Coxsackievirus and adenovirus receptor amino-terminal immunoglobulin V-related domain binds adenovirus type 2 and fiber knob from adenovirus type 12

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.

Adenoviruses in medicine: innocuous pathogen, predator, or partner

The consequences of human adenovirus (HAdV) infections are generally mild. However, despite the perception that HAdVs are harmless, infections can cause severe disease in certain individuals, including newborns, the immunocompromised, and those with pre-existing conditions, including respiratory or cardiac disease. In addition, HAdV outbreaks remain relatively common events and the recent emergence of more pathogenic genomic variants of various genotypes has been well documented. Coupled with evidence of zoonotic transmission, interspecies recombination, and the lack of approved AdV antivirals or widely available vaccines, HAdVs remain a threat to public health. At the same time, the detailed understanding of AdV biology garnered over nearly 7 decades of study has made this group of viruses a molecular workhorse for vaccine and gene therapy applications.

The IgCAM CAR Regulates Gap Junction-Mediated Coupling on Embryonic Cardiomyocytes and Affects Their Beating Frequency

The IgCAM coxsackie-adenovirus receptor (CAR) is essential for embryonic heart development and electrical conduction in the mature heart. However, it is not well-understood how CAR exerts these effects at the cellular level. To address this question, we analyzed the spontaneous beating of cultured embryonic hearts and cardiomyocytes from wild type and CAR knockout (KO) embryos. Surprisingly, in the absence of the CAR, cultured cardiomyocytes showed increased frequencies of beating and calcium cycling. Increased beatings of heart organ cultures were also induced by the application of reagents that bind to the extracellular region of the CAR, such as the adenovirus fiber knob. However, the calcium cycling machinery, including calcium extrusion via SERCA2 and NCX, was not disrupted in CAR KO cells. In contrast, CAR KO cardiomyocytes displayed size increases but decreased in the total numbers of membrane-localized Cx43 clusters. This was accompanied by improved cell-cell coupling between CAR KO cells, as demonstrated by increased intercellular dye diffusion. Our data indicate that the CAR may modulate the localization and oligomerization of Cx43 at the plasma membrane, which could in turn influence electrical propagation between cardiomyocytes via gap junctions.

Adenovirus entry: Stability, uncoating, and nuclear import

Adenoviruses (AdVs) are widespread in vertebrates. They infect the respiratory and gastrointestinal tracts, the eyes, heart, liver, and kidney, and are lethal to immunosuppressed people. Mastadenoviruses infecting mammals comprise several hundred different types, and many specifically infect humans. Human adenoviruses are the most widely used vectors in clinical applications, including cancer treatment and COVID-19 vaccination. AdV vectors are physically and genetically stable and generally safe in humans. The particles have an icosahedral coat and a nucleoprotein core with a DNA genome. We describe the concept of AdV cell entry and highlight recent advances in cytoplasmic transport, uncoating, and nuclear import of the viral DNA. We highlight a recently discovered "linchpin" function of the virion protein V ensuring cytoplasmic particle stability, which is relaxed at the nuclear pore complex by cues from the E3 ubiquitin ligase Mind bomb 1 (MIB1) and the proteasome triggering disruption. Capsid disruption by kinesin motor proteins and microtubules exposes the linchpin and renders protein V a target for MIB1 ubiquitination, which dissociates V from viral DNA and enhances DNA nuclear import. These advances uncover mechanisms controlling capsid stability and premature uncoating and provide insight into nuclear transport of nucleic acids.

Human Coxsackie- and adenovirus receptor is a putative target of neutrophil elastase-mediated shedding

Background

During viral-induced myocarditis, immune cells migrate towards the site of infection and secrete proteases, which in turn can act as sheddases by cleaving extracellular domains of transmembrane proteins. We were interested in the shedding of the Coxsackie- and adenovirus receptor (CAR) that acts as an entry receptor for both eponymous viruses, which cause myocarditis. CAR shedding by secreted immune proteases could result in a favourable outcome of myocarditis as CAR’s extracellular domain would be removed from the cardiomyocytes’ surface leading to decreased susceptibility to ongoing viral infections.

Methods and results

In this work, matrix metalloproteinases and serine proteinases were screened for their proteolytic activity towards human CAR. Whereas matrix metalloproteinases, proteinase 3, and cathepsin G did not cleave human recombinant CAR or only within long incubation times, neutrophil elastase showed a distinct cleavage pattern of CAR’s extracellular domain that was time- and dose-dependent. Neutrophil elastase cleaves CAR at its membrane-proximal immunoglobulin domain as we determined by nanoLC-MS/MS. Furthermore, neutrophil elastase treatment of cells reduced CAR surface levels as seen by flow cytometry and immunofluorescence microscopy.

Conclusions

With this study, we show that CAR might be a target for shedding by neutrophil elastase.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11033-022-07153-2.

Coxsackievirus and Adenovirus Receptor (CXADR): Recent Findings and Its Role and Regulation in Spermatogenesis

Coxsackievirus and adenovirus receptor (CXADR) belongs to immunoglobulin superfamily of cell adhesion molecules. It expresses in most tissues, but displays unique and indispensable functions in some tissues such as heart and testis. CXADR is a multifunctional protein that can serve as a viral receptor, a junction structural protein and a signalling molecule. Thus, it exerts a wide range of functions such as facilitating leukocyte transmigration, regulating barrier function and cell adhesion, promoting EMT transition, and mediating spermatogenesis. This review aims to provide an overview and highlights some recent findings on CXADR in the field with emphasis on studies in the testis, upon which future studies can be designed to delineate the roles and regulation of CXADR in spermatogenesis.

