Heptad-repeat regions of respiratory syncytial virus F1 protein form a six-membered coiled-coil complex

Therapeutic stapled peptides: Efficacy and molecular targets

Peptide stapling, by employing a stable, preformed alpha-helical conformation, results in the production of peptides with improved membrane permeability and enhanced proteolytic stability, compared to the original peptides, and provides an effective solution to accelerate the rapid development of peptide drugs. Various reviews present peptide stapling chemistries, anchoring residues and one- or two-component cyclization, however, therapeutic stapled peptides have not been systematically summarized, especially focusing on various disease-related targets. This review highlights the latest advances in therapeutic peptide drug development facilitated by the application of stapling technology, including different stapling techniques, synthetic accessibility, applicability to biological targets, potential for solving biological problems, as well as the current status of development. Stapled peptides as therapeutic drug candidates have been classified and analysed mainly by receptor- and ligand-based stapled peptide design against various diseases, including cancer, infectious diseases, inflammation, and diabetes. This review is expected to provide a comprehensive reference for the rational design of stapled peptides for different diseases and targets to facilitate the development of therapeutic peptides with enhanced pharmacokinetic and biological properties.

Evidence that an interaction between the respiratory syncytial virus F and G proteins at the distal ends of virus filaments mediates efficient multiple cycle infection

Evidence for a stable interaction between the respiratory syncytial virus (RSV) F and G proteins on the surface of virus filaments was provided using antibody immunoprecipitation studies on purified RSV particles, and by the in situ analysis on the surface of RSV-infected cells using the proximity ligation assay. Imaging of the F and G protein distribution on virus filaments suggested that this protein complex was localised at the distal ends of the virus filaments, and suggested that this protein complex played a direct role in mediating efficient localised cell-to-cell virus transmission. G protein expression was required for efficient localised cell-to-cell transmission of RSV in cell monolayers which provided evidence that this protein complex mediates efficient multiple cycle infection. Collectively, these data provide evidence that F and G proteins form a complex on the surface of RSV particles, and that a role for this protein complex in promoting virus transmission is suggested.

Prefusion-specific antibody-derived peptides trivalently presented on DNA-nanoscaffolds as an innovative strategy against RSV entry

Human respiratory syncytial virus (RSV) is the primary cause of acute lower respiratory tract infections in children and the elderly worldwide, for which neither a vaccine nor an effective therapy is approved. The entry of RSV into the host cell is mediated by stepwise structural changes in the surface RSV fusion (RSV-F) glycoprotein. Recent progress in structural and functional studies of RSV-F glycoprotein revealed conformation-dependent neutralizing epitopes which have become attractive targets for vaccine and therapeutic development. As RSV-F is present on viral surface in a trimeric form, a trivalent binding interaction between a candidate fusion inhibitor and the respective epitopes on each of the three monomers is expected to prevent viral infection at higher potency than a monovalent or bivalent inhibitor. Here we demonstrate a novel RSV entry inhibitory approach by implementing a trimeric DNA nanostructure as a template to display up to three linear peptide moieties that simultaneously target an epitope on the surface of the prefusion RSV-F protein. In order to design synthetic binding peptides that can be coupled to the DNA nanostructure, the prefusion RSV-F-specific monoclonal antibody (D25) was selected. Complementarity-determining region 3 (CDR3) derived peptides underwent truncation and alanine-scanning mutagenesis analysis, followed by systematic sequence modifications using non-canonical amino acids. The most effective peptide candidate was used as a binding moiety to functionalize the DNA nanostructure. The designed DNA-peptide construct was able to block RSV infection on cells more efficiently than the monomeric peptides, however a more moderate reduction of viral load was observed in the lungs of infected mice upon intranasal application, likely due to dissociation or absorption of the underlying DNA structure by cells in the lungs. Taken together, our results point towards the inhibitory potential of a novel trimeric DNA-peptide based approach against RSV and open the possibility to apply this platform to target other viral infections.

Labyrinthopeptins as virolytic inhibitors of respiratory syncytial virus cell entry

Acute lower respiratory tract infections (ALRI) caused by respiratory syncytial virus (RSV) are associated with a severe disease burden among infants and elderly patients. Treatment options are limited. While numerous drug candidates with different viral targets are under development, the utility of RSV entry inhibitors is challenged by a low resistance barrier and by single mutations causing cross-resistance against a wide spectrum of fusion inhibitor chemotypes. We developed a cell-based screening assay for discovery of compounds inhibiting infection with primary RSV isolates. Using this system, we identified labyrinthopeptin A1 and A2 (Laby A1/A2), lantibiotics isolated from Actinomadura namibiensis, as effective RSV cell entry inhibitors with IC₅₀s of 0.39 μM and 4.97 μM, respectively, and with favourable therapeutic index (>200 and > 20, respectively). Both molecules were active against multiple RSV strains including primary isolates and their antiviral activity against RSV was confirmed in primary human airway cells ex vivo and a murine model in vivo. Laby A1/A2 were antiviral in prophylactic and therapeutic treatment regimens and displayed synergistic activity when applied in combination with each other. Mechanistic studies showed that Laby A1/A2 exert virolytic activity likely by binding to phosphatidylethanolamine moieties within the viral membrane and by disrupting virus particle membrane integrity. Probably due to its specific mode of action, Laby A1/A2 antiviral activity was not affected by common resistance mutations to known RSV entry inhibitors. Taken together, Laby A1/A2 represent promising candidates for development as RSV inhibitors. Moreover, the cell-based screening system with primary RSV isolates described here should be useful to identify further antiviral agents.

