Characterization of the structural determinants required for the high affinity interaction of asparagine-linked oligosaccharides with immobilized Phaseolus vulgaris leukoagglutinating and erythroagglutinating lectins

An aberrant sugar modification of BACE1 blocks its lysosomal targeting in Alzheimer's disease

The β-site amyloid precursor protein cleaving enzyme-1 (BACE1), an essential protease for the generation of amyloid-β (Aβ) peptide, is a major drug target for Alzheimer's disease (AD). However, there is a concern that inhibiting BACE1 could also affect several physiological functions. Here, we show that BACE1 is modified with bisecting N-acetylglucosamine (GlcNAc), a sugar modification highly expressed in brain, and demonstrate that AD patients have higher levels of bisecting GlcNAc on BACE1. Analysis of knockout mice lacking the biosynthetic enzyme for bisecting GlcNAc, GnT-III (Mgat3), revealed that cleavage of Aβ-precursor protein (APP) by BACE1 is reduced in these mice, resulting in a decrease in Aβ plaques and improved cognitive function. The lack of this modification directs BACE1 to late endosomes/lysosomes where it is less colocalized with APP, leading to accelerated lysosomal degradation. Notably, other BACE1 substrates, CHL1 and contactin-2, are normally cleaved in GnT-III-deficient mice, suggesting that the effect of bisecting GlcNAc on BACE1 is selective to APP. Considering that GnT-III-deficient mice remain healthy, GnT-III may be a novel and promising drug target for AD therapeutics.

Chemical Characterization of N-Linked Oligosaccharide As the Antigen Epitope Recognized by an Anti-Sperm Auto-Monoclonal Antibody, Ts4

Ts4, an anti-sperm auto-monoclonal antibody, possesses immunoreactivity to the acrosomal region of mouse epididymal spermatozoa. In addition, the mAb shows specific immunoreactivity to reproduction-related regions such as testicular germ cells and early embryo. Our qualitative study previously showed that the antigen epitope for Ts4 contained a N-linked common oligosaccharide (OS) chain on testicular glycoproteins as determined by Western blotting for testicular glycoproteins after treatment with several glycohydrolases. Since the distribution of the Ts4-epitope is unique, the OS chain in Ts4-epitope may have role(s) in the reproductive process. The aim of this study was to clarify the molecular structure of the Ts4-epitope, particularly its OS moiety. Using Ts4 immunoprecipitation combined with liquid chromatography and multiple-stage mass spectrometry, the candidate carbohydrate structure in the Ts4-epitope is proposed to be N-linked fucosylated agalacto-biantennary with bisecting N-acetylglucosamine (GlcNAc) or with N-acetylgalactosamine-GlcNAc motif. Further binding analyses using various lectins against the mouse testicular Ts4-immunoprecipitants revealed that Phaseolus vulgaris erythroagglutinin and Pisum sativum agglutinin showed positive staining of the bands corresponding to Ts4 reactive proteins. Moreover, the immunoreactivity of Ts4 against the testicular extract was completely abrogated after digestion with β-N-acetylglucosaminidase. These results show that the Ts4-epitope contains agalacto-biantennary N-glycan with bisecting GlcNAc carrying fucose residues.

Glycoengineering of Chinese hamster ovary cells for enhanced erythropoietin N-glycan branching and sialylation

Sialic acid, a terminal residue on complex N-glycans, and branching or antennarity can play key roles in both the biological activity and circulatory lifetime of recombinant glycoproteins of therapeutic interest. In order to examine the impact of glycosyltransferase expression on the N-glycosylation of recombinant erythropoietin (rEPO), a human α2,6-sialyltransferase (ST6Gal1) was expressed in Chinese hamster ovary (CHO-K1) cells. Sialylation increased on both EPO and CHO cellular proteins as observed by SNA lectin analysis, and HPLC profiling revealed that the sialic acid content of total glycans on EPO increased by 26%. The increase in sialic acid content was further verified by detailed profiling of the N-glycan structures using mass spectra (MS) analysis. In order to enhance antennarity/branching, UDP-N-acetylglucosamine: α-1,3-D-mannoside β1,4-N-acetylglucosaminyltransferase (GnTIV/Mgat4) and UDP-N-acetylglucosamine:α-1,6-D-mannoside β1,6-N-acetylglucosaminyltransferase (GnTV/Mgat5), was incorporated into CHO-K1 together with ST6Gal1. Tri- and tetraantennary N-glycans represented approximately 92% of the total N-glycans on the resulting EPO as measured using MS analysis. Furthermore, sialic acid content of rEPO from these engineered cells was increased ∼45% higher with tetra-sialylation accounting for ∼10% of total sugar chains compared to ∼3% for the wild-type parental CHO-K1. In this way, coordinated overexpression of these three glycosyltransferases for the first time in model CHO-K1 cell lines provides a mean for enhancing both N-glycan branching complexity and sialylation with opportunities to generate tailored complex N-glycan structures on therapeutic glycoproteins in the future.

Prognostic significance of β1,6-N-acetylglucosaminyltransferase V expression in patients with hepatocellular carcinoma

OBJECTIVE

Alterations to the N-glycans in glycoproteins have been suggested to play important roles in the proliferation, differentiation, invasion and metastasis of hepatocellular carcinoma (HCC). This study aims to evaluate the potential prognostic value of β1,6-N-acetylglucosaminyltransferase V (Mgat5) in hepatocellular carcinoma patients after surgical resection.

METHODS

We retrospectively enrolled 300 patients (156 in the training cohort and 144 in the validation cohort) with hepatocellular carcinoma undergoing hepatectomy at a single institution. Mgat5 intensities were assessed by immunohistochemistry in the specimens of patients. The Kaplan-Meier method was applied to compare survival curves. Cox regression models were used to analyze the impact of prognostic factors on overall survival and recurrence-free survival. The concordance index was calculated to assess predictive accuracy.

RESULTS

Intratumoral Mgat5 expression was significantly higher than non-tumoral tissues (P < 0.001). In both cohorts, elevated Mgat5 expression in tumor tissues positively correlated with vascular invasion and advanced tumor-node-metastasis stage. High Mgat5 expression indicated poor survival (P < 0.001 in the training cohort and P < 0.001 in the validation cohort) and recurrence (P < 0.001 in both cohorts, respectively) in patients with hepatocellular carcinoma, particularly with early-stage disease. Mgat5 expression was identified as an independent adverse prognostic factor for survival and recurrence. The predictive accuracy of tumor-node-metastasis and Barcelona Clinic Liver Cancer prognostic models was improved when Mgat5 expression was added.

CONCLUSION

Mgat5 expression is a potential independent adverse prognostic biomarker for recurrence and survival of patients with hepatocellular carcinoma after hepatectomy.

