Apical localization of the coxsackie-adenovirus receptor by glycosyl-phosphatidylinositol modification is sufficient for adenovirus-mediated gene transfer through the apical surface of human airway epithelia

Effective viral-mediated lung gene therapy: is airway surface preparation necessary?

Gene-based therapeutics are actively being pursued for the treatment of lung diseases. While promising advances have been made over the last decades, the absence of clinically available lung-directed genetic therapies highlights the difficulties associated with this effort. Largely, progress has been hindered by the presence of inherent physical and physiological airway barriers that significantly reduce the efficacy of gene transfer. These barriers include surface mucus, mucociliary action, cell-to-cell tight junctions, and the basolateral cell membrane location of viral receptors for many commonly used gene vectors. Accordingly, airway surface preparation methods have been developed to disrupt these barriers, creating a more conducive environment for gene uptake into the target airway cells. The two major approaches have been chemical and physical methods. Both have proven effective for increasing viral-mediated gene transfer pre-clinically, although with variable effect depending on the specific strategy employed. While such methods have been explored extensively in experimental settings, they have not been used clinically. This review covers the airway surface preparation strategies reported in the literature, the advantages and disadvantages of each method, as well as a discussion about applying this concept in the clinic.

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.

Isoform specific editing of the coxsackievirus and adenovirus receptor

The Coxsackievirus and adenovirus receptor (CAR) is both a viral receptor and cell adhesion protein. CAR has two transmembrane isoforms that localize distinctly in polarized epithelial cells. Whereas the seven exon-encoded isoform (CAREx7) exhibits basolateral localization, the eight exon-encoded isoform (CAREx8) can localize to the apical epithelial surface where it can mediate luminal adenovirus infection. To further understand the distinct biological functions of these two isoforms, CRISPR/Cas9 genomic editing was used to specifically delete the eighth exon of the CXADR gene in a Madine Darby Canine Kidney (MDCK) cell line with a stably integrated lentiviral doxycycline-inducible CAREx8 cDNA. The gene-edited clone demonstrated a significant reduction in adenovirus susceptibility when both partially and fully polarized, and doxycycline-induction of CAREx8 restored sensitivity to adenovirus. These data reinforce the importance of CAREx8 in apical adenovirus infection and provide a new model cell line to probe isoform specific biological functions of CAR.

Cystic Fibrosis Gene Therapy: Looking Back, Looking Forward

Cystic fibrosis (CF) is an autosomal recessive disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene that encodes a cAMP-regulated anion channel. Although CF is a multi-organ system disease, most people with CF die of progressive lung disease that begins early in childhood and is characterized by chronic bacterial infection and inflammation. Nearly 90% of people with CF have at least one copy of the ΔF508 mutation, but there are hundreds of CFTR mutations that result in a range of disease severities. A CFTR gene replacement approach would be efficacious regardless of the disease-causing mutation. After the discovery of the CFTR gene in 1989, the in vitro proof-of-concept for gene therapy for CF was quickly established in 1990. In 1993, the first of many gene therapy clinical trials attempted to rescue the CF defect in airway epithelia. Despite the initial enthusiasm, there is still no FDA-approved gene therapy for CF. Here we discuss the history of CF gene therapy, from the discovery of the CFTR gene to current state-of-the-art gene delivery vector designs. While implementation of CF gene therapy has proven more challenging than initially envisioned; thanks to continued innovation, it may yet become a reality.

Targeted Adenoviral Vectors I

Human adenovirus (Ad) has been used extensively to develop gene transfer vectors for vaccine and gene therapy applications. A major factor limiting the efficacy of the current generation of Ad vectors is their inability to accomplish specific gene delivery to the cells of interest. Transductional targeting strategies seek to redirect virus binding to the appropriate cellular receptor to increase infection efficiency in selected cell types to achieve therapeutic intervention. These efforts mainly focused on incorporating targeting ligands by means of chemical conjugation or genetic modification of Ad capsid proteins and using bispecific adapter molecules to mediate virus recognition of target cells. This review summarizes current progress in Ad tropism modification maneuvers that embody genetic capsid modification and adapter-based approaches that have encouraging implications for further development of advanced vectors suitable for clinical translation.

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.

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.

