NRP2 and CD63 Are Host Factors for Lujo Virus Cell Entry

Establishment of replication-competent vesicular stomatitis virus recapitulating SADS-CoV entry

Zoonotic coronaviruses pose a continuous threat to human health, with newly identified bat-borne viruses like swine acute diarrhea syndrome coronavirus (SADS-CoV) causing high mortality in piglets. In vitro studies indicate that SADS-CoV can infect cell lines from diverse species, including humans, highlighting its potential risk to human health. However, the lack of tools to study viral entry, along with the absence of vaccines or antiviral therapies, perpetuates this threat. To address this, we engineered an infectious molecular clone of Vesicular Stomatitis Virus (VSV), replacing its native glycoprotein (G) with SADS-CoV spike (S) and inserting a Venus reporter at the 3' leader region to generate a replication-competent rVSV-Venus-SADS S virus. Serial passages of rVSV-Venus-SADS S led to the identification of an 11-amino-acid truncation in the cytoplasmic tail of the S protein, which allowed more efficient viral propagation due to increased cell membrane anchoring of the S protein. The S protein was integrated into rVSV-Venus-SADS SΔ11 particles, susceptible to neutralization by sera from SADS-CoV S1 protein-immunized rabbits. Additionally, we found that TMPRSS2 promotes SADS-CoV spike-mediated cell entry. Furthermore, we assessed the serum-neutralizing ability of mice vaccinated with rVSV-Venus-SADS SΔ11 using a prime-boost immunization strategy, revealing effective neutralizing antibodies against SADS-CoV infection. In conclusion, we have developed a safe and practical tool for studying SADS-CoV entry and exploring the potential of a recombinant VSV-vectored SADS-CoV vaccine.IMPORTANCEZoonotic coronaviruses, like swine acute diarrhea syndrome coronavirus (SADS-CoV), pose a continual threat to human and animal health. To combat this, we engineered a safe and efficient tool by modifying the Vesicular Stomatitis Virus (VSV), creating a replication-competent rVSV-Venus-SADS S virus. Through serial passages, we optimized the virus for enhanced membrane anchoring, a key factor in viral propagation. This modified virus, rVSV-Venus-SADS SΔ11, proved susceptible to neutralization, opening avenues for potential vaccines. Additionally, our study revealed the role of TMPRSS2 in SADS-CoV entry. Mice vaccinated with rVSV-Venus-SADS SΔ11 developed potent neutralizing antibodies against SADS-CoV. In conclusion, our work presents a secure and practical tool for studying SADS-CoV entry and explores the promise of a recombinant VSV-vectored SADS-CoV vaccine.

A comprehensive review on targeting cluster of differentiation: An attractive strategy for inhibiting viruses through host proteins

Viral infections continue to pose a significant global public health threat. Targeting host proteins, such as cluster of differentiation (CD) macromolecules, may offer a promising alternative approach to developing antiviral treatments. CDs are cell-surface biological macromolecules mainly expressed on leukocytes that viruses can use to enter cells, thereby evading immune detection and promoting their replication. The manipulation of CDs by viruses may represent an effective and clever means of survival through the prolonged co-evolution of hosts and viruses. Targeting of CDs is anticipated to hinder the invasion of related viruses, modulate the body's immune system, and diminish the incidence of subsequent inflammation. They have become crucial for biomedical diagnosis, and some have been used as valuable tools for resisting viral infections. However, a summary of the structures and functions of CDs involved in viral infection is currently lacking. The development of drugs targeting these biological macromolecules is restricted both in terms of their availability and the number of compounds currently identified. This review provides a comprehensive analysis of the critical role of CD proteins in virus invasion and a list of relevant targeted antiviral agents, which will serve as a valuable reference for future research in this field.

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

INTRODUCTION

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

AREAS COVERED

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

EXPERT OPINION

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

Entry inhibitors as arenavirus antivirals

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

Deficiency of inflammation-sensing protein neuropilin-2 in myeloid-derived macrophages exacerbates colitis via NF-κB activation

Neuropilin-2 (NRP2) is a multifunctional protein engaged in the regulation of angiogenesis, lymphangiogenesis, axon guidance, and tumor metastasis, but its function in colitis remains unclear. Here, we found that NRP2 was an inflammation-sensing protein rapidly and dramatically induced in myeloid cells, especially in macrophages, under inflammatory contexts. NRP2 deficiency in myeloid cells exacerbated dextran sulfate sodium salt-induced experimental colitis by promoting polarization of M1 macrophages and colon injury. Mechanistically, NRP2 could be induced via NF-κB activation by TNF-α in macrophages, but exerted an inhibitory effect on NF-κB signaling, forming a negative feedback loop with NF-κB to sense and alleviate inflammation. Deletion of NRP2 in macrophages broke this negative feedback circuit, leading to NF-κB overactivation, inflammatory exacerbation, and more severe colitis. Collectively, these findings reveal inflammation restriction as a role for NRP2 in macrophages under inflammation contexts and suggest that NRP2 in macrophages may relieve inflammation in inflammatory bowel disease. © 2023 The Pathological Society of Great Britain and Ireland.

In Silico Prediction and Molecular Docking of SNPs in NRP1 Gene Associated with SARS-COV-2

Neuropilin-1 (NRP1) which is a main transmembrane cell surface receptor acts as a host cell mediator resulting in increasing the SARS-Cov-2 infectivity and also plays a role in neuronal development, angiogenesis and axonal outgrowth. The goal of this study is to estimate the impact of single nucleotide polymorphisms (SNPs) in the NRP1 gene on the function, structure and stabilization of protein as well as on the miRNA-mRNA binding regions using bioinformatical tools. It is also aimed to investigate the changes caused by SNPs in NRP1 on interactions with drug molecule and spike protein. The missense type of SNPs was analyzed using SIFT, PolyPhen-2, SNAP2, PROVEAN, Mutation Assessor, SNPs&GO, PhD-SNP, I-Mutant 3.0, MUpro, STRING, Project HOPE, ConSurf, and PolymiRTS. Docking analyses were conducted by AutoDock Vina program. As a result, a total of 733 missense SNPs were determined within the NRP1 gene and nine SNPs were specified as damaging to the protein. The modelling results showed that wild and mutant type amino acids had some different properties such as size, charge, and hydrophobicity. Additionally, their three-dimensional structures of protein were utilized for confirmation of these differences. After evaluating the results, nine polymorphisms rs141633354, rs142121081, rs145954532, rs200028992, rs200660300, rs369312020, rs370117610, rs370551432, rs370641686 were determined to be damaging on the structure and function of NRP1 protein and located in conserved regions. The results of molecular docking showed that the binding affinity values are nearly the same for wild-type and mutant structures support that the mutations carried out are not in the focus of the binding site, therefore the ligand does not affect the binding energy. It is expected that the results will be useful for future studies.

The Arenaviridae Family: Knowledge Gaps, Animal Models, Countermeasures, and Prototype Pathogens

Lassa virus (LASV), Junin virus (JUNV), and several other members of the Arenaviridae family are capable of zoonotic transfer to humans and induction of severe viral hemorrhagic fevers. Despite the importance of arenaviruses as potential pandemic pathogens, numerous gaps exist in scientific knowledge pertaining to this diverse family, including gaps in understanding replication, immunosuppression, receptor usage, and elicitation of neutralizing antibody responses, that in turn complicates development of medical countermeasures. A further challenge to the development of medical countermeasures for arenaviruses is the requirement for use of animal models at high levels of biocontainment, where each model has distinct advantages and limitations depending on, availability of space, animals species-specific reagents, and most importantly the ability of the model to faithfully recapitulate human disease. Designation of LASV and JUNV as prototype pathogens can facilitate progress in addressing the public health challenges posed by members of this important virus family.

