Novel herpesvirus discovered in walrus liver.

A walrus (Odobenus rosmarus) born in an aquarium and hand-reared in Japan died at the age of 11 months. The affected animal showed fever and anorexia and had high levels of AST and ALT. Necropsy showed multiple necroses in the liver and adrenal glands and histological examination revealed necrotic lesions of the liver and adrenal cortex, both of which contained intranuclear inclusions. Electron microscopic analysis of the liver sample showed herpesvirus-like particles. High-throughput sequencing analysis of the liver sample and phylogenetic analysis of herpesvirus polymerase genes identified a new virus, Walrus alphaherpesvirus 1 (WaHV-1), which belonged to the subfamily Alphaherpesvirinae and had high homology with Phocid alphaherpesvirus 1. Phylogenetic analysis of the UL30 gene encoding glycoprotein B revealed that WaHV-1 was closely related to a cluster of phocid herpesviruses, including one that caused high mortality rates in harbor seals during past outbreaks. The mother walrus of the dead animal showed evidence of herpesvirus infection in the past and potentially harbored WaHV-1. As a result of hand-rearing, the dead animal might have acquired WaHV-1 from its infected mother and succumbed to WaHV-1 due to lack of maternal IgG, including those that could neutralize WaHV-1.

Microtubule-Dependent Trafficking of Alphaherpesviruses in the Nervous System: The Ins and Outs.

The Alphaherpesvirinae include the neurotropic pathogens herpes simplex virus and varicella zoster virus of humans and pseudorabies virus of swine. These viruses establish lifelong latency in the nuclei of peripheral ganglia, but utilize the peripheral tissues those neurons innervate for productive replication, spread, and transmission. Delivery of virions from replicative pools to the sites of latency requires microtubule-directed retrograde axonal transport from the nerve terminus to the cell body of the sensory neuron. As a corollary, during reactivation newly assembled virions must travel along axonal microtubules in the anterograde direction to return to the nerve terminus and infect peripheral tissues, completing the cycle. Neurotropic alphaherpesviruses can therefore exploit neuronal microtubules and motors for long distance axonal transport, and alternate between periods of sustained plus end- and minus end-directed motion at different stages of their infectious cycle. This review summarizes our current understanding of the molecular details by which this is achieved.

Human herpesvirus portal proteins: Structure, function, and antiviral prospects.

Herpesviruses (Herpesvirales) and tailed bacteriophages (Caudovirales) package their dsDNA genomes through an evolutionarily conserved mechanism. Much is known about the biochemistry and structural biology of phage portal proteins and the DNA encapsidation (viral genome cleavage and packaging) process. Although not at the same level of detail, studies on HSV-1, CMV, VZV, and HHV-8 have revealed important information on the function and structure of herpesvirus portal proteins. During dsDNA phage and herpesviral genome replication, concatamers of viral dsDNA are cleaved into single length units by a virus-encoded terminase and packaged into preformed procapsids through a channel located at a single capsid vertex (portal). Oligomeric portals are formed by the interaction of identical portal protein monomers. Comparing portal protein primary aa sequences between phage and herpesviruses reveals little to no sequence similarity. In contrast, the secondary and tertiary structures of known portals are remarkable. In all cases, function is highly conserved in that portals are essential for DNA packaging and also play a role in releasing viral genomic DNA during infection. Preclinical studies have described small molecules that target the HSV-1 and VZV portals and prevent viral replication by inhibiting encapsidation. This review summarizes what is known concerning the structure and function of herpesvirus portal proteins primarily based on their conserved bacteriophage counterparts and the potential to develop novel portal-specific DNA encapsidation inhibitors.

Making the case: married versus separate models of alphaherpes virus anterograde transport in axons.

Alphaherpesvirus virions infect neurons and are transported in axons for long distance spread within the host nervous system. The assembly state of newly made herpesvirus particles during anterograde transport in axons is an essential question in alphaherpesvirus biology. The structure of the particle has remained both elusive and controversial for the past two decades, with conflicting evidence from EM, immunofluorescence, and live cell imaging studies. Two opposing models have been proposed-the Married and Separate Models. Under the Married Model, infectious virions are assembled in the neuronal cell body before sorting into axons and then traffic inside a transport vesicle. Conversely, the Separate Model postulates that vesicles containing viral membrane proteins are sorted into axons independent of capsids, with final assembly of mature virions occurring at a distant egress site. Recently, a complementary series of studies employing high-resolution EM and live cell fluorescence microscopy have provided evidence consistent with the Married Model, whereas other studies offer evidence supporting the Separate Model. In this review, we compare and discuss the published data and attempt to reconcile divergent findings and interpretations as they relate to these models.

Completely assembled virus particles detected by transmission electron microscopy in proximal and mid-axons of neurons infected with herpes simplex virus type 1, herpes simplex virus type 2 and pseudorabies virus.

