Deubiquitinating activity of SARS-CoV-2 papain-like protease does not influence virus replication or innate immune responses in vivo

Attenuating Mutations in Usutu Virus: Towards Understanding Orthoflavivirus Virulence Determinants and Live Attenuated Vaccine Design

BACKGROUND/OBJECTIVES

Understanding virulence determinants can inform safer and more efficacious live attenuated vaccine design. However, applying this knowledge across related viruses does not always result in conserved phenotypes from similar mutants.

METHODS

Using Usutu virus (USUV), an emerging orthoflavivirus spreading through Europe, we assessed whether the attenuating effect of the mutations described for related orthoflaviviruses is conserved. Candidate attenuating mutations were selected based on previous studies in other orthoflaviviruses and incorporated into USUV.

RESULTS

Nine variants, with mutations in the USUV envelope, non-structural (NS) proteins NS1, NS2A, or NS4B were stable and selected for further characterisation. The variants with an attenuating phenotype in cell culture were then compared to the wild-type virus in an Ifnar-/- mouse model. Mutations of the envelope glycosylation sites and glycosaminoglycan binding sites, which were recognised as more-conserved mechanisms of orthoflavivirus attenuation, were attenuating in USUV as well. However, not all the mutations explored in the USUV non-structural proteins exhibited an attenuated phenotype. Instead, the attenuation was either less pronounced, or there was no change in phenotype relative to the wild-type virus at all.

CONCLUSIONS

In addition to improving our understanding of USUV virulence determinants, these results add to a growing body of literature highlighting the most promising mechanisms to target for the design of safe live attenuated vaccines against emerging orthoflaviviruses.

Human coronaviruses: activation and antagonism of innate immune responses

SUMMARYHuman coronaviruses cause a range of respiratory diseases, from the common cold (HCoV-229E, HCoV-NL63, HCoV-OC43, and SARS-CoV-2) to lethal pneumonia (SARS-CoV, SARS-CoV-2, and MERS-CoV). Coronavirus interactions with host innate immune antiviral responses are an important determinant of disease outcome. This review compares the host's innate response to different human coronaviruses. Host antiviral defenses discussed in this review include frontline defenses against respiratory viruses in the nasal epithelium, early sensing of viral infection by innate immune effectors, double-stranded RNA and stress-induced antiviral pathways, and viral antagonism of innate immune responses conferred by conserved coronavirus nonstructural proteins and genus-specific accessory proteins. The common cold coronaviruses HCoV-229E and -NL63 induce robust interferon signaling and related innate immune pathways, SARS-CoV and SARS-CoV-2 induce intermediate levels of activation, and MERS-CoV shuts down these pathways almost completely.

Perturbation of de novo lipogenesis hinders MERS-CoV assembly and release, but not the biogenesis of viral replication organelles

Coronaviruses hijack host cell metabolic pathways and resources to support their replication. They induce extensive host endomembrane remodeling to generate viral replication organelles and exploit host membranes for assembly and budding of their enveloped progeny virions. Because of the overall significance of host membranes, we sought to gain insight into the role of host factors involved in lipid metabolism in cells infected with Middle East respiratory syndrome coronavirus (MERS-CoV). We employed a single-cycle infection approach in combination with pharmacological inhibitors, biochemical assays, lipidomics, and light and electron microscopy. Pharmacological inhibition of acetyl-CoA carboxylase (ACC) and fatty acid synthase (FASN), key host factors in de novo fatty acid biosynthesis, led to pronounced inhibition of MERS-CoV particle release. Inhibition of ACC led to a profound metabolic switch in Huh7 cells, altering their lipidomic profile and inducing lipolysis. However, despite the extensive changes induced by the ACC inhibitor, the biogenesis of viral replication organelles remained unaffected. Instead, ACC inhibition appeared to affect the trafficking and post-translational modifications of the MERS-CoV envelope proteins. Electron microscopy revealed an accumulation of nucleocapsids in early budding stages, indicating that MERS-CoV assembly is adversely impacted by ACC inhibition. Notably, inhibition of palmitoylation resulted in similar effects, while supplementation of exogenous palmitic acid reversed the compound's inhibitory effects, possibly reflecting a crucial need for palmitoylation of the MERS-CoV spike and envelope proteins for their role in virus particle assembly.IMPORTANCEMiddle East respiratory syndrome coronavirus (MERS-CoV) is the etiological agent of a zoonotic respiratory disease of limited transmissibility between humans. However, MERS-CoV is still considered a high-priority pathogen and is closely monitored by WHO due to its high lethality rate of around 35% of laboratory-confirmed infections. Like other positive-strand RNA viruses, MERS-CoV relies on the host cell's endomembranes to support various stages of its replication cycle. However, in spite of this general reliance of MERS-CoV replication on host cell lipid metabolism, mechanistic insights are still very limited. In our study, we show that pharmacological inhibition of acetyl-CoA carboxylase (ACC), a key enzyme in the host cell's fatty acid biosynthesis pathway, significantly disrupts MERS-CoV particle assembly without exerting a negative effect on the biogenesis of viral replication organelles. Furthermore, our study highlights the potential of ACC as a target for the development of host-directed antiviral therapeutics against coronaviruses.

