Adenoviruses
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Adenoviruses are a frequent cause of acute upper respiratory tract infections, i.e. “colds”. In addition, they also cause a number of other types of infection. They were first isolated in 1953, but in 1962, some adenoviruses were shown to cause tumours in rodents – this caused a considerable panic! However adenovirus oncogenesis appears to be associated with abortive infections and has never been observed in humans. There are at least 51 human adenovirus serotypes which have been divided into subgroups A to F.
Detailed three dimensional structural models of adenovirus particles have been constructed based on a combination of cryoelectron microscopy and X-ray crystallography. There are at least 13 proteins in the adenovirus particle. All adenovirus particles are structurally similar: non-enveloped and 60-90 nm in diameter. They have icosahedral symmetry easily visible in the electron microscope by negative staining and are composed of 252 capsomers: 240 “hexons” + 12 “pentons” at vertices of icosahedron. The pentons have a toxin-like activity, purified pentons causing c.p.e. in the absence of any other virus components. A trimeric fibre protein extends from each of the 12 vertices (attached to the penton base proteins) and is responsible for recognition and binding to the cellular receptor. A globular domain at the end of the adenovirus fiber is responsible for recognition of the cellular receptor.
The adenovirus genome consists of linear, non-segmented, double-stranded DNA, 30-38 kbp long which has the capacity to encode about 40 genes. The terminal sequences of each strand are inverted repeats, hence the denatured single strands can form “panhandle” structures (100-140 bp long). There is a 55kD protein called the terminal protein (TP) covalently attached to the 5′ end of each strand.
The replication of all adenoviruses is similar and occurs in the nucleus of the host cell. Replication is divided into early and late phases, the late phase defined as beginning with the onset of DNA replication. This temporal division is characteristic of the replication of DNA viruses.
Attachment to cells is rather slow, taking several hours to reach a maximum. Uptake of the particle is a two stage process involving an initial interaction of the fibre protein with a range of cellular receptors, which include the MHC class I molecule and the coxsackievirus-adenovirus receptor. The penton base protein then binds to the integrin family of cell surface heterodimers allowing internalization via receptor-mediated endocytosis. Most cells express receptors for the adenovirus fibre coat protein, however internalisation is a more selective process. Penetration involves phagocytosis into phagocytic vacuoles, after which the toxic activity of the pentons is responsible for rupture of the phagocytic membrane and release of the particle into the cytoplasm. Uncoating follows an ordered sequence, first the pentons, releasing a spherical, partially uncoated particle into the cytoplasm. The core migrates to the nucleus where the DNA enters through nuclear pores, where it is converted into a virus DNA-cellular histone complex.
Before and independently of genome replication, immediate early and early mRNAs are transcribed from the input DNA. Transcription of the adenovirus genome is regulated by virus-encoded trans-acting regulatory factors. Products of the immediate early genes regulate expression of the early genes. Early genes are encoded at various locations on both strands of the DNA (l = “leftward strand” and r = “rightward strand”). Multiple protein products are made from each gene by alternative splicing of mRNA transcripts – splicing was first discovered in adenoviruses by Philip Sharp in 1977.
The first mRNA/protein to be made (~1h after infection) is E1A. This protein is a trans-acting transcriptional regulatory factor which is necessary for transcriptional activation of early genes. The protein is also capable of activating transcription from a variety of other virus and cellular promoters and shows no sequence-specificity, rather it causes a modification of the cellular environment. The second protein made is E1B. E1A and E1B together (and independently of other virus proteins) are capable of transforming primary cells in vitro (especially in Ad5, Ad12).
Adenovirus DNA replication has been studied extensively. At least three virus-encoded proteins are known to be involved in DNA replication:
- TP – acts as a primer for initiation of synthesis
- Ad DBP – a DNA-binding protein
- Ad DNA Pol – 140kD DNA-dependent polymerase
In addition, many cellular proteins in the nucleus also participate in replication of the genome (e.g. NFI, NFII, topoisomerase I). The adenovirus genome has inverted terminal repeats (ITRs) of about 100 bp. Located within the ITRs are the cis-acting DNA sequences which define ori, the origin of DNA replication. Covalently attached to each 5′ end is a terminal protein (TP) which is an additional cis-acting component of ori.
At the onset of DNA replication, the pattern of transcription changes radically from the early to the late genes. Late phase transcription is driven primarily by the major late promoter. Although transcription from this promoter is complex involving multiple polyadenylation signals and an elaborate usage of RNA splicing, five gene clusters can be defined (L1-L5). Late phase gene expression is primarily concerned with the synthesis of virion proteins.
Particle assembly occurs in the nucleus, but begins in the cytoplasm when individual monomers form into hexon and penton capsomers. Empty, immature capsids are assembled from these protomers in the nucleus, where the core is formed from genomic DNA plus its associated core proteins.
Certain types of adenovirus are commonly associated with particular clinical syndromes. This list includes relatively common infections (at the top) and some rare infections (at the bottom). Most adenovirus infections involve either the respiratory or gastrointestinal tracts or the eye. Adenovirus infections are very common, most are asymptomatic. Most people have been infected with at least 1 type at age 15. Virus can be isolated from the majority of tonsils/adenoids surgically removed, indicating latent infections. It is not known how long the virus can persist in the body, or whether it is capable of reactivation after long periods, causing disease (it is hard to isolate this occult virus as it may be present in only a few cells). It is known that virus is reactivated during immunosuppression, e.g. in AIDS and transplant patients.
A characteristic feature of adenoviruses is their ability to subvert the host immune response by using the early E3 cassette of genes. The E3 membrane proteins are able to downregulate critical recognition structures for the host immune system from the cell surface.
There is no treatment available for adenovirus infections. Inactivated vaccines have been developed and are routinely used for military recruits in some countries (notably the USA). This is because adolescents and others in close daily contact are at risk for epidemic spread of respiratory infections, but the risk to general population is so low that vaccination is not a viable proposition.
In recent years, there has been considerable interest in developing adenoviruses as defective vectors to carry and express foreign genes for therapeutic purposes. One reason for this is that the adenovirus genome is relatively easily manipulated in vitro (c.f. retroviruses) and the genes coupled to the late promoter are efficiently expressed in large amounts. Adenoviruses have been studied intensively for over 50 years as models of virus-cell interactions and more recently as gene vectors.
Related:
- MicrobiologyBytes: Adenoviruses
- DNA Viruses
- Adenovirus vectors – new genes, new vaccines
- Adenoviruses: update on structure and function. J Gen Virol. 2009 90(1): 1-20
Tags: Biology, Health, Medicine, Microbiology, Podcast, Science, Virology
