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

Rac1 and Nectin3 are essential for PCP-directed axon guidance in the peripheral auditory system

Our sense of hearing is critically dependent on the spiral ganglion neurons (SGNs) that connect the sound receptors in the organ of Corti (OC) to the cochlear nuclei of the hindbrain. Type I SGNs innervate inner hair cells (IHCs) to transmit sound signals, while type II SGNs (SGNIIs) innervate outer hair cells (OHCs) to detect moderate-to-intense sound. During development, SGNII afferents make a characteristic 90-degree turn toward the base of the cochlea and innervate multiple OHCs. It has been shown that the Planar Cell Polarity (PCP) pathway acts non-autonomously to mediate environmental cues in the cochlear epithelium for SGNII afferent turning towards the base. However, the underlying mechanisms are unknown. Here, we present evidence that PCP signaling regulates multiple downstream effectors to influence cell adhesion and the cytoskeleton in cochlear supporting cells (SCs), which serve as intermediate targets of SGNII afferents. We show that the core PCP gene Vangl2 regulates the localization of the small GTPase Rac1 and the cell adhesion molecule Nectin3 at SC-SC junctions through which SGNII afferents travel. Through in vivo genetic analysis, we also show that loss of Rac1 or Nectin3 partially phenocopied SGNII peripheral afferent turning defects in Vangl2 mutants, and that Rac1 plays a non-autonomous role in this process in part by regulating PCP protein localization at the SC-SC junctions. Additionally, epistasis analysis indicates that Nectin3 and Rac1 likely act in the same genetic pathway to control SGNII afferent turning. Together, these experiments identify Nectin3 and Rac1 as novel regulators of PCP-directed SGNII axon guidance in the cochlea.

Single amino acid mutation of nectin-1 provides remarkable resistance against lethal pseudorabies virus infection in mice

An approach to genetically engineered resistance to pseudorabies virus (PRV) infection was examined by using a mouse model with defined point mutation in primary receptor for alphaherpesviruses, nectin-1, by the CRISPR/Cas9 system. It has become clear that phenylalanine at position 129 of nectin-1 is important for binding to viral glycoprotein D (gD), and mutation of phenylalanine 129 to alanine (F129A) prevents nectin-1 binding to gD and virus entry in vitro. Here, to assess the antiviral potential of the single amino acid mutation of nectin-1, F129A, in vivo, we generated genome-edited mutant mouse lines; F129A and 135 knockout (KO). The latter, 135 KO used as a nectin-1 knockout line for comparison, expresses a carboxy-terminal deleted polypeptide consisting of 135 amino acids without phenylalanine 129. In the challenge with 10 LD50 PRV via intranasal route, perfect protection of disease onset was induced by expression of the mutation of nectin-1, F129A (survival rate: 100% in F129A and 135 KO versus 0% in wild type mice). Neither viral DNA/antigens nor pathological changes were detected in F129A, suggesting that viral entry was prevented at the primary site in natural infection. In the challenge with 50 LD50 PRV, lower but still strong protective effect against disease onset was observed (survival rate: 57% in F129A and 75% in 135 KO versus 0% in wild type mice). The present results indicate that single amino acid mutation of nectin-1 F129A provides significant resistance against lethal pseudorabies.

Hearing loss in mice with disruption of auditory epithelial patterning in the cochlea

In the cochlear auditory epithelia, sensory hair and supporting cells are arranged in a checkerboard-like mosaic pattern, which is conserved across a wide range of species. The cell adhesion molecules nectin-1 and nectin-3 are required for this pattern formation. The checkerboard-like pattern is thought to be necessary for auditory function, but has never been examined. Here, we showed the significance of checkerboard-like cellular pattern in the survival and function of sensory hair cells in the cochlear auditory epithelia of nectin-3 knockout (KO) mice. Nectin-3 KO mice showed progressive hearing loss associated with degeneration of aberrantly attached hair cells via apoptosis. Apoptotic hair cell death was due to the disorganization of tight junctions between the hair cells. Our study revealed that the checkerboard-like cellular pattern in the auditory epithelium provides a structural basis for ensuring the survival of cochlear hair cells and hearing function.

Nectin4 is a potential therapeutic target for asthma

BACKGROUND

Nectins comprise a family of cellular adhesion molecules involved in Ca2+-independent cellular adhesion. Neither the biological significance nor clinical potential of Nectin4 for asthma has been investigated.

OBJECTIVES

The aims of this study were to elucidate the role of Nectin4 in airway inflammation and to determine the relationship between Nectin4 and clinical variables in patients with asthma.

METHODS

The relationship between Nectin4 levels in the blood of asthmatic patients and clinical variables was examined. Dermatophagoides pteronyssinus 1 (Der p1)-exposed normal human bronchial epithelial (NHBE) cells, and Nectin4-deficient (Nectin4-/-) and wild-type (WT) mice sensitized/challenged with ovalbumin (OVA), were used to investigate the involvement of Nectin4 in the pathogenesis of bronchial asthma via the Src/Rac1 pathway.

RESULTS

Plasma Nectin4 levels were significantly higher in asthmatic patients than controls and correlated with specific IgE D1, D2, lung function. The ROC curves for Nectin4 levels differed between asthma patients and controls. Nectin4/Afadin and Src/Rac1 levels were significantly increased in NHBE cells exposed to Der p1, but decreased in NHBE cells treated with Nectin4 siRNA. Airway obstruction and inflammation, as well as the levels of Th2 cytokines, Nectin4, and Src/Rac1, were increased in WT OVA/OVA mice compared with WT sham mice. Nectin4 knockdown resulted in lower levels of Afadin and Src/Rac1 in Nectin4-/-OVA/OVA than WT OVA/OVA mice.

CONCLUSION

These results suggest that Nectin4 is involved in airway inflammation and may be a therapeutic target in patients with asthma.

