Antiviral activity and conformational features of an octapeptide derived from the membrane-proximal ectodomain of the feline immunodeficiency virus transmembrane glycoprotein

Tailoring the nuclear Overhauser effect for the study of small and medium-sized molecules by solvent viscosity manipulation

The nuclear Overhauser effect (NOE) is a consequence of cross-relaxation between nuclear spins mediated by dipolar coupling. Its sensitivity to internuclear distances has made it an increasingly important tool for the determination of through-space atom proximity relationships within molecules of sizes ranging from the smallest systems to large biopolymers. With the support of sophisticated FT-NMR techniques, the NOE plays an essential role in structure elucidation, conformational and dynamic investigations in liquid-state NMR. The efficiency of magnetization transfer by the NOE depends on the molecular rotational correlation time, whose value depends on solution viscosity. The magnitude of the NOE between ¹H nuclei varies from +50% when molecular tumbling is fast to -100% when it is slow, the latter case corresponding to the spin diffusion limit. In an intermediate tumbling regime, the NOE may be vanishingly small. Increasing the viscosity of the solution increases the motional correlation time, and as a result, otherwise unobservable NOEs may be revealed and brought close to the spin diffusion limit. The goal of this review is to report the resolution of structural problems that benefited from the manipulation of the negative NOE by means of viscous solvents, including examples of molecular structure determination, conformation elucidation and mixture analysis (the ViscY method).

Binding of the Anti-FIV Peptide C8 to Differently Charged Membrane Models: From First Docking to Membrane Tubulation

Gp36 is the virus envelope glycoproteins catalyzing the fusion of the feline immunodeficiency virus with the host cells. The peptide C8 is a tryptophan-rich peptide corresponding to the fragment ⁷⁷⁰W-I⁷⁷⁷ of gp36 exerting antiviral activity by binding the membrane cell and inhibiting the virus entry. Several factors, including the membrane surface charge, regulate the binding of C8 to the lipid membrane. Based on the evidence that imperceptible variation of membrane charge may induce a dramatic effect in several critical biological events, in the present work we investigate the effect induced by systematic variation of charge in phospholipid bilayers on the aptitude of C8 to interact with lipid membranes, the tendency of C8 to assume specific conformational states and the re-organization of the lipid bilayer upon the interaction with C8. Accordingly, employing a bottom-up multiscale protocol, including CD, NMR, ESR spectroscopy, atomistic molecular dynamics simulations, and confocal microscopy, we studied C8 in six membrane models composed of different ratios of zwitterionic/negatively charged phospholipids. Our data show that charge content modulates C8-membrane binding with significant effects on the peptide conformations. C8 in micelle solution or in SUV formed by DPC or DOPC zwitterionic phospholipids assumes regular β-turn structures that are progressively destabilized as the concentration of negatively charged SDS or DOPG phospholipids exceed 40%. Interaction of C8 with zwitterionic membrane surface is mediated by Trp1 and Trp4 that are deepened in the membrane, forming H-bonds and cation-π interactions with the DOPC polar heads. Additional stabilizing salt bridge interactions involve Glu2 and Asp3. MD and ESR data show that the C8-membrane affinity increases as the concentration of zwitterionic phospholipid increases. In the lipid membrane characterized by an excess of zwitterionic phospholipids, C8 is adsorbed at the membrane interface, inducing a stiffening of the outer region of the DOPC bilayer. However, the bound of C8 significantly perturbs the whole organization of lipid bilayer resulting in membrane remodeling. These events, measurable as a variation of the bilayer thickness, are the onset mechanism of the membrane fusion and vesicle tubulation observed in confocal microscopy by imaging zwitterionic MLVs in the presence of C8 peptide.

NMR Structure of the FIV gp36 C-Terminal Heptad Repeat and Membrane-Proximal External Region

