plasticity.

Mutagenesis of the epithelial polarity gene, discs large 1, perturbs nephrogenesis in the developing mouse kidney

BACKGROUND

During development of the permanent mammalian kidney (metanephros) several key epithelial events occur such as ureteric branching morphogenesis and nephrogenesis. One of the first stages of nephrogenesis involves the conversion of mesenchymal cells to epithelial cells, and thus the metanephros provides an excellent model to study epithelial polarization. The aim of this study was to investigate the role of the epithelial polarity gene, discs large 1 (dlg1), during development of the mouse kidney.

METHODS

We utilized mice with a gene trap vector insertion within dlg1 (dlg(gt)) resulting in a truncated Dlg1 protein, lacking the SH3, protein 4.1 and guanylate kinase-like (GUK) domains, fused to a LacZ reporter. These mice were used to analyze the expression of Dlg1 during kidney development, the subcellular localization of Dlg1 in epithelial cells, and the ability of Dlg1 to bind to calmodulin-associated serine/threonine kinase (CASK). Metanephric organ culture was used to study branching morphogenesis and nephrogenesis in wild-type and dlg(gt) mutant mice.

RESULTS

Dlg1 was expressed in ureteric and mesenchyme-derived epithelial cells during kidney development. Truncation of Dlg1 altered the normal basolateral localization of Dlg1 restricting it to the adherens junction. Due to the loss of the SH3 domain the binding capacity of Dlg1 to CASK was reduced. Nephrogenesis was altered in dlg(gt)/dlg(gt) metanephroi with a 30% decrease in nephron number.

CONCLUSION

Our results indicate that the loss of the SH3, protein 4.1 and/or GUK domains of Dlg1 disrupt epithelial polarity and perturb nephrogenesis either as a secondary consequence to a defect in ureteric branching morphogenesis and/or delay in mesenchyme-to- epithelial transition.

Involvement of nectins in the formation of puncta adherentia junctions and the mossy fiber trajectory in the mouse hippocampus

Synapses are specialized intercellular junctions whose specificity and plasticity are mediated by synaptic cell adhesion molecules. In hippocampus, the mossy fibers form synapses on the apical dendrites of the CA3 pyramidal cells where synaptic and puncta adherentia junctions (PAJs) are highly developed. Synaptic junctions are the sites of neurotransmission, while PAJs are regarded as mechanical adhesion sites. Cell-cell adhesion molecules nectin-1 and nectin-3 asymmetrically localize at the pre- and post-synaptic sides of PAJs, respectively. To reveal the definitive role of nectins, we analyzed nectin-1-/- and nectin-3(-/-) mice. In both the mutant mice, the number of PAJs at the synapses between the mossy fiber terminals and the dendrites of the CA3 pyramidal cells was reduced. In addition, the abnormal mossy fiber trajectory was observed. These results indicate that nectins are involved in the formation of PAJs, which maintain the proper mossy fiber trajectory.

The nectin-1alpha transmembrane domain, but not the cytoplasmic tail, influences cell fusion induced by HSV-1 glycoproteins

Nectin-1 is a receptor for herpes simplex virus (HSV), a member of the immunoglobulin superfamily, and a cellular adhesion molecule. To study domains of nectin-1alpha involved in cell fusion, we measured the ability of nectin-1alpha/nectin-2alpha chimeras, nectin-1alpha/CD4 chimeras, and transmembrane domain and cytoplasmic tail mutants of nectin-1alpha to promote cell fusion induced by HSV-1 glycoproteins. Our results demonstrate that only chimeras and mutants containing the entire V-like domain and a link to the plasma membrane conferred cell-fusion activity. The transmembrane domain and cytoplasmic tail of nectin-1 were not required for any viral receptor or cell adhesion function tested. Cellular cytoplasmic factors that bind to the nectin-1alpha cytoplasmic tail, therefore, did not influence virus entry or cell fusion. Interestingly, the efficiency of cell fusion was reduced when membrane-spanning domains of nectin-1alpha and gD were replaced by glycosylphosphatidylinositol tethers, indicating that transmembrane domains may play a modulatory role in the gD/nectin-1alpha interaction in fusion.

