Transport of UDP-galactose into the Golgi lumen regulates the biosynthesis of proteoglycans

Delivery of Nucleotide Sugars to the Mammalian Golgi: A Very Well (un)Explained Story

Nucleotide sugars (NSs) serve as substrates for glycosylation reactions. The majority of these compounds are synthesized in the cytoplasm, whereas glycosylation occurs in the endoplasmic reticulum (ER) and Golgi lumens, where catalytic domains of glycosyltransferases (GTs) are located. Therefore, translocation of NS across the organelle membranes is a prerequisite. This process is thought to be mediated by a group of multi-transmembrane proteins from the SLC35 family, i.e., nucleotide sugar transporters (NSTs). Despite many years of research, some uncertainties/inconsistencies related with the mechanisms of NS transport and the substrate specificities of NSTs remain. Here we present a comprehensive review of the NS import into the mammalian Golgi, which consists of three major parts. In the first part, we provide a historical view of the experimental approaches used to study NS transport and evaluate the most important achievements. The second part summarizes various aspects of knowledge concerning NSTs, ranging from subcellular localization up to the pathologies related with their defective function. In the third part, we present the outcomes of our research performed using mammalian cell-based models and discuss its relevance in relation to the general context.

A new adenine nucleotide transporter located in the ER is essential for maintaining the growth of Toxoplasma gondii

The lumen of the endoplasmic reticulum (ER) is the subcellular site where secretory protein folding, glycosylation and sulfation of membrane-bound proteins, proteoglycans, and lipids occur. The protein folding and degradation in the lumen of the ER require high levels of energy in the form of ATP. Biochemical and genetic approaches show that ATP must first be translocated across ER membrane by particular transporters before serving as substrates and energy sources in the lumenal reactions. Here we describe an ATP/ADP transporter residing in the ER membranes of T.gondii. Immunofluorescence (IFA) assay in transgenic TgANT1-HA tag revealed that TgANT1 is a protein specifically expressed in the ER. In vitro assays, functional integration of TgANT in the cytoplasmic membrane of intact E. coli cells reveals high specificity for an ATP/ADP antiport. The depletion of TgANT leads to fatal growth defects in T.gondii, including a significant slowdown in replication, no visible plaque formation, and reduced ability to invade. We also found that the amino acid mutations in two domains of TgANT lead to the complete loss of its function. Since these two domains are conserved in multiple species, they may share the same transport mechanism. Our results indicate that TgANT is the only ATP/ADP transporter in the ER of T. gondii, and the lack of ATP in the ER is the cause of the death of T. gondii.

The secretory protein of T. gondii is essential for its invasion and normal growth in host cells, all the secretory proteins are synthesized in the ER before being destined for these distinct organelles, such as apicoplast, microneme, dense granule and rhoptry. ER ATP is demanded to support secretory protein folding and trafficking, and the level of ER ATP determines which proteins are able to be directed to the distinct organelles. In theory, the supply of ATP in the ER is necessary for T. gondii. However, the transport mechanism and importance of the ER ATP in T. gondii are still unclear. In our study, we identified an ATP/ADP transporter (TgANT) located in the ER and verified its function through various methods. Unlike the ER ATP/ADP transporter in mammals, we proved that TgANT is functionally specific; the deletion of TgANT caused the interruption of the supply of ATP in the ER, which leads to fatal phenotypic defects of T. gondii. Our research further expands the understanding of the growth regulation in T. gondii.

Inherited Proteoglycan Biosynthesis Defects-Current Laboratory Tools and Bikunin as a Promising Blood Biomarker

Proteoglycans consist of proteins linked to sulfated glycosaminoglycan chains. They constitute a family of macromolecules mainly involved in the architecture of organs and tissues as major components of extracellular matrices. Some proteoglycans also act as signaling molecules involved in inflammatory response as well as cell proliferation, adhesion, and differentiation. Inborn errors of proteoglycan metabolism are a group of orphan diseases with severe and irreversible skeletal abnormalities associated with multiorgan impairments. Identifying the gene variants that cause these pathologies proves to be difficult because of unspecific clinical symptoms, hardly accessible functional laboratory tests, and a lack of convenient blood biomarkers. In this review, we summarize the molecular pathways of proteoglycan biosynthesis, the associated inherited syndromes, and the related biochemical screening techniques, and we focus especially on a circulating proteoglycan called bikunin and on its potential as a new biomarker of these diseases.

SLC35A2-CDG: Functional characterization, expanded molecular, clinical, and biochemical phenotypes of 30 unreported Individuals

Pathogenic de novo variants in the X-linked gene SLC35A2 encoding the major Golgi-localized UDP-galactose transporter required for proper protein and lipid glycosylation cause a rare type of congenital disorder of glycosylation known as SLC35A2-congenital disorders of glycosylation (CDG; formerly CDG-IIm). To date, 29 unique de novo variants from 32 unrelated individuals have been described in the literature. The majority of affected individuals are primarily characterized by varying degrees of neurological impairments with or without skeletal abnormalities. Surprisingly, most affected individuals do not show abnormalities in serum transferrin N-glycosylation, a common biomarker for most types of CDG. Here we present data characterizing 30 individuals and add 26 new variants, the single largest study involving SLC35A2-CDG. The great majority of these individuals had normal transferrin glycosylation. In addition, expanding the molecular and clinical spectrum of this rare disorder, we developed a robust and reliable biochemical assay to assess SLC35A2-dependent UDP-galactose transport activity in primary fibroblasts. Finally, we show that transport activity is directly correlated to the ratio of wild-type to mutant alleles in fibroblasts from affected individuals.

My journey in the discovery of nucleotide sugar transporters of the Golgi apparatus

Defects in protein glycosylation can have a dramatic impact on eukaryotic cells and is associated with mental and developmental pathologies in humans. The studies outlined below illustrate how a basic biochemical problem in the mechanisms of protein glycosylation, specifically substrate transporters of nucleotide sugars, including ATP and 3'-phosphoadenyl-5'-phosphosulfate (PAPS), in the membrane of the Golgi apparatus and endoplasmic reticulum, expanded into diverse biological systems from mammals, including humans, to yeast, roundworms, and protozoa. Using these diverse model systems allowed my colleagues and me to answer fundamental biological questions that enabled us to formulate far-reaching hypotheses and expanded our knowledge of human diseases caused by malfunctions in the metabolic processes involved.

An insight into the orphan nucleotide sugar transporter SLC35A4

SLC35A4 has been classified in the SLC35A subfamily based on amino acid sequence homology. Most of the proteins belonging to the SLC35 family act as transporters of nucleotide sugars. In this study, the subcellular localization of endogenous SLC35A4 was determined via immunofluorescence staining, and it was demonstrated that SLC35A4 localizes mainly to the Golgi apparatus. In silico topology prediction suggests that SLC35A4 has an uneven number of transmembrane domains and its N-terminus is directed towards the Golgi lumen. However, an experimental assay refuted this prediction: SLC35A4 has an even number of transmembrane regions with both termini facing the cytosol. In vivo interaction analysis using the FLIM-FRET approach revealed that SLC35A4 neither forms homomers nor associates with other members of the SLC35A subfamily except SLC35A5. Additional assays demonstrated that endogenous SLC35A4 is 10 to 40nm proximal to SLC35A2 and SLC35A3. To determine SLC35A4 function SLC35A4 knock-out cells were generated with the CRISPR-Cas9 approach. Although no significant changes in glycosylation were observed, the introduced mutation influenced the subcellular distribution of the SLC35A2/SLC35A3 complexes. Additional FLIM-FRET experiments revealed that overexpression of SLC35A4-BFP together with SLC35A3 and the SLC35A2-Golgi splice variant negatively affects the interaction between the two latter proteins. The results presented here strongly indicate a modulatory role for SLC35A4 in intracellular trafficking of SLC35A2/SLC35A3 complexes.

