Cooperative role of nectin-nectin and nectin-afadin interactions in formation of nectin-based cell-cell adhesion

Rac1 and Nectin3 are essential for planar cell polarity-directed axon guidance in the peripheral auditory system

Spiral ganglion neurons (SGNs) carry sound information from the cochlea to the hindbrain, and innervate either inner or outer hair cells. Type II SGNs (SGNIIs) extend peripheral afferents towards outer hair cells, which make a characteristic 90° turn towards the cochlear base and innervate multiple outer hair cells. It has been shown that the planar cell polarity (PCP) pathway acts non-autonomously in the cochlear epithelium to guide SGNII peripheral afferent turning. However, the underlying mechanisms are unknown. Here, we show that PCP signaling regulates junctional localization of the small GTPase Rac1 and the cell adhesion molecule Nectin3 in mouse cochlear supporting cells (SCs), which serve as intermediate targets of SGNII peripheral afferents. Loss of Rac1 or Nectin3 partially phenocopied SGNII peripheral afferent turning defects in Vangl2 mutants. We present evidence that Rac1 plays a non-autonomous role in part by regulating the localization of core PCP proteins Vangl2 and Dvl3 at the SC-SC junctions, while Nectin3 likely serves a cell adhesion function to control SGNII afferent turning. Together, these experiments identify Nectin3 and Rac1 as regulators of PCP-directed SGNII axon guidance in the cochlea.

Decoding the Nectin Interactome: Implications for Brain Development, Plasticity, and Neurological Disorders

The nectin family of cell adhesion molecules (CAMs) comprising nectins and nectin-like molecules has emerged as a key regulator of various pivotal neural processes, including neuronal development, migration, synapse formation, and plasticity. Nectins engage in homophilic and heterophilic interactions to mediate cell-cell adhesion, contributing to the establishment and maintenance of neural circuits. Their extracellular domains facilitate trans-synaptic interactions, while intracellular domains participate in signaling cascades influencing cytoskeletal dynamics and synaptic function. The exhibition of distinct localization patterns in neurons, astrocytes, and the blood-brain barrier underscores their diverse roles in the brain. The dysregulation of nectins has been implicated in several neurological disorders, such as neurodevelopmental disorders, depression, schizophrenia, and Alzheimer's disease. This review examines the structural and functional characteristics of nectins and their distribution and molecular mechanisms governing neural connectivity and cognition. It further discusses experimental studies unraveling nectin-mediated pathophysiology and potential therapeutic interventions targeting nectin-related pathways. Collectively, this comprehensive analysis highlights the significance of nectins in brain development, function, and disorders, paving the way for future research directions and clinical implications.

Targeted therapeutic strategies for Nectin-4 in breast cancer: Recent advances and future prospects

Nectin-4 is a cell adhesion molecule which has gained more and more attention as a therapeutic target in cancer recently. Overexpression of Nectin-4 has been observed in various tumors, including breast cancer, and is associated with tumor progression. Enfortumab vedotin(EV)is an antibody-drug conjugate (ADC) targeting Nectin-4, which has been approved by FDA for the treatment of urothelial carcinoma. Notably, Nectin-4 was also investigated as a target for breast cancer in preclinical and clinical settings. Nectin-4-targeted approaches, such as ADCs, oncolytic viruses, photothermal therapy and immunotherapy, have shown promising results in early-phase clinical trials. These therapies offer novel strategies for delivering targeted treatments to Nectin-4-expressing cancer cells, enhancing treatment efficacy and minimizing off-target effects. In conclusion, this review aims to provide an overview of the latest advances in understanding the role of Nectin-4 in breast cancer and discuss the future development prospects of Nectin-4 targeted agents.

Evidence that Nectin-3 is the soybean agglutinin binding protein on rat corneal endothelium cell surfaces

The means by which the lectin soybean agglutinin (SBA) binds to the corneal endothelium cell surface following explantation into organ culture was investigated using Sprague-Dawley rats. SBA binding does not occur in freshly isolated and fixed rat corneal endothelium. However, after 48 h in organ culture, SBA binding occurs in a punctate pattern that clearly outlines all endothelial cells of the tissue monolayer. To determine what cell surface component was responsible for this binding, a series of experiments were employed that focused on the possibility that SBA bound to a nectin molecule(s). To this extent we performed a series of immunocytochemical localizations using antibodies against either nectin-2, nectin-3 or nectin-4. Of these, only nectin-3 bound to the endothelium in a manner that mimicked SBA binding. To further verify that nectin-3 bound SBA, displacement experiments employing non-labeled SBA were undertaken. Following a 48 h organ culture, tissues were fixed and incubated with SBA followed by exposure to nectin-3 antibody. No subsequent immunofluorescence could be detected, indicating that anti-nectin-3 binding was prevented. Likewise, when organ-cultured tissues were fixed and incubated in anti-nectin-3 antibody, followed by SBA exposure, no SBA binding could be detected. These results suggest that stresses accompanying explantation of the tissue into organ culture promote the appearance of nectin-3 around the cell periphery. The emergence of nectin-3 along the peripheral endothelial cell membrane in organ culture may imply a necessary role for this molecule in maintaining monolayer integrity and barrier function during either a pathologic condition, wound repair, or in organ storage.

