Arenaviruses can cause fatal human haemorrhagic fever (HF) diseases for which vaccines and therapies are extremely limited. Both the New World (NW) and Old World (OW) groups of arenaviruses contain HF-causing pathogens. Although these two groups share many similarities, important differences with regard to pathogenicity and molecular mechanisms of virus infection exist. These closely related pathogens share many characteristics, including genome structure, viral assembly, natural host selection and the ability to interfere with innate immune signalling. However, members of the NW and OW viruses appear to use different receptors for cellular entry, as well as different mechanisms of virus internalization. General differences in disease signs and symptoms and pathological lesions in patients infected with either NW or OW arenaviruses are also noted and discussed herein. Whilst both the OW Lassa virus (LASV) and the NW Junin virus (JUNV) can cause disruption of the vascular endothelium, which is an important pathological feature of HF, the immune responses to these related pathogens seem to be quite distinct. Whereas LASV infection results in an overall generalized immune suppression, patients infected with JUNV seem to develop a cytokine storm. Additionally, the type of immune response required for recovery and clearance of the virus is different between NW and OW infections. These differences may be important to allow the viruses to evade host immune detection. Understanding these differences will aid the development of new vaccines and treatment strategies against deadly HF viral infections.
Host signaling pathways and cellular proteins play important roles in the influenza viral life cycle and can serve as antiviral targets. In this study, we report the engagement of host phosphoinositide-specific phospholipase γ1 (PLC-γ1) in mediating cell entry of influenza virus H1N1 but not H3N2 subtype. Both PLC-γ1-specific inhibitor and short hairpin RNA (shRNA) strongly suppress the replication of H1N1 but not H3N2 viruses in cell culture, suggesting that PLC-γ1 plays an important subtype-specific role in the influenza viral life cycle. Further analyses demonstrate that PLC-γ1 activation is required for viral postbinding cell entry. In addition, H1N1, but not H3N2, infection leads to the phosphorylation of PLC-γ1 at Ser 1248 immediately after infection and independent of viral replication. We have further shown that H1N1-induced PLC-γ1 activation is downstream of epidermal growth factor receptor (EGFR) signaling. Interestingly, both H1N1 and H3N2 infections activate EGFR, but only H1N1 infection leads to PLC-γ1 activation. Taking our findings together, we have identified for the first time the subtype-specific interplay of host PLC-γ1 signaling and H1N1 virus that is critical for viral uptake early in the infection. Our study provides novel insights into how virus interacts with the cellular signaling network by demonstrating that viral determinants can regulate how the host signaling pathways function in virally infected cells.
Primary progressive aphasia (PPA) represents a diverse group of language-led dementias most often due to frontotemporal lobar degeneration. We report clinical, neuropsychological and neuroimaging data in the case of a 47 year old woman presenting with non-fluent PPA due to a genetically confirmed pathogenic Presenilin 1 P264L mutation. This case highlights an unusual clinical presentation of familial Alzheimer’s disease and a novel presentation of the P264L mutation. The case adds to accumulating evidence that particular mutations can promote specific brain network degeneration, with wider implications for understanding the sporadic forms of Alzheimer’s disease and PPA.
Primary progressive aphasia; familial Alzheimer’s disease; Presenilin 1
Lassa virus (LASV) causes deadly hemorrhagic fever disease for which there are no vaccines and limited treatments. LASV-encoded L polymerase is required for viral RNA replication and transcription. The functional domains of L–a large protein of 2218 amino acid residues–are largely undefined, except for the centrally located RNA-dependent RNA polymerase (RdRP) motif. Recent structural and functional analyses of the N-terminal region of the L protein from lymphocytic choriomeningitis virus (LCMV), which is in the same Arenaviridae family as LASV, have identified an endonuclease domain that presumably cleaves the cap structures of host mRNAs in order to initiate viral transcription. Here we present a high-resolution crystal structure of the N-terminal 173-aa region of the LASV L protein (LASV L173) in complex with magnesium ions at 1.72 Å. The structure is highly homologous to other known viral endonucleases of arena- (LCMV NL1), orthomyxo- (influenza virus PA), and bunyaviruses (La Crosse virus NL1). Although the catalytic residues (D89, E102 and K122) are highly conserved among the known viral endonucleases, LASV L endonuclease structure shows some notable differences. Our data collected from in vitro endonuclease assays and a reporter-based LASV minigenome transcriptional assay in mammalian cells confirm structural prediction of LASV L173 as an active endonuclease. The high-resolution structure of the LASV L endonuclease domain in complex with magnesium ions should aid the development of antivirals against lethal Lassa hemorrhagic fever.