Feline immunodeficiency virus (FIV), a lentivirus causing an immunodeficiency syndrome in cats, represents a relevant model of pre-screening therapies for human immunodeficiency virus (HIV). The envelope glycoproteins gp36 in FIV and gp41 in HIV mediate the fusion of the virus with the host cell membrane. They have a common structural framework in the C-terminal region that includes a Trp-rich membrane-proximal external region (MPER) and a C-terminal heptad repeat (CHR). MPER is essential for the correct positioning of gp36 on the lipid membrane, whereas CHR is essential for the stabilization of the low-energy six-helical bundle (6HB) that is necessary for the fusion of the virus envelope with the cell membrane. Conformational data for gp36 are missing, and several aspects of the MPER structure of different lentiviruses are still debated. In the present work, we report the structural investigation of a gp36 construct that includes the MPER and part of the CHR domain (^737-786gp36 CHR–MPER). Using 2D and 3D homo and heteronuclear NMR spectra on ¹⁵N and ¹³C double-labelled samples, we solved the NMR structure in micelles composed of dodecyl phosphocholine (DPC) and sodium dodecyl sulfate (SDS) 90/10 M: M. The structure of ^737-786gp36 CHR–MPER is characterized by a helix–turn–helix motif, with a regular α-helix and a moderately flexible 3₁₀ helix, characterizing the CHR and the MPER domains, respectively. The two helices are linked by a flexible loop regulating their orientation at a ~43° angle. We investigated the positioning of ^737-786gp36 CHR–MPER on the lipid membrane using spin label-enhanced NMR and ESR spectroscopies. On a different scale, using confocal microscopy imaging, we studied the effect of ^737-786gp36 CHR–MPER on 1,2-dioleoyl-sn-glycero-3-phosphocholine/1,2-dioleoyl-sn-glycero-3-phospho-(1’-rac-glycerol) (DOPC/DOPG) multilamellar vesicles (MLVs). This effect results in membrane budding and tubulation that is reminiscent of a membrane-plasticizing role that is typical of MPER domains during the event in which the virus envelope merges with the host cell membrane.

Structural basis of antiviral activity of peptides from MPER of FIV gp36

Feline immunodeficiency virus (FIV) is a naturally occurring Lentivirus causing acquired immunodeficiency syndrome in felines. It is considered a useful non-primate model to study HIV infection, and to test anti-HIV vaccine. Similarly to HIV, FIV enters cells via a mechanism involving a surface glycoprotein named gp36. C8 is a short synthetic peptide corresponding to the residues 770WEDWVGWI777 of gp36 membrane proximal external region (MPER). It elicits antiviral activity by inhibiting the fusion of the FIV and host cell membrane. C8 is endowed with evident membrane binding property, inducing alteration of the phospholipid bilayer and membrane fusion. The presence and the position of tryptophan residues in C8 are important for antiviral activity: the C8 derivative C6a, obtained by truncating the N-terminal 770WE771 residues, exhibits conserved antiviral activity, while the C8 derivative C6b, derived from the truncation of the C-terminal 776WI777, is nearly inactive. To elucidate the structural factors that induce the different activity profiles of C6a and C6b, in spite of their similarity, we investigated the structural behaviour of the two peptides in membrane mimicking environments. Conformational data on the short peptides C6a and C6b, matched to those of their parent peptide C8, allow describing a pharmacophore model of antiviral fusion inhibitors. This includes the essential structural motifs to design new simplified molecules overcoming the pharmacokinetic and high cost limitations affecting the antiviral entry inhibitors that currently are in therapy.

HIV antivirals: targeting the functional organization of the lipid envelope

Most of the surface of the lipid bilayer covering the human immunodeficiency virus type 1 (HIV-1) particle is directly accessible from the aqueous medium. Its peculiar chemical composition and physical properties appear to be critical for infection and, therefore, may comprise a target for selective antiviral activity. The HIV-1 membrane is enriched in raft-type lipids and also displays aminophospholipids on its external leaflet. We contend here that a great deal of membrane-active compounds described to block HIV-1 infection can do so by following a common mechanism of action: alteration of the lateral heterogeneity that supports the functional organization of the lipid envelope. The confirmation of this hypothesis could lay new foundations for the rational development of compounds with anti-HIV activity.

