Deletion analysis of functional domains in baculovirus-expressed adenovirus type 2 fiber

Adenovirus Fibers as Ultra-Stable Vehicles for Intracellular Nanoparticle and Protein Delivery

Protein-based carriers are promising vehicles for the intracellular delivery of therapeutics. In this study, we designed and studied adenovirus protein fiber constructs with potential applications as carriers for the delivery of protein and nanoparticle cargoes. We used as a basic structural framework the fibrous shaft segment of the adenovirus fiber protein comprising of residues 61–392, connected to the fibritin foldon trimerization motif at the C-terminal end. A fourteen-amino-acid biotinylation sequence was inserted immediately after the N-terminal, His-tagged end of the construct in order to enable the attachment of a biotin moiety in vivo. We report herein that this His-tag biotinylated construct folds into thermally and protease-stable fibrous nanorods that can be internalized into cells and are not cytotoxic. Moreover, they can bind to proteins and nanoparticles through the biotin–streptavidin interaction and mediate their delivery to cells. We demonstrate that streptavidin-conjugated gold nanoparticles can be transported into NIH3T3 fibroblast and HeLa cancer cell lines. Furthermore, two streptavidin-conjugated model proteins, alkaline phosphatase and horseradish peroxidase can be delivered into the cell cytoplasm in their enzymatically active form. This work is aimed at establishing the proof-of-principle for the rational engineering of diverse functionalities onto the initial protein structural framework and the use of adenovirus fiber-based proteins as nanorods for the delivery of nanoparticles and model proteins. These constructs could constitute a stepping stone for the development of multifunctional and modular fibrous nanorod platforms that can be tailored to applications at the sequence level.

O-GlcNAcylation modulates HBV replication through regulating cellular autophagy at multiple levels

O-GlcNAcylation is a form of posttranslational modification, and serves various functions, including modulation of location, stability, and activity for the modified proteins. O-linked-N-acetylglucosamine (O-GlcNAc) transferase (OGT) is an essential cellular enzyme that posttranslationally modifies the cellular proteins with O-GlcNAc moiety. Early studies reported that the decreased O-GlcNAcylation regulates cellular autophagy, a process relevant for hepatitis B virus replication (HBV) and assembly. Therefore, we addressed the question how O-GlcNAcylation regulates cellular autophagy and HBV replication. Inhibition of OGT activity with a small molecule inhibitor OSMI-1 or silencing OGT significantly enhanced HBV replication and HBsAg production in hepatoma cells and primary human hepatocytes (PHHs). Western blotting analysis showed that inhibition of O-GlcNAcylation-induced endoplasmic reticulum (ER) stress and cellular autophagy, two processes subsequently leading to enhanced HBV replication. Importantly, the numbers of autophagosomes and the levels of autophagic markers LC3-II and SQSTM1/p62 in hepatoma cells were elevated after inhibition of O-GlcNAcylation. Further analysis revealed that inhibition of O-GlcNAcylation blocked autophagosome-lysosome fusion and thereby prevented autophagic degradation of HBV virions and proteins. Moreover, OSMI-1 further promoted HBV replication by inducing autophagosome formation via inhibiting the O-GlcNAcylation of Akt and mTOR. In conclusion, decreased O-GlcNAcylation enhanced HBV replication through increasing autophagosome formation at multiple levels, including triggering ER-stress, Akt/mTOR inhibition, and blockade of autophagosome-lysosome fusion.

Transient Chimeric Ad5/37 Fiber Enhances NK-92 Carrier Cell-Mediated Delivery of Oncolytic Adenovirus Type 5 to Tumor Cells

Methods for customizing and improving virus vector tropism are limited. In this study, we introduce a microRNA (miRNA)-regulated molecular method to enhance vector transduction without genome alteration. Based on the importance of adenovirus (Ad) vectors for cancer and gene treatment, we exemplified this technology for an Ad type 5 (Ad5) vector temporally carrying a knob from Ad37. We constructed a producer cell line stably expressing a fused Ad5/37 chimeric fiber comprising the Ad5 shaft-tail and the Ad37 knob and a miRNA inhibiting Ad5 knob expression (HEK293-Ad5/37-miRNA). The chimeric Ad5/37 vector resulted in enhanced transduction rates in Ad37 adequately and Ad5 poorly transduced cells. Particularly, encapsidation of the oncolytic Ad5-human telomerase reverse transcriptase (hTERT) vector genome into the chimeric Ad5/37 capsid showed efficient transduction of NK-92 carrier cells. These infected carrier cells then delivered the oncolytic vector to tumor cells, which resulted in enhanced Ad5-hTERT-mediated tumor cell killing. We show that this transiently capsid-modified chimeric vector carrying an Ad5 genome displayed higher transduction efficiencies of natural killer cell-derived NK-92 cells utilized as carriers in cancer immune therapy. In summary, transiently modified adenoviral vectors will have important implications for cancer and gene therapy.

Fiber knob domain lacking the shaft sequence but fused to a coiled coil is a candidate subunit vaccine against egg-drop syndrome

Egg-drop syndrome (EDS) virus is an avian adenovirus that causes a sudden drop in egg production and in the quality of the eggs when it infects chickens, leading to substantial economic losses in the poultry industry. Inactivated EDS vaccines produced in embryonated duck eggs or cell culture systems are available for the prophylaxis of EDS. However, recombinant subunit vaccines that are efficacious and inexpensive are a desirable alternative. In this study, we engineered chimeric fusion proteins in which the trimeric fiber knob domain lacking the triple β-spiral motif in the fiber shaft region was genetically fused to trimeric coiled coils, such as those of the engineered form of the GCN4 leucine zipper peptide or chicken cartilage matrix protein (CMP). The fusion proteins were expressed predominantly as soluble trimeric proteins in Escherichia coli at levels of 15-80mg/L of bacterial culture. The single immunization of chickens with the purified fusion proteins, at a dose equivalent to 10μg of the knob moiety, elicited serum antibodies with high hemagglutination inhibition (HI) activities, similar to those induced by an inactivated EDS vaccine. A dose-response analysis indicated that a single immunization with as little as 1μg of the knob moiety of the CMP-knob fusion protein was as effective as the inactivated vaccine in inducing antibodies with HI activity. The immunization of laying hens had no apparent adverse effects on egg production and effectively prevented clinical symptoms of EDS when the chickens were challenged with pathogenic EDS virus. This study demonstrates that the knob domain lacking the shaft sequence but fused to a trimeric coiled coil is a promising candidate subunit vaccine for the prophylaxis of EDS in chickens.

Inhibition of O-Linked N-Acetylglucosamine Transferase Reduces Replication of Herpes Simplex Virus and Human Cytomegalovirus

O-linked N-acetylglucosamine (O-GlcNAc) transferase (OGT) is an essential cellular enzyme that posttranslationally modifies nuclear and cytoplasmic proteins via O-linked addition of a single N-acetylglucosamine (GlcNAc) moiety. Among the many targets of OGT is host cell factor 1 (HCF-1), a transcriptional regulator that is required for transactivation of the immediate-early genes of herpes simplex virus (HSV). HCF-1 is synthesized as a large precursor that is proteolytically cleaved by OGT, which may regulate its biological function. In this study, we tested whether inhibition of the enzymatic activity of OGT with a small molecule inhibitor, OSMI-1, affects initiation of HSV immediate-early gene expression and viral replication. We found that inhibiting OGT's enzymatic activity significantly decreased HSV replication. The major effect of the inhibitor occurred late in the viral replication cycle, when it reduced the levels of late proteins and inhibited capsid formation. However, depleting OGT levels with small interfering RNA (siRNA) reduced the expression of HSV immediate-early genes, in addition to reducing viral yields. In this study, we identified OGT as a novel cellular factor involved in HSV replication. Our results obtained using a small molecule inhibitor and siRNA depletion suggest that OGT's glycosylation and scaffolding functions play distinct roles in the replication cycle of HSV.

IMPORTANCE

Antiviral agents can target viral or host gene products essential for viral replication. O-GlcNAc transferase (OGT) is an important cellular enzyme that catalyzes the posttranslational addition of GlcNAc sugar residues to hundreds of nuclear and cytoplasmic proteins, and this modification regulates their activity and function. Some of the known OGT targets are cellular proteins that are critical for the expression of herpes simplex virus (HSV) genes, suggesting a role for OGT in the replication cycle of HSV. In this study, we found that OGT is required for efficient expression of viral genes and for assembly of new virions. Thus, we identify OGT as a novel host factor involved in the replication of HSV and a potential target for antiviral therapy.

