Embryonic stem cells deficient in I beta1,6-N-acetylglucosaminyltransferase exhibit reduced expression of embryoglycan and the loss of a Lewis X antigen, 4C9

Glycans define the stemness of naïve and primed pluripotent stem cells

Cell surface glycans are tissue-specific and developmentally regulated. They function as essential modulators in cell-cell interactions, cell-extracellular matrix interactions, and ligand-receptor interactions, binding to various ligands, including Wnt, fibroblast growth factors, and bone morphogenetic proteins. Embryonic stem (ES) cells, originally derived from the inner cell mass of blastocysts, have the essential characteristics of pluripotency and self-renewal. Recently, it has been proposed that mouse and human conventional ES cells are present in different developmental stages, namely pre-implantation blastocyst and post-implantation blastocyst stages, also called the naïve state and the primed state, respectively. They therefore require different extrinsic signals for the maintenance of self-renewal and pluripotency, and also appear to require different surface glycans. Understanding of molecular mechanisms involving glycans in self-renewal and pluripotency of ES cells is increasingly important for potential clinical applications, as well as for basic research. This review focuses on the roles of glycans in the two different states of pluripotent stem cells, namely the naïve state and the primed state, and the transition between these two states.

Embryoglycan: a highly branched poly-N-acetyllactosamine in pluripotent stem cells and early embryonic cells

Embryonal carcinoma cells, stem cells of teratocarcinomas, are pluripotent stem cells and also prototypes of embryonic stem cells. Embryonal carcinoma cells contain large amounts of a highly branched poly-N-acetyllactosamine called embryoglycan, which has a molecular weight of approximately 10,000 or greater, and is asparagine-linked. This glycan was found by analyses of fucose-labeled glycopeptides, and its characteristics were established by biochemical analyses. The content of embryoglycan progressively decreases during the in vitro differentiation of embryonal carcinoma cells. Embryoglycan is also abundant in mouse embryonic stem cells and preimplantation mouse embryos, and decreases during embryogenesis. Embryoglycan carries a number of carbohydrate markers of murine pluripotent stem cells. Lewis x markers, such as SSEA-1, 4C9 antigen, and binding sites for Lotus tetragonolobus agglutinin are of particular importance. 4C9 antigenicity requires clustering of Lewis x, best accomplished by poly-N-acetyllactosamine branching, whereas SSEA-1 does not. Although in vivo evidence is lacking, these epitopes have been suggested to participate in cell-to-cell and cell-to-substratum adhesion. Other markers on embryoglycan include α-galactosyl antigens such as ECMA-2, and binding sites for Dolichos biflorus agglutinin, the epitope of which is considered to be identical to Sd^a antigen, namely, GalNAcβ1-4(NeuAcα2-3)Galβ1-4GlcNAc. While embryoglycan is also present in human teratocarcinoma cells, the carbohydrate markers characterized in human pluripotent stem cells to date are largely carried by glycolipids and keratan sulfate. Information on embryoglycan and markers carried by it may assist in the development of new markers of human pluripotent stem cells and their progenies.

I-branching N-acetylglucosaminyltransferase regulates prostate cancer invasiveness by enhancing α5β1 integrin signaling

Cell surface carbohydrates are important for cell migration and invasion of prostate cancer (PCa). Accordingly, the I‐branching N‐acetylglucosaminyltransferase (GCNT2) converts linear i‐antigen to I‐branching glycan, and its expression is associated with breast cancer progression. In the present study, we identified relationships between GCNT2 expression and clinicopathological parameters in patients with PCa. Paraffin‐embedded PCa specimens were immunohistochemically tested for GCNT2 expression, and the roles of GCNT2 in PCa progression were investigated using cell lines with high GCNT2 expression and low GCNT2 expression. GCNT2‐positive cells were significantly lesser in organ‐confined disease than in that with extra‐capsular extensions, and GCNT2‐negative tumors were associated with significantly better prostate‐specific antigen‐free survival compared with GCNT2‐positive tumors. Subsequent functional studies revealed that knockdown of GCNT2 expression in PCa cell lines significantly inhibited cell migration and invasion. GCNT2 regulated the expression of cell surface I‐antigen on the O‐glycan and glycolipid. Moreover, I‐antigen‐bearing glycolipids were subject to α5β1 integrin–fibronectin mediated protein kinase B phosphorylation. In conclusion, GCNT2 expression is closely associated with invasive potential of PCa.