The morphology of alphaherpesviruses during anterograde axonal transport from the neuron cell body towards the axon terminus is controversial. Reports suggest that transport of herpes simplex virus type 1 (HSV-1) nucleocapsids and envelope proteins occurs in separate compartments and that complete virions form at varicosities or axon termini (subassembly transport model), while transport of a related alphaherpesvirus, pseudorabies virus (PRV) occurs as enveloped capsids in vesicles (assembled transport model). Transmission electron microscopy of proximal and mid-axons of primary superior cervical ganglion (SCG) neurons was used to compare anterograde axonal transport of HSV-1, HSV-2 and PRV. SCG cell bodies were infected with HSV-1 NS and 17, HSV-2 2.12 and PRV Becker. Fully assembled virus particles were detected intracellularly within vesicles in proximal and mid-axons adjacent to microtubules after infection with each virus, indicating that assembled virions are transported anterograde within axons for all three alphaherpesviruses.

Entry of herpes simplex virus 1 and other alphaherpesviruses via the paired immunoglobulin-like type 2 receptor alpha.

Herpes simplex virus 1 (HSV-1) enters cells either via fusion of the virion envelope and host cell plasma membrane or via endocytosis, depending on the cell type. In the study reported here, we investigated a viral entry pathway dependent on the paired immunoglobulin-like type 2 receptor alpha (PILRalpha), a recently identified entry coreceptor for HSV-1 that associates with viral envelope glycoprotein B (gB). Experiments using inhibitors of endocytic pathways and ultrastructural analyses of Chinese hamster ovary (CHO) cells transduced with PILRalpha showed that HSV-1 entry into these cells was via virus-cell fusion at the cell surface. Together with earlier observations that HSV-1 uptake into normal CHO cells and those transduced with a receptor for HSV-1 envelope gD is mediated by endocytosis, these results indicated that expression of PILRalpha produced an alternative HSV-1 entry pathway in CHO cells. We also showed that human and murine PILRalpha were able to mediate entry of pseudorabies virus, a porcine alphaherpesvirus, but not of HSV-2. These results indicated that viral entry via PILRalpha appears to be conserved but that there is a PILRalpha preference among alphaherpesviruses.

A homologous in vitro model to study interactions between alphaherpesviruses and trigeminal ganglion neurons.

A key aspect in the life cycle of alphaherpesviruses is their neurotropic behaviour. Sensory neurons of the trigeminal ganglion (TG) are important target cells for many alphaherpesviruses (including herpes simplex virus 1, pseudorabies virus (PRV), bovine herpesvirus 1) and constitute major sites for latent infections. The aim of this study was to develop an in vitro model that simulates the in vivo infection pattern of TG neurons by alphaherpesviruses. To this end, we developed a homologous in vitro two-chamber model using PRV and porcine TG neurons. TG of 4- to 6-week-old piglets were dissociated and cultured in the inner chamber of the in vitro model, which is separated from the outer chamber by a medium- and virus-impermeable silicon barrier. Outgrowth of axons from neuronal cell bodies in the inner chamber through the silicon barrier into the outer chamber could be observed after 2-3 weeks of cultivation. Subsequent addition of PRV to the outer chamber resulted in exclusive infection of the TG neurons by transport of virus through the axons, subsequently giving rise to productively infected TG neurons that transmitted virus to contacting neurons and non-neuronal cells in the inner chamber. Thus, we established a homologous in vitro model that mimics the natural route of alphaherpesvirus infection of TG neurons that can be used to study interactions between these viruses and this pathogenetically very important cell type.

Proliferative dermatitis associated with a novel alphaherpesvirus in an Atlantic bottlenose dolphin (Tursiops truncatus).

Herpesviruses and herpes-like viruses have been reported in only a small number of species of cetaceans, and, to date, clinical manifestations have been either as a life-threatening, disseminated infection or as a non-life-threatening dermatitis. A stranded juvenile Atlantic bottlenose dolphin, Tursiops truncatus, was admitted to the Dolphin and Whale Hospital for rehabilitation. On initial physical examination, the rostral skin had multifocal regions of hyperplasia, and the skin of the dorsum contained a large number of small papules. Histologically, epithelial hyperplasia was evident, and clusters of epithelial cells contained 5-15-microm intranuclear inclusion bodies. Transmission electron microscopic investigation revealed numerous 170-190-nm enveloped virions in both the intracellular spaces and the cytoplasm of epithelial cells, with numerous nucleocapsids noted in epithelial cell nuclei. Consensus primer polymerase chain reaction identified the presence of a novel herpesvirus associated with the lesions. Phylogenetic analysis of the deduced amino acid sequences of the herpesvirus DNA polymerase gene fragment showed it to align with alphaherpesvirus sequences from humans and domestic animals. Although clearly distinct, it was most closely related to two previously described alphaherpesviruses of dolphins. This case represents the first documentation of herpesvirus dermatitis in the Atlantic bottlenose dolphin.

Electron microscopic studies of the morphogenesis of duck enteritis virus.

