Brain-specific expression of N-acetylglucosaminyltransferase IX (GnT-IX) is regulated by epigenetic histone modifications

Brain-specific glycosylation enzyme GnT-IX maintains levels of protein tyrosine phosphatase receptor PTPRZ, thereby mediating glioma growth

Gliomas are the most prevalent primary tumor of the central nervous system. Despite advances in imaging technologies, neurosurgical techniques, and radiotherapy, a cure for high-grade glioma remains elusive. Several groups have reported that protein tyrosine phosphatase receptor type Z (PTPRZ) is highly expressed in glioblastoma, and that targeting PTPRZ attenuates tumor growth in mice. PTPRZ is modified with diverse glycan, including the PTPRZ-unique human natural killer-1 capped O-mannosyl core M2 glycans. However, the regulation and function of these unique glycans are unclear. Using CRISPR genome-editing technology, we first demonstrated that disruption of the PTPRZ gene in human glioma LN-229 cells resulted in profoundly reduced tumor growth in xenografted mice, confirming the potential of PTPRZ as a therapeutic target for glioma. Furthermore, multiple glycan analyses revealed that PTPRZ derived from glioma patients and from xenografted glioma expressed abundant levels of human natural killer-1-capped O-Man glycans via extrinsic signals. Finally, since deficiency of O-Man core M2 branching enzyme N-acetylglucosaminyltransferase IX (GnT-IX) was reported to reduce PTPRZ protein levels, we disrupted the GnT-IX gene in LN-229 cells and found a significant reduction of glioma growth both in vitro and in the xenograft model. These results suggest that the PTPR glycosylation enzyme GnT-IX may represent a promising therapeutic target for glioma.

Possible regulation of ganglioside GD3 synthase gene expression with DNA methylation in human glioma cells

Gangliosides are expressed in nervous systems and some neuroectoderm-derived tumors at high levels and play pivotal roles. However, mechanisms for the regulation of glycosyltransferase genes responsible for the ganglioside synthesis are not well understood. In this study, we analyzed DNA methylation patterns of promoter regions of GD3 synthase (ST8SIA1) as well as mRNA levels and ganglioside expression using human glioma cell lines. Among 5 cell lines examined, 4 lines showed changes in the expression levels of related genes after treatment with 5-aza-dC. LN319 showed up-regulation of St8sia1 and increased b-series gangliosides after 5-aza-dC treatment, and an astrocytoma cell line, AS showed high expression of ST8SIA1 and b-series gangliosides persistently before and after 5-Aza-2'-deoxycytidine treatment. Using these 2 cell lines, DNA methylation patterns of the promoter regions of the gene were analyzed by bisulfite-sequencing. Consequently, 2 regions that were methylated before 5-Aza-2'-deoxycytidine treatment were demethylated in LN319 after the treatment, while those regions were persistently demethylated in AS. These 2 regions corresponded with sites defined as promoter regions by Luciferase assay. Taken together, it was suggested that ST8SIA1 gene is regulated by DNA methylation at the promoter regions, leading to the regulation of tumor phenotypes.

Epigenetics of Immunoglobulin G Glycosylation

Alternative glycosylation of immunoglobulin G (IgG) affects its effector functions during the immune response. IgG glycosylation is altered in many diseases, but also during a healthy life of an individual. Currently, there is limited knowledge of factors that alter IgG glycosylation in the healthy state and factors involved in specific IgG glycosylation patterns associated with pathophysiology. Genetic background plays an important role, but epigenetic mechanisms also contribute to the alteration of IgG glycosylation patterns in healthy life and in disease. It is known that the expression of many glycosyltransferases is regulated by DNA methylation and by microRNA (miRNA) molecules, but the involvement of other epigenetic mechanisms, such as histone modifications, in the regulation of glycosylation-related genes (glycogenes) is still poorly understood. Recent studies have identified several differentially methylated loci associated with IgG glycosylation, but the mechanisms involved in the formation of specific IgG glycosylation patterns remain poorly understood.