Plasmid DNA is internalized from the apical plasma membrane of the salivary gland epithelium in live animals

Non-viral-mediated gene delivery represents an alternative way to express the gene of interest without inducing immune responses or other adverse effects. Understanding the mechanisms by which plasmid DNAs are delivered to the proper target in vivo is a fundamental issue that needs to be addressed in order to design more effective strategies for gene therapy. As a model system, we have used the submandibular salivary glands in live rats and we have recently shown that reporter transgenes can be expressed in different cell populations of the glandular epithelium, depending on the modality of administration of plasmid DNA. Here, by using a combination of immunofluorescence and intravital microscopy, we have explored the relationship between the pattern of transgenes expression and the internalization of plasmid DNA. We found that plasmid DNA is internalized: (1) by all the cells in the salivary gland epithelium, when administered alone, (2) by large ducts, when mixed with empty adenoviral particles, and (3) by acinar cells upon stimulation of compensatory endocytosis. Moreover, we showed that plasmid DNA utilizes different routes of internalization, and evades both the lysosomal degradative pathway and the retrograde pathway towards the Golgi apparatus. This study clearly shows that in vivo approaches have the potential to address fundamental questions on the cellular mechanisms regulating gene delivery.

Accessibility of the coxsackievirus and adenovirus receptor and its importance in adenovirus gene transduction efficiency

Viruses are commonly investigated as vector systems for gene therapy. To be effective, virus-mediated gene-delivery systems require the presence of specific virus receptors to enter the target cell. One example is adenovirus and its primary receptor is the coxsackievirus and adenovirus receptor (CAR). Madin-Darby canine kidney (MDCK) cells have become a choice model system for studying CAR and adenovirus infection due to their ability to polarize rapidly into an epithelium with high transepithelial resistance. We show here that, whilst MDCK cells are resistant to adenovirus infection and hence appear functionally CAR-deficient, polarized MDCK cells express significant levels of CAR sequestered on the basolateral surface, where it is inaccessible for virus infection. Thus, although a cell type may be resistant to adenovirus infection, it is impossible to know whether it is due to a deficiency, as both CAR absence and inaccessibility are barriers to adenovirus-mediated gene transfer.

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.

Isoform-specific regulation and localization of the coxsackie and adenovirus receptor in human airway epithelia

Adenovirus is an important respiratory pathogen. Adenovirus fiber from most serotypes co-opts the Coxsackie-Adenovirus Receptor (CAR) to bind and enter cells. However, CAR is a cell adhesion molecule localized on the basolateral membrane of polarized epithelia. Separation from the lumen of the airways by tight junctions renders airway epithelia resistant to inhaled adenovirus infection. Although a role for CAR in viral spread and egress has been established, the mechanism of initial respiratory infection remains controversial. CAR exists in several protein isoforms including two transmembrane isoforms that differ only at the carboxy-terminus (CAR(Ex7) and CAR(Ex8)). We found low-level expression of the CAR(Ex8) isoform in well-differentiated human airway epithelia. Surprisingly, in contrast to CAR(Ex7), CAR(Ex8) localizes to the apical membrane of epithelia where it augments adenovirus infection. Interestingly, despite sharing a similar class of PDZ-binding domain with CAR(Ex7), CAR(Ex8) differentially interacts with PICK1, PSD-95, and MAGI-1b. MAGI-1b appears to stoichiometrically regulate the degradation of CAR(Ex8) providing a potential mechanism for the apical localization of CAR(Ex8) in airway epithelial. In summary, apical localization of CAR(Ex8) may be responsible for initiation of respiratory adenoviral infections and this localization appears to be regulated by interactions with PDZ-domain containing proteins.

Targeting of adenovirus serotype 5 pseudotyped with short fiber from serotype 41 to c-erbB2-positive cells using bispecific single-chain diabody

The purpose of the current study was to alter the broad native tropism of human adenovirus for virus targeting to c-erbB2-positive cancer cells. First, we engineered a single-chain antibody (scFv) against the c-erbB2 oncoprotein into minor capsid protein IX (pIX) of adenovirus serotype 5 (Ad5) in a manner commensurate with virion integrity and binding to the soluble extracellular c-erbB2 domain. To ablate native viral tropism and facilitate binding of the pIX-incorporated scFv to cellular c-erbB2, we replaced the Ad5 fiber with the Ad41 short (41s) fiber devoid of all known cell-binding determinants. The resultant Ad5F41sIX6.5 vector demonstrated increased cell binding and gene transfer as compared to the Ad5F41s control; however, this augmentation of virus infectivity was not c-erbB2 specific. Incorporation of a six-histidine (His(6)) peptide into the C-terminus of the 41s fiber protein resulted in markedly increased Ad5F41s6H infectivity in 293AR cells, which express a membrane-anchored scFv against the C-terminal oligohistidine tag, as compared to the Ad5F41s vector and the parental 293 cells. These data suggested that a 41s-fiber-incorporated His(6) tag could serve for attachment of an adapter protein designed to guide Ad5F41s6H infection in a c-erbB2-specific manner. We therefore engineered a bispecific scFv diabody (scDb) combining affinities for both c-erbB2 and the His(6) tag and showed its ability to provide up to 25-fold increase of Ad5F41s6H infectivity in c-erbB2-positive cells. Thus, Ad5 fiber replacement by a His(6)-tagged 41s fiber coupled with virus targeting mediated by an scDb adapter represents a promising strategy to confer Ad5 vector tropism for c-erbB2-positive cancer cells.