Inhibitors of cannabinoid receptor 1 suppress the cellular entry of Lujo virus

Lujo virus (LUJV), which belongs to Mammarenavirus, family Arenaviridae, has emerged as a pathogen causing severe hemorrhagic fever with high mortality. Currently, there are no effective treatments for arenaviruses, including LUJV. Here, we screened chemical compound libraries of Food and Drug Administration (FDA)-approved drugs and G protein-coupled receptor-associated drugs to identify effective antivirals against LUJV targeting cell entry using a vesicular stomatitis virus-based pseudotyped virus bearing the LUJV envelope glycoprotein (GP). Cannabinoid receptor 1 (CB1) antagonists, such as rimonabant, AM251 and AM281, have been identified as robust inhibitors of LUJV entry. The IC50 of rimonabant was 0.26 and 0.53 μM in Vero and Huh7 cells, respectively. Analysis of the cell fusion activity of the LUJV GP in the presence of CB1 inhibitors revealed that these inhibitors suppressed the fusion activity of the LUJV GP. Moreover, rimonabant, AM251 and AM281 reduced the infectivity of authentic LUJV in vitro, suggesting that the antiviral activity of CB1 antagonists against LUJV is mediated, at least in part, by inhibition of the viral entry, especially, membrane fusion. These findings suggest promising candidates for developing new therapies against LUJV infections.

A novel in vitro system of supported planar endosomal membranes (SPEMs) reveals an enhancing role for cathepsin B in the final stage of Ebola virus fusion and entry

Ebola virus (EBOV) causes a hemorrhagic fever with fatality rates up to 90%. The EBOV entry process is complex and incompletely understood. Following attachment to host cells, EBOV is trafficked to late endosomes/lysosomes where its glycoprotein (GP) is processed to a 19-kDa form, which binds to the EBOV intracellular receptor Niemann-Pick type C1. We previously showed that the cathepsin protease inhibitor, E-64d, blocks infection by pseudovirus particles bearing 19-kDa GP, suggesting that further cathepsin action is needed to trigger fusion. This, however, has not been demonstrated directly. Since 19-kDa Ebola GP fusion occurs in late endosomes, we devised a system in which enriched late endosomes are used to prepare supported planar endosomal membranes (SPEMs), and fusion of fluorescent (pseudo)virus particles is monitored by total internal reflection fluorescence microscopy. We validated the system by demonstrating the pH dependencies of influenza virus hemagglutinin (HA)-mediated and Lassa virus (LASV) GP-mediated fusion. Using SPEMs, we showed that fusion mediated by 19-kDa Ebola GP is dependent on low pH, enhanced by Ca2+, and augmented by the addition of cathepsins. Subsequently, we found that E-64d inhibits full fusion, but not lipid mixing, mediated by 19-kDa GP, which we corroborated with the reversible cathepsin inhibitor VBY-825. Hence, we provide both gain- and loss-of-function evidence that further cathepsin action enhances the fusion activity of 19-kDa Ebola GP. In addition to providing new insights into how Ebola GP mediates fusion, the approach we developed employing SPEMs can now be broadly used for studies of virus and toxin entry through endosomes. IMPORTANCE Ebola virus is the causative agent of Ebola virus disease, which is severe and frequently lethal. EBOV gains entry into cells via late endosomes/lysosomes. The events immediately preceding fusion of the viral and endosomal membranes are incompletely understood. In this study, we report a novel in vitro system for studying virus fusion with endosomal membranes. We validated the system by demonstrating the low pH dependencies of influenza and Lassa virus fusion. Moreover, we show that further cathepsin B action enhances the fusion activity of the primed Ebola virus glycoprotein. Finally, this model endosomal membrane system should be useful in studying the mechanisms of bilayer breaching by other enveloped viruses, by non-enveloped viruses, and by acid-activated bacterial toxins.

A structural perspective on the evolution of viral/cellular macromolecular complexes within the arenaviridae family of viruses

Viruses are obligatory parasites that can replicate only inside host cells. Therefore, the evolutionary drive to enter cells is immense, leading to diversification in the cell-entry strategies of viruses. One of the most critical steps for cell entry is the recognition of the target cell, a process driven by the formation of viral/host macromolecular complexes. The accumulation of recent structural data for viruses within the arenaviridae family allows us to examine how different viral species from the same viral family utilize evolutionarily-related viral glycoproteins to engage with a variety of different cellular receptors. These structural data, compared to other viruses from the coronaviridae family, hint about possible routes that such viruses use for evolving new receptor-binding capabilities, allowing them to switch from one receptor to another.

Replicating-Competent VSV-Vectored Pseudotyped Viruses

Vesicular stomatitis virus (VSV) is prototype virus in the family of Rhabdoviridae. Reverse genetic platform has enabled the genetic manipulation of VSV as a powerful live viral vector. Replicating-competent VSV is constructed by replacing the original VSV glycoprotein gene with heterologous envelope genes. The resulting recombinant viruses are able to replicate in permissive cells and incorporate the foreign envelope proteins on the surface of the viral particle without changing the bullet-shape morphology. Correspondingly, the cell tropism of replicating-competent VSV is determined by the foreign envelope proteins. Replicating-competent VSVs have been successfully used for selecting critical viral receptors or host factors, screening mutants that escape therapeutic antibodies, and developing VSV-based live viral vaccines.

Arenaviruses: Old viruses present new solutions for cancer therapy

Viral-based cancer therapies have tremendous potential, especially in the context of treating poorly infiltrated cold tumors. However, in tumors with intact anti-viral interferon (IFN) pathways, while some oncolytic viruses induce strong innate and adaptive immune responses, they are neutralized before exerting their therapeutic effect. Arenaviruses, particularly the lymphocytic choriomeningitis virus (LCMV) is a noncytopathic virus with preferential cancer tropism and evolutionary mechanisms to escape the immune system for longer and to block early clearance. These escape mechanisms include inhibition of the MAVS dependent IFN pathway and spike protein antigen masking. Regarding its potential for cancer treatment, LCMV is therefore able to elicit long-term responses within the tumor microenvironment (TME), boost anti-tumor immune responses and polarize poorly infiltrating tumors towards a hot phenotype. Other arenaviruses including the attenuated Junin virus vaccine also have anti-tumor effects. Furthermore, the LCMV and Pichinde arenaviruses are currently being used to create vector-based vaccines with attenuated but replicating virus. This review focuses on highlighting the potential of arenaviruses as anti-cancer therapies. This includes providing a molecular understanding of its tropism as well as highlighting past and present preclinical and clinical applications of noncytophatic arenavirus therapies and their potential in bridging the gap in the treatment of cancers weakly responsive or unresponsive to oncolytic viruses. In summary, arenaviruses represent promising new therapies to broaden the arsenal of anti-tumor therapies for generating an immunogenic tumor microenvironment.

LAMP3/CD63 Expression in Early and Late Endosomes in Human Vaginal Epithelial Cells Is Associated with Enhancement of HSV-2 Infection

Herpes simplex virus 2 (HSV-2) is a lifelong sexually transmitted virus that disproportionately infects women through heterosexual transmission in the vaginal tract. The vaginal epithelium is known to be highly susceptible to HSV-2 infection; however, the cellular mechanism of HSV-2 uptake and replication in vaginal epithelium has not been extensively studied. Previously, we observed that lysosomal-associated membrane protein-3 (LAMP3/CD63) was among the highly upregulated genes during HSV-2 infection of human vaginal epithelial cell line VK2, leading us to posit that LAMP3/CD63 may play a role in HSV-2 infection. Consequently, we generated two gene-altered VK2-derived cell lines, a LAMP3-overexpressed (OE) line and a LAMP3 knockout (KO) line. The wild-type VK2 and the LAMP3 OE and KO cell lines were grown in air-liquid interface (ALI) cultures for 7 days and infected with HSV-2. Twenty-four hours postinfection, LAMP3 OE cells produced and released significantly higher numbers of HSV-2 virions than wild-type VK2 cells, while virus production was greatly attenuated in LAMP3 KO cells, indicating a functional association between LAMP3/CD63 expression and HSV-2 replication. Fluorescence microscopy of HSV-2-infected cells revealed that HSV-2 colocalized with LAMP3 in both early endosomes and lysosomal compartments. In addition, blocking endosomal maturation or late endosomal/lysosomal fusion using specific inhibitors resulted in reduced HSV-2 replication in VK2 cells. Similarly, LAMP3 KO cells exhibited very low viral entry and association with endosomes, while LAMP3 OE cells demonstrated large amounts of virus that colocalized with LAMP3/CD63 in endosomes and lysosomes. IMPORTANCE Collectively, these results showed that HSV-2 is taken up by human vaginal epithelial cells through an endosomal-lysosomal pathway in association with LAMP3, which plays a crucial role in the enhancement of HSV-2 replication. These findings provide the basis for the future design of antiviral agents for prophylactic measures against HSV-2 infection.

