Modeling human congenital disorder of glycosylation type IIa in the mouse: conservation of asparagine-linked glycan-dependent functions in mammalian physiology and insights into disease pathogenesis

MHCII glycosylation modulates Bacteroides fragilis carbohydrate antigen presentation

N-linked glycans are thought to protect class II major histocompatibility complex (MHC) molecules (MHCII) from proteolytic cleavage and assist in arranging proteins within the immune synapse, but were not thought to directly participate in antigen presentation. Here, we report that antigen-presenting cells (APCs) lacking native complex N-glycans showed reduced MHCII binding and presentation of the T cell activating glycoantigen (GlyAg) polysaccharide A from Bacteroides fragilis but not conventional peptides. APCs lacking native N-glycans also failed to mediate GlyAg-driven T cell activation but activated T cells normally with protein antigen. Mice treated with the mannosidase inhibitor kifunensine to prevent the formation of complex N-glycans were unable to expand GlyAg-specific T cells in vivo upon immunization, yet adoptive transfer of normally glycosylated APCs into these animals overcame this defect. Our findings reveal that MHCII N-glycosylation directly impacts binding and presentation of at least one class of T cell-dependent antigen.

Alpha1,6-fucosyltransferase-deficient mice exhibit multiple behavioral abnormalities associated with a schizophrenia-like phenotype: importance of the balance between the dopamine and serotonin systems

Previously, we reported that α1,6-fucosyltransferase (Fut8)-deficient (Fut8(-/-)) mice exhibit emphysema-like changes in the lung and severe growth retardation due to dysregulation of TGF-β1 and EGF receptors and to abnormal integrin activation, respectively. To study the role of α1,6-fucosylation in brain tissue where Fut8 is highly expressed, we examined Fut8(-/-) mice using a combination of neurological and behavioral tests. Fut8(-/-) mice exhibited multiple behavioral abnormalities consistent with a schizophrenia-like phenotype. Fut8(-/-) mice displayed increased locomotion compared with wild-type (Fut8(+/+)) and heterozygous (Fut8(+/-)) mice. In particular, Fut8(-/-) mice showed strenuous hopping behavior in a novel environment. Working memory performance was impaired in Fut8(-/-) mice as evidenced by the Y-maze tests. Furthermore, Fut8(-/-) mice showed prepulse inhibition (PPI) deficiency. Intriguingly, although there was no significant difference between Fut8(+/+) and Fut8(+/-) mice in the PPI test under normal conditions, Fut8(+/-) mice showed impaired PPI after exposure to a restraint stress. This result suggests that reduced expression of Fut8 is a plausible cause of schizophrenia and related disorders. The levels of serotonin metabolites were significantly decreased in both the striatum and nucleus accumbens of the Fut8(-/-) mice. Likewise, treatment with haloperidol, which is an antipsychotic drug that antagonizes dopaminergic and serotonergic receptors, significantly reduced hopping behaviors. The present study is the first to clearly demonstrate that α1,6-fucosylation plays an important role in the brain, and that it might be related to schizophrenia-like behaviors. Thus, the results of the present study provide new insights into the underlying mechanisms responsible for schizophrenia and related disorders.

Boric acid gel enrichment of glycosylated proteins in human wound fluids

The enrichment of glycosylated proteins by glycocapturing materials plays a pivotal role for the investigation of polysaccharide containing proteins in disease pathogenesis. Hence, we investigated a boric acid gel as a binding material for glycoprotein enrichment. The bovine proteins alpha-1-acid-glycoprotein (A1AG) and alpha-2-HS-glycoprotein (fetuin A) were spiked in human chronic wound fluids and were subsequently enriched by a boric acid gel affinity chromatography (BAGAC). The enrichment efficiency was evaluated by western blot analysis and mass spectrometry. Additionally, glycoproteins of human wound fluids from diabetes mellitus patients with chronic foot ulcers were analyzed after BAGAC enrichments. In total 104 glycoproteins were identified, with reported glycosylation sites. 60 proteins were detected in at least 2 out of 3 biological replicates and were used for quantitative analysis between the bound and unbound fractions. Almost 80% of these glycoproteins were more prominent in the bound fraction. Only 2 glycoproteins revealed higher spectral counts in the flow through fraction compared to the bound fraction. These findings demonstrate the capability of the BAGAC material to enrich glycosylated proteins from complex human wound fluids.

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.

