Inhibition of Ebola virus entry by a C-peptide targeted to endosomes

Antiviral activity of the host defense peptide piscidin 1: investigating a membrane-mediated mode of action

Outbreaks of viral diseases are on the rise, fueling the search for antiviral therapeutics that act on a broad range of viruses while remaining safe to human host cells. In this research, we leverage the finding that the plasma membranes of host cells and the lipid bilayers surrounding enveloped viruses differ in lipid composition. We feature Piscidin 1 (P1), a cationic host defense peptide (HDP) that has antimicrobial effects and membrane activity associated with its N-terminal region where a cluster of aromatic residues and copper-binding motif reside. While few HDPs have demonstrated antiviral activity, P1 acts in the micromolar range against several enveloped viruses that vary in envelope lipid composition. Notably, it inhibits HIV-1, a virus that has an envelope enriched in cholesterol, a lipid associated with higher membrane order and stability. Here, we first document through plaque assays that P1 boasts strong activity against SARS-CoV-2, which has an envelope low in cholesterol. Second, we extend previous studies done with homogeneous bilayers and devise cholesterol-containing zwitterionic membranes that contain the liquid disordered (Ld; low in cholesterol) and ordered (Lo, rich in cholesterol) phases. Using dye leakage assays and cryo-electron microscopy on vesicles, we show that P1 has dramatic permeabilizing capability on the Lo/Ld, an effect matched by a strong ability to aggregate, fuse, and thin the membranes. Differential scanning calorimetry and NMR experiments demonstrate that P1 mixes the lipid content of vesicles and alters the stability of the Lo. Structural studies by NMR indicate that P1 interacts with the Lo/Ld by folding into an α-helix that lies parallel to the membrane surface. Altogether, these results show that P1 is more disruptive to phase-separated than homogenous cholesterol-containing bilayers, suggesting an ability to target domain boundaries. Overall, this multi-faceted research highlights how a peptide that interacts strongly with membranes through an aromatic-rich N-terminal motif disrupt viral envelope mimics. This represents an important step towards the development of novel peptides with broad-spectrum antiviral activity.

A pan-coronavirus peptide inhibitor prevents SARS-CoV-2 infection in mice by intranasal delivery

Coronaviruses (CoVs) have brought serious threats to humans, particularly severe acute respiratory syndrome Coronavirus 2 (SARS-CoV-2), which continually evolves into multiple variants. These variants, especially Omicron, reportedly escape therapeutic antibodies and vaccines, indicating an urgent need for new antivirals with pan-SARS-CoV-2 inhibitory activity. We previously reported that a peptide fusion inhibitor, P3, targeting heptad repeated-1 (HR1) of SARS-CoV-2 spike (S) protein, could inhibit viral infections. Here, we further designed multiple derivatives of the P3 based on structural analysis and found that one derivative, the P315V3, showed the most efficient antiviral activity against SARS-CoV-2 variants and several other sarbecoviruses, as well as other human-CoVs (HCoVs). P315V3 also exhibited effective prophylactic efficacy against the SARS-CoV-2 Delta and Omicron variants in mice via intranasal administration. These results suggest that P315V3, which is in Phase II clinical trial, is promising for further development as a nasal pan-SARS-CoV-2 or pan-CoVs inhibitor to prevent or treat CoV diseases.

Antiviral Activity against SARS-CoV-2 of Conformationally Constrained Helical Peptides Derived from Angiotensin-Converting Enzyme 2

Despite the availability of vaccines, COVID-19 continues to be aggressive, especially in immunocompromised individuals. Therefore, the development of a specific therapeutic agent with antiviral activity against SARS-CoV-2 is necessary. The infection pathway starts when the receptor binding domain of the viral spike protein interacts with the angiotensin converting enzyme 2 (ACE2), which acts as a host receptor for the RBD expressed on the host cell surface. In this scenario, ACE2 analogs binding to the RBD and preventing the cell entry can be promising antiviral agents. Most of the ACE2 residues involved in the interaction belong to the α1 helix, more specifically to the minimal fragment ACE2(24-42). In order to increase the stability of the secondary structure and thus antiviral activity, we designed different triazole-stapled analogs, changing the position and the number of bridges. The peptide called P3, which has the triazole-containing bridge in the positions 36-40, showed promising antiviral activity at micromolar concentrations assessed by plaque reduction assay. On the other hand, the double-stapled peptide P4 lost the activity, showing that excessive rigidity disfavors the interaction with the RBD.

A Review: The Antiviral Activity of Cyclic Peptides

In the design and development of therapeutic agents, macromolecules with restricted structures have stronger competitive edges than linear biological entities since cyclization can overcome the limitations of linear structures. The common issues of linear peptides include susceptibility to degradation of the peptidase enzyme, off-target effects, and necessity of routine dosing, leading to instability and ineffectiveness. The unique conformational constraint of cyclic peptides provides a larger surface area to interact with the target at the same time, improving the membrane permeability and in vivo stability compared to their linear counterparts. Currently, cyclic peptides have been reported to possess various activities, such as antifungal, antiviral and antimicrobial activities. To date, there is emerging interest in cyclic peptide therapeutics, and increasing numbers of clinically approved cyclic peptide drugs are available on the market. In this review, the medical significance of cyclic peptides in the defence against viral infections will be highlighted. Except for chikungunya virus, which lacks specific antiviral treatment, all the viral diseases targeted in this review are those with effective treatments yet with certain limitations to date. Thus, strategies and approaches to optimise the antiviral effect of cyclic peptides will be discussed along with their respective outcomes. Apart from isolated naturally occurring cyclic peptides, chemically synthesized or modified cyclic peptides with antiviral activities targeting coronavirus, herpes simplex viruses, human immunodeficiency virus, Ebola virus, influenza virus, dengue virus, five main hepatitis viruses, termed as type A, B, C, D and E and chikungunya virus will be reviewed herein.

Graphical Abstract

Ligand-based design of peptide entry inhibitors targeting the endosomal receptor binding site of filoviruses

Filoviruses enter cells through micropinocytosis and trafficking into the endosomes in which they bind to the receptor Niemann-Pick C1 protein (NPC1) for membrane fusion and entry into the cytoplasm. The endosomal receptor-binding is critical step for filovirus entry. Designing inhibitors to block receptor binding will prevent viral entry. Using available binding structural information from the co-crystal structures of the viral GP with the receptor NPC1 or with monoclonal antibodies, we have conducted structure-based design of peptide inhibitors to target the receptor binding site (RBS). The designed peptides were tested for their inhibition activity against pseudo-typed or replication-competent viruses in a cell-based assay. The results indicate that these peptides exhibited strong activities against both Ebola and Marburg virus infection. It is expected that these peptides can be further developed for therapeutic use to treat filovirus infection and combat the outbreaks.

