A role for the PDZ-binding domain of the coxsackie B virus and adenovirus receptor (CAR) in cell adhesion and growth

Liver-specific Coxsackievirus and adenovirus receptor deletion develop metabolic dysfunction-associated fatty liver disease

Metabolic dysfunction-associated fatty liver disease (MAFLD) is a common liver disease associated with obesity and is caused by the accumulation of ectopic fat without alcohol consumption. Coxsackievirus and adenovirus receptor (CAR) are vital for cardiac myocyte-intercalated discs and endothelial cell-to-cell tight junctions. CAR has also been reported to be associated with obesity and high blood pressure. However, its function in the liver is still not well understood. The liver of obese mice exhibit elevated CAR mRNA and protein levels. Furthermore, in the liver of patients with non-alcoholic steatohepatitis, CAR is reduced in hepatocyte cell-cell junctions compared to normal levels. We generated liver-specific CAR knockout (KO) mice to investigate the role of CAR in the liver. Body and liver weights were not different between wild-type (WT) and KO mice fed a paired or high-fat diet (HFD). However, HFD induced significant liver damage and lipid accumulation in CAR KO mice compared with WT mice. Additionally, inflammatory cytokines transcription, hepatic permeability, and macrophage recruitment considerably increased in CAR KO mice. We identified a new interaction partner of CAR using a protein pull-down assay and mass spectrometry. Apolipoprotein B mRNA editing enzyme catalytic polypeptide-like 3C (APOBEC3C) demonstrated a complex relationship with CAR, and hepatic CAR expression tightly regulated its level. Moreover, Apolipoprotein B (ApoB) and Low-density lipoprotein receptor (LDLR) levels correlated with APOBEC3C expression in the liver of CAR KO mice, suggesting that CAR may regulate lipid accumulation by controlling APOBEC3C activity. In this study, we showed that hepatic CAR deficiency increased cell-to-cell permeability. In addition, CAR deletion significantly increased hepatic lipid accumulation by inducing ApoB and LDLR expression. Although the underlying mechanism is unclear, CARs may be a target for the development of novel therapies for MAFLD.

Tight junction membrane proteins regulate the mechanical resistance of the apical junctional complex

Epithelia must be able to resist mechanical force to preserve tissue integrity. While intercellular junctions are known to be important for the mechanical resistance of epithelia, the roles of tight junctions (TJs) remain to be established. We previously demonstrated that epithelial cells devoid of the TJ membrane proteins claudins and JAM-A completely lack TJs and exhibit focal breakages of their apical junctions. Here, we demonstrate that apical junctions fracture when claudin/JAM-A-deficient cells undergo spontaneous cell stretching. The junction fracture was accompanied by actin disorganization, and actin polymerization was required for apical junction integrity in the claudin/JAM-A-deficient cells. Further deletion of CAR resulted in the disruption of ZO-1 molecule ordering at cell junctions, accompanied by severe defects in apical junction integrity. These results demonstrate that TJ membrane proteins regulate the mechanical resistance of the apical junctional complex in epithelial cells.

CXADR: From an Essential Structural Component to a Vital Signaling Mediator in Spermatogenesis

Canonical coxsackievirus and adenovirus receptor (CXADR) is a transmembrane component of cell junctions that is crucial for cardiac and testicular functions via its homophilic and heterophilic interaction. CXADR is expressed in both Sertoli cells and germ cells and is localized mainly at the interface between Sertoli-Sertoli cells and Sertoli-germ cells. Knockout of CXADR in mouse Sertoli cells specifically impairs male reproductive functions, including a compromised blood-testis barrier, apoptosis of germ cells, and premature loss of spermatids. Apart from serving as an important component for cell junctions, recent progress has showed the potential roles of CXADR as a signaling mediator in spermatogenesis. This review summarizes current research progress related to the regulation and role of CXADR in spermatogenesis as well as in pathological conditions. We hope this review provides some future directions and a blueprint to promote the further study on the roles of CXADR.

Epithelial coxsackievirus adenovirus receptor promotes house dust mite-induced lung inflammation

Airway inflammation and remodelling are important pathophysiologic features in asthma and other respiratory conditions. An intact epithelial cell layer is crucial to maintain lung homoeostasis, and this depends on intercellular adhesion, whilst damaged respiratory epithelium is the primary instigator of airway inflammation. The Coxsackievirus Adenovirus Receptor (CAR) is highly expressed in the epithelium where it modulates cell-cell adhesion stability and facilitates immune cell transepithelial migration. However, the contribution of CAR to lung inflammation remains unclear. Here we investigate the mechanistic contribution of CAR in mediating responses to the common aeroallergen, House Dust Mite (HDM). We demonstrate that administration of HDM in mice lacking CAR in the respiratory epithelium leads to loss of peri-bronchial inflammatory cell infiltration, fewer goblet-cells and decreased pro-inflammatory cytokine release. In vitro analysis in human lung epithelial cells confirms that loss of CAR leads to reduced HDM-dependent inflammatory cytokine release and neutrophil migration. Epithelial CAR depletion also promoted smooth muscle cell proliferation mediated by GSK3β and TGF-β, basal matrix production and airway hyperresponsiveness. Our data demonstrate that CAR coordinates lung inflammation through a dual function in leucocyte recruitment and tissue remodelling and may represent an important target for future therapeutic development in inflammatory lung diseases.

Understanding the molecular basis of cardiomyopathy

Inherited cardiomyopathies are a major cause of mortality and morbidity worldwide and can be caused by mutations in a wide range of proteins located in different cellular compartments. The present review is based on Dr. Ju Chen's 2021 Robert M. Berne Distinguished Lectureship of the American Physiological Society Cardiovascular Section, in which he provided an overview of the current knowledge on the cardiomyopathy-associated proteins that have been studied in his laboratory. The review provides a general summary of the proteins in different compartments of cardiomyocytes associated with cardiomyopathies, with specific focus on the proteins that have been studied in Dr. Chen's laboratory.

The Coxsackievirus and Adenovirus Receptor Has a Short Half-Life in Epithelial Cells

The coxsackievirus and adenovirus receptor (CAR) is an essential cellular protein that is involved in cell adhesion, cell signaling, and viral infection. The 8-exon encoded isoform (CAREx8) resides at the apical surface of polarized epithelia, where it is accessible as a receptor for adenovirus entering the airway lumen. Given its pivotal role in viral infection, it is a target for antiviral strategies. To understand the regulation of CAREx8 and determine the feasibility of receptor downregulation, the half-life of total and apical localized CAREx8 was determined and correlated with adenovirus transduction. Total and apical CAREx8 has a relatively short half-life of approximately 2 h. The half-life of apical CAREx8 correlates well with adenovirus transduction. These results suggest that antiviral strategies that aim to degrade the primary receptor for apical adenovirus infection will be effective within a relatively short time frame after application.