Epigenetic Regulation of Glycosylation in Cancer and Other Diseases

In the last few decades, the newly emerging field of epigenetic regulation of glycosylation acquired more importance because it is unraveling physiological and pathological mechanisms related to glycan functions. Glycosylation is a complex process in which proteins and lipids are modified by the attachment of monosaccharides. The main actors in this kind of modification are the glycoenzymes, which are translated from glycosylation-related genes (or glycogenes). The expression of glycogenes is regulated by transcription factors and epigenetic mechanisms (mainly DNA methylation, histone acetylation and noncoding RNAs). This review focuses only on these last ones, in relation to cancer and other diseases, such as inflammatory bowel disease and IgA1 nephropathy. In fact, it is clear that a deeper knowledge in the fine-tuning of glycogenes is essential for acquiring new insights in the glycan field, especially if this could be useful for finding novel and personalized therapeutics.

Zearalenone-Induced Interaction between PXR and Sp1 Increases Binding of Sp1 to a Promoter Site of the eNOS, Decreasing Its Transcription and NO Production in BAECs

Zearalenone (ZEN) is a non-steroidal mycotoxin that has various toxicological impacts on mammalian health. Here, we found that ZEN significantly affected the production of nitric oxide (NO) and the expression of endothelial NO synthase (eNOS) of bovine aortic endothelial cells (BAECs). A promoter analysis using 5′-serially deleted human eNOS promoter revealed that the proximal region (−135 to +22) was responsible for ZEN-mediated reduction of the human eNOS promoter activity. This effect was reversed by mutation of two specificity protein 1 (Sp1) binding elements in the human eNOS promoter. A chromatin immunoprecipitation assay revealed that ZEN increased Sp1 binding to the bovine eNOS promoter region (−113 to −12), which is homologous to −135 to +22 of the human eNOS promoter region. We also found that ZEN promoted the binding of the pregnane X receptor (PXR) to Sp1 of the bovine eNOS, consequently decreasing eNOS expression. This reduction of eNOS could have contributed to the decreased acetylcholine-induced vessel relaxation upon ZEN treatment in our ex vivo study using mouse aortas. In conclusion, our data demonstrate that ZEN decreases eNOS expression by enhancing the binding of PXR-Sp1 to the eNOS promoter, thereby decreasing NO production and potentially causing vessel dysfunction.

Hyperglycemia exacerbates colon cancer malignancy through hexosamine biosynthetic pathway

Hyperglycemia is a common feature of diabetes mellitus, considered as a risk factor for cancer. However, its direct effects in cancer cell behavior are relatively unexplored. Herein we show that high glucose concentration induces aberrant glycosylation, increased cell proliferation, invasion and tumor progression of colon cancer. By modulating the activity of the rate-limiting enzyme, glutamine-fructose-6-phosphate amidotransferase (GFAT), we demonstrate that hexosamine biosynthetic pathway (HBP) is involved in those processes. Biopsies from patients with colon carcinoma show increased levels of GFAT and consequently aberrant glycans’ expression suggesting an increase of HBP flow in human colon cancer. All together, our results open the possibility that HBP links hyperglycemia, aberrant glycosylation and tumor malignancy, and suggest this pathway as a potential therapeutic target for colorectal cancer.

Glyco-redox, a link between oxidative stress and changes of glycans: Lessons from research on glutathione, reactive oxygen and nitrogen species to glycobiology

Reduction-oxidation (redox) response is one of the most important biological phenomena. The concept introduced by Helmut Sies encouraged many researchers to examine oxidative stress under pathophysiological conditions. Our group has been interested in redox regulation under oxidative stress as well as glycobiology in relation to disease. Current studies by our group and other groups indicate that functional and structural changes of glycans are regulated by redox responses resulting from the generation of reactive oxygen species (ROS) or reactive nitrogen species (RNS) in various diseases including cancer, diabetes, neurodegenerative disease such as Parkinson disease, Alzheimer's disease and amyotrophic lateral sclerosis (ALS), and chronic obstructive pulmonary disease (COPD), even though very few investigators appear to be aware of these facts. Here we propose that the field "glyco-redox" will open the door to a more comprehensive understanding of the mechanism associated with diseases in relation to glycan changes under oxidative stress. A tight link between structural and functional changes of glycans and redox system under oxidative stress will lead to the recognition and interest of these aspects by many scientists. Helmut's contribution in this field facilitated our future perspectives in glycobiology.

