The identification and characterization of fusogenic domains in herpes virus glycoprotein B molecules

HSV-1 infection induces phosphorylated tau propagation among neurons via extracellular vesicles

Extracellular vesicles (EV), key players in cell-to-cell communication, may contribute to disease propagation in several neurodegenerative diseases, including Alzheimer's disease (AD), by favoring the dissemination of neurotoxic proteins within the brain. Interestingly, growing evidence supports the role of herpes simplex virus type 1 (HSV-1) infection in the pathogenesis of AD. Here, we investigated whether HSV-1 infection could promote the spread of phosphorylated tau (ptau) among neurons via EV. We analyzed the ptau species that were secreted via EV following HSV-1 infection in neuroblastoma cells and primary neurons, focusing particularly on T205, T181, and T217, the phosphorylation sites mainly associated with AD. Moreover, by overexpressing human tau tagged with GFP (htauGFP), we found that recipient tau knockout (KO) neurons uptook EV that are loaded with HSV-1-induced phtauGFP. Finally, we exploited an in vivo model of acute infection and assessed that cerebral HSV-1 infection promotes the release of ptau via EV in the brain of infected mice. Overall, our data suggest that, following HSV-1 infection, EV play a role in tau spreading within the brain, thus contributing to neurodegeneration.IMPORTANCEHerpes simplex virus type 1 (HSV-1) infection that reaches the brain has been repeatedly linked with the appearance of the pathognomonic markers of Alzheimer's disease (AD), including accumulation of amyloid beta and hyperphosphorylated tau proteins, and cognitive deficits. AD is a multifactorial neurodegenerative disease representing the most common form of dementia in the elderly, and no cure is currently available, thus prompting additional investigation on potential risk factors and pathological mechanisms. Here, we demonstrate that the virus exploits the extracellular vesicles (EV) to disseminate phosphorylated tau (ptau) among brain cells. Importantly, we provide evidence that the HSV-1-induced EV-bearing ptau can be undertaken by recipient neurons, thus likely contributing to misfolding and aggregation of native tau, as reported for other AD models. Hence, our data highlight a novel mechanism exploited by HSV-1 to propagate tau-related damage in the brain.

Herpesvirus-Mimicking DNAzyme-Loaded Nanoparticles as a Mitochondrial DNA Stress Inducer to Activate Innate Immunity for Tumor Therapy

Virus-based immunotherapy is a promising approach to treat tumor. Closely mimicking the structure and sequential infection processes of natural viruses is highly desirable for effective tumor immunotherapy but remains challenging. Here, inspired by the robust innate immunity induced by herpesvirus, a herpesvirus-mimicking nanoparticle (named Vir-ZM@TD) is engineered for tumor therapy by mimicking the structure and infection processes of herpesvirus. In this biomimetic system, DNAzyme-loaded manganese-doped zeolitic imidazolate framework-90 (ZIF-90) nanoparticles (ZM@TD) mimic the virus nucleocapsid containing the genome; the erythrocyte membrane mimics the viral envelope; and two functional peptides, RGD and HA2 peptides, resemble the surface glycoprotein spikes of herpesvirus. Vir-ZM@TD can both effectively evade rapid clearance in the blood circulation and closely mimic the serial infection processes of herpesvirus, including specific tumor targeting, membrane fusion-mediated endosomal escape, and TFAM (transcription factor A, mitochondrial) deficiency-triggered mitochondrial DNA stress, as well as the release of manganese ions (Mn²⁺ ) from organelles into the cytosol, ultimately effectively priming cGAS-STING pathway-mediated innate immunity with 68% complete regression of primary tumors and extending by 32 days the median survival time of 4T1-tumor-bearing mice.

Herpes Simplex Virus Cell Entry Mechanisms: An Update

Herpes simplex virus (HSV) can infect a broad host range and cause mild to life threating infections in humans. The surface glycoproteins of HSV are evolutionarily conserved and show an extraordinary ability to bind more than one receptor on the host cell surface. Following attachment, the virus fuses its lipid envelope with the host cell membrane and releases its nucleocapsid along with tegument proteins into the cytosol. With the help of tegument proteins and host cell factors, the nucleocapsid is then docked into the nuclear pore. The viral double stranded DNA is then released into the host cell’s nucleus. Released viral DNA either replicates rapidly (more commonly in non-neuronal cells) or stays latent inside the nucleus (in sensory neurons). The fusion of the viral envelope with host cell membrane is a key step. Blocking this step can prevent entry of HSV into the host cell and the subsequent interactions that ultimately lead to production of viral progeny and cell death or latency. In this review, we have discussed viral entry mechanisms including the pH-independent as well as pH-dependent endocytic entry, cell to cell spread of HSV and use of viral glycoproteins as an antiviral target.

An Improved DNA Vaccine Against Bovine Herpesvirus-1 Using CD40L and a Chemical Adjuvant Induces Specific Cytotoxicity in Mice

