Mice lacking N-acetylglucosaminyltransferase I activity die at mid-gestation, revealing an essential role for complex or hybrid N-linked carbohydrates

Involvement of murine β-1,4-galactosyltransferase V in lactosylceramide biosynthesis

Human β-1,4-galactosyltransferase (β-1,4-GalT) V was shown to be involved in the biosynthesis of N-glycans, O-glycans and lactosylceramide (Lac-Cer) by in vitro studies. To determine its substrate specificity, enzymatic activity and its products were analyzed using mouse embryonic fibroblast (MEF) cells from β-1,4-GalT V (B4galt5)-mutant mice. Analysis of expression levels of the β-1,4-GalT I-VI genes revealed that the expression of the β-1,4-GalT V gene in B4galt5 ( +/- ) - and B4galt5 ( -/- ) -derived MEF cells are a half and null when compared to that of B4galt5 ( +/+ )-derived MEF cells without altering the expression levels of other β-1,4-GalT genes. These MEF cells showed no apparent difference in their growth. When β-1,4-GalT activities were determined towards GlcNAcβ-S-pNP, no significant difference in its specific activity was obtained among B4galt5 ( +/+ )-, B4galt5 ( +/- ) - and B4galt5 ( -/- ) -derived MEF cells. No significant differences were obtained in structures and amounts of N-glycans and lectin bindings to membrane glycoproteins among B4galt5 ( +/+ )-, B4galt5 ( +/- ) - and B4galt5 ( -/- ) -derived MEF cells. However, when cell homogenates were incubated with glucosylceramide in the presence of UDP-[(3)H]Gal, Lac-Cer synthase activity in B4galt5 ( +/- ) - and B4galt5 ( -/- ) -derived MEF cells decreased to 41% and 11% of that of B4galt5 ( +/+ )-derived MEF cells. Consistent with this, amounts of Lac-Cer and its derivative GM3 in B4galt5 ( -/- ) -derived MEF cells decreased remarkably when compared with those of B4galt5 ( +/+ )-derived MEF cells. These results indicate that murine β-1,4-GalT V is involved in Lac-Cer biosynthesis.

Alteration of brain glycoproteins during aging

Glycosylation is the most common post-translational modification of proteins. Protein sequence data suggested that more than half of all proteins produced in mammalian cells are glycoproteins. Recent studies showed that glycans of secreted glycoproteins affect many protein properties, such as solubility, stability, protease sensitivity and polarity, whereas glycans on cell-surface glycoproteins are involved in various cellular functions, including cell-cell communication. Accordingly, the investigation of glycoprotein changes caused by aging is expected to help understand the aging process and to elucidate age-associated diseases. The present review will summarize our current knowledge of changes found in brain glycoproteins resulting from the aging process.

Cell surface N-glycans-mediated isolation of mouse neural stem cells

The isolation of neural stem cells (NSCs) has been hampered by the lack of valid cell-surface antigens on NSCs, and novel valuable markers have been proposed. Glycan (oligosaccharide chain) is a potential candidate as a marker to isolate NSCs, because the species and the combination order of saccharides in glycan generate remarkable structural diversity and specificity. At present, the expression of hundreds of glycoconjugates with glycans have been found in the NSCs; however, just a few glycan-epitopes have been identified as valuable cell-surface markers. This review focused on the isolation of NSC using glycoprotein, especially complex type N-glycans. The cell-surface N-glycan-mediated isolation of NSCs is therefore expected to provide a comprehensive understanding of the biologic characteristics of NSCs in the brain, and thereby help to develop novel strategies in the field of regenerative medicine.

New insights into cathepsin D in mammary tissue development and remodeling

Cathepsin D (CatD) is a lysosomal aspartyl endopeptidase originally considered a "house keeping enzyme" involved in the clearance of unwanted proteins. However, recent studies have revealed CatD's involvement in apoptosis and autophagy, thus signifying an important function in the proper development and maintenance of multi-cellular organs. In the mammary gland, matrix degradation and the remodeling process are orchestrated by proteolytic enzymes, but the role of CatD at distinct developmental stages has remained mostly unexplored. Based on our previous studies we sought to address the role of this endopeptidase in mammary gland development and remodeling. By employing a mouse model, we report a previously unidentified participation of CatD in different stages of mammary gland development. Our findings reveal that CatD undergoes distinct protein processing at different stages of mammary gland development, and this customized processing results in differential enzymatic activity (constitutive and low pH activatable) best fitting particular stage(s) of development. In addition, at the onset of involution the N-glycan structure of this endopeptidase switches from a mixed high mannose and hybrid structure to an almost exclusively high mannose type, but reverts back to the original N-glycan composition by day 4 of involution. Our findings illuminate (at least in part) the "raison d'être" for CatD's numerous and highly regulated proteolytic processing steps from the pro-form to the mature enzyme. In the mammary gland, specific cleavage product(s) perform specialized function(s) befitting each stage of remodeling. It is noteworthy that deregulated synthesis, secretion and glycosylation of CatD are hallmarks of cancer progression. Thus, identifying the role of CatD in a dynamic normal tissue undergoing highly regulated cycles of remodeling could provide valuable information illuminating the deregulation of CatD associated with cancer development and metastasis.

A testis-specific regulator of complex and hybrid N-glycan synthesis

GnT1IP inhibits GlcNAcT-1 to change N-glycosylation patterns on secretory proteins, potentially regulating germ cell adhesion to Sertoli cells during spermatogenesis.

Database analyses identified 4933434I20Rik as a glycosyltransferase-like gene expressed mainly in testicular germ cells and regulated during spermatogenesis. Expression of a membrane-bound form of the protein resulted in a marked and specific reduction in N-acetylglucosaminyltransferase I (GlcNAcT-I) activity and complex and hybrid N-glycan synthesis. Thus, the novel activity was termed GlcNAcT-I inhibitory protein (GnT1IP). Membrane-bound GnT1IP localizes to the ER, the ER-Golgi intermediate compartment (ERGIC), and the cis-Golgi. Coexpression of membrane-anchored GnT1IP with GlcNAcT-I causes association of the two proteins, inactivation of GlcNAcT-I, and mislocalization of GlcNAcT-I from the medial-Golgi to earlier compartments. Therefore, GnT1IP is a regulator of GlcNAcT-I and complex and hybrid N-glycan production. Importantly, the formation of high mannose N-glycans resulting from inhibition of GlcNAcT-I by GnT1IP markedly increases the adhesion of CHO cells to TM4 Sertoli cells. Testicular germ cells might use GnT1IP to induce the expression of high mannose N-glycans on glycoproteins, thereby facilitating Sertoli–germ cell attachment at a particular stage of spermatogenesis.