A number of arenaviruses are pathogenic for humans, but they differ significantly in virulence. Lassa virus, found in West Afri ca, causes severe hemorrhagic fever (HF), while the other principal Old World arenavirus, lymphocytic choriomeningitis virus, causes mild illness in persons with normal immune function, and poses a threat only to immunocompromised individuals. The New World agents, including Junin, Machupo and Sabia virus, are highly pathogenic for humans. Arenaviral HF is characterized by high viremia and general immune suppression, the mechanism of which is unknown. Studies using viral reverse genetics, cell-based assays, animal models and human genome-wide association analysis have revealed potential mechanisms by which arenaviruses cause severe disease in humans. Each of the four viral gene products (GPC, L polymerase, NP, and Z matrix protein) and several host-cell factors (e.g., α-dystroglycan) are responsible for mediating viral entry, genome replication, and the inhibition of apoptosis, translation and interferon-beta (IFNβ) production. This review summarizes current knowledge of the role of each viral protein and host factor in the pathogenesis of arenaviral HF. Insights from recent studies are being exploited for the development of novel therapies.
Several arenaviruses are responsible for causing viral hemorrhagic fevers (VHF) in humans. Lassa virus (LASV), the causative agent of Lassa fever, is a biosafety level 4 (BSL4) pathogen that requires handling in BSL4 facilities. In contrast, the Pichinde arenavirus (PICV) is a BSL2 pathogen that can cause hemorrhagic fever-like symptoms in guinea pigs that resemble those observed in human Lassa fever. Comparative sequence analysis of the avirulent P2 strain of PICV and the virulent P18 strain shows a high degree of sequence homology in the bisegmented genome between the two strains despite the polarized clinical outcomes noted for the infected animals. Using reverse genetics systems that we have recently developed, we have mapped the sequence changes in the large (L) segment of the PICV genome that are responsible for the heightened virulence phenotype of the P18 strain. By monitoring the degree of disease severity and lethality caused by the different mutant viruses, we have identified specific residues located within the viral L polymerase gene encoded on the L segment essential for mediating disease pathogenesis. Through quantitative reverse transcription-PCR (RT-PCR) analysis, we have confirmed that the same set of residues is responsible for the increased viral replicative potential of the P18 strain and its heightened disease severity in vivo. Our laboratory findings serve to reinforce field observations that a high level of viremia often correlates with severe disease outcomes in LASV-infected patients.
We use a small animal model, based on guinea pigs infected with a non-pathogenic Pichinde virus (PICV), to understand the virulence mechanisms of arenavirus infections in the hosts. PICV P2 strain causes a mild febrile reaction in guinea pigs, while P18 causes severe disease with clinical and pathological features reminiscent of Lassa hemorrhagic fever in humans. The envelope glycoproteins (GPC) of P2 and P18 viruses differ at positions 119, 140, and 164, all localized to the receptor-binding G1 subunit. We found that lentiviral pseudotyped virions (VLPs) bearing P18 GPC show more efficient cell entry than those with P2 GPC, and that the E140 residue plays a critical role in this process. Infection of guinea pigs with the recombinant viruses containing the E140K change demonstrated that E140 of GPC is a necessary virulence determinant of P18 infections, possibly by enhancing the ability of virus to enter target cells.
virulence; arenavirus; Pichinde virus; glycoprotein; entry
To assess regional patterns of gray and white matter atrophy in familial Alzheimer disease (FAD) mutation carriers.
A total of 192 participants with volumetric T1-weighted MRI, genotyping, and clinical diagnosis were available from the Dominantly Inherited Alzheimer Network. Of these, 69 were presymptomatic mutation carriers, 50 were symptomatic carriers (31 with Clinical Dementia Rating [CDR] = 0.5, 19 with CDR > 0.5), and 73 were noncarriers from the same families. Voxel-based morphometry was used to identify cross-sectional group differences in gray matter and white matter volume.
