Internalization of a sunflower mannose-binding lectin into phytopathogenic fungal cells induces cytotoxicity

Lectins CGL and MTL, representatives of mytilectin family, exhibit different antiproliferative activity in Burkitt's lymphoma cells

Lectins are carbohydrate-binding proteins, whose biological effects are exerted via binding to glycoconjugates expressed on the surface of cells. Exposure to lectins can lead not only to a change in the structure and properties of cells but also to their death. Here, we studied the biological activity of lectins from the mussels Crenomytilus graynus (CGL) and Mytilus trossulus (MTL) and showed that these proteins can affect the proliferation of human lymphoma cells. Both lectins suppressed the formation of colonies as well as cell cycle progression. The mechanism of action of these lectins was not mediated by reactive oxygen species but included damaging of mitochondria, inhibition of key cell cycle points, and activation of MAPK signaling pathway in tumor cells. Computer modeling suggested that various effects of CGL and MTL on lymphoma cells may be due to the difference in the energy of binding of these lectins to carbohydrate ligands on the cell surface. Thus, molecular recognition of residues of terminal carbohydrates on the surface of tumor cells is a key factor in the manifestation of the biological action of lectins.

The sunflower jacalin Helja: biological and structural insights of its antifungal activity against Candida albicans

The limited availability of efficient treatments for Candida infections and the increased emergence of antifungal-resistant strains stimulates the search for new antifungal agents. We have previously isolated a sunflower mannose-binding lectin (Helja) with antifungal activity against Candida albicans, capable of binding mannose-bearing oligosaccharides exposed on the cell surface. This work aimed to investigate the biological and biophysical basis of Helja's binding to C. albicans cell wall mannans and its influence on the fungicidal activity of the lectin. We evaluated the interaction of Helja with the cell wall mannans extracted from the isogenic parental strain (WT) and a glycosylation-defective C. albicans with altered cell wall phosphomannosylation (mnn4∆ null mutants) and investigated its antifungal effect. Helja exhibited stronger antifungal activity on the mutant strain, showing greater inhibition of fungal growth, loss of cell viability, morphological alteration, and formation of clusters with agglutinated cells. This differential biological activity of Helja was correlated with the biophysical parameters determined by solid phase assays and isothermal titration calorimetry, which demonstrated that the lectin established stronger interactions with the cell wall mannans of the mnn4∆ null mutant than with the WT strain. In conclusion, our results provide new evidence on the nature of the Helja molecular interactions with cell wall components, i.e. phosphomannan, and its impact on the antifungal activity. This study highlights the relevance of plant lectins in the design of effective antifungal therapies.

Antimicrobial Studies on Garlic Lectin

Allium sativum agglutinin (ASA) is an important lectin isolated from garlic bulbs and has shown promising therapeutic potential in earlier reports. It has a bulb-type lectin domain, and members of this protein family have been investigated for anti-cancer, antimicrobial and other effects. In our earlier study, we have reported ASA as an anti-cancer agent, and in the present study, we have evaluated it for its antifungal and antimicrobial effects. The effects of ASA on the opportunistic pathogens in humans Candida auris and Candida glabrata fungal strains have been evaluated, and efforts are made to evaluate the mechanistic basis of these antifungal effects. The antifungal activity of ASA on different strains of C. glabrata and C. auris was found with MIC50 concentration range of 30-70 µg/ml. Fungal growth was significantly suppressed upon treatment with ASA at MIC50 and 2MIC50. Hydrogen peroxide production was detected after ASA treatment in fungal cells and cell morphology, and integrity was affected when analysed through FE-SEM. Further, the anti-biofilm effect of ASA was investigated against Candida and three bacterial strains (Escherichia coli, Staphylococcus aureus and Klebsiella pneumoniae), and promising results were obtained with maximal effect in case of K. pneumoniae among the bacterial strains. These results can form the basis for the development of ASA as antimicrobial agent.