The Ashwell-Morell receptor

The Ashwell-Morell receptor (AMR) of hepatocytes, originally termed the hepatic asialoglycoprotein receptor, was the first cellular receptor to be identified and isolated and the first lectin to be detected in mammals. It is one of the multiple lectins of the C-type lectin family involved in recognition, binding, and clearance of asialoglycoproteins. We recently identified endogenous ligands of the AMR as desialylated prothrombotic components, including platelets and von Willebrand Factor [Ellies L. G., Ditto D., Levy G. G., Wahrenbrock M., Ginsburg D., Varki A., Le D. T., and Marth J. D. (2002). Sialyltransferase ST3Gal-IV operates as a dominant modifier of hemostasis by concealing asialoglycoprotein receptor ligands. Proc. Natl. Acad. Sci. USA 99: pp. 10042-10047; Grewal, P. K. Uchiyama, S., Ditto, D., Varki, N., Le, D. T., Nizet, V., Marth, J. D. (2008). The Ashwell receptor mitigates the lethal coagulopathy of sepsis. Nat. Medicine 14, pp. 648-655]. Among these components, clearance by the liver's AMR is enhanced by exposure of terminal galactose on the glycan chains. A physiological role for engaging the AMR in rapid clearance was identified as mitigating disseminating intravascular coagulopathy in sepsis to promote survival. This chapter overviews the endogenous ligands of the AMR as components of the coagulatory system, describes clearance mechanisms of the liver, and details hematology and coagulation assays used in mouse coagulation studies.

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.

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.

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.

Simultaneous glycoproteomics on the basis of structure using ion mobility-mass spectrometry

Simultaneous glycoproteomic characterization using rapid (mus to ms) structural separations provided by ion mobility-mass spectrometry (IM-MS) is described. Advantages from using both ESI and MALDI ion sources are presented with future implications toward high throughput glycan and glycoconjugate characterization.

The repertoire of glycan determinants in the human glycome

The number of glycan determinants that comprise the human glycome is not known. This uncertainty arises from limited knowledge of the total number of distinct glycans and glycan structures in the human glycome, as well as limited information about the glycan determinants recognized by glycan-binding proteins (GBPs), which include lectins, receptors, toxins, microbial adhesins, antibodies, and enzymes. Available evidence indicates that GBP binding sites may accommodate glycan determinants made up of 2 to 6 linear monosaccharides, together with their potential side chains containing other sugars and modifications, such as sulfation, phosphorylation, and acetylation. Glycosaminoglycans, including heparin and heparan sulfate, comprise repeating disaccharide motifs, where a linear sequence of 5 to 6 monosaccharides may be required for recognition. Based on our current knowledge of the composition of the glycome and the size of GBP binding sites, glycoproteins and glycolipids may contain approximately 3000 glycan determinants with an additional approximately 4000 theoretical pentasaccharide sequences in glycosaminoglycans. These numbers provide an achievable target for new chemical and/or enzymatic syntheses, and raise new challenges for defining the total glycome and the determinants recognized by GBPs.

Mass spectrometry in the characterization of human genetic N-glycosylation defects

Human genetic diseases that affect N-glycosylation result from the defective synthesis of the N-linked sugar moiety (glycan) of glycoproteins. The role of glycans for proper protein folding and biological functions is illustrated in the variety and severity of clinical manifestations shared by congenital disorders of glycosylation (CDG). This family of inherited metabolic disorders includes defects in the assembly of the oligosaccharide precursor that lead to an under-occupancy of N-glycosylation sites (CDG-I), and defects of glycan remodeling (CDG-II). Mass spectrometry constitutes a key tool for characterization of CDG-I defects by mass resolution of native protein glycoforms that differ for glycosylation-site occupancy. Glycan MS analyses in CDG-II is mandatory to detect whenever possible a repertoire of structures to pinpoint candidate enzymes and genes responsible for the abnormal N-glycan synthesis. In this manuscript, we review the MS applications in the area of CDG and related disorders with a special emphasis on those techniques that have been already applied or might become functional for diagnosis, characterization, and treatment monitoring in some specific conditions.

Differentiation between isomeric triantennary N-linked glycans by negative ion tandem mass spectrometry and confirmation of glycans containing galactose attached to the bisecting (beta1-4-GlcNAc) residue in N-glycans from IgG

Negative ion tandem mass spectrometry (MS/MS) spectra of three isomeric triantennary N-linked glycans provided clear differentiation between the isomers and confirmed the occurrence of an isomer that was substituted with galactose on a bisecting GlcNAc (1 --> 4-substituted on the core mannose) residue recently reported by Takegawa et al. from N-glycans released from human immunoglobulin G (IgG). We extend this analysis of human serum IgG to reveal an analogue of the fucosylated triantennary glycan reported by Takegawa et al. together with a third compound that lacked both the sialic acid and the fucose residues. In addition, we demonstrate the biosynthesis of bisected hybrid-type glycans with the galactose modification, with and without core fucose, on the stem cell marker glycoprotein, 19A, expressed in a partially ricin-resistant human embryonic kidney cell line. It would appear, therefore, that this modification of N-linked glycans containing a galactosylated bisecting GlcNAc residue may be more common than originally thought. Negative ion MS/MS analysis of glycans is likely to prove an invaluable tool in the analysis and monitoring of therapeutic glycoproteins.