Bovine herpesvirus-1 (BoHV-1) uses many mechanisms to elude the immune system; one of them is spreading intracellularly, even in the presence of specific antiviral antibodies. Cytotoxic T lymphocytes (CTLs) are necessary to eliminate the virus. The main preventive strategy is vaccination based on inactivated virus. These vaccines are poor inducers of cellular immune responses, and complicate serological diagnosis and determination of the real prevalence of infection. DNA vaccines are a good option because of the capacity of Differentiating Infected from Vaccinated Animals-(DIVA vaccine)-and may be the best way to induce cytotoxic responses. Although this type of vaccines leads to only weak "in vivo" expression and poor immune responses, incorporation of molecular and/or chemical adjuvants can improve the latter, both in magnitude and in direction. In this study, we have investigated the specific immune responses elicited in mice by DNA vaccines based on the BoHV-1 glycoprotein D (pCIgD) with and without two different adjuvants: a plasmid encoding for murine CD40L (pCD40L) or Montanide™ 1113101PR (101). Mice vaccinated with pCIgD⁺CD40L, pCIgD⁺101, and pCIgD⁺CD40L⁺101 developed significantly higher specific antibody titers against BoHV-1 than the pCIgD group (p < 0.01). The animals vaccinated with pCgD⁺pCD40L⁺101 raised significantly higher levels of IgG2a and IgG2b (p < 0.01 and p < 0.001, respectively) than mice vaccinated with pCIgD alone. On the contrary, when the activity of CTL against cells infected with BoHV-1 was measured, the vaccine pCgD⁺pCD40L⁺101 induced significantly higher levels of cytotoxicity activity (p < 0.001) than pCIgD alone. A significant increase in the CD4⁺ populations in the group receiving pCIgD⁺CD40L⁺101 in comparison with the pCIgD group was observed and, also, interferon gamma, interleukin (IL)-6, and IL-17A levels were higher. Considering the results obtained from this study for humoral and cellular responses in mice, the inclusion of pCD40L and 101 as adjuvants in a BoHV-1 DNA vaccine for cattle is highly recommendable.

A boost to the antiviral activity: Cholesterol tagged peptides derived from glycoprotein B of Herpes Simplex virus type I

Conformational changes of viral glycoproteins govern the fusion of viral and cellular membranes in the entry of enveloped viruses. Peptides mimicking domains of viral glycoproteins are apt to interfere with the fusion event, likely hampering the conformational rearrangements from the pre- to the post-fusion structures. We previously developed a peptide sequence with a high potential to inhibit the entry of herpes simplex type 1, which was able to trap glycoprotein B at an intermediate stage, arresting fusion. We propose that similarly to other viruses, membrane targeting through cholesterol conjugation may potently block fusion. The peptide conjugated to polyethylenglycol and cholesterol interacts with viral and cell membranes thanks to the presence of cholesterol and blocks the conformational rearrangements of the glycoprotein B. Here, we also probed the effect of the linker (polyethylenglycol) length on the activity. By targeting the peptide gBh1m to the membranes where fusion occurs and by engineering sequences with increased binding affinity for gB we have enhanced the antiviral potency of our prototype inhibitors. Our results provide proof of concept for the application of cholesterol tagging to develop inhibitors of HSV-1.

Alphaherpesvirus gB Homologs Are Targeted to Extracellular Vesicles, but They Differentially Affect MHC Class II Molecules

Herpesvirus envelope glycoprotein B (gB) is one of the best-documented extracellular vesicle (EVs)-incorporated viral proteins. Regarding the sequence and structure conservation between gB homologs, we asked whether bovine herpesvirus-1 (BoHV-1) and pseudorabies virus (PRV)-encoded gB share the property of herpes simplex-1 (HSV-1) gB to be trafficked to EVs and affect major histocompatibility complex (MHC) class II. Our data highlight some conserved and differential features of the three gBs. We demonstrate that mature, fully processed BoHV-1 and PRV gBs localize to EVs isolated from constructed stable cell lines and EVs-enriched fractions from virus-infected cells. gB also shares the ability to co-localize with CD63 and MHC II in late endosomes. However, we report here a differential effect of the HSV-1, BoHV-1, and PRV glycoprotein on the surface MHC II levels, and MHC II loading to EVs in stable cell lines, which may result from their adverse ability to bind HLA-DR, with PRV gB being the most divergent. BoHV-1 and HSV-1 gB could retard HLA-DR exports to the plasma membrane. Our results confirm that the differential effect of gB on MHC II may require various mechanisms, either dependent on its complex formation or on inducing general alterations to the vesicular transport. EVs from virus-infected cells also contained other viral glycoproteins, like gD or gE, and they were enriched in MHC II. As shown for BoHV-1 gB- or BoHV-1-infected cell-derived vesicles, those EVs could bind anti-virus antibodies in ELISA, which supports the immunoregulatory potential of alphaherpesvirus gB.

Membranotropic peptides mediating viral entry

The means used by enveloped viruses to bypass cellular membranes are well characterized; however, the mechanisms used by non-enveloped viruses to deliver their genome inside the cell remain unresolved and poorly defined. The discovery of short, membrane interacting, amphipathic or hydrophobic sequences (known as membranotropic peptides) in both enveloped and non-enveloped viruses suggests that these small peptides are strongly involved in breaching the host membrane and in the delivery of the viral genome into the host cell. Thus, in spite of noticeable differences in entry, this short stretches of membranotropic peptides are probably associated with similar entry-related events. This review will uncover the intrinsic features of viral membranotropic peptides involved in viral entry of both naked viruses and the ones encircled with a biological membrane with the objective to better elucidate their different functional properties and possible applications in the biomedical field.

Design, Synthesis and Characterization of Novel Co-Polymers Decorated with Peptides for the Selective Nanoparticle Transport across the Cerebral Endothelium

The development of new strategies for enhancing drug delivery to the brain represents a major challenge in treating cerebral diseases. In this paper, we report on the synthesis and structural characterization of a biocompatible nanoparticle (NP) made up of poly(lactic-co-glycolic acid) (PLGA)-polyethylene glycol (PEG) co-polymer (namely PELGA) functionalized with the membranotropic peptide gH625 (gH) and the iron-mimicking peptide CRTIGPSVC (CRT) for transport across the blood-brain barrier (BBB). gH possesses a high translocation potency of the cell membrane. Conversely, CRT selectively recognizes the brain endothelium, which interacts with transferrin (Tf) and its receptor (TfR) through a non-canonical ligand-directed mechanism. We hypothesize that the delivery across the BBB of PELGA NPs should be efficiently enhanced by the NP functionalization with both gH and CRT. Synthesis of peptides and their conjugation to the PLGA as well as NP physical-chemical characterization are performed. Moreover, NP uptake, co-localization, adhesion under dynamic conditions, and permeation across in vitro BBB model are evaluated as a function of gH/CRT functionalization ratio. Results establish that the cooperative effect of CRT and gH may change the intra-cellular distribution of NPs and strengthen NP delivery across the BBB at the functionalization ratio 33% gH–66% CRT.