Effects of HIV-1 gp41-Derived Virucidal Peptides on Virus-like Lipid Membranes

Membrane fusion induced by the envelope glycoprotein enables the intracellular replication of HIV-1; hence, this process constitutes a major target for antiretroviral compounds. It has been proposed that peptides having propensity to interact with membrane interfaces might exert broad antiviral activity against enveloped viruses. To test this hypothesis, in this contribution we have analyzed the antiviral effects of peptides derived from the membrane-proximal external region and the transmembrane domain of the envelope glycoprotein subunit gp41, which display different degrees of interfacial hydrophobicity. Our data support the virucidal activity of a region that combines hydrophobic-at-interface membrane-proximal external region aromatics with hydrophobic residues of the transmembrane domain, and contains the absolutely conserved ⁶⁷⁹LWYIK/R⁶⁸³ sequence, proposed to embody a "cholesterol recognition/interaction amino acid consensus" motif. We further sought to correlate the antiviral activity of these peptides and their effects on membranes that mimic lipid composition and biophysical properties of the viral envelope. The data revealed that peptides endowed with virucidal activity were membrane active and induced permeabilization and fusion of virus-like lipid vesicles. In addition, they modulated lipid packing and miscibility of laterally segregated liquid domains, two properties that depend on the high cholesterol content of the viral membrane. Thus, the overall experimental evidence is consistent with a pattern of HIV inhibition that involves direct alteration of the physical chemistry of the virus membrane. Furthermore, the sequence-dependent effects observed might guide the development of new virucidal peptides.

PepBio: predicting the bioactivity of host defense peptides

A large-scale QSAR study of host defense peptides sheds light on the origin of their bioactivities (antibacterial, anticancer, antiviral and antifungal).

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.

Pharmacological inhibition of feline immunodeficiency virus (FIV)

Feline immunodeficiency virus (FIV) is a member of the retroviridae family of viruses and causes an acquired immunodeficiency syndrome (AIDS) in domestic and non-domestic cats worldwide. Genome organization of FIV and clinical characteristics of the disease caused by the virus are similar to those of human immunodeficiency virus (HIV). Both viruses infect T lymphocytes, monocytes and macrophages, and their replication cycle in infected cells is analogous. Due to marked similarity in genomic organization, virus structure, virus replication and disease pathogenesis of FIV and HIV, infection of cats with FIV is a useful tool to study and develop novel drugs and vaccines for HIV. Anti-retroviral drugs studied extensively in HIV infection have targeted different steps of the virus replication cycle: (1) inhibition of virus entry into susceptible cells at the level of attachment to host cell surface receptors and co-receptors; (2) inhibition of fusion of the virus membrane with the cell membrane; (3) blockade of reverse transcription of viral genomic RNA; (4) interruption of nuclear translocation and viral DNA integration into host genomes; (5) prevention of viral transcript processing and nuclear export; and (6) inhibition of virion assembly and maturation. Despite much success of anti-retroviral therapy slowing disease progression in people, similar therapy has not been thoroughly investigated in cats. In this article we review current pharmacological approaches and novel targets for anti-lentiviral therapy, and critically assess potentially suitable applications against FIV infection in cats.

Feline immunodeficiency virus (FIV) neutralization: a review

One of the major obstacles that must be overcome in the design of effective lentiviral vaccines is the ability of lentiviruses to evolve in order to escape from neutralizing antibodies. The primary target for neutralizing antibodies is the highly variable viral envelope glycoprotein (Env), a glycoprotein that is essential for viral entry and comprises both variable and conserved regions. As a result of the complex trimeric nature of Env, there is steric hindrance of conserved epitopes required for receptor binding so that these are not accessible to antibodies. Instead, the humoral response is targeted towards decoy immunodominant epitopes on variable domains such as the third hypervariable loop (V3) of Env. For feline immunodeficiency virus (FIV), as well as the related human immunodeficiency virus-1 (HIV-1), little is known about the factors that lead to the development of broadly neutralizing antibodies. In cats infected with FIV and patients infected with HIV-1, only rarely are plasma samples found that contain antibodies capable of neutralizing isolates from other clades. In this review we examine the neutralizing response to FIV, comparing and contrasting with the response to HIV. We ask whether broadly neutralizing antibodies are induced by FIV infection and discuss the comparative value of studies of neutralizing antibodies in FIV infection for the development of more effective vaccine strategies against lentiviral infections in general, including HIV-1.

Design and synthesis of membrane fusion inhibitors against the feline immunodeficiency virus

Feline immunodeficiency virus (FIV) is a pathogenic virus that causes an AIDS-like syndrome in the domestic cats. For viral entry and infection, fusion between the virus and the cell membrane is the critical process and this process is mediated by an envelope glycoprotein gp40. We have identified fusion inhibitory peptides from the heptad repeat-2 (HR2) of gp40. Remodeling of the original sequences using alpha-helix-inducible motifs revealed the interactive residues of gp40. Comparative analysis of HR2 peptides derived from four FIV strains demonstrated that the interactive surface of the Shizuoka strain-derived HR2 peptides provides the highest affinity of all the FIV strains examined.

