Signal peptide of Lassa virus glycoprotein GP-C exhibits an unusual length

Defining bottlenecks and opportunities for Lassa virus neutralization by structural profiling of vaccine-induced polyclonal antibody responses

Lassa fever continues to be a major public health burden in West Africa, yet effective therapies or vaccines are lacking. The isolation of protective neutralizing antibodies against the Lassa virus glycoprotein complex (GPC) justifies the development of vaccines that can elicit strong neutralizing antibody responses. However, Lassa vaccine candidates have generally been unsuccessful at doing so, and the associated antibody responses to these vaccines remain poorly characterized. Here, we establish an electron microscopy-based epitope mapping workflow that enables high-resolution structural characterization of polyclonal antibodies to the GPC. By applying this method to rabbits vaccinated with a recombinant GPC vaccine and a GPC-derived virus-like particle, we reveal determinants of neutralization that involve epitopes of the GPC-A competition cluster. Furthermore, by identifying undescribed immunogenic off-target epitopes, we expose the challenges that recombinant GPC vaccines face. By enabling detailed polyclonal antibody characterization, our work ushers in a next generation of more rational Lassa vaccine design.

Deep mutational scanning reveals functional constraints and antibody-escape potential of Lassa virus glycoprotein complex

Lassa virus is estimated to cause thousands of human deaths per year, primarily due to spillovers from its natural host, Mastomys rodents. Efforts to create vaccines and antibody therapeutics must account for the evolutionary variability of the Lassa virus's glycoprotein complex (GPC), which mediates viral entry into cells and is the target of neutralizing antibodies. To map the evolutionary space accessible to GPC, we used pseudovirus deep mutational scanning to measure how nearly all GPC amino-acid mutations affected cell entry and antibody neutralization. Our experiments defined functional constraints throughout GPC. We quantified how GPC mutations affected neutralization with a panel of monoclonal antibodies. All antibodies tested were escaped by mutations that existed among natural Lassa virus lineages. Overall, our work describes a biosafety-level-2 method to elucidate the mutational space accessible to GPC and shows how prospective characterization of antigenic variation could aid the design of therapeutics and vaccines.

The structure of the Lujo virus spike complex

Lujo virus (LUJV) is a human pathogen that was the cause of a deadly hemorrhagic fever outbreak in Africa. LUJV is a divergent member of the Arenaviridae with some similarities to both the "Old World" and "New World" serogroups, but it uses a cell-entry receptor, neuropilin-2 (NRP2), that is distinct from the receptors of OW and NW viruses. Though the receptor binding domain of LUJV has been characterized structurally, the overall organization of the trimeric spike complex and how NRP2 is recognized in this context were unknown. Here, we present the structure of the membrane-embedded LUJV spike complex determined by cryo-electron microscopy. Analysis of the structure suggested that a single NRP2 molecule is bound at the apex of the trimeric spike and that multiple subunits of the trimer contact the receptor. The binding of NRP2 involves an intriguing arginine-methionine interaction, which we analyzed using quantum mechanical modeling methods. We compare the LUJV spike structure with the only other available structure of a complete arenaviral spike, which is the Lassa virus. The similarities and differences between them shed light on Arenavirus evolution, inform vaccine design, and provide information that will be useful in combating future Arenavirus outbreaks.

Restoration of virulence in the attenuated Candid#1 vaccine virus requires reversion at both positions 168 and 427 in the envelope glycoprotein GPC

Live-attenuated virus vaccines provide long-lived protection against viral disease but carry inherent risks of residual pathogenicity and genetic reversion. The live-attenuated Candid#1 vaccine was developed to protect Argentines against lethal infection by the Argentine hemorrhagic fever arenavirus, Junín virus. Despite its safety and efficacy in Phase III clinical study, the vaccine is not licensed in the US, in part due to concerns regarding the genetic stability of attenuation. Previous studies had identified a single F427I mutation in the transmembrane domain of the Candid#1 envelope glycoprotein GPC as the key determinant of attenuation, as well as the propensity of this mutation to revert upon passage in cell culture and neonatal mice. To ascertain the consequences of this reversion event, we introduced the I427F mutation into recombinant Candid#1 (I427F rCan) and investigated the effects in two validated small-animal models: in mice expressing the essential virus receptor (human transferrin receptor 1; huTfR1) and in the conventional guinea pig model. We report that I427F rCan displays only modest virulence in huTfR1 mice and appears attenuated in guinea pigs. Reversion at another attenuating locus in Candid#1 GPC (T168A) was also examined, and a similar pattern was observed. By contrast, virus bearing both revertant mutations (A168T+I427F rCan) approached the lethal virulence of the pathogenic Romero strain in huTfR1 mice. Virulence was less extreme in guinea pigs. Our findings suggest that genetic stabilization at both positions is required to minimize the likelihood of reversion to virulence in a second-generation Candid#1 vaccine.IMPORTANCELive-attenuated virus vaccines, such as measles/mumps/rubella and oral poliovirus, provide robust protection against disease but carry with them the risk of genetic reversion to the virulent form. Here, we analyze the genetics of reversion in the live-attenuated Candid#1 vaccine that is used to protect against Argentine hemorrhagic fever, an often-lethal disease caused by the Junín arenavirus. In two validated small-animal models, we find that restoration of virulence in recombinant Candid#1 viruses requires back-mutation at two positions specific to the Candid#1 envelope glycoprotein GPC, at positions 168 and 427. Viruses bearing only a single change showed only modest virulence. We discuss strategies to genetically harden Candid#1 GPC against these two reversion events in order to develop a safer second-generation Candid#1 vaccine virus.

Deep mutational scanning reveals functional constraints and antigenic variability of Lassa virus glycoprotein complex

Lassa virus is estimated to cause thousands of human deaths per year, primarily due to spillovers from its natural host, Mastomys rodents. Efforts to create vaccines and antibody therapeutics must account for the evolutionary variability of Lassa virus's glycoprotein complex (GPC), which mediates viral entry into cells and is the target of neutralizing antibodies. To map the evolutionary space accessible to GPC, we use pseudovirus deep mutational scanning to measure how nearly all GPC amino-acid mutations affect cell entry and antibody neutralization. Our experiments define functional constraints throughout GPC. We quantify how GPC mutations affect neutralization by a panel of monoclonal antibodies and show that all antibodies are escaped by mutations that exist among natural Lassa virus lineages. Overall, our work describes a biosafety-level-2 method to elucidate the mutational space accessible to GPC and shows how prospective characterization of antigenic variation could aid design of therapeutics and vaccines.

Defining bottlenecks and opportunities for Lassa virus neutralization by structural profiling of vaccine-induced polyclonal antibody responses

Lassa fever continues to be a major public health burden in endemic countries in West Africa, yet effective therapies or vaccines are lacking. The isolation of potent and protective neutralizing antibodies against the Lassa virus glycoprotein complex (GPC) justifies the development of vaccines that can elicit strong neutralizing antibody responses. However, Lassa vaccines candidates have generally been unsuccessful in doing so and the associated antibody responses to these vaccines remain poorly characterized. Here, we establish an electron-microscopy based epitope mapping pipeline that enables high-resolution structural characterization of polyclonal antibodies to GPC. By applying this method to rabbits vaccinated with a recombinant GPC vaccine and a GPC-derived virus-like particle, we reveal determinants of neutralization which involve epitopes of the GPC-C, GPC-A, and GP1-A competition clusters. Furthermore, by identifying previously undescribed immunogenic off-target epitopes, we expose challenges that recombinant GPC vaccines face. By enabling detailed polyclonal antibody characterization, our work ushers in a next generation of more rational Lassa vaccine design.