Enhanced Drug Delivery into Cell Cytosol via Glycoprotein H-Derived Peptide Conjugated Nanoemulsions

The key role of nanocarriers in improving the pharmacological properties of commonly used drugs is recognized worldwide. It is also known that in the development of new effective nanocarriers the use of targeting moieties integrated on their surface is essential. Herein, we propose a nanocarrier based on an oil in water nanoemulsion coated with a membranotropic peptide derived from the glycoprotein H of Herpes simplex virus 1, known as gH625, in order to reduce endolysosomal accumulation and to enhance cytosolic localization. In addition, we show an enhanced anti-inflammatory activity of curcumin, a bioactive compound isolated from the Curcuma longa plant, when loaded into our engineered nanocarriers. This effect is a consequence of a higher uptake combined with a high curcumin preservation exerted by the active nanocapsules compared to control ones. When loaded into our nanocapsules, indeed, curcumin molecules are directly internalized into the cytosol rather than into lysosomes. Further, in order to extend the in vitro experimental setting with a more complex model and to explore the possibility to use our nanocarriers for further biological applications, we tested their performance in a 3D sprouting angiogenesis model. Finally, we show promising preliminary in vivo results by assessing the anti-inflammatory properties of the proposed nanocarrier.

Infectivity inhibition by overlapping synthetic peptides derived from the gH/gL heterodimer of herpes simplex virus type 1

Herpes simplex virus (HSV) is a human pathogen that infects epithelial cells. The cutaneous lesions, caused by the virus, spread to the nervous system creating several complications. Fusion of host membranes with the viral envelope is mandatory and mediated by a group of glycoproteins conserved in all Herpesviridae subfamilies, such as the glycoproteins B (gB), H (gH), L (gL) and D (gD). We investigated the inhibitory activity mediated by synthetic overlapping peptides spanning the entire ectodomains of gH and gL glycoproteins. We have performed a brute analysis of the complete gH/gL heterodimer in order to explore the inhibitory activity of peptides modelled on these glycoproteins against HSV‐1 infection. Twenty‐four of the gH peptides at a concentration of 150 μM reached the 50% of inhibition cut‐off. Interestingly, they are mainly located in the gH carboxy‐terminal domain. None of the gL peptides had a clear inhibiting effect. No peptide toxicity was observed by lactate dehydrogenase assay at the concentrations used in our experimental conditions. HSV‐1 therapy is based on acyclovir treatment, but some resistant strains are emerging. In this scenario, innovative approaches for HSV‐1 treatment are necessary. Our data support the direct involvement of the described domains in the process of virus penetration; therefore, these results are of relevance to the potential development of novel therapeutic compounds to prevent HSV‐1 infections. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.

A DNA Vaccine Formulated with Chemical Adjuvant Provides Partial Protection against Bovine Herpes Virus Infection in Cattle

Bovine herpesvirus-1 (BoHV-1) is the causative agent of bovine infectious rhinotracheitis, an important disease worldwide. Although conventional BoHV-1 vaccines, including those based on the use of modified live virus and also inactivated vaccines, are currently used in many countries, they have several disadvantages. DNA vaccines have emerged as an attractive approach since they have the potential to induce both humoral and cellular immune response; nevertheless, it is largely known that potency of naked DNA vaccines is limited. We demonstrated previously, in the murine model, that the use of adjuvants in combination with a DNA vaccine against BoHV-1 is immunologically beneficial. In this study, we evaluate the immune response and protection against challenge elicited in bovines, by a DNA vaccine carrying the sequence of secreted version of glycoprotein D (gD) of BoHV-1 formulated with chemical adjuvants. Bovines were vaccinated with formulations containing the sequence of gD alone or in combination with adjuvants ESSAI 903110 or Montanide™ 1113101PR. After prime vaccination and two boosters, animals were challenged with infectious BoHV-1. Formulations containing adjuvants Montanide™ 1113101PR and ESSAI 903110 were both, capable of increasing humoral immune response against the virus and diminishing clinical symptoms. Nevertheless, only formulations containing adjuvant Montanide™ 1113101PR was capable of improving cellular immune response and diminishing viral excretion. To our knowledge, it is the first time that a BoHV-1 DNA vaccine is combined with adjuvants and tested in cattle. These results could be useful to design a vaccine for the control of bovine rhinotracheitis.

Rapid Screening by Cell-Based Fusion Assay for Identifying Novel Antivirals of Glycoprotein B-Mediated Herpes Simplex Virus Type 1 Infection

Herpes simplex virus type 1 (HSV-1) is a causative agent for a variety of diseases. Although antiherpetic drugs such as acyclovir have been developed to inhibit virus replication through interaction with DNA kinases, their continuous administration leads to an increase in the frequency of drug-resistant HSV-1, which is an important clinical issue that requires urgent solution. Recently, we reported that the sialylated O-linked sugar T antigen (sTn) and its attached peptide region (O-glycosylated sTn peptide) derived from the HSV-1 glycoprotein B (gB) protein inhibited HSV-1 infection by specifically targeting paired immunoglobulin-like type 2 receptor alpha (PILRα) in vitro. In this study, to further identify novel inhibitors of gB-mediated HSV-1 infection in vitro, we established a cell-based fusion assay for rapid drug screening. Chinese hamster ovary (CHO) cells were transfected with expression plasmids for HSV-1 gB, gD, gH, and gL, and T7 RNA polymerase, and were designated as the effector cells. The CHO-K1 cells stably expressing PILRα were transfected with the expression plasmid for firefly luciferase under the T7 promoter, and were designated as the target cells. The effector and target cells were co-cultured, and luminescence was measured when both cells were successfully fused. Importantly, we found that cell-to-cell fusion was specifically inhibited by O-glycosylated sTn peptide in a dose dependent manner. Our results suggested that this virus-free cell-based fusion assay system could be a useful and promising approach to identify novel inhibitors of gB-mediated HSV-1 infection, and will aid in the development of antiviral therapeutic strategies for HSV-1-associated diseases.

