Chinese hamster ovary cell mutants defective in glycosaminoglycan assembly and glucuronosyltransferase I

Regulation of morphogen pathways by a Drosophila chondroitin sulfate proteoglycan Windpipe

Morphogens provide quantitative and robust signaling systems to achieve stereotypic patterning and morphogenesis. Heparan sulfate (HS) proteoglycans (HSPGs) are key components of such regulatory feedback networks. In Drosophila, HSPGs serve as co-receptors for a number of morphogens, including Hedgehog (Hh), Wingless (Wg), Decapentaplegic (Dpp) and Unpaired (Upd, or Upd1). Recently, Windpipe (Wdp), a chondroitin sulfate (CS) proteoglycan (CSPG), was found to negatively regulate Upd and Hh signaling. However, the roles of Wdp, and CSPGs in general, in morphogen signaling networks are poorly understood. We found that Wdp is a major CSPG with 4-O-sulfated CS in Drosophila. Overexpression of wdp modulates Dpp and Wg signaling, showing that it is a general regulator of HS-dependent pathways. Although wdp mutant phenotypes are mild in the presence of morphogen signaling buffering systems, this mutant in the absence of Sulf1 or Dally, molecular hubs of the feedback networks, produces high levels of synthetic lethality and various severe morphological phenotypes. Our study indicates a close functional relationship between HS and CS, and identifies the CSPG Wdp as a novel component in morphogen feedback pathways.

Heparan sulfate regulates IL-21 bioavailability and signal strength that control germinal center B cell selection and differentiation

In antibody responses, mutated germinal center B (B_GC) cells are positively selected for reentry or differentiation. As the products from GCs, memory B cells and antibody-secreting cells (ASCs) support high-affinity and long-lasting immunity. Positive selection of B_GC cells is controlled by signals received through the B cell receptor (BCR) and follicular helper T (T_FH) cell-derived signals, in particular costimulation through CD40. Here, we demonstrate that the T_FH cell effector cytokine interleukin-21 (IL-21) joins BCR and CD40 in supporting B_GC selection and reveal that strong IL-21 signaling prioritizes ASC differentiation in vivo. B_GC cells, compared with non-B_GC cells, show significantly reduced IL-21 binding and attenuated signaling, which is mediated by low cellular heparan sulfate (HS) sulfation. Mechanistically, N-deacetylase and N-sulfotransferase 1 (Ndst1)-mediated N-sulfation of HS in B cells promotes IL-21 binding and signal strength. Ndst1 is down-regulated in B_GC cells and up-regulated in ASC precursors, suggesting selective desensitization to IL-21 in B_GC cells. Thus, specialized biochemical regulation of IL-21 bioavailability and signal strength sets a balance between the stringency and efficiency of GC selection.

Novel genes and variants associated with congenital pituitary hormone deficiency in the era of next-generation sequencing

Combined pituitary hormone deficiency (CPHD) is not a rare disorder, with a frequency of approximately 1 case per 4,000 live births. However, in most cases, a genetic diagnosis is not available. Furthermore, the diagnosis is challenging because no clear correlation exists between the pituitary hormones affected and the gene(s) responsible for the disorder. Next-generation sequencing (NGS) has recently been widely used to identify novel genes that cause (or putatively cause) CPHD. This review outlines causative genes for CPHD that have been newly reported in recent years. Moreover, novel variants of known CPHD-related genes (POU1F1 and GH1 genes) that contribute to CPHD through unique mechanisms are also discussed in this review. From a clinical perspective, variants in some of the recently identified causative genes result in extra-pituitary phenotypes. Clinical research on the related symptoms and basic research on pituitary formation may help in inferring the causative gene(s) of CPHD. Future NGS analysis of a large number of CPHD cases may reveal new genes related to pituitary development. Clarifying the causative genes of CPHD may help to understand the process of pituitary development. We hope that future innovations will lead to the identification of genes responsible for CPHD and pituitary development.

Research and Application of Chondroitin Sulfate/Dermatan Sulfate-Degrading Enzymes

Chondroitin sulfate (CS) and dermatan sulfate (DS) are widely distributed on the cell surface and in the extracellular matrix in the form of proteoglycan, where they participate in various biological processes. The diverse functions of CS/DS can be mainly attributed to their high structural variability. However, their structural complexity creates a big challenge for structural and functional studies of CS/DS. CS/DS-degrading enzymes with different specific activities are irreplaceable tools that could be used to solve this problem. Depending on the site of action, CS/DS-degrading enzymes can be classified as glycosidic bond-cleaving enzymes and sulfatases from animals and microorganisms. As discussed in this review, a few of the identified enzymes, particularly those from bacteria, have wildly applied to the basic studies and applications of CS/DS, such as disaccharide composition analysis, the preparation of bioactive oligosaccharides, oligosaccharide sequencing, and potential medical application, but these do not fulfill all of the needs in terms of the structural complexity of CS/DS.

Chondrocytes respond to an altered heparan sulfate composition with distinct changes of heparan sulfate structure and increased levels of chondroitin sulfate

Heparan sulfate (HS) regulates the activity of many signaling molecules critical for the development of endochondral bones. Even so, mice with a genetically altered HS metabolism display a relatively mild skeletal phenotype compared to the defects observed in other tissues and organs pointing to a reduced HS dependency of growth-factor signaling in chondrocytes. To understand this difference, we have investigated the glycosaminoglycan (GAG) composition in two mouse lines that produce either reduced levels of HS (Ext1^gt/gt mice) or HS lacking 2-O-sulfation (Hs2st1^-/- mice). Analysis by RPIP-HPLC revealed an increased level of sulfated disaccarides not affected by the mutation in both mouse lines indicating that chondrocytes attempt to restore a critical level of sulfation. In addition, in both mutant lines we also detected significantly elevated levels of CS. Size exclusion chromatography further demonstrated that Ext1^gt/gt mutants produce more but shorter CS chains, while the CS chains produced by (Hs2st1^-/- mice) mutants are of similar length to that of wild type littermates indicating that chondrocytes produce more rather than longer CS chains. Expression analysis revealed an upregulation of aggrecan, which likely carries most of the additionally produced CS. Together the results of this study demonstrate for the first time that not only a reduced HS synthesis but also an altered HS structure leads to increased levels of CS in mammalian tissues. Furthermore, as chondrocytes produce 100-fold more CS than HS the increased CS levels point to an active, precursor-independent mechanism that senses the quality of HS in a vast excess of CS. Interestingly, reducing the level of cell surface CS by chondroitinase treatment leads to reduced Bmp2 induced Smad1/5/9 phosphorylation. In addition, Erk phosphorylation is increased independent of Fgf18 treatment indicating that both, HS and CS, affect growth factor signaling in chondrocytes in distinct manners.

Imaging specific cellular glycan structures using glycosyltransferases via click chemistry

Heparan sulfate (HS) is a polysaccharide fundamentally important for biologically activities. T/Tn antigens are universal carbohydrate cancer markers. Here, we report the specific imaging of these carbohydrates using a mesenchymal stem cell line and human umbilical vein endothelial cells (HUVEC). The staining specificities were demonstrated by comparing imaging of different glycans and validated by either removal of target glycans, which results in loss of signal, or installation of target glycans, which results in gain of signal. As controls, representative key glycans including O-GlcNAc, lactosaminyl glycans and hyaluronan were also imaged. HS staining revealed novel architectural features of the extracellular matrix (ECM) of HUVEC cells. Results from T/Tn antigen staining suggest that O-GalNAcylation is a rate-limiting step for O-glycan synthesis. Overall, these highly specific approaches for HS and T/Tn antigen imaging should greatly facilitate the detection and functional characterization of these biologically important glycans.

A mutant-cell library for systematic analysis of heparan sulfate structure-function relationships

Heparan sulfate (HS) is a complex linear polysaccharide that modulates a wide range of biological functions. Elucidating the structure-function relationship of HS has been challenging. Here we report the generation of an HS-mutant mouse lung endothelial cell library by systematic deletion of HS genes expressed in the cell. We used this library to (1) determine that the strictly defined fine structure of HS, not its overall degree of sulfation, is more important for FGF2-FGFR1 signaling; (2) define the epitope features of commonly used anti-HS phage display antibodies; and (3) delineate the fine inter-regulation networks by which HS genes modify HS and chain length in mammalian cells at a cell-type-specific level. Our mutant-cell library will allow robust and systematic interrogation of the roles and related structures of HS in a cellular context.