The morphogenesis of duck enteritis virus (DEV) and distribution in vivo were observed by electron microscopy after ducks were infected experimentally with DEV virulent strain. The investigation showed that a few typical herpesvirus virions and nucleocapsids were first observed in the spleen, thymus, and bursa of Fabricius (BF), and many nucleocapsids, mature viruses, and viral inclusion bodies could be found in the nucleus and cytoplasm of infected liver, small intestine, spleen, thymus, and BF when the ducks died. Nucleocapsids assembled both in nucleus and cytoplasm and could be divided into four different types according to their structures. Typical herpesvirus, light particles (L-particles), and virions without tegument could be observed at the same time. With the replication, assembly, and maturation of the viruses, intracytoplasmic and intranuclear inclusion bodies, electron-density particles, microtubules, hollow tubes, and coated electron-density bodies were observed in infected cells.

Sorting and transport of alpha herpesviruses in axons.

The alpha herpesviruses, a subfamily of the herpesviruses, are neurotropic pathogens found associated with most mammalian species. The prototypic member of this subfamily is herpes simplex virus type 1, the causative agent of recurrent cold sores in humans. The mild nature of this disease is a testament to the complex and highly regulated life cycle of the alpha herpesviruses. The cellular mechanisms used by these viruses to disseminate infection in the nervous system are beginning to be understood. Here, we overview the life cycle of alpha herpesviruses with an emphasis on assembly and transport of viral particles in neurons.

Egress of alphaherpesviruses: comparative ultrastructural study.

Egress of four important alphaherpesviruses, equine herpesvirus 1 (EHV-1), herpes simplex virus type 1 (HSV-1), infectious laryngotracheitis virus (ILTV), and pseudorabies virus (PrV), was investigated by electron microscopy of infected cell lines of different origins. In all virus-cell systems analyzed, similar observations were made concerning the different stages of virion morphogenesis. After intranuclear assembly, nucleocapsids bud at the inner leaflet of the nuclear membrane, resulting in enveloped particles in the perinuclear space that contain a sharply bordered rim of tegument and a smooth envelope surface. Egress from the perinuclear cisterna primarily occurs by fusion of the primary envelope with the outer leaflet of the nuclear membrane, which has been visualized for HSV-1 and EHV-1 for the first time. The resulting intracytoplasmic naked nucleocapsids are enveloped at membranes of the trans-Golgi network (TGN), as shown by immunogold labeling with a TGN-specific antiserum. Virions containing their final envelope differ in morphology from particles within the perinuclear cisterna by visible surface projections and a diffuse tegument. Particularly striking was the addition of a large amount of tegument material to ILTV capsids in the cytoplasm. Extracellular virions were morphologically identical to virions within Golgi-derived vesicles, but distinct from virions in the perinuclear space. Studies with gB- and gH-deleted PrV mutants indicated that these two glycoproteins, which are essential for virus entry and direct cell-to-cell spread, are dispensable for egress. Taken together, our studies indicate that the deenvelopment-reenvelopment process of herpesvirus maturation also occurs in EHV-1, HSV-1, and ILTV and that membrane fusion processes occurring during egress are substantially different from those during entry and direct viral cell-to-cell spread.

[Structure and classification of alpha-herpesvirus].

Virion of herpesviruses (100 to 200 nm in diameter) consists of a core containing a linear double-stranded DNA, an icosahedral capsid, an amorphous layer designated as the tegument which surrounds the capsid, and a protein-containing lipid membrane designated as the envelope. Terminal and internal reiterated sequences are present on herpesvirus genomes, giving a diverse range of DNA structures. The family Herpesviridae is divided into three subfamily of Alpah-, Beta- and Gammaherpesvirinae. Herpesviruses seem to have diversified from a common ancestor, in a manner mediating co-speciation of herpesviruses with host species through species-specific latent infection.

Simian agent 8 (SA8): morphogenesis and ultrastructure.

Electron microscopic studies on the morphogenesis of SA8 in primary rabbit brain cell cultures revealed that in early stages of infection, envelopment of nucleocapsids commonly occurred at the inner nuclear membrane. From the perinuclear space, enveloped virus particles moved into the cisternae of the endoplasmic reticulum (ER) in which they were transported, through the cytoplasm, to the plasma membrane. Alternatively, de-envelopment at the outer nuclear membrane and egress of naked capsids into the cytoplasm were frequently observed. Non-enveloped cytoplasmic capsids were also a consistent feature of cells in late stages of infection, when nuclear membranes became ruptured. In these cases, the envelopment of naked capsids took place by budding either into the cisternae of ER or into cytoplasmic vesicles and vacuoles, in which transport to and exocytosis at the cell membrane occurred. Budding at the cell membrane was rarely found. Capsids of enveloped particles were asymmetrically surrounded by an electron-dense layer which may be identical to the tegument. Because only enveloped cytoplasmic and free virions were tegumented we suggested that the tegumentation must occur during the envelopment (budding) into cytoplasmic vesicles and at the plasma membrane.