Structure and sequence of the saimiriine herpesvirus 1 genome

Two Sides to Every Story: Herpes Simplex Type-1 Viral Glycoproteins gB, gD, gH/gL, gK, and Cellular Receptors Function as Key Players in Membrane Fusion

Herpes simplex virus type-1 (HSV-1) and type-2 (HSV-2) are prototypical alphaherpesviruses that are characterized by their unique properties to infect trigeminal and dorsal root ganglionic neurons, respectively, and establish life-long latent infections. These viruses initially infect mucosal epithelial tissues and subsequently spread to neurons. They are associated with a significant disease spectrum, including orofacial and ocular infections for HSV-1 and genital and neonatal infections for HSV-2. Viral glycoproteins within the virion envelope bind to specific cellular receptors to mediate virus entry into cells. This is achieved by the fusion of the viral envelope with the plasma membrane. Similarly, viral glycoproteins expressed on cell surfaces mediate cell-to-cell fusion and facilitate virus spread. An interactive complex of viral glycoproteins gB, gD/gH/gL, and gK and other proteins mediate these membrane fusion phenomena with glycoprotein B (gB), the principal membrane fusogen. The requirement for the virion to enter neuronal axons suggests that the heterodimeric protein complex of gK and membrane protein UL20, found only in alphaherpesviruses, constitute a critical determinant for neuronal entry. This hypothesis was substantiated by the observation that a small deletion in the amino terminus of gK prevents entry into neuronal axons while allowing entry into other cells via endocytosis. Cellular receptors and receptor-mediated signaling synergize with the viral membrane fusion machinery to facilitate virus entry and intercellular spread. Unraveling the underlying interactions among viral glycoproteins, envelope proteins, and cellular receptors will provide new innovative approaches for antiviral therapy against herpesviruses and other neurotropic viruses.

Sequence of the ateline alphaherpesvirus 1 (HVA1) genome

Here, we report the genome sequence of a spider monkey alphaherpesvirus (ateline alphaherpesvirus 1, HVA1) and compare it with that of other primate alphaherpesviruses. The HVA1 genome is 147,346 bp long and contains 67 predicted ORFs. The genetic layout of the HVA1 genome is similar to that of the squirrel monkey alphaherpesvirus (saimirine alphaherpesvirus 1, HVS1) in that it lacks inverted repeat regions flanking the unique long region and homologues of the UL43, UL49.5, US8.5 and US10-12 genes. Unlike HVS1, HVA1 also lacks a homologue of the RL1 (γ34.5) gene and a replication origin near the end of the genome. Consistent with previous phylogenetic analyses, all predicted proteins of HVA1 are most closely related to those of HVS1.

Understanding Primate Herpesviruses

Viruses related to the herpes simplex viruses of humans are present in all nonhuman primate (NHP) species tested and cross species transmission has been documented. The herpesvirus present in macaques, Herpes B virus (BV) rarely causes disease in its natural macaque host. However, when transmitted to a nonnative host, BV has occasionally caused severe and even fatal disease if not treated immediately. Here we present a comprehensive review of the taxonomy, molecular biology, physiology, epidemiology, diagnosis and treatment of BV. We also summarizes what is known about related herpesviruses of other NHP species and the zoonotic potential of these viruses.

Endemic Viruses of Squirrel Monkeys (Saimiri spp.)

Nonhuman primates are the experimental animals of choice for the study of many human diseases. As such, it is important to understand that endemic viruses of primates can potentially affect the design, methods, and results of biomedical studies designed to model human disease. Here we review the viruses known to be endemic in squirrel monkeys (Saimiri spp.). The pathogenic potential of these viruses in squirrel monkeys that undergo experimental manipulation remains largely unexplored but may have implications regarding the use of squirrel monkeys in biomedical research.

A Functional Interaction between Herpes Simplex Virus 1 Glycoprotein gH/gL Domains I and II and gD Is Defined by Using Alphaherpesvirus gH and gL Chimeras

Whereas most viruses require only a single protein to bind to and fuse with cells, herpesviruses use multiple glycoproteins to mediate virus entry, and thus communication among these proteins is required. For most alphaherpesviruses, the minimal set of viral proteins required for fusion with the host cell includes glycoproteins gD, gB, and a gH/gL heterodimer. In the current model of entry, gD binds to a cellular receptor and transmits a signal to gH/gL. This signal then triggers gB, the conserved fusion protein, to insert into the target membrane and refold to merge the viral and cellular membranes. We previously demonstrated that gB homologs from two alphaherpesviruses, herpes simplex virus 1 (HSV-1) and saimiriine herpesvirus 1 (SaHV-1), were interchangeable. In contrast, neither gD nor gH/gL functioned with heterotypic entry glycoproteins, indicating that gD and gH/gL exhibit an essential type-specific functional interaction. To map this homotypic interaction site on gH/gL, we generated HSV-1/SaHV-1 gH and gL chimeras. The functional interaction with HSV-1 gD mapped to the N-terminal domains I and II of the HSV-1 gH ectodomain. The core of HSV-1 gL that interacts with gH also was required for functional homotypic interaction. The N-terminal gH/gL domains I and II are the least conserved and may have evolved to support species-specific glycoprotein interactions.

