Unusual N-Glycan Structures in α-Mannosidase II/IIx Double Null Embryos Identified by a Systematic Glycomics Approach Based on Two-dimensional LC Mapping and Matrix-dependent Selective Fragmentation Method in MALDI-TOF/TOF Mass Spectrometry

alpha-Mannosidase IIx (MX) is an enzyme closely related to alpha-mannosidase II (MII), a key enzyme in N-glycan biosynthesis that catalyzes the first step in conversion of hybrid- to complex-type N-glycans in Golgi apparatus. Recently we generated MII/MX double knock-out mice and found that double nulls completely lack the complex-type N-glycans (Akama, T. O., Nakagawa, H., Wong, N. K., Sutton-Smith, M., Dell, A., Morris, H. R., Nakayama, J., Nishimura, S.-I., Pai, A., Moremen, K. W., Marth, J. D., and Fukuda, M. N. (2006) Essential and mutually compensatory roles of alpha-mannosidase II and alpha-mannosidase IIx in N-glycan processing in vivo in mice. Proc. Natl. Acad. Sci. U. S. A. 103, 8983-8988). In the present study, we determined minor but unusual N-glycan structures found in MII/MX double knock-out mice. We identified such N-glycans by a systematic glycomics approach applying a two-dimensional LC mapping database and matrix-dependent selective fragmentation technique in MALDI-TOF/TOF MS, a highly sensitive and reliable technique that provides specific fragmentations enabling the determination of precise oligosaccharide structures including regioisomers (Kurogochi, M., and Nishimura, S.-I. (2004) Structural characterization of N-glycopeptides by matrix-dependent selective fragmentation of MALDI-TOF/TOF tandem mass spectrometry. Anal. Chem. 76, 6097-6101). Quantitative profiling of all N-glycan structures including minor components from MII/MX nulls, MII nulls, MX nulls, and wild-type mice at embryonic day 15.5 yielded a total of 37 species when structural heterogeneity was reduced by the removal of the sialic acids. Among six unusual N-glycan structures, two glycoforms were novel and were found only in MII/MX double nulls. We characterize such structure as pseudocomplex-type N-glycans. The present study demonstrated that use of the versatile matrix-dependent selective fragmentation method in MALDI-TOF/TOF MS greatly accelerates detailed structural analysis of a trace amount of N-glycans.

Essential role of RGS-PX1/sorting nexin 13 in mouse development and regulation of endocytosis dynamics

RGS-PX1 (also known as sorting nexin 13) is a member of both the regulator of G protein signaling (RGS) and sorting nexin (SNX) protein families. Biochemical and cell culture studies have shown that RGS-PX1/SNX13 attenuates Galphas-mediated signaling through its RGS domain and regulates endocytic trafficking and degradation of the epidermal growth factor receptor. To understand the functions of RGS-PX1/SNX13 in vivo, we generated mice carrying targeted mutations of Snx13 and found that systemic Snx13-null mice were embryonic lethal around midgestation. Snx13-null embryos had significant overall growth retardation and defects in neural tube closure, blood vessel formation, and the formation of the placental labyrinthine layer. Moreover, the Snx13-null visceral yolk sac endoderm cells showed dramatic changes in the organization of endocytic compartments, abundant autophagic vacuoles, and abnormal localization of several endocytic markers, including megalin, a receptor for nutrients and proteins; ARH, a coat protein that binds megalin; LAMP2; and LC3. These changes suggest that Snx13-null embryos are defective in nutrient uptake and transport, which may contribute to the other developmental abnormalities observed. Taken together, our findings demonstrate an essential role for RGS-PX1/SNX13 in mouse development and provide previously undescribed insights into its cellular function in the regulation of endocytosis dynamics.

ST6Gal-I restrains CD22-dependent antigen receptor endocytosis and Shp-1 recruitment in normal and pathogenic immune signaling

