Regulation of zebrafish skeletogenesis by ext2/dackel and papst1/pinscher

Mutations in human Exostosin genes (EXTs) confer a disease called Hereditary Multiple Exostoses (HME) that affects 1 in 50,000 among the general population. Patients with HME have a short stature and develop osteochondromas during childhood. Here we show that two zebrafish mutants, dackel (dak) and pinscher (pic), have cartilage defects that strongly resemble those seen in HME patients. We have previously determined that dak encodes zebrafish Ext2. Positional cloning of pic reveals that it encodes a sulphate transporter required for sulphation of glycans (Papst1). We show that although both dak and pic are required during cartilage morphogenesis, they are dispensable for chondrocyte and perichondral cell differentiation. They are also required for hypertrophic chondrocyte differentiation and osteoblast differentiation. Transplantation analysis indicates that dak^−/− cells are usually rescued by neighbouring wild-type chondrocytes. In contrast, pic^−/− chondrocytes always act autonomously and can disrupt the morphology of neighbouring wild-type cells. These findings lead to the development of a new model to explain the aetiology of HME.

Hereditary Multiple Exostoses is a disease that causes the formation of benign bone tumours in children. Besides causing severe skeletal deformity, the bone tumours can compress nerves or other tissue resulting in chronic pain. Although the tumours can usually be surgically removed, they sometimes recur or are in positions that prevent surgery. We have identified two strains of zebrafish whose offspring have skeletal defects that resemble those of patients with Hereditary Multiple Exostoses. We have found that each strain carries a mutated form of an essential gene. Importantly, these two genes are also found in humans, and thus by analysing their function in zebrafish, we may shed light on their role in humans. Our study has elucidated the roles of these genes during normal skeletal development and has allowed us to generate a model for how genetic changes give rise to bone tumours in humans.

Functional roles of N-glycans in cell signaling and cell adhesion in cancer

Glycosylation is one of the most common post-translational modification reactions and nearly half of all known proteins in eukaryotes are glycosylated. In fact, changes in oligosaccharide structures are associated with many physiological and pathological events, including cell growth, migration, differentiation, tumor invasion, host-pathogen interactions, cell trafficking, and transmembrane signaling. Emerging roles of glycan functions have been highly attractive to scientists in various fields of life science as they open a field, "Functional Glycomics", that is a comprehensive study of the glycan structures in relation to functions. In particular, the N-glycans of signaling molecules including receptors or adhesion molecules are considered to be involved in cellular functions. This review will focus on the roles of glycosyltransferases involved in the biosynthesis of N-glycan branching and identification of cell surface receptors as their target proteins. We also suggest that the modulation of N-glycans of those receptors alters their important functions such as cell signaling and cell adhesion which are implicated in cancer invasion and metastasis.

Requirement for O-linked N-acetylglucosaminyltransferase in lymphocytes activation

The dynamic modification of nuclear and cytoplasmic proteins with O-linked beta-N-acetylglucosamine (O-GlcNAc) by the O-linked N-acetylglucosaminyltransferase (OGT) is a regulatory post-translational modification that is responsive to various stimuli. Here, we demonstrate that OGT is a central factor for T- and B-lymphocytes activation. SiRNA-mediated knockdown of OGT in T cells leads to an impaired activation of the transcription factors NFAT and NFkappaB. This results in a reduction of IL-2 production consistent with prevention of T-cell activation. OGT is also required for the early activation of B cells mediated by stimulation of the B-cell receptor. Mechanistically, we demonstrate that NFkappaB as well as NFAT are glycosylated with O-GlcNAc after direct binding to OGT. Moreover, kinetic experiments show that O-GlcNAc modification prominently increased shortly after activation of lymphoid cells and it might be required for nuclear translocation of the transcription factors NFkappaB and NFAT.

A novel strategy for mammalian cell surface glycome profiling using lectin microarray

The glycome represents the total set of glycans expressed in a cell. The glycome has been assumed to vary between cell types, stages of development and differentiation, and during malignant transformation. Analysis of the glycome provides a basis for understanding the functions of glycans in these cellular processes. Recently, a technique called lectin microarray was developed for rapid profiling of glycosylation, although its use was mainly restricted to glycoproteins of cell lysates, and thus unable to profile the intact cell surface glycans. Here we report a simple and sensitive procedure based on this technology for direct analysis of the live mammalian cell-surface glycome. Fluorescent-labeled live cells were applied in situ to the established lectin microarray consisting of 43 immobilized lectins with distinctive binding specificities. After washing, bound cells were directly detected by an evanescent-field fluorescence scanner in a liquid phase without fixing and permeabilization. The results obtained by differential profiling of CHO and its glycosylation-defective mutant cells, and splenocytes of wild-type and beta1-3-N-acetylglucosaminyltransferase II knockout mice performed as model experiments agreed well with their glycosylation phenotypes. We also compared cell surface glycans of K562 cells before and after differentiation and found a significant increase in the expression of O-glycans on differentiated cells. These results demonstrate that the technique provides a novel strategy for profiling global changes of the mammalian cell surface glycome.

