Griffithsin tandemers: flexible and potent lectin inhibitors of the human immunodeficiency virus

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.

The Antiviral Activity of the Lectin Griffithsin against SARS-CoV-2 Is Enhanced by the Presence of Structural Proteins

Although COVID-19 transmission has been reduced by the advent of vaccinations and a variety of rapid monitoring techniques, the SARS-CoV-2 virus itself has shown a remarkable ability to mutate and persist. With this long track record of immune escape, researchers are still exploring prophylactic treatments to curtail future SARS-CoV-2 variants. Specifically, much focus has been placed on the antiviral lectin Griffithsin in preventing spike protein-mediated infection via the hACE2 receptor (direct infection). However, an oft-overlooked aspect of SARS-CoV-2 infection is viral capture by attachment receptors such as DC-SIGN, which is thought to facilitate the initial stages of COVID-19 infection in the lung tissue (called trans-infection). In addition, while immune escape is dictated by mutations in the spike protein, coronaviral virions also incorporate M, N, and E structural proteins within the particle. In this paper, we explored how several structural facets of both the SARS-CoV-2 virion and the antiviral lectin Griffithsin can affect and attenuate the infectivity of SARS-CoV-2 pseudovirus. We found that Griffithsin was a better inhibitor of hACE2-mediated direct infection when the coronaviral M protein is present compared to when it is absent (possibly providing an explanation regarding why Griffithsin shows better inhibition against authentic SARS-CoV-2 as opposed to pseudotyped viruses, which generally do not contain M) and that Griffithsin was not an effective inhibitor of DC-SIGN-mediated trans-infection. Furthermore, we found that DC-SIGN appeared to mediate trans-infection exclusively via binding to the SARS-CoV-2 spike protein, with no significant effect observed when other viral proteins (M, N, and/or E) were present. These results provide etiological data that may help to direct the development of novel antiviral treatments, either by leveraging Griffithsin binding to the M protein as a novel strategy to prevent SARS-CoV-2 infection or by narrowing efforts to inhibit trans-infection to focus on DC-SIGN binding to SARS-CoV-2 spike protein.

The Antiviral Potential of Algal Lectins

Algae have emerged as fascinating subjects of study due to their vast potential as sources of valuable metabolites with diverse biotechnological applications, including their use as fertilizers, feed, food, and even pharmaceutical precursors. Among the numerous compounds found in algae, lectins have garnered special attention for their unique structures and carbohydrate specificities, distinguishing them from lectins derived from other sources. Here, a comprehensive overview of the latest scientific and technological advancements in the realm of algal lectins with a particular focus on their antiviral properties is provided. These lectins have displayed remarkable effectiveness against a wide range of viruses, thereby holding great promise for various antiviral applications. It is worth noting that several alga species have already been successfully commercialized for their antiviral potential. However, the discovery of a diverse array of lectins with potent antiviral capabilities suggests that the field holds immense untapped potential for further expansion. In conclusion, algae stand as a valuable and versatile resource, and their lectins offer an exciting avenue for developing novel antiviral agents, which may lead to the development of cutting-edge antiviral therapies.

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.

Therapeutic Potential of Marine Bioactive Peptides against Human Immunodeficiency Virus: Recent Evidence, Challenges, and Future Trends

Acquired immunodeficiency syndrome (AIDS) is a chronic and potentially fatal ailment caused by the human immunodeficiency virus (HIV) and remains a major health problem worldwide. In recent years, the research focus has shifted to a greater emphasis on complementing treatment regimens involving conventional antiretroviral (ARV) drug therapies with novel lead structures isolated from various marine organisms that have the potential to be utilized as therapeutics for the management of HIV-AIDS. The present review summarizes the recent developments regarding bioactive peptides sourced from various marine organisms. This includes a discussion encompassing the potential of these novel marine bioactive peptides with regard to antiretroviral activities against HIV, preparation, purification, and processing techniques, in addition to insight into the future trends with an emphasis on the potential of exploration and evaluation of novel peptides to be developed into effective antiretroviral drugs.

Lectins and lectibodies: potential promising antiviral agents

In nature, lectins are widely dispersed proteins that selectively recognize and bind to carbohydrates and glycoconjugates via reversible bonds at specific binding sites. Many viral diseases have been treated with lectins due to their wide range of structures, specificity for carbohydrates, and ability to bind carbohydrates. Through hemagglutination assays, these proteins can be detected interacting with various carbohydrates on the surface of cells and viral envelopes. This review discusses the most robust lectins and their rationally engineered versions, such as lectibodies, as antiviral proteins. Fusion of lectin and antibody’s crystallizable fragment (Fc) of immunoglobulin G (IgG) produces a molecule called a “lectibody” that can act as a carbohydrate-targeting antibody. Lectibodies can not only bind to the surface glycoproteins via their lectins and neutralize and clear viruses or infected cells by viruses but also perform Fc-mediated antibody effector functions. These functions include complement-dependent cytotoxicity (CDC), antibody-dependent cell-mediated cytotoxicity (ADCC), and antibody-dependent cell-mediated phagocytosis (ADCP). In addition to entering host cells, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike protein S1 binds to angiotensin-converting enzyme 2 (ACE2) and downregulates it and type I interferons in a way that may lead to lung disease. The SARS-CoV-2 spike protein S1 and human immunodeficiency virus (HIV) envelope are heavily glycosylated, which could make them a major target for developing vaccines, diagnostic tests, and therapeutic drugs. Lectibodies can lead to neutralization and clearance of viruses and cells infected by viruses by binding to glycans located on the envelope surface (e.g., the heavily glycosylated SARS-CoV-2 spike protein).

