Cis-acting elements in the antigenomic promoter of Nipah virus

A Comparative Assessment of the Pathogenic Potential of Newly Discovered Henipaviruses

Recent advances in high-throughput sequencing technologies have led to the discovery of a plethora of previously unknown viruses in animal samples. Some of these newly detected viruses are closely related to human pathogens. A prime example are the henipaviruses. Both Nipah (NiV) and Hendra virus (HeV) cause severe disease in humans. Henipaviruses are of zoonotic origin, and animal hosts, including intermediate hosts, play a critical role in viral transmission to humans. The natural reservoir hosts of NiV and HeV seem to be restricted to a few fruit bat species of the Pteropus genus in distinct geographic areas. However, the recent discovery of novel henipa- and henipa-like viruses suggests that these viruses are far more widespread than was originally thought. To date, these new viruses have been found in a wide range of animal hosts, including bats, shrews, and rodents in Asia, Africa, Europe, and South America. Since these viruses are closely related to human pathogens, it is important to learn whether they pose a threat to human health. In this article, we summarize what is known about the newly discovered henipaviruses, highlight differences to NiV and HeV, and discuss their pathogenic potential.

Tetracistronic Minigenomes Elucidate a Functional Promoter for Ghana Virus and Unveils Cedar Virus Replicase Promiscuity for all Henipaviruses

Batborne henipaviruses, such as Nipah virus and Hendra virus, represent a major threat to global health due to their propensity for spillover, severe pathogenicity, and high mortality rate in human hosts. Coupled with the absence of approved vaccines or therapeutics, work with the prototypical species and uncharacterized, emergent species is restricted to high biocontainment facilities. There is a scarcity of such specialized spaces for research, and often the scope and capacity of research which can be conducted at BSL-4 is limited. Therefore, there is a pressing need for innovative life-cycle modeling systems to enable comprehensive research within lower biocontainment settings. This work showcases tetracistronic, transcription and replication competent minigenomes for Nipah virus, Hendra virus, Cedar virus, and Ghana virus, which encode viral proteins facilitating budding, fusion, and receptor binding. We validate the functionality of all encoded viral proteins and demonstrate a variety of applications to interrogate the viral life cycle. Notably, we found that the Cedar virus replicase exhibits remarkable promiscuity, efficiently rescuing minigenomes from all tested henipaviruses. We also apply this technology to GhV, an emergent species which has so far not been isolated in culture. We demonstrate that the reported sequence of GhV is incomplete, but that this missing sequence can be substituted with analogous sequences from other henipaviruses. Use of our GhV system establishes the functionality of the GhV replicase and identifies two antivirals which are highly efficacious against the GhV polymerase.

The critical region for viral RNA encapsidation in leader promoter of Nipah virus

Encapsidation by nucleocapsid (N) protein is crucial for viral RNA to serve as a functional template for virus replication. However, the potential region that is vital for RNA encapsidation of Nipah virus (NiV) is still unknown. Thus, this study was aimed to identify these regions using a NiV minireplicon system. A series of broad range internal deletion mutations was generated in the 5' non-translated region (NTR) of the N gene mRNA region of NiV leader promoter via site-directed overlapping PCR-mediated mutagenesis. The mutation effects on synthesis and encapsidation of antigenome RNA, transcription, and RNA binding affinity of N protein were evaluated. The deletions of nucleotides 73-108, 79-108, and 85-108 from NiV leader promoter inhibited the encapsidation of antigenome RNA, while the deletion of nucleotides 103-108 suppressed the synthesis and encapsidation of antigenome RNA, implying that these regions are required for genome replication. Surprisingly, none of the mutations had detrimental effect on viral transcription. Using isothermal titration calorimetry, the binding of NiV N protein to genome or antigenome RNA transcript lacking of nucleotides 73-108 was found to be suppressed. Additionally, in silico analysis on secondary structure of genome RNA further supported the plausible cause of inefficient encapsidation of antigenome RNA by the loss of encapsidation signal in genome template. In conclusion, this study suggests that the nucleotides 73-90 within 5' NTR of the N gene mRNA region in NiV leader promoter contain cis-acting RNA element that is important for efficient encapsidation of antigenome RNA.