Membrane topology of the Alg14 endoplasmic reticulum UDP-GlcNAc transferase subunit

N-linked glycosylation begins in the endoplasmic reticulum with the synthesis of a highly conserved dolichol-linked oligosaccharide precursor. The UDP-GlcNAc glycosyltransferase catalyzing the second sugar addition of this precursor consists in most eukaryotes of at least two subunits, Alg14 and Alg13. Alg14 is a membrane protein that recruits the soluble Alg13 catalytic subunit from the cytosol to the face of the endoplasmic reticulum (ER) membrane where this reaction occurs. Here, we investigated the membrane topology of Saccharomyces cerevisiae Alg14 and its requirements for ER membrane association. Alg14 is predicted by most algorithms to contain one or more transmembrane spanning helices (transmembrane domains (TMDs)). We provide evidence that Alg14 contains a C-terminal cytosolic tail and an N terminus that resides within the ER lumen. However, we also demonstrate that Alg14 lacking this TMD is functional and remains peripherally associated with ER membranes, suggesting that additional domains can mediate ER association. These conclusions are based on the functional analysis of Alg13/Alg14 chimeras containing Alg13 fused at either end of Alg14 or truncated Alg14 variants lacking the predicted TMD; protease protection assays of Alg14 in intact ER membranes; and extraction of Alg14-containing ER membranes with high pH. These yeast Alg13-Alg14 chimeras recapitulate the phylogenetic diversity of Alg13-Alg14 domain arrangements that evolved in some protozoa. They encode single polypeptides containing an Alg13 domain fused to Alg14 domain in either orientation, including those lacking the Alg14 TMD. Thus, this Alg13-Alg14 UDP-GlcNAc transferase represents an unprecedented example of a bipartite glycosyltransferase that evolved by both fission and fusion.

Functional conservation of the human EXT1 tumor suppressor gene and its Drosophila homolog tout velu

Heparan sulfate proteoglycans play a vital role in signaling of various growth factors in both Drosophila and vertebrates. In Drosophila, mutations in the tout velu (ttv) gene, a homolog of the mammalian EXT1 tumor suppressor gene, leads to abrogation of glycosaminoglycan (GAG) biosynthesis. This impairs distribution and signaling activities of various morphogens such as Hedgehog (Hh), Wingless (Wg), and Decapentaplegic (Dpp). Mutations in members of the exostosin (EXT) gene family lead to hereditary multiple exostosis in humans leading to bone outgrowths and tumors. In this study, we provide genetic and biochemical evidence that the human EXT1 (hEXT1) gene is conserved through species and can functionally complement the ttv mutation in Drosophila. The hEXT1 gene was able to rescue a ttv null mutant to adulthood and restore GAG biosynthesis.

Adherens junction remodeling by the Notch pathway in Drosophila melanogaster oogenesis

Identifying genes involved in the control of adherens junction (AJ) remodeling is essential to understanding epithelial morphogenesis. During follicular epithelium development in Drosophila melanogaster, the main body follicular cells (MBFCs) are displaced toward the oocyte and become columnar. Concomitantly, the stretched cells (StCs) become squamous and flatten around the nurse cells. By monitoring the expression of epithelial cadherin and Armadillo, I have discovered that the rate of AJ disassembly between the StCs is affected in follicles with somatic clones mutant for fringe or Delta and Serrate. This results in abnormal StC flattening and delayed MBFC displacement. Additionally, accumulation of the myosin II heavy chain Zipper is delayed at the AJs that require disassembly. Together, my results demonstrate that the Notch pathway controls AJ remodeling between the StCs and that this role is crucial for the timing of MBFC displacement and StC flattening. This provides new evidence that Notch, besides playing a key role in cell differentiation, also controls cell morphogenesis.