Emerging tools to study proteoglycan function during skeletal development

In the past 20years, appreciation for the varied roles of proteoglycans (PGs), which are specific types of sugar-coated proteins, has increased dramatically. PGs in the extracellular matrix were long known to impart structural functions to many tissues, especially articular cartilage, which cushions bones and allows mobility at skeletal joints. Indeed, osteoarthritis is a debilitating disease associated with loss of PGs in articular cartilage. Today, however, PGs have a demonstrated role in cell biological processes, such as growth factor signalling, prompting new perspectives on the etiology of PG-associated diseases. Here, we review diseases associated with defects in PG synthesis and sulfation, also highlighting current understanding of the underlying genetics, biochemistry, and cell biology. Since most research has analyzed a class of PGs called heparan sulfate PGs, more attention is paid here to studies of chondroitin sulfate PGs (CSPGs), which are abundant in cartilage. Interestingly, CSPG synthesis is tightly linked to the cell biological processes of secretion and lysosomal degradation, suggesting that these systems may be linked genetically. Animal models of loss of CSPG function have revealed CSPGs to impact skeletal development. Specifically, our work from a mutagenesis screen in zebrafish led to the hypothesis that cartilage PGs normally delay the timing of endochondral ossification. Finally, we outline emerging approaches in zebrafish that may revolutionize the study of cartilage PG function, including transgenic methods and novel imaging techniques. Our recent work with X-ray fluorescent imaging, for example, enables direct correlation of PG function with PG-dependent biological processes.

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.

"GAG-ing with the neuron": The role of glycosaminoglycan patterning in the central nervous system

Proteoglycans (PGs) are a diverse family of proteins that consist of one or more glycosaminoglycan (GAG) chains, covalently linked to a core protein. PGs are major components of the extracellular matrix (ECM) and play critical roles in development, normal function and damage-response of the central nervous system (CNS). GAGs are classified based on their disaccharide subunits, into the following major groups: chondroitin sulfate (CS), heparan sulfate (HS), heparin (HEP), dermatan sulfate (DS), keratan sulfate (KS) and hyaluronic acid (HA). All except HA are modified by sulfation, giving GAG chains specific charged structures and binding properties. While significant neuroscience research has focused on the role of one PG family member, chondroitin sulfate proteoglycan (CSPG), there is ample evidence in support of a role for the other PGs in regulating CNS function in normal and pathological conditions. This review discusses the role of all the identified PG family members (CS, HS, HEP, DS, KS and HA) in normal CNS function and in the context of pathology. Understanding the pleiotropic roles of these molecules in the CNS may open the door to novel therapeutic strategies for a number of neurological conditions.

Short N-terminal region of UDP-galactose transporter (SLC35A2) is crucial for galactosylation of N-glycans

UDP-galactose transporter (UGT) and UDP-N-acetylglucosamine transporter (NGT) form heterologous complexes in the Golgi apparatus (GA) membrane. We aimed to identify UGT region responsible for galactosylation of N-glycans. Chimeric proteins composed of human UGT and either NGT or CMP-sialic acid transporter (CST) localized to the GA, and all but UGT/CST chimera corrected galactosylation defect in UGT-deficient cell lines, although at different efficiency. Importantly, short N-terminal region composed of 35 N-terminal amino-acid residues of UGT was crucial for galactosylation of N-glycans. The remaining molecule must be derived from NGT not CST, confirming that the role played by UGT and NGT is coupled.

The Golgi localized bifunctional UDP-rhamnose/UDP-galactose transporter family of Arabidopsis

Plant cells are surrounded by a cell wall that plays a key role in plant growth, structural integrity, and defense. The cell wall is a complex and diverse structure that is mainly composed of polysaccharides. The majority of noncellulosic cell wall polysaccharides are produced in the Golgi apparatus from nucleotide sugars that are predominantly synthesized in the cytosol. The transport of these nucleotide sugars from the cytosol into the Golgi lumen is a critical process for cell wall biosynthesis and is mediated by a family of nucleotide sugar transporters (NSTs). Numerous studies have sought to characterize substrate-specific transport by NSTs; however, the availability of certain substrates and a lack of robust methods have proven problematic. Consequently, we have developed a novel approach that combines reconstitution of NSTs into liposomes and the subsequent assessment of nucleotide sugar uptake by mass spectrometry. To address the limitation of substrate availability, we also developed a two-step reaction for the enzymatic synthesis of UDP-l-rhamnose (Rha) by expressing the two active domains of the Arabidopsis UDP-l-Rha synthase. The liposome approach and the newly synthesized substrates were used to analyze a clade of Arabidopsis NSTs, resulting in the identification and characterization of six bifunctional UDP-l-Rha/UDP-d-galactose (Gal) transporters (URGTs). Further analysis of loss-of-function and overexpression plants for two of these URGTs supported their roles in the transport of UDP-l-Rha and UDP-d-Gal for matrix polysaccharide biosynthesis.

Structure and function of nucleotide sugar transporters: Current progress

The proteomes of eukaryotes, bacteria and archaea are highly diverse due, in part, to the complex post-translational modification of protein glycosylation. The diversity of glycosylation in eukaryotes is reliant on nucleotide sugar transporters to translocate specific nucleotide sugars that are synthesised in the cytosol and nucleus, into the endoplasmic reticulum and Golgi apparatus where glycosylation reactions occur. Thirty years of research utilising multidisciplinary approaches has contributed to our current understanding of NST function and structure. In this review, the structure and function, with reference to various disease states, of several NSTs including the UDP-galactose, UDP-N-acetylglucosamine, UDP-N-acetylgalactosamine, GDP-fucose, UDP-N-acetylglucosamine/UDP-glucose/GDP-mannose and CMP-sialic acid transporters will be described. Little is known regarding the exact structure of NSTs due to difficulties associated with crystallising membrane proteins. To date, no three-dimensional structure of any NST has been elucidated. What is known is based on computer predictions, mutagenesis experiments, epitope-tagging studies, in-vitro assays and phylogenetic analysis. In this regard the best-characterised NST to date is the CMP-sialic acid transporter (CST). Therefore in this review we will provide the current state-of-play with respect to the structure–function relationship of the (CST). In particular we have summarised work performed by a number groups detailing the affect of various mutations on CST transport activity, efficiency, and substrate specificity.