Interaction of short modified peptides deriving from glycoprotein gp36 of feline immunodeficiency virus with phospholipid membranes

A tryptophan-rich octapeptide, C8 (Ac-Trp-Glu-Asp-Trp-Val-Gly-Trp-Ile-NH(2)), modelled on the membrane-proximal external region of the feline immunodeficiency virus (FIV) gp36 glycoprotein ectodomain, exhibits potent antiviral activity against FIV. A mechanism has been proposed by which the peptide, being positioned on the surface of the cell membrane, inhibits its fusion with the virus. In the present work, peptide-lipid interactions of C8 with dimyristoyl phosphatidylcholine liposomes are investigated using electron spin resonance spectroscopy of spin-labelled lipids. Three other peptides, obtained from modifications of C8, have also been investigated, in an attempt to clarify the essential molecular features of the interactions involving the tryptophan residues. The results show that C8 adsorbs strongly on the bilayer surface. Membrane binding requires not only the presence of the Trp residues in the sequence, but also their common orientation on one side of the peptide that is engendered by the WX(2) WX(2) W motif. Membrane interaction correlates closely with peptide antiviral activity, indicating that the membrane is essential in stabilizing the peptide conformation that will be able to inhibit viral infection.

Membrane insertion of the three main membranotropic sequences from SARS-CoV S2 glycoprotein

In order to complete the fusion process of SARS-CoV virus, several regions of the S2 virus envelope glycoprotein are necessary. Recent studies have identified three membrane-active regions in the S2 domain of SARS-CoV glycoprotein, one situated downstream of the minimum furin cleavage, which is considered the fusion peptide (SARS_FP), an internal fusion peptide located immediately upstream of the HR1 region (SARS_IFP) and the pre-transmembrane domain (SARS_PTM). We have explored the capacity of these selected membrane-interacting regions of the S2 SARS-CoV fusion protein, alone or in equimolar mixtures, to insert into the membrane as well as to perturb the dipole potential of the bilayer. We show that the three peptides interact with lipid membranes depending on lipid composition and experiments using equimolar mixtures of these peptides show that different segments of the protein may act in a synergistic way suggesting that several membrane-active regions could participate in the fusion process of the SARS-CoV.

Interfacial pre-transmembrane domains in viral proteins promoting membrane fusion and fission

Membrane fusion and fission underlie two limiting steps of enveloped virus replication cycle: access to the interior of the host-cell (entry) and dissemination of viral progeny after replication (budding), respectively. These dynamic processes proceed mediated by specialized proteins that disrupt and bend the lipid bilayer organization transiently and locally. We introduced Wimley–White membrane-water partitioning free energies of the amino acids as an algorithm for predicting functional domains that may transmit protein conformational energy into membranes. It was found that many viral products possess unusually extended, aromatic-rich pre-transmembrane stretches predicted to stably reside at the membrane interface. Here, we review structure–function studies, as well as data reported on the interaction of representative peptides with model membranes, all of which sustain a functional role for these domains in viral fusion and fission. Since pre-transmembrane sequences also constitute antigenic determinants in a membrane-bound state, we also describe some recent results on their recognition and blocking at membrane interface by neutralizing antibodies.

Differential activation of polymorphisms of the formyl peptide receptor by formyl peptides

We have investigated the role of two polymorphic sites (R190W and N192K) on the binding and activation of the formyl peptide receptor (FPR) by viral and formyl peptides. WEDWVGWI, a peptide with antiviral activity derived from the membrane proximal region of feline immunodeficiency virus, binds with high affinity to FPR. The three tryptophans in the peptide are all essential for FPR binding, just as they were essential for antiviral activity [S. Giannecchini, A. Di Fenza, A.M. D'Ursi, D. Matteucci, P. Rovero, M. Bendinelli, Antiviral activity and conformational features of an octapeptide derived from the membrane-proximal ectodomain of the feline immunodeficiency virus transmembrane glycoprotein, J. Virol. 77 (2003) 3724]. Formyl-NleWEDWVGWI behaved as a weak partial agonist with FPR W190/N192 but a stronger partial agonist with FPR R190/K192 and FPR R190/N192. Formyl-NleNleWEDWVGWI behaved as a full agonist toward all three FPRs but exhibited a much higher EC(50) with W190/N192 FPR (300+/-45 nM) than for R190/K192 FPR (40+/-3 nM) or R190/N192 (60+/-8 nM). Formyl-MYKWPWYVWL preferentially activated R190/K192 and R190/N192 FPRs by>5 fold compared to W190/N192 FPR. Formyl-MFEDAVAWF, a peptide derived from a protein in Mycobacterium avium subsp. paratuberculosis and formyl-MFTFEPFPTN, a peptide derived from the N-terminus of chemotaxis inhibitory protein of Staphylococcus aureus with an added N-terminal formyl-methionine exhibited the greatest selectivity for R190/K192 and R190/N192 FPRs with approximately 10 fold lower EC(50)s than that observed with FPR W190/N192. Thus, individuals with the W190 polymorphism may display a reduced ability to detect certain formyl peptides.