Adhesion-Based Self-Organization in Tissue Patterning

Since the proposal of the differential adhesion hypothesis, scientists have been fascinated by how cell adhesion mediates cellular self-organization to form spatial patterns during development. The search for molecular tool kits with homophilic binding specificity resulted in a diverse repertoire of adhesion molecules. Recent understanding of the dominant role of cortical tension over adhesion binding redirects the focus of differential adhesion studies to the signaling function of adhesion proteins to regulate actomyosin contractility. The broader framework of differential interfacial tension encompasses both adhesion and nonadhesion molecules, sharing the common function of modulating interfacial tension during cell sorting to generate diverse tissue patterns. Robust adhesion-based patterning requires close coordination between morphogen signaling, cell fate decisions, and changes in adhesion. Current advances in bridging theoretical and experimental approaches present exciting opportunities to understand molecular, cellular, and tissue dynamics during adhesion-based tissue patterning across multiple time and length scales.

Revisiting the embryogenesis of lip and palate development

Clefts of the lip and palate (CLP), the major causes of congenital facial malformation globally, result from failure of fusion of the facial processes during embryogenesis. With a prevalence of 1 in 500-2500 live births, CLP causes major morbidity throughout life as a result of problems with facial appearance, feeding, speaking, obstructive apnoea, hearing and social adjustment and requires complex, multi-disciplinary care at considerable cost to healthcare systems worldwide. Long-term outcomes for affected individuals include increased mortality compared with their unaffected siblings. The frequent occurrence and major healthcare burden imposed by CLP highlight the importance of dissecting the molecular mechanisms driving facial development. Identification of the genetic mutations underlying syndromic forms of CLP, where CLP occurs in association with non-cleft clinical features, allied to developmental studies using appropriate animal models is central to our understanding of the molecular events underlying development of the lip and palate and, ultimately, how these are disturbed in CLP.

Update in TIGIT Immune-Checkpoint Role in Cancer

The in-depth characterization of cross-talk between tumor cells and T cells in solid and hematological malignancies will have to be considered to develop new therapeutical strategies concerning the reactivation and maintenance of patient-specific antitumor responses within the patient tumor microenvironment. Activation of immune cells depends on a delicate balance between activating and inhibitory signals mediated by different receptors. T cell immunoreceptor with immunoglobulin and ITIM domain (TIGIT) is an inhibitory receptor expressed by regulatory T cells (Tregs), activated T cells, and natural killer (NK) cells. TIGIT pathway regulates T cell-mediated tumor recognition in vivo and in vitro and represents an exciting target for checkpoint blockade immunotherapy. TIGIT blockade as monotherapy or in combination with other inhibitor receptors or drugs is emerging in clinical trials in patients with cancer. The purpose of this review is to update the role of TIGIT in cancer progression, looking at TIGIT pathways that are often upregulated in immune cells and at possible therapeutic strategies to avoid tumor aggressiveness, drug resistance, and treatment side effects. However, in the first part, we overviewed the role of immune checkpoints in immunoediting, the TIGIT structure and ligands, and summarized the key immune cells that express TIGIT.

A Nectin1 Mutant Mouse Model Is Resistant to Pseudorabies Virus Infection

The present study generated nectin1-mutant mice with single amino acid substitution and tested the anti-pseudorabies virus (PRV) ability of the mutant mice, with the aim to establish a model for PRV-resistant livestock. A phenylalanine to alanine transition at position 129 (F129A) of nectin1 was introduced into the mouse genome to generate nectin1 (F129A) mutant mice. The mutant mice were infected with a field-isolated highly virulent PRV strain by subcutaneous injection of virus. We found that the homozygous mutant mice had significantly alleviated disease manifestations and decreased death rate and viral loading in serum and tissue compared with heterozygous mutant and wild-type mice. In addition to disease resistance, the homozygous mutant mice showed a defect in eye development, indicating the side effect on animals by only one amino acid substitution in nectin1. Results demonstrate that gene modification in nectin1 is an effective approach to confer PRV resistance on animals, but the mutagenesis pattern requires further investigation to increase viral resistance without negative effect on animal development.

Opportunities, Technology, and the Joy of Discovery

My grandparents were immigrants. My paternal grandfather was illiterate. Yet my parents were able to complete college and to become teachers. I had a conventional upbringing in a small town in Florida, graduating from high school in 1960. I was fortunate enough to graduate cum laude from Florida State University and to earn other credentials leading to faculty positions at outstanding institutions of higher education: the University of Chicago and Northwestern University. At a time when women were rarely the leaders of research groups, I was able to establish a well-funded research program and to make contributions to our understanding of viral entry into cells. My best research was done after I became confident enough to seek productive interactions with collaborators. I am grateful for the collaborators and collaborations that moved our field forward and for my trainees who have gone on to successes in many different careers. Expected final online publication date for the Annual Review of Virology, Volume 9 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

A numerical algorithm for modeling cellular rearrangements in tissue morphogenesis

Among morphological phenomena, cellular patterns in developing sensory epithelia have gained attention in recent years. Although physical models for cellular rearrangements are well-established thanks to a large bulk of experimental work, their computational implementation lacks solid mathematical background and involves experimentally unreachable parameters. Here we introduce a level set-based computational framework as a tool to rigorously investigate evolving cellular patterns, and study its mathematical and computational properties. We illustrate that a compelling feature of the method is its ability to correctly handle complex topology changes, including frequent cell intercalations. Combining this accurate numerical scheme with an established mathematical model, we show that the proposed framework features minimum possible number of parameters and is capable of reproducing a wide range of tissue morphological phenomena, such as cell sorting, engulfment or internalization. In particular, thanks to precise mathematical treatment of cellular intercalations, this method succeeds in simulating experimentally observed development of cellular mosaic patterns in sensory epithelia.

A numerical algorithm handles complex cell topology changes and reproduces tissue morphological phenomena without relying on nonphysical parameters.

Nectin cell adhesion molecule-4 (NECTIN-4): A potential target for cancer therapy

NECTIN-4 [a poliovirus receptor-related-4 (pvrl-4) encoded protein] is a Ca2+ independent immunoglobulin-like protein. Along with other Nectins (Nectin-1, -2 and -3), it is primarily involved in cell-cell adhesion. In contrast to other Nectins, Nectin-4 is specifically enriched in the embryonic and placental tissues but its expression significantly declines in adult life. In recent years, it has been found that Nectin-4 is especially overexpressed and served as a tumor associated inducer in various malignant tumors including breast, lung, colorectal, pancreatic, ovarian cancers etc. Over-expression of Nectin-4 is associated with various aspects of tumor progression like proliferation, angiogenesis, epithelial to mesenchymal transition, metastasis, DNA repair, tumor relapse, poor prognosis in several types of cancer. This review systematically highlights the implications of Nectin-4 in every possible aspect of cancer and the molecular mechanism of Nectin-4 mediated cancer progression. We have further emphasized on the therapeutic strategies that are being proposed to specifically target Nectin-4.

