Cyanovirin-N defines a new class of antiviral agent targeting N-linked, high-mannose glycans in an oligosaccharide-specific manner

Cyanovirin-N Binding to N-Acetyl-d-glucosamine Requires Carbohydrate-Binding Sites on Two Different Protomers

Cyanovirin-N (CV-N) binds high-mannose oligosaccharides on enveloped viruses with two carbohydrate-binding sites, one bearing high affinity and one low affinity to Manα(1-2)Man moieties. A tandem repeat of two CV-N molecules (CVN2) was tested for antiviral activity against human immunodeficiency virus type I (HIV-1) by using a domain-swapped dimer. CV-N was shown to bind N-acetylmannosamine (ManNAc) and N-acetyl-d-glucosamine (GlcNAc) when the carbohydrate-binding sites in CV-N were free to interact with these monosaccharides independently. CVN2 recognized ManNAc at a Kd of 1.4 μM and bound this sugar in solution, regardless of the lectin making amino acid side chain contacts on the targeted viral glycoproteins. An interdomain cross-contacting residue Glu41, which has been shown to be hydrogen bonding with dimannose, was substituted in the monomeric CV-N. The amide derivative of glucose, GlcNAc, achieved similar high affinity to the new variant CVN-E41T as high-mannose N-glycans, but binding to CVN2 in the nanomolar range with four binding sites involved or binding to the monomeric CVN-E41A. A stable dimer was engineered and expressed from the alanine-to-threonine-substituted monomer to confirm binding to GlcNAc. In summary, low-affinity binding was achieved by CVN2 to dimannosylated peptide or GlcNAc with two carbohydrate-binding sites of differing affinities, mimicking biological interactions with the respective N-linked glycans of interest and cross-linking of carbohydrates on human T cells for lymphocyte activation.

Lectibodies as antivirals

Growing concerns regarding the emergence of highly transmissible viral diseases highlight the urgent need to expand the repertoire of antiviral therapeutics. For this reason, new strategies for neutralizing and inhibiting these viruses are necessary. A promising approach involves targeting the glycans present on the surfaces of enveloped viruses. Lectins, known for their ability to recognize specific carbohydrate molecules, offer the potential for glycan-targeted antiviral strategies. Indeed, numerous studies have reported the antiviral effects of various lectins of both endogenous and exogenous origins. However, many lectins in their natural forms, are not suitable for use as antiviral therapeutics due to toxicity, other unfavorable pharmacological effects, and/or unreliable manufacturing sources. Therefore, improvements are crucial for employing lectins as effective antiviral therapeutics. A novel approach to enhance lectins' suitability as pharmaceuticals could be the generation of recombinant lectin-Fc fusion proteins, termed "lectibodies." In this review, we discuss the scientific rationale behind lectin-based antiviral strategies and explore how lectibodies could facilitate the development of new antiviral therapeutics. We will also share our perspective on the potential of these molecules to transcend their potential use as antiviral agents.

Mannose-specific plant and microbial lectins as antiviral agents: A review

Lectins are non-immunological carbohydrate-binding proteins classified on the basis of their structure, origin, and sugar specificity. The binding specificity of such proteins with the surface glycan moiety determines their activity and clinical applications. Thus, lectins hold great potential as diagnostic and drug discovery agents and as novel biopharmaceutical products. In recent years, significant advancements have been made in understanding plant and microbial lectins as therapeutic agents against various viral diseases. Among them, mannose-specific lectins have being proven as promising antiviral agents against a variety of viruses, such as HIV, Influenza, Herpes, Ebola, Hepatitis, Severe Acute Respiratory Syndrome Coronavirus-1 (SARS-CoV-1), Middle Eastern Respiratory Syndrome Coronavirus (MERS-CoV) and most recent Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2). The binding of mannose-binding lectins (MBLs) from plants and microbes to high-mannose containing N-glycans (which may be simple or complex) of glycoproteins found on the surface of viruses has been found to be highly specific and mainly responsible for their antiviral activity. MBLs target various steps in the viral life cycle, including viral attachment, entry and replication. The present review discusses the brief classification and structure of lectins along with antiviral activity of various mannose-specific lectins from plants and microbial sources and their diagnostic and therapeutic applications against viral diseases.

Irreversible Inactivation of SARS-CoV-2 by Lectin Engagement with Two Glycan Clusters on the Spike Protein

Host cell infection by SARS-CoV-2, similar to that by HIV-1, is driven by a conformationally metastable and highly glycosylated surface entry protein complex, and infection by these viruses has been shown to be inhibited by the mannose-specific lectins cyanovirin-N (CV-N) and griffithsin (GRFT). We discovered in this study that CV-N not only inhibits SARS-CoV-2 infection but also leads to irreversibly inactivated pseudovirus particles. The irreversibility effect was revealed by the observation that pseudoviruses first treated with CV-N and then washed to remove all soluble lectin did not recover infectivity. The infection inhibition of SARS-CoV-2 pseudovirus mutants with single-site glycan mutations in spike suggested that two glycan clusters in S1 are important for both CV-N and GRFT inhibition: one cluster associated with the RBD (receptor binding domain) and the second with the S1/S2 cleavage site. We observed lectin antiviral effects with several SARS-CoV-2 pseudovirus variants, including the recently emerged omicron, as well as a fully infectious coronavirus, therein reflecting the breadth of lectin antiviral function and the potential for pan-coronavirus inactivation. Mechanistically, observations made in this work indicate that multivalent lectin interaction with S1 glycans is likely a driver of the lectin infection inhibition and irreversible inactivation effect and suggest the possibility that lectin inactivation is caused by an irreversible conformational effect on spike. Overall, lectins' irreversible inactivation of SARS-CoV-2, taken with their breadth of function, reflects the therapeutic potential of multivalent lectins targeting the vulnerable metastable spike before host cell encounter.

