Plant lectins: tools for the study of complex carbohydrates

Recent Insights into Glucose-Responsive Concanavalin A-Based Smart Hydrogels for Controlled Insulin Delivery

Many efforts are continuously undertaken to develop glucose-sensitive biomaterials able of controlling glucose levels in the body and self-regulating insulin delivery. Hydrogels that swell or shrink as a function of the environmental free glucose content are suitable systems for monitoring blood glucose, delivering insulin doses adapted to the glucose concentration. In this context, the development of sensors based on reversible binding to glucose molecules represents a continuous challenge. Concanavalin A (Con A) is a bioactive protein isolated from sword bean plants (Canavalia ensiformis) and contains four sugar-binding sites. The high affinity for reversibly and specifically binding glucose and mannose makes Con A as a suitable natural receptor for the development of smart glucose-responsive materials. During the last few years, Con A was used to develop smart materials, such as hydrogels, microgels, nanoparticles and films, for producing glucose biosensors or drug delivery devices. This review is focused on Con A-based materials suitable in the diagnosis and therapeutics of diabetes. A brief outlook on glucose-derived theranostics of cancer is also presented.

Roles of the structural units, glycotopes / mammalian N-glycans for Con A-glycan interactions, their codes, and their recognition factors

The binding property of Con A has been studied intensively and applied widely to glycoconjugates / glycobiology for over 80 years. However, its role and functional relationship of Con A with these mammalian structural units, glycotopes, N-glycan chains, as well as their polyvalent forms in N-glycoproteins involved in the Con A-glycan interactions have not been well defined and organized. In this study, the recognition factors involved in these interactions were analyzed by our well developed method- the enzyme linked lectinosorbent (ELLSA) and inhibition assay. Based on all the data obtained, it is concluded that Con A, as previously reported, has a relatively broad and wide recognition ability of the Manα1 → and Glcα1 → related glycans. It reacted not only strongly with yeast mannan and glycogens, but also bound well with a large number of mammalian N-glycans, including the N-glycans of rat sublingual gp (RSL), human Tamm-Horsfall glycoprotein (THGP), thyroglobulin and lactoferrin. The recognition specificity of Con A towards ligands, expressed by Molar Relative Potency (Molar R.P.), in a decreasing order is as follows: α1 → 3, α1 → 6 Mannopentaose (M5) and Biantennary N-linked core pentasaccharide (MDi) ≥ α1 → 3, α1 → 6 Mannotriose (M3) > Manα1 → 3Man (α1 → 3Mannobiose), Manα1 → 2Man (α1 → 2Mannobiose), Manα1 → 6Man (α1 → 6Mannobiose), Manα1 → 4Man (α1 → 4Mannobiose) > GlcNAcβ1 → 2Man (β1 → 2 N-Acetyl glucosamine-mannose) > Manα1 → /Glcα1 → > Man > Glc, while Gal / GalNAc were inactive. Furthermore, the Man related code system, in this study, is proposed to express by both numbers of Man and GlcNAcβ1 → branches (M3 to M9 / MMono to Penta etc.) and a table of three Manα1 → and Glcα1 → related biomasses of six recognition factors involved in the Con A-glycan interactions has also been demonstrated. These themes should be one of the most valuable advances since 1980s.

Linking aberrant glycosylation of plasma glycoproteins with progression of myelodysplastic syndromes: a study based on plasmonic biosensor and lectin array

Aberrant glycosylation of glycoproteins has been linked with various pathologies. Therefore, understanding the relationship between aberrant glycosylation patterns and the onset and progression of the disease is an important research goal that may provide insights into cancer diagnosis and new therapy development. In this study, we use a surface plasmon resonance imaging biosensor and a lectin array to investigate aberrant glycosylation patterns associated with oncohematological disease-myelodysplastic syndromes (MDS). In particular, we detected the interaction between the lectins and glycoproteins present in the blood plasma of patients (three MDS subgroups with different risks of progression to acute myeloid leukemia (AML) and AML patients) and healthy controls. The interaction with lectins from Aleuria aurantia (AAL) and Erythrina cristagalli was more pronounced for plasma samples of the MDS and AML patients, and there was a significant difference between the sensor response to the interaction of AAL with blood plasma from low and medium-risk MDS patients and healthy controls. Our data also suggest that progression from MDS to AML is accompanied by sialylation of glycoproteins and increased levels of truncated O-glycans and that the number of lectins that allow discriminating different stages of disease increases as the disease progresses.

