Longer and shorter forms of Sendai virus C proteins play different roles in modulating the cellular antiviral response

Measles virus V protein blocks interferon (IFN)-alpha/beta but not IFN-gamma signaling by inhibiting STAT1 and STAT2 phosphorylation

Measles virus (MV), a member of the family Paramyxoviridae, encodes C and V non-structural proteins. To clarify the functions of MV C and V proteins, HeLa cell lines constitutively expressing C or V protein were established. We found that expression of V protein inhibited interferon (IFN)-alpha/beta signaling but not IFN-gamma signaling. C protein had no inhibitory effect on IFN signaling in our experimental condition. Degradation of selective signal transducers and activators of transcription (STAT) proteins was not observed in HeLa cells expressing V protein. In contrast, tyrosine phosphorylation of both STAT1 and STAT2 was inhibited in these cells after IFN-beta stimulation.

The STAT2 activation process is a crucial target of Sendai virus C protein for the blockade of alpha interferon signaling

Sendai virus (SeV) C protein functions as an interferon (IFN) antagonist and renders cells unresponsive to both alpha/beta IFN (IFN-alpha/beta) and IFN-gamma. We have recently found the physical association of the C protein with signal transducer and activator of transcription 1 (STAT1) in infected cells. However, involvement of the C-STAT1 interaction in the blockade of IFN signaling has remained unclear. We generated here a series of C mutant proteins that retained or lost the STAT1-binding capacity and examined their effects on IFN-alpha signaling. All of the C mutant proteins with no STAT1-binding capacity lost the ability to inhibit the IFN-alpha response. In contrast, the C mutant proteins retaining the STAT1-binding capacity suppressed IFN-alpha-stimulated tyrosine phosphorylation of both STAT2 and STAT1 to various degrees. Remarkably, their anti-IFN-alpha capacities correlated well with the inhibitory effect on phosphorylation of STAT2 rather than STAT1. In infected cells, the levels of tyrosine-phosphorylated (pY) STAT2 were below the detection level irrespective of duration of IFN-alpha stimulation, whereas the levels of pY-STAT1 strikingly increased after long-term IFN-alpha stimulation. These results suggest that the STAT2 activation process is a crucial target for the blockade of IFN-alpha signaling. An in vitro binding assay with extracts from (STAT1-deficient) U3A and (STAT1-expressing) U3A-ST1 cells suggested the requirement of STAT1 for the C-STAT2 interaction. Furthermore, expression of STAT1 enhanced the inhibitory effect of the C protein on STAT2 activation in U3A cells. The C protein thus appears to participate in the inhibitory process for STAT2 activation through the STAT1 interaction.

The amino-terminal extensions of the longer Sendai virus C proteins modulate pY701-Stat1 and bulk Stat1 levels independently of interferon signaling

The Sendai virus (SeV) C proteins are known to interact with Stat1 to prevent interferon (IFN)-induced pY701-Stat1 formation and IFN signaling. Nevertheless, pY701-Stat1 levels paradoxically increase during SeV infection. The C proteins also induce bulk Stat1 instability in some cells, similar to rubulavirus V proteins. We have found that SeV infection increases pY701-Stat1 levels even in cells in which bulk Stat1 levels strongly decrease. Remarkably, both the decrease in bulk Stat1 levels and the increase in pY701-Stat1 levels were found to be independent of the IFN signaling system, i.e., these events occur in mutant cells in which various components of the IFN signaling system have been disabled. Consistent with this, the C-induced decrease in Stat1 levels does not require Y701 of Stat1. We present evidence that C interacts with Stat1 in two different ways, one that prevents IFN-induced pY701-Stat1 formation and IFN signaling that has already been documented, and another that induces pY701-Stat1 formation (while decreasing bulk Stat1 levels) in a manner that does not require IFN signaling. These two types of Stat1 interaction are also distinguishable by C gene mutations. In particular, the IFN signaling-independent Stat1 interactions specifically require the amino-terminal extensions of the longer C proteins. The actions of the SeV C proteins in counteracting the cellular antiviral response are clearly more extensive than previously appreciated.