Alternative conformations of a major antigenic site on RSV F

The respiratory syncytial virus (RSV) fusion (F) glycoprotein is a major target of neutralizing antibodies arising from natural infection, and antibodies that specifically bind to the prefusion conformation of RSV F generally demonstrate the greatest neutralization potency. Prefusion-stabilized RSV F variants have been engineered as vaccine antigens, but crystal structures of these variants have revealed conformational differences in a key antigenic site located at the apex of the trimer, referred to as antigenic site Ø. Currently, it is unclear if flexibility in this region is an inherent property of prefusion RSV F or if it is related to inadequate stabilization of site Ø in the engineered variants. Therefore, we set out to investigate the conformational flexibility of antigenic site Ø, as well as the ability of the human immune system to recognize alternative conformations of this site, by determining crystal structures of prefusion RSV F bound to neutralizing human-derived antibodies AM22 and RSD5. Both antibodies bound with high affinity and were specific for the prefusion conformation of RSV F. Crystal structures of the complexes revealed that the antibodies recognized distinct conformations of antigenic site Ø, each diverging at a conserved proline residue located in the middle of an α-helix. These data suggest that antigenic site Ø exists as an ensemble of conformations, with individual antibodies recognizing discrete states. Collectively, these results have implications for the refolding of pneumovirus and paramyxovirus fusion proteins and should inform development of prefusion-stabilized RSV F vaccine candidates.

A pan-coronavirus fusion inhibitor targeting the HR1 domain of human coronavirus spike

Continuously emerging highly pathogenic human coronaviruses (HCoVs) remain a major threat to human health, as illustrated in past SARS-CoV and MERS-CoV outbreaks. The development of a drug with broad-spectrum HCoV inhibitory activity would address this urgent unmet medical need. Although previous studies have suggested that the HR1 of HCoV spike (S) protein is an important target site for inhibition against specific HCoVs, whether this conserved region could serve as a target for the development of broad-spectrum pan-CoV inhibitor remains controversial. Here, we found that peptide OC43-HR2P, derived from the HR2 domain of HCoV-OC43, exhibited broad fusion inhibitory activity against multiple HCoVs. EK1, the optimized form of OC43-HR2P, showed substantially improved pan-CoV fusion inhibitory activity and pharmaceutical properties. Crystal structures indicated that EK1 can form a stable six-helix bundle structure with both short α-HCoV and long β-HCoV HR1s, further supporting the role of HR1 region as a viable pan-CoV target site.

Potent MERS-CoV Fusion Inhibitory Peptides Identified from HR2 Domain in Spike Protein of Bat Coronavirus HKU4

The Middle East respiratory syndrome coronavirus (MERS-CoV) emerged in 2012 and caused continual outbreaks worldwide with high mortality. However, no effective anti-MERS-CoV drug is currently available. Recently, numerous evolutionary studies have suggested that MERS-CoV originated from bat coronavirus (BatCoV). We herein reported that three peptides derived from the HR2 region in spike protein of BatCoV HKU4, including HKU4-HR2P1, HKU4-HR2P2 and HKU4-HR2P3, could bind the MERS-CoV HR1-derived peptide to form a six-helix bundle (6-HB) with high stability. Moreover, these peptides, particularly HKU4-HR2P2 and HKU4-HR2P3, exhibited potent inhibitory activity against MERS-CoV S-mediated cell–cell fusion and viral infection, suggesting that these HKU4 HR2-derived peptides could be candidates for futher development as antiviral agents against MERS-CoV infection.

Generation of monoclonal antibodies specific of the postfusion conformation of the Pneumovirinae fusion (F) protein

Paramyxovirus entry into cells requires fusion of the viral and cell membranes mediated by one of the major virus glycoproteins, the fusion (F) glycoprotein which transits from a metastable pre-fusion conformation to a highly stable post-fusion structure during the membrane fusion process. F protein refolding involves large conformational changes of the protein trimer. One of these changes results in assembly of two heptad repeat sequences (HRA and HRB) from each protomer into a six-helix bundle (6HB) motif. To assist in distinguishing pre- and post-fusion conformations of the Pneumovirinae F proteins, and as extension of previous work (Palomo et al., 2014), a general strategy was designed to obtain polyclonal and particularly monoclonal antibodies specific of the 6HB motif of the Pneumovirinae fusion protein. The antibodies reported here should assist in the characterization of the structural changes that the F protein of human metapneumovirus or respiratory syncytial virus experiences during the process of membrane fusion.

HRA2pl peptide: a fusion inhibitor for human metapneumovirus produced in tobacco plants by transient transformation

MAIN CONCLUSION

The HRA2pl peptide expressed by transient transformation in N. tabacum plants is capable of inhibiting the binding of the human metapneumovirus to HEp-2 cells at the fusion stage. Human metapneumovirus (hMPV) is an agent responsible for acute respiratory infections that mainly affects children under 3 years, the elderly and immunocompromised patients. In children younger than 5 years, respiratory tract infections account for 20 % of deaths worldwide. However, there is currently no treatment or vaccine available against hMPV. The production of a safe, efficient and low cost treatment against this virus is a current challenge. Plants provide a system for recombinant protein production that is cost effective and is easier to scale up to an industrial level than other platforms; in addition, the plant tissue may be used as raw food, dried or, alternatively, proteins may be partially or fully purified and administered in aerosol or capsules as dry powder. In this study, we designed a gene expressing an antiviral peptide against hMPV based on the heptad repeat A domain of the F protein of the virus. We produced the recombinant peptide by a viral transient expression system (Magnifection(®)) in Nicotiana tabacum plants. The efficacy of this antiviral peptide was confirmed by in vitro assays in HEp-2 cell line. This is a promising result that can offer a prophylactic approach against hMPV.

