Microbial lectins and their prospective mitogenic potential

Microbial lectins as a potential therapeutics for the prevention of certain human diseases

Lectins are protein or glycoprotein molecules with a specific ability to bind to carbohydrates. From viruses to mammals, they are found in various organisms and exhibit remarkable diverse structures and functions. They are significant contributors to defense mechanisms against microbial attacks in plants. They are also involved in functions such as controlling lymphocyte migration, regulating glycoprotein biosynthesis, cell-cell recognition, and embryonic development in animals. In addition, lectins serve as invaluable molecular tools in various biological and medical disciplines due to their reversible binding ability and enable the monitoring of cell membrane changes in physiological and pathological contexts. Microbial lectins, often referred to as adhesins, play an important role in microbial colonization, pathogenicity, and interactions among microorganisms. Viral lectins are located in the bilayered viral membrane, whereas bacterial lectins are found intracellularly and on the bacterial cell surface. Microfungal lectins are typically intracellular and have various functions in host-parasite interaction, and in fungal growth and morphogenesis. Although microbial lectin studies are less extensive than those of plants and animals, they provide insights into the infection mechanisms and potential interventions. Glycan specificity, essential functions in infectious diseases, and applications in the diagnosis and treatment of viral and bacterial infections are critical aspects of microbial lectin research. In this review, we will discuss the application and therapeutic potential of viral, bacterial and microfungal lectins.

Recent Progress in Human Milk Oligosaccharides and Its Antiviral Efficacy

Gastrointestinal (GI)-associated viruses, including rotavirus (RV), norovirus (NV), and enterovirus, usually invade host cells, transmit, and mutate their genetic information, resulting in influenza-like symptoms, acute gastroenteritis, encephalitis, or even death. The unique structures of human milk oligosaccharides (HMOs) enable them to shape the gut microbial diversity and endogenous immune system of human infants. Growing evidence suggests that HMOs can enhance host resistance to GI-associated viruses but without a systematic summary to review the mechanism. The present review examines the lactose- and neutral-core HMOs and their antiviral effects in the host. The potential negative impacts of enterovirus 71 (EV-A71) and other GI viruses on children are extensive and include neurological sequelae, neurodevelopmental retardation, and cognitive decline. However, the differences in the binding affinity of HMOs for GI viruses are vast. Hence, elucidating the mechanisms and positive effects of HMOs against different viruses may facilitate the development of novel HMO derived oligosaccharides.

Sclerotinia sclerotiorum Agglutinin Modulates Sclerotial Development, Pathogenicity and Response to Abiotic and Biotic Stresses in Different Manners

Sclerotinia sclerotiorum is an important plant pathogenic fungus of many crops. Our previous study identified the S. sclerotiorum agglutinin (SSA) that can be partially degraded by the serine protease CmSp1 from the mycoparasite Coniothyrium minitans. However, the biological functions of SSA in the pathogenicity of S. sclerotiorum and in its response to infection by C. minitans, as well as to environmental stresses, remain unknown. In this study, SSA disruption and complementary mutants were generated for characterization of its biological functions. Both the wild-type (WT) of S. sclerotiorum and the mutants were compared for growth and sclerotial formation on potato dextrose agar (PDA) and autoclaved carrot slices (ACS), for pathogenicity on oilseed rape, as well as for susceptibility to chemical stresses (NaCl, KCl, CaCl₂, sorbitol, mannitol, sucrose, sodium dodecyl sulfate, H₂O₂) and to the mycoparasitism of C. minitans. The disruption mutants (ΔSSA-175, ΔSSA-178, ΔSSA-225) did not differ from the WT and the complementary mutant ΔSSA-178C in mycelial growth. However, compared to the WT and ΔSSA-178C, the disruption mutants formed immature sclerotia on PDA, and produced less but larger sclerotia on ACS; they became less sensitive to the eight investigated chemical stresses, but more aggressive in infecting leaves of oilseed rape, and more susceptible to mycoparasitism by C. minitans. These results suggest that SSA positively regulates sclerotial development and resistance to C. minitans mycoparasitism, but negatively regulates pathogenicity and resistance to chemical stresses.