DVL, lectin from Dioclea violacea seeds, has multiples mechanisms of action against Candida spp via carbohydrate recognition domain

Lectins are proteins of non-immunological origin with the ability to bind to carbohydrates reversibly. They emerge as an alternative to conventional antifungals, given the ability to interact with carbohydrates in the fungal cell wall inhibiting fungal growth. The lectin from D. violacea (DVL) already has its activity described as anti-candida in some species. Here, we observed the anti-candida effect of DVL on C. albicans, C. krusei and C. parapsilosis and its multiple mechanisms of action toward the yeasts. Additionally, it was observed that DVL induces membrane and cell wall damage and ROS overproduction. DVL was also able to cause an imbalance in the redox system of the cells, interact with ergosterol, inhibit ergosterol biosynthesis, and induce cytochrome c release from the mitochondrial membrane. These results endorse the potential application of DVL in developing a new antifungal drug to fight back against fungal resistance.

LSPpred Suite: Tools for Leaderless Secretory Protein Prediction in Plants

Plant proteins that are secreted without a classical signal peptide leader sequence are termed leaderless secretory proteins (LSPs) and are implicated in both plant development and (a)biotic stress responses. In plant proteomics experimental workflows, identification of LSPs is hindered by the possibility of contamination from other subcellar compartments upon purification of the secretome. Applying machine learning algorithms to predict LSPs in plants is also challenging due to the rarity of experimentally validated examples for training purposes. This work attempts to address this issue by establishing criteria for identifying potential plant LSPs based on experimental observations and training random forest classifiers on the putative datasets. The resultant plant protein database LSPDB and bioinformatic prediction tools LSPpred and SPLpred are available at lsppred.lspdb.org. The LSPpred and SPLpred modules are internally validated on the training dataset, with false positives controlled at 5%, and are also able to classify the limited number of established plant LSPs (SPLpred (3/4, LSPpred 4/4). Until such time as a larger set of bona fide (independently experimentally validated) LSPs is established using imaging technologies (light/fluorescence/electron microscopy) to confirm sub-cellular location, these tools represent a bridging method for predicting and identifying plant putative LSPs for subsequent experimental validation.

Lectins: Biological significance to biotechnological application

Lectins are a set of non-enzymatic carbohydrate binding proteins appearing in all domains of life. They function to recognize, interact and bring about reversible binding of a specific sugar moiety present in a molecule. Since glycans are ubiquitous in nature and are an essential part of various biological process, the lectins are been investigated to understand the profile of these versatile but complex glycan molecule. The knowledge gained can be used to explore and streamline the various mechanisms involving glycans and their conjugates. Thus, lectins have gained importance in carbohydrate-protein interactions contributing to the development in the field of glycobiology. This has led to a deeper understanding of the importance of saccharide recognition in life. Since their discovery, the lectins have become a great choice of research in the field of glycobiology and their biological significances have recently received considerable attention in the biocontrol field as well as medical sectors.

Arabidopsis GDSL1 overexpression enhances rapeseed Sclerotinia sclerotiorum resistance and the functional identification of its homolog in Brassica napus

Sclerotinia stem rot (SSR) caused by Sclerotinia sclerotiorum is a devastating disease of rapeseed (Brassica napus L.). To date, the genetic mechanisms of rapeseed' interactions with S. sclerotiorum are not fully understood, and molecular-based breeding is still the most effective control strategy for this disease. Here, Arabidopsis thaliana GDSL1 was characterized as an extracellular GDSL lipase gene functioning in Sclerotinia resistance. Loss of AtGDSL1 function resulted in enhanced susceptibility to S. sclerotiorum. Conversely, overexpression of AtGDSL1 in B. napus enhanced resistance, which was associated with increased reactive oxygen species (ROS) and salicylic acid (SA) levels, and reduced jasmonic acid levels. In addition, AtGDSL1 can cause an increase in lipid precursor phosphatidic acid levels, which may lead to the activation of downstream ROS/SA defence-related pathways. However, the rapeseed BnGDSL1 with highest sequence similarity to AtGDSL1 had no effect on SSR resistance. A candidate gene association study revealed that only one AtGDSL1 homolog from rapeseed, BnaC07g35650D (BnGLIP1), significantly contributed to resistance traits in a natural B. napus population, and the resistance function was also confirmed by a transient expression assay in tobacco leaves. Moreover, genomic analyses revealed that BnGLIP1 locus was embedded in a selected region associated with SSR resistance during the breeding process, and its elite allele type belonged to a minor allele in the population. Thus, BnGLIP1 is the functional equivalent of AtGDSL1 and has a broad application in rapeseed S. sclerotiorum-resistance breeding.