Antibodies generated in cats by a lipopeptide reproducing the membrane-proximal external region of the feline immunodeficiency virus transmembrane enhance virus infectivity

The immunogenicity of a lipoylated peptide (lipo-P59) reproducing the membrane-proximal external region (MPER) of the transmembrane glycoprotein of feline immunodeficiency virus (FIV) was investigated with cats. In the attempt to mimic the context in which MPER is located within intact virions, lipo-P59 was administered in association with membrane-like micelles. Analyses showed that in this milieu, lipo-P59 had a remarkable propensity to be positioned at the membrane interface, displayed a large number of ordered structures folded in turn helices, and was as active as lipo-P59 alone at inhibiting FIV infectivity in vitro. The antibodies developed differed from the ones previously obtained by immunizing cats with the nonlipoylated version of the peptide (G. Freer, S. Giannecchini, A. Tissot, M. F. Bachmann, P. Rovero, P. F. Serres, and M. Bendinelli, Virology 322:360-369, 2004) in epitope specificity and in the fact that they bound FIV virions. However, they too lacked virus-neutralizing activity and actually enhanced FIV infectivity for lymphoid cell cultures. It is concluded that the use of MPER-reproducing oligopeptides is not a viable approach for vaccinating against FIV.

FIV as a Model for HIV: An Overview

Animal models for human immunodeficiency virus (HIV) infection play a key role in understanding the pathogenesis of AIDS and the development of therapeutic agents and vaccines. As the only lentivirus that causes an immunodeficiency resembling that of HIV infection, in its natural host, feline immunodeficiency virus (FIV) has been a unique and powerful model for AIDS research. FIV was first described in 1987 by Niels Pedersen and co-workers as the causative agent for a fatal immunodeficiency syndrome observed in cats housed in a cattery in Petaluma, California. Since this landmark observation, multiple studies have shown that natural and experimental infection of cats with biological isolates of FIV produces an AIDS syndrome very similar in pathogenesis to that observed for human AIDS. FIV infection induces an acute viremia associated with Tcell alterations including depressed CD4 :CD8 T-cell ratios and CD4 T-cell depletion, peripheral lymphadenopathy, and neutropenia. In later stages of FIV infection, the host suffers from chronic persistent infections that are typically self-limiting in an immunocompetent host, as well as opportunistic infections, chronic diarrhea and wasting, blood dyscracias, significant CD4 T-cell depletion, neurologic disorders, and B-cell lymphomas. Importantly, chronic FIV infection induces a progressive lymphoid and CD4 T-cell depletion in the infected cat. The primary mode of natural FIV transmission appears to be blood-borne facilitated by fighting and biting. However, experimental infection through transmucosal routes (rectal and vaginal mucosa and perinatal) have been well documented for specific FIV isolates. Accordingly, FIV disease pathogenesis exhibits striking similarities to that described for HIV-1 infection.

Receptor-triggered but alkylation-arrested env of murine leukemia virus reveals the transmembrane subunit in a prehairpin conformation

A central feature of the prevailing model for retrovirus fusion is conversion of the transmembrane (TM) subunit from a prehairpin to a hairpin-like structure. The fusion inhibition of many retroviruses, except murine leukemia virus (MLV), with peptides corresponding to interacting regions in the hairpin supports the model. MLV fusion is controlled by isomerization of the intersubunit disulfide in Env. We show here that TM peptides bind to MLV Env that has been arrested at an intermediate stage of activation by alkylation of the isomerization-active thiol in the surface subunit. This inhibits fusion rescue by dithiothreitol-mediated reduction of the surface protein-TM disulfide.