O-GlcNAc transferase inhibits KSHV propagation and modifies replication relevant viral proteins as detected by systematic O-GlcNAcylation analysis

O-GlcNAcylation is an inducible, highly dynamic and reversible post-translational modification, mediated by a unique enzyme named O-linked N-acetyl-d-glucosamine (O-GlcNAc) transferase (OGT). In response to nutrients, O-GlcNAc levels are differentially regulated on many cellular proteins involved in gene expression, translation, immune reactions, protein degradation, protein-protein interaction, apoptosis and signal transduction. In contrast to eukaryotic cells, little is known about the role of O-GlcNAcylation in the viral life cycle. Here, we show that the overexpression of the OGT reduces the replication efficiency of Kaposi's sarcoma-associated herpesvirus (KSHV) in a dose-dependent manner. In order to investigate the global impact of O-GlcNAcylation in the KSHV life cycle, we systematically analyzed the 85 annotated KSHV-encoded open reading frames for O-GlcNAc modification. For this purpose, an immunoprecipitation (IP) strategy with three different approaches was carried out and the O-GlcNAc signal of the identified proteins was properly controlled for specificity. Out of the 85 KSHV-encoded proteins, 18 proteins were found to be direct targets for O-GlcNAcylation. Selected proteins were further confirmed by mass spectrometry for O-GlcNAc modification. Correlation of the functional annotation and the O-GlcNAc status of KSHV proteins showed that the predominant targets were proteins involved in viral DNA synthesis and replication. These results indicate that O-GlcNAcylation plays a major role in the regulation of KSHV propagation.

HER3 targeting of adenovirus by fiber modification increases infection of breast cancer cells in vitro, but not following intratumoral injection in mice

Despite the tremendous potential of adenovirus (Ad) as a delivery vector for cancer gene therapy, its use in clinical settings has been limited, mainly as a result of the limited infectivity in many tumors and the wide tissue tropism associated with Ad. To modify the tropism of the virus, we have inserted the epidermal growth factor-like domain of the human heregulin-α (HRG) into the HI loop of Ad5 fiber. This insertion had no adverse effect on fiber trimerization nor did it affect incorporation of the modified fiber into infectious viral particles. Virions bearing modified fiber displayed growth characteristics and viral yields indistinguishable from those of wild-type (wt) virus. Most importantly, HRG-tagged virions showed enhanced infection of cells expressing the cognate receptors HER3/ErbB3 and HER4/ErbB4. This was significantly reduced in the presence of soluble HRG. Furthermore, HER3-expressing Chinese hamster ovary (CHO) cells were transduced by the HRG-modified virus, but not by wt virus. In contrast, CHO cells expressing the coxsackie-Ad receptor were transduced with both viruses. However, infection of an in vivo breast cancer xenograft model after intratumoral injection was similar with both viruses, suggesting that the tumor microenvironment and/or the route of delivery have important roles in infection of target cells with fiber-modified Ads.

Her2-specific multivalent adapters confer designed tropism to adenovirus for gene targeting

Adenoviruses (Ads) hold great promise as gene vectors for diagnostic or therapeutic applications. The native tropism of Ads must be modified to achieve disease site-specific gene delivery by Ad vectors and this should be done in a programmable way and with technology that can realistically be scaled up. To this end, we applied the technologies of designed ankyrin repeat proteins (DARPins) and ribosome display to develop a DARPin that binds the knob domain of the Ad fiber protein with low nanomolar affinity (K(D) 1.35 nM) and fused this protein with a DARPin specific for Her2, an established cell-surface biomarker of human cancers. The stability of the complex formed by this bispecific targeting adapter and the Ad virion resulted in insufficient gene transfer and was subsequently improved by increasing the valency of adapter-virus binding. In particular, we designed adapters that chelated the knob in a bivalent or trivalent fashion and showed that the efficacy of gene transfer by the adapter-Ad complex increased with the functional affinity of these molecules. This enabled efficient transduction at low stoichiometric adapter-to-fiber ratios. We confirmed the Her2 specificity of this transduction and its dependence on the Her2-binding DARPin component of the adapters. Even the adapter molecules with four fused DARPins could be produced and purified from Escherichia coli at very high levels. In principle, DARPins can be generated against any target and this adapter approach provides a versatile strategy for developing a broad range of disease-specific gene vectors.

Adenovirus 5-fiber 35 chimeric vector mediates efficient apical correction of the cystic fibrosis transmembrane conductance regulator defect in cystic fibrosis primary airway epithelia

In vivo gene transfer to the human respiratory tract by adenovirus serotype 5 (Ad5) vectors has revealed their limitations related to inefficient gene transfer, host antiviral response, and innate adenoviral toxicity. In the present work, we compared the cytotoxicity and efficiency of Ad5 and a chimeric Ad5F35 vector with respect to CFTR gene transfer to cystic fibrosis (CF) and non-CF human airway epithelial cells. We found that high doses of Ad5 vector had an adverse effect on the function of exogenous and endogenous CFTR. Results obtained with Ad5 capsid mutants suggested that the RGD motifs on the penton base capsomers were responsible for the negative effect on CFTR function. This negative interference did not result from a lower level of biosynthesis and/or altered cellular trafficking of the CFTR protein, but rather from an indirect mechanism of functional blockage of CFTR, related to the RGD integrin-mediated endocytic pathway of Ad5. No negative interference with CFTR was observed for Ad5F35, an Ad5-based vector pseudotyped with fibers from Ad35, a serotype that uses another cell entry pathway. In vitro, Ad5F35 vector expressing the GFP-tagged CFTR (Ad5F35-GFP-CFTR) showed a 30-fold higher efficiency of transduction and chloride channel correction in CFTR-deficient cells, compared with Ad5GFP-CFTR. Ex vivo, Ad5F35-GFP-CFTR had the capacity to transduce efficiently reconstituted airway epithelia from patients with CF (CF-HAE) via the apical surface, restored chloride channel function at relatively low vector doses, and showed relatively stable expression of GFP-CFTR for several weeks.

Serotype 5 Adenovirus fiber (F7F41S) chimeric vectors incur packaging deficiencies when targeting peptides are inserted into Ad41 short fiber

Adenovirus is a well-established viral gene transfer model system that presents two major hurdles when being considered for cell-specific targeting applications. First is the need to detarget the vector from inherent host binding mechanisms, and second is the need to establish a productive and stable method to retarget the vector to a desired cell receptor. In previous studies we had generated an adenovirus vector platform that lacks the normal targeting attributes derived from the fiber and penton capsid proteins. In the current study we characterized our detargeted Ad5-based vectors (Ad5.F7F41S and Ad5.F7F41SDeltaRGD) as platforms for novel retargeted viruses. The experimental strategy relied on incorporating small peptide ligands into several sites of the Ad 41short fiber knob domain (AB, CD, HI, G and Cterm). Reengineering of Ad41 short fiber resulted either in a bypass to fiber 7 usage, or in a dominant negative packaging/production deficiency phenotype. Under specific growth conditions we could remedy some of the capsid deficiencies and generate high titer viruses. However when examined by Western blot analysis, the resulting viruses were still defective in capsid content. The tandem fiber F7F41S platform has revealed an unanticipated sensitivity of Adenovirus packaging to fiber 41short structural modifications. These studies indicate fiber assembly into an intact virion or fiber influenced capsid stability as a bottleneck to efficient particle production. We also demonstrate that virus particles characterized as mature virions following CsCl banding can vary significantly in capsid protein content. Considering the complexity of virus entry into a target cell, modified "mature virions" may be compromised at the level of transduction not only through the intended modification, but also by virtue of secondary structural packaging conflicts.