Initiation, extension, and termination of RNA synthesis by a paramyxovirus polymerase

Paramyxoviruses represent a family of RNA viruses causing significant human diseases. These include measles virus, the most infectious virus ever reported, in addition to parainfluenza virus, and other emerging viruses. Paramyxoviruses likely share common replication machinery but their mechanisms of RNA biosynthesis activities and details of their complex polymerase structures are unknown. Mechanistic and functional details of a paramyxovirus polymerase would have sweeping implications for understanding RNA virus replication and for the development of new antiviral medicines. To study paramyxovirus polymerase structure and function, we expressed an active recombinant Nipah virus (NiV) polymerase complex assembled from the multifunctional NiV L protein bound to its phosphoprotein cofactor. NiV is an emerging highly pathogenic virus that causes severe encephalitis and has been declared a global public health concern due to its high mortality rate. Using negative-stain electron microscopy, we demonstrated NiV polymerase forms ring-like particles resembling related RNA polymerases. We identified conserved sequence elements driving recognition of the 3′-terminal genomic promoter by NiV polymerase, and leading to initiation of RNA synthesis, primer extension, and transition to elongation mode. Polyadenylation resulting from NiV polymerase stuttering provides a mechanistic basis for transcription termination. It also suggests a divergent adaptation in promoter recognition between pneumo- and paramyxoviruses. The lack of available antiviral therapy for NiV prompted us to identify the triphosphate forms of R1479 and GS-5734, two clinically relevant nucleotide analogs, as substrates and inhibitors of NiV polymerase activity by delayed chain termination. Overall, these findings provide low-resolution structural details and the mechanism of an RNA polymerase from a previously uncharacterized virus family. This work illustrates important functional differences yet remarkable similarities between the polymerases of nonsegmented negative-strand RNA viruses.

RNA viruses replicate and transcribe their genomes using complex enzymatic machines known as RNA-dependent RNA polymerases. The chemical reactions driving nucleotide addition are shared among nucleic acid polymerases but the underlying mechanisms of RNA biosynthesis and the complex polymerase structures are diverse. Of these RNA viruses is the paramyxovirus family, which includes major human pathogens. Paramyxoviruses have common biological and genetic properties but little is known about their replication machinery. Insights into the structure, function, and mechanisms of RNA synthesis of one paramyxovirus polymerase will likely extend to the entire virus family. An emerging, highly pathogenic paramyxovirus is Nipah virus (NiV), which causes encephalitis in humans. We have purified NiV polymerase, probed its enzymatic and biophysical properties and developed it as a model paramyxovirus polymerase. We investigated template strand sequence elements driving RNA biosynthesis for NiV polymerase and obtained a snapshot of NiV polymerase molecular organization using electron microscopy to provide the first structural information on a paramyxovirus polymerase. This work extends previous knowledge by producing the first recombinant paramyxovirus polymerase and using this protein in enzymatic assays to highlight key functional and structural characteristics for the design of new medicines.

Polymerases of paramyxoviruses and pneumoviruses

The paramyxo- and pneumoviruses are members of the order Mononegavirales, a group of viruses with non-segmented, negative strand RNA genomes. The polymerases of these viruses are multi-functional complexes, capable of transcribing subgenomic capped and polyadenylated mRNAs and replicating the genome. Although there is no native structure available for any complete paramyxo- or pneumovirus polymerase, functional and structural studies of a fragment of a pneumovirus polymerase protein and mutation analyses and resistance profiling of small-molecule inhibitors have generated a wealth of mechanistic information. This review integrates these data with the structure of a related polymerase, identifying similarities, differences, gaps in knowledge, and avenues for antiviral drug development.

Initiation and regulation of paramyxovirus transcription and replication

The paramyxovirus family has a genome consisting of a single strand of negative sense RNA. This genome acts as a template for two distinct processes: transcription to generate subgenomic, capped and polyadenylated mRNAs, and genome replication. These viruses only encode one polymerase. Thus, an intriguing question is, how does the viral polymerase initiate and become committed to either transcription or replication? By answering this we can begin to understand how these two processes are regulated. In this review article, we present recent findings from studies on the paramyxovirus, respiratory syncytial virus, which show how its polymerase is able to initiate transcription and replication from a single promoter. We discuss how these findings apply to other paramyxoviruses. Then, we examine how trans-acting proteins and promoter secondary structure might serve to regulate transcription and replication during different phases of the paramyxovirus replication cycle.

Distinct and overlapping roles of Nipah virus P gene products in modulating the human endothelial cell antiviral response

Nipah virus (NiV) is a highly pathogenic zoonotic paramyxovirus that causes fatal encephalitis in up to 75% of infected humans. Like other paramyxoviruses, NiV employs co-transcriptional mRNA editing during transcription of the phosphoprotein (P) gene to generate additional mRNAs encoding the V and W proteins. The C protein is translated from the P mRNA, but in an alternative reading frame. There is evidence from both in vitro and in vivo studies to show that the P gene products play a role in NiV pathogenesis. We have developed a reverse genetic system to dissect the individual roles of the NiV P gene products in limiting the antiviral response in primary human microvascular lung endothelial cells, which represent important targets in human NiV infection. By characterizing growth curves and early antiviral responses against a number of recombinant NiVs with genetic modifications altering expression of the proteins encoded by the P gene, we observed that multiple elements encoded by the P gene have both distinct and overlapping roles in modulating virus replication as well as in limiting expression of antiviral mediators such as IFN-β, CXCL10, and CCL5. Our findings corroborate observations from in vivo hamster infection studies, and provide molecular insights into the attenuation and the histopathology observed in hamsters infected with C, V, and W-deficient NiVs. The results of this study also provide an opportunity to verify the results of earlier artificial plasmid expression studies in the context of authentic viral infection.