Leveraging the therapeutic, biological, and self-assembling potential of peptides for the treatment of viral infections

Peptides and peptide-based materials have an increasing role in the treatment of viral infections through their use as active pharmaceutical ingredients, targeting moieties, excipients, carriers, or structural components in drug delivery systems. The discovery of peptide-based therapeutic compounds, coupled with the development of new stabilization and formulation strategies, has led to a resurgence of antiviral peptide therapeutics over the past two decades. The ability of peptides to bind cell receptors and to facilitate membrane penetration and subsequent intracellular trafficking enables their use in various antiviral systems for improved targeting efficiency and treatment efficacy. Importantly, the self-assembly of peptides into well-defined nanostructures provides a vast library of discrete constructs and supramolecular biomaterials for systemic and local delivery of antiviral agents. We review here the recent progress in exploiting the therapeutic, biological, and self-assembling potential of peptides, peptide conjugates, and their supramolecular assemblies in treating human viral infections, with an emphasis on the treatment strategies for Human Immunodeficiency Virus (HIV).

Inhibition of Viral Membrane Fusion by Peptides and Approaches to Peptide Design

Fusion of lipid-enveloped viruses with the cellular plasma membrane or the endosome membrane is mediated by viral envelope proteins that undergo large conformational changes following binding to receptors. The HIV-1 fusion protein gp41 undergoes a transition into a "six-helix bundle" after binding of the surface protein gp120 to the CD4 receptor and a co-receptor. Synthetic peptides that mimic part of this structure interfere with the formation of the helix structure and inhibit membrane fusion. This approach also works with the S spike protein of SARS-CoV-2. Here we review the peptide inhibitors of membrane fusion involved in infection by influenza virus, HIV-1, MERS and SARS coronaviruses, hepatitis viruses, paramyxoviruses, flaviviruses, herpesviruses and filoviruses. We also describe recent computational methods used for the identification of peptide sequences that can interact strongly with protein interfaces, with special emphasis on SARS-CoV-2, using the PePI-Covid19 database.

Two Distinct Lysosomal Targeting Strategies Afford Trojan Horse Antibodies With Pan-Filovirus Activity

Multiple agents in the family Filoviridae (filoviruses) are associated with sporadic human outbreaks of highly lethal disease, while others, including several recently identified agents, possess strong zoonotic potential. Although viral glycoprotein (GP)-specific monoclonal antibodies have demonstrated therapeutic utility against filovirus disease, currently FDA-approved molecules lack antiviral breadth. The development of broadly neutralizing antibodies has been challenged by the high sequence divergence among filovirus GPs and the complex GP proteolytic cleavage cascade that accompanies filovirus entry. Despite this variability in the antigenic surface of GP, all filoviruses share a site of vulnerability-the binding site for the universal filovirus entry receptor, Niemann-Pick C1 (NPC1). Unfortunately, this site is shielded in extracellular GP and only uncovered by proteolytic cleavage by host proteases in late endosomes and lysosomes, which are generally inaccessible to antibodies. To overcome this obstacle, we previously developed a 'Trojan horse' therapeutic approach in which engineered bispecific antibodies (bsAbs) coopt viral particles to deliver GP:NPC1 interaction-blocking antibodies to their endo/lysosomal sites of action. This approach afforded broad protection against members of the genus Ebolavirus but could not neutralize more divergent filoviruses. Here, we describe next-generation Trojan horse bsAbs that target the endo/lysosomal GP:NPC1 interface with pan-filovirus breadth by exploiting the conserved and widely expressed host cation-independent mannose-6-phosphate receptor for intracellular delivery. Our work highlights a new avenue for the development of single therapeutics protecting against all known and newly emerging filoviruses.

Viral Entry Inhibitors Targeting Six-Helical Bundle Core Against Highly Pathogenic Enveloped Viruses with Class I Fusion Proteins

TypeⅠ enveloped viruses bind to cell receptors through surface glycoproteins to initiate infection or undergo receptor-mediated endocytosis. They also initiate membrane fusion in the acidic environment of endocytic compartments, releasing genetic material into the cell. In the process of membrane fusion, envelope protein exposes fusion peptide, followed by insertion into the cell membrane or endosomal membrane. Further conformational changes ensue in which the type 1 envelope protein forms a typical six-helix bundle structure, shortening the distance between viral and cell membranes so that fusion can occur. Entry inhibitors targeting viral envelope proteins, or host factors, are effective antiviral agents and have been widely studied. Some have been used clinically, such as T20 and Maraviroc for human immunodeficiency virus 1 (HIV-1) or Myrcludex B for hepatitis D virus (HDV). This review focuses on entry inhibitors that target the six-helical bundle core against highly pathogenic enveloped viruses with class I fusion proteins, including retroviruses, coronaviruses, influenza A viruses, paramyxoviruses, and filoviruses.

Antivirals with common targets against highly pathogenic viruses

Historically, emerging viruses appear constantly and have cost millions of human lives. Currently, climate change and intense globalization have created favorable conditions for viral transmission. Therefore, effective antivirals, especially those targeting the conserved protein in multiple unrelated viruses, such as the compounds targeting RNA-dependent RNA polymerase, are urgently needed to combat more emerging and re-emerging viruses in the future. Here we reviewed the development of antivirals with common targets, including those against the same protein across viruses, or the same viral function, to provide clues for development of antivirals for future epidemics.

IL-4/IL-13 polarization of macrophages enhances Ebola virus glycoprotein-dependent infection

BACKGROUND

Ebolavirus (EBOV) outbreaks, while sporadic, cause tremendous morbidity and mortality. No therapeutics or vaccines are currently licensed; however, a vaccine has shown promise in clinical trials. A critical step towards development of effective therapeutics is a better understanding of factors that govern host susceptibility to this pathogen. As macrophages are an important cell population targeted during virus replication, we explore the effect of cytokine polarization on macrophage infection.

METHODS/MAIN FINDINGS

We utilized a BSL2 EBOV model virus, infectious, recombinant vesicular stomatitis virus encoding EBOV glycoprotein (GP) (rVSV/EBOV GP) in place of its native glycoprotein. Macrophages polarized towards a M2-like anti-inflammatory state by combined IL-4 and IL-13 treatment were more susceptible to rVSV/EBOV GP, but not to wild-type VSV (rVSV/G), suggesting that EBOV GP-dependent entry events were enhanced by these cytokines. Examination of RNA expression of known surface receptors that bind and internalize filoviruses demonstrated that IL-4/IL-13 stimulated expression of the C-type lectin receptor DC-SIGN in human macrophages and addition of the competitive inhibitor mannan abrogated IL-4/IL-13 enhanced infection. Two murine DC-SIGN-like family members, SIGNR3 and SIGNR5, were upregulated by IL-4/IL-13 in murine macrophages, but only SIGNR3 enhanced virus infection in a mannan-inhibited manner, suggesting that murine SIGNR3 plays a similar role to human DC-SIGN. In vivo IL-4/IL-13 administration significantly increased virus-mediated mortality in a mouse model and transfer of ex vivo IL-4/IL-13-treated murine peritoneal macrophages into the peritoneal cavity of mice enhanced pathogenesis.