Engineering recombinantly expressed lectin-based antiviral agents

Cyanovirin-N (CV-N), a lectin from Nostoc ellipsosporum was found an infusion inhibitory protein for human immunodeficiency virus (HIV)-1. A tandem-repeat of the engineered domain-swapped dimer bound specific sites at hemagglutinin (HA), Ebola and HIV spike glycoproteins as well as dimannosylated HA peptide, N-acetyl-D-glucosamine and high-mannose containing oligosaccharides. Among these, CV-N bound the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) spike protein at a dissociation constant (K_D) of 18.6 µM (and K_D=260 µM to RBD), which was low-affinity carbohydrate-binding as compared with the recognition of the other viral spikes. Binding of dimannosylated peptide to homo-dimeric CVN2 and variants of CVN2 that were pairing Glu-Arg residues sterically located close to its high-affinity carbohydrate binding sites, was measured using surface plasmon resonance (SPR) and isothermal titration calorimetry (ITC). Binding affinity increased with polar interactions, when the mutated residues were used to substitute a single, or two disulfide bonds, in CVN2. Site-specific N-linked glycans on spikes were mediating the infection with influenza virus by broadly neutralizing antibodies to HA and lectin binding to HA was further investigated via modes of saturation transfer difference (STD)-NMR. Our findings showed that stoichiometry and the lectin’s binding affinity were revealed by an interaction of CVN2 with dimannose units and either the high- or low-affinity binding site. To understand how these binding mechanisms add to viral membrane fusion we compare our tested HA-derived peptides in affinity with SARS-CoV-2 glycoprotein and review lectins and their mechanisms of binding to enveloped viruses for a potential use to simulate neutralization ability.

Design of novel cyanovirin-N variants by modulation of binding dynamics through distal mutations

We develop integrated co-evolution and dynamic coupling (ICDC) approach to identify, mutate, and assess distal sites to modulate function. We validate the approach first by analyzing the existing mutational fitness data of TEM-1 β-lactamase and show that allosteric positions co-evolved and dynamically coupled with the active site significantly modulate function. We further apply ICDC approach to identify positions and their mutations that can modulate binding affinity in a lectin, cyanovirin-N (CV-N), that selectively binds to dimannose, and predict binding energies of its variants through Adaptive BP-Dock. Computational and experimental analyses reveal that binding enhancing mutants identified by ICDC impact the dynamics of the binding pocket, and show that rigidification of the binding residues compensates for the entropic cost of binding. This work suggests a mechanism by which distal mutations modulate function through dynamic allostery and provides a blueprint to identify candidates for mutagenesis in order to optimize protein function.

Structural Insight into the Binding of Cyanovirin-N with the Spike Glycoprotein, Mpro and PLpro of SARS-CoV-2: Protein-Protein Interactions, Dynamics Simulations and Free Energy Calculations

The emergence of COVID-19 continues to pose severe threats to global public health. The pandemic has infected over 171 million people and claimed more than 3.5 million lives to date. We investigated the binding potential of antiviral cyanobacterial proteins including cyanovirin-N, scytovirin and phycocyanin with fundamental proteins involved in attachment and replication of SARS-CoV-2. Cyanovirin-N displayed the highest binding energy scores (−16.8 ± 0.02 kcal/mol, −12.3 ± 0.03 kcal/mol and −13.4 ± 0.02 kcal/mol, respectively) with the spike protein, the main protease (M^pro) and the papainlike protease (PL^pro) of SARS-CoV-2. Cyanovirin-N was observed to interact with the crucial residues involved in the attachment of the human ACE2 receptor. Analysis of the binding affinities calculated employing the molecular mechanics-Poisson–Boltzmann surface area (MM-PBSA) approach revealed that all forms of energy, except the polar solvation energy, favourably contributed to the interactions of cyanovirin-N with the viral proteins. With particular emphasis on cyanovirin-N, the current work presents evidence for the potential inhibition of SARS-CoV-2 by cyanobacterial proteins, and offers the opportunity for in vitro and in vivo experiments to deploy the cyanobacterial proteins as valuable therapeutics against COVID-19.

Cyanovirin-N Binds Viral Envelope Proteins at the Low-Affinity Carbohydrate Binding Site without Direct Virus Neutralization Ability

Glycan-targeting antibodies and pseudo-antibodies have been extensively studied for their stoichiometry, avidity, and their interactions with the rapidly modifying glycan shield of influenza A. Broadly neutralizing antiviral agents bind in the same order when they neutralize enveloped viruses regardless of the location of epitopes to the host receptor binding site. Herein, we investigated the binding of cyanovirin-N (CV-N) to surface-expressed glycoproteins such as those of human immunodeficiency virus (HIV) gp120, hemagglutinin (HA), and Ebola (GP)1,2 and compared their binding affinities with the binding response to the trimer-folded gp140 using surface plasmon resonance (SPR). Binding-site knockout variants of an engineered dimeric CV-N molecule (CVN2) revealed a binding affinity that correlated with the number of (high-) affinity binding sites. Binding curves were specific for the interaction with N-linked glycans upon binding with two low-affinity carbohydrate binding sites. This biologically active assembly of a domain-swapped CVN2, or monomeric CV-N, bound to HA with a maximum KD of 2.7 nM. All three envelope spike proteins were recognized at a nanomolar KD, whereas binding to HIV neutralizing 2G12 by targeting HA and Ebola GP1,2 was measured in the µM range and specific for the bivalent binding scheme in SPR. In conclusion, invariant structural protein patterns provide a substrate for affinity maturation in the membrane-anchored HA regions, as well as the glycan shield on the membrane-distal HA top part. They can also induce high-affinity binding in antiviral CV-N to HA at two sites, and CVN2 binding is achieved at low-affinity binding sites.

Cyanobacterial natural products as sources for antiviral drug discovery against COVID-19

The recent Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), positive-sense RNA viruses, originated from Wuhan City in December 2019 and propagated widely globally. Hence, the disease caused by this virus has been declared as a global pandemic by the WHO. As of 18th February 2021, at least seven different vaccines across three platforms have been rolled out in countries and more than 200 additional vaccine candidates have been in development, of which more than 60 are at the stage of the clinical development. So far, Most of the approved vaccine manufacturers are Pfizer, AstraZeneca, and Serum Institute of India, which have been finalized by WHO. Synthetic drug-associated complications have evoked scientific attention for natural product-based drugs. There has been a surge in the antiviral compounds from natural resources along with some therapies. Cyanobacteria are the fruitful reservoir of many metabolites like sulfated polysaccharides and lectins that possess strong antiviral activities and immunity boosting effects. However, the research in this field has been relatively under-developed. The current research highlights important features of cyanobacterial antiviral biomaterials, benefits and drawbacks of cyanobacterial drugs, challenges, future perspectives as well as overview of drugs against COVID-19. In addition, we have described mutated variants and transmission rate of coronaviruses. The current research suggests that cyanobacterial species and their extracts have promising applications as potentially antiviral drug biomaterials against COVID-19.Communicated by Ramaswamy H. Sarma.