In search for effective and definitive treatment of herpes simplex virus type 1 (HSV-1) infections

Herpes Simplex Virus type 1 (HSV-1) is a nuclear replicating enveloped virus.

Identification of Peptide Inhibitors of Enveloped Viruses Using Support Vector Machine

The peptides derived from envelope proteins have been shown to inhibit the protein-protein interactions in the virus membrane fusion process and thus have a great potential to be developed into effective antiviral therapies. There are three types of envelope proteins each exhibiting distinct structure folds. Although the exact fusion mechanism remains elusive, it was suggested that the three classes of viral fusion proteins share a similar mechanism of membrane fusion. The common mechanism of action makes it possible to correlate the properties of self-derived peptide inhibitors with their activities. Here we developed a support vector machine model using sequence-based statistical scores of self-derived peptide inhibitors as input features to correlate with their activities. The model displayed 92% prediction accuracy with the Matthew's correlation coefficient of 0.84, obviously superior to those using physicochemical properties and amino acid decomposition as input. The predictive support vector machine model for self- derived peptides of envelope proteins would be useful in development of antiviral peptide inhibitors targeting the virus fusion process.

Membranotropic Cell Penetrating Peptides: The Outstanding Journey

The membrane bilayer delimits the interior of individual cells and provides them with the ability to survive and function properly. However, the crossing of cellular membranes constitutes the principal impediment to gaining entry into cells, and the potential therapeutic application of many drugs is predominantly dependent on the development of delivery tools that should take the drug to target cells selectively and efficiently with only minimal toxicity. Cell-penetrating peptides are short and basic peptides are widely used due to their ability to deliver a cargo across the membrane both in vitro and in vivo. It is widely accepted that their uptake mechanism involves mainly the endocytic pathway, the drug is catched inside endosomes and lysosomes, and only a small quantity is able to reach the intracellular target. In this wide-ranging scenario, a fascinating novel hypothesis is that membranotropic peptides that efficiently cross biological membranes, promote lipid-membrane reorganizing processes and cause a local and temporary destabilization and reorganization of the membrane bilayer, may also be able to enter cells circumventing the endosomal entrapment; in particular, by either favoring the escape from the endosome or by direct translocation. This review summarizes current data on membranotropic peptides for drug delivery.

Surface decoration with gH625-membranotropic peptides as a method to escape the endo-lysosomal compartment and reduce nanoparticle toxicity

The membranotropic peptide gH625 is able to transport different cargos (i.e., liposomes, quantum dots, polymeric nanoparticles) within and across cells in a very efficient manner. However, a clear understanding of the detailed uptake mechanism remains elusive. In this work, we investigate the journey of gH625-functionalized polystyrene nanoparticles in mouse-brain endothelial cells from their interaction with the cell membrane to their intracellular final destination. The aim is to elucidate how gH625 affects the behavior of the nanoparticles and their cytotoxic effect. The results indicate that the mechanism of translocation of gH625 dictates the fate of the nanoparticles, with a relevant impact on the nanotoxicological profile of positively charged nanoparticles.

Peptide gH625 enters into neuron and astrocyte cell lines and crosses the blood-brain barrier in rats

Peptide gH625, derived from glycoprotein H of herpes simplex virus type 1, can enter cells efficiently and deliver a cargo. Nanoparticles armed with gH625 are able to cross an in vitro model of the blood-brain barrier (BBB). In the present study, in vitro experiments were performed to investigate whether gH625 can enter and accumulate in neuron and astrocyte cell lines. The ability of gH625 to cross the BBB in vivo was also evaluated. gH625 was administered in vivo to rats and its presence in the liver and in the brain was detected. Within 3.5 hours of intravenous administration, gH625 can be found beyond the BBB in proximity to cell neurites. gH625 has no toxic effects in vivo, since it does not affect the maximal oxidative capacity of the brain or the mitochondrial respiration rate. Our data suggest that gH625, with its ability to cross the BBB, represents a novel nanocarrier system for drug delivery to the central nervous system. These results open up new possibilities for direct delivery of drugs into patients in the field of theranostics and might address the treatment of several human diseases.

gH625: a milestone in understanding the many roles of membranotropic peptides

Here, we review the current knowledge about viral derived membranotropic peptides, and we discuss how they may be used for many therapeutic applications. While they have been initially discovered in viral fusion proteins and have been involved in the mechanism of viral entry, it is now clear that their features and their mode of interaction with membrane bilayers can be exploited to design viral inhibitors as well as to favor delivery of cargos across the cell membrane and across the blood–brain barrier. The peptide gH625 has been extensively used for all these purposes and provides a significant contribution to the field. We describe the roles of this sequence in order to close the gap between the many functions that are now emerging for membranotropic peptides.

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Membranotropic peptides and their therapeutic applications

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Membrane fusion, viral inhibition, drug delivery

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gH625, a peptide derived from Herpes simplex virus type I: a case study

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gH625 in vitro and in vivo delivery across the blood–brain barrier

The three lives of viral fusion peptides

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The presence of a fusion peptide (FP) is a hallmark of viral fusion glycoproteins.

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Structure–function relationships underlying FP conservation remain greatly unknown.

