The gamma134.5 protein of herpes simplex virus 1 has the structural and functional attributes of a protein phosphatase 1 regulatory subunit and is present in a high molecular weight complex with the enzyme in infected cells

The herpes simplex virus 1 vhs protein enhances translation of viral true late mRNAs and virus production in a cell type-dependent manner

The herpes simplex virus 1 (HSV-1) virion host shutoff protein (vhs) degrades viral and cellular mRNAs. Here, we demonstrate for the first time that vhs also boosts translation of viral true late mRNAs in a cell type-dependent manner and that this effect determines the viral growth phenotype in the respective cell type. Our study was prompted by the detection of stress granules, indicators of stalled translation initiation, in cells infected with vhs mutants but not in wild-type-virus-infected cells. Accumulation of true late-gene products gC and US11 was strongly reduced in the absence of vhs in HeLa cells and several other restrictive cell lines but not in Vero and other permissive cells and was independent of phosphorylation of the α subunit of eukaryotic initiation factor 2 (eIF2α). Polysome analysis showed that gC and US11 transcripts were poorly translated in vhs-null-virus-infected HeLa cells, while translation of a cellular mRNA was not affected. Interestingly, hippuristanol, an eIF4A inhibitor, produced a similar phenotype in HeLa cells infected with wild-type HSV-1, while Vero cells were much more resistant to the inhibitor. These results suggest that translation of true late-gene transcripts is particularly sensitive to conditions of limited access to translation factors and that vhs is able either to prevent the limiting conditions or to facilitate translation initiation under these conditions. The varied permissivity of cell lines to vhs-null infection may stem from differences in the resilience of the translation machinery or the ability to control the accumulation of mRNAs.

Engineered herpes simplex virus vectors for antitumor therapy and vaccine delivery

Genetically modified herpes simplex viruses (HSVs) have been exploited for both antitumor therapy and vaccine delivery. These mutant viruses retain their ability to replicate and lyse permissive cells, including many tumor types, and are referred to as oncolytic HSVs. In addition, deletion of nonessential genes permits the introduction of foreign genes to augment the antitumor effect by either immune stimulation, targeting for select tumors, or expression of tumor or vaccine antigens. This article reviews the development of oncolytic HSVs as an anticancer therapy, as well as the application of HSV-1 vectors for delivery of targeted antigens or as vaccine adjuvants. The impact of these novel vectors with respect to enhanced antitumor activity and development of antitumor vaccination strategies is discussed.

A herpesvirus virulence factor inhibits dendritic cell maturation through protein phosphatase 1 and Ikappa B kinase

Dendritic cells are sentinels in innate and adaptive immunity. Upon virus infection, a complex program is in operation, which activates IκB kinase (IKK), a key regulator of inflammatory cytokines and costimulatory molecules. Here we show that the γ(1)34.5 protein, a virulence factor of herpes simplex viruses, blocks Toll-like receptor-mediated dendritic cell maturation. While the wild-type virus inhibits the induction of major histocompatibility complex (MHC) class II, CD86, interleukin-6 (IL-6), and IL-12, the γ(1)34.5-null mutant does not. Notably, γ(1)34.5 works in the absence of any other viral proteins. When expressed in mammalian cells, including dendritic cells, γ(1)34.5 associates with IKKα/β and inhibits NF-κB activation. This is mirrored by the inhibition of IKKα/β phosphorylation, p65/RelA phosphorylation, and nuclear translocation in response to lipopolysaccharide or poly(I:C) stimulation. Importantly, γ(1)34.5 recruits both IKKα/β and protein phosphatase 1, forming a complex that dephosphorylates two serine residues within the catalytic domains of IκB kinase. The amino-terminal domain of γ(1)34.5 interacts with IKKα/β, whereas the carboxyl-terminal domain binds to protein phosphatase 1. Deletions or mutations in either domain abolish the activity of γ(1)34.5. These results suggest that the control of IκB kinase dephosphorylation by γ(1)34.5 represents a critical viral mechanism to disrupt dendritic cell functions.

How viruses hijack cell regulation

Viruses, as obligate intracellular parasites, are the pathogens that have the most intimate relationship with their host, and as such, their genomes have been shaped directly by interactions with the host proteome. Every step of the viral life cycle, from entry to budding, is orchestrated through interactions with cellular proteins. Accordingly, viruses will hijack and manipulate these proteins utilising any achievable mechanism. Yet, the extensive interactions of viral proteomes has yielded a conundrum: how do viruses commandeer so many diverse pathways and processes, given the obvious spatial constraints imposed by their compact genomes? One important approach is slowly being revealed, the extensive mimicry of host protein short linear motifs (SLiMs).

APP processing induced by herpes simplex virus type 1 (HSV-1) yields several APP fragments in human and rat neuronal cells

Lifelong latent infections of the trigeminal ganglion by the neurotropic herpes simplex virus type 1 (HSV-1) are characterized by periodic reactivation. During these episodes, newly produced virions may also reach the central nervous system (CNS), causing productive but generally asymptomatic infections. Epidemiological and experimental findings suggest that HSV-1 might contribute to the pathogenesis of Alzheimer's disease (AD). This multifactorial neurodegenerative disorder is related to an overproduction of amyloid beta (Aβ) and other neurotoxic peptides, which occurs during amyloidogenic endoproteolytic processing of the transmembrane amyloid precursor protein (APP). The aim of our study was to identify the effects of productive HSV-1 infection on APP processing in neuronal cells. We found that infection of SH-SY5Y human neuroblastoma cells and rat cortical neurons is followed by multiple cleavages of APP, which result in the intra- and/or extra-cellular accumulation of various neurotoxic species. These include: i) APP fragments (APP-Fs) of 35 and 45 kDa (APP-F35 and APP-F45) that comprise portions of Aβ; ii) N-terminal APP-Fs that are secreted; iii) intracellular C-terminal APP-Fs; and iv) Aβ_1-40 and Aβ_1-42. Western blot analysis of infected-cell lysates treated with formic acid suggests that APP-F35 may be an Aβ oligomer. The multiple cleavages of APP that occur in infected cells are produced in part by known components of the amyloidogenic APP processing pathway, i.e., host-cell β-secretase, γ-secretase, and caspase-3-like enzymes. These findings demonstrate that HSV-1 infection of neuronal cells can generate multiple APP fragments with well-documented neurotoxic potentials. It is tempting to speculate that intra- and extracellular accumulation of these species in the CNS resulting from repeated HSV-1 reactivation could, in the presence of other risk factors, play a co-factorial role in the development of AD.

Circadian CLOCK histone acetyl transferase localizes at ND10 nuclear bodies and enables herpes simplex virus gene expression

Expression of herpes simplex virus genes at the initiation of replication involves two steps that take place at ND10 nuclear bodies. These are suppression of cellular repressors that attempt to silence viral DNA and remodeling of the viral chromatin to make it accessible for transcription. In earlier studies we reported on the mechanism by which viral proteins ICP0 and U(S)3 protein kinase modify and disrupt the HDAC1/CoREST/REST/LSD1 repressor complex. The remodeling step requires in addition acetylation of histones bound to DNA. In an attempt to identify the enzyme, we took note of the observation that ICP0 physically and functionally interacts with Bmal1, a partner of the CLOCK histone acetyl transferase, and key members of the bHLH-PAS family of transcriptional factors. The Bmal11 and CLOCK heterodimer is best known as a regulator of the circadian oscillation in the mammalian CLOCK system. In this article we report the following: (i) in infected cells both Bmal1 and CLOCK localize at ND10 bodies; (ii) wild-type virus stabilizes the CLOCK protein; (iii) overexpression of CLOCK partially compensates for the absence of ICP0 and enables higher yields in cells infected with a ΔICP0 mutant and this activity is not expressed by CLOCK mutants lacking histone acetyl transferase activity; and (iv) depletion of CLOCK in cells infected with wild-type virus results in significant decrease in the expression of all viral proteins tested. We conclude that ICP0 interacts with Bmal1 and by extension with CLOCK histone acetyl transferase to remodel viral chromatin.