SIGNIFICANCE

These studies highlight the ability of macrophage polarization to influence EBOV GP-dependent virus replication in vivo and ex vivo, with M2a polarization upregulating cell surface receptor expression and thereby enhancing virus replication. Our findings provide an increased understanding of the host factors in macrophages governing susceptibility to filoviruses and identify novel murine receptors mediating EBOV entry.

Cholesterol-conjugated stapled peptides inhibit Ebola and Marburg viruses in vitro and in vivo

Filoviridae currently includes five official and one proposed genera. Genus Ebolavirus includes five established and one proposed ebolavirus species for Bombali virus (BOMV), Bundibugyo virus (BDBV), Ebola virus (EBOV), Reston virus (RESTV), Sudan virus (SUDV) and Taï Forest virus (TAFV), and genus Marburgvirus includes a single species for Marburg virus (MARV) and Ravn virus (RAVV). Ebola virus (EBOV) has emerged as a significant public health concern since the 2013-2016 Ebola Virus Disease outbreak in Western Africa. Currently, there are no therapeutics approved and the need for Ebola-specific therapeutics remains a gap. In search for anti-Ebola therapies we tested the idea of using inhibitory properties of peptides corresponding to the C-terminal heptad-repeat (HR2) domains of class I fusion proteins against EBOV infection. The fusion protein GP₂ of EBOV belongs to class I, suggesting that a similar strategy to HIV may be applied to inhibit EBOV infection. The serum half-life of peptides was expanded by cholesterol conjugation to allow daily dosing. The peptides were further constrained to stabilize a helical structure to increase the potency of inhibition. The EC₅₀s of lead peptides were in low micromolar range, as determined by a high-content imaging test of EBOV-infected cells. Lead peptides were tested in an EBOV lethal mouse model and efficacy of the peptides were determined following twice-daily administration of peptides for 9 days. The most potent peptide was able to protect mice from lethal challenge of mouse-adapted Ebola virus. These data show that engineered peptides coupled with cholesterol can inhibit viral production, protect mice against lethal EBOV infection, and may be used to build novel therapeutics against EBOV.

Crystal Structure of Refolding Fusion Core of Lassa Virus GP2 and Design of Lassa Virus Fusion Inhibitors

The envelope glycoproteins GP1 and GP2 of Lassa virus (LASV) bind to the host cell receptors to mediate viral infection. So far, no approved vaccines and specific treatment options against LASV exist. To develop specific fusion inhibitors against LASV, we solved the crystal structure of the post-fusion 6 helix bundle (6-HB) formed by two heptad repeat domains (HR1 and HR2) of GP2. This fusion core contains a parallel trimeric coiled-coil of three HR1 helices, around which three HR2 helices are entwined in an antiparallel manner. Various hydrophobic and charged interactions form between HR1 and HR2 domains to stabilize the overall conformation of GP2 fusion core. Based on the structure, we designed several peptides spanning the HR2 domain and tested their antiviral activities. We found that the longer HR2 peptides were effective in inhibiting LASV GPC protein-mediated cell–cell fusion under low pH condition. These results not only suggest that LASV infects the target cell mainly through endocytosis, including micropinocytosis, and membrane fusion at low pH, but also provide an important basis for rational design of LASV fusion inhibitors.

Identification of Ebola Virus Inhibitors Targeting GP2 Using Principles of Molecular Mimicry

A key step in the Ebola virus (EBOV) replication cycle involves conformational changes in viral glycoprotein 2 (GP2) which facilitate host-viral membrane fusion and subsequent release of the viral genome. Ebola GP2 plays a critical role in virus entry and has similarities in mechanism and structure to the HIV gp41 protein for which inhibitors have been successfully developed. In this work, a putative binding pocket for the C-terminal heptad repeat in the N-terminal heptad repeat trimer was targeted for identification of small molecules that arrest EBOV-host membrane fusion. Two computational structure-based virtual screens of ∼1.7 M compounds were performed (DOCK program) against a GP2 five-helix bundle, resulting in 165 commercially available compounds purchased for experimental testing. Based on assessment of inhibitory activity, cytotoxicity, and target specificity, four promising candidates emerged with 50% inhibitory concentration values in the 3 to 26 μM range. Molecular dynamics simulations of the two most potent candidates in their DOCK-predicted binding poses indicate that the majority of favorable interactions involve seven highly conserved residues that can be used to guide further inhibitor development and refinement targeting EBOV.IMPORTANCE The most recent Ebola virus disease outbreak, from 2014 to 2016, resulted in approximately 28,000 individuals becoming infected, which led to over 12,000 causalities worldwide. The particularly high pathogenicity of the virus makes paramount the identification and development of promising lead compounds to serve as inhibitors of Ebola infection. To limit viral load, the virus-host membrane fusion event can be targeted through the inhibition of the class I fusion glycoprotein of Ebolavirus In the current work, several promising small-molecule inhibitors that target the glycoprotein GP2 were identified through systematic application of structure-based computational and experimental drug design procedures.

Ebola Virus Entry: From Molecular Characterization to Drug Discovery

Ebola Virus Disease (EVD) is one of the most lethal transmissible infections, characterized by a high fatality rate, and caused by a member of the Filoviridae family. The recent large outbreak of EVD in Western Africa (2013–2016) highlighted the worldwide threat represented by the disease and its impact on global public health and the economy. The development of highly needed anti-Ebola virus antivirals has been so far hampered by the shortage of tools to study their life cycle in vitro, allowing to screen for potential active compounds outside a biosafety level-4 (BSL-4) containment. Importantly, the development of surrogate models to study Ebola virus entry in a BSL-2 setting, such as viral pseudotypes and Ebola virus-like particles, tremendously boosted both our knowledge of the viral life cycle and the identification of promising antiviral compounds interfering with viral entry. In this context, the combination of such surrogate systems with large-scale small molecule compounds and haploid genetic screenings, as well as rational drug design and drug repurposing approaches will prove priceless in our quest for the development of a treatment for EVD.

De Novo Design of α-Helical Lipopeptides Targeting Viral Fusion Proteins: A Promising Strategy for Relatively Broad-Spectrum Antiviral Drug Discovery

Class I enveloped viruses share similarities in their apparent use of a hexameric coiled-coil assembly to drive the merging of virus and host cell membranes. Inhibition of coiled coil-mediated interactions using bioactive peptides that replicate an α-helical chain from the viral fusion machinery has significant antiviral potential. Here, we present the construction of a series of lipopeptides composed of a de novo heptad repeat sequence-based α-helical peptide plus a hydrocarbon tail. Promisingly, the constructs adopted stable α-helical conformations and exhibited relatively broad-spectrum antiviral activities against Middle East respiratory syndrome coronavirus (MERS-CoV) and influenza A viruses (IAVs). Together, these findings reveal a new strategy for relatively broad-spectrum antiviral drug discovery by relying on the tunability of the α-helical coiled-coil domains present in all class I fusion proteins and the amphiphilic nature of the individual helices from this multihelix motif.