Peste des petits ruminants virus infection of small ruminants: a comprehensive review

Peste des petits ruminants (PPR) is caused by a Morbillivirus that belongs to the family Paramyxoviridae. PPR is an acute, highly contagious and fatal disease primarily affecting goats and sheep, whereas cattle undergo sub-clinical infection. With morbidity and mortality rates that can be as high as 90%, PPR is classified as an OIE (Office International des Epizooties)-listed disease. Considering the importance of sheep and goats in the livelihood of the poor and marginal farmers in Africa and South Asia, PPR is an important concern for food security and poverty alleviation. PPR virus (PPRV) and rinderpest virus (RPV) are closely related Morbilliviruses. Rinderpest has been globally eradicated by mass vaccination. Though a live attenuated vaccine is available against PPR for immunoprophylaxis, due to its instability in subtropical climate (thermo-sensitivity), unavailability of required doses and insufficient coverage (herd immunity), the disease control program has not been a great success. Further, emerging evidence of poor cross neutralization between vaccine strain and PPRV strains currently circulating in the field has raised concerns about the protective efficacy of the existing PPR vaccines. This review summarizes the recent advancement in PPRV replication, its pathogenesis, immune response to vaccine and disease control. Attempts have also been made to highlight the current trends in understanding the host susceptibility and resistance to PPR.

Polyclonal and monoclonal antibodies specific for the six-helix bundle of the human respiratory syncytial virus fusion glycoprotein as probes of the protein post-fusion conformation

Human respiratory syncytial virus (hRSV) has two major surface glycoproteins (G and F) anchored in the lipid envelope. Membrane fusion promoted by hRSV_F occurs via refolding from a pre-fusion form to a highly stable post-fusion state involving large conformational changes of the F trimer. One of these changes results in assembly of two heptad repeat sequences (HRA and HRB) into a six-helix bundle (6HB) motif. To assist in distinguishing pre- and post-fusion conformations of hRSV_F, we have prepared polyclonal (α-6HB) and monoclonal (R145) rabbit antibodies specific for the 6HB. Among other applications, these antibodies were used to explore the requirements of 6HB formation by isolated protein segments or peptides and by truncated mutants of the F protein. Site-directed mutagenesis and electron microscopy located the R145 epitope in the post-fusion hRSV_F at a site distantly located from previously mapped epitopes, extending the repertoire of antibodies that can decorate the F molecule.

Mucosal delivery of a double-stapled RSV peptide prevents nasopulmonary infection

Respiratory syncytial virus (RSV) infection accounts for approximately 64 million cases of respiratory disease and 200,000 deaths worldwide each year, yet no broadly effective prophylactic or treatment regimen is available. RSV deploys paired, self-associating, heptad repeat domains of its fusion protein, RSV-F, to form a fusogenic 6-helix bundle that enables the virus to penetrate the host cell membrane. Here, we developed hydrocarbon double-stapled RSV fusion peptides that exhibit stabilized α-helical structure and striking proteolytic resistance. Pretreatment with double-stapled RSV peptides that specifically bound to the RSV fusion bundle inhibited infection by both laboratory and clinical RSV isolates in cells and murine infection models. Intranasal delivery of a lead double-stapled RSV peptide effectively prevented viral infection of the nares. A chitosan-based nanoparticle preparation markedly enhanced pulmonary delivery, further preventing progression of RSV infection to the lung. Thus, our results provide a strategy for inhibiting RSV infection by mucosal and endotracheal delivery of double-stapled RSV fusion peptides.

In search of a small-molecule inhibitor for respiratory syncytial virus

Respiratory syncytial virus has been an ongoing health problem for 50 years. Hospitalization rates due to virus-induced respiratory illness continue to be substantial for infants, small children, the elderly and the immunocompromised. The only currently available treatments are a broad-spectrum antiviral and two immunoprophylactic antibodies, all of which are reserved for high-risk patients. The combination of this limited therapeutic repertoire and the lack of a vaccine clearly demonstrates the need to continue the search for more efficacious and safe agents against respiratory syncytial virus. The following is a review on the current progress of that search.

Neutralization of human respiratory syncytial virus infectivity by antibodies and low-molecular-weight compounds targeted against the fusion glycoprotein

Human respiratory syncytial virus (HRSV) fusion (F) protein is an essential component of the virus envelope that mediates fusion of the viral and cell membranes, and, therefore, it is an attractive target for drug and vaccine development. Our aim was to analyze the neutralizing mechanism of anti-F antibodies in comparison with other low-molecular-weight compounds targeted against the F molecule. It was found that neutralization by anti-F antibodies is related to epitope specificity. Thus, neutralizing and nonneutralizing antibodies could bind equally well to virions and remained bound after ultracentrifugation of the virus, but only the former inhibited virus infectivity. Neutralization by antibodies correlated with inhibition of cell-cell fusion in a syncytium formation assay, but not with inhibition of virus binding to cells. In contrast, a peptide (residues 478 to 516 of F protein [F478-516]) derived from the F protein heptad repeat B (HRB) or the organic compound BMS-433771 did not interfere with virus infectivity if incubated with virus before ultracentrifugation or during adsorption of virus to cells at 4 degrees C. These inhibitors must be present during virus entry to effect HRSV neutralization. These results are best interpreted by asserting that neutralizing antibodies bind to the F protein in virions interfering with its activation for fusion. Binding of nonneutralizing antibodies is not enough to block this step. In contrast, the peptide F478-516 or BMS-433771 must bind to F protein intermediates generated during virus-cell membrane fusion, blocking further development of this process.

Pharmacologic advances in the treatment and prevention of respiratory syncytial virus

Currently, only 2 drugs have been approved for the treatment of respiratory syncytial virus (RSV). Palivizumab is a monoclonal antibody for the prevention of RSV in high-risk children. Ribavirin is approved for treatment of severe RSV disease; however, its effectiveness in improving outcomes is questionable. During the past 40 years, many obstacles have delayed the development of safe and effective vaccines and treatment regimens. This article reviews these obstacles and presents the novel development strategies used to overcome many of them. Also discussed are promising new antiviral treatment candidates and their associated mechanism of action, the significant advances made in vaccine development, and exciting, new studies directed at improving outcomes through pharmacologic manipulation of the host response to RSV disease.