The African swine fever virus DP71L protein recruits the protein phosphatase 1 catalytic subunit to dephosphorylate eIF2alpha and inhibits CHOP induction but is dispensable for these activities during virus infection

The African swine fever virus (ASFV) DP71L protein is present in all isolates as either a short form of 70 to 72 amino acids or a long form of about 184 amino acids, and both of these share sequence similarity to the C-terminal domain of the herpes simplex virus ICP34.5 protein and cellular protein GADD34. In the present study we expressed DP71L in different mammalian cells and demonstrated that DP71L causes dephosphorylation of eukaryotic translation initiation factor 2 alpha (eIF2α) in resting cells and during chemical-induced endoplasmic reticulum stress and acts to enhance expression of cotransfected reporter genes. We showed that DP71L binds to all the three isoforms (α, β, and γ) of the protein phosphatase 1 catalytic subunit (PP1c) and acts by recruiting PP1c to eIF2α. We also showed that DP71L inhibits the induction of ATF4 and its downstream target, CHOP. We investigated the eIF2α phosphorylation status and induction of CHOP in porcine macrophages infected by two ASFV field isolates, Malawi Lil20/1 and Benin 97/1, and two DP71L deletion mutants, MalawiΔNL and E70ΔNL. Our results showed that deletion of the DP71L gene did not cause an increase in the level of eIF2α phosphorylation or induction of CHOP, indicating that DP71L is not the only factor required by the virus to control the phosphorylation level of eIF2α during infection. We therefore hypothesize that ASFV has other mechanisms to prevent the eIF2α phosphorylation and the subsequent protein synthesis inhibition.

Herpesvirus vectors for therapy of brain tumors

Genetically modified, conditionally-replicating Herpes Simplex Virus Type 1 (HSV-1) vectors for the treatment of malignant glioma have provided encouraging results in the handful of Phase I and Phase II clinical trials conducted to date. In recent years, a number of new strategies have been developed to improve anti-tumor activity of these attenuated vectors, through either introduction of foreign gene inserts to enhance tumor killing through a variety of mechanisms, or through combination with existing treatment regimens, including radiation and/or chemotherapeutics. Another promising new approach has been the engineering of novel oncolytic HSV vectors that retain wildtype replication, but are targeted to tumor cells through a variety of mechanisms. This review summarizes the latest advances in herpesvirus-mediated oncolytic therapies from both preclinical results and clinical trials with oncolytic HSV vectors in patients, and their implication for design of future trials.

Apoptosis induction after herpes simplex virus infection differs according to cell type in vivo

We compared apoptosis induction in mice following three routes of infection. After intravenous infection, wild-type herpes simplex virus (HSV) types 1 and 2 and US3Delta mutants infected the adrenal gland and caused apoptosis. Corneal infection with wild-type virus resulted in apoptosis in a fraction of infected epithelium cells. Interestingly, many uninfected cells were apoptotic in the retina. Although neurons in the trigeminal ganglion were heavily infected, no apoptotic neurons were observed. Intracranial infection with wild-type virus resulted in HSV-infected cells inside the brain; however, most of the infected neurons escaped apoptosis. In contrast, infection with US3Delta and gamma(1)34.5Delta mutants caused apoptosis in infected neurons. Cleaved caspase-8 and p53 were detected in apoptotic cells in the adrenal gland and the brain; however, phospho-JNK was detected only in apoptotic cells of the brain. These results suggest that the activation of apoptotic signaling proteins differs depending on the host cell type and modulates the induction of apoptosis in HSV-infected cells.

Viral strategies for the evasion of immunogenic cell death

Viral strategies for the evasion of immunogenic cell death (Symposium). J Intern Med 2010; 267: 526-542. Driven by co-evolutionary forces, viruses have refined a wide arsenal of strategies to interfere with the host defences. On one hand, viruses can block/retard programmed cell death in infected cells, thereby suppressing one of the most ancient mechanisms against viral dissemination. On the other hand, multiple viral factors can efficiently trigger the death of infected cells and uninfected cells from the immune system, which favours viral spreading and prevents/limits an active antiviral response, respectively. Moreover, several viruses are able to inhibit the molecular machinery that drives the translocation of calreticulin to the surface of dying cells. Thereby, viruses block the exposure of an engulfment signal that is required for the efficient uptake of dying cells by dendritic cells and for the induction of the immune response. In this review, we discuss a variety of mechanisms by which viruses interfere with the cell death machinery and, in particular, by which they subvert immunogenic cell death.

GADD34 suppresses wound healing by upregulating expression of myosin IIA

Wound healing consists of sequential steps of tissue repair, and cell migration is particularly important. In order to analyze the potential function of growth arrest and DNA damage inducible protein 34 (GADD34) in tissue repair, we performed in vitro and in vivo wound healing experiments. In an in vitro scratch assay, GADD34 knockout (KO) mouse embryonic fibroblasts (MEFs) had higher migration rates than did wild type (WT) MEFs. Furthermore, the rate of wound closure was faster in GADD34 KO MEFs than in WT MEFs. Using in vivo punch biopsy assays, GADD34 KO mice had accelerated wound healing compared to WT mice. WT mice expressed higher amounts of myosin IIA in migrating macrophages and myofibroblasts than did GADD34 KO mice. These results indicate that GADD34 negatively regulates cell migration in wound healing via expression of myosin IIA.

Herpes simplex virus-1 infection causes the secretion of a type I interferon-antagonizing protein and inhibits signaling at or before Jak-1 activation

Host cells respond to viral infection by the production of type I interferons (IFNs), which induce the expression of antiviral genes. Herpes simplex virus I (HSV-1) encodes many mechanisms that inhibit the type I IFN response, including the ICP27-dependent inhibition of type I IFN signaling. Here we show inhibition of Stat-1 nuclear accumulation in cells that express ICP27. ICP27 expression also induces the secretion of a small, heat-stable type I IFN antagonizing protein that inhibits Stat-1 nuclear accumulation. We show that the inhibition of IFN-induced Stat-1 phosphorylation occurs at or upstream of Jak-1 phosphorylation. Finally, we show that ISG15 expression is induced after IFNalpha treatment in mock-infected cells, but not cells infected with WT HSV-1 or ICP27(-) HSV-1. These data suggest that HSV-1 has evolved multiple mechanisms to inhibit IFN signaling not only in infected cells, but also in neighboring cells, thereby allowing for increased viral replication and spread.

Dephosphorylation of eIF2alpha mediated by the gamma134.5 protein of herpes simplex virus 1 facilitates viral neuroinvasion

The gamma(1)34.5 protein, a virulence factor of herpes simplex viruses, redirects protein phosphatase 1 to dephosphorylate the alpha subunit of translation initiation factor 2 (eIF2alpha). Additionally, it inhibits the induction of antiviral genes by TANK-binding kinase 1. Nevertheless, its precise role in vivo remains to be established. Here we show that eIF2alpha dephosphorylation by gamma(1)34.5 is crucial for viral neuroinvasion. V(193)E and F(195)L substitutions in gamma(1)34.5 abrogate viral replication in the eye and spread to the trigeminal ganglia and brain. Intriguingly, inhibition of antiviral gene induction by gamma(1)34.5 is not sufficient to exhibit viral virulence.