Emerging and state of the art hemagglutinin-targeted influenza virus inhibitors

Introduction: Seasonal influenza vaccination, together with FDA-approved neuraminidase (NA) and polymerase acidic (PA) inhibitors, is the most effective way for prophylaxis and treatment of influenza infections. However, the low efficacy of prevailing vaccines to newly emerging influenza strains and increasing resistance to available drugs drives intense research to explore more effective inhibitors. Hemagglutinin (HA), one of the major surface proteins of influenza strains, represents an attractive therapeutic target to develop such new inhibitors.Areas covered: This review summarizes the current progress of HA-based influenza virus inhibitors and their mechanisms of action, which may facilitate further research in developing novel antiviral inhibitors for controlling influenza infections.Expert opinion: HA-mediated entry of influenza virus is an essential step for successful infection of the host, which makes HA a promising target for the development of antiviral drugs. Recent progress in delineating the crystal structures of HA, especially HA-inhibitors complexes, has revealed a number of key residues and conserved binding pockets within HA. This has opened up important insights for developing HA-based antiviral inhibitors that have a high resistance barrier and broad-spectrum activities.

Extending the Martini Coarse-Grained Force Field to N-Glycans

Glycans play a vital role in a large number of cellular processes. Their complex and flexible nature hampers structure-function studies using experimental techniques. Molecular dynamics (MD) simulations can help in understanding dynamic aspects of glycans if the force field parameters used can reproduce key experimentally observed properties. Here, we present optimized coarse-grained (CG) Martini force field parameters for N-glycans, calibrated against experimentally derived binding affinities for lectins. The CG bonded parameters were obtained from atomistic (ATM) simulations for different glycan topologies including high mannose and complex glycans with various branching patterns. In the CG model, additional elastic networks are shown to improve maintenance of the overall conformational distribution. Solvation free energies and octanol-water partition coefficients were also calculated for various N-glycan disaccharide combinations. When using standard Martini nonbonded parameters, we observed that glycans spontaneously aggregated in the solution and required down-scaling of their interactions for reproduction of ATM model radial distribution functions. We also optimized the nonbonded interactions for glycans interacting with seven lectin candidates and show that a relatively modest scaling down of the glycan-protein interactions can reproduce free energies obtained from experimental studies. These parameters should be of use in studying the role of glycans in various glycoproteins and carbohydrate binding proteins as well as their complexes, while benefiting from the efficiency of CG sampling.

Use of Circular Statistics To Model αMan-(1→2)-αMan and αMan-(1→3)-α/βMan O-Glycosidic Linkage Conformation in 13C-Labeled Disaccharides and High-Mannose Oligosaccharides

A new experimental method, MA' AT analysis, has been applied to investigate the conformational properties of O-glycosidic linkages in several biologically important mannose-containing di- and oligosaccharides. Methyl α-d-mannopyranosyl-(1→2)-α-d-mannopyranoside (2), methyl α-d-mannopyranosyl-(1→3)-α-d-mannopyranoside (3), and methyl α-d-mannopyranosyl-(1→3)-β-d-mannopyranoside (4) were prepared with selective ¹³C-enrichment to enable the measurement of NMR scalar couplings across their internal O-glycosidic linkages. Density functional theory (DFT) was used to parameterize equations for J_CH and J_CC values in 2-4 that are sensitive to phi (ϕ) and psi (ψ). The experimental J-couplings and parameterized equations were treated using a circular statistics algorithm encoded in the MA' AT program. Conformations about ϕ and ψ treated using single-state von Mises models gave excellent fits to the ensembles of redundant J-couplings. Mean values and circular standard deviations (CSDs) for each linkage torsion angle ϕ (CSD) and ψ (CSD) in 2, -29° (25°) and 20° (22°); in 3, -36° (36°) and 8° (27°); in 4, -37° (34°) and 10° (26°); ϕ = H1'-C1'-O1'-CX and ψ = C1'-O1'-CX-HX (CX = aglycone carbon) were compared to histograms obtained from 1 μs aqueous molecular dynamics (MD) simulations and X-ray database statistical analysis. MA' AT-derived models of ψ were in very good agreement with the MD and X-ray data, but not those of ϕ, suggesting a need for force field revision. The effect of structural context on linkage conformation was also investigated in four selectively ¹³C-labeled homomannose tri- and tetrasaccharides using the MA' AT method. In the cases examined, context effects were found to be small.

Lectins as Promising Therapeutics for the Prevention and Treatment of HIV and Other Potential Coinfections

Human immunodeficiency virus-acquired immunodeficiency syndrome (HIV/AIDS) remains a global health problem. Current therapeutics specifically target the viral pathogen at various stages of its life cycle, although complex interactions between HIV and other pathogenic organisms are evident. Targeting HIV and concomitant infectious pathogens simultaneously, both by therapeutic regimens and in prevention strategies, would help contain the AIDS pandemic. Lectins, a ubiquitous group of proteins that specifically bind glycosylated molecules, are interesting compounds that could be used for this purpose, with demonstrated anti-HIV properties. In addition, potential coinfecting pathogens, including other enveloped viruses, bacteria, yeasts and fungi, and protozoa, display sugar-coated macromolecules on their surfaces, making them potential targets of lectins. This review summarizes the currently available findings suggesting that lectins should be further developed to simultaneously fight the AIDS pandemic and concomitant infections in HIV infected individuals.

An update of the recombinant protein expression systems of Cyanovirin-N and challenges of preclinical development

Introduction: Human immunodeficiency virus (HIV) is a debilitating challenge and concern worldwide. Accessibility to highly active antiretroviral drugs is little or none for developing countries. Production of cost-effective microbicides to prevent the infection with HIV is a requirement. Cyanovirin-N (CVN) is known as a promising cyanobacterial lectin, capable of inhibiting the HIV cell entry in a highly specific manner.

Methods: This review article presents an overview of attempts conducted on different expression systems for the recombinant production of CVN. We have also assessed the potential of the final recombinant product, as an effective anti-HIV microbicide, comparing prokaryotic and eukaryotic expression systems.

Results: Artificial production of CVN is a challenging task because the desirable anti-HIV activity (CVN-gp120 interaction) depends on the correct formation of disulfide bonds during recombinant production. Thus, inexpensive and functional production of rCVN requires an effective expression system which must be found among the bacteria, yeast, and transgenic plants, for the subsequent satisfying medical application. Moreover, the strong anti-HIV potential of CVN in trace concentrations (micromolar to picomolar) was reported for the in vitro and in vivo tests.