The parasitic nematode Oesophagostomum dentatum synthesizes unusual glycosaminoglycan-like O-glycans

O-glycosylation is probably one of the most varied sets of post-translational modifications across all organisms, but amongst the most refractory to analyze. In animals, O-xylosylation of serine residues represents the first stage in the synthesis of glycosaminoglycans, whose repeat regions are generally analyzed as fragments resulting from enzymatic or chemical degradation, whereas their core regions can be isolated by β-elimination or endo-β-xylosidase digestion. In the present study, we show that hydrazinolysis can be employed for release of glycosaminoglycan-type oligosaccharides from nematodes prior to fluorescent labeling with 2-aminopyridine. While various [HexNAcHexA]nGal2Xyl oligosaccharides were isolated from the model organism Caenorhabditis elegans, more unusual glycosaminoglycan-type glycans were found to be present in the porcine parasite Oesophagostomum dentatum. In this case, as judged by MS/MS before and after hydrofluoric acid or β-galactosidase digestion, core sequences with extra galactose and phosphorylcholine residues were detected as [(±PC)HexNAcHexA]n(±PC)Galβ3-(±Galβ4)Galβ4Xyl. Thus, hydrazinolysis and fluorescent labeling can be combined to analyze unique forms of O-xylosylation, including new examples of zwitterionic glycan modifications.

Whole-Genome Sequencing of Invasion-Resistant Cells Identifies Laminin α2 as a Host Factor for Bacterial Invasion

To understand the role of glycosaminoglycans in bacterial cellular invasion, xylosyltransferase-deficient mutants of Chinese hamster ovary (CHO) cells were created using clustered regularly interspaced short palindromic repeat (CRISPR) and CRISPR-associated gene 9 (CRISPR-cas9) gene targeting. When these mutants were compared to the pgsA745 cell line, a CHO xylosyltransferase mutant generated previously using chemical mutagenesis, an unexpected result was obtained. Bacterial invasion of pgsA745 cells by group B Streptococcus (GBS), group A Streptococcus, and Staphylococcus aureus was markedly reduced compared to the invasion of wild-type cells, but newly generated CRISPR-cas9 mutants were only resistant to GBS. Invasion of pgsA745 cells was not restored by transfection with xylosyltransferase, suggesting that an additional mutation conferring panresistance to multiple bacteria was present in pgsA745 cells. Whole-genome sequencing and transcriptome sequencing (RNA-Seq) uncovered a deletion in the gene encoding the laminin subunit α2 (Lama2) that eliminated much of domain L4a. Silencing of the long Lama2 isoform in wild-type cells strongly reduced bacterial invasion, whereas transfection with human LAMA2 cDNA significantly enhanced invasion in pgsA745 cells. The addition of exogenous laminin-α2β1γ1/laminin-α2β2γ1 strongly increased bacterial invasion in CHO cells, as well as in human alveolar basal epithelial and human brain microvascular endothelial cells. Thus, the L4a domain in laminin α2 is important for cellular invasion by a number of bacterial pathogens.

IMPORTANCE

Pathogenic bacteria penetrate host cellular barriers by attachment to extracellular matrix molecules, such as proteoglycans, laminins, and collagens, leading to invasion of epithelial and endothelial cells. Here, we show that cellular invasion by the human pathogens group B Streptococcus, group A Streptococcus, and Staphylococcus aureus depends on a specific domain of the laminin α2 subunit. This finding may provide new leads for the molecular pathogenesis of these bacteria and the development of novel antimicrobial drugs.

Modular sub-monomeric cell-penetrating guanidine-rich peptoids – synthesis, assembly and biological evaluation

Development of a guanidinium-rich transporters toolset to study GAG-mediated cell permeation.

Neisserial Heparin Binding Antigen (NHBA) Contributes to the Adhesion of Neisseria meningitidis to Human Epithelial Cells

Neisserial Heparin Binding Antigen (NHBA) is a surface-exposed lipoprotein ubiquitously expressed by Neisseria meningitidis strains and an antigen of the Bexsero^® vaccine. NHBA binds heparin through a conserved Arg-rich region that is the target of two proteases, the meningococcal NalP and human lactoferrin (hLf). In this work, in vitro studies showed that recombinant NHBA protein was able to bind epithelial cells and mutations of the Arg-rich tract abrogated this binding. All N-terminal and C-terminal fragments generated by NalP or hLf cleavage, regardless of the presence or absence of the Arg-rich region, did not bind to cells, indicating that a correct positioning of the Arg-rich region within the full length protein is crucial. Moreover, binding was abolished when cells were treated with heparinase III, suggesting that this interaction is mediated by heparan sulfate proteoglycans (HSPGs). N. meningitidis nhba knockout strains showed a significant reduction in adhesion to epithelial cells with respect to isogenic wild-type strains and adhesion of the wild-type strain was inhibited by anti-NHBA antibodies in a dose-dependent manner. Overall, the results demonstrate that NHBA contributes to meningococcal adhesion to epithelial cells through binding to HSPGs and suggest a possible role of anti-Bexsero^® antibodies in the prevention of colonization.

Determinants of Glycosaminoglycan (GAG) Structure

Proteoglycans (PGs) are glycosylated proteins of biological importance at cell surfaces, in the extracellular matrix, and in the circulation. PGs are produced and modified by glycosaminoglycan (GAG) chains in the secretory pathway of animal cells. The most common GAG attachment site is a serine residue followed by a glycine (-ser-gly-), from which a linker tetrasaccharide extends and may continue as a heparan sulfate, a heparin, a chondroitin sulfate, or a dermatan sulfate GAG chain. Which type of GAG chain becomes attached to the linker tetrasaccharide is influenced by the structure of the protein core, modifications occurring to the linker tetrasaccharide itself, and the biochemical environment of the Golgi apparatus, where GAG polymerization and modification by sulfation and epimerization take place. The same cell type may produce different GAG chains that vary, depending on the extent of epimerization and sulfation. However, it is not known to what extent these differences are caused by compartmental segregation of protein cores en route through the secretory pathway or by differential recruitment of modifying enzymes during synthesis of different PGs. The topic of this review is how different aspects of protein structure, cellular biochemistry, and compartmentalization may influence GAG synthesis.

GNeosomes: Highly Lysosomotropic Nanoassemblies for Lysosomal Delivery

GNeosomes, lysosomotropic lipid vesicles decorated with guanidinoneomycin, can encapsulate and facilitate the cellular internalization and lysosomal delivery of cargo ranging from small molecules to high molecular weight proteins, in a process that is exclusively dependent on cell surface glycosaminoglycans. Their cellular uptake mechanism and co-localization with lysosomes, as well as the delivery, release, and activity of internalized cargo, are quantified. GNeosomes are proposed as a universal platform for lysosomal delivery with potential as a basic research tool and a therapeutic vehicle.

Macromolecular uptake of alkyl-chain-modified guanidinoglycoside molecular transporters

Guanidinoglycosides, a family of cellular transporters capable of delivering high Mw biopolymers, have previously been shown to display high selectivity for cell-surface heparan sulfate proteoglycans and promote their clustering. Herein, the internalization mechanism of amphiphilic guanidinoglycoside derivatives was investigated by cell-surface FRET analysis. Unexpectedly, although the heparan sulfate selectivity is maintained, the cellular uptake of these derivatives does not appear to involve clustering of the proteoglycans on the cell surface. This suggests a distinct uptake mechanism when compared to the parent guanidinoglycoside-based carriers.