Increased eIF2alpha phosphorylation attenuates replication of herpes simplex virus 2 vhs mutants in mouse embryonic fibroblasts and correlates with reduced accumulation of the PKR antagonist ICP34.5

Herpes simplex virus 2 (HSV-2) strains containing mutations in the virion host shutoff (vhs) protein are attenuated for replication compared with wild-type virus in mouse embryonic fibroblasts (MEFs). However, HSV-2 vhs mutants replicate to near wild-type levels in the absence of the RNA-activated protein kinase (PKR). PKR is one of several kinases that phosphorylates the eukaryotic initiation factor 2alpha (eIF2alpha) to inhibit translation initiation, and we previously found that more of the phosphorylated form of eIF2alpha accumulates in MEFs infected with HSV-2 vhs mutants than with wild-type virus. Here, we show that this increase in phosphorylated eIF2alpha is primarily PKR dependent. Using MEFs expressing nonphosphorylatable eIF2alpha, we demonstrate that phosphorylated eIF2alpha is the primary cause of attenuated replication of HSV-2 vhs mutants and that attenuation correlates with decreased accumulation of viral proteins. Normally, HSV antagonizes eIF2alpha phosphorylation through the action of ICP34.5, which redirects protein phosphatase 1alpha (PP1alpha) to dephosphorylate eIF2alpha during infection. We show that ICP34.5 does not accumulate efficiently in MEFs infected with HSV-2 vhs mutant viruses, suggesting that the accumulation of phosphorylated eIF2alpha and the attenuated phenotype of HSV-2 vhs mutants in MEFs result from a deficiency in ICP34.5.

HSV-2 ICP34.5 protein modulates herpes simplex virus glycoprotein processing

The ICP34.5 gene from HSV-2 strain 333 was cloned and, when expressed in Vero cells, enhanced the efficiency and extent of glycoprotein processing of glycoprotein C (gC1), a representative viral glycoprotein, during infection with HSV-1 SP7. The ICP34.5 from HSV-1 SP7 limits the extent and efficiency of viral glycoprotein processing. The ability of the HSV-2 ICP34.5 protein to enhance the efficiency and extent of HSV-1 SP7 glycoprotein processing indicates that modulation of viral glycoprotein processing is also a property of the HSV-2 ICP34.5 protein.

African swine fever virus

African swine fever virus (ASFV) is a large, intracytoplasmically-replicating DNA arbovirus and the sole member of the family Asfarviridae. It is the etiologic agent of a highly lethal hemorrhagic disease of domestic swine and therefore extensively studied to elucidate the structures, genes, and mechanisms affecting viral replication in the host, virus-host interactions, and viral virulence. Increasingly apparent is the complexity with which ASFV replicates and interacts with the host cell during infection. ASFV encodes novel genes involved in host immune response modulation, viral virulence for domestic swine, and in the ability of ASFV to replicate and spread in its tick vector. The unique nature of ASFV has contributed to a broader understanding of DNA virus/host interactions.

Novel less-abundant viral microRNAs encoded by herpes simplex virus 2 latency-associated transcript and their roles in regulating ICP34.5 and ICP0 mRNAs

We recently identified an acutely and latently expressed viral microRNA (miRNA), miR-I, encoded by herpes simplex virus 2 (HSV-2) latency-associated transcript (LAT) through small RNA cloning and two miRNAs encoded by HSV-1 LAT through prediction. We now report the use of high-throughput sequencing technology to identify two additional relatively less-abundant viral miRNAs, miR-II and miR-III, encoded by HSV-2 LAT exon 2. miR-II includes two miRNAs, miR-II-5p and miR-II-3p, which are processed from the same miRNA precursor. miR-II and miR-III map antisense to the 5' untranslated region of ICP34.5 and to the coding region of ICP0 exon 3, respectively. These novel miRNAs are conserved in different HSV-2 strains, and their presence in infected- and transfected-cell cultures was confirmed by Northern hybridization. All three HSV-2 LAT-encoded miRNAs map to genome locations similar to those of three out of four identified HSV-1 LAT-encoded miRNAs, but the sequences of these miRNAs are not conserved. The expression of LAT-encoded miRNAs is negatively regulated by ICP4, the major viral transactivator. We further show that, similar to miR-I, miR-II is able to efficiently silence the expression of ICP34.5, a key viral neurovirulence factor, and that miR-III is able to silence the expression of ICP0, a key viral transactivator. All these data suggest that LAT sequences likely contribute to HSV latency and reactivation through tight control of these LAT-encoded miRNAs and their viral targets.

Control of TANK-binding kinase 1-mediated signaling by the gamma(1)34.5 protein of herpes simplex virus 1

TANK-binding kinase 1 (TBK1) is a key component of Toll-like receptor-dependent and -independent signaling pathways. In response to microbial components, TBK1 activates interferon regulatory factor 3 (IRF3) and cytokine expression. Here we show that TBK1 is a novel target of the gamma(1)34.5 protein, a virulence factor whose expression is regulated in a temporal fashion. Remarkably, the gamma(1)34.5 protein is required to inhibit IRF3 phosphorylation, nuclear translocation, and the induction of antiviral genes in infected cells. When expressed in mammalian cells, the gamma(1)34.5 protein forms complexes with TBK1 and disrupts the interaction of TBK1 and IRF3, which prevents the induction of interferon and interferon-stimulated gene promoters. Down-regulation of TBK1 requires the amino-terminal domain. In addition, unlike wild type virus, a herpes simplex virus mutant lacking gamma(1)34.5 replicates efficiently in TBK1(-/-) cells but not in TBK1(+/+) cells. Addition of exogenous interferon restores the antiviral activity in both TBK1(-/-) and TBK(+/+) cells. Hence, control of TBK1-mediated cell signaling by the gamma(1)34.5 protein contributes to herpes simplex virus infection. These results reveal that TBK1 plays a pivotal role in limiting replication of a DNA virus.

ICP34.5-dependent and -independent activities of salubrinal in herpes simplex virus-1 infected cells

The small molecule salubrinal has antiviral activity against herpes simplex virus-1 (HSV-1) and inhibits dephosphorylation of eIF2 alpha mediated by the HSV-1 protein ICP34.5. We investigated whether salubrinal's activities in infected cells depend on ICP34.5. An ICP34.5 deletion mutant was as sensitive as wild type HSV-1 to salubrinal inhibition of plaque formation in Vero cells. However, salubrinal induced formation of syncytia in infected Vero cells, which was enhanced by ICP34.5 mutations. Expression of HSV-1 US11 with immediate early kinetics, which is known to suppress the effects of ICP34.5 mutations, resulted in slight resistance to salubrinal in murine embryonic fibroblasts, and substantial resistance in those cells when ICP34.5 was additionally mutated. ICP34.5 mutations, but not immediate early expression of US11, prevented salubrinal's ability to increase phosphorylation of eIF2 alpha during HSV-1 infection of Vero cells. Taken together, our data indicate that salubrinal has both ICP34.5-dependent and -independent activities in HSV-1 infected cells.

A conserved domain of herpes simplex virus ICP34.5 regulates protein phosphatase complex in mammalian cells

ICP34.5, encoded by herpes simplex virus 1, is a protein phosphatase 1 (PP1) regulatory subunit that mediates dephosphorylation of the alpha subunit of translation initiation factor 2 (eIF2alpha). However, the mechanism of its action remains poorly understood. Here, we show that amino acid substitutions in the arginine-rich motif have differential effects on ICP34.5 activity. The phenotypes parallel with viral protein synthesis and cytopathic effects in virus infected cells. Besides the consensus PP1 binding motif, the Arg-motif appears to enhance the interaction between ICP34.5 and PP1. These results suggest that concerted action between the PP1 binding domain and the effector domain of ICP34.5 is crucial for eIF2alpha dephosphorylation and viral protein synthesis.