The ST6Gal-I sialyltransferase produces Siglec ligands for the B-cell-specific CD22 lectin and sustains humoral immune responses. Using multiple experimental approaches to elucidate the mechanisms involved, we report that ST6Gal-I deficiency induces immunoglobulin M (IgM) antigen receptor endocytosis in the absence of immune stimulation. This coincides with increased antigen receptor colocalization with CD22 in both clathrin-deficient and clathrin-enriched membrane microdomains concurrent with diminished tyrosine phosphorylation of Igalpha/beta, Syk, and phospholipase C-gamma2 upon immune activation. Codeficiency with CD22 restores IgM antigen receptor half-life at the cell surface in addition to reversing alterations in membrane trafficking and immune signaling. Diminished immune responses due to ST6Gal-I deficiency further correlate with constitutive recruitment of Shp-1 to CD22 in unstimulated B cells independent of Lyn tyrosine kinase activity and prevent autoimmune disease pathogenesis in the Lyn-deficient model of systemic lupus erythematosus, resulting in a significant extension of life span. Protein glycosylation by ST6Gal-I restricts access of antigen receptors and Shp-1 to CD22 and operates by a CD22-dependent mechanism that decreases the basal rate of IgM antigen receptor endocytosis in altering the threshold of B-cell immune activation.

Essential and mutually compensatory roles of {alpha}-mannosidase II and {alpha}-mannosidase IIx in N-glycan processing in vivo in mice

Many proteins synthesized through the secretory pathway receive posttranslational modifications, including N-glycosylation. alpha-Mannosidase II (MII) is a key enzyme converting precursor high-mannose-type N-glycans to matured complex-type structures. Previous studies showed that MII-null mice synthesize complex-type N-glycans, indicating the presence of an alternative pathway. Because alpha-mannosidase IIx (MX) is a candidate enzyme for this pathway, we asked whether MX functions in N-glycan processing by generating MII/MX double-null mice. Some double-nulls died between embryonic days 15.5 and 18.5, but most survived until shortly after birth and died of respiratory failure, which represents a more severe phenotype than that seen in single-nulls for either gene. Structural analysis of N-glycans revealed that double-nulls completely lack complex-type N-glycans, demonstrating a critical role for at least one of these enzymes for effective N-glycan processing. Recombinant mouse MX and MII showed identical substrate specificities toward N-glycan substrates, suggesting that MX is an isozyme of MII. Thus, either MII or MX can biochemically compensate for the deficiency of the other in vivo, and either of two is required for late embryonic and early postnatal development.

Dietary and genetic control of glucose transporter 2 glycosylation promotes insulin secretion in suppressing diabetes

Pancreatic beta cell-surface expression of glucose transporter 2 (Glut-2) is essential for glucose-stimulated insulin secretion, thereby controlling blood glucose homeostasis in response to dietary intake. We show that the murine GlcNAcT-IVa glycosyltransferase is required for Glut-2 residency on the beta cell surface by constructing a cell-type- and glycoprotein-specific N-glycan ligand for pancreatic lectin receptors. Loss of GlcNAcT-IVa, or the addition of glycan-ligand mimetics, attenuates Glut-2 cell-surface half-life, provoking endocytosis with redistribution into endosomes and lysosomes. The ensuing impairment of glucose-stimulated insulin secretion leads to metabolic dysfunction diagnostic of type 2 diabetes. Remarkably, the induction of diabetes by chronic ingestion of a high-fat diet is associated with reduced GlcNAcT-IV expression and attenuated Glut-2 glycosylation coincident with Glut-2 endocytosis. We infer that beta cell glucose-transporter glycosylation mediates a link between diet and insulin production that typically suppresses the pathogenesis of type 2 diabetes.

Cellular and Molecular Analysis of Neural Development of Glycosyltransferase Gene Knockout Mice

Recent studies demonstrate that carbohydrates synthesized by specific glycosyltransferases play important roles in the development of the central nervous system. Among these carbohydrates, polysialic acid is a unique glycan that modulates functions of the neural cell adhesion molecule (NCAM) by attenuating NCAM-mediated interaction between neural cells. During brain development, polysialic acid is synthesized in a specific spatiotemporal pattern by two polysialyltransferases, ST8SiaII and ST8SiaIV. To study in vivo the roles of polysialic acid synthesized by each respective enzyme, we generated ST8SiaII and ST8SiaIV knockout mice. Single knockout ST8SiaII or ST8SiaIV mice show polysialic acid expression patterns differing from wild type, and those patterns indicate different roles of each gene during neural development. In this chapter, we discuss methods used to analyze polysialyltransferase knockout mice using immunohistochemistry of brain and primary cultures of neurons.