Nectin-2 in general and in the brain

Nectins are immunoglobulin-like cell adhesion molecules constituting a family with four members, nectin-1, nectin-2, nectin-3, and nectin-4. In the brain, nectin-2 as well as nectin-1 and nectin-3 are expressed whereas nectin-4 is hardly expressed. In the nervous system, physiological functions of nectin-1 and nectin-3, such as synapse formation, mossy fiber trajectory regulation, interneurite affinity, contextual fear memory formation, and stress-related mental disorders, have been revealed. Nectin-2 is ubiquitously expressed in non-neuronal tissues and various nectin-2 functions in non-nervous systems have been extensively investigated, but nectin-2 functions in the brain have not been revealed until recently. Recent findings have revealed that nectin-2 is expressed in the specific areas of the brain and plays important roles, such as homeostasis of astrocytes and neurons and the formation of synapses. Moreover, a single nucleotide polymorphism in the human NECTIN2 gene is associated with Alzheimer's disease. We here summarize recent progress in our understanding of nectin-2 functions in the brain.

NOTCH Signaling Controls Ciliary Body Morphogenesis and Secretion by Directly Regulating Nectin Protein Expression

Anterior segment dysgenesis is often associated with cornea diseases, cataracts, and glaucoma. In the anterior segment, the ciliary body (CB) containing inner and outer ciliary epithelia (ICE and OCE) secretes aqueous humor that maintains intraocular pressure (IOP). However, CB development and function remain poorly understood. Here, this study shows that NOTCH signaling in the CB maintains the vitreous, IOP, and eye structures by regulating CB morphogenesis, aqueous humor secretion, and vitreous protein expression. Notch2 and Notch3 function via RBPJ in the CB to control ICE-OCE adhesion, CB morphogenesis, aqueous humor secretion, and protein expression, thus maintaining IOP and eye structures. Mechanistically, NOTCH signaling transcriptionally controls Nectin1 expression in the OCE to promote cell adhesion for driving CB morphogenesis and to directly stabilize Cx43 for controlling aqueous humor secretion. Finally, NOTCH signaling directly controls vitreous protein secretion in the ICE. Therefore, this study provides important insight into CB functions and involvement in eye diseases.

Resistance to pseudorabies virus by knockout of nectin1/2 in pig cells

Pseudorabies virus (PRV) is a pig pathogen that causes substantial economic losses to the pig industry. Infection of host cells by PRV is mediated by the membrane proteins nectin1 and nectin2, which are presumed to be receptors for PRV infection. Here, we generated nectin1/2 knockout (KO) cells with the aim of establishing a PRV-resistant cell model. Nectin1 and 2 were ablated in PK15 cells by CRISPR/Cas9-mediated gene targeting. PRV infection in either nectin1 or nectin2 KO cells showed a significant reduction in viral growth compared with wild-type (WT) cells. We further simultaneously deleted nectin1 and nectin2 in PK15 cells and found that double KO cells showed no further increase in resistance to PRV compared with single gene-KO cells, despite being more resistant than WT. By investigating the cell entry steps of PRV infection, we found that nectin1 or/and nectin2 KO did not greatly affect virus attachment or internalization to cells but blocked cell-to-cell spread. Our results demonstrate that KO of either nectin1 or nectin2 confers PRV resistance to PK15 cells. This strategy could be applied to establish PRV-resistant pigs with nectin1/2 modifications to benefit the pig industry.

Disruption of the nectin-afadin complex recapitulates features of the human cleft lip/palate syndrome CLPED1

Cleft palate (CP), one of the most common congenital conditions, arises from failures in secondary palatogenesis during embryonic development. Several human genetic syndromes featuring CP and ectodermal dysplasia have been linked to mutations in genes regulating cell-cell adhesion, yet mouse models have largely failed to recapitulate these findings. Here, we use in utero lentiviral-mediated genetic approaches in mice to provide the first direct evidence that the nectin-afadin axis is essential for proper palate shelf elevation and fusion. Using this technique, we demonstrate that palatal epithelial conditional loss of afadin (Afdn) - an obligate nectin- and actin-binding protein - induces a high penetrance of CP, not observed when Afdn is targeted later using Krt14-Cre We implicate Nectin1 and Nectin4 as being crucially involved, as loss of either induces a low penetrance of mild palate closure defects, while loss of both causes severe CP with a frequency similar to Afdn loss. Finally, expression of the human disease mutant NECTIN1^W185X causes CP with greater penetrance than Nectin1 loss, suggesting this alteration may drive CP via a dominant interfering mechanism.

Interaction between nectin-1 and the human natural killer cell receptor CD96

Regulation of Natural Killer (NK) cell activity is achieved by the integration of both activating and inhibitory signals acquired at the immunological synapse with potential target cells. NK cells express paired receptors from the immunoglobulin family which share common ligands from the nectin family of adhesion molecules. The activating receptor CD226 (DNAM-1) binds to nectin-2 and CD155, which are also recognized by the inhibitory receptor TIGIT. The third receptor in this family is CD96, which is less well characterized and may have different functions in human and mouse models. Human CD96 interacts with CD155 and ligation of this receptor activates NK cells, while in mice the presence of CD96 correlates with decreased NK cell activation. Mouse CD96 also binds nectin-1, but the effect of this interaction has not yet been determined. Here we show that human nectin-1 directly interacts with CD96 in vitro. The binding site for CD96 is located on the nectin-1 V-domain, which comprises a canonical interface that is shared by nectins to promote cell adhesion. The affinity of nectin-1 for CD96 is lower than for other nectins such as nectin-3 and nectin-1 itself. However, the affinity of nectin-1 for CD96 is similar to its affinity for herpes simplex virus glycoprotein D (HSV gD), which binds the nectin-1 V-domain during virus entry. The affinity of human CD96 for nectin-1 is lower than for its known activating ligand CD155. We also found that human erythroleukemia K562 cells, which are commonly used as susceptible targets to assess NK cell cytotoxicity did not express nectin-1 on their surface and were resistant to HSV infection. When expressed in K562 cells, nectin-1-GFP accumulated at cell contacts and allowed HSV entry. Furthermore, overexpression of nectin-1-GFP led to an increased susceptibility of K562 cells to NK-92 cell cytotoxicity.