N-glycosylation gene DPAGT1 is a target of the Wnt/beta-catenin signaling pathway

Protein N-glycosylation and the Wnt/β-catenin signaling pathways play critical roles in development and cancer. Although N-glycosylation has been shown to influence Wnt signaling through its effects on Wnt ligands, it is unclear whether the Wnt/β-catenin pathway impacts protein N-glycosylation. In this study, we show that promoters of the first N-glycosylation gene, DPAGT1, from Chinese hamster ovary (CHO), Madin-Darby canine kidney (MDCK), and human epidermoid carcinoma (A253) cells contain the T-cell factor/lymphoid enhancer-binding factor (TCF/LEF) consensus sequence. Treatment of cells with a Wnt activator, lithium chloride, up-regulated DPAGT1 transcript levels that correlated with an increase in the β-catenin abundance. Furthermore, exposure of cells to a Wnt receptor ligand, Wnt3a, resulted in an increase in the DPAGT1 transcript levels that was abrogated by the Wnt inhibitor, Dickkopf-1. DNA mobility shift assays revealed specific protein complexes at the DPAGT1 TCF/LEF binding region that were competed off with antibodies to either Tcf3/4 or β-catenin. Chromatin immunoprecipitation analysis confirmed the presence of β-catenin at the DPAGT1 promoter in vivo. In addition, the DPAGT1 TCF/LEF sequence drove the expression of the luciferase reporter gene. Furthermore, up-regulation of DPAGT1 transcripts by Wnt3a led to altered N-glycosylation of E-cadherin. Interestingly, the DPAGT1 TCF/LEF sequence also interacted with γ-catenin, a close homologue of β-catenin, although not in a lithium chloride-dependent manner. Our results provide the first evidence that the Wnt/β-catenin signaling pathway regulates the metabolic pathway of protein N-glycosylation by targeting DPAGT1 expression. Moreover, they suggest the existence of another regulatory mechanism involving the interaction of Tcf with γ-catenin at the DPAGT1 promoter.

Phylogenetic conservation of the cell-type-specific Lan3-2 glycoepitope in Caenorhabditis elegans

The biological function of a cell-type-specific glycosylation of an adhesion molecule belonging to the L1CAM immunoglobulin superfamily was previously determined in the nervous system of the embryonic leech, Hirudo medicinalis. The Lan3-2 glycoepitope is a surface marker of sensory afferent neurons and is required for their appropriate developmental collateral branching and synaptogenesis in the CNS. The chemical structure of the Lan3-2 glycoepitope consists of beta-(1,4)-linked mannopyranose. Here, we show the conservation of the cell-type-specific expression of this mannose polymer in Caenorhabditis elegans. The Lan3-2 glycoepitope is expressed on the cell surface of a subset of dissociated embryonic neurons and, in the adult worm, by the pharyngeal motor neuron, M5, and the chemosensory afferents, the amphids. Additionally, the vulval epithelium expresses the Lan3-2 glycoepitope in late L4 larvae and in adult hermaphrodites. To investigate proteins carrying this restrictively expressed glycoepitope, worm extract was immunoaffinity purified with Lan3-2 monoclonal antibody and Western blotted. A polyclonal antibody reactive with the cytoplasmic tail of LAD-1/SAX-7, a C. elegans member of the L1CAM family, recognizes a 270 kDa protein band while Lan3-2 antibody also recognizes a 190 kDa glycoform, its putative Lan3-2 ectodomain. Thus, in C. elegans, as in leech, the Lan3-2 epitope is located on a L1CAM homologue. The cell-type-specific expression of the Lan3-2 glycoepitope shared by leech and C. elegans will be useful for understanding how cell-type-specific glycoepitopes mediate cell-cell interactions during development.

Neuronal expression of Mgat1 rescues the shortened life span of Drosophila Mgat11 null mutants and increases life span

The enzyme UDP-GlcNAc:alpha3-D-mannoside beta1,2-N-acetylglucosaminyltransferase I (GnT1, encoded by Mgat1) controls the synthesis of paucimannose N-glycans in Drosophila. We have previously reported that null mutations in Drosophila Mgat1 are viable but exhibit defects in locomotion, brain abnormalities, and a severely reduced life span. Here, we show that knockdown of Mgat1 in the central nervous system (CNS) of wild-type flies decreases locomotor activity and life span. This phenotype is similar to that observed in Drosophila Mgat1(1) null mutants, demonstrating that Mgat1 is required in the CNS. We also found that neuronal expression of a wild-type Mgat1 transgene rescued the shortened life span of Mgat1(1) null mutants and resulted in a dramatic 135% increase in mean life span relative to genetically identical controls. Neuronal expression of a wild-type Mgat1 transgene in wild-type flies resulted in a modest 9% increase in mean life span relative to genetically identical controls. In both Mgat1(1) null mutants and wild-type flies, neuronal expression of wild-type Mgat1 transgene resulted in a significant increase in GnT1 activity and resistance to oxidative stress. Whereas dietary restriction is not absolutely essential for the increased life span, it plays a role in the process. Interestingly, we observe a direct correlation between GnT1 activity and mean life span up to a maximum of appropriately 136 days, showing that the ability of GnT1 activity to increase life span is limited. Altogether, these observations suggest that Mgat1-dependent N-glycosylation plays an important role in the control of Drosophila life span.

Recombinant expression and characterization of N-acetylglucosaminyltransferase I derived from Nicotiana tabacum

The C-terminal catalytic domain of tobacco N-acetylglucosaminyltransferase I fused to maltose-binding protein was produced in Escherichia coli as a soluble form with significant activity. The protein was affinity-purified using amylose resin, and its enzymatic properties were investigated, including its divalent cation requirements, optimal temperature, optimal pH, and substrate specificity.

Effects of varying Notch1 signal strength on embryogenesis and vasculogenesis in compound mutant heterozygotes

Background

Identifying developmental processes regulated by Notch1 can be addressed in part by characterizing mice with graded levels of Notch1 signaling strength. Here we examine development in embryos expressing various combinations of Notch1 mutant alleles. Mice homozygous for the hypomorphic Notch1^12f allele, which removes the single O-fucose glycan in epidermal growth factor-like repeat 12 (EGF12) of the Notch1 ligand binding domain (lbd), exhibit reduced growth after weaning and defective T cell development. Mice homozygous for the inactive Notch1^lbd allele express Notch1 missing an ~20 kDa internal segment including the canonical Notch1 ligand binding domain, and die at embryonic day ~E9.5. The embryonic and vascular phenotypes of compound heterozygous Notch1^12f/lbd embryos were compared with Notch1^+/12f, Notch1^12f/12f, and Notch1^lbd/lbd embryos. Embryonic stem (ES) cells derived from these embryos were also examined in Notch signaling assays. While Notch1 signaling was stronger in Notch1^12f/lbd compound heterozygotes compared to Notch1^lbd/lbd embryos and ES cells, Notch1 signaling was even stronger in embryos carrying Notch1^12f and a null Notch1 allele.

Results

Mouse embryos expressing the hypomorphic Notch1^12f allele, in combination with the inactive Notch1^lbd allele which lacks the Notch1 ligand binding domain, died at ~E11.5-12.5. Notch1^12f/lbd ES cells signaled less well than Notch1^12f/12f ES cells but more strongly than Notch1^lbd/lbd ES cells. However, vascular defects in Notch1^12f/lbd yolk sac were severe and similar to Notch1^lbd/lbd yolk sac. By contrast, vascular disorganization was milder in Notch1^12f/lbd compared to Notch1^lbd/lbd embryos. The expression of Notch1 target genes was low in Notch1^12f/lbd yolk sac and embryo head, whereas Vegf and Vegfr2 transcripts were increased. The severity of the compound heterozygous Notch1^12f/lbd yolk sac phenotype suggested that the allelic products may functionally interact. By contrast, compound heterozygotes with Notch1^12f in combination with a Notch1 null allele (Notch1^tm1Con) were capable of surviving to birth.

Conclusions

Notch1 signaling in Notch1^12f/lbd compound heterozygous embryos is more defective than in compound heterozygotes expressing a hypomorphic Notch1^12f allele and a Notch1 null allele. The data suggest that the gene products Notch1^lbd and Notch1^12f interact to reduce the activity of Notch1^12f.

