Stephen A. Goldstein

Middle East Respiratory Syndrome Coronavirus NS4b Protein Inhibits Host RNase L Activation

ABSTRACT Middle East respiratory syndrome coronavirus (MERS-CoV) is the first highly pathogenic human coronavirus to emerge since severe acute respiratory syndrome coronavirus (SARS-CoV) in 2002. Like many coronaviruses, MERS-CoV carries genes that encode multiple accessory proteins that are not required for replication of the genome but are likely involved in pathogenesis. Evasion of host innate immunity through interferon (IFN) antagonism is a critical component of viral pathogenesis. The IFN-inducible oligoadenylate synthetase (OAS)-RNase L pathway activates upon sensing of viral double-stranded RNA (dsRNA). Activated RNase L cleaves viral and host single-stranded RNA (ssRNA), which leads to translational arrest and subsequent cell death, preventing viral replication and spread. Here we report that MERS-CoV, a lineage C Betacoronavirus, and related bat CoV NS4b accessory proteins have phosphodiesterase (PDE) activity and antagonize OAS-RNase L by enzymatically degrading 2′,5′-oligoadenylate (2-5A), activators of RNase L. This is a novel function for NS4b, which has previously been reported to antagonize IFN signaling. NS4b proteins are distinct from lineage A Betacoronavirus PDEs and rotavirus gene-encoded PDEs, in having an amino-terminal nuclear localization signal (NLS) and are localized mostly to the nucleus. However, the expression level of cytoplasmic MERS-CoV NS4b protein is sufficient to prevent activation of RNase L. Finally, this is the first report of an RNase L antagonist expressed by a human or bat coronavirus and provides a specific mechanism by which this occurs. Our findings provide a potential mechanism for evasion of innate immunity by MERS-CoV while also identifying a potential target for therapeutic intervention. IMPORTANCE Middle East respiratory syndrome coronavirus (MERS-CoV) is the first highly pathogenic human coronavirus to emerge since severe acute respiratory syndrome coronavirus (SARS-CoV). MERS-CoV, like other coronaviruses, carries genes that encode accessory proteins that antagonize the host antiviral response, often the type I interferon response, and contribute to virulence. We found that MERS-CoV NS4b and homologs from related lineage C bat betacoronaviruses BtCoV-SC2013 (SC2013) and BtCoV-HKU5 (HKU5) are members of the 2H-phosphoesterase (2H-PE) enzyme family with phosphodiesterase (PDE) activity. Like murine coronavirus NS2, a previously characterized PDE, MERS NS4b, can antagonize activation of the OAS-RNase L pathway, an interferon-induced potent antiviral activity. Furthermore, MERS-CoV mutants with deletion of genes encoding accessory proteins NS3 to NS5 or NS4b alone or inactivation of the PDE can activate RNase L during infection of Calu-3 cells. Our report may offer a potential target for therapeutic intervention if NS4b proves to be critical to pathogenesis in in vivo models of MERS-CoV infection. Middle East respiratory syndrome coronavirus (MERS-CoV) is the first highly pathogenic human coronavirus to emerge since severe acute respiratory syndrome coronavirus (SARS-CoV). MERS-CoV, like other coronaviruses, carries genes that encode accessory proteins that antagonize the host antiviral response, often the type I interferon response, and contribute to virulence. We found that MERS-CoV NS4b and homologs from related lineage C bat betacoronaviruses BtCoV-SC2013 (SC2013) and BtCoV-HKU5 (HKU5) are members of the 2H-phosphoesterase (2H-PE) enzyme family with phosphodiesterase (PDE) activity. Like murine coronavirus NS2, a previously characterized PDE, MERS NS4b, can antagonize activation of the OAS-RNase L pathway, an interferon-induced potent antiviral activity. Furthermore, MERS-CoV mutants with deletion of genes encoding accessory proteins NS3 to NS5 or NS4b alone or inactivation of the PDE can activate RNase L during infection of Calu-3 cells. Our report may offer a potential target for therapeutic intervention if NS4b proves to be critical to pathogenesis in in vivo models of MERS-CoV infection.

Activation of RNase L is dependent on OAS3 expression during infection with diverse human viruses

