Varicella-zoster virus does not significantly induce cell defence mechanism mediated by the 2-5A/RNase L pathway during its replication cycle

Transcriptional response to VZV infection is modulated by RNA polymerase III in lung epithelial cell lines

Ancestral RNA polymerase III (Pol III) is a multi-subunit polymerase responsible for transcription of short non-coding RNA, such as double-stranded short interspersed nuclear elements (SINEs). Although SINE ncRNAs are generally transcriptionally repressed, they can be induced in response to viral infections and can stimulate immune signaling pathways. Indeed, mutations in RNA Pol III have been associated with poor antiviral interferon response following infection with varicella zoster virus (VZV). In this study, we probed the role of Pol III transcripts in the detection and initial immune response to VZV by characterizing the transcriptional response following VZV infection of wild type A549 lung epithelial cells as well as A549 cells lacking specific RNA sensors MAVS and TLR3, or interferon-stimulated genes RNase L and PKR in presence or absence of functional RNA Pol III. Multiple components of the antiviral sensing and interferon signaling pathways were involved in restricting VZV replication in lung epithelial cells thus suggesting an innate defense system with built-in redundancy. In addition, RNA Pol III silencing altered the antiviral transcriptional program indicating that it plays an essential role in the sensing of VZV infection.

Specific inhibition of the PKR-mediated antiviral response by the murine cytomegalovirus proteins m142 and m143

Double-stranded RNA (dsRNA) produced during viral infection activates several cellular antiviral responses. Among the best characterized is the shutoff of protein synthesis mediated by the dsRNA-dependent protein kinase (PKR) and the oligoadenylate synthetase (OAS)/RNase L system. As viral replication depends on protein synthesis, many viruses have evolved mechanisms for counteracting the PKR and OAS/RNase L pathways. The murine cytomegalovirus (MCMV) proteins m142 and m143 have been characterized as dsRNA binding proteins that inhibit PKR activation, phosphorylation of the translation initiation factor eIF2alpha, and a subsequent protein synthesis shutoff. In the present study we analyzed the contribution of the PKR- and the OAS-dependent pathways to the control of MCMV replication in the absence or presence of m142 and m143. We show that the induction of eIF2alpha phosphorylation during infection with an m142- and m143-deficient MCMV is specifically mediated by PKR, not by the related eIF2alpha kinases PERK or GCN2. PKR antagonists of vaccinia virus (E3L) or herpes simplex virus (gamma34.5) rescued the replication defect of an MCMV strain with deletions of both m142 and m143. Moreover, m142 and m143 bound to each other and interacted with PKR. By contrast, an activation of the OAS/RNase L pathway by MCMV was not detected in the presence or absence of m142 and m143, suggesting that these viral proteins have little or no influence on this pathway. Consistently, an m142- and m143-deficient MCMV strain replicated to high titers in fibroblasts lacking PKR but did not replicate in cells lacking RNase L. Hence, the PKR-mediated antiviral response is responsible for the essentiality of m142 and m143.

ICP0 prevents RNase L-independent rRNA cleavage in herpes simplex virus type 1-infected cells

The classical interferon (IFN)-dependent antiviral response to viral infection involves the regulation of IFN-stimulated genes (ISGs), one being the gene encoding cellular endoribonuclease RNase L, which arrests protein synthesis and induces apoptosis by nonspecifically cleaving rRNA. Recently, the herpes simplex virus type 1 (HSV-1) protein ICP0 has been shown to block the induction of ISGs by subverting the IFN pathway upstream of the 2'-5'-oligoadenylate synthetase (OAS)/RNase L pathway. We report that ICP0 also prevents rRNA degradation at late stages of HSV-1 infection, independent of its E3 ubiquitin ligase activity, and that the resultant rRNA degradation is independent of the classical RNase L antiviral pathway. Moreover, the degradation is independent of the viral RNase vhs and is independent of IFN response factor 3. These studies indicate the existence of another, previously unidentified, RNase that is part of the host antiviral response to viral infection.

Varicella-zoster virus requires a functional PI3K/Akt/GSK-3alpha/beta signaling cascade for efficient replication

Successful replication of Varicella-zoster virus (VZV) relies upon strategies to counteract host defense mechanisms. This can be achieved by modulating host cell signaling pathways, which regulate apoptosis and cell survival. The Akt cascade is crucial for the regulation of cell survival since it controls factors such as Bad, FOXO1, mTor and GSK-3alpha/beta. These factors are involved in the regulation of cell death, cell cycle and translation. Here, we report i) that the VZV infection of MeWo cells caused a 9 to 18-fold increased phosphorylation of Akt. This phosphorylation was independent from PI3K inasmuch as the PI3K phosphorylation pattern differed strongly from the one of Akt. Bad, FOXO1 and mTor showed also variations in their phosphorylation patterns: phosphorylation of Bad (ser-136) decreased during the infection while phosphorylation of ser-2448 of mTor and of ser-256 of FOXO1 increased. The phosphorylation of GSK-3alpha/beta remained relatively stable during the infection. ii) Inhibition of PI3K, Akt or GSK-3alpha/beta prior to infection resulted in a severe decline of viral replication. The inhibition of Akt resulted also in an increased apoptotic response. iii) Transfection studies using plasmids coding for functional or inactive VZV protein kinases, pORFs 47 and 66, demonstrated an increase in Akt phosphorylation. Infection of MeWo cells with VZVDelta47 and VZVDelta66 resulted in a decline of Akt and GSK-3alpha/beta phosphorylation. These results suggest i) an essential role of PI3K/Akt/GSK-3alpha/beta signaling for a successful replication of VZV and ii) a key function of VZV kinases pORFs 47 and 66 to activate this pathway.