C3-Symmetric Aromatic Core of Griffithsin Is Essential for Potent Anti-HIV Activity

Lectins, carbohydrate-binding proteins of nonimmune origin, bind to carbohydrates and glycan shields present on the surfaces of cells and viral spike proteins. Lectins thus hold great promise as therapeutic and diagnostic proteins, exemplified by their potent antiviral activities and the desire to engineer synthetic carbohydrate receptors based on lectin recognition principles. Here, we describe a new carbohydrate-binding architectural motif─namely, a C₃-symmetric tyrosine-based aromatic core, present in the therapeutic lectin griffithsin (GRFT). By using structure-based amino acid substitutions, X-ray crystallography, molecular dynamics (MD) simulations, and HIV-1 neutralization assays, we show that this core is critical for potent (pM) antiviral activity and nanomolar binding to the glycan shield largely consisting of high mannose glycans. Crystal structures and MD simulations show that CH-π interactions stabilize the aromatic cluster to maintain the three pseudo-symmetric carbohydrate-binding sites, nonaromatic amino acid substitutions (Tyr to Ala) abrogate antiviral activity, and increasing the aromatic CH-π edge-to-centroid interface via a Tyr to Trp substitution yields a GRFT variant with improved potency and increased residence time of Man-9 observed in MD simulations. NMR titrations of a Tyr-to-Ala variant indicate that disruption of the aromatic prevents the intermolecular crosslinking between two equivalents of Man-9 and one carbohydrate-binding face observed in wild-type GRFT and known to be critical for picomolar potency of this lectin. This C₃-symmetric aromatic core defines a new recognition motif for the design of carbohydrate receptors and suggests principles for engineering known lectins to have increased affinity and stability.

Algal and Cyanobacterial Lectins and Their Antimicrobial Properties

Lectins are proteins with a remarkably high affinity and specificity for carbohydrates. Many organisms naturally produce them, including animals, plants, fungi, protists, bacteria, archaea, and viruses. The present report focuses on lectins produced by marine or freshwater organisms, in particular algae and cyanobacteria. We explore their structure, function, classification, and antimicrobial properties. Furthermore, we look at the expression of lectins in heterologous systems and the current research on the preclinical and clinical evaluation of these fascinating molecules. The further development of these molecules might positively impact human health, particularly the prevention or treatment of diseases caused by pathogens such as human immunodeficiency virus, influenza, and severe acute respiratory coronaviruses, among others.

Novel Antifungal Activity of Q-Griffithsin, a Broad-Spectrum Antiviral Lectin

There is a rising global incidence of Candida strains with high levels of resistance to fluconazole and other antifungal drugs, hence the need for novel antifungal treatment strategies. Here, we describe the first evidence of antifungal activity of Q-Griffithsin (Q-GRFT), a recombinant oxidation-resistant variant of Griffithsin, a marine red algal lectin with broad-spectrum antiviral activity. We demonstrated that Q-GRFT binds to α-mannan in the Candida albicans cell wall. We also observed that Q-GRFT binding disrupted cell wall integrity and induced reactive oxidative species (ROS) formation, resulting in cell death. Furthermore, we showed that Q-GRFT inhibited the growth of other Candida species C. glabrata, C. parapsilosis, and C. krusei and had modest activity against some strains of multi- and pandrug-resistant C. auris. We found that Q-GRFT induced differential expression of numerous genes involved in response to cell stress, including those responsible for neutralizing ROS production and cell cycle regulation. In conclusion, this novel antifungal activity suggests that Q-GRFT is potentially an ideal drug candidate and represents an alternative strategy for the prevention and treatment of candidiasis. IMPORTANCE Fungal infections contribute to morbidity and mortality annually, and the number of organisms that are nonresponsive to the current available drug regimens are on the rise. There is a need to develop new agents to counter these infections and to add to the limited arsenal available to treat fungal infections. Our study has identified Q-GRFT, a broad-spectrum antiviral protein that harbors growth-inhibitory activity against several Candida strains, as a potential candidate for the prevention and treatment of fungal infections.