Elevated temperature triggers human respiratory syncytial virus F protein six-helix bundle formation

Human respiratory syncytial virus (RSV) is a major cause of severe lower respiratory tract infection in infants, immunocompromised patients, and the elderly. The RSV fusion (F) protein mediates fusion of the viral envelope with the target cell membrane during virus entry and is a primary target for antiviral drug and vaccine development. The F protein contains two heptad repeat regions, HR1 and HR2. Peptides corresponding to these regions form a six-helix bundle structure that is thought to play a critical role in membrane fusion. However, characterization of six-helix bundle formation in native RSV F protein has been hindered by the fact that a trigger for F protein conformational change has yet to be identified. Here we demonstrate that RSV F protein on the surface of infected cells undergoes a conformational change following exposure to elevated temperature, resulting in the formation of the six-helix bundle structure. We first generated and characterized six-helix bundle-specific antibodies raised against recombinant peptides modeling the RSV F protein six-helix bundle structure. We then used these antibodies as probes to monitor RSV F protein six-helix bundle formation in response to a diverse array of potential triggers of conformational changes. We found that exposure of 'membrane-anchored' RSV F protein to elevated temperature (45-55 degrees C) was sufficient to trigger six-helix bundle formation. Antibody binding to the six-helix bundle conformation was detected by both flow cytometry and cell-surface immunoprecipitation of the RSV F protein. None of the other treatments, including interaction with a number of potential receptors, resulted in significant binding by six-helix bundle-specific antibodies. We conclude that native, untriggered RSV F protein exists in a metastable state that can be converted in vitro to the more stable, fusogenic six-helix bundle conformation by an increase in thermal energy. These findings help to better define the mechanism of RSV F-mediated membrane fusion and have important implications for the identification of therapeutic strategies and vaccines targeting RSV F protein conformational changes.

The host response and molecular pathogenesis associated with respiratory syncytial virus infection

Since the isolation of respiratory syncytial virus (RSV) in 1956, its significance as an important human pathogen in infants, the elderly and the immunocompromised has been established. Many important mechanisms contributing to RSV infection, replication and disease pathogenesis have been uncovered; however, there is still insufficient knowledge in these and related areas, which must be addressed to facilitate the development of safe and effective vaccines and therapeutic treatments. A better understanding of the molecular pathogenesis of RSV infection, particularly the host-cell response and transcription profiles to RSV infection, is required to advance disease intervention strategies. Substantial information is accumulating regarding how RSV proteins modulate molecular signaling and regulation of cytokine and chemokine responses to infection, molecular signals regulating programmed cell death, and innate and adaptive immune responses to infection. This review discusses RSV manipulation of the host response to infection and related disease pathogenesis.

Coiled-Coil Hydrogels. Effect of Grafted Copolymer Composition and Cyclization on Gelation

A mean-field theoretical approach was developed to model gelation of solutions of hydrophilic polymers with grafted peptide motifs capable of forming associates of coiled-coil type. The model addresses the competition between associates engaged in branching and cyclization. It results in relative concentrations of intra- and intermolecular associates in dependence on associate strength and motif concentration. The cyclization probability is derived from the model of equivalent Gaussian chain and takes into account all possible paths connecting the interacting motifs. Examination of the association-dissociation equilibria, controlled by the equilibrium constant for association taken as input information, determines the fractions of inter- and intramolecularly associated motifs. The gelation model is based on the statistical theory of branching processes and in combination with the cyclization model predicts the critical concentration delimiting the regions of gelled and liquid states of the system. A comparison between predictions of the model and experimental data available for aqueous solutions of poly[N-(2-hydroxypropyl)methacrylamide] grafted with oppositely charged pentaheptad peptides, CCE and CCK, indicates that the association constant of grafted motifs by four orders of magnitude lower than that of free motifs. It is predicted that at the critical concentration of each motif of about 6×10(-7) mol/cm(3), about half of motifs in associated state is engaged in intramolecular bonds.

Design of an engineered N-terminal HIV-1 gp41 trimer with enhanced stability and potency

HIV fusion is mediated by a conformational transition in which the C-terminal region (HR2) of gp41 interacts with the N-terminal region (HR1) to form a six-helix bundle. Peptides derived from the HR1 form a well-characterized, trimeric coiled-coil bundle in the presence of HR2 peptides, but there is little structural information on the isolated HR1 trimer. Using protein design, we have designed synthetic HR1 peptides that form soluble, thermostable HR1 trimers. In vitro binding of HR2 peptides to the engineered trimer suggests that the design strategy has not significantly impacted the ability to form the six-helix bundle. The peptides have enhanced antiviral activity compared to wild type, with up to 30-fold greater potency against certain viral isolates. In vitro passaging was used to generate HR1-resistant virus and the observed resistance mutations map to the HR2 region of gp41, demonstrating that the peptides block the fusion process by binding to the viral HR2 domain. Interestingly, the activity of the HR2 fusion inhibitor, enfuvirtide (ENF), against these resistant viruses is maintained or improved up to fivefold. The 1.5 A crystal structure of one of these designs has been determined, and we show that the isolated HR1 is very similar to the conformation of the HR1 in the six-helix bundle. These results provide an initial model of the pre-fusogenic state, are attractive starting points for identifying novel fusion inhibitors, and offer new opportunities for developing HIV therapeutics based on HR1 peptides.