Significant differences in gray matter (p < 0.05, family-wise error–corrected) were observed between noncarriers and mildly symptomatic (CDR = 0.5) carriers in the thalamus and putamen, as well as in the temporal lobe, precuneus, and cingulate gyrus; the same pattern, but with more extensive changes, was seen in those with CDR > 0.5. Significant white matter differences between noncarriers and symptomatic carriers were observed in the cingulum and fornix; these form input and output connections to the medial temporal lobe, cingulate, and precuneus. No differences between noncarriers and presymptomatic carriers survived correction for multiple comparisons, but there was a trend for decreased gray matter in the thalamus for carriers closer to their estimated age at onset. There were no significant increases of gray or white matter in asymptomatic or symptomatic carriers compared to noncarriers.
Atrophy in FAD is observed early, both in areas commonly associated with sporadic Alzheimer disease and also in the putamen and thalamus, 2 regions associated with early amyloid deposition in FAD mutation carriers.
Background: Arenaviral nucleoproteins play a critical role in innate immune suppression.
Results: Structures of Lassa nucleoprotein in complex with triphosphate dsRNA and Tacaribe virus nucleoprotein have been determined.
Conclusion: Both Lassa and Tacaribe nucleoproteins can strongly inhibit IFN-β production by degrading immune-stimulatory dsRNA.
Significance: A unique immune suppression mode of arenaviral nucleoproteins has been revealed.
A hallmark of severe Lassa fever is the generalized immune suppression, the mechanism of which is poorly understood. Lassa virus (LASV) nucleoprotein (NP) is the only known 3′-5′ exoribonuclease that can suppress type I interferon (IFN) production possibly by degrading immune-stimulatory RNAs. How this unique enzymatic activity of LASV NP recognizes and processes RNA substrates is unknown. We provide an atomic view of a catalytically active exoribonuclease domain of LASV NP (LASV NP-C) in the process of degrading a 5′ triphosphate double-stranded (ds) RNA substrate, a typical pathogen-associated molecular pattern molecule, to induce type I IFN production. Additionally, we provide for the first time a high-resolution crystal structure of an active exoribonuclease domain of Tacaribe arenavirus (TCRV) NP. Coupled with the in vitro enzymatic and cell-based interferon suppression assays, these structural analyses strongly support a unified model of an exoribonuclease-dependent IFN suppression mechanism shared by all known arenaviruses. New knowledge learned from these studies should aid the development of therapeutics against pathogenic arenaviruses that can infect hundreds of thousands of individuals and kill thousands annually.
Immunosuppression; Nucleic Acid Enzymology; Protein Structure; Protein-nucleic Acid Interaction; Viral Protein; 3′-5′ Exoribonuclease; Lassa Fever Virus; Tacaribe Virus; Type I Interferons; Immune Evasion
Arenaviruses can cause severe hemorrhagic fever diseases in humans, with limited prophylactic or therapeutic measures. A small RING-domain viral protein Z has been shown to mediate the formation of virus-like particles and to inhibit viral RNA synthesis, although its biological roles in an infectious viral life cycle have not been directly addressed. By taking advantage of the available reverse genetics system for a model arenavirus, Pichinde virus (PICV), we provide the direct evidence for the essential biological roles of the Z protein's conserved residues, including the G2 myristylation site, the conserved C and H residues of RING domain, and the poorly characterized C-terminal L79 and P80 residues. Dicodon substitutions within the late (L) domain (PSAPPYEP) of the PICV Z protein, although producing viable mutant viruses, have significantly reduced virus growth, a finding suggestive of an important role for the intact L domain in viral replication. Further structure-function analyses of both PICV and Lassa fever virus Z proteins suggest that arenavirus Z proteins have similar molecular mechanisms in mediating their multiple functions, with some interesting variations, such as the role of the G2 residue in blocking viral RNA synthesis. In summary, our studies have characterized the biological roles of the Z protein in an infectious arenavirus system and have shed important light on the distinct functions of its domains in virus budding and viral RNA regulation, the knowledge of which may lead to the development of novel antiviral drugs.