The glycosylation-dependent interaction of perlecan core protein with LDL: implications for atherosclerosis

Perlecan is a major heparan sulfate (HS) proteoglycan in the arterial wall. Previous studies have linked it to atherosclerosis. Perlecan contains a core protein and three HS side chains. Its core protein has five domains (DI–DV) with disparate structures and DII is highly homologous to the ligand-binding portion of LDL receptor (LDLR). The functional significance of this domain has been unknown. Here, we show that perlecan DII interacts with LDL. Importantly, the interaction largely relies on O-linked glycans that are only present in the secreted DII. Among the five repeat units of DII, most of the glycosylation sites are from the second unit, which is highly divergent and rich in serine and threonine, but has no cysteine residues. Interestingly, most of the glycans are capped by the negatively charged sialic acids, which are critical for LDL binding. We further demonstrate an additive effect of HS and DII on LDL binding. Unlike LDLR, which directs LDL uptake through endocytosis, this study uncovers a novel feature of the perlecan LDLR-like DII in receptor-mediated lipoprotein retention, which depends on its glycosylation. Thus, perlecan glycosylation may play a role in the early LDL retention during the development of atherosclerosis.

Determinants of versican-V1 proteoglycan processing by the metalloproteinase ADAMTS5

Proteolysis of the Glu(441)-Ala(442) bond in the glycosaminoglycan (GAG) β domain of the versican-V1 variant by a disintegrin-like and metalloproteinase domain with thrombospondin type 1 motif (ADAMTS) proteases is required for proper embryo morphogenesis. However, the processing mechanism and the possibility of additional ADAMTS-cleaved processing sites are unknown. We demonstrate here that if Glu(441) is mutated, ADAMTS5 cleaves inefficiently at a proximate upstream site but normally does not cleave elsewhere within the GAGβ domain. Chondroitin sulfate (CS) modification of versican is a prerequisite for cleavage at the Glu(441)-Ala(442) site, as demonstrated by reduced processing of CS-deficient or chondroitinase ABC-treated versican-V1. Site-directed mutagenesis identified the N-terminal CS attachment sites Ser(507) and Ser(525) as essential for processing of the Glu(441)-Ala(442) bond by ADAMTS5. A construct including only these two GAG chains, but not downstream GAG attachment sites, was cleaved efficiently. Therefore, CS chain attachment to Ser(507) and Ser(525) is necessary and sufficient for versican proteolysis by ADAMTS5. Mutagenesis of Glu(441) and an antibody to a peptide spanning Thr(432)-Gly(445) (i.e. containing the scissile bond) reduced versican-V1 processing. ADAMTS5 lacking the C-terminal ancillary domain did not cleave versican, and an ADAMTS5 ancillary domain construct bound versican-V1 via the CS chains. We conclude that docking of ADAMTS5 with two N-terminal GAG chains of versican-V1 via its ancillary domain is required for versican processing at Glu(441)-Ala(442). V1 proteolysis by ADAMTS1 demonstrated a similar requirement for the N-terminal GAG chains and Glu(441). Therefore, versican cleavage can be inhibited substantially by mutation of Glu(441), Ser(507), and Ser(525) or by an antibody to the region of the scissile bond.

Aggregation-mediated macromolecular uptake by a molecular transporter

Endocytosis is a key process in cellular delivery of macromolecules by molecular transporters, although the mechanism of internalization remains unclear. Here, we probe the cellular uptake of streptavidin using biotinylated guanidinoneomycin (biotinGNeo), a low molecular weight guanidinium-rich molecular transporter. Two distinct modes were explored: (i) incubation of cells with a preformed tetravalent streptavidin-(biotinGNeo)4 conjugate and (ii) preincubation of cells with the biotinGNeo before exposure to streptavidin. A significant enhancement in uptake was observed after preincubation with biotinGNeo. FRET studies showed that the enhanced uptake was accompanied by extensive aggregation of streptavidin on the cell surface. Because guanidinylated neomycin was previously found to exclusively bind to heparan sulfate, our observations suggest that heparan sulfate proteoglycan aggregation is a pivotal step for endocytic entry into cells by guanidinoglycosides. These observations put forward a practical and general pathway for the cellular delivery of diverse macromolecules.

Smad3 controls β-1,3-glucuronosyltransferase 1 expression in rat nucleus pulposus cells: implications of dysregulated expression in disc disease

OBJECTIVE

To study the regulation of expression of β-1,3-glucuronosyltransferase 1 (GlcAT-1), an important regulator of glycosaminoglycan (GAG) synthesis, by Smad3 in nucleus pulposus (NP) cells.

METHODS

GlcAT-1 expression was examined in rat NP and anulus fibrosus (AF) cells treated with transforming growth factor β (TGFβ). The effects of Smad signaling and Smad suppression on GlcAT-1 were examined in rat NP cells. GlcAT-1 expression was analyzed in the discs of Smad3-null mice and in degenerated human NP tissue.

RESULTS

TGFβ increased the expression of GlcAT-1 in rat NP but not rat AF cells. Suppression of GlcAT-1 promoter activity was evident with dominant-negative ALK-5 (DN-ALK-5). Cotransfection with Smad3 strongly induced promoter activity independent of TGFβ. Bioinformatics analysis indicated the presence of several Smad binding sites in the promoter; deletion analysis showed that the region between -274 and -123 bp was required for Smad3 response. DN-Smad3, Smad 3 small interfering RNA, and Smad7 strongly suppressed basal as well as TGFβ-induced promoter activity. Induction of promoter activity by Smad3 was significantly blocked by DN-Smad3; Smad7 had a very small effect. Lentiviral transduction of NP cells with short hairpin RNA Smad3 resulted in a decrease in GlcAT-1 expression and accumulation of GAG. Compared to wild-type mice, significantly lower expression of GlcAT-1 was seen in the discs of Smad3-null mice. Analysis of degenerated human NP tissue specimens showed no positive correlation between GlcAT-1 and TGFβ expression. Moreover, isolated cells from degenerated human tissue showed a lack of induction of GlcAT-1 expression following TGFβ treatment, suggesting an altered response.

CONCLUSION

Our findings demonstrate that in healthy NP cells, the TGFβ-Smad3 axis serves as a regulator of GlcAT-1 expression. However, an altered responsiveness to TGFβ during disc degeneration may compromise GAG synthesis.

Attachment of Chlamydia trachomatis L2 to host cells requires sulfation

Chlamydia trachomatis is a pathogen responsible for a prevalent sexually transmitted disease. It is also the most common cause of infectious blindness in the developing world. We performed a loss-of-function genetic screen in human haploid cells to identify host factors important in C. trachomatis L2 infection. We identified and confirmed B3GAT3, B4GALT7, and SLC35B2, which encode glucuronosyltransferase I, galactosyltransferase I, and the 3'-phosphoadenosine 5'-phosphosulfate transporter 1, respectively, as important in facilitating Chlamydia infection. Knockout of any of these three genes inhibits Chlamydia attachment. In complementation studies, we found that the introduction of functional copies of these three genes into the null clones restored full susceptibility to Chlamydia infection. The degree of attachment of Chlamydia strongly correlates with the level of sulfation of the host cell, not simply with the amount of heparan sulfate. Thus, other, as-yet unidentified sulfated macromolecules must contribute to infection. These results demonstrate the utility of screens in haploid cells to study interactions of human cells with bacteria. Furthermore, the human null clones generated can be used to investigate the role of heparan sulfate and sulfation in other settings not limited to infectious disease.

Metabolic engineering of Chinese hamster ovary cells: towards a bioengineered heparin

Heparin is the most widely used pharmaceutical to control blood coagulation in modern medicine. A health crisis that took place in 2008 led to a demand for production of heparin from non-animal sources. Chinese hamster ovary (CHO) cells, commonly used mammalian host cells for production of foreign pharmaceutical proteins in the biopharmaceutical industry, are capable of producing heparan sulfate (HS), a related polysaccharide naturally. Since heparin and HS share the same biosynthetic pathway, we hypothesized that heparin could be produced in CHO cells by metabolic engineering. Based on the expression of endogenous enzymes in the HS/heparin pathways of CHO-S cells, human N-deacetylase/N-sulfotransferase (NDST2) and mouse heparan sulfate 3-O-sulfotransferase 1 (Hs3st1) genes were transfected sequentially into CHO host cells growing in suspension culture. Transfectants were screened using quantitative RT-PCR and Western blotting. Out of 120 clones expressing NDST2 and Hs3st1, 2 clones, Dual-3 and Dual-29, were selected for further analysis. An antithrombin III (ATIII) binding assay using flow cytometry, designed to recognize a key sugar structure characteristic of heparin, indicated that Hs3st1 transfection was capable of increasing ATIII binding. An anti-factor Xa assay, which affords a measure of anticoagulant activity, showed a significant increase in activity in the dual-expressing cell lines. Disaccharide analysis of the engineered HS showed a substantial increase in N-sulfo groups, but did not show a pattern consistent with pharmacological heparin, suggesting that further balancing the expression of transgenes with the expression levels of endogenous enzymes involved in HS/heparin biosynthesis might be necessary.