Conclusion: To produce pharmaceutically effective CVN, we first need to identify the best expression system, with Escherichia coli, Pichia pastoris , Lactic acid bacteria and transgenic plants being possible candidates. For this reason, heterologous production of this valuable protein is a serious challenge. Since different obstacles influence clinical trials on microbicides in the field of HIV prevention, these items should be considered for evaluating the CVN activity in pre-clinical and clinical studies.

Antiviral lectins: Selective inhibitors of viral entry

Many natural lectins have been reported to have antiviral activity. As some of these have been put forward as potential development candidates for preventing or treating viral infections, we have set out in this review to survey the literature on antiviral lectins. The review groups lectins by structural class and class of source organism we also detail their carbohydrate specificity and their reported antiviral activities. The review concludes with a brief discussion of several of the pertinent hurdles that heterologous proteins must clear to be useful clinical candidates and cites examples where such studies have been reported for antiviral lectins. Though the clearest path currently being followed is the use of antiviral lectins as anti-HIV microbicides via topical mucosal administration, some investigators have also found systemic efficacy against acute infections following subcutaneous administration.

Cyanobacterial lectins characteristics and their role as antiviral agents

Lectins are ubiquitous proteins/glycoproteins of non-immune origin that bind reversibly to carbohydrates in non-covalent and highly specific manner. These lectin-glycan interactions could be exploited for establishment of novel therapeutics, targeting the adherence stage of viruses and thus helpful in eliminating wide spread viral infections. Here the review focuses on the haemagglutination activity, carbohydrate specificity and characteristics of cyanobacterial lectins. Cyanobacterial lectins exhibiting high specificity towards mannose or complex glycans have potential role as anti-viral agents. Prospective role of cyanobacterial lectins in targeting various diseases of worldwide concern such as HIV, hepatitis, herpes, influenza and ebola viruses has been discussed extensively. The review also lays emphasis on recent studies involving structural analysis of glycan-lectin interactions which in turn influence their mechanism of action. Altogether, the promising approach of these cyanobacterial lectins provides insight into their use as antiviral agents.

Structure and Glycan Binding of a New Cyanovirin-N Homolog

The HIV-1 envelope glycoprotein gp120 is heavily glycosylated and bears numerous high mannose sugars. These sugars can serve as targets for HIV-inactivating compounds, such as antibodies and lectins, which bind to the glycans and interfere with viral entry into the target cell. We determined the 1.6 Å x-ray structure of Cyt-CVNH, a recently identified lectin from the cyanobacterium Cyanothece(7424), and elucidated its glycan specificity by NMR. The Cyt-CVNH structure and glycan recognition profile are similar to those of other CVNH proteins, with each domain specifically binding to Manα(1-2)Manα units on the D1 and D3 arms of high mannose glycans. However, in contrast to CV-N, no cross-linking and precipitation of the cross-linked species in solution was observed upon Man-9 binding, allowing, for the first time, investigation of the interaction of Man-9 with a member of the CVNH family by NMR. HIV assays showed that Cyt-CVNH is able to inhibit HIV-1 with ∼4-fold higher potency than CV-N(P51G), a stabilized version of wild type CV-N. Therefore, Cyt-CVNH may qualify as a valuable lectin for potential microbicidal use.

Anti-influenza virus activity of high-mannose binding lectins derived from genus Pseudomonas

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Three Pseudomonas-derived lectins: PFL, PML, and PTL, have been examined for anti-influenza virus activity against several strains of influenza virus.

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These lectins would bind high-mannose glycan and blocked the entry of influenza virus into the host cells.

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It is expected that these lectins could have an antiviral activity against not only influenza virus but also other enveloped viruses including HIV as described by many other studies.

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These three lectins will be applicable to a novel microbicide.

Lectin PFL binding high-mannose glycan derived from Pseudomonas fluorescens and other homologous lectins: PML derived from Pseudomonas mandelii and PTL derived from Pseudomonas taiwanensis were examined for antiviral activity. The cDNA of these lectin genes were synthesized, cloned, expressed in Escherichia coli. The expressed lectins were purified by gel filtrations, and supplied to cultures infected with several strains of influenza virus. These three lectins have inhibited propagation of influenza viruses with a similar extent, 50% of inhibition-dose was around ten nanomolar concentration. An immunofluorescent microscopy, a microarray analysis, and several infection experiments with different time periods of lectin addition or using the competitor substrates indicated that binding of these lectins with high-mannose glycan on HA protein of influenza virus could block the virus entry into the host cells, thereby resulting in inhibition of the virus propagation. These Pseudomonas-derived lectins would be protential and attractive antiviral agents targeting glycoproteins of enveloped viruses including influenza virus.

Griffithsin binds to the glycosylated proteins (E and prM) of Japanese encephalitis virus and inhibit its infection

Griffithsin (GRFT) is a broad-spectrum antiviral protein against several glycosylated viruses. In our previous publication, we have shown that GRFT exerted antiviral activity against Japanese encephalitis virus (JEV) infection. Herein, we further elucidated the mechanism by which GRFT inhibits JEV infection in BHK-21 cells. In vitro experiments using Pull-down assay and Co-immunoprecipitation (CO-IP) assay showed that GRFT binds to the JEV glycosylated viral proteins, specifically the enveloped (E) and premature (prM) glycoproteins. The binding of GRFT to the JEV was competitively inhibited by increasing concentrations of mannose; in turns abolished anti-JEV activity of GRFT. We suggested that, the binding of GRFT to the glycosylated viral proteins may contribute to its anti-JEV activity. Collectively, our data indicated a possible mechanism by which GRFT exerted its anti-JEV activity. This observation suggests GRFT's potentials in the development of therapeutics against JEV or other flavivirus infection.

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.