Sevoflurane-induced learning deficits and spine loss via nectin-1/corticotrophin-releasing hormone receptor type 1 signaling

In recent years, the neurotoxicity of general anesthetics in the developing brain has been studied and raised great concern as a major health issue to the public and physicians. Sevoflurane inhalation may induce neurotoxicity expressed as memory and learning impairment in young animals. In the current study, we investigated the role of nectin-1 and corticotrophin-releasing hormone receptor type 1 (CRHR1) in sevoflurane-induced learning deficits and dendritic spines loss in neonatal mice. Neonatal mice (P7) were treated with 3% sevoflurane with 60% O₂ or 60% O₂ for 6 h. Cognitive function was evaluated by Y Maze, Object recognition test, and Morris Water Maze. Hippocampal nectin-1 and L-afadin expression assessed using western blot analysis. The dendritic spines morphology of the hippocampus was determined using Golgi impregnation on 7 d and 2 months old. Sevoflurane exposed to neonatal mice decreased hippocampal nectin-1 levels from 1 h to 2 months after sevoflurane inhalation and attenuated working and spatial memory and spinal number in adulthood, which could be reversed by nectin-1 overexpression and CRHR1 antagonist Antalarmin. Nectin-1 knockdown caused spatial learning deficits and dendritic spine loss and lower L-afadin protein expression. Sevoflurane-induced nectin-1 and L-afadin expression decrease was mediated by CRHR1 signaling in the hippocampus. This information can be used to develop targeted intervention aimed at decreasing the neurotoxicity of sevoflurane inhalation.

Implication of Soluble Forms of Cell Adhesion Molecules in Infectious Disease and Tumor: Insights from Transgenic Animal Models

Cell adhesion molecules (CAMs) are surface ligands, usually glycoproteins, which mediate cell-to-cell adhesion. They play a critical role in maintaining tissue integrity and mediating migration of cells, and some of them also act as viral receptors. It has been known that soluble forms of the viral receptors bind to the surface glycoproteins of the viruses and neutralize them, resulting in inhibition of the viral entry into cells. Nectin-1 is one of important CAMs belonging to immunoglobulin superfamily and herpesvirus entry mediator (HVEM) is a member of the tumor necrosis factor (TNF) receptor family. Both CAMs also act as alphaherpesvirus receptor. Transgenic mice expressing the soluble form of nectin-1 or HVEM showed almost complete resistance against the alphaherpesviruses. As another CAM, sialic acid-binding immunoglobulin-like lectins (Siglecs) that recognize sialic acids are also known as an immunoglobulin superfamily member. Siglecs play an important role in the regulation of immune cell functions in infectious diseases, inflammation, neurodegeneration, autoimmune diseases and cancer. Siglec-9 is one of Siglecs and capsular polysaccharide (CPS) of group B Streptococcus (GBS) binds to Siglec-9 on neutrophils, leading to suppress host immune response and provide a survival advantage to the pathogen. In addition, Siglec-9 also binds to tumor-produced mucins such as MUC1 to lead negative immunomodulation. Transgenic mice expressing the soluble form of Siglec-9 showed significant resistance against GBS infection and remarkable suppression of MUC1 expressing tumor proliferation. This review describes recent developments in the understanding of the potency of soluble forms of CAMs in the transgenic mice and discusses potential therapeutic interventions that may alter the outcomes of certain diseases.

Identification of OAF and PVRL1 as candidate genes for an ocular anomaly characterized by Peters anomaly type 2 and ectopia lentis

Keratolenticular dysgenesis (KLD) and ectopia lentis are congenital eye defects. The aim of this study is the identification of molecular genetic alterations responsible for those ocular anomalies with neurologic impairment in an individual with a de novo balanced chromosome translocation t(11;18)(q23.3;q11.2)dn. Disruption of OAF, the human orthologue of the Drosophila oaf, by the 11q23.3 breakpoint results in reduced expression of this transcriptional regulator. Furthermore, four most likely nonfunctional chimeric transcripts comprising up to OAF exon 3, derived from the der(11) allele, have also been identified. This locus has been implicated by publicly available genome-wide association data in corneal disease and corneal topography. The expression of the poliovirus receptor-related 1(PVRL1) or nectin cell adhesion molecule 1 (NECTIN1), a paralogue of nectin cell adhesion molecule 3 (PVRL3) associated with congenital ocular defects, situated 500 kb upstream from 11q23.3 breakpoint, is increased. The 18q11.2 breakpoint is localized between cutaneous T-cell lymphoma-associated antigen 1(CTAGE1) and retinoblastoma binding protein 8 (RBBP8) genes. Genomic imbalance that could contribute to the observed phenotype was excluded. Analysis of gene expression datasets throughout normal murine ocular lens embryogenesis suggests that OAF expression is significantly enriched in the lens from early stages of development through adulthood, whereas PVRL1 is lens-enriched until E12.5 and then down-regulated. This contrasts with the observation that the proposita's lymphoblastoid cell lines exhibit low OAF and high PVRL1 expression as compared to control, which offers further support that the alterations described above are most likely responsible for the clinical phenotype. Finally, gene interaction topology data for PVRL1 also agree with our proposal that disruption of OAF by the translocation breakpoint and misregulation of PVRL1 due to a position effect contribute to the observed ocular and neurological phenotype.

