Mannose-Binding Lectins from Brazilian Flora: Potential Antiviral Agents Against SARS-CoV-2

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Abstract:

The entry of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) into host cells primarily relies on interactions between the viral spike protein (S) and the angiotensin-converting enzyme 2 (ACE-2) cell receptor, both of which are extensively glycosylated. Consequently, agents with carbohydrate-binding properties hold promise as candidates to impede viral infection.

In this study, we assessed the in vitro anti-SARS-CoV-2 activity of two mannose-binding lectins, ConBR and DVL, isolated from Brazilian plants Canavalia brasiliensis and Dioclea violacea, respectively. These lectins demonstrated potent inhibition against SARS-CoV-2 Wuhan-Hu-1 strain and its variants Gamma and Omicron, with selectivity indexes (SI) of 7, 1.7, and 6.5 for ConBR, and 25, 16.8, and 22.3 for DVL.

ConBR and DVL effectively inhibited over 95% of the early stages of viral infection, exhibiting a strong virucidal effect, as well as protection against cell infection and post-entry inhibition.

Notably, the presence of mannose completely abrogated the anti-SARS-CoV-2 activity of ConBR and DVL, leading to the recovery of virus titers. Molecular investigations through ATR-FTIR, molecular docking, and dynamic simulations between SARS-CoV-2 S and these lectins revealed robust molecular interactions, with predicted binding energies of −85.4 and −72.0 Kcal/Mol, respectively. These findings underscore the potent anti-SARS-CoV-2 activity of ConBR and DVL lectins, potentially mediated through glycan interactions that block virus entry into cells, thus positioning them as promising candidates for the development of novel antiviral drugs.

The study….

In late 2019, the world witnessed the emergence of an acute respiratory disease, COVID-19, caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

This novel coronavirus rapidly spread across the globe, leading to the World Health Organization (WHO) declaring it a pandemic in March 2020. Since then, the virus has infected nearly 770 million people and claimed the lives of over 7 million individuals.

While vaccination campaigns have made significant progress in curbing the outbreak, the need for effective antiviral drugs remains crucial.

The first antiviral drug approved by the U.S. Food and Drug Administration (FDA) for COVID-19 treatment was VEKLURY®, also known as Remdesivir. However, the continuous evolution of the virus and the emergence of new variants have prompted researchers to explore alternative therapeutic options.

Among these options are two promising candidates: mannose-binding lectins ConBR and DVL, derived from Brazilian flora.

In this comprehensive article, we delve into the structure of SARS-CoV-2, the importance of glycosylation, and the potential antiviral properties of these lectins.

Structure of SARS-CoV-2

SARS-CoV-2 belongs to the Coronaviridae family and is characterized as an enveloped, single-stranded positive RNA virus. Its genome encodes a variety of proteins, with two main groups being structural and non-structural proteins. The structural proteins include the spike protein (S), matrix protein (M), and envelope protein (E), while the non-structural proteins comprise proteases and RNA-dependent RNA polymerase (RdRP).

The spike protein (S) is of particular significance as it plays a pivotal role in the virus’s attachment to and entry into host cells. This protein is a complex structure consisting of two subunits: S1 and S2. The S1 subunit comprises the receptor binding domain (RBD) and N-terminal domain (NTD), both of which interact with cellular receptors on the host cell membrane. The primary receptor for SARS-CoV-2 is the angiotensin-converting enzyme 2 (ACE-2), though other receptors such as CD147 and NRP1 have also been identified.

The S2 subunit, on the other hand, consists of several domains including a cytoplasmic tail, a transmembrane domain, heptad repeat 2, connector domain, central helix, heptad repeat 1, and fusion peptide. The cleavage site between S1 and S2 is crucial for the activation of the S protein, which facilitates the fusion of the viral envelope with the host cell membrane. Additionally, the presence of N-glycan sites on the S protein is essential for modulating SARS-CoV-2 membrane fusion and its interaction with host cell receptors.

Glycosylation and Immune Evasion

Glycans, or carbohydrate chains, are integral components of the SARS-CoV-2 S protein. Molecular analysis has revealed the presence of 22 N-glycan sites per monomer in the S protein. These glycans are critical for the modulation of membrane fusion and the dynamics of the RBD, affecting its interaction with host cell receptors. Importantly, N-linked glycans play a crucial role in the binding affinity, association, and dissociation of the RBD-ACE-2 interactions, influencing the virus’s ability to enter host cells.

Glycans also serve as a defense mechanism for the virus by shielding S protein epitopes. This shielding interferes with the recognition of the virus by neutralizing antibodies of the host immune system. Two N-Mannose glycan sites, N234 and N709, are predominantly oligomannose-type and are found in the NTD and FP regions of the spike protein. These glycan sites regulate the conformational dynamics of the RBD, influencing its interaction with host cell receptors.

Mannose-Binding Lectins as Potential Antiviral Agents

Lectins are proteins known for their carbohydrate-binding domains, which enable them to interact with specific sugar moieties on glycoproteins, glycolipids, and free monosaccharides. In the context of viral infections, lectins can recognize and interact with specific glycans present on the surface of viruses, altering their ability to infect host cells.

Two notable mannose-binding lectins, ConBR and DVL, derived from Brazilian flora, have shown promise as potential antiviral agents against SARS-CoV-2. ConBR, initially purified from Canavalia brasiliensis seeds, and DVL, obtained from Dioclea violacea seeds, both exhibit mannose-binding specificity.

Plant-derived lectins have demonstrated potent antibacterial and antiviral effects against various infectious diseases. For example, lectins from Bauhinia variegata seeds, with specificity for glucose and galactose, have shown activity against Coxsackievirus B3 and Rotavirus.