Development of antiviral fusion inhibitors: short modified peptides derived from the transmembrane glycoprotein of feline immunodeficiency virus

Feline immunodeficiency virus (FIV) is a naturally occurring pathogen that causes an AIDS-like syndrome in domestic cats and is a valuable model system by which criteria for antiviral vaccines and drugs development can be tested. The cell-entry step of the lentivirus life cycle is regarded as a promising target for the development of new generation inhibitors. We have previously described potent in vitro anti-FIV activity associated with a synthetic octapeptide, termed C8 (Ac-Trp-Glu-Asp-Trp-Val-Gly-Trp-Ile-NH2), containing the Trp-rich motif of FIV transmembrane glycoprotein, which shares a common structural framework with the corresponding molecule of HIV and appears to play a similar role in cell entry. In this report, in an attempt to develop simpler potential fusion inhibitors to be tested in vivo, we describe further studies focused on synthetic peptide analogues of C8. Since C8 inhibitory activity is dependent upon the Trp motif, we systematically replaced these residues with bulky and/or aromatic natural and unnatural amino acids, in order to develop a rational structure-activity relationship. Furthermore, the amino acids located between the Trp residues, which are not crucial for inhibitory activity, were replaced by simple alkyl spacers of appropriate length. Design, NMR structural analysis, in vitro anti-FIV activity in lymphoid cell cultures, and serum stability of these new analogues are reported. The final results indicate that a simpler hexapeptide (Ac-Nal2-Ape-Nal2-Ape-Nal2-Ile-NH2; Nal2 = 3-naphthalen-2-yl-L-alanine, Ape = 5-aminopentanoic acid), almost entirely made up of unnatural amino acid residues, has markedly increased enzymatic stability, while maintaining strong antiviral potency in vitro.

Peptides derived from HIV-1, HIV-2, Ebola virus, SARS coronavirus and coronavirus 229E exhibit high affinity binding to the formyl peptide receptor

Peptides derived from the membrane proximal region of fusion proteins of human immunodeficiency viruses 1 and 2, Coronavirus 229 E, severe acute respiratory syndrome coronavirus and Ebola virus were all potent antagonists of the formyl peptide receptor expressed in Chinese hamster ovary cells. Binding of viral peptides was affected by the naturally occurring polymorphisms at residues 190 and 192, which are located at second extracellular loop-transmembrane helix 5 interface. Substitution of R190 with W190 enhanced the affinity for a severe acute respiratory syndrome coronavirus peptide 6 fold but reduced the affinity for N-formyl-Nle-Leu-Phe by 2.5 fold. A 12 mer peptide derived from coronavirus 229E (ETYIKPWWVWL) was the most potent antagonist of the formyl peptide receptor W190 with a K(i) of 230 nM. Fluorescently labeled ETYIKPWWVWL was effectively internalized by all three variants with EC(50) of approximately 25 nM. An HKU-1 coronavirus peptide, MYVKWPWYVWL, was a potent antagonist but N-formyl-MYVKWPWYVWL was a potent agonist. ETYIKPWWVWL did not stimulate GTPgammaS binding but inhibited the stimulation by formyl-NleLeuPhe. It also blocked beta arrestin translocation and receptor downregulation induced by formyl-Nle-Leu-Phe. This indicates that formyl peptide receptor may be important in viral infections and that variations in its sequence among individuals may affect their likelihood of viral and bacterial infections.

Physicochemical characterization of a peptide deriving from the glycoprotein gp36 of the feline immunodeficiency virus and its lipoylated analogue in micellar systems

P59 is the Trp-rich 20-mer peptide ((767)L-G(786)), partial sequence of the membrane-proximal external region (MPER) of the FIV gp36. It has potent antiviral activity, possibly due to a mechanism that inhibits the fusion of the virus with the cell membranes. In the hypothesis that a lipophilic tail could enhance the adhesion of P59 to the membrane so improving its antiviral activity, we synthesized its lipoylated analogue lipo-P59. Fluorescence, CD and NMR investigations in membrane mimicking environments (such as SDS and DPC micelles) were aimed to assess the potential of the lipo-P59 lipophilic tail to affect the biophysical and conformational behaviour of the peptide. In vitro inhibitory assays using lymphoid cell cultures to check the antiviral activity of peptides were also performed. The data show that the biophysical properties and the conformational preferences of the peptides are not dramatically affected by the hydrophobic tail, suggesting that the lipopeptide is capable of preserving all the biophysical peculiarities. Similarly, antiviral experimental data show that the membrane-anchored lipo-P59 peptide is also effective in inhibiting virus replication. Moreover, the lipophilic tail allows P59 to preserve its antiviral activity even in conditions in which the non lipoylated peptide is devoid of activity. In accordance with the unusual high Trp presence, the peptides confirm the preference to be positioned on the membrane interface. Furthermore, the data point out a peculiarity of interaction of the peptides with SDS as compared with DPC.