Pseudomonas aeruginosa LecB suppresses immune responses by inhibiting transendothelial migration

Pseudomonas aeruginosa is a Gram-negative bacterium causing morbidity and mortality in immuno-compromised humans. It produces a lectin, LecB, that is considered a major virulence factor, however, its impact on the immune system remains incompletely understood. Here we show that LecB binds to endothelial cells in human skin and mice and disrupts the transendothelial passage of leukocytes in vitro. It impairs the migration of dendritic cells into the paracortex of lymph nodes leading to a reduced antigen-specific T cell response. Under the effect of the lectin, endothelial cells undergo profound cellular changes resulting in endocytosis and degradation of the junctional protein VE-cadherin, formation of an actin rim, and arrested cell motility. This likely negatively impacts the capacity of endothelial cells to respond to extracellular stimuli and to generate the intercellular gaps for allowing leukocyte diapedesis. A LecB inhibitor can restore dendritic cell migration and T cell activation, underlining the importance of LecB antagonism to reactivate the immune response against P. aeruginosa infection.

Heat shock transcription factor (Hsf) gene family in common bean (Phaseolus vulgaris): genome-wide identification, phylogeny, evolutionary expansion and expression analyses at the sprout stage under abiotic stress

BACKGROUND

Common bean (Phaseolus vulgaris) is an essential crop with high economic value. The growth of this plant is sensitive to environmental stress. Heat shock factor (Hsf) is a family of antiretroviral transcription factors that regulate plant defense system against biotic and abiotic stress. To date, few studies have identified and bio-analyzed Hsfs in common bean.

RESULTS

In this study, 30 Hsf transcription factors (PvHsf1-30) were identified from the PFAM database. The PvHsf1-30 belonged to 14 subfamilies with similar motifs, gene structure and cis-acting elements. The Hsf members in Arabidopsis, rice (Oryza sativa), maize (Zea mays) and common bean were classified into 14 subfamilies. Collinearity analysis showed that PvHsfs played a role in the regulation of responses to abiotic stress. The expression of PvHsfs varied across different tissues. Moreover, quantitative real-time PCR (qRT-PCR) revealed that most PvHsfs were differentially expressed under cold, heat, salt and heavy metal stress, indicating that PvHsfs might play different functions depending on the type of abiotic stress.

CONCLUSIONS

In this study, we identified 30 Hsf transcription factors and determined their location, motifs, gene structure, cis-elements, collinearity and expression patterns. It was found that PvHsfs regulates responses to abiotic stress in common bean. Thus, this study provides a basis for further analysis of the function of PvHsfs in the regulation of abiotic stress in common bean.

Lectins purified from medicinal and edible mushrooms: Insights into their antiviral activity against pathogenic viruses

For thousands of years, fungi have been a valuable and promising source of therapeutic agents for treatment of various diseases. Mushroom is a macrofungus which has been cultivated worldwide for its nutritional value and medicinal applications. Several bioactive molecules were extracted from mushroom such as polysaccharides, lectins and terpenoids. Lectins are carbohydrate-binding proteins with non-immunologic origin. Lectins were classified according to their structure, origin and sugar specificity. This protein has different binding specificity with surface glycan moiety which determines its activity and therapeutic applications. A wide range of medicinal activities such as antitumor, antiviral, antimicrobial, immunomodulatory and antidiabetic were reported from sugar-binding proteins. However, glycan-binding protein from mushroom is not well explored as antiviral agent. The discovery of novel antiviral agents is a public health emergency to overcome the current pandemic and be ready for the upcoming viral pandemics. The mechanism of action of lectin against viruses targets numerous steps in viral life cycle such as viral attachment, entry and replication. This review described the history, classification, purification techniques, structure-function relationship and different therapeutic applications of mushroom lectin. In addition, we focus on the antiviral activity, purification and physicochemical characteristics of some mushroom lectins.