Improved adenovirus type 5 vector-mediated transduction of resistant cells by piggybacking on coxsackie B-adenovirus receptor-pseudotyped baculovirus

Taking advantage of the wide tropism of baculoviruses (BVs), we constructed a recombinant BV (BV(CAR)) pseudotyped with human coxsackie B-adenovirus receptor (CAR), the high-affinity attachment receptor for adenovirus type 5 (Ad5), and used the strategy of piggybacking Ad5-green fluorescent protein (Ad5GFP) vector on BV(CAR) to transduce various cells refractory to Ad5 infection. We found that transduction of all cells tested, including human primary cells and cancer cell lines, was significantly improved using the BV(CAR)-Ad5GFP biviral complex compared to that obtained with Ad5GFP or BV(CAR)GFP alone. We determined the optimal conditions for the formation of the complex and found that a high level of BV(CAR)-Ad5GFP-mediated transduction occurred at relatively low adenovirus vector doses, compared with transduction by Ad5GFP alone. The increase in transduction was dependent on the direct coupling of BV(CAR) to Ad5GFP via CAR-fiber knob interaction, and the cell attachment of the BV(CAR)-Ad5GFP complex was mediated by the baculoviral envelope glycoprotein gp64. Analysis of the virus-cell binding reaction indicated that the presence of BV(CAR) in the complex provided kinetic benefits to Ad5GFP compared to the effects with Ad5GFP alone. The endocytic pathway of BV(CAR)-Ad5GFP did not require Ad5 penton base RGD-integrin interaction. Biodistribution of BV(CAR)-Ad5Luc complex in vivo was studied by intravenous administration to nude BALB/c mice and compared to Ad5Luc injected alone. No significant difference in viscerotropism was found between the two inocula, and the liver remained the preferred localization. In vitro, coagulation factor X drastically increased the Ad5GFP-mediated transduction of CAR-negative cells but had no effect on the efficiency of transduction by the BV(CAR)-Ad5GFP complex. Various situations in vitro or ex vivo in which our BV(CAR)-Ad5 duo could be advantageously used as gene transfer biviral vector are discussed.

Modification of adenovirus type 5 tropism for a preferential transduction of human papillomavirus-positive cancer cells

Human group C adenoviruses can infect many cell types, and this is due to the widespread expression of their receptor, the coxsackievirus and adenovirus receptor (CAR). Adenovirus vectors for cancer gene therapy could be significantly improved if their tropism were restricted to tumor cells. In this work, previously identified peptides that target human papillomaviruses (HPV)-transformed cells were inserted into the HI loop of a non-CAR-binding fiber. These modified fiber proteins were able to assemble into adenovirus particles. We demonstrated that these modifications ablated the native tropism of adenovirus type 5, and these modified adenoviruses were shown to preferentially transduce HPV-transformed cell lines.

Protein crystals in Adenovirus type 5-infected cells: requirements for intranuclear crystallogenesis, structural and functional analysis

Intranuclear crystalline inclusions have been observed in the nucleus of epithelial cells infected with Adenovirus serotype 5 (Ad5) at late steps of the virus life cycle. Using immuno-electron microscopy and confocal microscopy of cells infected with various Ad5 recombinants modified in their penton base or fiber domains, we found that these inclusions represented crystals of penton capsomers, the heteromeric capsid protein formed of penton base and fiber subunits. The occurrence of protein crystals within the nucleus of infected cells required the integrity of the fiber knob and part of the shaft domain. In the knob domain, the region overlapping residues 489-492 in the FG loop was found to be essential for crystal formation. In the shaft, a large deletion of repeats 4 to 16 had no detrimental effect on crystal inclusions, whereas deletion of repeats 8 to 21 abolished crystal formation without altering the level of fiber protein expression. This suggested a crucial role of the five penultimate repeats in the crystallisation process. Chimeric pentons made of Ad5 penton base and fiber domains from different serotypes were analyzed with respect to crystal formation. No crystal was found when fiber consisted of shaft (S) from Ad5 and knob (K) from Ad3 (heterotypic S5-K3 fiber), but occurred with homotypic S3K3 fiber. However, less regular crystals were observed with homotypic S35-K35 fiber. TB5, a monoclonal antibody directed against the Ad5 fiber knob was found by immunofluorescence microscopy to react with high efficiency with the intranuclear protein crystals in situ. Data obtained with Ad fiber mutants indicated that the absence of crystalline inclusions correlated with a lower infectivity and/or lower yields of virus progeny, suggesting that the protein crystals might be involved in virion assembly. Thus, we propose that TB5 staining of Ad-infected 293 cells can be used as a prognostic assay for the viability and productivity of fiber-modified Ad5 vectors.

Creation of hybrid nanorods from sequences of natural trimeric fibrous proteins using the fibritin trimerization motif

Stable, artificial fibrous proteins that can be functionalized open new avenues in fields such as bionanomaterials design and fiber engineering. An important source of inspiration for the creation of such proteins are natural fibrous proteins such as collagen, elastin, insect silks, and fibers from phages and viruses. The fibrous parts of this last class of proteins usually adopt trimeric, beta-stranded structural folds and are appended to globular, receptor-binding domains. It has been recently shown that the globular domains are essential for correct folding and trimerization and can be successfully substituted by a very small (27-amino acid) trimerization motif from phage T4 fibritin. The hybrid proteins are correctly folded nanorods that can withstand extreme conditions. When the fibrous part derives from the adenovirus fiber shaft, different tissue-targeting specificities can be engineered into the hybrid proteins, which therefore can be used as gene therapy vectors. The integration of such stable nanorods in devices is also a big challenge in the field of biomechanical design. The fibritin foldon domain is a versatile trimerization motif and can be combined with a variety of fibrous motifs, such as coiled-coil, collagenous, and triple beta-stranded motifs, provided the appropriate linkers are used. The combination of different motifs within the same fibrous molecule to create stable rods with multiple functions can even be envisioned. We provide a comprehensive overview of the experimental procedures used for designing, creating, and characterizing hybrid fibrous nanorods using the fibritin trimerization motif.

The 3-O-(3’,3’-dimethylsuccinyl) derivative of betulinic acid (DSB) inhibits the assembly of virus-like particles in HIV-1 Gag precursor-expressing cells

Background

The 3- O-(3’,3’-dimethylsuccinyl) derivative of betulinic acid (DSB) blocks HIV-1 maturation by interfering with viral protease (PR) at the capsid (CA)-SP1 cleavage site, a crucial region in HIV-1 morphogenesis.

Methods

We analysed the effect of DSB on the assembly of HIV-1 Gag precursor (Pr55GagHIV) into membrane-enveloped virus-like particles (VLP) in baculovirus-infected cells expressing Pr55GagHIV, in a cellular context devoid of viral PR.

Results

DSB showed a dose-dependent negative effect on VLP assembly, with an IC50∼10 μM. The DSB inhibitory effect was p6-independent and was also observed for intracellular assembly of non-N-myristoylated Gag core-like particles. HIV-1 VLP assembled in the presence of DSB exhibited a lower stability of their inner cores upon membrane delipidation compared with control VLP, suggesting weaker Gag-Gag interactions. DSB also inhibited the assembly of simian immunodeficiency virus SIVmac251 VLP, although with a twofold lower efficacy (IC50∼20 μM). No detectable inhibitory activity was observed for murine leukaemia virus (MLV) VLP; however, fusion of the SP1-NC-p6 domains from HIV-1 to the matrix (MA)-CA domains from MLV conferred DSB sensitivity to the chimaeric Gag precursor Pr72GagMLV–HIV (IC50=30 μM). This observation suggested that the main DSB target on Pr55Gag was the SP1 domain, but the higher degree of DSB resistance for Pr72GagMLV–HIV compared with Pr55GagHIV implied that other upstream Gag region(s) might contribute to DSB reactivity.

Conclusions

Sequence alignment and three-dimensional modelling by homology of the CA-SP1-NC junction in HIV-1, SIVmac251 and Pr72GagMLV–HIV suggested that a higher hydrophilic character of the CA region immediately upstream to the HIV-1 CA-SP1 junction, as occurred in Pr72GagMLV–HIV, correlated with a lower DSB sensitivity.

Characterization of the Adenovirus Fiber Protein

Entry of the adenovirus (Ad) capsids during the early stages of infection is a multistep process that includes initial attachment of the virus capsid to the cell surface followed by internalization of the virus into early endosomes. The Ad fiber protein, a complex of three apparently identical subunits, mediates the initial attachment step. In this chapter, methods for the purification and characterization of the Ad fiber protein are presented. Chromatographic methods for the isolation of the protein from infected cells can yield substantial quantities of protein for biochemical analysis. Protocols for characterization of the protein by Western blot and by indirect immunofluorescence of infected cells are also presented. The specificity of different monoclonal and polyclonal antibodies that recognize Ad fiber is also discussed. Ad fiber from a number of serotypes also contains a posttranslational modification, O-linked N-acetyl-glucosamine; methods for detection and characterization of this modification are also provided. With these tools and protocols, one can address important questions about this protein, which helps direct the tissue tropism of Ad.

