Screening of Botanical Drugs against Lassa Virus Entry

Identification of residues in Lassa virus glycoprotein 1 involved in receptor switch

Lassa virus (LASV) is an enveloped, negative-sense RNA virus that causes Lassa hemorrhagic fever. Successful entry of LASV requires the viral glycoprotein 1 (GP1) to undergo a receptor switch from its primary receptor alpha-dystroglycan (α-DG) to its endosomal receptor lysosome-associated membrane protein 1 (LAMP1). A conserved histidine triad in LASV GP1 has been reported to be responsible for receptor switch. To test the hypothesis that other non-conserved residues also contribute to receptor switch, we constructed a series of mutant LASV GP1 proteins and tested them for binding to LAMP1. Four residues, L84, K88, L107, and H170, were identified as critical for receptor switch. Substituting any of the four residues with the corresponding lymphocytic choriomeningitis virus (LCMV) residue (L84N, K88E, L10F, and H170S) reduced the binding affinity of LASV GP1 for LAMP1. Moreover, all mutations caused decreases in glycoprotein precursor (GPC)-mediated membrane fusion at both pH 4.5 and 5.2. The infectivity of pseudotyped viruses bearing either GPCL84N or GPCK88E decreased sharply in multiple cell types, while L107F and H170S had only mild effects on infectivity. Using biolayer light interferometry assay, we found that all four mutants had decreased binding affinity to LAMP1, in the order of binding affinity is L84N > L107F > K88E > H170S. The four amino acid loci identified for the first time in this study have important reference significance for the in-depth investigation of the mechanism of receptor switching and immune escape of LASV occurrence and the development of reserve anti-LASV infection drugs.

Hexestrol, an estrogen receptor agonist, inhibits Lassa virus entry

Lassa virus (LASV) is the causative agent of human Lassa fever which in severe cases manifests as hemorrhagic fever leading to thousands of deaths annually. However, no approved vaccines or antiviral drugs are currently available. Recently, we screened approximately 2,500 compounds using a recombinant vesicular stomatitis virus (VSV) expressing LASV glycoprotein GP (VSV-LASVGP) and identified a P-glycoprotein inhibitor as a potential LASV entry inhibitor. Here, we show that another identified candidate, hexestrol (HES), an estrogen receptor agonist, is also a LASV entry inhibitor. HES inhibited VSV-LASVGP replication with a 50% inhibitory concentration (IC50) of 0.63 µM. Importantly, HES also inhibited authentic LASV replication with IC50 values of 0.31 µM-0.61 µM. Time-of-addition and cell-based membrane fusion assays suggested that HES inhibits the membrane fusion step during virus entry. Alternative estrogen receptor agonists did not inhibit VSV-LASVGP replication, suggesting that the estrogen receptor itself is unlikely to be involved in the antiviral activity of HES. Generation of a HES-resistant mutant revealed that the phenylalanine at amino acid position 446 (F446) of LASVGP, which is located in the transmembrane region, conferred resistance to HES. Although mutation of F446 enhanced the membrane fusion activity of LASVGP, it exhibited reduced VSV-LASVGP replication, most likely due to the instability of the pre-fusion state of LASVGP. Collectively, our results demonstrated that HES is a promising anti-LASV drug that acts by inhibiting the membrane fusion step of LASV entry. This study also highlights the importance of the LASVGP transmembrane region as a target for anti-LASV drugs.IMPORTANCELassa virus (LASV), the causative agent of Lassa fever, is the most devastating mammarenavirus with respect to its impact on public health in West Africa. However, no approved antiviral drugs or vaccines are currently available. Here, we identified hexestrol (HES), an estrogen receptor agonist, as the potential antiviral candidate drug. We showed that the estrogen receptor itself is not involved in the antiviral activity. HES directly bound to LASVGP and blocked membrane fusion, thereby inhibiting LASV infection. Through the generation of a HES-resistant virus, we found that phenylalanine at position 446 (F446) within the LASVGP transmembrane region plays a crucial role in the antiviral activity of HES. The mutation at F446 caused reduced virus replication, likely due to the instability of the pre-fusion state of LASVGP. These findings highlight the potential of HES as a promising candidate for the development of antiviral compounds targeting LASV.