Allergen-induced airway remodeling in brown norway rats: structural and metabolic changes in glycosaminoglycans

Increased proteoglycan (PG) deposition is a feature of airway remodeling in asthma. Glycosaminoglycans (GAGs) mediate many of the biological and mechanical properties of PGs by providing docking sites through their carbohydrate chains to bioactive ligands; therefore, it is imperative to define structural and metabolic changes of GAGs in asthma. Using a Brown Norway (BN) ovalbumin (OVA)-sensitized and -challenged rat model to induce airway remodeling, we found excessive deposition of chondroitin/dermatan (CS/DS)-, heparan (HS), and keratan (KS) sulfate GAGs in the airways and bronchoalveolar lavage cells of OVA-challenged rats. Disaccharide composition of CS/DS of OVA-challenged rats was significantly different compared with saline-treated (SAL) control rats, with increased levels of 0-, 6-, and 4-sulfated disaccharides. Increases in the amount and a change in the proportion of CS/DS versus HS GAGs were noted in OVA-challenged rats. The higher content and sulfation of CS/DS disaccharides was reflected by the increased expression of xylosyltransferase-I, β1,3-glucuronosyltransferase-I, chondroitin-4, and chondroitin-6 sulfotransferase genes and protein expression of xylosyltransferase-I and β1,3-glucuronosyltransferase-I in OVA-challenged rats. Genes encoding the core proteins of the CS/DS and KS-containing PGs, such as versican, biglycan, decorin, and lumican, were overexpressed in OVA-challenged rats. Our results suggest that GAG biosynthetic enzymes may be involved in the altered expression of GAGs in the airways and are potential targets for inhibiting excess PG-GAG deposition and the airway remodeling process in asthma.

Proteoglycans and cartilage repair

Repair of damaged articular cartilage in osteoarthritis (OA) is a clinical challenge. Because cartilage is an avascular and aneural tissue, normal mechanisms of tissue repair through recruitment of cells to the site of tissue destruction are not feasible. Proteoglycan (PG) depletion induced by the proinflammatory cytokine interleukin-1β, a principal mediator in OA, is a major factor in the onset and progression of joint destruction. Current symptomatic treatments of OA by anti-inflammatory drugs do not alter the progression of the disease. Various therapeutic strategies have been developed to antagonize the effect of proinflammatory cytokines. However, relatively few studies were conducted to stimulate anabolic activity, in an attempt to enhance cartilage repair. To this aim, a nonviral gene transfer strategy of glycosyltransferases responsible for PG synthesis has been developed and tested for its capacity to promote cartilage PG synthesis and deposition. Transfection of chondrocytes or cartilage explants by the expression vector for the glycosyltransferase β-1,3-glucuronosyltransferase-I (GlcAT-I) enhanced PG synthesis and deposition in the ECM by promoting the synthesis of chondroitin sulfate GAG chains of the cartilage matrix. This indicates that therapy mediated through GT gene delivery may constitute a new strategy for the treatment of OA.

Hypoxic regulation of β-1,3-glucuronyltransferase 1 expression in nucleus pulposus cells of the rat intervertebral disc: role of hypoxia-inducible factor proteins

OBJECTIVE

To determine whether hypoxia and hypoxia-inducible factor (HIF) proteins regulate expression of β-1,3-glucuronyltransferase 1 (GlcAT-1), a key enzyme in glycosaminoglycan synthesis in nucleus pulposus cells.

METHODS

Real-time reverse transcriptase-polymerase chain reaction and Western blotting were used to measure GlcAT-1 expression. Transfections were performed to determine the effect of HIF-1α and HIF-2α on GlcAT-1 promoter activity.

RESULTS

Under hypoxic conditions there was an increase in GlcAT-1 expression; a significant increase in promoter activity was seen both in nucleus pulposus cells and in N1511 chondrocytes. We investigated whether HIF controlled GlcAT-1 expression. Suppression of HIF-1α and HIF-2α induced GlcAT-1 promoter activity and expression only in nucleus pulposus cells. Transfection with CA-HIF-1α as well as with CA-HIF-2α suppressed GlcAT-1 promoter activity only in nucleus pulposus cells, suggesting a cell type-specific regulation. Site-directed mutagenesis and deletion constructs were used to further confirm the suppressive effect of HIFs on GlcAT-1 promoter function in nucleus pulposus cells. Although it was evident that interaction of HIF with hypoxia-responsive elements resulted in suppression of basal promoter activity, it was not necessary for transcriptional suppression. This result suggested both a direct and an indirect mode of regulation, possibly through recruitment of a HIF-dependent repressor. Finally, we showed that hypoxic expression of GlcAT-1 was also partially dependent on MAPK signaling.

CONCLUSION

These studies demonstrate that hypoxia regulates GlcAT-1 expression through a signaling network comprising both activator and suppressor molecules, and that this regulation is unique to nucleus pulposus cells.

Small lytic peptides escape the inhibitory effect of heparan sulfate on the surface of cancer cells

Background

Several naturally occurring cationic antimicrobial peptides (CAPs), including bovine lactoferricin (LfcinB), display promising anticancer activities. These peptides are unaffected by multidrug resistance mechanisms and have been shown to induce a protective immune response against solid tumors, thus making them interesting candidates for developing novel lead structures for anticancer treatment. Recently, we showed that the anticancer activity by LfcinB was inhibited by the presence of heparan sulfate (HS) on the surface of tumor cells. Based on extensive structure-activity relationship studies performed on LfcinB, shorter and more potent peptides have been constructed. In the present study, we have investigated the anticancer activity of three chemically modified 9-mer peptides and the influence of HS and chondroitin sulfate (CS) on their cytotoxic activity.

Methods

Various cell lines and red blood cells were used to investigate the anticancer activity and selectivity of the peptides. The cytotoxic effect of the peptides against the different cell lines was measured by use of a colorimetric MTT viability assay. The influence of HS and CS on their cytotoxic activity was evaluated by using HS/CS expressing and HS/CS deficient cell lines. The ability of soluble HS and CS to inhibit the cytotoxic activity of the peptides and the peptides' affinity for HS and CS were also investigated.

Results

The 9-mer peptides displayed selective anticancer activity. Cells expressing HS/CS were equally or more susceptible to the peptides than cells not expressing HS/CS. The peptides displayed a higher affinity for HS compared to CS, and exogenously added HS inhibited the cytotoxic effect of the peptides.

Conclusions

In contrast to the previously reported inhibitory effect of HS on LfcinB, the present study shows that the cytotoxic activity of small lytic peptides was increased or not affected by cell surface HS.

