ADDovenom: Thermostable Protein-Based ADDomer Nanoparticles as New Therapeutics for Snakebite Envenoming.

Snakebite envenoming can be a life-threatening medical emergency that requires prompt medical intervention to neutralise the effects of venom toxins. Each year up to 138,000 people die from snakebites and threefold more victims suffer life-altering disabilities. The current treatment of snakebite relies solely on antivenom-polyclonal antibodies isolated from the plasma of hyperimmunised animals-which is associated with numerous deficiencies. The ADDovenom project seeks to deliver a novel snakebite therapy, through the use of an innovative protein-based scaffold as a next-generation antivenom. The ADDomer is a megadalton-sized, thermostable synthetic nanoparticle derived from the adenovirus penton base protein; it has 60 high-avidity binding sites to neutralise venom toxins. Here, we outline our experimental strategies to achieve this goal using state-of-the-art protein engineering, expression technology and mass spectrometry, as well as in vitro and in vivo venom neutralisation assays. We anticipate that the approaches described here will produce antivenom with unparalleled efficacy, safety and affordability.

2C protein of Enterovirus: key protein of viral replication and antiviral target.

Enteroviruses (EVs) include many human pathogens of increasing public health concern. These EVs are often associated with mild clinical manifestations, but they can lead to serious complications such as encephalitis, meningitis, pneumonia, myocarditis or poliomyelitis. Despite significant advances, there is no approved antiviral therapy for the treatment of enterovirus infections. Due to the high genotypic diversity of EVs, molecules targeting highly conserved viral proteins may be considered for developing a pan-EV treatment. In this regard, the ATPase/Helicase 2C, which is a highly conserved non-structural protein among EVs, has essential functions for viral replication and is therefore an attractive antiviral target. Recent functional and structural studies on the 2C protein led to the identification of molecules showing ex vivo anti-EV activity and associated with resistance mutations on the coding sequence of the 2C protein. This review presents the current state of knowledge about the 2C protein from an antiviral target perspective and the mode of action of specific inhibitors for this therapeutic target.

Interplay of RNA 2'-O-methylations with viral replication.

Viral RNAs (vRNAs) are decorated by post-transcriptional modifications, including methylation of nucleotides. Methylations regulate biological functions linked to the sequence, structure, and protein interactome of RNA. Several RNA viruses were found to harbor 2'-O-methylations, affecting the ribose moiety of RNA. This mark was initially shown to target the first and second nucleotides of the 5'-end cap structure of mRNA. More recently, nucleotides within vRNA were also reported to carry 2'-O-methylations. The consequences of such methylations are still puzzling since they were associated with both proviral and antiviral effects. Here, we focus on the mechanisms governing vRNA 2'-O-methylation and we explore the possible roles of this epitranscriptomic modification for viral replication.

Internal RNA 2'O-methylation in the HIV-1 genome counteracts ISG20 nuclease-mediated antiviral effect.

RNA 2'O-methylation is a 'self' epitranscriptomic modification allowing discrimination between host and pathogen. Indeed, human immunodeficiency virus 1 (HIV-1) induces 2'O-methylation of its genome by recruiting the cellular FTSJ3 methyltransferase, thereby impairing detection by RIG-like receptors. Here, we show that RNA 2'O-methylations interfere with the antiviral activity of interferon-stimulated gene 20-kDa protein (ISG20). Biochemical experiments showed that ISG20-mediated degradation of 2'O-methylated RNA pauses two nucleotides upstream of and at the methylated residue. Structure-function analysis indicated that this inhibition is due to steric clash between ISG20 R53 and D90 residues and the 2'O-methylated nucleotide. We confirmed that hypomethylated HIV-1 genomes produced in FTSJ3-KO cells were more prone to in vitro degradation by ISG20 than those produced in cells expressing FTSJ3. Finally, we found that reverse-transcription of hypomethylated HIV-1 was impaired in T cells by interferon-induced ISG20, demonstrating the direct antagonist effect of 2'O-methylation on ISG20-mediated antiviral activity.

Despite highly effective antiretroviral therapies, the human immunodeficiency virus (HIV-1) remains a major public health threat. Its pathogenesis depends on its ability to establish a persistent infection in cells of the immune system. Our study highlights a new insight into how HIV-1 evades early restriction by the immune system. We showed that 2'O-methylation marks found inside HIV-1 RNA promote viral evasion from the antiviral action of the interferon-stimulated gene 20-kDa protein (ISG20), an innate immune restriction factor with a nuclease activity. By disrupting the level of 2'O-methylation of the HIV-1 genome, we demonstrated that ISG20 impairs the reverse transcription process of hypomethylated viruses, as a result of viral RNA decay.

Identification of potent inhibitors of arenavirus and SARS-CoV-2 exoribonucleases by fluorescence polarization assay.

Viral exoribonucleases are uncommon in the world of RNA viruses. To date, they have only been identified in the Arenaviridae and the Coronaviridae families. The exoribonucleases of these viruses play a crucial role in the pathogenicity and interplay with host innate immune response. Moreover, coronaviruses exoribonuclease is also involved in a proofreading mechanism ensuring the genetic stability of the viral genome. Because of their key roles in virus life cycle, they constitute attractive target for drug design. Here we developed a sensitive, robust and reliable fluorescence polarization assay to measure the exoribonuclease activity and its inhibition in vitro. The effectiveness of the method was validated on three different viral exoribonucleases, including SARS-CoV-2, Lymphocytic Choriomeningitis and Machupo viruses. We performed a screening of a focused library consisting of 113 metal chelators. Hit compounds were recovered with an IC50 at micromolar level. We confirmed 3 hits in SARS-CoV-2 infected Vero-E6 cells.