Initiation of protein O glycosylation by the polypeptide GalNAcT-1 in vascular biology and humoral immunity

Core-type protein O glycosylation is initiated by polypeptide N-acetylgalactosamine (GalNAc) transferase (ppGalNAcT) activity and produces the covalent linkage of serine and threonine residues of proteins. More than a dozen ppGalNAcTs operate within multicellular organisms, and they differ with respect to expression patterns and substrate selectivity. These distinctive features imply that each ppGalNAcT may differentially modulate regulatory processes in animal development, physiology, and perhaps disease. We found that ppGalNAcT-1 plays key roles in cell and glycoprotein selective functions that modulate the hematopoietic system. Loss of ppGalNAcT-1 activity in the mouse results in a bleeding disorder which tracks with reduced plasma levels of blood coagulation factors V, VII, VIII, IX, X, and XII. ppGalNAcT-1 further supports leukocyte trafficking and residency in normal homeostatic physiology as well as during inflammatory responses, in part by providing a scaffold for the synthesis of selectin ligands expressed by neutrophils and endothelial cells of peripheral lymph nodes. Animals lacking ppGalNAcT-1 are also markedly impaired in immunoglobulin G production, coincident with increased germinal center B-cell apoptosis and reduced levels of plasma B cells. These findings reveal that the initiation of protein O glycosylation by ppGalNAcT-1 provides a distinctive repertoire of advantageous functions that support vascular responses and humoral immunity.

Glycomic studies of Drosophila melanogaster embryos

With the complete genome sequence of Drosophila melanogaster defined a systematic approach towards understanding the function of glycosylation has become possible. Structural assignment of the entire Drosophila glycome during specific developmental stages could provide information that would shed further light on the specific roles of different glycans during development and pinpoint the activity of certain glycosyltransferases and other glycan biosynthetic genes that otherwise might be missed through genetic analyses. In this paper the major glycoprotein N- and O-glycans of Drosophila embryos are described as part of our initial undertaking to characterize the glycome of Drosophila melanogaster. The N-glycans are dominated by high mannose and paucimannose structures. Minor amounts of mono-, bi- and tri-antennary complex glycans were observed with GlcNAc and Galbeta1-4GlcNAc non-reducing end termini. O-glycans were restricted to the mucin-type core 1 Galbeta1-3GalNAc sequence.

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.

N-glycan structures and associated gene expression reflect the characteristic N-glycosylation pattern of human hematopoietic stem and progenitor cells

OBJECTIVE

Cell surface glycans contribute to the adhesion capacity of cells and are essential in cellular signal transduction. Yet, the glycosylation of hematopoietic stem and progenitor cells (HSPC), such as CD133+ cells, is poorly explored.

MATERIALS AND METHODS

N-glycan structures of cord blood-derived CD133+ and CD133- cells were analyzed with mass spectrometric profiling and exoglycosidase digestion, cell surface glycan epitopes with lectin binding assay, and expression of N-glycan biosynthesis-related genes with microarray analysis.

RESULTS

Over 10% difference was demonstrated in the N-glycan profiles of CD133+ and CD133- cells. Biantennary complex-type N-glycans were enriched in CD133+ cells. Of the genes regulating the synthesis of these structures, CD133+ cells overexpressed MGAT2 and underexpressed MGAT4. Moreover, the amount of high-mannose type N-glycans and terminal alpha2,3-sialylation was increased in CD133+ cells. Elevated alpha2,3-sialylation was supported by the overexpression of ST3GAL6.

CONCLUSION

Our work presents new information on the characters of HSPCs. The new knowledge of HSPC-specific N-glycosylation advances their identification and provides tools to promote HSPC homing and mobilization or targeting to specific tissues.

Agm1/Pgm3-mediated sugar nucleotide synthesis is essential for hematopoiesis and development

Carbohydrate modification of proteins includes N-linked and O-linked glycosylation, proteoglycan formation, glycosylphosphatidylinositol anchor synthesis, and O-GlcNAc modification. Each of these modifications requires the sugar nucleotide UDP-GlcNAc, which is produced via the hexosamine biosynthesis pathway. A key step in this pathway is the interconversion of GlcNAc-6-phosphate (GlcNAc-6-P) and GlcNAc-1-P, catalyzed by phosphoglucomutase 3 (Pgm3). In this paper, we describe two hypomorphic alleles of mouse Pgm3 and show there are specific physiological consequences of a graded reduction in Pgm3 activity and global UDP-GlcNAc levels. Whereas mice lacking Pgm3 die prior to implantation, animals with less severe reductions in enzyme activity are sterile, exhibit changes in pancreatic architecture, and are anemic, leukopenic, and thrombocytopenic. These phenotypes are accompanied by specific rather than wholesale changes in protein glycosylation, suggesting that while universally required, the functions of certain proteins and, as a consequence, certain cell types are especially sensitive to reductions in Pgm3 activity.