A Rigid Hinge Region Is Necessary for High-Affinity Binding of Dimannose to Cyanovirin and Associated Constructs

Mutations in the hinge region of cyanovirin-N (CVN) dictate its preferential oligomerization state. Constructs with the Pro51Gly mutation preferentially exist as monomers, whereas wild-type cyanovirin can form domain-swapped dimers under certain conditions. Because the hinge region is an integral part of the high-affinity binding site of CVN, we investigated whether this mutation affects the shape, flexibility, and binding affinity of domain B for dimannose. Our studies indicate that the capability of monomeric wild-type CVN to resist mechanical perturbations is enhanced when compared to that of constructs in which the hinge region is more flexible. Our computational results also show that enhanced flexibility leads to blocking of the binding site by allowing different rotational isomeric states of Asn53. Moreover, at higher temperatures, this observed flexibility leads to an interaction between Asn53 and Asn42, further hindering access to the binding site. On the basis of these results, we predicted that binding affinity for dimannose would be more favorable for cyanovirin constructs containing a wild-type hinge region, whereas affinity would be impaired in the case of mutants containing Pro51Gly. Experimental characterization by isothermal titration calorimetry of a set of cyanovirin mutants confirms this hypothesis. Those possessing the Pro51Gly mutation are consistently inferior binders.

Antiviral drug discovery: broad-spectrum drugs from nature

Covering: up to April 2014. The development of drugs with broad-spectrum antiviral activities is a long pursued goal in drug discovery. It has been shown that blocking co-opted host-factors abrogates the replication of many viruses, yet the development of such host-targeting drugs has been met with scepticism mainly due to toxicity issues and poor translation to in vivo models. With the advent of new and more powerful screening assays and prediction tools, the idea of a drug that can efficiently treat a wide range of viral infections by blocking specific host functions has re-bloomed. Here we critically review the state-of-the-art in broad-spectrum antiviral drug discovery. We discuss putative targets and treatment strategies, with particular focus on natural products as promising starting points for antiviral lead development.

A flexible docking scheme efficiently captures the energetics of glycan-cyanovirin binding

Cyanovirin-N (CVN), a cyanobacterial lectin, exemplifies a class of antiviral agents that inhibit HIV by binding to the highly glycosylated envelope protein gp120. Here, we investigate the energetics of glycan recognition using a computationally inexpensive flexible docking approach, backbone perturbation docking (BP-Dock). We benchmarked our method using two mutants of CVN: P51G-m4-CVN, which binds dimannose with high affinity through domain B, and CVN((mutDB)), in which binding to domain B has been abolished through mutation of five polar residues to small nonpolar side chains. We investigated the energetic contribution of these polar residues along with the additional position 53 by docking dimannose to single-point CVN mutant models. Analysis of the docking simulations indicated that the E41A/G and T57A mutations led to a significant decrease in binding energy scores due to rearrangements of the hydrogen-bond network that reverberated throughout the binding cavity. N42A decreased the binding score to a level comparable to that of CVN((mutDB)) by affecting the integrity of the local protein structure. In contrast, N53S resulted in a high binding energy score, similar to P51G-m4-CVN. Experimental characterization of the five mutants by NMR spectroscopy confirmed the binding affinity pattern predicted by BP-Dock. Despite their mostly conserved fold and stability, E41A, E41G, and T57A displayed dissociation constants in the millimolar range. N53S showed a binding constant in the low micromolar range, similar to that observed for P51G-m4-CVN. No binding was observed for N42A. Our results show that BP-Dock is a useful tool for rapidly screening the relative binding affinity pattern of in silico-designed mutants compared with wild-type, supporting its use to design novel mutants with enhanced binding properties.

The antiviral lectin cyanovirin-N: probing multivalency and glycan recognition through experimental and computational approaches

CVN (cyanovirin-N), a small lectin isolated from cyanobacteria, exemplifies a novel class of anti-HIV agents that act by binding to the highly glycosylated envelope protein gp120 (glycoprotein 120), resulting in inhibition of the crucial viral entry step. In the present review, we summarize recent work in our laboratory and others towards determining the crucial role of multivalency in the antiviral activity, and we discuss features that contribute to the high specificity and affinity for the glycan ligand observed in CVN. An integrated approach that encompasses structural determination, mutagenesis analysis and computational work holds particular promise to clarify aspects of the interactions between CVN and glycans.

Expression, purification and identification of CtCVNH, a novel anti-HIV (Human Immunodeficiency Virus) protein from Ceratopteris thalictroides

CVN (cyanovirin-N) is an anti-HIV protein. CVNH (cyanovirin-N homology) represents its homology. In a previous study, we first reported the full-length sequences of the CVNH gene cloned from Ceratopteris thalictroides. Based on the finding, the coding sequence of CtCVNH was optimized in the study, and then a pET prokaryotic expression vector was constructed. The purification and identification of CtCVNH protein were investigated, as well. SDS-PAGE analysis indicated that a 31 kDa protein was overexpressed and mainly accumulated in the soluble fraction. Only a single protein was obtained after the Ni- nitrilotriacetic acid (NTA) affinity chromatography. The purified protein was identified to be the recombinant CtCVNH by both Western blot and peptide mass fingerprinting analysis.

Fluorinated carbohydrates as lectin ligands: dissecting glycan-cyanovirin interactions by using 19F NMR spectroscopy

NMR spectroscopy and isothermal titration calorimetry (ITC) are powerful methods to investigate ligand-protein interactions. Here, we present a versatile and sensitive fluorine NMR spectroscopic approach that exploits the (19)F nucleus of (19)F-labeled carbohydrates as a sensor to study glycan binding to lectins. Our approach is illustrated with the 11 kDa Cyanovirin-N, a mannose binding anti-HIV lectin. Two fluoro-deoxy sugar derivatives, methyl 2-deoxy-2-fluoro-α-D-mannopyranosyl-(1→2)-α-D-mannopyranoside and methyl 2-deoxy-2-fluoro-α-D-mannopyranosyl-(1→2)-α-D-mannopyranosyl-(1→2)-α-D-mannopyranoside were utilized. Binding was studied by (19)F NMR spectroscopy of the ligand and (1)H-(15)N HSQC NMR spectroscopy of the protein. The NMR data agree well with those obtained from the equivalent reciprocal and direct ITC titrations. Our study shows that the strategic design of fluorinated ligands and fluorine NMR spectroscopy for ligand screening holds great promise for easy and fast identification of glycan binding, as well as for their use in reporting structural and/or electronic perturbations that ensue upon interaction with a cognate lectin.

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.

