Structural organization and polypeptide composition of the avian adenovirus core

The Natural Large Genomic Deletion Is Unrelated to the Increased Virulence of the Novel Genotype Fowl Adenovirus 4 Recently Emerged in China

Since 2015, severe hydropericardium-hepatitis syndrome (HHS), caused by a highly pathogenic fowl adenovirus 4 (FAdV-4), emerged in China. In our previous study, the FAdV-4 has been identified as a novel genotype with a unique 1966-bp nucleotide deletion (1966Del) between open reading frame 42 and 43. In this study, the natural 1966Del was frequently identified among 17 clinical isolates and other reported Chinese clinical strains. To investigate the relationship between 1966Del and the increased virulence of the novel FAdV-4, a CRISPR/Cas9 operating platform for FAdV-4 was developed for the first time in this study. Based on this platform, a Re1966 strain was rescued, inserted the relative 1966Del sequence of a nonpathogenic strain KR5. In the pathogenicity study, the Re1966 strain retained high virulence for specific-pathogen-free chickens, similar to the parental wild-type HLJFAd15, although the survival time of chickens infected with Re1966 was much longer. Therefore, the natural 1966Del was identified as a non-essential site for the increased virulence of the emerged novel FAdV-4. Although further research on the virulence-determining region or point within the genome of the novel FAdV-4 is needed, the CRISPR/Cas9 operating platform for the novel FAdV-4 was developed and successfully applied to edit the genomic DNA for the first time, and it provides a novel powerful tool for both basic virology studies and vaccine vector development of FAdVs.

Development of nonhuman adenoviruses as vaccine vectors

Human adenoviral (HAd) vectors have demonstrated great potential as vaccine vectors. Preclinical and clinical studies have demonstrated the feasibility of vector design, robust antigen expression and protective immunity using this system. However, clinical use of adenoviral vectors for vaccine purposes is anticipated to be limited by vector immunity that is either preexisting or develops rapidly following the first inoculation with adenoviral vectors. Vector immunity inactivates the vector particles and rapidly removes the transduced cells, thereby limiting the duration of transgene expression. Due to strong vector immunity, subsequent use of the same vector is usually less efficient. In order to circumvent this limitation, nonhuman adenoviral vectors have been proposed as alternative vectors. In addition to eluding HAd immunity, these vectors possess most of the attractive features of HAd vectors. Several replication-competent or replication-defective nonhuman adenoviral vectors have been developed and investigated for their potential as vaccine-delivery vectors. Here, we review recent advances in the design and characterization of various nonhuman adenoviral vectors, and discuss their potential applications for human and animal vaccination.

Structural studies on adenoviruses

The adenovirus genome encodes more than 40 proteins, of which 11 combine with the viral DNA to form an icosahedral capsid of approximately 150 MDa molecular weight and approximately 900 A in diameter. This chapter reviews the information that structural biology techniques have provided about the adenovirus proteins and capsid. The structures of two capsid proteins (hexon and fiber) and two non-structural polypeptides (DNA-binding protein and protease) have been solved by X-ray crystallography. Fiber and its knob have been the focus of the latest structural studies, due to their role in host recognition and consequently in virus targeting for human gene therapy. The current model for the large capsid comes from a combination of electron microscopy and crystallography. The resultant images have revealed a surprising similarity between adenovirus and a bacterial virus, which suggests their common evolutionary origin.

Construction of avian adenovirus CELO recombinants in cosmids

The avian adenovirus CELO is a promising vector for gene transfer applications. In order to study this potentiality, we developed an improved method for construction of adenovirus vectors in cosmids that was used to engineer the CELO genome. For all the recombinant viruses constructed by this method, the ability to produce infectious particles and the stability of the genome were evaluated in a chicken hepatocarcinoma cell line (LMH cell line). Our aim was to develop a replication-competent vector for vaccination of chickens, so we first generated knockout point mutations into 16 of the 22 unassigned CELO open reading frames (ORFs) to determine if they were essential for virus replication. As the 16 independent mutant viruses replicated in our cellular system, we constructed CELO genomes with various deletions in the regions of these nonessential ORFs. An expression cassette coding for the enhanced green fluorescent protein (eGFP) was inserted in place of these deletions to easily follow expression of the transgene and propagation of the vector in cell monolayers. Height-distinct GFP-expressing CELO vectors were produced that were all replication competent in our system. We then retained the vector backbone with the largest deletion (i.e., 3.6 kb) for the construction of vectors carrying cDNA encoding infectious bursal disease virus proteins. These CELO vectors could be useful for vaccination in the chicken species.