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.

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.

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.

N-linked neutral oligosaccharides in the stratum corneum of normal and ichthyotic skin

N-Glycan oligosaccharides are thought to play multiple, important roles in a variety of biological events. However, N-glycan profiles in the stratum corneum of human skin have not yet been studied in detail. To clarify the N-glycan profiles in the stratum corneum of normal and ichthyotic epidermis, N-glycan profiles were studied by high-performance liquid chromatography using normal human epidermal samples and scales from hyperkeratotic skin of ichthyosis patients. Chromatograms of patient scale samples showed unique alterations in three peaks eluted at 15.8, 18.8 and 26.9 min. The N-glycan profiles were significantly altered in ichthyotic hyperkeratotic skin compared with normal non-hyperkeratotic controls. These findings indicate the reduction of N-acetylglucosaminyltransferase II and fucosyltransferase 8 activities. Alteration of N-glycan structures in hyperkeratotic skin suggests the biological role of N-glycans in keratinization.

Comparative analysis by frontal affinity chromatography of oligosaccharide specificity of GlcNAc-binding lectins, Griffonia simplicifolia lectin-II (GSL-II) and Boletopsis leucomelas lectin (BLL)

Lectin-based structural glycomics requires a search for useful lectins and their biochemical characterization to profile complex features of glycans. In this paper, two GlcNAc-binding lectins are reported with their detailed oligosaccharide specificity. One is a classic plant lectin, Griffonia simplicifolia lectin-II (GSL-II), and the other is a novel fungal lectin, Boletopsis leucomelas lectin (BLL). Their sugar-binding specificity was analyzed by frontal affinity chromatography using 146 glycans (125 pyridylaminated and 21 p-nitrophenyl saccharides). As a result, it was found that both GSL-II and BLL showed significant affinity toward complex-type N-glycans, which are either partially or completely agalactosylated. However, their branch-specific features differed significantly: GSL-II strongly bound to agalacto-type, tri- or tetra-antennary N-glycans with its primary recognition of a GlcNAc residue transferred by GlcNAc-transferase IV, while BLL preferred N-glycans with fewer branches. In fact, the presence of a GlcNAc residue transferred by GlcNAc-transferase V abolishes the binding of BLL. Thus, GSL-II and BLL forms a pair of complementally probes to profile a series of agalacto-type N-glycans.

Genetic defects in the human glycome

The spectrum of all glycan structures--the glycome--is immense. In humans, its size is orders of magnitude greater than the number of proteins that are encoded by the genome, one percent of which encodes proteins that make, modify, localize or bind sugar chains, which are known as glycans. In the past decade, over 30 genetic diseases have been identified that alter glycan synthesis and structure, and ultimately the function of nearly all organ systems. Many of the causal mutations affect key biosynthetic enzymes, but more recent discoveries point to defects in chaperones and Golgi-trafficking complexes that impair several glycosylation pathways. As more glycosylation disorders and patients with these disorders are identified, the functions of the glycome are starting to be revealed.

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.

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.

Carriers and patients with muscle–eye–brain disease can be rapidly diagnosed by enzymatic analysis of fibroblasts and lymphoblasts

We report a new fibroblast and lymphoblast based protein O-mannosyl beta-1,2-N-acetylglucosaminyltransferase 1 enzymatic assay, which allows rapid and accurate diagnosis of carriers and patients with muscle-eye-brain type of congenital muscular dystrophy. Seven patients with genetically confirmed muscle-eye-brain disease were assayed for protein O-mannosyl beta-1,2-N-acetylglucosaminyltransferase 1 enzyme activity. In three patients and their heterozygous parents, the assays were done on EBV-transformed lymphoblasts, in another three patients they were done on cultured fibroblasts and in the last patient on both fibroblasts and lymphoblasts. Cultured fibroblasts and lymphoblasts from the muscle-eye-brain patients showed a highly significant decrease in protein O-mannosyl beta-1,2-N-acetylglucosaminyltransferase 1 activity relative to controls. The residual protein O-mannosyl beta-1,2-N-acetylglucosaminyltransferase 1 level in fibroblasts (average 0.11 nmoles/h per mg) was about 13% of normal controls. The ratio of protein O-mannosyl beta-1,2-N-acetylglucosaminyltransferase 1 activity to the activity of a glycosyltransferase control (N-acetylglucosaminyltransferase 1; GnT1) in fibroblasts was on average 0.006 in muscle-eye-brain patients and 0.045 in controls. The average residual protein O-mannosyl beta-1,2-N-acetylglucosaminyltransferase 1 level in lymphoblasts was 15% of normal controls. The average ratio of protein O-mannosyl beta-1,2-N-acetylglucosaminyltransferase 1/GnT1 activity was 0.007 in muscle-eye-brain patients, 0.026 in heterozygous carriers and 0.046 in normal controls. Assay of protein O-mannosyl beta-1,2-N-acetylglucosaminyltransferase 1 activity in fibroblasts and lymphoblasts from muscle-eye-brain carriers and patients provides a rapid and relatively simple diagnostic test for this disease and could be used as a screening test in carriers and patients with complex congenital muscular dystrophy.