UDP-N-acetylglucosamine transporter (SLC35A3) regulates biosynthesis of highly branched N-glycans and keratan sulfate

SLC35A3 is considered the main UDP-N-acetylglucosamine transporter (NGT) in mammals. Detailed analysis of NGT is restricted because mammalian mutant cells defective in this activity have not been isolated. Therefore, using the siRNA approach, we developed and characterized several NGT-deficient mammalian cell lines. CHO, CHO-Lec8, and HeLa cells deficient in NGT activity displayed a decrease in the amount of highly branched tri- and tetraantennary N-glycans, whereas monoantennary and diantennary ones remained unchanged or even were accumulated. Silencing the expression of NGT in Madin-Darby canine kidney II cells resulted in a dramatic decrease in the keratan sulfate content, whereas no changes in biosynthesis of heparan sulfate were observed. We also demonstrated for the first time close proximity between NGT and mannosyl (α-1,6-)-glycoprotein β-1,6-N-acetylglucosaminyltransferase (Mgat5) in the Golgi membrane. We conclude that NGT may be important for the biosynthesis of highly branched, multiantennary complex N-glycans and keratan sulfate. We hypothesize that NGT may specifically supply β-1,3-N-acetylglucosaminyl-transferase 7 (β3GnT7), Mgat5, and possibly mannosyl (α-1,3-)-glycoprotein β-1,4-N-acetylglucosaminyltransferase (Mgat4) with UDP-GlcNAc.

UDP-Gal/UDP-GlcNAc chimeric transporter complements mutation defect in mammalian cells deficient in UDP-Gal transporter

The role of UDP-galactose transporter (UGT; SLC35A2) and UDP-N-acetylglucosamine transporter (NGT; SLC35A3) in glycosylation of macromolecules may be coupled and either of the transporters may partially replace the function played by its partner. The aim of this study was to construct chimeric transporters composed of the N-terminal portion of human NGT and the C-terminal portion of human UGT1 or UGT2 (NGT-UGT1 or NGT-UGT2, respectively), as well as of the N-terminal portion of UGT and C-terminal portion of NGT (UGT-NGT), overexpress them stably in UGT-deficient CHO-Lec8 and MDCK-RCA(r) cells, and characterize their involvement in glycosylation. Two chimeric proteins, NGT-UGT1 and NGT-UGT2, did not overexpress properly. In contrast, UGT-NGT chimeric protein was successfully overexpressed and localized properly to the Golgi apparatus. UGT-NGT chimeric transporter delivered UDP-Gal to the Golgi vesicles of UGT-deficient cells, which resulted in the increased content of galactosylated structures to such an extent that the wild-type phenotypes were completely restored. Our data further support our hypothesis that UGT and NGT cooperate in the UDP-Gal delivery for glycosyltransferases located in the Golgi apparatus. In a wider context, the results gained in this study add to our understanding of glycosylation, one of the basic posttranslational modifications, which affects the majority of macromolecules.

UDP-N-acetylglucosamine transporter and UDP-galactose transporter form heterologous complexes in the Golgi membrane

UDP-galactose transporter (UGT; SLC35A2) and UDP-N-acetylglucosamine transporter (NGT; SLC35A3) are evolutionarily related. We hypothesize that their role in glycosylation may be coupled through heterologous complex formation. Coimmunoprecipitation analysis and FLIM-FRET measurements performed on living cells showed that NGT and UGT form complexes when overexpressed in MDCK-RCA(r) cells. We also postulate that the interaction of NGT and UGT may explain the dual localization of UGT2. For the first time we demonstrated in vivo homodimerization of the NGT nucleotide sugar transporter. In conclusion, we suggest that NGT and UGT function in glycosylation is combined via their mutual interaction.

Proteoglycan synthesis and Golgi organization in polarized epithelial cells

A large number of complex glycosylation mechanisms take place in the Golgi apparatus. In epithelial cells, glycosylated protein molecules are transported to both the apical and the basolateral surface domains. Although the prevailing view is that the Golgi apparatus provides the same lumenal environment for glycosylation of apical and basolateral cargo proteins, there are indications that proteoglycans destined for the two opposite epithelial surfaces are exposed to different conditions in transit through the Golgi apparatus. We will here review data relating proteoglycan and glycoprotein synthesis to characteristics of the apical and basolateral secretory pathways in epithelial cells.

Comparative analysis of involvement of UGT1 and UGT2 splice variants of UDP-galactose transporter in glycosylation of macromolecules in MDCK and CHO cell lines

Nucleotide sugar transporters deliver nucleotide sugars into the Golgi apparatus and endoplasmic reticulum. This study aimed to further characterize mammalian UDP-galactose transporter (UGT) in MDCK and CHO cell lines. MDCK-RCA^r and CHO-Lec8 mutant cell lines are defective in UGT transporter, although they exhibit some level of galactosylation. Previously, only single forms of UGT were identified in both cell lines, UGT1 in MDCK cells and UGT2 in CHO cells. We have identified the second UGT splice variants in CHO (UGT1) and MDCK (UGT2) cells. Compared to UGT1, UGT2 is more abundant in nearly all examined mammalian tissues and cell lines, but MDCK cells exhibit different relative distribution of both splice variants. Complementation analysis demonstrated that both UGT splice variants are necessary for N- and O-glycosylation of proteins. Both mutant cell lines produce chondroitin-4-sulfate at only a slightly lower level compared to wild-type cells. This defect is corrected by overexpression of both UGT splice variants. MDCK-RCA^r mutant cells do not produce keratan sulfate and this effect is not corrected by either UGT splice variant, overexpressed either singly or in combination. Here we demonstrate that both UGT splice variants are important for glycosylation of proteins. In contrast to MDCK cells, MDCK-RCA^r mutant cells may possess an additional defect within the keratan sulfate biosynthesis pathway.

Overexpression of UDP-GlcNAc transporter partially corrects galactosylation defect caused by UDP-Gal transporter mutation

Nucleotide sugar transporters deliver substrates for glycosyltransferases into the endoplasmic reticulum and the Golgi apparatus. We demonstrated that overexpression of UDP-GlcNAc transporter (NGT) in MDCK-RCA(r) and CHO-Lec8 mutant cells defective in UDP-Gal transporter (UGT) restored galactosylation of N-glycans. NGT overexpression resulted in decreased transport of UDP-GlcNAc into the Golgi vesicles. This effect resembled the phenotype of mutant cells corrected by UGT1 overexpression. The transport of UDP-Gal was not significantly changed. Our data suggest that the biological function of UGT and NGT in galactosylation of macromolecules may be coupled.

A novel approach for the characterisation of proteoglycans and biosynthetic enzymes in a snail model

Proteoglycans encompass a heterogeneous group of glycoconjugates where proteins are substituted with linear, highly negatively charged glycosaminoglycan chains. Sulphated glycosaminoglycans are ubiquitous to the animal kingdom of the Eukarya domain. Information on the distribution and characterisation of proteoglycans in invertebrate tissues is limited and restricted to a few species. By the use of multidimensional protein identification technology and immunohistochemistry, this study shows for the first time the presence and tissue localisation of different proteoglycans, such as perlecan, aggrecan, and heparan sulphate proteoglycan, amongst others, in organs of the gastropoda Achatina fulica. Through a proteomic analysis of Golgi proteins and immunohistochemistry of tissue sections, we detected the machinery involved in glycosaminoglycan biosynthesis, related to polymer formation (polymerases), as well as secondary modifications (sulphation and uronic acid epimerization). Therefore, this work not only identifies both the proteoglycan core proteins and glycosaminoglycan biosynthetic enzymes in invertebrates but also provides a novel method for the study of glycosaminoglycan and proteoglycan evolution.