Glycosylation-Dependent Induction of Programmed Cell Death in Murine Adenocarcinoma Cells

Altered surface glycosylation is a major hallmark of tumor cells associated with aggressive phenotype and poor prognosis. By recognizing specific carbohydrate motifs, lectins can be applied to distinguish tumor from healthy cells based on the expression of glycosylation-dependent markers. Through their ability to bind to specific carbohydrates, lectins induce cell agglutination and cross-link surface glycoproteins, thereby mediating mitogenic and death-inducing effects in various cell types. The carbohydrate-selective cytotoxic effect of lectins also enables their possible application in therapies targeting cancer cells. To clarify the intracellular pathways mediating cell death induced by a group of plant and fungal lectins, we investigated mouse adenocarcinoma MC-38 cells harboring inactive genes involved in apoptosis, necroptosis and pyroptosis. Treatment of MC-38 cells with wheat germ agglutinin, Maackia amurensis lectin I, and Aleuria aurantia lectin induced multiple cell death pathways through reactions that relied on the autophagy machinery without depending on caspase activation. Furthermore, inhibition of de novo protein synthesis by cycloheximide strongly decreased the cytotoxic response, indicating that the lectins investigated induced cell death via effector molecules that are not expressed under normal circumstances and supporting the non-apoptotic nature of cell death. The broad cytotoxic response to lectins can be beneficial for the development of combination therapies targeting tumor cells. Given that tumors acquire resistance to various cytotoxic treatments because of mutations in cell death pathways, compounds inducing broad cytotoxic responses, such as lectins, represent potent sensitizers to promote tumor cell killing.

Endolysosomal N-glycan processing is critical to attain the most active form of the enzyme acid alpha-glucosidase

Acid alpha-glucosidase (GAA) is a lysosomal glycogen-catabolizing enzyme, the deficiency of which leads to Pompe disease. Pompe disease can be treated with systemic recombinant human GAA (rhGAA) enzyme replacement therapy (ERT), but the current standard of care exhibits poor uptake in skeletal muscles, limiting its clinical efficacy. Furthermore, it is unclear how the specific cellular processing steps of GAA after delivery to lysosomes impact its efficacy. GAA undergoes both proteolytic cleavage and glycan trimming within the endolysosomal pathway, yielding an enzyme that is more efficient in hydrolyzing its natural substrate, glycogen. Here, we developed a tool kit of modified rhGAAs that allowed us to dissect the individual contributions of glycan trimming and proteolysis on maturation-associated increases in glycogen hydrolysis using in vitro and in cellulo enzyme processing, glycopeptide analysis by MS, and high-pH anion-exchange chromatography with pulsed amperometric detection for enzyme kinetics. Chemical modifications of terminal sialic acids on N-glycans blocked sialidase activity in vitro and in cellulo, thereby preventing downstream glycan trimming without affecting proteolysis. This sialidase-resistant rhGAA displayed only partial activation after endolysosomal processing, as evidenced by reduced catalytic efficiency. We also generated enzymatically deglycosylated rhGAA that was shown to be partially activated despite not undergoing proteolytic processing. Taken together, these data suggest that an optimal rhGAA ERT would require both N-glycan and proteolytic processing to attain the most efficient enzyme for glycogen hydrolysis and treatment of Pompe disease. Future studies should examine the amenability of next-generation ERTs to both types of cellular processing.

Lectins in Cervical Screening

Cervical screening in low-resource settings remains an unmet need. Lectins are naturally occurring sugar-binding glycoproteins whose binding patterns change as cancer develops. Lectins discriminate between dysplasia and normal tissue in several precancerous conditions. We explored whether lectins could be developed for cervical screening via visual inspection. Discovery work comprised lectin histochemistry using a panel of candidate lectins on fixed-human cervix tissue (high-grade cervical intraepithelial neoplasia (CIN3, n = 20) or normal (n = 20)), followed by validation in a separate cohort (30 normal, 25 CIN1, 25 CIN3). Lectin binding was assessed visually according to staining intensity. To validate findings macroscopically, near-infra red fluorescence imaging was conducted on freshly-resected cervix (1 normal, 7 CIN3), incubated with topically applied fluorescently-labelled lectin. Fluorescence signal was compared for biopsies and whole specimens according to regions of interest, identified by the overlay of histopathology grids. Lectin histochemistry identified two lectins-wheat germ agglutinin (WGA) and Helix pomatia agglutinin (HPA)-with significantly decreased binding to CIN3 versus normal in both discovery and validation cohorts. Findings at the macroscopic level confirmed weaker WGA binding (lower signal intensity) in CIN3 vs. normal for biopsies (p = 0.0308) and within whole specimens (p = 0.0312). Our findings confirm proof-of-principle and indicate that WGA could potentially be developed further as a probe for high-grade cervical disease.