Closing the Gap: Mouse Models to Study Adhesion in Secondary Palatogenesis

Secondary palatogenesis occurs when the bilateral palatal shelves (PS), arising from maxillary prominences, fuse at the midline, forming the hard and soft palate. This embryonic phenomenon involves a complex array of morphogenetic events that require coordinated proliferation, apoptosis, migration, and adhesion in the PS epithelia and underlying mesenchyme. When the delicate process of craniofacial morphogenesis is disrupted, the result is orofacial clefting, including cleft lip and cleft palate (CL/P). Through human genetic and animal studies, there are now hundreds of known genetic alternations associated with orofacial clefts; so, it is not surprising that CL/P is among the most common of all birth defects. In recent years, in vitro cell-based assays, ex vivo palate cultures, and genetically engineered animal models have advanced our understanding of the developmental and cell biological pathways that contribute to palate closure. This is particularly true for the areas of PS patterning and growth as well as medial epithelial seam dissolution during palatal fusion. Here, we focus on epithelial cell-cell adhesion, a critical but understudied process in secondary palatogenesis, and provide a review of the available tools and mouse models to better understand this phenomenon.

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.

P-Cadherin is necessary for retinal stem cell behavior in vitro, but not in vivo

Adult retinal stem cells (RSCs) are rare quiescent cells within the ciliary epithelium of the eye, which is made up of non-pigmented N-Cadherin^+ve inner and pigmented P-Cadherin^+ve outer cell layers. Through FACs and single cell analyses, we have shown that RSCs arise from single cells from within the pigmented CE and express P-Cadherin. However, whether the expression of P-Cadherin is required for maintenance of the stem cell in vivo or in the formation of the clonal stem cell spheres in vitro is not known. Using cadherin functional blocking antibody experiments and a P-Cadherin ^-/- mouse to test whether the RSC population is affected by the loss of P-Cadherin expression, our experiments demonstrate that the RSCs reside in the pigmented CE layer and express P-Cadherin, which is important to the formation of adherent sphere colonies in vitro, however P-Cadherin is not required for maintenance of RSCs in vivo.

Differential and Cooperative Cell Adhesion Regulates Cellular Pattern in Sensory Epithelia

Animal tissues are composed of multiple cell types arranged in complex and elaborate patterns. In sensory epithelia, including the auditory epithelium and olfactory epithelium, different types of cells are arranged in unique mosaic patterns. These mosaic patterns are evolutionarily conserved, and are thought to be important for hearing and olfaction. Recent progress has provided accumulating evidence that the cellular pattern formation in epithelia involves cell rearrangements, movements, and shape changes. These morphogenetic processes are largely mediated by intercellular adhesion systems. Differential adhesion and cortical tension have been proposed to promote cell rearrangements. Many different types of cells in tissues express various types of cell adhesion molecules. Although cooperative mechanisms between multiple adhesive systems are likely to contribute to the production of complex cell patterns, our current understanding of the cooperative roles between multiple adhesion systems is insufficient to entirely explain the complex mechanisms underlying cellular patterning. Recent studies have revealed that nectins, in cooperation with cadherins, are crucial for the mosaic cellular patterning in sensory organs. The nectin and cadherin systems are interacted with one another, and these interactions provide cells with differential adhesive affinities for complex cellular pattern formations in sensory epithelia, which cannot be achieved by a single mechanism.

Localization of nectin-2δ at perivascular astrocytic endfoot processes and degeneration of astrocytes and neurons in nectin-2 knockout mouse brain

Nectins are Ca²⁺-independent immunoglobulin-like cell-cell adhesion molecules. In the nervous system, among four members (nectin-1, -2, -3, and -4), nectin-1 and -3 are asymmetrically localized at puncta adherentia junctions formed between the mossy fiber terminals and the dendrites of CA3 pyramidal neurons in the mouse hippocampus and heterophilic trans-interactions between nectin-1 and nectin-3 are involved in the selective interaction of axons and dendrites of cultured neurons. By contrast, nectin-2, which has two splicing variants, nectin-2α and -2δ, has not been well characterized in the brain. We showed here that nectin-2α was expressed in both cultured mouse neurons and astrocytes whereas nectin-2δ was selectively expressed in the astrocytes. Nectin-2δ was localized at the adhesion sites between adjacent cultured astrocytes, but in the brain it was localized on the plasma membranes of astrocytic perivascular endfoot processes facing the basement membrane of blood vessels. Genetic ablation of nectin-2 caused degeneration of astrocytic perivascular endfoot processes and neurons in the cerebral cortex. These results uncovered for the first time the localization and critical functions of nectin-2 in the brain.

Host Nectin-1 Promotes Chlamydial Infection in the Female Mouse Genital Tract, but Is Not Required for Infection in a Novel Male Murine Rectal Infection Model

Chlamydia trachomatis is the most common bacterial sexually transmitted pathogen, but more than 70% of patients fail to seek treatment due to the asymptomatic nature of these infections. Women suffer from numerous complications from chronic chlamydial infections, which include pelvic inflammatory disease and infertility. We previously demonstrated in culture that host cell nectin-1 knockdown significantly reduced chlamydial titers and inclusion size. Here, we sought to determine whether nectin-1 was required for chlamydial development in vivo by intravaginally infecting nectin-1^-/- mice with Chlamydia muridarum and monitoring chlamydial shedding by chlamydial titer assay. We observed a significant reduction in chlamydial shedding in female nectin-1^-/- mice compared to nectin-1^+/+ control mice, an observation that was confirmed by PCR. Immunohistochemical staining in mouse cervical tissue confirmed that there are fewer chlamydial inclusions in Chlamydia-infected nectin-1^-/- mice. Notably, anorectal chlamydial infections are becoming a substantial health burden, though little is known regarding the pathogenesis of these infections. We therefore established a novel male murine model of rectal chlamydial infection, which we used to determine whether nectin-1 is required for anorectal chlamydial infection in male mice. In contrast to the data from vaginal infection, no difference in rectal chlamydial shedding was observed when male nectin-1^+/+ and nectin-1^-/- mice were compared. Through the use of these two models, we have demonstrated that nectin-1 promotes chlamydial infection in the female genital tract but does not appear to contribute to rectal infection in male mice. These models could be used to further characterize tissue and sex related differences in chlamydial infection.

