Measles virus V protein inhibits p53 family member p73

Insights into the different mechanisms of Autophagy and Apoptosis mediated by Morbilliviruses

Viruses are obligate intracellular parasites that have co-evolved with the host. During the course of evolution, viruses have acquired abilities to abrogate the host's immune responses by modulating the host proteins which play a pivotal role in various biological processes. One such process is the programmed cell death in virus-infected cells, which can occur via autophagy or apoptosis. Morbilliviruses are known to modulate both autophagy and apoptosis. Upon infecting a cell, the morbilliviruses can utilize autophagosomes as their nest and delay the host defense apoptotic response, and/or can promote apoptosis to escalate the virus dissemination. Moreover, there is an active interplay between these two pathways which eventually decides the fate of a virus-infected cell. Recent advances in our understanding of these processes provide a potential rationale to further explore morbilliviruses for therapeutic purposes.

Non-Structural Protein V of Canine Distemper Virus Induces Autophagy via PI3K/AKT/mTOR Pathway to Facilitate Viral Replication

Canine distemper (CD) is a highly infectious disease of dogs which is caused by canine distemper virus (CDV). Previous studies have demonstrated that CDV infection can induce autophagy in cells. However, the mechanism underlying CDV-induced autophagy remains not fully understood. The CDV non-structural protein V plays a vital role in viral replication and pathogenicity in the host. In this study, we investigated the relationship between the CDV-V protein and autophagy induction and further explored its impact on the viral replication and the mechanism behind this. Our results showed that the V protein induced autophagy via inhibiting the phosphorylation of PI3K, AKT, and mTOR to promote viral replication. The activation or inhibition of PI3K phosphorylation resulted in enhancing or reducing viral replication, respectively. Further studies revealed that the V protein interacted with PI3K to induce cellular autophagy. The present study demonstrated that the CDV-V protein can induce cellular autophagy by inhibiting the PI3K/AKT signaling pathway to enhance viral replication. The results improve the understanding of the molecular mechanism of CDV infection and offer new perspectives for the development of effective treatment and prevention strategies.

Insights into the mechanism of Morbillivirus induced immune suppression

Viruses enter the host cell, and various strategies are employed to evade the host immune system. These include overcoming the various components of the immune system, including modulation of the physical and chemical barriers, non-specific innate response and specific adaptive immune response. Morbilliviruses impose immune modulation by utilizing various approaches including hindering antigen presentation to T-Helper (TH) cells, hematopoiesis and suppression of effector molecule activities. These viruses can also impede the early stages of T cell activation. Despite the availability of effective vaccines, morbilliviruses are still a significant threat to mankind. After infection, they also induce a state of immune suppression in the host. The molecular mechanisms employed by morbilliviruses to induce the state of immune suppression in the infected host are still being investigated. This review is an attempt to summarize insights into some of the strategies adopted by morbilliviruses to mediate immune modulation in the host.

Evasion of Host Antiviral Innate Immunity by Paramyxovirus Accessory Proteins

For efficient replication, viruses have developed multiple strategies to evade host antiviral innate immunity. Paramyxoviruses are a large family of enveloped RNA viruses that comprises diverse human and animal pathogens which jeopardize global public health and the economy. The accessory proteins expressed from the P gene by RNA editing or overlapping open reading frames (ORFs) are major viral immune evasion factors antagonizing type I interferon (IFN-I) production and other antiviral innate immune responses. However, the antagonistic mechanisms against antiviral innate immunity by accessory proteins differ among viruses. Here, we summarize the current understandings of immune evasion mechanisms by paramyxovirus accessory proteins, specifically how accessory proteins directly or indirectly target the adaptors in the antiviral innate immune signaling pathway to facilitate virus replication. Additionally, some cellular responses, which are also involved in viral replication, will be briefly summarized.

Adaptive homeostasis and the p53 isoform network

All living organisms have developed processes to sense and address environmental changes to maintain a stable internal state (homeostasis). When activated, the p53 tumour suppressor maintains cell and organ integrity and functions in response to homeostasis disruptors (stresses) such as infection, metabolic alterations and cellular damage. Thus, p53 plays a fundamental physiological role in maintaining organismal homeostasis. The TP53 gene encodes a network of proteins (p53 isoforms) with similar and distinct biochemical functions. The p53 network carries out multiple biological activities enabling cooperation between individual cells required for long‐term survival of multicellular organisms (animals) in response to an ever‐changing environment caused by mutation, infection, metabolic alteration or damage. In this review, we suggest that the p53 network has evolved as an adaptive response to pathogen infections and other environmental selection pressures.

Proteomic Analysis Uncovers Measles Virus Protein C Interaction With p65-iASPP Protein Complex

Viruses manipulate the central machineries of host cells to their advantage. They prevent host cell antiviral responses to create a favorable environment for their survival and propagation. Measles virus (MV) encodes two nonstructural proteins MV-V and MV-C known to counteract the host interferon response and to regulate cell death pathways. Several molecular mechanisms underlining MV-V regulation of innate immunity and cell death pathways have been proposed, whereas MV-C host-interacting proteins are less studied. We suggest that some cellular factors that are controlled by MV-C protein during viral replication could be components of innate immunity and the cell death pathways. To determine which host factors are targeted by MV-C, we captured both direct and indirect host-interacting proteins of MV-C protein. For this, we used a strategy based on recombinant viruses expressing tagged viral proteins followed by affinity purification and a bottom-up mass spectrometry analysis. From the list of host proteins specifically interacting with MV-C protein in different cell lines, we selected the host targets that belong to immunity and cell death pathways for further validation. Direct protein interaction partners of MV-C were determined by applying protein complementation assay and the bioluminescence resonance energy transfer approach. As a result, we found that MV-C protein specifically interacts with p65–iASPP protein complex that controls both cell death and innate immunity pathways and evaluated the significance of these host factors on virus replication.