Recruitment of E-cadherin associated with alpha- and beta-catenins and p120ctn to the nectin-based cell-cell adhesion sites by the action of 12-O-tetradecanoylphorbol-13-acetate in MDCK cells

The formation of tight junctions (TJs) is dependent on the formation of adherens junctions (AJs) in MDCK cells. E-Cadherin and nectin are major cell-cell adhesion molecules (CAMs) at AJs, whereas claudin, occludin and junctional adhesion molecule (JAM) are major CAMs at TJs. When MDCK cells precultured at 2 microm Ca(2+) are cultured at 2 mm Ca(2+), nectin first forms cell-cell adhesion and recruits E-cadherin to the nectin-based cell-cell adhesion sites to form AJs. Thereafter, nectin recruits first JAM-A and then claudin-1 and occludin to the apical side of AJs to form TJs. In contrast, when MDCK cells precultured at 2 microm Ca(2+) are cultured at 2 microm Ca(2+) in the presence of a phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), a TJ-like structure is formed without the formation of the E-cadherin-based AJs. We showed here that GFP-E-cadherin, which did not trans-interact due to 2 microm Ca(2+) but associated with alpha- and beta-catenins and p120(ctn), was recruited to the nectin-based cell-cell adhesion sites by the action of TPA. The nectin inhibitors, which inhibited the trans-interaction of nectin, inhibited the recruitment of GFP-E-cadherin and their associating catenins by the action of TPA. Microbeads coated with the extracellular fragment of nectin recruited not only cellular nectin but also GFP-E-cadherin and their associating catenins by the action of TPA. These results indicate that when the TJ-like structure is formed by the action of TPA, non-trans-interacting E-cadherin and its associating catenins are recruited to the nectin-based cell-cell adhesion sites and that the trans-interaction of E-cadherin is not essential for the formation of TJs.

Delta-catenin at the synaptic-adherens junction

Delta-catenin belongs to the p120-catenin (p120(ctn)) protein family, which is characterized by ten, characteristically spaced Armadillo repeats that bind to the juxtamembrane segment of the classical cadherins. Delta-catenin is the only member of this family that is expressed specifically in neurons, where it binds to PDZ domain proteins in the post-synaptic compartment. As a component of both adherens and synaptic junctions, delta-catenin can link the adherens junction to the synapse and, thereby, coordinate synaptic input with changes in the adherens junction. By virtue of its restriction to the post-synaptic area, delta-catenin creates an asymmetric adherens junction in the region of the synapse. The crucial nature of the specialized function of delta-catenin in neurons is demonstrated by a targeted gene mutation, which causes deficits in learning and in synaptic plasticity. Taken together, recent evidence indicates that delta-catenin is a sensor of synaptic activity and implements activity-related morphological changes at the synapse.

Expression of nectin-2 in mouse granulosa cells

OBJECTIVE

The development and maturation of the ovarian follicles are characterized by structural changes that require components involved in cell-cell adhesion systems. Recently a novel group of cell adhesion molecules named nectins has been identified. The present study examined expression and cell-specific localization of nectins during mouse follicular development.

STUDY DESIGN

Expression of nectins in mouse ovary was investigated by immnuoblot analysis and immunohistochemistry. More precise localization was determined by electron microscopy.

RESULTS

Immunoblot analysis revealed expression of nectin-2 and nectin-3 but not nectin-1 in ovarian granulosa cells. Immunohistochemistry demonstrated expression of nectin-2 at cell-cell adhesion sites of granulosa cell layer in the primary and preantral follicles. Especially, intense immunoreactivity of nectin-2 accumulated around the zona pellucida. In antral follicles, the intensity of nectin-2 expression on granulosa cells was decreased. By electron microscopy nectin-2 was detected not only on thin extensions of granulosa cells penetrating the zona pellucida, but also on the attachment sites between thin extensions of granulosa cells and oocyte surface.

CONCLUSION

The restricted expression of nectin-2 in the granulosa cells of primary and preantral follicles might reflect some of the molecular changes in cell-cell adhesion during early follicular development.