Epigenetic regulation of ganglioside expression in neural stem cells and neuronal cells

The structural diversity and localization of cell surface glycosphingolipids (GSLs), including gangliosides, in glycolipid-enriched microdomains (GEMs, also known as lipid rafts) render them ideally suited to play important roles in mediating intercellular recognition, interactions, adhesion, receptor function, and signaling. Gangliosides, sialic acid-containing GSLs, are most abundant in the nerve tissues. The quantity and expression pattern of gangliosides in brain change drastically throughout development and these changes are mainly regulated through stage-specific expression of glycosyltransferase genes. We previously demonstrated for the first time that efficient histone acetylation of the glycosyltransferase genes in mouse brain contributes to the developmental alteration of ganglioside expression. We further demonstrated that acetylation of histones H3 and H4 on the N-acetylgalactosaminyltransferase I (GalNAcT, GA2/GM2/GD2/GT2-synthase; B4galnt1) gene promoter resulted in recruitment of trans-activation factors. In addition, we showed that epigenetic activation of the GalNAcT gene was detected and accompanied by an apparent induction of neuronal differentiation of neural stem cells (NSCs) responding to an exogenous supplement of ganglioside GM1. Most recently, we found that nuclear GM1 binds with acetylated histones on the promoters of the GalNAcT as well as on the NeuroD1 genes in differentiated neurons. Here, we will introduce epigenetic regulation of ganglioside synthase genes in neural development and neuronal differentiation of NSCs.

Enzymes for N-Glycan Branching and Their Genetic and Nongenetic Regulation in Cancer

N-glycan, a fundamental and versatile protein modification in mammals, plays critical roles in various physiological and pathological events including cancer progression. The formation of N-glycan branches catalyzed by specific N-acetylglucosaminyltransferases [GnT-III, GnT-IVs, GnT-V, GnT-IX (Vb)] and a fucosyltransferase, Fut8, provides functionally diverse N-glycosylated proteins. Aberrations of these branches are often found in cancer cells and are profoundly involved in cancer growth, invasion and metastasis. In this review, we focus on the GlcNAc and fucose branches of N-glycans and describe how their expression is dysregulated in cancer by genetic and nongenetic mechanisms including epigenetics and nucleotide sugar metabolisms. We also survey the roles that these N-glycans play in cancer progression and therapeutics. Finally, we discuss possible applications of our knowledge on basic glycobiology to the development of medicine and biomarkers for cancer therapy.

GM1 Ganglioside is Involved in Epigenetic Activation Loci of Neuronal Cells

Gangliosides are sialic acid-containing glycosphingolipids that are most abundant in the nerve tissues. The quantity and expression pattern of gangliosides in brain change drastically throughout development and are mainly regulated through stage-specific expression of glycosyltransferase (ganglioside synthase) genes. We previously demonstrated that acetylation of histones H3 and H4 on the N-acetylgalactosaminyltransferase I (GalNAcT, GA2/GM2/GD2/GT2-synthase) gene promoter resulted in recruitment of trans-activation factors. In addition, we reported that epigenetic activation of the GalNAcT gene was also detected as accompanied by an apparent induction of neuronal differentiation in neural stem cells responding to an exogenous supplement of ganglioside GM1. Here, we present evidence supporting the concept that nuclear GM1 is associated with gene regulation in neuronal cells. We found that nuclear GM1 binds acetylated histones on the promoters of the GalNAcT and NeuroD1 genes in differentiated neurons. Our study demonstrates for the first time that GM1 interacts with chromatin via acetylated histones at the nuclear periphery of neuronal cells.

Glycans and cancer: role of N-glycans in cancer biomarker, progression and metastasis, and therapeutics

Glycosylation is catalyzed by various glycosyltransferase enzymes which are mostly located in the Golgi apparatus in cells. These enzymes glycosylate various complex carbohydrates such as glycoproteins, glycolipids, and proteoglycans. The enzyme activity of glycosyltransferases and their gene expression are altered in various pathophysiological situations including cancer. Furthermore, the activity of glycosyltransferases is controlled by various factors such as the levels of nucleotide sugars, acceptor substrates, nucleotide sugar transporters, chaperons, and endogenous lectin in cancer cells. The glycosylation results in various functional changes of glycoproteins including cell surface receptors and adhesion molecules such as E-cadherin and integrins. These changes confer the unique characteristic phenotypes associated with cancer cells. Therefore, glycans play key roles in cancer progression and treatment. This review focuses on glycan structures, their biosynthetic glycosyltransferases, and their genes in relation to their biological significance and involvement in cancer, especially cancer biomarkers, epithelial-mesenchymal transition, cancer progression and metastasis, and therapeutics. Major N-glycan branching structures which are directly related to cancer are β1,6-GlcNAc branching, bisecting GlcNAc, and core fucose. These structures are enzymatic products of glycosyltransferases, GnT-V, GnT-III, and Fut8, respectively. The genes encoding these enzymes are designated as MGAT5 (Mgat5), MGAT3 (Mgat3), and FUT8 (Fut8) in humans (mice in parenthesis), respectively. GnT-V is highly associated with cancer metastasis, whereas GnT-III is associated with cancer suppression. Fut8 is involved in expression of cancer biomarker as well as in the treatment of cancer. In addition to these enzymes, GnT-IV and GnT-IX (GnT-Vb) will be also discussed in relation to cancer.