Similarly, lectins from Momordica charantia, which bind to galactose and N-Acetylgalactosamine, have exhibited antiviral effects against HIV, HSV-1, H1N1, H3N2, and H5N1.

In the case of SARS-CoV-2, lectins from Senna tora, with specificity for mannose and galactose, have demonstrated antiviral activity, highlighting the potential of mannose-binding lectins as a therapeutic strategy.

Evaluating the Anti-SARS-CoV-2 Effects of ConBR and DVL

To investigate the potential antiviral activity of ConBR and DVL against SARS-CoV-2, researchers conducted a series of experiments. Initially, the lectins were screened using a VSV-SARS-CoV-2 pseudotyped virus model to assess their in vitro activity against the virus.

Subsequently, the antiviral effects of ConBR and DVL were validated against infectious SARS-CoV-2 Wuhan-Hu-1 (SARS-CoV-2WT) and two SARS-CoV-2 variants, Omicron and Gamma. These studies aimed to determine the lectins’ ability to interfere with virus cell entry and, potentially, inhibit SARS-CoV-2 infection.

Conclusion

As the world continues to grapple with the COVID-19 pandemic, the search for effective antiviral treatments remains a top priority. Mannose-binding lectins, such as ConBR and DVL, derived from Brazilian flora, hold promise as potential antiviral agents against SARS-CoV-2.

These lectins’ ability to bind to glycans on the virus’s spike protein may interfere with the virus’s ability to enter host cells, providing a novel avenue for therapeutic intervention.

Further research and clinical trials are necessary to fully understand the potential of ConBR and DVL as antiviral agents against SARS-CoV-2 and its variants. If successful, these lectins could contribute to the arsenal of treatments available to combat COVID-19 and mitigate the impact of future viral outbreaks.


in deep…

Origins of ConBR and DVL

ConBR: ConBR is a mannose-binding lectin initially purified from the seeds of Canavalia brasiliensis, commonly known as the Brazilian jackbean. This plant, native to Brazil, belongs to the Leguminosae family and has been a valuable source of bioactive compounds, including lectins. ConBR, isolated from this plant, has gained attention due to its unique binding specificity for mannose, a sugar molecule crucial in the context of viral infections.

DVL: DVL, another mannose-binding lectin with promising antiviral potential, is obtained from the seeds of Dioclea violacea, a plant abundant in Brazilian cerrado vegetation, a tropical savanna ecoregion. Like ConBR, DVL exhibits a strong affinity for mannose, making it an intriguing candidate for combating SARS-CoV-2.

Chemical Properties of ConBR and DVL

Understanding the chemical properties of ConBR and DVL is essential to unravel their antiviral mechanisms. These lectins possess distinctive features that contribute to their mannose-binding specificity and potential antiviral activity:

Carbohydrate-Binding Specificity: Both ConBR and DVL are categorized as lectins, a class of proteins known for their carbohydrate-binding domains. Their unique specificity for mannose allows them to selectively target viral glycoproteins, particularly the spike protein of SARS-CoV-2, which is rich in mannose residues. This selective binding forms the foundation of their antiviral activity.

Oligomerization State: Lectins like ConBR and DVL often exist as multimeric structures, with multiple subunits binding together. This oligomerization enhances their affinity for carbohydrates, including mannose. The multivalent binding capacity of these lectins increases their effectiveness in interacting with the viral surface, potentially hindering viral entry into host cells.

Glycan-Binding Sites: The chemical structure of lectins is intricately designed to accommodate specific sugar moieties. In the case of ConBR and DVL, their glycan-binding sites are tailored to interact with mannose residues, facilitating strong and specific binding to the glycosylated surface of SARS-CoV-2.

Physical Activities of ConBR and DVL

Interaction with SARS-CoV-2 Spike Protein: One of the key physical activities of ConBR and DVL is their interaction with the spike protein of SARS-CoV-2. This interaction occurs through the lectins’ mannose-binding domains, which recognize and bind to mannose-rich glycans on the viral spike protein. This binding can interfere with the virus’s ability to attach to host cells, a critical step in viral infection.

Inhibition of Viral Entry: By binding to the spike protein, ConBR and DVL may block the attachment of SARS-CoV-2 to host cell receptors, such as angiotensin-converting enzyme 2 (ACE-2). This interference can prevent viral entry into host cells, effectively halting the infection cycle.

Potential Immunomodulation: Lectins like ConBR and DVL may also possess immunomodulatory properties. Their interactions with viral glycoproteins can trigger immune responses, potentially enhancing the host’s ability to combat the virus. This dual action on both the virus and the host immune system makes them intriguing candidates for antiviral therapy.

Promising Antiviral Candidates:

The chemical and physical activities of ConBR and DVL make them promising antiviral candidates against SARS-CoV-2. Their specificity for mannose-rich glycans on the viral spike protein and their potential to inhibit viral entry highlight their significance in the fight against COVID-19.

Future Directions and Challenges:

While the potential of ConBR and DVL as antiviral agents is encouraging, further research is needed to validate their efficacy and safety in clinical settings. Challenges such as drug delivery, stability, and dosage optimization must be addressed to harness the full potential of these lectins as therapeutics against SARS-CoV-2.

Conclusion

ConBR and DVL, two mannose-binding lectins derived from Brazilian flora, hold great promise as antiviral agents against SARS-CoV-2. Their unique chemical properties, including carbohydrate-binding specificity, oligomerization state, and glycan-binding sites, enable them to interact with the viral spike protein and potentially inhibit viral entry into host cells. These lectins represent a fascinating avenue of research in the ongoing battle against the COVID-19 pandemic, offering hope for the development of innovative antiviral treatments.


reference link: https://www.mdpi.com/1999-4915/15/9/1886

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