Metabolism, cell surface organization, and disease

Genetic information flows from DNA to macromolecular structures-the dominant force in the molecular organization of life. However, recent work suggests that metabolite availability to the hexosamine and Golgi N-glycosylation pathways exerts control over the assembly of macromolecular complexes on the cell surface and, in this capacity, acts upstream of signaling and gene expression. The structure and number of N-glycans per protein molecule cooperate to regulate lectin binding and thereby the distribution of glycoproteins at the cell surface. Congenital disorders of glycosylation provide insight as extreme hypomorphisms, whereas milder deficiencies may encompass many common chronic conditions, including autoimmunity, metabolic syndrome, and aging.

Targeted genetic inactivation of N-acetylglucosaminyltransferase-IVa impairs insulin secretion from pancreatic beta cells and evokes type 2 diabetes

The biological significance of protein N-glycosylation has been elucidated using a mouse model bearing a genetic mutation of N-acetylglucosaminyltransferases (GnTs), which initiate the formation of specific branch structures on the mannose core of N-glycans. These glycosylation defects evoked a variety of abnormalities and disorders in specific cell types, tissues, and the whole body, reflecting functional requirements. N-Acetylglucosaminyltransferase-IVa (GnT-IVa) initiates the GlcNAcbeta1-4 branch synthesis on the Manalpha1-3 arm of the N-glycan core thereby increasing N-glycan branch complexity. To investigate the physiological function of GnT-IVa, we engineered and characterized GnT-IVa-deficient mice. GnT-IVa-deficient mice showed a metabolic disorder subsequently diagnosed as type 2 diabetes. In this chapter, methods for characterizing GnT-IVa-deficient mice by physiological analyses to detect metabolic alterations and biochemical analyses using primary isolated pancreatic beta cells are summarized and discussed.

Roles of glycosylation in Notch signaling

Notch and the DSL Notch ligands Delta and Serrate/Jagged are glycoproteins with a single transmembrane domain. The extracellular domain (ECD) of both Notch receptors and Notch ligands contains numerous epidermal growth factor (EGF)-like repeats which are post-translationally modified by a variety of glycans. Inactivation of a subset of genes that encode glycosyltransferases which initiate and elongate these glycans inhibits Notch signaling. In the formation of developmental boundaries in Drosophila and mammals, in mouse T-cell and marginal zone B-cell development, and in co-culture Notch signaling assays, the regulation of Notch signaling by glycans is to date a cell-autonomous effect of the Notch-expressing cell. The regulation of Notch signaling by glycans represents a new paradigm of signal transduction. O-fucose glycans modulate the strength of Notch binding to DSL Notch ligands, while O-glucose glycans facilitate juxta-membrane cleavage of Notch, generating the substrate for intramembrane cleavage and Notch activation. Identifying precisely how the addition of particular sugars at specific locations on Notch modifies Notch signaling is a challenge for the future.

Mass spectrometric analysis of mutant mice

Mass spectrometry (MS) has proven to be the preeminent tool for the rapid, high-sensitivity analysis of the primary structure of glycans derived from diverse biological sources including cells, fluids, secretions, tissues, and organs. These analyses are anchored by matrix-assisted laser desorption ionization time of flight (MALDI-TOF) analysis of permethylated derivatives of glycan pools released from the samples, to produce glycomic mass fingerprints. The application of complimentary techniques, such as chemical and enzymatic digestions, GC-MS linkage analysis, and tandem mass spectrometry (MS/MS) utilizing both electrospray (ES) and MALDI-TOF/TOF, together with bioinformatic tools allows the elucidation of incrementally more detailed structural information from the sample(s) of interest. The mouse as a model organism offers many advantages in the study of human biology, health, and disease; it is a mammal, shares 99% genetic homology with humans and its genome supports targeted mutagenesis in specific genes to produce knockouts efficiently and precisely. Glycomic analyses of tissues and organs from mice genetically deficient in one or more glycosylation gene and comparison with data collected from wild-type samples enables the facile identification of changes and perturbations within the glycome. The Consortium for Functional Glycomics (CFG) has been applying such MS-based glycomic analyses to a range of murine tissues from both wild-type and glycosylation-knockout mice in order to provide a repository of structural data for the glycobiology community. In this chapter, we describe in detail the methodologies used to prepare, derivatize, purify, and analyze glycan pools from mouse organs and tissues by MS. We also present a summary of data produced from the CFG systematic structural analysis of wild-type and knockout mouse tissues, together with a detailed example of a glycomic analysis of the Mgat4a knockout mouse.

Manipulating cell surface glycoproteins by targeting N-glycan-galectin interactions

The interaction of cell surface receptors and transporters with cognate ligands depends on their concentration, distribution, and organization at the cellular surface. The majority of cell surface receptors and transporters are co- and/or post-translationally modified with asparagine (N)-linked oligosaccharides (glycans). N-Glycan number and structure combine to control the concentration of glycoproteins at the cell surface through interactions with endogenous lectins such as galectins. ER/Golgi enzyme activity and hexosamine pathway supply of Golgi metabolites co-dependently regulate N-glycan biosynthesis and combine to provide adaptive control over cell growth and differentiation. Studies in mice and humans have revealed metabolic and genetic dysregulation of N-glycosylation in T-cell-mediated autoimmunity. In this chapter, we describe methods used to analyze N-glycan-galectin interactions in controlling the distribution and organization of cell surface receptors and transporters.

Physiological and glycomic characterization of N-acetylglucosaminyltransferase-IVa and -IVb double deficient mice

N-Acetylglucosaminyltransferase-IV (GnT-IV) has two isoenzymes, GnT-IVa and GnT-IVb, which initiate the GlcNAcbeta1-4 branch synthesis on the Manalpha1-3 arm of the N-glycan core thereby increasing N-glycan branch complexity and conferring endogenous lectin binding epitopes. To elucidate the physiological significance of GnT-IV, we engineered and characterized GnT-IVb-deficient mice and further generated GnT-IVa/-IVb double deficient mice. In wild-type mice, GnT-IVa expression is restricted to gastrointestinal tissues, whereas GnT-IVb is broadly expressed among organs. GnT-IVb deficiency induced aberrant GnT-IVa expression corresponding to the GnT-IVb distribution pattern that might be attributed to increased Ets-1, which conceivably activates the Mgat4a promoter, and thereafter preserved apparent GnT-IV activity. The compensative GnT-IVa expression might contribute to amelioration of the GnT-IVb-deficient phenotype. GnT-IVb deficiency showed mild phenotypic alterations in hematopoietic cell populations and hemostasis. GnT-IVa/-IVb double deficiency completely abolished GnT-IV activity that resulted in the disappearance of the GlcNAcbeta1-4 branch on the Manalpha1-3 arm that was confirmed by MALDI-TOF MS and GC-MS linkage analyses. Comprehensive glycomic analyses revealed that the abundance of terminal moieties was preserved in GnT-IVa/-IVb double deficiency that was due to the elevated expression of glycosyltransferases regarding synthesis of terminal moieties. Thereby, this may maintain the expression of glycan ligands for endogenous lectins and prevent cellular dysfunctions. The fact that the phenotype of GnT-IVa/-IVb double deficiency largely overlapped that of GnT-IVa single deficiency can be attributed to the induced glycomic compensation. This is the first report that mammalian organs have highly organized glycomic compensation systems to preserve N-glycan branch complexity.