Creation and implications of a phenome-genome network

Although gene and protein measurements are increasing in quantity and comprehensiveness, they do not characterize a sample's entire phenotype in an environmental or experimental context. Here we comprehensively consider associations between components of phenotype, genotype and environment to identify genes that may govern phenotype and responses to the environment. Context from the annotations of gene expression data sets in the Gene Expression Omnibus is represented using the Unified Medical Language System, a compendium of biomedical vocabularies with nearly 1-million concepts. After showing how data sets can be clustered by annotative concepts, we find a network of relations between phenotypic, disease, environmental and experimental contexts as well as genes with differential expression associated with these concepts. We identify novel genes related to concepts such as aging. Comprehensively identifying genes related to phenotype and environment is a step toward the Human Phenome Project.

N-Glycan structure analysis using lectins and an alpha-mannosidase activity assay

Alpha-mannosidase IIx (MX) and alpha-mannosidase II (MII) are homologous enzymes whose critical roles in N-glycan processing were established in large part by analysis of the MII/MX double-knockout mouse. To analyze the structures of N-glycans synthesized in the mutant mice, we employed lectin blot and lectin histochemistry in addition to mass spectrometry analysis and two-dimensional high-performance liquid chromatography (HPLC) mapping. We also produced soluble MII and MX by transfecting mammalian cells with expression vectors and determined substrate specificity of MX. This chapter describes methods using lectins to analyze N-glycans in knockout mice and provides a protocol to assay alpha-mannosidase activity using soluble MX.

N-glycans are involved in the response of Caenorhabditis elegans to bacterial pathogens

Caenorhabditis elegans is becoming a popular tool for the study of glycan function particularly as it applies to development. More than 150 C. elegans genes have been identified as homologs of vertebrate genes involved in glycan metabolism. However, only a relatively small number of these genes have been expressed and studied in any detail. Oligomannose N-glycans (Man5-9GlcNAc2Asn), major components of the N-glycans of all eukaryotes including C. elegans, are essential, at least in part, for eukaryote survival, because they play an important role in protein quality control. In addition, vertebrates make hybrid (GlcNAcMan3-5GlcNAc2Asn) and complex (XGlcNAc2-6Man3GlcNAc2Asn) but little or no paucimannose (Man3-4GlcNAc2Asn)N-glycans, whereas plants, insects, and C. elegans make paucimannose but little or no hybrid nor complex N-glycans. UDP-GlcNAc:alpha3-D-mannoside beta1,2-N-acetylglucosaminyltransferase I (encoded by the gene Mgat1) controls the synthesis of hybrid, complex, and paucimannose N-glycans in all eukaryotes. C. elegans has three genes encoding beta1,2-N-acetylglucosaminyltransferase I (gly-12, gly-13, gly-14). To determine the functional requirement for this enzyme in worms, we generated seven worm strains with mutations in these three genes (gly-12, dpy-6 gly-13, gly-14, gly-12 gly-13, gly-14;gly-12, gly-14;dpy-6 gly-13 and gly-14;gly-12 gly-13). Whereas mice and Drosophila melanogaster with null mutations in Mgat1 suffer severe developmental abnormalities, all seven C. elegans strains with null mutations in the genes encoding beta1,2-N-acetylglucosaminyltransferase I develop normally and seem to have a wild-type phenotype. We now present evidence that beta1,2-N-acetylglucosaminyltransferase I-dependent N-glycans (consisting mainly of paucimannose N-glycans) play a role in the interaction of C. elegans with pathogenic bacteria, suggesting that these N-glycans are components of the worm's innate immune system.