Griffithsin and Carrageenan Combination Results in Antiviral Synergy against SARS-CoV-1 and 2 in a Pseudoviral Model

Over 182 million confirmed cases of COVID-19 and more than 4 million deaths have been reported to date around the world. It is essential to identify broad-spectrum antiviral agents that may prevent or treat infections by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) but also by other coronaviruses that may jump the species barrier in the future. We evaluated the antiviral selectivity of griffithsin and sulfated and non-sulfated polysaccharides against SARS-CoV-1 and SARS-CoV-2 using a cytotoxicity assay and a cell-based pseudoviral model. The half-maximal cytotoxic concentration (CC₅₀) and half-maximal effective concentration (EC₅₀) were determined for each compound, using a dose-response-inhibition analysis on GraphPad Prism v9.0.2 software (San Diego, CA, USA). The therapeutic index (TI = CC₅₀/EC₅₀) was calculated for each compound. The potential synergistic, additive, or antagonistic effect of different compound combinations was determined by CalcuSyn v1 software (Biosoft, Cambridge, UK), which estimated the combination index (CI) values. Iota and lambda carrageenan showed the most potent antiviral activity (EC₅₀ between 3.2 and 7.5 µg/mL). Carrageenan and griffithsin combinations exhibited synergistic activity (EC₅₀ between 0.2 and 3.8 µg/mL; combination index <1), including against recent SARS-CoV-2 mutations. The griffithsin and carrageenan combination is a promising candidate to prevent or treat infections by SARS-CoV-1 and SARS-CoV-2.

Exploring lectin-glycan interactions to combat COVID-19: Lessons acquired from other enveloped viruses

The emergence of a new human coronavirus (SARS-CoV-2) has imposed great pressure on the health system worldwide. The presence of glycoproteins on the viral envelope opens a wide range of possibilities for application of lectins to address some urgent problems involved in this pandemic. In this work, we discuss the potential contributions of lectins from non-mammalian sources in the development of several fields associated with viral infections, most notably COVID-19. We review the literature on the use of non-mammalian lectins as a therapeutic approach against members of the Coronaviridae family, including recent advances in strategies of protein engineering to improve their efficacy. The applications of lectins as adjuvants for antiviral vaccines are also discussed. Finally, we present some emerging strategies employing lectins for the development of biosensors, microarrays, immunoassays and tools for purification of viruses from whole blood. Altogether, the data compiled in this review highlights the importance of structural studies aiming to improve our knowledge about the basis of glycan recognition by lectins and its repercussions in several fields, providing potential solutions for complex aspects that are emerging from different health challenges.

Potential of algal metabolites for the development of broad-spectrum antiviral therapeutics: Possible implications in COVID-19 therapy

Covid-19 pandemic severely affected human health worldwide. Till October 19, 2020, total confirmed patients of COVID-19 are 39,944,882, whereas 1,111,998 people died across the globe. Till to date, we do not have any specific medicine and/or vaccine to treat COVID-19; however, research is still going on at war footing. So far vaccine development is concerned, here it is noteworthy that till now three major variants (named A, B, and C) of severe acute respiratory syndrome-coronavirus2 (SARS-CoV-2) have been recognized. Increased mutational rate and formation of new viral variants may increase the attrition rate of vaccines and/or candidate chemotherapies. Herbal remedies are chemical cocktails, thus open another avenue for effective antiviral therapeutics development. In fact, India is a large country, which is densely populated, but the overall severity of COVID-19 per million populations is lesser than any other country of the world. One of the major reasons for the aforesaid difference is the use of herbal remedies by the Government of India as a preventive measure for COVID-19. Therefore, the present review focuses on the epidemiology and molecular pathogenesis of COVID-19 and explores algal metabolites for their antiviral properties.

Hantavirus Infection Is Inhibited by Griffithsin in Cell Culture

Andes virus (ANDV) and Sin Nombre virus (SNV), highly pathogenic hantaviruses, cause hantavirus pulmonary syndrome in the Americas. Currently no therapeutics are approved for use against these infections. Griffithsin (GRFT) is a high-mannose oligosaccharide-binding lectin currently being evaluated in phase I clinical trials as a topical microbicide for the prevention of human immunodeficiency virus (HIV-1) infection (ClinicalTrials.gov Identifiers: NCT04032717, NCT02875119) and has shown broad-spectrum in vivo activity against other viruses, including severe acute respiratory syndrome coronavirus, hepatitis C virus, Japanese encephalitis virus, and Nipah virus. In this study, we evaluated the in vitro antiviral activity of GRFT and its synthetic trimeric tandemer 3mGRFT against ANDV and SNV. Our results demonstrate that GRFT is a potent inhibitor of ANDV infection. GRFT inhibited entry of pseudo-particles typed with ANDV envelope glycoprotein into host cells, suggesting that it inhibits viral envelope protein function during entry. 3mGRFT is more potent than GRFT against ANDV and SNV infection. Our results warrant the testing of GRFT and 3mGRFT against ANDV infection in animal models.