PDGF beta-receptor kinase activity and ERK1/2 mediate glycosaminoglycan elongation on biglycan and increases binding to LDL

The initiation of atherosclerosis involves the subendothelial retention of lipoproteins by proteoglycans (PGs). Structural characteristics of glycosaminoglycan (GAG) chains on PGs influence lipoprotein binding and are altered adversely by platelet-derived growth factor (PDGF). The signaling pathway for PDGF-mediated GAG elongation via the PDGF receptor (PDGFR) was investigated. In human vascular smooth muscle cells, PDGF significantly increased (35)S-sulfate incorporation into PGs and GAG chain size. PGs from PDGF-stimulated cells showed increased binding low-density lipoprotein (P < 0.001) in gel mobility shift assays. Knockdown of PDGFRbeta using small interfering RNA demonstrated that PDGF mediated changes in PGs via PDGFRbeta. GAG synthesis and hyperelongation was blocked by inhibition of receptor tyrosine kinase autophosphorylation site Tyr857 activity using Ki11502 or imatinib. Downstream signaling to GAG hyperelongation was mediated through ERK MAPK and not phosphatidylinositol-3 kinase or phospholipase Cgamma. In high-fat-fed apolipoprotein E(-/-) mice, inhibition of PDGFRbeta activity by imatinib reduced aortic total lipid staining area by 35% (P < 0.05). Inhibition of PDGFRbeta tyrosine kinase activity leads to inhibition of GAG synthesis on vascular PGs and aortic lipid area in vivo. PDGFRbeta and its signaling pathways are potential targets for novel therapeutic agents to prevent the earliest stages atherosclerosis.

Inhibition of Golgi apparatus glycosylation causes endoplasmic reticulum stress and decreased protein synthesis

Nucleotide sugar transporters of the Golgi apparatus play an essential role in the glycosylation of proteins, lipids, and proteoglycans. Down-regulation of expression of the transporters for CMP-sialic acid, GDP-fucose, or both unexpectedly resulted in accumulation of glycoconjugates in the Golgi apparatus rather than in the plasma membrane. Pulse-chase experiments with radiolabeled sugars and amino acids showed decreased synthesis and secretion of both nonglycoproteins and glycoproteins. Further studies revealed that the above silencing induced endoplasmic reticulum stress and inhibited protein translation initiation. Together these results suggest that global inhibition of Golgi apparatus glycosylation may lead to important secondary metabolic changes, unrelated to glycosylation.

Growth factor-mediated hyper-elongation of glycosaminoglycan chains on biglycan requires transcription and translation

The mechanism through which growth factors cause glycosaminoglycan (GAG) hyper-elongation is unclear. We have investigated the role of transcription and translation on the GAG hyper-elongation effect of platelet-derived growth factor (PDGF) in human vascular smooth muscle cells (VSMCs). To determine if the response involves specific signalling pathways or the process of GAG hyper-elongation we have also investigated the effects of epidermal growth factor (EGF), transforming growth factor-beta (TGF-beta) and thrombin. We report that both actinomycin D and cycloheximide completely abolished the ability of PDGF to stimulate radiosulphate incorporation and GAG elongation into secreted proteoglycans, and to increase the size of xyloside GAGs. Blocking de novo protein synthesis completely prevented the action of all growth factors tested to elongate GAG chains. These results lay a foundation for further investigation into the genes and proteins implicated in this response.

Expression of genes encoding for proteins involved in heparan sulphate and chondroitin sulphate chain synthesis and modification in normal and malignant plasma cells

Syndecan-1 is a proteoglycan that concentrates heparin-binding factors on the surface of multiple myeloma cells, and probably plays a major role in multiple myeloma biology. As heparan sulphate and chondroitin sulphate are the bioactive components of syndecan-1, we analysed the signature of genes encoding 100 proteins involved in synthesis of these chains, i.e. from precursor uptake to post-translational modifications, using Affymetrix microarrays. The expression of enzymes required for heparan sulphate and chondroitin sulphate biosynthesis was shown to increase in parallel with syndecan-1 expression, throughout the differentiation of memory B cells into plasmablasts and normal bone marrow plasma cells. Sixteen genes were significantly different between normal and malignant plasma cells, nine of these genes -EXT2, CHSY3, CSGALNACT1, HS3ST2, HS2ST1, CHST11, CSGALNACT2, HPSE, SULF2 - encode proteins involved in glycosaminoglycan chain synthesis or modifications. Kaplan-Meier analysis was performed in two independent series of patients: B4GALT7, CSGALNACT1, HS2ST1 were associated with a good prognosis whereas EXT1 was linked to a bad prognosis. This study provides an overall picture of the major genes encoding for proteins involved in heparan sulphate and chondroitin sulphate synthesis and modifications that can be implicated in normal and malignant plasma cells.

Glucosamine inhibits the synthesis of glycosaminoglycan chains on vascular smooth muscle cell proteoglycans by depletion of ATP

Glucosamine via GlcNAc is a precursor for the synthesis of glycosaminoglycan (GAG) chains on proteoglycans. We previously found that proteoglycans synthesized and secreted by vascular smooth muscle cells (VSMC) in the presence of supplementary glucosamine had GAG of decreased not increased size. We investigated the possibility that the inhibition of GAG chains synthesis on proteoglycans might be related to cellular ATP depletion. Confluent primate VSMCs were exposed to glucosamine, azide, or 2-deoxyglucose (2-DG). Each of these agents depleted cell ATP content by 25-30%. All agents decreased (35)S-SO(4) incorporation and reduced the size of the proteoglycans, decorin and biglycan as assessed by SDS-PAGE. On withdrawal of the glucosamine, azide or 2-DG ATP levels and proteoglycan synthesis returned towards baseline values. Glucosamine decreased glucose uptake and consumption suggesting that ATP depletion was due preferential phosphorylation of glucosamine over glucose. Thus, glucosamine inhibition of proteoglycan synthesis is due, at least in part, to depletion of cellular ATP content.

Nucleotide sugar transporters of the Golgi apparatus

The Golgi apparatus is the major site of protein, lipid and proteoglycan glycosylation. The glycosylation enzymes, as well as kinases and sulfatases that catalyze phosphorylation and sulfation, are localized within the Golgi cisternae in characteristic distributions that frequently reflect their order in a particular pathway (Kornfeld and Kornfeld 1985; Colley 1997). The glycosyl-transferases, sulfotransferases and kinases are “transferases” that require activated donor molecules for the reactions they catalyze. For eukaryotic, fungal and protozoan glycosyltransferases these are the nucleotide sugars UDP-N-acetylglucosamine (UDP-GlcNAc), UDP-galactose (UDP-Gal), GDP-fucose (GDP-Fuc), CMP-sialicacid (CMP-Sia), UDP-glucuronicacid (UDP-GlcA), GDP-mannose (GDP-Man), and UDP-xylose (UDP-Xyl) (Hirschberg et al. 1998). For the kinases, ATP functions as the donor, while for the sulfotransferases, adenosine 3′-phosphate 5′-phosphate (PAPS) acts as the donor (Hirschberg et al. 1998). The active sites of all these enzymes are oriented towards the lumen of the Golgi cisternae. This necessitates the translocation of their donors from the cytosol into the lumenal Golgi compartments. In this chapter we will focus on the structure, function and localization of the Golgi nucleotide sugar transporters (NSTs), and highlight the diseases and developmental defects associated with defective transporters. We direct the reader to several excellent reviews on Golgi transporters for additional details and references (Hirschberg et al. 1998; Berninsone and Hirschberg 2000; Gerardy-Schahn et al. 2001; Handford et al. 2006; Caffaro and Hirschberg 2006).