N-glycans and metastasis in galectin-3 transgenic mice

Poly-N-acetyl-lactosamine (polyLacNAc) on N-glycans facilitate lung specific metastasis of melanoma cells by serving as high affinity ligands for galectin-3, expressed in highest amounts in the lungs, on almost all its tissue compartments including on the surface of vascular endothelium. PolyLacNAc not only aids in initial arrest on the organ endothelium but in all the events of extravasation. Inhibition of polyLacNAc synthesis, or competitive inhibition of its interaction with galectin-3 all inhibited these processes and experimental metastasis. Transgenic galectin-3 mice, viz., gal-3(+/+) (wild type), gal-3(+/-) (hemizygous) and gal-3(-/-) (null) have been used to prove that galectin-3/polyLacNAc interactions are indeed critical for lung specific metastasis. Gal-3(+/-) mice which showed <50% expression of galectin-3 on the lungs also showed proportionate decrease in the number of B16F10 melanoma metastatic colonies affirming that galectin-3 and polyLacNAc interactions are indeed key determinants of lung metastasis. However, surprisingly, the number and size of metastatic colonies in gal-3(-/-) mice was very similar as that seen in gal-3(+/+) mice. The levels of lactose binding lectins on the lungs and the transcripts of other galectins (galectin-1, -8 and -9) which are expressed on lungs and have similar sugar binding specificities as galectins-3, remain unchanged in gal-3(+/+) and gal-3(-/-) mice. Further, inhibition of N-glycosylation with Swainsonine (SW) which drastically reduces metastasis of B16F10 cells in gal-3(+/+) mice, did not affect lung metastasis when assessed in gal-3(-/-) mice. Together, these results rule out the possibility of some other galectin taking over the function of galectin-3 in gal-3(-/-) mice. Chimeric mice generated to assess if absence of any effect on metastasis is due to compromised tumor immunity by replacing bone marrow of gal-3(-/-) mice with that from gal-3(+/+) mice, also failed to impact melanoma metastasis. As galectin-3 regulates several immune functions including maturation of different immune cells, compromised tumor immunity could be the major determinant of melanoma metastasis in gal-3(-/-) mice and warrants thorough investigation.

RNAi screens for genes involved in Golgi glycosylation

RNAi screening has gained popularity in recent years, due to its usefulness in systematic investigations of biological pathways. Combined with high-content screening and advances in imaging and analysis methods, it can enable detailed genetic characterization of cellular processes such as protein glycosylation, a major function of the Golgi apparatus. Glycosylation concerns about one third of all human proteins and regulates various cellular behaviors. Yet the methods available to study it are limited and not easily accessible. In this chapter, we detail a step-by-step method to systematically and quantitatively investigate glycosylation using fluorescent lectin staining, following high-throughput RNAi-based downregulation of gene activities. We also provide a workflow for downstream analysis of the data generated.

N-Glycans in Xenopus laevis testis characterised by lectin histochemistry

Analysis of glycan chains of glycoconjugates is difficult because of their considerable variety. Despite this, several functional roles for these glycans have been reported. N-Glycans are oligosaccharides linked to asparagine residues of proteins. They are synthesised in the endoplasmic reticulum (ER) in a unique way, and later modified in both the ER and Golgi apparatus, developing different oligosaccharide chains. An essential role for complex N-glycans in mammalian spermatogenesis has been reported. The aim of the present study was to analyse the N-glycans of the Xenopus laevis testis by means of lectin histochemistry. Five lectins were used that specifically recognise mannose-containing and complex glycans, namely Galanthus nivalis agglutinin (GNA) from snowdrops, concanavalin A (Con A) from the Jack bean, Lens culinaris agglutinin (LCA) from lentils and Phaseolus vulgaris erythroagglutinin (PHA-E) and P. vulgaris leukoagglutinin (PHA-L) from the common bean. GNA and Con A labelled the interstitium and most of the germ cell types, whereas LCA and PHA-E showed affinity only for the interstitium. A granular cytoplasmic region was labelled in spermatogonia and spermatocytes by GNA and PHA-L, whereas GNA and LCA labelled a spermatid region that is probably associated with the centriolar basal body of the nascent flagellum. There was no specific labelling in the acrosome. Some unexpected results were found when deglycosylative pretreatments were used: pre-incubation of tissue sections with peptide N glycosidase F, which removes N-linked glycans, reduced or removed labelling with most lectins, as expected. However, after this pretreatment, the intensity of labelling remained or increased for Con A in the follicle (Sertoli) and post-meiotic germ cells. The β-elimination procedure, which removes O-linked glycans, revealed new labelling patterns with GNA, LCA and PHA-L, suggesting that some N-glycans were masked by O-glycans, and thus they became accessible to these lectins only after removal of the O-linked oligosaccharides. The functional role of the glycan chains identified could be related to the role of N-glycans involved in mammalian spermatogenesis reported previously.

Detection of distinct glycosylation patterns on human γ-glutamyl transpeptidase 1 using antibody-lectin sandwich array (ALSA) technology

Background

γ-Glutamyl transpeptidase 1 (GGT1) is an N-glycosylated membrane protein that catabolizes extracellular glutathione and other γ-glutamyl-containing substrates. In a variety of disease states, including tumor formation, the enzyme is shed from the surface of the cell and can be detected in serum. The structures of the N-glycans on human GGT1 (hGGT1) have been shown to be tissue-specific. Tumor-specific changes in the glycans have also been observed, suggesting that the N-glycans on hGGT1 would be an important biomarker for detecting tumors and monitoring their progression during treatment. However, the large quantities of purified protein required to fully characterize the carbohydrate content poses a significant challenge for biomarker development. Herein, we investigated a new antibody-lectin sandwich array (ALSA) platform to determine whether this microanalytical technique could be applied to the characterization of N-glycan content of hGGT1 in complex biological samples.

Results

Our data show that hGGT1 can be isolated from detergent extracted membrane proteins by binding to the ALSA platform. Probing hGGT1 with lectins enables characterization of the N-glycans. We probed hGGT1 from normal human liver tissue, normal human kidney tissue, and hGGT1 expressed in the yeast Pichia pastoris. The lectin binding patterns obtained with the ALSA platform are consistent with the hGGT1 N-glycan composition obtained from previous large-scale hGGT1 N-glycan characterizations from these sources. We also validate the implementation of the Microcystis aeruginosa lectin, microvirin, in this platform and provide refined evidence for its efficacy in specifically recognizing high-mannose-type N-glycans, a class of carbohydrate modification that is distinctive of hGGT1 expressed by many tumors.

Conclusion

Using this microanalytical approach, we provide proof-of-concept for the implementation of ALSA in conducting high-throughput studies aimed at investigating disease-related changes in the glycosylation patterns on hGGT1 with the goal of enhancing clinical diagnoses and targeted treatment regimens.

Electronic supplementary material

The online version of this article (doi:10.1186/s12896-014-0101-0) contains supplementary material, which is available to authorized users.

Shedding of glycan-modifying enzymes by signal peptide peptidase-like 3 (SPPL3) regulates cellular N-glycosylation

Protein N-glycosylation is involved in a variety of physiological and pathophysiological processes such as autoimmunity, tumour progression and metastasis. Signal peptide peptidase-like 3 (SPPL3) is an intramembrane-cleaving aspartyl protease of the GxGD type. Its physiological function, however, has remained enigmatic, since presently no physiological substrates have been identified. We demonstrate that SPPL3 alters the pattern of cellular N-glycosylation by triggering the proteolytic release of active site-containing ectodomains of glycosidases and glycosyltransferases such as N-acetylglucosaminyltransferase V, β-1,3 N-acetylglucosaminyltransferase 1 and β-1,4 galactosyltransferase 1. Cleavage of these enzymes leads to a reduction in their cellular activity. In line with that, reduced expression of SPPL3 results in a hyperglycosylation phenotype, whereas elevated SPPL3 expression causes hypoglycosylation. Thus, SPPL3 plays a central role in an evolutionary highly conserved post-translational process in eukaryotes.