Increased deposition of chondroitin/dermatan sulfate glycosaminoglycan and upregulation of β1,3-glucuronosyltransferase I in pulmonary fibrosis

Pulmonary fibrosis (PF) is characterized by increased deposition of proteoglycans (PGs), in particular core proteins. Glycosaminoglycans (GAGs) are key players in tissue repair and fibrosis, and we investigated whether PF is associated with changes in the expression and structure of GAGs as well as in the expression of β1,3-glucuronosyltransferase I (GlcAT-I), a rate-limiting enzyme in GAG synthesis. Lung biopsies from idiopathic pulmonary fibrosis (IPF) patients and lung tissue from a rat model of bleomycin (BLM)-induced PF were immunostained for chondroitin sulfated-GAGs and GlcAT-I expression. Alterations in disaccharide composition and sulfation of chondroitin/dermatan sulfate (CS/DS) were evaluated by fluorophore-assisted carbohydrate electrophoresis (FACE) in BLM rats. Lung fibroblasts isolated from control (saline-instilled) or BLM rat lungs were assessed for GAG structure and GlcAT-I expression. Disaccharide analysis showed that 4- and 6-sulfated disaccharides were increased in the lungs and lung fibroblasts obtained from fibrotic rats compared with controls. Fibrotic lung fibroblasts and transforming growth factor-β(1) (TGF-β(1))-treated normal lung fibroblasts expressed increased amounts of hyaluronan and 4- and 6-sulfated chondroitin, and neutralizing anti-TGF-β(1) antibody diminished the same. TGF-β(1) upregulated GlcAT-I and versican expression in lung fibroblasts, and signaling through TGF-β type I receptor/p38 MAPK was required for TGF-β(1)-mediated GlcAT-I and CS-GAG expression in fibroblasts. Our data show for the first time increased expression of CS-GAGs and GlcAT-I in IPF, fibrotic rat lungs, and fibrotic lung fibroblasts. These data suggest that alterations of sulfation isomers of CS/DS and upregulation of GlcAT-I contribute to the pathological PG-GAG accumulation in PF.

Loss of the heparan sulfate sulfotransferase, Ndst1, in mammary epithelial cells selectively blocks lobuloalveolar development in mice

BACKGROUND

Considerable evidence indicates that heparan sulfate is essential for the development of tissues consisting of branching ducts and tubules. However, there are few examples where specific sulfate residues regulate a specific stage in the formation of such tissues.

METHODOLOGY/PRINCIPAL FINDINGS

We examined the role of heparan sulfation in mammary gland branching morphogenesis, lactation and lobuloalveolar development by inactivation of heparan sulfate GlcNAc N-deacetylase/N-sulfotransferase genes (Ndst) in mammary epithelial cells using the Cre-loxP system. Ndst1 deficiency resulted in an overall reduction in glucosamine N-sulfation and decreased binding of FGF to mammary epithelial cells in vitro and in vivo. Mammary epithelia lacking Ndst1 underwent branching morphogenesis, filling the gland with ductal tissue by sexual maturity to the same extent as wildtype epithelia. However, lobuloalveolar expansion did not occur in Ndst1-deficient animals, resulting in insufficient milk production to nurture newly born pups. Lactational differentiation of isolated mammary epithelial cells occurred appropriately via stat5 activation, further supporting the notion that the lack of milk production was due to lack of expansion of the lobuloalveoli.

CONCLUSIONS/SIGNIFICANCE

These findings demonstrate a selective, highly penetrant, cell autonomous effect of Ndst1-mediated sulfation on lobuloalveolar development.

BMP-2 and TGF-beta stimulate expression of beta1,3-glucuronosyl transferase 1 (GlcAT-1) in nucleus pulposus cells through AP1, TonEBP, and Sp1: role of MAPKs

The goal of the study was to investigate bone morphogenetic protein 2 (BMP-2) and transforming growth factor beta (TGF-beta) control of the expression of beta1,3-glucuronosyl transferase 1 (GlcAT-1), an important regulator of chondroitin sulfate synthesis in cells of the nucleus pulposus. Treatment with both growth factors resulted in induction of GlcAT-1 expression and promoter activity. Deletion analysis indicated that promoter constructs lacking AP1 and TonE sites were unresponsive to growth factor treatment. Experiments using dominant-negative proteins showed that these transcription factors along with Sp1 were required for induction of GlcAT-1 promoter activity. Moreover, when either AP1 or TonE binding sites were mutated, induction was suppressed. Both BMP-2 and TGF-beta increased c-Jun and TonEBP expression and phosphorylation of transactivation domains. We investigated the role of the mitogen-activated protein kinase (MAPK) signaling pathway following growth factor treatment; a robust and transient activation of ERK1/2, p38, and JNK was noted. Treatment with MAPK inhibitors blocked BMP-2- and TGF-beta-induced AP1 reporter function, GlcAT-1 expression, and GAG accumulation. We found that DN-ERK1 but not DN-ERK2 resulted in suppression of growth factor-mediated induction of GlcAT-1 promoter activity; we also showed that p38 delta was important in GlcAT-1 activation. Results of these studies demonstrate that BMP-2 and TGF-beta regulate GlcAT-1 expression in nucleus pulposus cells through a signaling network comprising MAPK, AP1, Sp1, and TonEBP. It is concluded that by controlling both GAG and aggrecan synthesis, these growth factors positively influence disk cell function.

FAM20B is a kinase that phosphorylates xylose in the glycosaminoglycan–protein linkage region

2-O-phosphorylation of xylose has been detected in the glycosaminoglycan–protein linkage region, GlcAβ1-3Galβ1-3Galβ1-4Xylβ1-O-Ser, of proteoglycans. Recent mutant analyses in zebrafish suggest that xylosyltransferase I and FAM20B, a protein of unknown function that shows weak similarity to a Golgi kinase encoded by four-jointed, operate in a linear pathway for proteoglycan production. In the present study, we identified FAM20B as a kinase that phosphorylates the xylose residue in the linkage region. Overexpression of FAM20B increased the amount of both chondroitin sulfate and heparan sulfate in HeLa cells, whereas the RNA interference of FAM20B resulted in a reduction of their amount in the cells. Gel-filtration analysis of the glycosaminoglycan chains synthesized in the overexpressing cells revealed that the glycosaminoglycan chains had a similar length to those in mock-transfected cells. These results suggest that FAM20B regulates the number of glycosaminoglycan chains by phosphorylating the xylose residue in the glycosaminoglycan–protein linkage region of proteoglycans.

The anticancer activity of lytic peptides is inhibited by heparan sulfate on the surface of the tumor cells

BACKGROUND

Cationic antimicrobial peptides (CAPs) with antitumor activity constitute a promising group of novel anticancer agents. These peptides induce lysis of cancer cells through interactions with the plasma membrane. It is not known which cancer cell membrane components influence their susceptibility to CAPs. We have previously shown that CAPs interact with the two glycosaminoglycans (GAGs), heparan sulfate (HS) and chondroitin sulfate (CS), which are present on the surface of most cells. The purpose of this study was to investigate the role of the two GAGs in the cytotoxic activity of CAPs.

METHODS

Various cell lines, expressing different levels of cell surface GAGs, were exposed to bovine lactoferricin (LfcinB) and the designer peptide, KW5. The cytotoxic effect of the peptides was investigated by use of the colorimetric MTT viability assay. The cytotoxic effect on wild type CHO cells, expressing normal amounts of GAGs on the cell surface, and the mutant pgsA-745, that has no expression of GAGs on the cell surface, was also investigated.

RESULTS

We show that cells not expressing HS were more susceptible to CAPs than cells expressing HS at the cell surface. Further, exogenously added heparin inhibited the cytotoxic effect of the peptides. Chondroitin sulfate had no effect on the cytotoxic activity of KW5 and only minor effects on LfcinB cytotoxicity.

CONCLUSION

Our results show for the first time that negatively charged molecules at the surface of cancer cells inhibit the cytotoxic activity of CAPs. Our results indicate that HS at the surface of cancer cells sequesters CAPs away from the phospholipid bilayer and thereby impede their ability to induce cytolysis.

