Structural Basis for Rabbit Hemorrhagic Disease Virus Antibody Specificity.

Rabbit hemorrhagic disease virus (RHDV) typically causes a fatal disease in rabbits. In Australia, RHDV was imported to control the feral rabbit population, while it poses a severe threat to native rabbits in other countries. RHDV variants are genetically diverse and serological studies using antibodies isolated from infected rabbits or raised against RHDV virus-like particles (VLPs) have found RHDV variants antigenically distinct. In this study, we determined the X-ray crystal structure of an RHDV GI.2 (N11 strain) protruding (P) domain in complex with a diagnostic monoclonal antibody (2D9) Fab. We showed that 2D9 interacted with conserved and variable residues on top of the P domain with nanomolar affinity. To better illustrate 2D9 specificity, we determined the X-ray crystal structure of an RHDV GI.1b (Ast89 strain) that was a 2D9 non-binder. Structural analysis indicated that amino acid substitutions on the GI.1b P domain likely restricted 2D9 binding. Interestingly, a model of the GI.2 P domain-Fab complex superimposed onto a cryo-EM structure of an RHDV VLP revealed that 2D9 Fab molecules clashed with neighboring Fabs and indicated that there was a reduced antibody binding occupancy. Moreover, the RHDV GI.2 histo-blood group antigen (HBGA) co-factor binding site appeared obstructed when 2D9 was modeled on the VLP and suggested that 2D9 might also function by blocking HBGA attachment. Overall, this new data provides the first structural basis of RHDV antibody specificity and explains how amino acid variation at the binding site likely restricts 2D9 cross-reactivity. IMPORTANCE Isolated RHDV antibodies have been used for decades to distinguish between antigenic variants, monitor temporal capsid evolution, and examine neutralizing capacities. In this study, we provided the structural basis for an RHDV GI.2 specific diagnostic antibody (2D9) binding and reveal that a small number of amino acid substitutions at the binding site could differentiate between RHDV GI.2 and GI.1b. This novel structural information provides a framework for understanding how RHDV displays a specific antigenic epitope and engages an antibody at the atomic level. Importantly, part of the 2D9 binding region was earlier reported to contain a neutralizing epitope and our structural modeling as well as recent human norovirus antibody-mediated neutralization studies, suggest that the 2D9 antibody has the potential to block HBGA attachment. These new findings should aid in characterizing antigenic variants and advance the development of novel monoclonal antibodies for diagnostics and therapeutics.

Dual role of neddylation in transcription of hepatitis B virus RNAs from cccDNA and production of viral surface antigen.

Background & Aims: HBV persistence is maintained by both an episomal covalently closed circular (ccc)DNA reservoir and genomic integration of HBV DNA fragments. While cccDNA transcription is regulated by Cullin4A-DDB1-HBx-mediated degradation of the SMC5/6 complex, HBsAg expression from integrants is largely SMC5/6 independent. Inhibiting neddylation of Cullin-RING ubiquitin ligases impairs degradation of substrates. Herein, we show that targeting neddylation pathway components by small-interfering (si)RNAs or the drug MLN4924 (pevonedistat) suppresses expression of HBV proteins from both cccDNA and integrants. Methods: An siRNA screen targeting secretory pathway regulators and neddylation genes was performed. Activity of MLN4924 was assessed in infection and integration models. Trans-complementation assays were used to study HBx function in cccDNA-driven expression. Results: siRNA screening uncovered neddylation pathway components (Nedd8, Ube2m) that promote HBsAg production post-transcriptionally. Likewise, MLN4924 inhibited production of HBsAg encoded by integrants and reduced intracellular HBsAg levels, independent of HBx. MLN4924 also profoundly inhibited cccDNA transcription in three infection models. Using the HBV inducible cell line HepAD38 as a model, we verified the dual action of MLN4924 on both cccDNA and integrants with sustained suppression of HBV markers during 42 days of treatment. Conclusions: Neddylation is required both for transcription of a cccDNA reservoir and for the genomic integration of viral DNA. Therefore, blocking neddylation might offer an attractive approach towards functional cure of chronic hepatitis B. Lay summary: Current treatments for chronic hepatitis B are rarely able to induce a functional cure. This is partly because of the presence of a pool of circular viral DNA in the host nucleus, as well as viral DNA fragments that are integrated into the host genome. Herein, we show that a host biological pathway called neddylation could play a key role in infection and viral DNA integration. Inhibiting this pathway could hold therapeutic promise for patients with chronic hepatitis B.

Discovery of potent benzoxaborole inhibitors against SARS-CoV-2 main and dengue virus proteases.