Targeted disruption of pi-pi stacking in Malaysian banana lectin reduces mitogenicity while preserving antiviral activity

Lectins, carbohydrate-binding proteins, have been regarded as potential antiviral agents, as some can bind glycans on viral surface glycoproteins and inactivate their functions. However, clinical development of lectins has been stalled by the mitogenicity of many of these proteins, which is the ability to stimulate deleterious proliferation, especially of immune cells. We previously demonstrated that the mitogenic and antiviral activities of a lectin (banana lectin, BanLec) can be separated via a single amino acid mutation, histidine to threonine at position 84 (H84T), within the third Greek key. The resulting lectin, H84T BanLec, is virtually non-mitogenic but retains antiviral activity. Decreased mitogenicity was associated with disruption of pi-pi stacking between two aromatic amino acids. To examine whether we could provide further proof-of-principle of the ability to separate these two distinct lectin functions, we identified another lectin, Malaysian banana lectin (Malay BanLec), with similar structural features as BanLec, including pi-pi stacking, but with only 63% amino acid identity, and showed that it is both mitogenic and potently antiviral. We then engineered an F84T mutation expected to disrupt pi-pi stacking, analogous to H84T. As predicted, F84T Malay BanLec (F84T) was less mitogenic than wild type. However, F84T maintained strong antiviral activity and inhibited replication of HIV, Ebola, and other viruses. The F84T mutation disrupted pi-pi stacking without disrupting the overall lectin structure. These findings show that pi-pi stacking in the third Greek key is a conserved mitogenic motif in these two jacalin-related lectins BanLec and Malay BanLec, and further highlight the potential to rationally engineer antiviral lectins for therapeutic purposes.

Plant-Derived Lectins as Potential Cancer Therapeutics and Diagnostic Tools

Cancer remains a global health challenge, with high morbidity and mortality, despite the recent advances in diagnosis and treatment. Multiple compounds assessed as novel potential anticancer drugs derive from natural sources, including microorganisms, plants, and animals. Lectins, a group of highly diverse proteins of nonimmune origin with carbohydrate-binding abilities, have been detected in virtually all kingdoms of life. These proteins can interact with free and/or cell surface oligosaccharides and might differentially bind cancer cells, since malignant transformation is tightly associated with altered cell surface glycans. Therefore, lectins could represent a valuable tool for cancer diagnosis and be developed as anticancer therapeutics. Indeed, several plant lectins exert cytotoxic effects mainly by inducing apoptotic and autophagic pathways in malignant cells. This review summarizes the current knowledge regarding the basis for the use of lectins in cancer diagnosis and therapy, providing a few examples of plant-derived carbohydrate-binding proteins with demonstrated antitumor effects.

Sclerotium rolfsii lectin induces opposite effects on normal PBMCs and leukemic Molt-4 cells by recognising TF antigen and its variants as receptors

Sclerotium rolfsii lectin (SRL) exerts apoptotic effect against various cancer cells and an antitumor activity on mice with colon and breast cancer xenografts. The current study aimed to explore its exquisite carbohydrate specificity on human peripheral blood mononuclear cells (PBMCs) and leukemic T-cells. SRL, showed strong binding (>98%) to resting/activated PBMCs, leukemic Molt-4 and Jurkat cell lines. The glycans mediated binding to these cells was effectively blocked by mucin and fetuin, exhibiting 97% and 94% inhibition respectively. SRL showed mitogenic stimulation of PBMCs at 10 μg/ml as determined by thymidine incorporation assay. In contrast, lectin induced a dose dependent growth inhibition of Molt-4 cells with 58% inhibition at 25 μg/ml. Many common membrane receptors in activated PBMCs, Molt 4 and Jurkat cells were identified by lectin blotting. However, membrane receptors that are recognized by SRL in normal resting PBMCs were totally different and are high molecular weight glycoproteins. Treatment of membrane receptors with glycosidases prior to lectin probing, revealed that fucosylated Thomsen-Friedenreich(TF) antigen glycans are increasingly expressed on transformed Molt-4 leukemic cells compared to other cells. The findings highlight the opposite effects of SRL on transformed and normal hematopoietic cells by recognizing different glycan-receptors. SRL has promising potential for diagnostics and therapeutic applications in leukaemia.