MAGI-1 PDZ2 Domain Blockade Averts Adenovirus Infection via Enhanced Proteolysis of the Apical Coxsackievirus and Adenovirus Receptor

Adenoviruses (AdVs) are etiological agents of gastrointestinal, heart, eye, and respiratory tract infections that can be lethal for immunosuppressed people. Many AdVs use the coxsackievirus and adenovirus receptor (CAR) as a primary receptor. The CAR isoform resulting from alternative splicing that includes the eighth exon, CAR^Ex8, localizes to the apical surface of polarized epithelial cells and is responsible for the initiation of AdV infection. We have shown that the membrane level of CAR^Ex8 is tightly regulated by two MAGI-1 PDZ domains, PDZ2 and PDZ4, resulting in increased or decreased AdV transduction, respectively. We hypothesized that targeting the interactions between the MAGI-1 PDZ2 domain and CAR^Ex8 would decrease the apical CAR^Ex8 expression level and prevent AdV infection. Decoy peptides that target MAGI-1 PDZ2 were synthesized (TAT-E6 and TAT-NET1). PDZ2 binding peptides decreased CAR^Ex8 expression and reduced AdV transduction. CAR^Ex8 degradation was triggered by the activation of the regulated intramembrane proteolysis (RIP) pathway through a disintegrin and metalloproteinase (ADAM17) and γ-secretase. Further analysis revealed that ADAM17 interacts directly with the MAGI-1 PDZ3 domain, and blocking the PDZ2 domain enhanced the accessibility of ADAM17 to the substrate (CAR^Ex8). Finally, we validated the efficacy of TAT-PDZ2 peptides in protecting the epithelia from AdV transduction in vivo using a novel transgenic animal model. Our data suggest that TAT-PDZ2 binding peptides are novel anti-AdV molecules that act by enhanced RIP of CAR^Ex8 and decreased AdV entry. This strategy has additional translational potential for targeting other viral receptors that have PDZ binding domains, such as the angiotensin-converting enzyme 2 receptor. IMPORTANCE Adenovirus is a common threat in immunosuppressed populations and military recruits. There are no currently approved treatments/prophylactic agents that protect from most AdV infections. Here, we developed peptide-based small molecules that can suppress AdV infection of polarized epithelia by targeting the AdV receptor, coxsackievirus and adenovirus receptor (CAR^Ex8). The newly discovered peptides target a specific PDZ domain of the CAR^Ex8-interacting protein MAGI-1 and decrease AdV transduction in multiple polarized epithelial models. Peptide-induced CAR^Ex8 degradation is triggered by extracellular domain (ECD) shedding through ADAM17 followed by γ-secretase-mediated nuclear translocation of the C-terminal domain. The enhanced shedding of the CAR^Ex8 ECD further protected the epithelium from AdV infection. Taken together, these novel molecules protect the epithelium from AdV infection. This approach may be applicable to the development of novel antiviral molecules against other viruses that use a receptor with a PDZ binding domain.

Identification of chicken CAR homology as a cellular receptor for the emerging highly pathogenic fowl adenovirus 4 via unique binding mechanism

Since 2015, the prevalence of severe hepatitis-hydropericardium syndrome, which is caused by the novel genotype fowl adenovirus serotype 4 (FAdV-4), has increased in China and led to considerable economic losses. The replication cycle of FAdV-4, especially the emerging highly pathogenic novel genotype FAdV-4, remains largely unknown. The adenovirus fibre interacts with the cellular receptor as the initial step in adenovirus (AdV) infection. In our previous studies, the complete genome sequence showed that the fibre patterns of FAdV-4 were distinct from all other AdVs. Here, protein-blockage and antibody-neutralization assays were performed to confirm that the novel FAdV-4 short fibre was critical for binding to susceptible leghorn male hepatocellular (LMH) cells. Subsequently, fibre 1 was used as bait to investigate the receptor on LMH cells via mass spectrometry. The chicken coxsackie and adenovirus receptor (CAR) protein was confirmed as the novel FAdV-4 receptor in competition assays. We further identified the D2 domain of CAR (D2-CAR) as the active domain responsible for binding to the short fibre of the novel FAdV-4. Taken together, these findings demonstrate for the first time that the chicken CAR homolog is a cellular receptor for the novel FAdV-4, which facilitates viral entry by interacting with the viral short fibre through the D2 domain. Collectively, these findings provide an in-depth understanding of the mechanisms of the emerging novel genotype FAdV-4 invasion and pathogenesis.

Virus-Receptor Interactions: Structural Insights For Oncolytic Virus Development

Recent advancements in oncolytic virotherapy commend a special attention to developing new strategies for targeting cancer cells with oncolytic viruses (OVs). Modifications of the viral envelope or coat proteins serve as a logical mean of repurposing viruses for cancer treatment. In this review, we discuss how detailed structural knowledge of the interactions between OVs and their natural receptors provide valuable insights into tumor specificity of some viruses and re-targeting of alternate receptors for broad tumor tropism or improved tumor selectivity.

Glycomics and Proteomics Approaches to Investigate Early Adenovirus-Host Cell Interactions

Adenoviruses as most viruses rely on glycan and protein interactions to attach to and enter susceptible host cells. The Adenoviridae family comprises more than 80 human types and they differ in their attachment factor and receptor usage, which likely contributes to the diverse tropism of the different types. In the past years, methods to systematically identify glycan and protein interactions have advanced. In particular sensitivity, speed and coverage of mass spectrometric analyses allow for high-throughput identification of glycans and peptides separated by liquid chromatography. Also, developments in glycan microarray technologies have led to targeted, high-throughput screening and identification of glycan-based receptors. The mapping of cell surface interactions of the diverse adenovirus types has implications for cell, tissue, and species tropism as well as drug development. Here we review known adenovirus interactions with glycan- and protein-based receptors, as well as glycomics and proteomics strategies to identify yet elusive virus receptors and attachment factors. We finally discuss challenges, bottlenecks, and future research directions in the field of non-enveloped virus entry into host cells.

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Adenovirus entry into cells is guided by specific glycan and protein interactions.

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Glycan arrays and shotgun glycomics methods are valuable technologies to identify virus–glycan interactions.

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Shotgun proteomics and ligand-based receptor capture are powerful methods for proteinaceous receptor discovery.

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A combination of shotgun glycomics and proteomics with CRISPR/Cas9 and RNAi validation holds the promise of generating a systems biology view of virus entry processes.

Adeno-associated Virus (AAV) Serotypes Have Distinctive Interactions with Domains of the Cellular AAV Receptor

Adeno-associated virus (AAV) entry is determined by its interactions with specific surface glycans and a proteinaceous receptor(s). Adeno-associated virus receptor (AAVR) (also named KIAA0319L) is an essential cellular receptor required for the transduction of vectors derived from multiple AAV serotypes, including the evolutionarily distant serotypes AAV2 and AAV5. Here, we further biochemically characterize the AAV-AAVR interaction and define the domains within the ectodomain of AAVR that facilitate this interaction. By using a virus overlay assay, it was previously shown that the major AAV2 binding protein in membrane preparations of human cells corresponds to a glycoprotein with a molecular mass of 150 kDa. By establishing a purification procedure, performing further protein separation by two-dimensional electrophoresis, and utilizing mass spectrometry, we now show that this glycoprotein is identical to AAVR. While we find that AAVR is an N-linked glycosylated protein, this glycosylation is not a strict requirement for AAV2 binding or functional transduction. Using a combination of genetic complementation with deletion constructs and virus overlay assays with individual domains, we find that AAV2 functionally interacts predominantly with the second Ig-like polycystic kidney disease (PKD) repeat domain (PKD2) present in the ectodomain of AAVR. In contrast, AAV5 interacts primarily through the first, most membrane-distal, PKD domain (PKD1) of AAVR to promote transduction. Furthermore, other AAV serotypes, including AAV1 and -8, require a combination of PKD1 and PKD2 for optimal transduction. These results suggest that despite their shared dependence on AAVR as a critical entry receptor, different AAV serotypes have evolved distinctive interactions with the same receptor.