Testing therapeutics in cell-based assays: Factors that influence the apparent potency of drugs

Identifying effective antivirals for treating Ebola virus disease (EVD) and minimizing transmission of such disease is critical. A variety of cell-based assays have been developed for evaluating compounds for activity against Ebola virus. However, very few reports discuss the variable assay conditions that can affect the results obtained from these drug screens. Here, we describe variable conditions tested during the development of our cell-based drug screen assays designed to identify compounds with anti-Ebola virus activity using established cell lines and human primary cells. The effect of multiple assay readouts and variable assay conditions, including virus input, time of infection, and the cell passage number, were compared, and the impact on the effective concentration for 50% and/ or 90% inhibition (EC50, EC90) was evaluated using the FDA-approved compound, toremifene citrate. In these studies, we show that altering cell-based assay conditions can have an impact on apparent drug potency as measured by the EC50. These results further support the importance of developing standard operating procedures for generating reliable and reproducible in vitro data sets for potential antivirals.

Discovery of Hydrocarbon-Stapled Short α-Helical Peptides as Promising Middle East Respiratory Syndrome Coronavirus (MERS-CoV) Fusion Inhibitors

The hexameric α-helical coiled-coil formed between the C-terminal and N-terminal heptad repeat (CHR and NHR) regions of class I viral fusion proteins plays an important role in mediating the fusion of the viral and cellular membranes and provides a clear starting point for molecular mimicry that drives viral fusion inhibitor design. Unfortunately, such peptide mimicry of the short α-helical region in the CHR of Middle East respiratory syndrome coronavirus (MERS-CoV) spike protein has been thwarted by the loss of the peptide's native α-helical conformation when taken out of the parent protein structure. Here, we describe that appropriate all-hydrocarbon stapling of the short helical portion-based peptide to reinforce its bioactive secondary structure remarkably improves antiviral potency. The resultant stapled peptide P21S10 could effectively inhibit infection by MERS-CoV pseudovirus and its spike protein-mediated cell-cell fusion; additionally, P21S10 exhibits improved pharmacokinetic properties than HR2P-M2, suggesting strong potential for development as an anti-MERS-CoV therapeutic.

General Staining and Segmentation Procedures for High Content Imaging and Analysis

Automated quantitative fluorescence microscopy, also known as high content imaging (HCI), is a rapidly growing analytical approach in cell biology. Because automated image analysis relies heavily on robust demarcation of cells and subcellular regions, reliable methods for labeling cells is a critical component of the HCI workflow. Labeling of cells for image segmentation is typically performed with fluorescent probes that bind DNA for nuclear-based cell demarcation or with those which react with proteins for image analysis based on whole cell staining. These reagents, along with instrument and software settings, play an important role in the successful segmentation of cells in a population for automated and quantitative image analysis. In this chapter, we describe standard procedures for labeling and image segmentation in both live and fixed cell samples. The chapter will also provide troubleshooting guidelines for some of the common problems associated with these aspects of HCI.

Identification of a peptide derived from the heptad repeat 2 region of the porcine epidemic diarrhea virus (PEDV) spike glycoprotein that is capable of suppressing PEDV entry and inducing neutralizing antibodies

Heptad repeat (HR) regions are highly conserved motifs located in the glycoproteins of enveloped viruses that form a six-helix bundle structure and is important in the process of virus fusion. Peptides derived from the HR regions of some viruses have also been shown to inhibit viral entry. Porcine epidemic diarrhea virus (PEDV) was predicted to have HR regions (HR1 and HR2) in the spike glycoprotein S2 subunit. Based on this analysis, six peptides derived from HR1 and HR2 were selected, expressed in Escherichia coli, purified, and characterized. Three peptides (HR2M, HR2L and HR2P) were identified as potential competitive inhibitors in PEDV in vitro infection assays, with the HR2P peptide representing the most potent inhibitor. Further study indicated that immunization of HR2P in mice elicited antibodies capable of neutralizing PEDV infection in vitro. These results demonstrate that the HR2P peptide and anti-HR2P antibody can serve as a tool for dissecting the fusion mechanism of PEDV, guiding the search for potent inhibitors with therapeutic value against PEDV infection.

•

Six peptides derived from heptad repeat (HR) 1 and 2 regions of PEDV S glycoprotein were expressed and characterized.

•

Three peptides (HR2M, HR2L and HR2P) exhibited antiviral activity in vitro.

•

Immunization of the HR2P peptide in mice elicited antibodies capable of neutralizing PEDV infection in vitro.

•

HR2P peptide can serve as a potential antiviral drug against PEDV infection.

Development of small-molecule viral inhibitors targeting various stages of the life cycle of emerging and re-emerging viruses

In recent years, unexpected outbreaks of infectious diseases caused by emerging and re-emerging viruses have become more frequent, which is possibly due to environmental changes. These outbreaks result in the loss of life and economic hardship. Vaccines and therapeutics should be developed for the prevention and treatment of infectious diseases. In this review, we summarize and discuss the latest progress in the development of small-molecule viral inhibitors against highly pathogenic coronaviruses, including severe acute respiratory syndrome coronavirus and Middle East respiratory syndrome coronavirus, Ebola virus, and Zika virus. These viruses can interfere with the specific steps of viral life cycle by blocking the binding between virus and host cells, disrupting viral endocytosis, disturbing membrane fusion, and interrupting viral RNA replication and translation, thereby demonstrating potent therapeutic effect against various emerging and re-emerging viruses. We also discuss some general strategies for developing small-molecule viral inhibitors.

Screening of commercial cyclic peptide conjugated to HIV-1 Tat peptide as inhibitor of N-terminal heptad repeat glycoprotein-2 ectodomain Ebola virus through in silico analysis

Ebola Hemorrhagic Fever (EHF) is a disease caused by viruses from genus Ebolavirus. Zaire ebolavirus (EBOV) is the deadliest species which has 76% case fatality rate. Up until now, there is no U.S. Food and Drug Administration (FDA) approved drugs to treat EHF. Antiviral drug based on EBOV N-terminal heptad repeat glycoprotein-2 (NHR GP2) Ectodomain inhibitor is one kind of treatment that has not well developed. NHR GP2 Ectodomain has an important role in the process of EBOV entry into the cell through endocytosis mechanism. In this study, we used in silico methods to investigate the activity of commercial cyclic peptide conjugated to Human Immunodeficiency Virus type 1 Trans-activator of the transcription (HIV-1 Tat) peptide as ligands which act as an inhibitor of EBOV NHR GP2 Ectodomain. The commercial cyclic peptides which we used in this study were obtained from the selected chemical companies. Conjugation of the commercial cyclic peptides to HIV-1 Tat peptide was done in order to accumulate it inside the endosome. The ligands which had the best inhibition properties were screened using molecular docking and molecular dynamics simulation. Prediction of pharmacological properties of the peptides was done to choose the best drug candidate. The result of screening processes shows that Ligand 023 has the highest potency as the drug lead. The ligand needs to undergo further analysis through in vitro, in vivo, and a clinical trial to ensure that this ligand has a therapeutic ability as an antiviral drug for Ebola virus infection.