Control of Glycosylation-Related Genes by DNA Methylation: the Intriguing Case of the B3GALT5 Gene and Its Distinct Promoters

Glycosylation is a metabolic pathway consisting of the enzymatic modification of proteins and lipids through the stepwise addition of sugars that gives rise to glycoconjugates. To determine the full complement of glycoconjugates that cells produce (the glycome), a variety of genes are involved, many of which are regulated by DNA methylation. The aim of the present review is to briefly describe some relevant examples of glycosylation-related genes whose DNA methylation has been implicated in their regulation and to focus on the intriguing case of a glycosyltransferase gene (B3GALT5). Aberrant promoter methylation is frequently at the basis of their modulation in cancer, but in the case of B3GALT5, at least two promoters are involved in regulation, and a complex interplay is reported to occur between transcription factors, chromatin remodelling and DNA methylation of typical CpG islands or even of other CpG dinucleotides. Transcription of the B3GALT5 gene underwent a particular evolutionary fate, so that promoter hypermethylation, acting on one transcript, and hypomethylation of other sequences, acting on the other, cooperate on one gene to obtain full cancer-associated silencing. The findings may also help in unravelling the complex origin of serum CA19.9 antigen circulating in some patients.

Epigenetic regulation of a brain-specific glycosyltransferase N-acetylglucosaminyltransferase-IX (GnT-IX) by specific chromatin modifiers

Expression of glycosyltransferase genes is essential for glycosylation. However, the detailed mechanisms of how glycosyltransferase gene expression is regulated in a specific tissue or during disease progression are poorly understood. In particular, epigenetic studies of glycosyltransferase genes are limited, although epigenetic mechanisms, such as histone and DNA modifications, are central to establish tissue-specific gene expression. We previously found that epigenetic histone activation is essential for brain-specific expression of N-acetylglucosaminyltransferase-IX (GnT-IX, also designated GnT-Vb), but the mechanism of brain-specific chromatin activation around GnT-IX gene (Mgat5b) has not been clarified. To reveal the mechanisms regulating the chromatin surrounding GnT-IX, we have investigated the epigenetic factors that are specifically involved with the mouse GnT-IX locus by comparing their involvement with other glycosyltransferase loci. We first found that a histone deacetylase (HDAC) inhibitor enhanced the expression of GnT-IX but not of other glycosyltransferases tested. By overexpression and knockdown of a series of HDACs, we found that HDAC11 silenced GnT-IX. We also identified the O-GlcNAc transferase (OGT) and ten-eleven translocation-3 (TET3) complex as a specific chromatin activator of GnT-IX gene. Moreover, chromatin immunoprecipitation (ChIP) analysis in combination with OGT or TET3 knockdown showed that this OGT-TET3 complex facilitates the binding of a potent transactivator, NeuroD1, to the GnT-IX promoter, suggesting that epigenetic chromatin activation by the OGT-TET3 complex is a prerequisite for the efficient binding of NeuroD1. These results reveal a new epigenetic mechanism of brain-specific GnT-IX expression regulated by defined chromatin modifiers, providing new insights into the tissue-specific expression of glycosyltransferases.