Structural characterization of the feline-immunodeficiency-virus envelope glycoprotein 36 ectodomain for the development of new antivirals

In the fight against the human HIV, new targets are being explored, such as the proteins involved in the process of fusion of the virus with the host cell. Recently, the first generation of fusion inhibitors (enfuvirtide), targeting gp41 (virus envelope glycoprotein 41), has become commercially available. However, this promising class of drugs has to be improved in respect of their efficacy and bioavailability. Considering the strong homologies between HIV and FIV (feline immunodeficiency virus), as well as the highly conserved structure of the transmembrane envelope protein among species, FIV represents a relevant model of pre-screening studies for HIV. Taking into account (i) sequence homologies between the ectodomain of HIV gp41 and FIV gp36 (envelope glycoprotein 36), (ii) structural data available for gp41 and (iii) the fact that synthetic peptides derived from gp36 are effective inhibitors of FIV infection, we designed several peptides derived from gp36 sequence. We checked that these peptides had the same structural features as the corresponding peptides from gp41 HIV by CD, analytical ultracentrifugation and 1H-2H (hydrogen-deuterium) exchange combined with MS. By combining this latter technique with surface-plasmon-resonance assays, we identified the amino acid residues of the C-terminal region of the ectodomain of gp36 that are critical for interaction with the N-terminal region. This gave clues for therapy and vaccines against FIV, thus providing helpful data for treatments against HIV.

Feline Immunodeficiency Virus Plasma Load Reduction by a Retroinverso Octapeptide Reproducing the Trp-Rich Motif of the Transmembrane Glycoprotein

The Trp-rich motif (TrpM) of the transmembrane glycoprotein (TM) of lentiviruses is an attractive domain on which to design new potential cell entry peptide inhibitors. We recently demonstrated that an octapeptide reproducing the TrpM of feline immunodeficiency virus (FIV), designated C8, broadly inhibited this virus in vitro and that the retroinverso analogue of this peptide (riC8) was almost as inhibitory and exhibited features suggestive of a much increased stability. Here, we demonstrated that riC8 is indeed highly stable, maintaining its concentration unchanged for at least 24 h in cat serum in vitro. Furthermore, once inoculated into cats, riC8 produced no major acute toxic effects and exhibited satisfactory pharmacokinetic properties. Finally, we report the results of a short-term monotherapy experiment in chronically FIV-infected cats showing that riC8 is well tolerated and also has substantial antiviral activity in vivo. In particular, the mean viral load of riC8-treated animals declined progressively with increasing time of treatment, whereas that of control animals given C8 or solvent alone did not. These results provide the first evidence that clinically useful inhibition of virus replication with a small peptide derived from a functional domain of the TM of a lentivirus can be achieved in vivo.

Dissection of seroreactivity against the tryptophan-rich motif of the feline immunodeficiency virus transmembrane glycoprotein

Immunogenicity of the tryptophan-rich motif (TrpM) in the membrane-proximal ectodomain of the transmembrane (TM) glycoprotein of feline immunodeficiency virus (FIV) was investigated. Peptide 59, a peptide containing the TrpM of the TM of FIV, was covalently coupled to Qbeta phage virus-like particles (Qbeta-59) in the attempt to induce potent anti-TrpM B cell responses in cats. All Qbeta-59 immunized cats, but not cats that received a mixture of uncoupled Qbeta and peptide 59, developed antibodies that reacted with a same epitope in extensive binding and binding competition assays. The epitope recognized was composed of three amino acids, two of which are adjacent. However, Qbeta-59-immune sera failed to recognize whole FIV in all binding and neutralization assays performed. Furthermore, no reactivity against the TrpM was detected by screening sera from FIV-infected cats that had reacted with TM peptides, confirming that this epitope does not seem to be serologically functional in the FIV virion. The data suggest that TrpM may not be a suitable target for antiviral vaccine design.