Our understanding of the molecular mechanisms of many neurological disorders has been greatly enhanced by the discovery of mutations in genes linked to familial forms of these diseases. These have facilitated the generation of cell and animal models that can be used to understand the underlying molecular pathology. Recently, there has been a surge of interest in the use of patient-derived cells, due to the development of induced pluripotent stem cells and their subsequent differentiation into neurons and glia. Access to patient cell lines carrying the relevant mutations is a limiting factor for many centres wishing to pursue this research. We have therefore generated an open-access collection of fibroblast lines from patients carrying mutations linked to neurological disease. These cell lines have been deposited in the National Institute for Neurological Disorders and Stroke (NINDS) Repository at the Coriell Institute for Medical Research and can be requested by any research group for use in in vitro disease modelling. There are currently 71 mutation-defined cell lines available for request from a wide range of neurological disorders and this collection will be continually expanded. This represents a significant resource that will advance the use of patient cells as disease models by the scientific community.
We have previously reported that two receptor tyrosine kinase inhibitors (RTKIs), called AG879 and tyrphostin A9 (A9), can each block the replication of influenza A virus in cultured cells. In this study, we further characterized the in vitro antiviral efficacies and specificities of these agents. The 50% effective concentration (EC50) of each against influenza A was found to be in the high nanomolar range, and the selectivity index (SI = 50% cytotoxic concentration [CC50]/EC50) was determined to be >324 for AG879 and 50 for A9, indicating that therapeutically useful concentrations of each drug produce only low levels of cytotoxicity. Each compound showed efficacy against representative laboratory strains of both human influenza A (H1N1 or H3N2) and influenza B viruses. Importantly, no drug-resistant influenza virus strains emerged even after 25 viral passages in the presence of AG879, whereas viruses resistant to amantadine appeared after only 3 passages. AG879 and A9 each also exhibited potent inhibitory activity against a variety of other RNA and DNA viruses, including Sendai virus (Paramyxoviridae), herpes simplex virus (Herpesviridae), mouse hepatitis virus (Coronaviridae), and rhesus rotavirus (Reoviridae), but not against Pichinde virus (Arenaviridae). These results together suggest that RTKIs may be useful as therapeutics against viral pathogens, including but not limited to influenza, due to their high selectivity indices, low frequency of drug resistance, and broad-spectrum antiviral activities.
Influenza A viral polymerase is a heterotrimeric complex that consists of PA, PB1, and PB2 subunits. We previously reported that a di-codon substitution mutation (G507A-R508A), denoted J10, in the C-terminal half of PA had no apparent effect on viral RNA synthesis but prevented infectious virus production, indicating that PA may have a novel role independent of its polymerase activity. To further examine the roles of PA in the viral life cycle, we have now generated and characterized additional mutations in regions flanking the J10 site from residues 497 to 518. All tested di-codon mutations completely abolished or significantly reduced viral infectivity, but they did so through disparate mechanisms. Several showed effects resembling those of J10, in that the mutant polymerase supported normal levels of viral RNA synthesis but nonetheless failed to generate infectious viral particles. Others eliminated polymerase activity, in most cases by perturbing the normal nuclear localization of PA protein in cells. We also engineered single-codon mutations that were predicted to pack near the J10 site in the crystal structure of PA, and found that altering residues K378 or D478 each produced a J10-like phenotype. In further studies of J10 itself, we found that this mutation does not affect the formation and release of virion-like particles per se, but instead impairs the ability of those particles to incorporate each of the eight essential RNA segments (vRNAs) that make up the viral genome. Taken together, our analysis identifies mutations in the C-terminal region of PA that differentially affect at least three distinct activities: protein nuclear localization, viral RNA synthesis, and a trans-acting function that is required for efficient packaging of all eight vRNAs.
Host signaling pathways play important roles in the replication of influenza virus, but their functional effects remain to be characterized at the molecular level. Here we identify two receptor tyrosine kinase inhibitors (RTKIs) of the tyrphostin class that exhibit robust antiviral activity against influenza A virus replication in cultured cells. One of these (AG879) is a selective inhibitor of the nerve growth factor receptor and human epidermal growth factor receptor 2 (TrkA/HER2) signaling; the other, tyrphostin A9 (A9), inhibits the platelet-derived growth factor receptor (PDGFR) pathway. We find that each inhibits at least three postentry steps of the influenza virus life cycle: AG879 and A9 both strongly inhibit the synthesis of all three influenza virus RNA species, block Crm1-dependent nuclear export, and also prevent the release of viral particles through a pathway that is modulated by the lipid biosynthesis enzyme farnesyl diphosphate synthase (FPPS). Tests of short hairpin RNA (shRNA) knockdown and additional small-molecule inhibitors confirmed that interventions targeting TrkA can suppress influenza virus replication. Our study suggests that host cell receptor tyrosine kinase signaling is required for maximal influenza virus RNA synthesis, viral ribonucleoprotein (vRNP) nuclear export, and virus release and that specific RTKIs hold promise as novel anti-influenza virus therapeutics.