ACE2-Independent Alternative Receptors for SARS-CoV-2

Severe acute respiratory syndrome-related coronavirus (SARS-CoV-2), the causative agent of coronavirus disease 2019 (COVID-19), is highly contagious and remains a major public health challenge despite the availability of effective vaccines. SARS-CoV-2 enters cells through the binding of its spike receptor-binding domain (RBD) to the human angiotensin-converting enzyme 2 (ACE2) receptor in concert with accessory receptors/molecules that facilitate viral attachment, internalization, and fusion. Although ACE2 plays a critical role in SARS-CoV-2 replication, its expression profiles are not completely associated with infection patterns, immune responses, and clinical manifestations. Additionally, SARS-CoV-2 infects cells that lack ACE2, and the infection is resistant to monoclonal antibodies against spike RBD in vitro, indicating that some human cells possess ACE2-independent alternative receptors, which can mediate SARS-CoV-2 entry. Here, we discuss these alternative receptors and their interactions with SARS-CoV-2 components for ACE2-independent viral entry. These receptors include CD147, AXL, CD209L/L-SIGN/CLEC4M, CD209/DC-SIGN/CLEC4L, CLEC4G/LSECtin, ASGR1/CLEC4H1, LDLRAD3, TMEM30A, and KREMEN1. Most of these receptors are known to be involved in the entry of other viruses and to modulate cellular functions and immune responses. The SARS-CoV-2 omicron variant exhibits altered cell tropism and an associated change in the cell entry pathway, indicating that emerging variants may use alternative receptors to escape the immune pressure against ACE2-dependent viral entry provided by vaccination against RBD. Understanding the role of ACE2-independent alternative receptors in SARS-CoV-2 viral entry and pathogenesis may provide avenues for the prevention of infection by SARS-CoV-2 variants and for the treatment of COVID-19.

Primate hemorrhagic fever-causing arteriviruses are poised for spillover to humans

Simian arteriviruses are endemic in some African primates and can cause fatal hemorrhagic fevers when they cross into primate hosts of new species. We find that CD163 acts as an intracellular receptor for simian hemorrhagic fever virus (SHFV; a simian arterivirus), a rare mode of virus entry that is shared with other hemorrhagic fever-causing viruses (e.g., Ebola and Lassa viruses). Further, SHFV enters and replicates in human monocytes, indicating full functionality of all of the human cellular proteins required for viral replication. Thus, simian arteriviruses in nature may not require major adaptations to the human host. Given that at least three distinct simian arteriviruses have caused fatal infections in captive macaques after host-switching, and that humans are immunologically naive to this family of viruses, development of serology tests for human surveillance should be a priority.

Correlations between Cytokine Levels, Liver Function Markers, and Neuropilin-1 Expression in Patients with COVID-19

Aim: The study evaluated the correlations between cytokine levels, liver function markers, and neuropilin-1 (NRP-1) expression in patients with COVID-19 in Egypt. The study also aimed to evaluate the accuracy sensitivity, specificity, and area under the curve (AUC) of the tested laboratory parameters in identifying COVID-19 infection and its severity. Patients and Methods: Fifty healthy subjects and 100 confirmed patients with COVID-19 were included in this study. COVID-19 patients were separated into two groups based on the severity of their symptoms. Serum ALT, AST, albumin, C-reactive protein (CRP), interleukin (IL)-1β, IL-4, IL-6, IL-18, IL-35, prostaglandin E2 (PGE2), and thromboxane A2 (TXA2) were estimated. We measured the gene expression for nuclear factor-kappa B p50 (NF-κB p50) and nuclear factor-kappa B p65 (NF-κB p65) and NRP-1 in blood samples using quantitative real-time polymerase chain reaction (qRT-PCR). AUC and sensitivity and specificity for cytokine levels and NF-κB p50 and NF-κB p65 and NRP-1 in identifying COVID-19 infection were also determined in both moderate and severe patient groups using receiver-operating characteristic curve (ROC) analysis. Results: All patients with COVID-19 showed higher serum activities of liver enzymes, levels of CRP, IL-1β, IL-4, IL-6, IL-18, IL-35 PGE2, and TXA2, and mRNA expression of NF-κB p50, NF-κB p65, and NRP-1 than healthy subjects. The severe group exhibited a significant increase in serum ALT, AST and IL-6 and a significant decrease in albumin, IL-1β, TXA2, and NF-κB p65 levels compared to the moderate group. In all patients (moderate and severe), all cytokines were positively correlated with NF-κB p50, NF-κB p65 and NRP-1 expression levels. Serum ALT and AST were positively correlated with CRP, cytokines (IL-4, IL-6, IL-18, IL-35 and TXA2), and NF-κB p50 and NF-κB p65 expression levels in both moderate and severe groups. They were also positively correlated with serum IL-1β level in the severe COVID-19 patient group and with NRP-1 expression in the moderate group. Using the logistic regression analysis, the most important four statistically significant predictors associated with COVID-19 infection in the study were found to be IL-6, TAX2, NF-κB p50 and NF-κB p65. ROC analysis of these variables revealed that three of them had AUC > 0.8. In moderate cases, AUC of the serum TXA2 level and NF-κB p65 expression were 0.843 (95% CI 0.517−0.742, p < 0.001) and 0.806 (95% CI 0.739−0.874, p < 0.001), respectively. In the severe group, AUC of serum IL-6 level was 0.844 (95% CI 0.783−0.904, p < 0.001). Moreover, Il-6 had a sensitivity of 100% in both moderate and severe groups. Conclusions: This study concluded that liver injury in patients with COVID-19 may be strongly attributed to the cytokines storm, especially IL-6, which was positively correlated to NF-κB p50, NF-κB p65 and NRP-1 mRNA expression levels. Moreover, ROC analysis revealed that IL-6, TXA2, and NF-κB p65 could be useful in predicting the possibility of infection with COVID-19, and IL-6 could be of possible significance as a good predictor of the severity and disease progress. However, RT-qPCR for SARS-CoV-2 detection is essential to confirm infection and further clinical studies are required to confirm this elucidation.

The Bovine Seminal Plasma Protein PDC-109 Possesses Pan-Antiviral Activity

Mammalian seminal plasma contains a multitude of bioactive components, including lipids, glucose, mineral elements, metabolites, proteins, cytokines, and growth factors, with various functions during insemination and fertilization. The seminal plasma protein PDC-109 is one of the major soluble components of the bovine ejaculate and is crucially important for sperm motility, capacitation, and acrosome reaction. A hitherto underappreciated function of seminal plasma is its anti-microbial and antiviral activity, which may limit the sexual transmission of infectious diseases during intercourse. We have recently discovered that PDC-109 inhibits the membrane fusion activity of influenza virus particles and significantly impairs viral infections at micromolar concentrations. Here we investigated whether the antiviral activity of PDC-109 is restricted to Influenza or if other mammalian viruses are similarly affected. We focused on Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2), the etiological agent of the Coronavirus Disease 19 (COVID-19), thoroughly assessing PDC-109 inhibition with SARS-CoV-2 Spike (S)-pseudotyped reporter virus particles, but also live-virus infections. Consistent with our previous publications, we found significant virus inhibition, albeit accompanied by substantial cytotoxicity. However, using time-of-addition experiments we discovered a treatment regimen that enables virus suppression without affecting cell viability. We furthermore demonstrated that PDC-109 is also able to impair infections mediated by the VSV glycoprotein (VSVg), thus indicating a broad pan-antiviral activity against multiple virus species and families.