Overexpression of the 3'-phosphoadenosine 5'-phosphosulfate (PAPS) transporter 1 increases sulfation of chondroitin sulfate in the apical pathway of MDCK II cells

The canine 3'-phosphoadenosine 5'-phosphosulfate (PAPS) transporter 1 fused to GFP was stably expressed with a typical Golgi localization in MDCK II cells (MDCK II-PAPST1). The capacity for PAPS uptake into Golgi vesicles was enhanced to almost three times that of Golgi vesicles isolated from untransfected cells. We have previously shown that chondroitin sulfate proteoglycans (CSPGs) are several times more intensely sulfated in the basolateral than the apical secretory pathway in MDCK II cells (Tveit H, Dick G, Skibeli V, Prydz K. 2005. A proteoglycan undergoes different modifications en route to the apical and basolateral surfaces of Madin-Darby canine kidney cells. J Biol Chem. 280:29596-29603). Here we demonstrate that increased availability of PAPS in the Golgi lumen enhances the sulfation of CSPG in the apical pathway several times, while sulfation of CSPGs in the basolateral pathway shows minor changes. Sulfation of heparan sulfate proteoglycans is essentially unchanged. Our data indicate that CSPG sulfation in the apical pathway of MDCK II cells occurs at suboptimal conditions, either because the sulfotransferases involved have high K(m) values, or there is a lower PAPS concentration in the lumen of the apical secretory route than in the basolateral counterpart.

Nucleotide-sugar transporters: structure, function and roles in vivo

The glycosylation of glycoconjugates and the biosynthesis of polysaccharides depend on nucleotide-sugars which are the substrates for glycosyltransferases. A large proportion of these enzymes are located within the lumen of the Golgi apparatus as well as the endoplasmic reticulum, while many of the nucleotide-sugars are synthesized in the cytosol. Thus, nucleotide-sugars are translocated from the cytosol to the lumen of the Golgi apparatus and endoplasmic reticulum by multiple spanning domain proteins known as nucleotide-sugar transporters (NSTs). These proteins were first identified biochemically and some of them were cloned by complementation of mutants. Genome and expressed sequence tag sequencing allowed the identification of a number of sequences that may encode for NSTs in different organisms. The functional characterization of some of these genes has shown that some of them can be highly specific in their substrate specificity while others can utilize up to three different nucleotide-sugars containing the same nucleotide. Mutations in genes encoding for NSTs can lead to changes in development in Drosophila melanogaster or Caenorhabditis elegans, as well as alterations in the infectivity of Leishmania donovani. In humans, the mutation of a GDP-fucose transporter is responsible for an impaired immune response as well as retarded growth. These results suggest that, even though there appear to be a fair number of genes encoding for NSTs, they are not functionally redundant and seem to play specific roles in glycosylation.

The CMP-sialic acid transporter is localized in the medial-trans Golgi and possesses two specific endoplasmic reticulum export motifs in its carboxyl-terminal cytoplasmic tail

The addition of sialic acid to glycoproteins and glycolipids requires Golgi sialyltransferases to have access to their glycoconjugate substrates and nucleotide sugar donor, CMP-sialic acid. CMP-sialic acid is transported into the lumen of the Golgi complex through the CMP-sialic acid transporter, an antiporter that also functions to transport CMP into the cytosol. We localized the transporter using immunofluorescence and deconvolution microscopy to test the prediction that it is broadly distributed across the Golgi stack to serve the many sialyltransferases involved in glycoconjugate sialylation. The transporter co-localized with ST6GalI in the medial and trans Golgi, showed partial overlap with a medial Golgi marker and little overlap with early Golgi or trans Golgi network markers. Endoplasmic reticulum-retained forms of sialyltransferases did not redistribute the transporter from the Golgi to the endoplasmic reticulum, suggesting that transporter-sialyltransferase complexes are not involved in transporter localization. Next we evaluated the role of the transporter's N- and C-terminal cytoplasmic tails in its trafficking and localization. The N-tail was not required for either endoplasmic reticulum export or Golgi localization. The C-tail was required for endoplasmic reticulum export and contained di-Ile and terminal Val motifs at its very C terminus that function as independent endoplasmic reticulum export signals. Deletion of the last four amino acids of the C-tail (IIGV) eliminated these export signals and prevented endoplasmic reticulum export of the transporter. This form of the transporter supplied limited amounts of CMP-sialic acid to Golgi sialyltransferases but was unable to completely rescue the transporter defect of Lec2 Chinese hamster ovary cells.

Characterization of a mutation and an alternative splicing of UDP-galactose transporter in MDCK-RCAr cell line

The UDP-galactose (UDP-Gal) transporter present in the Golgi apparatus is a member of a transporter family comprising hydrophobic proteins with multiple transmembrane domains. Co-immunoprecipitation experiments showed that the full-length UDP-Gal transporter protein forms oligomeric structures in the MDCK cell. A ricin-resistant mutant of the MDCK cell line (MDCK-RCA(r)) is deficient in galactose linked to macromolecules because of a lower UDP-Gal transport rate into the Golgi apparatus. We cloned this mutated protein and found that it contains a stop codon close to the 5' terminus of its open reading frame. We also detected a shorter splicing variant of the UDP-Gal transporter which contains a 183-nt in-frame deletion in both the wild-type and the mutant mRNA. We showed that the protein, when overexpressed, is localized in the Golgi apparatus and could partially correct the phenotype of the MDCK-RCA(r) and CHO-Lec8 mutant cell lines. The level of mRNA of the UDP-Gal transporter is much lower (25-30 copies per cell) than those of the CMP-sialic acid transporter (100 copies per cell), UDP-N-acetylglucosamine transporter (80 copies per cell), and GDP-fucose transporter (65 copies per cell). The transcript level of the shorter splicing variant of the UDP-Gal transporter is extremely rare in wild-type MDCK cells (a few copies per cell), but it is significantly increased in the mutant, RCA-resistant cells.

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

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

Core fucosylation of N-linked glycans in leukocyte adhesion deficiency/congenital disorder of glycosylation IIc fibroblasts

Leukocyte adhesion deficiency/congenital disorder of glycosylation IIc (LAD II/CDG IIc) is a genetic disease characterized by a decreased expression of fucose in glycoconjugates, resulting in leukocyte adhesion deficiency and severe morphological and neurological abnormalities. The biochemical defect is a reduced transport of guanosine diphosphate-L-fucose (GDP-L-fucose) from cytosol into the Golgi compartment, which reduces its availability as substrate for fucosyltransferases. The aim of this study was to determine the effects of a limited supply of GDP-L-fucose inside the Golgi on core fucosylation (alpha1,6-fucose linked to core N-acetylglucosamine [GlcNAc]) of N-linked glycans in LAD II fibroblasts. The results showed that, although [3H]fucose incorporation was generally reduced in LAD II cells, core fucosylation was affected to a greater extent compared with other types of fucosylation of N-linked oligosaccharides. In particular, core fucosylation was found to be nearly absent in biantennary negatively charged oligosaccharides, whereas other types of structures, in particular triantennary neutral species, were less affected by the reduction. Expression and activity of alpha1,6-fucosyltransferase (FUT8) in control and LAD II fibroblasts were comparable, thus excluding the possibility of a decreased activity of the transferase. The data obtained confirm that the concentration of GDP-L-fucose inside the Golgi can differentially affect the various types of fucosylation in vivo and also indicate that core fucosylation is not dependent only on the availability of GDP-L-fucose, but it is significantly influenced by the type of oligosaccharide structure. The relevant reduction in core fucosylation observed in some species of oligosaccharides could also provide clues for the identification of glycans involved in the severe developmental abnormalities observed in LAD II.