Candidate medical countermeasures targeting Ebola virus cell entry

Medical countermeasures (MCMs) against virus infections ideally prevent the adsorption or entry of virions into target cells, thereby circumventing infection. Recent significant advances in elucidating the mechanism of Ebola virus (EBOV) host-cell penetration include the involvement of two-pore channels at the early stage of entry, and identification of cellular proteases for EBOV spike glycoprotein maturation and the intracellular EBOV receptor, Niemann–Pick type C1. This improved understanding of the initial steps of EBOV infection is now increasingly applied to rapid development of candidate MCMs, some of which have already entered the clinic. Candidate MCMs discussed include antibodies, small molecules and peptides that target various stages of the described EBOV cell-entry pathway. In this review, we summarize the currently known spectrum of EBOV cell-entry inhibitors, describe their mechanism of action and evaluate their potential for future development.

Mechanisms of Filovirus Entry

Filovirus entry into cells is complex, perhaps as complex as any viral entry mechanism identified to date. However, over the past 10 years, the important events required for filoviruses to enter into the endosomal compartment and fuse with vesicular membranes have been elucidated (Fig. 1). Here, we highlight the important steps that are required for productive entry of filoviruses into mammalian cells.

Filovirus proteins for antiviral drug discovery: A structure/function analysis of surface glycoproteins and virus entry

This review focuses on the recent progress in our understanding of filovirus protein structure/function and its impact on antiviral research. Here we focus on the surface glycoprotein GP_1,2 and its different roles in filovirus entry. We first describe the latest advances on the characterization of GP gene-overlapping proteins sGP, ssGP and Δ-peptide. Then, we compare filovirus surface GP_1,2 proteins in terms of structure, synthesis and function. As they bear potential in drug-design, the discovery of small organic compounds inhibiting filovirus entry is a currently very active field. Although it is at an early stage, the development of antiviral drugs against Ebola and Marburg virus entry might prove essential to reduce outbreak-associated fatality rates through post-exposure treatment of both suspected and confirmed cases.

•

The filovirus surface glycoprotein is the key player protein responsible for viral entry.

•

Secreted forms of the glycoprotein have been suggested to participate to filovirus virus pathogenicity.

•

Recent structural insights of the filovirus surface glycoprotein highlight new antiviral perspectives.

•

Interesting compounds and innovative antiviral strategies emerge from research and development to inhibit filovirus entry.

Large-Scale Screening and Identification of Novel Ebola Virus and Marburg Virus Entry Inhibitors

Filoviruses are highly infectious, and no FDA-approved drug therapy for filovirus infection is available. Most work to find a treatment has involved only a few strains of Ebola virus and testing of relatively small drug libraries or compounds that have shown efficacy against other virus types. Here we report the findings of a high-throughput screening of 319,855 small molecules from the Molecular Libraries Small Molecule Repository library for their activities against Marburg virus and Ebola virus. Nine of the most potent, novel compounds that blocked infection by both viruses were analyzed in detail for their mechanisms of action. The compounds inhibited known key steps in the Ebola virus infection mechanism by blocking either cell surface attachment, macropinocytosis-mediated uptake, or endosomal trafficking. To date, very few specific inhibitors of macropinocytosis have been reported. The 2 novel macropinocytosis inhibitors are more potent inhibitors of Ebola virus infection and less toxic than ethylisopropylamiloride, one commonly accepted macropinocytosis inhibitor. Each compound blocked infection of primary human macrophages, indicating their potential to be developed as new antifiloviral therapies.

Cholesterol conjugation potentiates the antiviral activity of an HIV immunoadhesin

Immunoadhesins are engineered proteins combining the constant domain (Fc) of an antibody with a ligand-binding (adhesion) domain. They have significant potential as therapeutic agents, because they maintain the favourable pharmacokinetics of antibodies with an expanded repertoire of ligand-binding domains: proteins, peptides, or small molecules. We have recently reported that the addition of a cholesterol group to two HIV antibodies can dramatically improve their antiviral potency. Cholesterol, which can be conjugated at various positions in the antibody, including the constant (Fc) domain, endows the conjugate with affinity for the membrane lipid rafts, thus increasing its concentration at the site where viral entry occurs. Here, we extend this strategy to an HIV immunoadhesin, combining a cholesterol-conjugated Fc domain with the peptide fusion inhibitor C41. The immunoadhesin C41-Fc-chol displayed high affinity for Human Embryonic Kidney (HEK) 293 cells, and when tested on a panel of HIV-1 strains, it was considerably more potent than the unconjugated C41-Fc construct. Potentiation of antiviral activity was comparable to what was previously observed for the cholesterol-conjugated HIV antibodies. Given the key role of cholesterol in lipid raft formation and viral fusion, we expect that the same strategy should be broadly applicable to enveloped viruses, for many of which it is already known the sequence of a peptide fusion inhibitor similar to C41. Moreover, the sequence of heptad repeat-derived fusion inhibitors can often be predicted from genomic information alone, opening a path to immunoadhesins against emerging viruses.

Enfuvirtide-PEG conjugate: A potent HIV fusion inhibitor with improved pharmacokinetic properties

Enfuvirtide (ENF) is a clinically used peptide drug for the treatment of HIV infections, but its poor pharmacokinetic profile (T1/2 = 1.5 h in rats) and low aqueous solubility make the therapy expensive and inconvenience. In this study, we present a simple and practical strategy to address these problems by conjugating ENF with polyethylene glycol (PEG). Site-specific attachment of a 2 kDa PEG at the N-terminus of ENF resulted in an ENF-PEG (EP) conjugate with high solubility (≥3 mg/mL) and long half-life in rats (T1/2 = 16.1 h). This conjugate showed similar antiviral activity to ENF against various primary HIV-1 isolates (EC50 = 6-91 nM). Mechanistic studies suggested the sources of the antiviral potency. The conjugate bound to a functional domain of the HIV gp41 protein in a helical conformation with high affinity (Kd = 307 nM), thereby inhibiting the gp41-mediated fusion of viral and host-cell membranes. As PEG conjugation has advanced many bioactive proteins and peptides into clinical applications, the EP conjugate described here represents a potential new treatment for HIV infections that may address the unmet medical needs associated with the current ENF therapy.