Hepatitis C virus entry and neutralization

The processes of hepatitis C virus (HCV) entry and antibody-mediated neutralization are intimately linked. The high frequency of neutralizing antibodies (nAbs) that inhibit E2-CD81 interaction(s) suggests that this is a major target for the humoral immune response. The observation that HCV can transmit to naive cells by means of CD81-dependent and -independent routes in vitro awaits further investigation to assess the significance in vivo but may offer new strategies for HCV to escape nAbs. The identification of claudins in the entry process highlights the importance of cell polarity in defining routes of HCV entry and release, with recent experiments suggesting a polarized route of viral entry into cells in vitro. In this review, the authors summarize the current understanding of the mechanism(s) defining HCV entry and the role of nAbs in controlling HCV replication.

Adenovirus serotype 5 hexon is critical for virus infection of hepatocytes in vivo

Human species C adenovirus serotype 5 (Ad5) is the most common viral vector used in clinical studies worldwide. Ad5 vectors infect liver cells in vivo with high efficiency via a poorly defined mechanism, which involves virus binding to vitamin K-dependent blood coagulation factors. Here, we report that the major Ad5 capsid protein, hexon, binds human coagulation factor X (FX) with an affinity of 229 pM. This affinity is 40-fold stronger than the reported affinity of Ad5 fiber for the cellular receptor coxsackievirus and adenovirus receptor, CAR. Cryoelectron microscopy and single-particle image reconstruction revealed that the FX attachment site is localized to the central depression at the top of the hexon trimer. Hexon-mutated virus bearing a large insertion in hexon showed markedly reduced FX binding in vitro and failed to deliver a transgene to hepatocytes in vivo. This study describes the mechanism of FX binding to Ad5 and demonstrates the critical role of hexon for virus infection of hepatocytes in vivo.

Human CAR gene expression in nonpermissive hamster cells boosts entry of type 12 adenovirions and nuclear import of viral DNA

Adenovirus type 12 (Ad12) propagation in hamster BHK21 cells is blocked prior to viral DNA replication. The amounts of Ad12 DNA in the nuclei or cytoplasm of hamster cells are about 2 orders of magnitude (2 h postinfection [p.i.]) and 4 to 5 orders of magnitude (48 h p.i.) lower than in permissive human cells. Cell line BHK21-hCAR is transgenic for and expresses the human coxsackie- and adenovirus receptor (hCAR) gene. Nuclear uptake of Ad12 DNA in BHK21-hCAR cells is markedly increased compared to that in naïve BHK21 cells. Ad12 elicits a cytopathic effect in BHK21-hCAR cells but not in BHK21 cells. Quantitative PCR or [(3)H]thymidine labeling followed by zone velocity sedimentation fails to detect Ad12 DNA replication in BHK21 or BHK21-hCAR cells. Newly assembled Ad12 virions cannot be detected. Thus, the block in Ad12 DNA replication in hamster cells is not released by enhanced nuclear import of Ad12 DNA.

Effect of cell polarization on hepatitis C virus entry

The primary reservoir for hepatitis C virus (HCV) replication in vivo is believed to be hepatocytes within the liver. Three host cell molecules have been reported to be important entry factors for receptors for HCV: the tetraspanin CD81, scavenger receptor BI (SR-BI), and the tight-junction (TJ) protein claudin 1 (CLDN1). The recent discovery of a TJ protein as a critical coreceptor highlighted the importance of studying the effect(s) of TJ formation and cell polarization on HCV entry. The colorectal adenocarcinoma Caco-2 cell line forms polarized monolayers containing functional TJs and was found to express the CD81, SR-BI, and CLDN1 proteins. Viral receptor expression levels increased upon polarization, and CLDN1 relocalized from the apical pole of the lateral cell membrane to the lateral cell-cell junction and basolateral domains. In contrast, expression and localization of the TJ proteins ZO-1 and occludin 1 were unchanged upon polarization. HCV infected polarized and nonpolarized Caco-2 cells to comparable levels, and entry was neutralized by anti-E2 monoclonal antibodies, demonstrating glycoprotein-dependent entry. HCV pseudoparticle infection and recombinant HCV E1E2 glycoprotein interaction with polarized Caco-2 cells occurred predominantly at the apical surface. Disruption of TJs significantly increased HCV entry. These data support a model where TJs provide a physical barrier for viral access to receptors expressed on lateral and basolateral cellular domains.