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FPs establish interactions satisfying their folding within pre-fusion glycoproteins.

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Upon fusion activation FPs insert into and restructure target membranes.

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FPs can finally combine with transmembrane domains to form integral membrane bundles.

Fusion peptides comprise conserved hydrophobic domains absolutely required for the fusogenic activity of glycoproteins from divergent virus families. After 30 years of intensive research efforts, the structures and functions underlying their high degree of sequence conservation are not fully elucidated. The long-hydrophobic viral fusion peptide (VFP) sequences are structurally constrained to access three successive states after biogenesis. Firstly, the VFP sequence must fulfill the set of native interactions required for (meta) stable folding within the globular ectodomains of glycoprotein complexes. Secondly, at the onset of the fusion process, they get transferred into the target cell membrane and adopt specific conformations therein. According to commonly accepted mechanistic models, membrane-bound states of the VFP might promote the lipid bilayer remodeling required for virus-cell membrane merger. Finally, at least in some instances, several VFPs co-assemble with transmembrane anchors into membrane integral helical bundles, following a locking movement hypothetically coupled to fusion-pore expansion. Here we review different aspects of the three major states of the VFPs, including the functional assistance by other membrane-transferring glycoprotein regions, and discuss briefly their potential as targets for clinical intervention.

Conformational modifications of gB from herpes simplex virus type 1 analyzed by synthetic peptides

Entry of enveloped viruses requires fusion of viral and cellular membranes, driven by conformational changes of viral glycoproteins. The crystallized trimeric glycoprotein gB of herpes simplex virus has been described as a postfusion conformation, and several studies prove that like other class III fusion proteins, gB undergoes a pH-dependent switch between the pre- and postfusion conformations. Using several biophysical techniques, we show that peptides corresponding to the long helix of the gB postfusion structure interfere with the membrane fusion event, likely hampering the conformational rearrangements from the pre- to the postfusion structures. Those peptides represent good candidates for further design of peptidomimetic antagonists capable of blocking the fusion process.

Peptide-lipid interactions: experiments and applications

The interactions between peptides and lipids are of fundamental importance in the functioning of numerous membrane-mediated cellular processes including antimicrobial peptide action, hormone-receptor interactions, drug bioavailability across the blood-brain barrier and viral fusion processes. Moreover, a major goal of modern biotechnology is obtaining new potent pharmaceutical agents whose biological action is dependent on the binding of peptides to lipid-bilayers. Several issues need to be addressed such as secondary structure, orientation, oligomerization and localization inside the membrane. At the same time, the structural effects which the peptides cause on the lipid bilayer are important for the interactions and need to be elucidated. The structural characterization of membrane active peptides in membranes is a harsh experimental challenge. It is in fact accepted that no single experimental technique can give a complete structural picture of the interaction, but rather a combination of different techniques is necessary.

Shuttle-mediated nanoparticle delivery to the blood-brain barrier

Many therapeutic drugs are excluded from entering the brain due to their lack of transport through the blood-brain barrier (BBB). The development of new strategies for enhancing drug delivery to the brain is of great importance in diagnostics and therapeutics of central nervous diseases. To overcome this problem, a viral fusion peptide (gH625) derived from the glycoprotein gH of Herpes simplex virus type 1 is developed, which possesses several advantages including high cell translocation potency, absence of toxicity of the peptide itself, and the feasibility as an efficient carrier for delivering therapeutics. Therefore, it is hypothesized that brain delivery of nanoparticles conjugated with gH625 should be efficiently enhanced. The surface of fluorescent aminated polystyrene nanoparticles (NPs) is functionalized with gH625 via a covalent binding procedure, and the NP uptake mechanism and permeation across in vitro BBB models are studied. At early incubation times, the uptake of NPs with gH625 by brain endothelial cells is greater than that of the NPs without the peptide, and their intracellular motion is mainly characterized by a random walk behavior. Most importantly, gH625 peptide decreases NP intracellular accumulation as large aggregates and enhances the NP BBB crossing. In summary, these results establish that surface functionalization with gH625 may change NP fate by providing a good strategy for the design of promising carriers to deliver drugs across the BBB for the treatment of brain diseases.

Glycoprotein targeted therapeutics: a new era of anti-herpes simplex virus-1 therapeutics

Herpes simplex virus type-1 (HSV-1) is among the most common human pathogens worldwide. Its entry into host cells is an intricate process that relies heavily on the ability of the viral glycoproteins to bind host cellular proteins and to efficiently mediate fusion of the virus envelope with the cell membrane. Acquisition of HSV-1 results in a lifelong latent infection. Because of the cycles of reactivation from a latent state, much emphasis has been placed on the management of infection through the use of DNA synthesis inhibitors. However, new methods are needed to provide more effective treatment at earlier phases of the viral infection and to prevent the development of drug resistance by the virus. This review outlines the infection process and the common therapeutics currently used against the fundamental stages of HSV-1 replication and fusion. The remainder of this article will focus on a new approach for HSV-1 infection control and management, the concept of glycoprotein-receptor targeting.

Peptide inhibitors against herpes simplex virus infections

Herpes simplex virus (HSV) is a significant human pathogen causing mucocutaneous lesions primarily in the oral or genital mucosa. Although acyclovir (ACV) and related nucleoside analogs provide successful treatment, HSV remains highly prevalent worldwide and is a major cofactor for the spread of human immunodeficiency virus. Encephalitis, meningitis, and blinding keratitis are among the most severe diseases caused by HSV. ACV resistance poses an important problem for immunocompromised patients and highlights the need for new safe and effective agents; therefore, the development of novel strategies to eradicate HSV is a global public health priority. Despite the continued global epidemic of HSV and extensive research, there have been few major breakthroughs in the treatment or prevention of the virus since the introduction of ACV in the 1980s. A therapeutic strategy at the moment not fully addressed is the use of small peptide molecules. These can be either modeled on viral proteins or derived from antimicrobial peptides. Any peptide that interrupts protein-protein or viral protein-host cell membrane interactions is potentially a novel antiviral drug and may be a useful tool for elucidating the mechanisms of viral entry. This review summarizes current knowledge and strategies in the development of synthetic and natural peptides to inhibit HSV infectivity.

