The virion host shutoff protein of herpes simplex virus type 1 has RNA degradation activity in primary neurons

The ESCRT-Related ATPase Vps4 Is Modulated by Interferon during Herpes Simplex Virus 1 Infection

Interferons (IFNs) and autophagy are critical neuronal defenses against viral infection. IFNs alter neuronal autophagy by promoting the accumulation of IFN-dependent LC3-decorated autophagic structures, termed LC3 clusters. Here, we analyzed LC3 clusters in sensory ganglia following herpes simplex virus 1 (HSV-1) infection. In the vicinity of acutely infected neurons, antigen-negative neurons contained structures resembling accumulated autophagosomes and autolysosomes that culminated in LC3 clusters. This accumulation reflects a delayed completion of autophagy. The endosomal sorting complexes required for transport (ESCRT) machinery participates in autophagosome closure and is also required for HSV-1 replication. In this study, our results showed that HSV-1 infection in vivo and in primary neurons caused a decrease in Vps4 (a key ESCRT pathway ATPase) RNA and protein with concomitant Stat1 activation and LC3 cluster induction. We also observed that IFNs were sufficient to decrease RNA and protein levels of Vps4 in primary neurons and in other cell types. The accumulation of ubiquitin was also observed at the LC3 cluster sites. Together, our results show that IFNs modulate the ESCRT machinery in neurons in response to HSV-1 infections.IMPORTANCE Neurons rely on IFNs and autophagy as major defenses against viral infections, and HSV must overcome such defenses in order to replicate. In addition to controlling host immunity, HSV must also control host membranes in order to complete its life cycle. HSV uses the host ESCRT membrane scission machinery for viral production and transport. Here we present evidence of a new IFN-dependent mechanism used by the host to prevent ESCRT subversion by HSV. This activity also impacts the dynamics of autophagy, possibly explaining the presence of recently described LC3 clusters in the HSV-infected nervous system. The induced accumulations of ubiquitin observed in these LC3 clusters resembled those observed in certain neurodegenerative diseases, suggesting possible mechanistic parallels between these conditions.

Deletion of the Virion Host Shut-off Gene Enhances Neuronal-Selective Transgene Expression from an HSV Vector Lacking Functional IE Genes

The ability of herpes simplex virus (HSV) to establish lifelong latency in neurons suggests that HSV-derived vectors hold promise for gene delivery to the nervous system. However, vector toxicity and transgene silencing have created significant barriers to vector applications to the brain. Recently, we described a vector defective for all immediate-early gene expression and deleted for the joint region between the two unique genome segments that proved capable of extended transgene expression in non-neuronal cells. Sustained expression required the proximity of boundary elements from the latency locus. As confirmed here, we have also found that a transgene cassette introduced into the ICP4 locus is highly active in neurons but silent in primary fibroblasts. Remarkably, we observed that removal of the virion host shutoff (vhs) gene further improved transgene expression in neurons without inducing expression of viral genes. In rat hippocampus, the vhs-deleted vector showed robust transgene expression exclusively in neurons for at least 1 month without evidence of toxicity or inflammation. This HSV vector design holds promise for gene delivery to the brain, including durable expression of large or complex transgene cassettes.

Frequent release of low amounts of herpes simplex virus from neurons: results of a mathematical model

Herpes simplex virus-2 (HSV-2) is a sexually transmitted infection that is the leading cause of genital ulcers worldwide. Infection is life long and is characterized by repeated asymptomatic and symptomatic shedding episodes of virus that are initiated when virus is released from neurons into the genital tract. The pattern of HSV-2 release from neurons into the genital tract is poorly understood. We fit a mathematical model of HSV-2 pathogenesis to curves generated from daily quantification of HSV in mucosal swabs performed from patients with herpetic genital ulcers. We used virologic parameters derived from model fitting for stochastic model simulations. These simulations reproduced previously documented estimates for shedding frequency, and herpetic lesion diameter and frequency. The most realistic model output occurred when we assumed minimal amounts of daily neuronal virus introduction. In our simulations, small changes in average total quantity of HSV-2 released from neurons influenced detectable shedding frequency, while changes in frequency of neuronal HSV-2 release had little effect. Frequent HSV-2 shedding episodes in humans are explained by nearly constant release of small numbers of viruses from neurons that terminate in the genital tract.