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Measles virus controls immune response and cell death pathways to achieve replication.

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Host proteins interaction network with measles virulence factor C protein.

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Cellular p65–iASPP complex is targeted by measles virus C protein.

Autophagy enhances the replication of Peste des petits ruminants virus and inhibits caspase-dependent apoptosis in vitro

Peste des petits ruminants (PPR) is an acute and highly contagious disease in small ruminants that causes significant economic losses in developing countries. An increasing number of studies have demonstrated that both autophagy and apoptosis are important cellular mechanisms for maintaining homeostasis, and they participate in the host response to pathogens. However, the crosstalk between apoptosis and autophagy in host cells during PPRV infection has not been clarified. In this study, autophagy was induced upon virus infection in caprine endometrial epithelial cells (EECs), as determined by the appearance of double- and single-membrane autophagy-like vesicles, LC3-I/LC3-II conversion, and p62 degradation. We also found that PPRV infection triggered a complete autophagic response, most likely mediated by the non-structural protein C and nucleoprotein N. Moreover, our results suggest that autophagy not only promotes the replication of PPRV in EECs but also provides a potential mechanism for inhibiting PPRV-induced apoptosis. Inhibiting autophagosome formation by wortmannin and knocking down the essential autophagic proteins Beclin-1 and ATG7 induces caspase-dependent apoptosis in EECs in PPRV infection. However, inhibiting autophagosome and lysosome fusion by NH₄Cl and chloroquine did not increase the number of apoptotic cells. Collectively, these data are the first to indicate that PPRV-induced autophagy inhibits caspase-dependent apoptosis and thus contributes to the enhancement of viral replication and maturity in host cells.

Vy-PER: eliminating false positive detection of virus integration events in next generation sequencing data

Several pathogenic viruses such as hepatitis B and human immunodeficiency viruses may integrate into the host genome. These virus/host integrations are detectable using paired-end next generation sequencing. However, the low number of expected true virus integrations may be difficult to distinguish from the noise of many false positive candidates. Here, we propose a novel filtering approach that increases specificity without compromising sensitivity for virus/host chimera detection. Our detection pipeline termed Vy-PER (Virus integration detection bY Paired End Reads) outperforms existing similar tools in speed and accuracy. We analysed whole genome data from childhood acute lymphoblastic leukemia (ALL), which is characterised by genomic rearrangements and usually associated with radiation exposure. This analysis was motivated by the recently reported virus integrations at genomic rearrangement sites and association with chromosomal instability in liver cancer. However, as expected, our analysis of 20 tumour and matched germline genomes from ALL patients finds no significant evidence for integrations by known viruses. Nevertheless, our method eliminates 12,800 false positives per genome (80× coverage) and only our method detects singleton human-phiX174-chimeras caused by optical errors of the Illumina HiSeq platform. This high accuracy is useful for detecting low virus integration levels as well as non-integrated viruses.

Phylogenetically Distant Viruses Use the Same BH3-Only Protein Puma to Trigger Bax/Bak-Dependent Apoptosis of Infected Mouse and Human Cells

Viruses can trigger apoptosis of infected host cells if not counteracted by cellular or viral anti-apoptotic proteins. These protective proteins either inhibit the activation of caspases or they act as Bcl-2 homologs to prevent Bax/Bak-mediated outer mitochondrial membrane permeabilization (MOMP). The exact mechanism by which viruses trigger MOMP has however remained enigmatic. Here we use two distinct types of viruses, a double stranded DNA virus, herpes simplex virus-1 (HSV-1) and a positive sense, single stranded RNA virus, Semliki Forest virus (SFV) to show that the BH3-only protein Puma is the major mediator of virus-induced Bax/Bak activation and MOMP induction. Indeed, when Puma was genetically deleted or downregulated by shRNA, mouse embryonic fibroblasts and IL-3-dependent monocytes as well as human colon carcinoma cells were as resistant to virus-induced apoptosis as their Bax/Bak double deficient counterparts (Bax/Bak-/-). Puma protein expression started to augment after 2 h postinfection with both viruses. Puma mRNA levels increased as well, but this occurred after apoptosis initiation (MOMP) because it was blocked in cells lacking Bax/Bak or overexpressing Bcl-xL. Moreover, none of the classical Puma transcription factors such as p53, p73 or p65 NFκB were involved in HSV-1-induced apoptosis. Our data suggest that viruses use a Puma protein-dependent mechanism to trigger MOMP and apoptosis in host cells.

How do viruses control mitochondria-mediated apoptosis?

There is no doubt that viruses require cells to successfully reproduce and effectively infect the next host. The question is what is the fate of the infected cells? All eukaryotic cells can "sense" viral infections and exhibit defence strategies to oppose viral replication and spread. This often leads to the elimination of the infected cells by programmed cell death or apoptosis. This "sacrifice" of infected cells represents the most primordial response of multicellular organisms to viruses. Subverting host cell apoptosis, at least for some time, is therefore a crucial strategy of viruses to ensure their replication, the production of essential viral proteins, virus assembly and the spreading to new hosts. For that reason many viruses harbor apoptosis inhibitory genes, which once inside infected cells are expressed to circumvent apoptosis induction during the virus reproduction phase. On the other hand, viruses can take advantage of stimulating apoptosis to (i) facilitate shedding and hence dissemination, (ii) to prevent infected cells from presenting viral antigens to the immune system or (iii) to kill non-infected bystander and immune cells which would limit viral propagation. Hence the decision whether an infected host cell undergoes apoptosis or not depends on virus type and pathogenicity, its capacity to oppose antiviral responses of the infected cells and/or to evade any attack from immune cells. Viral genomes have therefore been adapted throughout evolution to satisfy the need of a particular virus to induce or inhibit apoptosis during its life cycle. Here we review the different strategies used by viruses to interfere with the two major apoptosis as well as with the innate immune signaling pathways in mammalian cells. We will focus on the intrinsic mitochondrial pathway and discuss new ideas about how particular viruses could activately engage mitochondria to induce apoptosis of their host.