Genetic Ablation of Polysialic Acid Causes Severe Neurodevelopmental Defects Rescued by Deletion of the Neural Cell Adhesion Molecule

Poly-alpha2,8-sialic acid (polySia) is a unique modification of the neural cell adhesion molecule, NCAM, tightly associated with neural development and plasticity. However, the vital role attributed to this carbohydrate polymer has been challenged by the mild phenotype of mice lacking polySia due to NCAM-deficiency. To dissect polySia and NCAM functions, we generated polySia-negative but NCAM-positive mice by simultaneous deletion of the two polysialyltransferase genes, St8sia-II and St8sia-IV. Beyond features shared with NCAM-null animals, a severe phenotype with specific brain wiring defects, progressive hydrocephalus, postnatal growth retardation, and precocious death was observed. These drastic defects were selectively rescued by additional deletion of NCAM, demonstrating that they originate from a gain of NCAM functions because of polySia deficiency. The data presented in this study reveal that the essential role of polySia resides in the control and coordination of NCAM interactions during mouse brain development. Moreover, this first demonstration in vivo that a highly specific glycan structure is more important than the glycoconjugate as a whole provides a novel view on the relevance of protein glycosylation for the complex process of building the vertebrate brain.

Sialylation regulates peripheral tolerance in CD4+ T cells

Decreased binding by the 6C10 auto-antibody serves as a unique marker for CD4+ T cell unresponsiveness after the induction of T cell tolerance in Vbeta8.1 TCR transgenic mice. We further define the nature of the epitope recognized by the 6C10 antibody to be a subset of Thy-1 bearing incompletely sialylated N-linked glycans, and furthermore, we demonstrate that tolerant CD4+ T cells have an increased degree of cell-surface sialylation. To test the significance of the altered glycosylation state identified by the 6C10 auto-antibody in the tolerant CD4+ T cell population, surface sialic acid was cleaved enzymatically. Treatment of purified peripheral CD4+ T cells with Vibrio cholerae sialidase (VCS) leads to increased 6C10 binding, significantly enhances proliferation in the tolerant CD4+ population and corrects defects in phosphotyrosine signaling observed in the tolerant CD4+ T cell. Furthermore, in vivo administration of VCS enhances proliferation in both tolerant and naive CD4+ T cell subsets. These studies suggest that sialylation of glycoproteins on the surface of the CD4+ T cell contributes to the regulation of T cell responsiveness in the tolerant state.

A crucial role for T-bet in selectin ligand expression in T helper 1 (Th1) cells

Proinflammatory T helper 1 (Th1) cells express high levels of carbohydrate ligands for the endothelial selectins, but the molecular basis for this phenotype is incompletely understood. We document here a significant role in selectin ligand formation for the recently described Th1 transcription factor T-bet. Th1 cells generated from T-bet-/- mice showed significantly lower levels of ligands for both E-selectin and P-selectin, compared with wild-type (WT) Th1 cells. Enforced expression of T-bet in WT Th0 cells only modestly up-regulated P-selectin ligands and had no effect on E-selectin ligands. To define a mechanism for the defects observed in T-bet-/- mice, we examined expression of glycosyltransferases involved in selectin ligand biosynthesis. T-bet-/- Th1 cells expressed significantly lower levels of core 2 beta1,6 N-acetylglucosaminyltransferase I (C2GlcNAcT-I), but no differences in levels of alpha 2,3-sialyltransferase IV (ST3Gal-IV). Further, we show that T-bet is responsible for the signal transducer and activator of transcription 4 (Stat4)-independent increase in Th1 cells of fucosyltransferase VII (FucT-VII). We also identify ST3Gal-VI, which is thought to play an important role in E- and P-selectin ligand formation, as an interleukin 12 (IL-12)-regulated, T-bet-dependent gene. These data show that T-bet controls selectin ligand formation in Th1 cells via control of expression of multiple key enzymes in response to IL-12 signaling and establishes an independent transcriptional pathway for control of Th1 cell traffic.

Neuronal LRP1 functionally associates with postsynaptic proteins and is required for normal motor function in mice

The LDL receptor-related protein 1 (LRP1) is a multifunctional cell surface receptor that is highly expressed on neurons. Neuronal LRP1 in vitro can mediate ligand endocytosis, as well as modulate signal transduction processes. However, little is known about its role in the intact nervous system. Here, we report that mice that lack LRP1 selectively in differentiated neurons develop severe behavioral and motor abnormalities, including hyperactivity, tremor, and dystonia. Since their central nervous systems appear histoanatomically normal, we suggest that this phenotype is likely attributable to abnormal neurotransmission. This conclusion is supported by studies of primary cultured neurons that show that LRP1 is present in close proximity to the N-methyl-D-aspartate (NMDA) receptor in dendritic synapses and can be coprecipitated with NMDA receptor subunits and the postsynaptic density protein PSD-95 from neuronal cell lysates. Moreover, treatment with NMDA, but not dopamine, reduces the interaction of LRP1 with PSD-95, indicating that LRP1 participates in transmitter-dependent postsynaptic responses. Together, these findings suggest that LRP1, like other ApoE receptors, can modulate synaptic transmission in the brain.