Significance RNase L, an antiviral enzyme activated during infection, degrades viral and cellular RNAs, inhibits protein synthesis, and restricts the replication and spread of diverse viruses. RNase L activation depends on 2′,5′-oligoadenylates synthesized by different oligoadenylate synthetases (OASs), i.e., OAS1, OAS2, and OAS3. OASs are induced by interferon and are activated by viral dsRNA. It has been unclear which of these OAS proteins is necessary and/or sufficient to activate RNase L during viral infections. We show that OAS3, but not OAS1 or OAS2, is required to activate RNase L and to restrict the replication of four different human viruses. These findings suggest that OAS3 may provide a target for antiviral therapies and that OAS1 and OAS2 may have alternative roles. The 2′,5′-oligoadenylate (2-5A) synthetase (OAS)–RNase L system is an IFN-induced antiviral pathway. RNase L activity depends on 2-5A, synthesized by OAS. Although all three enzymatically active OAS proteins in humans—OAS1, OAS2, and OAS3—synthesize 2-5A upon binding dsRNA, it is unclear which are responsible for RNase L activation during viral infection. We used clustered regularly interspaced short palindromic repeats (CRISPR)–CRISPR-associated protein-9 nuclease (Cas9) technology to engineer human A549-derived cell lines in which each of the OAS genes or RNase L is knocked out. Upon transfection with poly(rI):poly(rC), a synthetic surrogate for viral dsRNA, or infection with each of four viruses from different groups (West Nile virus, Sindbis virus, influenza virus, or vaccinia virus), OAS1-KO and OAS2-KO cells synthesized amounts of 2-5A similar to those synthesized in parental wild-type cells, causing RNase L activation as assessed by rRNA degradation. In contrast, OAS3-KO cells synthesized minimal 2-5A, and rRNA remained intact, similar to infected RNase L-KO cells. All four viruses replicated to higher titers in OAS3-KO or RNase L-KO A549 cells than in parental, OAS1-KO, or OAS2-KO cells, demonstrating the antiviral effects of OAS3. OAS3 displayed a higher affinity for dsRNA in intact cells than either OAS1 or OAS2, consistent with its dominant role in RNase L activation. Finally, the requirement for OAS3 as the major OAS isoform responsible for RNase L activation was not restricted to A549 cells, because OAS3-KO cells derived from two other human cell lines also were deficient in RNase L activation.

Laser Facial Nerve Welding in a Rabbit Model

OBJECTIVE To assess the feasibility of laser tissue welding for repair of facial nerve injury. METHODS In a prospective in vivo animal survival surgery model, rabbit facial nerve injury was followed by either standard suture neurorrhaphy or laser tissue welding using a diode laser (808 ± 1 nm) to weld biological solder. Rabbits were evaluated at 4, 8, 12, and 16 weeks by facial videography and electromyography. Histopathological analysis of the repair was performed at 4 and 16 weeks. RESULTS Videographic analysis demonstrated the laser tissue welding repair trended toward superior outcomes compared with suture neurorrhaphy at all 4 time points. Electrophysiological analysis demonstrated similar or better results, with statistically significant improvement at week 16 (P < .05). Histologic analysis demonstrated no difference in axon organization or extravasation between groups; however, the laser nerve repair created a greater initial inflammatory reaction. An analysis of operative time demonstrated significantly decreased time and ease of use for laser tissue welding. CONCLUSIONS This pilot study demonstrates that laser nerve welding may be an expedient, feasible, and safe method for facial nerve repair in a rabbit model. Further experiments with larger numbers are needed to provide additional evidence that laser tissue welding produces a neurorrhaphy that has functional, electrophysiological, and histological results that could rival traditional suture neurorrhaphy.

Ribonuclease L mediates the cell-lethal phenotype of double-stranded RNA editing enzyme ADAR1 deficiency in a human cell line

ADAR1 isoforms are adenosine deaminases that edit and destabilize double-stranded RNA reducing its immunostimulatory activities. Mutation of ADAR1 leads to a severe neurodevelopmental and inflammatory disease of children, Aicardi-Goutiéres syndrome. In mice, Adar1 mutations are embryonic lethal but are rescued by mutation of the Mda5 or Mavs genes, which function in IFN induction. However, the specific IFN regulated proteins responsible for the pathogenic effects of ADAR1 mutation are unknown. We show that the cell-lethal phenotype of ADAR1 deletion in human lung adenocarcinoma A549 cells is rescued by CRISPR/Cas9 mutagenesis of the RNASEL gene or by expression of the RNase L antagonist, murine coronavirus NS2 accessory protein. Our result demonstrate that ablation of RNase L activity promotes survival of ADAR1 deficient cells even in the presence of MDA5 and MAVS, suggesting that the RNase L system is the primary sensor pathway for endogenous dsRNA that leads to cell death. DOI: http://dx.doi.org/10.7554/eLife.25687.001

Origins and pathogenesis of Middle East respiratory syndrome-associated coronavirus: recent advances

Middle East respiratory syndrome-associated coronavirus (MERS-CoV) has been a significant research focus since its discovery in 2012. Since 2012, 2,040 cases and 712 deaths have been recorded (as of August 11, 2017), representing a strikingly high case fatality rate of 36%. Over the last several years, MERS-CoV research has progressed in several parallel and complementary directions. This review will focus on three particular areas: the origins and evolution of MERS-CoV, the challenges and achievements in the development of MERS-CoV animal models, and our understanding of how novel proteins unique to MERS-CoV counter the host immune response. The origins of MERS-CoV, likely in African bats, are increasingly clear, although important questions remain about the establishment of dromedary camels as a reservoir seeding human outbreaks. Likewise, there have been important advances in the development of animal models, and both non-human primate and mouse models that seem to recapitulate human disease are now available. How MERS-CoV evades and inhibits the host innate immune response remains less clear. Although several studies have identified MERS-CoV proteins as innate immune antagonists, little of this work has been conducted using live virus under conditions of actual infection, but rather with ectopically expressed proteins. Accordingly, considerable space remains for major contributions to understanding unique ways in which MERS-CoV interacts with and modulates the host response. Collectively, these areas have seen significant advances over the last several years but continue to offer exciting opportunities for discovery.