Activation of TonEBP by calcium controls {beta}1,3-glucuronosyltransferase-I expression, a key regulator of glycosaminoglycan synthesis in cells of the intervertebral disc

The goal of this investigation was to study the expression and regulation of beta1,3-Glucuronosyltransferase-I (GlcAT-I), a key enzyme regulating GAG synthesis in cells of the intervertebral disc. There was a robust expression of GlcAT-I in the nucleus pulposus in vivo. Treatment with the calcium ionophore ionomycin resulted in increased GlcAT-I expression, whereas GlcAT-I promoter constructs lacking TonE site or a mutant TonE were unresponsive to the ionophore. Experiments using TonEBP and DN-TonEBP constructs showed that TonEBP positively regulated GlcAT-I promoter activity. ChIP analysis confirmed binding of TonEBP to the promoter. We further validated the role of TonEBP in controlling GlcAT-I expression using mouse embryo fibroblasts from TonEBP null mice. GlcAT-I promoter activity in null cells was significantly lower than the wild type cells. In contrast to wild type cells, treatment with ionomycin failed to increase GlcAT-I promoter activity in null cells. We then investigated if calcineurin (Cn)-NFAT signaling played a regulatory role in GlcAT-I expression. Inhibition of Cn following ionomycin treatment did not block GlcAT-I and tauT, a TonEBP-responsive reporter activity. GlcAT-I promoter activity was suppressed by co-expression of Cn, NFAT2, NFAT3, and NFAT4. Moreover, following ionomycin treatment, fibroblasts from CnAalpha and CnAbeta null mice exhibited robust induction in GlcAT-I promoter activity compared with wild type cells. Results of these studies demonstrate that calcium regulates GlcAT-I expression in cells of the nucleus pulposus through a signaling network comprising both activator and suppressor molecules. The results suggest that by controlling both GAG and aggrecan synthesis, disc cells can autoregulate their osmotic environment and accommodate mechanical loading.

Functional UDP-xylose transport across the endoplasmic reticulum/Golgi membrane in a Chinese hamster ovary cell mutant defective in UDP-xylose Synthase

In mammals, xylose is found as the first sugar residue of the tetrasaccharide GlcAbeta1-3Galbeta1-3Galbeta1-4Xylbeta1-O-Ser, initiating the formation of the glycosaminoglycans heparin/heparan sulfate and chondroitin/dermatan sulfate. It is also found in the trisaccharide Xylalpha1-3Xylalpha1-3Glcbeta1-O-Ser on epidermal growth factor repeats of proteins, such as Notch. UDP-xylose synthase (UXS), which catalyzes the formation of the UDP-xylose substrate for the different xylosyltransferases through decarboxylation of UDP-glucuronic acid, resides in the endoplasmic reticulum and/or Golgi lumen. Since xylosylation takes place in these organelles, no obvious requirement exists for membrane transport of UDP-xylose. However, UDP-xylose transport across isolated Golgi membranes has been documented, and we recently succeeded with the cloning of a human UDP-xylose transporter (SLC25B4). Here we provide new evidence for a functional role of UDP-xylose transport by characterization of a new Chinese hamster ovary cell mutant, designated pgsI-208, that lacks UXS activity. The mutant fails to initiate glycosaminoglycan synthesis and is not capable of xylosylating Notch. Complementation was achieved by expression of a cytoplasmic variant of UXS, which proves the existence of a functional Golgi UDP-xylose transporter. A approximately 200 fold increase of UDP-glucuronic acid occurred in pgsI-208 cells, demonstrating a lack of UDP-xylose-mediated control of the cytoplasmically localized UDP-glucose dehydrogenase in the mutant. The data presented in this study suggest the bidirectional transport of UDP-xylose across endoplasmic reticulum/Golgi membranes and its role in controlling homeostasis of UDP-glucuronic acid and UDP-xylose production.

Small changes in lymphocyte development and activation in mice through tissue-specific alteration of heparan sulphate

We have examined the role of heparan sulphate in lymphocyte development and activation in mice by conditionally deleting the genes encoding the heparan sulphate biosynthetic enzymes N-deacetylase/N-sulphotransferase-1 and -2 (Ndst1 and Ndst2) and glucuronic acid/N-acetylglucosamine co-polymerase-1 (Ext1) in T cells and B cells, respectively. Ndst1 and Ndst2 are the only Ndst isoforms in T cells. In T-cell Ndst-deficient mice there were normal ratios of CD4(+)/CD8(+) cells in the blood, spleen and thymus, indicating no dramatic effect on development. However, Ndst-deficient T cells were hyperresponsive to low-level activation, suggesting that cell surface heparan sulphate plays a role in T-cell proliferation. The hyperresponsive state correlated with a decrease in cell surface heparan sulphate that occurs in response to activation in wild-type cells. There was a slight change in the number of developing B cells in B-cell Ext1-deficient mice, but the alteration did not cause a change in antibody production. These findings demonstrate that cell surface heparan sulphate may not play a crucial role in lymphocyte development, but can modulate the sensitivity of T cells to activation.

Heparan sulfate and syndecan-1 are essential in maintaining murine and human intestinal epithelial barrier function

Patients with protein-losing enteropathy (PLE) fail to maintain intestinal epithelial barrier function and develop an excessive and potentially fatal efflux of plasma proteins. PLE occurs in ostensibly unrelated diseases, but emerging commonalities in clinical observations recently led us to identify key players in PLE pathogenesis. These include elevated IFN-gamma, TNF-alpha, venous hypertension, and the specific loss of heparan sulfate proteoglycans from the basolateral surface of intestinal epithelial cells during PLE episodes. Here we show that heparan sulfate and syndecan-1, the predominant intestinal epithelial heparan sulfate proteoglycan, are essential in maintaining intestinal epithelial barrier function. Heparan sulfate- or syndecan-1-deficient mice and mice with intestinal-specific loss of heparan sulfate had increased basal protein leakage and were far more susceptible to protein loss induced by combinations of IFN-gamma, TNF-alpha, and increased venous pressure. Similarly, knockdown of syndecan-1 in human epithelial cells resulted in increased basal and cytokine-induced protein leakage. Clinical application of heparin has been known to alleviate PLE in some patients but its unknown mechanism and severe side effects due to its anticoagulant activity limit its usefulness. We demonstrate here that non-anticoagulant 2,3-de-O-sulfated heparin could prevent intestinal protein leakage in syndecan-deficient mice, suggesting that this may be a safe and effective therapy for PLE patients.

Molecular basis for acceptor substrate specificity of the human β1,3-glucuronosyltransferases GlcAT-I and GlcAT-P involved in glycosaminoglycan and HNK-1 carbohydrate epitope biosynthesis, respectively

The human beta1,3-glucuronosyltransferases galactose-beta1,3-glucuronosyltransferase I (GlcAT-I) and galactose-beta1,3-glucuronosyltransferase P (GlcAT-P) are key enzymes involved in proteoglycan and HNK-1 carbohydrate epitope synthesis, respectively. Analysis of their acceptor specificity revealed that GlcAT-I was selective toward Galbeta1,3Gal (referred to as Gal2-Gal1), whereas GlcAT-P presented a broader profile. To understand the molecular basis of acceptor substrate recognition, we constructed mutants and chimeric enzymes based on multiple sequence alignment and structural information. The drastic effect of mutations of Glu227, Arg247, Asp252, and Glu281 on GlcAT-I activity indicated a key role for the hydrogen bond network formed by these four conserved residues in dictating Gal2 binding. Investigation of GlcAT-I determinants governing Gal1 recognition showed that Trp243 could not be replaced by its counterpart Phe in GlcAT-P. This result combined with molecular modeling provided evidence for the importance of stacking interactions with Trp at position 243 in the selectivity of GlcAT-I toward Galbeta1,3Gal. Mutation of Gln318 predicted to be hydrogen-bonded to 6-hydroxyl of Gal1 had little effect on GlcAT-I activity, reinforcing the role of Trp243 in Gal1 binding. Substitution of Phe245 in GlcAT-P by Ala selectively abolished Galbeta1,3Gal activity, also highlighting the importance of an aromatic residue at this position in defining the specificity of GlcAT-P. Finally, substituting Phe245, Val320, or Asn321 in GlcAT-P predicted to interact with N-acetylglucosamine (GlcNAc), by their counterpart in GlcAT-I, moderately affected the activity toward the reference substrate of GlcAT-P, N-acetyllactosamine, indicating that its active site tolerates amino acid substitutions, an observation that parallels its promiscuous substrate profile. Taken together, the data clearly define key residues governing the specificity of beta1,3-glucuronosyltransferases.