Core 2 branching beta1,6-N-acetylglucosaminyltransferase and high endothelial cell N-acetylglucosamine-6-sulfotransferase exert differential control over B- and T-lymphocyte homing to peripheral lymph nodes

Blood-borne lymphocyte trafficking to peripheral lymph nodes (PLNs) depends on the successful initiation of rolling interactions mediated by L-selectin binding to sialomucin ligands in high endothelial venules (HEVs). Biochemical analysis of purified L-selectin ligands has identified posttranslational modifications mediated by Core2GlcNAcT-I and high endothelial cell GlcNAc-6-sulfotransferase (HECGlcNAc6ST). Consequently, lymphocyte migration to PLNs of C2GlcNAcT-I(-/-) and HEC-GlcNAc6ST(-/-) mice was reduced; however, B-cell homing was more severely compromised than T-cell migration. Accordingly, intravital microscopy (IVM) of PLN HEVs revealed a defect in B-cell tethering and increased rolling velocity (V(roll)) in C2GlcNAcT-I(-/-) mice that was more pronounced than it was for T cells. By contrast, B- and T-cell tethering was normal in HEC-GlcNAc6ST(-/-) HEVs, but V(roll) was accelerated, especially for B cells. The increased sensitivity of B cells to glycan deficiencies was caused by lower expression levels of L-selectin; L-selectin(+/-) T cells expressing L-selectin levels equivalent to those of B cells exhibited intravascular behavior similar to that of B cells. These results demonstrate distinct functions for C2GlcNAcT-I and HEC-GlcNAc6ST in the differential elaboration of HEV glycoproteins that set a threshold for the amount of L-selectin needed for lymphocyte homing.

Inactivation of the Mgat1 gene in oocytes impairs oogenesis, but embryos lacking complex and hybrid N-glycans develop and implant

Complex and hybrid N-glycans contain sugar residues that have been implicated in fertilization, compaction of the embryo, and implantation. Inactivation of the Mgat1 gene responsible for their synthesis is embryonic lethal, but homozygous mutant blastocysts are phenotypically normal due to the presence of maternal Mgat1 gene transcripts. To identify roles for complex and hybrid N-glycans in oogenesis and preimplantation development, the Mgat1 gene in oocytes was deleted by using a ZP3Cre recombinase transgene. All mutant oocytes had an altered zona pellucida (ZP) that was thinner than the control ZP, and they did not possess complex N-glycans but contained ZP1, ZP2, and ZP3 glycoproteins. Mutant eggs were fertilized, all embryos implanted, and heterozygotes developed to birth. However, mutant females had decreased fertility, yielded fewer eggs after stimulation with gonadotropins, and produced a reduced number of preimplantation embryos and less progeny than controls. About 25% of embryonic day 3.5 (E3.5) embryos derived from mutant eggs were severely retarded in development, even when they were heterozygous and expressed complex N-glycans. Thus, a proportion of Mgat1(-)(/)(-) oocytes were developmentally compromised. Surprisingly, mutant eggs also gave rise to Mgat1(-)(/)(-) embryos that developed normally, implanted, and progressed to E9.5. Therefore, complex or hybrid N-glycans are required at some stage of oogenesis for the generation of a developmentally competent oocyte, but fertilization, blastogenesis, and implantation may proceed in their absence.

Sialyltransferase ST8Sia-II Assembles a Subset of Polysialic Acid That Directs Hippocampal Axonal Targeting and Promotes Fear Behavior