The RNA viruses SARS-CoV-2 and dengue pose a major threat to human health worldwide and their proteases (Mpro; NS2B/NS3) are considered as promising targets for drug development. We present the synthesis and biological evaluation of novel benzoxaborole inhibitors of these two proteases. The most active compound achieves single-digit micromolar activity against SARS-CoV-2 Mpro in a biochemical assay. The most active substance against dengue NS2B/NS3 protease has submicromolar activity in cells (EC50 0.54 µM) and inhibits DENV-2 replication in cell culture. Most benzoxaboroles had no relevant cytotoxicity or significant off-target inhibition. Furthermore, the class demonstrated passive membrane penetration and stability against the evaluated proteases. This compound class may contribute to the development of antiviral agents with activity against DENV or SARS-CoV-2.

Efficiency Improvements and Discovery of New Substrates for a SARS-CoV-2 Main Protease FRET Assay.

The COVID-19 pandemic, caused by the SARS-CoV-2 virus, has a huge impact on the world. Although several vaccines have recently reached the market, the development of specific antiviral drugs against SARS-CoV-2 is an important additional strategy in fighting the pandemic. One of the most promising pharmacological targets is the viral main protease (Mpro). Here, we present an optimized biochemical assay procedure for SARS-CoV-2 Mpro. We have comprehensively investigated the influence of different buffer components and conditions on the assay performance and characterized Förster resonance energy transfer (FRET) substrates with a preference for 2-Abz/Tyr(3-NO2) FRET pairs. The substrates 2-AbzSAVLQSGTyr(3-NO2)R-OH, a truncated version of the established DABCYL/EDANS FRET substrate, and 2-AbzVVTLQSGTyr(3-NO2)R-OH are promising candidates for screening and inhibitor characterization. In the latter substrate, the incorporation of Val at position P5 improved the catalytic efficiency. Based on the obtained results, we present here a reproducible, reliable assay protocol using highly affordable buffer components.

Beyond Basicity: Discovery of Nonbasic DENV-2 Protease Inhibitors with Potent Activity in Cell Culture.

The viral serine protease NS2B-NS3 is one of the promising targets for drug discovery against dengue virus and other flaviviruses. The molecular recognition preferences of the protease favor basic, positively charged moieties as substrates and inhibitors, which leads to pharmacokinetic liabilities and off-target interactions with host proteases such as thrombin. We here present the results of efforts that were aimed specifically at the discovery and development of noncharged, small-molecular inhibitors of the flaviviral proteases. A key factor in the discovery of these compounds was a cellular reporter gene assay for the dengue protease, the DENV2proHeLa system. Extensive structure-activity relationship explorations resulted in novel benzamide derivatives with submicromolar activities in viral replication assays (EC50 0.24 µM), selectivity against off-target proteases, and negligible cytotoxicity. This structural class has increased drug-likeness compared to most of the previously published active-site-directed flaviviral protease inhibitors and includes promising candidates for further preclinical development.

Peptide-ß-lactam Inhibitors of Dengue and West Nile Virus NS2B-NS3 Protease Display Two Distinct Binding Modes.

The ß-lactam ring represents a valuable moiety that can induce covalent binding of an inhibitor to its target. In this study, we explored di- and tripeptides with ß-lactam electrophilic warheads as inhibitors of dengue and West Nile virus NS2B-NS3 protease. Tripeptides with a (3S)-ß-lactam moiety displayed the highest activity, with IC50 and EC50 values in the lower micromolar range in biochemical and cellular assays. The activity against dengue protease was in general higher than against West Nile virus protease. The compounds were inactive against the off-targets thrombin and trypsin. Liquid chromatography-mass spectrometry experiments revealed that tripeptide-ß-lactam inhibitors bind to the protease in two distinct binding modes. Only one binding mode leads to a covalent, but reversible, interaction of the ß-lactam ring with the catalytic serine, followed by release of the inhibitor with opened ß-lactam ring. The other binding mode leads to the cleavage of the peptide backbone. This observation provides the first experimental evidence that benzyloxyphenylglycine in flaviviral protease inhibitors is positioned in the prime site of the enzyme.

Prodrug Activation by a Viral Protease: Evaluating Combretastatin Peptide Hybrids To Selectively Target Infected Cells.

