HSV has been a model virus for many studies in molecular biology. For instance, one of the first functional
promoters in
eukaryotes was discovered in HSV (of the
thymidine kinase gene) and the virion protein
VP16 is one of the most-studied
transcriptional activators.
Viral structure Animal herpes viruses all share some common properties. The structure of herpes viruses consists of a relatively large, double-stranded, linear
DNA genome encased within an
icosahedral protein cage called the
capsid, which is wrapped in a
lipid bilayer called the
envelope. The envelope is joined to the capsid through a
tegument. This complete particle is known as the
virion. HSV-1 and HSV-2 each contain at least 74 genes (or
open reading frames, ORFs) within their genomes, although speculation over gene crowding allows as many as 84 unique protein coding genes by 94 putative ORFs. These genes encode a variety of proteins involved in forming the capsid, tegument and envelope of the virus, as well as controlling the replication and infectivity of the virus. These genes and their functions are summarized in the table below. The genomes of HSV-1 and HSV-2 are complex and contain two unique regions called the long unique region (UL) and the short unique region (US). Of the 74 known ORFs, UL contains 56 viral genes, whereas US contains only 12. that regulate the expression of early and late viral genes, are the first to be expressed following infection.
Early gene expression follows, to allow the synthesis of
enzymes involved in
DNA replication and the production of certain
envelope glycoproteins. Expression of late genes occurs last; this group of genes predominantly encodes proteins that form the virion particle. The envelope covering the virus particle then fuses with the cell membrane, creating a pore through which the contents of the viral envelope enters the host cell. The sequential stages of HSV entry are analogous to
those of other viruses. At first, complementary receptors on the virus and the cell surface bring the viral and cell membranes into proximity. Interactions of these molecules then form a stable entry pore through which the viral envelope contents are introduced to the host cell. The virus can also be
endocytosed after binding to the receptors, and the fusion could occur at the
endosome. In electron micrographs, the outer leaflets of the viral and cellular lipid bilayers have been seen merged; this hemifusion may be on the usual path to entry or it may usually be an arrested state more likely to be captured than a transient entry mechanism. In the case of a herpes virus, initial interactions occur when two viral envelope glycoproteins called glycoprotein C (gC) and glycoprotein B (gB) bind to a cell surface
polysaccharide called
heparan sulfate. Next, the major receptor binding protein, glycoprotein D (gD), binds specifically to at least one of three known entry receptors. These cell receptors include herpesvirus entry mediator (
HVEM),
nectin-1 and 3-O sulfated heparan sulfate. The nectin receptors usually produce cell-cell adhesion, to provide a strong point of attachment for the virus to the host cell. Once attached to the nucleus at a nuclear entry pore, the capsid ejects its DNA contents via the capsid portal. The capsid portal is formed by 12 copies of the portal protein, UL6, arranged as a ring; the proteins contain a
leucine zipper sequence of
amino acids, which allow them to adhere to each other. Each
icosahedral capsid contains a single portal, located in one
vertex. The DNA exits the capsid in a single linear segment.
Host cell chromatin changes and molecular hijacking During lytic infection, HSV-1 can substantially alter host chromatin organization. Infections have been shown to induce global compaction of host chromatin and its redistribution toward the nuclear periphery. While viral DNA accumulates in viral replication compartments (VRCs). Within these compartments the virus recruits host transcriptional and topological machinery, including RNA polymerase II, Topoisomerase I, and the Cohesin complex, effectively sequestering them from host genes. This redistribution correlates with reduced host transcription and disruption of higher-order chromatin features such as chromatin loops and Topologically associating domains (TADs), although the broader separation between euchromatin and heterochromatin compartments is largely maintained.
Immune evasion HSV evades the immune system through interference with
MHC class I antigen presentation on the cell surface, by blocking the
transporter associated with antigen processing (TAP) induced by the secretion of
ICP-47 by HSV. In the host cell, TAP transports digested viral antigen epitope peptides from the cytosol to the endoplasmic reticulum, allowing these epitopes to be combined with MHC class I molecules and presented on the surface of the cell. Viral epitope presentation with MHC class I is a requirement for the activation of cytotoxic T-lymphocytes (CTLs), the major effectors of the cell-mediated immune response against virally infected cells. ICP-47 prevents the initiation of a CTL-response against HSV, allowing the virus to survive for a protracted period in the host. HSV usually produces cytopathic effect (CPE) within 24–72 hours post-infection in permissive cell lines which is observed by classical plaque formation. However, HSV-1 clinical isolates have also been reported that did not show any CPE in Vero and A549 cell cultures over several passages with low levels of virus protein expression. Probably these HSV-1 isolates are evolving towards a more "cryptic" form to establish chronic infection thereby unravelling yet another strategy to evade the host immune system, besides neuronal latency.