Detection of Core2 β-1,6-N-Acetylglucosaminyltransferase in Post-Digital Rectal Examination Urine Is a Reliable Indicator for Extracapsular Extension of Prostate Cancer

To identify appropriate candidates for aggressive treatment such as radical prostatectomy or radiation therapy of localized prostate cancer (PCa), novel predictive biomarkers of PCa aggressiveness are essential. Core2 β-1,6-N-acetylglucosaminyltransferase-1 (GCNT1) is a key enzyme that forms core 2-branched O-glycans. Its expression is associated with the progression of several cancers. We established a mouse IgG monoclonal antibody (mAb) against GCNT1 and examined the relationship of GCNT1 expression to the clinicopathological status of PCa. Paraffin-embedded PCa specimens were analyzed by immunohistochemistry for GCNT1 expression using a newly established mouse anti-GCNT1 mAb by ourselves. GCNT1-positive tumor showed significantly higher Gleason score and larger tumor volume. The number of GCNT1-positive cases was significantly lower in cases of organ-confined disease than in cases of extracapsular extension. GCNT1-negative tumors were associated with significantly better prostate-specific antigen (PSA)-free survival compared with GCNT1-positive tumors. Multivariate analysis revealed that detection of GCNT1 expression was an independent risk factor for PSA recurrence. We established new methods for GCNT1 detection from PCa specimens. Immunoblotting was used to examine post-digital rectal examination (DRE) urine from PCa patients. Over 90% of GCNT1-positive PCa patients with high concentrations of PSA showed extracapsular extension. In conclusion, GCNT1 expression closely associates with the aggressive potential of PCa. Further research aims to develop GCNT1 detection in post-DRE urine as a marker for PCa aggressiveness.

Polylactosaminoglycan glycomics: enhancing the detection of high-molecular-weight N-glycans in matrix-assisted laser desorption ionization time-of-flight profiles by matched filtering

For over 30 years, protocols based on the mass spectrometry (MS) of permethylated derivatives, complemented by enzymatic degradations, have underpinned glycomic experiments aimed at defining the structures of individual glycans present in the complex mixtures that are characteristic of biological samples. Both MS instrumentation and sample handling have improved markedly in recent years, enabling greater sensitivity and better signal-to-noise ratios, thereby facilitating the detection of glycans at much higher masses than could be achieved in the past. The latter is especially important for the characterization of the biologically important class of N-glycans that carry polylactosaminoglycan chains. Such advances in data acquisition heighten the need for informatics tools to assist in glycan structure assignment. Here, utilizing mouse lung tissue as a model system, we present evidence of polylactosaminoglycan-containing N-glycans with permethylated molecular weights exceeding 13 kDa. We show that antennae branching patterns and lengths can be successfully determined at these high masses via MS/MS experiments, even when MS ion counts are very low. We also describe the development and application of a matched filtering algorithm for assisting high-molecular-weight glycan detection and structure assignment.

Helicobacter pylori β1,3-N-acetylglucosaminyltransferase for versatile synthesis of type 1 and type 2 poly-LacNAcs on N-linked, O-linked and I-antigen glycans

Poly-N-acetyllactosamine extensions on N- and O-linked glycans are increasingly recognized as biologically important structural features, but access to these structures has not been widely available. Here, we report a detailed substrate specificity and catalytic efficiency of the bacterial β3-N-acetylglucosaminyltransferase (β3GlcNAcT) from Helicobacter pylori that can be adapted to the synthesis of a rich diversity of glycans with poly-LacNAc extensions. This glycosyltransferase has surprisingly broad acceptor specificity toward type-1, -2, -3 and -4 galactoside motifs on both linear and branched glycans, found commonly on N-linked, O-linked and I-antigen glycans. This finding enables the production of complex ligands for glycan-binding studies. Although the enzyme shows preferential activity for type 2 (Galβ1-4GlcNAc) acceptors, it is capable of transferring N-acetylglucosamine (GlcNAc) in β1-3 linkage to type-1 (Galβ1-3GlcNAc) or type-3/4 (Galβ1-3GalNAcα/β) sequences. Thus, by alternating the use of the H. pylori β3GlcNAcT with galactosyltransferases that make the β1-4 or β1-3 linkages, various N-linked, O-linked and I-antigen acceptors could be elongated with type-2 and type-1 LacNAc repeats. Finally, one-pot incubation of di-LacNAc biantennary N-glycopeptide with the β3GlcNAcT and GalT-1 in the presence of uridine diphosphate (UDP)-GlcNAc and UDP-Gal, yielded products with 15 additional LacNAc units on the precursor, which was seen as a series of sequential ion peaks representing alternative additions of GlcNAc and Gal residues, on matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF MS) analysis. Overall, our data demonstrate a broader substrate specificity for the H. pylori β3GlcNAcT than previously recognized and demonstrate its ability as a potent resource for preparative chemo-enzymatic synthesis of complex glycans.