N-glycosylation bidirectionally extends the boundaries of thymocyte positive selection by decoupling Lck from Ca²⁺ signaling

Positive selection of diverse yet self-tolerant thymocytes is vital to immunity and requires a limited degree of T cell antigen receptor (TCR) signaling in response to self peptide-major histocompatibility complexes (self peptide-MHCs). Affinity of newly generated TCR for peptide-MHC primarily sets the boundaries for positive selection. We report that N-glycan branching of TCR and the CD4 and CD8 coreceptors separately altered the upper and lower affinity boundaries from which interactions between peptide-MHC and TCR positively select T cells. During thymocyte development, N-glycan branching varied approximately 15-fold. N-glycan branching was required for positive selection and decoupled Lck signaling from TCR-driven Ca(2+) flux to simultaneously promote low-affinity peptide-MHC responses while inhibiting high-affinity ones. Therefore, N-glycan branching imposes a sliding scale on interactions between peptide-MHC and TCR that bidirectionally expands the affinity range for positive selection.

Cytotoxic and antiproliferative effect of tepary bean lectins on C33-A, MCF-7, SKNSH, and SW480 cell lines

For many years, several studies have been employing lectin from vegetables in order to prove its toxic effect on various cell lines. In this work, we analyzed the cytotoxic, antiproliferative, and post-incubatory effect of pure tepary bean lectins on four lines of malignant cells: C33-A; MCF-7; SKNSH, and SW480. The tests were carried out employing MTT and 3[H]-thymidine assays. The results showed that after 24 h of lectin exposure, the cells lines showed a dose-dependent cytotoxic effect, the effect being higher on MCF-7, while C33-A showed the highest resistance. Cell proliferation studies showed that the toxic effect induced by lectins is higher even when lectins are removed, and in fact, the inhibition of proliferation continues after 48 h. Due to the use of two techniques to analyze the cytotoxic and antiproliferative effect, differences were observed in the results, which can be explained by the fact that one technique is based on metabolic reactions, while the other is based on the 3[H]-thymidine incorporated in DNA by cells under division. These results allow concluding that lectins exert a cytotoxic effect after 24 h of exposure, exhibiting a dose-dependent effect. In some cases, the cytotoxic effect is higher even when the lectins are eliminated, however, in other cases, the cells showed a proliferative effect.

Altered β1,6-GlcNAc branched N-glycans impair TGF-β-mediated epithelial-to-mesenchymal transition through Smad signalling pathway in human lung cancer

The change of oligosaccharide structure has been revealed to be crucial for glycoproteins' biological functions and cell biological characteristics. N-acetylglucosaminy transferase V (GnT-V), a key enzyme catalysing the reaction of adding β1, 6-N-acetylglucosamine (GlcNAc) on asparagine-linked oligosaccharides of cell proteins, has been implicated to a metastastic-promoting oncoprotein in some carcinomas. However, this correlation might not be subjected to all types of cancers, for example, in non-small cell lung cancers, low level of GnT-V expression is associated with relatively short survival time and poor prognosis. To explain the role of GnT-V in lung cancer progression, we studied the association of GnT-V expression with lung cancer EMT behaviour. We found that GnT-V expression was correlated with epithelial marker positively and mesenchymal marker negatively. GnT-V levels, as well as β1,6-GlcNAc branched N-glycans, were strongly reduced in TGF-β1-induced EMT of human lung adenocarcinoma A549 cells. Further studies showed that suppression of β1,6-GlcNAc branched N-glycans by inhibitor or GnT-V silencing in A549 cells could promote TGF-β1-induced EMT-like changes, cell migration and invasion. Meanwhile, overexpression of GnT-V impaired TGF-β1-induced EMT, migration and invasion. It suggests that GnT-V suppresses the EMT process of lung cancer cells through inhibiting the TGF-β/Smad signalling and its downstream transcription factors in a GnT-V catalytic activity–dependent manner. Taken together, the present study reveals a novel mechanism of GnT-V as a suppressor of both EMT and invasion in human lung cancer cells, which may be useful for fully understanding N-glycan's biological roles in lung cancer progression.

N-glycosylation of asparagine 8 regulates surface expression of major histocompatibility complex class I chain-related protein A (MICA) alleles dependent on threonine 24

NKG2D is an activating receptor expressed on several types of human lymphocytes. NKG2D ligands can be induced upon cell stress and are frequently targeted post-translationally in infected or transformed cells to avoid immune recognition. Virus infection and inflammation alter protein N-glycosylation, and we have previously shown that changes in cellular N-glycosylation are involved in regulation of NKG2D ligand surface expression. The specific mode of regulation through N-glycosylation is, however, unknown. Here we investigated whether direct N-glycosylation of the NKG2D ligand MICA itself is critical for cell surface expression and sought to identify the essential residues. We found that a single N-glycosylation site (Asn(8)) was important for MICA018 surface expression. The frequently expressed MICA allele 008, with an altered transmembrane and intracellular domain, was not affected by mutation of this N-glycosylation site. Mutational analysis revealed that a single amino acid (Thr(24)) in the extracellular domain of MICA018 was essential for the N-glycosylation dependence, whereas the intracellular domain was not involved. The HHV7 immunoevasin, U21, was found to inhibit MICA018 surface expression by affecting N-glycosylation, and the retention was rescued by T24A substitution. Our study reveals N-glycosylation as an allele-specific regulatory mechanism important for regulation of surface expression of MICA018, and we pinpoint the residues essential for this N-glycosylation dependence. In addition, we show that this regulatory mechanism of MICA surface expression is likely targeted during different pathological conditions.

The role of glycans in immune evasion: the human fetoembryonic defence system hypothesis revisited

Emerging data suggest that mechanisms to evade the human immune system may be shared by the conceptus, tumour cells, persistent pathogens and viruses. It is therefore timely to revisit the human fetoembryonic defense system (Hu-FEDS) hypothesis that was proposed in two papers in the 1990s. The initial paper suggested that glycoconjugates expressed in the human reproductive system inhibited immune responses directed against gametes and the developing human by employing their carbohydrate sequences as functional groups. These glycoconjugates were proposed to block specific binding interactions and interact with lectins linked to signal transduction pathways that modulated immune cell functions. The second article suggested that aggressive tumour cells and persistent pathogens (HIV, H. pylori, schistosomes) either mimicked or acquired the same carbohydrate functional groups employed in this system to evade immune responses. This subterfuge enabled these pathogens and tumour cells to couple their survival to the human reproductive imperative. The Hu-FEDS model has been repeatedly tested since its inception. Data relevant to this model have also been obtained in other studies. Herein, the Hu-FEDS hypothesis is revisited in the context of these more recent findings. Far more supportive evidence for this model now exists than when it was first proposed, and many of the original predictions have been validated. This type of subterfuge by pathogens and tumour cells likely applies to all sexually reproducing metazoans that must protect their gametes from immune responses. Intervention in these pathological states will likely remain problematic until this system of immune evasion is fully understood and appreciated.