Neuropilin (NRPs) Related Pathological Conditions and Their Modulators

Neuropilin 1 (NRP1) represents one of the two homologous neuropilins (NRP, splice variants of neuropilin 2 are the other) found in all vertebrates. It forms a transmembrane glycoprotein distributed in many human body tissues as a (co)receptor for a variety of different ligands. In addition to its physiological role, it is also associated with various pathological conditions. Recently, NRP1 has been discovered as a coreceptor for the SARS-CoV-2 viral entry, along with ACE2, and has thus become one of the COVID-19 research foci. However, in addition to COVID-19, the current review also summarises its other pathological roles and its involvement in clinical diseases like cancer and neuropathic pain. We also discuss the diversity of native NRP ligands and perform a joint analysis. Last but not least, we review the therapeutic roles of NRP1 and introduce a series of NRP1 modulators, which are typical peptidomimetics or other small molecule antagonists, to provide the medicinal chemistry community with a state-of-the-art overview of neuropilin modulator design and NRP1 druggability assessment.

Generation of Reporter-Expressing New World Arenaviruses: A Systematic Comparison

Replication-competent reporter-expressing viruses are crucial tools in molecular virology with applications that range from antiviral screening to live-cell imaging of protein spatiotemporal dynamics. However, there is currently little information available regarding viable strategies to develop reporter-expressing arenaviruses. To address this, we used Tacaribe virus (TCRV), an apathogenic BSL2 arenavirus, to assess the feasibility of different reporter expression approaches. We first generated trisegmented TCRV viruses with either the glycoprotein (GP) or nucleoprotein (NP) replaced by a reporter (GFP, mCherry, or nanoluciferase). These viruses were all viable, but showed marked differences in brightness and attenuation. Next, we generated terminal fusions with each of the TCRV proteins (i.e., NP, GP, polymerase (L), matrix protein (Z)) either with or without a T2A self-cleavage site. We tested both the function of the reporter-fused proteins alone, and the viability of corresponding recombinant TCRVs. We successfully rescued viruses with both direct and cleavable reporter fusions at the C-terminus of Z, as well as cleavable N-terminal fusions with NP. These viruses all displayed detectable reporter activity, but were also moderately attenuated. Finally, reporter proteins were inserted into a flexible hinge region within L. These viruses were also viable and showed moderate attenuation; however, reporter expression was only detectable for the luminescent virus. These strategies provide an exciting range of new tools for research into the molecular biology of TCRV that can likely also be adapted to other arenaviruses.

Genome-Wide Knockout Screen Identifies Human Sialomucin CD164 as an Essential Entry Factor for Lymphocytic Choriomeningitis Virus

Lymphocytic choriomeningitis virus (LCMV) is a well-studied mammarenavirus that can be fatal in congenital infections. However, our understanding of LCMV and its interactions with human host factors remains incomplete. Here, host determinants affecting LCMV infection were investigated through a genome-wide CRISPR knockout screen in A549 cells, a human lung adenocarcinoma line. We identified and validated a variety of novel host factors that play a functional role in LCMV infection. Among these, knockout of the sialomucin CD164, a heavily glycosylated transmembrane protein, was found to ablate infection with multiple LCMV strains but not other hemorrhagic mammarenaviruses in several cell types. Further characterization revealed a dependency of LCMV entry on the cysteine-rich domain of CD164, including an N-linked glycosylation site at residue 104 in that region. Given the documented role of LCMV with respect to transplacental human infections, CD164 expression was investigated in human placental tissue and placental cell lines. CD164 was found to be highly expressed in the cytotrophoblast cells, an initial contact site for pathogens within the placenta, and LCMV infection in placental cells was effectively blocked using a monoclonal antibody specific to the cysteine-rich domain of CD164. Together, this study identifies novel factors associated with LCMV infection of human tissues and highlights the importance of CD164, a sialomucin that previously had not been associated with viral infection. IMPORTANCE Lymphocytic choriomeningitis virus (LCMV) is a human-pathogenic mammarenavirus that can be fatal in congenital infections. Although frequently used in the study of persistent infections in the field of immunology, aspects of this virus's life cycle remain incomplete. For example, while viral entry has been shown to depend on a cell adhesion molecule, DAG1, genetic knockout of this gene allows for residual viral infection, implying that additional receptors can mediate cell entry. The significance of our study is the identification of host factors important for successful infection, including the sialomucin CD164, which had not been previously associated with viral infection. We demonstrated that CD164 is essential for LCMV entry into human cells and can serve as a possible therapeutic target for treatment of congenital infection.

The Tetraspanin CD81 Is a Host Factor for Chikungunya Virus Replication

Chikungunya virus (CHIKV) is an arthritogenic reemerging virus replicating in plasma membrane-derived compartments termed "spherules." Here, we identify the human transmembrane protein CD81 as host factor required for CHIKV replication. Ablation of CD81 results in decreased CHIKV permissiveness, while overexpression enhances infection. CD81 is dispensable for virus uptake but critically required for viral genome replication. Likewise, murine CD81 is crucial for CHIKV permissiveness and is expressed in target cells such as dermal fibroblasts, muscle and liver cells. Whereas related alphaviruses, including Ross River virus (RRV), Semliki Forest virus (SFV), Sindbis virus (SINV) and Venezuelan equine encephalitis virus (VEEV), also depend on CD81 for infection, RNA viruses from other families, such as coronaviruses, replicate independently of CD81. Strikingly, the replication-enhancing function of CD81 is linked to cholesterol binding. These results define a mechanism exploited by alphaviruses to hijack the membrane microdomain-modeling protein CD81 for virus replication through interaction with cholesterol. IMPORTANCE In this study, we discover the tetraspanin CD81 as a host factor for the globally emerging chikungunya virus and related alphaviruses. We show that CD81 promotes replication of viral genomes in human and mouse cells, while virus entry into cells is independent of CD81. This provides novel insights into how alphaviruses hijack host proteins to complete their life cycle. Alphaviruses replicate at distinct sites of the plasma membrane, which are enriched in cholesterol. We found that the cholesterol-binding ability of CD81 is important for its function as an alphavirus host factor. This discovery thus broadens our understanding of the alphavirus replication process and the use of host factors to reprogram cells into virus replication factories.

CD164 is a host factor for lymphocytic choriomeningitis virus entry

Lymphocytic choriomeningitis virus (LCMV) is the prototypic arenavirus and has been utilized for decades as a model to understand the host immune response against viral infection. LCMV infection can lead to fatal meningitis in immunocompromised people and can lead to congenital birth defects and spontaneous abortion if acquired during pregnancy. Using a genetic screen, we uncover host factors involved in LCMV entry that were previously unknown and are candidate therapeutic targets to combat LCMV infection. This study expands our understanding of the entry pathway of LCMV, revealing that its glycoprotein switches from utilizing the known receptor α-DG and heparan sulfate at the plasma membrane to binding the lysosomal mucin CD164 at pH levels found in endolysosomal compartments, facilitating membrane fusion.

Lymphocytic choriomeningitis virus (LCMV) is a rodent-borne zoonotic arenavirus that causes congenital abnormalities and can be fatal for transplant recipients. Using a genome-wide loss-of-function screen, we identify host factors required for LCMV entry into cells. We identify the lysosomal mucin CD164, glycosylation factors, the heparan sulfate biosynthesis machinery, and the known receptor alpha-dystroglycan (α-DG). Biochemical analysis revealed that the LCMV glycoprotein binds CD164 at acidic pH and requires a sialylated glycan at residue N104. We demonstrate that LCMV entry proceeds by the virus switching binding from heparan sulfate or α-DG at the plasma membrane to CD164 prior to membrane fusion, thus identifying additional potential targets for therapeutic intervention.