Paramyxovirus strategies for evading the interferon response

Two genera, the Respirovirus (Sendai virus (SeV) and human parainfluenza virus (hPIV3) and the Rubulavirus (simian virus (SV) 5, SV41, mumps virus and hPIV2), of the three in the subfamily Paramyxovirinae inhibit interferon (IFN) signalling to circumvent the IFN response. The viral protein responsible for the inhibition is the C protein for respirovirus SeV and the V protein for the rubulaviruses, both of which are multifunctional accessory proteins expressed from the P gene. SeV suppresses IFN-stimulated tyrosine phosphorylation of signal transducers and activators of transcription (STATs) at an early phase of infection and further inhibits the downstream signalling without degrading any of the signalling components in most cell lines. On the contrary, the Rubulavirus V protein targets Stat1 or Stat2 for degradation. Proteasome-mediated degradation appears to be involved in most cases. Studies on the molecular mechanisms by which paramyxoviruses evade the IFN response will offer important information for modulating the JAK-STAT pathway, designing novel antiviral drugs and recombinant live vaccines, and improving paramyxovirus expression vectors for gene therapy.

A decade after the generation of a negative-sense RNA virus from cloned cDNA - what have we learned?

Since the first generation of a negative-sense RNA virus entirely from cloned cDNA in 1994, similar reverse genetics systems have been established for members of most genera of the Rhabdo- and Paramyxoviridae families, as well as for Ebola virus (Filoviridae). The generation of segmented negative-sense RNA viruses was technically more challenging and has lagged behind the recovery of nonsegmented viruses, primarily because of the difficulty of providing more than one genomic RNA segment. A member of the Bunyaviridae family (whose genome is composed of three RNA segments) was first generated from cloned cDNA in 1996, followed in 1999 by the production of influenza virus, which contains eight RNA segments. Thus, reverse genetics, or the de novo synthesis of negative-sense RNA viruses from cloned cDNA, has become a reliable laboratory method that can be used to study this large group of medically and economically important viruses. It provides a powerful tool for dissecting the virus life cycle, virus assembly, the role of viral proteins in pathogenicity and the interplay of viral proteins with components of the host cell immune response. Finally, reverse genetics has opened the way to develop live attenuated virus vaccines and vaccine vectors.

Roles of nonstructural protein nsP2 and Alpha/Beta interferons in determining the outcome of Sindbis virus infection

Alphaviruses productively infect a variety of vertebrate and insect cell lines. In vertebrate cells, Sindbis virus redirects cellular processes to meet the needs of virus propagation. At the same time, cells respond to virus replication by downregulating virus growth and preventing dissemination of the infection. The balance between these two mechanisms determines the outcome of infection at the cellular and organismal levels. In this report, we demonstrate that a viral nonstructural protein, nsP2, is a significant regulator of Sindbis virus-host cell interactions. This protein not only is a component of the replicative enzyme complex required for replication and transcription of viral RNAs but also plays a role in suppressing the antiviral response in Sindbis virus-infected cells. nsP2 most likely acts by decreasing interferon (IFN) production and minimizing virus visibility. Infection of murine cells with Sindbis virus expressing a mutant nsP2 leads to higher levels of IFN secretion and the activation of 170 cellular genes that are induced by IFN and/or virus replication. Secreted IFN protects naive cells against Sindbis virus infection and also stops viral replication in productively infected cells. Mutations in nsP2 can also attenuate Sindbis virus cytopathogenicity. Such mutants can persist in mammalian cells with defects in the alpha/beta IFN (IFN-alpha/beta) system or when IFN activity is neutralized by anti-IFN-alpha/beta antibodies. These findings provide new insight into the alphavirus-host cell interaction and have implications for the development of improved alphavirus expression systems with better antigen-presenting potential.