Lassa fever virus (LASV) causes thousands of deaths yearly and is a biological threat agent, for which there is no vaccine and limited therapy1. The nucleoprotein (NP) of LASV plays essential roles in viral RNA synthesis and immune suppression2-6, the molecular mechanisms of which are poorly understood. Here, we report the crystal structure of LASV NP at 1.80 Angstrom resolution, which reveals N- and C-domains with structures unlike any of the reported viral NPs7-10. The N domain folds into a novel structure with a deep cavity for binding the m7GpppN cap structure that is required for viral RNA transcription, whereas the C domain contains 3′-5′ exoribonuclease activity involved in suppressing interferon induction. This is the first X-ray crystal structure solved for an arenaviral NP, which reveals its unexpected functions and suggests unique mechanisms in cap binding and immune evasion. These findings provide great potential for vaccine and drug development.
The newly identified retrovirus—the xenotropic murine leukemia virus-related virus (XMRV)—has recently been shown to be strongly associated with familial prostate cancer in humans (A. Urisman et al., PLoS Pathog. 2:e25, 2006). While that study showed evidence of XMRV infection exclusively in the prostatic stromal fibroblasts, a recent study found XMRV protein antigens mainly in malignant prostate epithelial cells (R. Schlaberg et al., Proc. Natl. Acad. Sci. U. S. A. 106:16351-16356, 2009). To help elucidate the mechanisms behind XMRV infection, we show that prostatic fibroblast cells express Xpr1, a known receptor of XMRV, but its expression is absent in other cell lines of the prostate (i.e., epithelial and stromal smooth muscle cells). We also show that certain amino acid residues located within the predicted extracellular loop (ECL3 and ECL4) sequences of Xpr1 are required for efficient XMRV entry. Although we found strong evidence to support XMRV infection of prostatic fibroblast cell lines via Xpr1, we learned that XMRV was indeed capable of infecting cells that did not necessarily express Xpr1, such as those of the prostatic epithelial and smooth muscle origins. Further studies suggest that the expression of Xpr1 and certain genotypes of the RNASEL gene, which could restrict XMRV infection, may play important roles in defining XMRV tropisms in certain cell types. Collectively, our data reveal important cellular determinants required for XMRV entry into different human prostate cells in vitro, which may provide important insights into the possible role of XMRV as an etiologic agent in human prostate cancer.
Arenaviruses are enveloped single-strand RNA viruses that mostly have natural hosts in rodents. Upon infection of humans, several arenaviruses can cause severe hemorrhagic fever diseases, including Lassa fever that is endemic in West Africa. The virulence mechanism of these deadly arenaviruses can be studied in a safe and economical small animal model - guinea pigs infected by a non-pathogenic arenavirus Pichinde virus (PICV), a virulent strain of which can cause similar disease syndromes in guinea pigs as arenaviral hemorrhagic fevers in humans. We have recently developed molecular clones for both the virulent and avirulent strains of PICV. Using the available reverse genetics tools, we are characterizing the molecular determinants of virulent arenavirus infections in vivo.
viral hemorrhagic fever; arenavirus; reverse genetics system; animal model; virulence factors; Lassa fever; Pichinde virus; guinea pig
Several arenaviruses can cause hemorrhagic fever diseases (VHFs) in humans, the pathogenic mechanism of which is poorly understood due to their virulent nature and the lack of molecular clones. A safe, convenient, and economical small animal model of arenavirus hemorrhagic fever is based on guinea pigs infected by the arenavirus Pichinde (PICV). PICV does not cause disease in humans, but an adapted strain of PICV (P18) causes a disease in guinea pigs that mimics arenavirus hemorrhagic fever in humans in many aspects, while a low-passaged strain (P2) remains avirulent in infected animals. In order to identify the virulence determinants within the PICV genome, we developed the molecular clones for both the avirulent P2 and virulent P18 viruses. Recombinant viruses were generated by transfecting plasmids that contain the antigenomic L and S RNA segments of PICV under the control of the T7 promoter into BSRT7-5 cells, which constitutively express T7 RNA polymerase. By analyzing viral growth kinetics in vitro and virulence in vivo, we show that the recombinant viruses accurately recapitulate the replication and virulence natures of their respective parental viruses. Both parental and recombinant virulent viruses led to high levels of viremia and titers in different organs of the infected animals, whereas the avirulent viruses were effectively controlled and cleared by the hosts. These novel infectious clones for the PICV provide essential tools to identify the virulence factors that are responsible for the severe VHF-like disease in infected animals.