Interaction domain of glycoproteins gB and gH of Marek's disease virus and identification of an antiviral peptide with dual functions

Our previous study reported that both glycoproteins gB and gH of the herpesvirus Marek's disease virus (MDV) contain eleven potential heptad repeat domains. These domains overlap with α-helix-enriched hydrophobic regions, including the gH-derived HR1 (gHH1) and HR3 (gHH3) and gB-derived HR1 (gBH1) regions, which demonstrate effective antiviral activity, with 50% inhibitory concentrations (IC(50)) of less than 12 µM. Plaque formation and chicken embryo infection assays confirmed these results. In this study, biochemical and biophysical analyses detected potential interactions between these peptides. gHH1, gHH3, and gBH1 were found to interact with each other in pairs. The complex formed by gHH3 and gBH1 showed the most stable interaction at a molar ratio of 1:3, the binding between gHH1 and gBH1 was relatively weak, and no interaction was observed between the three HR peptides. These results indicate that gHH3 and gBH1 are likely the key contributors to the interaction between gB and gH. Furthermore, each HR peptide from herpesvirus glycoproteins did not effectively inhibit virus infection compared with peptides from a class I enveloped virus. In this report, the HR mimic peptide modified with a double glutamic acid (EE) or a double lysine (KK) at the non-interactive sites (i.e., solvent-accessible sites) did not noticeably affect the antiviral activity compared with the wild-type HR peptide, whereas tandem peptides from gH-derived gHH1 and gB-derived gBH1 (i.e., gBH1-Linker-gHH1) produced efficient antiviral effects, unlike the individual peptides. The proposed interpretation of inhibition of entry has been addressed. Our results support the hypothesis that the interaction domain between glycoproteins gH and gB is a critical target in the design of inhibitors of herpesvirus infection.

Residues within the C-terminal arm of the herpes simplex virus 1 glycoprotein B ectodomain contribute to its refolding during the fusion step of virus entry

Herpesvirus entry into cells requires coordinated interactions among several viral glycoproteins. The final membrane fusion step of entry is executed by glycoprotein B (gB), a class III viral fusion protein that is conserved across all herpesviruses. Fusion proteins are metastable proteins that mediate fusion by inserting into a target membrane and refolding from a prefusion to postfusion conformation to bring the viral and cell membranes together. Although the structure of gB has been solved in a conformation that likely represents its postfusion form, its prefusion structure and the details of how it refolds to execute fusion are unknown. The postfusion gB structure contains a trimeric coiled-coil at its core and a long C-terminal arm within the ectodomain packs against this coil in an antiparallel manner. This coil-arm complex is reminiscent of the six-helix bundle that provides the energy for fusion in class I fusogens. To determine the role of the coil-arm complex, we individually mutated residues in the herpes simplex virus 1 gB coil-arm complex to alanine and assessed the contribution of each residue to cell-cell and virus-cell fusion. Several coil mutations resulted in a loss of cell surface expression, indicating that the coil residues are important for proper processing of gB. Three mutations in the arm region (I671A, H681A, and F683A) reduced fusion without affecting expression. Combining these three arm mutations drastically reduced the ability of gB to execute fusion; however, fusion function could be restored by adding known hyperfusogenic mutations to the arm mutant. We propose that the formation of the coil-arm complex drives the gB transition to a postfusion conformation and the coil-arm complex performs a function similar to that of the six-helix bundle in class I fusion. Furthermore, we suggest that these specific mutations in the arm may energetically favor the prefusion state of gB.

Biophysical characterization and membrane interaction of the two fusion loops of glycoprotein B from herpes simplex type I virus

The molecular mechanism of entry of herpesviruses requires a multicomponent fusion system. Cell invasion by Herpes simplex virus (HSV) requires four virally encoded glycoproteins: namely gD, gB and gH/gL. The role of gB has remained elusive until recently when the crystal structure of HSV-1 gB became available and the fusion potential of gB was clearly demonstrated. Although much information on gB structure/function relationship has been gathered in recent years, the elucidation of the nature of the fine interactions between gB fusion loops and the membrane bilayer may help to understand the precise molecular mechanism behind herpesvirus-host cell membrane fusion. Here, we report the first biophysical study on the two fusion peptides of gB, with a particular focus on the effects determined by both peptides on lipid bilayers of various compositions. The two fusion loops constitute a structural subdomain wherein key hydrophobic amino acids form a ridge that is supported on both sides by charged residues. When used together the two fusion loops have the ability to significantly destabilize the target membrane bilayer, notwithstanding their low bilayer penetration when used separately. These data support the model of gB fusion loops insertion into cholesterol enriched membranes.

Herpes simplex virus infects most cell types in vitro: clues to its success

Herpes simplex virus (HSV) type-1 and type-2 have evolved numerous strategies to infect a wide range of hosts and cell types. The result is a very successful prevalence of the virus in the human population infecting 40-80% of people worldwide. HSV entry into host cell is a multistep process that involves the interaction of the viral glycoproteins with various cell surface receptors. Based on the cell type, HSV enter into host cell using different modes of entry. The combination of various receptors and entry modes has resulted in a virus that is capable of infecting virtually all cell types. Identifying the common rate limiting steps of the infection may help the development of antiviral agents that are capable of preventing the virus entry into host cell. In this review we describe the major features of HSV entry that have contributed to the wide susceptibility of cells to HSV infection.

