Altered glycan structures: the molecular basis of congenital disorders of glycosylation

Deficiencies in subunits of the Conserved Oligomeric Golgi (COG) complex define a novel group of Congenital Disorders of Glycosylation

Processing of the glycan structures on glycoproteins by different glycosylation enzymes depends on, among other, the non-uniform distribution of these enzymes within the Golgi stacks. This compartmentalization is achieved by a balance between anterograde and retrograde vesicular trafficking. If the balance is disturbed, the glycosylation machinery is mislocalized, which can cause Congenital Disorders of Glycosylation type II (CDG-II), as illustrated by the identification of congenital defects in the Conserved Oligomeric Golgi (COG) complex in humans. We collected findings from different COG deficient cell types, such as CHO, yeast and human fibroblasts to hypothesize about structure and function of the COG complex, and compared the phenotypes and genotypes of the patients known with a COG deficiency. Among 35 CDG-II patients we found 5 patients with a COG defect. COG defects are a novel group of CDG-II with deficient N- as well as O-glycosylation.

Proteomics Reveals N-Linked Glycoprotein Diversity in Caenorhabditis elegans and Suggests an Atypical Translocation Mechanism for Integral Membrane Proteins

Protein glycosylation is one of the most common post-translational modifications in eukaryotes and affects various aspects of protein structure and function. To facilitate studies of protein glycosylation, we paired glycosylation site-specific stable isotope tagging of lectin affinity-captured N-linked glycopeptides with mass spectrometry and determined 1,465 N-glycosylated sites on 829 proteins expressed in Caenorhabditis elegans. The analysis shows the diversity of protein glycosylation in eukaryotes in terms of glycosylation sites and oligosaccharide structures attached to polypeptide chains and suggests the substrate specificity of oligosaccharyltransferase, a single multienzyme complex in C. elegans that incorporates an oligosaccharide moiety en bloc to newly synthesized polypeptides. In addition, topological analysis of 257 N-glycosylated proteins containing a putative single transmembrane segment that were identified based on the relative positions of glycosylation sites and transmembrane segments suggests that an atypical non-cotranslational mechanism translocates large N-terminal segments from the cytosol to the endoplasmic reticulum lumen in the absence of signal sequence function.

A la carte proteomics with an emphasis on gel-free techniques

Since the introduction of the proteome term somewhat more than a decade ago the field of proteomics witnessed a rapid growth mainly fueled by instrumental analytical improvements. Of particular notice is the advent of a diverse set of gel-free proteomics techniques. In this review, we discuss several of these gel-free techniques both for monitoring protein concentration changes and protein modifications, in particular protein phosphorylation, glycosylation, and protein processing. Furthermore, different approaches for (multiplexed) gel-free proteome analysis are discussed.

N-glycosylation site occupancy in serum glycoproteins using multiple reaction monitoring liquid chromatography-mass spectrometry

Congenital disorders of glycosylation (CDGs) are a family of N-linked glycosylation defects associated with severe clinical manifestations. In CDG type-I, deficiency of lipid-linked oligosaccharide assembly leads to the underoccupancy of N-glycosylation sites on glycoproteins. Although the level of residual glycosylation activity is known to correlate with the clinical phenotype linked to individual CDG mutations, it is not known whether the degree of N-glycosylation site occupancy by itself correlates with the severity of the disease. To quantify the extent of underglycosylation in healthy control and in CDG samples, we developed a quantitative method of N-glycosylation site occupancy based on multiple reaction monitoring LC-MS/MS. Using isotopically labeled standard peptides, we directly quantified the level of N-glycosylation site occupancy on selected serum proteins. In healthy control samples, we determined 98-100% occupancy for all N-glycosylation sites of transferrin and alpha(1)-antitrypsin. In CDG type-I samples, we observed a reduction in N-glycosylation site occupancy that correlated with the severity of the disease. In addition, we noticed a selective underglycosylation of N-glycosylation sites, indicating preferential glycosylation of acceptor sequons of a given glycoprotein. In transferrin, a preferred occupancy for the first N-glycosylation site was observed, and a decreasing preference for the first, third, and second N-glycosylation sites was observed in alpha(1)-antitrypsin. This multiple reaction monitoring LC-MS/MS method can be extended to multiple glycoproteins, thereby enabling a glycoproteomics survey of N-glycosylation site occupancies in biological samples.