Polysialic acid (PSA) is a post-translational protein modification that is widely expressed among neural cell types during development. Found predominantly on the neural cell adhesion molecule (NCAM), PSA becomes restricted to regions of neurogenesis and neuroplasticity in the adult. In the mammalian genome, two polysialyltransferases termed ST8Sia-II and ST8Sia-IV have been hypothesized to be responsible for the production of PSA in vivo. Approaches to discover PSA function have involved the application of endoneuraminidase-N to remove PSA and genetic manipulations in the mouse to deplete either NCAM or ST8Sia-IV. Here we report the production and characterization of mice deficient in the ST8Sia-II polysialyltransferase. We observed alterations in brain PSA expression unlike those observed in mice lacking ST8Sia-IV. This included a PSA deficit in regions of neurogenesis but without changes in the frequency of mitotic neural progenitor cells. In further contrast with ST8Sia-IV deficiency, loss of ST8Sia-II did not impair hippocampal synaptic plasticity but instead resulted in the misguidance of infrapyramidal mossy fibers and the formation of ectopic synapses in the hippocampus. Consistent with studies of animal models bearing these morphological changes, ST8Sia-II-deficient mice exhibited higher exploratory drive and reduced behavioral responses to Pavlovian fear conditioning. PSA produced by the ST8Sia-II polysialyltransferase modifies memory and behavior processes that are distinct from the neural roles reported for ST8Sia-IV. This genetic partitioning of PSA formation engenders discrete neurological processes and reveals that this post-translational modification forms the predominant basis for the multiple functions attributed to the NCAM glycoprotein.

Dynamic O-GlcNAc modification of nucleocytoplasmic proteins in response to stress. A survival response of mammalian cells

Cellular response to environmental, physiological, or chemical stress is key to survival following injury or disease. Here we describe a unique signaling mechanism by which cells detect and respond to stress in order to survive. A wide variety of stress stimuli rapidly increase nucleocytoplasmic protein modification by O-linked beta-N-acetylglucosamine (O-GlcNAc), an essential post-translational modification of Ser and Thr residues of metazoans. Blocking this post-translational modification, or reducing it, renders cells more sensitive to stress and results in decreased cell survival; and increasing O-GlcNAc levels protects cells. O-GlcNAc regulates both the rates and extent of the stress-induced induction of heat shock proteins, providing a molecular basis for these findings.

Masking of CD22 by cis ligands does not prevent redistribution of CD22 to sites of cell contact

CD22, a negative regulator of B cell signaling, is a member of the siglec family that binds to alpha2-6-linked sialic acids on glycoproteins. Previous reports demonstrated that binding of multivalent sialoside probes to CD22 is blocked, or "masked," by endogenous (cis) ligands, unless they are first destroyed by sialidase treatment. These results suggest that cis ligands on B cells make CD22 functionally unavailable for binding to ligands in trans. Through immunofluorescence microscopy, however, we observed that CD22 on resting B cells redistributes to the site of contact with other B or T lymphocytes. Redistribution is mediated by interaction with trans ligands on the opposing cell because it does not occur with ligand-deficient lymphocytes from ST6GalI-null mice. Surprisingly, CD45, proposed as both a cis and trans ligand of CD22, was not required for redistribution to sites of cell contact, given that redistribution of CD22 was independent of CD45 and was observed with lymphocytes from CD45-deficient mice. Furthermore, CD45 is not required for CD22 masking as similar levels of masking were observed in the WT and null mice. Comparison of the widely used sialoside-polyacrylamide probe with a sialoside-streptavidin probe revealed that the latter bound a subset of B cells without sialidase treatment, suggesting that cis ligands differentially impacted the binding of these two probes in trans. The combined results suggest that equilibrium binding to cis ligands does not preclude binding of CD22 to ligands in trans, and allows for its redistribution to sites of contact between lymphocytes.

Ogt-dependent X-chromosome-linked protein glycosylation is a requisite modification in somatic cell function and embryo viability

The Ogt gene encodes a glycosyltransferase that links N-acetylglucosamine to serine and threonine residues (O-GlcNAc) on nuclear and cytosolic proteins. Efforts to study a mammalian model of Ogt deficiency have been hindered by the requirement for this X-linked gene in embryonic stem cell viability, necessitating the use of conditional mutagenesis in vivo. We have extended these observations by segregating Ogt mutation to distinct somatic cell types, including neurons, thymocytes, and fibroblasts, the latter by an approach developed for inducible Ogt mutagenesis. We show that Ogt mutation results in the loss of O-GlcNAc and causes T-cell apoptosis, neuronal tau hyperphosphorylation, and fibroblast growth arrest with altered expression of c-Fos, c-Jun, c-Myc, Sp1, and p27. We further segregated the mutant Ogt allele to parental gametes by oocyte- and spermatid-specific Cre-loxP mutagenesis. By this we established an in vivo genetic approach that supports the ontogeny of female heterozygotes bearing mutant X-linked genes required during embryogenesis. Successful production and characterization of such female heterozygotes further indicates that mammalian cells commonly require a functional Ogt allele. We find that O-GlcNAc modulates protein phosphorylation and expression among essential and conserved cell signaling pathways.