Treatment of highly virulent mammarenavirus infections-status quo and future directions

INTRODUCTION

Mammarenaviruses are negative-sense bisegmented enveloped RNA viruses that are endemic in Africa, the Americas, and Europe. Several are highly virulent, causing acute human diseases associated with high case fatality rates, and are considered to be significant with respect to public health impact or bioterrorism threat.

AREAS COVERED

This review summarizes the status quo of treatment development, starting with drugs that are in advanced stages of evaluation in early clinical trials, followed by promising candidate medical countermeasures emerging from bench analyses and investigational animal research.

EXPERT OPINION

Specific therapeutic treatments for diseases caused by mammarenaviruses remain limited to the off-label use of ribavirin and transfusion of convalescent sera. Progress in identifying novel candidate medical countermeasures against mammarenavirus infection has been slow in part because of the biosafety and biosecurity requirements. However, novel methodologies and tools have enabled increasingly efficient high-throughput molecular screens of regulatory-agency-approved small-molecule drugs and led to the identification of several compounds that could be repurposed for the treatment of infection with several mammarenaviruses. Unfortunately, most of them have not yet been evaluated in vivo. The most promising treatment under development is a monoclonal antibody cocktail that is protective against multiple lineages of the Lassa virus in nonhuman primate disease models.

Entry inhibitors as arenavirus antivirals

Arenaviruses belonging to the Arenaviridae family, genus mammarenavirus, are enveloped, single-stranded RNA viruses primarily found in rodent species, that cause severe hemorrhagic fever in humans. With high mortality rates and limited treatment options, the search for effective antivirals is imperative. Current treatments, notably ribavirin and other nucleoside inhibitors, are only partially effective and have significant side effects. The high lethality and lack of treatment, coupled with the absence of vaccines for all but Junín virus, has led to the classification of these viruses as Category A pathogens by the Centers for Disease Control (CDC). This review focuses on entry inhibitors as potential therapeutics against mammarenaviruses, which include both New World and Old World arenaviruses. Various entry inhibition strategies, including small molecule inhibitors and neutralizing antibodies, have been explored through high throughput screening, genome-wide studies, and drug repurposing. Notable progress has been made in identifying molecules that target receptor binding, internalization, or fusion steps. Despite promising preclinical results, the translation of entry inhibitors to approved human therapeutics has faced challenges. Many have only been tested in in vitro or animal models, and a number of candidates showed efficacy only against specific arenaviruses, limiting their broader applicability. The widespread existence of arenaviruses in various rodent species and their potential for their zoonotic transmission also underscores the need for rapid development and deployment of successful pan-arenavirus therapeutics. The diverse pool of candidate molecules in the pipeline provides hope for the eventual discovery of a broadly effective arenavirus antiviral.

Bergamottin Inhibits PRRSV Replication by Blocking Viral Non-Structural Proteins Expression and Viral RNA Synthesis

The porcine reproductive and respiratory syndrome virus (PRRSV) causes economic losses in the swine industry worldwide. However, current vaccines cannot provide effective protection against PRRSV, and PRRSV-specific treatments for infected herds are still unavailable. In this study, we found that bergamottin showed strong inhibitory effects against PRRSV replication. Bergamottin inhibited PRRSV at the stage of the replication cycle. Mechanically, bergamottin promoted the activation of IRF3 and NF-κB signaling, leading to the increased expression of proinflammatory cytokines and interferon, which inhibited viral replication to some extent. In addition, bergamottion could reduce the expression of the non-structural proteins (Nsps), leading to the interruption of replication and transcription complex (RTC) formation and viral dsRNA synthesis, ultimately restraining PRRSV replication. Our study identified that bergamottin possesses potential value as an antiviral agent against PRRSV in vitro.

Deletion of the first glycosylation site promotes Lassa virus glycoprotein-mediated membrane fusion

The Lassa virus is endemic in West Africa and causes severe hemorrhagic Lassa fever in humans. The glycoprotein complex (GPC) of LASV is highly glycosylation-modified, with 11 N-glycosylation sites. All 11 N-linked glycan chains play critical roles in GPC cleavage, folding, receptor binding, membrane fusion, and immune evasion. In this study, we focused on the first glycosylation site because its deletion mutant (N79Q) results in an unexpected enhanced membrane fusion, whereas it exerts little effect on GPC expression, cleavage, and receptor binding. Meanwhile, the pseudotype virus bearing GPC_N79Q was more sensitive to the neutralizing antibody 37.7H and was attenuated in virulence. Exploring the biological functions of the key glycosylation site on LASV GPC will help elucidate the mechanism of LASV infection and provide strategies for the development of attenuated vaccines against LASV infection.