Inhibition of Arenavirus Entry and Replication by the Cell-Intrinsic Restriction Factor ZMPSTE24 Is Enhanced by IFITM Antiviral Activity

In the absence of effective vaccines and treatments, annual outbreaks of severe human haemorrhagic fever caused by arenaviruses, such as Lassa virus, continue to pose a significant human health threat. Understanding the balance of cellular factors that inhibit or promote arenavirus infection may have important implications for the development of effective antiviral strategies. Here, we identified the cell-intrinsic zinc transmembrane metalloprotease, ZMPSTE24, as a restriction factor against arenaviruses. Notably, CRISPR-Cas9-mediated knockout of ZMPSTE24 in human alveolar epithelial A549 cells increased arenavirus glycoprotein-mediated viral entry in pseudoparticle assays and live virus infection models. As a barrier to viral entry and replication, ZMPSTE24 may act as a downstream effector of interferon-induced transmembrane protein (IFITM) antiviral function; though through a yet poorly understood mechanism. Overexpression of IFITM1, IFITM2, and IFITM3 proteins did not restrict the entry of pseudoparticles carrying arenavirus envelope glycoproteins and live virus infection. Furthermore, gain-of-function studies revealed that IFITMs augment the antiviral activity of ZMPSTE24 against arenaviruses, suggesting a cooperative effect of viral restriction. We show that ZMPSTE24 and IFITMs affect the kinetics of cellular endocytosis, suggesting that perturbation of membrane structure and stability is likely the mechanism of ZMPSTE24-mediated restriction and cooperative ZMPSTE24-IFITM antiviral activity. Collectively, our findings define the role of ZMPSTE24 host restriction activity in the early stages of arenavirus infection. Moreover, we provide insight into the importance of cellular membrane integrity for productive fusion of arenaviruses and highlight a novel avenue for therapeutic development.

Structure and receptor recognition by the Lassa virus spike complex

Lassa virus (LASV) is a human pathogen, causing substantial morbidity and mortality^1,2. Similar to other Arenaviridae, it presents a class-I spike complex on its surface that facilitates cell entry. The virus's cellular receptor is matriglycan, a linear carbohydrate that is present on α-dystroglycan^3,4, but the molecular mechanism that LASV uses to recognize this glycan is unknown. In addition, LASV and other arenaviruses have a unique signal peptide that forms an integral and functionally important part of the mature spike^5-8; yet the structure, function and topology of the signal peptide in the membrane remain uncertain^9-11. Here we solve the structure of a complete native LASV spike complex, finding that the signal peptide crosses the membrane once and that its amino terminus is located in the extracellular region. Together with a double-sided domain-switching mechanism, the signal peptide helps to stabilize the spike complex in its native conformation. This structure reveals that the LASV spike complex is preloaded with matriglycan, suggesting the mechanism of binding and rationalizing receptor recognition by α-dystroglycan-tropic arenaviruses. This discovery further informs us about the mechanism of viral egress and may facilitate the rational design of novel therapeutics that exploit this binding site.

Glycosaminoglycan binding by arboviruses: a cautionary tale

Arboviruses are medically important arthropod-borne viruses that cause a range of diseases in humans from febrile illness to arthritis, encephalitis and hemorrhagic fever. Given their transmission cycles, these viruses face the challenge of replicating in evolutionarily divergent organisms that can include ticks, flies, mosquitoes, birds, rodents, reptiles and primates. Furthermore, their cell attachment receptor utilization may be affected by the opposing needs for generating high and sustained serum viremia in vertebrates such that virus particles are efficiently collected during a hematophagous arthropod blood meal but they must also bind sufficiently to cellular structures on divergent organisms such that productive infection can be initiated and viremia generated. Sulfated polysaccharides of the glycosaminoglycan (GAG) groups, primarily heparan sulfate (HS), have been identified as cell attachment moieties for many arboviruses. Original identification of GAG binding as a phenotype of arboviruses appeared to involve this attribute arising solely as a consequence of adaptation of virus isolates to growth in cell culture. However, more recently, naturally circulating strains of at least one arbovirus, eastern equine encephalitis, have been shown to bind HS efficiently and the GAG binding phenotype continues to be associated with arbovirus infection in published studies. If GAGs are attachment receptors for many naturally circulating arboviruses, this could lead to development of broad-spectrum antiviral therapies through blocking of the virus-GAG interaction. This review summarizes the available data for GAG/HS binding as a phenotype of naturally circulating arbovirus strains emphasizing the importance of avoiding tissue culture amplification and artifactual phenotypes during their isolation.

A Two-Color Haploid Genetic Screen Identifies Novel Host Factors Involved in HIV-1 Latency

To identify novel host factors as putative targets to reverse HIV-1 latency, we performed an insertional mutagenesis genetic screen in a latent HIV-1 infected pseudohaploid KBM7 cell line (Hap-Lat). Following mutagenesis, insertions were mapped to the genome, and bioinformatic analysis resulted in the identification of 69 candidate host genes involved in maintaining HIV-1 latency. A select set of candidate genes was functionally validated using short hairpin RNA (shRNA)-mediated depletion in latent HIV-1 infected J-Lat A2 and 11.1 T cell lines. We confirmed ADK, CHD9, CMSS1, EVI2B, EXOSC8, FAM19A, GRIK5, IRF2BP2, NF1, and USP15 as novel host factors involved in the maintenance of HIV-1 latency. Chromatin immunoprecipitation assays indicated that CHD9, a chromodomain helicase DNA-binding protein, maintains HIV-1 latency via direct association with the HIV-1 5′ long terminal repeat (LTR), and its depletion results in increased histone acetylation at the HIV-1 promoter, concomitant with HIV-1 latency reversal. FDA-approved inhibitors 5-iodotubercidin, trametinib, and topiramate, targeting ADK, NF1, and GRIK5, respectively, were characterized for their latency reversal potential. While 5-iodotubercidin exhibited significant cytotoxicity in both J-Lat and primary CD4⁺ T cells, trametinib reversed latency in J-Lat cells but not in latent HIV-1 infected primary CD4⁺ T cells. Importantly, topiramate reversed latency in cell line models, in latently infected primary CD4⁺ T cells, and crucially in CD4⁺ T cells from three people living with HIV-1 (PLWH) under suppressive antiretroviral therapy, without inducing T cell activation or significant toxicity. Thus, using an adaptation of a haploid forward genetic screen, we identified novel and druggable host factors contributing to HIV-1 latency.

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.

VLDLR and ApoER2 are receptors for multiple alphaviruses

Alphaviruses, like many other arthropod-borne viruses, infect vertebrate species and insect vectors separated by hundreds of millions of years of evolutionary history. Entry into evolutionarily divergent host cells can be accomplished by recognition of different cellular receptors in different species, or by binding to receptors that are highly conserved across species. Although multiple alphavirus receptors have been described^1–3, most are not shared among vertebrate and invertebrate hosts. Here we identify the very low-density lipoprotein receptor (VLDLR) as a receptor for the prototypic alphavirus Semliki forest virus. We show that the E2 and E1 glycoproteins (E2–E1) of Semliki forest virus, eastern equine encephalitis virus and Sindbis virus interact with the ligand-binding domains (LBDs) of VLDLR and apolipoprotein E receptor 2 (ApoER2), two closely related receptors. Ectopic expression of either protein facilitates cellular attachment, and internalization of virus-like particles, a VLDLR LBD–Fc fusion protein or a ligand-binding antagonist block Semliki forest virus E2–E1-mediated infection of human and mouse neurons in culture. The administration of a VLDLR LBD–Fc fusion protein has protective activity against rapidly fatal Semliki forest virus infection in mouse neonates. We further show that invertebrate receptor orthologues from mosquitoes and worms can serve as functional alphavirus receptors. We propose that the ability of some alphaviruses to infect a wide range of hosts is a result of their engagement of evolutionarily conserved lipoprotein receptors and contributes to their pathogenesis.

Studies using viral coat glycoproteins show that alphaviruses can enter cells via the very low-density lipoprotein receptor (VLDLR) and apolipoprotein E receptor 2 (ApoER2), members of an evolutionarily conserved family of lipoprotein receptors.

A widespread viral entry mechanism: The C-end Rule motif-neuropilin receptor interaction

Many phylogenetically distant animal viruses, including the new coronavirus severe acute respiratory syndrome coronavirus 2, have surface proteins with polybasic sites that are cleaved by host furin and furin-like proteases. Other than priming certain viral surface proteins for fusion, cleavage generates a carboxy-terminal RXXR sequence. This C-end Rule (CendR) motif is known to bind to neuropilin (NRP) receptors on the cell surface. NRPs are ubiquitously expressed, pleiotropic cell surface receptors with important roles in growth factor signaling, vascular biology, and neurobiology, as well as immune homeostasis and activation. The CendR–NRP receptor interaction promotes endocytic internalization and tissue spreading of different cargo, including viral particles. We propose that the interaction between viral surface proteins and NRPs plays an underappreciated and prevalent role in the transmission and pathogenesis of diverse viruses and represents a promising broad-spectrum antiviral target.