Epstein-Barr virus nuclear antigen 3C interact with p73: Interplay between a viral oncoprotein and cellular tumor suppressor

The p73 protein has structural and functional homology with the tumor suppressor p53, which plays an important role in cell cycle regulation, apoptosis, and DNA repair. The p73 locus encodes both a tumor suppressor (TAp73) and a putative oncogene (ΔNp73). p73 May play a significant role in p53-deficient lymphomas infected with Epstein-Barr virus (EBV). EBV produces an asymptomatic infection in the majority of the global population, but it is associated with several human B-cell malignancies. The EBV-encoded Epstein-Barr virus nuclear antigen 3C (EBNA3C) is thought to disrupt the cell cycle checkpoint by interacting directly with p53 family proteins. Doxorubicin, a commonly used chemotherapeutic agent, induces apoptosis through p53 and p73 signaling such that the lowΔNp73 level promotes the p73-mediated intrinsic pathway of apoptosis. In this report, we investigated the mechanism by which EBV infection counters p73α-induced apoptosis through EBNA3C.

Human bocavirus NP1 inhibits IFN-β production by blocking association of IFN regulatory factor 3 with IFNB promoter

Human bocavirus (HBoV) mainly infects young children. Although many infected children suffer from respiratory or gastroenteric tract diseases, an association between HBoV and these diseases is not definite. Because modulation of type I IFN is crucial for viruses to establish efficient replication, in this study, we tested whether HBoV modulates type I IFN production. We observed that a nearly full-length HBoV clone significantly reduced both Sendai virus (SeV)- and poly(deoxyadenylic-thymidylic) acid-induced IFN-β production. Further study showed that NP1 blocked IFN-β activation in response to SeV, poly(deoxyadenylic-thymidylic) acid, and IFN-β pathway inducers, including retinoic acid-inducible protein I, mitochondrial antiviral signaling protein, inhibitor of κB kinase ε, and TANK-binding kinase 1. In addition, NP1 interfered with IRF-3-responsive PRD(III-I) promoter activated by SeV and a constitutively active mutant of IRF-3 (IRF-3/5D). Although NP1 suppressed the IRF-3 pathway, it did not affect IRF-3 activation processes, including phosphorylation, dimerization, and nuclear translocation. Coimmunoprecipitation assays confirmed the interaction between NP1 and IRF-3. Additional deletion mutagenesis and coimmunoprecipitation assays revealed that NP1 bound to the DNA-binding domain of IRF-3, resulting in the interruption of an association between IRF-3 and IFNB promoter. Altogether, our results indicate that HBoV NP1 blocks IFN production through a unique mechanism. To our knowledge, this is the first study to investigate the modulation of innate immunity by HBoV. Our findings suggest a potential immune-evasion mechanism used by HBoV and provide a basis for better understanding HBoV pathogenesis.

Measles virus C protein interferes with Beta interferon transcription in the nucleus

Transcriptional induction of beta interferon (IFN-β) through pattern recognition receptors is a key event in the host defense against invading viruses. Infection of cells by paramyxoviruses, like measles virus (MV) (genus Morbillivirus), is sensed predominantly by the ubiquitous cytoplasmic helicase RIG-I, recognizing viral 5'-triphosphate RNAs, and to some degree by MDA5. While MDA5 activation is effectively prevented by the MV V protein, the viral mechanisms for inhibition of MDA5-independent induction of IFN-β remained obscure. Here, we identify the 186-amino-acid MV C protein, which shuttles between the nucleus and the cytoplasm, as a major viral inhibitor of IFN-β transcription in human cells. Activation of the transcription factor IRF3 by upstream kinases and nuclear import of activated IRF3 were not affected in the presence of C protein, suggesting a nuclear target. Notably, C proteins of wild-type MV isolates, which are poor IFN-β inducers, were found to comprise a canonical nuclear localization signal (NLS), whereas the NLSs of all vaccine strains, irrespective of their origins, were mutated. Site-directed mutagenesis of the C proteins from an MV wild-type isolate and from the vaccine virus strain Schwarz confirmed a correlation of nuclear localization and inhibition of IFN-β transcription. A functional NLS and efficient nuclear accumulation are therefore critical for MV C to retain its potential to downregulate IFN-β induction. We suggest that a defect in efficient nuclear import of C protein contributes to attenuation of MV vaccine strains.

Proteomic analysis of virus-host interactions in an infectious context using recombinant viruses

RNA viruses exhibit small-sized genomes encoding few proteins, but still establish complex networks of interactions with host cell components to achieve replication and spreading. Ideally, these virus-host protein interactions should be mapped directly in infected cell culture, but such a high standard is often difficult to reach when using conventional approaches. We thus developed a new strategy based on recombinant viruses expressing tagged viral proteins to capture both direct and indirect physical binding partners during infection. As a proof of concept, we engineered a recombinant measles virus (MV) expressing one of its virulence factors, the MV-V protein, with a One-STrEP amino-terminal tag. This allowed virus-host protein complex analysis directly from infected cells by combining modified tandem affinity chromatography and mass spectrometry analysis. Using this approach, we established a prosperous list of 245 cellular proteins interacting either directly or indirectly with MV-V, and including four of the nine already known partners of this viral factor. These interactions were highly specific of MV-V because they were not recovered when the nucleoprotein MV-N, instead of MV-V, was tagged. Besides key components of the antiviral response, cellular proteins from mitochondria, ribosomes, endoplasmic reticulum, protein phosphatase 2A, and histone deacetylase complex were identified for the first time as prominent targets of MV-V and the critical role of the later protein family in MV replication was addressed. Most interestingly, MV-V showed some preferential attachment to essential proteins in the human interactome network, as assessed by centrality and interconnectivity measures. Furthermore, the list of MV-V interactors also showed a massive enrichment for well-known targets of other viruses. Altogether, this clearly supports our approach based on reverse genetics of viruses combined with high-throughput proteomics to probe the interaction network that viruses establish in infected cells.