The Systems Biology of Glycosylation

Glycosylation can have a profound influence on the function of a variety of eukaryotic cells. In particular, it can affect signal transduction and cell-cell communication properties and thus shape critical cell decisions, including the regulation of differentiation and apoptosis. Regulation of glycosylation has multiple layers of complexity, both structural and functional, which make its experimental and theoretical analysis difficult to perform and interpret. Novel research methodologies provided by systems biology can help to address many outstanding issues and integrate glycosylation with other metabolic and cell regulation processes. Here we review the toolbox available for biochemical systems analysis of glycosylation.

Microarray analysis of the rat lacrimal gland following the loss of parasympathetic control of secretion

Previous studies showed that loss of muscarinic parasympathetic input to the lacrimal gland (LG) leads to a dramatic reduction in tear secretion and profound changes to LG structure. In this study, we used DNA microarrays to examine the regulation of the gene expression of the genes for secretory function and organization of the LG. Long-Evans rats anesthetized with a mixture of ketamine/xylazine (80:10 mg/kg) underwent unilateral sectioning of the greater superficial petrosal nerve, the input to the pterygopalatine ganglion. After 7 days, tear secretion was measured, the animals were killed, and structural changes in the LG were examined by light microscopy. Total RNA from control and experimental LGs (n = 5) was used for DNA microarray analysis employing the U34A GeneChip. Three statistical algorithms (detection, change call, and signal log ratio) were used to determine differential gene expression using the Microarray Suite (5.0) and Data Mining Tools (3.0). Tear secretion was significantly reduced and corneal ulcers developed in all experimental eyes. Light microscopy showed breakdown of the acinar structure of the LG. DNA microarray analysis showed downregulation of genes associated with the endoplasmic reticulum and Golgi, including genes involved in protein folding and processing. Conversely, transcripts for cytoskeleton and extracellular matrix components, inflammation, and apoptosis were upregulated. The number of significantly upregulated genes (116) was substantially greater than the number of downregulated genes (49). Removal of the main secretory input to the rat LG resulted in clinical symptoms associated with severe dry eye. Components of the secretory pathway were negatively affected, and the increase in cell proliferation and inflammation may lead to loss of organization in the parasympathectomized lacrimal gland.

Molecular physiology and pathology of the nucleotide sugar transporter family (SLC35)

The solute carrier family SLC35 consists of at least 17 molecular species in humans. The family members so far characterized encode nucleotide sugar transporters localizing at the Golgi apparatus and/or the endoplasmic reticulum (ER). These transporters transport nucleotide sugars pooled in the cytosol into the lumen of these organelles, where most glycoconjugate synthesis occurs. Pathological analyses and developmental studies of small, multicellular organisms deficient in nucleotide sugar transporters have shown these transporters to be involved in tumour metastasis, cellular immunity, organogenesis and morphogenesis. Leukocyte adhesion deficiency type II (LAD II) or the congenital disorder of glycosylation type IIc (CDG IIc) are the sole human congenital disorders known to date that are caused by a defect of GDP-fucose transport. Along with LAD II, the possible involvement of nucleotide sugar transporters in disorders of connective tissues and muscles is also discussed.

Age-related changes in the response of human articular cartilage to IL-1alpha and transforming growth factor-beta (TGF-beta): chondrocytes exhibit a diminished sensitivity to TGF-beta

Cartilage glycosaminoglycan (GAG) synthesis and composition, upon which its structural integrity depends, varies with age, is modified by anabolic and catabolic stimuli, and is regulated by UDP-glucuronate availability. However, how such stimuli, prototypically represented by transforming growth factor-beta1 (TGF-beta1) and IL-1alpha, modify GAG synthesis during aging of normal human articular cartilage is not known. Using explants, we show that chondroitin sulfate (CS):total GAG ratios decrease, whereas C6S:C4S ratios increase with cartilage maturation, and that chondrocytes in the cartilage mid-zone, but not the superficial or deep zones, exhibit uridine 5'-diphosphoglucose dehydrogenase (UDPGD) activity, which is also increased in mature cartilage. We also show that IL-1alpha treatment reduces both total GAG and CS synthesis, decreases C6S:C4S ratios (less C6S), but fails to modify chondrocyte UDPGD activity at all ages. On the other hand, TGF-beta1 increases total GAG synthesis in immature, but not mature, cartilage (stimulates CS but not non-CS), age-independently decreases C6S:C4S (more C4S), and increases chondrocyte UDPGD activity in a manner inversely correlated with age. Our findings show that TGF-beta1, but not IL-1alpha, modifies matrix synthesis such that its composition more closely resembles "less mature" articular cartilage. These effects of TGF-beta1, which appear to be restricted to periods of skeletal immaturity, are closely associated although not necessarily mechanistically linked with increases in chondrocyte UDPGD activity. The antianabolic effects of IL-1alpha are, on the other hand, likely to be independent of any direct modification in UDPGD activity and manifest equally in human cartilage of all ages.

Differential terminal fucosylation of N-linked glycans versus protein O-fucosylation in leukocyte adhesion deficiency type II (CDG IIc)

LAD II/CDG IIc is a rare autosomal recessive disease characterized by a decreased expression of fucosylated antigens on cell surfaces that results in leukocyte adhesion deficiency and severe neurological and developmental abnormalities. Its molecular basis has been identified as a defect in the transporter of GDP-l-fucose into the Golgi lumen, which reduces the availability of the substrate for fucosyltransferases. During metabolic radiolabeling experiments using [3H]fucose, LAD II fibroblasts incorporated significantly less radiolabel compared with control cells. However, fractionation and analysis of the different classes of glycans indicated that the decrease in [3H]fucose incorporation is not generalized and is mainly confined to terminal fucosylation of N-linked oligosaccharides. In contrast, the total levels of protein O-fucosylation, including that observed in Notch protein, were unaffected. This finding demonstrates that the decrease in GDP-l-fucose levels in the fibroblast Golgi caused by the LAD II defect does not impair bulk protein O-fucosylation, but severely affects the bulk addition of fucose as a terminal modification of N-linked glycans. These data suggest that the severe clinical abnormalities including neurological and developmental ones observed in at least some of the LAD II patients may be related to alteration in recognition systems involving terminal fucose modifications of N-glycans and not be due to a defective O-fucosylation of proteins such as Notch.

