Structure-activity relationship optimization for lassa virus fusion inhibitors targeting the transmembrane domain of GP2

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.

Purification and characterization of the Lassa virus transmembrane domain

Lassa virus (LASV) is the most prevalent arenavirus afflicting humans and has high potential to become a threat to global public health. The transmembrane domain (TM) of the LASV glycoprotein complex forms critical interactions with the LASV stable signal peptide that are important for the maturation and fusion activity of the virus. A further study of the structure-based molecular mechanisms is required to understand the role of the TM in the lifecycle of LASV in greater detail. However, it is challenging to obtain the TM in high quantity and purity due to its hydrophobic nature which results in solubility issues that makes it prone to aggregation in typical buffer systems. Here, we designed a purification and detergent screen protocol for the highly insoluble TM to enhance the yield and purity for structural studies. Based on the detergents tested, the TM had the highest incorporation in LMPG. Circular dichroism (CD) and nuclear magnetic resonance (NMR) spectroscopy were utilized to confirm the best detergent system for structural studies. Through CD spectroscopy, we were able to characterize the secondary structure of the TM as largely alpha-helical, while NMR spectroscopy showed a well-structured and stable TM in LMPG. From these results, LMPG was determined to be the optimal detergent for further structural studies.

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.

Lassa Virus Infection: a Summary for Clinicians

OBJECTIVES

This summary on Lassa virus (LASV) infection and Lassa fever disease (LF) was developed from a clinical perspective to provide clinicians a condensed, accessible understanding of the current literature. The information provided highlights pathogenesis, clinical features, and diagnostics with an emphasis on therapies and vaccines that have demonstrated potential value for use in clinical or research environments.

METHODS

An integrative literature review was conducted on the clinical and pathological features, vaccines, and treatments for LASV infection, with a focus on recent studies and in vivo evidence from humans and/or non-human primates (NHPs), when available.

RESULTS

Two antiviral medications with potential benefit for the treatment of LASV infection and one for post-exposure prophylaxis were identified, although a larger number of potential candidates are currently being evaluated. Multiple vaccine platforms are in pre-clinical development for LASV prevention, but data from human clinical trials are not yet available.

CONCLUSION

We provide succinct summaries of medical countermeasures against LASV to give the busy clinician a rapid reference. Although there are no approved drugs or vaccines for LF, we provide condensed information from a literature review for measures that can be taken when faced with a suspected infection, including investigational treatment options and hospital engineering controls.

Screening and identification of Lassa virus endonuclease-targeting inhibitors from a fragment-based drug discovery library

Lassa virus (LASV) belongs to the Old World genus Mammarenavirus, family Arenaviridae, and order Bunyavirales. Arenavirus contains a segmented negative-sense RNA genome, which is in line with the bunyavirus and orthomyxoviruses. The segmented negative-sense RNA viruses utilize a cap-snatching strategy to provide primers cleavaged from the host capped mRNA for viral mRNA transcription. As a similar strategy and the conformational conservation shared with these viruses, the endonuclease (EN) would serve as an attractive target for developing broad-spectrum inhibitors. Using the LASV minigenome (MG) system, we screened a fragment-based drug discovery library and found that two hits, F1204 and F1781, inhibited LASV MG activity. Both hits also inhibited the prototype arenavirus Lymphocytic choriomeningitis virus (LCMV) MG activity. Furthermore, both hits effectively inhibited authentic LCMV and severe fever with thrombocytopenia syndrome virus (SFTSV) infections. Similarly, both hits could inhibit the activity of LASV, LCMV, and SFTSV EN. The combination of either compound with an arenavirus entry inhibitor had significant synergistic antiviral effects. Moreover, both hits were found to be capable of binding to LASV EN with a binding affinity at the micromolar level. These findings provide a basis for developing the hits as potential candidates for the treatment of segmented negative-sense RNA virus infections.

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.