Design, Synthesis, and Biological Evaluation of Benzimidazole Derivatives as Potential Lassa Virus Inhibitors

The Lassa virus (LASV) causes Lassa fever, a highly infectious and lethal agent of acute viral hemorrhagic fever. At present, there are still no effective treatments available, creating an urgent need to develop novel therapeutics. Some benzimidazole compounds targeting the arenavirus envelope glycoprotein complex (GPC) are promising inhibitors of LASV. In this study, we synthesized two series of LASV inhibitors based on the benzimidazole structure. Lentiviral pseudotypes bearing the LASV GPC were established to identify virus entry inhibitors. Surface plasmon resonance (SPR) was further used to verify the binding activities of the potential compounds. Compounds 7d-Z, 7h-Z, 13c, 13d, and 13f showed relatively excellent antiviral activities with IC₅₀ values ranging from 7.58 to 15.46 nM and their SI values above 1251. These five representative compounds exhibited stronger binding affinity with low equilibrium dissociation constants (K_D < 8.25 × 10^-7 M) in SPR study. The compound 7h-Z displayed the most potent antiviral activity (IC₅₀ = 7.58 nM) with a relatively high SI value (2496), which could be further studied as a lead compound. The structure-activity relationship indicated that the compounds with lipophilic and spatially larger substituents might possess higher antiviral activity and a much larger safety margin. This study will provide some good guidance for the development of highly active compounds with a novel skeleton against LASV.

Bergamottin: location, aggregation and interaction with the plasma membrane

Bioactive furanocoumarins, a group of natural secondary metabolites common in higher plants, are recognized for their benefits to human health and have been shown to have numerous biological properties. However, the knowledge of its biomolecular mechanism is not known. One of the main furanocoumarins is bergamottin (BGM), which is characterized by a planar three-ringed structure and a hydrocarbon chain, which give BGM its high lipid/water partition coefficient. Because of that, and although the biological mechanism of BGM is not known, BGM bioactive properties could be ascribed to its potential to interact with the biological membrane, modulating its structure, changing its dynamics and at the same time that it might interact with lipids. For our goal, we have applied molecular dynamics to determine the position of BGM in a complex membrane and discern the possibility of certain interactions with membrane lipids. Our findings establish that BGM tends to locate in the middle of the hydrocarbon layer of the membrane, inserts in between the hydrocarbon chains of the phospholipids in an oblique position with respect to the membrane plane, increasing the fluidity of the membrane. Significantly, BGM tends to be surrounded by POPC molecules but exclude the molecule of CHOL. Outstandingly, BGM molecules associate spontaneously creating aggregates, which does not preclude them from interacting with and inserting into the membrane. The bioactive properties of BGM could be ascribed to its membranotropic effects and support the improvement of these molecules as therapeutic molecules, giving place to new opportunities for potential medical improvements.Communicated by Ramaswamy H. Sarma.

Screening and Identification of Lassa Virus Entry Inhibitors from a Fragment-Based Drug Discovery Library

Lassa virus (LASV) is a highly pathogenic virus that is categorized as a biosafety level-4 pathogen. Currently, there are no approved drugs or vaccines specific to LASV. In this study, high-throughput screening of a fragment-based drug discovery library was performed against LASV entry using a pseudotype virus bearing the LASV envelope glycoprotein complex (GPC). Two compounds, F1920 and F1965, were identified as LASV entry inhibitors that block GPC-mediated membrane fusion. Analysis of adaptive mutants demonstrated that the transient mutants L442F and I445S, as well as the constant mutant F446L, were located on the same side on the transmembrane domain of the subunit GP2 of GPC, and all the mutants conferred resistance to both F1920 and F1965. Furthermore, F1920 antiviral activity extended to other highly pathogenic mammarenaviruses, whereas F1965 was LASV-specific. Our study showed that both F1920 and F1965 provide a potential backbone for the development of lead drugs for preventing LASV infection.

