A novel variant form of murine beta-1, 6-N-acetylglucosaminyltransferase forming branches in poly-N-acetyllactosamines

Elucidating Human Milk Oligosaccharide biosynthetic genes through network-based multi-omics integration

Human Milk Oligosaccharides (HMOs) are abundant carbohydrates fundamental to infant health and development. Although these oligosaccharides were discovered more than half a century ago, their biosynthesis in the mammary gland remains largely uncharacterized. Here, we use a systems biology framework that integrates glycan and RNA expression data to construct an HMO biosynthetic network and predict glycosyltransferases involved. To accomplish this, we construct models describing the most likely pathways for the synthesis of the oligosaccharides accounting for >95% of the HMO content in human milk. Through our models, we propose candidate genes for elongation, branching, fucosylation, and sialylation of HMOs. Our model aggregation approach recovers 2 of 2 previously known gene-enzyme relations and 2 of 3 empirically confirmed gene-enzyme relations. The top genes we propose for the remaining 5 linkage reactions are consistent with previously published literature. These results provide the molecular basis of HMO biosynthesis necessary to guide progress in HMO research and application with the goal of understanding and improving infant health and development.

Human milk oligosaccharides are fundamental to infant health. Here the authors deploy a multi-omics systems biology approach to elucidate their biosynthetic network, including the associated enzymes and likely structures of ambiguous oligosaccharides.

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.

Structural characterization of multibranched oligosaccharides from seal milk by a combination of off-line high-performance liquid chromatography-matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry and sequential exoglycosidase digestion

A complex mixture of diverse oligosaccharides related to the carbohydrates in glycoconjugates involved in various biological events is found in animal milk/colostrum and has been challenging targets for separation and structural studies. In the current study, we isolated oligosaccharides having high molecular masses (MW approximately 3800) from the milk samples of bearded and hooded seals and analyzed their structures by off-line normal-phase-high-performance liquid chromatography-matrix-assisted laser desorption/ionization-time-of-flight (NP-HPLC-MALDI-TOF) mass spectrometry (MS) by combination with sequential exoglycosidase digestion. Initially, a mixture of oligosaccharides from the seal milk was reductively aminated with 2-aminobenzoic acid and analyzed by a combination of HPLC and MALDI-TOF MS. From MS data, these oligosaccharides contained different numbers of lactosamine units attached to the nonreducing lactose (Galbeta1-4Glc) and fucose residue. The isolated oligosaccharides were sequentially digested with exoglycosidases and characterized by MALDI-TOF MS. The data revealed that oligosaccharides from both seal species were composed from lacto-N-neohexaose (LNnH, Galbeta1-4GlcNAcbeta1-6[Galbeta1-4GlcNAcbeta1-3]Galbeta1-4Glc) as the common core structure, and most of them contained Fucalpha1-2 residues at the nonreducing ends. Furthermore, the oligosaccharides from both samples contained multibranched oligosaccharides having two Galbeta1-4GlcNAc (N-acetyllactosamine, LacNAc) residues on the Galbeta1-4GlcNAcbeta1-3 branch or both branches of LNnH. Elongation of the chains was observed at 3-OH positions of Gal residues, but most of the internal Gal residues were also substituted with an N-acetyllactosamine at the 6-OH position.

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

Embryoglycan is a class of branched high-molecular-weight poly-N-acetyllactosamines characteristically expressed in early embryonic cells and has been shown to be involved in the intercellular adhesion of early embryonic cells in vitro. Branching of poly-N-acetyllactosamine chains is performed by beta1,6-N-acetylglucosaminylation of the galactosyl residue. We previously knocked out the gene encoding I beta1, 6-N-acetylglucosaminyltransferase (IGnT), and the resultant deficient mice were born without any abnormality, although the mice exhibited various deficits in later life. In the present investigation, we produced embryonic stem (ES) cells from IGnT-deficient embryos. The mutant ES cells exhibited a reduced capability in embryoglycan synthesis. Thus, IGnT is a major enzyme involved in the branching of poly-N-acetyllactosamine chains in embryoglycan. Since ES cells are equivalent to multipotential cells of the embryonic ectoderm in early postimplantation embryos, this result indicates that an abundance of embryoglycan in these cells is not essential for normal embryogenesis. The IGnT-deficient ES cells continued to express SSEA-1, but lacked the expression of 4C9 antigen, although the epitope of 4C9 antigen was confirmed to be Lewis X by a transfection experiment. The result establishes the distinct nature of 4C9 antigenicity, which requires either Lewis X epitope on I-branch or clustering of Lewis X epitope, best accomplished by poly-N-acetyllactosamine branching. Alpha6-integrin was newly identified as a carrier of embryoglycan. The IGnT-deficient ES cells adhered to dishes coated with laminin, which is a ligand for alpha6-integrin, significantly less than wild-type ES cells, raising the possibility that embryoglycan in ES cells enhances alpha6-integrin-dependent adhesion in vitro.