From Plasmid to Pure Protein: Production and Characterization of SARS-CoV-2 PLpro

Papain-like protease (PLpro) from SARS-CoV-2 is a high-priority target for COVID-19 antiviral drug development. We present protocols for PLpro production in Escherichia coli. PLpro expressed as a fusion with the Saccharomyces cerevisiae Smt3 protein (SUMO), is purified and obtained in its native form upon hydrolysis, with yields as high as 38 mg L-1. The protocol also provides isotope-enriched samples suitable for NMR studies. Protocols are also presented for PLpro characterization by mass spectrometry, 1D 19F-NMR and 2D heteronuclear NMR, and a fluorescence-based enzyme assay.

Design of quinoline SARS-CoV-2 papain-like protease inhibitors as oral antiviral drug candidates

The ever-evolving SARS-CoV-2 variants necessitate the development of additional oral antivirals. This study presents the systematic design of quinoline-containing SARS-CoV-2 papain-like protease (PLpro) inhibitors as potential oral antiviral drug candidates. By leveraging the recently discovered Val70Ub binding site in PLpro, we designed a series of quinoline analogs demonstrating potent PLpro inhibition and antiviral activity. Notably, the X-ray crystal structures of 6 lead compounds reveal that the 2-aryl substitution can occupy either the Val70Ub site as expected or the BL2 groove in a flipped orientation. The in vivo lead Jun13296 exhibits favorable pharmacokinetic properties and potent inhibition against SARS-CoV-2 variants and nirmatrelvir-resistant mutants. In a mouse model of SARS-CoV-2 infection, oral treatment with Jun13296 significantly improves survival, reduces body weight loss and lung viral titers, and prevents lung tissue damage. These results underscore the potential of quinoline PLpro inhibitors as promising oral SARS-CoV-2 antiviral candidates, instilling hope for the future of SARS-CoV-2 treatment.

Dose and strain dependent lethality of Usutu virus in an Ifnar-/- mouse model

Usutu virus (USUV) is a mosquito-borne zoonotic flavivirus with a geographic range that has expanded over recent years. Maintained in a transmission cycle between mosquito vectors and avian reservoirs the virus can cause large seasonal outbreaks in bird populations, but spillover into mammalian hosts has also been reported. While usually mild or asymptomatic in humans, neurological disorders are increasingly observed, which has boosted interest and the need for better understanding of the pathogenesis of various USUV lineages. In this study we inoculated interferon α/β receptor knockout (Ifnar-/-) mice with decreasing doses of USUV, monitoring symptoms and survival to determine a less lethal dose, and we directly compared isolates from three different viral lineages. We found that a Dutch isolate of USUV Africa-3 lineage is lethal at a dose of 20 pfu per mouse, which is considerably lower than what was anticipated based upon the literature. A Europe-2 strain showed an even higher virulence in this mouse model, compared to strains from Africa-3 and Europe-3 lineages-though this was not reflected in in vitro studies. These results enhance our understanding of the pathogenicity of different USUV strains and provide guidance for the use of low doses for inoculation in an Ifnar-/- animal model.