Modeling the complete kinetics of coxsackievirus B3 reveals human determinants of host-cell feedback

Complete kinetic models are pervasive in chemistry but lacking in biological systems. We encoded the complete kinetics of infection for coxsackievirus B3 (CVB3), a compact and fast-acting RNA virus. The model consists of separable, detailed modules describing viral binding-delivery, translation-replication, and encapsidation. Specific module activities are dampened by the type I interferon response to viral double-stranded RNAs (dsRNAs), which is itself disrupted by viral proteinases. The experimentally validated kinetics uncovered that cleavability of the dsRNA transducer mitochondrial antiviral signaling protein (MAVS) becomes a stronger determinant of viral outcomes when cells receive supplemental interferon after infection. Cleavability is naturally altered in humans by a common MAVS polymorphism, which removes a proteinase-targeted site but paradoxically elevates CVB3 infectivity. These observations are reconciled with a simple nonlinear model of MAVS regulation. Modeling complete kinetics is an attainable goal for small, rapidly infecting viruses and perhaps viral pathogens more broadly. A record of this paper's transparent peer review process is included in the Supplemental information.

Naturally occurring variants in the transmembrane and cytoplasmic domains of the human Coxsackie- and adenovirus receptor have no impact on virus internalisation

The Coxsackie- and adenovirus receptor (CAR) mediates homophilic cell-cell contacts and susceptibility to both human pathogenic viruses through its membrane-distal immunoglobulin domain. In the present study, we screened five missense variants of the human CAR gene for their influence on adenovector or Coxsackievirus entry into Chinese hamster ovary cells. The CAR variants facilitated virus internalisation to a similar extent as wild type CAR. This underlines CAR's presumed invariance and essential physiological role in embryogenesis.

Location of the Cell Adhesion Molecule "Coxsackievirus and Adenovirus Receptor" in the Adult Mouse Brain

The coxsackievirus and adenovirus receptor (CAR) is a single-pass transmembrane cell adhesion molecule (CAM). CAR is expressed in numerous mammalian tissues including the brain, heart, lung, and testes. In epithelial cells, CAR functions are typical of the quintessential roles of numerous CAMs. However, in the brain the multiple roles of CAR are poorly understood. To better understand the physiological role of CAR in the adult brain, characterizing its location is a primordial step to advance our knowledge of its functions. In addition, CAR is responsible for the attachment, internalization, and retrograde transport of canine adenovirus type 2 (CAV-2) vectors, which have found a niche in the mapping of neuronal circuits and gene transfer to treat and model neurodegenerative diseases. In this study, we used immunohistochemistry and immunofluorescence to document the global location of CAR in the healthy, young adult mouse brain. Globally, we found that CAR is expressed by maturing and mature neurons in the brain parenchyma and located on the soma and on projections. While CAR occasionally colocalizes with glial fibrillary acidic protein, this overlap was restricted to areas that are associated with adult neurogenesis.

The coxsackievirus and adenovirus receptor: virological and biological beauty

The coxsackievirus and adenovirus receptor (CAR) is an essential multifunctional cellular protein that is only beginning to be understood. CAR serves as a receptor for many adenoviruses, human group B coxsackieviruses, swine vesicular disease virus, and possibly other viruses. While named for its function as a viral receptor, CAR is also involved in cell adhesion, immune cell activation, synaptic transmission, and signaling. Knockout mouse models were first to identify some of these biological functions; however, tissue-specific model systems have shed light on the complexity of different CAR isoforms and their specific activities. Many of these functions are mediated by the large number of interacting proteins described so far, and several new putative interactions have recently been discovered. As antiviral and gene therapy strategies that target CAR continue to emerge, future work poised to understand the biological implications of manipulating CAR in vivo is critical.

The cell adhesion protein CAR is a negative regulator of synaptic transmission

The Coxsackievirus and adenovirus receptor (CAR) is essential for normal electrical conductance in the heart, but its role in the postnatal brain is largely unknown. Using brain specific CAR knockout mice (KO), we discovered an unexpected role of CAR in neuronal communication. This includes increased basic synaptic transmission at hippocampal Schaffer collaterals, resistance to fatigue, and enhanced long-term potentiation. Spontaneous neurotransmitter release and speed of endocytosis are increased in KOs, accompanied by increased expression of the exocytosis associated calcium sensor synaptotagmin 2. Using proximity proteomics and binding studies, we link CAR to the exocytosis machinery as it associates with syntenin and synaptobrevin/VAMP2 at the synapse. Increased synaptic function does not cause adverse effects in KO mice, as behavior and learning are unaffected. Thus, unlike the connexin-dependent suppression of atrioventricular conduction in the cardiac knockout, communication in the CAR deficient brain is improved, suggesting a role for CAR in presynaptic processes.

Unexpected subcellular distribution of a specific isoform of the Coxsackie and adenovirus receptor, CAR-SIV, in human pancreatic beta cells

AIMS/HYPOTHESIS

The Coxsackie and adenovirus receptor (CAR) is a transmembrane cell-adhesion protein that serves as an entry receptor for enteroviruses and may be essential for their ability to infect cells. Since enteroviral infection of beta cells has been implicated as a factor that could contribute to the development of type 1 diabetes, it is often assumed that CAR is displayed on the surface of human beta cells. However, CAR exists as multiple isoforms and it is not known whether all isoforms subserve similar physiological functions. In the present study, we have determined the profile of CAR isoforms present in human beta cells and monitored the subcellular localisation of the principal isoform within the cells.

METHODS

Formalin-fixed, paraffin-embedded pancreatic sections from non-diabetic individuals and those with type 1 diabetes were studied. Immunohistochemistry, confocal immunofluorescence, electron microscopy and western blotting with isoform-specific antisera were employed to examine the expression and cellular localisation of the five known CAR isoforms. Isoform-specific qRT-PCR and RNA sequencing (RNAseq) were performed on RNA extracted from isolated human islets.

RESULTS

An isoform of CAR with a terminal SIV motif and a unique PDZ-binding domain was expressed at high levels in human beta cells at the protein level. A second isoform, CAR-TVV, was also present. Both forms were readily detected by qRT-PCR and RNAseq analysis in isolated human islets. Immunocytochemical studies indicated that CAR-SIV was the principal isoform in islets and was localised mainly within the cytoplasm of beta cells, rather than at the plasma membrane. Within the cells it displayed a punctate pattern of immunolabelling, consistent with its retention within a specific membrane-bound compartment. Co-immunofluorescence analysis revealed significant co-localisation of CAR-SIV with zinc transporter protein 8 (ZnT8), prohormone convertase 1/3 (PC1/3) and insulin, but not proinsulin. This suggests that CAR-SIV may be resident mainly in the membranes of insulin secretory granules. Immunogold labelling and electron microscopic analysis confirmed that CAR-SIV was localised to dense-core (insulin) secretory granules in human islets, whereas no immunolabelling of the protein was detected on the secretory granules of adjacent exocrine cells. Importantly, CAR-SIV was also found to co-localise with protein interacting with C-kinase 1 (PICK1), a protein recently demonstrated to play a role in insulin granule maturation and trafficking.