Abnormalities caused by carbohydrate alterations in Ibeta6-N-acetylglucosaminyltransferase-deficient mice

Ibeta6-N-acetylglucosaminyltransferase (IGnT) catalyzes the branching of poly-N-acetyllactosamine carbohydrate chains. In both humans and mice, three spliced forms of IGnT have been identified, and a common exon is present in all of them. We generated mice deficient in the common exon to understand the physiological function of poly-N-acetyllactosamine branching. IGnT activity was abolished in the stomach, kidney, bone marrow, and cerebellum of the deficient mice, while a low level of the activity persisted in the small intestine. Immunohistochemical analysis confirmed the loss of I antigen from the lung, stomach, and kidney. The deficient mice had reduced spontaneous locomotive activity. The number of peripheral blood lymphocytes was also reduced and renal function decreased in the deficient mice. Furthermore, in aged mice, vacuolization occurred in the kidney, and epidermoid cysts were frequently formed. However, cataracts did not develop earlier in the deficient mice. Decreased levels of lysosomal proteins, LAMP-2 and synaptotagmin VII, were found in the kidney of the deficient mice and correlated with renal abnormalities.

Carbohydrate antigens expressed on stem cells and early embryonic cells

Lewis X antigen (Le(X)) is a marker of embryonic stem cells, embryonal carcinoma cells and multipotential cells of early embryos in the mouse. Le(X) is carried by branched, high-molecular weight poly-N-acetyllactosamines (embryoglycan). While embryoglycan is present in human embryonal carcinoma cells, Le(X) is not expressed in human embryonic stem cells, embryonal carcinoma cells or inner cell mass cells. Instead, these cells express SSEA-3 and SSEA-4, both of which are carried by globo-series glycolipids. Le(X) is a marker of primordial germ cells or multipotential stem cells derived from primordial germ cells both in the mouse and human. In other species of vertebrates, Le(X) is widely expressed in early embryonic cells and primordial germ cells, but the mode of expression is not completely conserved among species. Le(X) is expressed in neural stem cells from both humans and mice. Hematopoietic stem cells are not reported to express the above carbohydrate markers. A marker of these cells is CD34, a membrane-bound sialomucin. Another sialomucin, CD164 (MGC-24v) is expressed in hemotopoietic progenitor cells. As a function of Le(X) in stem cells, the promotion of integrin action is proposed, based on analyses of glycoproteins with the marker, cDNA transfection experiments and the inhibitory effects of an anti-Le(X) antibody. Most probably, Le(X) antigen as well as poly-N-acetyllactosamines play roles in the interactions on the same membrane. On the other hand, O-linked oligosaccharides on CD34 and CD164 are probably involved in the regulation of cell adhesion and proliferation via intercellular recognition.

Expression profile of mouse BWF1, a protein with a BEACH domain, WD40 domain and FYVE domain

We isolated a mouse cDNA encoding a protein that contains a BEACH domain, 5 WD40 repeats and a FYVE domain, which we designated as BWF1. The mRNA is approximately 10 kb in size and encodes a protein consisting of 3508 amino acids with a predicted molecular weight of 385 kDa. BWF1 has 45% homology with the Drosophila protein, blue cheese (BCHS). The BWF1 gene consists of 67 exons, which span 270 kb of genomic sequence, and has been mapped to mouse chromosome 5. Northern blot analysis revealed that it was strongly expressed in the liver, moderately in the kidney and testis, and weakly in the brain of adult mice. During the development of the mouse brain, BWF1 mRNA was abundant on embryonic day (E) 14-16; after birth, the level of BWF1 mRNA expression decreased markedly to reach the adult level at postnatal day 3. In situ hybridization analysis revealed that the expressed BWF1 mRNA was restricted to the marginal region both in E14 and E16 embryonic brain, but became diffuse after birth. Confocal microscopy studies of the epitope-tagged BWF1 protein showed that the protein was a cytoplasmic one.