Adenovirus type 5 fiber knob domain has a critical role in fiber protein synthesis and encapsidation

Adenovirus serotype 5 (Ad5) vectors carrying knobless fibers designed to remove their natural tropism were found to have a lower fiber content than recombinant Ad5 with wild-type (WT) capsid, implying a role for the knob-coding sequence or/and the knob domain in fiber encapsidation. Experimental data using a variety of fiber gene constructs showed that the defect did not occur at the fiber mRNA level, but at the protein level. Knobless fiber proteins were found to be synthesized at a significant slower rate compared with knob-carrying fibers, and the trimerization process of knobless fibers paralleled their slow rate of synthesis. A recombinant Ad5 diploid for the fiber gene (referred to as Ad5/R7-ZZwt/E1 : WT-fiber) was constructed to analyse the possible rescue of the knobless low-fiber-content phenotype by co-expression of WT fiber. Ad5/R7-ZZwt/E1 : WT-fiber contained a knobless fiber gene in its natural location (L5) in the viral genome and an additional WT fiber gene in an ectopic position in E1. Knobless fiber was still synthesized at low levels compared with the co-expressed E1 : WT fiber and the recovery of the two fiber species in virus progeny reflected their respective amounts in the infected cells. Our results suggested that deletion of the fiber knob domain had a negative effect on the translation of the fiber mRNA and on the intracellular concentration of fiber protein. They also suggested that the knob control of fiber protein synthesis and encapsidation occurred as aciseffect, which was not modified by WT fiber protein providedin transby the same Ad5 genome.

Adenovirus fiber shaft contains a trimerization element that supports peptide fusion for targeted gene delivery

Adenoviral (Ad) vectors have been widely used in human gene therapy clinical trials. However, their application has frequently been restricted by the unfavorable expression of cell surface receptors critical for Ad infection. Infections by Ad2 and Ad5 are largely regulated by the elongated fiber protein that mediates its attachment to a cell surface receptor, coxsackie and adenovirus receptor (CAR). The fiber protein is a homotrimer consisting of an N-terminal tail, a long shaft, and a C-terminal knob region that is responsible for high-affinity receptor binding and Ad tropism. Consequently, the modification of the knob region, including peptide insertion and C-terminal fusion of ligands for cell surface receptors, has become a major research focus for targeting gene delivery. Such manipulation tends to disrupt fiber assembly since the knob region contains a stabilization element for fiber trimerization. We report here the identification of a novel trimerization element in the Ad fiber shaft. We demonstrate that fiber fragments containing the N-terminal tail and shaft repeats formed stable trimers that assembled onto Ad virions independently of the knob region. This fiber shaft trimerization element (FSTE) exhibited a capacity to support peptide fusion. We showed that Ad, modified with a chimeric protein by direct fusion of the FSTE with a growth factor ligand or a single-chain antibody, delivered a reporter gene selectively. Together, these results indicate that the shaft region of Ad fiber protein contains a trimerization element that allows ligand fusion, which potentially broadens the basis for Ad vector development.

Typical and atypical trafficking pathways of Ad5 penton base recombinant protein: implications for gene transfer

The adenovirus (Ad) penton base protein facilitates viral infection by binding cell surface integrins, triggering receptor-mediated endocytosis and mediating endosomal penetration. Given these multiple functions, recombinant penton base proteins have been utilized as non-viral vehicles for gene transfer by our lab and others. Although we have previously demonstrated that penton base-derived vectors undergo integrin-specific binding and cell entry, less than desirable levels of gene expression have led us to re-evaluate the recombinant penton base as an agent for gene delivery. To do so, we have examined here the intracellular trafficking of an Ad serotype 5 (Ad5) recombinant penton base protein (PB). Here, we not only observed that PB utilizes a similar, typical trafficking pathway of whole Ad, but also found that PB entered HeLa cells through pathways not yet identified as contributing to cell entry by the whole virus. We show by high-resolution confocal microscopy and biochemical methods that binding to alphav-integrins is a requirement for cell entry, but that early internalization stages did not substantially pass through clathrin-positive and early endosomal compartments. Moreover, a subpopulation of internalized protein localized with caveolin-positive compartments and Golgi markers, suggesting that a certain percentage of proteins pass through non-clathrin-mediated pathways. Similar to the virus, trafficking toward the nucleus was affected by disruption of microtubules and dynein. The majority of penton base molecules avoided the lysosome while facilitating early vesicle release of low molecular weight dextran molecules. In further support of a vesicle escape capacity, a subpopulation of internalized penton base appeared to enter the nucleus, as observed by high-resolution confocal microscopy and cell fractionation. As a confirmation of these findings, we demonstrate that a recombinant penton base facilitated cytosolic entry of an siRNA molecule as observed by RNA interference of a marker gene. Based on our findings here, we suggest that whereas soluble penton base proteins may enter cells through clathrin- and non-clathrin-mediated pathways, vesicle escape and nuclear delivery appear to be supported by a clathrin-mediated pathway. As our previous efforts have focused on utilizing recombinant penton base proteins as delivery agents for therapeutics, these findings allow us to evaluate the use of the penton base as a cell entry and intracellular trafficking agent, and may be of interest concerning the development of vectors for efficient delivery of therapeutics to cells.

Natural Triple β‐Stranded Fibrous Folds1

A distinctive family of beta-structured folds has recently been described for fibrous proteins from viruses. Virus fibers are usually involved in specific host-cell recognition. They are asymmetric homotrimeric proteins consisting of an N-terminal virus-binding tail, a central shaft or stalk domain, and a C-terminal globular receptor-binding domain. Often they are entirely or nearly entirely composed of beta-structure. Apart from their biological relevance and possible gene therapy applications, their shape, stability, and rigidity suggest they may be useful as blueprints for biomechanical design. Folding and unfolding studies suggest their globular C-terminal domain may fold first, followed by a "zipping-up" of the shaft domains. The C-terminal domains appear to be important for registration because peptides corresponding to shaft domains alone aggregate into nonnative fibers and/or amyloid structures. C-terminal domains can be exchanged between different fibers and the resulting chimeric proteins are useful as a way to solve structures of unknown parts of the shaft domains. The following natural triple beta-stranded fibrous folds have been discovered by X-ray crystallography: the triple beta-spiral, triple beta-helix, and T4 short tail fiber fold. All have a central longitudinal hydrophobic core and extensive intermonomer polar and nonpolar interactions. Now that a reasonable body of structural and folding knowledge has been assembled about these fibrous proteins, the next challenge and opportunity is to start using this information in medical and industrial applications such as gene therapy and nanotechnology.

The 100K-chaperone protein from adenovirus serotype 2 (Subgroup C) assists in trimerization and nuclear localization of hexons from subgroups C and B adenoviruses

Recombinant hexons from subgroup C adenoviruses (Ad2 and Ad5) and from a member of subgroup B (Ad3) adenoviruses have been expressed in insect cells. When expressed alone, all three hexons were found to be insoluble and accumulated as inclusion bodies in the cytoplasm. However, co-expression of recombinant Ad2, Ad5 or Ad3 hexon with Ad2 L4-100K protein resulted in the formation of soluble trimeric hexons. EM analysis of hexons revealed that they were indistinguishable from native hexon capsomers isolated from Ad2-infected human cells, or released from partially disrupted adenovirions. This suggests that 100K acts as a chaperone for hexon folding and self-assembly into capsomer in insect cells. Since 100K protein assists in the trimerization of subgroup C hexon, and of subgroup B hexon protein, it implies that it functions in a manner that is both homo- and heterotypic. During the course of recombinant protein expression, the 100K protein was found in association with hexon monomers and trimers within the cytoplasm. In the nucleus, however, 100K was found in complexes with hexon trimers exclusively. EM observation of purified 100K protein samples showed a dumb-bell-shaped molecule compatible with a monomeric protein. EM analysis of hexon-100K protein complexes showed that interaction of hexon with the 100K protein occurred via one of the globular domains of the 100K protein molecule. Our data confirm the role of the 100K protein as a scaffold protein for hexon, and provide evidence suggesting its function in hexon nuclear import in insect cells.