Structural conservation of Lassa virus glycoproteins and recognition by neutralizing antibodies

Lassa fever is an acute hemorrhagic fever caused by the zoonotic Lassa virus (LASV). The LASV glycoprotein complex (GPC) mediates viral entry and is the sole target for neutralizing antibodies. Immunogen design is complicated by the metastable nature of recombinant GPCs and the antigenic differences among phylogenetically distinct LASV lineages. Despite the sequence diversity of the GPC, structures of most lineages are lacking. We present the development and characterization of prefusion-stabilized, trimeric GPCs of LASV lineages II, V, and VII, revealing structural conservation despite sequence diversity. High-resolution structures and biophysical characterization of the GPC in complex with GP1-A-specific antibodies suggest their neutralization mechanisms. Finally, we present the isolation and characterization of a trimer-preferring neutralizing antibody belonging to the GPC-B competition group with an epitope that spans adjacent protomers and includes the fusion peptide. Our work provides molecular detail information on LASV antigenic diversity and will guide efforts to design pan-LASV vaccines.

Life after Cleavage: The Story of a β-Retroviral (MMTV) Signal Peptide-From Murine Lymphoma to Human Breast Cancer

An increasing body of evidence in recent years supports an association of the betaretrovirus mouse mammary tumor virus (MMTV) with human breast cancer. This is an issue that still raises heated controversy. We have come to address this association using the signal peptide p14 of the MMTV envelope precursor protein as a key element of our strategy. In addition to its signal peptide function, p14 has some significant post endoplasmic reticulum (ER)-targeting characteristics: (1) it localizes to nucleoli where it binds key proteins (RPL5 and B23) involved (among other activities) in the regulation of nucleolar stress response, ribosome biogenesis and p53 stabilization; (2) p14 is a nuclear export factor; (3) it is expressed on the cell surface of infected cells, and as such, is amenable to, and successfully used, in preventive vaccination against experimental tumors that harbor MMTV; (4) the growth of such tumors is impaired in vivo using a combination of monoclonal anti-p14 antibodies or adoptive T-cell transfer treatments; (5) p14 is a phospho-protein endogenously phosphorylated by two different serine kinases. The phosphorylation status of the two sites determines whether p14 will function in an oncogenic or tumor-suppressing capacity; (6) transcriptional activation of genes (RPL5, ErbB4) correlates with the oncogenic potential of MMTV; (7) finally, polyclonal anti-p14 antibodies have been applied in immune histochemistry analyses of breast cancer cases using formalin fixed paraffin-embedded sections, supporting the associations of MMTV with the disease. Taken together, the above findings constitute a road map towards the diagnosis and possible prevention and treatment of MMTV-associated breast cancer.

The Bombyx mori Nucleopolyhedrovirus GP64 Retains the Transmembrane Helix of Signal Peptide to Contribute to Secretion across the Cytomembrane

Bombyx mori nucleopolyhedrovirus (BmNPV) is the primary pathogen of silkworms that causes severe economic losses in sericulture. GP64 is the key membrane fusion protein that mediates budded virus (BV) fusion with the host cell membrane. Previously, we found that the n-region of the GP64 signal peptide (SP) is required for protein secretion and viral pathogenicity; however, our understanding of BmNPV GP64 remains limited. Here, we first reported that BmNPV GP64 retained its SP in the mature protein and virion in only host cells but did not retain in nonhost cells. Uncleaved SP mediates protein targeting to the cytomembrane or secretion in Bombyx mori cells. The exitance of the n-region extended the transmembrane helix length, which resulted in the cleavage site to be located in the helix structure and thus blocked cleavage from signal peptidase (SPase). Without the n-region, the protein fails to be transported to the cytomembrane, but this failure can be rescued by the cleavage site mutation of SP. Helix-breaking mutations in SP abolished protein targeting to the cytomembrane and secretion. Our results revealed a previously unrecognized mechanism by which SP of membrane fusion not only determines protein localization but also determines viral pathogenicity, which highlights the escape mechanism of SP from the cleavage by SPase. IMPORTANCE BmNPV is the primary pathogen of silkworms, which causes severe economic losses in sericulture. BmNPV and Autographa californica multiple nucleopolyhedrovirus (AcMNPV) are closely related group I alphabaculoviruses, but they exhibit nonoverlapping host specificity. Recent studies suppose that GP64 is a determinant of host range, while knowledge remains limited. In this study, we revealed that BmNPV GP64 retained its SP in host cells but not in nonhost cells, and the SP retention is required for GP64 secretion across the cytomembrane. This is the first report that a type I membrane fusion protein retained its SP in mature proteins and virions. Our results unveil the mechanism by which SP GP64 escapes cleavage and the role of SP in protein targeting. This study will help elucidate an important mechanistic understanding of BmNPV infection and host range specificity.

Surveillance of Class I Newcastle Disease Virus at Live Bird Markets in China and Identification of Variants with Increased Virulence and Replication Capacity

In this study, 232 class I Newcastle disease viruses (NDVs) were identified from multiple bird species at nationwide live bird markets (LBMs) from 2017 to 2019 in China. Phylogenetic analysis indicated that all 232 isolates were clustered into genotype 1.1.2 of class I on the basis of the fusion (F) gene sequences, which were distinct from the genotypes identified in other countries. Most of the isolates (212/232) were shown to have the typical F gene molecular characteristics of class I NDVs, while a few (20/232) contained mutations at the site of the conventional start codon of the F gene, which resulted in open reading frames (ORFs) altered in length. The isolates with ACG, CTA, and ATA mutations showed different levels of increased virulence and replication capacity, suggesting that these viruses may be transitional types during the evolution of class I NDVs from avirulent to virulent. Further evaluation of biological characteristics with recombinant viruses obtained by reverse genetics demonstrated that the ATG located at genomic positions 4523 to 4525 was the authentic start codon in the F gene of class I NDV, and the specific ATA mutations which contributed to the expression of F protein on the surface of infected cells were the key determinants of increased replication capacity and virulence. Interestingly, the mutation at the corresponding site of genotype II LaSota of class II had no effects on the virulence and replication capacity in chickens. Our results suggest that the alteration of virulence and replication capacity caused by specific mutations in the F gene could be a specific characteristic of class I NDVs and indicate the possibility of the emergence of virulent NDVs due to the persistent circulation of class I NDVs. IMPORTANCE The available information on the distribution, genetic diversity, evolution, and biological characteristics of class I Newcastle disease viruses (NDVs) in domestic poultry is currently very limited. Here, identification of class I NDVs at nationwide live bird markets (LBMs) in China was performed and representative isolates were characterized. A widespread distribution of genotype 1.1.2 of class I NDVs was found in multiple bird species at LBMs in China. Though most isolates demonstrated typical molecular characteristics of class I NDVs, a few that contained specific mutations at the site of the conventional start codon of the fusion gene with increased virulence and replication capacity were identified for the first time. Our findings indicate that the virulence of class I NDVs could have evolved, and the widespread transmission and circulation of class I NDVs may represent a potential threat for disease outbreaks in poultry.

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.

Pathogenicity and virulence mechanisms of Lassa virus and its animal modeling, diagnostic, prophylactic, and therapeutic developments

Lassa fever (LF) is a deadly viral hemorrhagic disease that is endemic to West Africa. The causative agent of LF is Lassa virus (LASV), which causes approximately 300,000 infections and 5,000 deaths annually. There are currently no approved therapeutics or FDA-approved vaccines against LASV. The high genetic variability between LASV strains and immune evasion mediated by the virus complicate the development of effective therapeutics and vaccines. Here, we aim to provide a comprehensive review of the basic biology of LASV and its mechanisms of disease pathogenesis and virulence in various animal models, as well as an update on prospective vaccines, therapeutics, and diagnostics for LF. Until effective vaccines and/or therapeutics are available for use to prevent or treat LF, a better level of understanding of the basic biology of LASV, its natural genetic variations and immune evasion mechanisms as potential pathogenicity factors, and of the rodent reservoir-vector populations and their geographical distributions, is necessary for the development of accurate diagnostics and effective therapeutics and vaccines against this deadly human viral pathogen.