Coxsackievirus and adenovirus receptor expression in non-malignant lung tissues and clinical lung cancers

Adenoviral vector mediated gene delivery has been applied in clinical trials and mechanistic studies to explore new treatment approaches for lung cancers. The expression of coxsackievirus adenovirus receptor (CAR), the primary receptor for the most commonly used adenovirus serotype 5 (Ad5)-based vectors, predominantly determines the permissiveness of lung cancer cells. CAR expression is also suggested to modulate tumor cell proliferation capacity. Here, we studied CAR expression in archival lung cancer specimens by using well-characterized CAR 72 antibodies. High levels of CAR expression were observed in most of the 32 cases of squamous cell carcinoma lung cancers and in all the five cases of small cell lung cancers investigated. In contrast, high levels of CAR expression were detected only in 6 of 22 adenocarcinoma lung cancers. The relative levels of CAR expression did not correlate with the pathologic grade in lung cancers, and was thus inconsistent with a role of modulating cancer cell proliferation. Of note, CAR expression was not detected in non-malignant alveolar cells. Our data suggest a preferred utility of Ad5 vector mediated gene delivery to squamous cell carcinoma lung cancers, small cell lung cancers, but not to the majority of adenocarcinoma lung cancers.

The coxsackie- and adenovirus receptor (CAR) is an in vivo marker for epithelial tight junctions, with a potential role in regulating permeability and tissue homeostasis

The coxsackie- and adenovirus receptor (CAR) is a transmembrane protein belonging to the immunoglobulin superfamily. The function of CAR as a virus receptor has been extensively analyzed, while its physiological role and expression pattern in adult tissues have remained less clear. CAR associates with epithelial tight junctions in vitro and mediates cell-cell adhesion. Using a set of affinity-purified antibodies, we show that CAR is predominantly expressed in epithelial cells lining the body cavities in adult mice, where it specifically co-localizes with the tight junction components ZO-1 and occludin. Notably, CAR could not be detected in endothelial cells of the vasculature, including brain capillaries. CAR expression correlated positively with the maturity of tight junctions and inversely with permeability. With a few exceptions, the two known CAR isoforms were co-expressed in most epithelial cells analyzed. A CAR mutant lacking the intracellular tail over-expressed in transgenic mice was diffusely localized over the plasma membrane, showing the importance of this domain for correct subcellular localization in vivo. We conclude that CAR is localized to epithelial tight junctions in vivo where it may play a role in the regulation of epithelial permeability and tissue homeostasis.

Adenovirus-mediated erythropoietin production by airway epithelia is enhanced by apical localization of the coxsackie-adenovirus receptor in vivo

In well-differentiated human airway epithelia, the coxsackie B and adenovirus types 2 and 5 receptor (CAR) resides on the basolateral membrane. Replacing the transmembrane and cytoplasmic tail of CAR with a glycosyl-phosphatidylinositol anchor (GPI-CAR) allows apical localization of GPI-CAR, where it can bind adenovirus and enhance gene transfer in vitro. To test this hypothesis further and to investigate requirements and barriers we developed an in vivo model that quantitatively assesses gene transfer of erythropoietin (EPO) to mouse airway epithelia. Our data suggest that erythropoietin is secreted basolaterally, allowing possible access to the bloodstream. The data also suggest that basolateral adenovirus-mediated airway epithelia EPO secretion persists for long periods and could be used to study persistence in vivo. Additionally, the increase in hematocrit in response to the increased serum EPO could be used for therapeutic purposes. Finally, we tested the ability of apically localized CAR to enhance the infection of AdEPO in mouse airway epithelia in vivo. The data suggest that apical receptors in airway epithelia may be sufficient to improve adenovirus infection of airway epithelia in vivo.