Multiple Receptors Involved in Invasion and Neuropathogenicity of Canine Distemper Virus: A Review

The canine distemper virus (CDV) is a morbillivirus that infects a broad range of terrestrial carnivores, predominantly canines, and is associated with high mortality. Similar to another morbillivirus, measles virus, which infects humans and nonhuman primates, CDV transmission from an infected host to a naïve host depends on two cellular receptors, namely, the signaling lymphocyte activation molecule (SLAM or CD150) and the adherens junction protein nectin-4 (also known as PVRL4). CDV can also invade the central nervous system by anterograde spread through olfactory nerves or in infected lymphocytes through the circulation, thus causing chronic progressive or relapsing demyelination of the brain. However, the absence of the two receptors in the white matter, primary cultured astrocytes, and neurons in the brain was recently demonstrated. Furthermore, a SLAM/nectin-4-blind recombinant CDV exhibits full cell-to-cell transmission in primary astrocytes. This strongly suggests the existence of a third CDV receptor expressed in neural cells, possibly glial cells. In this review, we summarize the recent progress in the study of CDV receptors, highlighting the unidentified glial receptor and its contribution to pathogenicity in the host nervous system. The reviewed studies focus on CDV neuropathogenesis, and neural receptors may provide promising directions for the treatment of neurological diseases caused by CDV. We also present an overview of other neurotropic viruses to promote further research and identification of CDV neural receptors.

Afadin (AF6) in cancer progression: A multidomain scaffold protein with complex and contradictory roles

Adherens (AJ) and tight junctions (TJ) maintain cell-cell adhesions and cellular polarity in normal tissues. Afadin, a multi-domain scaffold protein, is commonly found in both adherens and tight junctions, where it plays both structural and signal-modulating roles. Afadin is a complex modulator of cellular processes implicated in cancer progression, including signal transduction, migration, invasion, and apoptosis. In keeping with the complexities associated with the roles of adherens and tight junctions in cancer, afadin exhibits both tumor suppressive and pro-metastatic functions. In this review, we will explore the dichotomous roles that afadin plays during cancer progression.

Identification of amino acid residues involved in the interaction between peste-des-petits-ruminants virus haemagglutinin protein and cellular receptors

Peste-des-petits-ruminants virus (PPRV) haemagglutinin (H) protein mediates binding to cellular receptors and then initiates virus entry. To identify the key residues of PPRV H (Hv) protein of the Nigeria 75/1 strain involved in binding to receptors, interaction of the Hv and mutated Hv (mHv) proteins with receptors (SLAM and Nectin 4) and their mutants (mSLAM1, mSLAM2, mSLAM3 and mNectin 4) was investigated using surface plasmon resonance imaging (SPRi) and coimmunoprecipitation (co-IP) assays. The results showed that the Hv protein failed to interact with mSLAM3, but interacted at a strong or medium intensity with SLAM, mSLAM2, Nectin 4 and mNectin 4, and at a low level with mSLAM1. The mHv protein was unable to interact with SLAM and its mutants, but bound to Nectin 4 and mNectin 4 with medium and weak intensity, respectively. Further analysis showed that the Hv protein could precipitate mSLAM1, mSLAM2 and mNectin 4, but not mSLAM3. The mHv protein failed to coprecipitate with SLAM and its mutants. The binding activities of mNectin 4 and Nectin 4 to mHv were less than 30.36 and 51.94 % of the wild-type levels, respectively. Based on the results obtained, amino acids at positions R389, L464, I498, R503, R533, Y541, Y543, F552 and Y553 of H protein and I61, H62, L64, K76, K78, E123, H130, I210, A211, S226 and R227 in SLAM were identified to be essential for the specificity of H-SLAM interaction, while the critical residues of H-Nectin 4 interaction require further study. These findings would improve our understanding of the invasive mechanisms of PPRV.

Trans-endocytosis elicited by nectins transfers cytoplasmic cargo, including infectious material, between cells

Here, we show that cells expressing the adherens junction protein nectin-1 capture nectin-4-containing membranes from the surface of adjacent cells in a trans-endocytosis process. We find that internalized nectin-1-nectin-4 complexes follow the endocytic pathway. The nectin-1 cytoplasmic tail controls transfer: its deletion prevents trans-endocytosis, while its exchange with the nectin-4 tail reverses transfer direction. Nectin-1-expressing cells acquire dye-labeled cytoplasmic proteins synchronously with nectin-4, a process most active during cell adhesion. Some cytoplasmic cargo remains functional after transfer, as demonstrated with encapsidated genomes of measles virus (MeV). This virus uses nectin-4, but not nectin-1, as a receptor. Epithelial cells expressing nectin-4, but not those expressing another MeV receptor in its place, can transfer infection to nectin-1-expressing primary neurons. Thus, this newly discovered process can move cytoplasmic cargo, including infectious material, from epithelial cells to neurons. We name the process nectin-elicited cytoplasm transfer (NECT). NECT-related trans-endocytosis processes may be exploited by pathogens to extend tropism. This article has an associated First Person interview with the first author of the paper.