Definitive evidence that a single N-glycan among three glycans on inducible costimulator is required for proper protein trafficking and ligand binding

Glycosylation is a widespread post-translational modification found in glycoproteins. Glycans play key roles in protein folding, quality control in the endoplasmic reticulum (ER) and protein trafficking within cells. However, it remains unclear whether all positions of protein glycosylation are involved in glycan functions, or if specific positions have individual roles. Here we demonstrate the integral involvement of a specific N-glycan from amongst the three glycans present on inducible costimulator (ICOS), a T-cell costimulatory molecule, in proper protein folding and intracellular trafficking to the cell surface membrane. We found that glycosylation-defective mutant proteins lacking N-glycan at amino-acid position 89 (N89), but not proteins lacking either N23 or N110, were retained within the cell and were not detected on the cell surface membrane. Additional evidence suggested that N89 glycosylation was indirectly involved in ICOS ligand binding. These data suggest that amongst the three putative ICOS glycosylation sites, N89 is required for proper ICOS protein folding in the ER, intracellular trafficking and ligand binding activity. This study represents a substantial contribution to the current mechanistic understanding of the necessity and potential functions of a specific N-glycan among the multiple glycans of glycoproteins.

Golgi linked protein glycosylation and associated diseases

One of the Golgi's main functions is the glycosylation of secreted proteins. A large variety of glycan chains can be synthesized in the Golgi, and it is increasingly clear that these are critical in basic cellular functions as well as the development of multicellular organisms. The structurally best-documented glycans are N-glycans, yet these are also the most enigmatic in their function. In contrast, O-glycan function is far better understood, but here the structures and biosynthetic pathways are very incomplete. The critical importance of glycans is highlighted by the broad spectrum of diseases they are associated with, such as a number of inherited diseases, but also cancers or diabetes. The molecular clues to these, however, are only just being elucidated. Although some glycan structures are known to be involved in signaling or adhesion to the extracellular matrix, for most the functions are not yet known. This review aims at summarizing current knowledge as much as to point out critical areas key for future progress.

Novel Kv3 glycoforms differentially expressed in adult mammalian brain contain sialylated N-glycans

The N-glycan pool of mammalian brain contains remarkably high levels of sialylated N-glycans. This study provides the first evidence that voltage-gated K+ channels Kv3.1, Kv3.3, and Kv3.4, possess distinct sialylated N-glycan structures throughout the central nervous system of the adult rat. Electrophoretic migration patterns of Kv3.1, Kv3.3, and Kv3.4 glycoproteins from spinal cord, hypothalamus, thalamus, cerebral cortex, hippocampus, and cerebellum membranes digested with glycosidases were used to identify the various glycoforms. Differences in the migration of Kv3 proteins were attributed to the desialylated N-glycans. Expression levels of the Kv3 proteins were highest in cerebellum, whereas those of Kv3.1 and Kv3.3 were much lower in the other 5 regions. The lowest level of Kv3.1 was expressed in the hypothalamus, whereas the lowest levels of Kv3.3 were expressed in both thalamus and hypothalamus. The other regions expressed intermediate levels of Kv3.3, with spinal cord expressing the highest. The expression level of Kv3.4 in the hippocampus was slightly lower than that in cerebellum, and was closely followed by the other 4 regions, with spinal cord expressing the lowest level. We suggest that novel Kv3 glycoforms may endow differences in channel function and expression among regions throughout the central nervous system.

T cell receptor signaling co-regulates multiple Golgi genes to enhance N-glycan branching

T cell receptor (TCR) signaling enhances beta1,6GlcNAc-branching in N-glycans, a phenotype that promotes growth arrest and inhibits autoimmunity by increasing surface retention of cytotoxic T lymphocyte antigen-4 (CTLA-4) via interactions with galectins. N-Acetylglucosaminyltransferase V (MGAT5) mediates beta1,6GlcNAc-branching by transferring N-acetylglucosamine (GlcNAc) from UDP-GlcNAc to N-glycan substrates produced by the sequential action of Golgi alpha1,2-mannosidase I (MIa,b,c), MGAT1, alpha1,2-mannosidase II (MII, IIx), and MGAT2. Here we report that TCR signaling enhances mRNA levels of MIa,b,c and MII,IIx in parallel with MGAT5, whereas limiting levels of MGAT1 and MGAT2. Blocking the increase in MI or MII enzyme activity induced by TCR signaling with deoxymannojirimycin or swainsonine, respectively, limits beta1,6GlcNAc-branching, suggesting that enhanced MI and MII activity are both required for this phenotype. MGAT1 and MGAT2 have an approximately 250- and approximately 20-fold higher affinity for UDP-GlcNAc than MGAT5, respectively, and increasing MGAT1 expression paradoxically inhibits beta1,6GlcNAc branching by limiting UDP-GlcNAc supply to MGAT5, suggesting that restricted changes in MGAT1 and MGAT2 mRNA levels in TCR-stimulated cells serves to enhance availability of UDP-GlcNAc to MGAT5. Together, these data suggest that TCR signaling differentially regulates multiple N-glycan-processing enzymes at the mRNA level to cooperatively promote beta1,6GlcNAc branching, and by extension, CTLA-4 surface expression, T cell growth arrest, and self-tolerance.

Regulation and pathophysiological implications of UDP-GlcNAc 2-epimerase/ManNAc kinase (GNE) as the key enzyme of sialic acid biosynthesis

The key enzyme for the biosynthesis ofN-acetylneuraminic acid, from which all other sialic acids are formed, is the bifunctional enzyme UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE). GNE is a highly conserved protein found throughout the animal kingdom. Its highest expression is seen in the liver and placenta. GNE is regulated by a variety of biochemical means, including tetramerization promoted by the substrate UDP-GlcNAc, phosphorylation by protein kinase C and feedback inhibition by CMP-Neu5Ac, which is defect in the human disease sialuria. GNE knock-out in mice leads to embryonic lethality, emphasizing the crucial role of this key enzyme for sialic acid biosynthesis. The metabolic capacity to synthesize sialic acid and CMP-sialic acid upon ManNAc loads is amazingly high. An additional characteristic of GNE is its interaction with proteins involved in the regulation of development, which might play a crucial role in the hereditary inclusion body myopathy. Due to the importance of increased concentrations of tumor-surface sialic acid, first attempts to find inhibitors of GNE have been successful.

Cell surface N-glycans mediated isolation of mouse neural stem cells

The isolation of neural stem cells (NSCs) from the brain has been hampered by the lack of valid cell surface markers and the requirement for long-term in vitro cultivation that may lead to phenotype deterioration. However, few suitable specific cell surface antigens are available on NSCs that could be used for their prospective isolation. The present study demonstrated that the expression of complex type asparagine-linked oligosaccharide (N-glycans) was detected on brain cells dissociated from embryonic and adult brain using Phaseolus vulgaris erythroagglutinating lectin (E-PHA) which binds to biantennary complex type N-glycans, and demonstrated that E-PHA bound preferentially to purified NSCs, but not to neurons, microglia, or oligodendrocyte precursor cells. The labeling of dissociated mouse embryonic brain cells or adult brain cells with E-PHA enabled the enrichment of NSCs by 25-fold or 9-fold of the number of neurosphere-forming cells in comparison to that of unsorted cells, respectively. Furthermore, a lectin blot analysis revealed the presence of several glycoproteins which were recognized by E-PHA in the membrane fraction of the proliferating NSCs, but not in the differentiated cells. These results indicate that complex type N-glycans is a valuable cell surface marker for living mouse NSCs from both the embryonic and adult brain.