CONCLUSIONS/INTERPRETATION

The SIV isoform of CAR is abundant in human beta cells and is localised mainly to insulin secretory granules, implying that it may be involved in granule trafficking and maturation. We propose that this subcellular localisation of CAR-SIV contributes to the unique sensitivity of human beta cells to enteroviral infection.

Structural Basis of Highly Specific Interaction between Nephrin and MAGI1 in Slit Diaphragm Assembly and Signaling

BACKGROUND

The slit diaphragm is a specialized adhesion junction between opposing podocytes, establishing the final filtration barrier that prevents passage of proteins from the capillary lumen into the urinary space. Nephrin, the key structural and signaling adhesion molecule expressed in the slit diaphragm, contains an evolutionally conserved, atypical PDZ-binding motif (PBM) reported to bind to a variety of proteins in the slit diaphragm. Several mutations in NPHS1 (the gene encoding nephrin) that result in nephrin lacking an intact PBM are associated with glomerular diseases. However, the molecular basis of nephrin-PBM-mediated protein complexes is still unclear.

METHODS

Using a combination of biochemic, biophysic, and cell biologic approaches, we systematically investigated the interactions between nephrin-PBM and PDZ domain-containing proteins in the slit diaphragm.

RESULTS

We found that nephrin-PBM specifically binds to one member of the membrane-associated guanylate kinase family of scaffolding proteins, MAGI1, but not to another, MAGI2. The complex structure of MAGI1-PDZ3/nephrin-PBM reveals that the Gly at the -3 position of nephrin-PBM is the determining feature for MAGI1-PDZ3 recognition, which sharply contrasts with the typical PDZ/PBM binding mode. A single gain-of-function mutation within MAGI2 enabled nephrin-PBM binding. In addition, using our structural analysis, we developed a highly efficient inhibitory peptide capable of specifically blocking the nephrin/MAGI1 interaction.

CONCLUSIONS

MAGI1 interacts with nephrin-PBM with exquisite specificity. A newly developed, potent inhibitory peptide that blocks this interaction may be useful for future functional investigations in vivo. Our findings also provide possible explanations for the diseases caused by NPHS1 mutations.

Poliovirus Receptor: More than a simple viral receptor

The human poliovirus receptor (PVR) is a cell surface protein with a multitude of functions in human biology. PVR was initially identified as the receptor for the human poliovirus and recent discoveries have given a greater insight into both its morphology and its function. Alternative splicing of the PVR gene results in a total of 4 alternatively spliced isoforms. Two of these isoforms lack a complete transmembrane domain and are considered soluble and block viral infection; the remaining two transmembrane isoforms differ only at their extreme C-terminal domains resulting in differential localization in epithelia and polarity of viral infection. In addition to its role as a receptor for the human poliovirus, several native biological functions have also been uncovered. PVR is an important cell adhesion protein and is involved in the transendothelial migration of leukocytes. Through its interactions with CD226 and TIGIT, transmembrane proteins found on leukocytes, PVR is a key regulator of the cell-mediated immune response. As PVR is differentially regulated in a broad spectrum of cancers, it has a strong potential for clinical use as a biomarker. PVR is also a possible target for novel cancer therapies. Utilizing its natural tropism for PVR, a genetically modified form of the live attenuated poliovirus vaccine is currently being tested for its ability to locate and destroy certain tumors. These recent studies emphasize the importance of PVR in human biology and demonstrate its utility beyond being a viral receptor protein.

Adenovirus transduction: More complicated than receptor expression

The abundance and accessibility of a primary virus receptor are critical factors that impact the susceptibility of a host cell to virus infection. The Coxsackievirus and adenovirus receptor (CAR) has two transmembrane isoforms that occur due to alternative splicing and differ in localization and function in polarized epithelia. To determine the relevance of isoform-specific expression across cell types, the abundance and localization of both isoforms were determined in ten common cell lines, and correlated with susceptibility to adenovirus transduction relative to polarized primary human airway epithelia. Data show that the gene and protein expression for each isoform of CAR varies significantly between cell lines and polarization, as indicated by high transepithelial resistance, is inversely related to adenovirus transduction. In summary, the variability of polarity and isoform-specific expression among model cells are critical parameters that must be considered when evaluating the clinical relevance of potential adenovirus-mediated gene therapy and anti-adenovirus strategies.

Cell-cell communication mediated by the CAR subgroup of immunoglobulin cell adhesion molecules in health and disease

The immunoglobulin superfamily represents a diverse set of cell-cell contact proteins and includes well-studied members such as NCAM1, DSCAM, L1 or the contactins which are strongly expressed in the nervous system. In this review we put our focus on the biological function of a less understood subgroup of Ig-like proteins composed of CAR (coxsackievirus and adenovirus receptor), CLMP (CAR-like membrane protein) and BT-IgSF (brain and testis specific immunoglobulin superfamily). The CAR-related proteins are type I transmembrane proteins containing an N-terminal variable (V-type) and a membrane proximal constant (C2-type) Ig domain in their extracellular region which are implicated in homotypic adhesion. They are highly expressed during embryonic development in a variety of tissues including the nervous system whereby in adult stages the protein level of CAR and CLMP decreases, only BT-IgSF expression increases within age. CAR-related proteins are concentrated at specialized cell-cell communication sites such as gap or tight junctions and are present at the plasma membrane in larger protein complexes. Considerable progress has been made on the molecular structure and interactions of CAR while research on CLMP and BT-IgSF is at an early stage. Studies on mouse mutants revealed biological functions of CAR in the heart and for CLMP in the gastrointestinal and urogenital systems. Furthermore, CAR and BT-IgSF appear to regulate synaptic function in the hippocampus.

TNFα promotes CAR-dependent migration of leukocytes across epithelial monolayers

Trans-epithelial migration (TEpM) of leukocytes during inflammation requires engagement with receptors expressed on the basolateral surface of the epithelium. One such receptor is Coxsackie and Adenovirus Receptor (CAR) that binds to Junctional Adhesion Molecule-like (JAM-L) expressed on leukocytes. Here we provide the first evidence that efficient TEpM of monocyte-derived THP-1 cells requires and is controlled by phosphorylation of CAR. We show that TNFα acts in a paracrine manner on epithelial cells via a TNFR1-PI3K-PKCδ pathway leading to CAR phosphorylation and subsequent transmigration across cell junctions. Moreover, we show that CAR is hyper-phosphorylated in vivo in acute and chronic lung inflammation models and this response is required to facilitate immune cell recruitment. This represents a novel mechanism of feedback between leukocytes and epithelial cells during TEpM and may be important in controlling responses to pro-inflammatory cytokines in pathological settings.