Nck-1 selectively modulates eIF2alphaSer51 phosphorylation by a subset of eIF2alpha-kinases

Phosphorylation of the alpha-subunit of the eukaryotic initiation factor 2 (eIF2) on Ser51 is an early event associated with the down-regulation of protein synthesis at the level of translation and initiation of a transcriptional program. This constitutes a potent mechanism to overcome various stress conditions. In mammals, four eIF2alpha-kinases [PKR-like endoplasmic reticulum kinase (PERK), dsRNA-activated protein kinase (PKR), heme regulated inhibitor (HRI) and general control nonderepressible-2 (GCN2)], activated following specific stresses, have been shown to be involved in this process. In this article, we report that the ubiquitously expressed adaptor protein Nck, composed only of Src homology domains and classically implicated in cell signaling by activated plasma membrane receptor tyrosine kinases, modulates eIF2alpha-kinase-mediated eIF2alphaSer51 phosphorylation in a specific manner. Our results show that Nck not only prevents eIF2alpha phosphorylation upon PERK activation, as reported previously, but also reduces eIF2alpha phosphorylation in conditions leading to PKR and HRI activation. By contrast, the overexpression of Nck in mammalian cells fails to attenuate eIF2alphaSer51 phosphorylation in response to amino acid starvation, a stress well known to activate GCN2. This observation is further confirmed by showing that Nck fails to alter eIF2alphaSer51 phosphorylation in Saccharomyces cerevisiae, for which the sole eIF2alpha-kinase is Gcn2p. Our results suggest the existence of a novel mechanism that specifically modulates the phosphorylation of eIF2alpha on Ser51 under various stress conditions.

The dsRNA protein kinase PKR: virus and cell control

The IFN-induced double-stranded RNA-dependent protein kinase (PKR) is one of the four mammalian serine-threonine kinases (the three others being HRI, GCN2 and PERK) that phosphorylate the eIF2 alpha translation initiation factor, in response to stress signals, mainly as a result of viral infections. eIF2 alpha phosphorylation results in arrest of translation of both cellular and viral mRNAs, an efficient way to inhibit virus replication. The particularity of PKR is to activate by binding to dsRNA through two N terminal dsRNA binding motifs (dsRBM). PKR activation during a viral infection represents a threat for several viruses, which have therefore evolved to express PKR inhibitors, such as the Vaccinia E3L and K3L proteins. The function of PKR can also be regulated by cellular proteins, either positively (RAX/PACT; Mda7) or negatively (p58IPK, TRBP, nucleophosmin, Hsp90/70). PKR can provoke apoptosis, in part through its ability to control protein translation, but the situation appears to be more complex, as NF-kappaB, ATF-3 and p53 have also been implicated. PKR-induced apoptosis involves mainly the FADD/caspase 8 pathway, while the mitochondrial APAF/caspase 9 pathway is also engaged. As a consequence of the effects of PKR on translation, transcription and apoptosis, PKR can function to control cell growth and cell differentiation, and its activity can be controlled by the action of several oncogenes.

Impact of protein kinase PKR in cell biology: from antiviral to antiproliferative action

The double-stranded RNA-dependent protein kinase PKR is a critical mediator of the antiproliferative and antiviral effects exerted by interferons. Not only is PKR an effector molecule on the cellular response to double-stranded RNA, but it also integrates signals in response to Toll-like receptor activation, growth factors, and diverse cellular stresses. In this review, we provide a detailed picture on how signaling downstream of PKR unfolds and what are the ultimate consequences for the cell fate. PKR activation affects both transcription and translation. PKR phosphorylation of the alpha subunit of eukaryotic initiation factor 2 results in a blockade on translation initiation. However, PKR cannot avoid the translation of some cellular and viral mRNAs bearing special features in their 5' untranslated regions. In addition, PKR affects diverse transcriptional factors such as interferon regulatory factor 1, STATs, p53, activating transcription factor 3, and NF-kappaB. In particular, how PKR triggers a cascade of events involving IKK phosphorylation of IkappaB and NF-kappaB nuclear translocation has been intensively studied. At the cellular and organism levels PKR exerts antiproliferative effects, and it is a key antiviral agent. A point of convergence in both effects is that PKR activation results in apoptosis induction. The extent and strength of the antiviral action of PKR are clearly understood by the findings that unrelated viral proteins of animal viruses have evolved to inhibit PKR action by using diverse strategies. The case for the pathological consequences of the antiproliferative action of PKR is less understood, but therapeutic strategies aimed at targeting PKR are beginning to offer promising results.

The MyD116 African swine fever virus homologue interacts with the catalytic subunit of protein phosphatase 1 and activates its phosphatase activity

The DP71L protein of African swine fever virus (ASFV) shares sequence similarity with the herpes simplex virus ICP34.5 protein over a C-terminal domain. We showed that the catalytic subunit of protein phosphatase 1 (PP1) interacts specifically with the ASFV DP71L protein in a yeast two-hybrid screen. The chimeric full-length DP71L protein, from ASFV strain Badajoz 71 (BA71V), fused to glutathione S-transferase (DP71L-GST) was expressed in Escherichia coli and shown to bind specifically to the PP1-alpha catalytic subunit expressed as a histidine fusion protein (6xHis-PP1alpha) in E. coli. The functional effects of this interaction were investigated by measuring the levels of PP1 and PP2A in ASFV-infected Vero cells. This showed that infection with wild-type ASFV strain BA71V activated PP1 between two- and threefold over that of mock-infected cells. This activation did not occur in cells infected with the BA71V isolate in which the DP71L gene had been deleted, suggesting that expression of DP71L leads to PP1 activation. In contrast, no effect was observed on the activity of PP2A following ASFV infection. We showed that infection of cells with wild-type BA71V virus resulted in decreased phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF-2alpha). ICP34.5 recruits PP1 to dephosphorylate the alpha subunit of eukaryotic translational initiation factor 2 (also known as eIF-2alpha); possibly the ASFV DP71L protein has a similar function.

Interferon-gamma mRNA attenuates its own translation by activating PKR: a molecular basis for the therapeutic effect of interferon-beta in multiple sclerosis

PKR, the interferon (IFN)-inducible protein kinase activated by double-stranded RNA, inhibits translation by phosphorylating the initiation factor eIF2alpha chain. Uniquely, human IFN-gamma mRNA uses local activation of PKR in the cell to control its own translation yield. IFN-gamma mRNA activates PKR through a structure in its 5'- region harboring a pseudoknot which is critical for PKR activation. Mutations that impair pseudoknot stability reduce the ability of IFN-gamma mRNA to activate PKR and strongly increase its translation efficiency. The cis-acting RNA element in IFN-gamma mRNA functions as a biological sensor of intracellular PKR levels. During an immune response, as IFN-gamma and other inflammatory cytokines build up in the cell's microenvironment, they act to induce higher levels of PKR in the cell, resulting in a more extensive activation of PKR by IFN-gamma mRNA. With the resulting phosphorylation of eIF2alpha, a negative feedback loop is created and the production of IFN-gamma is progressively attenuated. We propose that the therapeutic effect of IFN-beta in multiple sclerosis may rest, at least in part, on its exquisite ability to induce high levels of PKR in the cell and thereby to limit IFN-gamma mRNA translation through this negative feedback loop, blocking the excessive IFN-gamma gene expression that precedes clinical attacks.

Endoplasmic reticulum stress signaling in disease

The extracellular space is an environment hostile to unmodified polypeptides. For this reason, many eukaryotic proteins destined for exposure to this environment through secretion or display at the cell surface require maturation steps within a specialized organelle, the endoplasmic reticulum (ER). A complex homeostatic mechanism, known as the unfolded protein response (UPR), has evolved to link the load of newly synthesized proteins with the capacity of the ER to mature them. It has become apparent that dysfunction of the UPR plays an important role in some human diseases, especially those involving tissues dedicated to extracellular protein synthesis. Diabetes mellitus is an example of such a disease, since the demands for constantly varying levels of insulin synthesis make pancreatic beta-cells dependent on efficient UPR signaling. Furthermore, recent discoveries in this field indicate that the importance of the UPR in diabetes is not restricted to the beta-cell but is also involved in peripheral insulin resistance. This review addresses aspects of the UPR currently understood to be involved in human disease, including their role in diabetes mellitus, atherosclerosis, and neoplasia.