Atypical sialylated N-glycan structures are attached to neuronal voltage-gated potassium channels

Mammalian brains contain relatively high amounts of common and uncommon sialylated N-glycan structures. Sialic acid linkages were identified for voltage-gated potassium channels, Kv3.1, 3.3, 3.4, 1.1, 1.2 and 1.4, by evaluating their electrophoretic migration patterns in adult rat brain membranes digested with various glycosidases. Additionally, their electrophoretic migration patterns were compared with those of NCAM (neural cell adhesion molecule), transferrin and the Kv3.1 protein heterologously expressed in B35 neuroblastoma cells. Metabolic labelling of the carbohydrates combined with glycosidase digestion reactions were utilized to show that the N-glycan of recombinant Kv3.1 protein was capped with an oligo/poly-sialyl unit. All three brain Kv3 glycoproteins, like NCAM, were terminated with α2,3-linked sialyl residues, as well as atypical α2,8-linked sialyl residues. Additionally, at least one of their antennae was terminated with an oligo/poly-sialyl unit, similar to recombinant Kv3.1 and NCAM. In contrast, brain Kv1 glycoproteins consisted of sialyl residues with α2,8-linkage, as well as sialyl residues linked to internal carbohydrate residues of the carbohydrate chains of the N-glycans. This type of linkage was also supported for Kv3 glycoproteins. To date, such a sialyl linkage has only been identified in gangliosides, not N-linked glycoproteins. We conclude that all six Kv channels (voltage-gated K+ channels) contribute to the α2,8-linked sialylated N-glycan pool in mammalian brain and furthermore that their N-glycan structures contain branched sialyl residues. Identification of these novel and unique sialylated N-glycan structures implicate a connection between potassium channel activity and atypical sialylated N-glycans in modulating and fine-tuning the excitable properties of neurons in the nervous system.

Role of unusual O-glycans in intercellular signaling

In the last two decades, our knowledge of the role of glycans in development and signal transduction has expanded enormously. While most work has focused on the importance of N-linked or mucin-type O-linked glycosylation, recent work has highlighted the importance of several more unusual forms of glycosylation that are the focus of this review. In particular, the ability of O-fucose glycans on the epidermal growth factor-like (EGF) repeats of Notch to modulate signaling places glycosylation alongside phosphorylation as a means to modulate protein-protein interactions and their resultant downstream signals. The recent discovery that O-glucose modification of Notch EGF repeats is also required for Notch function has further expanded the range of glycosylation events capable of modulating Notch signaling. The prominent role of Notch during development and in later cell-fate decisions underscores the importance of these modifications in human biology. The role of glycans in intercellular signaling events is only beginning to be understood and appears ready to expand into new areas with the discovery that thrombospondin type 1 repeats are also modified with O-fucose glycans. Finally, a rare form of glycosylation called C-mannosylation modifies tryptophans in some signaling competent molecules and may be a further layer of complexity in the field. We will review each of these areas focusing on the glycan structures produced, the consequence of their presence, and the enzymes responsible.

Lectin panning method: the prospective isolation of mouse neural progenitor cells by the attachment of cell surface N-glycans to Phaseolus vulgaris erythroagglutinating lectin-coated dishes

Retrospective isolation of neural progenitor cells (NPCs) may cause deterioration of the phenotype during the long-term cultivation. Therefore, prospective isolation is essential for understanding the exact characteristics of intact NPCs in the brain. However, few suitable specific cell surface antigens on NPCs that could be used for their prospective isolation are available. The present study demonstrated that within 60 min after initial plating, embryonic day 12 (E12) brain cells firmly attach to several types of lectin-coated culture wells, including Phaseolus vulgaris erythroagglutinating lectin (E-PHA), concanavalin A (Con A) and wheat germ agglutinin (WGA). Approximately 80% of the cells isolated from E-PHA-coated wells expressed the nestin antigen, which is a specific intracellular marker for NPCs and the ratio of 5-bromo-2'-deoxyuridine (BrdU)-positive/nestin-positive cells to the cells attached on the E-PHA-coated wells was significantly higher than that of the cells attached on the wells coated with other adhesive substrates. The cells that were isolated from the E-PHA-coated wells continued to attach to the well for 1 week, while those isolated from Con A- and WGA-coated wells lost their attachment after 6 days and 1 day, respectively. Furthermore, the cells isolated from the E-PHA-coated wells grew quite satisfactorily and formed numerous attached neurospheres. Their growth rate was almost equal to that observed in suspension cultures. These results indicate that the lectin panning method enables the prospective, quick and easy isolation of mouse NPCs without requiring a fluorescence-activated cell sorter (FACS) system.

N-Glycans in cancer progression

N-Glycan branching in the medial-Golgi generates ligands for lattice-forming lectins (e.g., galectins) that regulate surface levels of glycoproteins including epidermal growth factor (EGF) and transforming growth factor-beta (TGF-beta) receptors. Moreover, functional classes of glycoproteins differ in N-glycan multiplicities (number of N-glycans/peptide), a genetically encoded feature of glycoproteins that interacts with metabolic flux (UDP-GlcNAc) and N-glycan branching to differentially regulate surface levels. Oncogenesis increases beta1,6-N-acetylglucosaminyltransferase V (encoded by Mgat5) expression, and its high-affinity galectin ligands promote surface retention of growth receptors with a reduced dependence on UDP-GlcNAc. Mgat5(-/-) tumor cells are less metastatic in vivo and less responsive to cytokines in vitro, but undergo secondary changes that support tumor cell proliferation. These include loss of Caveolin-1, a negative regulator of EGF signaling, and increased reactive oxygen species, an inhibitor of phosphotyrosine phosphatases. These studies suggest a systems approach to cancer treatment where the surface distribution of receptors is targeted through metabolism and N-glycan branching to induce growth arrest.

In vivo site-specific recombination using the β-rec/six system

The prokaryotic β serine recombinase (β-rec) catalyzes site-specific recombination between two directly oriented six sites (93 bp) in mammalian cells, both in episomal and in chromosomally integrated substrates. The β-rec/six exclusive intramolecular site-specific recombination (SSR) system has been proposed as a suitable approach when several independently controlled recombination events are needed in a single cell. Here we explored the use of the β-rec/six system for selective induction of genome-targeted modifications. We generated and analyzed mouse transgenic lines (Tgβ) expressing β-rec under the control of the Lck promoter. β-rec activity was demonstrated, and there was no evidence of alterations to thymic or peripheral T cell development. We developed two transgenic mouse lines harboring different target sequences (Tgrec and KOsix) and analyzed the effect of β-rec expression on these animals. The results indicate that the β-rec/six SSR system is functional for in vivo gene-targeting applications.

In vivo consequences of deleting EGF repeats 8-12 including the ligand binding domain of mouse Notch1

BACKGROUND

Notch signaling is highly conserved in the metazoa and is critical for many cell fate decisions. Notch activation occurs following ligand binding to Notch extracellular domain. In vitro binding assays have identified epidermal growth factor (EGF) repeats 11 and 12 as the ligand binding domain of Drosophila Notch. Here we show that an internal deletion in mouse Notch1 of EGF repeats 8-12, including the putative ligand binding domain (lbd), is an inactivating mutation in vivo. We also show that maternal and zygotic Notch1(lbd/lbd) mutant embryos develop through gastrulation to mid-gestation.

RESULTS

Notch1(lbd/lbd) embryos died at mid-gestation with a phenotype indistinguishable from Notch1 null mutants. In embryonic stem (ES) cells, Notch1(lbd) was expressed on the cell surface at levels equivalent to wild type Notch1, but Delta1 binding was reduced to the same level as in Notch1 null cells. In an ES cell co-culture assay, Notch signaling induced by Jagged1 or Delta1 was reduced to a similar level in Notch1(lbd) and Notch1 null cells. However, the Notch1(lbd/lbd) allele was expressed similarly to wild type Notch1 in Notch1(lbd/lbd) ES cells and embryos at E8.75, indicating that Notch1 signaling is not essential for the Notch1 gene to be expressed. In addition, maternal and zygotic Notch1 mutant blastocysts developed through gastrulation.