Reduction in Golgi apparatus dimension in the absence of a residential protein, N-acetylglucosaminyltransferase V

Various proteins are involved in the generation and maintenance of the membrane complex known as the Golgi apparatus. We have used mutant Chinese hamster ovary (CHO) cell lines Lec4 and Lec4A lacking N-acetylglucosaminyltransferase V (GlcNAcT-V, MGAT5) activity and protein in the Golgi apparatus to study the effects of the absence of a single glycosyltransferase on the Golgi apparatus dimension. Quantification of immunofluorescence in serial confocal sections for Golgi α-mannosidase II and electron microscopic morphometry revealed a reduction in Golgi volume density up to 49 % in CHO Lec4 and CHO Lec4A cells compared to parental CHO cells. This reduction in Golgi volume density could be reversed by stable transfection of Lec4 cells with a cDNA encoding Mgat5. Inhibition of the synthesis of β1,6-branched N-glycans by swainsonine had no effect on Golgi volume density. In addition, no effect on Golgi volume density was observed in CHO Lec1 cells that contain enzymatically active GlcNAcT-V, but cannot synthesize β1,6-branched glycans due to an inactive GlcNAcT-I in their Golgi apparatus. These results indicate that it may be the absence of the GlcNAcT-V protein that is the determining factor in reducing Golgi volume density. No dimensional differences existed in cross-sectioned cisternal stacks between Lec4 and control CHO cells, but significantly reduced Golgi stack hits were observed in cross-sectioned Lec4 cells. Therefore, the Golgi apparatus dimensional change in Lec4 and Lec4A cells may be due to a compaction of the organelle.

Lectin-probed western blot analysis

Lectin-probed western blot analysis, the so-called lectin blot analysis, is a useful method to yield basic information on the glycan structures of glycoproteins, based on the carbohydrate-binding specificities of lectins. By lectin blot analysis, researchers can directly analyze the glycan structures without releasing the glycans from glycoproteins. Here, the author describes protocols for standard analysis, and applies analysis in combination with glycosidase digestion of blot.

Recognition of bisecting N-acetylglucosamine: structural basis for asymmetric interaction with the mouse lectin dendritic cell inhibitory receptor 2

Dendritic cell inhibitory receptor 2 (DCIR2) is a C-type lectin expressed on classical dendritic cells. We recently identified the unique ligand specificity of mouse DCIR2 (mDCIR2) toward biantennary complex-type glycans containing bisecting N-acetylglucosamine (GlcNAc). Here, we report the crystal structures of the mDCIR2 carbohydrate recognition domain in unliganded form as well as in complex with an agalactosylated complex-type N-glycan unit carrying a bisecting GlcNAc residue. Bisecting GlcNAc and the α1-3 branch of the biantennary oligosaccharide asymmetrically interact with canonical and non-canonical mDCIR2 residues. Ligand-protein interactions occur directly through mDCIR2-characteristic amino acid residues as well as via a calcium ion and water molecule. Our structural and biochemical data elucidate for the first time the unique binding mode of mDCIR2 for bisecting GlcNAc-containing glycans, a mode that contrasts sharply with that of other immune C-type lectin receptors such as DC-SIGN.

Galectin-3 guides intracellular trafficking of some human serotransferrin glycoforms

Transferrin internalization via clathrin-mediated endocytosis and subsequent recycling after iron delivery has been extensively studied. Here we demonstrate a previously unrecognized parameter regulating this recycling, the binding of galectin-3 to particular glycoforms of transferrin. Two fractions of transferrin, separated by affinity chromatography based on their binding or not to galectin-3, are targeted to kinetically different endocytic pathways in HFL-1 cells expressing galectin-3 but not in SKBR3 cells lacking galectin-3; the SKBR3 cells, however, can acquire the ability to target these transferrin glycoforms differently after preloading with exogenously added galectin-3. In all, this study provides the first evidence of a functional role for transferrin glycans, in intracellular trafficking after uptake. Moreover, the galectin-3-bound glycoform increased in cancer, suggesting a pathophysiological regulation. These are novel aspects of transferrin cell biology, which has previously considered only a degree of iron loading, but not other forms of heterogeneity.

Galectin-3 induces clustering of CD147 and integrin-β1 transmembrane glycoprotein receptors on the RPE cell surface

Proliferative vitreoretinopathy (PVR) is a blinding disease frequently occurring after retinal detachment surgery. Adhesion, migration and matrix remodeling of dedifferentiated retinal pigment epithelial (RPE) cells characterize the onset of the disease. Treatment options are still restrained and identification of factors responsible for the abnormal behavior of the RPE cells will facilitate the development of novel therapeutics. Galectin-3, a carbohydrate-binding protein, was previously found to inhibit attachment and spreading of retinal pigment epithelial cells, and thus bares the potential to counteract PVR-associated cellular events. However, the identities of the corresponding cell surface glycoprotein receptor proteins on RPE cells are not known. Here we characterize RPE-specific Gal-3 containing glycoprotein complexes using a proteomic approach. Integrin-β1, integrin-α3 and CD147/EMMPRIN, a transmembrane glycoprotein implicated in regulating matrix metalloproteinase induction, were identified as potential Gal-3 interactors on RPE cell surfaces. In reciprocal immunoprecipitation experiments we confirmed that Gal-3 associated with CD147 and integrin-β1, but not with integrin-α3. Additionally, association of Gal-3 with CD147 and integrin-β1 was observed in co-localization analyses, while integrin-α3 only partially co-localized with Gal-3. Blocking of CD147 and integrin-β1 on RPE cell surfaces inhibited binding of Gal-3, whereas blocking of integrin-α3 failed to do so, suggesting that integrin-α3 is rather an indirect interactor. Importantly, Gal-3 binding promoted pronounced clustering and co-localization of CD147 and integrin-β1, with only partial association of integrin-α3. Finally, we show that RPE derived CD147 and integrin-β1, but not integrin-α3, carry predominantly β-1,6-N-actyl-D-glucosamine-branched glycans, which are high-affinity ligands for Gal-3. We conclude from these data that extracellular Gal-3 triggers clustering of CD147 and integrin-β1 via interaction with β1,6-branched N-glycans on RPE cells and hypothesize that Gal-3 acts as a positive regulator for CD147/integrin-β1 clustering and therefore modifies RPE cell behavior contributing to the pathogenesis of PVR. Further investigations at this pathway may aid in the development of specific therapies for PVR.

Knockdown of N-acetylglucosaminyl transferase V ameliorates hepatotoxin-induced liver fibrosis in mice

Aberrant N-glycosylation caused by altered N-acetyl glucosaminyltransferase V (GnT-V) expression is known to regulate tumor invasion and metastasis by modulating multiple cytokine signaling pathways. However, the exact role of GnT-V in the development of liver fibrosis has not been clearly defined. Here, we induced mouse liver fibrosis by ip injections of carbon tetrachloride (CCl4) or thioacetamide (TAA) and observed significant increase of hepatic GnT-V during the processes of liver fibrogenesis. Meanwhile, upregulations of GnT-V were detected in the activated hepatic stellate cells (HSCs) and injured hepatocytes. To knock down hepatic GnT-V expression, adenovirus that expressed the GnT-V siRNA was injected via the tail vein. Adenovirus-mediated delivery of GnT-V siRNA dramatically reduced the GnT-V expression in fibrotic liver and activated HSC in vivo and consequently alleviated CCl4- or TAA-induced liver fibrosis as assessed through collagen deposition and profiles of profibrogenic markers. Furthermore, knockdown of GnT-V in HSCs reduced transforming growth factor beta (TGF-β)/Smad signaling and blunted the activated HSC phenotype. The suppression of TGF-β/Smad signaling in HSCs correlated with the decrease of GnT-V-modified β1,6-branched N-glycan on TGF-β receptors. Knockdown of GnT-V also suppressed platelet-derived growth factor (PDGF)-induced HSC proliferation and migration through inhibiting PDGF/Erk signaling. Finally, we demonstrated that knockdown of GnT-V profoundly suppressed TGF-β1-induced epithelial-mesenchymal transition (EMT) in hepatocytes by morphological assessment and reversal of EMT markers. In conclusion, this study demonstrates that GnT-V is implicated in hepatotoxin-induced liver fibrosis, and targeting GnT-V may be a feasible and promising approach for treating liver fibrosis.