Infections with flaviviruses such as dengue virus (DENV) are prevalent throughout tropical regions worldwide. Replication of these viruses depends on tubulin, a host cell factor that can be targeted to obtain broad-spectrum antiviral agents. Targeting of tubulin does, however, require specific measures to avoid toxic side-effects. Herein, we report the synthesis and biological evaluation of combretastatin peptide hybrids that incorporate the cleavage site of the DENV protease to allow activation of the tubulin ligand within infected cells. The prodrug candidates have no effect on tubulin polymerization in vitro and are 20-2000-fold less toxic than combretastatin A-4. Several of the prodrug candidates were cleaved by the DENV protease in vitro with similar efficiency as the natural viral substrates. Selected compounds were studied in DENV and Zika virus replication assays and had antiviral activity at subcytotoxic concentrations.

De Novo Discovery of Nonstandard Macrocyclic Peptides as Noncompetitive Inhibitors of the Zika Virus NS2B-NS3 Protease.

The Zika virus presents a major public health concern due to severe fetal neurological disorders associated with infections in pregnant women. In addition to vaccine development, the discovery of selective antiviral drugs is essential to combat future epidemic Zika virus outbreaks. The Zika virus NS2B-NS3 protease, which performs replication-critical cleavages of the viral polyprotein, is a promising drug target. We report the first macrocyclic peptide-based inhibitors of the NS2B-NS3 protease, discovered de novo through in vitro display screening of a genetically reprogrammed library including noncanonical residues. Six compounds were selected, resynthesized, and isolated, all of which displayed affinities in the low nanomolar concentration range. Five compounds showed significant protease inhibition. Two of these were validated as hits with submicromolar inhibition constants and selectivity toward Zika over the related proteases from dengue and West Nile viruses. The compounds were characterized as noncompetitive inhibitors, suggesting allosteric inhibition.

Structural Constraints on Human Norovirus Binding to Histo-Blood Group Antigens.

Human norovirus interacts with the polymorphic human histo-blood group antigens (HBGAs), and this interaction is thought to be important for infection. The genogroup II genotype 4 (GII.4) noroviruses are the dominant cluster, evolve every other year, and are thought to modify their binding interactions with different HBGA types. Most human noroviruses bind HBGAs, while some strains were found to have minimal or no HBGA interactions. Here, we explain some possible structural constraints for several noroviruses that were found to bind poorly to HBGAs by using X-ray crystallography. We showed that one aspartic acid was flexible or positioned away from the fucose moiety of the HBGAs and this likely hindered binding, although other fucose-interacting residues were perfectly oriented. Interestingly, a neighboring loop also appeared to influence the loop hosting the aspartic acid. These new findings might explain why some human noroviruses bound HBGAs poorly, although further studies are required.

Production of Human Norovirus Protruding Domains in E. coli for X-ray Crystallography.

The norovirus capsid is composed of a single major structural protein, termed VP1. VP1 is subdivided into a shell (S) domain and a protruding (P) domain. The S domain forms a contiguous scaffold around the viral RNA, whereas the P domain forms viral spikes on the S domain and contains determinants for antigenicity and host-cell interactions. The P domain binds carbohydrate structures, i.e., histo-blood group antigens, which are thought to be important for norovirus infections. In this protocol, we describe a method for producing high quality norovirus P domains in high yields. These proteins can then be used for X-ray crystallography and ELISA in order to study antigenicity and host-cell interactions. The P domain is firstly cloned into an expression vector and then expressed in bacteria. The protein is purified using three steps that involve immobilized metal-ion affinity chromatography and size exclusion chromatography. In principle, it is possible to clone, express, purify, and crystallize proteins in less than four weeks, which makes this protocol a rapid system for analyzing newly emerging norovirus strains.

The sweet quartet: Binding of fucose to the norovirus capsid.

Human noroviruses bind histo-blood group antigens (HBGAs) and this interaction is thought to be important for an infection. We identified two additional fucose-binding pockets (termed fucose-3/4 sites) on a genogroup II human (GII.10) norovirus-protruding (P) dimer using X-ray crystallography. Fucose-3/4 sites were located between two previously determined HBGA binding pockets (termed fucose-1/2 sites). We found that four fucose molecules were capable of binding altogether at fucose-1/2/3/4 sites on the P dimer, though the fucose molecules bound in a dose-dependent and step-wise manner. We also showed that HBGA B-trisaccharide molecules bound in a similar way at the fucose-1/2 sites. Interestingly, we discovered that the monomers of the P dimer were asymmetrical in an unliganded state and when a single B-trisaccharide molecule bound, but were symmetrical when two B-trisaccharide molecules bound. We postulate that the symmetrical dimers might favor HBGA binding interactions at fucose-1/2 sites.

Structural analysis of a feline norovirus protruding domain.