The virion host shut-off (vhs) protein blocks a TLR-independent pathway of herpes simplex virus type 1 recognition in human and mouse dendritic cells

Molecular pathways underlying the activation of dendritic cells (DCs) in response to Herpes Simplex Virus type 1 (HSV-1) are poorly understood. Removal of the HSV virion host shut-off (vhs) protein relieves a block to DC activation observed during wild-type infection. In this study, we utilized a potent DC stimulatory HSV-1 recombinant virus lacking vhs as a tool to investigate the mechanisms involved in the activation of DCs by HSV-1. We report that the release of pro-inflammatory cytokines by conventional DC (cDC) during HSV-1 infection is triggered by both virus replication-dependent and replication-independent pathways. Interestingly, while vhs is capable of inhibiting the release of cytokines during infection of human and mouse cDCs, the secretion of cytokines by plasmacytoid DC (pDC) is not affected by vhs. These data prompted us to postulate that infection of cDCs by HSV triggers a TLR independent pathway for cDC activation that is susceptible to blockage by the vhs protein. Using cDCs isolated from mice deficient in both the TLR adaptor protein MyD88 and TLR3, we show that HSV-1 and the vhs-deleted virus can activate cDCs independently of TLR signaling. In addition, virion-associated vhs fails to block cDC activation in response to treatment with TLR agonists, but it efficiently blocked cDC activation triggered by the paramyxoviruses Sendai Virus (SeV) and Newcastle Disease Virus (NDV). This block to SeV- and NDV-induced activation of cDC resulted in elevated SeV and NDV viral gene expression indicating that infection with HSV-1 enhances the cell's susceptibility to other pathogens through the action of vhs. Our results demonstrate for the first time that a viral protein contained in the tegument of HSV-1 can block the induction of DC activation by TLR-independent pathways of viral recognition.

Selective ablation of virion host shutoff protein RNase activity attenuates herpes simplex virus 2 in mice

The virion host shutoff (vhs) protein of herpes simplex virus (HSV) has endoribonuclease activity and rapidly reduces protein synthesis in infected cells through mRNA degradation. Herpes simplex virus 1 (HSV-1) and HSV-2 vhs mutants are highly attenuated in vivo, but replication and virulence are largely restored to HSV-2 vhs mutants in the absence of a type I interferon (IFN) response. The role of vhs in pathogenesis and the hindrance of the type I IFN response have classically been examined with viruses that completely lack vhs or express a truncated vhs protein. To determine whether RNase activity is the principal mechanism of vhs-mediated type I IFN resistance and virulence, we constructed a HSV-2 point mutant that synthesizes full-length vhs protein lacking RNase activity (RNase(-) virus). Wild-type and mutant HSV-2 vhs proteins coimmunoprecipitated with VP16 and VP22. vhs protein bearing the point mutation was packaged into the virion as efficiently as the wild-type vhs protein. Like a mutant encoding truncated vhs, the RNase(-) virus showed IFN-dependent replication that was restricted compared with that of the wild-type virus. The RNase(-) virus was highly attenuated in wild-type mice infected intravaginally, with reduced mucosal replication, disease severity, and spread to the nervous system comparable to those of the vhs truncation mutant. Surprisingly, in alpha/beta interferon (IFN-alpha/beta) receptor knockout mice, the vhs RNase mutant was more attenuated than the vhs truncation mutant in terms of disease severity and virus titer in vaginal swabs and central nervous system samples, suggesting that non-enzymatically active vhs protein interferes with efficient virus replication. Our results indicate that vhs enzymatic activity plays a complex role in vhs-mediated type I IFN resistance during HSV-2 infection.