Amyloid fibril formation from sequences of a natural beta-structured fibrous protein, the adenovirus fiber

Amyloid fibrils are fibrous beta-structures that derive from abnormal folding and assembly of peptides and proteins. Despite a wealth of structural studies on amyloids, the nature of the amyloid structure remains elusive; possible connections to natural, beta-structured fibrous motifs have been suggested. In this work we focus on understanding amyloid structure and formation from sequences of a natural, beta-structured fibrous protein. We show that short peptides (25 to 6 amino acids) corresponding to repetitive sequences from the adenovirus fiber shaft have an intrinsic capacity to form amyloid fibrils as judged by electron microscopy, Congo Red binding, infrared spectroscopy, and x-ray fiber diffraction. In the presence of the globular C-terminal domain of the protein that acts as a trimerization motif, the shaft sequences adopt a triple-stranded, beta-fibrous motif. We discuss the possible structure and arrangement of these sequences within the amyloid fibril, as compared with the one adopted within the native structure. A 6-amino acid peptide, corresponding to the last beta-strand of the shaft, was found to be sufficient to form amyloid fibrils. Structural analysis of these amyloid fibrils suggests that perpendicular stacking of beta-strand repeat units is an underlying common feature of amyloid formation.

The Ad5 fiber mediates nonviral gene transfer in the absence of the whole virus, utilizing a novel cell entry pathway

The interesting discovery reported here that soluble adenovirus serotype 5 (Ad5) fiber proteins enter cells without the virus was a serendipitous result during our development of Ad5 capsid proteins as nonviral gene transfer vectors. The Ad5 capsid fiber and penton proteins mediate infection. The fiber docks to a noninternalizing cell surface protein called the coxsackievirus-Ad receptor (CAR), followed by penton binding to integrins, triggering integrin-mediated endocytosis of the virus. In our previous work, we assembled the nonviral complex, 3PO, which utilized the penton to mediate gene transfer through integrin binding and endocytosis. Here, we tested whether incorporating the fiber targets 3PO to CAR, thus recapitulating the Ad5 infection pathway. As CAR is not an endocytic receptor, we were surprised to find that the fiber alone, without the penton, enabled gene transfer by binding CAR, but internalizing through an unknown mechanism. We show here that the fiber distributes to the nucleus and cytoplasm after temperature-independent uptake, whereas the penton accumulates around the nucleus after temperature-dependent uptake. Fiber uptake by HeLa cells is also actin-dependent, requires the fiber tail/shaft region, and is largely inhibited by heparin. This study raises the possibility that alternative pathways may enable both viral and nonviral cell entry.

Adenovirus Fibre Shaft Sequences Fold into the Native Triple Beta-Spiral Fold when N-terminally Fused to the Bacteriophage T4 Fibritin Foldon Trimerisation Motif

Adenovirus fibres are trimeric proteins that consist of a globular C-terminal domain, a central fibrous shaft and an N-terminal part that attaches to the viral capsid. In the presence of the globular C-terminal domain, which is necessary for correct trimerisation, the shaft segment adopts a triple beta-spiral conformation. We have replaced the head of the fibre by the trimerisation domain of the bacteriophage T4 fibritin, the foldon. Two different fusion constructs were made and crystallised, one with an eight amino acid residue linker and one with a linker of only two residues. X-ray crystallographic studies of both fusion proteins shows that residues 319-391 of the adenovirus type 2 fibre shaft fold into a triple beta-spiral fold indistinguishable from the native structure, although this is now resolved at a higher resolution of 1.9 A. The foldon residues 458-483 also adopt their natural structure. The intervening linkers are not well ordered in the crystal structures. This work shows that the shaft sequences retain their capacity to fold into their native beta-spiral fibrous fold when fused to a foreign C-terminal trimerisation motif. It provides a structural basis to artificially trimerise longer adenovirus shaft segments and segments from other trimeric beta-structured fibre proteins. Such artificial fibrous constructs, amenable to crystallisation and solution studies, can offer tractable model systems for the study of beta-fibrous structure. They can also prove useful for gene therapy and fibre engineering applications.

Gene transduction and cell entry pathway of fiber-modified adenovirus type 5 vectors carrying novel endocytic peptide ligands selected on human tracheal glandular cells

Monolayers of cystic fibrosis transmembrane conductance regulator (CFTR)-deficient human tracheal glandular cells (CF-KM4) were subjected to phage biopanning, and cell-internalized phages were isolated and sequenced, in order to identify CF-KM4-specific peptide ligands that would confer upon adenovirus type 5 (Ad5) vector a novel cell target specificity and/or higher efficiency of gene delivery into airway cells of patients with cystic fibrosis (CF). Three different ligands, corresponding to prototypes of the most represented families of phagotopes recovered from intracellular phages, were designed and individually inserted into Ad5-green fluorescent protein (GFP) (AdGFP) vectors at the extremities of short fiber shafts (seven repeats [R7]) terminated by scissile knobs. Only one vector, carrying the decapeptide GHPRQMSHVY (abbreviated as QM10), showed an enhanced gene transduction of CF-KM4 cells compared to control nonliganded vector with fibers of the same length (AdGFP-R7-knob). The enhancement in gene transfer efficiency was not specific to CF-KM4 cells but was observed in other mammalian cell lines tested. The QM10-liganded vector was referred to as AdGFP-QM10-knob in its knobbed version and as AdGFP-QM10 in its proteolytically deknobbed version. AdGFP-QM10 was found to transduce cells with a higher efficiency than its knob-bearing version, AdGFP-QM10-knob. Consistent with this, competition experiments indicated that the presence of knob domains was not an absolute requirement for cell attachment of the QM10-liganded vector and that the knobless AdGFP-QM10 used alternative cell-binding domains on its capsid, including penton base capsomer, via a site(s) different from its RGD motifs. The QM10-mediated effect on gene transduction seemed to take place at the step of endocytosis in both quantitative and qualitative manners. Virions of AdGFP-QM10 were endocytosed in higher numbers than virions of the control vector and were directed to a compartment different from the early endosomes targeted by members of species C Ad. AdGFP-QM10 was found to accumulate in late endosomal and low-pH compartments, suggesting that QM10 acted as an endocytic ligand of the lysosomal pathway. These results validated the concept of detargeting and retargeting Ad vectors via our deknobbing system and redirecting Ad vectors to an alternative endocytic pathway via a peptide ligand inserted in the fiber shaft domain.

Abolition of hCAR-dependent cell tropism using fiber knobs of Atadenovirus serotypes

Most adenoviral vectors use in gene therapy protocols derive from species C. However, expression of the primary receptor (human Coxsackievirus and Adenovirus receptor, hCAR) for these AdV is variable on cancer cells. In vivo targeting of a therapeutic gene to specific cells has then become a major issue in gene therapy. The Ad fiber protein largely determines viral tropism through interaction with specific receptors. Hereto, we constructed a set of HAdV5 vectors carrying chimeric fibers with knob domains from nonhuman AdV, namely from the FAdV-1 (Aviadenovirus), DAdV-1, and BAdV-4 (Atadenovirus). Correspondents viruses were produced using an established new HEK293 cell line, which express the HAdV2 fiber. Recombinant HAdV harboring chimeric fibers constituted of the N-terminal domain of HAdV2, and knob domain of bovine adenovirus type 4 (BAdV-4) demonstrated the greatest reduction in fiber-mediated gene transfer into human cells expressing the hCAR. Moreover, this vector infects with a better efficiency than vector with wild-type fiber, the Chinese Hamster Ovarian (CHO) and SKOV3 cell lines, both from ovarian origin, hamster and human, respectively. These studies support the concept that changing the fiber knob domain to ablate hCAR interaction should result in a de- or retargeted adenoviral vector. The adenoviral vector with the chimeric HAdV2/BAdV-4 fiber lacking hCAR interaction and with an ovarian cell tropism could be a nice candidate to elaborate vectors for ovarian tumor therapy.

Formation of Highly Stable Chimeric Trimers by Fusion of an Adenovirus Fiber Shaft Fragment with the Foldon Domain of Bacteriophage T4 Fibritin

The folding of beta-structured, fibrous proteins is a largely unexplored area. A class of such proteins is used by viruses as adhesins, and recent studies revealed novel beta-structured motifs for them. We have been studying the folding and assembly of adenovirus fibers that consist of a globular C-terminal domain, a central fibrous shaft, and an N-terminal part that attaches to the viral capsid. The globular C-terminal, or "head" domain, has been postulated to be necessary for the trimerization of the fiber and might act as a registration signal that directs its correct folding and assembly. In this work, we replaced the head of the fiber by the trimerization domain of the bacteriophage T4 fibritin, termed "foldon." Two chimeric proteins, comprising the foldon domain connected at the C-terminal end of four fiber shaft repeats with or without the use of a natural linker sequence, fold into highly stable, SDS-resistant trimers. The structural signatures of the chimeric proteins as seen by CD and infrared spectroscopy are reported. The results suggest that the foldon domain can successfully replace the fiber head domain in ensuring correct trimerization of the shaft sequences. Biological implications and implications for engineering highly stable, beta-structured nanorods are discussed.