Nonhelical leash and alpha-helical structures determine the potency of a peptide antagonist of human T-cell leukemia virus entry

Viral fusion proteins mediate the entry of enveloped viral particles into cells by inducing fusion of the viral and target cell membranes. Activated fusion proteins undergo a cascade of conformational transitions and ultimately resolve into a compact trimer of hairpins or six-helix bundle structure, which pulls the interacting membranes together to promote lipid mixing. Significantly, synthetic peptides based on a C-terminal region of the trimer of hairpins are potent inhibitors of membrane fusion and viral entry, and such peptides are typically extensively alpha-helical. In contrast, an atypical peptide inhibitor of human T-cell leukemia virus (HTLV) includes alpha-helical and nonhelical leash segments. We demonstrate that both the C helix and C-terminal leash are critical to the inhibitory activities of these peptides. Amino acid side chains in the leash and C helix extend into deep hydrophobic pockets at the membrane-proximal end of the HTLV type 1 (HTLV-1) coiled coil, and these contacts are necessary for potent antagonism of membrane fusion. In addition, a single amino acid substitution within the inhibitory peptide improves peptide interaction with the core coiled coil and yields a peptide with enhanced potency. We suggest that the deep pockets on the coiled coil are ideal targets for small-molecule inhibitors of HTLV-1 entry into cells. Moreover, the extended nature of the HTLV-1-inhibitory peptide suggests that such peptides may be intrinsically amenable to modifications designed to improve inhibitory activity. Finally, we propose that leash-like mimetic peptides may be of value as entry inhibitors for other clinically important viral infections.

Identification and evaluation of a highly effective fusion inhibitor for human metapneumovirus

Human metapneumovirus (hMPV) can cause acute upper and lower respiratory tract infections that are particularly severe in young children, elderly subjects, and immunocompromised patients. To date, no treatments or vaccines are available for hMPV infections. Our objective was to assess the inhibitory potential of several peptides derived from the heptad repeat A and B (HRA and HRB) domains of the hMPV fusion protein. Nine candidate peptides were expressed in Escherichia coli or obtained synthetically and tested in vitro and in an animal model. Excellent in vitro inhibition of an hMPV strain of the A1 subgroup was obtained with five peptides, with 50% inhibitory concentrations ranging from 1.4 nM to 3.3 microM. One peptide, HRA2, displayed very potent activity against all four hMPV subgroups. It was also moderately active against human respiratory syncytial virus (strain A2) but displayed no activity against human parainfluenza virus type 3. BALB/c mice that received the HRA2 peptide and a lethal hMPV intranasal challenge simultaneously were completely protected from clinical symptoms and mortality. On day 5 postinfection, HRA2-treated mice had undetectable lung viral loads which were significantly less than those of untreated mice (3 x 10(4) 50% tissue culture infective doses/lung). Pulmonary inflammation, levels of proinflammatory cytokines/chemokines (RANTES, gamma interferon, and monocyte chemoattractant protein 1) and airway obstruction were also significantly decreased in HRA2-treated mice. The results of this study demonstrate that potent antivirals can be derived from the hMPV fusion protein HR domains. Moreover, hMPV, compared to other paramyxoviruses and to the human immunodeficiency virus, seems to be more susceptible to HRA- than HRB-derived peptides.

Human metapneumovirus fusion protein vaccines that are immunogenic and protective in cotton rats

Human metapneumovirus (hMPV) is a recently described paramyxovirus that is a major cause of upper and lower respiratory infection in children and adults worldwide. A safe and effective vaccine could decrease the burden of disease associated with this novel pathogen. We previously reported the development of the cotton rat model of hMPV infection and pathogenesis (J. V. Williams et al., J. Virol. 79:10944-10951, 2005). We report here the immunogenicity of an hMPV fusion (F) protein in this model. We constructed DNA plasmids that exhibited high levels of expression of hMPV F in mammalian cells (DNA-F). These constructs were used to develop a novel strategy to produce highly pure, soluble hMPV F protein lacking the transmembrane domain (FDeltaTM). We then immunized cotton rats at 0 and 14 days with either control vector, DNA-F alone, DNA-F followed by FDeltaTM protein, or FDeltaTM alone. All groups were challenged intranasally at 28 days with live hMPV. All three groups that received some form of hMPV F immunization mounted neutralizing antibody responses and exhibited partial protection against virus shedding in the lungs compared to controls. The FDeltaTM-immunized animals showed the greatest degree of protection (>1,500-fold reduction in lung virus titer). All three immunized groups showed a modest reduction of nasal virus shedding. Neither evidence of a Th2-type response nor increased lung pathology were present in the immunized animals. We conclude that sequence-optimized hMPV F protein protects against hMPV infection when delivered as either a DNA or a protein vaccine in cotton rats.

Examination of a fusogenic hexameric core from human metapneumovirus and identification of a potent synthetic peptide inhibitor from the heptad repeat 1 region

Paramyxoviruses are a leading cause of childhood illness worldwide. A recently discovered paramyxovirus, human metapneumovirus (hMPV), has been studied by our group in order to determine the structural relevance of its fusion (F) protein to other well-characterized viruses utilizing type I integral membrane proteins as fusion aids. Sequence analysis and homology models suggested the presence of requisite heptad repeat (HR) regions. Synthetic peptides from HR regions 1 and 2 (HR-1 and -2, respectively) were induced to form a thermostable (melting temperature, approximately 90 degrees C) helical structure consistent in mass with a hexameric coiled coil. Inhibitory studies of hMPV HR-1 and -2 indicated that the synthetic HR-1 peptide was a significant fusion inhibitor with a 50% inhibitory concentration and a 50% effective concentration of approximately 50 nM. Many viral fusion proteins are type I integral membrane proteins utilizing the formation of a hexameric coiled coil of HR peptides as a major driving force for fusion. Our studies provide evidence that hMPV also uses a coiled-coil structure as a major player in the fusion process. Additionally, viral HR-1 peptide sequences may need further investigation as potent fusion inhibitors.