Congenital Disorders of Glycosylation: A Rapidly Expanding Disease Family

Congenital disorders of glycosylation (CDG) are a large family of genetic diseases resulting from defects in the synthesis of glycans and in the attachment of glycans to other compounds. These disorders cause a wide range of human diseases, with examples emanating from all medical subspecialties. Since our 2001 review on CDG ( 36 ), this field has seen substantial growth: The number of N-glycosylation defects has doubled (from 6 to 12), five new O-glycosylation defects have been added to the two previously known ones, three combined N- and O-glycosylation defects have been identified, the first lipid glycosylation defects have been discovered, and a new domain, that of the hyperglycosylation defects, has been introduced. A number of CDG are due to defects in enzymes with a putative glycosyltransferase function. There is also a growing group of patients with unidentified defects (CDG-x), some with typical clinical presentations and others with presentations not seen before in CDG. This review focuses on the clinical, biochemical, and genetic characteristics of CDG and on advances expected in their future study and clinical management.

The liver in congenital disorders of glycosylation: ultrastructural features

A new group of genetic diseases characterized by defective glycoprotein biosynthesis was recently described. Transferrin isoelectric focusing enabled identification of several types of patients with congenital disorders of glycosylation (CDG). The authors report on the liver involvement in two siblings with CDG type Ix presenting with failure to thrive and hypertransaminasemia who developed cardiomyopathy. In the initially affected infant, liver biopsy at 13 months of age showed increased periportal cellularity, steatosis, and mild fibrosis. Ultrastructurally, the hepatocytes displayed numerous myelinosomes, mostly with a pericanalicular polarization. No myelinosomes were seen in the bile canaliculi, Kupffer cells, and sinusoidal lining cells. Focal large droplet steatosis was also noticed. These ultrastructural findings represent another diagnostic element in this heterogenic group of conditions. Electron microscopy can contribute to the elucidation of hypertransaminasemia and differentiate some types of CDG from other lysosomal diseases.

A rapid mass spectrometric strategy for the characterization of N- and O-glycan chains in the diagnosis of defects in glycan biosynthesis

Glycosylation of proteins is a very complex process which involves numerous factors such as enzymes or transporters. A defect in one of these factors in glycan biosynthetic pathways leads to dramatic disorders named congenital disorders of glycosylation (CDG). CDG can affect the biosynthesis of not only protein N-glycans but also O-glycans. The structural analysis of glycans on serum glycoproteins is essential to solving the defect. For this reason, we propose in this paper a strategy for the simultaneous characterization of both N- and O-glycan chains isolated from the serum glycoproteins. The serum (20 microL) is used for the characterization of N-glycans which are released by enzymatic digestion with PNGase F. O-glycans are chemically released by reductive elimination from whole serum glycoproteins using 10 microL of the serum. Using strategies based on mass spectrometric analysis, the structures of N- and O-glycan chains are defined. These strategies were applied on the sera from one patient with CDG type IIa, and one patient with a mild form of congenital disorder of glycosylation type II (CDG-II) that is caused by a deficiency in the Cog1 subunit of the complex.

High-Throughput Quantitative Profiling of Serum N-Glycome by MALDI-TOF Mass Spectrometry and N-Glycomic Fingerprint of Liver Fibrosis

Background: The use of MALDI-TOF mass spectrometry (MS) in quantitative glycan profiling has not been reported. In this study, we attempted to establish a high-throughput quantitative assay for profiling serum N-glycome, and we applied the new assay to identifying serum N-glycans for diagnosis of liver fibrosis and cirrhosis.

Methods: N-glycans from whole serum proteins in 2 μL serum were released by enzymatic digestion, cleaned up by hydrophilic chromatography, and subsequently quantitatively profiled with a linear MALDI-TOF MS system, which was originally designed for quantitative proteomic profiling. Serum N-glycome profiles from 46 patients with chronic hepatitis B infection and with different degrees of liver fibrosis were examined.

Results: The intra- and interassay CVs of peak intensities of the standard N-glycans were <8% and <17%, respectively. When the assay was applied to the analysis of serum N-glycome profiles, 17 peaks were found to be potential biomarkers for detection of liver fibrosis/cirrhosis. Linear regression analysis revealed that 4 peaks of 1341.5, 1829.7, 1933.3, and 2130.3 m/z (all P <0.005) had complementary value in detecting liver fibrosis and included them, but not any serological markers, in the diagnostic model. Leave-one-out cross-validation showed the diagnostic model could identify significant fibrosis (Ishak score ≥3) and cirrhosis (Ishak score ≥5), both at 85% accuracy.