ZEC, a zinc finger protein with novel binding specificity and transcription regulatory activity

A novel 114-kDa zinc finger protein, ZEC, has been found by cDNA cloning and characterized. ZEC was strongly expressed in the testis, liver and kidney, and also in embryonic stem cells. Epitope-tagged experiments indicated nuclear localization of ZEC. ZEC contained 18 C2H2 zinc fingers which were organized in two clusters. A ZEC binding DNA sequence, C/GA/TA/TGGTTGGTTGC, which we have designated the GT box, was identified by random oligonucleotide binding selection assay. The GT box did not contain binding sites for other previously characterized transcription factors and thus represented a potentially novel DNA target sequence. Electrophoretic mobility shift assay (EMSA) showed that both clusters of zinc fingers bound to the same DNA sequence. Site-directed mutagenesis revealed that the core sequence TTGGTT within the GT box was essential to ZEC binding, while DNA sequences outside of the core sequence enhanced this interaction. Furthermore, co-transfection assays demonstrated that ZEC could activate a reporter luciferase gene driven by this DNA sequence.

The molecular genetics of the mouse I beta-1,6-N-acetylglucosaminyltransferase locus

The I antigen and its precursor, the i antigen, are carbohydrate structures and are found on the surface of most mammalian cells. Conversion of the i to the I structure requires I beta-1,6-N-acetylglucosaminyltransferase activity. The present investigation demonstrates a novel transcript form expressed from the mouse I locus and elucidates the molecular genetics and the genomic organization of the mouse I locus. The mouse I locus was demonstrated to express three transcript forms, one newly identified and two previously reported, which have a different exon 1 but identical exons 2 and 3. The three transcripts were shown to express differentially in various mouse tissues, and all their protein products demonstrated GlcNAc-transferring activity in enzyme function assay. The molecular genetics proposed for the mouse I locus shows that it is homologous to the human I locus. It has been established recently that a defect in the human I locus may lead to the development of congenital cataracts. It was demonstrated that the mouse and the human I transcripts expressed in the epithelium cells of the mouse and human lens, respectively, are homologous forms.

A novel I-branching beta-1,6-N-acetylglucosaminyltransferase involved in human blood group I antigen expression

The human blood group i and I antigens are determined by linear and branched poly-N-acetyllactosamine structures, respectively. In erythrocytes, the fetal i antigen is converted to the adult I antigen by I-branching beta-1,6-N-acetylglucosaminyltransferase (IGnT) during development. Dysfunction of the I-branching enzyme may result in the adult i phenotype in erythrocytes. However, the I gene responsible for blood group I antigen has not been fully confirmed. We report here a novel human I-branching enzyme, designated IGnT3. The genes for IGnT1 (reported in 1993), IGnT2 (also presented in this study), and IGnT3 consist of 3 exons and share the second and third exons. Bone marrow cells preferentially expressed IGnT3 transcript. During erythroid differentiation using CD34(+) cells, IGnT3 was markedly up-regulated with concomitant decrease in IGnT1/2. Moreover, reticulocytes expressed the IGnT3 transcript, but IGnT1/2 was below detectable levels. By molecular genetic analyses of an adult i pedigree, individuals with the adult i phenotype were revealed to have heterozygous alleles with mutations in exon 2 (1006G>A; Gly336Arg) and exon 3 (1049G>A; Gly350Glu), respectively, of the IGnT3 gene. Chinese hamster ovary (CHO) cells transfected with each mutated IGnT3 cDNA failed to express I antigen. These findings indicate that the expression of the blood group I antigen in erythrocytes is determined by a novel IGnT3, not by IGnT1 or IGnT2.