N-glycan branching requirement in neuronal and postnatal viability

The structural variations among extracellular N-glycans reflect the activity of glycosyltransferases and glycosidases that operate in the Golgi apparatus. More than other types of vertebrate glycans, N-glycans are highly branched oligosaccharides with multiple antennae linked to an underlying mannose core structure. The branching patterns of N-glycans consist of three types, termed high-mannose, hybrid, and complex. Though most extracellular mammalian N-glycans are of the complex type, some cells variably express hybrid and high-mannose forms. Nevertheless, a requirement for hybrid and complex N-glycan branching exists in embryonic development and postnatal function among mice and humans inheriting defective Mgat1 or Mgat2 alleles. The resulting defects in formation N-glycan branching patterns cause multiple abnormalities, including neurologic defects, and have inferred the presence of distinct functions for hybrid and complex N-glycan branches among different cell lineages. We have further explored N-glycan structure-function relationships in vivo by using Cre-loxP conditional mutagenesis to abolish hybrid and complex N-glycan branching specifically among neuronal cells. Our findings show that hybrid N-glycan branching is an essential posttranslational modification among neurons. Loss of Mgat1 resulted in a unique pattern of neuronal glycoprotein deficiency concurrent with caspase 3 activation and apoptosis. Such animals exhibited severe locomotor deficits, tremors, paralysis, and early postnatal death. Unexpectedly, neuronal Mgat2 deletion resulting in the loss of complex but not hybrid N-glycan branching was well tolerated without phenotypic markers of neuronal or locomotor dysfunction. Structural features associated with hybrid N-glycan branching comprise a requisite posttranslational modification to neuronal glycoproteins that permits normal cellular function and viability.

Core 2 Branching β1,6-N-Acetylglucosaminyltransferase and High Endothelial Venule-restricted Sulfotransferase Collaboratively Control Lymphocyte Homing

L-selectin mediates lymphocyte homing by facilitating lymphocyte adhesion to carbohydrate ligands expressed on high endothelial venules (HEV) of the secondary lymphoid organs. Previous studies demonstrated that L-selectin ligand sulfotransferase (LSST) forms 6-sulfo sialyl Lewis x (sLe(x)) on both core 2 branch and MECA-79-positive extended core 1 O-glycans, but the chemical nature and roles of HEV ligands elaborated by LSST and core 2 beta1,6-N-acetylglucosaminyltransferase-1 (Core2GlcNAcT) have been undefined. In the present study, we have generated mutant mice with deficient LSST and show that inactivation of LSST gene alone leads to only partial impairment of lymphocyte homing to peripheral lymph nodes and moderate reduction in lymphocyte counts in the peripheral lymph nodes, despite the fact that L-selectin ligands that contain 6-sulfo sLe(x) are reduced at HEV. By contrast, LSST/Core2GlcNAcT double null mice exhibited a markedly reduced lymphocyte homing and reduced lymphocyte counts as a result of significantly decreased 6-sulfo sLe(x) on HEV L-selectin counterreceptors, relative to LSST- or Core2GlcNAcT-single null mice. Moreover, induction of LSST and Core2GlcNAcT transcripts was observed in HEV-like structure formed in the salivary gland of the non-obese diabetic mouse, which displays chronic inflammation. These results indicate that LSST and Core2GlcNAcT cooperatively synthesize HEV-specific L-selectin ligands required for lymphocyte homing and suggest that LSST and Core2GlcNAcT play a critical role in lymphocyte trafficking during chronic inflammation.

A genetic approach to Mammalian glycan function

The four essential building blocks of cells are proteins, nucleic acids, lipids, and glycans. Also referred to as carbohydrates, glycans are composed of saccharides that are typically linked to lipids and proteins in the secretory pathway. Glycans are highly abundant and diverse biopolymers, yet their functions have remained relatively obscure. This is changing with the advent of genetic reagents and techniques that in the past decade have uncovered many essential roles of specific glycan linkages in living organisms. Glycans appear to modulate biological processes in the development and function of multiple physiologic systems, in part by regulating protein-protein and cell-cell interactions. Moreover, dysregulation of glycan synthesis represents the etiology for a growing number of human genetic diseases. The study of glycans, known as glycobiology, has entered an era of renaissance that coincides with the acquisition of complete genome sequences for multiple organisms and an increased focus upon how posttranslational modifications to protein contribute to the complexity of events mediating normal and disease physiology. Glycan production and modification comprise an estimated 1% of genes in the mammalian genome. Many of these genes encode enzymes termed glycosyltransferases and glycosidases that reside in the Golgi apparatus where they play the major role in constructing the glycan repertoire that is found at the cell surface and among extracellular compartments. We present a review of the recently established functions of glycan structures in the context of mammalian genetic studies focused upon the mouse and human species. Nothing tends so much to the advancement of knowledge as the application of a new instrument. The native intellectual powers of men in different times are not so much the causes of the different success of their labours, as the peculiar nature of the means and artificial resources in their possession. T. Hager: Force of Nature (1)