Modification and optimization of the inhibition of HIV-1 cell-to-cell transmission assay

HIV-1 infection is caused by cell-free and cell-associated viruses. Currently most of the assays used to screen potential HIV-1 entry inhibitors focus on the inhibition of cell-free viruses. One assay that is widely employed is the TZM-bl neutralization assay that uses pseudotyped viruses. However, a study by Abela et al. showed that many inhibitors that potently inhibit cell-free HIV-1 in this assay can be less effective against the cell-to-cell transmission of the virus. These researchers then designed a method to screen entry inhibitors for activity against cell-associated HIV-1, using pseudotyped viruses. The main limitation of this method, however, was that it can only be reliably employed against viruses that cannot infect target cells as cell-free virion in the absence of a polycation supplement such as DEAE (diethylaminoethyl). Thus, in the current study we provide modifications to this method that solves the problem and makes it possible to study entry inhibitors against cell-to-cell infection of both polycation depend and independent viruses. The main modification involves the introduction of the relative light unit (RLU) vs. virus producing 293-T cells / corresponding supernatants graph. This graph is used to select a virus input that only allows for the detection of cell-associated viruses infection.•The method is a modification of the cell-to-cell transmission assay published by Abela et al.•The method allows for the study of the inhibition of cell-to-cell transmission of both polycation dependent and independent HIV-1 pseudoviruses.

Griffithsin Inhibits Nipah Virus Entry and Fusion and Can Protect Syrian Golden Hamsters From Lethal Nipah Virus Challenge

Nipah virus (NiV) is a highly pathogenic zoonotic paramyxovirus that causes fatal encephalitis and respiratory disease in humans. There is currently no approved therapeutic for human use against NiV infection. Griffithsin (GRFT) is high-mannose oligosaccharide binding lectin that has shown in vivo broad-spectrum activity against viruses, including severe acute respiratory syndrome coronavirus, human immunodeficiency virus 1, hepatitis C virus, and Japanese encephalitis virus. In this study, we evaluated the in vitro antiviral activities of GRFT and its synthetic trimeric tandemer (3mG) against NiV and other viruses from 4 virus families. The 3mG had comparatively greater potency than GRFT against NiV due to its enhanced ability to block NiV glycoprotein-induced syncytia formation. Our initial in vivo prophylactic evaluation of an oxidation-resistant GRFT (Q-GRFT) showed significant protection against lethal NiV challenge in Syrian golden hamsters. Our results warrant further development of Q-GRFT and 3mG as potential NiV therapeutics.

An off-the-Shelf Approach for the Production of Fc Fusion Proteins by Protein Trans-Splicing towards Generating a Lectibody In Vitro

Monoclonal antibodies, engineered antibodies, and antibody fragments have become important biological therapeutic platforms. The IgG format with bivalent binding sites has a modular structure with different biological roles, i.e., effector and binding functions, in different domains. We demonstrated the reconstruction of an IgG-like domain structure in vitro by protein ligation using protein trans-splicing. We produced various binding domains to replace the binding domain of IgG from Escherichia coli and the Fc domain of human IgG from Brevibacillus choshinensis as split-intein fusions. We showed that in vitro protein ligation could produce various Fc-fusions at the N-terminus in vitro from the independently produced domains from different organisms. We thus propose an off-the-shelf approach for the combinatorial production of Fc fusions in vitro with several distinct binding domains, particularly from naturally occurring binding domains. Antiviral lectins from algae are known to inhibit virus entry of HIV and SARS coronavirus. We demonstrated that a lectin could be fused with the Fc-domain in vitro by protein ligation, producing an IgG-like molecule as a “lectibody”. Such an Fc-fusion could be produced in vitro by this approach, which could be an attractive method for developing potential therapeutic agents against rapidly emerging infectious diseases like SARS coronavirus without any genetic fusion and expression optimization.

Comparison of the antiviral activity of the microbicide candidate griffithsin and its tandemers derivatives against different modes of HIV-1 transmission

Tandemers 2MG, 2MG3, 3MG and 4MG are derivatives of the potent anti-HIV-1 microbicide candidate griffithsin (GRFT). We compared these compounds anti-HIV-1 activity to GRFT using the viruses CAP206.08 and CAAN5342.A2 that have decreased sensitivity to this lectin. The 2MG and 2MG3 tandemers had similar activity to GRFT against cell-free and cell-associated viruses, while 3MG and 4MG were significantly more potent. Furthermore, the restoration of the 234N or 295N glycan in these viruses, known to increase sensitivity to GRFT, also increased sensitivity to 2MG and 2MG3, and not to 3MG and 4MG. In addition, GRFT resistant viruses generated in-vitro were equally resistant to 2MG and 2MG3 while they had considerably low resistance to 3MG and 4MG. Lastly, all five compounds showed increased inhibitory activity in seminal and vaginal simulants although the effect was more pronounced in the former. These data support further studies of tandemers as potential microbicides.