Genetic alteration of endothelial heparan sulfate selectively inhibits tumor angiogenesis

To examine the role of endothelial heparan sulfate during angiogenesis, we generated mice bearing an endothelial-targeted deletion in the biosynthetic enzyme N-acetylglucosamine N-deacetylase/N-sulfotransferase 1 (Ndst1). Physiological angiogenesis during cutaneous wound repair was unaffected, as was growth and reproductive capacity of the mice. In contrast, pathological angiogenesis in experimental tumors was altered, resulting in smaller tumors and reduced microvascular density and branching. To simulate the angiogenic environment of the tumor, endothelial cells were isolated and propagated in vitro with proangiogenic growth factors. Binding of FGF-2 and VEGF(164) to cells and to purified heparan sulfate was dramatically reduced. Mutant endothelial cells also exhibited altered sprouting responses to FGF-2 and VEGF(164), reduced Erk phosphorylation, and an increase in apoptosis in branching assays. Corresponding changes in growth factor binding to tumor endothelium and apoptosis were also observed in vivo. These findings demonstrate a cell-autonomous effect of heparan sulfate on endothelial cell growth in the context of tumor angiogenesis.

Biosynthesis of chondroitin and heparan sulfate in chinese hamster ovary cells depends on xylosyltransferase II

Xylosyltransferase (XylT) catalyzes the initial enzymatic reaction in the glycosaminoglycan assembly pathway for proteoglycan biosynthesis. Its activity is thought to be rate-limiting. Two xylosyltransferases have been found using genomic analyses, and one of these, XylT1, has been shown to have xylosyltransferase activity. On the other hand, the less studied XylT2 in recombinant form lacks xylosyltransferase activity and has no known function. Wild-type Chinese hamster ovary cells express abundant Xylt2 mRNA levels and lack detectable Xylt1 mRNA levels. Analysis of a previously described Chinese hamster ovary cell xylosyltransferase mutant (psgA-745) shows that it harbors an Xylt2 nonsense mutation and fails to assemble glycosaminoglycans onto recombinant biglycan. Transfection of this cell line with a murine Xylt2 minigene results in the production of recombinant chondroitin sulfate-modified biglycan core protein and restoration of fibroblast growth factor binding to cell surface-associated heparan sulfate. Expression analyses on 10 different human transformed cell lines detect exclusive XYLT2 expression in two and co-expression of XYLT1 and XYLT2 in the others but at disparate ratios where XYLT2 expression is greater than XYLT1 in most cell lines. These results indicate that XylT2 has a significant role in proteoglycan biosynthesis and that cell type may control which family member is utilized.

Human xylosyltransferase II is involved in the biosynthesis of the uniform tetrasaccharide linkage region in chondroitin sulfate and heparan sulfate proteoglycans

Human xylosyltransferase I (XT-I) initiates the biosynthesis of the glycosaminoglycan (GAG) linkage tetrasaccharide in proteoglycans. Xylosyltransferase II (XT-II) is a protein homologous to XT-I but with hitherto unknown activity or physiological function. Here, we report the enzymatic activity of XT-II and provide evidence that XT-II initiates the biosynthesis of both heparan sulfate and chondroitin sulfate GAGs. Transfection of the xylosyltransferase-deficient Chinese hamster ovary mutant pgsA-745 with XT-I or XT-II coding cDNA completely restored GAG biosynthesis. GAG disaccharide analysis revealed that XT-I- and XT-II-transfected pgsA-745 cells produced similar amounts of chondroitin sulfate and heparan sulfate. Furthermore, a high xylosyltransferase activity was measured after transfection with cDNAs encoding either isozyme. Analysis of the enzyme activity revealed that XT-II catalyzes the transfer of xylose to similar peptide acceptors as XT-I but with different efficiency. The optimal XT-II acceptor was observed using a bikunin-related peptide (K(m) 5.2 microM). Analysis of XT-I and XT-II mRNA expression in murine tissues showed a differential expression pattern for both enzymes. In particular, XT-II is highly expressed in liver tissue, where XT-I transcripts were not detected. This is the first report on the enzyme activity of XT-II and its involvement in chondroitin sulfate and heparan sulfate biosynthesis.

Guanidinylated neomycin delivers large, bioactive cargo into cells through a heparan sulfate-dependent pathway

Facilitating the uptake of molecules into living cells is of substantial interest for basic research and drug delivery applications. Arginine-rich peptides have been shown to facilitate uptake of high molecular mass cargos into cells, but the mechanism of uptake is complex and may involve multiple receptors. In this report, we show that a derivative of the aminoglycoside antibiotic neomycin, in which all of the ammonium groups have been converted into guanidinium groups, can carry large (>300 kDa) bioactive molecules across cell membranes. Delivery occurs at nanomolar transporter concentrations and under these conditions depends entirely on cell surface heparan sulfate proteoglycans. Conjugation of guanidinoneomycin to the plant toxin saporin, a ribosome-inactivating agent, results in proteoglycan-dependent cell toxicity. In contrast, an arginine-rich peptide shows both heparan sulfate-dependent and -independent cellular uptake. The high selectivity of guanidinoneomycin for heparan sulfate suggests the possibility of exploiting differences in proteoglycan compositions to target delivery to different cell types.

Differential usage of carbohydrate co-receptors influences cellular tropism of Theiler's murine encephalomyelitis virus infection of the central nervous system

Theiler's murine encephalomyelitis viruses (TMEV) are ubiquitous pathogens of mice, producing either rapidly fatal encephalitis (high-neurovirulence strains) or persistent central nervous system infection and inflammatory demyelination (low-neurovirulence strains). Although a protein entry receptor has not yet been identified, carbohydrate co-receptors that effect docking and concentration of the virus on the cell surface are known for both TMEV neurovirulence groups. Low-neurovirulence TMEV use alpha2,3-linked N-acetylneuramic acid (sialic acid) on an N-linked glycoprotein, whereas high-neurovirulence TMEV use the proteoglycan heparan sulfate (HS) as a co-receptor. While the binding of low-neurovirulence TMEV to sialic acid can be inhibited completely, only a third of the binding of high-neurovirulence TMEV to HS is inhibitable, suggesting that high-neurovirulence strains use another co-receptor or bind directly to the putative protein entry receptor. Four amino acids on the surface (VP2 puff B) of low-neurovirulence strains make contact with sialic acid through non-covalent hydrogen bonds. Since these virus residues are conserved in all TMEV strains, the capsid conformation of this region is probably responsible for sialic acid binding. A persistence determinant that maps within the virus coat using recombinant TMEV is also conformational in nature. Low-neurovirulence virus variants that do not bind to sialic acid fail to persist in the central nervous system of mice, indicating a role for sialic acid binding in TMEV persistence. Analysis of high-neurovirulence variants that do not bind HS demonstrates that HS co-receptor usage influences neuronal tropism in brain, whereas, the HS co-receptor use is not required for the infection of spinal cord anterior horn cells associated with poliomyelitis.

Heparan sulfate biosynthetic gene Ndst1 is required for FGF signaling in early lens development

Multiple signaling molecules, including bone morphogenic proteins (BMP) and fibroblast growth factors (FGF), play important roles in early lens development. However, how these morphogens are regulated is still largely unknown. Heparan sulfate participates in both morphogen transport and morphogen-receptor interaction. In this study, we demonstrate that inactivation of the heparan sulfate biosynthetic gene Ndst1 resulted in invagination defects of the early lens and in the disruption of lens-determination gene expression, leading to severe lens hypoplasia or anophthalmia. Ndst1 mutants exhibited reduced sulfation of heparan sulfate, but both BMP- and Wnt-signaling remained unchanged. Instead, these embryos showed diminished binding of a subset of FGF proteins to FGF receptors. Consistent with disruption of FGF signaling, expression of phospho-Erk and ERM were also downregulated in Ndst1-mutant lenses. Taken together, these results establish an important role of Ndst1 function in FGF signaling during lens development.