An effective strategy for recapitulating N-terminal heptad repeat trimers in enveloped virus surface glycoproteins for therapeutic applications

Sequestering peptides derived from the N-terminal heptad repeat (NHR) of class I viral fusion proteins into a non-aggregating trimeric coiled-coil conformation remains a major challenge. Here, we implemented a synthetic strategy to stabilize NHR-helical trimers, with the human immunodeficiency virus type 1 (HIV-1) gp41 fusion protein as the initial focus. A set of trimeric scaffolds was realized in a synthetic gp41 NHR-derived peptide sequence by relying on the tractability of coiled-coil structures and an additional isopeptide bridge-tethering strategy. Among them, (N36M)3 folded as a highly stable helical trimer and exhibited promising inhibitory activity against HIV-1 infection, exceptional resistance to proteolysis, and effective native ligand-binding capability. We anticipate that the trimeric coiled-coil recapitulation methodology described herein may have broader applicability to yield NHR trimers of other class I enveloped viruses and to prepare helical tertiary structure mimetics of certain natural protein-protein interactions for biomedical applications.

Host-Primed Ebola Virus GP Exposes a Hydrophobic NPC1 Receptor-Binding Pocket, Revealing a Target for Broadly Neutralizing Antibodies

The filovirus surface glycoprotein (GP) mediates viral entry into host cells. Following viral internalization into endosomes, GP is cleaved by host cysteine proteases to expose a receptor-binding site (RBS) that is otherwise hidden from immune surveillance. Here, we present the crystal structure of proteolytically cleaved Ebola virus GP to a resolution of 3.3 Å. We use this structure in conjunction with functional analysis of a large panel of pseudotyped viruses bearing mutant GP proteins to map the Ebola virus GP endosomal RBS at molecular resolution. Our studies indicate that binding of GP to its endosomal receptor Niemann-Pick C1 occurs in two distinct stages: the initial electrostatic interactions are followed by specific interactions with a hydrophobic trough that is exposed on the endosomally cleaved GP1 subunit. Finally, we demonstrate that monoclonal antibodies targeting the filovirus RBS neutralize all known filovirus GPs, making this conserved pocket a promising target for the development of panfilovirus therapeutics.

IMPORTANCE

Ebola virus uses its glycoprotein (GP) to enter new host cells. During entry, GP must be cleaved by human enzymes in order for receptor binding to occur. Here, we provide the crystal structure of the cleaved form of Ebola virus GP. We demonstrate that cleavage exposes a site at the top of GP and that this site binds the critical domain C of the receptor, termed Niemann-Pick C1 (NPC1). We perform mutagenesis to find parts of the site essential for binding NPC1 and map distinct roles for an upper, charged crest and lower, hydrophobic trough in cleaved GP. We find that this 3-dimensional site is conserved across the filovirus family and that antibody directed against this site is able to bind cleaved GP from every filovirus tested and neutralize viruses bearing those GPs.

Development of Small-molecule HIV Entry Inhibitors Specifically Targeting gp120 or gp41

Human immunodeficiency virus type 1 (HIV-1) envelope (Env) glycoprotein surface subunit gp120 and transmembrane subunit gp41 play important roles in HIV-1 entry, thus serving as key targets for the development of HIV-1 entry inhibitors. T20 peptide (enfuvirtide) is the first U.S. FDA-approved HIV entry inhibitor; however, its clinical application is limited by the lack of oral availability. Here, we have described the structure and function of the HIV-1 gp120 and gp41 subunits and reviewed advancements in the development of small-molecule HIV entry inhibitors specifically targeting these two Env glycoproteins. We then compared the advantages and disadvantages of different categories of HIV entry inhibitor candidates and further predicted the future trend of HIV entry inhibitor development.

Design and characterization of ebolavirus GP prehairpin intermediate mimics as drug targets

Ebolaviruses are highly lethal filoviruses that cause hemorrhagic fever in humans and nonhuman primates. With no approved treatments or preventatives, the development of an anti-ebolavirus therapy to protect against natural infections and potential weaponization is an urgent global health need. Here, we describe the design, biophysical characterization, and validation of peptide mimics of the ebolavirus N-trimer, a highly conserved region of the GP2 fusion protein, to be used as targets to develop broad-spectrum inhibitors of ebolavirus entry. The N-trimer region of GP2 is 90% identical across all ebolavirus species and forms a critical part of the prehairpin intermediate that is exposed during viral entry. Specifically, we fused designed coiled coils to the N-trimer to present it as a soluble trimeric coiled coil as it appears during membrane fusion. Circular dichroism, sedimentation equilibrium, and X-ray crystallography analyses reveal the helical, trimeric structure of the designed N-trimer mimic targets. Surface plasmon resonance studies validate that the N-trimer mimic binds its native ligand, the C-peptide region of GP2. The longest N-trimer mimic also inhibits virus entry, thereby confirming binding of the C-peptide region during viral entry and the presence of a vulnerable prehairpin intermediate. Using phage display as a model system, we validate the suitability of the N-trimer mimics as drug screening targets. Finally, we describe the foundational work to use the N-trimer mimics as targets in mirror-image phage display, which will be used to identify D-peptide inhibitors of ebolavirus entry.

Structural and Functional Studies on the Marburg Virus GP2 Fusion Loop

Marburg virus (MARV) and the ebolaviruses belong to the family Filoviridae (the members of which are filoviruses) that cause severe hemorrhagic fever. Infection requires fusion of the host and viral membranes, a process that occurs in the host cell endosomal compartment and is facilitated by the envelope glycoprotein fusion subunit, GP2. The N-terminal fusion loop (FL) of GP2 is a hydrophobic disulfide-bonded loop that is postulated to insert and disrupt the host endosomal membrane during fusion. Here, we describe the first structural and functional studies of a protein corresponding to the MARV GP2 FL. We found that this protein undergoes a pH-dependent conformational change, as monitored by circular dichroism and nuclear magnetic resonance. Furthermore, we report that, under low pH conditions, the MARV GP2 FL can induce content leakage from liposomes. The general aspects of this pH-dependent structure and lipid-perturbing behavior are consistent with previous reports on Ebola virus GP2 FL. However, nuclear magnetic resonance studies in lipid bicelles and mutational analysis indicate differences in structure exist between MARV and Ebola virus GP2 FL. These results provide new insight into the mechanism of MARV GP2-mediated cell entry.

Chemical and Structural Aspects of Ebola Virus Entry Inhibitors

The Ebolaviruses are members of the family Filoviridae (“filoviruses”) and cause severe hemhorragic fever with human case fatality rates as high as 90%. Infection requires attachment of the viral particle to cells and triggering of membrane fusion between the host and viral membranes, a process that occurs in the host endosome and is facilitated by the envelope glycoprotein (GP). One potential strategy for therapeutic intervention is the development of agents (antibodies, peptides, and small molecules) that can interfere with viral entry aspects such as attachment, uptake, priming, or membrane fusion. This paper highlights recent developments in the discovery and evaluation of therapeutic entry inhibitors and identifies opportunities moving forward.