Gene Transfer to Respiratory Epithelia with Lentivirus Pseudotyped with Jaagsiekte Sheep Retrovirus Envelope Glycoprotein

A feline immunodeficiency virus (FIV)-based lentiviral vector was pseudotyped to identify envelope (env) glycoproteins that direct efficient gene transfer to pulmonary epithelia for the treatment or prevention of lung diseases. The envelope glycoprotein from the Jaagsiekte sheep retrovirus (JSRV) is a candidate under investigation. We utilized high titer FIV vector (>10(8) TU/ml) pseudotyped with the JSRV env glycoprotein (JSRVFIV) to study the transduction of polarized primary cultures of human airway epithelia and receptor/vector interactions. The reported receptor for JSRV, hyaluronidase 2 (HYAL2), is a GPI-linked protein. We expressed FLAG-tagged HYAL2 in polarized airway epithelia using an adenoviral vector and documented that the HYAL2 protein sorts predominantly to the apical surface. Of interest, the efficiency of gene transfer with apically applied JSRV-FIV was markedly less than FIV pseudotyped with VSV-G, even in Ad-HYAL2 complemented epithelia. The inefficient gene transfer with JSRV-FIV in HYAL2 complemented cells suggests that factors other than receptor abundance limit apical gene transfer efficiency with this envelope. JSRV-FIV transduced the distal lung epithelia of rabbits in vivo and transduced primary cultures of rabbit type II cells with 100-fold greater efficiency than primary cultures of rabbit tracheal cells. These data indicate that a lentivirus pseudotyped with the JSRV envelope glycoprotein transduces type II cells with greater efficiency than conducting airway epithelia and provides an example of glycoprotein-mediated cell-specific tropism within a tissue with a widely heterogeneous cell population.

The role of the extracellular domain in the biology of the coxsackievirus and adenovirus receptor

The Coxsackievirus B and Adenovirus Receptor (CAR) plays a dual role as a homotypic junctional adhesion protein and as a viral receptor. CAR is a transmembrane protein and a member of the Immunoglobulin (Ig) superfamily with two extracellular Ig-like domains. The most distal Ig-like domain (D1) mediates the homophilic interaction and is also responsible for the high-affinity binding of the adenovirus (Ad) fiber protein. Currently, no activity has been ascribed to the proximal Ig-like domain (D2). To further understand the function of the extracellular domain in the biological activities of CAR, we created extracellular deletion mutants and evaluated cellular localization, adhesion, and viral infection. Deletion of any segment of the extracellular domain results in loss of adhesion and mislocalization as explained by a model, termed "diffusion trapping," that suggests adhesion is the driving force in junctional localization. Loss of junctional localization and adhesion was particularly apparent in polarized human airway epithelia, where mutant CAR expression was basolateral but not limited to the lateral junctions between cells. Surprisingly, the D2 domain was required for adenovirus fiber-knob binding and infection. In summary, the entire extracellular domain of CAR is of vital importance to the biology of this highly conserved and important protein.

Glycocalyx restricts adenoviral vector access to apical receptors expressed on respiratory epithelium in vitro and in vivo: role for tethered mucins as barriers to lumenal infection

Inefficient adenoviral vector (AdV)-mediated gene transfer to the ciliated respiratory epithelium has hindered gene transfer strategies for the treatment of cystic fibrosis lung disease. In part, the inefficiency is due to an absence of the coxsackie B and adenovirus type 2 and 5 receptor (CAR) from the apical membranes of polarized epithelia. In this study, using an in vitro model of human ciliated airway epithelium, we show that providing a glycosylphosphatidylinositol (GPI)-linked AdV receptor (GPI-CAR) at the apical surface did not significantly improve AdV gene transfer efficiency because the lumenal surface glycocalyx limited the access of AdV to apical GPI-CAR. The highly glycosylated tethered mucins were considered to be significant glycocalyx components that restricted AdV access because proteolytic digestion and inhibitors of O-linked glycosylation enhanced AdV gene transfer. To determine whether these in vitro observations are relevant to the in vivo situation, we generated transgenic mice expressing GPI-CAR at the surface of the airway epithelium, crossbred these mice with mice that were genetically devoid of tethered mucin type 1 (Muc1), and tested the efficiency of gene transfer to murine airways expressing apical GPI-human CAR (GPI-hCAR) in the presence and absence of Muc1. We determined that AdV gene transfer to the murine airway epithelium was inefficient even in GPI-hCAR transgenic mice but that the gene transfer efficiency improved in the absence of Muc1. However, the inability to achieve a high gene transfer efficiency, even in mice with a deletion of Muc1, suggested that other glycocalyx components, possibly other tethered mucin types, also provide a significant barrier to AdV interacting with the airway lumenal surface.