Modulation of apoptosis by V protein mumps virus

Background

The Urabe AM9 vaccine strain of mumps virus contains two variants of V protein: VWT (of HN-A1081 viral population) and VGly (of HN-G1081). The V protein is a promoting factor of viral replication by blocking the IFN antiviral pathway.

Findings

We studied the relationship between V protein variants and IFN-α2b-induced apoptosis. V proteins decrease activation of the extrinsic IFN-α2b-induced apoptotic pathway monitored by the caspase 8 activity, being the effect greater with the VWT protein. Both V proteins decrease the activity of caspase 9 of the intrinsic apoptotic pathway. In a system without IFN, the VWT and VGly proteins expression promotes activation of caspases 3 and 7. However, when the cellular system was stimulated with IFN-α, this activity decreased partially. TUNEL assay shows that for treatment with IFN-α and ibuprofen of cervical adenocarcinoma cells there is nuclear DNA fragmentation but the V protein expression reduces this process.

Conclusions

The reduction in the levels of caspases and DNA fragmentation, suggesting that V protein, particularly VWT protein of Urabe AM9 vaccine strain, modulates apoptosis. In addition, the VWT protein shows a protective role for cell proliferation in the presence of antiproliferative signals.

Use of attenuated paramyxoviruses for cancer therapy

Paramyxoviruses, measles virus (MV), mumps virus (MuV) and Newcastle disease virus (NDV), are well known for causing measles and mumps in humans and Newcastle disease in birds. These viruses have been tamed (attenuated) and successfully used as vaccines to immunize their hosts. Remarkably, pathogenic MuV and vaccine strains of MuV, MV and NDV efficiently infect and kill cancer cells and are consequently being investigated as novel cancer therapies (oncolytic virotherapy). Phase I/II clinical trials have shown promise but treatment efficacy needs to be enhanced. Technologies being developed to increase treatment efficacy include: virotherapy in combination with immunosuppressive drugs (cyclophosphamide); retargeting of viruses to specific tumor types or tumor vasculature; using infected cell carriers to protect and deliver the virus to tumors; and genetic manipulation of the virus to increase viral spread and/or express transgenes during viral replication. Transgenes have enabled noninvasive imaging or tracking of viral gene expression and enhancement of tumor destruction.

The measles virus V protein binds to p65 (RelA) to suppress NF-kappaB activity

Nuclear factor κB (NF-κB) transcription factors are involved in controlling numerous cellular processes, including inflammation, innate and adaptive immunity, and cell survival. Here we show that the immunosuppressive measles virus (MV; Morbillivirus genus, Paramyxoviridae) has evolved multiple functions to interfere with canonical NF-κB signaling in epithelial cells. The MV P, V, and C proteins, also involved in preventing host cell interferon responses, were found to individually suppress NF-κB-dependent reporter gene expression in response to activation of the tumor necrosis factor (TNF) receptor, RIG-I-like receptors, or Toll-like receptors. NF-κB activity was most efficiently suppressed in the presence of V, while expression of P or C resulted in moderate inhibition. As indicated by reporter gene assays involving overexpression of the IκB kinase (IKK) complex, which phosphorylates the inhibitor of κB to liberate NF-κB, V protein targets a downstream step in the signaling cascade. Coimmunoprecipitation experiments revealed that V specifically binds to the Rel homology domain of the NF-κB subunit p65 but not of p50. Notably, the short C-terminal domain of the V protein, which is also involved in binding STAT2, IRF7, and MDA5, was sufficient for the interaction and for preventing reporter gene activity. As observed by confocal microscopy, the presence of V abolished nuclear translocation of p65 upon TNF-α stimulation. Thus, MV V appears to prevent NF-κB-dependent gene expression by retaining p65 in the cytoplasm. These findings reveal NF-κB as a key target of MV and stress the importance of the V protein as the major viral immune-modulatory factor.

Dissociation of paramyxovirus interferon evasion activities: universal and virus-specific requirements for conserved V protein amino acids in MDA5 interference

The V protein of the paramyxovirus subfamily Paramyxovirinae is an important virulence factor that can interfere with host innate immunity by inactivating the cytosolic pathogen recognition receptor MDA5. This interference is a result of a protein-protein interaction between the highly conserved carboxyl-terminal domain of the V protein and the helicase domain of MDA5. The V protein C-terminal domain (CTD) is an evolutionarily conserved 49- to 68-amino-acid region that coordinates two zinc atoms per protein chain. Site-directed mutagenesis of conserved residues in the V protein CTD has revealed both universal and virus-specific requirements for zinc coordination in MDA5 engagement and has also identified other conserved residues as critical for MDA5 interaction and interference. Mutation of these residues produces V proteins that are specifically defective for MDA5 interference and not impaired in targeting STAT1 for proteasomal degradation via the VDC ubiquitin ligase complex. Results demonstrate that mutation of conserved charged residues in the V proteins of Nipah virus, measles virus, and mumps virus also abolishes MDA5 interaction. These findings clearly define molecular determinants for MDA5 inhibition by the paramyxovirus V proteins.