CONCLUSION

Mouse Notch1 lacking the ligand binding domain is expressed at the cell surface but does not signal in response to the canonical Notch ligands Delta1 and Jagged1. Homozygous Notch1(lbd/lbd) mutant embryos die at approximately E10 similar to Notch1 null embryos. While Notch1 is expressed in oocytes and blastocysts, Notch1 signaling via canonical ligands is dispensable during oogenesis, blastogenesis, implantation and gastrulation.

Bitter sweetness of complexity

Glycosylation of proteins, lipids and mucins has gained increasing complexity in the course of evolution. Metazoans and mammals exhibit extensively exploited pathways of N-glycan biosynthesis, with unique features that are not found in plants or protozoans.Paralleling the complexity of N-glycan structure, their impact on regulatory processes has become very diverse and has evolved into a multidimensional lattice imprinting modes of cellular communication. Processes that are regulated by N-glycans are cellular adhesion and motility, growth factor and cytokine signalling, metabolic homeostasis, and binding of certain pathogens. Consequently, alterations in N-glycan biosynthesis interfere with cellular proliferation and differentiation and may produce disturbances in embryonic development, trigger inflammatory processes, favour tumour development and enhance the metastastic dissemination of primary tumours. Particular N-glycans that have been causally related to these pathological scenarios are the complex-type N-glycans, branching from oligomannosidic core structures into β-glycosidic linkages, connected to acetylated glucosamine and galactose, and yield extended lactosamine chains of variable length. These N-acetyllactosamines are preferred building blocks for further modification by fucosylation, sialylation, and sulphation, thus creating binding sites for different galectins or selectins. The focus of this review will be on the b1,6-N -acetylglucosaminyltransferase-V/GnT-V/MGAT5, a phylogenically conserved enzyme that is required for the synthesis of β1,6-branched complex-type oligosaccharides in the medial Golgi compartment, and its implications in metabolism and cancer progression.

Beta N-acetylglucosaminyltransferase V (Mgat5) deficiency reduces the depression-like phenotype in mice

The central nervous system (CNS) is rich in glycoconjugates, located on cell surface and in extracellular matrix. The products of Golgi UDP-GlcNAc:N-acetylglucosaminyltransferases (encoded by Mgat1, Mgat2, Mgat4 and Mgat5) act sequentially to generate the GlcNAc-branched complex-type N-glycans on glycoprotein receptors. While elimination of all the branched N-glycans in Mgat1(-/-) mouse embryos is lethal at neural tube fold stage, decreased branching is associated with late developmental defects similar to type 2 of congenital disorders of glycosylation, with developmental and psychomotor abnormalities. To study the role of complex-type N-glycans in brain function, we tested Mgat5(-/-) mice in a battery of neurological and behavioral tests. Despite the absence of tri- and tetra-antennary products, Mgat5(-/-) mice were not different from their wild-type littermates in physical and neurological assessments, anxiety level, startle reactivity and sensorimotor gating. However, they displayed a robust decrease in the immobility time in the forced swim test and the tail suspension test independent of locomotor activity, interpreted as a change in depression-like behavior. This effect was accentuated after chronic mild stress. Comparable increase in plasma corticosterone of Mgat5(+/+) and Mgat5(-/-) mice in response to acute stress shows an intact function of the hypothalamus-pituitary-adrenal axis. A change in social interactions was also observed. Our results indicate that Mgat5 modification of complex-type N-glycans on CNS glycoproteins is involved in the regulation of depression-like behavior.

The autoimmune regulator directly controls the expression of genes critical for thymic epithelial function

The autoimmune regulator (Aire) gene plays an essential role in negative selection of T cells and deletion of autoreactive T cells in the thymus. The defect in thymic selection in Aire(-/-) mice was attributed to the repressed expression of tissue-specific Ags in the thymic epithelial cells and defective Ag presentation; however, the molecular mechanism underlying these functions has been elusive. Using the chromatin immunoprecipitation technique, we demonstrate here that Aire binds in vivo to specific DNA sequence motifs and directly regulates thymic expression of genes important for thymic functions including expression of autoantigens, cytokines, transcription factors, and posttranslational modifiers. These results unambiguously established Aire as a key transcriptional regulator of the immune system.

Complex N-glycan number and degree of branching cooperate to regulate cell proliferation and differentiation

The number of N-glycans (n) is a distinct feature of each glycoprotein sequence and cooperates with the physical properties of the Golgi N-glycan-branching pathway to regulate surface glycoprotein levels. The Golgi pathway is ultrasensitive to hexosamine flux for the production of tri- and tetra-antennary N-glycans, which bind to galectins and form a molecular lattice that opposes glycoprotein endocytosis. Glycoproteins with few N-glycans (e.g., TbetaR, CTLA-4, and GLUT4) exhibit enhanced cell-surface expression with switch-like responses to increasing hexosamine concentration, whereas glycoproteins with high numbers of N-glycans (e.g., EGFR, IGFR, FGFR, and PDGFR) exhibit hyperbolic responses. Computational and experimental data reveal that these features allow nutrient flux stimulated by growth-promoting high-n receptors to drive arrest/differentiation programs by increasing surface levels of low-n glycoproteins. We have identified a mechanism for metabolic regulation of cellular transition between growth and arrest in mammals arising from apparent coevolution of N-glycan number and branching.

Disruption of alpha-mannosidase processing induces non-canonical hybrid-type glycosylation

Golgi alpha-mannosidase II is essential for the efficient formation of complex-type glycosylation. Here, we demonstrate that the disruption of Golgi alpha-mannosidase II activity by swainsonine in human embryonic kidney cells is capable of inducing a novel class of hybrid-type glycosylation containing a partially processed mannose moiety. The discovery of 'Man(6)-based' hybrid-type glycans reveals a broader in vivo specificity of N-acetylglucosaminyltransferase I, further defines the arm-specific tolerance of core alpha1-6 fucosyltransferase to terminal alpha1-2 mannose residues, and suggests that disruption of Golgi alpha-mannosidase II activity is capable of inducing potentially 'non-self' structures.

Molecular cloning and characterization of the Caenorhabditis elegans alpha1,3-fucosyltransferase family

The genome of Caenorhabditis elegans encodes five genes with homology to known alpha1,3 fucosyltransferases (alpha1,3FTs), but their expression and functions are poorly understood. Here we report the molecular cloning and characterization of these C. elegans alpha1,3FTs (CEFT-1 through -5). The open-reading frame for each enzyme predicts a type II transmembrane protein and multiple potential N-glycosylation sites. We prepared recombinant epitope-tagged forms of each CEFT and found that they had unusual acceptor specificity, cation requirements, and temperature sensitivity. CEFT-1 acted on the N-glycan pentasaccharide core acceptor to generate Manalpha1-3(Manalpha1-6)Manbeta1-4GlcNAcbeta1-4(Fucalpha1-3)GlcNAcbeta1-Asn. In contrast, CEFT-2 did not act on the pentasaccharide acceptor, but instead utilized a LacdiNAc acceptor to generate GalNAcbeta1-4(Fucalpha1-3)GlcNAcbeta1-3Galbeta1-4Glc, which is a novel activity. CEFT-3 utilized a LacNAc acceptor to generate Galbeta1-4(Fucalpha1-3)GlcNAcbeta1-3Galbeta1-4Glc without requiring cations. CEFT-4 was similar to CEFT-3, but its activity was enhanced by some divalent cations. Recombinant CEFT-5 was well expressed, but did not act on available acceptors. Each CEFT was optimally active at room temperature and rapidly lost activity at 37 degrees C. Promoter analysis showed that CEFT-1 is expressed in C. elegans eggs and adults, but its expression was restricted to a few neuronal cells at the head and tail. We prepared deletion mutants for each enzyme for phenotypic analysis. While loss of CEFT-1 correlated with loss of pentasaccharide core activity and core alpha1,3-fucosylated glycans in worms, loss of other enzymes did not correlate with any phenotypic changes. These results suggest that each of the alpha1,3FTs in C. elegans has unique specificity and expression patterns.