The Golgi protein RCAS1 controls cell surface expression of tumor-associated O-linked glycan antigens

Tumor immunology has received a large impetus from the identification of tumor-associated antigens. Among them, a monoclonal antibody, 22.1.1, was instrumental in defining a novel tumor-associated antigen that was termed "receptor binding cancer antigen expressed on SiSo cells" (RCAS1). RCAS1 was proposed to induce growth arrest and apoptosis on activated immune cells, mediated by a putative death receptor. Structurally, RCAS1 was predicted to exist as a type II transmembrane protein and in a soluble form. Here, we analyzed occurrence, membrane topology, and subcellular localization of the RCAS1-encoded gene product. RCAS1 was shown to be a ubiquitously expressed type III transmembrane protein with a Golgi-predominant localization. Monoclonal antibody 22.1.1 failed to recognize RCAS1, as demonstrated by confocal microscopy. Instead, we showed that the cognate 22.1.1 epitope is identical with the tumor-associated O-linked glycan Tn (N-acetyl-d-galactosamine, GalNAc). Overexpression of RCAS1 in cell lines that are negative for 22.1.1 surface staining led to the generation of Tn and the closely related TF (Thomsen-Friedenreich, Galbeta1-3GalNAc) antigen, thus providing a functional link to the generation of the 22.1.1 epitope. We suggest that RCAS1 modulates surface expression of tumor-associated, normally cryptic O-linked glycan structures and contributes indirectly to the antigenicity of tumor cells.

Transport of UDP-galactose in plants. Identification and functional characterization of AtUTr1, an Arabidopsis thaliana UDP-galactos/UDP-glucose transporter

The synthesis of non-cellulosic polysaccharides and glycoproteins in the plant cell Golgi apparatus requires UDP-galactose as substrate. The topology of these reactions is not known, although the orientation of a plant galactosyltransferase involved in the biosynthesis of galactomannans in fenugreek is consistent with a requirement for UDP-galactose in the lumen of the Golgi cisternae. Here we provide evidence that sealed, right-side-out Golgi vesicles isolated from pea stems transport UDP-galactose into their lumen and transfer galactose, likely to polysaccharides and other acceptors. In addition, we identified and cloned AtUTr1, a gene from Arabidopsis thaliana that encodes a multitransmembrane hydrophobic protein similar to nucleotide sugar transporters. Northern analysis showed that AtUTr1 is indeed expressed in Arabidopsis. AtUTr1 is able to complement the phenotype of MDCK ricin-resistant cells; a mammalian cell line deficient in transport of UDP-galactose into the Golgi. In vitro assays using a Golgi-enriched vesicle fraction obtained from Saccharomyces cerevisiae expressing AtUTr1-MycHis is able to transport UDP-galactose but also UDP-glucose. AtUTr1- MycHis does not transport GDP-mannose, GDP-fucose, CMP-sialic acid, UDP-glucuronic acid, or UDP-xylose when expressed in S. cerevisiae. AtUTr1 is the first transporter described that is able to transport UDP-galactose and UDP-glucose. Thus AtUTr1 may play an important role in the synthesis of glycoconjugates in Arabidopsis that contain galactose and glucose.

Conditional control of selectin ligand expression and global fucosylation events in mice with a targeted mutation at the FX locus

Glycoprotein fucosylation enables fringe-dependent modulation of signal transduction by Notch transmembrane receptors, contributes to selectin-dependent leukocyte trafficking, and is faulty in leukocyte adhesion deficiency (LAD) type II, also known as congenital disorder of glycosylation (CDG)-IIc, a rare human disorder characterized by psychomotor defects, developmental abnormalities, and leukocyte adhesion defects. We report here that mice with an induced null mutation in the FX locus, which encodes an enzyme in the de novo pathway for GDP-fucose synthesis, exhibit a virtually complete deficiency of cellular fucosylation, and variable frequency of intrauterine demise determined by parental FX genotype. Live-born FX(-/-) mice exhibit postnatal failure to thrive that is suppressed with a fucose-supplemented diet. FX(-/-) adults suffer from an extreme neutrophilia, myeloproliferation, and absence of leukocyte selectin ligand expression reminiscent of LAD-II/CDG-IIc. Contingent restoration of leukocyte and endothelial selectin ligand expression, general cellular fucosylation, and normal postnatal physiology is achieved by modulating dietary fucose to supply a salvage pathway for GDP-fucose synthesis. Conditional control of fucosylation in FX(-/-) mice identifies cellular fucosylation events as essential concomitants to fertility, early growth and development, and leukocyte adhesion.

Metabolic control of recombinant monoclonal antibody N-glycosylation in GS-NS0 cells

Variable N-glycosylation at Asn(297) in the Fc region of recombinant therapeutic immunoglobulin G (IgG) molecules, specifically terminal galactosylation and sialylation, may affect both pharmacokinetic behavior and effector functions of recombinant therapeutic antibodies. We investigated the hypothesis that IgG Fc glycosylation can be controlled by manipulation of cellular nucleotide-sugar metabolism. In control cultures, N-glycans associated with the Fc domain of a recombinant humanized IgG1 produced by GS-NS0 cells in culture were predominantly biantennary, variably beta-galactosylated (average 0.3 mol galactose complex N-glycan(-1)) structures with no bisecting N-acetylglucosamine residues, sialylation, or alpha1,3-linked galactosylation evident. However, a variable proportion (5% to 15%) of high-mannose (Man5 to Man9) oligosaccharides were present. To manipulate the cellular content of the nucleotide sugar precursor required for galactosylation, UDP-Gal, we included either 10 mM glucosamine or 10 mM galactose in the culture medium. In the case of the former, a 17-fold increase in cellular UDP-N-acetylhexosamine content was observed, with a concomitant reduction (33%) in total UDP-hexose, although the ratio of UDP-Glc:UDP-Gal (4:1) was unchanged. Associated with these alterations in cellular UDP-sugar content was a significant reduction (57%) in the galactosylation of Fc-derived oligosaccharides. The proportion of high-mannose-type N-glycans (specifically Man5, the substrate for N-acetylglucosaminyltransferase I) at Asn(297) was unaffected. In contrast, inclusion of 10 mM galactose in culture specifically stimulated UDP-Gal content almost five-fold. However, this resulted in only a minimal, insignificant increase (6%) in beta1,4-galactosylation of Fc N-glycans. Sialylation was not improved upon the addition of the CMP-sialic acid (CMP-SA) precursor N-acetylmannosamine (20 mM), even with an associated 44-fold increase in cellular CMP-SA content. Analysis of recombinant IgG1 Fc glycosylation during batch culture showed that beta1,4-linked galactosylation declined slightly during culture, although, in the latter stages of culture, the release of proteases and glycosidases by lysed cells were likely to have contributed to the more dramatic drop in galactosylation. These data demonstrate: (i) the effect of steric hindrance on Fc N-glycan processing; (ii) the extent to which alterations in cellular nucleotide-sugar content may affect Fc N-glycan processing; and (iii) the potential for direct metabolic control of Fc N-glycosylation.

Nucleotide sugar transporters: biological and functional aspects

The Golgi apparatus serves as the major site of glycosylation reactions. Nucleotide sugars which are substrates of the Golgi localized glycosyltransferases are synthesized in the cytoplasm (cell nucleus in case of CMP-sialic acid) and must be transported into the compartment lumen. This transport function is carried out by nucleotide sugar transporters. The first genes were cloned in the year 1996 and revealed a family of structurally conserved multi-transmembrane-spanning proteins. Due to the high structural and functional conservation, the identification of many putative nucleotide sugar transporter sequences has become possible in the existing gene data bases and accelerates the increase in knowledge on structure-function-relationships. Recent developments in the nucleotide sugar transporter field are discussed in this article.