Molecular basis of the adult i phenotype and the gene responsible for the expression of the human blood group I antigen

The human blood group i and I antigens are characterized as linear and branched repeats of N-acetyllactosamine, respectively. Conversion of the i to the I structure requires the activity of I-branching beta-1,6-N-acetylglucosaminyltransferase (IGnT). Thus the blood group I gene is assigned to encode a beta-1,6-N-acetylglucosaminyltransferase; however, its identity has not been confirmed. The null phenotype of I, the adult i phenotype, provides a means to identify the I gene. Interestingly, the adult i phenotype has been noted to be associated with congenital cataracts in Asians. Molecular genetic studies of 3 adult i pedigrees are reported here. The results obtained on mutation detection within the 2 I-branching enzyme encoding genes, segregation analyses, and enzyme function assays identify molecular changes associated with the adult i phenotype. The adult i phenotype in 2 of the pedigrees studied resulted from 1043G-->A and 1148G-->A mutations, which predict Gly348Glu and Arg383His alterations, respectively, in the IGnT gene. These amino acid changes abolished the original GlcNAc-transferase activity. Deletion of the IGnT gene was observed in the person with adult i phenotype in the third pedigree. These findings suggest that the IGnT gene, first reported in 1993, is the candidate for the blood group I gene. Confirmation of the blood group I gene will further assist in the investigations of the molecular genetics that control I antigen expression in secretions and the molecular basis for the association of the adult i phenotype with congenital cataracts in Asians.

Functional analysis of promoter activity of murine beta-1,6-N-acetylglucosaminyltransferase

In the mouse, beta-1,6-N-acetylglucosaminyltransferase (IGnT) forms branches in poly-N-acetyllactosamines, which are good scaffolds for oncodevelopmental cell-surface antigens and recognition markers. There are two isoforms of IGnT, IGnT A and B, which are produced by alternative splicing of the IGnT gene; the unique portion is encoded by exon 1 and common portion is encoded by exons 2 and 3. Thus, the expression of each isoform is controlled by a different promoter. Here, we identified the regulatory regions of the mouse IGnT A and B genes. The promoter regions for IGnT A and B did not contain putative TATA or CAAT boxes, but each contained GT boxes. The upstream regulatory region of each gene was examined by transient luciferase reporter gene transfection experiments and gel mobility shift assay. Promoter activity for each gene was detected in F9 embryonal carcinoma cells, which express IGnT A and B, but not in N2a cells, which do not express the gene. Deletion analysis demonstrated that the regions 308 bp upstream from the transcriptional initiation site of IGnT A and 430 bp upstream from the transcriptional initiation site of IGnT B showed minimal promoter activity. Mutation of the single GT box in IGnT A and two GT boxes in IGnT B caused marked reduction of the promoter activity. These findings provided strong evidence that the GT boxes play crucial roles in transcriptional regulation of the genes.

Protein-bound carbohydrates on cell-surface as targets of recognition: an odyssey in understanding them

Multidisciplinary approaches by a number of investigators have established that cell-surface carbohydrates are integral components of recognition systems regulating survival, migration, adhesion, growth and differentiation of various cells. Our own experience and contributions to this exciting field are described. We discovered Endo D as the first endoglycosidase acting on glycoproteins, found complementary specificity of two endoglycosidases (Endo D and Endo H), and applied these enzymes for glycoprotein research. Endo-beta-galactosidase C, which hydrolyzes Galalpha1-3Galbeta1-4GlcNAc xenoantigenic determinant, was later found and molecularly cloned. We also found highly branched poly-N-acetyllactosamines in early embryonic cells, and demonstrated developmentally regulated carbohydrate changes during early mammalian development. The binding site for Dolichos biflorus agglutinin was introduced as a new differentiation marker. Basigin and embigin, two related members of the immunoglobulin superfamily, a sialomucin MGC-24 and other glycoproteins were discovered as carriers of developmentally regulated carbohydrate markers. We proposed enhancement of integrin action as a function of sugar chains with Lewis X epitope, and observed a relationship between the expression of carbohydrate markers and invasive properties of human carcinoma. Midkine, a heparin-binding growth factor, was discovered more recently and its interaction with heparin and oversulfated chondroitin sulfate was elucidated. N-Acetylglucosamine-6-sulfotransferase was cloned and used to reconstitute L-selectin ligands. Gene knockout was applied to reveal in vivo function of basigin, syndecan-4 and chondroitin 6-sulfate. Throughout my research on all these subjects, I have been fortunate in obtaining unexpected observations and enjoying fruitful collaborations.