A missense mutation in the bovine SLC35A3 gene, encoding a UDP-N-acetylglucosamine transporter, causes complex vertebral malformation

The extensive use of a limited number of elite bulls in cattle breeding can lead to rapid spread of recessively inherited disorders. A recent example is the globally distributed syndrome Complex Vertebral Malformation (CVM), which is characterized by misshapen and fused vertebrae around the cervico-thoracic junction. Here, we show that CVM is caused by a mutation in the Golgi-resident nucleotide-sugar transporter encoded by SLC35A3. Thus, the disease showed complete cosegregation with the mutation in a homozygous state, and proteome patterns indicated abnormal protein glycosylation in tissues of affected animals. In addition, a yeast mutant that is deficient in the transport of UDP-N-acetylglucosamine into its Golgi lumen can be rescued by the wild-type SLC35A3 gene, but not by the mutated gene. These results provide the first demonstration of a genetic disorder associated with a defective SLC35A3 gene, and reveal a new mechanism for malformation of the vertebral column caused by abnormal nucleotide-sugar transport into the Golgi apparatus.

Hair barbering in mice: implications for neurobehavioural research

Barbering (fur/whisker trimming, the Dalila effect) is a behaviour-associated hair and whisker loss frequently seen in laboratory rodents, including mice. Here we analyse barbering behaviour in 129S1, NMRI, C57BL/6 and BALB/c mouse strains and some of their F1 hybrids. Our study shows that barbering in mice, depending on their genotype, is a complex behaviour with several distinct contexts or domains. We observed social (dominant) barbering in NMRI and C57BL/6 mice, sexual over-grooming in 129S1 and C57BL/6 mice, maternal barbering in lactating 129S1 and C57BL/6 mice, and stress-evoked barbering in F1 (NMRIx129S1) hybrids. In contrast, aggressive BALB/c mice and their F1 progeny do not use barbering in their behaviour. We suggest that barbering may be an important complex multi-domain behaviour sensitive to various manipulations, and represent a useful index in neurobehavioural research.

Current models and future directions for understanding the neural circuitries of maternal behaviors in rodents

Maternal behaviors in rodents include a number of subcomponents, such as nursing, nest building, licking and grooming of pups, pup retrieval, and maternal aggression. Because each behavior involves a unique motor pattern, a unique ensemble neural circuitry must underlie each behavior. To what extent there is overlap in terms of brain regions and specific neurons for each circuit is being actively investigated. This review will first examine overlapping and separate components of pup retrieval and maternal aggression circuitries while examining a central role for medial preoptic area (MPA) in both behaviors. With an emphasis on experimental approaches, the review will then highlight recent findings and propose future directions for understanding maternal behavior regulation. Finally, examples for why studying the neural basis of maternal behaviors can bring insights to other areas of neuroscience, such as feeding, addiction, and anxiety and aggression regulation will be provided.

Introduction of extended LEC14-type branching into core-fucosylated biantennary N-glycan

A series of enzymatic substitutions modifies the basic structure of complex-type biantennary N-glycans. Among them, a beta1,2-linked N-acetylglucosamine residue is introduced to the central mannose moiety of the core-fucosylated oligosaccharide by N-acetylglucosaminyltransferase VII. This so-called LEC14 epitope can undergo galactosylation at the beta1,2-linked N-acetylglucosamine residue. Guided by the hypothesis that structural modifications in the N-glycan alter its capacity to serve as ligand for lectins, we prepared a neoglycoprotein with the extended LEC14 N-glycan and tested its properties in three different assays. In order to allow comparison to previous results on other types of biantennary N-glycans the functionalization of the glycans for coupling and assay conditions were deliberately kept constant. Compared to the core-fucosylated N-glycan no significant change in affinity was seen when testing three galactoside-specific proteins. However, cell positivity in flow cytofluorimetry was enhanced in six of eight human tumor lines. Analysis of biodistribution in tumor-bearing mice revealed an increase of blood clearance by about 40%, yielding a favorable tumor/blood ratio. Thus, the extended LEC14 motif affects binding properties to cellular lectins on cell surfaces and organs when compared to the core-fucosylated biantennary N-glycan. The results argue in favor of the concept of viewing substitutions as molecular switches for lectin-binding affinity. Moreover, they have potential relevance for glycoengineering of reagents in tumor imaging.

Caenorhabditis elegans triple null mutant lacking UDP-N-acetyl-D-glucosamine:alpha-3-D-mannoside beta1,2-N-acetylglucosaminyltransferase I

We have previously reported, from the nematode worm Caenor-habditis elegans, three genes (gly-12, gly-13 and gly-14) encoding enzymically active UDP-N-acetyl-D-glucosamine:alpha-3-D-mannoside beta1,2-N-acetylglucosaminyltransferase I (GnT I), an enzyme essential for hybrid, paucimannose and complex N-glycan synthesis. We now describe a worm with null mutations in all three GnT I genes, gly-14 (III);gly-12 gly-13 (X) (III and X refer to the chromosome number). The triple-knock-out (TKO) worms have a normal phenotype, although they do not express GnT I activity and do not synthesize 31 paucimannose, complex and fucosylated oligomannose N-glycans present in the wild-type worm. The TKO worm has increased amounts of non-fucosylated oligomannose N-glycan structures, a finding consistent with the site of GnT I action. Five fucosylated oligomannose N-glycan structures were observed in TKO, but not wild-type, worms, indicating the presence of unusual GnT I-independent fucosyltransferases. It is concluded that wild-type C. elegans makes a large number of GnT I-dependent N-glycans that are not essential for normal worm development under laboratory conditions. The TKO worm may be more susceptible to mutations in other genes, thereby providing an approach for the identification of genes that interact with GnT I.