Sequence elements of the fusion peptide of human respiratory syncytial virus fusion protein required for activity

We have reported previously the expression and purification of an anchorless form of the human respiratory syncytial virus (HRSV) F protein (F(TM-)) representing the ectodomain of the full-length F. F(TM-) molecules are seen as unaggregated cones by electron microscopy but completion of proteolytic cleavage of the F0 monomers in the F(TM-) trimer leads to a change in shape from cones to lollipops that aggregate into rosettes. This aggregation apparently occurs by interaction of the fusion peptides of F(TM-) molecules that are exposed after cleavage. Since exposure of the fusion peptide is a key event in the process of membrane fusion, changes associated with F(TM-) cleavage may reflect those occurring in full-length F during membrane fusion. Deletions or substitutions that changed either the length, charge or hydrophobicity of the fusion peptide inhibited aggregation of F(TM-), and these mutants remained as unaggregated cones after cleavage. In contrast, more conservative changes did not inhibit the change of shape and aggregation of F(TM-). When the same changes were introduced in the fusion peptide of full-length F, only the mutations that inhibited aggregation of F(TM-) prevented membrane fusion. Thus, the conformational changes that follow completion of cleavage of the F(TM-) protein require a functional fusion peptide. These sequence constraints may restrict accumulation of sequence changes in the fusion peptide of HRSV F when compared with other hydrophobic regions of the molecule.

Contribution of cysteine residues in the extracellular domain of the F protein of human respiratory syncytial virus to its function

The mature F protein of all known isolates of human respiratory syncytial virus (HRSV) contains fifteen absolutely conserved cysteine (C) residues that are highly conserved among the F proteins of other pneumoviruses as well as the paramyxoviruses. To explore the contribution of the cysteines in the extracellular domain to the fusion activity of HRSV F protein, each cysteine was changed to serine. Mutation of cysteines 37, 313, 322, 333, 343, 358, 367, 393, 416, and 439 abolished or greatly reduced cell surface expression suggesting these residues are critical for proper protein folding and transport to the cell surface. As expected, the fusion activity of these mutations was greatly reduced or abolished. Mutation of cysteine residues 212, 382, and 422 had little to no effect upon cell surface expression or fusion activity at 32 degrees C, 37 degrees C, or 39.5 degrees C. Mutation of C37 and C69 in the F2 subunit either abolished or reduced cell surface expression by 75% respectively. None of the mutations displayed a temperature sensitive phenotype.

Small molecules VP-14637 and JNJ-2408068 inhibit respiratory syncytial virus fusion by similar mechanisms

Here we present data on the mechanism of action of VP-14637 and JNJ-2408068 (formerly R-170591), two small-molecule inhibitors of respiratory syncytial virus (RSV). Both inhibitors exhibited potent antiviral activity with 50% effective concentrations (EC50s) of 1.4 and 2.1 nM, respectively. A similar inhibitory effect was observed in a RSV-mediated cell fusion assay (EC50=5.4 and 0.9 nM, respectively). Several drug-resistant RSV variants were selected in vitro in the presence of each compound. All selected viruses exhibited significant cross-resistance to both inhibitors and contained various single amino acid substitutions in two distinct regions of the viral F protein, the heptad repeat 2 (HR2; mutations D486N, E487D, and F488Y), and the intervening domain between HR1 and HR2 (mutation K399I and T400A). Studies using [3H]VP-14637 revealed a specific binding of the compound to RSV-infected cells that was efficiently inhibited by JNJ-2408068 (50% inhibitory concentration=2.9 nM) but not by the HR2-derived peptide T-118. Further analysis using a transient T7 vaccinia expression system indicated that RSV F protein is sufficient for this interaction. F proteins containing either the VP-14637 or JNJ-2408068 resistance mutations exhibited greatly reduced binding of [3H]VP-14637. Molecular modeling analysis suggests that both molecules may bind into a small hydrophobic cavity in the inner core of F protein, interacting simultaneously with both the HR1 and HR2 domains. Altogether, these data indicate that VP-14637 and JNJ-2408068 interfere with RSV fusion through a mechanism involving a similar interaction with the F protein.

Use of a novel cell-based fusion reporter assay to explore the host range of human respiratory syncytial virus F protein

Human respiratory syncytial virus (HRSV) is an important respiratory pathogen primarily affecting infants, young children, transplant recipients and the elderly. The F protein is the only virion envelope protein necessary and sufficient for virus replication and fusion of the viral envelope membrane with the target host cell. During natural infection, HRSV replication is limited to respiratory epithelial cells with disseminated infection rarely, if ever, occurring even in immunocompromised patients. However, in vitro infection of multiple human and non-human cell types other than those of pulmonary tract origin has been reported. To better define host cell surface molecules that mediate viral entry and dissect the factors controlling permissivity for HRSV, we explored the host range of HRSV F protein mediated fusion. Using a novel recombinant reporter gene based fusion assay, HRSV F protein was shown to mediate fusion with cells derived from a wide range of vertebrate species including human, feline, equine, canine, bat, rodent, avian, porcine and even amphibian (Xenopus). That finding was extended using a recombinant HRSV engineered to express green fluorescent protein (GFP), to confirm that viral mRNA expression is limited in several cell types. These findings suggest that HRSV F protein interacts with either highly conserved host cell surface molecules or can use multiple mechanisms to enter cells, and that the primary determinants of HRSV host range are at steps post-entry.

Thermostability of the human respiratory syncytial virus fusion protein before and after activation: implications for the membrane-fusion mechanism

Anchorless fusion (F) proteins () of human respiratory syncytial virus (RSV) are seen by electron microscopy as unaggregated cones when the proteolytic cleavage at two furin sites required for membrane-fusion activity is incomplete, but aggregate into rosettes of lollipop-shaped spikes following cleavage. To show that this aggregation occurred by interactions of the fusion peptide, a deletion mutant of lacking the first half of the fusion peptide was generated. This mutant remained unaggregated even after completion of cleavage, supporting the notion that aggregation of involved the fusion peptide. As exposure of the fusion peptide is a key event that occurs after activation of F proteins, the uncleaved and cleaved forms of may represent the pre- and post-active forms of RSV F protein. In an analysis of the structural differences between the two forms, their thermostability before and after proteolytic cleavage was examined. In contrast to other viral proteins involved in membrane fusion (e.g. influenza haemagglutinin), the pre-active (uncleaved) and post-active (cleaved) forms of were equally resistant to heat denaturation, assessed by spectrofluorimetry, circular dichroism or antibody binding. These results are interpreted in terms of the proposed structural changes associated with the process of membrane fusion mediated by RSV F protein.