The protective effects of PUGNAc on cardiac function after trauma-hemorrhage are mediated via increased protein O-GlcNAc levels

We have previously shown that administration of glucosamine after trauma-hemorrhage (TH) improved cardiac output and organ perfusion, and this was associated with increased levels of O-linked N-acetylglucosamine (O-GlcNAc) on proteins in the heart and brain. An alternative means of increasing O-GlcNAc levels is by inhibition of O-linked N-acetylglucosaminidase, which catalyzes the removal of N-acetylglucosamine from proteins, with O-(2-acetamido-2-deoxy-d-glucopyranosylidene) amino-N-phenylcarbamate (PUGNAc). The goal of this study, therefore, was to determine whether PUGNAc administration after TH also improves recovery of organ perfusion and function. Fasted male rats were bled to and maintained at a mean arterial blood pressure of 40 mmHg for 90 min, followed by fluid resuscitation. Intravenous administration of PUGNAc (200 micromol/kg body weight) 30 min after the onset of resuscitation significantly improved cardiac output compared with the vehicle controls (12.3 +/- 1.3 mL/min per 100 g body weight vs. 25.5 +/- 2.0 mL/min per 100 g body weight; P < 0.05), decreased total peripheral resistance (6.6 +/- 0.8 mmHg/mL per minute per 100 g body weight vs. 3.7 +/- 0.3 mmHg/mL per minute per 100 g body weight; P < 0.05), and increased perfusion of critical organ systems, including the kidney and liver, determined at 2 h after the end of resuscitation. Treatment with PUGNAc also attenuated the TH-induced increase in plasma IL-6 levels (864 +/- 112 pg/mL vs. 392 +/- 188 pg/mL; P < 0.05) and TNF-alpha levels (216 +/- 21 pg/mL vs. 94 +/- 11 pg/mL; P < 0.05) and significantly increased O-GlcNAc levels in the heart, liver, and kidney. Thus, PUGNAc, like glucosamine, improves cardiac function and organ perfusion and reduced the level of circulating IL-6 and TNF-alpha after TH. The similar effects of glucosamine and PUGNAc support the notion that the protection associated with both interventions is mediated via increased protein O-GlcNAc levels.

Mgat5 and Pten interact to regulate cell growth and polarity

Phosphatase and tensin homolog (Pten) phosphatase opposes intracellular phosphoinositide 3-kinase (PI3K)/Akt signaling and is a potent tumor suppressor, while Golgi beta1,6 N-acetylglucosaminyltransferase V (Mgat5) is positively associated with cancer progression and metastasis. beta1,6GlcNAc-branched N-glycans on receptor glycoproteins promote their surface residency and sensitizes cells to growth factor signaling. Here we demonstrate that the Pten heterozygosity in mouse embryonic fibroblasts enhances cell adhesion-dependent PI3K/Akt signaling, cell spreading, and proliferation, while Pten/Mgat5 double mutant cells are normalized. However, planar asymmetry typical of fibroblasts and invasive carcinomas is not fully rescued, suggesting that Mgat5 and Pten function together to regulate the membrane dynamics of PI3K/Akt signaling typical of motile cells. Pten heterozygosity was associated with increased surface beta1,6GlcNAc-branched N-glycans, suggesting positive feedback from PI3K signaling to N-glycan branching. In vivo, Mgat5(-/-) Pten(+/-) and Mgat5(+/-)Pten(+/-)mutant mice showed a small but significant increase in longevity compared with Pten(+/-) mice. Taken together, our results reveal that Mgat5 and Pten interact in an opposing manner to regulate cellular sensitivities to extracelluar growth cues.

Dorsal-ventral midline signaling in the developing Drosophila eye

Boundaries between different cell types play key roles in many developmental patterning processes. They can be established by various mechanisms, and signaling between the different cell types can occur in a number of ways. One mechanism of crossboundary signaling is controlled by the Notch (N)-modifying protein Fringe (Fng). At the Drosophila wing dorsal-ventral (D-V) border, the mechanism by which an Fng(+)-Fng(-) interface controls local N activation has been well characterized. A similar N-activating Fng(+)-Fng(-) interface has also been described at the D-V border of the fly eye, but the mechanisms that establish and regulate it are different from those in the wing. Here we describe the ventral role of the Sloppy-paired (Slp) transcription factor, and its interactions with dorsally expressed Iroquois (Iro) transcription factors in the regulation of signaling about the Fng(+)-Fng(-) interface in the developing eye. The two transcription factors are mutually repressive and initially abut at the D-V midline. However, N signaling at the interface downregulates Slp expression, and a gap opens between the two expression domains in which Serrate (Ser, an N ligand) is upregulated.