Evidence of calcium‐dependent pathway in the regulation of human β1,3‐glucuronosyltransferase‐1 (GlcAT‐I) gene expression: a key enzyme in proteoglycan synthesis

The importance of heparan- and chondroitin-sulfate proteoglycans in physiological and pathological processes led to the investigation of the regulation of beta1,3-glucuronosyltransferase I (GlcAT-I), responsible for the completion of glycosaminoglycan-protein linkage tetrasaccharide, a key step prior to polymerization of chondroitin- and heparan-sulfate chains. We have cloned and functionally characterized GlcAT-I 5'-flanking regulatory region. Mutation analysis and electrophoretic mobility shift assays demonstrated the importance of Sp1 motif located at -65/-56 position in promoter activity. Furthermore, we found that elevation of intracellular calcium concentration by the calcium ionophore ionomycin stimulated GlcAT-I gene expression as well as glycosaminoglycan chain synthesis in HeLa cells. Bisanthracycline, an anti-Sp1 compound, inhibited GlcAT-I basal promoter activity and suppressed ionomycin induction, suggesting the importance of Sp1 in calcium induction of GlcAT-I gene expression. Nuclear protein extracts from ionomycin-induced cells exhibited an increased DNA binding of Sp1 factor to the consensus sequence at position -65/-56. Signaling pathway analysis and MEK inhibition studies revealed the important role of p42/p44 MAPK in the stimulation of GlcAT-I promoter activity by ionomycin. The present study identifies, for the first time, GlcAT-I as a target of calcium-dependent signaling pathway and evidences the critical role of Sp1 transcription factor in the activation of GlcAT-I expression.

Interaction of Chlamydia trachomatis with mammalian cells is independent of host cell surface heparan sulfate glycosaminoglycans

The hypothesis that host cell surface heparan sulfate is required to promote chlamydial infection was tested using a cell line (CHO-18.4) containing a single retroviral insertion and the concomitant loss of heparan sulfate biosynthesis. Tests of chlamydial infectivity of heparan sulfate-deficient CHO-18.4 cells and parental cells, CHO-22, demonstrated that both were equally sensitive to infection by Chlamydia trachomatis serovars L2 and D. These data do not support the hypothesis and demonstrate that host cell surface heparan sulfate does not serve an essential functional role in chlamydial infectivity.

CHO glycosylation mutants: proteoglycans

Most glycosaminoglycan (GAG)-defective mutants have been isolated and characterized from Chinese hamster ovary (CHO) cells. Wild-type and GAG-defective CHO cells have been used by several hundreds of laboratories to study how altering the GAG structure of proteoglycans affects fundamental properties of cells, such as bacterial/viral infection, signaling, protein degradation, and cell adhesion. This chapter describes methods used to construct and characterize new CHO cell lines with gain-of-function GAG structures. These novel CHO cell lines allow herpes simplex virus (HSV) entry or have anticoagulant properties that are not possessed by wild-type CHO cells. The method used to study GAG biosynthetic mechanisms that control specific GAG sequence assembly is also described.

Purification and characterization of a soluble form of the recombinant human galactose-beta1,3-glucuronosyltransferase I expressed in the yeast Pichia pastoris

The galactose-beta1,3-glucuronosyltransferase I (GlcAT-I) catalyzes the transfer of glucuronic acid from UDP-alpha-D-glucuronic acid onto the terminal galactose of the trisaccharide glycosaminoglycan-protein linker region of proteoglycans. This enzyme plays a key role in the process of proteoglycan assembly since the completion of the linkage region is essential for the conversion of a core protein into a functional proteoglycan. To investigate the enzymatic properties of human GlcAT-I, we established an expression system for producing a soluble form of enzyme in the methylotrophic yeast Pichia pastoris and developed a three-step purification procedure using a combination of anion exchange, cation exchange and heparin chromatographies. This procedure yielded 1.6 mg homogeneous enzyme from 200 ml yeast cell culture, with a specific activity value of 1.5 micromol/min/mg protein. Analysis of the specificity of GlcAT-I towards Galbeta1-3Gal and Galbeta1-4GlcNAc derivatives known as substrates of the beta1,3-glucuronosyltransferases, showed that the enzyme exhibited a strict selectivity towards Galbeta1-3Gal structures. Thus, the large source of purified active enzyme allowed the determination of the kinetic parameters of GlcAT-I towards the donor substrate UDP-GlcA and the acceptor substrate digalactoside Galbeta1-3Gal.

Decorin core protein secretion is regulated by N-linked oligosaccharide and glycosaminoglycan additions

Expression of decorin using the vaccinia virus/T7 expression system resulted in secretion of two distinct glycoforms: a proteoglycan substituted with a single chondroitin sulfate chain and N-linked oligosaccharides and a core protein glycoform substituted with N-linked glycans but without a glycosaminoglycan chain. In this report, we have addressed two distinct questions. What is the rate-limiting step in glycosaminoglycan synthesis? Is glycosylation with either N-linked oligosaccharides or glycosaminoglycan required for secretion of decorin? N-terminal sequencing of the core protein glycoform, the addition of benzyl-beta-d-xyloside, and a UDP-xylose: core protein beta-d-xylosyltransferase activity assay show that xylosylation is a rate-limiting step in chondroitin sulfate biosynthesis. Decorin can be efficiently secreted with N-linked oligosaccharides alone or with a single chondroitin sulfate chain alone; however, there is severely impaired secretion of core protein devoid of any glycosylation. A decorin core protein mutant devoid of N-linked oligosaccharide attachment sites will not be secreted by Chinese hamster ovary cells deficient in xylosyltransferase or by parental Chinese hamster ovary wild type cells if the xylosyltransferase recognition sequence is disrupted. This finding suggests that quality control mechanisms sensitive to an absence of N-linked oligosaccharides can be abrogated by interaction of the core protein with the glycosaminoglycan synthetic machinery. We propose a model of regulation of decorin secretion that has several components, including appropriate substitution with N-linked oligosaccharides and factors involved in glycosaminoglycan synthesis.

Cell surface heparan sulfate promotes replication of Toxoplasma gondii

Previous work suggests that cell surface heparan sulfate acts as a receptor for the Apicomplexan parasite Toxoplasma gondii. Using Chinese hamster ovary cell mutants defective in heparan sulfate biosynthesis, we show that heparan sulfate is necessary and sufficient for infectivity. Further, we demonstrate that the parasite requires N sulfation of heparan sulfate initiated by N-deacetylase/N-sulfotransferase-1, but 2-O sulfation and 6-O sulfation appear to be dispensable. In order to study the role of heparan sulfate in other cell types, we created a conditional allele for N-deacetylase/N-sulfotransferase-1 by using Cre-loxP technology. Mammary tumor cells lacking N-deacetylase/N-sulfotransferase-1 exhibited reduced toxoplasma infectivity like Chinese hamster ovary cell mutants. Surprisingly, heparin, chemically modified heparinoids, and monoclonal antibodies to heparan sulfate had no effect on toxoplasma infection. T. gondii attachment and invasion were unchanged in N-deacetylase/N-sulfotransferase-1-inactivated cells as well, but replication was reduced. Thus, heparan sulfate does not appear to function as a receptor for T. gondii but instead facilitates parasite replication postinvasion.

Quantification of glycosaminoglycans by reversed-phase HPLC separation of fluorescent isoindole derivatives

Glycosaminoglycans (GAGs) are linear polysaccharides made by all animal cells. GAGs bind to hundreds of proteins, such as growth factors, cytokines, chemokines, extracellular matrix components, protease inhibitors, proteases, and lipoprotein lipase, through carbohydrate and protein interactions. These interactions control many multicellular processes. The increased use of GAGs isolated from cells and small tissue samples in bioassays and binding experiments demands a sensitive and robust quantification method. We have developed such a method, which is based on a popular assay for amino acid analysis. We have refined it to enhance GAG quantification. It allows the quantification of glucosamine- and galactosamine-containing GAGs after the reversed-phase separation of their fluorescent isoindole derivatives. The derivatives are created by the reaction of o-phthaldialdehyde and 3-mercaptopropionic acid (3MPA) with the amino group of hexosaminitol monosaccharides generated from GAG acid hydrolysis and sodium borohydride reduction. The advantages of our method include automatic derivitization, a simple chromatograph with clean separation of glucosaminitol and galactosaminitol derivatives from contaminating amino acids, excellent sensitivity with 0.04 pmol detection, and linearity from 2.5 to 1280 pmol. A major advantage is that it can be readily implemented in any laboratory with typical reversed-phase high performance liquid chromatography (HPLC) equipment.