Lectin microarrays: concept, principle and applications

The lectin microarray is a novel platform for glycan analysis, having emerged only in recent years. Unlike other conventional methods, e.g., liquid chromatography and mass spectrometry, it enables rapid and high-sensitivity profiling of complex glycan features without the need for liberation of glycans. Target samples include an extensive range of glycoconjugates involved in cells, tissues, body fluids, as well as synthetic glycans and their mimics. Various procedures for rapid differential glycan profiling have been developed for glycan-related biomarkers. Such glycoproteomics targeting allows precise diagnosis of chronic diseases potentially related to cancer. Application of this method to evaluation of various types of stem cells resulted in the discovery of a new pluripotent cell-specific glycan marker. To explore this technology a more fundamental and extensive understanding of lectins is necessary in relation to the structural uniqueness of glycans. In this chapter, the essence of the lectin microarray is described with some focus on an evanescent-field-activated fluorescence detection principle as a system to achieve in situ (i.e., washing free) aqueous-phase observation under equilibrium conditions. The developed lectin microarray system allows even researchers with poor experience in glycan profiling to perform extensive high-throughput analysis targeting various forms of glycans and even cells.

Application of microarrays for deciphering the structure and function of the human glycome

Glycan structures were defined historically using multiple methods to determine composition, sequence, linkage, and anomericity of component monosaccharides. Such approaches have been replaced by more sensitive MS methods to profile or predict glycan structures, but these methods are limited in their ability to completely define glycan structures. Glycan-binding proteins, including lectins and antibodies, have been found to have exquisite binding specificities that can provide information about glycan structures. Here, we show glycan-binding proteins can be used along with MS to help define glycan linkages and other determinants in unknown glycans printed as shotgun glycan microarrays.

Hypomorphic MGAT5 polymorphisms promote multiple sclerosis cooperatively with MGAT1 and interleukin-2 and 7 receptor variants

Deficiency of the Golgi N-glycan branching enzyme Mgat5 in mice promotes T cell hyperactivity, endocytosis of CTLA-4 and autoimmunity, including a spontaneous multiple sclerosis (MS)-like disease. Multiple genetic and environmental MS risk factors lower N-glycan branching in T cells. These include variants in interleukin-2 receptor-α (IL2RA), interleukin-7 receptor-α (IL7RA), and MGAT1, a Golgi branching enzyme upstream of MGAT5, as well as vitamin D3 deficiency and Golgi substrate metabolism. Here we describe linked intronic variants of MGAT5 that are associated with reduced N-glycan branching, CTLA-4 surface expression and MS (p=5.79×10(-9), n=7,741), the latter additive with the MGAT1, IL2RA and IL7RA MS risk variants (p=1.76×10(-9), OR=0.67-1.83, n=3,518).

Identification of compounds that augment the lectin-sensitivity of Chinese Hamster Ovary cells

Glycosylation is now recognized as one of the most important modifications of eukaryotic proteins. In cancer biology, alterations in cell surface glycosylation have been exploited as valuable biomarkers, and the relationship of this modification to the metastatic characteristics of cancer cells has also been well-documented. Chemicals that can alter cell surface glycosylation patterns will therefore become attractive lead compounds for controlling the metastatic characteristics of cancer cells, one of the critical factors in their malignancy and prognosis of the disease. In this study, we established a system for screening compounds that have the potential to alter cell surface glycosylation by taking advantage of the susceptibility of cells toward various lectins. Through our screening of a chemical library, we were able to identify two compounds that augment the sensitivity of Chinese Hamster Ovary (CHO-K1) cells against the L4-PHA lectin. Surprisingly, these compounds did not result in alterations in cell surface glycan structures. Instead, they appeared to render the cells to be more sensitive to various lectins with distinct carbohydrate specificities. These compounds promise to be valuable, not only as tools for providing insights into the intracellular signaling of lectin-mediated growth arrest, but also as potential lead compounds for use as therapeutic, anti-cancer drugs.

Loss of effector function of human cytolytic T lymphocytes is accompanied by major alterations in N- and O-glycosylation

Most human tumors are not eliminated by the immune system, and therapeutic vaccination shows poor results, a fact that can be explained at least partially by an immunosuppressive tumor microenvironment that is abundant in galectin-3. On cytolytic T lymphocyte (CTL) clones, maintained in culture by regular stimulation, recently activated CTLs present low effector functions. However, these functions are restored after a short treatment with LacNAc. The latter, which is in agreement with the glycoprotein-galectin lattice concept involving reduced motility, poses the question why galectin-3 ligands improve effector functions. We employed ultrasensitive MALDI-TOF-MS on resting and recently activated CTL clones combined with various glycosidase digestions and GC-MS linkage analyses. Our results showed that compared with the resting CTLs, the N-glycans of the recently activated CTLs consisted of (i) larger LacNAc oligomers of which a significant portion was longer than four-units and (ii) more multi-antennary structures. Interestingly, our results showed that the poly-LacNAc appeared to be equally distributed on all available N-glycan branches and not selectively enriched on a specific branch. The above structural alterations in the recently activated CTLs are expected to increase the galectin-3-LacNAc lattices and multivalent interactions and, therefore, reduce the motility of surface glycoproteins, such as the T-cell receptor. These findings suggest that the loss of effector functions on CTLs may be linked to reduced motility of surface glycoproteins. In addition, our results showed that recently activated CTLs had a reduced abundance of NeuAcα2,6-linked N-glycans and an increased abundance of disialylated core 1 and monosialylated core 2 O-glycan structures.

Human prostate cancer in a clinically relevant xenograft mouse model: identification of β(1,6)-branched oligosaccharides as a marker of tumor progression

PURPOSE

To establish xenograft mouse models of metastatic and nonmetastatic human prostate cancer and to apply these models to the search for aberrant glycosylation patterns associated with tumor progression in vivo and in patients.

EXPERIMENTAL DESIGN

Prostate cancer cells (LNCaP, PC-3, LuCaP 23.1, and DU-145) were xenografted subcutaneously into immunodeficient pfp(-/-)/rag2(-/-) mice. Tumor growth and metastasis formation were quantified and as altered glycosylation patterns have been associated with metastasis formation in several other malignancies, prostate cancer cells were profiled by a quantitative real-time PCR (qRT-PCR) glycosylation array and compared with normal human prostate cells. The activity of upregulated glycosyltransferases was analyzed by their sugar residues end products using lectin histochemistry on primary tumors and metastases in the animal experiments and on 2,085 clinical samples.

RESULTS

PC-3 cells produced the largest number of spontaneous lung metastases, followed by LNCaP and LuCaP 23.1, whereas DU-145 was nonmetastatic. qRT-PCR revealed an upregulation of β1,6-N-acetylglucosaminyltransferase-5b (Mgat5b) in all prostate cancer cell lines. Mgat5b products [β(1,6)-branched oligosaccharides] were predominantly detectable in metastatic xenografts as shown by increased binding of Phaseolus vulgaris leukoagglutinin (PHA-L). The percentage of prostate cancer patients who were PHA-L positive was 86.5. PHA-L intensity correlated with serum prostate-specific antigen and a cytoplasmic staining negatively affected disease-free survival.