Targeting a binding pocket within the trimer-of-hairpins: small-molecule inhibition of viral fusion

Trimeric class I virus fusion proteins undergo a series of conformational rearrangements that leads to the association of C- and N-terminal heptad repeat domains in a "trimer-of-hairpins" structure, facilitating the apposition of viral and cellular membranes during fusion. This final fusion hairpin structure is sustained by protein-protein interactions, associations thought initially to be refractory to small-molecule inhibition because of the large surface area involved. By using a photoaffinity analog of a potent respiratory syncytial virus fusion inhibitor, we directly probed the interaction of the inhibitor with its fusion protein target. Studies have shown that these inhibitors bind within a hydrophobic cavity formed on the surface of the N-terminal heptad-repeat trimer. In the fusogenic state, this pocket is occupied by key amino acid residues from the C-terminal heptad repeat that stabilize the trimer-of-hairpins structure. The results indicate that a low-molecular-weight fusion inhibitor can interfere with the formation or consolidation of key structures within the hairpin moiety that are essential for membrane fusion. Because analogous cavities are present in many class I viruses, including HIV, these results demonstrate the feasibility of this approach as a strategy for drug discovery.

Orally active fusion inhibitor of respiratory syncytial virus

BMS-433771 was found to be a potent inhibitor of respiratory syncytial virus (RSV) replication in vitro. It exhibited excellent potency against multiple laboratory and clinical isolates of both group A and B viruses, with an average 50% effective concentration of 20 nM. Mechanism-of-action studies demonstrated that BMS-433771 inhibits the fusion of lipid membranes during both the early virus entry stage and late-stage syncytium formation. After isolation of resistant viruses, resistance was mapped to a series of single amino acid mutations in the F1 subunit of the fusion protein. Upon oral administration, BMS-433771 was able to reduce viral titers in the lungs of mice infected with RSV. This new class of orally active RSV fusion inhibitors offers potential for clinical development.

Neuraminidase treatment of respiratory syncytial virus-infected cells or virions, but not target cells, enhances cell-cell fusion and infection

Respiratory syncytial virus (RSV) infection of HeLa cells induces fusion, but transient expression of the three viral glycoproteins induces fusion poorly, if at all. We found that neuraminidase treatment of RSV-infected cells to remove sialic acid (SA) increases fusion dramatically and that the same treatment of transiently transfected cells expressing the three viral glycoproteins, or even cells expressing the fusion (F) protein alone, results in easily detectable fusion. Neuraminidase treatment of the effector cells, expressing the viral glycoproteins, enhanced fusion while treatment of the target cells did not. Likewise, infectivity was increased by treating virions with neuraminidase, but not by treating target cells. Reduction of charge repulsion by removal of the negatively charged SA is unlikely to explain this effect, since removal of negative charges from either membrane would reduce charge repulsion. Infection with neuraminidase-treated virus remained heparan-sulfate-dependent, indicating that a novel attachment mechanism is not revealed by SA removal. Interestingly, neuraminidase enhancement of RSV infectivity was less pronounced in a virus expressing both the G and the F glycoproteins, compared to virus expressing only the F glycoprotein, possibly suggesting that the G protein sterically hinders access of the neuraminidase to its fusion-enhancing target.

Structural characterization of respiratory syncytial virus fusion inhibitor escape mutants: homology model of the F protein and a syncytium formation assay

Respiratory syncytial virus (RSV) is a ubiquitous human pathogen and the leading cause of lower respiratory tract infections in infants. Infection of cells and subsequent formation of syncytia occur through membrane fusion mediated by the RSV fusion protein (RSV-F). A novel in vitro assay of recombinant RSV-F function has been devised and used to characterize a number of escape mutants for three known inhibitors of RSV-F that have been isolated. Homology modeling of the RSV-F structure has been carried out on the basis of a chimera derived from the crystal structures of the RSV-F core and a fragment from the orthologous fusion protein from Newcastle disease virus (NDV). The structure correlates well with the appearance of RSV-F in electron micrographs, and the residues identified as contributing to specific binding sites for several monoclonal antibodies are arranged in appropriate solvent-accessible clusters. The positions of the characterized resistance mutants in the model structure identify two promising regions for the design of fusion inhibitors.

Inhibition of respiratory syncytial virus fusion by the small molecule VP-14637 via specific interactions with F protein

Human respiratory syncytial virus (RSV) is a major cause of respiratory tract infections worldwide. Several novel small-molecule inhibitors of RSV have been identified, but they are still in preclinical or early clinical evaluation. One such inhibitor is a recently discovered triphenol-based molecule, VP-14637 (ViroPharma). Initial experiments suggested that VP-14637 acted early and might be an RSV fusion inhibitor. Here we present studies demonstrating that VP-14637 does not block RSV adsorption but inhibits RSV-induced cell-cell fusion and binds specifically to RSV-infected cells with an affinity corresponding to its inhibitory potency. VP-14637 is capable of specifically interacting with the RSV fusion protein expressed by a T7 vaccinia virus system. RSV variants resistant to VP-14637 were selected; they had mutations localized to two distinct regions of the RSV F protein, heptad repeat 2 (HR2) and the intervening domain between heptad repeat 1 (HR1) and HR2. No mutations arose in HR1, suggesting a mechanism other than direct disruption of the heptad repeat interaction. The F proteins containing the resistance mutations exhibited greatly reduced binding of VP-14637. Despite segregating with the membrane fraction following incubation with intact RSV-infected cells, the compound did not bind to membranes isolated from RSV-infected cells. In addition, binding of VP-14637 was substantially compromised at temperatures of < or =22 degrees C. Therefore, we propose that VP-14637 inhibits RSV through a novel mechanism involving an interaction between the compound and a transient conformation of the RSV F protein.