Griffithsin Retains Anti-HIV-1 Potency with Changes in gp120 Glycosylation and Complements Broadly Neutralizing Antibodies PGT121 and PGT126

Griffithsin (Grft) is an antiviral lectin that has been shown to potently inhibit HIV-1 by binding high-mannose N-linked glycosylation sites on HIV-1 gp120. A key factor for Grft potency is glycosylation at N295 of gp120, which is directly adjacent to N332, a target glycan for an entire class of broadly neutralizing antibodies (bNAbs). Here, we unify previous work on the importance of other glycans to Grft potency against HIV-1 and Grft's role in mediating the conformational change of gp120 by mutating nearly every glycosylation site in gp120. In addition to a significant loss of Grft activity by the removal of glycosylation at N295, glycan absence at N332 or N448 was found to have moderate effects on Grft potency. Interestingly, in the absence of N295, Grft effectiveness could be improved by a mutation that results in the glycan at N448 shifting to N446, indicating that the importance of individual glycans may be related to their effect on glycosylation density. Grft's ability to alter the structure of gp120, exposing the CD4 binding site, correlated with the presence of glycosylation at N295 only in clade B strains, not clade C strains. We further demonstrate that Grft can rescue the activity of the bNAbs PGT121 and PGT126 in the event of a loss or a shift of glycosylation at N332, where the bNAbs suffer a drastic loss of potency. Despite targeting the same region, Grft in combination with PGT121 and PGT126 produced additive effects. This indicates that Grft could be an important combinational therapeutic.

Griffithsin, a Highly Potent Broad-Spectrum Antiviral Lectin from Red Algae: From Discovery to Clinical Application

Virus entry into a susceptible host cell is the first step in the formation of all viral diseases. Controlling viral infections by disrupting viral entry is advantageous for antibody-mediated neutralization by the host’s immune system and as a preventive and therapeutic antiviral strategy. Recently, several plant-derived carbohydrate-binding proteins (lectins) have emerged as a new class of antiviral biologics by taking advantage of a unique glycosylation pattern only found on the surface of viruses. In particular, a red algae-derived griffithsin (GRFT) protein has demonstrated superior in vitro and in vivo antiviral activity with minimum host toxicity against a variety of clinically relevant, enveloped viruses. This review examines the structural characteristics of GRFT, focusing on its carbohydrate-binding capability. Its in vitro antiviral profiles against human immunodeficiency virus (HIV) are also discussed followed by a description of the results from a combination study using anti-HIV drugs. The results of several studies regarding its novel antiviral mechanism of action are provided in conjunction with an explanation of viral resistance profiles to GRFT. In addition, its in vitro and in vivo host toxicity profiles are summarized with its pharmacokinetic behavior using in vivo efficacy study results. Also, a large-scale production and formulation strategy, as well as a drug delivery strategy, for GRFT as a new class of broad-spectrum microbicides is discussed. Finally, results from two ongoing clinical studies examining GRFT’s effects on viruses are presented.

Mannose-Specific Lectins from Marine Algae: Diverse Structural Scaffolds Associated to Common Virucidal and Anti-Cancer Properties

To date, a number of mannose-specific lectins have been isolated and characterized from seaweeds, especially from red algae. In fact, man-specific seaweed lectins consist of different structural scaffolds harboring a single or a few carbohydrate-binding sites which specifically recognize mannose-containing glycans. Depending on the structural scaffold, man-specific seaweed lectins belong to five distinct structurally-related lectin families, namely (1) the griffithsin lectin family (β-prism I scaffold); (2) the Oscillatoria agardhii agglutinin homolog (OAAH) lectin family (β-barrel scaffold); (3) the legume lectin-like lectin family (β-sandwich scaffold); (4) the Galanthus nivalis agglutinin (GNA)-like lectin family (β-prism II scaffold); and, (5) the MFP2-like lectin family (MFP2-like scaffold). Another algal lectin from Ulva pertusa, has been inferred to the methanol dehydrogenase related lectin family, because it displays a rather different GlcNAc-specificity. In spite of these structural discrepancies, all members from the five lectin families share a common ability to specifically recognize man-containing glycans and, especially, high-mannose type glycans. Because of their mannose-binding specificity, these lectins have been used as valuable tools for deciphering and characterizing the complex mannose-containing glycans from the glycocalyx covering both normal and transformed cells, and as diagnostic tools and therapeutic drugs that specifically recognize the altered high-mannose N-glycans occurring at the surface of various cancer cells. In addition to these anti-cancer properties, man-specific seaweed lectins have been widely used as potent human immunodeficiency virus (HIV-1)-inactivating proteins, due to their capacity to specifically interact with the envelope glycoprotein gp120 and prevent the virion infectivity of HIV-1 towards the host CD4+ T-lymphocyte cells in vitro.

Marine Algae Metabolites as Promising Therapeutics for the Prevention and Treatment of HIV/AIDS

This review presents an analysis of works devoted to the anti-human immunodeficiency virus (HIV) activity of algae metabolites-sulfated polysaccharides (fucoidans, carrageenans), lectins, laminarans, and polyphenols. Despite the presence of a significant number of antiretroviral drugs, the development of new therapeutic and prophylactic agents against this infection remains very urgent problem. This is due to the variability of HIV, the absence of an animal model (except monkeys) and natural immunity to this virus and the toxicity of therapeutic agents and their high cost. In this regard, the need for new therapeutic approaches and broad-spectrum drugs, which in addition to antiviral effects can have anti-inflammatory, antioxidant, and immunomodulatory effects, and to which the minimum resistance of HIV strains would be formed. These requirements meet the biologically active substances of marine algae. The results of experimental and clinical studies conducted in vitro and in vivo are presented, and the issues of the anti-HIV activity of these compounds are considered depending on their structural features. On the whole, the presented data prove the high efficiency of seaweed metabolites and justify the possibility of their use as a potential basis for the development of new drugs with a wide spectrum of activity.