Sialic acid capping of CD8beta core 1-O-glycans controls thymocyte-major histocompatibility complex class I interaction

Bidentate interaction of a T-cell receptor and CD8alphabeta heterodimer with a peptide-MHCI complex is required for the generation of cytotoxic T-lymphocytes. During thymic development, the modification of CD8beta glycans influences major histocompatibility complex class I binding to T-cell precursors called thymocytes. ES mass spectrometry (MS) and tandem MS/MS analysis were used to identify the changes occurring in the CD8beta-glycopeptides during T-cell development. Several threonine residues proximal to the CD8beta Ig headpiece are glycosylated with core-type 1 O-glycans. Non-sialylated glycoforms are present in immature thymocytes but are virtually absent in mature thymocytes. These results suggest how sialylation in a discrete segment of the CD8beta stalk by ST3Gal-1 sialyltransferase creates a molecular developmental switch that affects ligand binding.

Cloning and characterization of a new human UDP-N-acetyl-alpha-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase, designated pp-GalNAc-T13, that is specifically expressed in neurons and synthesizes GalNAc alpha-serine/threonine antigen

To date, 10 members of the UDP-N-acetyl-alpha-d-galactosamine:polypeptide N-acetylgalactosaminyltransferase (pp-GalNAc-T) family have been cloned and analyzed in human. In this study, we cloned and analyzed a novel human pp-GalNAc-T from an NT2 cell cDNA library, and we named it pp-GalNAc-T13. In amino acid sequences, pp-GalNAc-T13 was highly homologous, showing 84.3% identity, to pp-GalNAc-T1. Real time PCR analysis revealed pp-GalNAc-T13 to be highly and restrictively expressed in the brain and present at very low or undetectable levels in other tissues, in contrast to the ubiquitous expression of pp-GalNAc-T1. pp-GalNAc-T13 was abundantly expressed in all neuroblastoma cells examined and primary cultured neurons but not in glioblastoma cells and primary cultured astrocytes. pp-GalNAc-T13 exhibited much stronger activity to transfer GalNAc to mucin peptides, such as Muc5Ac and MUC7, than did pp-GalNAc-T1. In addition, pp-GalNAc-T13 differed in substrate specificity to pp-GalNAc-T1. pp-GalNAc-T13 was able to form a triplet Tn epitope, three consecutive GalNAc-Ser/Thr structures, on peptides encoded in syndecan-3, a proteoglycan expressed in neurons. pp-GalNAc-T13-deficient mice have been established in a previous work. Immunohistochemical study showed a remarkable decrease in Tn antigen expression in the cerebellum of the pp-GalNAc-T13 knockout mouse. pp-GalNAc-T13 would be a major enzyme responsible for the synthesis of O-glycan and specifically the Tn antigen epitope in neurons.

Mice with a homozygous deletion of the Mgat2 gene encoding UDP-N-acetylglucosamine:alpha-6-D-mannoside beta1,2-N-acetylglucosaminyltransferase II: a model for congenital disorder of glycosylation type IIa