O-GlcNAc integrates the proteasome and transcriptome to regulate nuclear hormone receptors

Mechanisms controlling nuclear hormone receptors are a central question to mammalian developmental and disease processes. Herein, we show that a subtle increase in O-GlcNAc levels inhibits activation of nuclear hormone receptors. In vivo, increased levels of O-GlcNAc impair estrogen receptor activation and cause a decrease in mammary ductal side-branching morphogenesis associated with loss of progesterone receptors. Increased O-GlcNAc levels suppress transcriptional expression of coactivators and of the nuclear hormone receptors themselves. Surprisingly, increased O-GlcNAc levels are also associated with increased transcription of genes encoding corepressor proteins NCoR and SMRT. The association of the enzyme O-GlcNAc transferase with these corepressors contributes to specific regulation of nuclear hormone receptors by O-GlcNAc. Overall, transcriptional inhibition is related to the integrated effect of O-GlcNAc by direct modification of critical elements of the transcriptome and indirectly through O-GlcNAc modification of the proteasome.

EXT-related pathways are not involved in the pathogenesis of dysplasia epiphysealis hemimelica and metachondromatosis

Dysplasia epiphysealis hemimelica (DEH) and metachondromatosis (MC) are considered in the differential diagnosis of solitary and hereditary osteochondromas. Both are rare disorders with DEH demonstrating cartilaginous overgrowth of an epiphysis and MC exhibiting synchronous enchondromas and osteochondromas. Ten cases of DEH and two of MC were compared with osteochondromas at the histological and molecular level. Histologically, clumping of chondrocytes within a fibrillary chondroid matrix is characteristic of DEH, while osteochondromas and MC display the characteristic growth plate architecture. Using cDNA microarray analysis we demonstrate that DEH and MC cluster separately from osteochondromas and growth plates. The EXT genes, involved in the hereditary multiple osteochondromas syndrome, and downregulated in osteochondroma, were normally expressed in DEH and MC as shown by quantitative reverse transcriptase-polymerase chain reaction (qPCR). EXT is involved in heparan sulphate biosynthesis, important for Indian Hedgehog/ParaThyroid Hormone Like Hormone (IHH/PTHLH) growth plate signalling pathways. IHH/PTHLH signalling molecules were expressed in DEH and MC as shown by both qPCR and immunohistochemistry, suggesting that this pathway is active. This is in contrast to osteochondroma, in which PTHLH signalling is downregulated. Thus, lesions of DEH and MC are separate entities from osteochondroma as confirmed by their different cDNA and protein expression profiles. Downstream targets of EXT, which are downregulated in osteochondroma, are expressed in DEH and MC, suggesting that EXT signalling is not disturbed.

Blocking of N-acetylglucosaminyltransferase V induces cellular endoplasmic reticulum stress in human hepatocarcinoma 7,721 cells

N-acetylglucosaminyltransferase V (GnT-V) is an important tumorigenesis and metastasis-associated enzyme. To study its biofunction, the GnT-V stably suppressed cell line (GnT-V-AS/7,721) was constructed from 7,721 hepatocarcinoma cells in previous study. In this study, cDNA array gene expression profiles were compared between GnT-V-AS/7,721 and parental 7,721 cells. The data indicated that GnT-V-AS/7,721 showed a characteristic expression pattern consistent with the ER stress. The molecular mechanism of the ER stress was explored in GnT-V-AS/7,721 by the analysis on key molecules in both two unfolded protein response (UPR) pathways. For ATF6 and Ire1/XBP-1 pathway, it was evidenced by the up-regulation of BIP at mRNA and protein level, and the appearance of the spliced form of XBP-1. As for PERK/eIF2alpha pathway, the activation of ER eIF2alpha kinase PERK was observed. To confirm the results from GnT-V-AS/7,721 cells, the key molecules in the UPR were examined again in 7,721 cells interfered with the GnT-V by the specific RNAi treatment. The results were similar with those from GnT-V-AS/7721, indicating that blocking of GnT-V can specifically activate ER stress in 7,721 cells. Rate of (3)H-Man incorporation corrected with rate of (3)H-Leu incorporation in GnT-V-AS/7,721 was down-regulated greatly compared with the control, which demonstrated the deficient function of the enzyme synthesizing N-glycans after GnT-V blocking. Moreover, the faster migrating form of chaperone GRP94 associated with the underglycosylation, and the extensively changed N-glycans structures of intracellular glycoproteins were also detected in GnT-V-AS/7,721. These results supported the mechanism that blocking of GnT-V expression impaired functions of chaperones and N-glycan-synthesizing enzymes, which caused UPR in vivo.