IMPORTANCE Over the past decade, AAV vectors have emerged as leading gene delivery tools for therapeutic applications and biomedical research. However, fundamental aspects of the AAV life cycle, including how AAV interacts with host cellular factors to facilitate infection, are only partly understood. In particular, AAV receptors contribute significantly to AAV vector transduction efficiency and tropism. The recently identified AAV receptor (AAVR) is a key host receptor for multiple serotypes, including the most studied serotype, AAV2. AAVR binds directly to AAV2 particles and is rate limiting for viral transduction. Defining the AAV-AAVR interface in more detail is important to understand how AAV engages with its cellular receptor and how the receptor facilitates the entry process. Here, we further define AAV-AAVR interactions, genetically and biochemically, and show that different AAV serotypes have discrete interactions with the Ig-like PKD domains of AAVR. These findings reveal an unexpected divergence of AAVR engagement within these parvoviruses.

The novel asymmetric entry intermediate of a picornavirus captured with nanodiscs

Many nonenveloped viruses engage host receptors that initiate capsid conformational changes necessary for genome release. Structural studies on the mechanisms of picornavirus entry have relied on in vitro approaches of virus incubated at high temperatures or with excess receptor molecules to trigger the entry intermediate or A-particle. We have induced the coxsackievirus B3 entry intermediate by triggering the virus with full-length receptors embedded in lipid bilayer nanodiscs. These asymmetrically formed A-particles were reconstructed using cryo-electron microscopy and a direct electron detector. These first high-resolution structures of a picornavirus entry intermediate captured at a membrane with and without imposing icosahedral symmetry (3.9 and 7.8 Å, respectively) revealed a novel A-particle that is markedly different from the classical A-particles. The asymmetric receptor binding triggers minimal global capsid expansion but marked local conformational changes at the site of receptor interaction. In addition, viral proteins extrude from the capsid only at the site of extensive protein remodeling adjacent to the nanodisc. Thus, the binding of the receptor triggers formation of a unique site in preparation for genome release.

Single-step concentration and purification of adenoviruses by coxsackievirus-adenovirus receptor-binding capture and elastin-like polypeptide-mediated precipitation

A single-step method for quick concentration and purification of adenoviruses (Ads) was established by combining coxsackievirus and adenovirus receptor (CAR)-binding capture with elastin-like polypeptide (ELP)-mediated precipitation. The soluble ELP-CAR fusion protein was expressed in vector-transformed E. coli and purified to high purity by two rounds of inverse transition cycling (ITC). After demonstration of the specific binding of fusion protein, a recombinant Ad (rAd), namely rAd/GFP, was pulled down from the culture medium and extract of rAd-transduced cells using ELP-CAR protein, with recovery of 76.2 % and 73.3 %, respectively. The rAd was eluted from the ELP-CAR protein and harvested by one round of ITC, with recoveries ranging from 30.6 % to 34.5 % (virus titration assay). Both ELP-CAR-bound and eluted rAds were able to transduce CAR-positive cells, but not CAR-negative cells (fluorescent microscopy). A further viral titration assay showed that the ELP-CAR-bound rAd/GFP had significantly lower transduction efficiency than the eluted rAd, and there was less of a decrease when tested in the presence of fetal bovine serum. In addition, rAd/GFP was efficiently recovered from the "spiked" PBS and tap water with recovery of ~74 % or ~60 %. This work demonstrates the usefulness of the ELP-CAR-binding capture method for concentration and/or purification of Ads in cellular and environmental samples.

1. Alternative splicing of viral receptors: A review of the diverse morphologies and physiologies of adenoviral receptors

Understanding the biology of cell surface proteins is important particularly when they are utilized as viral receptors for viral entry. By manipulating the expression of cell surface receptors that have been coopted by viruses, the susceptibility of an individual to virus-induced disease or, alternatively, the effectiveness of viral-based gene therapy can be modified. The most commonly studied vector for gene therapy is adenovirus. The majority of adenovirus types utilize the coxsackievirus and adenovirus receptor (CAR) as a primary receptor to enter cells. Species B adenovirus do not interact with CAR, but instead interact with the cell surface proteins desmoglein-2 (DSG-2) and cluster of differentiation 46 (CD46). These cell surface proteins exhibit varying degrees of alternative mRNA splicing, creating an estimated 20 distinct protein isoforms. It is likely that alternative splice forms have allowed these proteins to optimize their effectiveness in a plethora of niches, including roles as cell adhesion proteins and regulators of the innate immune system. Interestingly, there are soluble isoforms of these viral receptors, which lack the transmembrane domain. These soluble isoforms can potentially bind to the surface of a virus in the extracellular compartment, blocking the ability of the virus to bind to the host cell, reducing viral infectivity. Finally, the diversity of viral receptor isoforms appears to facilitate an assortment of interactions between viral receptor proteins and cytosolic proteins, leading to differential sorting in polarized cells. Using adenoviral receptors as a model system, the purpose of this review is to highlight the role that isoform-specific protein localization plays in the entry of pathogenic viruses from the apical surface of polarized epithelial cells.

Nonenveloped capsid: mechanisms of viral entry

ABSTRACT 

Viruses are a diverse class of nanoparticles. However, they have evolved a few common mechanisms that enable successful infection of their host cells. The first stage of this process involves entry into the cell. For enveloped viruses this process has been well characterized. For nonenveloped viruses, the focus of this review, the entry mechanisms are less well understood. For these viruses, a typical pathway involves receptor attachment followed by internalization into cellular vesicles and subsequent viral escape to the cytosol and transport to the site of genome replication. Significantly, these viruses have evolved numerous mechanisms to fulfill this seemingly simple infection scheme. We focus on the latest observations for several families of nonenveloped viruses and highlight specific members for eukaryotic families: Adenoviridae, Papillomaviridae, Parvoviridae, Picornaviridae, Polyomaviridae and Reoviridae; and prokaryotic families: Microviridae, Myoviridae, Podoviridae and Siphoviridae.