Ligand binding and retention in snake gourd seed lectin (SGSL). A crystallographic, thermodynamic and molecular dynamics study

Snake gourd seed lectin (SGSL) is a non-toxic homolog of type II ribosome-inactivating proteins (RIPs) which contain a catalytic domain and a lectin domain. Isothermal titration calorimetry (ITC) measurements of the interactions of the protein with LacNAc, Lac, Gal, Me-α-Gal were carried out and the crystal structures of the native protein and its complex with Lac were determined. The crystal structure of the Me-α-Gal complex has already been determined. While the crystal structure showed the presence of two-sugar-binding sites, one on each of the two domains of the lectin chain, ITC measurements indicated the presence of only one binding site. In order to resolve this anomaly, molecular dynamics (MD) simulations were carried out on the native protein and on its complexes with Me-α-Gal and Lac. Simulations were also performed on the protein after reducing the inter-chain disulfide bridge between the two chains. The crystal structures and the simulations confirmed the robustness of the protein structure, irrespective of the presence or absence of the disulfide bridge. The simulations indicated that although two sites can bind sugar, only the ligand at one site is retained in a dynamic situation. The studies thus bring out the subtle relationship between binding and retention of the ligand.

Induction of Recombinant Lectin Expression by an Artificially Constructed Tandem Repeat Structure: A Case Study Using Bryopsis plumosa Mannose-Binding Lectin

Lectin is an important protein in medical and pharmacological applications. Impurities in lectin derived from natural sources and the generation of inactive proteins by recombinant technology are major obstacles for the use of lectins. Expressing recombinant lectin with a tandem repeat structure can potentially overcome these problems, but few studies have systematically examined this possibility. This was investigated in the present study using three distinct forms of recombinant mannose-binding lectin from Bryopsis plumosa (BPL2)-i.e., the monomer (rD1BPL2), as well as the dimer (rD2BPL2), and tetramer (rD4BPL2) arranged as tandem repeats. The concentration of the inducer molecule isopropyl β-D-1-thiogalactopyranoside and the induction time had no effect on the efficiency of the expression of each construct. Of the tested constructs, only rD4BPL2 showed hemagglutination activity towards horse erythrocytes; the activity of towards the former was 64 times higher than that of native BPL2. Recombinant and native BPL2 showed differences in carbohydrate specificity; the activity of rD4BPL2 was inhibited by the glycoprotein fetuin, whereas that of native BPL2 was also inhibited by d-mannose. Our results indicate that expression as tandem repeat sequences can increase the efficiency of lectin production on a large scale using a bacterial expression system.

Surface-displayed porcine reproductive and respiratory syndrome virus from cell culture onto gram-positive enhancer matrix particles

Vaccine immunization is now one of the most effective ways to control porcine reproductive and respiratory syndrome virus (PRRSV) infection. Impurity is one of the main factors affecting vaccine safety and efficacy. Here we present a novel innovative PRRSV purification approach based on surface display technology. First, a bifunctional protein PA-GRFT (protein anchor-griffithsin), the crucial factor in the purification process, was successfully produced in Escherichia coli yielding 80 mg/L of broth culture. Then PRRSV purification was performed by incubation of PA-GRFT with PRRSV and gram-positive enhancer matrix (GEM) particles, followed by centrifugation to collect virions loaded onto GEM particles. Our results showed that most of the bulk impurities had been removed, and PA-GRFT could capture PRRSV onto GEM particles. Our lactic acid bacteria-based purification method, which is promising as ease of operation, low cost and easy to scale-up, may represent a candidate method for the large-scale purification of this virus for vaccine production.

Gene-silencing suppressors for high-level production of the HIV-1 entry inhibitor griffithsin in Nicotiana benthamiana

The exploration of emerging host organisms for the economic and efficient production of protein microbicides against HIV is urgently needed in resource-poor areas worldwide. In this study, the production of the novel HIV entry inhibitor candidate, griffithsin (GRFT), was investigated using Nicotiana benthamiana as the expression platform based on a non-viral vector. To increase the yield of recombinant GRFT, the RNA silencing defense mechanism of N. benthamiana was abolished by using three gene silencing suppressors. A transient expression system was used by transferring the GRFT gene, which encodes 122 amino acids, under the control of the enhanced CaMV 35S promoter. The presence of correctly assembled GRFT in transgenic leaves was confirmed using immunoglobulin-specific sandwich ELISA. The data demonstrated that the use of three gene silencing suppressors allowed the highest accumulation of GRFT, with a yield of 400 μg g-1 fresh weight, and this amount was reduced to 287 μg g-1 after purification, representing a recovery of 71.75%. The analysis also showed that the ability of GRFT expressed in N. benthamiana to bind to glycoprotein 120 is close to that of the GRFT protein purified from E. coli. Whole-cell assays using purified GRFT showed that our purified GRFT was potently active against HIV. This study provides the first high-level production of the HIV-1 entry inhibitor griffithsin with a non-viral expression system and illustrates the robustness of the co-agroinfiltration expression system improved through the use of three gene silencing suppressors.