Nck in a Complex Containing the Catalytic Subunit of Protein Phosphatase 1 Regulates Eukaryotic Initiation Factor 2α Signaling and Cell Survival to Endoplasmic Reticulum Stress

Stress imposed on the endoplasmic reticulum (ER) induces the phosphorylation of the alpha-subunit of the eukaryotic initiation factor 2 (eIF2) on Ser51. This results in transient inhibition of general translation initiation while concomitantly activating a signaling pathway that promotes the expression of genes whose products improve ER function. Conversely, dephosphorylation of eIF2alphaSer51 is accomplished by protein phosphatase 1 (PP1c) complexes containing either the protein CReP or GADD34, which target PP1c to eIF2. Here, we demonstrate that the Src homology (SH) domain-containing adaptor Nck is a key component of a molecular complex that controls eIF2alpha phosphorylation and signaling in response to ER stress. We show that overexpression of Nck decreases basal and ER stress-induced eIF2alpha phosphorylation and the attendant induction of ATF4 and CHOP. In contrast, we demonstrate that the mouse embryonic fibroblasts lacking both isoforms of Nck (Nck1-/-Nck2-/-) show higher levels of eIF2alpha phosphorylation and premature induction of ATF4, CHOP, and GADD34 in response to ER stress and finally, are more resistant to cell death induced by prolonged ER stress conditions. We establish that a significant amount of Nck protein localizes at the ER and is in a complex with eIF2 subunits. Further analysis of this complex revealed that it also contains the Ser/Thr phosphatase PP1c, its regulatory subunit CReP, and dephosphorylates eIF2alpha on Ser51 in vitro. Overall, we demonstrate that Nck as a component of the CReP/PP1c holophosphatase complex contributes to maintain eIF2alpha in a hypophosphorylated state. In this manner, Nck modulates translation and eIF2alpha signaling in response to ER stress.

Functional genomic analysis of herpes simplex virus type 1 counteraction of the host innate response

Herpes simplex virus type 1 (HSV-1) mutants lacking the ICP34.5 gene are severely attenuated in mouse models and have a significant growth defect in confluent mouse embryo fibroblasts. Previously, ICP34.5 was demonstrated to have a crucial role in evading the innate immune response to infection by mediating the dephosphorylation of eIF2alpha, a translation initiation factor phosphorylated by PKR during the antiviral response. To further understand the role of ICP34.5 in evasion of the antiviral response, we used transcriptional profiling to examine host cell gene expression in both wild-type and ICP34.5-null virus-infected mouse embryo fibroblasts over a time course of infection. Our study revealed that cells responded to infection within 3 h through PKR-dependent eIF2alpha phosphorylation and that the majority of up-regulated genes at 3 h postinfection were involved in the antiviral response. HSV-1 counters this response through early expression of ICP34.5 and dephosphorylation of eIF2alpha. By 12 h postinfection, the differences between the number and functional classification of genes differentially up- and down-regulated between wild-type and ICP34.5-null virus-infected cells were maximal. Specifically, in wild-type virus-infected cells, the majority of changed genes were involved in metabolic and biosynthetic processes, while in ICP34.5-null virus-infected cells, mostly antiviral genes were up-regulated. Further, ICP34.5-null virus-infected cells produced greater amounts of beta interferon than wild-type virus-infected cells. These results indicate that ICP34.5 expression and function at early times postinfection have a pivotal role in the ability of HSV-1 to gain control of the host cell and maintain an environment for successful viral replication.

Viruses, endoplasmic reticulum stress, and interferon responses

Viral infection induces endoplasmic reticulum (ER) stress and interferon responses. While viral double-stranded RNA intermediates trigger interferon responses, viral polypeptides synthesized during infection stimulate ER stress. Among the interferon-regulated gene products, the double-stranded RNA-dependent protein kinase (PKR) plays a key role in limiting viral replication. Thus, to establish productive infection, viruses have evolved mechanisms to overcome the deleterious effects of PKR. It has become clear that ER stress causes translational attenuation and transcriptional upregulation of genes encoding proteins that facilitate folding or degradation of proteins. Notably, prolonged ER stress triggers apoptosis. Therefore, viruses are confronted with the consequences of ER stress. Emerging evidence suggests that viruses not only interfere with the interferon system involving PKR but also manipulate the programs emanating from the ER in a complex way, which may facilitate viral replication or pathogenesis. This review highlights recent progress in these areas.

Ionizing Radiation Activates Late Herpes Simplex Virus 1 Promoters via the p38 Pathway in Tumors Treated with Oncolytic Viruses

Ionizing radiation potentiates the oncolytic activity of attenuated herpes simplex viruses in tumors exposed to irradiation at specific time intervals by inducing higher virus yields. Cell culture studies have shown that an attenuated virus lacking the viral gamma(1)34.5 genes underproduces late proteins whose synthesis depends on sustained synthesis of viral DNA. Here we report that ionizing radiation enhances gene expression from late viral promoters in transduced cells in the absence of other viral gene products. Consistent with this result, we show that in tumors infected with the attenuated virus, ionizing radiation increases 13.6-fold above baseline the gene expression from a late viral promoter as early as 2 hours after virus infection, an interval too short to account for viral DNA synthesis. The radiation-dependent up-regulation of late viral genes is mediated by the p38 pathway, inasmuch as the enhancement is abolished by p38 inhibitors or a p38 dominant-negative construct. The p38 pathway is not essential for wild-type virus gene expression. The results suggest that ionizing radiation up-regulates late promoters active in the course of viral DNA synthesis and provide a rationale for use of radiation to up-regulate cytotoxic genes introduced into tumor cells by viral vectors for cytoreductive therapy.

Nuclear Targeting of Protein Phosphatase-1 by HIV-1 Tat Protein

Transcription of human immunodeficiency virus (HIV)-1 genes is activated by HIV-1 Tat protein, which induces phosphorylation of the C-terminal domain of RNA polymerase-II by CDK9/cyclin T1. We previously showed that Tat-induced HIV-1 transcription is regulated by protein phosphatase-1 (PP1). In the present study we demonstrate that Tat interacts with PP1 and that disruption of this interaction prevents induction of HIV-1 transcription. We show that PP1 interacts with Tat in part through the binding of Val36 and Phe38 of Tat to PP1 and that Tat is involved in the nuclear and subnuclear targeting of PP1. The PP1 binding mutant Tat-V36A/F38A displayed a decreased affinity for PP1 and was a poor activator of HIV-1 transcription. Surprisingly, Tat-Q35R mutant that had a higher affinity for PP1 was also a poor activator of HIV-1 transcription, because strong PP1 binding competed out binding of Tat to CDK9/cyclin T1. Our results suggest that Tat might function as a nuclear regulator of PP1 and that interaction of Tat with PP1 is critical for activation of HIV-1 transcription by Tat.

Herpesviruses and the innate immune response

Herpesvirus infection leads to the rapid induction of an innate immune response. A central aspect of this host response is the production and secretion of type I interferon. The current model of virus-mediated interferon production includes three stages: sensitization, induction, and amplification. A key mediator of all three stages is the cellular transcription factor interferon regulatory factor 3 (IRF3). Although the precise details of IRF3 activation and interferon production in response to herpesvirus infection are still being elucidated, viral proteins that block components of the interferon pathway, particularly IRF3, have been identified and characterized. In vivo studies have shown that in addition to type I interferon, interleukin-15 (IL-15) and natural killer (NK) cells also play an important role in mediating resistance to herpesvirus infection. Recent investigations have demonstrated a strong association between IRF3, interferon, IL-15, and NK cells. This review will focus on herpesvirus-mediated induction of innate immunity, the central role of the type I interferon response and mechanisms used by herpesviruses to block host antiviral immunity.

Characterization of the triplet repeats in the central domain of the gamma134.5 protein of herpes simplex virus 1

The gamma134.5 protein of herpes simplex virus 1 (HSV-1) consists of an amino-terminal domain, a central domain with triplet repeats (Ala-Thr-Pro) and a carboxyl-terminal domain. The triplet repeats are a unique feature of the gamma134.5 protein encoded by HSV-1, but the number of repeats varies among different strains. Notably, the central domain containing the triplet repeats is implicated in neuroinvasion. In this report, it has been shown that partial or full deletion of triplet repeats, i.e. from ten to either three or zero, in the gamma134.5 protein has no effect on the virus response to interferon. The triplet deletion mutants replicate efficiently in CV-1 and mouse 10T1/2 cells. However, in mouse 3T6 cells, these mutants grow with delayed growth kinetics. This decrease in growth, compared with wild-type HSV-1(F), does not result from failure of the virus to suppress the RNA-dependent protein kinase response, but rather from a delay in virus release or egress. Accordingly, these mutant viruses are predominantly present within infected cells. These results indicate that deletions in the central domain of the gamma134.5 protein impair virus egress, but not virus response to interferon.