[Ebola virus host cell entry]

Ebola virus is an enveloped virus with filamentous structure and causes a severe hemorrhagic fever in human and nonhuman primates. Host cell entry is the first essential step in the viral life cycle, which has been extensively studied as one of the therapeutic targets. A virus factor of cell entry is a surface glycoprotein (GP), which is an only essential viral protein in the step, as well as the unique particle structure. The virus also interacts with a lot of host factors to successfully enter host cells. Ebola virus at first binds to cell surface proteins and internalizes into cells, followed by trafficking through endosomal vesicles to intracellular acidic compartments. There, host proteases process GPs, which can interact with an intracellular receptor. Then, under an appropriate circumstance, viral and endosomal membranes are fused, which is enhanced by major structural changes of GPs, to complete host cell entry. Recently the basic research of Ebola virus infection mechanism has markedly progressed, largely contributed by identification of host factors and detailed structural analyses of GPs. This article highlights the mechanism of Ebola virus host cell entry, including recent findings.

Development of therapeutics for treatment of Ebola virus infection

Ebola virus infection can cause Ebola virus disease (EVD). Patients usually show severe symptoms, and the fatality rate can reach up to 90%. No licensed medicine is available. In this review, development of therapeutics for treatment of Ebola virus infection and EVD will be discussed.

Cholesterol-conjugated peptide antivirals: a path to a rapid response to emerging viral diseases

While it is now possible to identify and genetically fingerprint the causative agents of emerging viral diseases, often with extraordinary speed, suitable therapies cannot be developed with equivalent speed, because drug discovery requires information that goes beyond knowledge of the viral genome. Peptides, however, may represent a special opportunity.

For all enveloped viruses, fusion between the viral and the target cell membrane is an obligatory step of the life cycle. Class I fusion proteins harbor regions with a repeating pattern of amino acids, the heptad repeats (HRs), that play a key role in fusion, and HR‐derived peptides such as enfuvirtide, in clinical use for HIV, can block the process. Because of their characteristic sequence pattern, HRs are easily identified in the genome by means of computer programs, providing the sequence of candidate peptide inhibitors directly from genomic information.

Moreover, a simple chemical modification, the attachment of a cholesterol group, can dramatically increase the antiviral potency of HR‐derived inhibitors and simultaneously improve their pharmacokinetics. Further enhancement can be provided by dimerization of the cholesterol‐conjugated peptide. The examples reported so far include inhibitors of retroviruses, paramyxoviruses, orthomyxoviruses, henipaviruses, coronaviruses, and filoviruses. For some of these viruses, in vivo efficacy has been demonstrated in suitable animal models.

The combination of bioinformatic lead identification and potency/pharmacokinetics improvement provided by cholesterol conjugation may form the basis for a rapid response strategy, where development of an emergency cholesterol‐conjugated therapeutic would immediately follow the availability of the genetic information of a new enveloped virus. Copyright © 2014 European Peptide Society and John Wiley & Sons, Ltd.

An update on the use of antibodies against the filoviruses

Multiple recent, independent studies have confirmed that passively administered antibodies can provide effective postexposure therapy in nonhuman primates after exposure to an otherwise lethal dose of Ebola virus or Marburg virus. In this article, we review composition and performance of the antibody cocktails tested thus far, what is known about antibody epitopes on the viral glycoprotein target and ongoing research questions in further development of such cocktails for pre-exposure or emergency postexposure use.

Role of electrostatic repulsion in controlling pH-dependent conformational changes of viral fusion proteins

Viral fusion proteins undergo dramatic conformational transitions during membrane fusion. For viruses that enter through the endosome, these conformational rearrangements are typically pH sensitive. Here, we provide a comprehensive review of the molecular interactions that govern pH-dependent rearrangements and introduce a paradigm for electrostatic residue pairings that regulate progress through the viral fusion coordinate. Analysis of structural data demonstrates a significant role for side-chain protonation in triggering conformational change. To characterize this behavior, we identify two distinct residue pairings, which we define as Histidine-Cation (HisCat) and Anion-Anion (AniAni) interactions. These side-chain pairings destabilize a particular conformation via electrostatic repulsion through side-chain protonation. Furthermore, two energetic control mechanisms, thermodynamic and kinetic, regulate these structural transitions. This review expands on the current literature by identification of these residue clusters, discussion of data demonstrating their function, and speculation of how these residue pairings contribute to the energetic controls.

Peptides corresponding to the predicted heptad repeat 2 domain of the feline coronavirus spike protein are potent inhibitors of viral infection

Background

Feline infectious peritonitis (FIP) is a lethal immune-mediated disease caused by feline coronavirus (FCoV). Currently, no therapy with proven efficacy is available. In searching for agents that may prove clinically effective against FCoV infection, five analogous overlapping peptides were designed and synthesized based on the putative heptad repeat 2 (HR2) sequence of the spike protein of FCoV, and the antiviral efficacy was evaluated.

Methods

Plaque reduction assay and MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) cytotoxicity assay were performed in this study. Peptides were selected using a plaque reduction assay to inhibit Feline coronavirus infection.

Results

The results demonstrated that peptide (FP5) at concentrations below 20 μM inhibited viral replication by up to 97%. The peptide (FP5) exhibiting the most effective antiviral effect was further combined with a known anti-viral agent, human interferon-α (IFN-α), and a significant synergistic antiviral effect was observed.

Conclusion

Our data suggest that the synthetic peptide FP5 could serve as a valuable addition to the current FIP prevention methods.

Emerging targets and novel approaches to Ebola virus prophylaxis and treatment

Ebola is a highly virulent pathogen causing severe hemorrhagic fever with a high case fatality rate in humans and non-human primates (NHPs). Although safe and effective vaccines or other medicinal agents to block Ebola infection are currently unavailable, a significant effort has been put forth to identify several promising candidates for the treatment and prevention of Ebola hemorrhagic fever. Among these, recombinant adenovirus-based vectors have been identified as potent vaccine candidates, with some affording both pre- and post-exposure protection from the virus. Recently, Investigational New Drug (IND) applications have been approved by the US Food and Drug Administration (FDA) and phase I clinical trials have been initiated for two small-molecule therapeutics: anti-sense phosphorodiamidate morpholino oligomers (PMOs: AVI-6002, AVI-6003) and lipid nanoparticle/small interfering RNA (LNP/siRNA: TKM-Ebola). These potential alternatives to vector-based vaccines require multiple doses to achieve therapeutic efficacy, which is not ideal with regard to patient compliance and outbreak scenarios. These concerns have fueled a quest for even better vaccination and treatment strategies. Here, we summarize recent advances in vaccines or post-exposure therapeutics for prevention of Ebola hemorrhagic fever. The utility of novel pharmaceutical approaches to refine and overcome barriers associated with the most promising therapeutic platforms are also discussed.