PICK-1: a scaffold protein that interacts with Nectins and JAMs at cell junctions

Nectin adhesion molecules are involved in the early steps of cell junction formation. Later during the polarisation process, Nectins are components of epithelial adherens junctions where they are indirectly associated with the E-cadherin/Catenins complex via the adaptator AF-6. To have a better understanding of Nectin-based cell junctions, we looked for some new Nectins' partners. We demonstrate that the scaffold molecule PICK-1, involved in the clustering of junctional receptors in synaptic junctions, interacts directly with Nectins in a PSD-95/Dlg/ZO-1 domain-dependent manner and is localised at adherens junctions in epithelial cells. Finally, we observed that protein interacting with C-kinase-1 (PICK-1) also interacts directly with the junctional adhesion molecules, and we suggest that PICK-1 could be involved in the regulation of both adherens and tight junctions in epithelial cells.

Roles of cell-adhesion molecules nectin 1 and nectin 3 in ciliary body development

Nectins are Ca2+-independent immunoglobulin-like cell-cell-adhesion molecules consisting of four members. Nectins homophilically and heterophilically trans-interact to form a variety of cell-cell junctions, including cadherin-based adherens junctions in epithelial cells and fibroblasts in culture, synaptic junctions in neurons, and Sertoli cell-spermatid junctions in the testis, in cooperation with, or independently of, cadherins. To further explore the function of nectins, we generated nectin 1–/– and nectin 3–/– mice. Both nectin 1–/– and nectin 3–/– mice showed a virtually identical ocular phenotype, microphthalmia, accompanied by a separation of the apex-apex contact between the pigment and non-pigment cell layers of the ciliary epithelia. Immunofluorescence and immunoelectron microscopy revealed that nectin 1 and nectin 3, but not nectin 2, localized at the apex-apex junctions between the pigment and non-pigment cell layers of the ciliary epithelia. However, nectin 1–/– and nectin 3–/– mice showed no impairment of the apicolateral junctions between the pigment epithelia where nectin 1, nectin 2 and nectin 3 localized, or of the apicolateral junctions between the non-pigment epithelia where nectin 2 and nectin 3, but not nectin 1, localized. These results indicate that the heterophilic trans-interaction between nectin 1 and nectin 3 plays a sentinel role in establishing the apex-apex adhesion between the pigment and non-pigment cell layers of the ciliary epithelia that is essential for the morphogenesis of the ciliary body.

Nectin-like molecule-1/TSLL1/SynCAM3: a neural tissue-specific immunoglobulin-like cell-cell adhesion molecule localizing at non-junctional contact sites of presynaptic nerve terminals, axons and glia cell processes

Nectins are Ca2+-independent immunoglobulin-like cell-cell adhesion molecules and comprise a family of four members. At the mossy fiber terminals of hippocampus, nectin-1 and nectin-3 localize at the presynaptic and postsynaptic sides of synaptic junctions, respectively, and their trans-interactions play a role in formation of synapses in cooperation with N-cadherin. Nectins are associated with the actin cytoskeleton through afadin, a nectin- and actin-filament-binding protein. Five nectin-like molecules (Necls) which have domain structures similar to those of nectins have been identified and here we characterize Necl-1/TSLL1/SynCAM3, from now on referred to as Necl-1. Tissue distribution analysis showed that Necl-1 was specifically expressed in the neural tissue. Immunofluorescence and immunoelectron microscopy revealed that Necl-1 localized at the contact sites among axons, their terminals, and glia cell processes that cooperatively formed synapses, axon bundles and myelinated axons. Necl-1 showed Ca2+-independent homophilic cell-cell adhesion activity. It furthermore showed Ca2+-independent heterophilic cell-cell adhesion activity with Necl-2/IGSF4/RA175/SgIGSF/TSLC1/SynCAM1 from now on referred to as Necl-2, nectin-1 and nectin-3, but not with Necl-5 or nectin-2. The C-terminal cytoplasmic region of Necl-1 did not bind afadin but bound membrane-associated guanylate kinase subfamily members that contain the L27 domain, including Dlg3, Pals2 and CASK. These results indicate that Necl-1 is a neural-tissue-specific Ca2+-independent immunoglobulin-like cell-cell adhesion molecule which potentially has membrane-associated guanylate kinase subfamily member-binding activity and localizes at the non-junctional cell-cell contact sites.