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

The coxsackie and adenovirus receptor (CAR) plays a role in viral infection, maintenance of the junction adhesion complex in polarized epithelia, and modulation of cellular growth properties. As a viral receptor, the C-terminus appears to play no role indicating that the major function of CAR is to tether the virus to the cell. By contrast, the C-terminus is known to play a role in cellular localization and probably has a significant function in CAR-mediated adhesion and cell growth properties. We hypothesized that the CAR PDZ (PSD-95/Disc-large/ZO-1) binding motif interacts with PDZ-domain-containing proteins to modulate the cellular phenotype. CAR was modified by deleting the last four amino acids (CARΔGSIV) and evaluated for cell-cell adhesion in polarized primary human airway epithelia and growth characteristics in stably transfected L-cells. Although ablation of the CAR PDZ-binding motif did not affect adenoviral infection, it did have a significant effect both on cell-cell adhesion and on cell growth. Expression of CARΔGSIV failed to increase the transepithelial resistance in polarized epithelia to the same degree as wild-type CAR and failed to act as a growth modulator in L-cells. Furthermore, we provide evidence for three new CAR interacting partners, including MAGI-1b, PICK1 and PSD-95. CAR appears to interact with several distinct PDZ-domain-containing proteins and may exert its biological function through these interactions.

Digitoxin mimics gene therapy with CFTR and suppresses hypersecretion of IL-8 from cystic fibrosis lung epithelial cells

Cystic fibrosis (CF) is a fatal, autosomal, recessive genetic disease that is characterized by profound lung inflammation. The inflammatory process is believed to be caused by massive overproduction of the proinflammatory protein IL-8, and the high levels of IL-8 in the CF lung are therefore believed to be the central mechanism behind CF lung pathophysiology. We show here that digitoxin, at sub nM concentrations, can suppress hypersecretion of IL-8 from cultured CF lung epithelial cells. Certain other cardiac glycosides are also active but with much less potency. The specific mechanism of digitoxin action is to block phosphorylation of the inhibitor of NF-kappa B (I kappa B alpha). I kappa B alpha phosphorylation is a required step in the activation of the NF-kappa B signaling pathway and the subsequent expression of IL-8. Digitoxin also has effects on global gene expression in CF cells. Of the informative genes expressed by the CF epithelial cell line IB-3, 58 are significantly (P < 0.05) affected by gene therapy with wild-type (CFTR CF transmembrane conductance regulator). Of these 58 genes, 36 (62%) are similarly affected by digitoxin and related active analogues. We interpret this result to suggest that digitoxin can also partially mimic the genomic consequences of gene therapy with CF transmembrane conductance regulator. We therefore suggest that digitoxin, with its lengthy history of human use, deserves consideration as a candidate drug for suppressing IL-8-dependent lung inflammation in CF.

Jaagsiekte sheep retrovirus envelope efficiently pseudotypes human immunodeficiency virus type 1-based lentiviral vectors

Jaagsiekte sheep retrovirus (JSRV) infects lung epithelial cells in sheep, and oncoretroviral vectors bearing JSRV Env can mediate transduction of human cells, suggesting that such vectors might be useful for lung-directed gene therapy. Here we show that JSRV Env can also efficiently pseudotype a human immunodeficiency virus type 1-based lentiviral vector, a more suitable vector for transduction of slowly dividing lung epithelial cells. We created several chimeric Env proteins that, unlike the parental Env, do not transform rodent fibroblasts but are still capable of pseudotyping lentiviral and oncoretroviral vectors.

The Coxsackie-Adenovirus Receptor—A New Receptor in the Immunoglobulin Family Involved in Cell Adhesion

The physiological and cell biological aspects of the Coxsackie-Adenovirus Receptor (CAR) is discussed in this review. The receptor obviously recognizes the group C adenoviruses in vivo, but also fibers from other groups except group B in vitro. The latter viruses seem to utilize a different receptor. The receptor accumulates at, or close to, the tight junction in polarized epithelial cells and probably functions as a cell-cell adhesion molecule. The cytoplasmic tail of the receptor is not required for virus attachment and uptake. Although there is a correlation between CAR and uptake of adenoviruses in several human tumor cells, evidence of an absolute requirement for integrins has not been forthcoming. The implication of these findings for adenovirus gene therapy is discussed.

Virus interactions with mucosal surfaces: alternative receptors, alternative pathways

Viruses attach to specific receptors, but identified receptors are often absent from the epithelial surfaces where infections begin. Viruses can bind to alternative receptor molecules present on epithelial surfaces and they can enter hosts in ways that bypass mucosal barriers to infection. To understand how viruses cross the mucosa to initiate infection we need new information about the mechanisms of attachment and entry into cells, but we also need in vivo studies to define precisely where infection occurs.