Epigenetic activation of mouse ganglioside synthase genes: implications for neurogenesis

The quantity and expression pattern of gangliosides in mammalian brain change drastically during development and are mainly regulated through stage-specific expression of ganglioside synthase genes. Despite extensive investigations in the past, it remains largely unclear how the transcriptional activation of the genes encoding glycosyltransferases is regulated. Here, we show that in the neuronogenic cultures of mouse embryonic brain-derived neuroepithelial cells, histone modifications including acetylated histone H3 and histone H4, but not histone H3 trimethylation at lysine 27 of two genes encoding two key regulatory GTs, namely, N-acetylgalactosaminyltransferase I and sialyltransferase II, were extensively and gradually enhanced, respectively. As a consequence, the level of each GT mRNA was increased correspondingly. Hyperacetylation of histones on the GalNAcT promoter resulted in recruitment of the trans-activation factors Sp2 and AP-1 when cellular histone deacetylases 1 and 2 were knocked down with RNA interference or inhibited by treatment with valproic acid. Moreover, epigenetic activation of GalNAcT was also detected, as accompanied by a pronounced induction of neural differentiation in primary neuroepithelium culture responding to an exogenous supplement of ganglioside GM1, a downstream product of the gene's encoding enzyme. Our findings thus provide direct evidence of novel pathways for ganglioside expression via the epigenetic up-regulation of ganglioside synthase genes during neural development.

Epigenetic regulation of glycosylation is the quantum mechanics of biology

BACKGROUND

Most proteins are glycosylated, with glycans being integral structural and functional components of a glycoprotein. In contrast to polypeptides, which are fully encoded by the corresponding gene, glycans result from a dynamic interaction between the environment and a network of hundreds of genes.

SCOPE OF REVIEW

Recent developments in glycomics, genomics and epigenomics are discussed in the context of an evolutionary advantage for higher eukaryotes over microorganisms, conferred by the complexity and adaptability which glycosylation adds to their proteome.

MAJOR CONCLUSIONS

Inter-individual variation of glycome composition in human population is large; glycome composition is affected by both genes and environment; epigenetic regulation of "glyco-genes" has been demonstrated; and several mechanisms for transgenerational inheritance of epigenetic marks have been documented.

GENERAL SIGNIFICANCE

Epigenetic recording of acquired characteristics and their transgenerational inheritance could be important mechanisms used by higher organisms to compete or collaborate with microorganisms.

Loss of branched O-mannosyl glycans in astrocytes accelerates remyelination

In demyelinating diseases such as multiple sclerosis, a critical problem is failure of remyelination, which is important for protecting axons against degeneration and restoring conduction deficits. However, the underlying mechanism of demyelination/remyelination remains unclear. N-acetylglucosaminyltransferase-IX (GnT-IX; also known as GnT-Vb) is a brain-specific glycosyltransferase that catalyzes the branched formation of O-mannosyl glycan structures. O-Mannosylation of α-dystroglycan is critical for its function as an extracellular matrix receptor, but the biological significance of its branched structures, which are exclusively found in the brain, is unclear. In this study, we found that GnT-IX formed branched O-mannosyl glycans on receptor protein tyrosine phosphatase β (RPTPβ) in vivo. Since RPTPβ is thought to play a regulatory role in demyelinating diseases, GnT-IX-deficient mice were subjected to cuprizone-induced demyelination. Cuprizone feeding for 8 weeks gradually promoted demyelination in wild-type mice. In GnT-IX-deficient mice, the myelin content in the corpus callosum was reduced after 4 weeks of treatment, but markedly increased at 8 weeks, suggesting enhanced remyelination under GnT-IX deficiency. Furthermore, astrocyte activation in the corpus callosum of GnT-IX-deficient mice was significantly attenuated, and an oligodendrocyte cell lineage analysis indicated that more oligodendrocyte precursor cells differentiated into mature oligodendrocytes. Together, branched O-mannosyl glycans in the corpus callosum in the brain are a necessary component of remyelination inhibition in the cuprizone-induced demyelination model, suggesting that modulation of O-mannosyl glycans is a likely candidate for therapeutic strategies.

Identification of ectonucleotide pyrophosphatase/phosphodiesterase 3 (ENPP3) as a regulator of N-acetylglucosaminyltransferase GnT-IX (GnT-Vb)