Purification of a potent mitogenic homodimeric Penicillium griseoroseum lectin and its characterisation

Penicillium griseoroseum lectin was 80-fold purified by successive DEAE Sepharose anion exchange and Sephadex G-100 gel permeation chromatography. P. griseoroseum lectin exhibited haemagglutination activity towards protease-treated rabbit erythrocytes. It showed specificity towards various carbohydrates such as d-mannose, N-acetyl-d-glucosamine, mucins, and so forth. P. griseoroseum lectin was found as a glycoprotein with glycan content of 4.33%. Purified P. griseoroseum lectin is homodimeric having a molecular mass of 57 kDa with subunit molecular mass of 28.6 kDa. Haemagglutination activity of purified P. griseoroseum lectin was completely stable from 25°C to 35°C at a pH range of 6-7.5. Lectin activity was not influenced by divalent metal ions and denaturants. P. griseoroseum lectin manifested mitogenicity towards mice splenocytes and activity reached a peak at 75 μg/ml of lectin concentration. P. griseoroseum lectin in microgram concentrations stimulated proliferation of mice splenocytes. Thus, P. griseoroseum lectin exhibits potential mitogenicity, which can be exploited for further biomedical applications.

The lectin domain containing proteins with mucosal immunity and digestive functions in oyster Crassostrea gigas

Lectins are carbohydrate-binding proteins with lectin domains, which are extensively studied for their numerous roles in biological recognition. However, the lectin domain containing proteins (LDCPs) chimerized with other non-lectin domains have not received sufficient attention. In the present study, a genome-wide survey of LDCPs in oyster Crassostrea gigas was conducted, and an expansive 640 LDCPs derived from ten lectin domains were identified and functionally explored. In these LDCPs, a total of 282 kinds of domains were predicted, and 90% of the LDCPs contained more than one kind of domain. The lectin domains were frequently fused with non-lectin domains, such as epidermal growth factor domain and peptidase related domains, which supplied LDCPs with more diversity in structures and functions. The C-type lectin domains were the most abundant domains in LDCPs, and they were largely co-existed with non-lectin domains of complement activation-related domains (such as CUB domain and PAN-1 domain) but relative independence with other lectin domains. Furthermore, the C-type lectin domain containing proteins (CTLPs) found to mainly act as pattern immune recognition receptors and were highly expressed in mucosal tissues (digestive gland, male gonad and labial palp) to provide mucosal immune protections. The Concanavalin A-like lectin domains were the second richest domains in LDCPs, and they were mostly constructed into chimeric proteins with epidermal growth factor domain and peptidase related domains. The Concanavalin A-like lectin domain containing proteins (CALPs) were significantly enriched with peptidase activities and mainly expressed in digestive tissues. All the results suggested the mucosal immunity and digestive functions of oyster LDCPs, which provided a fresh idea about the functions of invertebrate lectin family.

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.

Gallic acid binding to Spatholobus parviflorus lectin provides insight to its quaternary structure forming

Therapeutic effects of gallic acid (GA) have already been extensively studied. However, its interaction with lectins has not gained much attention. It is of interest to validate the binding profile of GA with Spatholobus parviflorus seed lectin. A combination of Isothermal Titration Calorimetry (ITC), haemagglutination assay and molecular docking was applied on SPL-GA interaction. ITC results showed four binding sites, stoichiometry, n=4, irrespective of the ratio of SPL:GA taken for titration. Difference among the four binding sites of a single molecule of SPL with regard to GA binding kinetic parameters was consistently varying. Similarly, the glide scores obtained for GA in the four different binding clefts of SPL were also conformed to the ITC. The binding of GA on SPL without affecting its sugar binding property could be considered as a boon for glycobiological research. From the presented studies, it could be proposed that the SPL-GA interactions may facilitate drug delivery by specific targeting/attachment by profiling of cell-surface glycans, followed by controlled release of drugs.