Effects of the binding of a Helianthus annuus lectin to Candida albicans cell wall on biofilm development and adhesion to host cells

BACKGROUND

In our previous study, we isolated and characterized a lectin called Helja from Helianthus annuus (sunflower) and then, in a further study, demonstrated its antifungal activity against Candida spp. Since Candida infections are a major health concern due to the increasing emergence of antifungal resistant strains, the search for new antifungal agents offers a promising opportunity for improving the treatment strategies against candidiasis.

PURPOSE

The aim of this work was to get insights about the mechanism of action of Helja, an antifungal lectin of H. annuus, and to explore its ability to inhibit Candida albicans biofilm development and adherence to buccal epithelial cells (BEC).

STUDY DESIGN/METHODS

Yeast viability was evaluated by Evans Blue uptake and counting of colony forming units (CFU). The yeast cell integrity was assessed using Calcofluor White (CFW) as a cell wall perturbing agent and sorbitol as osmotic protectant. The induction of oxidative stress was evaluated using 3,3'-diaminobenzidine (DAB) for detection of hydrogen peroxide. The adherence was determined by counting the yeast cells attached to BEC after methylene blue staining. The biofilms were developed on polystyrene microplates, visualized by confocal laser scanning microscopy and the viable biomass was quantified by CFU counting. The binding lectin-Candida was assessed using Helja conjugated to fluorescein isothiocyanate (Helja-FITC) and simultaneous staining with CFW. The cellular surface hydrophobicity (CSH) was determined using a microbial adhesion to hydrocarbons method.

RESULTS

C. albicans cells treated with 0.1 µg/µl of Helja showed a drastic decrease in yeast survival. The lectin affected the fungal cell integrity, induced the production of hydrogen peroxide and inhibited the morphological transition from yeast to filamentous forms. Helja caused a significant reduction of adherent cells and a decrease in biofilm biomass and coverage area. The treatment with the protein also reduced the surface hydrophobicity of fungal cells. We show the binding of Helja-FITC to yeast cells distributed as a thin outer layer to the CFW signal, and this interaction was displaced by mannose and Concanavalin A.

CONCLUSION

The results demonstrate the interaction of Helja with the mannoproteins of C. albicans cell wall, the disruption of the cell integrity, the induction of oxidative stress, the inhibition of the morphological transition from yeast to filamentous forms and the fungal cell viability loss. The binding Helja-Candida also provides a possible explanation of the lectin effect on cell adherence, biofilm development and CSH, relevant features related to virulence of the pathogen.

Histochemical localization of N-acetylhexosamine-binding lectin HOL-18 in Halichondria okadai (Japanese black sponge), and its antimicrobial and cytotoxic anticancer effects

We studied localization and physiological activities of a lectin showing specific binding to N-acetylhexosamines, termed HOL-18, purified from Japanese black sponge (Halichondria okadai). Antiserum against the lectin was generated in rabbit and applied for immunohistochemical analyses. HOL-18 was expressed specifically around water pores and on spicules of sponge tissues. It showed strong binding to a variety of N-acetylhexosamines: N-acetyl D-glucosamine, N-acetyl D-galactosamine, N-acetyl mannosamine, N-acetyl muramic acid, and N-acetyl neuraminic acid. Hemagglutination induced by the lectin was inhibited by lipopolysaccharides and a peptidoglycan. HOL-18 inhibited growth of a gram-positive bacterium (Listeria monocytogenes), gram-negative bacteria (Escherichia coli, Shigella boydii, Pseudomonas aeruginosa), and a fungus (Aspergillus niger). It displayed anti-biofilm activity against P. aeruginosa. HOL-18 was internalized into conidiophores of A. niger, and displayed notable antifungal activity. Fluorescence microscopy revealed binding and incorporation of the lectin into human cancer cell lines HeLa, MCF-7, and T47D, but not Caco-2. HOL-18 displayed dose-dependent cytotoxic effects against HeLa, MCF-7, and T47D, with respective IC₅₀ values 40, 52, and 63 μg/mL. In HeLa cells, it activated phosphorylation of MAPK pathway molecule (ERK_1/2) and activated caspase-3 to trigger apoptosis. HOL-18 thus has the potential to upregulate metabolic pathways in higher animal cells through binding to N-acetylhexosamines.