Second-Generation Live-Attenuated Candid#1 Vaccine Virus Resists Reversion and Protects against Lethal Junín Virus Infection in Guinea Pigs

Live-attenuated virus vaccines are highly effective in preventing viral disease but carry intrinsic risks of residual virulence and reversion to pathogenicity. The classically derived Candid#1 virus protects seasonal field workers in Argentina against zoonotic infection by Junín virus (JUNV) but is not approved in the United States, in part due to the potential for reversion at the attenuating locus, a phenylalanine-to-isoleucine substitution at position 427 in the GP2 subunit of the GPC envelope glycoprotein. Previously, we demonstrated facile reversion of recombinant Candid#1 (rCan) in cell culture and identified an epistatic interaction between the attenuating I427 and a secondary K33S mutation in the stable signal peptide (SSP) subunit of GPC that imposes an evolutionary barrier to reversion. The magnitude of this genetic barrier is manifest in our repeated failures to rescue the hypothetical revertant virus. In this study, we show that K33S rCan is safe and attenuated in guinea pigs and capable of eliciting potent virus-neutralizing antibodies. Immunized animals are fully protected against lethal challenge with virulent JUNV. In addition, we employed a more permissive model of infection in neonatal mice to investigate genetic reversion. RNA sequence analysis of the recovered virus identified revertant viruses in pups inoculated with the parental rCan virus and none in mice receiving K33S rCan (P < 0.0001). Taken together, our findings support the further development of K33S rCan as a safe second-generation JUNV vaccine. IMPORTANCE Our most successful vaccines comprise weakened strains of virus that initiate a limited and benign infection in immunized persons. The live-attenuated Candid#1 strain of Junín virus (JUNV) was developed to protect field workers in Argentina from rodent-borne hemorrhagic fever but is not licensed in the United States, in part due to the likelihood of genetic reversion to virulence. A single-amino-acid change in the GPC envelope glycoprotein of the virus is responsible for attenuation, and a single nucleotide change may regenerate the pathogenic virus. Here, we take advantage of a unique genetic interaction between GPC subunits to design a mutant Candid#1 virus that establishes an evolutionary barrier to reversion. The mutant virus (K33S rCan) is fully attenuated and protects immunized guinea pigs against lethal JUNV infection. We find no instances of reversion in mice inoculated with K33S rCan. This work supports the further development of K33S rCan as a second-generation JUNV vaccine.

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.

Adjuvant formulated virus-like particles expressing native-like forms of the Lassa virus envelope surface glycoprotein are immunogenic and induce antibodies with broadly neutralizing activity

Lassa mammarenavirus (LASV) is a rodent-borne arenavirus endemic to several West African countries. It is the causative agent of human Lassa fever, an acute viral hemorrhagic fever disease. To date, no therapeutics or vaccines against LASV have obtained regulatory approval. Polyclonal neutralizing antibodies derived from hyperimmunized animals may offer a useful strategy for prophylactic and therapeutic intervention to combat human LASV infections. The LASV envelope surface glycoprotein complex (GP) is the major target for neutralizing antibodies, and it is the main viral antigen used for the design of an LASV vaccine. Here, we assessed the immunogenic potential of mammalian cell-derived virus-like particles (VLPs) expressing GP from the prototypic LASV strain Josiah in a native-like conformation as the sole viral antigen. We demonstrate that an adjuvanted prime-boost immunization regimen with GP-derived VLPs elicited neutralizing antibody responses in rabbits, suggesting that effective antigenic epitopes of GP were displayed. Notably, these antibodies exhibited broad reactivity across five genetic lineages of LASV. VLP-based immunization strategies may represent a powerful approach for generating polyclonal sera containing cross-reactive neutralizing antibodies against LASV.

Distinct Molecular Mechanisms of Host Immune Response Modulation by Arenavirus NP and Z Proteins

Endemic to West Africa and South America, mammalian arenaviruses can cross the species barrier from their natural rodent hosts to humans, resulting in illnesses ranging from mild flu-like syndromes to severe and fatal haemorrhagic zoonoses. The increased frequency of outbreaks and associated high fatality rates of the most prevalent arenavirus, Lassa, in West African countries, highlights the significant risk to public health and to the socio-economic development of affected countries. The devastating impact of these viruses is further exacerbated by the lack of approved vaccines and effective treatments. Differential immune responses to arenavirus infections that can lead to either clearance or rapid, widespread and uncontrolled viral dissemination are modulated by the arenavirus multifunctional proteins, NP and Z. These two proteins control the antiviral response to infection by targeting multiple cellular pathways; and thus, represent attractive targets for antiviral development to counteract infection. The interplay between the host immune responses and viral replication is a key determinant of virus pathogenicity and disease outcome. In this review, we examine the current understanding of host immune defenses against arenavirus infections and summarise the host protein interactions of NP and Z and the mechanisms that govern immune evasion strategies.

50 Years of Lassa Fever Research

Lassa fever was first described as a clinical entity fifty years ago. The causative agent Lassa virus was isolated from these first known cases. This chapter reviews the key publications on Lassa fever research that appeared in the scientific literature at that time and over the ensuing decades.

Lassa Virus Genetics

In a pattern repeated across a range of ecological niches, arenaviruses have evolved a compact four-gene genome to orchestrate a complex life cycle in a narrow range of susceptible hosts. A number of mammalian arenaviruses cross-infect humans, often causing a life-threatening viral hemorrhagic fever. Among this group of geographically bound zoonoses, Lassa virus has evolved a unique niche that leads to significant and sustained human morbidity and mortality. As a biosafety level 4 pathogen, direct study of the pathogenesis of Lassa virus is limited by the sparse availability, high operating costs, and technical restrictions of the high-level biocontainment laboratories required for safe experimentation. In this chapter, we introduce the relationship between genome structure and the life cycle of Lassa virus and outline reverse genetic approaches used to probe and describe functional elements of the Lassa virus genome. We then review the tools used to obtain viral genomic sequences used for phylogeny and molecular diagnostics, before shifting to a population perspective to assess the contributions of phylogenetic analysis in understanding the evolution and ecology of Lassa virus in West Africa. We finally consider the future outlook and clinical applications for genetic study of Lassa virus.

Comprehensive Interactome Analysis Reveals that STT3B Is Required for N-Glycosylation of Lassa Virus Glycoprotein

Glycoproteins play vital roles in the arenavirus life cycle by facilitating virus entry and participating in the virus budding process. N-glycosylation of GPs is responsible for their proper functioning; however, little is known about the host factors on which the virus depends for this process. In this study, a comprehensive LASV GP interactome was characterized, and further study revealed that STT3B-dependent N-glycosylation was preferentially required by arenavirus GPs and critical for virus infectivity. The two specific thioredoxin subunits of STT3B-OST MAGT1 and TUSC3 were found to be essential for the N-glycosylation of viral GP. NGI-1, a small-molecule inhibitor of OST, also showed a robust inhibitory effect on arenavirus. Our study provides new insights into LASV GP-host interactions and extends the potential targets for the development of novel therapeutics against Lassa fever in the future.

Lassa virus (LASV) is the causative agent of a fatal hemorrhagic fever in humans. The glycoprotein (GP) of LASV mediates viral entry into host cells, and correct processing and modification of GP by host factors is a prerequisite for virus replication. Here, using an affinity purification-coupled mass spectrometry (AP-MS) strategy, 591 host proteins were identified as interactors of LASV GP. Gene ontology analysis was performed to functionally annotate these proteins, and the oligosaccharyltransferase (OST) complex was highly enriched. Functional studies conducted by using CRISPR-Cas9-mediated knockouts showed that STT3A and STT3B, the two catalytically active isoforms of the OST complex, are essential for the propagation of the recombinant arenavirus rLCMV/LASV glycoprotein precursor, mainly via affecting virus infectivity. Knockout of STT3B, but not STT3A, caused hypoglycosylation of LASV GP, indicating a preferential requirement of LASV for the STT3B-OST isoform. Furthermore, double knockout of magnesium transporter 1 (MAGT1) and tumor suppressor candidate 3 (TUSC3), two specific subunits of STT3B-OST, also caused hypoglycosylation of LASV GP and affected virus propagation. Site-directed mutagenesis analysis revealed that the oxidoreductase CXXC active-site motif of MAGT1 or TUSC3 is essential for the glycosylation of LASV GP. NGI-1, a small-molecule OST inhibitor, can effectively reduce virus infectivity without affecting cell viability. The STT3B-dependent N-glycosylation of GP is conserved among other arenaviruses, including both the Old World and New World groups. Our study provided a systematic view of LASV GP-host interactions and revealed the preferential requirement of STT3B for LASV GP N-glycosylation.