IMPORTANCE

The first step of the herpesvirus life cycle is entry into a host cell. A coordinated interaction among multiple viral glycoproteins is required to mediate fusion of the viral envelope with the cell membrane. The details of how these glycoproteins interact to trigger fusion are unclear. By swapping the entry glycoproteins of two alphaherpesviruses (HSV-1 and SaHV-1), we previously demonstrated a functional homotypic interaction between gD and gH/gL. To define the gH and gL requirements for homotypic interaction, we evaluated the function of a panel of HSV-1/SaHV-1 gH and gL chimeras. We demonstrate that domains I and II of HSV-1 gH are sufficient to promote a functional, albeit reduced, interaction with HSV-1 gD. These findings contribute to our model of how the entry glycoproteins cooperate to mediate herpesvirus entry into the cell.

Genome sequence of a pathogenic isolate of monkey B virus (species Macacine herpesvirus 1)

The only genome sequence for monkey B virus (BV; species Macacine herpesvirus 1) is that of an attenuated vaccine strain originally isolated from a rhesus monkey (BVrh). Here we report the genome sequence of a virulent BV strain isolated from a cynomolgus macaque (BVcy). The overall genome organization is the same, although sequence differences exist. The greatest sequence divergence is located in non-coding areas of the long and short repeat regions. Like BVrh, BVcy has duplicated Ori elements and lacks an ORF corresponding to the γ34.5 gene of herpes simplex virus. Nine of ten miRNAs and the majority of ORFs are conserved between BVrh and BVcy. The most divergent genes are several membrane-associated proteins and those encoding immediate early proteins.

Substitution of herpes simplex virus 1 entry glycoproteins with those of saimiriine herpesvirus 1 reveals a gD-gH/gL functional interaction and a region within the gD profusion domain that is critical for fusion

To gain insight into the mechanism of herpesvirus entry into cells, the four glycoproteins that are necessary for herpes simplex virus (HSV) fusion were cloned from the saimiriine herpesvirus 1 (SaHV-1) genome, a primate member of the alphaherpesvirus family. Cell-cell fusion assays indicate that SaHV-1 entry glycoproteins function with the previously identified alphaherpesvirus entry receptors nectin-1 and CD155 but not with herpesvirus entry mediator (HVEM) or paired immunoglobulin-like type 2 receptor alpha (PILRα). Replacement of HSV-1 gD with the SaHV-1 gD homolog resulted in a complete loss of fusion function when coexpressed with HSV-1 gB and gH/gL. HSV-1 gD was also unable to substitute for SaHV-1 gD when coexpressed with SaHV-1 gB and gH/gL. Similarly, the gH/gL heterodimers from HSV-1 and SaHV-1 were not interchangeable. In contrast, both the HSV-1 and SaHV-1 gB homologs retained function in a heterotypic context. These results suggest that an essential interaction between homotypic gD and gH/gL occurs during both HSV-1 and SaHV-1 entry. To map the site of this homotypic interaction, we created a series of gD chimeras, focusing on the "profusion domain" (PFD) that consists of HSV-1 gD residues 261 to 305 or SaHV-1 gD residues 264 to 307. We identified a seven-amino-acid stretch (264 RTLPPPK 270) at the N terminus of the SaHV-1 gD PFD that contributes to homotypic fusion. Finally, we found that the gD receptor-binding region and PFD cannot function independently but that both can inhibit the function of wild-type gD.

IMPORTANCE

The herpesvirus entry machinery requires the concerted action of at least four glycoproteins; however, details of the interactions among these glycoproteins are not well understood. Like HSV-1, SaHV-1 belongs to the alphaherpesvirus subfamily. Using cell-cell fusion experiments, we found that SaHV-1 uses the entry receptors nectin-1 and CD155 but not HVEM or PILRα. By swapping the entry glycoproteins between HSV-1 and SaHV-1, we revealed a functional interaction between gD and gH/gL. To examine the homotypic interaction site on gD, we evaluated the function of a panel of HSV-1/SaHV-1 gD chimeras and identified a small region in the SaHV-1 gD profusion domain that is critical for SaHV-1 fusion. This study contributes to our understanding of the molecular mechanisms of herpesvirus entry and membrane fusion.