A dual mechanism of action of AT-527 against SARS-CoV-2 polymerase.

The guanosine analog AT-527 represents a promising candidate against Severe Acute Respiratory Syndrome coronavirus type 2 (SARS-CoV-2). AT-527 recently entered phase III clinical trials for the treatment of COVID-19. Once in cells, AT-527 is converted into its triphosphate form, AT-9010, that presumably targets the viral RNA-dependent RNA polymerase (RdRp, nsp12), for incorporation into viral RNA. Here we report a 2.98 Å cryo-EM structure of the SARS-CoV-2 nsp12-nsp7-nsp82-RNA complex, showing AT-9010 bound at three sites of nsp12. In the RdRp active-site, one AT-9010 is incorporated at the 3' end of the RNA product strand. Its modified ribose group (2'-fluoro, 2'-methyl) prevents correct alignment of the incoming NTP, in this case a second AT-9010, causing immediate termination of RNA synthesis. The third AT-9010 is bound to the N-terminal domain of nsp12 - known as the NiRAN. In contrast to native NTPs, AT-9010 is in a flipped orientation in the active-site, with its guanine base unexpectedly occupying a previously unnoticed cavity. AT-9010 outcompetes all native nucleotides for NiRAN binding, inhibiting its nucleotidyltransferase activity. The dual mechanism of action of AT-527 at both RdRp and NiRAN active sites represents a promising research avenue against COVID-19.

Fluoxetine targets an allosteric site in the enterovirus 2C AAA+ ATPase and stabilizes a ring-shaped hexameric complex.

Enteroviruses are globally prevalent human pathogens responsible for many diseases. The nonstructural protein 2C is a AAA+ helicase and plays a key role in enterovirus replication. Drug repurposing screens identified 2C-targeting compounds such as fluoxetine and dibucaine, but how they inhibit 2C is unknown. Here, we present a crystal structure of the soluble and monomeric fragment of coxsackievirus B3 2C protein in complex with (S)-fluoxetine (SFX), revealing an allosteric binding site. To study the functional consequences of SFX binding, we engineered an adenosine triphosphatase (ATPase)­competent, hexameric 2C protein. Using this system, we show that SFX, dibucaine, HBB [2-(α-hydroxybenzyl)-benzimidazole], and guanidine hydrochloride inhibit 2C ATPase activity. Moreover, cryo­electron microscopy analysis demonstrated that SFX and dibucaine lock 2C in a defined hexameric state, rationalizing their mode of inhibition. Collectively, these results provide important insights into 2C inhibition and a robust engineering strategy for structural, functional, and drug-screening analysis of 2C proteins.

Structure-function analysis of the nsp14 N7-guanine methyltransferase reveals an essential role in Betacoronavirus replication.

As coronaviruses (CoVs) replicate in the host cell cytoplasm, they rely on their own capping machinery to ensure the efficient translation of their messenger RNAs (mRNAs), protect them from degradation by cellular 5' exoribonucleases (ExoNs), and escape innate immune sensing. The CoV nonstructural protein 14 (nsp14) is a bifunctional replicase subunit harboring an N-terminal 3'-to-5' ExoN domain and a C-terminal (N7-guanine)-methyltransferase (N7-MTase) domain that is presumably involved in viral mRNA capping. Here, we aimed to integrate structural, biochemical, and virological data to assess the importance of conserved N7-MTase residues for nsp14's enzymatic activities and virus viability. We revisited the crystal structure of severe acute respiratory syndrome (SARS)-CoV nsp14 to perform an in silico comparative analysis between betacoronaviruses. We identified several residues likely involved in the formation of the N7-MTase catalytic pocket, which presents a fold distinct from the Rossmann fold observed in most known MTases. Next, for SARS-CoV and Middle East respiratory syndrome CoV, site-directed mutagenesis of selected residues was used to assess their importance for in vitro enzymatic activity. Most of the engineered mutations abolished N7-MTase activity, while not affecting nsp14-ExoN activity. Upon reverse engineering of these mutations into different betacoronavirus genomes, we identified two substitutions (R310A and F426A in SARS-CoV nsp14) abrogating virus viability and one mutation (H424A) yielding a crippled phenotype across all viruses tested. Our results identify the N7-MTase as a critical enzyme for betacoronavirus replication and define key residues of its catalytic pocket that can be targeted to design inhibitors with a potential pan-coronaviral activity spectrum.

Fluoxetine Inhibits Enterovirus Replication by Targeting the Viral 2C Protein in a Stereospecific Manner.

Enteroviruses (family Picornaviridae) comprise a large group of human pathogens against which no licensed antiviral therapy exists. Drug-repurposing screens uncovered the FDA-approved drug fluoxetine as a replication inhibitor of enterovirus B and D species. Fluoxetine likely targets the nonstructural viral protein 2C, but detailed mode-of-action studies are missing because structural information on 2C of fluoxetine-sensitive enteroviruses is lacking. We here show that broad-spectrum anti-enteroviral activity of fluoxetine is stereospecific concomitant with binding to recombinant 2C. (S)-Fluoxetine inhibits with a 5-fold lower 50% effective concentration (EC50) than racemic fluoxetine. Using a homology model of 2C of the fluoxetine-sensitive enterovirus coxsackievirus B3 (CVB3) based upon a recently elucidated structure of a fluoxetine-insensitive enterovirus, we predicted stable binding of (S)-fluoxetine. Structure-guided mutations disrupted binding and rendered coxsackievirus B3 (CVB3) resistant to fluoxetine. The study provides new insights into the anti-enteroviral mode-of-action of fluoxetine. Importantly, using only (S)-fluoxetine would allow for lower dosing in patients, thereby likely reducing side effects.