Conservation, Compensation, and Evolution of N-Linked Glycans in the HIV-1 Group M Subtypes and Circulating Recombinant Forms

The “glycan shield” exposed on the surface of the HIV-1 gp120 env glycoprotein has been previously proposed as a novel target for anti-HIV treatments. While such targeting of these glycans provides an exciting prospect for HIV treatment, little is known about the conservation and variability of glycosylation patterns within and between the various HIV-1 group M subtypes and circulating recombinant forms. Here, we present evidence of strong strain-specific glycosylation patterns and show that the epitope for the 2G12 neutralising antibody is poorly conserved across HIV-1 group M. The unique glycosylation patterns within the HIV-1 group M subtypes and CRFs appear to explain their varying susceptibility to neutralisation by broadly cross-neutralising (BCN) antibodies. Compensatory glycosylation at linearly distant yet three-dimensionally proximal amino acid positions appears to maintain the integrity of the glycan shield while conveying resistance to neutralisation by BCN antibodies. We find that highly conserved clusters of glycosylated residues do exist on the gp120 trimer surface and suggest that these positions may provide an exciting target for the development of BCN anticarbohydrate therapies.

Griffithsin inhibits Japanese encephalitis virus infection in vitro and in vivo

Griffithsin (GRFT) is a broad-spectrum antiviral protein that is effective against several glycosylated viruses. Here, we have evaluated the in vitro and in vivo antiviral activities of GRFT against Japanese encephalitis virus (JEV) infection. In vitro experiments showed that treatment of JEV with GRFT before inoculation of BHK-21 cells inhibited infection in a dose-dependent manner, with 99 % inhibition at 100 μg/ml and a 50 % inhibitory concentration (IC₅₀) of 265 ng/ml (20 nM). Binding assays suggested that binding of GRFT to JEV virions inhibited JEV infection. In vivo experiment showed that GRFT (5 mg/kg) administered intraperitoneally before virus infection could completely prevent mortality in mice challenged intraperitoneally with a lethal dose of JEV. Our study also suggested that GRFT prevents JEV infection at the entry phase by targeting the virus. Collectively, our data demonstrate that GRFT is an antiviral agent with potential application in the development of therapeutics against JEV or other flavivirus infections.

Carbohydrate biomarker recognition using synthetic lectin mimics

Carbohydrate biomarkers play very important roles in a wide range of biological and pathological processes. Compounds that can specifically recognize a carbohydrate biomarker are useful for targeted delivery of imaging agents and for development of new diagnostics. Furthermore, such compounds could also be candidates for the development of therapeutic agents. A tremendous amount of active work on synthetic lectin mimics has been reported in recent years. Amongst all the synthetic lectins, boronic-acid-based lectins (boronolectins) have shown great promise. Along this line, four classes of boronolectins including peptide-, nucleic-acid-, polymer-, and small-molecule-based ones are discussed with a focus on the design principles and recent advances. We hope that by presenting the potentials of this field, this review will stimulate more research in this area.

Direct measurement of glyconanoparticles and lectin interactions by isothermal titration calorimetry

Glyconanomaterials have shown high potential in applications including bioanalysis and nanomedicine. Here, a quantitative analytical technique, based on isothermal titration calorimetry, was developed to characterize the interactions between glyconanoparticles and lectins. By titrating lectins into the glyconanoparticle solution, the apparent dissociation constant, thermodynamic parameters, and the number of binding sites were derived simultaneously. For the glyconanoparticles-lectin binding pairs investigated, a 3-5 order of magnitude affinity enhancement over the free ligand-lectin interactions was observed which can be attributed to the multivalent ligand presentation on the nanoparticles. The impact of ligand density was also studied, and results showed that the affinity increased with the number of glycans on the nanoparticle.

The lectins griffithsin, cyanovirin-N and scytovirin inhibit HIV-1 binding to the DC-SIGN receptor and transfer to CD4(+) cells

It is generally believed that during the sexual transmission of HIV-1, the glycan-specific DC-SIGN receptor binds the virus and mediates its transfer to CD4(+) cells. The lectins griffithsin (GRFT), cyanovirin-N (CV-N) and scytovirin (SVN) inhibit HIV-1 infection by binding to mannose-rich glycans on gp120. We measured the ability of these lectins to inhibit both the HIV-1 binding to DC-SIGN and the DC-SIGN-mediated HIV-1 infection of CD4(+) cells. While GRFT, CV-N and SVN were moderately inhibitory to DC-SIGN binding, they potently inhibited DC-SIGN-transfer of HIV-1. The introduction of the 234 glycosylation site abolished HIV-1 sensitivity to lectin inhibition of binding to DC-SIGN and virus transfer to susceptible cells. However, the addition of the 295 glycosylation site increased the inhibition of transfer. Our data suggest that GRFT, CV-N and SVN can block two important stages of the sexual transmission of HIV-1, DC-SIGN binding and transfer, supporting their further development as microbicides.

Designed oligomers of cyanovirin-N show enhanced HIV neutralization

Cyanovirin-N (CV-N) is a small, cyanobacterial lectin that neutralizes many enveloped viruses, including human immunodeficiency virus type I (HIV-1). This antiviral activity is attributed to two homologous carbohydrate binding sites that specifically bind high mannose glycosylation present on envelope glycoproteins such as HIV-1 gp120. We created obligate CV-N oligomers to determine whether increasing the number of binding sites has an effect on viral neutralization. A tandem repeat of two CV-N molecules (CVN(2)) increased HIV-1 neutralization activity by up to 18-fold compared to wild-type CV-N. In addition, the CVN(2) variants showed extensive cross-clade reactivity and were often more potent than broadly neutralizing anti-HIV antibodies. The improvement in activity and broad cross-strain HIV neutralization exhibited by these molecules holds promise for the future therapeutic utility of these and other engineered CV-N variants.

Multivalent glyconanoparticles with enhanced affinity to the anti-viral lectin Cyanovirin-N

Low-mannose (LM) structures were coupled to gold nanoparticles (Au NPs) to amplify the affinity of LMs with Cyanovirin-N (CV-N) lectins and to study the structures of CV-N variants CVN(Q50C) and CVN(MutDB).