Complex oligosaccharides are N-linked to Kv3 voltage-gated K+ channels in rat brain

Neuronal Kv3 voltage-gated K(+) channels have two absolutely conserved N-glycosylation sites. Here, it is shown that Kv3.1, 3.3, and 3.4 channels are N-glycosylated in rat brain. Digestion of total brain membranes with peptide N glycosidase F (PNGase F) produced faster migrating immunobands than those of undigested membranes. Additionally, partial PNGase F digests showed that both sites are occupied by oligosaccharides. Neuraminidase treatment produced a smaller immunoband shift relative to PNGase F treatment. These results indicate that both sites are highly available and occupied by N-linked oligosaccharides for Kv3.1, 3.3, and 3.4 in rat brain, and furthermore that at least one oligosaccharide is of complex type. Additionally, these results point to an extracytoplasmic S1-S2 linker in Kv3 proteins expressed in native membranes. We suggest that N-glycosylation processing of Kv3 channels is critical for the expression of K(+) currents at the surface of neurons, and perhaps contributes to the pathophysiology of congenital disorders of glycosylation.

Glycosylation in cellular mechanisms of health and disease

Glycosylation produces an abundant, diverse, and highly regulated repertoire of cellular glycans that are frequently attached to proteins and lipids. The past decade of research on glycan function has revealed that the enzymes responsible for glycosylation-the glycosyltransferases and glycosidases-are essential in the development and physiology of living organisms. Glycans participate in many key biological processes including cell adhesion, molecular trafficking and clearance, receptor activation, signal transduction, and endocytosis. This review discusses the increasingly sophisticated molecular mechanisms being discovered by which mammalian glycosylation governs physiology and contributes to disease.

Respiratory distress and neonatal lethality in mice lacking Golgi alpha1,2-mannosidase IB involved in N-glycan maturation

There are three mammalian Golgi alpha1,2-mannosidases, encoded by different genes, that form Man5GlcNAc2 from Man(8-9)GlcNAc2 for the biosynthesis of hybrid and complex N-glycans. Northern blot analysis and in situ hybridization indicate that the three paralogs display distinct developmental and tissue-specific expression. The physiological role of Golgi alpha1,2-mannosidase IB was investigated by targeted gene ablation. The null mice have normal gross appearance at birth, but they display respiratory distress and die within a few hours. Histology of fetal lungs the day before birth indicate some delay in development, whereas neonatal lungs show extensive pulmonary hemorrhage in the alveolar region. No significant histopathological changes occur in other tissues. No remarkable ultrastructural differences are detected between wild type and null lungs. The membranes of a subset of bronchiolar epithelial cells are stained with lectins from Phaseolus vulgaris (leukoagglutinin and erythroagglutinin) and Datura stramonium in wild type lungs, but this staining disappears in lungs from null mice. Mass spectrometry of N-glycans from different tissues shows no significant changes in global N-glycans of null mice. Therefore, only a few glycoproteins required for normal lung function depend on alpha1,2-mannosidase IB for maturation. There are no apparent differences in the expression of several lung epithelial cell and endothelial cell markers between null and wild type mice. The alpha1,2-mannosidase IB null phenotype differs from phenotypes caused by ablation of other enzymes in N-glycan biosynthesis and from other mouse gene disruptions that affect pulmonary development and function.

Protein Glycosylation, Conserved from Yeast to Man: A Model Organism Helps Elucidate Congenital Human Diseases

Proteins can be modified by a large variety of covalently linked saccharides. The present review concentrates on two types, protein N-glycosylation and protein O-mannosylation, which, with only a few exceptions, are evolutionary conserved from yeast to man. They are also distinguished by some special features: The corresponding glycosylation processes start in the endoplasmatic reticulum, are continued in the Golgi apparatus, and require dolichol-activated precursors for the initial biosynthetic steps. With respect to the molecular biology of both types of protein glycosylation, the pathways and the genetic background of the reactions have most successfully been studied with the genetically easy-to-handle baker's yeast, Saccharomyces cerevisae. Many of the severe developmental disturbances in children are related to protein glycosylation, for example, the CDG syndrome (congenital disorders of glycosylation) as well as congenital muscular dystrophies with neuronal-cell-migration defects have been elucidated with the help of yeast.

Conditional deletion of the colony stimulating factor-1 receptor (c-fms proto-oncogene) in mice

Colony stimulating factor-1 (CSF-1) is the primary regulator of the mononuclear phagocytic lineage acting through its transmembrane tyrosine kinase receptor, CSF-1R, that is the product of the c-fms proto-oncogene. Null mutations in either the ligand or the receptor genes result in a severe osteopetrosis as well as a number of other phenotypes, including reproductive defects and perturbations in organ development. The CSF-1R is also expressed in oocytes, myoblast progenitors, decidual, and trophoblastic cells. To distinguish cell type specific phenotypes, we have created a conditional allele of the Csf1r by placing LoxP sites around Exon 5 of the Csf1r gene in mice. Excision of this floxed sequence results in a null allele that in the homozygous state gives a phenotype indistinguishable of the complete Csf1r null mutant mouse. This conditional allele will prove extremely valuable to study the spatial and temporal roles of CSF-1R.

A deletion in the golgi alpha-mannosidase II gene of Caenorhabditis elegans results in unexpected non-wild-type N-glycan structures

The processing of N-linked oligosaccharides by alpha-mannosidases in the endoplasmic reticulum and Golgi is a process conserved in plants and animals. After the transfer of a GlcNAc residue to Asn-bound Man(5)GlcNAc(2) by N-acetylglucosaminyltransferase I, an alpha-mannosidase (EC 3.2.1.114) removes one alpha1,3-linked and one alpha1,6-linked mannose residue. In this study, we have identified the relevant alpha-mannosidase II gene (aman-2; F58H1.1) from Caenorhabditis elegans and have detected its activity in both native and recombinant forms. For comparative studies, the two other cDNAs encoding class II mannosidases aman-1 (F55D10.1) and aman-3 (F48C1.1) were cloned; the corresponding enzymes are, respectively, a putative lysosomal alpha-mannosidase and a Co(II)-activated alpha-mannosidase. The analysis of the N-glycan structures of an aman-2 mutant strain demonstrates that the absence of alpha-mannosidase II activity results in a shift to structures not seen in wild-type worms (e.g. N-glycans with the composition Hex(5-7)HexNAc(2-3)Fuc(2)Me) and an accumulation of hybrid oligosaccharides. Paucimannosidic glycans are almost absent from aman-2 worms, indicative also of a general lack of alpha-mannosidase III activity. We hypothesize that there is a tremendous flexibility in the glycosylation pathway of C. elegans that does not impinge, under standard laboratory conditions, on the viability of worms with glycotypes very unlike the wild-type pattern.