A single viral gene determines lethal cross-species neurovirulence of baboon herpesvirus HVP2

Alpha-herpesviruses can produce more severe infections in non-natural host species than in their natural host. Isolates of the baboon alpha-herpesvirus Papiine herpesvirus 2 (HVP2) are either very neurovirulent in mice (subtype nv) or non-virulent (subtype ap), but no such difference is evident in the natural baboon host. Comparative genome sequencing was used to identify subtype-specific sequence differences (SSDs) between HVP2nv and HVP2ap isolates. Some genes were identified that despite exhibiting sequence variation among isolates did not have any SSDs, while other genes had comparatively high levels of SSDs. Construction of genomic recombinants between HVP2nv and HVP2ap isolates mapped the mouse neurovirulence determinant to within three genes. Construction of gene-specific recombinants demonstrated that the UL39 ORF is responsible for determining the lethal neurovirulence phenotype of HVP2 in mice. These results demonstrate that differences in a single viral gene can determine the severity of herpesvirus infection in a non-natural host species.

Genome sequence of a chimpanzee herpesvirus and its relation to other primate alphaherpesviruses

This study reports the complete genome sequence of chimpanzee herpesvirus (ChHV), an alphaherpesvirus isolated from a chimpanzee. Although closely related to human herpes simplex virus type 2 (HSV2), the level of sequence diversity confirms that ChHV is sufficiently distinct to be considered a member of a different virus species rather than a variant strain of HSV2. Phylogenetic comparison with other simplexviruses at several levels supports the hypothesis that HSV2 and ChHV co-evolved with their respective human and chimpanzee hosts and raises questions regarding the evolutionary origins of HSV1.

The evolution of infectious agents in relation to sex in animals and humans: brief discussions of some individual organisms

The following series of concise summaries addresses the evolution of infectious agents in relation to sex in animals and humans from the perspective of three specific questions: (1) what have we learned about the likely origin and phylogeny, up to the establishment of the infectious agent in the genital econiche, including the relative frequency of its sexual transmission; (2) what further research is needed to provide additional knowledge on some of these evolutionary aspects; and (3) what evolutionary considerations might aid in providing novel approaches to the more practical clinical and public health issues facing us currently and in the future?

A wide extent of inter-strain diversity in virulent and vaccine strains of alphaherpesviruses

Alphaherpesviruses are widespread in the human population, and include herpes simplex virus 1 (HSV-1) and 2, and varicella zoster virus (VZV). These viral pathogens cause epithelial lesions, and then infect the nervous system to cause lifelong latency, reactivation, and spread. A related veterinary herpesvirus, pseudorabies (PRV), causes similar disease in livestock that result in significant economic losses. Vaccines developed for VZV and PRV serve as useful models for the development of an HSV-1 vaccine. We present full genome sequence comparisons of the PRV vaccine strain Bartha, and two virulent PRV isolates, Kaplan and Becker. These genome sequences were determined by high-throughput sequencing and assembly, and present new insights into the attenuation of a mammalian alphaherpesvirus vaccine strain. We find many previously unknown coding differences between PRV Bartha and the virulent strains, including changes to the fusion proteins gH and gB, and over forty other viral proteins. Inter-strain variation in PRV protein sequences is much closer to levels previously observed for HSV-1 than for the highly stable VZV proteome. Almost 20% of the PRV genome contains tandem short sequence repeats (SSRs), a class of nucleic acids motifs whose length-variation has been associated with changes in DNA binding site efficiency, transcriptional regulation, and protein interactions. We find SSRs throughout the herpesvirus family, and provide the first global characterization of SSRs in viruses, both within and between strains. We find SSR length variation between different isolates of PRV and HSV-1, which may provide a new mechanism for phenotypic variation between strains. Finally, we detected a small number of polymorphic bases within each plaque-purified PRV strain, and we characterize the effect of passage and plaque-purification on these polymorphisms. These data add to growing evidence that even plaque-purified stocks of stable DNA viruses exhibit limited sequence heterogeneity, which likely seeds future strain evolution.

Alphaherpesviruses such as herpes simplex virus (HSV) are ubiquitous in the human population. HSV causes oral and genital lesions, and has co-morbidities in acquisition and spread of human immunodeficiency virus (HIV). The lack of a vaccine for HSV hinders medical progress for both of these infections. A related veterinary alphaherpesvirus, pseudorabies virus (PRV), has long served as a model for HSV vaccine development, because of their similar pathogenesis, neuronal spread, and infectious cycle. We present here the first full genome characterization of a live PRV vaccine strain, Bartha, and reveal a spectrum of unique mutations that are absent from two divergent wild-type PRV strains. These mutations can now be examined individually for their contribution to vaccine strain attenuation and for potential use in HSV vaccine development. These inter-strain comparisons also revealed an abundance of short repetitive elements in the PRV genome, a pattern which is repeated in other herpesvirus genomes and even the unrelated Mimivirus. We provide the first global characterization of repeats in viruses, comparing both their presence and their variation among different viral strains and species. Repetitive elements such as these have been shown to serve as hotspots of variation between individuals or strains of other organisms, generating adaptations or even disease states through changes in length of DNA-binding sites, protein folding motifs, and other structural elements. These data suggest for the first time that similar mechanisms could be widely distributed in viral biology as well.