Stats: transcriptional control and biological impact

Extracellular proteins bound to cell-surface receptors can change nuclear gene expression patterns in minutes, with far-reaching consequences for development, cell growth and homeostasis. The signal transducer and activator of transcription (STAT) proteins are among the most well studied of the latent cytoplasmic signal-dependent transcription-factor pathways. In addition to several roles in normal cell decisions, dysregulation of STAT function contributes to human disease, making the study of these proteins an important topic of current research.

Paramyxovirus accessory proteins as interferon antagonists

A new role of the Paramyxovirus accessory proteins has been uncovered. The P gene of the subfamily Paramyxovirinae encodes accessory proteins including the V and/or C protein by means of pseudotemplated nucleotide addition (RNA editing) or by overlapping open reading frame. The Respirovirus (Sendai virus and human parainfluenza virus (hPIV)3) and Rubulavirus (simian virus (SV)5, SV41, mumps virus and hPIV2) circumvent the interferon (IFN) response by inhibiting IFN signaling. The responsible genes were mapped to the C gene for SeV and the V gene for rubulaviruses. On the other hand, wild type measles viruses isolated from clinical specimens suppress production of IFN, although responsible viral factors remain to be identified. Both human and bovine respiratory syncytial viruses (RSVs) counteract the antiviral effect of IFN with inhibiting neither IFN signaling nor IFN production. Bovine RSV NS1 and NS2 proteins cooperatively antagonize the antiviral effect of IFN. Studies on the molecular mechanism by which viruses circumvent the host IFN response will not only illustrate co-evolution of virus strategies of immune evasion but also provide basic information useful for engineering novel antiviral drugs as well as recombinant live vaccine.

The amino-terminal half of Sendai virus C protein is not responsible for either counteracting the antiviral action of interferons or down-regulating viral RNA synthesis

The Sendai virus C proteins, C', C, Y1, and Y2, are a nested set of independently initiated carboxy-coterminal proteins translated from a reading frame overlapping the P frame on the P mRNA. The C proteins are extremely versatile and have been shown to counteract the antiviral action of interferons (IFNs), to down-regulate viral RNA synthesis, and to promote virus assembly. Using the stable cell lines expressing the C, Y1, Y2, or truncated C protein, we investigated the region responsible for anti-IFN action and for down-regulating viral RNA synthesis. Truncation from the amino terminus to the middle of the C protein maintained the inhibition of the signal transduction of IFNs, the formation of IFN-stimulated gene factor 3 (ISGF3) complex, the generation of the anti-vesicular stomatitis virus state, and the synthesis of viral RNA, but further truncation resulted in the simultaneous loss of all of these inhibitory activities. A relatively small truncation from the carboxy terminus also abolished all of these inhibitory activities. These data indicated that the activities of the C protein to counteract the antiviral action of IFNs and to down-regulate viral RNA synthesis were not encoded within a region of at least 98 amino acids in its amino-terminal half.

STAT2 acts as a host range determinant for species-specific paramyxovirus interferon antagonism and simian virus 5 replication

The antiviral state induced by alpha/beta interferon (IFN-alpha/beta) is a powerful selective pressure for virus evolution of evasive strategies. The paramyxoviruses simian virus 5 (SV5) and human parainfluenza virus 2 (HPIV2) overcome IFN-alpha/beta responses through the actions of their V proteins, which induce proteasomal degradation of cellular IFN-alpha/beta-activated signal transducers and activators of transcription STAT1 and STAT2. SV5 infection induces STAT1 degradation and IFN-alpha/beta inhibition efficiently in human cells but not in mouse cells, effectively restricting SV5 host range. Here, the cellular basis for this species specificity is demonstrated to result from differences between human and murine STAT2. Expression in mouse cells of full-length or truncated human STAT2 cDNA is sufficient to permit antagonism of endogenous murine IFN-alpha/beta signaling by SV5 and HPIV2 V proteins. Furthermore, virus-induced STAT protein degradation is observed in mouse cells only in the presence of ectopically expressed human STAT2. The results indicate that STAT2 acts as an intracellular determinant of paramyxovirus host range restriction, which contributes to the species specificity of virus replication, and that human STAT2 can confer a growth advantage for SV5 in the murine host.