Separation Force Measurements Reveal Different Types of Modulation of E-cadherin-based Adhesion by Nectin-1 and -3

Nectins are Ca2+-independent cell adhesion molecules found at cadherin-based adherens junctions. We used a dual pipette assay that measures the forces required to separate cell doublets to determine how nectins affect the formation and strength of cell-cell adhesion. Less force was required to separate doublets of L cells expressing nectin-1 or nectin-3 than to separate doublets of E-cadherin-expressing cells. Heterodimers formed between cells expressing nectin-1 or nectin-3 adhered more strongly than homodimers. Nectin-3 that does not trans-interact with nectin-1 inhibited E-cadherin-mediated adhesion. However, the extracellular fragment of nectin-1 did not have an agonistic effect on E-cadherin-dependent cell adhesion when it trans-interacted with nectin-3, expressed at high levels in cells. In contrast, the extracellular fragment of nectin-3 had a significant agonistic effect on cadherin-based adhesion when it interacted with endogenous nectin-1, expressed at low levels in cells. Our results indicate that E-cadherin is the key molecule involved in cell adhesion and that the regulation of E-cadherin-based adhesion involving cellular nectin-1 trans-interacting with nectin-3 is qualitatively different from that involving cellular nectin-3 trans-interacting with nectin-1 and depends on the nectin levels expressed by cells.

Solution structure of AF-6 PDZ domain and its interaction with the C-terminal peptides from Neurexin and Bcr

AF-6 is a key molecule essential for structure organization of cell-cell junction of polarized epithelia. It belongs to a novel cell-cell adhesion system. The AF-6 PDZ domain mediates interactions by binding to a specific amino acid sequence in target proteins. Here we report the solution structure of the AF-6 PDZ domain determined by NMR. Previously, the AF-6 PDZ domain was considered to be a class II PDZ domain. However we found that a unique hydrophilic amino acid, Gln70, at position alphaB1 makes the alphaB/betaB groove of the AF-6 PDZ domain significantly different from that of the canonical class II PDZ domain. The AF-6 PDZ domain does not have the second hydrophobic binding pocket, and the N-terminal end of alphaB is closer to betaB. Using BIACORE and NMR chemical shift perturbation experiments, we have studied the binding characteristics of the PDZ domain to the C-terminal peptide of Neurexin, KKNKDKEYYV, and that of Bcr, KRQSILFSTEV. The C-terminal peptide of Neurexin is a class II ligand, whereas that of Bcr is a class I ligand. The dissociation constants of these ligands were 4.08 x 10(-7) and 2.23 x 10(-6) m, respectively. Each of the four C-terminal positions in Neurexin and Bcr may contribute to the interaction. The three-dimensional models of the AF-6 PDZ-Neurexin C-terminal peptide complex and the AF-6 PDZ-Bcr C-terminal peptide complex were built up by molecular dynamics simulations. Unlike the canonical class II PDZ domain, Ala74 at alphaB5 rather than the residue at alphaB1 makes direct hydrophobic contact with the side chain of Tyr at the -2 position of the ligand.

Rap1 GTPase inhibits leukocyte transmigration by promoting endothelial barrier function

The passage of leukocytes out of the blood circulation and into tissues is necessary for the normal inflammatory response, but it also occurs inappropriately in many pathological situations. This process is limited by the barrier presented by the junctions between adjacent endothelial cells that line blood vessels. Here we show that activation of the Rap1 GTPase in endothelial cells accelerated de novo assembly of endothelial cell-cell junctions and increased the barrier function of endothelial monolayers. In contrast, depressing Rap1 activity by expressing Rap1GAP led to disassembly of these junctions and increased their permeability. We also demonstrate that endogenous Rap1 was rapidly activated at early stages of junctional assembly, confirming the involvement of Rap1 during junctional assembly. Intriguingly, elevating Rap1 activity selectively within endothelial cells decreased leukocyte transendothelial migration, whereas inhibiting Rap1 activity by expression of Rap1GAP increased leukocyte transendothelial migration, providing physiological relevance to our hypothesis that Rap1 augments barrier function of inter-endothelial cell junctions. Furthermore, these results suggest that Rap1 may be a novel therapeutic target for clinical conditions in which an inappropriate inflammatory response leads to disease.