Upregulation of nectin-4 is associated with ITGB1 and vasculogenic mimicry and may serve as a predictor of poor prognosis in colorectal cancer

Colorectal cancer (CRC) is one of the most common malignancies worldwide. Unlike endothelium-dependent vasculature, vasculogenic mimicry (VM) is an alternative type of blood supply in tumors that is frequently associated with poor patient outcome. Nectin-4 serves a vital role in the formation and maintenance of adherens junctions; integrin β-1 (ITGB1) promotes tumor invasion, metastasis and VM formation. In the present study, the analysis of nectin-4 mRNA expression in a database of The Cancer Genome Atlas (TCGA) was combined with that of another non-overlapping cohort of 68 patients with CRC. TCGA data were used to examine nectin-4 mRNA expression in CRC and its correlation with the clinicopathological features of patients. Data from the non-overlapping cohort of patients was used to determine nectin-4 and ITGB1 protein expression in CRC by immunohistochemical (IHC) staining. Cluster of differentiation 34/periodic acid-Schiff double staining was performed to validate the presence of VM formation. The association with, and significance of combining nectin-4, ITGB1 protein expression and VM formation for predicting patient prognosis was evaluated. The TCGA dataset demonstrated that nectin-4 mRNA was upregulated in CRC, which was significantly relate to lymph node metastasis (P=0.0017), distant metastasis (P=0.0045), and tumor-node-metastasis (TNM) stage (P=0.0015). Of the 68 patients analyzed by IHC staining, 48 (70.6%) were positive for nectin-4, 46 (67.6%) for ITGB1 and 17 (25%) for VM formation. Nectin-4 protein expression was associated with ITGB1 protein expression (P<0.01) and VM formation (P<0.05). Nectin-4, ITGB1 expression and VM formation were associated with distant metastasis stage (P<0.05) and TNM stage (P<0.05). Based on these findings it was concluded that nectin-4 was upregulated in CRC tissues compared with normal mucosal tissues, and was associated with ITGB1 expression and VM formation. Furthermore, nectin-4 and ITGB1 protein expression, together with VM formation may be used to predict poor prognosis in CRC.

Dynamic spatiotemporal patterns of alternative splicing of an F-actin scaffold protein, afadin, during murine development

An F-actin scaffold protein, afadin, comprises two splice variants called l-afadin (a long isoform) and s-afadin (a short isoform). It is known that in adult tissues, l-afadin is ubiquitously expressed while s-afadin is restrictedly expressed in brain. In cultured cortical neurons, l-afadin potentiates axonal branching whereas s-afadin blocks axonal branching by functioning as a naturally occurring dominant-negative isoform that forms a heterodimer with l-afadin. However, the temporal and spatial expression pattern of s-afadin during development or across multiple tissues and organs has not been fully understood. In this study, using Western blotting and RT-qPCR techniques and the fluorescent splicing reporters, we examined the detailed expression patterns of l- and s-afadin isoforms across various tissues and cell types, including rat organs at developmental stages, primary cultured neuronal and non-neuronal cells prepared from the developing rat brain, and in neurons in vitro generated from P19 embryonal carcinoma (EC) cells. Both mRNA and protein of s-afadin were abundantly expressed in various regions of rat neuronal tissues, and their expression dynamically changed during development in vivo. The expression of s-afadin was also detected in primary rat cortical neurons, but not in astrocytes or fibroblasts, and the neuronal expression increased during neuronal maturation in vitro. The dynamic alternative splicing event of afadin during development was successfully visualized with the newly developed fluorescent splicing reporter plasmids at a single cell level. Moreover, s-afadin was undetectable in undifferentiated EC cells, and the expression was induced upon differentiation of the cells into neurons. These data suggest that spatiotemporal and dynamic alternative splicing produces differential expression patterns of afadin isoforms and that alternative splicing of afadin is controlled by the neuron-specific regulator(s) whose activity is triggered and dynamically altered during neuronal differentiation and maturation.

Oncogenic Deregulation of Cell Adhesion Molecules in Leukemia

Numerous cell⁻cell and cell⁻matrix interactions within the bone marrow microenvironment enable the controlled lifelong self-renewal and progeny of hematopoietic stem and progenitor cells (HSPCs). On the cellular level, this highly mutual interaction is granted by cell adhesion molecules (CAMs) integrating differentiation, proliferation, and pro-survival signals from the surrounding microenvironment to the inner cell. However, cell⁻cell and cell⁻matrix interactions are also critically involved during malignant transformation of hematopoietic stem/progenitor cells. It has become increasingly apparent that leukemia-associated gene products, such as activated tyrosine kinases and fusion proteins resulting from chromosomal translocations, directly regulate the activation status of adhesion molecules, thereby directing the leukemic phenotype. These observations imply that interference with adhesion molecule function represents a promising treatment strategy to target pre-leukemic and leukemic lesions within the bone marrow niche. Focusing on myeloid leukemia, we provide a current overview of the mechanisms by which leukemogenic gene products hijack control of cellular adhesion to subsequently disturb normal hematopoiesis and promote leukemia development.

Differential regional and subcellular localization patterns of afadin splice variants in the mouse central nervous system

AF6/afadin is an F-actin scaffold protein that plays essential roles in the organization of cell-cell junctions of polarized epithelia. Afadin comprises two major isoforms produced by alternative splicing - a longer isoform l-afadin, having the F-actin-binding domain, and a shorter isoform s-afadin, harboring the amino acid sequences unique to the isoform but lacking the F-actin-binding domain. We recently identified functional differences between l- and s-afadin isoforms in the regulation of axon branching in primary cultured cortical neurons; the former potentiates and the latter blocks axon branching. Previous biochemical reports indicate differences in tissue and cell-type distributions of isoforms, and it was shown that l-afadin is ubiquitously expressed in various tissues and cell-types, while s-afadin is predominantly expressed in neuronal tissues and cultured neurons. However, the spatial expression pattern of s-afadin across neuronal tissues or within neurons has not been revealed because no antibody specific for s-afadin is yet available. In this study, we report the generation and characterization of an antibody that specifically distinguishes s-afadin from l-afadin, and its application to investigate the expression profile of s-afadin in primary cultured neurons and tissue cryosections of adult mouse brain and retina. We describe differential regional and subcellular localization patterns of l- and s-afadin isoforms in the mouse central nervous system.