ADAM10 Is Involved in Cell Junction Assembly in Early Porcine Embryo Development

ADAM10 (A Disintegrin and Metalloprotease domain-containing protein 10) is a cell surface protein with a unique structure possessing both potential adhesion and protease domains. However, the role of ADAM10 in preimplantation stage embryos is not clear. In this study, we examined the expression patterns and functional roles of ADAM10 in porcine parthenotes during preimplantation development. The transcription level of ADAM10 dramatically increased from the morula stage onward. Immunostaining revealed that ADAM10 was present in both the nucleus and cytoplasm in early cleavage stage embryos, and localized to the apical region of the outer cells in morula and blastocyst embryos. Knockdown (KD) of ADAM10 using double strand RNA did not alter preimplantation embryo development until morula stage, but resulted in significantly reduced development to blastocyst stage. Moreover, the KD blastocyst showed a decrease in gene expression of adherens and tight junction (AJ/TJ), and an increase in trophectoderm TJ permeability by disrupting TJ assembly. Treatment with an ADAM10 specific chemical inhibitor, GI254023X, at the morula stage also inhibited blastocyst development and led to disruption of TJ assembly. An in situ proximity ligation assay demonstrated direct interaction of ADAM10 with coxsackie virus and adenovirus receptor (CXADR), supporting the involvement of ADAM10 in TJ assembly. In conclusion, our findings strongly suggest that ADADM10 is important for blastocyst formation rather than compaction, particularly for TJ assembly and stabilization in preimplantation porcine parthenogenetic development.

Membrane Dynamics and Signaling of the Coxsackievirus and Adenovirus Receptor

The coxsackievirus and adenovirus receptor (CAR) belongs to the immunoglobulin superfamily and acts as a receptor for some adenovirus types and group B coxsackieviruses. Its role is best described in epithelia where CAR participates to tight junction integrity and maintenance. Recently, several studies aimed to characterize its potential interaction with intracellular signaling pathways and highlighted several features linking CAR to gene expression. In addition, the molecular mechanisms leading to CAR-specific membrane targeting via the secretory pathway in polarized cells and its internalization are starting to be unraveled. This chapter discusses the interaction between membrane dynamics, intracellular trafficking, and signaling of CAR.

Protein interacting with C kinase 1 suppresses invasion and anchorage-independent growth of astrocytic tumor cells

Astrocytic tumors are the most common form of primary brain tumor. Astrocytic tumor cells infiltrate the surrounding CNS tissue, allowing them to evade removal upon surgical resection of the primary tumor. Dynamic changes to the actin cytoskeleton are crucial to cancer cell invasion, but the specific mechanisms that underlie the particularly invasive phenotype of astrocytic tumor cells are unclear. Protein interacting with C kinase 1 (PICK1) is a PDZ and BAR domain-containing protein that inhibits actin-related protein 2/3 (Arp2/3)-dependent actin polymerization and is involved in regulating the trafficking of a number of cell-surface receptors. Here we report that, in contrast to other cancers, PICK1 expression is down-regulated in grade IV astrocytic tumor cell lines and also in clinical cases of the disease in which grade IV tumors have progressed from lower-grade tumors. Exogenous expression of PICK1 in the grade IV astrocytic cell line U251 reduces their capacity for anchorage-independent growth, two-dimensional migration, and invasion through a three-dimensional matrix, strongly suggesting that low PICK1 expression plays an important role in astrocytic tumorigenesis. We propose that PICK1 negatively regulates neoplastic infiltration of astrocytic tumors and that manipulation of PICK1 is an attractive possibility for therapeutic intervention.

Simultaneous FRAP, FLIM and FAIM for measurements of protein mobility and interaction in living cells

We present a novel integrated multimodal fluorescence microscopy technique for simultaneous fluorescence recovery after photobleaching (FRAP), fluorescence lifetime imaging (FLIM) and fluorescence anisotropy imaging (FAIM). This approach captures a series of polarization-resolved fluorescence lifetime images during a FRAP recovery, maximizing the information available from a limited photon budget. We have applied this method to analyse the behaviour of GFP-labelled coxsackievirus and adenovirus receptor (CAR) in living human epithelial cells. Our data reveal that CAR exists in oligomeric states throughout the cell, and that these complexes occur in conjunction with high immobile fractions of the receptor at cell-cell junctions. These findings shed light on previously unknown molecular associations between CAR receptors in intact cells and demonstrate the power of combined FRAP, FLIM and FAIM microscopy as a robust method to analyse complex multi-component dynamics in living cells.

NGL-2 Is a New Partner of PAR Complex in Axon Differentiation

Neuronal polarization is pivotal for neural network formation during brain development. Axon differentiation is a hallmark of initial neuronal polarization. Here, we report that the leucine-rich repeat-containing protein netrin-G ligand-2 (NGL-2) as a polarity regulator that localizes asymmetrically in rat hippocampal neurons and is required for differentiation of the future axon. NGL-2 was associated with PAR complex, and this interaction resulted in local stabilization of axonal microtubules. Further study showed that the C terminal of NGL-2 binds to the PDZ domain of PAR6, and NGL-2 interacts with PAR3 and atypical PKCζ (aPKCζ), with PAR6 acting as a bridge or modifier. Then, NGL-2 regulates the local stabilization of microtubules and promotes axon differentiation by the aPKCζ/microtubule affinity-regulating kinase 2 pathway. These findings reveal the critical role of NGL-2 in regulating axon differentiation in rat hippocampal neurons and reveal a novel partner of the PAR complex.

The Intracellular Domain of the Coxsackievirus and Adenovirus Receptor Differentially Influences Adenovirus Entry

The coxsackievirus and adenovirus receptor (CAR) is a cell adhesion molecule used as a docking molecule by some adenoviruses (AdVs) and group B coxsackieviruses. We previously proposed that the preferential transduction of neurons by canine adenovirus type 2 (CAV-2) is due to CAR-mediated internalization. Our proposed pathway of CAV-2 entry is in contrast to that of human AdV type 5 (HAdV-C5) in nonneuronal cells, where internalization is mediated by auxiliary receptors such as integrins. We therefore asked if in fibroblast-like cells the intracellular domain (ICD) of CAR plays a role in the internalization of the CAV-2 fiber knob (FK(CAV)), CAV-2, or HAdV-C5 when the capsid cannot engage integrins. Here, we show that in fibroblast-like cells, the CAR ICD is needed for FK(CAV) entry and efficient CAV-2 transduction but dispensable for HAdV-C5 and an HAdV-C5 capsid lacking the RGD sequence (an integrin-interacting motif) in the penton. Moreover, the deletion of the CAR ICD further impacts CAV-2 intracellular trafficking, highlighting the crucial role of CAR in CAV-2 intracellular dynamics. These data demonstrate that the CAR ICD contains sequences important for the recruitment of the endocytic machinery that differentially influences AdV cell entry.