Glycopolymer Brushes by Reversible Deactivation Radical Polymerization: Preparation, Applications, and Future Challenges

The cellular surface contains specific proteins, also known as lectins, that are carbohydrates receptors involved in different biological events, such as cell-cell adhesion, cell recognition and cell differentiation. The synthesis of well-defined polymers containing carbohydrate units, known as glycopolymers, by reversible deactivation radical polymerization (RDRP) methods allows the development of tailor-made materials with high affinity for lectins because of their multivalent interaction. These polymers are promising candidates for the biomedical field, namely as novel diagnostic disease markers, biosensors, or carriers for tumor-targeted therapy. Although linear glycopolymers are extensively studied for lectin recognition, branched glycopolymeric structures, such as polymer brushes can establish stronger interactions with lectins. This specific glycopolymer topology can be synthesized in a bottlebrush form or grafted to/from surfaces by using RDRP methods, allowing a precise control over molecular weight, grafting density, and brush thickness. Here, the preparation and application of glycopolymer brushes is critically discussed and future research directions on this topic are suggested.

Chitin is a functional component of the larval adhesive of barnacles

Barnacles are the only sessile crustaceans, and their larva, the cyprid, is supremely adapted for attachment to surfaces. Barnacles have a universal requirement for strong adhesion at the point of larval attachment. Selective pressure on the cyprid adhesive has been intense and led to evolution of a tenacious and versatile natural glue. Here we provide evidence that carbohydrate polymers in the form of chitin provide stability to the cyprid adhesive of Balanus amphitrite. Chitin was identified surrounding lipid-rich vesicles in the cyprid cement glands. The functional role of chitin was demonstrated via removal of freshly attached cyprids from surfaces using a chitinase. Proteomic analysis identified a single cement gland-specific protein via its association with chitin and localized this protein to the same vesicles. The role of chitin in cyprid adhesion raises intriguing questions about the evolution of barnacle adhesion, as well as providing a new target for antifouling technologies.

Nick Aldred et al. show that chitin provides stability in the cyprid adhesive of the barnacle Balanus amphitrite. They show that a single cement gland-specific protein associates with chitin, and that freshly attached cyprids can be removed from surfaces using chitinase.

Lectins for histochemical demonstration of glycans

Lectins have been proven to be invaluable reagents for the histochemical detection of glycans in cells and tissues by light and electron microscopy. This technical review deals with the conditions of tissue fixation and embedding for lectin labeling, as well as various markers and related labeling techniques. Furthermore, protocols for lectin labeling of sections from paraffin and resin-embedded tissues are detailed together with various controls to demonstrate the specificity of the labeling by lectins.

Mushroom lectins: current status and future perspectives

Lectins are nonimmune proteins or glycoproteins that bind specifically to cell surface carbohydrates, culminating in cell agglutination. These are known to play key roles in host defense system and also in metastasis. Many new sources have been explored for the occurrence of lectins during the last few years. Numerous novel lectins with unique specificities and exploitable properties have been discovered. Mushrooms have attracted a number of researchers in food and pharmaceuticals. Many species have long been used in traditional Chinese medicines or functional foods in Japan and other Asian countries. A number of bioactive constituents have been isolated from mushrooms including polysaccharides, polysaccharopeptides, polysaccharide-protein complexes, proteases, ribonucleases, ribosome inactivating proteins, antifungal proteins, immunomodulatory proteins, enzymes, lectins, etc. Mushroom lectins are endowed with mitogenic, antiproliferative, antitumor, antiviral, and immune stimulating potential. In this review, an attempt has been made to collate the information on mushroom lectins, their blood group and sugar specificities, with an emphasis on their biomedical potential and future perspectives.

Use of lectins for probing differentiated human embryonic stem cells for carbohydrates

The carbohydrates present on the surface of differentiated human embryonic stem cells (hESCs) are not yet well established. Here, we have employed a panel of lectins and several anti-carbohydrate antibodies to determine the carbohydrates that are present at day 12 of hESC differentiation as embryoid bodies (EBs). On the basis of staining with fluorescein-labeled lectins, we have determined the presence of both terminal and internally linked alpha-d-mannopyranosyl groups, poly-N-acetyllactosaminyl chains, both alpha2,3- and alpha2,6-linked N-acetylneuraminic acid (Neu5Ac), alpha1,6-linked l-fucosyl, and beta-D-galactosyl groups, and more specifically, the T, Tn, and sialyl-Tn antigens. However, no alpha1,2-linked l-fucosyl, terminal nonreducing alpha-D-galactosyl, N-acetyl-beta-D-glucosaminyl, nor N-acetyl-alpha-D-galactosaminyl groups were found by this approach. We also established the presence of Neu5Acalpha2,3/2,6-Galbeta1,4 GlcNAc-terminated chains on the surfaces of 12-day-old EBs, as indicated by the great enhancement of staining by Erythrina cristagalli agglutinin (ECA) after treatment with neuraminidase. In each case, inhibition of binding by a haptenic sugar or treatment with neuraminidase was used to eliminate the possibility of nonspecific binding of the lectins. A comparison with undifferentiated cell staining revealed an increase in alpha2,3-linked Neu5Ac as well as a change to exclusively alpha1,6-linked l-fucose upon differentiation.