The role of viral and host genes in corneal infection with herpes simplex virus type 1

Herpes simplex virus infection of the eye is the leading cause of blindness due to infection in the US despite the availability of several antiviral drugs. Studies with animal models have shown that three factors, innate host resistance, the host adaptive immune response, and the strain of virus interact to determine whether an infection is asymptomatic or proceeds to the development of blinding keratitis (HSK). Of these, the role of adaptive immunity has received the most attention. This work has clearly shown that stromal keratitis is an immunopathological disease, most likely due to the induction of a delayed type hypersensitivity response. Substantially less is known about the role of specific host genes in resistance to HSK. The fact that different strains of virus display different disease phenotypes indicates that viral 'virulence' genes are critical. Of the 80 plus HSV genes, few have been formally tested for their role in HSV keratitis. Most studies of virulence genes to date have focused on a single gene or protein and large changes in disease phenotypes are usually measured. Large changes in the ability to cause disease are likely to reduce the fitness of the virus, thus such studies, although useful, do not mimic the natural situation. Viral gene products are known to interact with each other, and with host proteins and these interactions are critical in determining the outcome of infection. In reality, the 'constellation' of genes encoded by each particular strain is critical, and how this constellation of genes works together and with host proteins determines the outcome of an infection. The goal of this review is to discuss the current state of knowledge regarding the role of host and viral genes in HSV keratitis. The roles of specific genes that have been shown to influence keratitis are discussed. Recent data showing that different viral genes cooperate to influence disease severity and confirming that the constellation of genes within a particular strain determines the disease phenotype are also discussed, as are the methods used to test the role of viral genes in virulence. It will become apparent that there is a paucity of information regarding the function of many viral genes in keratitis. Improving our knowledge of the role of viral genes is critical for devising more effective treatments for this disease.

Synergistic inhibition of human cytomegalovirus replication by interferon-alpha/beta and interferon-gamma

BACKGROUND

Recent studies have shown that gamma interferon (IFN-gamma) synergizes with the innate IFNs (IFN-alpha and IFN-beta) to inhibit herpes simplex virus type 1 (HSV-1) replication in vitro. To determine whether this phenomenon is shared by other herpesviruses, we investigated the effects of IFNs on human cytomegalovirus (HCMV) replication.

RESULTS

We have found that as with HSV-1, IFN-gamma synergizes with the innate IFNs (IFN-alpha/beta) to potently inhibit HCMV replication in vitro. While pre-treatment of human foreskin fibroblasts (HFFs) with IFN-alpha, IFN-beta or IFN-gamma alone inhibited HCMV plaque formation by approximately 30 to 40-fold, treatment with IFN-alpha and IFN-gamma or IFN-beta and IFN-gamma inhibited HCMV plaque formation by 163- and 662-fold, respectively. The generation of isobole plots verified that the observed inhibition of HCMV plaque formation and replication in HFFs by IFN-alpha/beta and IFN-gamma was a synergistic interaction. Additionally, real-time PCR analyses of the HCMV immediate early (IE) genes (IE1 and IE2) revealed that IE mRNA expression was profoundly decreased in cells stimulated with IFN-alpha/beta and IFN-gamma (approximately 5-11-fold) as compared to vehicle-treated cells. Furthermore, decreased IE mRNA expression was accompanied by a decrease in IE protein expression, as demonstrated by western blotting and immunofluorescence.

CONCLUSION

These findings suggest that IFN-alpha/beta and IFN-gamma synergistically inhibit HCMV replication through a mechanism that may involve the regulation of IE gene expression. We hypothesize that IFN-gamma produced by activated cells of the adaptive immune response may potentially synergize with endogenous type I IFNs to inhibit HCMV dissemination in vivo.

Herpes simplex virus 1 infection activates the endoplasmic reticulum resident kinase PERK and mediates eIF-2alpha dephosphorylation by the gamma(1)34.5 protein

The gamma(1)34.5 protein of herpes simplex virus (HSV) plays a crucial role in virus infection. Although the double-stranded RNA-dependent protein kinase (PKR) is activated during HSV infection, the gamma(1)34.5 protein inhibits the activity of PKR by mediating dephosphorylation of the translation initiation factor eIF-2alpha. Here we show that HSV infection also induces phosphorylation of an endoplasmic reticulum (ER) resident kinase PERK, a hallmark of ER stress response. The virus-induced phosphorylation of PERK is blocked by cycloheximide but not by phosphonoacetic acid, suggesting that the accumulation of viral proteins in the ER is essential. Notably, the maximal phosphorylation of PERK is delayed in PKR+/+ cells compared to that seen in PKR-/- cells. Further analysis indicates that hyperphosphorylation of eIF-2alpha caused by HSV is greater in PKR+/+ cells than in PKR-/- cells. However, expression of the gamma(1)34.5 protein suppresses the ER stress response caused by virus, dithiothreitol, and thapsigargin as measured by global protein synthesis. Interestingly, the expression of GADD34 stimulated by HSV infection parallels the status of eIF-2alpha phosphorylation. Together, these observations suggest that regulation of eIF-2alpha phosphorylation by the gamma(1)34.5 protein is an efficient way to antagonize the inhibitory activity of PKR as well as PERK during productive infection.

Cells lacking NF-kappaB or in which NF-kappaB is not activated vary with respect to ability to sustain herpes simplex virus 1 replication and are not susceptible to apoptosis induced by a replication-incompetent mutant virus

Earlier we reported that NF-kappaB is activated by protein kinase R (PKR) in herpes simplex virus 1-infected cells. Here we report that in PKR(-/-) cells the yields of wild-type virus are 10-fold higher than in PKR(+/+) cells. In cells lacking NF-kappaB p50 (nfkb1), p65 (relA), or both p50 and p65, the yields of virus were reduced 10-fold. Neither wild-type nor mutant cells undergo apoptosis following infection with wild-type virus. Whereas PKR(+/+) and NF-kappaB(+/+) control cell lines undergo apoptosis induced by the d120 (Deltaalpha4) mutant of HSV-1, the mutant PKR(-/-) and NF-kappaB(-/-) cell lines were resistant. The evidence suggests that the stress-induced apoptosis resulting from d120 infection requires activation of NF-kappaB and that this proapoptotic pathway is blocked in cells in which NF-kappaB is not activated or absent. Activation of NF-kappaB in the course of viral infection may have dual roles of attempting to curtain viral replication by rendering the cell susceptible to apoptosis induced by the virus and by inducing the synthesis of proteins that enhance viral replication.