Mice homozygous for a deletion of the Mgat2 gene encoding UDP-N-acetylglucosamine:alpha-6-D-mannoside beta1,2-N-acetylglucosaminyltransferase II (GlcNAcT-II, EC 2.4.1.143) have been reported. GlcNAcT-II is essential for the synthesis of complex N-glycans. The Mgat2-null mice were studied in a comparison with the symptoms of congenital disorder of glycosylation type IIa (CDG-IIa) in humans. Mutant mouse tissues were shown to be deficient in GlcNAcT-II enzyme activity and complex N-glycan synthesis, resulting in severe gastrointestinal, hematologic and osteogenic abnormalities. All mutant mice died in early post-natal development. However, crossing the Mgat2 mutation into a distinct genetic background resulted in a low frequency of survivors exhibiting additional and novel disease signs of CDG-IIa. Analysis of N-glycan structures in the kidneys of Mgat2-null mice showed a novel bisected hybrid N-glycan structure in which the bisecting GlcNAc residue was substituted with a beta1,4-linked galactose or the Lewis(x) structure. These studies suggest that some of the functions of complex N-glycan branches are conserved in mammals and that human disease due to aberrant protein N-glycosylation may be modeled in the mouse, with the expectation in this case of gaining insights into CDG-IIa disease pathogenesis. Further analyses of the Mgat2-deficient phenotype in the mouse have been accomplished involving cells in which the Mgat2 gene is dispensable, as well as other cell lineages in which a severe defect is present. Pre-natal defects appear in a significant number of embryos, and likely reflect a limited window of time in which a future therapeutic approach might effectively operate.

Sialyltransferase specificity in selectin ligand formation

Selectin ligands are glycan structures that participate in leukocyte trafficking and inflammation. At least 6 ST3Gal sialyltransferases (I-VI) have been identified that may contribute to selectin ligand formation. However, it is not known which of these sialyltransferases are involved in vivo and whether they may differentially regulate selectin function. We have produced and characterized mice genetically deficient in ST3Gal-I, ST3Gal-II, ST3Gal-III, and ST3Gal-IV. Unlike mice bearing severe defects in selectin ligand formation, there was no finding of leukocytosis with these single ST3Gal deficiencies. Among neutrophils, only ST3Gal-IV was found to play a role in the synthesis of selectin ligands. In vitro rolling of marrow-derived neutrophils on E- or P-selectins presented by Chinese hamster ovary cells was reduced in the absence of ST3Gal-IV. However, in a tumor necrosis factor alpha (TNF-alpha)-induced inflammation model in vivo, no defect among P-selectin ligands was observed. Nevertheless, the number of leukocytes rolling on postcapillary venules in an E-selectin-dependent manner was decreased while E-selectin-dependent rolling velocity was increased. We propose that multiple ST3Gal sialyltransferases contribute to selectin ligand formation, as none of these ST3Gal deficiencies recapitulated the degree of E- and P-selectin ligand deficit observed on neuraminidase treatment of intact neutrophils. Our findings indicate a high degree of functional specificity among sialyltransferases and a substantial role for ST3Gal-IV in selectin ligand formation.

Genetically altered mice with different sialyltransferase deficiencies show tissue-specific alterations in sialylation and sialic acid 9-O-acetylation

Glycan chains on glycoconjugates traversing the Golgi apparatus are often terminated by sialic acid residues, which can also be 9-O-acetylated. This process involves competition between multiple Golgi enzymes. Expression levels of Golgi enzyme mRNAs do not always correlate with enzyme activity, which in turn cannot accurately predict glycan sequences found on cell surfaces. Here we examine the cell type-specific expression of terminal glycans in tissues of normal mice in comparison with animals deficient in ST6Gal-I (transfers alpha2-6-linked sialic acid to Galbeta1-4GlcNAc) or ST3Gal-I (transfers alpha2-3-linked sialic acid to Galbeta1-3GalNAc). Tissues of ST6Gal-I null mice showed minimal binding of an alpha2-6-sialic acid-specific lectin, indicating that no other enzyme generates Siaalpha2-6Galbeta1-4GlcNAc and that Siaalpha2-6GalNAc (sialyl-Tn) is rare in mice. However, exposed Galbeta1-4GlcNAc termini were only moderately increased, indicating that these can be partially capped by other enzymes. Indeed, Galalpha1-3Galbeta1-4GlcNAc and Fucalpha1-2Galbeta1-4GlcNAc termini were enhanced in some tissues. Many tissues of ST3Gal-I null animals showed increases in Galbeta1-3GalNAc termini, and some increases in poly-N-acetyllactosamines. However, overall expression of alpha2-3-linked sialic acid was selectively reduced only in a few instances, indicating that other ST3Gal enzymes can generate this linkage in most tissues. Highly selective losses of 9-O-acetylation of sialic acid residues were also observed, with ST6Gal-I deficiency causing loss on endothelium and ST3Gal-I deficiency giving a marked decrease on CD4(+) lymphocytes. These data demonstrate selective regulation of sialylation and 9-O-acetylation, point to cell types with potential physiological defects in null animals, and show in vivo evidence for competition between Golgi enzymes.