CONCLUSION

We show a novel xenograft mouse model for human prostate cancer respecting the complete metastatic cascade. Specific glycosylation patterns reveal Mgat5b products as relevant markers of both metastatic competence in mice and disease-free survival in patients. This is the first description of Mgat5b in prostate cancer indicating a significant biologic importance of β(1,6)-branched oligosaccharides for prostate cancer progression.

A multiplex lectin-channel monitoring method for human serum glycoproteins by quantitative mass spectrometry

A mass profiling method and multiple reaction monitoring (MRM)-based quantitative approach were used to analyze multiple lectin-captured fractions of human serum using different lectins such as aleuria aurantia lectin (AAL), phytohemagglutinin-L(4) (L-PHA), concanavalin A (Con A), and Datura stramonium agglutinin (DSA) to quantitatively monitor protein glycosylation diversity. Each fraction, prepared by multiple lectin-fractionation and tryptic digestion, was analyzed by 1-D LC-MS/MS. Semi-quantitative profiling showed that the list of glycoproteins identified from each lectin-captured fraction is significantly different according to the used lectin. Thus, it was confirmed that the multiplex lectin-channel monitoring (LCM) using multiple lectins is useful for investigating protein glycosylation diversity in a proteome sample. Based on the semi-quantitative mass profiling, target proteins showing lectin-specificity among each lectin-captured fraction were selected and analyzed by the MRM-based method in triplicate using each lectin-captured fraction (average CV 7.9%). The MRM-based analysis for each lectin-captured fraction was similar to those obtained by the profiling experiments. The abundance of each target protein measured varied dramatically, based on the lectin-specificity. The multiplex LCM approach using MRM-based analyses is useful for quantitatively monitoring target protein glycoforms selectively fractionated by multiple lectins. Thus through multiplex LCM rather than single, we could inquire minutely into protein glycosylation states.

Differential glycosylation of MUC1 and CEACAM5 between normal mucosa and tumour tissue of colon cancer patients

Altered glycosylation in epithelial cancers may play an important role in tumour progression, as it may affect tumour cell migration and antigen presentation by antigen presenting cells. We specifically characterise the glycosylation patterns of two tumour antigens that are highly expressed in cancer tissue and often detected in their secreted form in serum: the epithelial mucin MUC1 and carcinoembryonic antigen (CEA, also called CEACAM5). We analysed 48 colorectal cancer patients, comparing normal colon and tumour epithelium within each patient. Lectin binding was studied by a standardised CEA/MUC1 capture ELISA, using several plant lectins, and the human C-type lectins MGL and DC-SIGN, and Galectin-3. Peanut agglutinin (PNA) bound to MUC1 from tumour tissue in particular, suggests increased expression of the Thomsen-Friedenreich antigen (TF-antigen) (Core 1, Galβ1-3GalNAc-Ser/Thr). Only small amounts of Tn-antigen (GalNAcα-Ser/Thr) expression was observed, but the human C-type lectin MGL showed increased binding to tumour-associated MUC1. Furthermore, sialylation was greatly enhanced. In sharp contrast, tumour-associated CEA (CEACAM5) contained high levels of the blood-group related carbohydrates, Lewis X and Lewis Y. This correlated strongly with the interaction of the human C-type lectin DC-SIGN to tumour-associated CEA, suggesting that CEA can be recognized and taken up by antigen presenting cells. In addition, increased mannose expression was observed and branched N-glycans were prominent, and this correlated well with human Galectin-3 binding. These data demonstrate that individual tumour antigens contain distinct glycan structures associated with cancer and, since glycans affect cellular interactions with its microenvironment, this may have consequences for progression of the disease.

N-acetylglucosamine inhibits T-helper 1 (Th1)/T-helper 17 (Th17) cell responses and treats experimental autoimmune encephalomyelitis

Current treatments and emerging oral therapies for multiple sclerosis (MS) are limited by effectiveness, cost, and/or toxicity. Genetic and environmental factors that alter the branching of Asn (N)-linked glycans result in T cell hyperactivity, promote spontaneous inflammatory demyelination and neurodegeneration in mice, and converge to regulate the risk of MS. The sugar N-acetylglucosamine (GlcNAc) enhances N-glycan branching and inhibits T cell activity and adoptive transfer experimental autoimmune encephalomyelitis (EAE). Here, we report that oral GlcNAc inhibits T-helper 1 (Th1) and T-helper 17 (Th17) responses and attenuates the clinical severity of myelin oligodendrocyte glycoprotein (MOG)-induced EAE when administered after disease onset. Oral GlcNAc increased expression of branched N-glycans in T cells in vivo as shown by high pH anion exchange chromatography, MALDI-TOF mass spectroscopy and FACS analysis with the plant lectin l-phytohemagglutinin. Initiating oral GlcNAc treatment on the second day of clinical disease inhibited MOG-induced EAE as well as secretion of interferon-γ, tumor necrosis factor-α, interleukin-17, and interleukin-22. In the more severe 2D2 T cell receptor transgenic EAE model, oral GlcNAc initiated after disease onset also inhibits clinical disease, except for those with rapid lethal progression. These data suggest that oral GlcNAc may provide an inexpensive and nontoxic oral therapeutic agent for MS that directly targets an underlying molecular mechanism causal of disease.

Genetics and the environment converge to dysregulate N-glycosylation in multiple sclerosis

How environmental factors combine with genetic risk at the molecular level to promote complex trait diseases such as multiple sclerosis (MS) is largely unknown. In mice, N-glycan branching by the Golgi enzymes Mgat1 and/or Mgat5 prevents T cell hyperactivity, cytotoxic T-lymphocyte antigen 4 (CTLA-4) endocytosis, spontaneous inflammatory demyelination and neurodegeneration, the latter pathologies characteristic of MS. Here we show that MS risk modulators converge to alter N-glycosylation and/or CTLA-4 surface retention conditional on metabolism and vitamin D₃, including genetic variants in interleukin-7 receptor-α (IL7RA*C), interleukin-2 receptor-α (IL2RA*T), MGAT1 (IV_AV_T−T) and CTLA-4 (Thr17Ala). Downregulation of Mgat1 by IL7RA*C and IL2RA*T is opposed by MGAT1 (IV_AV_T−T) and vitamin D₃, optimizing branching and mitigating MS risk when combined with enhanced CTLA-4 N-glycosylation by CTLA-4 Thr17. Our data suggest a molecular mechanism in MS whereby multiple environmental and genetic inputs lead to dysregulation of a final common pathway, namely N-glycosylation.

Complex diseases such as multiple sclerosis have both genetic and environmental components. This study demonstrates that variants of genes implicated in multiple sclerosis, and alterations in cellular metabolism and vitamin D3 levels, alter N-glycosylation, a post-translational modification causal of the disease in mice.

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.