African swine fever virus proteins involved in evading host defence systems

African swine fever virus (ASFV) can cause an acutely fatal haemorrhagic fever in domestic pigs although in its natural hosts, warthogs, bushpigs and the soft tick vector, Ornithodoros moubata, ASFV causes inapparent persistent infections. The virus is a large, cytoplasmic, double-stranded DNA virus which has a tropism for macrophages. As it is the only member of the Asfarviridae family, ASFV encodes many novel genes not encoded by other virus families. The ability of the virus to persist in its natural hosts and in domestic pigs, which recover from infection with less virulent isolates, shows that the virus has effective mechanisms to evade host defence systems. This review focuses on recent progress made in understanding the function of ASFV-encoded proteins, which are involved in modulating the host response to infection. Growing evidence suggests that a major strategy used by the virus is to modulate signalling pathways in infected macrophages, thus interfering with the expression of a large number of immunomodulatory genes. One potent immunomodulatory protein, A238L, inhibits both activation of the host NFkappaB transcription factor and inhibits calcineurin phosphatase activity. Calcineurin-dependent pathways, including activation of the NFAT transcription factor, are therefore inhibited. Another ASFV-encoded protein, CD2v, resembles the host CD2 protein, which is expressed on T cells and NK cells. This virus protein causes the adsorption of red blood cells around virus-infected cells and extracellular virus particles. Expression of the CD2v protein aids virus dissemination in pigs and the protein also has a role in impairing bystander lymphocyte function. This may be mediated either by a direct interaction of CD2v extracellular domain with ligands on lymphocytes or by an indirect mechanism involving interaction of the CD2v cytoplasmic tail with host proteins involved in signalling or trafficking pathways. Two ASFV proteins, an IAP and a Bcl2 homologue, inhibit apoptosis in infected cells and thus facilitate production of progeny virions. The prediction is that half to two-thirds of the approximately 150 genes encoded by ASFV are not essential for replication in cells but have an important role for virus survival and transmission in its hosts. These genes provide an untapped repository, and will be valuable tools for deciphering not only how the virus manipulates the host response to infection to avoid elimination, but also useful for understanding important host anti-viral mechanisms. In addition, they may provide leads for discovery of novel immunomodulatory drugs.

GADD34 induces p53 phosphorylation and p21/WAF1 transcription

Recently, others and we have shown that one of the functions of GADD34 is a recovery from a shutoff of protein synthesis induced by endoplasmic reticulum stress. GADD34 has been shown to induce growth arrest and apoptosis. Main protein of apoptosis is p53, especially phosphorylation of p53. And one of the main proteins of growth arrest is p21/WAF1. Here we analyzed the effects of GADD34 on p53 phosphorylation and p21/WAF1 transcription. Transfected Myc-tagged p53 was dose-dependently phosphorylated at Ser15 by increasing the amount of GADD34. Transfection of GADD34 also induced the endogenous phosphorylation of p53 and enhanced p21 protein expression. Transfection of GADD34 induced p21/WAF1 promoter activity. This activity was dependent on p53, because GADD34 transfection to p53-deficient cells produced only a slight increase of p21/WAF1 promoter activity. The p21/WAF1 promoter activity was greatly enhanced by the transfection of p53. Both GADD34 and p53 transfection induced much higher p21/WAF1 promoter activity. The promoter activity of p21/WAF1 was very low in GADD34 deficient MEF. The transfection of GADD34 increased the p21/WAF1 promoter activity in GADD34 deficient MEF.

Replication of herpes simplex virus 1 depends on the gamma 134.5 functions that facilitate virus response to interferon and egress in the different stages of productive infection

The ability of the gamma(1)34.5 protein to suppress the PKR response plays a crucial role in herpes simplex virus pathogenesis. In this process, the gamma(1)34.5 protein associates with protein phosphatase 1 to form a large complex that dephosphorylates eIF-2alpha and thereby prevents translation shutoff mediated by PKR. Accordingly, gamma(1)34.5 null mutants are virulent in PKR-knockout mice but not in wild-type mice. However, gamma(1)34.5 deletion mutants, with an extragenic compensatory mutation, inhibit PKR activity but remain avirulent, suggesting that the gamma(1)34.5 protein has additional functions. Here, we show that a substitution of the gamma(1)34.5 gene with the NS1 gene from influenza A virus renders viral resistance to interferon involving PKR. The virus replicates as efficiently as wild-type virus in SK-N-SH and CV-1 cells. However, in mouse 3T6 cells, the virus expressing the NS1 protein grows at an intermediate level between the wild-type virus and the gamma(1)34.5 deletion mutant. This decrease in growth, compared to that of the wild-type virus, is due not to an inhibition of viral protein synthesis but rather to a block in virus release or egress. Virus particles are predominantly present in the nucleus and cytoplasm. Notably, deletions in the amino terminus of the gamma(1)34.5 protein lead to a significant decrease in virus growth in mouse 3T6 cells, which is independent of eIF-2alpha dephosphorylation. In correlation, a series of deletions in the amino-terminal domain impair nuclear as well as cytoplasmic egress. These results indicate that efficient viral replication depends on the gamma(1)34.5 functions required to prevent the PKR response and to facilitate virus egress in the different stages during virus infection.

Herpes simplex virus 1 gene products occlude the interferon signaling pathway at multiple sites

Earlier studies have shown that herpes simplex virus 1 (HSV-1) blocks the interferon response pathways, at least at two sites, by circumventing the effects of activation of protein kinase R by double-stranded RNA and interferon and through the degradation of promyelocytic leukemia protein (PML) since interferon has no antiviral effects in PML(-/-) cells. Here we report on two effects of viral genes on other sites of the interferon signaling pathway. (i) In infected cells, Jak1 kinase associated with interferon receptors and Stat2 associated with the interferon signaling pathway rapidly disappear from infected cells. The level of interferon alpha receptor is also reduced, albeit less drastically at times after 4 h postinfection. Other members of the Stat family of proteins were either decreased in amount or posttranslationally processed in a manner different from those of mock-infected cells. The decrease in the levels of Jak1 and Stat2 may account for the decrease in the formation of complexes consisting of Stat1 or ISGF3 and DNA sequences containing the interferon-stimulated response elements after exposure to interferon. (ii) The disappearance of Jak1 and Stat2 was related at least in part to the function of the virion host shutoff protein, the product of the viral U(L)41 gene. Consistent with this observation, a mutant lacking the U(L)41 gene and treated with interferon produced lesser amounts of a late protein (U(L)38) than the wild-type parent. We conclude that HSV-1 blocks the interferon signaling pathways at several sites.

Neutralizing innate host defenses to control viral translation in HSV-1 infected cells

Lytic replication of many viruses activates an innate host response designed to prevent the completion of the viral lifecycle, thus impeding the spread of the infection. One branch of the host's complex reaction functions to incapacitate the cellular translational machinery on which the synthesis of viral polypeptides completely depends. This is achieved through the activation of specific protein kinases that phosphorylate eIF2 on its alpha subunit and inactivate this critical translation initiation factor. However, as continued synthesis of viral proteins is required to assemble the viral progeny necessary to transmit the infection to neighboring cells, viruses have developed a variety of strategies to counter this cellular response. Genetic and biochemical studies with herpes simplex virus type 1 (HSV-1) have revealed that the virus produces at least two discrete products at different times during its replicative program that act to prevent the accumulation of phosphorylated eIF2alpha. The gamma(1)34.5 gene product is expressed first, encoding a regulatory subunit that binds the cellular protein phosphatase 1alpha and regenerates pools of active eIF2 by removing the inhibitory phosphate from the alpha subunit. The second function, encoded by the product of the Us11 gene, specifies a double-stranded RNA-binding protein that prevents activation of PKR, a cellular eIF2alpha kinase. Together, both proteins cooperate to overcome the antiviral response of the host and properly regulate translation in HSV-1-infected cells.

Functional diversity of protein phosphatase-1, a cellular economizer and reset button

The protein serine/threonine phosphatase protein phosphatase-1 (PP1) is a ubiquitous eukaryotic enzyme that regulates a variety of cellular processes through the dephosphorylation of dozens of substrates. This multifunctionality of PP1 relies on its association with a host of function-specific targetting and substrate-specifying proteins. In this review we discuss how PP1 affects the biochemistry and physiology of eukaryotic cells. The picture of PP1 that emerges from this analysis is that of a "green" enzyme that promotes the rational use of energy, the recycling of protein factors, and a reversal of the cell to a basal and/or energy-conserving state. Thus PP1 promotes a shift to the more energy-efficient fuels when nutrients are abundant and stimulates the storage of energy in the form of glycogen. PP1 also enables the relaxation of actomyosin fibers, the return to basal patterns of protein synthesis, and the recycling of transcription and splicing factors. In addition, PP1 plays a key role in the recovery from stress but promotes apoptosis when cells are damaged beyond repair. Furthermore, PP1 downregulates ion pumps and transporters in various tissues and ion channels that are involved in the excitation of neurons. Finally, PP1 promotes the exit from mitosis and maintains cells in the G1 or G2 phases of the cell cycle.