Our previous studies on a β1,6-N-acetylglucosaminyltransferase, GnT-IX (GnT-Vb), a homolog of GnT-V, indicated that the enzyme has a broad GlcNAc transfer activity toward N-linked and O-mannosyl glycan core structures and that its brain-specific gene expression is regulated by epigenetic histone modifications. In this study, we demonstrate the existence of an endogenous inhibitory factor for GnT-IX that functions as a key regulator for GnT-IX enzymatic activity in Neuro2a (N2a) cells. We purified this factor from N2a cells and found that it is identical to ectonucleotide pyrophosphatase/phosphodiesterase 3 (ENPP3), as evidenced by mass spectrometry and by the knockdown and overexpression of ENPP3 in cultured cells. Kinetic analyses revealed that the mechanism responsible for the inhibition of GnT-IX caused by ENPP3 is the ENPP3-mediated hydrolysis of the nucleotide sugar donor substrate, UDP-GlcNAc, with the resulting generation of UMP, a potent and competitive inhibitor of GnT-IX. Indeed, ENPP3 knockdown cells had significantly increased levels of intracellular nucleotide sugars and displayed changes in the total cellular glycosylation profile. In addition to chaperones or other known regulators of glycosyltransferases, the ENPP3-mediated hydrolysis of nucleotide sugars would have widespread and significant impacts on glycosyltransferase activities and would be responsible for altering the total cellular glycosylation profile and modulating cellular functions.

Regulated expression and neural functions of human natural killer-1 (HNK-1) carbohydrate

Human natural killer-1 (HNK-1) carbohydrate, comprising a unique trisaccharide HSO(3)-3GlcAβ1-3Galβ1-4GlcNAc, shows well-regulated expression and unique functions in the nervous system. Recent studies have revealed sophisticated and complicated expression mechanisms for HNK-1 glycan. Activities of biosynthetic enzymes are controlled through the formation of enzyme-complexes and regulation of subcellular localization. Functional aspects of HNK-1 carbohydrate were examined by overexpression, knockdown, and knockout studies of these enzymes. HNK-1 is involved in several neural functions such as synaptic plasticity, learning and memory, and the underlying molecular mechanisms have been illustrated upon identification of the target carrier glycoproteins of HNK-1 such as the glutamate receptor subunit GluA2 or tenascin-R. In this review, we describe recent findings about HNK-1 carbohydrate that provide further insights into the mechanism of its expression and function in the nervous system.

The TCF4/β-catenin pathway and chromatin structure cooperate to regulate D-glucuronyl C5-epimerase expression in breast cancer

D-glucuronyl C5-epimerase (GLCE) is a potential tumor-suppressor gene involved in heparan sulfate biosynthesis. GLCE expression is significantly decreased in breast tumors; however, the underlying molecular mechanisms remain unclear. This study examined the possible epigenetic mechanisms for GLCE inactivation in breast cancer. Very little methylation of the GLCE promoter region was detected in breast tumors in vivo and in breast cancer cells (MCF7 and T47D) in vitro and GLCE expression in breast cancer cells was not altered by 5-deoxyazacytidine (5-aza-dC) treatment, suggesting that promoter methylation is not involved in regulating GLCE expression. Chromatin activation by Trichostatin A (TSA) or 5-aza-dC/TSA treatment increased GLCE expression by two to 3-fold due to an increased interaction between the GLCE promoter and the TCF4/β-catenin transactivation complex, or H3K9ac and H3K4Me3 histone modifications. However, ectopic expression of TCF4/β-catenin was not sufficient to activate GLCE expression in MCF7 cells, suggesting that chromatin structure plays a key role in GLCE regulation. Although TSA treatment significantly repressed canonical WNT signaling in MCF7 cells, it did not influence endogenous TCF4/β-catenin mRNA levels and activated TCF4/β-catenin-driven transcription from the GLCE promoter, indicating GLCE as a novel target for TCF4/β-catenin complex in breast cancer cells. A correlation was observed between GLCE, TCF4 and β-catenin expression in breast cancer cells and primary tumors, suggesting an important role for TCF4/β-catenin in regulating GLCE expression both in vitro and in vivo. Taken together, the results indicate that GLCE expression in breast cancer is regulated by a combination of chromatin structure and TCF4/β-catenin complex activity.

Developmental expression of the neuron-specific N-acetylglucosaminyltransferase Vb (GnT-Vb/IX) and identification of its in vivo glycan products in comparison with those of its paralog, GnT-V