Adenovirus signalling in entry

Viruses carry nucleic acids between and within host cells. Invariably, virus attachment to host cells leads to activation of cell signalling. These so-called forward signals emerge from interactions with cell surface receptors or cytosolic proteins and elicit profound responses in the cells, for example induction of growth or innate immunity responses. They can enhance or suppress infection. In addition, viruses receive signals from the cell. These reverse signals can impact on the structure of the virus leading to genome uncoating. They can enhance infection or inactivate virus, for example by facilitating degradation. Here we discuss the nature and mechanisms by which forward and reverse signals emerge and affect the outcome of human adenovirus infections. We describe how human adenoviruses use cell surface receptors for forward signalling to activate cell growth, intracellular transport or innate immune response. We also discuss how adenoviruses use acto-myosin, integrins or microtubule-based kinesin motors for reverse signalling to facilitate their stepwise uncoating programme.

Altering adenoviral tropism via click modification with ErbB specific ligands

Methods for targeting oncolytic viruses can increase efficacy and accelerate development. Genetic engineering, the predominant method for changing vector tropism, is limited in scope and often represents the bottleneck for vector development. Metabolic incorporation of an unnatural azido sugar, O-GlcNAz, at a specific site on the adenoviral surface allows chemoselective attachment of affibodies for Her2 or EGF receptors. Modification with these high-affinity, high-selectivity proteins is straightforward and readily generalizable, demonstrates minimal impact on virus physiology, and affords significant increases in gene delivery to cancer cells. As a result, this method has significant potential to increase the efficacy of next-generation viral vectors.

Emerging roles for ubiquitin in adenovirus cell entry

Adenovirus relies on numerous interactions between viral and host cell proteins to efficiently enter cells. Undoubtedly, post-translational modifications of host and cellular proteins can impact the efficiency of this cell entry process. Ubiquitylation, once simply thought of as a modification targeting proteins for proteasomal degradation, is now known to regulate protein trafficking within cells, protein-protein interactions and cell signalling pathways. Accumulating evidence suggests that protein ubiquitylation can influence all stages of the life cycle of other viruses such as cell entry, replication and egress. Until recently, the influence of ubiquitylation has only been documented during adenovirus replication. This review highlights the most recent evidence demonstrating direct engagement of host ubiquitylation and SUMOylation machinery by adenovirus during cell entry. Additionally, potential roles for host protein ubiquitylation and the potential for adenovirus regulation of host ubiquitylation machinery during cell entry are discussed.

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.

Specificity switching in virus-receptor complexes

Several structures of complexes between viral attachment proteins and their cellular receptors have been determined recently, enhancing our understanding of the molecular recognition processes that guide formation of virus-receptor complexes. Moreover, these structures also highlight strategies by which highly similar viral proteins within a single virus family can adapt to engage different receptors. Consequences of such differences are altered tropism and pathogenicity. An improved understanding of the molecular details of this specificity switching in receptor binding will help to establish links between receptor tropism, spread, and disease. Moreover, it also has relevance for the design and use of viruses as gene delivery vehicles with altered properties as well as for the identification of target viral epitopes of new vaccines.

Adenovirus receptors and their implications in gene delivery

Adenoviruses (Ads) have gained popularity as gene delivery vectors for therapeutic and prophylactic applications. Ad entry into host cells involves specific interactions between cell surface receptors and viral capsid proteins. Several cell surface molecules have been identified as receptors for Ad attachment and entry. Tissue tropism of Ad vectors is greatly influenced by their receptor usage. A variety of strategies have been investigated to modify Ad vector tropism by manipulating the receptor-interacting moieties. Many such strategies are aimed at targeting and/or detargeting of Ad vectors. In this review, we discuss the various cell surface molecules that are implicated as receptors for virus attachment and internalization. Special emphasis is given to Ad types that are utilized as gene delivery vectors. Various strategies to modify Ad tropism using the knowledge of Ad receptors are also discussed.

Infectious adenovirus type 2 transport through early but not late endosomes

Receptor-mediated endocytosis is a major gate for pathogens into cells. In this study, we analyzed the trafficking of human adenovirus type 2 and 5 (Ad2/5) and the escape-defective temperature-sensitive Ad2-ts1 mutant in epithelial cancer cells. Ad2/5 and Ad2-ts1 uptake into endosomes containing transferrin, major histocompatibility antigen 1 and the Rab5 effector early endosome antigen 1 (EEA1) involved dynamin, amphiphysin, clathrin and Eps15. Cointernalization experiments showed that most of the Ad2/5 and Ad2-ts1 visited the same EEA1-positive endosomes. In contrast to Ad2/5, Ad2-ts1 required functional Rab5 for endocytosis and lysosomal transport and was sensitive to the phosphatidyl-inositol-3 (PI3)-kinase inhibitor wortmannin or the ubiquitin-binding protein Hrs for sorting from early to late endosomes. Endosomal escape of Ad2 was not affected by incubation at 19 degrees C, which blocked membrane sorting in early endosomes and inhibited Ad2-ts1 transport to lysosomes. Unlike Semliki Forest Virus (SFV), sorting of Ad2-ts1 to late endosomes was independent of Rab7 and Ad2/5 infection independent of EEA1. The data indicate that Ad2/5 and Ad2-ts1 use an invariant machinery for clathrin-mediated uptake to early endosomes. We suggest that the infectious Ad2 particles are either directly released from early endosomes to the cytosol or sorted by a temperature-insensitive and PI3-kinase-independent mechanism to an escape compartment different from late endosomes or lysosomes.