Characterization of CELO virus proteins that modulate the pRb/E2F pathway

The avian adenovirus CELO can, like the human adenoviruses, transform several mammalian cell types, yet it lacks sequence homology with the transforming, early regions of human adenoviruses. In an attempt to identify how CELO virus activates the E2F-dependent gene expression important for S phase in the host cell, we have identified two CELO virus open reading frames that cooperate in activating an E2F-inducible reporter system. The encoded proteins, GAM-1 and Orf22, were both found to interact with the retinoblastoma protein (pRb), with Orf22 binding to the pocket domain of pRb, similar to other DNA tumor virus proteins, and GAM-1 interacting with pRb regions outside the pocket domain. The motif in Orf22 responsible for the pRb interaction is essential for Orf22-mediated E2F activation, yet it is remarkably unlike the E1A LxCxD and may represent a novel form of pRb-binding peptide.

Mutational analysis of the avian adenovirus CELO, which provides a basis for gene delivery vectors

The avian adenovirus CELO is being developed as a gene transfer tool. Using homologous recombination in Escherichia coli, the CELO genome was screened for regions that could be deleted and would tolerate the insertion of a marker gene (luciferase or enhanced green fluorescent protein). For each mutant genome, the production of viable virus able to deliver the transgene to target cells was monitored. A series of mutants in the genome identified a set of open reading frames that could be deleted but which must be supplied in trans for virus replication. A region of the genome which is dispensable for viral replication and allows the insertion of an expression cassette was identified and a vector based on this mutation was evaluated as a gene delivery reagent. Transduction of avian cells occurs at 10- to 100-fold greater efficiency (per virus particle) than with an adenovirus type 5 (Ad5)-based vector carrying the same expression cassette. Most important for gene transfer applications, the CELO vector transduced mammalian cells as efficiently as an Ad5 vector. The CELO vector is exceptionally stable, can be grown inexpensively in chicken embryos, and provides a useful alternative to Ad5-based vectors.

Transcriptional organization of the avian adenovirus CELO

A detailed map of the transcriptional organization of the CELO virus genome was produced. Recent computer analysis of CELO virus has indicated the presence of 38 putative open reading frames (ORFs). This study, based on analysis of the transcriptional products of CELO in vitro, confirmed the presence of RNAs for 26 of these 38 ORFs. All of the results were obtained by cDNA isolation or specific reverse transcriptase PCR. Observation of ORF transcription kinetics postinfection revealed the existence of early and late expression, with the earliest starting at 2 h postinfection. The 5' untranslated regions of some RNAs were also studied, and this revealed the existence of a bipartite leader sequence, comparable in structure to the tripartite leader of mastadenovirus. The leader most probably involved in transcriptional activity was observed in most of the structural protein genes of the CELO virus genome. This suggests some homology in transcriptional organization between the avian adenovirus CELO and known mastadenoviruses such as human adenovirus.

The complete DNA sequence and genomic organization of the avian adenovirus CELO

The complete DNA sequence of the avian adenovirus chicken embryo lethal orphan (CELO) virus (FAV-1) is reported here. The genome was found to be 43,804 bp in length, approximately 8 kb longer than those of the human subgenus C adenoviruses (Ad2 and Ad5). This length is supported by pulsed-field gel electrophoresis analysis of genomes isolated from several related FAV-1 isolates (Indiana C and OTE). The genes for major viral structural proteins (Illa, penton base, hexon, pVI, and pVIII), as well as the 52,000-molecular-weight (52K) and 100K proteins and the early-region 2 genes and IVa2, are present in the expected locations in the genome. CELO virus encodes two fiber proteins and a different set of the DNA-packaging core proteins, which may be important in condensing the longer CELO virus genome. No pV or pIX genes are present. Most surprisingly, CELO virus possesses no identifiable E1, E3, and E4 regions. There is 5 kb at the left end of the CELO virus genome and 15 kb at the right end with no homology to Ad2. The sequences are rich in open reading frames, and it is likely that these encode functions that replace the missing El, E3, and E4 functions.

Characterization of the structural proteins of hemorrhagic enteritis virus

The structural proteins of hemorrhagic enteritis (HEV), a turkey adenovirus, were analyzed by polyacrylamide gel electrophoresis (PAGE) and Western blotting using polyspecific, monospecific and monoclonal antibodies for detection. In purified HEV preparations, eleven polypeptides with apparent molecular weights ranging from 96,000 to 9,500 (96k to 9.5k), were specifically recognized by convalescent turkey serum. Six of these polypeptides were further characterized by PAGE, Western blotting, ELISA, sucrose gradient centrifugation and electron microscopy. The 96k polypeptide was identified as the hexon polypeptide which is a monomer of the major outer capsid or hexon protein. The 51/52k and 29k polypeptides, identified as the penton base and fiber polypeptides respectively, were the components of the vertex or penton protein. The 57k polypeptide was identified as a homologue of the human adenovirus type 2 (Ad 2) IIIa protein with which it shares a common epitope. Two core proteins with molecular weights of 12.5 and 9.5k were present in purified HEV nucleoprotein cores. The proteins of two HEV isolates, one apathogenic (HEV-A) and one virulent (HEV-V), resembled each other in most respects. However, differences between HEV-A and HEV-V were found in electrophoretic migration of the penton base protein both under native and denatured conditions, and in the electrophoretic migration of the 43/44k polypeptide. Moreover, homologous antiserum against the fiber protein reacted stronger than heterologous antiserum in an ELISA. Single fibers were detected by electron microscopy attached to the penton base proteins of HEV virions and in isolated pentons. The feature of having single fibers is shared with the mammalian adenoviruses and the avian egg drop syndrome 1976 virus (EDS 76 V), but not with the fowl adenoviruses which have double fibers attached to their penton base proteins.