Haploinsufficiency of C2GnT-I glycosyltransferase renders T lymphoma cells resistant to cell death

Neoplastic T cells in mycosis fungoides (MF) are resistant to apoptotic agents, including galectin-1 that is abundant in skin. Although MF cells are typically CD7-, and thus galectin-1 resistant, CD7+ HH cells, derived from a patient with MF, were also resistant to galectin-1. HH cells demonstrate altered cell surface glycosylation, with loss of core 2 O-glycan ligands for galectin-1 created by core 2 beta1,6-N-acetylglucosaminyltransferase (C2GnT-I). Loss of core 2 O-glycans on tumor cells was also seen in primary CD7+ MF lesions. Surprisingly, HH cells are heterozygous for a C2GnT-I point mutation, yet this mutation resulted in a dramatic reduction in cellular glycosyltransferase activity. Expression of wild-type C2GnT-I in human HH cells, or murine lymphoma cells that lack C2GnT-I, restored core 2 O-glycan expression and susceptibility to galectin-1, whereas mutant enzyme lacked activity and did not restore core 2 O-glycan expression or susceptibility to galectin-1. Mutant enzyme did not have a dominant negative effect by affecting dimerization or activity of wild-type enzyme; rather, C2GnT-I haploinsufficiency is sufficient for loss of core 2 O-glycan expression and galectin-1 resistance. Thus, glycosyltransferase haploinsufficiency results in altered cellular glycosylation and resistance to cell death, identifying a new survival mechanism for T-lymphoma cells.

Does O-GlcNAc play a role in neurodegenerative diseases?

There are several lines of evidence that the modification of proteins by cytosolic- and nuclear-specific O-linked N-acetylglucosamine (O-GlcNAc) glycosylation is closely related to neuropathologies, particularly Alzheimer's disease. Several neuronal proteins have been identified as being modified with O-GlcNAc; these proteins could form part of the inclusion bodies found, for example, in the most frequently observed neurologic disorder (i.e., Alzheimer's disease; Tau protein and beta-amyloid peptide are the well known aggregated proteins). O-GlcNAc proteins are also implicated in synaptosomal transport (e.g., synapsins and clathrin-assembly proteins). Inclusion bodies are partly characterized by a deficiency in the ubiquitin-proteasome system, avoiding the degradation of aggregated proteins. From this perspective, it appears interesting that substrate proteins could be protected against proteasomal degradation by being covalently modified with single N-acetylglucosamine on serine or threonine, and that the proteasome itself is modified and regulated by O-GlcNAc (in this case the turnover of neuronal proteins correlates with extracellular glucose). Interestingly, glucose uptake and metabolism are impaired in neuronal disorders, and this phenomenon is linked to increased phosphorylation. In view of the existence of the dynamic interplay between O-GlcNAc and phosphorylation, it is tempting to draw a parallel between the use of glucose, O-GlcNAc glycosylation and phosphorylation. Lastly, the two enzymes responsible for O-GlcNAc dynamism (i.e., O-GlcNAc transferase and glucosaminidase) are both enriched in the brain and genes that encode the two enzymes are located in two regions that are found to be frequently mutated in neurologic disorders. The data presented in this review strongly suggest that O-GlcNAc could play an active role in neurodegenerative diseases.

beta1,4-N-Acetylglucosaminyltransferase III potentiates beta1 integrin-mediated neuritogenesis induced by serum deprivation in Neuro2a cells

Aspects of the biological significance of the bisecting N-acetylglucosamine (GlcNAc) structure on N-glycans introduced by beta1,4-N-acetylglucosaminyltransferase III (GnT-III) in Neuro2a cell differentiation are demonstrated. The overexpression of GnT-III in the cells led to the induction of axon-like processes with numerous neurites and swellings, in which beta1 integrin was localized, under conditions of serum starvation. This enhancement in neuritogenesis was suppressed by either the addition of a bisecting GlcNAc-containing N-glycan or erythroagglutinating phytohemagglutinin (E(4)-PHA), which preferentially recognizes the bisecting GlcNAc. GnT-III-promoted neuritogenesis was also significantly perturbed by treatment with a functional blocking anti-beta1 integrin antibody. In fact, beta1 integrin was found to be one of the target proteins of GnT-III, as confirmed by a pull-down assay with E(4)-PHA. These data suggest that N-glycans with a bisecting GlcNAc on target molecules, such as beta1 integrin, play important roles in the regulation of neuritogenesis.