Identification of regions and residues in feline junctional adhesion molecule required for feline calicivirus binding and infection

The feline junctional adhesion molecule A (fJAM-A) is a functional receptor for feline calicivirus (FCV). fJAM-A is a member of the immunoglobulin superfamily (IgSF) and consists of two Ig-like extracellular domains (D1 and D2), a membrane-spanning domain, and a short cytoplasmic tail. To identify regions of fJAM-A that interact with FCV, we purified recombinant fJAM-A ectodomain and D1 and D2 domains. We found that preincubation of FCV with the ectodomain or D1 was sufficient to inhibit FCV infection in plaque reduction assays. In enzyme-linked immunosorbent assays, FCV binding to fJAM-A ectodomain was concentration dependent and saturable; however, FCV bound D1 alone weakly and was unable to bind D2. To characterize FCV binding to surface-expressed fJAM-A, we transfected truncated and chimeric forms of fJAM-A into a nonpermissive cell line and assayed binding by flow cytometry. Only D1 was necessary for FCV binding to cells; all other domains could be replaced. Using a structure-guided mutational approach, we identified three mutants of fJAM-A within D1 (D42N, K43N, and S97A) that exhibited significantly decreased capacities to bind FCV. In contrast to our finding that D1 mediated FCV binding, we found that all domains of fJAM-A were necessary to confer susceptibility to FCV infection. Furthermore, surface expression of fJAM-A was not sufficient to permit FCV infection by all of the isolates we investigated. This indicates that (i) other cellular factors are required to permit productive FCV infection and (ii) individual FCV isolates differ in the factors they require.

Functionalized carbon nanotubes for detecting viral proteins

We investigated the biocompatibility, specificity, and activity of a ligand-receptor-protein system covalently bound to oxidized single-walled carbon nanotubes (SWNTs) as a model proof-of-concept for employing such SWNTs as biosensors. SWNTs were functionalized under ambient conditions with either the Knob protein domain from adenovirus serotype 12 (Ad 12 Knob) or its human cellular receptor, the CAR protein, via diimide-activated amidation. We confirmed the biological activity of Knob protein immobilized on the nanotube surfaces by using its labeled conjugate antibody and evaluated the activity and specificity of bound CAR on SWNTs, first, in the presence of fluorescently labeled Knob, which interacts specifically with CAR, and second, with a negative control protein, YieF, which is not recognized by biologically active CAR proteins. In addition, current-gate voltage (I-V(g)) measurements on a dozen nanotube devices explored the effect of protein binding on the intrinsic electronic properties of the SWNTs, and also demonstrated the devices' high sensitivity in detecting protein activity. All data showed that both Knob and CAR immobilized on SWNT surfaces fully retained their biological activities, suggesting that SWNT-CAR complexes can serve as biosensors for detecting environmental adenoviruses.

Novel vectors forin vivogene delivery to vascular tissue

Although some success has been achieved with gene delivery in animal models of vascular disorders, the results from some clinical trials have been less promising, possibly due, in part, to the use of suboptimal vectors for in vivo gene transfer. Non-viral vectors have a very low transfection efficiency so are largely unsuitable for most in vivo applications, and the relatively broad tropism of many of the commonly used viral vectors can limit efficient gene delivery specifically to target vascular tissues. However, characterisation of novel virus serotypes and advances in techniques that enable vectors to be targeted to the required tissue have led to progress in the development of novel vectors that could be utilised for gene delivery for vascular disorders.

Positive and negative regulation of adenovirus infection by CAR-like soluble protein, CLSP

Coxsackievirus and adenovirus receptor (CAR) is a member of the immunoglobulin (Ig) superfamily and a component of epithelial tight junction. CAR also functions as a primary receptor for coxsackievirus B and adenovirus (Ad) infection. In this study, we report the identification of a novel protein, CAR-like soluble protein (CLSP), which is closely related to CAR. Mouse CLSP (mCLSP) was composed of 390 amino acids, including three Ig domains, and showed strong homology to the IgV domain of CAR. Interestingly, mCLSP lacks a transmembrane domain, indicating that this is a soluble protein. mCLSP mRNA was detected primarily in the brain and ovary. When mCLSP cDNA was introduced into SK HEP-1 cells, which were known to be CAR positive and easily infected with Ad vector, the infection with Ad vector was severely inhibited. On the other hand, mCLSP promoted the infection with Ad vector in CAR-negative NIH3T3 cells. Furthermore, recombinant CLSP directly bound to Ad and inhibited the Ad vector-mediated transduction in SK HEP-1 cells. Computational analysis for a genome database showed that the CLSP gene is rodent-specific, and that human and bovine lack this gene. These results suggest that CLSP may play a role in the antiviral defense of the host in rodent animals.

The Coxsackie and Adenovirus Receptor Binds Microtubules and Plays a Role in Cell Migration

The Coxsackie and adenovirus receptor (CAR), a cell adhesion molecule of the immunoglobulin superfamily, inhibits cell growth of a variety of tumors. The cytoplasmic domain of CAR has been implicated in decreased invasion and intracerebral growth of human U87 glioma cells. Using affinity binding, we identified tubulin as an interaction partner for the cytoplasmic domain of CAR. The interaction was specific; CAR and tubulin co-immunoprecipitated in cells expressing endogenous CAR and partially co-localized in situ. The binding of CAR to tubulin heterodimers and to microtubules was direct, with dissociation constants of approximately 1 mum for tubulin and approximately 32 nm for in vitro assembled microtubules. Whereas CAR-expressing U87 glioma cells had decreased migration in a chemotactic assay in Boyden chambers as compared with control cells, an effect that depended on the presence of the cytoplasmic domain of CAR, the difference was abrogated at low, non-cytotoxic doses of the taxane paclitaxel, a microtubule-stabilizing agent. These results indicate that CAR may affect cell migration through its interaction with microtubules.

Synapses: sites of cell recognition, adhesion, and functional specification

Synapses are specialized adhesive contacts characteristic of many types of cell-cell interactions involving neurons, immune cells, epithelial cells, and even pathogens and host cells. Cell-cell adhesion is mediated by structurally diverse classes of cell-surface glycoproteins, which form homophilic or heterophilic interactions across the intercellular space. Adhesion proteins bind to a cytoplasmic network of scaffolding proteins, regulators of the actin cytoskeleton, and signal transduction pathways that control the structural and functional organization of synapses. The themes of this review are to compare the organization of synapses in different cell types and to understand how different classes of cell adhesion proteins and cytoplasmic protein networks specify the assembly of functionally distinct synapses in different cell contexts.

Protein aggregation and bioprocessing

Protein aggregation is a common issue encountered during manufacture of biotherapeutics. It is possible to influence the amount of aggregate produced during the cell culture and purification process by carefully controlling the environment (eg, media components) and implementing appropriate strategies to minimize the extent of aggregation. Steps to remove aggregates have been successfully used at a manufacturing scale. Care should be taken when developing a process to monitor the compatibility of the equipment and process with the protein to ensure that potential aggregation is minimized.