Screening of Botanical Drugs against Lassa Virus Entry

Lassa virus (LASV) belongs to the Old World Mammarenavirus genus (family Arenaviridae). At present, there are no approved drugs or vaccines specific for LASV. In this study, high-throughput screening of a botanical drug library was performed against LASV entry using a pseudotype virus bearing the LASV envelope glycoprotein complex (GPC). Two hit compounds, bergamottin and casticin, were identified as micromolar range inhibitors of LASV entry. A mechanistic study revealed that casticin inhibited LASV entry by blocking low pH-induced membrane fusion. Analysis of adaptive mutants demonstrated that the F446L mutation, located in the transmembrane domain of GP2, conferred resistance to casticin. Furthermore, casticin antiviral activity extends to the New World (NW) pathogenic mammarenaviruses, and mutation of the conserved F446 also conferred resistance to casticin in these viruses. Unlike casticin, bergamottin showed little effect on LASV GPC-mediated membrane fusion, instead inhibiting LASV entry by blocking endocytic trafficking. Notably, both compounds showed inhibitory effects on authentic lymphocytic choriomeningitis virus. Our study shows that both casticin and bergamottin are candidates for LASV therapy and that the conserved F446 in LASV GPC is important in drug resistance in mammarenaviruses.IMPORTANCE: Currently, there is no approved therapy to treat Lassa fever (LASF). Our goal was to identify potential candidate molecules for LASF therapy. Herein, we screened a botanical drug library and identified two compounds, casticin and bergamottin, that inhibited LASV entry via different mechanisms.

Characterizing the Lassa Virus Envelope Glycoprotein Membrane Proximal External Region for Its Role in Fusogenicity

The membrane-proximal external region (MPER) of Lassa virus (LASV) glycoprotein complex (GPC) is critical in modulating its functionality. Till now, the high-resolution structure of the intact GPC, including MPER is not available. In this study, we used alanine substitution to scan all 16 residues located in LASV MPER. Western blotting and quantification fusion assay showed that the residues located at the C terminus of the HR2 (M414 and L415) and N terminus of the MPER (K417 and Y419) are critical for GPC-mediated membrane fusion function. Furthermore, cell surface biotinylation experiments revealed that M414A, K417A and Y419A expressed similar levels as WT, whereas L415A mutant led to a reduction of mature GPC on the cell surface. Moreover, substitution of these residues with the similar residue such as M414L, L415I, K417R and Y419F would partly compensate the loss of the fusion activity caused by the alanine mutant in these sites. Results from this study showed that several key residues in the MPER region are indispensable to promote the conformational changes that drive fusion events and shed light on the structure analysis of LASV GPC and anti-LASV therapeutics.

Identification of the dietary supplement capsaicin as an inhibitor of Lassa virus entry

The limited treatment options for the increasing occurrence of Lassa hemorrhagic fever in West Africa poses an urgent need for the discovery and development of novel therapeutics. Dietary supplements, especially natural products that are edible and safe for human use, are a good source of drug discovery with potential for uncovering novel applications. In this study, we tested 40 natural products of dietary supplements and identified capsaicin, a common dietary supplement abundant in chili peppers, as an inhibitor of Lassa virus (LASV) entry with EC50 of 6.9-10.0 μmol/L using an HIV based pseudovirus platform. Capsaicin inhibits the entry of five LASV strains but not against the Old World arenavirus lymphocytic choriomeningitis virus (LCMV), showing a preferential activity against LASV. Capsaicin inhibits LASV entry by blocking the pH dependent viral fusion through affecting the stable signal peptide (SSP)-GP2 transmembrane (GP2TM) region of the LASV surface glycoprotein. Mutational study revealed the key residues Ala25, Val431, Phe434 and Val435 in SSP-GP2TM region in capsaicin's antiviral effect. This study for the first time reveals a direct acting antiviral effect of capsaicin against the hemorrhagic fever causing LASV, providing detailed interaction hot spots in the unique SSP-GP2TM interface of LASV glycoprotein that is crucial in fusion inhibition, and offering a new strategy in discovering and developing antivirals from natural products that are safe for human use.