Structural and Intermolecular Associations Between Connexin36 and Protein Components of the Adherens Junction-Neuronal Gap Junction Complex

Intimate structural and functional relationships between gap junctions and adherens junctions have been demonstrated in peripheral tissues, but have not been thoroughly examined in the central nervous system, where adherens junctions are often found in close proximity to neuronal gap junctions. Here, we used immunofluorescence approaches to document the localization of various protein components of adherens junctions in relation to those that we have previously reported to occur at electrical synapses formed by neuronal gap junctions composed of connexin36 (Cx36). The adherens junction constituents N-cadherin and nectin-1 were frequently found to localize near or overlap with Cx36-containing gap junctions in several brain regions examined. This was also true of the adherens junction-associated proteins α-catenin and β-catenin, as well as the proteins zonula occludens-1 and AF6 (aka, afadin) that were reported constituents of both adherens junctions and gap junctions. The deployment of the protein constituents of these junctions was especially striking at somatic contacts between primary afferent neurons in the mesencephalic trigeminal nucleus (MesV), where the structural components of adherens junctions appeared to be maintained in connexin36 null mice. These results support emerging views concerning the multi-molecular composition of electrical synapses and raise possibilities for various structural and functional protein-protein interactions at what now can be considered the adherens junction-neuronal gap junction complex. Further, the results point to intracellular signaling pathways that could potentially contribute to the assembly, maintenance and turnover of this complex, as well as to the dynamic nature of neuronal communication at electrical synapses.

Nectin spot: a novel type of nectin-mediated cell adhesion apparatus

Nectins are Ca(2+)-independent immunoglobulin (Ig) superfamily cell adhesion molecules constituting a family with four members, all of which have three Ig-like loops at their extracellular regions. Nectins play roles in the formation of a variety of cell-cell adhesion apparatuses. There are at least three types of nectin-mediated cell adhesions: afadin- and cadherin-dependent, afadin-dependent and cadherin-independent, and afadin- and cadherin-independent. In addition, nectins trans-interact with nectin-like molecules (Necls) with three Ig-like loops and other Ig-like molecules with one to three Ig-like loops. Furthermore, nectins and Necls cis-interact with membrane receptors and integrins, some of which are associated with the nectin-mediated cell adhesions, and play roles in the regulation of many cellular functions, such as cell polarization, movement, proliferation, differentiation, and survival, co-operatively with these cell surface proteins. The nectin-mediated cell adhesions are implicated in a variety of diseases, including genetic disorders, neural disorders, and cancers. Of the three types of nectin-mediated cell adhesions, the afadin- and cadherin-dependent apparatus has been most extensively investigated, but the examples of the third type of apparatus independent of afadin and cadherin are recently increasing and its morphological and functional properties have been well characterized. We review here recent advances in research on this type of nectin-mediated cell adhesion apparatus, which is named nectin spot.

Multiple roles of afadin in the ultrastructural morphogenesis of mouse hippocampal mossy fiber synapses

A hippocampal mossy fiber synapse, which is implicated in learning and memory, has a complex structure in which mossy fiber boutons attach to the dendritic shaft by puncta adherentia junctions (PAJs) and wrap around a multiply-branched spine, forming synaptic junctions. Here, we electron microscopically analyzed the ultrastructure of this synapse in afadin-deficient mice. Transmission electron microscopy analysis revealed that typical PAJs with prominent symmetrical plasma membrane darkening undercoated with the thick filamentous cytoskeleton were observed in the control synapse, whereas in the afadin-deficient synapse, atypical PAJs with the symmetrical plasma membrane darkening, which was much less in thickness and darkness than those of the control typical PAJs, were observed. Immunoelectron microscopy analysis revealed that nectin-1, nectin-3, and N-cadherin were localized at the control typical PAJs, whereas nectin-1 and nectin-3 were localized at the afadin-deficient atypical PAJs to extents lower than those in the control synapse and N-cadherin was localized at their nonjunctional flanking regions. These results indicate that the atypical PAJs are formed by nectin-1 and nectin-3 independently of afadin and N-cadherin and that the typical PAJs are formed by afadin and N-cadherin cooperatively with nectin-1 and nectin-3. Serial block face-scanning electron microscopy analysis revealed that the complexity of postsynaptic spines and mossy fiber boutons, the number of spine heads, the area of postsynaptic densities, and the density of synaptic vesicles docked to active zones were decreased in the afadin-deficient synapse. These results indicate that afadin plays multiple roles in the complex ultrastructural morphogenesis of hippocampal mossy fiber synapses.