Interaction of adenovirus type 5 fiber with the coxsackievirus and adenovirus receptor activates inflammatory response in human respiratory cells

The innate immune response to adenovirus (Ad)-derived gene transfer vectors has been shown to initiate immediately after interaction of Ad with respiratory epithelial cells, through the induction of extracellular signal-regulated kinase 1 and 2 (ERK1/2) and JNK mitogen-activated protein kinase (MAPK), nuclear factor kappaB (NF-kappaB), and different proinflammatory genes. Ad serotypes 2 or 5 (Ad2/5) enter respiratory epithelia after initial binding of fiber with the coxsackievirus-adenovirus receptor (CAR) or, alternatively, with cell surface heparan sulfate glycosaminoglycans. Ad2/5 internalization is triggered by binding of penton base to cellular RGD-binding integrins. Here we investigated the role of the Ad5 surface domain proteins constituting the vector capsid, namely, the fiber, the penton base, and the hexon, on the transmembrane signals leading to the transcription of the different proinflammatory genes in the human respiratory A549 cell line. Interaction of Ad fiber with CAR activates both ERK1/2 and JNK MAPK and the nuclear translocation of NF-kappaB, whereas no activation was observed after exposing A549 cells to penton base and hexon proteins. Moreover, interaction of Ad fiber with CAR, but not heparan sulfate proteoglycans, promotes transcription of the chemokines interleukin-8, GRO-alpha, GRO-gamma, RANTES, and interferon-inducible protein 10. These results identify the binding of Ad5 fiber with the cellular CAR as a key proinflammatory activation event in epithelial respiratory cells that is independent of the transcription of Ad5 genes.

Adenovirus-induced alterations in host cell gene expression prior to the onset of viral gene expression

In this report, we have studied gene expression profiles in human primary lung fibroblasts (IMR-90) during the very early phase of an adenovirus infection. Eight out of twelve genes with known functions encoded transcription factors linked to two major cellular processes; inhibition of cell growth (ATF3, ATF4, KLF4, KLF6 and ELK3) and immune response (NR4A1 and CEBPB), indicating that the earliest consequences of an adenovirus infection are growth arrest and induction of an immune response. A time course analysis showed that the induction of these immediate-early response genes was transient and suppressed after the onset of the adenovirus early gene expression.

Adenoviral gene vector tethering to nanoparticle surfaces results in receptor-independent cell entry and increased transgene expression

The present studies investigated the hypothesis that affinity immobilization of replication-defective adenoviruses (Ad) on the surfaces of biodegradable nanoparticles (NP) can improve transduction through uncoupling cellular uptake from the coxsackie-adenovirus receptor (CAR). Ad was tethered to the surfaces of polylactide-based NP that were surface-activated using a photoreactive polyallylamine-benzophenone-pyridyldithiocarboxylate polymer, which enabled (via thiol chemistry) the covalent attachment of Ad-binding proteins, either the recombinant D1 domain of CAR or an adenoviral knob-specific monoclonal antibody. Gene transfer by NP-Ad complexes was studied in relation to cellular uptake as a function of cell type and the character of NP-Ad binding. NP-Ad complexes, but not Ad applied with or without control nonimmune IgG-modified NP, significantly increased green fluorescent protein reporter expression in endothelioma and endothelial and arterial smooth muscle cells (SMC) in direct correlation to the extent of NP-Ad internalization. CAR-independent uptake of NP-Ad was confirmed by demonstrating inhibition of free Ad- but not NP-Ad complex-mediated transduction by knob protein. Complexes formulated with an Ad encoding inducible nitric oxide synthase inhibited growth of cultured SMC to a significantly greater extent than those with (GFP)Ad or (NULL)Ad or free vector. It is concluded that Ad-specific affinity tethering to biodegradable NP can significantly increase the level of gene expression via a CAR-independent uptake mechanism.

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.

Identification of adipocyte adhesion molecule (ACAM), a novel CTX gene family, implicated in adipocyte maturation and development of obesity

Few cell adhesion molecules have been reported to be expressed in mature adipocytes, and the significance of cell adhesion process in adipocyte biology is also unknown. In the present study, we identified ACAM (adipocyte adhesion molecule), a novel homologue of the CTX (cortical thymocyte marker in Xenopus) gene family. ACAM cDNA was isolated during PCR-based cDNA subtraction, and its mRNA was shown to be up-regulated in WATs (white adipose tissues) of OLETF (Otsuka Long-Evans Tokushima fatty) rats, an animal model for Type II diabetes and obesity. ACAM, 372 amino acids in total, has a signal peptide, V-type (variable) and C2-type (constant) Ig domains, a single transmembrane segment and a cytoplasmic tail. The amino acid sequence in rat is highly homologous to mouse (94%) and human (87%). ACAM mRNA was predominantly expressed in WATs in OLETF rats, and increased with the development of obesity until 30 weeks of age, which is when the peak of body mass is reached. Western blot analysis revealed that ACAM protein, approx. 45 kDa, was associated with plasma membrane fractions of mature adipocytes isolated from mesenteric and subdermal adipose deposits of OLETF rats. Up-regulation of ACAM mRNAs in obesity was also shown in WATs of genetically obese db/db mice, diet-induced obese ICR mice and human obese subjects. In primary cultured mouse and human adipocytes, ACAM mRNA expression was progressively up-regulated during differentiation. Several stably transfected Chinese-hamster ovary K1 cell lines were established, and the quantification of ACAM mRNA and cell aggregation assay revealed that the degree of homophilic aggregation correlated well with ACAM mRNA expression. In summary, ACAM may be the critical adhesion molecule in adipocyte differentiation and development of obesity.

What Does It Take to Bind CAR?

Recombinant adenoviruses (Ads) have been used as reagents for biological studies and therapeutic protocols for the treatment of human patients. The two most commonly used Ads, Ad2 and Ad5, infect a broad range of tissues through interaction with the coxsackie and adenovirus receptor CAR. Both mutational analyses and crystal structure data have established residues in the fiber knob and shaft critical for Ad-CAR binding. In this report we review the contributions of various residues to Ad-CAR binding, taking into consideration the documented cellular tropism of other Ad serotypes.