Mode of action of two inhibitory peptides from heptad repeat domains of the fusion protein of Newcastle disease virus

Peptides derived from heptad repeat (HR) sequences of viral fusion proteins from several enveloped viruses have been shown to inhibit virus-mediated membrane fusion but the mechanism remains unknown. To further investigate this, the inhibition mechanism of two HR-derived peptides from the fusion protein of the paramyxovirus Newcastle disease virus (NDV) was investigated. Peptide N24 (residues 145-168) derived from HR1 was found to be 145-fold more inhibitory in a syncytium assay than peptide C24 (residues 474-496), derived from HR2. Both peptides failed to block lipid-mixing between R18-labeled virus and cells. None of the peptides interfered with the binding of hemagglutinin-neuraminidase (HN) protein to the target cells, as demonstrated by hemagglutining assays. When both peptides were mixed at equimolar concentrations, their inhibitory effect was abolished. In addition, both peptides induced the aggregation of negatively charged and zwitterionic phospholipid membranes. The ability of the peptides to interact with each other in solution suggests that these peptides may bind to the opposite HR region on the protein whereas their ability to interact with membranes as well as their failure to block lipid transfer suggest a second binding site. Taken together these results, suggest a mode of action for C24 and N24 in which both peptides have two different targets on the F protein: the opposite HR sequence and their corresponding domains.

RFI-641, a potent respiratory syncytial virus inhibitor

Human respiratory syncytial virus (RSV), a paramyxovirus, is a major cause of acute upper and lower respiratory tract infections in infants, young children, and adults. RFI-641 is a novel anti-RSV agent with potent in vitro and in vivo activity. RFI-641 is active against both RSV type A and B strains. The viral specificity and the large therapeutic window of RFI-641 (>100-fold) indicate that the antiviral activity of the compound is not due to adverse effects on normal cells. The potent in vitro activity of RFI-641 can be translated to efficacy in vivo: RFI-641 is efficacious when administered prophylactically by the intranasal route in mice, cotton rats, and African green monkeys. RFI-641 is also efficacious when administered therapeutically (24 h postinfection) in the monkey model. Mechanism of action studies indicate that RFI-641 blocks viral F protein-mediated fusion and cell syncytium formation.

Mutations in the fusion peptide and adjacent heptad repeat inhibit folding or activity of the Newcastle disease virus fusion protein

Paramyxovirus fusion proteins have two heptad repeat domains, HR1 and HR2, which have been implicated in the fusion activity of the protein. Peptides with sequences from these two domains form a six-stranded coiled coil, with the HR1 sequences forming a central trimer (K. A. Baker, R. E. Dutch, R. A. Lamb, and T. S. Jardetzky, Mol. Cell 3:309-319, 1999; X. Zhao, M. Singh, V. N. Malashkevich, and P. S. Kim, Proc. Natl. Acad. Sci. USA 97:14172-14177, 2000). We have extended our previous mutational analysis of the HR1 domain of the Newcastle disease virus fusion protein, focusing on the role of the amino acids forming the hydrophobic core of the trimer, amino acids in the "a" and "d" positions of the helix from amino acids 123 to 182. Both conservative and nonconservative point mutations were characterized for their effects on synthesis, stability, proteolytic cleavage, and surface expression. Mutant proteins expressed on the cell surface were characterized for fusion activity by measuring syncytium formation, content mixing, and lipid mixing. We found that all mutations in the "a" position interfered with proteolytic cleavage and surface expression of the protein, implicating the HR1 domain in the folding of the F protein. However, mutation of five of seven "d" position residues had little or no effect on surface expression but, with one exception at residue 175, did interfere to various extents with the fusion activity of the protein. One of these "d" mutations, at position 154, interfered with proteolytic cleavage, while the rest of the mutants were cleaved normally. That most "d" position residues do affect fusion activity argues that a stable HR1 trimer is required for formation of the six-stranded coiled coil and, therefore, optimal fusion activity. That most of the "d" position mutations do not block folding suggests that formation of the core trimer may not be required for folding of the prefusion form of the protein. We also found that mutations within the fusion peptide, at residue 128, can interfere with folding of the protein, implicating this region in folding of the molecule. No characterized mutation enhanced fusion.

Structural characterization of the human respiratory syncytial virus fusion protein core

Human respiratory syncytial virus (HRSV) is a major cause of a number of severe respiratory diseases, including bronchiolitis and pneumonia, in infants and young children. The HRSV F protein, a glycoprotein essential for viral entry, is a primary target for vaccine and drug development. Two heptad-repeat regions within the HRSV F sequence were predicted by the computer program learncoil-vmf. These regions are thought to form trimer-of-hairpins-like structures, similar to those found in the fusion proteins of several enveloped viruses. The hairpin structure likely brings the viral and cellular membranes into close apposition, thereby facilitating membrane fusion and subsequent viral entry. Here, we show that peptides, denoted HR-N and HR-C, corresponding to the heptad-repeat regions from the N-terminal and C-terminal segments of the HRSV F protein, respectively, form a stable alpha-helical trimer of heterodimers. The HRSV N/C complex was crystallized and its x-ray structure was determined at 2.3-A resolution. As anticipated, the complex is a six-helix bundle in which the HR-N peptides form a three-stranded, central coiled coil, and the HR-C peptides pack in an antiparallel manner into hydrophobic grooves on the coiled-coil surface. There is remarkable structural similarity between the HRSV N/C complex and the fusion protein core of other viruses, including HIV-1 gp41. In addition, earlier work has shown that HRSV HR-C peptides, like the HIV-1 gp41 C peptides, inhibit viral infection. Thus, drug discovery and vaccine development strategies aimed at inhibiting viral entry by blocking hairpin formation may be applied to the inhibition of HRSV.