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.

High-sensitivity O-glycomic analysis of mice deficient in core 2 {beta}1,6-N-acetylglucosaminyltransferases

Core 2 β1,6-N-acetylglucosaminyltransferase (C2GnT), which exists in three isoforms, C2GnT1, C2GnT2 and C2GnT3, is one of the key enzymes in the O-glycan biosynthetic pathway. These isoenzymes produce core 2 O-glycans and have been correlated with the biosynthesis of core 4 O-glycans and I-branches. Previously, we have reported mice with single and multiple deficiencies of C2GnT isoenzyme(s) and have evaluated the biological and structural consequences of the loss of core 2 function. We now present more comprehensive O-glycomic analyses of neutral and sialylated glycans expressed in the colon, small intestine, stomach, kidney, thyroid/trachea and thymus of wild-type, C2GnT2 and C2GnT3 single knockouts and the C2GnT1-3 triple knockout mice. Very high-quality data have emerged from our mass spectrometry techniques with the capability of detecting O-glycans up to at least 3500 Da. We were able to unambiguously elucidate the types of O-glycan core, branching location and residue linkages, which allowed us to exhaustively characterize structural changes in the knockout tissues. The C2GnT2 knockout mice suffered a major loss of core 2 O-glycans as well as glycans with I-branches on core 1 antennae especially in the stomach and the colon. In contrast, core 2 O-glycans still dominated the O-glycomic profile of most tissues in the C2GnT3 knockout mice. Analysis of the C2GnT triple knockout mice revealed a complete loss of both core 2 O-glycans and branched core 1 antennae, confirming that the three known isoenzymes are entirely responsible for producing these structures. Unexpectedly, O-linked mannosyl glycans are upregulated in the triple deficient stomach. In addition, our studies have revealed an interesting terminal structure detected on O-glycans of the colon tissues that is similar to the RM2 antigen from glycolipids.

Phosphorylation status of transcription factor C/EBPalpha determines cell-surface poly-LacNAc branching (I antigen) formation in erythropoiesis and granulopoiesis

The cell-surface straight and branched repeats of N-acetyllactosamine (LacNAc) units, called poly-LacNAc chains, characterize the histo-blood group i and I antigens, respectively. The transition of straight to branched poly-LacNAc chain (i to I) is determined by the I locus, which expresses 3 IGnT transcripts, IGnTA, IGnTB, and IGnTC. Our previous investigation demonstrated that the i-to-I transition in erythroid differentiation is regulated by the transcription factor CCAAT/enhancer binding protein alpha (C/EBPalpha). In the present investigation, the K-562 cell line was used as a model to show that the i-to-I transition is determined by the phosphorylation status of the C/EBPalpha Ser-21 residue, with dephosphorylated C/EBPalpha Ser-21 stimulating the transcription of the IGnTC gene, consequently resulting in I branching. Results from studies using adult erythropoietic and granulopoietic progenitor cells agreed with those derived using the K-562 cell model, with lentiviral expression of C/EBPalpha in CD34(+) hematopoietic cells demonstrating that the dephosphorylated form of C/EBPalpha Ser-21 induced the expression of I antigen, granulocytic CD15, and also erythroid CD71 antigens. Taken together, these results demonstrate that the regulation of poly-LacNAc branching (I antigen) formation in erythropoiesis and granulopoiesis share a common mechanism, with dephosphorylation of the Ser-21 residue on C/EBPalpha playing the critical role.