Molecular virology of the henipaviruses

Nipah (NiV) and Hendra (HeV) viruses comprise the genus Henipavirus and are highly pathogenic paramyxoviruses, which cause fatal encephalitis and respiratory disease in humans. Since their respective initial outbreaks in 1998 and 1994, they have continued to cause sporadic outbreaks resulting in fatal disease. Due to their designation as Biosafety Level 4 pathogens, the level of containment required to work with live henipaviruses is available only to select laboratories around the world. This chapter provides an overview of the molecular virology of NiV and HeV including comparisons to other, well-characterized paramyxoviruses. This chapter also describes the sequence diversity present among the henipaviruses.

Analysis of nucleotides 13-96 of the human parainfluenza virus type 3 antigenomic promoter reveals positive- and negative-acting replication elements

During replication of human parainfluenza virus type 3 (HPIV3), the 96-nucleotide antigenomic promoter (AGP) of HPIV3 directs the synthesis of genomic RNA. Previous work showed that nucleotides 1-12 were critical in promoting replication of an HPIV3 minireplicon, but nucleotides 13-96 were not investigated. In this study, the role of nucleotides 13-96 in AGP function was analyzed by creating and assaying mutations in an HPIV3 minireplicon. A replication promoting element known as promoter element II (nt 79-96) was confirmed in the HPIV3 AGP. Additionally, nucleotides 13-39 were found to constitute an additional positive-acting cis-element. However, detailed analysis of the 13-39 element revealed a complicated control element with both stimulatory and repressing elements. Specifically, nucleotides 21-28 were shown to repress RNA replication, while flanking sequences had a stimulatory effect.

Vaccine potential of Nipah virus-like particles

Nipah virus (NiV) was first recognized in 1998 in a zoonotic disease outbreak associated with highly lethal febrile encephalitis in humans and a predominantly respiratory disease in pigs. Periodic deadly outbreaks, documentation of person-to-person transmission, and the potential of this virus as an agent of agroterror reinforce the need for effective means of therapy and prevention. In this report, we describe the vaccine potential of NiV virus-like particles (NiV VLPs) composed of three NiV proteins G, F and M. Co-expression of these proteins under optimized conditions resulted in quantifiable amounts of VLPs with many virus-like/vaccine desirable properties including some not previously described for VLPs of any paramyxovirus: The particles were fusogenic, inducing syncytia formation; PCR array analysis showed NiV VLP-induced activation of innate immune defense pathways; the surface structure of NiV VLPs imaged by cryoelectron microscopy was dense, ordered, and repetitive, and consistent with similarly derived structure of paramyxovirus measles virus. The VLPs were composed of all the three viral proteins as designed, and their intracellular processing also appeared similar to NiV virions. The size, morphology and surface composition of the VLPs were consistent with the parental virus, and importantly, they retained their antigenic potential. Finally, these particles, formulated without adjuvant, were able to induce neutralizing antibody response in Balb/c mice. These findings indicate vaccine potential of these particles and will be the basis for undertaking future protective efficacy studies in animal models of NiV disease.

The marburg virus 3' noncoding region structurally and functionally differs from that of ebola virus

We have previously shown that the first transcription start signal (TSS) of Zaire Ebola virus (ZEBOV) is involved in formation of an RNA secondary structure regulating VP30-dependent transcription activation. Interestingly, transcription of Marburg virus (MARV) minigenomes occurs independently of VP30. In this study, we analyzed the structure of the MARV 3' noncoding region and its influence on VP30 necessity. Secondary structure formation of the TSS of the first gene was experimentally determined and showed substantial differences from the structure formed by the ZEBOV TSS. Chimeric MARV minigenomes mimicking the ZEBOV-specific RNA secondary structure were neither transcribed nor replicated. Mapping of the MARV genomic replication promoter revealed that the region homologous to the sequence involved in formation of the regulatory ZEBOV RNA structure is part of the MARV promoter. The MARV promoter is contained within the first 70 nucleotides of the genome and consists of two elements separated by a spacer region, comprising the TSS of the first gene. Mutations within the spacer abolished transcription activity and led to increased replication, indicating competitive transcription and replication initiation. The second promoter element is located within the nontranslated region of the first gene and consists of a stretch of three UN(5) hexamers. Recombinant full-length MARV clones, in which the three conserved U residues were substituted, could not be rescued, underlining the importance of the UN(5) hexamers for replication activity. Our data suggest that differences in the structure of the genomic replication promoters might account for the different transcription strategies of Marburg and Ebola viruses.