The endocytic pathway taken by cationic substances requires Rab14 but not Rab5 and Rab7

Endocytosis and endosome dynamics are controlled by proteins of the small GTPase Rab family. Besides possible recycling routes to the plasma membrane and various organelles, previously described endocytic pathways (e.g., clathrin-mediated endocytosis, macropinocytosis, CLIC/GEEC pathway) all appear to funnel the endocytosed material to Rab5-positive early endosomes that then mature into Rab7-positive late endosomes/lysosomes. By studying the uptake of a series of cell-penetrating peptides (CPPs), we identify an endocytic pathway that moves material to nonacidic Lamp1-positive late endosomes. Trafficking via this endocytic route is fully independent of Rab5 and Rab7 but requires the Rab14 protein. The pathway taken by CPPs differs from the conventional Rab5-dependent endocytosis at the stage of vesicle formation already, as it is not affected by a series of compounds that inhibit macropinocytosis or clathrin-mediated endocytosis. The Rab14-dependent pathway is also used by physiological cationic molecules such as polyamines and homeodomains found in homeoproteins.

The late endosome-resident lipid bis(monoacylglycero)phosphate is a cofactor for Lassa virus fusion

Arenavirus entry into host cells occurs through a low pH-dependent fusion with late endosomes that is mediated by the viral glycoprotein complex (GPC). The mechanisms of GPC-mediated membrane fusion and of virus targeting to late endosomes are not well understood. To gain insights into arenavirus fusion, we examined cell-cell fusion induced by the Old World Lassa virus (LASV) GPC complex. LASV GPC-mediated cell fusion is more efficient and occurs at higher pH with target cells expressing human LAMP1 compared to cells lacking this cognate receptor. However, human LAMP1 is not absolutely required for cell-cell fusion or LASV entry. We found that GPC-induced fusion progresses through the same lipid intermediates as fusion mediated by other viral glycoproteins–a lipid curvature-sensitive intermediate upstream of hemifusion and a hemifusion intermediate downstream of acid-dependent steps that can be arrested in the cold. Importantly, GPC-mediated fusion and LASV pseudovirus entry are specifically augmented by an anionic lipid, bis(monoacylglycero)phosphate (BMP), which is highly enriched in late endosomes. This lipid also specifically promotes cell fusion mediated by Junin virus GPC, an unrelated New World arenavirus. We show that BMP promotes late steps of LASV fusion downstream of hemifusion–the formation and enlargement of fusion pores. The BMP-dependence of post-hemifusion stages of arenavirus fusion suggests that these viruses evolved to use this lipid as a cofactor to selectively fuse with late endosomes.

Pathogenic arenaviruses pose a serious health threat. The viral envelope glycoprotein GPC mediates attachment to host cells and drives virus entry via endocytosis and low pH-dependent fusion within late endosomes. Understanding the host factors and processes that are essential for arenavirus fusion may identify novel therapeutic targets. To delineate the mechanism of arenavirus entry, we examined cell-cell fusion induced by the Old World Lassa virus GPC proteins at low pH. Lassa GPC-mediated fusion was augmented by the human LAMP1 receptor and progressed through lipid curvature-sensitive intermediates, such as hemifusion (merger of contacting leaflets of viral and cell membrane without the formation of a fusion pore). We found that most GPC-mediated fusion events were off-path hemifusion structures and that the transition from hemifusion to full fusion and fusion pore enlargement were specifically promoted by an anionic lipid, bis(monoacylglycero)phosphate, which is highly enriched in late endosomes. This lipid also specifically promotes fusion of unrelated New World Junin arenavirus. Our results imply that arenaviruses evolved to use bis(monoacylglycero)phosphate to enter cells from late endosomes.

Molecular cloning and characterization of CD63 in common carp infected with koi herpesvirus

CD63 is a member of the four-transmembrane-domain protein superfamily and is the first characterized tetraspanin protein. In the present study, we cloned the common carp (Cyprinus Carpio) CD63 (ccCD63) sequence and found that the ccCD63 ORF contained 711 bp and encoded a protein of 236 amino acids. Homology analysis revealed that the complete ccCD63 sequence had 84.08% amino acid similarity to CD63 of Sinocyclocheilus anshuiensis. Subcellular localization analysis revealed that ccCD63 was localized in the cytoplasm. Quantitative real-time PCR (qRT-PCR) analysis indicated that ccCD63 was expressed in the gill, intestine, liver, spleen, brain and kidney, with higher expression in spleen and brain tissues than in the other examined tissues. After koi herpesvirus (KHV) infection, these tissues exhibited various expression levels of ccCD63. The expression level was the lowest in the liver and highest in the brain; the expression level in the brain was 8.7-fold higher than that in the liver. Furthermore, knockdown of ccCD63 promoted KHV infection. Moreover, ccCD63 was correlated with the regulation of RIG-I/MAVS/TRAF3/TBK1/IRF3 and may be involved in the antiviral response through the RIG-I viral recognition signalling pathway in a TRAF3/TBK1-dependent manner. Taken together, our results suggested that ccCD63 upregulated the interaction of KHV with the host immune system and suppressed the dissemination of KHV.

Progress in Anti-Mammarenavirus Drug Development

Mammarenaviruses are prevalent pathogens distributed worldwide, and several strains cause severe cases of human infections with high morbidity and significant mortality. Currently, there is no FDA-approved antiviral drugs and vaccines against mammarenavirus and the potential treatment option is limited to an off-label use of ribavirin that shows only partial protective effect and associates with side effects. For the past few decades, extensive research has reported potential anti-mammarenaviral drugs and their mechanisms of action in host as well as vaccine candidates. This review describes current knowledge about mammarenavirus virology, progress of antiviral drug development, and technical strategies of drug screening.

Special Issue "Arenaviruses 2020"

Rodent-borne arenaviruses have been traditionally predominantly associated with certain muroid species from Mastomys/Praomys genera (African arenaviruses) or with species that belong to murid subfamily Cricetidae (New World arenaviruses) [...].

Generation of cd63-deficient zebrafish to analyze the role of cd63 in viral infection

The tetraspanin superfamily proteins are transmembrane proteins identified in a diverse range of eukaryotic organisms. Tetraspanins are involved in a variety of essential biological functions, including cell differentiation, adhesion, migration, signal transduction, intracellular trafficking, and immune responses. For an infection to occur, viruses must interact with various cell surface components, including receptors and signaling molecules. Tetraspanin CD63 is involved in the organization of the cell membrane and trafficking of cellular transmembrane proteins that interact with many viruses. In this study, the cd63 gene was characterized by studying its expression and function in a zebrafish model. The functional domains and structural features of Cd63, such as the Cys-Cys-Gly (CCG) motif in the large extracellular loop and cysteine residues, are conserved in zebrafish. We confirmed that cd63 was expressed in immune system organs, such as the axial vein and pronephric duct, during the embryonic development of zebrafish. To better understand the role of cd63 in the zebrafish immune system, we established cd63-deficient zebrafish lines using the clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 (CRISPR/Cas9) system. A 19 bp insertion mutation was generated in single guide RNA (sgRNA) target sequence of exon 3 of the cd63 gene, to create a pre-mature stop codon. We then analyzed the expression of cd63-related genes cxcr4a and cxcr4b in wild type (WT) and cd63-deficient zebrafish. We believe our study provides an important model that could be used to investigate the roles of cd63 in viral infection in vivo.

Neuropilins: C-end rule peptides and their association with nociception and COVID-19

Viral internalization is aided by host cell surface receptors. In the case of SARS-CoV-2 and SARS-CoV, the primary host receptor is the angiotensin-converting enzyme 2 (ACE2). Considering the disparities in the transmission rate and viral tropism of the two coronaviruses, additional host factors were suspected. Recently, a novel host factor for SARS-CoV-2 entry, neuropilin-1 (NRP-1) has been identified. These receptors potentiate viral infection in the presence of other host factors like ACE2. Through its C-end rule (CendR) motif exposed following furin processing, the SARS-CoV-2 spike protein binds to the CendR pocket of NRP-1 and achieves cell entry through endocytosis. The binding of SARS-CoV-2 spike protein to the NRP-1 receptor interferes with the docking of its endogenous ligand VEGF-A, signaling that would otherwise promote nociception. This review looks at the function of neuropilins and how it contributes to SARS-CoV-2 infection and nociception.