Molecular and cellular insights into the coxsackie-adenovirus receptor: role in cellular interactions in the stem cell niche

In recent years, progress has been made in characterizing the molecular and cellular elements that are responsible for the regeneration in the damaged brain and highlighting the key role of the stromal-vascular 'environment' to orchestrate secondary neurogenesis and repair. Indeed, the ability of the stem cells to self-renew and differentiate is tightly regulated by stromal ependymal cells and endothelial cells expressing molecular cues that constitute the extracellular stem cell 'niche'. Several soluble growth factors such as EGF, TGFbeta, FGF2, SDF-1alpha and Noggin are important signals for the stem cell niche but little is known about the role of membrane-bound molecules in intercellular communications between the niche and the stem cells. In this mini-review, we highlight the emerging role of a family of adhesion molecules in the control of secondary neurogenesis. The coxsackie-adenovirus receptor (CAR) is a 46 kDa transmembrane protein and a member of the immunoglobulin super family. It is close structurally and evolutionary to other adhesion molecules involved in cell-cell interactions during embryogenesis, broadly expressed in the developing central nervous system but restricted to ependymal cells in the adult brain. This unique location and its newly established signalling properties further support the role of CAR in intercellular communications. Elucidating the other signalling molecules and manipulating the stromal-vascular niche for example by adenovirus gene therapy remain important goals for future clinical applications.

Neutrophil migration across tight junctions is mediated by adhesive interactions between epithelial coxsackie and adenovirus receptor and a junctional adhesion molecule-like protein on neutrophils

Neutrophil (polymorphonuclear leukocytes [PMN]) transepithelial migration during inflammatory episodes involves a complex series of adhesive interactions and signaling events. Previous studies have shown that key adhesive interactions between leukocyte CD11b/CD18 and basally expressed fucosylated glycoproteins followed by binding to desmosomal-associated JAM-C are key elements of the transmigration response. Here we provide the first evidence that PMN-expressed junctional adhesion molecule-like protein (JAML) regulates transmigration via binding interactions with epithelial coxsackie and adenovirus receptor (CAR). Experiments with a JAML fusion protein revealed specific binding of JAML to epithelial CAR expressed at tight junctions in T84 cell monolayers and normal human colonic mucosa. Furthermore, JAML-CAR binding is mediated via the membrane distal immunoglobulin (Ig) loop of CAR and the membrane proximal Ig loop of JAML. PMN bound to immobilized CAR but not JAML in a divalent cation-independent manner. Lastly, in assays of PMN transepithelial migration, JAML/CAR fusion proteins and their antibodies significantly inhibited transmigration in a specific manner. Taken together, these results indicate that JAML and CAR are a novel pair of adhesion molecules that play an important role in modulating PMN migration cross epithelial tight junctions. These findings add a new element to a multistep model of PMN transepithelial migration and may provide new targets for anti-inflammatory therapies.

Protein aggregation during overexpression limited by peptide extensions with large net negative charge

Folding of the human coxsackie and adenovirus receptor immunoglobulin (Ig) variable-type domain (CAR D1) during overexpression in the Escherichia coli cytoplasm was shown previously to be partially rescued by fusion to a 22-residue C-terminal peptide. Here, peptide sequence features required for solubilization and folding of CAR D1 and similar Ig variable-type domains from two other human membrane proteins were investigated. Peptide extensions with net negative charge > -6 fully solubilized CAR D1, and approximately half of the peptide-solubilized protein was correctly folded. The Ig variable-type domains from human A33 antigen and myelin P-zero proteins were only partially solubilized by peptide extensions with net charge of -12, however, and only the solubilized P-zero domain appeared to fold correctly whereas the A33 domain formed soluble microaggregates of misfolded protein. Our results suggest a model where the large net charge of peptide extensions increases electrostatic repulsion between nascent polypeptides. The resulting decrease in aggregation rate can enable some polypeptides to fold spontaneously into their native protein conformations. Analysis of the solubility and folding status of sets of structurally homologous proteins, such as the Ig variable-type domains described here, during overexpression could provide insights into how amino acid and gene sequences influence the efficiency of spontaneous protein folding.

Breaking into the epithelial apical-junctional complex--news from pathogen hackers

The epithelial apical-junctional complex is a key regulator of cellular functions. In addition, it is an important target for microbial pathogens that manipulate the cell to survive, proliferate and sometimes persist within a host. Out of a myriad of potential molecular targets, some bacterial and viral pathogens have selected a subset of protein targets at the apical-junctional complex of epithelial cells. Studying how microbes use these targets also teaches us about the inherent physiological properties of host molecules in the context of normal junctional structure and function. Thus, we have learned that three recently uncovered components of the apical-junctional complex of the Ig superfamily--junctional adhesion molecule, Nectin and the coxsackievirus and adenovirus receptor--are important regulators of junction structure and function and represent critical targets of microbial virulence gene products.

A soluble CAR-SCF fusion protein improves adenoviral vector-mediated gene transfer to c-Kit-positive hematopoietic cells

BACKGROUND

Although adenoviral vectors primarily derived from the adenovirus serotype 5 (Ad5) are widely used for many gene transfer applications, they cannot efficiently infect hematopoietic cells, since these cells do not express the coxsackie-adenoviral receptor (CAR).

METHODS

We have developed a soluble fusion protein that bridges adenoviral fibers and the c-Kit receptor to alter Ad5 tropism to immature hematopoietic cells. The CAR-SCF fusion protein consists of the extracellular domains of CAR and stem cell factor (SCF). The human megakaryoblastic leukemia cell lines UT-7 and M07e, human chronic myelogenous leukemia cell line K-562, and erythroleukemia cell line TF-1 were used to assess CAR-SCF-assisted Ad5-mediated gene transfer. Hematopoietic cell lines were infected with an Ad5 vector (Ad5-eGFP) or a fiber-mutant Ad5/F35 (Ad5/F35-eGFP) expressing the enhanced green fluorescent protein gene in the presence or absence of CAR-SCF.

RESULTS

Twenty-four hours after infection, more than 80% of M07e cells infected in the presence of CAR-SCF were eGFP-positive, compared with very few eGFP-positive cells following Ad5-eGFP infection in the absence of CAR-SCF. The enhancement of Ad5-eGFP infection by CAR-SCF was greater than that caused by Ad5/F35-eGFP (50%). The ability of CAR-SCF to enhance Ad5-eGFP infectivity was highly dependent on cellular c-Kit expression levels. Furthermore, CAR-SCF also enhanced Ad5-mediated gene transfer into human primary CD34(+) cells.

CONCLUSIONS

The CAR-SCF fusion protein assists Ad5-mediated transduction to c-Kit(+) CAR(-) hematopoietic cells. The use of this fusion protein would enhance a utility of Ad5-mediated hematopoietic cell transduction strategies.