Ectodomain shedding of the cell adhesion molecule Nectin-4 in ovarian cancer is mediated by ADAM10 and ADAM17

We previously showed that the cell adhesion molecule Nectin-4 is overexpressed in ovarian cancer tumors, and its cleaved extracellular domain can be detected in the serum of ovarian cancer patients. The ADAM (adisintegrin and metalloproteinase) proteases are involved in ectodomain cleavage of transmembrane proteins, and ADAM17 is known to cleave Nectin-4 in breast cancer. However, the mechanism of Nectin-4 cleavage in ovarian cancer has not yet been determined. Analysis of ovarian cancer gene microarray data showed that higher expression of Nectin-4, ADAM10, and ADAM17 is associated with significantly decreased progression-free survival. We quantified Nectin-4 shedding from the surface of ovarian cancer cells after stimulation with lysophosphatidic acid. We report that ADAM17 and ADAM10 cleave Nectin-4 and release soluble Nectin-4 (sN4). Small molecule inhibitors and siRNA knockdown of both ADAM proteases confirmed these results. In matched samples from 11 high-grade serous ovarian cancer patients, we detected 2-20-fold more sN4 in ascites fluid than serum. Co-incubation of ovarian cancer cells with ascites fluid significantly increased sN4 shedding, which could be blocked using a dual inhibitor of ADAM10 and ADAM17. Furthermore, we detected RNA for Nectin-4, ADAM10, and ADAM17 in primary ovarian carcinoma tumors, secondary omental metastases, and ascites cells isolated from serous ovarian cancer patients. In a signaling pathway screen, lysophosphatidic acid increased phosphorylation of AKT, EGF receptor, ERK1/2, JNK1/2/3, and c-Jun. Understanding the function of Nectin-4 shedding in ovarian cancer progression is critical to facilitate its development as both a serum biomarker and a therapeutic target for ovarian cancer.

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.

Nectin-2 (CD112) Is Expressed on Outgrowth Endothelial Cells and Regulates Cell Proliferation and Angiogenic Function

Outgrowth endothelial cells (OECs) are a subpopulation of endothelial progenitor cells (EPCs) that have the capacity for proliferation and the ability to promote angiogenesis. In this study, we identified Nectin-2 as a surface protein of OECs through unbiased quantitative proteomics analysis. Using immunocytochemistry and flow cytometry, we confirmed that Nectin-2 is highly expressed on OECs. Nectin-2 (CD112) expression was limited or lower on mononuclear cells (MNCs) and mature tube-forming endothelial cells (ECs). Blocking Nectin-2 with a neutralizing monoclonal antibody significantly increased the trans-well migration and tube forming capacity of OECs. Similarly, Nectin-2 knockdown resulted in enhanced tube formation, cell migration and proliferation with p-Erk activation. Moreover, Nectin-2 deficiency resulted in compensatory increase of other Nectin family genes including Nectin-3 and Necl-4 which promote VEGFR signaling. These results indicate that Nectin-2 is a surface marker and an important regulator of OECs, with significant implications for the isolation of OECs and blocking Nectin-2 on OECs by an antibody for angiogenic applications.

A Novel Nectin-mediated Cell Adhesion Apparatus That Is Implicated in Prolactin Receptor Signaling for Mammary Gland Development

Mammary gland development is induced by the actions of various hormones to form a structure consisting of collecting ducts and milk-secreting alveoli, which comprise two types of epithelial cells known as luminal and basal cells. These cells adhere to each other by cell adhesion apparatuses whose roles in hormone-dependent mammary gland development remain largely unknown. Here we identified a novel cell adhesion apparatus at the boundary between the luminal and basal cells in addition to desmosomes. This apparatus was formed by the trans-interaction between the cell adhesion molecules nectin-4 and nectin-1, which were expressed in the luminal and basal cells, respectively. Nectin-4 of this apparatus further cis-interacted with the prolactin receptor in the luminal cells to enhance the prolactin-induced prolactin receptor signaling for alveolar development with lactogenic differentiation. Thus, a novel nectin-mediated cell adhesion apparatus regulates the prolactin receptor signaling for mammary gland development.

The Nectin-4/Afadin Protein Complex and Intercellular Membrane Pores Contribute to Rapid Spread of Measles Virus in Primary Human Airway Epithelia

The discovery that measles virus (MV) uses the adherens junction protein nectin-4 as its epithelial receptor provides a new vantage point from which to characterize its rapid spread in the airway epithelium. We show here that in well-differentiated primary cultures of airway epithelial cells from human donors (HAE), MV infectious centers form rapidly and become larger than those of other respiratory pathogens: human respiratory syncytial virus, parainfluenza virus 5, and Sendai virus. While visible syncytia do not form after MV infection of HAE, the cytoplasm of an infected cell suddenly flows into an adjacent cell, as visualized through wild-type MV-expressed cytoplasmic green fluorescent protein (GFP). High-resolution video microscopy documents that GFP flows through openings that form on the lateral surfaces between columnar epithelial cells. To assess the relevance of the protein afadin, which connects nectin-4 to the actin cytoskeleton, we knocked down its mRNA. This resulted in more-limited infectious-center formation. We also generated a nectin-4 mutant without the afadin-binding site in its cytoplasmic tail. This mutant was less effective than wild-type human nectin-4 at promoting MV infection in primary cultures of porcine airway epithelia. Thus, in airway epithelial cells, MV spread requires the nectin-4/afadin complex and is based on cytoplasm transfer between columnar cells. Since the viral membrane fusion apparatus may open the passages that allow cytoplasm transfer, we refer to them as intercellular membrane pores. Virus-induced intercellular pores may contribute to extremely efficient measles contagion by promoting the rapid spread of the virus through the upper respiratory epithelium.