Antagonism of innate immunity by paramyxovirus accessory proteins

Paramyxovirinae, a subfamily of Paramyxoviridae, are negative strand RNA viruses comprised of many important human and animal pathogens, which share a high degree of genetic and structural homology. The accessory proteins expressed from the P/V/C gene are major factors in the pathogenicity of the viruses, because of their ability to abrogate various facets of type I interferon (IFN) induction and signaling. Most of the paramyxoviruses exhibit a commonality in their ability to antagonize innate immunity by blocking IFN induction and the Jak/STAT pathway. However, the manner in which the accessory proteins inhibit the pathway differs among viruses. Similarly, there are variations in the capability of the viruses to counteract intracellular detectors (RNA helicases, mda-5 and RIG-I). Furthermore, a functional specificity in the antagonism of the IFN response has been reported, suggesting that specificity in the circumvention of innate immunity restricts viral host range. Available evidence indicates that paramyxoviruses employ specific strategies to antagonize the IFN response of their specific hosts, which is one of the major factors that determine viral pathogenicity and host range.

Lymphotropism and host responses during acute wild-type canine distemper virus infections in a highly susceptible natural host

The mechanisms behind the in vivo virulence of immunosuppressive wild-type morbillivirus infections are still not fully understood. To investigate lymphotropism and host responses, we have selected the natural host model of canine distemper virus (CDV) infection in mink. This model displays multisystemic infection, similar to measles virus and rinderpest virus infections in their susceptible natural hosts. The wild-type CDVs investigated provoked marked virulence differences, inducing mild versus marked to severe acute disease. The mildly virulent wild-type virus induced transient lymphopenia, despite the development of massive infection of peripheral blood mononuclear cells (PBMCs) exceeding that determined for the highly virulent wild-type virus, indicating an inverse relationship between acute virulence and the extent of viraemia in the investigated wild-type viruses. Single-cell cytokine production in PBMCs was investigated throughout the acute infections. We observed Th1- and Th2-type cytokine responses beginning in the prodromal phase, and late inflammatory responses were shared between the wild-type infections.

Severe acute respiratory syndrome coronavirus papain-like protease ubiquitin-like domain and catalytic domain regulate antagonism of IRF3 and NF-kappaB signaling

The outcome of a viral infection is regulated in part by the complex coordination of viral and host interactions that compete for the control and optimization of virus replication. Severe acute respiratory syndrome coronavirus (SARS-CoV) intimately engages and regulates the host innate immune responses during infection. Using a novel interferon (IFN) antagonism screen, we show that the SARS-CoV proteome contains several replicase, structural, and accessory proteins that antagonize the IFN pathway. In this study, we focus on the SARS-CoV papain-like protease (PLP), which engages and antagonizes the IFN induction and NF-kappaB signaling pathways. PLP blocks these pathways by affecting activation of the important signaling proteins in each pathway, IRF3 and NF-kappaB. We also show that the ubiquitin-like domain of PLP is necessary for pathway antagonism but not sufficient by itself to block these pathways regardless of the enzymatic activity of the protease. The potential mechanism of PLP antagonism and its role in pathogenesis are discussed.

Canine distemper virus selectively inhibits apoptosis progression in infected immune cells

Morbillivirus infections are characterized by severe leukopenia and immune suppression that develop even before the onset of clinical signs. To characterize in more detail the fate of the immune cells during the critical first week, we evaluated the overall viability, level of apoptosis, cell cycle status, and extent of infection in different immune tissues of ferrets inoculated with a lethal canine distemper virus (CDV) strain. Initial experiments with MDCK cells, a canine epithelial cell line, revealed that CDV infection resulted in only a marginal increase in apoptosis at high infection levels and that infected cells were more resistant to chemically induced apoptosis. In ferrets, levels of viability and early and late apoptosis remained stable in thymus and lymph node, where more than 80% of cells were infected, whereas a gradual albeit small increase in apoptosis was observed in peripheral blood mononuclear cells and spleen. Furthermore, the progression of spontaneous apoptosis in infected cells was inhibited, while the proportion of apoptotic noninfected "bystander" cells increased. The distribution of cells in the different stages of the cell cycle in the bone marrow was not affected, but dividing cells in the thymus decreased by 50%, and a 10-fold increase in cell division was noted in the spleen. It is unlikely that the extent of infection-induced cell death and cell cycle alterations alone can account for the dramatic leukopenia observed in this model. The investigation of additional mechanisms is therefore warranted.

Protein X of Borna disease virus inhibits apoptosis and promotes viral persistence in the central nervous systems of newborn-infected rats

Borna disease virus (BDV) is a neurotropic member of the order Mononegavirales with noncytolytic replication and obligatory persistence in cultured cells and animals. Here we show that the accessory protein X of BDV represents the first mitochondrion-localized protein of an RNA virus that inhibits rather than promotes apoptosis induction. Rat C6 astroglioma cells persistently infected with wild-type BDV were significantly more resistant to death receptor-dependent and -independent apoptotic stimuli than uninfected cells or cells infected with a BDV mutant expressing reduced amounts of X. Confocal microscopy demonstrated that X colocalizes with mitochondria and expression of X from plasmid DNA rendered human 293T and mouse L929 cells resistant to apoptosis induction. A recombinant virus encoding a mutant X protein unable to associate with mitochondria (BDV-X(A6A7)) failed to block apoptosis in C6 cells. Furthermore, Lewis rats neonatally infected with BDV-X(A6A7) developed severe neurological symptoms and died around day 30 postinfection, whereas all animals infected with wild-type BDV remained healthy and became persistently infected. TUNEL (terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling) staining revealed a significant increase in the number of apoptotic cells in the brain of BDV-X(A6A7)-infected animals, whereas the numbers of CD3(+) T lymphocytes were comparable to those detected in animals infected with wild-type BDV. Our data thus indicate that inhibition of apoptosis by X promotes noncytolytic viral persistence and is required for the survival of cells in the central nervous system of BDV-infected animals.