The severe phenotypic effects of altered glycosylation in the congenital muscular dystrophies, including Walker-Warburg syndrome, muscle-eye-brain disease, Fukuyama congenital muscular dystrophy, and congenital muscular dystrophy 1D, are caused by mutations resulting in altered glycans linked to proteins through O-linked mannose. A glycosyltransferase that branches O-Man, N-acetylglucosaminyltransferase Vb (GnT-Vb), is highly expressed in neural tissues. To understand the expression and function of GnT-Vb, we studied its expression during neuromorphogenesis and generated GnT-Vb null mice. A paralog of GnT-Vb, N-acetylglucosaminyltransferase (GnT-V), is expressed in many tissues and brain, synthesizing N-linked, β1,6-branched glycans, but its ability to synthesize O-mannosyl-branched glycans is unknown; conversely, although GnT-Vb can synthesize N-linked glycans in vitro, its contribution to their synthesis in vivo is unknown. Our results showed that deleting both GnT-V and GnT-Vb results in the total loss of both N-linked and O-Man-linked β1,6-branched glycans. GnT-V null brains lacked N-linked, β1,6-glycans but had normal levels of O-Man β1,6-branched structures, showing that GnT-Vb could not compensate for the loss of GnT-V. By contrast, GnT-Vb null brains contained normal levels of N-linked β1,6-glycans but low levels of some O-Man β1,6-branched glycans. Therefore, GnT-V could partially compensate for GnT-Vb activity in vivo. We found no apparent change in α-dystroglycan binding of glycan-specific antibody IIH6C4 or binding to laminin in GnT-Vb null mice. These results demonstrate that GnT-V is involved in synthesizing branched O-mannosyl glycans in brain, but the function of these branched O-mannosyl structures is unresolved using mice that lack these glycosyltransferases.

Sensitivity of heterozygous α1,6-fucosyltransferase knock-out mice to cigarette smoke-induced emphysema: implication of aberrant transforming growth factor-β signaling and matrix metalloproteinase gene expression

We previously demonstrated that a deficiency in core fucosylation caused by the genetic disruption of α1,6-fucosyltransferase (Fut8) leads to lethal abnormalities and the development of emphysematous lesions in the lung by attenuation of TGF-β1 receptor signaling. Herein, we investigated the physiological relevance of core fucosylation in the pathogenesis of emphysema using viable heterozygous knock-out mice (Fut8(+/-)) that were exposed to cigarette smoke (CS). The Fut8(+/-) mice exhibited a marked decrease in FUT8 activity, and matrix metalloproteinase (MMP)-9 activities were elevated in the lung at an early stage of exposure. Emphysema developed after a 3-month CS exposure, accompanied by the recruitment of large numbers of macrophages to the lung. CS exposure substantially and persistently elevated the expression level of Smad7, resulting in a significant reduction of Smad2 phosphorylation (which controls MMP-9 expression) in Fut8(+/-) mice and Fut8-deficient embryonic fibroblast cells. These in vivo and in vitro studies show that impaired core fucosylation enhances the susceptibility to CS and constitutes at least part of the disease process of emphysema, in which TGF-β-Smad signaling is impaired and the MMP-mediated destruction of lung parenchyma is up-regulated.

Functional roles of gangliosides in neurodevelopment: an overview of recent advances

Gangliosides are sialic acid-containing glycosphingolipids that are most abundant in the nervous system. They are localized primarily in the outer leaflets of plasma membranes and participated in cell-cell recognition, adhesion, and signal transduction and are integral components of cell surface microdomains or lipid rafts along with proteins, sphingomyelin and cholesterol. Ganglioside-rich lipid rafts play an important role in signaling events affecting neural development and the pathogenesis of certain diseases. Disruption of gangloside synthase genes in mice induces developmental defects and neural degeneration. Targeting ganglioside metabolism may represent a novel therapeutic strategy for intervention in certain diseases. In this review, we focus on recent advances on metabolic and functional studies of gangliosides in normal brain development and in certain neurological disorders.

Integrated approach toward the discovery of glyco-biomarkers of inflammation-related diseases

Glycobiology has contributed tremendously to the discovery and characterization of cancer-related biomarkers containing glycans (i.e., glyco-biomarkers) and a more detailed understanding of cancer biology. It is now recognized that most chronic diseases involve some elements of chronic inflammation; these include cancer, Alzheimer's disease, and metabolic syndrome (including consequential diabetes mellitus and cardiovascular diseases). By extending the knowledge and experience of the glycobiology community regarding cancer biomarker discovery, we should be able to contribute to the discovery of diagnostic/prognostic glyco-biomarkers of other chronic diseases that involve chronic inflammation. Future integration of large-scale "omics"-type data (e.g., genomics, epigenomics, transcriptomics, proteomics, and glycomics) with computational model building, or a systems glycobiology approach, will facilitate such efforts.