Mechanism of feline immunodeficiency virus envelope glycoprotein-mediated fusion

Feline immunodeficiency virus (FIV) shares remarkable homology to primate lentiviruses, human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV). The process of lentiviral env glycoprotein-mediated fusion of membranes is essential for viral entry and syncytia formation. A detailed understanding of this phenomenon has helped identify new targets for antiviral drug development. Using a model based on syncytia formation between FIV env-expressing cells and a feline CD4+ T cell line we have studied the mechanism of FIV env-mediated fusion. Using this model we show that FIV env-mediated fusion mechanism and kinetics are similar to HIV env. Syncytia formation could be blocked by CXCR4 antagonist AMD3100, establishing the importance of this receptor in FIV gp120 binding. Interestingly, CXCR4 alone was not sufficient to allow fusion by a primary isolate of FIV, as env glycoprotein from FIV-NCSU(1) failed to induce syncytia in several feline cell lines expressing CXCR4. Syncytia formation could be inhibited at a post-CXCR4 binding step by synthetic peptide T1971, which inhibits interaction of heptad repeat regions of gp41 and formation of the hairpin structure. Finally, using site-directed mutagenesis, we also show that a conserved tryptophan-rich region in the membrane proximal ectodomain of gp41 is critical for fusion, possibly at steps post hairpin structure formation.

The membrane-proximal tryptophan-rich region in the transmembrane glycoprotein ectodomain of feline immunodeficiency virus is important for cell entry

The mechanisms whereby feline immunodeficiency virus (FIV) adsorbs and enters into susceptible cells are poorly understood. Here, we investigated the role exerted in such functions by the tryptophan (Trp)-rich motif present membrane-proximally in the ectodomain of the FIV transmembrane glycoprotein. Starting from p34TF10, which encodes the entire genome of FIV Petaluma, we produced 11 mutated clones having the Trp-rich motif scrambled or variously deleted or substituted. All mutated progenies adsorbed normally to cells, but the ones with severe disruptions of the motif failed to generate proviral DNA. In the latter mutants, proviral DNA formation was restored by providing an independent source of intact FIV envelope glycoproteins or by addition of the fusing agent polyethylene glycol, thus clearly indicating that their defect resided primarily at the level of cell entry. In addition, the replication-competent mutants exhibited a generally enhanced susceptibility to selected entry inhibitory synthetic peptides, suggestive of a reduced efficiency of the entry step.

Cloaked similarity between HIV-1 and SARS-CoV suggests an anti-SARS strategy

Background

Severe acute respiratory syndrome (SARS) is a febrile respiratory illness. The disease has been etiologically linked to a novel coronavirus that has been named the SARS-associated coronavirus (SARS-CoV), whose genome was recently sequenced. Since it is a member of the Coronaviridae, its spike protein (S2) is believed to play a central role in viral entry by facilitating fusion between the viral and host cell membranes. The protein responsible for viral-induced membrane fusion of HIV-1 (gp41) differs in length, and has no sequence homology with S2.

Results

Sequence analysis reveals that the two viral proteins share the sequence motifs that construct their active conformation. These include (1) an N-terminal leucine/isoleucine zipper-like sequence, and (2) a C-terminal heptad repeat located upstream of (3) an aromatic residue-rich region juxtaposed to the (4) transmembrane segment.

Conclusions

This study points to a similar mode of action for the two viral proteins, suggesting that anti-viral strategy that targets the viral-induced membrane fusion step can be adopted from HIV-1 to SARS-CoV. Recently the FDA approved Enfuvirtide, a synthetic peptide corresponding to the C-terminal heptad repeat of HIV-1 gp41, as an anti-AIDS agent. Enfuvirtide and C34, another anti HIV-1 peptide, exert their inhibitory activity by binding to a leucine/isoleucine zipper-like sequence in gp41, thus inhibiting a conformational change of gp41 required for its activation. We suggest that peptides corresponding to the C-terminal heptad repeat of the S2 protein may serve as inhibitors for SARS-CoV entry.

Retroinverso analogue of the antiviral octapeptide C8 inhibits feline immunodeficiency virus in serum

We described the antiviral activity of an octapeptide corresponding to a Trp-rich domain of feline immunodeficiency virus (FIV) transmembrane glycoprotein. To overcome the limited enzymatic stability of short peptides, the retroinverso analogue was prepared and tested for inhibitory activity of FIV in the presence or absence of normal cat serum. Differently from the unmodified peptide, the retroinverso analogue maintains strong inhibitory activity in serum. NMR studies showed that it displays crucial conformational features believed to be important for antiviral activity.