Biodistribution of radioiodinated adenovirus fiber protein knob domain after intravenous injection in mice

The knob domains from the fiber proteins of adenovirus serotypes 2 and 12 were labeled with radioiodine and then injected into the bloodstreams of mice. Knob proteins with functional binding sites for the coxsackie and adenovirus receptor (CAR) were cleared rapidly from the circulation, with radioactivity appearing predominantly in the stomach, while knob mutants unable to bind to CAR remained in the blood circulation for a prolonged period. The clearance of radiolabeled wild-type knob from the blood was slowed by coinjecting an excess of unlabeled wild-type knob protein. An earlier study showed that (99m)Tc-labeled knob protein with intact CAR-binding activity also cleared rapidly from the blood circulation of mice, with radioactivity accumulating predominantly in the liver (K. R. Zinn et al., Gene Ther. 5:798-808, 1998). Together these results suggest that rapid clearance of knob protein from the blood results from specific binding to CAR in the liver and that the bound knob then enters a degradative pathway. The elevated levels of radioiodine in the stomach observed in our experiments are consistent with deiodination of labeled knob by dehalogenases in hepatocyte microsomes and uptake of the resultant free radioiodine by Na/I symporters in the gastric mucosa. Although CAR has been shown to localize in tight junctions of polarized epithelial cells, where it functions in intercellular adhesion, the results of our study suggest that a subset of CAR molecules in the liver is highly accessible to ligands in the blood and able to rapidly deliver bound ligand to an intracellular degradative compartment.

Structural Similarities in the Cellular Receptors Used by Adenovirus and Reovirus

Adenovirus and reovirus are nonenveloped viruses that engage cell-surface receptors using filamentous attachment proteins with head-and-tail morphology. The coxsackievirus and adenovirus receptor (CAR) and reovirus receptor junctional adhesion molecule 1 (JAM1) are immunoglobulin superfamily members that form homodimers stabilized by ionic and hydrophobic contacts between their N-terminal immunoglobulin-like domains. Both proteins are expressed at regions of cell-cell contact and contain sequences in their cytoplasmic tails that anchor the proteins to the actin cytoskeleton. Like CAR and JAM1, the attachment proteins of adenovirus and reovirus, fiber and sigma1, respectively, also share key structural features. Both fiber and sigma1 have defined regions of flexibility within the tail, which is constructed in part using an unusual triple beta-spiral motif. The head domains of both proteins are formed by an 8-stranded beta-barrel with identical beta-strand connectivity. Strikingly, both adenovirus fiber and reovirus 1 engage their receptors by interacting with sequences that also mediate formation of receptor homodimers. Therefore, while adenovirus and reovirus belong to different virus families and have few overall properties in common, the observed similarities between the receptors and attachment proteins of these viruses suggest a conserved mechanism of attachment and an evolutionary relationship.

Membrane cofactor protein is a receptor for adenoviruses associated with epidemic keratoconjunctivitis

Subgroup D adenovirus (Ad) types 8, 19, and 37 (Ad8, -19, and -37, respectively) are causative agents of epidemic keratoconjunctivitis and genital tract infections. Previous studies showed that Ad37 binds to a 50-kDa membrane glycoprotein expressed on human ocular (conjunctival) cells. To identify and characterize the role of the 50-kDa glycoprotein in Ad37 infection, we partially purified this molecule from solubilized Chang C conjunctival cell membranes by using lentil lectin chromatography and preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Liquid chromatography coupled to nano-electrospray ionization-tandem mass spectrometry was subsequently used to identify four Ad37 receptor candidates: CD46, CD87, CD98, and CD147. Immunodepletion analyses demonstrated that the 50-kDa protein is identical to CD46 (also known as membrane cofactor protein). The Ad37, but not Ad5, fiber knob bound to the extracellular domain of CD46, demonstrating a direct interaction of an Ad37 capsid protein with CD46. An antibody specific for the N-terminal 19 amino acids of CD46 also blocked Ad37 infection of human cervical carcinoma and conjunctival cells, indicating a requirement for CD46 in infection. Finally, expression of a 50-kDa isoform of human CD46 in a CD46-null cell line increased cell binding by wild-type Ad37 and gene delivery by an Ad vector pseudotyped with the Ad37 fiber, but not by a vector bearing the Ad5 fiber. Together, these studies demonstrate that CD46 serves as an attachment receptor for Ad37 and shed further light on the cell entry pathway of subgroup D Ads.

Virotherapeutics: conditionally replicative adenoviruses for viral oncolysis

Viral oncolysis, or virotherapy, is an endeavor to use viruses as therapeutic agents in an effort to exploit their highly evolved qualities of host cell killing and simultaneous multiplication and spread. This review describes the concept of oncolytic adenoviruses, also called conditionally replicative adenoviruses (CRAds), and recent developments--inspired by early clinical results--that aim at the optimization of CRAd efficacy. Molecular strategies applied for the development of oncolytic adenoviruses include (i) the genetic manipulation of the expression and/or function of key regulatory viral proteins in order to restrict viral replication and spread to tumor cells, (ii) the engineering of the adenoviral capsid for efficient and tumor-targeted infection, and (iii) the incorporation of heterologous genes to facilitate combination therapies or tracking of the virus. Initial clinical trials have provided proof-of-concept for adenoviral oncolysis in patients and a favorable safety profile for oncolytic adenoviruses has been demonstrated. In conclusion, adenoviral oncolysis, with its distinct therapeutic mechanism, shows remarkable therapeutic potential. Advanced generations of virotherapeutics are currently in development.

Evolution of cell recognition by viruses: a source of biological novelty with medical implications

The picture beginning to form from genome analyses of viruses, unicellular organisms, and multicellular organisms is that viruses have shared functional modules with cells. A process of coevolution has probably involved exchanges of genetic information between cells and viruses for long evolutionary periods. From this point of view present-day viruses show flexibility in receptor usage and a capacity to alter through mutation their receptor recognition specificity. It is possible that for the complex DNA viruses, due to a likely limited tolerance to generalized high mutation rates, modifications in receptor specificity will be less frequent than for RNA viruses, albeit with similar biological consequences once they occur. It is found that different receptors, or allelic forms of one receptor, may be used with different efficiency and receptor affinities are probably modified by mutation and selection. Receptor abundance and its affinity for a virus may modulate not only the efficiency of infection, but also the capacity of the virus to diffuse toward other sites of the organism. The chapter concludes that receptors may be shared by different, unrelated viruses and that one virus may use several receptors and may expand its receptor specificity in ways that, at present, are largely unpredictable.

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.