Galectin-3 protein modulates cell surface expression and activation of vascular endothelial growth factor receptor 2 in human endothelial cells

Angiogenesis is heavily influenced by VEGF-A and its family of receptors, particularly VEGF receptor 2 (VEGF-R2). Like most cell surface proteins, VEGF-R2 is glycosylated, although the function of VEGF-R2 with respect to its glycosylation pattern is poorly characterized. Galectin-3, a glycan binding protein, interacts with the EGF and TGFβ receptors, retaining them on the plasma membrane and altering their signal transduction. Because VEGF-R2 is glycosylated and both galectin-3 and VEGF-R2 are involved with angiogenesis, we hypothesized that galectin-3 binds VEGF-R2 and modulates its signal transduction as well. Employing a Western blot analysis approach, we found that galectin-3 induces phosphorylation of VEGF-R2 in endothelial cells. Knockdown of galectin-3 and Mgat5, an enzyme that synthesizes high-affinity glycan ligands of galectin-3, reduced VEGF-A mediated angiogenesis in vitro. A direct interaction on the plasma membrane was detected between galectin-3 and VEGF-R2, and this interaction was dependent on the expression of Mgat5. Using immunofluorescence and cell surface labeling, we found an increase in the level of internalized VEGF-R2 in both Mgat5 and galectin-3 knockdown cells, suggesting that galectin-3 retains the receptor on the plasma membrane. Finally, we observed reduced suture-induced neovascularization in the corneas of Gal3(-/-) and Mgat5(-/-) mice. These findings are consistent with the hypothesis that, like its role with the EGF and TGFβ receptors, galectin-3 contributes to the plasma membrane retention and proangiogenic function of VEGF-R2.

Enhanced epithelial-mesenchymal transition-like phenotype in N-acetylglucosaminyltransferase V transgenic mouse skin promotes wound healing

N-Acetylglucosaminyltransferase V (GnT-V) catalyzes the β1,6 branching of N-acetylglucosamine on N-glycans. GnT-V expression is elevated during malignant transformation in various types of cancer. However, the mechanism by which GnT-V promotes cancer progression is unclear. To characterize the biological significance of GnT-V, we established GnT-V transgenic (Tg) mice, in which GnT-V is regulated by a β-actin promoter. No spontaneous cancer was detected in any organs of the GnT-V Tg mice. However, GnT-V expression was up-regulated in GnT-V Tg mouse skin, and cultured keratinocytes derived from these mice showed enhanced migration, which was associated with changes in E-cadherin localization and epithelial-mesenchymal transition (EMT). Further, EMT-associated factors snail, twist, and N-cadherin were up-regulated, and cutaneous wound healing was accelerated in vivo. We further investigated the detailed mechanisms of EMT by assessing EGF signaling and found up-regulated EGF receptor signaling in GnT-V Tg mouse keratinocytes. These findings indicate that GnT-V overexpression promotes EMT and keratinocyte migration in part through enhanced EGF receptor signaling.

MS-based glycomic strategies for probing the structural details of polylactosaminoglycan chain on N-glycans and glycoproteomic identification of its protein carriers

Most MS-based glycomic and glycoproteomic analyses focus on identifying changes in terminal glyco-epitopes represented by sialylation and fucosylation at specific positions of the terminal N-acetyllactosamine units. Much less attention was accorded to the underlying linear or branched poly-N-acetyllactosamine extension from the N-glycan trimannosyl core other than a simple inference of its presence due to mass data and hence glycosyl compositional assignment. Using the EA.hy926 cell line derived from human umbilical vein endothelial cells (HUVEC), we have systematically investigated the MALDI- and ESI-MS-based methodologies for probing the structural details of endothelial polylactosaminoglycans at both MS and MS(2) levels in conjunction with the use of endo-β-galactosidase to identify branching motifs and initiation sites. We showed that the polylactosaminoglycan chains on the N-glycans of EA.hy926 were less sialylated and fucosylated but more extended and branched than those of human umbilical vein endothelial cells, thus demonstrating a fundamental glycomic difference. For EA.hy926 that was investigated in more details, its polylactosaminoglycan chains were shown to be not restricted to extending from a specific antenna including the biologically important 6-arm position. Finally, experimental conditions for glycopeptide enrichment by tomato lectin were further optimized, which led to identification of over 40 candidate endothelial membrane protein carriers of polylactosaminoglycans by proteomic analysis.

Characterisation of the carbohydrate fraction of the temporary adhesive secreted by the tube feet of the sea star Asterias rubens

In sea stars, adhesion takes place at the level of a multitude of small appendages, the tube feet. It involves the secretion of an adhesive material which, after tube foot detachment, remains on the substratum as a footprint. It was previously reported that the two main organic components of this material are proteins and carbohydrates. The carbohydrate moiety of the adhesive secretion of Asterias rubens was investigated using a set of 16 lectins which were used on sections through tube feet, on footprints, and on the proteins extracted from these footprints. After gel electrophoresis, these proteins separate into eight protein bands which were named sea star footprint proteins (Sfps). Eleven lectins label the tube foot epidermis at the level of the adhesive cells, four react with footprints, and eight with two of the extracted footprint proteins, which are therefore classified as glycoproteins. Sfp-290 appears to bear mostly N-linked oligosaccharides and Sfp-210 principally O-linked oligosaccharides. The outer chains of both glycoproteins enclose galactose, N-acetylgalactosamine, fucose, and sialic acid residues. Another part of the carbohydrate fraction of the footprints would be in the form of larger molecules, such as sialylated proteoglycans. These two types of glycoconjugates are presumably key components of the sea star temporary adhesive providing both cohesive and adhesive contributions through electrostatic interactions by the polar and hydrogen-bonding functional groups of their glycan chains.

Lectin microarray profiling of metastatic breast cancers

Altered protein glycosylation compared with the disease-free state is a universal feature of cancer cells. It has long been established that distinct glycan structures are associated with specific forms of cancer, but far less is known about the complete array of glycans associated with certain tumors. The cancer glycome has great potential as a source of biomarkers, but progress in this field has been hindered by a lack of available techniques for the elucidation of disease-associated glycosylation. In the present study, lectin microarrays consisting of 45 lectins with different binding preferences covering N- and O-linked glycans were coupled with evanescent-field activated fluorescent detection in the glycomic analysis of primary breast tumors and the serum and urine of patients with metastatic breast cancer. A single 50 µm section of a primary breast tumor or <1 µL of breast cancer patient serum or urine was sufficient to detect glycosylation alterations associated with metastatic breast cancer, as inferred from lectin-binding patterns. The high-throughput, sensitive and relatively simple nature of the simultaneous analysis of N- and O-linked glycosylation following minimal sample preparation and without the need for protein deglycosylation makes the lectin microarray analysis described a valuable tool for discovery phase glycomic profiling.

Purification, biochemical characterization, and bioactive properties of a lectin purified from the seeds of white tepary bean (phaseolus acutifolius variety latifolius)

The present work shows the characterization of Phaseolus acutifolius variety latifolius, on which little research has been published, and provides detailed information on the corresponding lectin. This protein was purified from a semi-domesticated line of white tepary beans from Sonora, Mexico, by precipitation of the aqueous extract with ammonium sulfate, followed by affinity chromatography on an immobilized fetuin matrix. MALDI TOF analysis of Phaseolus acutifolius agglutinin (PAA) showed that this lectin is composed of monomers with molecular weights ranging between 28 and 31 kDa. At high salt concentrations, PAA forms a dimer of 63 kDa, but at low salt concentrations, the subunits form a tetramer. Analysis of PAA on 2D-PAGE showed that there are mainly three types of subunits with isoelectric points of 4.2, 4.4, and 4.5. The partial sequence obtained by LC/MS/MS of tryptic fragments from the PAA subunits showed 90-100% identity with subunits from genus Phaseolus lectins in previous reports. The tepary bean lectin showed lower hemagglutination activity than Phaseolus vulgaris hemagglutinin (PHA-E) toward trypsinized human A and O type erythrocytes. The hemagglutination activity was inhibited by N-glycans from glycoproteins. Affinity chromatography with the immobilized PAA showed a high affinity to glycopeptides from thyroglobulin, which also has N-glycans with a high content of N-acetylglucosamine. PAA showed less mitogenic activity toward human lymphocytes than PHA-L and Con A. The cytotoxicity of PAA was determined by employing three clones of the 3T3 cell line, demonstrating variability among the clones as follows: T4 (DI₅₀ 51.5 µg/mL); J20 (DI₅₀ 275 µg/mL), and N5 (DI₅₀ 72.5 µg/mL).