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.

Contributions of the Four Essential Entry Glycoproteins to HSV-1 Tropism and the Selection of Entry Routes

Herpes simplex viruses (HSV-1 and HSV-2) encode up to 16 envelope proteins, four of which are essential for entry. However, whether these four proteins alone are sufficient to dictate the broad cellular tropism of HSV-1 and the selection of different cell type-dependent entry routes is unknown. To begin addressing this, we previously pseudotyped vesicular stomatitis virus (VSV), lacking its native glycoprotein G, with only the four essential entry glycoproteins of HSV-1: gB, gH, gL, and gD. This novel VSVΔG-BHLD pseudotype recapitulated several important features of HSV-1 entry: the requirement for gB, gH, gL, gD, and a cellular receptor and sensitivity to anti-gB and anti-gH/gL neutralizing antibodies. However, due to the use of a single cell type in that study, the tropism of the VSVΔG-BHLD pseudotype was not investigated. Here, we show that the cellular tropism of the pseudotype is severely limited compared to that of wild-type HSV-1 and that its entry pathways differ from the native HSV-1 entry pathways. To test the hypothesis that other HSV-1 envelope proteins may contribute to HSV-1 tropism, we generated a derivative pseudotype containing the HSV-1 glycoprotein C (VSVΔG-BHLD-gC) and observed a gC-dependent increase in entry efficiency in two cell types. We propose that the pseudotyping platform developed here has the potential to uncover functional contributions of HSV-1 envelope proteins to entry in a gain-of-function manner.IMPORTANCE Herpes simplex viruses (HSV-1 and HSV-2) contain up to 16 different proteins in their envelopes. Four of these, glycoproteins gB, gD, gH, and gL, are termed essential with regard to entry, whereas the rest are typically referred to as nonessential based on the entry phenotypes of the respective single genetic deletions. However, the single-gene deletion approach, which relies on robust loss-of-function phenotypes, may be confounded by functional redundancies among the many HSV-1 envelope proteins. We have developed a pseudotyping platform in which the essential four entry glycoproteins are isolated from the rest, which can be added back individually for systematic gain-of-function entry experiments. Here, we show the utility of this platform for dissecting the contributions of HSV envelope proteins, both the essential four and the remaining dozen (using gC as an example), to HSV entry.

Recent Advances in Bunyavirus Glycoprotein Research: Precursor Processing, Receptor Binding and Structure

The Bunyavirales order accommodates related viruses (bunyaviruses) with segmented, linear, single-stranded, negative- or ambi-sense RNA genomes. Their glycoproteins form capsomeric projections or spikes on the virion surface and play a crucial role in virus entry, assembly, morphogenesis. Bunyavirus glycoproteins are encoded by a single RNA segment as a polyprotein precursor that is co- and post-translationally cleaved by host cell enzymes to yield two mature glycoproteins, Gn and Gc (or GP1 and GP2 in arenaviruses). These glycoproteins undergo extensive N-linked glycosylation and despite their cleavage, remain associated to the virion to form an integral transmembrane glycoprotein complex. This review summarizes recent advances in our understanding of the molecular biology of bunyavirus glycoproteins, including their processing, structure, and known interactions with host factors that facilitate cell entry.

Molecular Mechanisms Underlying the Cellular Entry and Host Range Restriction of Lujo Virus

Like other human-pathogenic arenaviruses, Lujo virus (LUJV) is a causative agent of viral hemorrhagic fever in humans. LUJV infects humans with high mortality rates, but the susceptibilities of other animal species and the molecular determinants of its host specificity remain unknown. We found that mouse- and hamster-derived cell lines (NIH 3T3 and BHK, respectively) were less susceptible to a replication-incompetent recombinant vesicular stomatitis virus (Indiana) pseudotyped with the LUJV glycoprotein (GP) (VSVΔG*-LUJV/GP) than were human-derived cell lines (HEK293T and Huh7). To determine the cellular factors involved in the differential susceptibilities between the human and mouse cell lines, we focused on the CD63 molecule, which is required for pH-activated GP-mediated membrane fusion during LUJV entry into host cells. The exogenous introduction of human CD63, but not mouse or hamster CD63, into BHK cells significantly increased susceptibility to VSVΔG*-LUJV/GP. Using chimeric human-mouse CD63 proteins, we found that the amino acid residues at positions 141 to 150 in the large extracellular loop (LEL) region of CD63 were important for the cellular entry of VSVΔG*-LUJV/GP. By site-directed mutagenesis, we further determined that a phenylalanine at position 143 in human CD63 was the key residue for efficient membrane fusion and VSVΔG*-LUJV/GP infection. Our data suggest that the interaction of LUJV GP with the LEL region of CD63 is essential for cell susceptibility to LUJV, thus providing new insights into the molecular mechanisms underlying the cellular entry of LUJV and the host range restriction of this virus. IMPORTANCE Lujo virus (LUJV) infects humans with high mortality rates, but the host range of LUJV remains unknown. We found that rodent-derived cell lines were less susceptible to LUJV infection than were human-derived cell lines, and the differential susceptibilities were determined by the difference of CD63, the intercellular receptor of LUJV. We further identified an amino acid residue on human CD63 important for efficient LUJV infection. These results suggest that the interaction between LUJV glycoprotein and CD63 is one of the important factors determining the host range of LUJV. Our findings on the CD63-regulated susceptibilities of the cell lines to LUJV infection provide important information for the development of anti-LUJV drugs as well as the identification of natural hosts of LUJV. Importantly, our data support a concept explaining the molecular mechanism underlying viral tropisms controlled by endosomal receptors.

Effects of Cocaine on Human Glial-Derived Extracellular Vesicles

BACKGROUND

Microglia are important myeloid cells present in the brain parenchyma that serve a surveillance function in the central nervous system. Microglial cell activation results in neuroinflammation that, when prolonged, can disrupt immune homeostasis and neurogenesis. Activated microglia-derived extracellular vesicles (EVs) may be involved in the propagation of inflammatory responses and modulation of cell-to-cell communication. However, a complete understanding of how EVs are regulated by drugs of abuse, such as cocaine, is still lacking.

FINDINGS

Cocaine exposure reduced human microglial cell (HMC3) viability, decreased expression of CD63 and dectin-1 in HMC3-derived EVs, and increased expression of the apoptotic marker histone H2A.x in HMC3-derived EVs.

CONCLUSION

Cocaine impacts HMC3 cell viability and specific EV protein expression, which could disrupt cellular signaling and cell-to-cell communication.

A novel circulating tamiami mammarenavirus shows potential for zoonotic spillover

A detailed understanding of the mechanisms underlying the capacity of a virus to break the species barrier is crucial for pathogen surveillance and control. New World (NW) mammarenaviruses constitute a diverse group of rodent-borne pathogens that includes several causative agents of severe viral hemorrhagic fever in humans. The ability of the NW mammarenaviral attachment glycoprotein (GP) to utilize human transferrin receptor 1 (hTfR1) as a primary entry receptor plays a key role in dictating zoonotic potential. The recent isolation of Tacaribe and lymphocytic choriominingitis mammarenaviruses from host-seeking ticks provided evidence for the presence of mammarenaviruses in arthropods, which are established vectors for numerous other viral pathogens. Here, using next generation sequencing to search for other mammarenaviruses in ticks, we identified a novel replication-competent strain of the NW mammarenavirus Tamiami (TAMV-FL), which we found capable of utilizing hTfR1 to enter mammalian cells. During isolation through serial passaging in mammalian immunocompetent cells, the quasispecies of TAMV-FL acquired and enriched mutations leading to the amino acid changes N151K and D156N, within GP. Cell entry studies revealed that both substitutions, N151K and D156N, increased dependence of the virus on hTfR1 and binding to heparan sulfate proteoglycans. Moreover, we show that the substituted residues likely map to the sterically constrained trimeric axis of GP, and facilitate viral fusion at a lower pH, resulting in viral egress from later endosomal compartments. In summary, we identify and characterize a naturally occurring TAMV strain (TAMV-FL) within ticks that is able to utilize hTfR1. The TAMV-FL significantly diverged from previous TAMV isolates, demonstrating that TAMV quasispecies exhibit striking genetic plasticity that may facilitate zoonotic spillover and rapid adaptation to new hosts.