Preferential reduction of the α-2-6-sialylation from cell surface N-glycans of human diploid fibroblastic cells by in vitro aging

Human diploid fibroblastic cell line, TIG-3, has a finite life span of about 80 population doubling levels (PDL), and is used for in vitro aging studies. Young cells (PDL 23) grew to higher cell densities at a higher growth rate than aged cells (PDL 77). When the electrophoretic mobility of cells was determined, the negative surface charge of the aged cells decreased significantly when compared to that of young cells. Lectin blot analysis of membrane glycoproteins showed that the alpha-2-6-sialylation but not the alpha-2-3-sialylation of N-glycans decreases markedly in the aged cells when compared to the young cells. In support of this observation, the cDNA microarray assay and reverse transcription-polymerase chain reaction (RT-PCR) analysis showed that the gene expression of the alpha-2,6-sialyltransferase I (ST6Gal I), which transfers sialic acid to galactose residues of N-glycans, decreases in the aged cells. These results indicate that the concordant decrease of the alpha-2,6-sialylation of N-glycans with the ST6Gal I gene expression is induced in TIG-3 cells by in vitro aging.

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.

X-ray Crystal Structures of Rabbit N-acetylglucosaminyltransferase I (GnT I) in Complex with Donor Substrate Analogues

The Golgi-resident glycosyltransferase, UDP-N-acetyl-d-glucosamine:alpha-3-d-mannoside beta-1,2-N-acetylglucosaminyltransferase I (GnT I), initiates the conversion of high-mannose oligosaccharides to complex and hybrid structures in the biosynthesis of N-linked glycans. Reported here are the X-ray crystal structures of GnT I in complex with UDP-CH2-GlcNAc (a non-hydrolyzable C-glycosidic phosphonate), UDP-2-deoxy-2-fluoro-glucose, UDP-glucose and UDP. Collectively, these structures provide evidence for the importance of the GlcNAc moiety and its N-acetyl group in donor substrate binding, as well as insight into the role played by the flexible 318-330 loop in substrate binding and product release. In addition, the UDP-CH2-GlcNAc complex reveals a well-defined glycerol molecule poised for nucleophilic attack on the C1 atom of the donor substrate analogue. The position and orientation of this glycerol molecule have allowed us to model the binding of the Manalpha1,3Manbeta1 moiety of the acceptor substrate and, based on the model, to suggest a rationalization for the main determinants of GnT I acceptor specificity.

A beta1,4-galactosyltransferase is required for convergent extension movements in zebrafish

Our understanding of how complex carbohydrates function during embryonic development is still very limited, primarily due to the large number of glycosyltransferases now known to be involved in their synthesis. To overcome these limitations, we have taken advantage of the zebrafish system to analyze the function of complex carbohydrates during development by down-regulating the expression of specific glycosyltransferases. Herein, we report the identification of the zebrafish ortholog of mammalian beta1,4-galactosyltransferase I, beta4GalT1, and its requirement for proper convergent extension movements during gastrulation. beta4GalT1 is expressed in the oocyte and throughout the embryo during the first 24 h of development. Knockdown of zebrafish beta4GalT1 by two independent morpholino oligonucleotides results in embryos with a truncated anterior-posterior axis, as well as elongated somites and moderate defects in the patterning of the head mesenchyme. Co-injection of zebrafish beta4GalT1 mRNA returns galactosyltransferase activity to control levels and rescues the defects produced by morpholino oligonucleotides. In situ hybridizations of various molecular markers reveal that the axial mesoderm of epiboly stage embryos is abnormally widened in beta4GalT1 morphants, indicative of abnormal convergent extension. Consistent with this, the rate of anterior-posterior axis elongation is reduced relative to control-injected embryos, similar to that seen in known convergent extension mutants. Among the many potential substrates for beta4GalT1 is laminin, a principle component of the extracellular matrix that supports cell movements such as those that occur during convergent extension. Previous in vitro studies have shown that the galactosylation status of laminin directly influences its ability to support cell spreading and migration. In this regard, laminin isolated from beta4GalT1 morphant embryos is poorly galactosylated, which may contribute to defective cell migration during convergent extension movements. This work demonstrates that zebrafish can be used to identify critical developmental roles for specific glycosyltransferases that would not be obvious otherwise, such as an absolute requirement for beta4GalT1 during convergent extension movements.

A beta1,4-galactosyltransferase is required for Bmp2-dependent patterning of the dorsoventral axis during zebrafish embryogenesis

Complex carbohydrates are highly polymorphic macromolecules that are involved in diverse biological processes; however, a detailed understanding of their function remains obscure. To better define the roles of complex carbohydrates during vertebrate embryogenesis, we have initiated an analysis of glycosyltransferase function using the zebrafish system. In this study, we report the characterization of a zebrafish beta1,4-galactosyltransferase (GalT), which has substantial homology with mammalian beta4GalT5 and is expressed zygotically throughout the zebrafish embryo. Downregulating the expression of beta4GalT5 by injection of specific morpholino oligonucleotides results in dorsalized zebrafish embryos, suggesting a role of beta4GalT5 in Bmp2-mediated specification of the dorsoventral axis. Consistent with this, morpholino-injected embryos have ventrally expanded chordin expression and reduced activation of the Bmp-dependent transcription factors Smad1/5/8. Because other growth factors, such as Egf and Fgf, require binding to extracellular proteoglycans for delivery and/or binding to their cognate receptors, we examined whether proteoglycans isolated from control and morpholino-injected embryos show differential binding affinities for Bmp2. In this regard, proteoglycans isolated from beta4GalT5 morphant embryos are underglycosylated and are unable to bind recombinant Bmp2 as efficiently as proteoglycans from control-injected embryos, whereas the binding of Bmp7 is relatively unaffected. These results suggest that beta4GalT5 is a previously unidentified zebrafish galactosyltransferase that is essential for proper patterning of the dorsoventral axis by regulating Bmp2 signaling. Furthermore, this work demonstrates that a relatively simple carbohydrate modification to endogenous proteoglycans can modulate the specificity of cytokine signaling.

Null Mutations in Drosophila N-Acetylglucosaminyltransferase I Produce Defects in Locomotion and a Reduced Life Span

UDP-GlcNAc:alpha3-D-mannoside beta1,2-N-acetylglucosaminyltransferase I (encoded by Mgat1) controls the synthesis of hybrid, complex, and paucimannose N-glycans. Mice make hybrid and complex N-glycans but little or no paucimannose N-glycans. In contrast, Drosophila melanogaster and Caenorhabditis elegans make paucimannose N-glycans but little or no hybrid or complex N-glycans. To determine the functional requirement for beta1,2-N-acetylglucosaminyltransferase I in Drosophila, we generated null mutations by imprecise excision of a nearby transposable element. Extracts from Mgat1(1)/Mgat1(1) null mutants showed no beta1,2-N-acetylglucosaminyltransferase I enzyme activity. Moreover, mass spectrometric analysis of these extracts showed dramatic changes in N-glycans compatible with lack of beta1,2-N-acetylglucosaminyltransferase I activity. Interestingly, Mgat1(1)/Mgat1(1) null mutants are viable but exhibit pronounced defects in adult locomotory activity when compared with Mgat1(1)/CyO-GFP heterozygotes or wild type flies. In addition, in null mutants males are sterile and have a severely reduced mean and maximum life span. Microscopic examination of mutant adult fly brains showed the presence of fused beta lobes. The removal of both maternal and zygotic Mgat1 also gave rise to embryos that no longer express the horseradish peroxidase antigen within the central nervous system. Taken together, the data indicate that beta1,2-N-acetylglucosaminyltransferase I-dependent N-glycans are required for locomotory activity, life span, and brain development in Drosophila.