IMPORTANCE

Understanding how viruses interact with the host cell surface and reach the intracellular space is of crucial importance for applied and fundamental virology. Here, we compare the role of a cell adhesion molecule (CAR) in the internalization of adenoviruses that naturally infect humans and Canidae. We show that the intracellular domain of CAR differentially regulates AdV entry and trafficking. Our study highlights the mechanistic differences that a receptor can have for two viruses from the same family.

Podocyte-Specific Deletion of Murine CXADR Does Not Impair Podocyte Development, Function or Stress Response

The coxsackie- and adenovirus receptor (CXADR) is a member of the immunoglobulin protein superfamily, present in various epithelial cells including glomerular epithelial cells. Beside its known function as a virus receptor, it also constitutes an integral part of cell-junctions. Previous studies in the zebrafish pronephros postulated a potential role of CXADR for the terminal differentiation of glomerular podocytes and correct patterning of the elaborated foot process architecture. However, due to early embryonic lethality of constitutive Cxadr knockout mice, mammalian data on kidney epithelial cells have been lacking. Interestingly, Cxadr is robustly expressed during podocyte development and in adulthood in response to glomerular injury. We therefore used a conditional transgenic approach to elucidate the function of Cxadr for podocyte development and stress response. Surprisingly, we could not discern a developmental phenotype in podocyte specific Cxadr knock-out mice. In addition, despite a significant up regulation of CXADR during toxic, genetic and immunologic podocyte injury, we could not detect any impact of Cxadr on these injury models. Thus these data indicate that in contrast to lower vertebrate models, mammalian podocytes have acquired molecular programs to compensate for the loss of Cxadr.

High-Throughput Differentiation and Screening of a Library of Mutant Stem Cell Clones Defines New Host-Based Genes Involved in Rabies Virus Infection

We used a genomic library of mutant murine embryonic stem cells (ESCs) and report the methodology required to simultaneously culture, differentiate, and screen more than 3,200 heterozygous mutant clones to identify host-based genes involved in both sensitivity and resistance to rabies virus infection. Established neuronal differentiation protocols were miniaturized such that many clones could be handled simultaneously, and molecular markers were used to show that the resultant cultures were pan-neuronal. Next, we used a green fluorescent protein (GFP) labeled rabies virus to develop, validate, and implement one of the first host-based, high-content, high-throughput screens for rabies virus. Undifferentiated cell and neuron cultures were infected with GFP-rabies and live imaging was used to evaluate GFP intensity at time points corresponding to initial infection/uptake and early and late replication. Furthermore, supernatants were used to evaluate viral shedding potential. After repeated testing, 63 genes involved in either sensitivity or resistance to rabies infection were identified. To further explore hits, we used a completely independent system (siRNA) to show that reduction in target gene expression leads to the observed phenotype. We validated the immune modulatory gene Unc13d and the dynein adapter gene Bbs4 by treating wild-type ESCs and primary neurons with siRNA; treated cultures were resistant to rabies infection/replication. Overall, the potential of such in vitro functional genomics screens in stem cells adds additional value to other libraries of stem cells. This technique is applicable to any bacterial or virus interactome and any cell or tissue types that can be differentiated from ESCs.

The PDZ3 domain of the cellular scaffolding protein MAGI-1 interacts with the Coxsackievirus and adenovirus receptor (CAR)

The Coxsackievirus and adenovirus receptor (CAR) is an essential cellular protein that is involved in cell-cell adhesion, protein trafficking, and viral infection. The major isoform of CAR is selectively sorted to the basolateral membrane of polarized epithelial cells where it co-localizes with the cellular scaffolding protein membrane-associated guanylate kinase with inverted domain structure-1 (MAGI-1). Previously, we demonstrated CAR interacts with MAGI-1 through a PDZ-domain dependent interaction. Here, we show that the PDZ3 domain of MAGI-1 is exclusively responsible for the high affinity interaction between the seven exon isoform of CAR and MAGI-1 using yeast-two-hybrid analysis and confirming this interaction biochemically and in cellular lysates by in vitro pull down assay and co-immunoprecipitation. The high affinity interaction between the PDZ3 domain and CAR C-terminus was measured by fluorescence resonance energy transfer. Further, we investigated the biological relevance of this high affinity interaction between CAR and the PDZ3 domain of MAGI-1 and found that it does not alter CAR-mediated adenovirus infection. By contrast, interruption of this high affinity interaction altered the localization of MAGI-1 indicating that CAR is able to traffic MAGI-1 to cell junctions. These data deepen the molecular understanding of the interaction between CAR and MAGI-1 and indicate that although CAR plays a role in trafficking PDZ-based scaffolding proteins to cellular junctions, association with a high affinity intracellular binding partner does not significantly alter adenovirus binding and entry via CAR.

Regulation of TGF-β Signal Transduction

Transforming growth factor-β (TGF-β) signaling regulates diverse cellular processes, including cell proliferation, differentiation, apoptosis, cell plasticity, and migration. TGF-β signaling can be mediated by Smad proteins or other signaling proteins such as MAP kinases and Akt. TGF-β signaling is tightly regulated at different levels along the pathways to ensure its proper physiological functions in different cells and tissues. Deregulation of TGF-β signaling has been associated with various kinds of diseases, such as cancer and tissue fibrosis. This paper focuses on our recent work on regulation of TGF-β signaling.

Neutrophil-derived JAML inhibits repair of intestinal epithelial injury during acute inflammation

Neutrophil transepithelial migration (TEM) during acute inflammation is associated with mucosal injury. Using models of acute mucosal injury in vitro and in vivo, we describe a new mechanism by which neutrophils infiltrating the intestinal mucosa disrupt epithelial homeostasis. We report that junctional adhesion molecule-like protein (JAML) is cleaved from neutrophil surface by zinc metalloproteases during TEM. Neutrophil-derived soluble JAML binds to the epithelial tight junction protein coxsackie-adenovirus receptor (CAR) resulting in compromised barrier and inhibition of wound repair, through decreased epithelial proliferation. The deleterious effects of JAML on barrier and wound repair are reversed with an anti-JAML monoclonal antibody that inhibits JAML-CAR binding. JAML released from transmigrating neutrophils across inflamed epithelia may thus promote recruitment of leukocytes and aid in clearance of invading microorganisms. However, sustained release of JAML under pathologic conditions associated with persistence of large numbers of infiltrated neutrophils would compromise intestinal barrier and inhibit mucosal healing. Thus, targeting JAML-CAR interactions may improve mucosal healing responses under conditions of dysregulated neutrophil recruitment.