Mammarenaviruses include emergent pathogens responsible of severe disease in humans in zoonotic events. The ability to use the human Transferrin receptor 1 (hTfR1) strongly correlates with their pathogenicity in humans. We isolated a new infectious Tamiami virus strain (TAMV-FL) from host-seeking ticks, which, contrary to the previous rodent-derived reference strain, can use hTfR1 to enter human cells. Moreover, serial passaging of TAMV-FL in human immunocompetent cells selected for two substitutions in the viral envelope glycoprotein: N151K and D156N. These substitutions increase the ability to highjack hTfR1 and the binding capacity to heparan sulfate proteoglycans and cause delayed endosomal escape. Our findings provide insight into the acquisition of novel traits by currently circulating TAMV that increase its potential to trespass the inter-species barrier.

Enhancing host cell infection by SARS-CoV-2

Neuropilin-1 binds the furin-processed spike protein of SARS-CoV-2 to promote virus entry

Neuropilin-1 facilitates SARS-CoV-2 cell entry and infectivity

Virus-host interactions determine cellular entry and spreading in tissues. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and the earlier SARS-CoV use angiotensin-converting enzyme 2 (ACE2) as a receptor; however, their tissue tropism differs, raising the possibility that additional host factors are involved. The spike protein of SARS-CoV-2 contains a cleavage site for the protease furin that is absent from SARS-CoV (see the Perspective by Kielian). Cantuti-Castelvetri et al. now show that neuropilin-1 (NRP1), which is known to bind furin-cleaved substrates, potentiates SARS-CoV-2 infectivity. NRP1 is abundantly expressed in the respiratory and olfactory epithelium, with highest expression in endothelial and epithelial cells. Daly et al. found that the furin-cleaved S1 fragment of the spike protein binds directly to cell surface NRP1 and blocking this interaction with a small-molecule inhibitor or monoclonal antibodies reduced viral infection in cell culture. Understanding the role of NRP1 in SARS-CoV-2 infection may suggest potential targets for future antiviral therapeutics.

Science, this issue p. 856, p. 861; see also p. 765

The causative agent of coronavirus disease 2019 (COVID-19) is the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). For many viruses, tissue tropism is determined by the availability of virus receptors and entry cofactors on the surface of host cells. In this study, we found that neuropilin-1 (NRP1), known to bind furin-cleaved substrates, significantly potentiates SARS-CoV-2 infectivity, an effect blocked by a monoclonal blocking antibody against NRP1. A SARS-CoV-2 mutant with an altered furin cleavage site did not depend on NRP1 for infectivity. Pathological analysis of olfactory epithelium obtained from human COVID-19 autopsies revealed that SARS-CoV-2 infected NRP1-positive cells facing the nasal cavity. Our data provide insight into SARS-CoV-2 cell infectivity and define a potential target for antiviral intervention.

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.

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.

A global lipid map defines a network essential for Zika virus replication

Zika virus (ZIKV), an arbovirus of global concern, remodels intracellular membranes to form replication sites. How ZIKV dysregulates lipid networks to allow this, and consequences for disease, is poorly understood. Here, we perform comprehensive lipidomics to create a lipid network map during ZIKV infection. We find that ZIKV significantly alters host lipid composition, with the most striking changes seen within subclasses of sphingolipids. Ectopic expression of ZIKV NS4B protein results in similar changes, demonstrating a role for NS4B in modulating sphingolipid pathways. Disruption of sphingolipid biosynthesis in various cell types, including human neural progenitor cells, blocks ZIKV infection. Additionally, the sphingolipid ceramide redistributes to ZIKV replication sites, and increasing ceramide levels by multiple pathways sensitizes cells to ZIKV infection. Thus, we identify a sphingolipid metabolic network with a critical role in ZIKV replication and show that ceramide flux is a key mediator of ZIKV infection.

A Replication-Competent Vesicular Stomatitis Virus for Studies of SARS-CoV-2 Spike-Mediated Cell Entry and Its Inhibition

There is an urgent need for vaccines and therapeutics to prevent and treat COVID-19. Rapid SARS-CoV-2 countermeasure development is contingent on the availability of robust, scalable, and readily deployable surrogate viral assays to screen antiviral humoral responses, define correlates of immune protection, and down-select candidate antivirals. Here, we generate a highly infectious recombinant vesicular stomatitis virus (VSV) bearing the SARS-CoV-2 spike glycoprotein S as its sole entry glycoprotein and show that this recombinant virus, rVSV-SARS-CoV-2 S, closely resembles SARS-CoV-2 in its entry-related properties. The neutralizing activities of a large panel of COVID-19 convalescent sera can be assessed in a high-throughput fluorescent reporter assay with rVSV-SARS-CoV-2 S, and neutralization of rVSV-SARS-CoV-2 S and authentic SARS-CoV-2 by spike-specific antibodies in these antisera is highly correlated. Our findings underscore the utility of rVSV-SARS-CoV-2 S for the development of spike-specific therapeutics and for mechanistic studies of viral entry and its inhibition.

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Highly infectious recombinant VSV expressing SARS-CoV-2 spike (S) was generated

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rVSV-SARS-CoV-2 S resembles SARS-CoV-2 in entry and inhibitor or antibody sensitivity

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rVSV-SARS-CoV-2 S affords rapid screens and forward-genetic analyses of antivirals

A replication-competent vesicular stomatitis virus for studies of SARS-CoV-2 spike-mediated cell entry and its inhibition

There is an urgent need for vaccines and therapeutics to prevent and treat COVID-19. Rapid SARS-CoV-2 countermeasure development is contingent on the availability of robust, scalable, and readily deployable surrogate viral assays to screen antiviral humoral responses, and define correlates of immune protection, and to down-select candidate antivirals. Here, we describe a highly infectious recombinant vesicular stomatitis virus bearing the SARS-CoV-2 spike glycoprotein S as its sole entry glycoprotein that closely resembles the authentic agent in its entry-related properties. We show that the neutralizing activities of a large panel of COVID-19 convalescent sera can be assessed in high-throughput fluorescent reporter assay with rVSV-SARS-CoV-2 S and that neutralization of the rVSV and authentic SARS-CoV-2 by spike-specific antibodies in these antisera is highly correlated. Our findings underscore the utility of rVSV-SARS-CoV-2 S for the development of spike-specific vaccines and therapeutics and for mechanistic studies of viral entry and its inhibition.

Differences in Tissue and Species Tropism of Reptarenavirus Species Studied by Vesicular Stomatitis Virus Pseudotypes

Reptarenaviruses cause Boid Inclusion Body Disease (BIBD), and co-infections by several reptarenaviruses are common in affected snakes. Reptarenaviruses have only been found in captive snakes, and their reservoir hosts remain unknown. In affected animals, reptarenaviruses appear to replicate in most cell types, but their complete host range, as well as tissue and cell tropism are unknown. As with other enveloped viruses, the glycoproteins (GPs) present on the virion's surface mediate reptarenavirus cell entry, and therefore, the GPs play a critical role in the virus cell and tissue tropism. Herein, we employed single cycle replication, GP deficient, recombinant vesicular stomatitis virus (VSV) expressing the enhanced green fluorescent protein (scrVSV∆G-eGFP) pseudotyped with different reptarenavirus GPs to study the virus cell tropism. We found that scrVSV∆G-eGFPs pseudotyped with reptarenavirus GPs readily entered mammalian cell lines, and some mammalian cell lines exhibited higher, compared to snake cell lines, susceptibility to reptarenavirus GP-mediated infection. Mammarenavirus GPs used as controls also mediated efficient entry into several snake cell lines. Our results confirm an important role of the virus surface GP in reptarenavirus cell tropism and that mamma-and reptarenaviruses exhibit high cross-species transmission potential.