The NF-κB signaling pathway has previously been shown to be required for efficient influenza A virus replication, although the molecular mechanism is not well understood. In this study, we identified a specific step of the influenza virus life cycle that is influenced by NF-κB signaling by using two known NF-κB inhibitors and a variety of influenza virus-specific assays. The results of time course experiments suggest that the NF-κB inhibitors Bay11-7082 and ammonium pyrrolidinedithiocarbamate inhibited an early postentry step of viral infection, but they did not appear to affect the nucleocytoplasmic trafficking of the viral ribonucleoprotein complex. Instead, we found that the levels of influenza virus genomic RNA (vRNA), but not the corresponding cRNA or mRNA, were specifically reduced by the inhibitors in virus-infected cells, indicating that NF-κB signaling is intimately involved in the vRNA synthesis. Furthermore, we showed that the NF-κB inhibitors specifically diminished influenza virus RNA transcription from the cRNA promoter but not from the vRNA promoter in a reporter assay, a result which is consistent with data obtained from virus-infected cells. The overexpression of the p65 NF-κB molecule could not only eliminate the inhibition but also activate influenza virus RNA transcription from the cRNA promoter. Finally, using p65-specific small interfering RNA, we have shown that p65 knockdown reduced the levels of influenza virus replication and vRNA synthesis. In summary, we have provided evidence showing, for the first time, that the NF-κB host signaling pathway can differentially regulate influenza virus RNA synthesis, which may also offer some new perspectives into understanding the host regulation of RNA synthesis by other RNA viruses.
A virulent (P18) strain of the Pichinde arenavirus produces a disease in guinea pigs that somewhat mimics human Lassa fever, whereas an avirulent (P2) strain of this virus is attenuated in infected animals. It has been speculated that the composition of viral genomes may confer the degree of virulence in an infected host; the complete sequence of the viral genomes, however, is not known. Here, we provide for the first time genomic sequences of both S and L segments for both the P2 and P18 strains. Sequence comparisons identify three mutations in the GP1 subunit of the viral glycoprotein, one in the nucleoprotein NP, and five in the viral RNA polymerase L protein. These mutations, alone or in combination, may contribute to the acquired virulence of Pichinde infection in animals. The 3 amino acid changes in the variable region of the GP1 glycoprotein subunit may affect viral entry by altering its receptor-binding activity. While NP has previously been shown to modulate the host immune responses to viral infection, we found that the R374K change in this protein does not affect the NP function in suppressing interferon-β expression. Four out of the five amino acid changes in the L protein occur in a small region of the protein that may contribute to viral virulence by enhancing its function on viral genomic RNA synthesis.
The influenza A virus genome consists of eight negative-sense RNA segments that must each be packaged to produce an infectious virion. We have previously mapped the minimal cis-acting regions necessary for efficient packaging of the PA, PB1, and PB2 segments, which encode the three protein subunits of the viral RNA polymerase. The packaging signals in each of these RNAs lie within two separate regions at the 3′ and 5′ termini, each encompassing the untranslated region and extending up to 80 bases into the adjacent coding sequence. In this study, we introduced scanning mutations across the coding regions in each of these RNA segments in order to finely define the packaging signals. We found that mutations producing the most severe defects were confined to a few discrete 5′ sites in the PA or PB1 coding regions but extended across the entire (80-base) 5′ coding region of PB2. In sequence comparisons among more than 580 influenza A strains from diverse hosts, these highly deleterious mutations were each found to affect one or more conserved bases, though they did not all lie within the most broadly conserved portions of the regions that we interrogated. We have introduced silent and conserved mutations to the critical packaging sites, which did not affect protein function but impaired viral replication at levels roughly similar to those of their defects in RNA packaging. Interestingly, certain mutations showed strong tendencies to revert to wild-type sequences, which implies that these putative packaging signals are critical for the influenza life cycle.