Lectins from red algae and their biomedical potential

Lectins are unique proteins or glycoproteins of non-immune origin that bind specifically to carbohydrates. They recognise and interact reversibly to either free carbohydrates or glycoconjugates, without modifying their structure. Lectins are highly diverse and widely distributed in nature and have been extensively reported from various red algae species. Numerous red algae species have been reported to possess lectins having carbohydrate specificity towards complex glycoproteins or high-mannose N-glycans. These lectin-glycan interactions further trigger many biochemical responses which lead to their extensive use as valuable tools in biomedical research. Thus, owing to their exceptional glycan recognition property, red algae lectins are potential candidate for inhibition of various viral diseases. Hence, the present report integrates existing information on the red algae lectins, their carbohydrate specificity, and characteristics of purified lectins. Further, the review also reports the current state of research into their anti-viral activity against various enveloped viruses such as HIV, hepatitis, influenza, encephalitis, coronavirus and herpes simplex virus and other biomedical activities such as anti-cancer, anti-microbial, anti-inflammatory, anti-nociceptive and acaricidal activities.

Intracellular Crosslinking of Filoviral Nucleoproteins with Xintrabodies Restricts Viral Packaging

Viruses assemble large macromolecular repeat structures that become part of the infectious particles or virions. Ribonucleocapsids (RNCs) of negative strand RNA viruses are a prime example where repetition of nucleoprotein (NP) along the genome creates a core polymeric helical scaffold that accommodates other nucleocapsid proteins including viral polymerase. The RNCs are transported through the cytosol for packaging into virions through association with viral matrix proteins at cell membranes. We hypothesized that RNC would be ideal targets for crosslinkers engineered to promote aberrant protein–protein interactions, thereby blocking their orderly transport and packaging. Previously, we had generated single-domain antibodies (sdAbs) against Filoviruses that have all targeted highly conserved C-terminal regions of NP known to be repetitively exposed along the length of the RNCs of Marburgvirus (MARV) and Ebolavirus (EBOV). Our crosslinker design consisted of dimeric sdAb expressed intracellularly, which we call Xintrabodies (X- for crosslinking). Electron microscopy of purified NP polymers incubated with purified sdAb constructs showed NP aggregation occurred in a genus-specific manner with dimeric and not monomeric sdAb. A virus-like particle (VLP) assay was used for initial evaluation where we found that dimeric sdAb inhibited NP incorporation into VP40-based VLPs whereas monomeric sdAb did not. Inhibition of NP packaging was genus specific. Confocal microscopy revealed dimeric sdAb was diffuse when expressed alone but focused on pools of NP when the two were coexpressed, while monomeric sdAb showed ambivalent partition. Infection of stable Vero cell lines expressing dimeric sdAb specific for either MARV or EBOV NP resulted in smaller plaques and reduced progeny of cognate virus relative to wild-type Vero cells. Though the impact was marginal at later time-points, the collective data suggest that viral replication can be reduced by crosslinking intracellular NP using relatively small amounts of dimeric sdAb to restrict NP packaging. The stoichiometry and ease of application of the approach would likely benefit from transitioning away from intracellular expression of crosslinking sdAb to exogenous delivery of antibody. By retuning sdAb specificity, the approach of crosslinking highly conserved regions of assembly critical proteins may well be applicable to inhibiting replication processes of a broad spectrum of viruses.

Pharmacokinetics of the Protein Microbicide 5P12-RANTES in Sheep following Single-Dose Vaginal Gel Administration