Conclusion: This is the first study to illustrate the quantitative aspect of MALDI-TOF MS in N-glycome profiling and the first study to reveal the potential value of the serum N-glycan profile for identifying liver fibrosis.

Translation attenuation by PERK balances ER glycoprotein synthesis with lipid-linked oligosaccharide flux

Endoplasmic reticulum (ER) homeostasis requires transfer and subsequent processing of the glycan Glc₃Man₉GlcNAc₂ (G₃M₉Gn₂) from the lipid-linked oligosaccharide (LLO) glucose₃mannose₉N-acetylglucosamine₂-P-P-dolichol (G₃M₉Gn₂-P-P-Dol) to asparaginyl residues of nascent glycoprotein precursor polypeptides. However, it is unclear how the ER is protected against dysfunction from abnormal accumulation of LLO intermediates and aberrant N-glycosylation, as occurs in certain metabolic diseases. In metazoans phosphorylation of eukaryotic initiation factor 2α (eIF2α) on Ser⁵¹ by PERK (PKR-like ER kinase), which is activated by ER stress, attenuates translation initiation. We use brief glucose deprivation to simulate LLO biosynthesis disorders, and show that attenuation of polypeptide synthesis by PERK promotes extension of LLO intermediates to G₃M₉Gn₂-P-P-Dol under these substrate-limiting conditions, as well as counteract abnormal N-glycosylation. This simple mechanism requires eIF2α Ser⁵¹ phosphorylation by PERK, and is mimicked by agents that stimulate cytoplasmic stress-responsive Ser⁵¹ kinase activity. Thus, by sensing ER stress from defective glycosylation, PERK can restore ER homeostasis by balancing polypeptide synthesis with flux through the LLO pathway.

The Asn418-linked N-glycan of ErbB3 plays a crucial role in preventing spontaneous heterodimerization and tumor promotion

ErbB2 and ErbB3, two members of the ErbB family, form a high-affinity heregulin coreceptor that elicits potent mitogenic and transforming signals, and clinical studies indicate that these receptors play an important role in tumor incidence and progression. To determine whether N-glycosylation is involved in the function of ErbB3, a series of human ErbB3 molecules devoid of N-glycans were prepared and transfected to Flp-In-CHO cells for stable expression. A cross-linking study showed that the Asn(418) to Gln mutant (N418Q) of ErbB3 underwent autodimerization without its ligand, heregulin. The wild-type or N418Q mutant of ErbB3 was next coexpressed with ErbB2 in Flp-In-CHO cells, and the effect of N-glycan on heterodimerization was examined. The N418Q mutant of ErbB3 was autodimerized with ErbB2 without ligand stimulation, and receptor tyrosine phosphorylation and subsequent extracellular signal-regulated kinase (ERK) and Akt phosphorylation were promoted in the absence of heregulin. A cell proliferation assay and a soft agar colony formation assay showed that the N418Q mutant of ErbB3 coexpressed with ErbB2 promoted cell proliferation and colony formation in soft agar in an ERK- and Akt-dependent manner. The mutation also promoted the growth of tumors in athymic mice when injected s.c. These findings suggest that the Asn(418)-linked N-glycan in ErbB3 plays an essential role in regulating receptor heterodimerization with ErbB2 and might have an effect on transforming activity.

Dolichol-linked oligosaccharide selection by the oligosaccharyltransferase in protist and fungal organisms

The dolichol-linked oligosaccharide Glc₃Man₉GlcNAc₂-PP-Dol is the in vivo donor substrate synthesized by most eukaryotes for asparagine-linked glycosylation. However, many protist organisms assemble dolichol-linked oligosaccharides that lack glucose residues. We have compared donor substrate utilization by the oligosaccharyltransferase (OST) from Trypanosoma cruzi, Entamoeba histolytica, Trichomonas vaginalis, Cryptococcus neoformans, and Saccharomyces cerevisiae using structurally homogeneous dolichol-linked oligosaccharides as well as a heterogeneous dolichol-linked oligosaccharide library. Our results demonstrate that the OST from diverse organisms utilizes the in vivo oligo saccharide donor in preference to certain larger and/or smaller oligosaccharide donors. Steady-state enzyme kinetic experiments reveal that the binding affinity of the tripeptide acceptor for the protist OST complex is influenced by the structure of the oligosaccharide donor. This rudimentary donor substrate selection mechanism has been refined in fungi and vertebrate organisms by the addition of a second, regulatory dolichol-linked oligosaccharide binding site, the presence of which correlates with acquisition of the SWP1/ribophorin II subunit of the OST complex.