Viruses and the type I interferon antiviral system: induction and evasion

The type I interferon (IFN) system responds to viral infection and induces an "antiviral state" in cells, providing an important first line of defense against virus infection. Interaction of type I IFNs (IFN alpha and IFN beta) with their receptor induces hundreds of cellular genes. Of the proteins induced by IFN, the antiviral function of only a few is known, and their mechanisms of action are only partly understood. Additionally, although viral-encoded mechanisms that counteract specific components of the type I IFN response have been known for some time, it has recently become clear that many (if not most) viruses encode some form of IFN-antagonist. Understanding the interplay between viral-encoded IFN antagonists and the interferon response will be essential if the therapeutic potential of IFNs is to be fully exploited.

Ambisense sendai viruses are inherently unstable but are useful to study viral RNA synthesis

Ambisense Sendai virus (SeV) was prepared in order to study the control of viral RNA synthesis. In these studies, we found that the relative ratios of genomes/antigenomes formed during infection are largely determined by the relative strengths of the replication promoters, independent of the presence of a functional mRNA start site. We also found that the ability of the viral polymerase (vRdRP) to respond to an mRNA editing site requires prior (re)initiation at an mRNA start site, similar to the acquisition of vRdRP processivity in the absence of nascent chain coassembly. During these studies, the inherent instability of ambisense SeV upon passage in embryonated chicken eggs was noted and was found to be associated with a point mutation in the ambisense mRNA (ambi-mRNA) start site that severely limited its expression. Since the interferon (IFN)-induced antiviral state is mediated in part via double-stranded RNA (dsRNA), the efficiency of the ambi-mRNA poly(A)/stop site was examined. This site was found to operate in a manner similar to that of other SeV mRNA poly(A)/stop sites, i.e., at approximately 95% efficiency. This modest level of vRdRP read-through is apparently tolerable for natural SeV because the potential to form dsRNA during infection remains limited. However, when mRNAs are expressed from ambisense SeV antigenomes, vRdRP read-through of the ambi-mRNA poly(A)/stop site creates a capped transcript that can potentially extend the entire length of the antigenome, since there are no further poly(A)/stop sites here. In support of this hypothesis, loss of ambi-mRNA expression during passage of ambisense SeV stocks in eggs is also characterized by conversion of virus that grows poorly in IFN-sensitive cultures and is relatively IFN sensitive to virus that grows well even in IFN-pretreated cells that restrict vesicular stomatitis virus replication, i.e., the wild-type SeV phenotype. The selection of mutants unable to express ambi-mRNA on passage in chicken eggs is presumably due to increased levels of dsRNA during infection. How natural ambisense viruses may deal with this dilemma is discussed.

The interferon antiviral response: from viral invasion to evasion

One of the initial responses of an organism to infection by pathogenic viruses is the synthesis of antiviral cytokines such as the type I interferons (interferon-alpha/beta), interleukins, and other proinflammatory cytokines and chemokines. Interferons provide a first line of defence against virus infections by generating an intracellular environment that restricts virus replication and signals the presence of a viral pathogen to the adaptive arm of the immune response. Interferons stimulate cells in the local environment to activate a network of interferon-stimulated genes, which encode proteins that have antiviral, antiproliferative and immunomodulatory activities. The present review focuses on recent reports that describe the activation of multiple signalling pathways following virus infection, new candidate genes that are implicated in the establishment of the antiviral state, and the strategies used by viruses and their specific viral products to antagonize and evade the host antiviral response.