Measles virus interaction with host cells and impact on innate immunity

Because viruses are obligate parasites, numerous partnerships between measles virus and cellular molecules can be expected. At the entry level, measles virus uses at least two cellular receptors, CD150 and a yet to be identified epithelial receptor to which the virus H protein binds. This dual receptor strategy illuminates the natural infection and inter-human propagation of this lymphotropic virus. The attenuated vaccine strains use CD46 as an additional receptor, which results in a tropism alteration. Surprisingly, the intracellular viral and cellular protein partnership leading to optimal virus life cycle remains mostly a black box, while the interactions between viral proteins that sustain the RNA-dependant RNA polymerase activity (i.e., transcription and replication), the particle assembly and the polarised virus budding are documented. Hsp72 is the only cellular protein that is known to regulate the virus transcription and replication through its interaction with the viral N protein. The viral P protein is phosphorylated by the casein kinase II with undetermined functional consequences. The cellular partnership that controls the intracellular trafficking of viral components, the assembly and/or the budding of measles virus, remains unknown. The virus to cell innate immunity war is better documented. The 5' triphosphate-ended virus leader transcript is recognised by RIG-I, a cellular helicase, and induces the interferon response. Measles virus V protein binds to the MDAS helicase and prevents the MDA5-mediated activation of interferon. By interacting with STAT1 and Jak1, the viral P and V proteins prevent the type I interferon receptor (IFNAR) signalling. The virus N protein interacts with eIF3-p40 to inhibit the translation of cellular mRNA. The H protein binds to TLR2, which then transduces an activation signal and CD150 expression in monocytes. The P protein activates the expression of the ubiquitin modifier A20, thus blocking the TLR4-mediated signalling. Few other partnerships between measles virus components and cellular proteins have been postulated or demonstrated, and they need further investigations to understand their physiopathological outcome.

Mechanisms of severe acute respiratory syndrome pathogenesis and innate immunomodulation

The modulation of the immune response is a common practice of many highly pathogenic viruses. The emergence of the highly pathogenic coronavirus severe acute respiratory virus (SARS-CoV) serves as a robust model system to elucidate the virus-host interactions that mediate severe end-stage lung disease in humans and animals. Coronaviruses encode the largest positive-sense RNA genome of approximately 30 kb, encode a variety of replicase and accessory open reading frames that are structurally unique, and encode novel enzymatic functions among RNA viruses. These viruses have broad or specific host ranges, suggesting the possibility of novel strategies for targeting and regulating host innate immune responses following virus infection. Using SARS-CoV as a model, we review the current literature on the ability of coronaviruses to interact with and modify the host intracellular environment during infection. These studies are revealing a rich set of novel viral proteins that engage, modify, and/or disrupt host cell signaling and nuclear import machinery for the benefit of virus replication.

Measles virus V protein is a decoy substrate for IkappaB kinase alpha and prevents Toll-like receptor 7/9-mediated interferon induction

The central role of plasmacytoid dendritic cells (pDC) in activating host immune responses stems from their high capacity to express alpha interferon (IFN-alpha) after stimulation of Toll-like receptors 7 and 9 (TLR7 and -9). This involves the adapter MyD88 and the kinases interleukin-1 receptor-associated kinase 1 (IRAK1), IRAK4, and IkappaB kinase alpha (IKKalpha), which activates IFN regulatory factor 7 (IRF7) and is independent of the canonical kinases TBK1 and IKKepsilon. We have recently shown that the immunosuppressive measles virus (MV) abolishes TLR7/9/MyD88-dependent IFN induction in human pDC (Schlender et al., J. Virol. 79:5507-5515, 2005), but the molecular mechanisms remained elusive. Here, we have reconstituted the pathway in cell lines and identified IKKalpha and IRF7 as specific targets of the MV V protein (MV-V). Binding of MV-V to IKKalpha resulted in phosphorylation of V on the expense of IRF7 phosphorylation by IKKalpha in vitro and in living cells. This corroborates the role of IKKalpha as the kinase phosphorylating IRF7. MV-V in addition bound to IRF7 and to phosphomimetic IRF7 and inhibited IRF7 transcriptional activity. Binding to both IKKalpha and IRF7 required the 68-amino-acid unique C-terminal domain of V. Inhibition of TLR/MyD88-dependent IFN induction by MV-V is unique among paramyxovirus V proteins and should contribute to the unique immunosuppressive phenotype of measles. The mechanisms employed by MV-V inspire strategies to interfere with immunopathological TLR/MyD88 signaling.

Protein kinase PKR mediates the apoptosis induction and growth restriction phenotypes of C protein-deficient measles virus

The measles virus (MV) accessory proteins V and C play important roles in MV replication and pathogenesis. Infection with recombinant MV lacking either V or C causes more cell death than infection with the parental vaccine-equivalent virus (MVvac), and C-deficient virus grows poorly relative to the parental virus. Here, we show that a major effector of the C phenotype is the RNA-dependent protein kinase PKR. Using human HeLa cells stably deficient in PKR as a result of RNA interference-mediated knockdown (PKR(kd) cells), we demonstrated that a reduction in PKR partially rescued the growth defect of C knockout (C(ko)) virus but had no effect on the growth of either wild-type (WT) or V knockout (V(ko)) virus. Increased growth of the C(ko) virus in PKR(kd) cells correlated with increased viral protein expression, while defective growth and decreased protein expression in PKR-sufficient cells correlated with increased phosphorylation of PKR and the alpha subunit of eukaryotic initiation factor 2. Furthermore, infection with WT, V(ko), or especially C(ko) virus caused significantly less apoptosis in PKR(kd) cells than in PKR-sufficient cells. Although apoptosis induced by C(ko) virus infection in PKR-sufficient cells was blocked by a caspase antagonist, the growth of C(ko) virus was not restored to the WT level by treatment with this pharmacologic inhibitor. Taken together, these results indicate that PKR plays an important antiviral role during MV infection but that the virus growth restriction by PKR is not dependent upon the induction of apoptosis. Furthermore, the results establish that a principal function of the MV C protein is to antagonize the proapoptotic and antiviral activities of PKR.