Differential lectin recognition of glycoproteins in choanomastigote-shaped trypanosomatids: taxonomic implications

The glycoprotein profiles of seven choanomastigote-shaped trypanosomatids (six Crithidia spp. and one Herpetomonas sp.), which have been suggested to form three distinct taxonomic groups (Crithidia, Angomonas and Strigomonas), were analyzed by Western blotting using the lectins Limax flavus (LFA), Sambucus nigra (SNA) and Maackia amurensis (MAA), which specifically recognize sialic acid residues, and concanavalin A (Con A) that recognizes mannose-like residues in glycoconjugates. All lectins showed a sugar-inhibited recognition with the parasite extracts, with the exception of LFA, which did not show any reactivity with the studied species. The SNA agglutinin presented a characteristic and specific pattern for each taxonomic group. The MAA lectin showed an identical profile for all species analyzed, while Con A grouped the choanomastigote-shaped species in two different patterns, one specific for the Angomonas group, and the other comprehending both Strigomonas and Crithidia groups. These results illustrate the heterogeneity of the genus Crithidia. The possible taxonomic redistribution of the choanomastigote-shaped trypanosomatids is also discussed.

Detection of sialoglycomolecules in five plant trypanosomatids and in an insect phytophagous isolate

The sialoglycoprotein profiles of five plant trypanosomatids (Phytomonas spp.) and of one flagellate (Herpetomonas sp.) isolated from the salivary gland of a phytophagous insect (Phthia picta) were analyzed by Western blotting using three distinct lectins (LFA, SNA and MAA), which recognize specifically sialic acid residues in glycoconjugates. All six flagellates presented at least one polypeptide recognized by the lectins, with the exception of Phytomonas françai, which did not show any reactivity with SNA agglutinin. Phytomonas serpens and P. françai showed the most distinct pattern of sialoglycoproteins. Phytomonas mcgheei, Herpetomonas sp. and the two other Phytomonas spp., isolated from latex, displayed an identical sialomolecule profile. We discuss the possible role of the sialoglycoproteins in the physiology of these trypanosomatids.

Screening of a unique lectin from 16 cultivable mushrooms with hybrid glycoprotein and neoproteoglycan probes and purification of a novel N-acetylglucosamine-specific lectin from Oudemansiella platyphylla fruiting body

Hybrid glycoprotein and neoproteoglycan probes were prepared by coupling various glycoproteins or polysaccharides to peroxidase or biotinyl bovine serum albumin, respectively. Lectins recognizable by the neoglycoconjugate probes were extracted from 16 cultivable mushrooms. Dot-blot assay revealed five extracts to be reactive with only hybrid glycoprotein probes, but others also reacted with neoproteoglycan probes. According to the reactivity pattern with probe screening, the one lectin from Oudemansiella platyphylla extract (OPL) bound best with asialotransferrin-- and asialoagalactotransferrin--peroxidase probes and was isolated using an asialotransferrin column, but it did not bind with other hybrid glycoprotein or neoproteoglycan probes. OPL, consisting of two polypeptides with high homology in the N-terminal amino acid sequences, exhibited weak hemagglutinating activity. Purified OPL specifically bound the beta-GlcNAc probe among various biotinylated polymeric sugar probes, while it exhibited essentially the same binding specificity toward neoglycoconjugate probes as that of the crude extract, showing a preference for the asialobiantennary complex type of N-linked glycans. These results indicate that the neoglycoconjugate probes are valuable in lectin screening.

Characterization of the carbohydrate binding specificity and kinetic parameters of lectins by using surface plasmon resonance

An accurate, rapid, and sensitive method for characterizing the carbohydrate binding properties of lectins using a BIAcore apparatus and the detection method of surface plasmon resonance is described. As a model study, the sialic acid binding lectins from Sambucus nigra and Maackia amurensis, which are specific for the epitopes Neu5Ac(alpha2-6)Gal and Neu5Ac(alpha2-3)Gal, respectively, were chosen as suitable candidates. Two systems, one for the analysis of oligosaccharides and the other for glycoproteins, were developed after a rigorous analysis and evaluation of such parameters as binding conditions, buffers, and regeneration conditions. The systems take into account nonspecific binding, using the respective denatured lectin as negative blank, and avoid loss of activity: regeneration of the surface using either 10 mM NaOAc (pH 4.3) buffer (oligosaccharide system) or 20 mM HCl (glycoprotein system). The specificity of the lectins is well illustrated, while the kinetics parameters are shown to be sensitive to subtle changes in the recognized epitopes, and to be affected by steric hindrance. Surface plasmon resonance is a suitable technique for the analysis and characterization of lectins.