Removal of two high-mannose N-linked glycans on gp120 renders human immunodeficiency virus 1 largely resistant to the carbohydrate-binding agent griffithsin

High-mannose N-linked glycans recognized by carbohydrate-binding agents (CBAs) are potential targets for topical microbicides. To better understand the mechanisms by which CBAs inhibit human immunodeficiency virus (HIV)-1 infection at the molecular level, we systematically analysed the contribution of site-specific glycans to the anti-HIV activity of CBAs by site-directed mutagenesis. Our results demonstrate that a single deglycosylation at N295 or N448 in a range of primary and T-cell-line-adapted HIV-1 isolates resulted in marked resistance to griffithsin (GRFT) but maintained the sensitivity to cyanovirin (CV-N), Galanthus nivalis agglutinin (GNA) and a range of neutralizing antibodies. Unlike CV-N and GNA, the interaction between GRFT and gp120 appeared to be dependent on the specific trimeric 'sugar tower' including N295 and N448. This was further strengthened by the results of GRFT-Env binding experiments. Our study identifies GRFT-specific gp120 glycans and may provide information for the design of novel CBA antiviral strategies.

Cantilever array sensors detect specific carbohydrate-protein interactions with picomolar sensitivity

Advances in carbohydrate sequencing technologies have revealed the tremendous complexity of the glycome. This complexity reflects the structural and chemical diversity of carbohydrates and is greater than that of proteins and oligonucleotides. The next step in understanding the biological function of carbohydrates requires the identification and quantification of carbohydrate interactions with other biomolecules, in particular, with proteins. To this end, we have developed a cantilever array biosensor with a self-assembling carbohydrate-based sensing layer that selectively and sensitively detects carbohydrate-protein binding interactions. Specifically, we examined binding of mannosides and the protein cyanovirin-N, which binds and blocks the human immunodeficiency virus (HIV). Cyanovirin-N binding to immobilized oligomannosides on the cantilever resulted in mechanical surface stress that is transduced into a mechanical force and cantilever bending. The degree and duration of cantilever deflection correlates with the interaction's strength, and comparative binding experiments reveal molecular binding preferences. This study establishes that carbohydrate-based cantilever biosensors are a robust, label-free, and scalable means to analyze carbohydrate-protein interactions and to detect picomolar concentrations of carbohydrate-binding proteins.

Combinatorial approaches to the prevention and treatment of HIV-1 infection

The discovery of the human immunodeficiency virus type 1 (HIV-1) in 1982 soon led to the identification and development of antiviral compounds to be used in treatment strategies for infected patients. Early in the epidemic, drug monotherapies frequently led to treatment failures because the virus quickly developed resistance to the single drug. Following the advent of highly active antiretroviral therapy (HAART) in 1995, dramatic improvements in HIV-1-infected patient health and survival were realized as more refined combination therapies resulted in reductions in viral loads and increases in CD4+ T-cell counts. In the absence of an effective vaccine, prevention of HIV-1 infection has also gained traction as an approach to curbing the pandemic. The development of compounds as safe and effective microbicides has intensified and has focused on blocking the transmission of HIV-1 during all forms of sexual intercourse. Initial preclinical investigations and clinical trials of microbicides focused on single compounds effective against HIV-1. However, the remarkable successes achieved using combination therapy to treat systemic HIV-1 infection have subsequently stimulated the study and development of combination microbicides that will simultaneously inhibit multiple aspects of the HIV-1 transmission process by targeting incoming viral particles, virus-infected cells, and cells susceptible to HIV-1 infection. This review focuses on existing and developing combination therapies, covering preclinical development, in vitro and in vivo efficacy studies, and subsequent clinical trials. The shift in focus within the microbicide development field from single compounds to combination approaches is also explored.

Antiviral activity of recombinant cyanovirin-N against HSV-1

In this study, a standard strain of HSV-1 (strain SM(44)) was used to investigate the antiviral activity of the recombinant Cyanovirin-N (CV-N) against Herpes simplex virus type 1 (HSV-1) in vitro and in vivo. Cytopathic effect (CPE) and MTT assays were used to evaluate the effect of CV-N on HSV-1 in Vero cells. The number of copies of HSV-DNA was detected by real-time fluorescence quantitative PCR (FQ-PCR). The results showed that CV-N had a low cytotoxicity on Vero cells with a CC(50) of 359.03 ± 0.56 μg/mL, and that it could not directly inactivate HSV-1 infectivity. CV-N not only reduced the CPE of HSV-1 when added before or after viral infection, with a 50% inhibitory concentration (IC(50)) with 2.26 and 30.16 μg/mL respectively, but it also decreased the copies of HSV-1 DNA in infected host cells. The encephalitis model for HSV-1 infection was conducted in Kunming mice, and treated with three dosages of CV-N (0.5, 5 & 10 mg/kg) which was administered intraperitoneally at 2h, 3d, 5d, 7d post infection. The duration for the appearance of symptoms of encephalitis and the survival days were recorded and brain tissue samples were obtained for pathological examination (HE staining). Compared with the untreated control group, in the 5mg/kg CV-N and 10mg/kg CV-N treated groups, the mice suffered light symptoms and the number of survival days were more than 9 d and 14 d respectively. HE staining also showed that in 5mg/kg CV-N and 10mg/kg CV-N treated groups, the brain cells did not show visible changes, except for a slight inflammation. Our results demonstrated that CV-N has pronounced antiviral activity against HSV-1 both in vitro and in vivo, and it would be a promising new candidate for anti-HSV therapeutics.

Mannose-rich glycosylation patterns on HIV-1 subtype C gp120 and sensitivity to the lectins, Griffithsin, Cyanovirin-N and Scytovirin

Griffithsin (GRFT), Cyanovirin-N (CV-N) and Scytovirin (SVN) are lectins that inhibit HIV-1 infection by binding to multiple mannose-rich glycans on the HIV-1 envelope glycoproteins (Env). Here we show that these lectins neutralize subtype C primary virus isolates in addition to Env-pseudotyped viruses obtained from plasma and cervical vaginal lavages. Among 15 subtype C pseudoviruses, the median IC(50) values were 0.4, 1.8 and 20.1nM for GRFT, CV-N and SVN, respectively, similar to what was found for subtype B and A. Analysis of Env sequences suggested that concomitant lack of glycans at positions 234 and 295 resulted in natural resistance to these compounds, which was confirmed by site-directed mutagenesis. Furthermore, the binding sites for these lectins overlapped that of the 2G12 monoclonal antibody epitope, which is generally absent on subtype C Env. This data support further research on these lectins as potential microbicides in the context of HIV-1 subtype C infection.