Respiratory syncytial virus (RSV) nonstructural (NS) proteins as host range determinants: a chimeric bovine RSV with NS genes from human RSV is attenuated in interferon-competent bovine cells

Bovine respiratory syncytial virus (BRSV) escapes from cellular responses to alpha/beta interferon (IFN-alpha/beta) by a concerted action of the two viral nonstructural proteins, NS1 and NS2. Here we show that the NS proteins of human RSV (HRSV) are also able to counteract IFN responses and that they have the capacity to protect replication of an unrelated rhabdovirus. Even combinations of BRSV and HRSV NS proteins showed a protective activity, suggesting common mechanisms and cellular targets of HRSV and BRSV NS proteins. Although able to cooperate, NS proteins from BRSV and HRSV showed differential protection capacity in cells from different hosts. A chimeric BRSV with HRSV NS genes (BRSV h1/2) was severely attenuated in bovine IFN competent MDBK and Klu cells, whereas it replicated like BRSV in IFN-incompetent Vero cells or in IFN-competent human HEp-2 cells. After challenge with exogenous IFN-alpha, BRSV h1/2 was better protected than wild-type BRSV in human HEp-2 cells. In contrast, in cells of bovine origin, BRSV h1/2 was much less resistant to exogenous IFN than wild-type BRSV. These data demonstrate that RSV NS1 and NS2 proteins are major determinants of host range. The differential IFN escape capacity of RSV NS proteins in cells from different hosts provides a basis for rational development of attenuated live RSV vaccines.

All four Sendai Virus C proteins bind Stat1, but only the larger forms also induce its mono-ubiquitination and degradation

Sendai virus infection strongly induces interferon (IFN) production and has recently been shown to interdict the subsequent IFN signaling through the Jak/Stat pathway. This anti-IFN activity of SeV is due to its "C" proteins, a nested set of four proteins (C', C, Y1, Y2) that carry out a nested set of functions in countering the innate immune response. We previously reported that all four C proteins interact with Stat1 to prevent IFN signaling through the Jak/Stat pathway. Nevertheless, only the longer C proteins reduced Stat1 levels and prevented IFN from inducing an antiviral (VSV) state, or apoptosis, in IFN-competent murine cells. Here, we investigate the mechanism by which the various C proteins differentially affect the host antiviral defenses. All four C proteins were found to physically associate with Stat1 during cell culture infections, and in vitro in the absence of other viral gene products (as evidenced by co-immunoprecipitation). In addition, the inability of a null mutant (C(F170S)) to bind Stat1 suggests that this interaction is physiologically relevant. We have also shown that the proteasomal inhibitor MG132 can prevent the C protein-induced dismantling of the antiviral (VSV) state in murine cells; thus, the turnover of Stat1 correlates with the C protein-mediated counteraction of the antiviral (VSV) state. The C protein-induced instability of Stat1 was accompanied by a clear increase in the level of mono-ubiquinated Stat1, an unexpected hallmark of protein degradation. Finally, we show that a rSeV with mutant C proteins but wild-type Y proteins (CDelta10-15, that does not counteract the endogenous antiviral (VSV) state of MEFs even though their C proteins bind Stat1 and prevent its activity) is also unable to decrease bulk Stat1 levels or to increase the level of ubiquinated Stat1.

Sendai virus C protein impairs both phosphorylation and dephosphorylation processes of Stat1

Sendai virus expresses C protein that blocks interferon (IFN) signaling. We previously reported suppression of IFN-stimulated tyrosine phosphorylation of signal transducers and activators of transcription (Stats) in infected cells. However this conclusion has remained controversial. To settle it, we re-examined the effect of C protein expression on phosphorylation of Stat1 in detail. IFN-stimulated tyrosine phosphorylation of Stat1 was doubtlessly suppressed early in infection, but the suppression was incomplete, suggesting the importance of the unknown blocking mechanism that inactivates the tyrosine-phosphorylated (pY)-Stat1 generated as the signaling leak. Interestingly, the dephosphorylation process of pY-Stat1 was also impaired. These effects on both phosphorylation and dephosphorylation processes were attributable to the function of the C protein.