Enhanced 3-O-sulfation of galactose in Asn-linked glycans and Maackia amurensis lectin binding in a new Chinese hamster ovary cell line

We report the characterization of two Chinese hamster ovary cell lines that produce large amounts of sulfated N-linked oligosaccharides. Clones 26 and 489 were derived by stable transfection of the glycosaminoglycan-deficient cell mutant pgsA-745 with a cDNA library prepared from wild-type cells. Peptide:N-glycanase F released nearly all of the sulfate label, indicating that sulfation had occurred selectively on the Asn-linked glycans. Hydrazinolysis followed by nitrous acid treatment at pH 4 and borohydride reduction yielded reduced sulfated disaccharides that comigrated with standard Gal3SO4beta1-4anhydromannitol. The disaccharides were resistant to periodate oxidation but became sensitive after the sulfate group was removed by methanolysis, indicating that the sulfate was located at C3 of the galactose residues. Maackia amurensis lectin bound to the sulfated glycopeptides on the cell surface and in free form, even after sialidase treatment. This finding indicates that the lectin requires only a charged group at C3 of the galactose unit and not an intact sialic acid. Growth of cells with chlorate restored sialidase sensitivity to lectin binding, indicating that sulfation and sialylation occurred largely at the same sites. The enhanced sulfation was due to elevated sulfotransferase activity that catalyzed transfer of sulfate from phosphoadenosine-5'-phosphosulfate to Galbeta1-4(3)GlcNAcbeta-O-naphthalenemethanol.

Point mutations identified in Lec8 Chinese hamster ovary glycosylation mutants that inactivate both the UDP-galactose and CMP-sialic acid transporters

Nucleotide-sugar transporters (NSTs) are critical components of glycosylation pathways in eukaryotes. The identification of structural elements that are involved in NST functions provides an important task. Chinese hamster ovary glycosylation mutants defective in nucleotide-sugar transport provide access to inactive transporters that can define such structure/function relationships. In this study, we have cloned the hamster UDP-galactose transporter (UGT) and identified defects in UGT gene transcripts from nine independent Chinese hamster ovary mutants that belong to the Lec8 complementation group. Reverse transcription polymerase chain reaction with primers that span the UGT open reading frame showed that three Lec8 mutants express a full-length open reading frame, while six Lec8 mutants predominantly express truncated UGT gene transcripts. Sequencing identified different single or triplet nucleotide changes in full-length UGT transcripts from three of the mutants. These mutations translate into three different amino acid changes at positions that are highly conserved in all the known mammalian NSTs. Transfection of a cDNA encoding either of the mutations Delta serine 213 or G281D failed to correct the UDP-galactose transport defect in Lec8 transfectants. Most importantly, introducing these same mutations into the homologous region of the murine CMP-sialic acid transporter caused inactivation of this transporter. Thus, identifying point mutations that inactivate UGT in Lec8 mutants resulted in the discovery of amino acids that are critical to the activity of both UGT and CST, the two most divergent mammalian NSTs.

Determination of Nucleotides and Sugar Nucleotides Involved in Protein Glycosylation by High-Performance Anion-Exchange Chromatography: Sugar Nucleotide Contents in Cultured Insect Cells and Mammalian Cells

We have developed a simple and highly sensitive HPLC method for determination of cellular levels of sugar nucleotides and related nucleotides in cultured cells. Separation of 9 sugar nucleotides (CMP-Neu5Ac, CMP-Neu5Gc, CMP-KDN, UDP-Gal, UDP-Glc, UDP-GalNAc, UDP-GlcNAc, GDP-Fuc, GDP-Man) and 12 nucleotides (AMP, ADP, ATP, CMP, CDP, CTP, GMP, GDP, GTP, UMP, UDP, and UTP) was examined by reversed-phase HPLC and high-performance anion-exchange chromatography (HPAEC). Although the reversed-phase HPLC, using an ion-pairing reagent, gave a good separation of the 12 nucleotides, it did not separate sufficiently the sugar nucleotides for quantification. On the other hand, the HPAEC method gave an excellent and reproducible separation of all nucleotides and sugar nucleotides with high sensitivity and reproducibility. We applied the HPAEC method to determine the intracellular sugar nucleotide levels of cultured Spodoptera frugiperda (Sf9) and Trichoplusia ni (High Five, BTN-TN-5B1-4) insect cells, and compared them with those in Chinese hamster ovary (CHO-K1) cells. Sf9 and High Five cells showed concentrations of UDP-GlcNAc, UDP-Gal, UDP-Glc, GDP-Fuc, and GDP-Man equal to or higher than those in CHO cells. CMP-Neu5Ac was detected in CHO cells, but it was not detected in Sf9 and High Five cells. In conclusion, the newly developed HPAEC method could provide valuable information necessary for generating sialylated complex-type N-glycans in insect or other cells, either native or genetically manipulated.

Overexpression of HUT1 gene stimulates in vivo galactosylation by enhancing UDP-galactose transport activity in Saccharomyces cerevisiae

Transfer of activated sugar-nucleotides from the cytoplasm to the lumen of the Golgi is an essential requirement for glycosylation of glycoproteins, proteoglycans and glycosphingolipids. Although mannosylation is the major modification in the yeast Saccharomyces cerevisiae, several reports suggest the presence of galactose residues on yeast proteins and sphingolipids. We have detected alpha-galactosylated O-linked chitinase by lectin blotting from cells that functionally express the gma12(+) gene, encoding alpha 1,2-galactosyltransferase from Schizosaccharomyces pombe. This result implies the presence of a UDP-galactose transporter in S. cerevisiae. A conserved gene, HUT1, which encodes a putative multi-transmembrane protein, was cloned and characterized for its possible involvement in galactosylation. The HUT1 gene is not essential and is expressed at a relatively low level under the physiological conditions we examined. The disruption of this gene did not show any apparent impairments in glycosylation. However, a temperature- and concentration-dependent increase in UDP--galactose transport activity was detected from cells overexpressing HUT1 in the presence of gma12(+). The surface of these cells was confirmed to carry galactose residues by staining with FITC-conjugated alpha-galactose-specific lectin. These results suggest a role for Hut1p in the transport of UDP--galactose from the cytosol into the Golgi lumen in S. cerevisiae.

Impairment of the Golgi GDP-L-fucose transport and unresponsiveness to fucose replacement therapy in LAD II patients

Leukocyte adhesion deficiency type II is an autosomal recessive syndrome characterized by generalized reduction of L-fucose in glycoconjugates; the specific molecular defect is still undefined. The most important clinical symptoms include severe growth and mental retardation and severe immunodeficiency. Patients from two ethnic groups have been reported, i.e. Arab and Turkish. We have observed that GDP-L-fucose transport into Golgi vesicles was specifically impaired in an Arab patient, with a significant reduction of the V:(max) but no significant differences in the K:(m) from control and parents. GDP-L-fucose transport showed simple saturation kinetics in all samples. Transport of UDP-galactose, UDP-N:-acetylglucosamine, and CMP-sialic acid was comparable into vesicles from the Arab patient, parents, and control. These kinetic parameters probably account for the failure to obtain any clinical and biochemical response to fucose therapy in Arab patients. This contrasts both with the distinctive kinetic properties of GDP-L-fucose transport and with the success of fucose therapy, which have been recently reported in one patient of Turkish origin. Accordingly, the biochemical properties of GDP-L-fucose transport into the Golgi are consistent with different variants of leukocyte adhesion deficiency type II that are probably the result of different molecular defects.