C-peptide inhibitors of Ebola virus glycoprotein-mediated cell entry: effects of conjugation to cholesterol and side chain-side chain crosslinking

We previously described potent inhibition of Ebola virus entry by a 'C-peptide' based on the GP2 C-heptad repeat region (CHR) targeted to endosomes ('Tat-Ebo'). Here, we report the synthesis and evaluation of C-peptides conjugated to cholesterol, and Tat-Ebo analogs containing covalent side chain-side chain crosslinks to promote α-helical conformation. We found that the cholesterol-conjugated C-peptides were potent inhibitors of Ebola virus glycoprotein (GP)-mediated cell entry (~10(3)-fold reduction in infection at 40 μM). However, this mechanism of inhibition is somewhat non-specific because the cholesterol-conjugated peptides also inhibited cell entry mediated by vesicular stomatitis virus glycoprotein G. One side chain-side chain crosslinked peptide had moderately higher activity than the parent compound Tat-Ebo. Circular dichroism revealed that the cholesterol-conjugated peptides unexpectedly formed a strong α-helical conformation that was independent of concentration. Side chain-side chain crosslinking enhanced α-helical stability of the Tat-Ebo variants, but only at neutral pH. These result provide insight into mechanisms of C-peptide inhibiton of Ebola virus GP-mediated cell entry.

Development of novel entry inhibitors targeting emerging viruses

Emerging viral diseases pose a unique risk to public health, and thus there is a need to develop therapies. A current focus of funding agencies, and hence research, is the development of broad-spectrum antivirals, and in particular, those targeting common cellular pathways. The scope of this article is to review screening strategies and recent advances in this area, with a particular emphasis on antivirals targeting the step of viral entry for emerging lipid-enveloped viruses such as Ebola virus and SARS-coronavirus.

Conformational properties of peptides corresponding to the ebolavirus GP2 membrane-proximal external region in the presence of micelle-forming surfactants and lipids

Ebola virus and Sudan virus are members of the family Filoviridae of nonsegmented negative-strand RNA viruses ("filoviruses") that cause severe hemorrhagic fever with fatality rates as high as 90%. Infection by filoviruses requires membrane fusion between the host and the virus; this process is facilitated by the two subunits of the envelope glycoprotein, GP1 (the surface subunit) and GP2 (the transmembrane subunit). The membrane-proximal external region (MPER) is a Trp-rich segment that immediately precedes the transmembrane domain of GP2. In the analogous glycoprotein for HIV-1 (gp41), the MPER is critical for membrane fusion and is the target of several neutralizing antibodies. However, the role of the MPER in filovirus GP2 and its importance in membrane fusion have not been established. Here, we characterize the conformational properties of peptides representing the GP MPER segments of Ebola virus and Sudan virus in the presence of micelle-forming surfactants and lipids, at pH 7 and 4.6. Circular dichroism spectroscopy and tryptophan fluorescence indicate that the GP2 MPER peptides bind to micelles of sodium dodecyl sulfate and dodecylphosphocholine (DPC). Nuclear magnetic resonance spectroscopy of the Sudan virus MPER peptide revealed that residues 644-651 interact directly with DPC, and that this interaction enhances the helical conformation of the peptide. The Sudan virus MPER peptide was found to moderately inhibit cell entry by a GP-pseudotyped vesicular stomatitis virus but did not induce leakage of a fluorescent molecule from a large unilammellar vesicle comprised of 1-palmitoyl-2-oleoylphosphatidylcholine or cause hemolysis. Taken together, this analysis suggests the filovirus GP2 MPER binds and inserts shallowly into lipid membranes.

Host cell factors in filovirus entry: novel players, new insights

Filoviruses cause severe hemorrhagic fever in humans with high case-fatality rates. The cellular factors exploited by filoviruses for their spread constitute potential targets for intervention, but are incompletely defined. The viral glycoprotein (GP) mediates filovirus entry into host cells. Recent studies revealed important insights into the host cell molecules engaged by GP for cellular entry. The binding of GP to cellular lectins was found to concentrate virions onto susceptible cells and might contribute to the early and sustained infection of macrophages and dendritic cells, important viral targets. Tyrosine kinase receptors were shown to promote macropinocytic uptake of filoviruses into a subset of susceptible cells without binding to GP, while interactions between GP and human T cell Ig mucin 1 (TIM-1) might contribute to filovirus infection of mucosal epithelial cells. Moreover, GP engagement of the cholesterol transporter Niemann-Pick C1 was demonstrated to be essential for GP-mediated fusion of the viral envelope with a host cell membrane. Finally, mutagenic and structural analyses defined GP domains which interact with these host cell factors. Here, we will review the recent progress in elucidating the molecular interactions underlying filovirus entry and discuss their implications for our understanding of the viral cell tropism.

Peptides as the next generation of anti-infectives

Synthesis and large-scale manufacturing technologies are now available for the commercial production of even the most complex peptide anti-infectives. Married with the potential of this class of molecule as the next generation of effective, resistance-free and safe antimicrobials, and a much better understanding of their biology, pharmacology and pharmacodynamics, the first regulatory approvals and introduction into clinical practice of these promising drug candidates will likely be soon. This is a key juncture in the history/life cycle of peptide anti-infectives and, perhaps, their commercial and therapeutic potential is about to be realized. This review highlights the promise of these agents as the next generation of therapeutics and summarizes the challenges faced in, and lessons learned from, the past.

Potential vaccines and post-exposure treatments for filovirus infections

Viruses of the family Filoviridae represent significant health risks as emerging infectious diseases as well as potentially engineered biothreats. While many research efforts have been published offering possibilities toward the mitigation of filoviral infection, there remain no sanctioned therapeutic or vaccine strategies. Current progress in the development of filovirus therapeutics and vaccines is outlined herein with respect to their current level of testing, evaluation, and proximity toward human implementation, specifically with regard to human clinical trials, nonhuman primate studies, small animal studies, and in vitro development. Contemporary methods of supportive care and previous treatment approaches for human patients are also discussed.

A general strategy to endow natural fusion-protein-derived peptides with potent antiviral activity

Fusion between the viral and target cell membranes is an obligatory step for the infectivity of all enveloped virus, and blocking this process is a clinically validated therapeutic strategy.

Viral fusion is driven by specialized proteins which, although specific to each virus, act through a common mechanism, the formation of a complex between two heptad repeat (HR) regions. The HR regions are initially separated in an intermediate termed “prehairpin”, which bridges the viral and cell membranes, and then fold onto each other to form a 6-helical bundle (6HB), driving the two membranes to fuse. HR-derived peptides can inhibit viral infectivity by binding to the prehairpin intermediate and preventing its transition to the 6HB.

The antiviral activity of HR-derived peptides differs considerably among enveloped viruses. For weak inhibitors, potency can be increased by peptide engineering strategies, but sequence-specific optimization is time-consuming. In seeking ways to increase potency without changing the native sequence, we previously reported that attachment to the HR peptide of a cholesterol group (”cholesterol-tagging”) dramatically increases its antiviral potency, and simultaneously increases its half-life in vivo. We show here that antiviral potency may be increased by combining cholesterol-tagging with dimerization of the HR-derived sequence, using as examples human parainfluenza virus, Nipah virus, and HIV-1. Together, cholesterol-tagging and dimerization may represent strategies to boost HR peptide potency to levels that in some cases may be compatible with in vivo use, possibly contributing to emergency responses to outbreaks of existing or novel viruses.