Endocytosis of adenovirus and adenovirus capsid proteins

Key proteins of the icosahedral-shaped adenovirus (Ad) capsid mediate infection, and interact with cellular proteins to coordinate stepwise events of cell entry that produce successful gene transfer. Infection is mediated predominantly by the penton and fiber capsid proteins. The fiber initiates cell binding while the penton binds integrin coreceptors, triggering integrin-mediated endocytosis. Penton integrin signaling precedes viral escape from the endosomal vesicle. After cell binding, the virus undergoes stepwise disassembly of the capsid, shedding proteins during cell entry. Intracellular trafficking of the remaining capsid shell is mediated by the interaction of naked particles with the cytoskeleton. The capsid translocates toward the nucleus, with the majority of capsid proteins accumulating at the nuclear periphery, while viral DNA and associated protein VII are extruded through the nuclear pore. This discussion will encompass the current knowledge on Ad cell entry and trafficking, with an emphasis on the contribution of Ad capsid proteins to these processes. A greater understanding of the highly effective Ad cell entry pathway may lend itself to the development of safer drug and gene delivery alternatives utilizing similar pathways.

The maturation of murine dendritic cells induced by human adenovirus is mediated by the fiber knob domain

We investigated the mechanism of adenovirus serotype 5 (Ad5)-mediated maturation of bone marrow-derived murine dendritic cells (DC) using (i) Ad5 vectors with wild-type capsid (AdE1 degrees, AdGFP); (ii) Ad5 vector mutant deleted of the fiber C-terminal knob domain (AdGFPDeltaknob); and (iii) capsid components isolated from Ad5-infected cells or expressed as recombinant proteins, hexon, penton, penton base, full-length fiber, fiber knob, and fiber mutants. We found that penton capsomer (penton base linked to its fiber projection), full-length fiber protein, and its isolated knob domain were all capable of inducing DC maturation, whereas no significant DC maturation was observed for hexon or penton base alone. This capacity was severely reduced for AdGFPDeltaknob and for fiber protein deletion mutants lacking the beta-stranded region F of the knob (residues Leu-485-Thr-486). The DC maturation effect was fully retained in a recombinant fiber protein deleted of the HI loop (FiDeltaHI), a fiber (Fi) deletion mutant that failed to trimerize, suggesting that the fiber knob-mediated DC activation did not depend on the integrity of the HI loop and on the trimeric status of the fiber. Interestingly, peptide-pulsed DC that had been stimulated with Ad5 knob protein induced a potent CD8+ T cell response in vivo.

Shortening adenovirus type 5 fiber shaft decreases the efficiency of postbinding steps in CAR-expressing and nonexpressing cells

The coxsackie B virus and adenovirus receptor (CAR) functions as an attachment receptor for multiple adenovirus serotypes. It has been shown that apart from virus-cellular receptor interactions, the fiber shaft length also influences viral tropism. We therefore generated Ad5FbDelta639 virus with 8beta-repeats in the shaft, instead of the 22beta-repeats present in the wild-type. Here, we show that the extent of attachment of the virus with shortened fiber to CAR-expressing cells was three- to fivefold lower than that of the wild-type. Transduction studies, however, clearly showed that infection of CAR-expressing cells with Ad5FbDelta639 was strongly impaired by comparison with the wild-type virus. Since this impairment was not linked to a proportional reduction in binding to cells, it appeared to be linked to subsequent/later events in infection. A similar decrease in efficacy of postbinding steps was also evidenced in cells that did not express CAR.

Adenovirus stripping: a versatile method to generate adenovirus vectors with new cell target specificity

We developed a new type of adenovirus type 5 (Ad5)-derived vector with genetically modified fiber proteins whose knob domains could be stripped off due to the insertion of a single Factor Xa cleavage site in the fiber shaft, between a cellular ligand and the knob domain. This Ad vector did not require a specific cell line for propagation and could be grown in HEK-293 cells. Stripping off the knob domains removed the endogenous cell-binding moiety of Ad but retained the new cell ligand for retargeting purposes. As experimental models for cell ligands, we used two peptides with different sequence complexities: (i) the integrin-binding tripeptide RGD and (ii) a 58-residue oligopeptide termed affibody (Zwt). Zwt binds specifically to the human IgG1 Fc domain or to its Fc3(1) homolog. The modified fibers were efficiently encapsidated into virions, and the Factor Xa sites were fully accessible to proteolysis. In vitro binding assays using recombinant Fc3(1) protein and Ad5-mediated gene transduction of Fc3(1)-expressing cells demonstrated that the proteolytically deknobbed Ad5-Zwt vector was functional and specific for receptor targeting.

Using GFP--ligand fusions to measure receptor-mediated endocytosis in living cells

Recombinant DNA technology has enabled the production of many types of chimeric proteins containing heterologous functional domains that have served a variety of useful capacities for cell biology research. Among proteins gaining wide use as a fusion partner is Aequorea victoria green fluorescent protein (GFP). GFP has been employed by numerous groups as a reporter gene for cell transfection and as an autofluorescent tag by recombinant fusion to foreign sequences. Here we describe the use of GFP as a tag for ligands, and provide examples of how purified recombinant GFP-ligand fusion proteins may be used to detect ligand-receptor interactions, including receptor-mediated endocytosis. Both its utility and limitations are discussed.

Mechanism of restriction of normal and cystic fibrosis transmembrane conductance regulator-deficient human tracheal gland cells to adenovirus infection and ad-mediated gene transfer

CF-KM4 (cystic fibrosis transmebrane conductance regulator-deficient) and MM-39 (healthy) cells, two serous cell lines from submucosal tracheal glands, were found to be poorly susceptible to adenovirus (Ad)5 infection and Ad5-mediated gene transduction. The major limiting steps apparently resided in the primary events of Ad5 interaction, i.e., cell attachment and entry. Both CF-KM4 and MM-39 cells failed to express the Coxsackie-Ad receptor (CAR), and experimental data suggested that alpha[2-->6]-linked sialic acid residues of sialoglycoproteins (SAGP) in CF-KM4 cells, and heparan sulfate glycosaminoglycans (HS-GAG) in MM-39, were used as receptors by Ad5 virions. Ad5 attached to SAGP and HS-GAG receptors via its fiber knob domain, but entered the cells via a penton base- and Arg-Gly-Asp (RGD)-integrin-independent pathway. The block to Ad5-mediated gene transfer in MM-39 and KM4 cells could be overcome by conferring to the vector a novel cell-binding specificity. Thus, Ad5 vectors carrying a stretch of 7-lysine residues genetically inserted at the C-terminus of the fiber knob were found to transduce MM-39 cells with a 10- to 20-fold higher efficiency than the original vectors, but the transduction of CF-KM4 was not significantly improved. Retargeting Ad5 to integrin receptors via RGD peptide ligands, inserted at the extremity of the fiber shaft, resulted in a transducing efficiency of 20- and 50-fold higher in MM-39 and KM4 cells, respectively, compared with Ad5 vectors carrying fibers terminated by their natural knob domain.

Double modification of adenovirus fiber with RGD and polylysine motifs improves coxsackievirus-adenovirus receptor-independent gene transfer efficiency

Adenoviral vectors based on serotype 5 (Ad5) have been widely used to deliver therapeutic genes to different organs and tissues. However, many tissues are poorly infected with Ad5 because of low-level expression of its primary receptor, coxsackievirus-adenovirus receptor (CAR). Two motifs, RGD and polylysine (pK7), have been shown to enhance Ad5 infection via CAR-independent pathways when incorporated into fiber separately. Because the two motifs bind to different cell surface proteins (RGD motif binds to integrins, and pK7 binds to heparan sulfate-containing receptors), we hypothesized that the two motifs function additively to improve gene transfer efficiency. In this study, we sought to improve infectivity of Ad5 by incorporating both RGD and pK7 motifs into fiber. We created an Ad5 vector containing an RGD motif in the HI loop and a pK7 motif at the C terminus of fiber (Ad5.RGD.pK7). Compared with unmodified and singly modified Ad5 vectors Ad5.RGD and Ad5.pK7, the doubly modified Ad5 demonstrated the highest infectivity in both CAR-positive and CAR-negative cells. The enhanced infectivity appeared to be mediated by additive effects of the two motifs. More importantly, Ad5.RGD.pK7 lost the natural CAR-dependent pathway while employing novel targeting mechanisms. This strategy thus may be used to overcome CAR deficiency and to achieve vector retargeting.