Lineage A betacoronavirus NS2 proteins and the homologous torovirus Berne pp1a carboxy-terminal domain are phosphodiesterases that antagonize activation of RNase L

ABSTRACT Viruses in the family Coronaviridae, within the order Nidovirales, are etiologic agents of a range of human and animal diseases, including both mild and severe respiratory diseases in humans. These viruses encode conserved replicase and structural proteins as well as more diverse accessory proteins, encoded in the 3′ ends of their genomes, that often act as host cell antagonists. We previously showed that 2′,5′-phosphodiesterases (2′,5′-PDEs) encoded by the prototypical Betacoronavirus, mouse hepatitis virus (MHV), and by Middle East respiratory syndrome-associated coronavirus antagonize the oligoadenylate-RNase L (OAS-RNase L) pathway. Here we report that additional coronavirus superfamily members, including lineage A betacoronaviruses and toroviruses infecting both humans and animals, encode 2′,5′-PDEs capable of antagonizing RNase L. We used a chimeric MHV system (MHVMut) in which exogenous PDEs were expressed from an MHV backbone lacking the gene for a functional NS2 protein, the endogenous RNase L antagonist. With this system, we found that 2′,5′-PDEs encoded by the human coronavirus HCoV-OC43 (OC43; an agent of the common cold), human enteric coronavirus (HECoV), equine coronavirus (ECoV), and equine torovirus Berne (BEV) are enzymatically active, rescue replication of MHVMut in bone marrow-derived macrophages, and inhibit RNase L-mediated rRNA degradation in these cells. Additionally, PDEs encoded by OC43 and BEV rescue MHVMut replication and restore pathogenesis in wild-type (WT) B6 mice. This finding expands the range of viruses known to encode antagonists of the potent OAS-RNase L antiviral pathway, highlighting its importance in a range of species as well as the selective pressures exerted on viruses to antagonize it. IMPORTANCE Viruses in the family Coronaviridae include important human and animal pathogens, including the recently emerged viruses severe acute respiratory syndrome-associated coronavirus (SARS-CoV) and Middle East respiratory syndrome-associated coronavirus (MERS-CoV). We showed previously that two viruses within the genus Betacoronavirus, mouse hepatitis virus (MHV) and MERS-CoV, encode 2′,5′-phosphodiesterases (2′,5′-PDEs) that antagonize the OAS-RNase L pathway, and we report here that these proteins are furthermore conserved among additional coronavirus superfamily members, including lineage A betacoronaviruses and toroviruses, suggesting that they may play critical roles in pathogenesis. As there are no licensed vaccines or effective antivirals against human coronaviruses and few against those infecting animals, identifying viral proteins contributing to virulence can inform therapeutic development. Thus, this work demonstrates that a potent antagonist of host antiviral defenses is encoded by multiple and diverse viruses within the family Coronaviridae, presenting a possible broad-spectrum therapeutic target.

ID: 203: Middle East respiratory syndrome coronavirus accessory protein NS4b is a 2H-Phosphoesterase that degrades 2′,5′-oligoadenylate activators of RNase L

Efficient and productive virus infection often requires viral countermeasures that block innate immunity. The interferon-inducible 2′,5′-oligoadenylate (2-5A) synthetases (OAS) and ribonuclease L (RNase L) are components of a potent host antiviral pathway. We previously showed that murine coronavirus (MHV) accessory protein ns2, group A rotavirus (RVA) VP3 carboxy-terminal domain (VP3-CTD), and mammalian AKAP7 are members of 2H phosphoesterase superfamily with 2′-phosphodiesterase (2′-PDE) activity that potently cleaves 2-5A thereby preventing activation of RNase L (Zhao, Jha et al., PMID: 22704621; Zhang, Jha et al., PMID: 23878220 and Gusho, Zhang, Jha et al., PMID: 24987090). Here, we will demonstrate that Middle East respiratory syndrome (MERS)-CoV gene NS4b encodes a homologous and similar PDE that cleaves 2-5A in vitro (km/Kcat=12.1M−1 s−1), inhibits 2-5A accumulation in cell culture and prevents ribosomal (r) RNA degradation in murine bone marrow macrophages (BMM), a hallmark of RNase L antagonism, and rescues an MHV mutant virus with a catalytically inactive NS2a protein unable to antagonize RNase L in vivo. Interestingly, NS4b has a nuclear localization signal however there is a mixed nuclear/cytoplasmic localization when overexpressed in human airway cell line A549 and in BMM when expressed from chimeric MHV. Viral evasion of OAS/RNase L pathway is a critical hepatovirulence determinant for lineage A Betacoronavirus mouse hepatitis virus (MHV). Taken together, our data suggest that RNase L antagonism may be a critical component of MERS-CoV pathogenesis. Additionally, this is the first evidence of RNase L antagonism by a lineage C Betacoronavirus.