Automatic annotation of matrix-assisted laser desorption/ionizationN-glycan spectra

Matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS) is the pre-eminent technique for mass mapping of glycans. In order to make this technique practical for high-throughput screening, reliable automatic methods of annotating peaks must be devised. We describe an algorithm called Cartoonist that labels peaks in MALDI spectra of permethylated N-glycans with cartoons which represent the most plausible glycans consistent with the peak masses and the types of glycans being analyzed. There are three main parts to Cartoonist. (i) It selects annotations from a library of biosynthetically plausible cartoons. The library we currently use has about 2800 cartoons, but was constructed using only about 300 archetype cartoons entered by hand. (ii) It determines the precision and calibration of the machine used to generate the spectrum. It does this automatically based on the spectrum itself. (iii) It assigns a confidence score to each annotation. In particular, rather than making a binary yes/no decision when annotating a peak, it makes all plausible annotations and associates them with scores indicating the probability that they are correct.

Glycomics: a pathway to a class of new and improved therapeutics

Complex glycans that are located at the surface of cells, deposited in the extracellular matrix and attached to soluble signalling molecules have a crucial role in the phenotypic expression of cellular genotypes. However, owing to their structural complexity and some redundancy in terms of structures that elicit a function, the therapeutic potential of complex glycans has not been well exploited, with a few notable exceptions. This review outlines recent advances that promise to increase our ability to use complex glycans as therapeutics. Opportunities for the development of further structure-function relationships for these complex molecules are also discussed.

Regulation of cytokine receptors by Golgi N-glycan processing and endocytosis

The Golgi enzyme beta1,6 N-acetylglucosaminyltransferase V (Mgat5) is up-regulated in carcinomas and promotes the substitution of N-glycan with poly N-acetyllactosamine, the preferred ligand for galectin-3 (Gal-3). Here, we report that expression of Mgat5 sensitized mouse cells to multiple cytokines. Gal-3 cross-linked Mgat5-modified N-glycans on epidermal growth factor and transforming growth factor-beta receptors at the cell surface and delayed their removal by constitutive endocytosis. Mgat5 expression in mammary carcinoma was rate limiting for cytokine signaling and consequently for epithelial-mesenchymal transition, cell motility, and tumor metastasis. Mgat5 also promoted cytokine-mediated leukocyte signaling, phagocytosis, and extravasation in vivo. Thus, conditional regulation of N-glycan processing drives synchronous modification of cytokine receptors, which balances their surface retention against loss via endocytosis.

Role of glycosylation in development

Researchers have long predicted that complex carbohydrates on cell surfaces would play important roles in developmental processes because of the observation that specific carbohydrate structures appear in specific spatial and temporal patterns throughout development. The astounding number and complexity of carbohydrate structures on cell surfaces added support to the concept that glycoconjugates would function in cellular communication during development. Although the structural complexity inherent in glycoconjugates has slowed advances in our understanding of their functions, the complete sequencing of the genomes of organisms classically used in developmental studies (e.g., mice, Drosophila melanogaster, and Caenorhabditis elegans) has led to demonstration of essential functions for a number of glycoconjugates in developmental processes. Here we present a review of recent studies analyzing function of a variety of glycoconjugates (O-fucose, O-mannose, N-glycans, mucin-type O-glycans, proteoglycans, glycosphingolipids), focusing on lessons learned from human disease and genetic studies in mice, D. melanogaster, and C. elegans.

Sugar-mediated ligand–receptor interactions in the immune system

Most molecules involved in the recognition and elimination of pathogens by the immune system are glycoproteins. Oligosaccharides attached to glycoproteins initiate biological functions through mechanisms that involve multiple interactions of the monosaccharide residues with receptors. For example, calreticulin, a quality-control lectin-like chaperone, interacts with glucosylated mannose glycans presented by empty major histocompatibility complex (MHC) class I molecules, retaining them in the endoplasmic reticulum (ER) until antigenic peptide is loaded. Clusters of specific IgG glycoforms, present in increased amounts in rheumatoid arthritis, bind mannose-binding lectin (MBL), providing a potential route to inflammation through activation of the complement pathway. Secretory IgA glycans bind gut bacteria, and an unusual cluster of mannose residues on gp120, the surface coat protein of the HIV virus, is recognized by the novel 'domain-swapped' IgG 2G12 serum antibody.