Novel anti-carbohydrate antibodies reveal the cooperative function of sulfated N- and O-glycans in lymphocyte homing

Cell surface glycans play pivotal roles in immune cell trafficking and immunity. Here we present an efficient method for generating anti-carbohydrate monoclonal antibodies (mAbs) using gene-targeted mice and describe critical glycans in lymphocyte homing. We immunized sulfotransferase GlcNAc6ST-1 and GlcNAc6ST-2 doubly deficient mice with sulfotransferase-overexpressing Chinese hamster ovary cells and generated two mAbs, termed S1 and S2. Both S1 and S2 bound high endothelial venules (HEVs) in the lymphoid organs of humans and wild-type mice, but not in those of doubly deficient mice. Glycan array analysis indicated that both S1 and S2 specifically bound 6-sulfo sialyl Lewis X and its defucosylated structure. Interestingly, S2 inhibited lymphocyte homing to peripheral lymph nodes by 95%, whereas S1 inhibited it by only 25%. S2 also significantly inhibited contact hypersensitivity responses and L-selectin-dependent leukocyte adhesion to HEVs. Immunohistochemical and Western blot analyses indicated that S1 preferentially bound sulfated O-glycans, whereas S2 bound both sulfated N- and O-glycans in HEVs. Furthermore, S2 strongly inhibited the N-glycan-dependent residual lymphocyte homing in mutant mice lacking sulfated O-glycans, indicating the importance of both sulfated N- and O-glycans in lymphocyte homing. Thus, the two mAbs generated by a novel method revealed the cooperative function of sulfated N- and O-glycans in lymphocyte homing and immune surveillance.

Sucrose co-administration reduces the toxic effect of lectin on gut permeability and intestinal bacterial colonization

INTRODUCTION

Legume lectins can have toxic effects when consumed without adequate cooking, occasionally leading to an acute gastroenteritis. Lectins are sugar binging proteins and may use this property to execute their toxic effects; these toxic effects may be secondary to increased gut bacteria and intestinal permeability. However, whether or not sucrose rescues these toxic effects by decreasing gut bacterial concentration and intestinal permeability is unknown.

AIM

Our aim was to test the hypothesis that sucrose may reduce toxic effects of legume lectins by protecting barrier function, bacterial overgrowth and bacterial translocation.

METHODS

Twenty-four rats were randomized to an ad libitum diet of either standard rat chow, a chow containing 26% crude red kidney beans or a chow containing 26% crude red kidney beans supplemented with 1 mM sucrose in drinking water for 24 h. After 12-h fast, rats were gavaged with sugar probes; breath gas and urine were collected for 5 h. Intestine and liver tissues were then collected. Mucosa-associated total bacterial count were measured by targeting the 16s rRNA gene. Four groups of in vitro Caco-2 cell lines were treated with PBS, 200 μg/ml phytohemagglutinin (PHA), 1 mM sucrose and both 200 μg/ml PHA and 1 mM sucrose, respectively, and trans-epithelial resistance was measured.

RESULTS

Rats fed crude red kidney beans for 24 h showed significant weight loss when compared to controls (P < 0.05), as well as increased intestinal permeability (P < 0.05), increased bacterial load (P < 0.05) and increased bacterial translocation to the liver (P < 0.05). Sucrose rescues the drop in trans-epithelial resistance due to PHA in CaCO2-cells (P < 0.05).

CONCLUSIONS

Sucrose reduced crude red kidney beans induced increase in intestinal permeability, bacterial load and translocation. Since red kidney beans are an important source of dietary protein in the world, their potential toxicity when inadequately cooked may be rescued by a suitable complementary diet.

Comparative quantitation of aberrant glycoforms by lectin-based glycoprotein enrichment coupled with multiple-reaction monitoring mass spectrometry

Lectin enrichment-coupled multiple-reaction monitoring (MRM) mass spectrometry was employed to quantitatively monitor the variation of aberrant glycoforms produced under pathological states. For this, aberrant glycoforms of the tissue inhibitor of metalloproteinase 1 (TIMP1) and protein tyrosine phosphatase kappa (PTPkappa), previously known target proteins for N-acetylglucosaminyltransferase-V (GnT-V), were enriched by phytohemagglutinin-L(4) (L-PHA) lectin and comparatively analyzed in the conditioned medium of the WiDr colon cancer cell line and its GnT-V-overexpressing transfectant cells. Enriched glycoforms were digested, and the resultant peptides were comparatively quantified by MRM analysis. MRM quantitation data for the L-PHA-enriched samples revealed that the abundance of aberrant glycoforms of TIMP1 and PTPkappa was greatly increased (11.7- and 16.5-fold, respectively) in GnT-V-treated cells compared to the control cells, although the abundance of total TIMP1 and PTPkappa in GnT-V-treated cells was slightly different (1.1- and 0.5-fold, respectively) for unenriched samples compared to that in control cells. The dramatic variation in abundance of the aberrant glycoforms due to overexpressed GnT-V was confirmed quantitatively by comparative MRM analysis of lectin-enriched samples. This method is capable of comparatively quantitating the abundance of a protein of interest and its aberrant glycoform and will be useful for studying pathological mechanisms of cancer or verifying biomarker candidates.

Galectin-3 is an important mediator of VEGF- and bFGF-mediated angiogenic response

Recent studies have shown that a carbohydrate-binding protein, galectin-3, is a novel pro-angiogenic molecule. The mechanism by which galectin-3 promotes angiogenesis remains unknown. We demonstrate here that galectin-3 is a mediator of vascular endothelial growth factor (VEGF)- and basic fibroblast growth factor (bFGF)-mediated angiogenic response. Angiogenesis assays revealed that galectin-3 inhibitors, β-lactose and dominant-negative galectin-3, reduce VEGF- and bFGF-mediated angiogenesis in vitro and that VEGF- and bFGF-mediated angiogenic response is reduced in galectin-3 knockdown cells and Gal3^−/− animals. Integrin αvβ3 was identified as the major galectin-3–binding protein and anti-αv, -β3, and -αvβ3 integrin function-blocking antibodies significantly inhibited the galectin-3–induced angiogenesis. Furthermore, galectin-3 promoted the clustering of integrin αvβ3 and activated focal adhesion kinase. Knockdown of GnTV, an enzyme that synthesizes high-affinity glycan ligands for galectin-3, substantially reduced: (a) complex N-glycans on αvβ3 integrins and (b) VEGF- and bFGF-mediated angiogenesis. Collectively, these data suggest that galectin-3 modulates VEGF- and bFGF-mediated angiogenesis by binding via its carbohydrate recognition domain, to the GnTV synthesized N-glycans of integrin αvβ3, and subsequently activating the signaling pathways that promote the growth of new blood vessels. These findings have broad implications for developing novel, carbohydrate-based therapeutic agents for inhibition of angiogenesis.