IMPORTANCE Glycoproteins play vital roles in the arenavirus life cycle by facilitating virus entry and participating in the virus budding process. N-glycosylation of GPs is responsible for their proper functioning; however, little is known about the host factors on which the virus depends for this process. In this study, a comprehensive LASV GP interactome was characterized, and further study revealed that STT3B-dependent N-glycosylation was preferentially required by arenavirus GPs and critical for virus infectivity. The two specific thioredoxin subunits of STT3B-OST MAGT1 and TUSC3 were found to be essential for the N-glycosylation of viral GP. NGI-1, a small-molecule inhibitor of OST, also showed a robust inhibitory effect on arenavirus. Our study provides new insights into LASV GP-host interactions and extends the potential targets for the development of novel therapeutics against Lassa fever in the future.

Identification of Residues in Lassa Virus Glycoprotein Subunit 2 That Are Critical for Protein Function

Lassa virus (LASV) is an Old World arenavirus, endemic to West Africa, capable of causing hemorrhagic fever. Currently, there are no approved vaccines or effective antivirals for LASV. However, thorough understanding of the LASV glycoprotein and entry into host cells could accelerate therapeutic design. LASV entry is a two-step process involving the viral glycoprotein (GP). First, the GP subunit 1 (GP1) binds to the cell surface receptor and the viral particle is engulfed into an endosome. Next, the drop in pH triggers GP rearrangements, which ultimately leads to the GP subunit 2 (GP2) forming a six-helix-bundle (6HB). The process of GP2 forming 6HB fuses the lysosomal membrane with the LASV envelope, allowing the LASV genome to enter the host cell. The aim of this study was to identify residues in GP2 that are crucial for LASV entry. To achieve this, we performed alanine scanning mutagenesis on GP2 residues. We tested these mutant GPs for efficient GP1-GP2 cleavage, cell-to-cell membrane fusion, and transduction into cells expressing α-dystroglycan and secondary LASV receptors. In total, we identified seven GP2 mutants that were cleaved efficiently but were unable to effectively transduce cells: GP-L280A, GP-L285A/I286A, GP-I323A, GP-L394A, GP-I403A, GP-L415A, and GP-R422A. Therefore, the data suggest these residues are critical for GP2 function in LASV entry.

Review of Mammarenavirus Biology and Replication

The family Arenaviridae is divided into three genera: Mammarenavirus, Reptarenavirus, and Hartmanivirus. The Mammarenaviruses contain viruses responsible for causing human hemorrhagic fever diseases including New World viruses Junin, Machupo, Guanarito, Sabia, and Chapare virus and Old World viruses Lassa, and Lujo virus. These two groups of arenaviruses share the same genome organization composed of two ambisense RNA segments. These segments contain four open reading frames that encode for four proteins: the nucleoprotein, glycoprotein precursor, L protein, and Z. Despite their genome similarities, these groups exhibit marked differences in their replication life cycles. This includes differences in attachment, entry, and immune evasion. By understanding the intricacy of replication in each of these viral species we can work to develop counter measures against human diseases. This includes the development of vaccines and antivirals for these emerging viral threats. Currently only the vaccine against Junin virus, Candid#1, is in use as well as Ribavirin for treatment of Lassa Fever. In addition, small molecule inhibitors can be developed to target various aspects of the virus life cycle. In these ways an understanding of the arenavirus replication cycle can be used to alleviate the mortality and morbidity of these infections worldwide.

Screening and Identification of Lassa Virus Entry Inhibitors from an FDA-Approved Drug Library

Lassa virus (LASV) belongs to the Mammarenavirus genus (family Arenaviridae) and causes severe hemorrhagic fever in humans. At present, there are no Food and Drug Administration (FDA)-approved drugs or vaccines specific for LASV. Here, high-throughput screening of an FDA-approved drug library was performed against LASV entry by using pseudotype virus bearing LASV envelope glycoprotein (GPC). Two hit compounds, lacidipine and phenothrin, were identified as LASV entry inhibitors in the micromolar range. A mechanistic study revealed that both compounds inhibited LASV entry by blocking low-pH-induced membrane fusion. Accordingly, lacidipine showed virucidal effects on the pseudotype virus of LASV. Adaptive mutant analyses demonstrated that replacement of T40, located in the ectodomain of the stable-signal peptide (SSP), with lysine (K) conferred LASV resistance to lacidipine. Furthermore, lacidipine showed antiviral activity against LASV, the closely related Mopeia virus (MOPV), and the New World arenavirus Guanarito virus (GTOV). Drug-resistant variants indicated that V36M in the ectodomain of the SSP mutant and V436A in the transmembrane domain of the GP2 mutant conferred GTOV resistance to lacidipine, suggesting the interface between SSP and GP2 is the target of lacidipine. This study shows that lacidipine is a candidate for LASV therapy, reinforcing the notion that the SSP-GP2 interface provides an entry-targeted platform for arenavirus inhibitor design.IMPORTANCE Currently, there is no approved therapy to treat Lassa fever; therefore, repurposing of approved drugs will accelerate the development of a therapeutic stratagem. In this study, we screened an FDA-approved library of drugs and identified two compounds, lacidipine and phenothrin, which inhibited Lassa virus entry by blocking low-pH-induced membrane fusion. Additionally, both compounds extended their inhibition against the entry of Guanarito virus, and the viral targets were identified as the SSP-GP2 interface.

Lassa virus glycoprotein: stopping a moving target

The structure of a prefusion arenavirus GPC was enigmatic for many years, owing to the metastable and non-covalent nature of the association between the receptor binding and fusion subunits. Recent engineering efforts to stabilize the glycoprotein of the Old World arenavirus Lassa in a native, yet cleaved state, allowed the first structure of any arenavirus prefusion GPC trimer to be determined. Comparison of this structure with the structures of other arenavirus glycoprotein subunits reveals surprising findings: that the receptor binding subunit, GP1, of Lassa virus is conformationally labile, while the GP1 subunit of New World arenaviruses is not, and that the arenavirus GPC adopts a trimeric state unlike other glycoproteins with similar fusion machinery. Structural analysis, combined with recent biochemical data regarding antibody epitopes and receptor binding requirements, provides a basis for rational vaccine design.

A new immunization and treatment strategy for mouse mammary tumor virus (MMTV) associated cancers

Mouse Mammary Tumor Virus (MMTV) causes mammary carcinoma or lymphoma in mice. An increasing body of evidence in recent years supports its involvement also in human sporadic breast cancer. It is thus of importance to develop new strategies to impair the development, growth and metastasis of MMTV-associated cancers. The signal peptide of the envelope precursor protein of this virus: MMTV-p14 (p14) is an excellent target for such strategies, due to unique characteristics distinct from its regular endoplasmic reticulum targeting function. These include cell surface expression in: murine cancer cells that harbor the virus, human breast cancer (MCF-7) cells that ectopically express p14, as well as cultured human cells derived from an invasive ductal breast carcinoma positive for MMTV sequences. These findings support its use in signal peptide-based immune targeting. Indeed, priming and boosting mice with p14 elicits a specific anti-signal peptide immune response sufficient for protective vaccination against MMTV-associated tumors. Furthermore, passive immunization using a combination of anti-p14 monoclonal antibodies or the transfer of T-cells from immunized mice (Adoptive Cell Transfer) is also therapeutically effective. With reports demonstrating involvement of MMTV in human breast cancer, we propose the immune-mediated targeting of p14 as a strategy for prevention, treatment and diagnosis of MMTV-associated cancers.