IMPORTANCE

Measles virus (MV), while targeted for eradication, still causes about 120,000 deaths per year worldwide. The recent reemergence of measles in insufficiently vaccinated populations in Europe and North America reminds us that measles is extremely contagious, but the processes favoring its spread in the respiratory epithelium remain poorly defined. Here we characterize wild-type MV spread in well-differentiated primary cultures of human airway epithelial cells. We observed that viral infection promotes the flow of cytoplasmic contents from infected to proximal uninfected columnar epithelial cells. Cytoplasm flows through openings that form on the lateral surfaces. Infectious-center growth is facilitated by afadin, a protein connecting the adherens junction and the actin cytoskeleton. The viral fusion apparatus may open intercellular pores, and the cytoskeleton may stabilize them. Rapid homogenization of cytoplasmic contents in epithelial infectious centers may favor rapid spread and contribute to the extremely contagious nature of measles.

A short splicing isoform of afadin suppresses the cortical axon branching in a dominant-negative manner

Suppression of surplus axon branching is crucial for formation of proper neuronal networks; however, the molecular mechanisms have been poorly understood. In a novel mechanism, s-afadin, a short splicing isoform of afadin lacking the F-actin–binding domain, acts as a dominant-negative suppressor of cortical axon branching.

Precise wiring patterns of axons are among the remarkable features of neuronal circuit formation, and establishment of the proper neuronal network requires control of outgrowth, branching, and guidance of axons. R-Ras is a Ras-family small GTPase that has essential roles in multiple phases of axonal development. We recently identified afadin, an F-actin–binding protein, as an effector of R-Ras mediating axon branching through F-actin reorganization. Afadin comprises two isoforms—l-afadin, having the F-actin–binding domain, and s-afadin, lacking the F-actin–binding domain. Compared with l-afadin, s-afadin, the short splicing variant of l-afadin, contains RA domains but lacks the F-actin–binding domain. Neurons express both isoforms; however, the function of s-afadin in brain remains unknown. Here we identify s-afadin as an endogenous inhibitor of cortical axon branching. In contrast to the abundant and constant expression of l-afadin throughout neuronal development, the expression of s-afadin is relatively low when cortical axons branch actively. Ectopic expression and knockdown of s-afadin suppress and promote branching, respectively. s-Afadin blocks the R-Ras–mediated membrane translocation of l-afadin and axon branching by inhibiting the binding of l-afadin to R-Ras. Thus s-afadin acts as a dominant-negative isoform in R-Ras-afadin–regulated axon branching.

Nectins and nectin-like molecules in development and disease

Nectins and nectin-like molecules (Necls)/Cadms are Ca(2+)-independent immunoglobulin superfamily cell adhesion molecules, expressed in most cell types. Nectins mediate not only homotypic but also heterotypic cell-cell adhesion, in contrast to classic cadherins which participate only in homophilic adhesion. Nectins and Necls function in organogenesis of the eye, inner ear, tooth, and cerebral cortex and in a variety of developmental processes including spermatogenesis, axon guidance, synapse formation, and myelination. They are also involved in various diseases, such as viral infection, hereditary ectodermal dysplasia, Alzheimer's disease, autism spectrum disorder, and cancer. Thus, nectins and Necls are crucial for both physiology and pathology. This review summarizes recent advances in research on these cell adhesion molecules in development and pathogenesis.

Crystal structure of afadin PDZ domain-nectin-3 complex shows the structural plasticity of the ligand-binding site

Afadin, a scaffold protein localized in adherens junctions (AJs), links nectins to the actin cytoskeleton. Nectins are the major cell adhesion molecules of AJs. At the initial stage of cell-cell junction formation, the nectin-afadin interaction plays an indispensable role in AJ biogenesis via recruiting and tethering other components. The afadin PDZ domain (AFPDZ) is responsible for binding the cytoplasmic C-terminus of nectins. AFPDZ is a class II PDZ domain member, which prefers ligands containing a class II PDZ-binding motif, X-Φ-X-Φ (Φ, hydrophobic residues); both nectins and other physiological AFPDZ targets contain this class II motif. Here, we report the first crystal structure of the AFPDZ in complex with the nectin-3 C-terminal peptide containing the class II motif. We engineered the nectin-3 C-terminal peptide and AFPDZ to produce an AFPDZ-nectin-3 fusion protein and succeeded in obtaining crystals of this complex as a dimer. This novel dimer interface was created by forming an antiparallel β sheet between β2 strands. A major structural change compared with the known AFPDZ structures was observed in the α2 helix. We found an approximately 2.5 Å-wider ligand-binding groove, which allows the PDZ to accept bulky class II ligands. Apparently, the last three amino acids of the nectin-3 C-terminus were sufficient to bind AFPDZ, in which the two hydrophobic residues are important.

The tumor-associated marker, PVRL4 (nectin-4), is the epithelial receptor for morbilliviruses

PVRL4 (nectin-4) was recently identified as the epithelial receptor for members of the Morbillivirus genus, including measles virus, canine distemper virus and peste des petits ruminants virus. Here, we describe the role of PVRL4 in morbillivirus pathogenesis and its promising use in cancer therapies. This discovery establishes a new paradigm for the spread of virus from lymphocytes to airway epithelial cells and its subsequent release into the environment. Measles virus vaccine strains have emerged as a promising oncolytic platform for cancer therapy in the last ten years. Given that PVRL4 is a well-known tumor-associated marker for several adenocarcinoma (lung, breast and ovary), the measles virus could potentially be used to specifically target, infect and destroy cancers expressing PVRL4.