Histamine alters E-cadherin cell adhesion to increase human airway epithelial permeability

During the immediate response to an inhaled allergen, there is an increase in the paracellular permeability of the airway epithelium.1 Histamine is an important agonist released during the immediate response to inhaled allergen. We hypothesized that histamine would increase human airway epithelial paracellular permeability and that it would do this by interrupting E-cadherin-based cell adhesion. Histamine, applied to the basolateral surface, increased the paracellular permeability of cultured human airway epithelia, and this effect of histamine was blocked by the histamine receptor antagonist promethazine. ECV304 cells express a histamine receptor, N-cadherin, and elements of the tight junction, including claudins, but they do not express E-cadherin. Histamine increased the paracellular permeability of ECV304 cells transfected with a vector and expressing E-cadherin but not ECV304 cells expressing lac-Z in the same vector. L cells do not express the histamine receptor, cadherins, or claudins. Histamine decreased adhesion of L cells expressing the human histamine receptor and E-cadherin to an E-cadherin-Fc fusion protein. Histamine did not alter the adhesion to the E-cadherin fusion protein of L cells expressing either the histamine receptor or E-cadherin alone. When applied to the apical surface, adenovirus poorly infects airway epithelial cells because its receptor, CAR, is restricted to the basolateral surface of the cells. When histamine was applied to the basolateral surface of airway epithelial cells, infection of the cells by adenovirus increased by approximately one log. This effect of histamine was also blocked by promethazine. Histamine increases airway paracellular permeability and increases susceptibility of airway epithelial cells to infection by adenovirus by interrupting E-cadherin adhesion.

Adenovirus endocytosis

Pathogen entry into cells occurs by direct penetration of the plasma membrane, clathrin-mediated endocytosis, caveolar endocytosis, pinocytosis or macropinocytosis. For a particular agent, the infectious pathways are typically restricted, reflecting a tight relationship with the host. Here, we survey the uptake process of human adenovirus (Ad) type 2 and 5 and integrate it into the cell biology of endocytosis. Ad2 and Ad5 naturally infect respiratory epithelial cells. They bind to a primary receptor, the coxsackie virus B Ad receptor (CAR). The CAR-docked particles activate integrin coreceptors and this triggers a variety of cell responses, including endocytosis. Ad2/Ad5 endocytosis is clathrin-mediated and involves the large GTPase dynamin and the adaptor protein 2. A second endocytic process is induced simultaneously with viral uptake, macropinocytosis. Together, these pathways are associated with viral infection. Macropinocytosis requires integrins, F-actin, protein kinase C and small G-proteins of the Rho family, but not dynamin. Macropinocytosis per se is not required for viral uptake into epithelial cells, but it appears to be a productive entry pathway of Ad artificially targeted to the high-affinity Fcgamma receptor CD64 of hematopoietic cells lacking CAR. In epithelial and hematopoietic cells, the macropinosomal contents are released to the cytosol. This requires viral signalling from the surface and coincides with particle escape from endosomes and infection. It emerges that incoming Ad2 and Ad5 distinctly modulate the endocytic trafficking and disrupt selective cellular compartments. These features can be exploited for effective artificial targeting of Ad vectors to cell types of interest.

Laser fluorescence bronchoscopy for detection of fluorescent reporter genes in airway epithelia

Current methods for detecting successful gene transfer to airway epithelia involve obtaining a sample of the target tissue. This may affect the longevity of expression of the transgene under evaluation. We describe a laser fluorescence bronchoscopic system that can detect the expression of the fluorescent protein, green fluorescence protein (GFP), in the airway of monkeys that have been transfected with adenovirus, without the need for obtaining tissue. This technique will have applications in pre-clinical and clinical studies of gene transfer to airway epithelia and other surface epithelia accessible by endoscopy.

Inflammatory response in airway epithelial cells isolated from patients with cystic fibrosis

The concept that inflammatory gene expression is dysregulated in airway epithelial cells from patients with cystic fibrosis (CF) is controversial. To examine this possibility systematically, responses to inflammatory stimuli were compared in CF airway epithelial cell lines without versus with wild-type CF transmembrane conductance regulator (CFTR) complementation and in tracheobronchial epithelial cells from patients with versus without CF. Epithelial cell expression of the leukocyte adhesion glycoprotein intercellular adhesion molecule-1 (ICAM-1) and release of the neutrophil chemoattractant interleukin (IL)-8 were determined under basal conditions or after exposure to stimuli important in CF airway inflammatory responses. We found that uncorrected CF airway epithelial cell lines inconsistently expressed higher ICAM-1 and IL-8 levels. Human CF tracheobronchial epithelial cells in primary culture released moderately increased IL-8 only after exposure to Pseudomonas aeruginosa. In CF cells with higher IL-8 release, transient expression of wild-type CFTR using an adenoviral vector did not specifically affect cytokine levels. The results indicate that there is considerable variability in airway epithelial cell responses to inflammatory stimuli among different individuals and cell models systems. Although increased ICAM-1 and IL-8 expression are observed in some CF airway epithelial cell models, many CF cells do not exhibit significant dysregulation of these important inflammatory genes.