Norovirus infects different animals, including humans, mice, dogs, and cats. Here, we show an X-ray crystal structure of a feline GIV.2 norovirus capsid-protruding (P) domain to 2.35Å resolution. The feline GIV.2 P domain was reminiscent of human norovirus P domains, except for a novel P2 subdomain α-helix and an extended P1 subdomain interface loop. These new structural features likely obstructed histo-blood group antigens, which are attachment factors for human norovirus, from binding at the equivalent sites on the feline GIV.2 P domain. Additionally, an ELISA showed that the feline GIV.2 was antigenically distinct from a human GII.10 norovirus.

Human noroviruses' fondness for histo-blood group antigens.

UNLABELLED: Human noroviruses are the dominant cause of outbreaks of gastroenteritis around the world. Human noroviruses interact with the polymorphic human histo-blood group antigens (HBGAs), and this interaction is thought to be important for infection. Indeed, synthetic HBGAs or HBGA-expressing enteric bacteria were shown to enhance norovirus infection in B cells. A number of studies have found a possible relationship between HBGA type and norovirus susceptibility. The genogroup II, genotype 4 (GII.4) noroviruses are the dominant cluster, evolve every other year, and are thought to modify their binding interactions with different HBGA types. Here we show high-resolution X-ray crystal structures of the capsid protruding (P) domains from epidemic GII.4 variants from 2004, 2006, and 2012, cocrystallized with a panel of HBGA types (H type 2, Lewis Y, Lewis B, Lewis A, Lewis X, A type, and B type). Many of the HBGA binding interactions were found to be complex, involving capsid loop movements, alternative HBGA conformations, and HBGA rotations. We showed that a loop (residues 391 to 395) was elegantly repositioned to allow for Lewis Y binding. This loop was also slightly shifted to provide direct hydrogen- and water-mediated bonds with Lewis B. We considered that the flexible loop modulated Lewis HBGA binding. The GII.4 noroviruses have dominated outbreaks over the past decade, which may be explained by their exquisite HBGA binding mechanisms, their fondness for Lewis HBGAs, and their temporal amino acid modifications. IMPORTANCE: Our data provide a comprehensive picture of GII.4 P domain and HBGA binding interactions. The exceptionally high resolutions of our X-ray crystal structures allowed us to accurately recognize novel GII.4 P domain interactions with numerous HBGA types. We showed that the GII.4 P domain-HBGA interactions involved complex binding mechanisms that were not previously observed in norovirus structural studies. Many of the GII.4 P domain-HBGA interactions we identified were negative in earlier enzyme-linked immunosorbent assay (ELISA)-based studies. Altogether, our data show that the GII.4 norovirus P domains can accommodate numerous HBGA types.

Structural analysis of a rabbit hemorrhagic disease virus binding to histo-blood group antigens.

UNLABELLED: Rabbit hemorrhagic disease virus (RHDV) is a member of the Caliciviridae family (Lagovirus genus). RHDV is highly contagious and attaches to epithelial cells in the digestive or respiratory tract, leading to massive lesions with high mortality rates. A new variant of RHDV (termed RHDVb) recently has emerged, and previously vaccinated rabbits appear to have little protection against this new strain. Similar to human norovirus (Caliciviridae, Norovirus genus), RHDV binds histo-blood group antigens (HBGAs), and this is thought to be important for infection. Here, we report the HBGA binding site on the RHDVb capsid-protruding domain (P domain) using X-ray crystallography. The HBGA binding pocket was located in a negatively charged patch on the side of the P domain and at a dimeric interface. Residues from both monomers contributed to the HBGA binding and involved a network of direct hydrogen bonds and water-mediated interactions. An amino acid sequence alignment of different RHDV strains indicated that the residues directly interacting with the ABH-fucose of the HBGAs (Asp472, Asn474, and Ser479) were highly conserved. This result suggested that different RHDV strains also could bind HBGAs at the equivalent pocket. Moreover, several HBGA binding characteristics between RHDVb and human genogroup II norovirus were similar, which indicated a possible convergent evolution of HBGA binding interactions. Further structural studies with other RHDV strains are needed in order to better understand the HBGA binding mechanisms among the diverse RHDV strains. IMPORTANCE: We identified, for the first time, the HBGA binding site on an RHDVb P domain using X-ray crystallography. Our results showed that RHDVb and human genogroup II noroviruses had similar HBGA binding interactions. Recently, it was discovered that synthetic HBGAs or HBGA-expressing enteric bacteria could enhance human genogroup II norovirus infection in B cells. Considering that RHDVb and genogroup II norovirus similarly interacted with HBGAs, it may be possible that a comparable cell culture system also could work with RHDVb. Taken together, these new findings will extend our understanding of calicivirus HBGA interactions and may help to elucidate the specific roles of HBGAs in calicivirus infections.