Evidence for alternative splicing and developmental regulation of the Drosophila melanogaster Mgat2 (N-acetylglucosaminyltransferase II) gene

We present a molecular study of the Drosophila melanogaster Mgat2 gene that codes for the N-acetylglucosaminyltransferase II. Isolation and analysis of two cDNA clones indicated that the gene is alternatively spliced, generating two transcripts of 3.3 and 2.7kb. Developmental specificity was observed between the two alternative transcripts during the major developmental stages of D. melanogaster. The deduced amino acid sequences of the two proteins show significant homology to the equivalent mammalian proteins, especially in the carboxyterminal region. In situ hybridizations in embryos and embryonic imaginal discs showed that the gene is expressed mainly but not exclusively in the brain.

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)

Glycosylation in congenital muscular dystrophies

Mammalian cells produce many glycoproteins, i.e., proteins with covalently attached sugar chains. Recent advances in glycobiology have revealed the importance of sugar chains as biosignals for multi-cellular organisms including cell-cell communication, intracellular signaling, protein folding, and targeting of proteins within cells. The O-mannosyl linkage, which used to be considered specific to yeast, has recently been found in mammals. One of the best known O-mannosyl-modified glycoproteins is alpha-dystroglycan, which is a central component of the dystrophin-glycoprotein complex isolated from skeletal muscle membranes. We have identified and characterized a glycosyltransferase, UDP-N-acetylglucosamine: protein O-mannose beta1,2-N-acetylglucosaminyltransferase (POMGnT1), involved in the biosynthesis of O-mannosyl glycans. We subsequently found that loss of function of the POMGnT1 gene is responsible for muscle-eye-brain disease (MEB). MEB is an autosomal recessive disorder characterized by congenital muscular dystrophy, ocular abnormalities and brain malformation (type II lissencephaly). Moreover, recent data suggest that aberrant protein glycosylation of alpha-dystroglycan is the primary cause of some forms of congenital muscular dystrophy. Here we review new insights into the glycobiology of muscular dystrophy and neuronal migration disorder.

Detailed glycan analysis of serum glycoproteins of patients with congenital disorders of glycosylation indicates the specific defective glycan processing step and provides an insight into pathogenesis

The fundamental importance of correct protein glycosylation is abundantly clear in a group of diseases known as congenital disorders of glycosylation (CDGs). In these diseases, many biological functions are compromised, giving rise to a wide range of severe clinical conditions. By performing detailed analyses of the total serum glycoproteins as well as isolated transferrin and IgG, we have directly correlated aberrant glycosylation with a faulty glycosylation processing step. In one patient the complete absence of complex type sugars was consistent with ablation of GlcNAcTase II activity. In another CDG type II patient, the identification of specific hybrid sugars suggested that the defective processing step was cell type-specific and involved the mannosidase III pathway. In each case, complementary serum proteome analyses revealed significant changes in some 31 glycoproteins, including components of the complement system. This biochemical approach to charting diseases that involve alterations in glycan processing provides a rapid indicator of the nature, severity, and cell type specificity of the suboptimal glycan processing steps; allows links to genetic mutations; indicates the expression levels of proteins; and gives insight into the pathways affected in the disease process.

Enzymatic diagnostic test for Muscle-Eye-Brain type congenital muscular dystrophy using commercially available reagents

OBJECTIVES

Mutations disrupting the interaction of extra-cellular ligands and alpha-dystroglycan are responsible for an etiologically heterogeneous group of autosomal recessive congenital muscular dystrophies (CMD) that can have associated brain and eye abnormalities. The objective is to develop a diagnostic test for one of these CMDs, Muscle-Eye-Brain disease (MEB), due to mutations in the gene encoding Protein O-Mannosyl beta-1,2-N-acetylglucosaminyltransferase 1 (POMGnT1).

DESIGN AND METHODS

POMGnT1 enzyme activity was determined in extracts of muscle biopsies from four MEB patients and various controls using commercially available reagents.

RESULTS

All four MEB muscle samples showed a highly significant decrease in POMGnT1 activity relative to controls.

CONCLUSIONS

The assay of POMGnT1 activity in MEB muscle provides a rapid and relatively simple diagnostic test for this disease. CMDs associated with brain malformations such as MEB, WWS and FCMD are heterogenous in clinical presentation and on radiologic examination, suggesting that POMGnT1 assays of muscle biopsies should be used as a screening procedure for MEB in all CMD patients associated with brain malformations.