A Cell-Cell Fusion Assay to Assess Arenavirus Envelope Glycoprotein Membrane-Fusion Activity

For many viruses that enter their target cells through pH-dependent fusion of the viral and endosomal membranes, cell-cell fusion assays can provide an experimental platform for investigating the structure-function relationships that promote envelope glycoprotein membrane-fusion activity. Typically, these assays employ effector cells expressing the recombinant envelope glycoprotein on the cell surface and target cells engineered to quantitatively report fusion with the effector cell. In the protocol described here, Vero cells are transfected with a plasmid encoding the arenavirus envelope glycoprotein complex GPC and infected with the vTF7-3 vaccinia virus expressing the bacteriophage T7 RNA polymerase. These effector cells are mixed with target cells infected with the vCB21R-lacZ vaccinia virus encoding a β-galactosidase reporter under the control of the T7 promoter. Cell-cell fusion is induced upon exposure to low-pH medium (pH 5.0), and the resultant expression of the β-galactosidase reporter is quantitated using a chemiluminescent substrate. We have utilized this robust microplate cell-cell fusion assay extensively to study arenavirus entry and its inhibition by small-molecule fusion inhibitors.

Cleavage of the Glycoprotein of Arenaviruses

The arenaviruses are a large family of emerging negative-stranded RNA viruses that include several severe human pathogens causing hemorrhagic fevers with high mortality. During the arenavirus life cycle, processing of the viral envelope glycoprotein precursor (GPC) by the cellular subtilisin kexin isozyme-1 (SKI-1)/site-1 protease (S1P) is crucial for productive infection. The ability of newly emerging arenaviruses to hijack human SKI-1/S1P is a key factor for zoonotic transmission and human disease potential. Apart from being an essential host factor for arenavirus infection, SKI-1/S1P is involved in the regulation of important physiological processes and linked to major human diseases. This chapter provides an overview of the mechanisms of arenavirus GPC processing by SKI-1/S1P including recent findings. We will highlight to what extent the molecular mechanisms of SKI-1/S1P cleavage of viral GPC differ from processing of SKI-1/S1P’s cellular substrates and discuss the implications for virus-host interaction and coevolution. Moreover, we will show how the use of the viral GPC as a “molecular probe” uncovered novel and unusual aspects of SKI-1/S1P biosynthesis and maturation. The crucial role of SKI-1/S1P in arenavirus infection and other major human diseases combined with its nature as an enzyme makes SKI-1/S1P further an attractive target for therapeutic intervention. In the last part, we will therefore cover past and present efforts to identify specific SKI-1/S1P inhibitors.

Epistastic Interactions within the Junín Virus Envelope Glycoprotein Complex Provide an Evolutionary Barrier to Reversion in the Live-Attenuated Candid#1 Vaccine

The Candid#1 strain of Junín virus was developed using a conventional attenuation strategy of serial passage in nonhost animals and cultured cells. The live-attenuated Candid#1 vaccine is used in Argentina to protect at-risk individuals against Argentine hemorrhagic fever, but it has not been licensed in the United States. Recent studies have revealed that Candid#1 attenuation is entirely dependent on a phenylalanine-to-isoleucine substitution at position 427 in the fusion subunit (GP2) of the viral envelope glycoprotein complex (GPC), thereby raising concerns regarding the potential for reversion to virulence. In this study, we report the identification and characterization of an intragenic epistatic interaction between the attenuating F427I mutation in GP2 and a lysine-to-serine mutation at position 33 in the stable signal peptide (SSP) subunit of GPC, and we demonstrate the utility of this interaction in creating an evolutionary barrier against reversion to the pathogenic genotype. In the presence of the wild-type F427 residue, the K33S mutation abrogates the ability of ectopically expressed GPC to mediate membrane fusion at endosomal pH. This defect is rescued by the attenuating F427I mutation. We show that the recombinant Candid#1 (rCan) virus bearing K33S GPC is viable and retains its attenuated genotype under cell culture conditions that readily select for reversion in the parental rCan virus. If back-mutation to F427 offers an accessible pathway to increase fitness in rCan, reversion in K33S-GPC rCan is likely to be lethal. The epistatic interaction between K33S and F427I thus may minimize the likelihood of reversion and enhance safety in a second-generation Candid#1 vaccine.IMPORTANCE The live-attenuated Candid#1 vaccine strain of Junín virus is used to protect against Argentine hemorrhagic fever. Recent findings that a single missense mutation in the viral envelope glycoprotein complex (GPC) is responsible for attenuation raise the prospect of facile reversion to pathogenicity. Here, we characterize a genetic interaction between GPC subunits that evolutionarily forces retention of the attenuating mutation. By incorporating this secondary mutation into Candid#1 GPC, we hope to minimize the likelihood of reversion and enhance safety in a second-generation Candid#1 vaccine. A similar approach may guide the design of live-attenuated vaccines against Lassa and other arenaviral hemorrhagic fevers.

Mutational Analysis of Lassa Virus Glycoprotein Highlights Regions Required for Alpha-Dystroglycan Utilization

Lassa virus (LASV) is an enveloped RNA virus endemic to West Africa and responsible for severe cases of hemorrhagic fever. Virus entry is mediated by the glycoprotein complex consisting of a stable-signal peptide, a receptor-binding subunit, GP1, and a viral-host membrane fusion subunit, GP2. Several cellular receptors can interact with the GP1 subunit and mediate viral entry, including alpha-dystroglycan (αDG) and lysosome-associated membrane protein 1 (LAMP1). In order to define the regions within GP1 that interact with the cellular receptors, we implemented insertional mutagenesis, carbohydrate shielding, and alanine scanning mutagenesis. Eighty GP constructs were engineered and evaluated for GP1-GP2 processing, surface expression, and the ability to mediate cell-to-cell fusion after low-pH exposure. To examine virus-to-cell entry, 49 constructs were incorporated onto vesicular stomatitis virus (VSV) pseudoparticles and transduction efficiencies were monitored in HAP1 and HAP1-ΔDAG1 cells that differentially produce the αDG cell surface receptor. Seven constructs retained efficient transduction in HAP1-ΔDAG1 cells yet poorly transduced HAP1 cells, suggesting that they are involved in αDG utilization. Residues H141, N146, F147, and Y150 cluster at the predicted central core of the trimeric interface and are important for GP-αDG interaction. Additionally, H92A-H93A, 150HA, 172HA, and 230HA displayed reduced transduction in both HAP1 and HAP1-ΔDAG1 cells, despite efficient cell-to-cell fusion activity. These mutations may interfere with interactions with the endosomal receptor LAMP1 or interfere at another stage in entry that is common to both cell lines. Insight gained from these data can aid in the development of more-effective entry inhibitors by blocking receptor interactions.IMPORTANCE Countries in which Lassa virus is endemic, such as Nigeria, Sierra Leone, Guinea, and Liberia, usually experience a seasonal outbreak of the virus from December to March. Currently, there is neither a preventative vaccine nor a therapeutic available to effectively treat severe Lassa fever. One way to thwart virus infection is to inhibit interaction with cellular receptors. It is known that the GP1 subunit of the Lassa glycoprotein complex plays a critical role in receptor recognition. Our results highlight a region within the Lassa virus GP1 protein that interacts with the cellular receptor alpha-dystroglycan. This information may be used for future development of new Lassa virus antivirals.

The N-terminal extension of the P. falciparum GBP130 signal peptide is irrelevant for signal sequence function

The malaria parasite P. falciparum exports a large number of proteins to its host cell, the mature human erythrocyte. Although the function of the majority of these proteins is not well understood, many exported proteins appear to play a role in modification of the erythrocyte following invasion. Protein export to the erythrocyte is a secretory process that begins with entry to the endoplasmic reticulum. For most exported proteins, this step is mediated by hydrophobic signal peptides found towards the N-terminal end of proteins. The signal peptides present on P. falciparum exported proteins often differ in length from those found in other systems, and generally contain a highly extended N-terminal region. Here we have investigated the function of these extended N-terminal regions, using the exported parasite protein GBP130 as a model. Surprisingly, several deletions of the extended N-terminal regions of the GBP130 signal peptide have no effect on the ability of the signal peptide to direct a fluorescent reporter to the secretory pathway. Addition of the same N-terminal extension to a canonical signal peptide does not affect transport of either soluble or membrane proteins to their correct respective subcellular localisations. Finally, we show that extended signal peptides are able to complement canonical signal peptides in driving protein traffic to the apicoplast of the parasite, and are also functional in a mammalian cell system. Our study is the first detailed analysis of an extended P. falciparum signal peptide and suggests that N-terminal extensions of exported Plasmodium falciparum proteins are not required for entry to the secretory system, and are likely to be involved in other, so far unknown, processes.