Insertion of an exogenous domain in the adenovirus type 2 fiber globular region

Adenoviruses have been used for gene therapy or immunization due to their ability to efficiently infect a broad range of cells and tissues. These applications as well as specificity could be improved further by redirecting binding of the virus to specific cell types. In this regard, modification of viral genes encoding coat proteins is an option to achieve improvement in retargeting. In this report, we describe a substitution in the adenovirus type 2 fiber globular region by the 44 amino acid C4 domain of human immunodeficiency virus type 1 gp120. In vitro translation analysis and immunoprecipitation assays show that the incorporation of the C4 domain into the fiber protein does not ablate its trimerization property and demonstrates the availability of the C4 epitope for interaction with monoclonal anti-C4 antibody. The recombinant adenovirus containing this modified fiber was also characterized by immunoprecipitation with the same antibody, showing the viability of such kind of modification.

Genetic retargeting of adenovirus vectors: functionality of targeting ligands and their influence on virus viability

BACKGROUND

We studied the ability of adenovirus type 5 (Ad5) to encapsidate new cellular ligands carried by their fibers to yield functional retargeted vectors for gene therapy. Recombinant Ad5 fibers containing shaft repeats 1 to 7 and an extrinsic trimerization motif, and terminated by its native knob or amino acid motifs containing RGD, have been rescued into infectious virions.

METHODS

Polypeptide ligands of cell surface molecules, including single-chain antibodies or epidermal growth factor, were cloned into recombinant fibers. Phenotypic analysis of fiber constructs and rescuing into the Ad5 genome were performed. Recombinant viruses were characterized with reference to fiber content, growth rate and infectivity.

RESULTS

A major limiting factor for recovering viable recombinant Ad5 carrying fiber-fused polypeptide ligands was apparently the ability of the ligand to fold correctly within the cellular cytoplasm. This constraint has previously not been systematically evaluated in the literature. Phenotypic analysis of the fiber-ligand fusions showed that their degree of cytoplasmic solubility correlated with their ability to yield viable Ad5 vectors. Our results suggested that the fiber manipulations diminish virus growth rate, probably through different, opposing effects: (i) the reduced shaft length increases fiber solubility in the absence of the knob but (ii) diminishes virus entry, and (iii) the absence of the knob alters the overall protein composition of the virion and decreases its fiber copy number.

CONCLUSIONS

Based on our findings, cytoplasmic solubility and cytoplasmic ligand reactivity of fiber-ligand fusion proteins are the best prediction criterion for viability and recovery of genetically retargeted Ad vectors.

Targeted adenoviral vectors

Replication-defective vectors based on human adenovirus serotypes 2 and 5 (Ad2 and Ad5) possess a number of attributes which favor their use as gene delivery vehicles in gene therapy applications. However, the widespread distribution of the primary cellular receptor for Ad, the coxsackievirus and adenovirus receptor (CAR), allows Ad vectors to infect a broad range of cells in the host. Conversely, a number of tissues which represent important targets for gene therapy, such as the airway epithelium and cancer cells, are refractory to Ad infection due a paucity of CAR. Thus, there is a strong rationale for the development of CAR-independent Ad vectors capable of enhanced specificity and efficiency of gene transfer to target cells. In this article we review the approaches which have been employed to generate tropism-modified Ad vectors. These targeting strategies have led to improvements in the safety and efficacy of Ad vectors and have the potential to yield an increased therapeutic benefit in the human clinical context.

Adenovirus hexon protein is a potent adjuvant for activation of a cellular immune response

The capacity of recombinant adenoviruses (rAd) to induce immunization against their transgene products has been well documented. In the present study, we evaluated the vaccinal adjuvant role of rAd independently of its vector function. BALB/c mice received one subcutaneous injection of a mixture of six lipopeptides (LP6) used as a model immunogen, along with AdE1 degrees (10(9) particles), a first-generation rAd empty vector. Although coinjected with a suboptimal dose of lipopeptides, AdE1 degrees significantly improved the effectiveness of the vaccination, even in the absence of booster immunization. In contrast to mice that received LP6 alone or LP6 plus a mock adjuvant, mice injected with AdE1 degrees plus LP6 developed both a polyspecific T-helper type 1 response and an effector CD8 T-cell response specific to at least two class I-restricted epitopes. The helper response was still observed when immunization was performed using LP6 plus a mixture of soluble capsid components released from detergent-disrupted virions. When mice were immunized with LP6 and each individual capsid component, i.e., hexon, penton base, or fiber, the results obtained suggested that hexon protein was responsible for the adjuvant effect exerted by disrupted Ad particles on the helper response to the immunogen. Our results thus have some important implications not only in vaccinology but also for gene therapy using rAd vectors.

Review: conformation and folding of novel beta-structural elements in viral fiber proteins: the triple beta-spiral and triple beta-helix

Apart from alpha-helical coiled coils and the collagen triple helices, fibrous proteins can contain beta-structure in various conformations. Elongated enzymes such as pectate lyase and the bacteriophage P22 tailspike protein contain single-stranded beta-helices. Virus and bacteriophage fibers, which are often trimeric, have been shown to contain novel triple-stranded beta-structures such as the triple beta-spiral and the triple beta-helix. The conformation and folding of viral fibers containing beta-structure are discussed.

Genetic retargeting of adenovirus: novel strategy employing "deknobbing" of the fiber

For efficient and versatile use of adenovirus (Ad) as an in vivo gene therapy vector, modulation of the viral tropism is highly desirable. In this study, a novel method to genetically alter the Ad fiber tropism is described. The knob and the last 15 shaft repeats of the fiber gene were deleted and replaced with an external trimerization motif and a new cell-binding ligand, in this case the integrin-binding motif RGD. The corresponding recombinant fiber retained the basic biological functions of the natural fiber, i.e., trimerization, nuclear import, penton formation, and ligand binding. The recombinant fiber bound to integrins but failed to react with antiknob antibody. For virus production, the recombinant fiber gene was rescued into the Ad genome at the exact position of the wild-type (WT) fiber to make use of the native regulation of fiber expression. The recombinant virus Ad5/FibR7-RGD yielded plaques on 293 cells, but the spread through the monolayer was two to three times delayed compared to WT, and the ratio of infectious to physical particles was 20 times lower. Studies on virus tropism showed that Ad5/FibR7-RGD was able to infect cells which did not express the coxsackie-adenovirus receptor (CAR), but did express integrins. Ad5/FibR7-RGD virus infectivity was unchanged in the presence of antiknob antibody, which neutralized the WT virus. Ad5/FibR7-RGD virus showed an expanded tropism, which is useful when gene transfer to cells not expressing CAR is needed. The described method should also make possible the construction of Ad genetically retargeted via ligands other than RGD.

3PO, a novel nonviral gene delivery system using engineered Ad5 penton proteins

This study describes the development of 3PO, a nonviral, protein-based gene delivery vector which utilizes the highly evolved cell-binding, cell-entry and intracellular transport functions of the adenovirus serotype 5 (Ad5) capsid penton protein. A penton fusion protein containing a polylysine sequence was produced by recombinant methods and tested for gene delivery capability. As the protein itself is known to bind integrins through a conserved consensus motif, the penton inherently possesses the ability to bind and enter cells through receptor-mediated internalization. The ability to lyse the cellular endosome encapsulating internalized receptors is also attributed to the penton. The recombinant protein gains the additional function of DNA binding and transport with the appendage of a polylysine motif. This protein retains the ability to form pentamers and mediates delivery of a reporter gene to cultured cells. Interference by oligopeptides bearing the integrin binding motif suggests that delivery is mediated specifically through integrin receptor binding and internalization. The addition of protamine to penton-DNA complexes allows gene delivery in the presence of serum.