Bronchoalveolar fluid is not a major hindrance to virus-mediated gene therapy in cystic fibrosis

Successfully targeting the airway epithelium is essential for gene therapy of some pulmonary diseases. However, the airway epithelium is resistant to virus-mediated gene transfer with commonly used vectors. Vectors that interact with endogenously expressed receptors on the apical surface significantly increase gene transfer efficiency. However, other endogenous components involved in host immunity may hinder virus-mediated gene transfer. We tested the effect of bronchoalveolar lavage liquid (BAL) from patients with cystic fibrosis (CF), BAL from subjects without CF (non-CF BAL), Pseudomonas aeruginosa-derived proteins, and an array of inflammatory proteins on gene transfer mediated by adeno-associated virus type 5 (AAV5) and adenovirus targeted to an apically expressed glycosylphosphatidylinositol-modified coxsackie-adenovirus receptor. We found that neither CF BAL nor its components had a significant effect on gene transfer to human airway epithelium by these vectors. Non-CF BAL significantly impaired adenovirus-mediated gene transfer. Removal of immunoglobulins in non-CF BAL restored gene transfer efficiency. As virus vectors are improved and mechanisms of humoral immunity are elucidated, barriers to successful gene therapy found in the complex environment of the human lung can be circumvented.

Interaction with decay-accelerating factor facilitates coxsackievirus B infection of polarized epithelial cells

All coxsackie B (CB) viruses can initiate infection by attaching to the coxsackievirus and adenovirus receptor (CAR). Although some CB isolates also bind to decay-accelerating factor (DAF), the role of DAF interaction during infection remains uncertain. We recently observed that CAR in polarized epithelial cells is concentrated at tight junctions, where it is relatively inaccessible to virus. In the experiments reported here we found that, unlike CAR, DAF was present on the apical surface of polarized cells and that DAF-binding isolates of CB3 and CB5 infected polarized epithelial cells more efficiently than did isolates incapable of attaching to DAF. Virus attachment and subsequent infection of polarized cells by DAF-binding isolates were prevented in the presence of anti-DAF antibody. Serial passage on polarized cell monolayers selected for DAF-binding virus variants. Taken together, these results indicate that interaction with DAF on the apical surface of polarized epithelial cells facilitates infection by a subset of CB virus isolates. The results suggest a possible role for DAF in infection of epithelial cells at mucosal surfaces.

Fatty acid modification of the coxsackievirus and adenovirus receptor

Membrane-proximal cysteines 259 and 260 in the cytoplasmic tail of the coxsackievirus and adenovirus receptor (CAR) are known to be essential for the tumor suppression activity of CAR. We demonstrate that these residues provide an S-acylation motif for modification of CAR with the fatty acid palmitate. Substitution of alanine for cysteines 259 and 260 results in the additional localization of CAR in perinuclear compartments with no effect on the efficiency of adenovirus infection. The results indicate that palmitylation is important for stable plasma membrane expression and biological activity of CAR but is not critical for adenovirus receptor performance.

Apical barriers to airway epithelial cell gene transfer with amphotropic retroviral vectors

Gene transfer to airway epithelia with amphotropic pseudotyped retroviral vectors is inefficient following apical vector application. To better understand this inefficiency, we localized the expression of Pit2, the amphotropic receptor, in polarized human airway epithelia. Pit2 was expressed on both the apical and basolateral surfaces of the cells, suggesting that factors other than receptor abundance may limit apical gene transfer efficiency. Binding studies performed with radiolabeled amphotropic MuLV suggested that the apically applied virus binds to Pit2. Hypothetical barriers to retroviral gene transfer include the apical glycocalyx and other secreted products of epithelia. In this study, we demonstrated that sialic acid, keratan sulfate and collagen type V are present on the apical surface of well-differentiated human airway epithelia. While enzyme treatment reduced the abundance of these components, the treatment also decreased the transepithelial resistance to approximately 35% of the controls, suggesting that the epithelial integrity was impaired. To attain an airway epithelial culture with a modified apical surface and intact epithelial integrity, we utilized 100 mM 2-deoxy-D-glucose, a glycosylation inhibitor, to prevent the glycocalyx from reforming following enzyme treatment. This approach allowed the resistance, but not the apical glycocalyx to recover. Despite this physical modification of the cell surface, the amphotropic retroviral vector failed to transduce airway epithelia following apical application. These results suggest that factors other than apical receptor abundance and the glycocalyx inhibit amphotropic retroviral gene transfer in human airway epithelia.