Allosteric Modulation of the Main Protease (MPro) of SARS-CoV-2 by Casticin—Insights from Molecular Dynamics Simulations

Inhibition of the main protease (M^pro) of SARS-CoV-2 has been suggested to be vital in shutting down viral replication in a host. Most efforts aimed at inhibiting M^Pro activity have been channeled into competitive inhibition at the active site, but this strategy will require a high inhibitor concentration and impressive inhibitor-M^Pro binding affinity. Allosteric inhibition can potentially serve as an effective strategy for alleviating these limitations. In this study, the ability of antiviral natural products to inhibit M^Pro in an allosteric fashion was explored with in silico techniques. Molecular docking revealed a strong interaction between casticin, an antiviral flavonoid, and M^pro at a site distant from the active site. This site, characterized as a distal site, has been shown to have an interdependent dynamic effect with the active site region. M^pro only, M^pro-peptide (binary) and M^pro-peptide-casticin (ternary) complexes were subjected to molecular dynamics simulations for 50 ns to investigate the modulatory activity of casticin binding on M^pro. Molecular dynamic simulations revealed that binding of casticin at the distal site interferes with the proper orientation of the peptide substrate in the oxyanion hole of the active site, and this could lead to a halt or decrease in catalytic activity. This study therefore highlights casticin as a potential allosteric modulator of the SARS-CoV-2 main protease, which could be optimized and developed into a potential lead compound for anti-SARS-CoV-2 chemotherapy.

Bergamottin, a bioactive component of bergamot, inhibits SARS-CoV-2 infection in golden Syrian hamsters

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has caused an ongoing pandemic, coronavirus disease-2019 (COVID-19), which has become a major global public health event. Antiviral compounds remain the predominant means of treating COVID-19. Here, we reported that bergamottin, a furanocoumarin originally found in bergamot, exhibited inhibitory activity against SARS-CoV-2 in vitro, ex vivo, and in vivo. Bergamottin interfered with multiple stages of virus life cycles, specifically blocking the SARS-CoV-2 spike-mediated membrane fusion and effectively reducing viral RNA synthesis. Oral delivery of bergamottin to golden Syrian hamsters at dosages of both 50 mg/kg and 75 mg/kg reduced the SARS-CoV-2 load in nasal turbinates and lung tissues. Pathological damage caused by viral infection was also ameliorated after bergamottin treatment. Overall, our study provides evidence of bergamottin as a promising natural compound, with broad-spectrum anti-coronavirus activity, that could be further developed in the fight against COVID-19 infection during the current pandemic.

Screening of Botanical Drugs against SARS-CoV-2 Entry Reveals Novel Therapeutic Agents to Treat COVID-19

An escalating pandemic caused by the novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has severely impacted global health. There is a severe lack of specific treatment options for diseases caused by SARS-CoV-2. In this study, we used a pseudotype virus (pv) containing the SARS-CoV-2 S glycoprotein to screen a botanical drug library containing 1037 botanical drugs to identify agents that prevent SARS-CoV-2 entry into the cell. Our study identified four hits, including angeloylgomisin O, schisandrin B, procyanidin, and oleanonic acid, as effective SARS-CoV-2 S pv entry inhibitors in the micromolar range. A mechanistic study revealed that these four agents inhibited SARS-CoV-2 S pv entry by blocking spike (S) protein-mediated membrane fusion. Furthermore, angeloylgomisin O and schisandrin B inhibited authentic SARS-CoV-2 with a high selective index (SI; 50% cytotoxic concentration/50% inhibition concentration). Our drug combination studies performed in cellular antiviral assays revealed that angeloylgomisin O has synergistic effects in combination with remdesivir, a drug widely used to treat SARS-CoV-2-mediated infections. We also showed that two hits could inhibit the newly emerged alpha (B.1.1.7) and beta (B.1.351) variants. Our findings collectively indicate that angeloylgomisin O and schisandrin B could inhibit SARS-CoV-2 efficiently, thereby making them potential therapeutic agents to treat the coronavirus disease of 2019.