Structural basis for antibody-mediated neutralization of Lassa virus

The arenavirus Lassa causes severe hemorrhagic fever and a significant disease burden in West Africa every year. The glycoprotein, GPC, is the sole antigen expressed on the viral surface and the critical target for antibody-mediated neutralization. Here we present the crystal structure of the trimeric, prefusion ectodomain of Lassa GP bound to a neutralizing antibody from a human survivor at 3.2-angstrom resolution. The antibody extensively anchors two monomers together at the base of the trimer, and biochemical analysis suggests that it neutralizes by inhibiting conformational changes required for entry. This work illuminates pH-driven conformational changes in both receptor-binding and fusion subunits of Lassa virus, illustrates the unique assembly of the arenavirus glycoprotein spike, and provides a much-needed template for vaccine design against these threats to global health.

Myristoylation of the Arenavirus Envelope Glycoprotein Stable Signal Peptide Is Critical for Membrane Fusion but Dispensable for Virion Morphogenesis

Arenaviruses are responsible for severe and often fatal hemorrhagic disease. In the absence of effective antiviral therapies and vaccines, these viruses pose serious threats to public health and biodefense. Arenaviruses enter the host cell by fusion of the viral and endosomal membranes, a process mediated by the virus envelope glycoprotein GPC. Unlike other class I viral fusion proteins, GPC retains its stable signal peptide (SSP) as an essential third subunit in the mature complex. SSP spans the membrane twice and is myristoylated at its cytoplasmic N terminus. Mutations that abolish SSP myristoylation have been shown to reduce pH-induced cell-cell fusion activity of ectopically expressed GPC to ∼20% of wild-type levels. In order to examine the role of SSP myristoylation in the context of the intact virus, we used reverse genetics to generate Junín viruses (Candid #1 isolate) in which the critical glycine-2 residue in SSP was either replaced by alanine (G2A) or deleted (ΔG2). These mutant viruses produced smaller foci of infection in Vero cells and showed an ∼5-fold reduction in specific infectivity, commensurate with the defect in cell-cell fusion. However, virus assembly and GPC incorporation into budded virions were unaffected. Our findings suggest that the myristate moiety is cryptically disposed in the prefusion GPC complex and may function late in the fusion process to promote merging of the viral and cellular membranes.

IMPORTANCE

Hemorrhagic fever arenaviruses pose significant threats to public health and biodefense. Arenavirus entry into the host cell is promoted by the virus envelope glycoprotein GPC. Unlike other viral envelope glycoproteins, GPC contains a myristoylated stable signal peptide (SSP) as an essential third subunit. Myristoylation has been shown to be important for the membrane fusion activity of recombinantly expressed GPC. Here, we use reverse genetics to study the role of SSP myristoylation in the context of the intact virion. We find that nonmyristoylated GPC mutants of the Candid #1 strain of Junín virus display a commensurate deficiency in their infectivity, albeit without additional defects in virion assembly and budding. These results suggest that SSP myristoylation may function late in the fusion process to facilitate merging of the viral and cellular membranes. Antiviral agents that target this novel aspect of GPC membrane fusion may be useful in the treatment of arenavirus hemorrhagic fevers.

Structure-function relationship of the mammarenavirus envelope glycoprotein

Mammarenaviruses, including lethal pathogens such as Lassa virus and Junín virus, can cause severe hemorrhagic fever in humans. Entry is a key step for virus infection, which starts with binding of the envelope glycoprotein (GP) to receptors on target cells and subsequent fusion of the virus with target cell membranes. The GP precursor is synthesized as a polypeptide, and maturation occurs by two cleavage events, yielding a tripartite GP complex (GPC) formed by a stable signal peptide (SSP), GP1 and GP2. The unique retained SSP interacts with GP2 and plays essential roles in virion maturation and infectivity. GP1 is responsible for binding to the cell receptor, and GP2 is a class I fusion protein. The native structure of the tripartite GPC is unknown. GPC is critical for the receptor binding, membrane fusion and neutralization antibody recognition. Elucidating the molecular mechanisms underlining the structure-function relationship of the three subunits is the key for understanding their function and can facilitate novel avenues for combating virus infections. This review summarizes the basic aspects and recent research of the structure-function relationship of the three subunits. We discuss the structural basis of the receptor-binding domain in GP1, the interaction between SSP and GP2 and its role in virion maturation and membrane fusion, as well as the mechanism by which glycosylation stabilizes the GPC structure and facilitates immune evasion. Understanding the molecular mechanisms involved in these aspects will contribute to the development of novel vaccines and treatment strategies against mammarenaviruses infection.

Small-Molecule Fusion Inhibitors Bind the pH-Sensing Stable Signal Peptide-GP2 Subunit Interface of the Lassa Virus Envelope Glycoprotein

Arenavirus species are responsible for severe life-threatening hemorrhagic fevers in western Africa and South America. Without effective antiviral therapies or vaccines, these viruses pose serious public health and biodefense concerns. Chemically distinct small-molecule inhibitors of arenavirus entry have recently been identified and shown to act on the arenavirus envelope glycoprotein (GPC) to prevent membrane fusion. In the tripartite GPC complex, pH-dependent membrane fusion is triggered through a poorly understood interaction between the stable signal peptide (SSP) and the transmembrane fusion subunit GP2, and our genetic studies have suggested that these small-molecule inhibitors act at this interface to antagonize fusion activation. Here, we have designed and synthesized photoaffinity derivatives of the 4-acyl-1,6-dialkylpiperazin-2-one class of fusion inhibitors and demonstrate specific labeling of both the SSP and GP2 subunits in a native-like Lassa virus (LASV) GPC trimer expressed in insect cells. Photoaddition is competed by the parental inhibitor and other chemically distinct compounds active against LASV, but not those specific to New World arenaviruses. These studies provide direct physical evidence that these inhibitors bind at the SSP-GP2 interface. We also find that GPC containing the uncleaved GP1-GP2 precursor is not susceptible to photo-cross-linking, suggesting that proteolytic maturation is accompanied by conformational changes at this site. Detailed mapping of residues modified by the photoaffinity adducts may provide insight to guide the further development of these promising lead compounds as potential therapeutic agents to treat Lassa hemorrhagic fever.

IMPORTANCE

Hemorrhagic fever arenaviruses cause lethal infections in humans and, in the absence of licensed vaccines or specific antiviral therapies, are recognized to pose significant threats to public health and biodefense. Lead small-molecule inhibitors that target the arenavirus envelope glycoprotein (GPC) have recently been identified and shown to block GPC-mediated fusion of the viral and cellular endosomal membranes, thereby preventing virus entry into the host cell. Genetic studies suggest that these inhibitors act through a unique pH-sensing intersubunit interface in GPC, but atomic-level structural information is unavailable. In this report, we utilize novel photoreactive fusion inhibitors and photoaffinity labeling to obtain direct physical evidence for inhibitor binding at this critical interface in Lassa virus GPC. Future identification of modified residues at the inhibitor-binding site will help elucidate the molecular basis for fusion activation and its inhibition and guide the development of effective therapies to treat arenaviral hemorrhagic fevers.