Lectin activity in mycelial extracts of Fusarium species

Lectins are non-immunogenic carbohydrate-recognizing proteins that bind to glycoproteins, glycolipids, or polysaccharides with high affinity and exhibit remarkable ability to agglutinate erythrocytes and other cells. In the present study, ten Fusarium species previously not explored for lectins were screened for the presence of lectin activity. Mycelial extracts of F. fujikuroi, F. beomiformii, F. begoniae, F. nisikadoi, F. anthophilum, F. incarnatum, and F. tabacinum manifested agglutination of rabbit erythrocytes. Neuraminidase treatment of rabbit erythrocytes increased lectin titers of F. nisikadoi and F. tabacinum extracts, whereas the protease treatment resulted in a significant decline in agglutination by most of the lectins. Results of hapten inhibition studies demonstrated unique carbohydrate specificity of Fusarium lectins toward O-acetyl sialic acids. Activity of the majority of Fusarium lectins exhibited binding affinity to d-ribose, l-fucose, d-glucose, l-arabinose, d-mannitol, d-galactosamine hydrochloride, d-galacturonic acid, N-acetyl-d-galactosamine, N-acetyl-neuraminic acid, 2-deoxy-d-ribose, fetuin, asialofetuin, and bovine submaxillary mucin. Melibiose and N-glycolyl neuraminic acid did not inhibit the activity of any of the Fusarium lectins. Mycelial extracts of F. begoniae, F. nisikadoi, F. anthophilum, and F. incarnatum interacted with most of the carbohydrates tested. F. fujikuroi and F. anthophilum extracts displayed strong interaction with starch. The expression of lectin activity as a function of culture age was investigated. Most species displayed lectin activity on the 7th day of cultivation, and it varied with progressing of culture age.

Development and Applications of Lectins as Biological Tools in Biomedical Research

As a new and burgeoning area following genomics and proteomics, glycomics has become a hot issue due to its pivotal roles in many physiological and pathological processes. Glycans are much more complicated than genes or proteins since glycans are highly branched and dynamic. Antibodies and lectins are the two major molecular tools applied for glycan profiling. Though the study of antibodies and lectins started at almost the same time in 1880s, lectins gained much less attention than the antibodies until recent decades when the importance and difficulties of glycomics were realized. The present review summarizes the discovery history of lectins and their biological functions with a special emphasis on their various applications as biological tools. Both older techniques that had been developed in the last century and new technologies developed in recent years, especially lectin microarrays and lectin-based biosensors, are included in this account.

Purification and characterization of a mucin specific mycelial lectin from Aspergillus gorakhpurensis: application for mitogenic and antimicrobial activity

BACKGROUND

Lectins are carbohydrate binding proteins or glycoproteins that bind reversibly to specific carbohydrates present on the apposing cells, which are responsible for their ability to agglutinate red blood cells, lymphocytes, fibroblasts, etc. Interest in lectins has been intensified due to their carbohydrate specificity as they can be valuable reagents for the investigation of cell surface sugars, purification and characterization of glycoproteins. The present study reports the purification, characterization and evaluation of mitogenic and antimicrobial potential of a mycelial lectin from Aspergillus gorakhpurensis.

METHODS

Affinity chromatography on mucin-sepharose column was carried out for purification of Aspergillus gorakhpurensis lectin. The lectin was characterized for physico-chemical parameters. Mitogenic potential of the lectin was evaluated against splenocytes of Swiss albino mice by MTT assay. Antimicrobial activity of the purified lectin has also been evaluated by disc diffusion assay.

RESULTS

Single-step affinity purification resulted in 18.6-fold purification of the mycelial lectin. The molecular mass of the lectin was found to be 70 kDa and it was composed of two subunits of 34.8 kDa as determined by gel filtration chromatography, SDS-PAGE and MALDI-TOF analysis. pH optima of the lectin was found to be 6.5-9.5, while optimum temperature for lectin activity was 20-30 °C. Lectin was stable within a pH range of 7.0-10.5 and showed fair thermostability. EDTA did not affect lectin activity whereas it was found susceptible to the denaturants tested. MTT assay revealed strong mitogenic potential of A. gorakhpurensis lectin at a concentration upto 150 µg/mL. Antimicrobial activity assay showed its potent antibacterial activity against Bacillus cereus, Staphylococcous aureus and Escherichia coli and marginal antifungal activity against Saccharomyces cerevisiae.

CONCLUSION

This is the first report on the mitogenic and antimicrobial potential of Aspergillus gorakhpurensis lectin. The results will provide useful guidelines for further research in clinical applications of this lectin.