Identification of novel allosteric sites of SARS-CoV-2 papain-like protease (PLpro) for the development of COVID-19 antivirals

Coronaviruses such as SARS-CoV-2 encode a conserved papain-like protease (PLpro) that is crucial for viral replication and immune evasion, making it a prime target for antiviral drug development. In this study, three surface pockets on SARS-CoV-2 PLpro that may function as sites for allosteric inhibition were computationally identified. To evaluate the effects of these pockets on proteolytic activity, 52 residues were separately mutated to alanine. In Pocket 1, located between the Ubl and thumb domains, the introduction of alanine at T10, D12, T54, Y72, or Y83 reduced PLpro activity to <12% of that of WT. In Pocket 2, situated at the interface of the thumb, fingers, and palm domains, Q237A, S239A, H275A, and S278A inactivated PLpro. Finally, introducing alanine at five residues in Pocket 3, between the fingers and palm domains, inactivated PLpro: S212, Y213, Y251, K254, and Y305. Pocket 1 has a higher druggability score than Pockets 2 and 3. MD simulations showed that interactions within and between domains play critical roles in PLpro activity and thermal stability. The essential residues in Pockets 1 and 2 participate in a combination of intra- and inter-domain interactions. By contrast, the essential residues in Pocket three predominantly participate in inter-domain interactions. The most promising targets for therapeutic development are Pockets one and 3, which have the highest druggability score and the largest number of essential residues, respectively. Non-competitive inhibitors targeting these pockets may be antiviral agents against COVID-19 and related coronaviruses.

Arylamines QSAR-Based Design and Molecular Dynamics of New Phenylthiophene and Benzimidazole Derivatives with Affinity for the C111, Y268, and H73 Sites of SARS-CoV-2 PLpro Enzyme

A non-structural SARS-CoV-2 protein, PLpro, is involved in post-translational modifications in cells, allowing the evasion of antiviral immune response mechanisms. In this study, potential PLpro inhibitory drugs were designed using QSAR, molecular docking, and molecular dynamics. A combined QSAR equation with physicochemical and Free-Wilson descriptors was formulated. The r2, q2, and r2test values were 0.833, 0.770, and 0.721, respectively. From the equation, it was found that the presence of an aromatic ring and a basic nitrogen atom is crucial for obtaining good antiviral activity. Then, a series of structures for the binding sites of C111, Y268, and H73 of PLpro were created. The best compounds were found to exhibit pIC50 values of 9.124 and docking scoring values of -14 kcal/mol. The stability of the compounds in the cavities was confirmed by molecular dynamics studies. A high number of stable contacts and good interactions over time were exhibited by the aryl-thiophenes Pred14 and Pred15, making them potential antiviral candidates.

Preclinical and Clinical Investigations of Potential Drugs and Vaccines for COVID-19 Therapy: A Comprehensive Review With Recent Update

The COVID-19 pandemic-led worldwide healthcare crisis necessitates prompt societal, ecological, and medical efforts to stop or reduce the rising number of fatalities. Numerous mRNA based vaccines and vaccines for viral vectors have been licensed for use in emergencies which showed 90% to 95% efficacy in preventing SARS-CoV-2 infection. However, safety issues, vaccine reluctance, and skepticism remain major concerns for making mass vaccination a successful approach to treat COVID-19. Hence, alternative therapeutics is needed for eradicating the global burden of COVID-19 from developed and low-resource countries. Repurposing current medications and drug candidates could be a more viable option for treating SARS-CoV-2 as these therapies have previously passed a number of significant checkpoints for drug development and patient care. Besides vaccines, this review focused on the potential usage of alternative therapeutic agents including antiviral, antiparasitic, and antibacterial drugs, protease inhibitors, neuraminidase inhibitors, and monoclonal antibodies that are currently undergoing preclinical and clinical investigations to assess their effectiveness and safety in the treatment of COVID-19. Among the repurposed drugs, remdesivir is considered as the most promising agent, while favipiravir, molnupiravir, paxlovid, and lopinavir/ritonavir exhibited improved therapeutic effects in terms of elimination of viruses. However, the outcomes of treatment with oseltamivir, umifenovir, disulfiram, teicoplanin, and ivermectin were not significant. It is noteworthy that combining multiple drugs as therapy showcases impressive effectiveness in managing individuals with COVID-19. Tocilizumab is presently employed for the treatment of patients who exhibit COVID-19-related pneumonia. Numerous antiviral drugs such as galidesivir, griffithsin, and thapsigargin are under clinical trials which could be promising for treating COVID-19 individuals with severe symptoms. Supportive treatment for patients of COVID-19 may involve the use of corticosteroids, convalescent plasma, stem cells, pooled antibodies, vitamins, and natural substances. This study provides an updated progress in SARS-CoV-2 medications and a crucial guide for inventing novel interventions against COVID-19.