Crystal structure of the prefusion surface glycoprotein of the prototypic arenavirus LCMV

Arenaviruses exist worldwide and can cause hemorrhagic fever and neurologic disease. A single glycoprotein expressed on the viral surface mediates entry into target cells. This glycoprotein, termed GPC, contains a membrane-associated signal peptide, a receptor-binding subunit termed GP1 and a fusion-mediating subunit termed GP2. Although GPC is a critical target of antibodies and vaccines, the structure of the metastable GP1-GP2 prefusion complex has remained elusive for all arenaviruses. Here we describe the crystal structure of the fully glycosylated prefusion GP1-GP2 complex of the prototypic arenavirus LCMV at 3.5 Å. This structure reveals the conformational changes that the arenavirus glycoprotein must undergo to cause fusion and illustrates the fusion regions and potential oligomeric states.

Acidic pH-Induced Conformations and LAMP1 Binding of the Lassa Virus Glycoprotein Spike

Lassa virus is an enveloped, bi-segmented RNA virus and the most prevalent and fatal of all Old World arenaviruses. Virus entry into the host cell is mediated by a tripartite surface spike complex, which is composed of two viral glycoprotein subunits, GP1 and GP2, and the stable signal peptide. Of these, GP1 binds to cellular receptors and GP2 catalyzes fusion between the viral envelope and the host cell membrane during endocytosis. The molecular structure of the spike and conformational rearrangements induced by low pH, prior to fusion, remain poorly understood. Here, we analyzed the three-dimensional ultrastructure of Lassa virus using electron cryotomography. Sub-tomogram averaging yielded a structure of the glycoprotein spike at 14-Å resolution. The spikes are trimeric, cover the virion envelope, and connect to the underlying matrix. Structural changes to the spike, following acidification, support a viral entry mechanism dependent on binding to the lysosome-resident receptor LAMP1 and further dissociation of the membrane-distal GP1 subunits.

Human hemorrhagic Fever causing arenaviruses: molecular mechanisms contributing to virus virulence and disease pathogenesis

Arenaviruses include multiple human pathogens ranging from the low-risk lymphocytic choriomeningitis virus (LCMV) to highly virulent hemorrhagic fever (HF) causing viruses such as Lassa (LASV), Junin (JUNV), Machupo (MACV), Lujo (LUJV), Sabia (SABV), Guanarito (GTOV), and Chapare (CHPV), for which there are limited preventative and therapeutic measures. Why some arenaviruses can cause virulent human infections while others cannot, even though they are isolated from the same rodent hosts, is an enigma. Recent studies have revealed several potential pathogenic mechanisms of arenaviruses, including factors that increase viral replication capacity and suppress host innate immunity, which leads to high viremia and generalized immune suppression as the hallmarks of severe and lethal arenaviral HF diseases. This review summarizes current knowledge of the roles of each of the four viral proteins and some known cellular factors in the pathogenesis of arenaviral HF as well as of some human primary cell-culture and animal models that lend themselves to studying arenavirus-induced HF disease pathogenesis. Knowledge gained from these studies can be applied towards the development of novel therapeutics and vaccines against these deadly human pathogens.

Current and emerging strategies for the diagnosis, prevention and treatment of Lassa fever

ABSTRACT 

Lassa fever (LF) is a potentially fatal disease that affects an estimated 300,000–500,000 people in endemic areas of west Africa each year. Though past studies have identified fatality rates of 5–20% in patients suspected to have contracted Lassa virus (LASV), new studies using more precise clinical diagnoses and modern diagnostic assays show fatalities rates above 60% in acutely ill patients from endemic regions. Currently, there are no approved vaccines or therapeutics, and only one Comformité Européenne (CE) marked rapid immunodiagnostic for acute LASV infection. Therefore, preventing LASV transmission is the primary goal in endemic regions. Development of rapid immunodiagnostics and research into the efficacy of current treatment options continues toward saving lives in west Africa as well as creating a line of defense against the nefarious use of LASV in bioterrorism settings.

Comparative analysis of disease pathogenesis and molecular mechanisms of New World and Old World arenavirus infections

Arenaviruses can cause fatal human haemorrhagic fever (HF) diseases for which vaccines and therapies are extremely limited. Both the New World (NW) and Old World (OW) groups of arenaviruses contain HF-causing pathogens. Although these two groups share many similarities, important differences with regard to pathogenicity and molecular mechanisms of virus infection exist. These closely related pathogens share many characteristics, including genome structure, viral assembly, natural host selection and the ability to interfere with innate immune signalling. However, members of the NW and OW viruses appear to use different receptors for cellular entry, as well as different mechanisms of virus internalization. General differences in disease signs and symptoms and pathological lesions in patients infected with either NW or OW arenaviruses are also noted and discussed herein. Whilst both the OW Lassa virus (LASV) and the NW Junin virus (JUNV) can cause disruption of the vascular endothelium, which is an important pathological feature of HF, the immune responses to these related pathogens seem to be quite distinct. Whereas LASV infection results in an overall generalized immune suppression, patients infected with JUNV seem to develop a cytokine storm. Additionally, the type of immune response required for recovery and clearance of the virus is different between NW and OW infections. These differences may be important to allow the viruses to evade host immune detection. Understanding these differences will aid the development of new vaccines and treatment strategies against deadly HF viral infections.

Current drug discovery strategies against arenavirus infections

Arenaviruses are a large group of emerging viruses including several causative agents of severe hemorrhagic fevers with high mortality in man. Considering the number of people affected and the currently limited therapeutic options, novel efficacious therapeutics against arenaviruses are urgently needed. Over the past decade, significant advances in knowledge about the basic virology of arenaviruses have been accompanied by the development of novel therapeutics targeting different steps of the arenaviral life cycle. High-throughput, small-molecule screens identified potent and broadly active inhibitors of arenavirus entry that were instrumental for the dissection of unique features of arenavirus fusion. Novel inhibitors of arenavirus replication have been successfully tested in animal models and hold promise for application in humans. Late in the arenavirus life cycle, the proteolytic processing of the arenavirus envelope glycoprotein precursor and cellular factors critically involved virion assembly and budding provide further promising 'druggable' targets for novel therapeutics to combat human arenavirus infection.

Viral envelope glycoprotein processing by proprotein convertases

The proprotein convertases (PCs) are a family of nine mammalian enzymes that play key roles in the maintenance of cell homeostasis by activating or inactivating proteins via limited proteolysis under temporal and spatial control. A wide range of pathogens, including major human pathogenic viruses can hijack cellular PCs for their own purposes. In particular, productive infection with many enveloped viruses critically depends on the processing of their fusion-active viral envelope glycoproteins by cellular PCs. Based on their crucial role in virus-host interaction, PCs can be important determinants for viral pathogenesis and represent promising targets of therapeutic antiviral intervention. In the present review we will cover basic aspects and recent developments of PC-mediated maturation of viral envelope glycoproteins of selected medically important viruses. The molecular mechanisms underlying the recognition of PCs by viral glycoproteins will be described, including recent findings demonstrating differential PC-recognition of viral and cellular substrates. We will further discuss a possible scenario how viruses during co-evolution with their hosts adapted their glycoproteins to modulate the activity of cellular PCs for their own benefit and discuss the consequences for virus-host interaction and pathogenesis. Particular attention will be given to past and current efforts to evaluate cellular PCs as targets for antiviral therapeutic intervention, with emphasis on emerging highly pathogenic viruses for which no efficacious drugs or vaccines are currently available.

Arenavirus budding: a common pathway with mechanistic differences

The Arenaviridae is a diverse and growing family of viruses that includes several agents responsible for important human diseases. Despite the importance of this family for public health, particularly in Africa and South America, much of its biology remains poorly understood. However, in recent years significant progress has been made in this regard, particularly relating to the formation and release of new enveloped virions, which is an essential step in the viral lifecycle. While this process is mediated chiefly by the viral matrix protein Z, recent evidence suggests that for some viruses the nucleoprotein (NP) is also required to enhance the budding process. Here we highlight and compare the distinct budding mechanisms of different arenaviruses, concentrating on the role of the matrix protein Z, its known late domain sequences, and the involvement of cellular endosomal sorting complex required for transport (ESCRT) pathway components. Finally we address the recently described roles for the nucleoprotein NP in budding and ribonucleoprotein complex (RNP) incorporation, as well as discussing possible mechanisms related to its involvement.