Immunoreactive domains and integrin-binding motifs in adenovirus penton base capsomer

A panel of nine independent mouse monoclonal antibodies (MAbs) against penton base capsomers of subgenus C adenovirus serotypes 2 (Ad2) and 5 (Ad5) were isolated and characterized. Two of them (1D2 and 5A5), raised against Ad5 virion as the immunogen, bound to sodium dodecyl sulfate (SDS)-resistant and subgenus C-specific epitopes that were not present in subgenus B Ad3 penton base. The 1D2 and 5A5 epitopes were mapped to two distinct regions that did not belong to the main variable region carrying the integrin-binding RGD motif at position 340. For the other seven MAbs, raised against recombinant Ad2 penton base protein (9S-pentamers), the epitopes were sensitive to SDS-denaturation, but reacted with native Ad2, Ad5, and Ad3 penton base. The epitopes recognized by the nine MAbs and by polyclonal antipenton base antibodies defined three major immunoreactive regions. One (I) mapped to the N-terminal domain (residues 116-165); the other two regions were almost symmetrically disposed on both sides of the integrin-binding RGD motif at position 340, within residues 248-270 (II), and within residues 368-427 (III) in the C-terminal domain. Region II overlapped the fiber-binding site in penton base (residues 254-260). None of the MAbs showed any detectable virus neutralization effect, but they all slightly augmented the efficiency of Ad-mediated gene transfer. Although none of their epitopes included the RGD-340 tripeptide, substitutions of the arginine residue in the RGD motif abolished the reactivity of six individual and distant epitopes, suggesting a major conformational role for the RGD-containing domain.

Adenovirus type 5 viral particles pseudotyped with mutagenized fiber proteins show diminished infectivity of coxsackie B-adenovirus receptor-bearing cells

A major limitation of adenovirus type 5 (Ad5)-based gene therapy, the inability to target therapeutic genes to selected cell types, is attributable to the natural tropism of the virus for the widely expressed coxsackievirus-adenovirus receptor (CAR) protein. Modifications of the Ad5 fiber knob domain have been shown to alter the tropism of the virus. We have developed a novel system to rapidly evaluate the function of modified fiber proteins in their most relevant context, the adenoviral capsid. This transient transfection/infection system combines transfection of cells with plasmids that express high levels of the modified fiber protein and infection with Ad5.beta gal.Delta F, an E1-, E3-, and fiber-deleted adenoviral vector encoding beta-galactosidase. We have used this system to test the adenoviral transduction efficiency mediated by a panel of fiber protein mutants that were proposed to influence CAR interaction. A series of amino acid modifications were incorporated via mutagenesis into the fiber expression plasmid, and the resulting fiber proteins were subsequently incorporated onto adenoviral particles. Mutations located in the fiber knob AB and CD loops demonstrated the greatest reduction in fiber-mediated gene transfer in HeLa cells. We also observed effects on transduction efficiency with mutations in the FG loop, indicating that the binding site may extend to the adjacent monomer in the fiber trimer and in the HI loop. These studies support the concept that modification of the fiber knob domain to diminish or ablate CAR interaction should result in a detargeted adenoviral vector that can be combined simultaneously with novel ligands for the development of a systemically administered, targeted adenoviral vector.

Recombinant adenovirus vectors with knobless fibers for targeted gene transfer

Adenoviral vector systems for gene therapy can be much improved by targeting vectors to specific cell types. This requires both the complete ablation of native adenovirus tropism and the introduction of a novel binding affinity in the viral capsid. We reasoned that these requirements could be fulfilled by deleting the entire knob domain of the adenovirus fiber protein and replacing it with two distinct moieties that provide a trimerization function for the knobless fiber and specific binding to the target cell, respectively. To test this concept, we constructed adenoviral vectors carrying knobless fibers comprising the alpha-helix trimerization domain from MoMuLV envelope glycoprotein. Two mimic targeting ligands, a Myc-epitope and a 6His-tag, were attached via a flexible linker peptide. The targeted knobless fiber molecules were properly expressed and imported into the nucleus of adenovirus packaging cells, where they were incorporated as functional trimers into the adenovirus capsid. Both ligands were exposed on the surface of the virion and were available for specific binding to their target molecules. Moreover, the knobless fibers mediated gene delivery into cells displaying receptors for the coupled ligand. Hence, these knobless fibers are prototype substrates for versatile addition of targeting ligands to generate truly targeted adenoviruses.

Targetable gene delivery vectors

Adenoviral vectors, which have targeting ligands for tumor cells on the capsid, no natural tropism, and carry a therapeutic payload should be constructed soon and tested in pre-clinical models. Nevertheless, there are still important considerations for the design and therapeutic use of targetable vectors. Perhaps the single greatest challenge in the future, as it was in the past, will be finding ligands that have a higher apparent affinity for tumor and/or tumor endothelial cells then normal cells. However, the advent of many rapidly advancing technologies and information including the sequencing of the human genome, in vivo and in vitro phage display, rapid analysis of gene and protein expression in any context, and new cellular targets such as angiogenic endothelial cells, may provide many opportunities for the discovery of novel and useful ligands. In addition, the interests in targeting vectors are rapidly growing with new journals and meetings solely devoted to this subject increasing annually. Within the next 5 years, we should have meaningful clinical data on targetable vectors to reassess our progress.

Strategies to adapt adenoviral vectors for targeted delivery

The utility of current generation adenoviral vectors for targeted, cell-specific gene delivery is limited by the promiscuous tropism of the parent virus. To address this issue, we have developed both genetic and immunologic methods to alter viral tropism. Immunologic retargeting has been achieved via conjugates comprised of an antifiber knob Fab and a targeting moiety consisting of a ligand or antireceptor antibody. Gene delivery by this approach has been accomplished via a variety of cellular pathways including receptors for folate, FGF, and EGF. In addition to cell-specific gene delivery, this strategy has allowed enhanced gene delivery to target cells lacking the native adenoviral receptor, CAR. Of note, this specific and extended gene delivery allowed enhanced survival in murine models of human carcinoma via cancer gene therapy. Genetic strategies to alter adenoviral tropism have included both fiber modification and fiber replacement. In the former, we have identified the HI loop of fiber as a propitious locale for introduction of heterologous peptides. Incorporation of an RGDC peptide at this locale allowed gene delivery via cellular integrins with dramatic efficiency augmentations. As a strategy to achieve both new tropism as well as to ablate native tropism, methods have been developed to replace the fiber protein with heterologous motif which preserves the key trimeric quaternary structure of fiber and allows for propagation. Such a fiber-replacement virus has been rescued and has demonstrated capacities consistent with its utility as a novel vector agent. These strategies have allowed the achievement of cell-specific gene delivery via adenoviral vectors and thus have the potential to enhance the utility of this vector agent.

Identification of contact residues and definition of the CAR-binding site of adenovirus type 5 fiber protein

The binding of adenovirus (Ad) fiber knob to its cellular receptor, the coxsackievirus and Ad receptor (CAR), promotes virus attachment to cells and is a major determinant of Ad tropism. Analysis of the kinetics of binding of Ad type 5 (Ad5) fiber knob to the soluble extracellular domains of CAR together (sCAR) and each immunoglobulin (Ig) domain (IgV and IgC2) independently by surface plasmon resonance demonstrated that the IgV domain is necessary and sufficient for binding, and no additional membrane components are required to confer high-affinity binding to Ad5 fiber knob. Four Ad5 fiber knob mutations, Ser408Glu and Pro409Lys in the AB loop, Tyr477Ala in the DG loop, and Leu485Lys in beta strand F, effectively abolished high-affinity binding to CAR, while Ala406Lys and Arg412Asp in the AB loop and Arg481Glu in beta strand E significantly reduced the level of binding. Circular dichroism spectroscopy showed that these mutations do not disorder the secondary structure of the protein, implicating Ser408, Pro409, Tyr477, and Leu485 as contact residues, with Ala406, Arg412, and Arg481 being peripherally or indirectly involved in CAR binding. The critical residues have exposed side chains that form a patch on the surface, which thus defines the high-affinity interface for CAR. Additional site-directed mutagenesis of Ad5 fiber knob suggests that the binding site does not extend to the adjacent subunit or toward the edge of the R sheet. These findings have implications for our understanding of the biology of Ad infection, the development of novel Ad vectors for targeted gene therapy, and the construction of peptide inhibitors of Ad infection.

A peptide from the adenovirus fiber shaft forms amyloid-type fibrils

The fiber protein of adenovirus consists of a C-terminal globular head, a shaft and a short N-terminal tail. The crystal structure of a stable domain comprising the head plus a part of the shaft of human adenovirus type 2 fiber has recently been solved at 2.4 A resolution [van Raaij et al. (1999) Nature 401, 935-938]. A peptide corresponding to the portion of the shaft immediately adjacent to the head (residues 355-396) has been synthesized chemically. The peptide failed to assemble correctly and instead formed amyloid-type fibrils as assessed by electron microscopy, Congo red binding and X-ray diffraction. Peptides corresponding to the fiber shaft could provide a model system to study mechanisms of amyloid fibril formation.