Interspecies differences in virus uptake versus cardiac function of the coxsackievirus and adenovirus receptor

The coxsackievirus and adenovirus receptor (CAR) is a cell contact protein with an important role in virus uptake. Its extracellular immunoglobulin domains mediate the binding to coxsackievirus and adenovirus as well as homophilic and heterophilic interactions between cells. The cytoplasmic tail links CAR to the cytoskeleton and intracellular signaling cascades. In the heart, CAR is crucial for embryonic development, electrophysiology, and coxsackievirus B infection. Noncardiac functions are less well understood, in part due to the lack of suitable animal models. Here, we generated a transgenic mouse that rescued the otherwise embryonic-lethal CAR knockout (KO) phenotype by expressing chicken CAR exclusively in the heart. Using this rescue model, we addressed interspecies differences in coxsackievirus uptake and noncardiac functions of CAR. Survival of the noncardiac CAR KO (ncKO) mouse indicates an essential role for CAR in the developing heart but not in other tissues. In adult animals, cardiac activity was normal, suggesting that chicken CAR can replace the physiological functions of mouse CAR in the cardiomyocyte. However, chicken CAR did not mediate virus entry in vivo, so that hearts expressing chicken instead of mouse CAR were protected from infection and myocarditis. Comparison of sequence homology and modeling of the D1 domain indicate differences between mammalian and chicken CAR that relate to the sites important for virus binding but not those involved in homodimerization. Thus, CAR-directed anticoxsackievirus therapy with only minor adverse effects in noncardiac tissue could be further improved by selectively targeting the virus-host interaction while maintaining cardiac function.

IMPORTANCE

Coxsackievirus B3 (CVB3) is one of the most common human pathogens causing myocarditis. Its receptor, the coxsackievirus and adenovirus receptor (CAR), not only mediates virus uptake but also relates to cytoskeletal organization and intracellular signaling. Animals without CAR die prenatally with major cardiac malformations. In the adult heart, CAR is important for virus entry and electrical conduction, but its nonmuscle functions are largely unknown. Here, we show that chicken CAR expression exclusively in the heart can rescue the otherwise embryonic-lethal CAR knockout phenotype but does not support CVB3 infection of adult cardiomyocytes. Our findings have implications for the evolution of virus-host versus physiological interactions involving CAR and could help to improve future coxsackievirus-directed therapies inhibiting virus replication while maintaining CAR's cellular functions.

Coxsackie and adenovirus receptor is a critical regulator for the survival and growth of oral squamous carcinoma cells

Coxsackie and adenovirus receptor (CAR) is essential for adenovirus infection to target cells, and its constitutive expression in various cancerous and normal tissues has been reported. Recently, the biological role of CAR in human cancers of several different origins has been investigated with respect to tumor progression, metastasis and tumorigenesis. However, its biological function in tumor cells remains controversial. Here we report the critical role of CAR in growth regulation of oral squamous cell carcinomas (SCCs) in vitro and in vivo via the specific interaction with Rho-associated protein kinase (ROCK). Loss of endogenous CAR expression by knockdown using specific small interfering RNA (siRNA) against CAR facilitates growth suppression of SCC cells due to cell dissociation, followed by apoptosis. The consequent morphological reaction was reminiscent of anoikis, rather than epithelial-mesenchymal transition, and the dissociation of oral SCC cells was triggered not by lack of contact with extracellular matrix, but by loss of cell-to-cell contact caused by abnormal translocation of E-cadherin from surface membrane to cytoplasm. Immunoprecipitation assays of the CAR-transfected oral SCC cell line, HSC-2, with or without ROCK inhibitor (Y-27632) revealed that CAR directly associates with ROCKI and ROCKII, which results in inhibition of ROCK activity and contributes to maintenance of cell-to-cell adhesion for their growth and survival. Based on these findings, in vivo behavior of CAR-downregulated HSC-2 cells from siRNA knockdown was compared with that of normally CAR-expressing cells in intraperitoneally xenografted mouse models. The mice engrafted with CAR siRNA-pretreated HSC-2 cells showed poor formation of metastatic foci in contrast to those implanted with the control siRNA-pretreated cells. Thus, CAR substantially has an impact on growth and survival of oral SCC cells as a negative regulator of ROCK in vitro and in vivo.

Regulation of adhesion by flexible ectodomains of IgCAMs

To perform their diverse biological functions the adhesion activities of the cell adhesion molecules of the immunoglobulin superfamily (IgCAMs) might be regulated by local clustering, proteolytical shedding of their ectodomains or rapid recycling to and from the plasma membrane. Another form of regulation of adhesion might be obtained through flexible ectodomains of IgCAMs which adopt distinct conformations and which in turn modulate their adhesion activity. Here, we discuss variations in the conformation of the extracellular domains of CEACAM1 and CAR that might influence their binding and signaling activities. Furthermore, we concentrate on alternative splicing of single domains and short segments in the extracellular regions of L1 subfamily members that might affect the organization of the N-terminal located Ig-like domains. In particular, we discuss variations of the linker sequence between Ig-like domains 2 and 3 (D2 and D3) that is required for the horseshoe conformation.

Coxsackievirus and adenovirus receptor (CAR) mediates trafficking of acid sensing ion channel 3 (ASIC3) via PSD-95

We have previously shown that the Coxsackievirus and adenovirus receptor (CAR) can interact with post-synaptic density 95 (PSD-95) and localize PSD-95 to cell-cell junctions. We have also shown that activity of the acid sensing ion channel (ASIC3), a H(+)-gated cation channel that plays a role in mechanosensation and pain signaling, is negatively modulated by PSD-95 through a PDZ-based interaction. We asked whether CAR and ASIC3 simultaneously interact with PSD-95, and if so, whether co-expression of these proteins alters their cellular distribution and localization. Results indicate that CAR and ASIC3 co-immunoprecipitate only when co-expressed with PSD-95. CAR also brings both PSD-95 and ASIC3 to the junctions of heterologous cells. Moreover, CAR rescues PSD-95-mediated inhibition of ASIC3 currents. These data suggest that, in addition to activity as a viral receptor and adhesion molecule, CAR can play a role in trafficking proteins, including ion channels, in a PDZ-based scaffolding complex.

The PDZ1 and PDZ3 domains of MAGI-1 regulate the eight-exon isoform of the coxsackievirus and adenovirus receptor

Epithelial integrity is essential for homeostasis and poses a formidable barrier to pathogen entry. Major factors for viral entry into epithelial cells are the localization and abundance of the primary receptor. The coxsackievirus and adenovirus receptor (CAR) is a primary receptor for these two pathogenic groups of viruses. In polarized epithelia, a low-abundance, alternatively spliced eight-exon isoform of CAR, CAR(Ex8), is localized apically where it can support viral infection from the air-exposed surface. Using biochemical, cell biology, genetic, and spectroscopic approaches, we show that the levels of apical CAR(Ex8) are negatively regulated by the PDZ domain-containing protein MAGI-1 (membrane-associated guanylate kinase with inverted orientation protein-1) and that two MAGI-1 PDZ domains, PDZ1 and PDZ3, regulate CAR(Ex8) levels in opposing ways. Similar to full-length MAGI-1, expression of the isolated PDZ3 domain significantly reduces cell surface CAR(Ex8) abundance and adenovirus infection. In contrast, the PDZ1 domain is able to rescue CAR(Ex8) and adenovirus infection from MAGI-1-mediated suppression. These data suggest a novel cell-based strategy to either suppress viral infection or augment adenovirus-based gene therapy.