Structural characteristics and antiviral activity of multiple peptides derived from MDV glycoproteins B and H

Background

Marek's disease virus (MDV), which is widely considered to be a natural model of virus-induced lymphoma, has the potential to cause tremendous losses in the poultry industry. To investigate the structural basis of MDV membrane fusion and to identify new viral targets for inhibition, we examined the domains of the MDV glycoproteins gH and gB.

Results

Four peptides derived from the MDV glycoprotein gH (gHH1, gHH2, gHH3, and gHH5) and one peptide derived from gB (gBH1) could efficiently inhibit plaque formation in primary chicken embryo fibroblast cells (CEFs) with 50% inhibitory concentrations (IC₅₀) of below 12 μM. These peptides were also significantly able to reduce lesion formation on chorioallantoic membranes (CAMs) of infected chicken embryos at a concentration of 0.5 mM in 60 μl of solution. The HR2 peptide from Newcastle disease virus (NDVHR2) exerted effects on MDV specifically at the stage of virus entry (i.e., in a cell pre-treatment assay and an embryo co-treatment assay), suggesting cross-inhibitory effects of NDV HR2 on MDV infection. None of the peptides exhibited cytotoxic effects at the concentrations tested. Structural characteristics of the five peptides were examined further.

Conclusions

The five MDV-derived peptides demonstrated potent antiviral activity, not only in plaque formation assays in vitro, but also in lesion formation assays in vivo. The present study examining the antiviral activity of these MDV peptides, which are useful as small-molecule antiviral inhibitors, provides information about the MDV entry mechanism.

Fusing structure and function: a structural view of the herpesvirus entry machinery

Herpesviruses are double-stranded DNA, enveloped viruses that infect host cells through fusion with either the host cell plasma membrane or endocytic vesicle membranes. Efficient infection of host cells by herpesviruses is remarkably more complex than infection by other viruses, as it requires the concerted effort of multiple glycoproteins and involves multiple host receptors. The structures of the major viral glycoproteins and a number of host receptors involved in the entry of the prototypical herpesviruses, the herpes simplex viruses (HSVs) and Epstein-Barr virus (EBV), are now known. These structural studies have accelerated our understanding of HSV and EBV binding and fusion by revealing the conformational changes that occur on virus-receptor binding, depicting potential sites of functional protein and lipid interactions, and identifying the probable viral fusogen.

The presence of a single N-terminal histidine residue enhances the fusogenic properties of a Membranotropic peptide derived from herpes simplex virus type 1 glycoprotein H

Herpes simplex virus type 1 (HSV-1)-induced membrane fusion remains one of the most elusive mechanisms to be deciphered in viral entry. The structure resolution of glycoprotein gB has revealed the presence of fusogenic domains in this protein and pointed out the key role of gB in the entry mechanism of HSV-1. A second putative fusogenic glycoprotein is represented by the heterodimer comprising the membrane-anchored glycoprotein H (gH) and the small secreted glycoprotein L, which remains on the viral envelope in virtue of its non-covalent interaction with gH. Different domains scattered on the ectodomain of HSV-1 gH have been demonstrated to display membranotropic characteristics. The segment from amino acid 626 to 644 represents the most fusogenic region identified by studies with synthetic peptides and model membranes. Herein we have identified the minimal fusogenic sequence present on gH. An enlongation at the N terminus of a single histidine (His) has proved to profoundly increase the fusogenic activity of the original sequence. Nuclear magnetic resonance (NMR) studies have shown that the addition of the N-terminal His contributes to the formation and stabilization of an alpha-helical domain with high fusion propensity.

Role of membranotropic sequences from herpes simplex virus type I glycoproteins B and H in the fusion process

The entry of enveloped viruses involves attachment followed by close apposition of the viral and plasma membranes. Then, either on the cell surface or in an endocytotic vesicle, the two membranes fuse by an energetically unfavourable process requiring the destabilisation of membrane microenvironment in order to release the viral nucleocapsid into the cytoplasm. The core fusion machinery, conserved throughout the herpesvirus family, involves glycoprotein B (gB) and the non-covalently associated complex of glycoproteins H and L (gH/gL). Both gB and gH possess several hydrophobic domains necessary for efficient induction of fusion, and synthetic peptides corresponding to these regions are able to associate to membranes and induce fusion of artificial liposomes. Here, we describe the first application of surface plasmon resonance (SPR) to the study of the interaction of viral membranotropic peptides with model membranes in order to enhance our molecular understanding of the mechanism of membrane fusion. SPR spectroscopy data are supported by tryptophan fluorescence, circular dichroism and electron spin resonance spectroscopy (ESR). We selected peptides from gB and gH and also analysed the behaviour of HIV gp41 fusion peptide and the cationic antimicrobial peptide melittin. The combined results of SPR and ESR showed a marked difference between the mode of action of the HSV peptides and the HIV fusion peptide compared to melittin, suggesting that viral-derived membrane interacting peptides all act via a similar mechanism, which is substantially different from that of the non-cell selective lytic peptide melittin.

Viral entry mechanisms: cellular and viral mediators of herpes simplex virus entry

Herpes simplex virus type-1 and type-2 are highly prevalent human pathogens causing life-long infections. The process of infection begins when the virions bind heparan sulfate moieties present on host cell surfaces. This initial attachment then triggers a cascade of molecular interactions involving multiple viral and host cell proteins and receptors, leading to penetration of the viral nucleocapsid and tegument proteins into the cytoplasm. The nucleocapsid is then transported to the nuclear membrane and the viral DNA is released for replication in the nucleus. Recent studies have revealed that herpes simplex virus entry or penetration into cells may be a highly complex process and the mechanism of entry may demonstrate unique cell-type specificities. Although specificities clearly exist, past and ongoing studies demonstrate that herpes simplex virus may share certain common receptors and pathways that are also used by many other human viruses. This minireview helps to shed light on recent revelations on the herpes simplex virus entry process.