Microvirin, a novel alpha(1,2)-mannose-specific lectin isolated from Microcystis aeruginosa, has anti-HIV-1 activity comparable with that of cyanovirin-N but a much higher safety profile

Microvirin (MVN), a recently isolated lectin from the cyanobacterium Microcystis aeruginosa PCC7806, shares 33% identity with the potent anti-human immunodeficiency virus (HIV) protein cyanovirin-N (CV-N) isolated from Nostoc ellipsosporum, and both lectins bind to similar carbohydrate structures. MVN is able to inhibit infection by a wide variety of HIV-1 laboratory-adapted strains and clinical isolates of different tropisms and subtypes in peripheral blood mononuclear cells. MVN also inhibits syncytium formation between persistently HIV-1-infected T cells and uninfected CD4(+) T cells and inhibits DC-SIGN-mediated HIV-1 binding and transmission to CD4(+) T cells. Long term passaging of HIV-1 exposed to dose-escalating concentrations of MVN resulted in the selection of a mutant virus with four deleted high mannose-type glycans in the envelope gp120. The MVN-resistant virus was still highly sensitive to various other carbohydrate binding lectins (e.g. CV-N, HHA, GNA, and UDA) but not anymore to the carbohydrate-specific 2G12 monoclonal antibody. Importantly, MVN is more than 50-fold less cytotoxic than CV-N. Also in sharp contrast to CV-N, MVN did not increase the level of the activation markers CD25, CD69, and HLA-DR in CD4(+) T lymphocytes, and subsequently, MVN did not enhance viral replication in pretreated peripheral blood mononuclear cells. Therefore, MVN may qualify as a useful lectin for potential microbicidal use based on its broad and potent antiviral activity and virtual lack of any stimulatory properties and cellular toxicity.

Carbohydrate recognition by boronolectins, small molecules, and lectins

Carbohydrates are known to mediate a large number of biological and pathological events. Small and macromolecules capable of carbohydrate recognition have great potentials as research tools, diagnostics, vectors for targeted delivery of therapeutic and imaging agents, and therapeutic agents. However, this potential is far from being realized. One key issue is the difficulty in the development of "binders" capable of specific recognition of carbohydrates of biological relevance. This review discusses systematically the general approaches that are available in developing carbohydrate sensors and "binders/receptors," and their applications. The focus is on discoveries during the last 5 years.

Anti-HIV activity of defective cyanovirin-N mutants is restored by dimerization

Cyanovirin-N (CV-N) is a two-domain, cyanobacterial protein that inhibits human immunodeficiency virus (HIV) at nanomolar concentrations by binding to high mannose sugars on the HIV envelope glycoprotein gp120. The wild type protein can exist as a monomer or a domain-swapped dimer with the monomer and dimer containing two or four sugar binding sites, respectively, one on each domain. Here we demonstrate that monomeric, single binding site mutants are completely inactive and that a single site, whether located on domain A or B, is insufficient to impart the antiviral activity. Linking inactive, monomeric proteins in a head-to-head fashion by an intermolecular disulfide bond or by creating an exclusively domain-swapped dimer via a hinge residue deletion restored antiviral activity to levels similar to that of wild type CV-N. These findings demonstrate unequivocally that multisite binding by CV-N type lectins is necessary for viral inhibition.

Carbohydrate biomarkers for future disease detection and treatment

Carbohydrates are considered as one of the most important classes of biomarkers for cell types, disease states, protein functions, and developmental states. Carbohydrate "binders" that can specifically recognize a carbohydrate biomarker can be used for developing novel types of site specific delivery methods and imaging agents. In this review, we present selected examples of important carbohydrate biomarkers and how they can be targeted for the development of therapeutic and diagnostic agents. Examples are arranged based on disease categories including (1) infectious diseases, (2) cancer, (3) inflammation and immune responses, (4) signal transduction, (5) stem cell transformation, (6) embryo development, and (7) cardiovascular diseases, though some issues cross therapeutic boundaries.

Mechanism by which the lectin actinohivin blocks HIV infection of target cells

Various lectins have attracted attention as potential microbicides to prevent HIV transmission. Their capacity to bind glycoproteins has been suggested as a means to block HIV binding and entry into susceptible cells. The previously undescribed lectin actinohivin (AH), isolated by us from an actinomycete, exhibits potent in vitro anti-HIV activity by binding to high-mannose (Man) type glycans (HMTGs) of gp120, an envelope glycoprotein of HIV. AH contains 114 aa and consists of three segments, all of which need to show high affinity to gp120 for the anti-HIV characteristic. To generate the needed mechanistic understanding of AH binding to HIV in anticipation of seeking approval for human testing as a microbicide, we have used multiple molecular tools to characterize it. AH showed a weak affinity to Man alpha(1-2)Man, Man alpha(1-2)Man alpha(1-2)Man, of HMTG (Man8 or Man9) or RNase B (which has a single HMTG), but exhibited a strong and highly specific affinity (K(d) = 3.4 x 10(-8) M) to gp120 of HIV, which contains multiple Man8 and/or Man9 units. We have compared AH to an alternative lectin, cyanovirin-N, which did not display similar levels of discrimination between high- and low-density HMTGs. X-ray crystal analysis of AH revealed a 3D structure containing three sugar-binding pockets. Thus, the strong specific affinity of AH to gp120 is considered to be due to multivalent interaction of the three sugar-binding pockets with three HMTGs of gp120 via the "cluster effect" of lectin. Thus, AH is a good candidate for investigation as a safe microbicide to help prevent HIV transmission.

A sugar binding protein cyanovirin-N blocks herpes simplex virus type-1 entry and cell fusion

Herpes simplex virus type-1 (HSV-1) causes significant health problems from periodic skin and corneal lesions to encephalitis. It is also considered a cofactor in the development of age-related secondary glaucoma. Inhibition of HSV-1 at the stage of viral entry generates a unique opportunity for preventative and/or therapeutic intervention. Here we provide evidence that a sugar binding antiviral protein, cyanovirin-N (CV-N), can act as a potent inhibitor of HSV-1 entry into natural target cells. Inhibition of entry was independent of HSV-1 gD receptor usage and it was observed in transformed as well as primary cell cultures. Evidence presented herein suggests that CV-N can not only block virus entry to cells but also, it is capable of significantly inhibiting membrane fusion mediated by HSV glycoproteins. While CV-N treated virions were significantly deficient in entering into cells, HSV-1 glycoproteins-expressing cells pretreated with CV-N demonstrated reduced cell-to-cell fusion and polykaryocytes formation. The observation that CV-N can block both entry as well as membrane fusion suggests a stronger potential for this compound in antiviral therapy against HSV-1.