Replication showing the viral
cytopathic effect of HSV (multinucleation, ground glass chromatin) Following the infection of a cell, a cascade of herpes virus proteins, called immediate-early,
early, and late, is produced. Research using
flow cytometry on another member of the herpes virus family,
Kaposi's sarcoma-associated herpesvirus, indicates the possibility of an additional
lytic stage, delayed-late. These stages of lytic infection, particularly late lytic, are distinct from the latency stage. In the case of HSV-1, no protein products are detected during latency, whereas they are detected during the lytic cycle. The early proteins transcribed are used in the regulation of genetic replication of the virus. On entering the cell, an α-TIF protein joins the viral particle and aids in immediate-early
transcription. The virion host shutoff protein (VHS or UL41) is very important to viral replication. This enzyme shuts off protein synthesis in the host, degrades host
mRNA, helps in viral replication, and regulates
gene expression of viral proteins. The viral genome immediately travels to the nucleus, but the VHS protein remains in the cytoplasm. The late proteins form the capsid and the receptors on the surface of the virus. Packaging of the viral particles — including the
genome, core, and capsid - occurs in the nucleus of the cell. Here,
concatemers of the viral genome are separated by cleavage and are placed into formed capsids. HSV-1 undergoes a process of primary and secondary envelopment. The primary envelope is acquired by budding into the inner nuclear membrane of the cell. This then fuses with the outer nuclear membrane. The virus acquires its final envelope by budding into cytoplasmic
vesicles.
Latent infection HSVs may persist in a quiescent but persistent form known as latent infection, notably in
neural ganglia. HSV-1 tends to reside in the
trigeminal ganglia, while HSV-2 tends to reside in the
sacral ganglia, but these are historical tendencies only. During latent infection of a cell, HSVs express
latency-associated transcript (LAT)
RNA. LAT regulates the host cell genome and interferes with natural cell death mechanisms. By maintaining the host cells, LAT expression preserves a reservoir of the virus, which allows subsequent, usually symptomatic, periodic recurrences or "outbreaks" characteristic of non-latency. Whether or not recurrences are symptomatic, viral shedding occurs to infect a new host. A protein found in neurons may bind to herpes virus DNA and regulate
latency. Herpes virus DNA contains a gene for a protein called ICP4, which is an important
transactivator of genes associated with lytic infection in HSV-1. Elements surrounding the gene for ICP4 bind a protein known as the human neuronal protein neuronal restrictive silencing factor (NRSF) or
human repressor element silencing transcription factor (REST). When bound to the viral DNA elements,
histone deacetylation occurs atop the
ICP4 gene sequence to prevent initiation of transcription from this gene, thereby preventing transcription of other viral genes involved in the lytic cycle. Another HSV protein reverses the inhibition of ICP4 protein synthesis.
ICP0 dissociates NRSF from the
ICP4 gene and thus prevents silencing of the viral DNA.
Genome The HSV genome spans about 150,000 bp and consists of two unique segments, named unique long (UL) and unique short (US), as well as
terminal inverted repeats found to the two ends of them named repeat long (RL) and repeat short (RS). There are also minor "terminal redundancy" (α) elements found on the further ends of RS. The overall arrangement is RL-UL-RL-α-RS-US-RS-α with each pair of repeats inverting each other. The whole sequence is then encapsulated in a terminal direct repeat. The long and short parts each have their own
origins of replication, with OriL located between UL28 and UL30 and OriS located in a pair near the RS. As the L and S segments can be assembled in any direction, they can be inverted relative to each other freely, forming various linear isomers.
Gene expression HSV genes are expressed in 3 temporal classes: immediate early (IE or α), early (E or ß), and late (γ) genes. However, the progression of viral
gene expression is rather gradual than in clearly distinct stages. Immediate early genes are transcribed right after infection and their gene products activate transcription of the early genes. Early gene products help to replicate the viral DNA. Viral
DNA replication, in turn, stimulates the expression of the late genes, encoding the structural proteins. Transcription of the immediate early (IE) genes begins right after virus DNA enters the nucleus. All virus genes are transcribed by the host
RNA polymerase II. Although host proteins are sufficient for virus transcription, viral proteins are necessary for the transcription of certain genes. For instance, VP16 plays an important role in IE transcription and the virus particle brings it into the host cell, so that it does not need to be produced first. Similarly, the IE proteins RS1 (ICP4), UL54 (ICP27), and ICP0 promote the transcription of the early (E) genes. Like IE genes, early gene promoters contain binding sites for cellular transcription factors. One early protein, ICP8, is necessary for both transcription of late genes and DNA replication. Later in the life cycle of HSV, the expression of immediate early and early genes is shut down. This is mediated by specific virus proteins, e.g. ICP4, which represses itself by binding to elements in its promoter. As a consequence, the down-regulation of ICP4 levels leads to a reduction of early and late gene expression, as ICP4 is important for both. Importantly, HSV shuts down host cell RNA, DNA, and protein synthesis to direct cellular resources to virus production. First, the virus protein VHS induces the degradation of existing
mRNAs early in infection. Other viral genes impede cellular transcription and translation. For instance, ICP27 inhibits
RNA splicing, so that virus mRNAs (which are usually not spliced) gain an advantage over host mRNAs. Finally, virus proteins destabilize certain cellular proteins involved in the host
cell cycle, so that both cell division and host cell DNA replication are disturbed in favor of virus replication. ==Evolution==