Expression of fowl adenovirus type 10 antigens in Escherichia coli

In an attempt to construct a genetic map of the fowl adenovirus (FAV) and to determine which viral proteins are natural immunogens in chickens and hence may be relevant to protective immunity we have constructed an expression library of FAV type 10 DNA. The genomic DNA was partially digested with the restriction endonuclease Sau3A, and this DNA was inserted into the 3' terminal end of the beta-galactosidase gene in a plasmid vector. To date, approximately 600 clones have been identified that express FAV type 10 antigens as determined by immunological screening with rabbit antisera to purified virus, including one that has amino acid homology with the 100 kDa protein of human adenovirus type 5. These antigen positive clones were found to contain DNA from FAV type 10 genome as determined by hybridisation to FAV DNA. These clones will allow the further characterisation of FAV and possibly the identification of potential vaccine molecules.

Isolation and characterization of adenovirus core nucleoprotein subunits

Digestion of adenovirus type 2 (Ad2) or Ad5 cores with micrococcal nuclease generated four nucleoprotein species that could be resolved by electrophoresis in low-ionic-strength polyacrylamide gels: these nucleoproteins displayed mobilities equivalent to those of DNA fragments of 900 to 1,025, 775 to 850, 650 to 725, and 525 to 600 base pairs (bp) and thus were readily distinguishable from HeLa cell mononucleosomes. The DNA fragments associated with the core nucleoprotein species were more than 250 to 90 bp long. Nucleoproteins containing 150, 120, or 90 bp of DNA were the most stable. Polypeptide VII was associated with each of the nucleoprotein species liberated from Ad2 cores. These data suggest that polypeptide VII and viral DNA of 90 to 150 bp comprise the unit particle of the Ad2 or Ad5 core nucleoproteins.

VA RNAs from avian and human adenoviruses: dramatic differences in length, sequence, and gene location

Human adenoviruses encode low-molecular-weight RNAs, so-called VA RNAs, which are transcribed by RNA polymerase III. These RNAs are required for an efficient translation of viral mRNAs late after infection. The genes for the VA RNAs in the genome of CELO virus were mapped and characterized. The results showed a number of surprising differences between CELO virus and human adenovirus type 2 (Ad2). Thus, the CELO virus genome encoded only one VA RNA species, in contrast to human Ad2, which encoded two distinct species. The VA RNA from CELO virus was much shorter than the Ad2 VA RNAs (90 nucleotides compared with 160 nucleotides), and there existed no detectable primary sequence homology between them. The predicted secondary structure of CELO virus VA RNA was, however, similar to that of the Ad2 VA RNAs, implying that the folding rather than the primary sequence was the important feature for biological activity. CELO VA RNA also stimulated translation in a transient expression assay, as did the Ad2 counterparts, albeit with a much lower efficiency. The location of the gene for CELO VA RNA also differed from all previously characterized serotypes, suggesting that the genome organization of avian and human adenoviruses are different. Finally, termination of CELO VA RNA transcription occurred in a TTATT sequence which is unique as a stop signal for RNA polymerase III transcription.

Ionic effects on the structure of nucleoprotein cores from adenovirus

Nucleoprotein cores, prepared from adenovirus type 5 with a deoxycholate/heat treatment, consist of the viral DNA and two major internal proteins. The core particles exhibit structural characteristics that are highly reproducible and dependent on their ionic environment. In low-ionic-strength buffer, the cores had a sedimentation coefficient of 180 S and appeared in the electron microscope as homogeneous particles with distinct centers from which numerous arms and loops radiated. Condensation of the cores was induced by Mg2+ or Ca2+ over the range 0 to 1 mM. The sedimentation coefficient increased monotonically with divalent cation concentration, reaching a maximum of 405 S in 1 mM Mg2+. A corresponding condensation in the core structure was observed by electron microscopy. Increasing concentrations of NaCl also produced a conformational change in the cores, with an almost linear increase in sedimentation velocity up to 274 S in 0.04 M NaCl. Between 0.05 and 1.0 M NaCl, the cores were insoluble. In 2.0 M NaCl, the cores were again soluble with an s20,w of 228 S. Under all ionic strength conditions in which the cores were soluble, both core proteins remained bound to the DNA.