The RNA-dependent RNA polymerase of influenza A virus is composed of three subunits that together synthesize all viral mRNAs and also replicate the viral genomic RNA segments (vRNAs) through intermediates known as cRNAs. Here we describe functional characterization of 16 site-directed mutants of one polymerase subunit, termed PA. In accord with earlier studies, these mutants exhibited diverse, mainly quantitative impairments in expressing one or more classes of viral RNA, with associated infectivity defects of varying severity. One PA mutant, however, targeting residues 507 and 508, caused only modest perturbations of RNA expression yet completely eliminated the formation of plaque-forming virus. Polymerases incorporating this mutant, designated J10, proved capable of synthesizing translationally active mRNAs and of replicating diverse cRNA or vRNA templates at levels compatible with viral infectivity. Both the mutant protein and its RNA products were appropriately localized in the cytoplasm, where influenza virus assembly occurs. Nevertheless, J10 failed to generate infectious particles from cells in a plasmid-based influenza virus assembly assay, and hemagglutinating material from the supernatants of such cells contained little or no nuclease-resistant genomic RNA. These findings suggest that PA has a previously unrecognized role in assembly or release of influenza virus virions, perhaps influencing core structure or the packaging of vRNAs or other essential components into nascent influenza virus particles.
The influenza A virus genome consists of eight negative-sense RNA segments. The cis-acting signals that allow these viral RNA segments (vRNAs) to be packaged into influenza virus particles have not been fully elucidated, although the 5′ and 3′ untranslated regions (UTRs) of each vRNA are known to be required. Efficient packaging of the NA, HA, and NS segments also requires coding sequences immediately adjacent to the UTRs, but it is not yet known whether the same is true of other vRNAs. By assaying packaging of genetically tagged vRNA reporters during plasmid-directed influenza virus assembly in cells, we have now mapped cis-acting sequences that are sufficient for packaging of the PA, PB1, and PB2 segments. We find that each involves portions of the distal coding regions. Efficient packaging of the PA or PB1 vRNAs requires at least 40 bases of 5′ and 66 bases of 3′ coding sequences, whereas packaging of the PB2 segment requires at least 80 bases of 5′ coding region but is independent of coding sequences at the 3′ end. Interestingly, artificial reporter vRNAs carrying mismatched ends (i.e., whose 5′ and 3′ ends are derived from different vRNA segments) were poorly packaged, implying that the two ends of any given vRNA may collaborate in forming specific structures to be recognized by the viral packaging machinery.
The Kaposi's sarcoma-associated herpesvirus (KSHV) gene product virally encoded G protein-coupled receptor (vGPCR) is a homolog of cellular GPCRs and has been proposed to play important roles in KSHV-induced angiogenesis. The most abundant vGPCR-containing transcripts are K14/vGPCR bicistronic RNAs that are strongly induced during lytic reactivation. Here we show that the promoter governing this transcript is strongly responsive to activation by the viral lytic switch protein RTA. By deletion mapping and scanning mutation analyses, we have identified three putative RTA response elements (A, B, and C) in this promoter. However, none of these sites appear to directly bind RTA in electrophoretic mobility shift assays (EMSA). Site C corresponds to a canonical binding site for RBP-J, a sequence-specific transcriptional repressor that is normally the target of Notch signaling. RBP-J can bind RTA and recruit it to its cognate recognition site; when this happens, the activation function of RTA can relieve RBP-J-mediated repression and upregulate expression of the targeted gene. EMSA studies reveal that both sites A and C can bind to RBP-J; sequence inspection reveals that site A is a novel functional variant of known RBP-J recognition sites. (Site B corresponds to an as-yet-unknown host DNA-binding protein.) The importance of sites A and C in vivo is underscored by the observation that K14/vGPCR promoter function is dramatically inhibited in cells genetically deficient in RBP-J. The regulation of K14/vGPCR transcripts by RBP-J raises the possibility that other modulators of Notch signaling might be able to induce expression of this RNA outside the context of lytic KSHV replication.