Inhibition of IFN-alpha/beta signaling by two discrete peptides within measles virus V protein that specifically bind STAT1 and STAT2

The V protein of measles virus (MV-V) is a potent inhibitor of IFN-alpha/beta signaling pathway. We previously reported that when physically dissociated, the N-terminal and C-terminal regions of MV-V (PNT and VCT, respectively) could independently impair signal transduction. The PNT region inhibited IFN-alpha/beta signaling by interacting with at least two components of this pathway: Jak1 and STAT1. Here we report a direct interaction between the VCT of MV-V and STAT2, a third component of IFN-alpha/beta transduction machinery. This interaction with STAT2 is carried by the cysteine-constrained peptide of 49 amino acids localized in the VCT region, and is essential to the inhibition of IFN-alpha/beta signaling. In parallel, we also mapped STAT1 binding site in the PNT region and identified a minimal peptide of only 11 amino acids. IFN-alpha/beta signaling was impaired in human cells treated with this MV-V peptide fused to a cell-penetrating sequence. Finally, we show that signaling downstream of IFN-lambda, a recently identified cytokine that also relies on STAT1, STAT2 and Jak1 to transduce, is blocked by MV-V. Altogether, our results illustrate how a single viral protein has evolved to achieve a robust inhibition of the antiviral response by interacting with several signaling molecules.

A hyperfusogenic F protein enhances the oncolytic potency of a paramyxovirus simian virus 5 P/V mutant without compromising sensitivity to type I interferon

Viral fusogenic membrane proteins have been proposed as tools to increase the potency of oncolytic viruses, but there is a need for mechanisms to control the spread of fusogenic viruses in normal versus tumor cells. We have previously shown that a mutant of the paramyxovirus simian virus 5 (SV5) that harbors mutations in the P/V gene from the canine parainfluenza virus (P/V-CPI(-)) is a potent inducer of type I interferon (IFN) and apoptosis and is restricted for spread through normal but not tumor cells in vitro. Here, we have used the cytopathic P/V-CPI(-) as a backbone vector to test the hypothesis that a virus expressing a hyperfusogenic glycoprotein will be a more effective oncolytic vector but will retain sensitivity to IFN. A P/V mutant virus expressing an F protein with a glycine-to-alanine substitution in the fusion peptide (P/V-CPI(-)-G3A) was more fusogenic than the parental P/V-CPI(-) mutant. In two model prostate tumor cell lines which are defective in IFN production (LNCaP and DU145), the hyperfusogenic P/V-CPI(-)-G3A mutant had normal growth properties at low multiplicities of infection and was more effective than the parental P/V-CPI(-) mutant at cell killing in vitro. However, in PC3 cells which produce and respond to IFN, the hyperfusogenic P/V-CPI(-)-G3A mutant was attenuated for growth and spread. Killing of PC3 cells was equivalent between the parental P/V-CPI(-) mutant and the hyperfusogenic P/V-CPI(-)-G3A mutant. In a nude mouse model using LNCaP cells, the hyperfusogenic P/V-CPI(-)-G3A mutant was more effective than P/V-CPI(-) at reducing tumor burden. In the case of DU145 tumors, the two vectors based on P/V-CPI(-) were equally effective at limiting tumor growth. Together, our results provide proof of principle that a cytopathic SV5 P/V mutant can serve as an oncolytic virus and that the oncolytic effectiveness of P/V mutants can be enhanced by a fusogenic membrane protein without compromising sensitivity to IFN. The potential advantages of SV5-based oncolytic vectors are discussed.

Interaction of Helicobacter pylori with gastric epithelial cells is mediated by the p53 protein family

BACKGROUND & AIMS

Although the p53 tumor suppressor has been extensively studied, many critical questions remain unanswered about the biological functions of p53 homologs, p73 and p63. Accumulating evidence suggests that both p73 and p63 play important roles in regulation of apoptosis, cell differentiation, and therapeutic drug sensitivity.

METHODS

Gastric epithelial cells were cocultured with Helicobacter pylori, and the roles of p63 and p73 proteins were assessed by luciferase reporter, real-time polymerase chain reaction, immunoblotting, and cell survival assays. Short hairpin RNA and dominant-negative mutants were used to inhibit activity of p73 and p63 isoforms. Human and murine gastric tissues were analyzed by immunohistochemistry with p73 and p63 antibodies and modified Steiner's silver method.

RESULTS

Interaction of H pylori with gastric epithelial cells leads to robust up-regulation of p73 protein in vitro and in vivo in human gastritis specimens and H pylori-infected mice. The p73 increase resulted in up-regulation of pro-apoptotic genes, NOXA, PUMA, and FAS receptor in gastric epithelial cells. Down-regulation of p73 activity suppressed cell death and Fas receptor induced by H pylori. Bacterial virulence factors within the cag pathogenicity island, c-Abl tyrosine kinase, and interaction with p63 isoforms control the activity of p73.

CONCLUSION

Our findings implicate p73 in H pylori-induced apoptosis and more generally suggest that the p53 family may play a role in the epithelial cell response to H pylori infection.