5P12-RANTES, a chemokine analogue that potently blocks the HIV CCR5 coreceptor, is being developed as both a vaginal and rectal microbicide for prevention of sexual transmission of HIV. Here, we report the first pharmacokinetic data for 5P12-RANTES following single-dose vaginal gel administration in sheep. Aqueous gel formulations containing low (1.24-mg/ml), intermediate (6.18-mg/ml), and high (32.0-mg/ml; suspension-type gel) concentrations of 5P12-RANTES were assessed via rheology, syringeability, and in vitro release testing. Following vaginal gel administration to sheep, 5P12-RANTES concentrations were measured in vaginal fluid, vaginal tissue, and serum over a 96-h period. All gels showed non-Newtonian pseudoplastic behavior, with the high-concentration gels exhibiting a greater viscosity and cohesive structure than the intermediate- and low-concentration gels. In in vitro release testing, >90% 5P12-RANTES was released from the low- and intermediate-concentration gels after 72 h. For the high-concentration gel, ∼50% 5P12-RANTES was detected, attributed to protein denaturation during lyophilization and/or subsequent solvation of the protein within the gel matrix. In sheep, 5P12-RANTES concentrations in vaginal fluid, vaginal tissue, and serum increased in a dose-dependent manner. The highest concentrations were measured in vaginal fluid (10⁵ to 10⁷ ng/ml), followed by vaginal tissue (10⁴ to 10⁶ ng/ml). Both of these concentration ranges are several orders of magnitude above the reported half-maximal inhibitory concentrations. The lowest concentration was measured in serum (<10² ng/ml). The 5P12-RANTES pharmacokinetic data are similar to those reported previously for other candidate microbicides. These data, coupled with 5P12-RANTES's potency at picomolar concentrations, its strong barrier to resistance, and the full protection that it was observed to provide in a rhesus macaque vaginal challenge model, support the continued development of 5P12-RANTES as a microbicide.

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.

Binding Site Geometry and Subdomain Valency Control Effects of Neutralizing Lectins on HIV-1 Viral Particles

Carbohydrate binding proteins such as griffithsin, cyanovirin-N, and BanLec are potent HIV entry inhibitors and promising microbicides. Each binds to high-mannose glycans on the surface envelope glycoprotein gp120, yet the mechanisms by which they engage viral spikes and exhibit inhibition constants ranging from nanomolar to picomolar are not understood. To determine the structural and mechanistic basis for recognition and potency, we selected a panel of lectins possessing different valencies per subunit, oligomeric states, and relative orientations of carbohydrate binding sites to systematically probe their contributions to inhibiting viral entry. Cryo-electron micrographs and immuno gold staining of lectin-treated viral particles revealed two distinct effects-namely, viral aggregation or clustering of the HIV-1 envelope on the viral membrane-that were dictated by carbohydrate binding site geometry and valency. "Sandwich" surface plasmon resonance experiments revealed that a second binding event occurs only for those lectins that could aggregate viral particles. Furthermore, picomolar K_d values were observed for the second binding event, providing a mechanism by which picomolar IC₅₀ values are achieved. We suggest that these binding and aggregation phenomena translate to neutralization potency.

Griffithsin: An Antiviral Lectin with Outstanding Therapeutic Potential

Griffithsin (GRFT), an algae-derived lectin, is one of the most potent viral entry inhibitors discovered to date. It is currently being developed as a microbicide with broad-spectrum activity against several enveloped viruses. GRFT can inhibit human immunodeficiency virus (HIV) infection at picomolar concentrations, surpassing the ability of most anti-HIV agents. The potential to inhibit other viruses as well as parasites has also been demonstrated. Griffithsin's antiviral activity stems from its ability to bind terminal mannoses present in high-mannose oligosaccharides and crosslink these glycans on the surface of the viral envelope glycoproteins. Here, we review structural and biochemical studies that established mode of action and facilitated construction of GRFT analogs, mechanisms that may lead to resistance, and in vitro and pre-clinical results that support the therapeutic potential of this lectin.

Microbicide vaginal rings: Technological challenges and clinical development

Vaginal rings (VRs) are flexible, torus-shaped, polymeric devices designed to sustain delivery of pharmaceutical drugs to the vagina for clinical benefit. Following first report in a 1970 patent application, several steroid-releasing VR products have since been marketed for use in hormone replacement therapy and contraception. Since 2002, there has been growing interest in the use of VR technology for delivery of drugs that can reduce the risk of sexual acquisition of human immunodeficiency virus type 1 (HIV-1), the causative agent of acquired immunodeficiency syndrome (AIDS). Although no vaginally-administered product has yet been approved for HIV reduction/prevention, extensive research efforts are continuing and a number of VR devices offering sustained release of so-called 'HIV microbicide' compounds are currently being evaluated in late-stage clinical studies. This review article provides an overview of the published scientific literature within this important field of research, focusing primarily on articles published within peer-reviewed journal publications. Many important aspects of microbicide-releasing VR technology are discussed, with a particular emphasis on the technological, manufacturing and clinical challenges that have emerged in recent years.

Proteins, peptides, polysaccharides, and nucleotides with inhibitory activity on human immunodeficiency virus and its enzymes

Human immunodeficiency virus (HIV), the causative agent of acquired immune deficiency syndrome, has claimed innumerable lives in the past. Many biomolecules which suppress HIV replication and also other biomolecules that inhibit enzymes essential to HIV replication have been reported. Proteins including a variety of milk proteins, ribosome-inactivating proteins, ribonucleases, antifungal proteins, and trypsin inhibitors; peptides comprising cathelicidins, defensins, synthetic peptides, and others; polysaccharides and polysaccharopeptides; nucleosides, nucleotides, and ribozymes, demonstrated anti-HIV activity. In many cases, the mechanism of anti-HIV action has been elucidated. Strategies have been devised to augment the anti-HIV potency of these compounds.