Disrupted interaction between CFTR and AF-6/afadin aggravates malignant phenotypes of colon cancer

How mutations or dysfunction of CFTR may increase the risk of malignancies in various tissues remains an open question. Here we report the interaction between CFTR and an adherens junction molecule, AF-6/afadin, and its involvement in the development of colon cancer. We have found that CFTR and AF-6/afadin are co-localized at the cell-cell contacts and physically interact with each other in colon cancer cell lines. Knockdown of CFTR results in reduced epithelial tightness and enhanced malignancies, with increased degradation and reduced stability of AF-6/afadin protein. The enhanced invasive phenotype of CFTR-knockdown cells can be completely reversed by either AF-6/afadin over-expression or ERK inhibitor, indicating the involvement of AF-6/MAPK pathway. More interestingly, the expression levels of CFTR and AF-6/afadin are significantly downregulated in human colon cancer tissues and lower expression of CFTR and/or AF-6/afadin is correlated with poor prognosis of colon cancer patients. The present study has revealed a previously unrecognized interaction between CFTR and AF-6/afadin that is involved in the pathogenesis of colon cancer and indicated the potential of the two as novel markers of metastasis and prognostic predictors for human colon cancer.

The V domain of dog PVRL4 (nectin-4) mediates canine distemper virus entry and virus cell-to-cell spread

The entry of canine distemper virus (CDV) is a multistep process that involves the attachment of CDV hemagglutinin (H) to its cellular receptor, followed by fusion between virus and cell membranes. Our laboratory recently identified PVRL4 (nectin-4) to be the epithelial receptor for measles and canine distemper viruses. In this study, we demonstrate that the V domain of PVRL4 is critical for CDV entry and virus cell-to-cell spread. Furthermore, four key amino acid residues within the V domain of dog PVRL4 and two within the CDV hemagglutinin were shown to be essential for receptor-mediated virus entry.

Deciphering teneurin domains that facilitate cellular recognition, cell-cell adhesion, and neurite outgrowth using atomic force microscopy-based single-cell force spectroscopy

Teneurins are evolutionarily conserved transmembrane receptors that function as axon guidance and target selection molecules in the developing nervous system. How teneurins recognize each other, whether they establish neuronal adhesion, and which teneurin specific interactions guide neurons remains to be determined. To reveal insight into these pertinent questions we combine atomic force microscopy-based single-cell force spectroscopy with genetic engineering and quantify the interactions teneurins establish between animal cells. Using a combinatorial approach of deletions and swaps of teneurin-1 and teneurin-2 domains, we unravel that teneurins use their NHL (NCL-1, HT2A, and Lin-41) domain to select homophilic teneurins from adjacent cells. This homophilic recognition of teneurins initiates cell-cell adhesion that, dependent on the intracellular domain, strengthens over time. Neurite outgrowth assays show that establishing and strengthening of teneurin-mediated homophilic cell-cell adhesion is required to stop outgrowth. On the basis of the results, we introduce a molecular model of teneurin domains that specify cellular recognition, adhesion strengthening, and neuronal pathfinding. The combined force spectroscopy and genetic approach can be applied to quantitatively decipher the contribution of any neuronal receptor domain and more generally of a given cell surface receptor domain to cell-cell recognition and adhesion.

De novo lumen formation and elongation in the developing nephron: a central role for afadin in apical polarity

A fundamental process in biology is the de novo formation and morphogenesis of polarized tubules. Although these processes are essential for the formation of multiple metazoan organ systems, little is known about the molecular mechanisms that regulate them. In this study, we have characterized several steps in tubule formation and morphogenesis using the mouse kidney as a model system. We report that kidney mesenchymal cells contain discrete Par3-expressing membrane microdomains that become restricted to an apical domain, coinciding with lumen formation. Once lumen formation has been initiated, elongation occurs by simultaneous extension and additional de novo lumen generation. We demonstrate that lumen formation and elongation require afadin, a nectin adaptor protein implicated in adherens junction formation. Mice that lack afadin in nephron precursors show evidence of Par3-expressing membrane microdomains, but fail to develop normal apical-basal polarity and generate a continuous lumen. Absence of afadin led to delayed and diminished integration of nectin complexes and failure to recruit R-cadherin. Furthermore, we demonstrate that afadin is required for Par complex formation. Together, these results suggest that afadin acts upstream of the Par complex to regulate the integration and/or coalescence of membrane microdomains, thereby establishing apical-basal polarity and lumen formation/elongation during kidney tubulogenesis.

Drebrin preserves endothelial integrity by stabilizing nectin at adherens junctions

Regulation of cell-cell contacts is essential for integrity of the vascular endothelium. Here, a critical role of the F-actin binding protein drebrin in maintaining endothelial integrity is revealed under conditions mimicking vascular flow. Drebrin knockdown leads to weakening of cell-cell contacts, characterized by loss of nectin from adherens junctions and its subsequent lysosomal degradation. Immunoprecipitation, FRAP and mitochondrial retargeting experiments show that nectin stabilization occurs through a chain of interactions: drebrin binding to F-actin, interaction of drebrin and afadin through their polyproline and PR1-2 regions, and recruitment of nectin through afadin's PDZ region. Key elements are drebrin's modules that confer binding to afadin and F-actin. Evidence is provided by constructs containing afadin's PDZ region coupled to drebrin's F-actin binding region or to lifeact, which restore junctional nectin under knockdown of drebrin or of both drebrin and afadin. Drebrin, containing binding sites for both afadin and F-actin, is thus uniquely equipped to stabilize nectin at endothelial junctions and to preserve endothelial integrity under vascular flow.