PICK1 promotes caveolin-dependent degradation of TGF-β type I receptor

Protein that interacts with C kinase 1 (PICK1) is a critical mediator of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) trafficking in neural synapses. However, its ubiquitous expression suggests that it may have other non-neural functions. Here we show that PICK1 antagonizes transforming growth factor beta (TGF-β) signaling by targeting TGF-β type I receptor (TβRI) for degradation. Biochemical analyses reveal that PICK1 directly interacts with the C-terminus of TβRI via its PDZ domain and acts as a scaffold protein to enhance the interaction between TβRI and caveolin-1, leading to enhanced lipid raft/caveolae localization. Therefore, PICK1 increases caveolin-mediated endocytosis, ubiquitination and degradation of TβRI. Moreover, a negative correlation between PICK1 expression and TβRI or phospho-Smad2 levels is observed in human breast tumors, indicating that PICK1 may participate in breast cancer development through inhibition of TGF-β signaling. Our findings reveal a non-neural function of PICK1 as an important negative regulator of TGF-β signaling.

Biochemical and computational analysis of LNX1 interacting proteins

PDZ (Post-synaptic density, 95 kDa, Discs large, Zona Occludens-1) domains are protein interaction domains that bind to the carboxy-terminal amino acids of binding partners, heterodimerize with other PDZ domains, and also bind phosphoinositides. PDZ domain containing proteins are frequently involved in the assembly of multi-protein complexes and clustering of transmembrane proteins. LNX1 (Ligand of Numb, protein X 1) is a RING (Really Interesting New Gene) domain-containing E3 ubiquitin ligase that also includes four PDZ domains suggesting it functions as a scaffold for a multi-protein complex. Here we use a human protein array to identify direct LNX1 PDZ domain binding partners. Screening of 8,000 human proteins with isolated PDZ domains identified 53 potential LNX1 binding partners. We combined this set with LNX1 interacting proteins identified by other methods to assemble a list of 220 LNX1 interacting proteins. Bioinformatic analysis of this protein list was used to select interactions of interest for future studies. Using this approach we identify and confirm six novel LNX1 binding partners: KCNA4, PAK6, PLEKHG5, PKC-alpha1, TYK2 and PBK, and suggest that LNX1 functions as a signalling scaffold.

Increased expression of the coxsackie and adenovirus receptor downregulates αvβ3 and αvβ5 integrin expression and reduces cell adhesion and migration

AIMS

Coxsackie and adenovirus receptor (CAR) is a tumor suppressor and a primary receptor for adenovirus type 5 (Ad5). Our study aims to examine the influence of forced expression of CAR in rhabdomyosarcoma cells (RD) on expression levels of integrins implicated in Ad5 entry, and the effect of CAR on cell-extracellular matrix adhesion and migration.

MAIN METHODS

CAR expressing clones were established from RD cells by stable transfection. Flow cytometry was used to evaluate the expression of CAR and integrins. Adhesion was measured in plates previously coated with vitronectin or fibronectin. Boyden chambers were used to investigate migration. Transfection of cells with siRNA was used to achieve integrin silencing. Ad5-mediated transgene expression was measured by β-gal staining.

KEY FINDINGS

Increased expression of CAR in RD cells reduces the expression of αvβ3 and αvβ5 integrins. Cells overexpressing CAR exhibit significantly reduced adhesion to vitronectin and fibronectin, and reduced cell migration. Specifically silencing αvβ3 integrin in RD cells reduced cell migration indicating that reduced migration could be the consequence of αvβ3 integrin downregulation. This study also demonstrates the negative effect of reduced levels of αvβ3 and αvβ5 integrins on Ad5-mediated transgene expression with Ad5 retargeted to αv integrins.

SIGNIFICANCE

The pharmacological upregulation of CAR aimed to increase Ad5-mediated transgene expression may actually downregulate αvβ3 and αvβ5 integrins and thus alter Ad5-mediated gene transfer. The mechanism of decreased cell migration, a prerequisite for metastasis and invasion, due to increased CAR expression may be explained by reduced αvβ3 integrin expression.

Multiple phenotypes in adult mice following inactivation of the Coxsackievirus and Adenovirus Receptor (Car) gene

To determine the normal function of the Coxsackievirus and Adenovirus Receptor (CAR), a protein found in tight junctions and other intercellular complexes, we constructed a mouse line in which the CAR gene could be disrupted at any chosen time point in a broad spectrum of cell types and tissues. All knockouts examined displayed a dilated intestinal tract and atrophy of the exocrine pancreas with appearance of tubular complexes characteristic of acinar-to-ductal metaplasia. The mice also exhibited a complete atrio-ventricular block and abnormal thymopoiesis. These results demonstrate that CAR exerts important functions in the physiology of several organs in vivo.

Impact of the coxsackievirus and adenovirus receptor on the adenoma-carcinoma sequence of colon cancer

Background:

Coxsackie and adenovirus receptor (CAR) has been suggested to function as a tumour suppressor. Its impact on the adenoma–carcinoma sequence of the colon, however, is unclear.

Methods:

Coxsackie and adenovirus receptor was analysed in non-cancerous and neoplastic colon samples using immunohistochemistry and quantitative RT–PCR. The function of CAR in colon cancer cell lines was determined following application of CAR siRNA or ectopic expression of a human full-length CAR cDNA.

Results:

Compared with healthy mucosa, increased CAR-mRNA expression was found in adenomas, whereas primary cancers and metastases displayed a marked decline. At the plasma membrane, CAR was present in normal mucosa samples (93%), adenomas, and metastases (100% ea.), whereas in colon cancers, it was found less frequently (49%, P<0.0001). Cytoplasmic CAR immunopositivity increased from normal mucosa (22%), to adenomas (73%, P=0.0006), primary cancers (83%, P<0.0001), and metastases (67%, P=0.0019). In cancer cell lines, CAR inhibition resulted in increased proliferation, whereas enforced ectopic CAR expression led to opposite results. Blocking the extracellular portion of CAR increased cell invasion in vitro. In mice, xenotransplants of colon cancer cells with enforced CAR expression formed significantly smaller tumours, whereas CAR inhibition increased the formation of liver metastases.

Conclusion:

We conclude that CAR facilitates complex effects during colon carcinogenesis, potentially mediated by its stage-dependent subcellular distribution; high CAR expression potentially prevents apoptosis in adenomas, loss of CAR at the plasma membrane promotes growth, and dissemination of primary cancers, and high membranous CAR presence may support the establishment of distant metastases.