Targeting virulence mechanisms for the prevention and therapy of arenaviral hemorrhagic fever

A number of arenaviruses are pathogenic for humans, but they differ significantly in virulence. Lassa virus, found in West Africa, causes severe hemorrhagic fever (HF), while the other principal Old World arenavirus, lymphocytic choriomeningitis virus, causes mild illness in persons with normal immune function, and poses a threat only to immunocompromised individuals. The New World agents, including Junin, Machupo and Sabia virus, are highly pathogenic for humans. Arenaviral HF is characterized by high viremia and general immune suppression, the mechanism of which is unknown. Studies using viral reverse genetics, cell-based assays, animal models and human genome-wide association analysis have revealed potential mechanisms by which arenaviruses cause severe disease in humans. Each of the four viral gene products (GPC, L polymerase, NP, and Z matrix protein) and several host-cell factors (e.g., α-dystroglycan) are responsible for mediating viral entry, genome replication, and the inhibition of apoptosis, translation and interferon-beta (IFNβ) production. This review summarizes current knowledge of the role of each viral protein and host factor in the pathogenesis of arenaviral HF. Insights from recent studies are being exploited for the development of novel therapies.

The role of proteolytic processing and the stable signal peptide in expression of the Old World arenavirus envelope glycoprotein ectodomain

Maturation of the arenavirus GP precursor (GPC) involves proteolytic processing by cellular signal peptidase and the proprotein convertase subtilisin kexin isozyme 1 (SKI-1)/site 1 protease (S1P), yielding a tripartite complex comprised of a stable signal peptide (SSP), the receptor-binding GP1, and the fusion-active transmembrane GP2. Here we investigated the roles of SKI-1/S1P processing and SSP in the biosynthesis of the recombinant GP ectodomains of lymphocytic choriomeningitis virus (LCMV) and Lassa virus (LASV). When expressed in mammalian cells, the LCMV and LASV GP ectodomains underwent processing by SKI-1/S1P, followed by dissociation of GP1 from GP2. The GP2 ectodomain spontaneously formed trimers as revealed by chemical cross-linking. The endogenous SSP, known to be crucial for maturation and transport of full-length arenavirus GPC was dispensable for processing and secretion of the soluble GP ectodomain, suggesting a specific role of SSP in the stable prefusion conformation and transport of full-length GPC.

Multifunctional nature of the arenavirus RING finger protein Z

Arenaviruses are a family of enveloped negative-stranded RNA viruses that can cause severe human disease ranging from encephalitis symptoms to fulminant hemorrhagic fever. The bi‑segmented RNA genome encodes four polypeptides: the nucleoprotein NP, the surface glycoprotein GP, the polymerase L, and the RING finger protein Z. Although it is the smallest arenavirus protein with a length of 90 to 99 amino acids and a molecular weight of approx. 11 kDa, the Z protein has multiple functions in the viral life cycle including (i) regulation of viral RNA synthesis, (ii) orchestration of viral assembly and budding, (iii) interaction with host cell proteins, and (iv) interferon antagonism. In this review, we summarize our current understanding of the structural and functional role of the Z protein in the arenavirus replication cycle.

Envelope glycoprotein of arenaviruses

Arenaviruses include lethal human pathogens which pose serious public health threats. So far, no FDA approved vaccines are available against arenavirus infections, and therapeutic options are limited, making the identification of novel drug targets for the development of efficacious therapeutics an urgent need. Arenaviruses are comprised of two RNA genome segments and four proteins, the polymerase L, the envelope glycoprotein GP, the matrix protein Z, and the nucleoprotein NP. A crucial step in the arenavirus life-cycle is the biosynthesis and maturation of the GP precursor (GPC) by cellular signal peptidases and the cellular enzyme Subtilisin Kexin Isozyme-1 (SKI-1)/Site-1 Protease (S1P) yielding a tripartite mature GP complex formed by GP1/GP2 and a stable signal peptide (SSP). GPC cleavage by SKI-1/S1P is crucial for fusion competence and incorporation of mature GP into nascent budding virion particles. In a first part of our review, we cover basic aspects and newer developments in the biosynthesis of arenavirus GP and its molecular interaction with SKI-1/S1P. A second part will then highlight the potential of SKI-1/S1P-mediated processing of arenavirus GPC as a novel target for therapeutic intervention to combat human pathogenic arenaviruses.

Drug discovery technologies and strategies for Machupo virus and other New World arenaviruses

INTRODUCTION

Seven arenaviruses cause viral hemorrhagic fever in humans: the Old World arenaviruses Lassa and Lujo, and the New World Clade B arenaviruses Machupo (MACV), Junín (JUNV), Guanarito (GTOV), Sabiá (SABV), and Chapare (CHPV). All of these viruses are Risk Group 4 biosafety pathogens. MACV causes human disease outbreak with high case-fatality rates. To date, at least 1,200 cases with ≈200 fatalities have been recorded.

AREAS COVERED

This review summarizes available systems and technologies for the identification of antivirals against MACV. Furthermore, the article summarizes animal models that have been used for the in vivo evaluation of novel inhibitors. The article highlights present treatments for arenaviral diseases and provides an overview of efficacious small molecules and other therapeutics reported to date. Finally, the article summarizes strategies to identify novel inhibitors for anti-arenaviral therapy.

EXPERT OPINION

New high-throughput approaches to quantitate infection rates of arenaviruses, as well as viruses modified to carry reporter genes, will accelerate compound screens and drug discovery efforts. RNAi, gene expression profiling and proteomics studies will identify host targets for therapeutic intervention. New discoveries in the cell entry mechanism of MACV and other arenaviruses as well as extensive structural studies of arenaviral L and NP could facilitate the rational design of antivirals effective against all pathogenic New World arenaviruses.

Dissection of the role of the stable signal peptide of the arenavirus envelope glycoprotein in membrane fusion

The arenavirus envelope glycoprotein (GPC) retains a stable signal peptide (SSP) as an essential subunit in the mature complex. The 58-amino-acid residue SSP comprises two membrane-spanning hydrophobic regions separated by a short ectodomain loop that interacts with the G2 fusion subunit to promote pH-dependent membrane fusion. Small-molecule compounds that target this unique SSP-G2 interaction prevent arenavirus entry and infection. The interaction between SSP and G2 is sensitive to the phylogenetic distance between New World (Junín) and Old World (Lassa) arenaviruses. For example, heterotypic GPC complexes are unable to support virion entry. In this report, we demonstrate that the hybrid GPC complexes are properly assembled, proteolytically cleaved, and transported to the cell surface but are specifically defective in their membrane fusion activity. Chimeric SSP constructs reveal that this incompatibility is localized to the first transmembrane segment of SSP (TM1). Genetic changes in TM1 also affect sensitivity to small-molecule fusion inhibitors, generating resistance in some cases and inhibitor dependence in others. Our studies suggest that interactions of SSP TM1 with the transmembrane domain of G2 may be important for GPC-mediated membrane fusion and its inhibition.

Diversity and phylogenetic relationships among the North American Tacaribe serocomplex viruses (family Arenaviridae)

The purpose of this study was to extend our knowledge of the genetic diversity and phylogenetic relationships among the North American Tacaribe serocomplex viruses. Analyses of glycoprotein precursor gene sequence data separated the North American arenaviruses into 7 major phylogenetic groups. The results of analyses of Z gene and nucleocapsid protein gene sequence data were not remarkably different from the glycoprotein precursor gene tree. In contrast, the tree generated from RNA-dependent RNA polymerase gene sequences differed from the glycoprotein precursor gene tree with regard to phylogenetic relationships among the viruses associated with woodrats captured in the western United States, Texas, or northern Mexico. Further analyses of the polymerase gene sequence data set suggested that the difference in topology was a consequence of incongruence among the gene tree data sets or chance rather than genetic reassortment or recombination between arenaviruses.