Role for the phosphoprotein P subunit of the paramyxovirus polymerase in limiting induction of host cell antiviral responses

Six amino acid substitutions in the shared N-terminal region of the P subunit of the viral polymerase and the accessory V protein convert the noncytopathic paramyxovirus simian virus 5 (SV5), which is a poor inducer of host cell responses, into a P/V mutant (P/V-CPI-) that induces high levels of apoptosis, interferon-beta (IFN-beta), and proinflammatory cytokines. In this study, we addressed the question of whether these new mutant phenotypes are due to the presence of an altered P protein or of an altered V protein or of both proteins. By the use of the P/V-CPI- mutant as a backbone, new mutant viruses were engineered to express the wild-type (WT) V protein (+V-wt) or WT P protein (+P-wt) from an additional gene inserted between the HN and L genes. In human epithelial cell lines, the +V-wt virus showed reduced activation of apoptosis and lower secretion of IFN-beta and proinflammatory cytokines compared to the parental P/V-CPI- virus. The presence of a V protein lacking the C-terminal cysteine-rich domain (corresponding to the SV5 I protein) did not reduce these host cell responses to P/V-CPI- infection. Unexpectedly, the +P-wt virus, which expressed a WT P subunit of the viral polymerase, also induced much lower levels of host cell responses than the parental P/V-CPI- mutant. For both +V-wt and +P-wt viruses, reduced levels of IFN-beta synthesis correlated with reduced IRF-3 dimerization and nuclear localization of IRF-3 and NF-kappaB, suggesting that the WT P and V proteins acted at an early stage in antiviral pathways. Host cell responses induced by the various P/V mutants directly correlated with levels of viral mRNA accumulation but not with steady-state levels of genomic RNA. Our results support the hypothesis that WT P and V proteins limit induction of antiviral responses by controlling the production of key viral inducers. A model is presented for the mechanism by which both the P subunit of the viral polymerase and the V accessory protein contribute to the ability of a paramyxovirus to limit activation of antiviral responses.

Measles virus V protein blocks Jak1-mediated phosphorylation of STAT1 to escape IFN-alpha/beta signaling

Viruses have evolved various strategies to escape the antiviral activity of type I interferons (IFN-alpha/beta). For measles virus, this function is carried by the polycistronic gene P that encodes, by an unusual editing strategy, for the phosphoprotein P and the virulence factor V (MV-V). MV-V prevents STAT1 nuclear translocation by either sequestration or phosphorylation inhibition, thereby blocking IFN-alpha/beta pathway. We show that both the N- and C-terminal domains of MV-V (PNT and VCT) contribute to the inhibition of IFN-alpha/beta signaling. Using the two-hybrid system and co-affinity purification experiments, we identified STAT1 and Jak1 as interactors of MV-V and demonstrate that MV-V can block the direct phosphorylation of STAT1 by Jak1. A deleterious mutation within the PNT domain of MV-V (Y110H) impaired its ability to interact and block STAT1 phosphorylation. Thus, MV-V interacts with at least two components of IFN-alpha/beta receptor complex to block downstream signaling.

SARS coronavirus and innate immunity

The emergence of the highly pathogenic SARS coronavirus (SARS-CoV) has reignited interest in coronavirus biology and pathogenesis. An emerging theme in coronavirus pathogenesis is that the interaction between specific viral genes and the host immune system, specifically the innate immune system, functions as a key determinant in regulating virulence and disease outcomes. Using SARS-CoV as a model, we will review the current knowledge of the interplay between coronavirus infection and the host innate immune system in vivo, and then discuss the mechanisms by which specific gene products antagonize the host innate immune response in cell culture models. Our data suggests that the SARS-CoV uses specific strategies to evade and antagonize the sensing and signaling arms of the interferon pathway. We summarize by identifying future points of consideration that will contribute greatly to our understanding of the molecular mechanisms governing coronavirus pathogenesis and virulence, and the development of severe disease in humans and animals.

Measles virus infection of SLAM (CD150) knockin mice reproduces tropism and immunosuppression in human infection

The human signaling lymphocyte activation molecule (SLAM, also called CD150), a regulator of antigen-driven T-cell responses and macrophage functions, acts as a cellular receptor for measles virus (MV), and its V domain is necessary and sufficient for receptor function. We report here the generation of SLAM knockin mice in which the V domain of mouse SLAM was replaced by that of human SLAM. The chimeric SLAM had an expected distribution and normal function in the knockin mice. Splenocytes from the SLAM knockin mice permitted the in vitro growth of a virulent MV strain but not that of the Edmonston vaccine strain. Unlike in vitro infection, MV could grow only in SLAM knockin mice that also lacked the type I interferon receptor (IFNAR). After intraperitoneal or intranasal inoculation, MV was detected in the spleen and lymph nodes throughout the body but not in the thymus. Notably, the virus appeared first in the mediastinal lymph node after intranasal inoculation. Splenocytes from MV-infected IFNAR(-/-) SLAM knockin mice showed suppression of proliferative responses to concanavalin A. Thus, MV infection of SLAM knockin mice reproduces lymphotropism and immunosuppression in human infection, serving as a useful small animal model for measles.

Tyrosine 110 in the measles virus phosphoprotein is required to block STAT1 phosphorylation

The measles virus (MV) P gene encodes three proteins: P, an essential polymerase cofactor, and C and V, which have multiple functions including immune evasion. We show here that the MV P protein also contributes to immune evasion, and that tyrosine 110 is required to block nuclear translocation of the signal transducer and activator of transcription factors (STAT) after interferon type I treatment. In particular, MV P inhibits STAT1 phosphorylation. This is shown not only by transient expression but also by reverse genetic analyses based on a new functional infectious cDNA derived from a MV vaccine vial (Moraten strain). Our study also identifies a conserved sequence around P protein tyrosine 110 as a candidate interaction site with a cellular protein.