PDZ binding to the BAR domain of PICK1 is elucidated by coarse-grained molecular dynamics

A key regulator of α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor traffic, PICK1 is known to interact with over 40 other proteins, including receptors, transporters and ionic channels, and to be active mostly as a homodimer. The current lack of a complete PICK1 structure determined at atomic resolution hinders the elucidation of its functional mechanisms. Here, we identify interactions between the component PDZ and BAR domains of PICK1 by calculating possible binding sites for the PDZ domain of PICK1 (PICK1-PDZ) to the homology-modeled, crescent-shaped dimer of the PICK1-BAR domain using multiplexed replica-exchange molecular dynamics (MREMD) and canonical molecular dynamics simulations with the coarse-grained UNRES force field. The MREMD results show that the preferred binding site for the single PDZ domain is the concave cavity of the BAR dimer. A second possible binding site is near the N-terminus of the BAR domain that is linked directly to the PDZ domain. Subsequent short canonical molecular dynamics simulations used to determine how the PICK1-PDZ domain moves to the preferred binding site on the BAR domain of PICK1 revealed that initial hydrophobic interactions drive the progress of the simulated binding. Thus, the concave face of the BAR dimer accommodates the PDZ domain first by weak hydrophobic interactions and then the PDZ domain slides to the center of the concave face, where more favorable hydrophobic interactions take over.

Protein interacting with C alpha kinase 1 (PICK1) is involved in promoting tumor growth and correlates with poor prognosis of human breast cancer

Protein interacting with C alpha kinase 1 (PICK1), which interacts with multiple different proteins in a variety of cellular contexts, is believed to play important roles in diverse pathological conditions including cancer. In this study, we attempted to investigate the correlation of PICK1 with clinicopathological features as well as prognosis of human breast cancer. In addition, we aimed at a better understanding of the biological function of PICK1 in breast cancer cell biology. As judged by semi- quantitative RT-PCR and western blotting, PICK1 was overexpressed in tumor cells as compared to adjacent normal epithelia in breast, lung, gastric, colorectal, and ovarian cancer. As judged by immunostaining breast cancer tissue microarrays, high levels of PICK1 expression correlated with shortened span of overall survival (OS). Protein interacting with C alpha kinase 1 (PICK1) expression seemed to be specifically associated with reduced OS in lymph node-positive, Her/neu-2 positive, and the basal-like type subgroups, respectively. Consistently, the expression of PICK1 correlated with histological grade, lymph node metastasis, Her-2/neu-positivity, and triple-negative basal-like breast cancer. Protein interacting with C alpha kinase 1 (PICK1) was not correlated with menopausal status, tumor size, or hormone receptor status. In a complementary study, transfection of MDA-MB-231 cells with PICK1 siRNA decreased cell proliferation and colony formation in vitro and inhibited tumorigenicity in nude mice. Our clinical and experimental evidence supports an oncogenic role of PICK1 in human breast cancer. In particular, our data suggest that PICK1 promotes tumor cell proliferation. Taken together, PICK1 may serve not only as a marker for poor prognosis, but also as a therapeutic target in breast cancer.

Tight junctions in the testis: new perspectives

In the testis, tight junctions (TJs) are found between adjacent Sertoli cells at the level of the blood-testis barrier (BTB) where they coexist with basal ectoplasmic specializations and desmosome-gap junctions. The BTB physically divides the seminiferous epithelium into two distinct compartments: a basal compartment where spermatogonia and early spermatocytes are found, and an adluminal compartment where more developed germ cells are sequestered from the systemic circulation. In order for germ cells (i.e. preleptotene spermatocytes) to enter the adluminal compartment, they must cross the BTB, a cellular event requiring the participation of several molecules and signalling pathways. Still, it is not completely understood how preleptotene spermatocytes traverse the BTB at stage VIII of the seminiferous epithelial cycle. In this review, we discuss largely how TJ proteins are exploited by viruses and cancer cells to cross endothelial and epithelial cells. We also discuss how this information may apply to future studies investigating the movement of preleptotene spermatocytes across the BTB.

Sertoli-germ cell junctions in the testis: a review of recent data

Spermatogenesis is a process that involves an array of cellular and biochemical events, collectively culminating in the formation of haploid spermatids from diploid precursor cells known as spermatogonia. As germ cells differentiate from spermatogonia into elongated spermatids, they also progressively migrate across the entire length of the seminiferous epithelium until they reach the luminal edge in anticipation of spermiation at late stage VIII of spermatogenesis. At the same time, these germ cells must maintain stable attachment with Sertoli cells via testis-unique intermediate filament- (i.e. desmosome-like junctions) and actin- (i.e. ectoplasmic specializations, ESs) based cell junctions to prevent sloughing of immature germ cells from the seminiferous epithelium, which may result in infertility. In essence, both desmosome-like junctions and basal ESs are known to coexist between Sertoli cells at the level of the blood-testis barrier where they cofunction with the well-studied tight junction in maintaining the immunological barrier. However, the type of anchoring device that is present between Sertoli and germ cells depends on the developmental stage of the germ cell, i.e. desmosome-like junctions are present between Sertoli and germ cells up to, but not including, step 8 spermatids after which this junction type is replaced by the apical ES. While little is known about the biology of the desmosome-like junction in the testis, we have a relatively good understanding of the molecular architecture and the regulation of the ES. Here, we discuss recent findings relating to these two junction types in the testis, highlighting prospective areas that should be investigated in future studies.

The coxsackievirus-adenovirus receptor reveals complex homophilic and heterophilic interactions on neural cells

The coxsackievirus-adenovirus receptor (CAR) is a member of the Ig superfamily strongly expressed in the developing nervous system. Our histological investigations during development reveal an initial uniform distribution of CAR on all neural cells with a concentration on membranes that face the margins of the nervous system (e.g., the basal laminae and the ventricular side). At more advanced stages, CAR becomes downregulated and restricted to specific regions including areas rich in axonal and dendritic surfaces. To study the function of CAR on neural cells, we used the fiber knob of the adenovirus, extracellular CAR domains, blocking antibodies to CAR, as well as CAR-deficient neural cells. Blocking antibodies were found to inhibit neurite extension in retina organ and retinal explant cultures, whereas the application of the recombinant fiber knob of the adenovirus subtype Ad2 or extracellular CAR domains promoted neurite extension and adhesion to extracellular matrices. We observed a promiscuous interaction of CAR with extracellular matrix glycoproteins, which was deduced from analytical ultracentrifugation experiments, affinity chromatography, and adhesion assays. The membrane proximal Ig domain of CAR, termed D2, was found to bind to a fibronectin fragment, including the heparin-binding domain 2, which promotes neurite extension of wild type, but not of CAR-deficient neural cells. In contrast to heterophilic interactions, homophilic association of CAR involves both Ig domains, as was revealed by ultracentrifugation, chemical cross-linking, and adhesion studies. The results of these functional and binding studies are correlated to a U-shaped homodimer of the complete extracellular domains of CAR detected by x-ray crystallography.