Cross talk among the glycoproteins involved in herpes simplex virus entry and fusion: the interaction between gB and gH/gL does not necessarily require gD

The gD, gB, and gH/gL glycoprotein quartet constitutes the basic apparatus for herpes simplex virus (HSV) entry into the cell and fusion. gD serves as a receptor binding glycoprotein and trigger of fusion. The conserved gB and gH/gL execute fusion. Central to understanding HSV entry/fusion has become the dissection of how the four glycoproteins engage in cross talk. While the independent interactions of gD with gB and gD with gH/gL have been documented, less is known of the interaction of gB with gH/gL. So far, this interaction has been detected only in the presence of gD by means of a split green fluorescent protein complementation assay. Here, we show that gB interacts with gH/gL in the absence of gD. The gB-gH/gL complex was best detected with a form of gB in which the endocytosis and phosphorylation motif have been deleted; this form of gB persists in the membranes of the exocytic pathway and is not endocytosed. The gB-gH/gL interaction was detected both in whole transfected cells by means of a split yellow fluorescent protein complementation assay and, biochemically, by a pull-down assay. Results with a panel of chimeric forms of gB, in which portions of the glycoprotein bracketed by consecutive cysteines were replaced with the corresponding portions from human herpesvirus 8 gB, favor the view that gB carries multiple sites for interaction with gH/gL, and one of these sites is located in the pleckstrin-like domain 1 carrying the bipartite fusion loop.

Herpes simplex virus glycoprotein B associates with target membranes via its fusion loops

Herpes simplex virus (HSV) glycoproteins gB, gD, and gH/gL are necessary and sufficient for virus entry into cells. Structural features of gB are similar to those of vesicular stomatitis virus G and baculovirus gp64, and together they define the new class III group of fusion proteins. Previously, we used mutagenesis to show that three hydrophobic residues (W174, Y179, and A261) within the putative gB fusion loops are integral to gB function. Here we expanded our analysis, using site-directed mutagenesis of each residue in both gB fusion loops. Mutation of most of the nonpolar or hydrophobic amino acids (W174, F175, G176, Y179, and A261) had severe effects on gB function in cell-cell fusion and null virus complementation assays. Of the six charged amino acids, mutation of H263 or R264 also negatively affected gB function. To further analyze the mutants, we cloned the ectodomains of the W174R, Y179S, H263A, and R264A mutants into a baculovirus expression system and compared them with the wild-type (WT) form, gB730t. As shown previously, gB730t blocks virus entry into cells, suggesting that gB730t competes with virion gB for a cell receptor. All four mutant proteins retained this function, implying that fusion loop activity is separate from gB-receptor binding. However, unlike WT gB730t, the mutant proteins displayed reduced binding to cells and were either impaired or unable to bind naked, cholesterol-enriched liposomes, suggesting that it may be gB-lipid binding that is disrupted by the mutations. Furthermore, monoclonal antibodies with epitopes proximal to the fusion loops abrogated gB-liposome binding. Taken together, our data suggest that gB associates with lipid membranes via a fusion domain of key hydrophobic and hydrophilic residues and that this domain associates with lipid membranes during fusion.

Multiple peptides homologous to herpes simplex virus type 1 glycoprotein B inhibit viral infection

The 773-residue ectodomain of the herpes simplex virus type 1 (HSV-1) glycoprotein B (gB) has been resistant to the use of mutagenic strategies because the majority of the induced mutations result in defective proteins. As an alternative strategy for the identification of functionally important regions and novel inhibitors of infection, we prepared a library of overlapping peptides homologous to the ectodomain of gB and screened for the ability of the peptides to block infection. Seven of 138 15-mer peptides inhibited infection by more than 50% at a concentration of 100 microM. Three peptides (gB94, gB122, and gB131) with 50% effective concentrations (EC(50)s) below 20 microM were selected for further studies. The gB131 peptide (residues 681 to 695 in HSV-1 gB [gB-1]) was a specific entry inhibitor (EC(50), approximately 12 microM). The gB122 peptide (residues 636 to 650 in gB-1) blocked viral entry (EC(50), approximately 18 microM), protected cells from infection (EC(50), approximately 72 microM), and inactivated virions in solution (EC(50), approximately 138 microM). We were unable to discern the step or steps inhibited by the gB94 peptide, which is homologous to residues 496 to 510 in gB-1. Substitution of a tyrosine in the gB122 peptide (Y640 in full-length gB-1) reduced the antiviral activity eightfold, suggesting that this residue is critical for inhibition. This peptide-based strategy could lead to the identification of functionally important regions of gB or other membrane proteins and identify novel inhibitors of HSV-1 entry.

Analysis of a membrane interacting region of herpes simplex virus type 1 glycoprotein H

Glycoprotein H (gH) of herpes simplex virus type I (HSV-1) is involved in the complex mechanism of membrane fusion of the viral envelope with the host cell. Membrane interacting regions and potential fusion peptides have been identified in HSV-1 gH as well as glycoprotein B (gB). Because of the complex fusion mechanism of HSV-1, which requires four viral glycoproteins, and because there are only structural data for gB and glycoprotein D, many questions regarding the mechanism by which HSV-1 fuses its envelope with the host cell membrane remain unresolved. Previous studies have shown that peptides derived from certain regions of gH have the potential to interact with membranes, and based on these findings we have generated a set of peptides containing mutations in one of these domains, gH-(626-644), to investigate further the functional role of this region. Using a combination of biochemical, spectroscopic, and nuclear magnetic resonance techniques, we showed that the alpha-helical nature of this stretch of amino acids in gH is important for membrane interaction and that the aromatic residues, tryptophan and tyrosine, are critical for induction of fusion.