The herpes simplex virus type 1 vhs-UL41 gene secures viral replication by temporarily evading apoptotic cellular response to infection: Vhs-UL41 activity might require interactions with elements of cellular mRNA degradation machinery

We have previously shown that herpes simplex virus type 1 (HSV-1) infection is associated with early destabilization/degradation of infected cell mRNAs and consequent shutoff of host protein synthesis by the activity of the virion-associated host shutoff (vhs) UL41 protein. Wild-type (wt) virus destabilized/degraded the housekeeping beta-actin and alpha-tubulin mRNAs as well host stress functions, like the heat shock 70 protein induced postinfection. vhs mutants did not degrade the mRNAs. Elaborate studies by others have been concerned with the mode of mRNA degradation and the mRNAs affected. We now describe vhs activity in primary cultures of mouse cerebellar granule neurons (CGNs). Specifically, (i) upon infection in the presence of actinomycin D to test activity of input viral particles, there was a generalized inhibition of protein synthesis, which depended on the input multiplicity of infection (MOI). (ii) Low-MOI infection with vhs-1 mutant virus was associated with increased synthesis of all apparent proteins. Higher MOIs caused some shutoff, albeit significantly lower than that of wt virus. This pattern could reflect an interaction(s) of vhs-1 protein with host machinery involved in cellular mRNA destabilization/degradation, sequestering this activity. (iii) wt virus infection was associated with cell survival, at least for a while, whereas mutant virus induced apoptotic cell death at earlier times. (iv) wt virus replicated well in the CGNs, whereas there was no apparent replication of the vhs-1 mutant virus. (v) The vhs-1 mutant could serve as helper virus for composite amplicon vectors carrying marker genes and the human p53 gene. Ongoing studies test the use of vhs-1-based composite oncolytic vectors towards cancer gene therapy.

Improved HSV-1 amplicon packaging using virion host shutoff mutants lacking mRNAse activity

Given their generous transgene capacity and inherent neurotropism, herpes simplex virus (HSV-1)-based viral vectors are promising tools for gene delivery to the central nervous system. Despite their widespread pre-clinical use, vector toxicity remains a concern with regard to the use of herpes vectors in humans. One potential source of toxicity stems from the tegument-associated virion host shutoff protein (vhs), which induces translational arrest in the host cell through non-specific mRNAse activity. In the current study we utilized a series of HSV-1 viruses containing a deletion in the U(L)41 open reading frame to investigate: (1) the requirement of intact vhs function in amplicon packaging and (2) whether vhs influences the post-transduction survival of dissociated cortical neurons. Our results demonstrate that while amplicon yield was reduced an order of magnitude, U(L)41 deletion was associated with reduced vector toxicity. Furthermore, partial reconstitution of vhs function using mRNAse-inactive point mutants improved amplicon titers without imparting the toxicity observed with wild-type controls. These findings offer a novel approach to improving the titer and toxicity profiles of HSV-based viral vectors.

Phosphorylation of the VP16 transcriptional activator protein during herpes simplex virus infection and mutational analysis of putative phosphorylation sites

VP16 is a virion phosphoprotein of herpes simplex virus and a transcriptional activator of the viral immediate-early (IE) genes. We identified four novel VP16 phosphorylation sites (Ser18, Ser353, Ser411, and Ser452) at late times in infection but found no evidence of phosphorylation of Ser375, a residue reportedly phosphorylated when VP16 is expressed from a transfected plasmid. A virus carrying a Ser375Ala mutation of VP16 was viable in cell culture but with a slow growth rate. The association of the mutant VP16 protein with IE gene promoters and subsequent IE gene expression was markedly reduced during infection, consistent with prior transfection and in vitro results. Surprisingly, the association of Oct-1 with IE promoters was also diminished during infection by the mutant strain. We propose that Ser375 is important for the interaction of VP16 with Oct-1, and that the interaction is required to enable both proteins to bind to IE promoters.