Synergistic action of nectins and cadherins generates the mosaic cellular pattern of the olfactory epithelium

Cellular rearrangements between olfactory cells and supporting cells, driven by the different expression and distribution of nectins and cadherins, are required for mosaic cellular patterning in the olfactory epithelium.

In the olfactory epithelium (OE), olfactory cells (OCs) and supporting cells (SCs), which express different cadherins, are arranged in a characteristic mosaic pattern in which OCs are enclosed by SCs. However, the mechanism underlying this cellular patterning is unclear. Here, we show that the cellular pattern of the OE is established by cellular rearrangements during development. In the OE, OCs express nectin-2 and N-cadherin, and SCs express nectin-2, nectin-3, E-cadherin, and N-cadherin. Heterophilic trans-interaction between nectin-2 on OCs and nectin-3 on SCs preferentially recruits cadherin via α-catenin to heterotypic junctions, and the differential distributions of cadherins between junctions promote cellular intercalations, resulting in the formation of the mosaic pattern. These observations are confirmed by model cell systems, and various cellular patterns are generated by the combinatorial expression of nectins and cadherins. Collectively, the synergistic action of nectins and cadherins generates mosaic pattern, which cannot be achieved by a single mechanism.

Brg1 coordinates multiple processes during retinogenesis and is a tumor suppressor in retinoblastoma

Retinal development requires precise temporal and spatial coordination of cell cycle exit, cell fate specification, cell migration and differentiation. When this process is disrupted, retinoblastoma, a developmental tumor of the retina, can form. Epigenetic modulators are central to precisely coordinating developmental events, and many epigenetic processes have been implicated in cancer. Studying epigenetic mechanisms in development is challenging because they often regulate multiple cellular processes; therefore, elucidating the primary molecular mechanisms involved can be difficult. Here we explore the role of Brg1 (Smarca4) in retinal development and retinoblastoma in mice using molecular and cellular approaches. Brg1 was found to regulate retinal size by controlling cell cycle length, cell cycle exit and cell survival during development. Brg1 was not required for cell fate specification but was required for photoreceptor differentiation and cell adhesion/polarity programs that contribute to proper retinal lamination during development. The combination of defective cell differentiation and lamination led to retinal degeneration in Brg1-deficient retinae. Despite the hypocellularity, premature cell cycle exit, increased cell death and extended cell cycle length, retinal progenitor cells persisted in Brg1-deficient retinae, making them more susceptible to retinoblastoma. ChIP-Seq analysis suggests that Brg1 might regulate gene expression through multiple mechanisms.

Summary: The SWI/SNF protein Brg1 controls cell cycle length, cell cycle exit and cell survival, and is required for cell differentiation and retinal lamination, in the developing mouse retina.

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 old and new face of craniofacial research: How animal models inform human craniofacial genetic and clinical data

The craniofacial skeletal structures that comprise the human head develop from multiple tissues that converge to form the bones and cartilage of the face. Because of their complex development and morphogenesis, many human birth defects arise due to disruptions in these cellular populations. Thus, determining how these structures normally develop is vital if we are to gain a deeper understanding of craniofacial birth defects and devise treatment and prevention options. In this review, we will focus on how animal model systems have been used historically and in an ongoing context to enhance our understanding of human craniofacial development. We do this by first highlighting "animal to man" approaches; that is, how animal models are being utilized to understand fundamental mechanisms of craniofacial development. We discuss emerging technologies, including high throughput sequencing and genome editing, and new animal repository resources, and how their application can revolutionize the future of animal models in craniofacial research. Secondly, we highlight "man to animal" approaches, including the current use of animal models to test the function of candidate human disease variants. Specifically, we outline a common workflow deployed after discovery of a potentially disease causing variant based on a select set of recent examples in which human mutations are investigated in vivo using animal models. Collectively, these topics will provide a pipeline for the use of animal models in understanding human craniofacial development and disease for clinical geneticist and basic researchers alike.

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.

Epidermal cell junctions and their regulation by p63 in health and disease

As the outermost tissue of the body, the epidermis is the first physical barrier for any pressure, stress or trauma. Several specialized cell-matrix and cell-cell adhesion structures, together with an intracellular network of dedicated intermediate filaments, are required to confer critical resilience to mechanical stress. The transcription factor p63 is a master regulator of gene expression in the epidermis and in other stratified epithelia. It has been extensively demonstrated that p63 positively controls a large number of tissue-specific genes, including those encoding a large fraction of tissue-restricted cell adhesion molecules. Consistent with p63 functions in cell adhesion and in epidermal differentiation, heterozygous mutations clustered mainly in the p63 C-terminus are causative of AEC syndrome, an autosomal dominant disorder characterized by cleft palate, ankyloblepharon and ectodermal dysplasia associated with severe skin erosions, bleeding and infections. The molecular basis of skin erosions in AEC patients is not fully understood, although defects in desmosomes and in other cell junctions are likely to be involved. Here, we provide an extensive review of the different epidermal cell junctions that cooperate to withstand mechanical stress and on the mechanisms by which p63 regulates gene expression of their components in healthy skin and in AEC syndrome. Collectively, advancement in understanding the molecular mechanisms by which epidermal cell junctions precisely exert their functions and how p63 orchestrates their coordinated expression, will ultimately lead to insight into developing future strategies for the treatment of AEC syndrome and more in generally for diseases that share an overlapping phenotype.