Nck-1 Antagonizes the Endoplasmic Reticulum Stress-induced Inhibition of Translation

Eukaryotic cells have developed specific mechanisms to overcome environmental stress. Here we show that the Src homology 2/3 (SH2/SH3) domain-containing protein Nck-1 prevents the unfolded protein response normally induced by pharmacological endoplasmic reticulum (ER) stress agents. Overexpression of Nck-1 enhances protein translation, whereas it abrogates eukaryotic initiation factor 2alpha (eIF2alpha) phosphorylation and inhibition of translation in response to tunicamycin or thapsigargin treatment. Nck-1 overexpression also attenuates induction of the ER chaperone, the immunoglobulin heavy chain-binding protein (BiP), and impairs cell survival in response to thapsigargin. We provided evidence that in these conditions, the effects of Nck on the unfolded protein response (UPR) involve its second SH3 domain and a calyculin A-sensitive phosphatase activity. In addition, we demonstrated that protein translation is reduced in mouse embryonic fibroblasts lacking both Nck isoforms and is enhanced in similar cells expressing high levels of Nck-1. In these various mouse embryonic fibroblasts, we also provided evidence that Nck modulates the activation of the ER resident eIF2alpha kinase PERK and consequently the phosphorylation of eIF2alpha on Ser-51 in response to stress. Our study establishes that Nck is required for optimal protein translation and demonstrates that, in addition to its adaptor function in mediating signaling from the plasma membrane, Nck also mediates signaling from the ER membrane compartment.

Inhibition of a constitutive translation initiation factor 2alpha phosphatase, CReP, promotes survival of stressed cells

Phosphorylation of eukaryotic translation initiation factor 2α (eIF2α) on serine 51 is effected by specific stress-activated protein kinases. eIF2α phosphorylation inhibits translation initiation promoting a cytoprotective gene expression program known as the integrated stress response (ISR). Stress-induced activation of GADD34 feeds back negatively on this pathway by promoting eIF2α dephosphorylation, however, GADD34 mutant cells retain significant eIF2α-directed phosphatase activity. We used a somatic cell genetic approach to identify a gene encoding a novel regulatory subunit of a constitutively active holophosphatase complex that dephosphorylates eIF2α. RNAi of this gene, which we named constitutive repressor of eIF2α phosphorylation (CReP, or PPP1R15B), repressed the constitutive eIF2α-directed phosphatase activity and activated the ISR. CReP RNAi strongly protected mammalian cells against oxidative stress, peroxynitrite stress, and more modestly against accumulation of malfolded proteins in the endoplasmic reticulum. These findings suggest that therapeutic inhibition of eIF2α dephosphorylation by targeting the CReP-protein–phosphatase-1 complex may be used to access the salubrious qualities of the ISR.

GADD34-PP1c recruited by Smad7 dephosphorylates TGFbeta type I receptor

The cascade of phosphorylation is a pivotal event in transforming growth factor β (TGFβ) signaling. Reversible phosphorylation regulates fundamental aspects of cell activity. TGFβ-induced Smad7 binds to type I receptor (TGFβ type I receptor; TβRI) functioning as a receptor kinase antagonist. We found Smad7 interacts with growth arrest and DNA damage protein, GADD34, a regulatory subunit of the protein phosphatase 1 (PP1) holoenzyme, which subsequently recruits catalytic subunit of PP1 (PP1c) to dephosphorylate TβRI. Blocking Smad7 expression by RNA interference inhibits association of GADD34–PP1c complex with TβRI, indicating Smad7 acts as an adaptor protein in the formation of the PP1 holoenzyme that targets TβRI for dephosphorylation. SARA (Smad anchor for receptor activation) enhances the recruitment PP1c to the Smad7–GADD34 complex by controlling the specific subcellular localization of PP1c. Importantly, GADD34–PP1c recruited by Smad7 inhibits TGFβ-induced cell cycle arrest and mediates TGFβ resistance in responding to UV light irradiation. The dephosphorylation of TβRI mediated by Smad7 is an effective mechanism for governing negative feedback in TGFβ signaling.

Regulation of eIF2alpha phosphorylation by different functions that act during discrete phases in the herpes simplex virus type 1 life cycle

Multiple herpes simplex virus type 1 functions control translation by regulating phosphorylation of the initiation factor eIF2 on its alpha subunit. Both of the two known regulators, the gamma(1)34.5 and Us11 gene products, are produced late in the viral life cycle, although the gamma(1)34.5 gene is expressed prior to the gamma(2) Us11 gene, as gamma(2) genes require viral DNA replication for their expression while gamma(1) genes do not. The gamma(1)34.5 protein, through a GADD34-related domain, binds a cellular phosphatase (PP1alpha), maintaining pools of active, unphosphorylated eIF2. Infection of a variety of cultured cells with a gamma(1)34.5 mutant virus results in the accumulation of phosphorylated eIF2alpha and the inhibition of translation prior to the completion of the viral lytic program. Ectopic, immediate-early Us11 expression prevents eIF2alpha phosphorylation and the inhibition of translation observed in cells infected with a gamma(1)34.5 mutant by inhibiting activation of the cellular kinase PKR and the subsequent phosphorylation of eIF2alpha; however, a requirement for the Us11 protein, produced in its natural context as a gamma(2) polypeptide, remains to be demonstrated. To determine if Us11 regulates late translation, we generated two Us11 null viruses. In cells infected with a Us11 mutant, elevated levels of activated PKR and phosphorylated eIF2alpha were detected, viral translation rates were reduced 6- to 7-fold, and viral replication was reduced 13-fold compared to replication in cells infected with either wild-type virus or a virus in which the Us11 mutation was repaired. This establishes that the Us11 protein is critical for proper late translation rates. Moreover, it demonstrates that the shutoff of protein synthesis observed in cells infected with a gamma(1)34.5 mutant virus, previously ascribed solely to the gamma(1)34.5 mutation, actually results from the combined loss of gamma(1)34.5 and Us11 functions, as the gamma(2) Us11 mRNA is not translated in cells infected with a gamma(1)34.5 mutant.

Dephosphorylation of eIF-2alpha mediated by the gamma(1)34.5 protein of herpes simplex virus type 1 is required for viral response to interferon but is not sufficient for efficient viral replication

The gamma(1)34.5 protein of herpes simplex virus type 1 (HSV-1) functions to block the shutoff of protein synthesis involving double-stranded RNA-dependent protein kinase (PKR). In this process, the gamma(1)34.5 protein recruits cellular protein phosphatase 1 (PP1) to form a high-molecular-weight complex that dephosphorylates eIF-2alpha. Here we show that the gamma(1)34.5 protein is capable of mediating eIF-2alpha dephosphorylation without any other viral proteins. While deletion of amino acids 1 to 52 from the gamma(1)34.5 protein has no effect on eIF-2alpha dephosphorylation, further truncations up to amino acid 146 dramatically reduce the activity of the gamma(1)34.5 protein. An additional truncation up to amino acid 188 is deleterious, indicating that the carboxyl-terminal domain alone is not functional. Like wild-type HSV-1, the gamma(1)34.5 mutant with a truncation of amino acids 1 to 52 is resistant to interferon, and resistance to interferon is coupled to eIF-2alpha dephosphorylation. Intriguingly, this mutant exhibits a similar growth defect seen for the gamma(1)34.5 null mutant in infected cells. Restoration of the wild-type gamma(1)34.5 gene in the recombinant completely reverses the phenotype. These results indicate that eIF-2alpha dephosphorylation mediated by the gamma(1)34.5 protein is required for HSV response to interferon but is not sufficient for viral replication. Additional functions or activities of the gamma(1)34.5 protein contribute to efficient viral infection.