Dog nectin-4 is an epithelial cell receptor for canine distemper virus that facilitates virus entry and syncytia formation

Canine distemper virus (CDV) was shown to use dog nectin-4 as a receptor to gain entry into epithelial cells. RNA from dog placenta or MDCK kidney cells was isolated and cDNAs were prepared. Two splice variants of dog nectin-4 were identified. A deletion of 25 amino acids was found in the cytoplasmic domain of dog nectin-4 from MDCK cells, corresponding to a splice variant that is also seen in murine nectin-4, and did not affect its role as a receptor. Both dog nectin-4 and human nectin-4 could function as an entry factor for CDV containing an EGFP reporter gene. Inhibition of dog nectin-4 expression by RNAi or nectin-4 antibodies decreased CDV titers and EGFP fluorescence. Finally, dog nectin-4 also promotes syncytia formation, which could be inhibited by siRNA treatment. These data confirm that dog nectin-4 can be used by CDV to gain entry into epithelial cells, and facilitate virus spread.

R-Ras controls axon branching through afadin in cortical neurons

Regulation of axon growth, guidance, and branching is essential for constructing a correct neuronal network. R-Ras, a Ras-family small GTPase, has essential roles in axon formation and guidance. During axon formation, R-Ras activates a series of phosphatidylinositol 3-kinase signaling, inducing activation of a microtubule-assembly promoter-collapsin response mediator protein-2. However, signaling molecules linking R-Ras to actin cytoskeleton-regulating axonal morphology remain obscure. Here we identify afadin, an actin-binding protein harboring Ras association (RA) domains, as an effector of R-Ras inducing axon branching through F-actin reorganization. We observe endogenous interaction of afadin with R-Ras in cortical neurons during the stage of axonal development. Ectopic expression of afadin increases axon branch number, and the RA domains and the carboxyl-terminal F-actin binding domain are required for this action. RNA interference knockdown experiments reveal that knockdown of endogenous afadin suppressed both basal and R-Ras-mediated axon branching in cultured cortical neurons. Subcellular localization analysis shows that active R-Ras-induced translocation of afadin and its RA domains is responsible for afadin localizing to the membrane and inducing neurite development in Neuro2a cells. Overall, our findings demonstrate a novel signaling pathway downstream of R-Ras that controls axon branching.

Crystal structure of cell adhesion molecule nectin-2/CD112 and its binding to immune receptor DNAM-1/CD226

The nectin and nectin-like molecule (Necl) family includes important cell adhesion molecules (CAMs) characterized by their Ig-like nature. Such CAMs regulate a broad spectrum of cell-cell interactions, including the interaction between NK cells and cytotoxic T lymphocytes (CTLs) and their target cells. CAM members nectin-2 (CD112) and Necl-5 (CD155) are believed to form homodimers (for nectin-2) or heterodimers in their functions for cell adhesion, as well as to interact with immune costimulatory receptor DNAX accessory molecule 1 (DNAM-1) (CD226) to regulate functions of both NK and CTL cells. However, the structural basis of the interactive mode of DNAM-1 with nectin-2 or Necl-5 is not yet understood. In this study, a soluble nectin-2 Ig-like V-set domain (nectin-2v) was successfully prepared and demonstrated to bind to both soluble ectodomain and cell surface-expressed full-length DNAM-1. The 1.85-Å crystal structure of nectin-2v displays a perpendicular homodimer arrangement, revealing the homodimer characteristics of the nectin and Necls. Further mutational analysis indicated that disruption of the homodimeric interface of nectin-2v led to a failure of the homodimer formation, as confirmed by crystal structure and biochemical properties of the mutant protein of nectin-2v. Interestingly, the monomer mutant also loses DNAM-1 binding, as evidenced by cell staining with tetramers and surface plasmon resonance assays. The data indicate that interaction with DNAM-1 requires either the homodimerization or engagement of the homodimeric interface of nectin-2v. These results have implications for immune intervention of tumors or autoimmune diseases in the DNAM-1/nectin-2-dependent pathway.

Immunoglobulin superfamily receptors and adherens junctions

The immunogroblin (Ig) superfamily proteins characterized by the presence of Ig-like domains are involved in various cellular functions. The properties of the Ig-like domains to form rod-like structures and to bind specifically to other proteins make them ideal for cell surface receptors and cell adhesion molecules (CAMs). Ig-CAMs, nectins in mammals and Echinoid in Drosophila, are crucial components of cadherin-based adherens junctions in the epithelium. Nectins form cell-cell adhesion by their trans-interactions and recruit cadherins to the nectin-initiated cell-cell adhesion site to establish adherens junctions. Thereafter junction adhesion molecules, occludin, and claudins, are recruited to the apical side of adherens junctions to establish tight junctions. The recruitment of these molecules by nectins is mediated both by the direct and indirect interactions of afadin with many proteins, such as catenins, and zonula occludens proteins, and by the nectin-induced reorganization of the actin cytoskeleton. Nectins contribute to the formation of both homotypic and heterotypic types of cell-cell junctions, such as synapses in the brain, contacts between pigment and non-pigment cell layers of the ciliary epithelium in the eye, Sertoli cell-spermatid junctions in the testis, and sensory cells and supporting cells in the sensory organs. In addition, cis- and trans-interactions of nectins with various cell surface proteins, such as integrins, growth factor receptors, and nectin-like molecules (Necls) play important roles in the regulation of many cellular functions, such as cell polarization, movement, proliferation, differentiation, survival, and cell sorting. Furthermore, the Ig-CAMs are implicated in many human diseases including viral infections, ectodermal dysplasia, cancers, and Alzheimer's disease.