Varicella-zoster virus influences the activities of components and targets of the ERK signalling pathway

Varicella-zoster virus (VZV) is ultimately dependent upon its host cell for replication. To ensure its reproduction, VZV reorganizes various cellular functions by taking advantage of pre-existing signalling pathways. Recently, it was demonstrated that the activation of stress-related mitogen-activated protein kinase pathways following infection led to increased phosphorylation of cellular transcription factors involved in VZV gene expression. Here, it was shown that members of the extracellular signal-regulated kinase (ERK) pathway are also influenced following VZV infection: c-Raf remained inactive in infected MeWo cells, whereas MEK1/2 and ERK1/2 were phosphorylated transiently, reaching their highest level of phosphorylation at between 10 and 12 h post-infection. Inhibition of this pathway resulted in a severe reduction in viral progeny and in an increased apoptotic response, indicating that the functionality of this cascade is essential for successful high-rate replication. In addition, the activities of Bad, a cytoplasmic target of ERK via ribosomal S6 kinase, and the nuclear-localized target c-Myc were analysed. Bad is a member of the Bcl-2 family and has a key function in regulating apoptosis. Pro-apoptotic functions of Bad are repressed by phosphorylation. A 10-fold increase in Bad phosphorylation at Ser-112 was detected following infection, which was suppressed after inhibition of ERK. The transcription factor c-Myc is involved in the regulation of cell growth and apoptosis. By performing immunoblots and quantitative RT-PCR, suppression of c-Myc expression was demonstrated at both the transcriptional and translational levels in VZV-infected cells. These results suggest that VZV optimizes the conditions for its replication in different ways: upregulation of proviral-acting systems and suppression of potentially antiviral-acting systems.

Varicella-zoster virus infection influences expression and organization of actin and alpha-tubulin but does not affect lamin A and vimentin

OBJECTIVE

The aim of this study was to examine the effects of varicella-zoster virus (VZV) infection on the cytoskeletal components actin, lamin A, alpha-tubulin and vimentin.

METHODS

The expression patterns of these four proteins during VZV infection were studied by Northern and Western blotting. The filaments were also studied in their cellular environment by immunofluorescence using confocal microscopy. Treatment with nocodazole and cytochalasin B was performed to examine the effects of the destruction of actin or tubulin networks on the VZV replicative cycle.

RESULTS

The amounts of the mRNAs of actin, lamin A, alpha-tubulin and vimentin decreased slightly at 48 h post infection (p.i.) with VZV. The cellular content of the lamin A protein appeared to remain stable during the time period analyzed, whereas the amounts of actin, alpha-tubulin and vimentin decreased slightly at 24 h p.i. until the end of the viral cycle. Rearrangement of microfilaments and microtubules was observed at 24 h p.i. The addition of nocodazole or cytochalasin B decreased viral replication.

CONCLUSIONS

During the VZV replicative cycle, tubulin and actin networks undergo significant changes including fiber elongation. If destroyed intentionally, viral replication is diminished, suggesting that these systems are vital for an efficient infection and viral replication.

The varicella-zoster virus-mediated delayed host shutoff: open reading frame 17 has no major function, whereas immediate-early 63 protein represses heterologous gene expression

We reported that varicella-zoster virus (VZV) causes a delayed host shutoff during its replicative cycle. VZV open reading frame 17 (ORF17) is the homologue of the herpes simplex virus (HSV) UL41 gene encoding the virion host shutoff (vhs) protein which is responsible for the shutoff effect observed in HSV-infected cells. In the present study, we demonstrated that ORF17 is expressed as a late protein during the VZV replicative cycle in different infected permissive cell lines which showed a delayed shutoff of cellular RNA. A cell line with stable expression of VZV ORF17 was infected with VZV. In these cells, VZV replication and delayed host shutoff remained unchanged when compared to normal infected cells. ORF17 was not capable of repressing the expression of the beta-gal reporter gene under the control of the human cytomegalovirus immediate-early gene promoter or to inhibit the expression of a CAT reporter gene under the control of the human GAPDH promoter, indicating that ORF17 has no major function in the VZV-mediated delayed host shutoff. To determine whether other viral factors are involved in the host shutoff, a series of cotransfection assays was performed. We found that the immediate-early 63 protein (IE63) was able to downregulate the expression of reporter genes under the control of the two heterologous promoters, indicating that this viral factor can be involved in the VZV-mediated delayed host shutoff. Other factors can be also implicated to modulate the repressing action of IE63 to achieve a precise balance between the viral and cellular gene expression.

Herpes simplex virus and varicella-zoster virus: why do these human alphaherpesviruses behave so differently from one another?

Members of the Herpesviridae family of viruses are classified into the alpha, beta and gamma subfamilies. The alpha subfamily is estimated to have diverged from the beta and gamma subfamilies 200-220 million years ago. The ancestors of the herpes simplex virus (HSV) and the varicella-zoster virus (VZV), two ubiquitous and clinically important human pathogens, appeared 70-80 million years ago. As these viruses coevolved with their specific primate hosts, genetic rearrangements led to the development of the contemporary alphaherpesviruses and their distinct complement of genes. Here the distinct features of HSV and VZV are discussed in terms of their transmissibility, clinical picture, tissue tropism, establishment of latency/reactivation and immune evasion, which can, at least in part, be explained by differences in their genomes.