Nectin family of cell-adhesion molecules: structural and molecular aspects of function and specificity

Cell-cell adhesive processes are central to the physiology of multicellular organisms. A number of cell surface molecules contribute to cell-cell adhesion, and the dysfunction of adhesive processes underlies numerous developmental defects and inherited diseases. The nectins, a family of four immunoglobulin superfamily members (nectin-1 to -4), interact through their extracellular domains to support cell-cell adhesion. While both homophilic and heterophilic interactions among the nectins are implicated in cell-cell adhesion, cell-based and biochemical studies suggest heterophilic interactions are stronger than homophilic interactions and control a range of physiological processes. In addition to interactions within the nectin family, heterophilic associations with nectin-like molecules, immune receptors, and viral glycoproteins support a wide range of biological functions, including immune modulation, cancer progression, host-pathogen interactions and immune evasion. We review current structural and molecular knowledge of nectin recognition processes, with a focus on the biochemical and biophysical determinants of affinity and selectivity that drive distinct nectin associations. These proteins and interactions are discussed as potential targets for immunotherapy.

p63-dependent and independent mechanisms of nectin-1 and nectin-4 regulation in the epidermis

Nectins are immunoglobulin-like cell adhesion molecules mainly localized in adherens junctions. The transcription factor p63 is a master regulator of gene expression in stratified epithelia and controls several molecular processes. As mutations in the Pvrl1 and Pvrl4 genes encoding for nectins cause genetic disorders with phenotypes similar to p63-related syndromes, we investigated whether these proteins might be under p63 transcriptional control. Here, we show that in p63-null skin, Pvrl1 gene expression is strongly reduced, whereas Pvrl4 expression is unaffected. In human and mouse primary keratinocytes p63 depletion leads to a specific downregulation of the Pvrl1 gene. Consistent with a direct regulation, chromatin immunoprecipitation experiments (ChIP) indicate that p63 binds to two conserved intronic Pvrl1 enhancer regions. Ankyloblepharon-ectodermal defects-cleft lip/palate (AEC) syndrome is a rare autosomal dominant disorder, caused by mutations in p63 gene, mainly characterized by skin fragility. To test whether nectins may be affected in AEC syndrome, their expression was measured in keratinocytes obtained from patients with AEC or from a conditional mouse model for AEC syndrome. Pvrl1 expression was reduced in AEC keratinocytes, consistent with impaired p63 function. Surprisingly, Pvrl4 expression was similarly affected, in parallel with decreased expression of the transcription factor Irf6. Consistent with the well-characterized role of Irf6 in keratinocyte differentiation and its strong downregulation in AEC syndrome, Irf6 depletion caused reduced expression of Pvrl4 in wild-type keratinocytes. Taken together, our results indicate that Pvrl1 is a bona fide target gene of the transcription factor p63, whereas Pvrl4 regulation is linked to epidermal differentiation and is under Irf6 control.

Afadin requirement for cytokine expressions in keratinocytes during chemically induced inflammation in mice

Afadin is a filamentous actin-binding protein and a mediator of nectin signaling. Nectins are Ig-like cell adhesion molecules, and the nectin family is composed of four members, nectin-1 to nectin-4. Nectins show homophilic and heterophilic interactions with other nectins or proteins on adjacent cells. Nectin signaling induces formation of cell–cell junctions and is required for the development of epithelial tissues, including skin. This study investigated the role of afadin in epithelial tissue development and established epithelium-specific afadin-deficient (CKO) mice. Although showing no obvious abnormality in the skin development and homeostasis, the mice showed the reduced neutrophil infiltration into the epidermis during chemical-induced inflammation with 12-O-tetradecanoylphorbol 13-acetate (TPA). Immunohistochemical and quantitative real-time PCR analyses showed that the expression levels of cytokines including Cxcl2, Il-1β and Tnf-α were reduced in CKO keratinocytes compared with control keratinocytes during TPA-induced inflammation. Primary-cultured skin keratinocytes from CKO mice also showed reduced expression of these cytokines and weak activation of Rap1 compared with those from control mice after the TPA treatment. These results suggested a remarkable function of afadin, which was able to enhance cytokine expression through Rap1 activation in keratinocytes during inflammation.

Aberrant cochlear hair cell attachments caused by Nectin-3 deficiency result in hair bundle abnormalities

The organ of Corti consists of sensory hair cells (HCs) interdigitated with nonsensory supporting cells (SCs) to form a checkerboard-like cellular pattern. HCs are equipped with hair bundles on their apical surfaces. We previously reported that cell-adhesive nectins regulate the checkerboard-like cellular patterning of HCs and SCs in the mouse auditory epithelium. Nectin-1 and -3 are differentially expressed in normal HCs and SCs, respectively, and in Nectin-3-deficient mice a number of HCs are aberrantly attached to each other. We show here that these aberrantly attached HCs in Nectin-3-deficient mice, but not unattached ones, show disturbances of the orientation and morphology of the hair bundles and the positioning of the kinocilium, with additional abnormal localisation of cadherin-catenin complexes and the apical-basal polarity proteins Pals1 and Par-3. These results indicate that, owing to the loss of Nectin-3, hair cells contact each other inappropriately and form abnormal junctions, ultimately resulting in abnormal hair bundle orientation and morphology.

Cajal-Retzius cells instruct neuronal migration by coincidence signaling between secreted and contact-dependent guidance cues

Cajal-Retzius (CR) cells are a transient cell population of the CNS that is critical for brain development. In the neocortex, CR cells secrete reelin to instruct the radial migration of projection neurons. It has remained unexplored, however, whether CR cells provide additional molecular cues important for brain development. Here, we show that CR cells express the immunoglobulin-like adhesion molecule nectin1, whereas neocortical projection neurons express its preferred binding partner, nectin3. We demonstrate that nectin1- and nectin3-mediated interactions between CR cells and migrating neurons are critical for radial migration. Furthermore, reelin signaling to Rap1 promotes neuronal Cdh2 function via nectin3 and afadin, thus directing the broadly expressed homophilic cell adhesion molecule Cdh2 toward mediating heterotypic cell-cell interactions between neurons and CR cells. Our findings identify nectins and afadin as components of the reelin signaling pathway and demonstrate that coincidence signaling between CR cell-derived secreted and short-range guidance cues direct neuronal migration.

The role of nectins in different types of cell-cell adhesion

Mammalian tissues and organs are composed of different types of cells that adhere to each other homotypically (i.e. interactions between cells of the same cell type) or heterotypically (i.e. interactions between different cell types), forming a variety of cellular patterns, including mosaic patterns. At least three types of cell-cell adhesion have been observed: symmetric homotypic, asymmetric homotypic and heterotypic cell adhesions. Cadherins and nectins, which are known cell-cell adhesion molecules, mediate these cell adhesions. Cadherins comprise a family of more than 100 members, but they are primarily involved in homophilic trans-interactions (i.e. interactions between the same cadherin members) between opposing cells. By contrast, the nectin family comprises only four members, and these proteins form both homophilic and heterophilic trans-interactions (i.e. interactions between the same and different nectin members on opposing cells). In addition, heterophilic trans-interactions between nectins are much stronger than homophilic trans-interactions. Because of these unique properties, nectins have crucial roles in asymmetric homotypic cell-cell adhesion at neuronal synapses and in various types of heterotypic cell-cell adhesions. We summarize recent progress in our understanding of the biology of nectins and discuss their roles in heterotypic cell-cell adhesions, whose formation cannot be solely explained by the action of cadherins.