Lassa virus glycoprotein complex review: insights into its unique fusion machinery

Lassa virus (LASV), an arenavirus endemic to West Africa, causes Lassa fever—a lethal hemorrhagic fever. Entry of LASV into the host cell is mediated by the glycoprotein complex (GPC), which is the only protein located on the viral surface and comprises three subunits: glycoprotein 1 (GP1), glycoprotein 2 (GP2), and a stable signal peptide (SSP). The LASV GPC is a class one viral fusion protein, akin to those found in viruses such as human immunodeficiency virus (HIV), influenza, Ebola virus (EBOV), and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). These viruses are enveloped and utilize membrane fusion to deliver their genetic material to the host cell. Like other class one fusion proteins, LASV-mediated membrane fusion occurs through an orchestrated sequence of conformational changes in its GPC. The receptor-binding subunit, GP1, first engages with a host cell receptor then undergoes a unique receptor switch upon delivery to the late endosome. The acidic pH and change in receptor result in the dissociation of GP1, exposing the fusion subunit, GP2, such that fusion can occur. These events ultimately lead to the formation of a fusion pore so that the LASV genetic material is released into the host cell. Interestingly, the mature GPC retains its SSP as a third subunit—a feature that is unique to arenaviruses. Additionally, the fusion domain contains two separate fusion peptides, instead of a standard singular fusion peptide. Here, we give a comprehensive review of the LASV GPC components and their unusual features.

Screening and Identification of Lujo Virus Entry Inhibitors From an Food and Drug Administration-Approved Drugs Library

Lujo virus (LUJV) belongs to the Old World (OW) genus Mammarenavirus (family Arenaviridae). It is categorized as a biosafety level (BSL) 4 agent. Currently, there are no U.S. Food and Drug Administration (FDA)-approved drugs or vaccines specifically for LUJV or other pathogenic OW mammarenaviruses. Here, a high-throughput screening of an FDA-approved drug library was conducted using pseudotype viruses bearing LUJV envelope glycoprotein (GPC) to identify inhibitors of LUJV entry. Three hit compounds, trametinib, manidipine, and lercanidipine, were identified as LUJV entry inhibitors in the micromolar range. Mechanistic studies revealed that trametinib inhibited LUJV GPC-mediated membrane fusion by targeting C410 [located in the transmembrane (TM) domain], while manidipine and lercanidipine inhibited LUJV entry by acting as calcium channel blockers. Meanwhile, all three hits extended their antiviral spectra to the entry of other pathogenic mammarenaviruses. Furthermore, all three could inhibit the authentic prototype mammarenavirus, lymphocytic choriomeningitis virus (LCMV), and could prevent infection at the micromolar level. This study shows that trametinib, manidipine, and lercanidipine are candidates for LUJV therapy and highlights the critical role of calcium in LUJV infection. The presented findings reinforce the notion that the key residue(s) located in the TM domain of GPC provide an entry-targeted platform for designing mammarenavirus inhibitors.

Target specificity of selective bioactive compounds in blocking α-dystroglycan receptor to suppress Lassa virus infection: an in silico approach

Lassa hemorrhagic fever, caused by Lassa mammarenavirus (LASV) infection, accumulates up to 5000 deaths every year. Currently, there is no vaccine available to combat this disease. In this study, a library of 200 bioactive compounds was virtually screened to study their drug-likeness with the capacity to block the α-dystroglycan (α-DG) receptor and prevent LASV influx. Following rigorous absorption, distribution, metabolism, and excretion (ADME) and quantitative structure-activity relationship (QSAR) profiling, molecular docking was conducted with the top ligands against the α-DG receptor. The compounds chrysin, reticuline, and 3-caffeoylshikimic acid emerged as the top three ligands in terms of binding affinity. Post-docking analysis revealed that interactions with Arg76, Asn224, Ser259, and Lys302 amino acid residues of the receptor protein were important for the optimum binding affinity of ligands. Molecular dynamics simulation was performed comprehensively to study the stability of the protein-ligand complexes. In-depth assessment of root-mean-square deviation (RMSD), root mean square fluctuation (RMSF), polar surface area (PSA), B-Factor, radius of gyration (Rg), solvent accessible surface area (SASA), and molecular surface area (MolSA) values of the protein-ligand complexes affirmed that the candidates with the best binding affinity formed the most stable protein-ligand complexes. To authenticate the potentialities of the ligands as target-specific drugs, an in vivo study is underway in real time as the continuation of the research.