Herpesvirus entry mediator is a serotype specific determinant of pathogenesis in ocular herpes

Infection with herpes simplex virus type 1 (HSV-1) and HSV-2 is initiated by viral glycoprotein D (gD) binding to a receptor on the host cell. Two receptors, herpesvirus entry mediator (HVEM) and nectin-1, mediate entry in murine models of HSV-1 and HSV-2. HVEM is dispensable for HSV-2 infection of the vagina and brain, but is required for WT pathogenesis of HSV-1 infection of the cornea. By challenging WT and HVEM KO mice with multiple strains of HSV-1 and HSV-2, we demonstrate that without HVEM, all HSV-1 strains tested do not replicate well in the cornea and infection does not result in severe symptoms, as observed in WT mice. In contrast, all HSV-2 strains tested had no requirement for HVEM to replicate to WT levels in the cornea and still cause severe disease. These findings imply that HSV-2 does not require HVEM to cause disease regardless of route of entry, but HVEM must be present for HSV-1 to cause full pathogenesis in the eye. These findings uncover a unique role for HVEM in mediating HSV-1 infection in an area innervated by the trigeminal ganglion and may explain why the presence of HVEM can lead to severe inflammation in the cornea. Thus, the dependence on HVEM is a dividing point between HSV-1 and HSV-2 that evolved to infect areas innervated by different sensory ganglia.

New insights into the mechanism of lens development using zebra fish

On the basis of recent advances in molecular biology, genetics, and live-embryo imaging, direct comparisons between zebra fish and human lens development are being made. The zebra fish has numerous experimental advantages for investigation of fundamental biomedical problems that are often best studied in the lens. The physical characteristics of visible light can account for the highly coordinated cell differentiation during formation of a beautifully transparent, refractile, symmetric optical element, the biological lens. The accessibility of the zebra fish lens for direct investigation during rapid development will result in new knowledge about basic functional mechanisms of epithelia-mesenchymal transitions, cell fate, cell-matrix interactions, cytoskeletal interactions, cytoplasmic crowding, membrane transport, cell adhesion, cell signaling, and metabolic specialization. The lens is well known as a model for characterization of cell and molecular aging. We review the recent advances in understanding vertebrate lens development conducted with zebra fish.

Modulation of cell-cell junctional complexes by matrix metalloproteinases

The ameloblast cell layer of the enamel organ is in contact with the forming enamel as it develops into the hardest substance in the body. Ameloblasts move in groups that slide by one another as the enamel layer thickens. Each ameloblast is responsible for the formation of one enamel rod, and the rods are the mineralized trail that moving ameloblasts leave behind. Matrix metalloproteinases (MMPs) facilitate cell movement in various tissues during development, and in this review we suggest that the tooth-specific MMP, enamelysin (MMP20), facilitates ameloblast movements during enamel development. Mmp20 null mice have thin brittle enamel with disrupted rod patterns that easily abrades from the underlying dentin. Strikingly, the Mmp20 null mouse enamel organ morphology is noticeably dysplastic during late-stage development, when MMP20 is no longer expressed. We suggest that in addition to its role of cleaving enamel matrix proteins, MMP20 also cleaves junctional complexes present on ameloblasts to foster the cell movement necessary for formation of the decussating enamel rod pattern. Therefore, inactivation of MMP20 would result in tight ameloblast cell-cell attachments that may cause maturation-stage enamel organ dysplasia. The tight ameloblast attachments would also preclude the ameloblast movement necessary to form decussating enamel rod patterns.

Nectin ectodomain structures reveal a canonical adhesive interface

Nectins are immunoglobulin superfamily glycoproteins that mediate intercellular adhesion in many vertebrate tissues. Homophilic and heterophilic interactions between nectin family members help to mediate tissue patterning. We determined homophilic binding affinities and heterophilic specificities of all four nectins and the related protein nectin-like 5 from human and mouse, revealing a range of homophilic strengths and a defined heterophilic specificity pattern. To understand the molecular basis of adhesion and specificity, we determined crystal structures of natively glycosylated full ectodomains or adhesive fragments of nectins 1–4 and nectin-like 5. All crystal structures reveal dimeric nectins bound through a stereotyped interface previously proposed to represent a cis dimer. However, conservation of this interface and results of targeted cross-linking experiments show that this dimer likely represents the adhesive trans interaction. Its structure provides a simple molecular explanation for the adhesive binding specificity of nectins.

Periderm cells covering palatal shelves have tight junctions and their desquamation reduces the polarity of palatal shelf epithelial cells in palatogenesis

In palatogenesis, bilateral palatal shelves grow and fuse with each other to establish mesenchyme continuity across the horizontal palate. The palatal shelves are covered with the medial edge epithelium (MEE) in which most apical cells are periderm cells. We investigated localization and roles of tight junction (TJ) and adherens junction (AJ) components and an apical membrane marker in the MEE in palatogenesis. Immunofluorescence and immunoelectron microscopy analyses revealed that TJs were located at the boundary between neighboring periderm cells, whereas AJ components were localized at the boundary between all epithelial cells in the MEE. Specifically, typical AJs were observed at the boundaries between neighboring periderm cells and between periderm cells and underlying epithelial cells where the signal for nectin-1 was observed. The TGF-β-induced desquamation of periderm cells reduced the polarity of remaining epithelial cells as estimated by changes of epithelial cell morphology and the staining of the polarity marker and the AJ components. These less polarized epithelial cells then intermingled and finally disappeared at least partly by apoptosis. These results indicate that periderm cells covering growing palatal shelves have bona fide TJs and their desquamation reduces the polarity of palatal shelf epithelial cells in palatogenesis.