Simplification of Mass Spectral Analysis of Acidic Glycopeptides Using GlycoPep ID

Mass spectral analysis is an increasingly common method used to characterize glycoproteins. When more than one glycosylation site is present on a protein, obtaining MS data of glycopeptides is a highly effective way of obtaining glycosylation information because this approach can be used to identify not only what the carbohydrates are but also at which glycosylation site they are attached. Unfortunately, this is not yet a routine analytical approach, in part because data analysis can be quite challenging. We are developing strategies to simplify this analysis. Presented herein is a novel mass spectrometry technique that identifies the peptide moiety of either sulfated, sialylated, or both sialylated and sulfated glycopeptides. This technique correlates product ions in collision-induced dissociation (CID) experiments of suspected glycopeptides to a peptide composition using a newly developed web-based tool, GlycoPep ID. After identifying the peptide portion of glycopeptides with GlycoPep ID, the process of assigning the rest of the glycopeptide composition to the MS data is greatly facilitated because the "unknown" portion of the mass assignment that remains can be directly attributed to the carbohydrate component. Several examples of the utility and reliability of this method are presented herein.

Introduction of bisecting GlcNAc in N-glycans of adenylyl cyclase III enhances its activity

Adenylyl cyclases (ACs) catalyze the synthesis of cAMP in response to extracellular and intracellular signals and are responsible for a wide variety of biological activities including cell growth, differentiation, and metabolism. There are nine, currently known, isoforms of transmembrane ACs, and the primary structure of the catalytic unit and the potential N-glycosylation sites are highly conserved among them. The enzyme beta1,4-N-acetylglucosaminyltransferase III (GnT-III) catalyzes the addition of a bisecting N-acetylglucosamine (GlcNAc) to N-glycans. We have been studying the function of GnT-III on signaling molecules. In this study, we report on the effects of a bisecting GlcNAc on AC signaling. We established GnT-III stable expressing cell lines of Neuro-2a mouse neuroblastoma cells and B16 mouse melanoma cells. Forskolin-induced AC activation and downstream signaling, such as the synthesis of cAMP and the phosphorylation of transcriptional factor CRE-binding protein were upregulated in the GnT-III transfectants compared with mock transfectants or a dominant negative mutant of GnT-III-transfected cells. Since endogenous AC expression levels in Neuro-2a and B16 cells were too low to permit the glycosylation status to be examined, AC type III (ACIII) was overexpressed in a stable expression system using Flp-In-293 cells. The N-glycans of ACIII in the GnT-III transfectants were confirmed to be modified by the introduction of a bisecting GlcNAc, and AC activity was found to be significantly up-regulated in the GnT-III transfectants. Thus, the structure of N-glycans of ACIII regulates its enzymatic activity and downstream signaling.

Mass spectrometric identification of N-linked glycopeptides using lectin-mediated affinity capture and glycosylation site–specific stable isotope tagging

Protein post-translational modifications (PTMs), such as glycosylation and phosphorylation, are crucial for various signaling and regulatory events, and are therefore an important objective of proteomics research. We describe here a protocol for isotope-coded glycosylation site-specific tagging (IGOT), a method for the large-scale identification of N-linked glycoproteins from complex biological samples. The steps of this approach are: (1) lectin column-mediated affinity capture of glycopeptides generated by protease digestion of protein mixtures; (2) purification of the enriched glycopeptides by hydrophilic interaction chromatography (HIC); (3) peptide-N-glycanase-mediated incorporation of a stable isotope tag, 18O18O, specifically at the N-glycosylation site; and (4) identification of 18O-tagged peptides by liquid chromatography-coupled mass spectrometry (LC/MS)-based proteomics technology. The application of this protocol to the characterization of N-linked glycoproteins from crude extracts of the nematode Caenorhabditis elegans or mouse liver provides a list of hundreds to a thousand glycoproteins and their sites of glycosylation within a week.

Ribophorin I acts as a substrate-specific facilitator of N-glycosylation

The mammalian oligosaccharyltransferase (OST) complex is composed of about eight subunits and mediates the N-glycosylation of nascent polypeptide chains entering the endoplasmic reticulum (ER). The conserved STT3 subunit of eukaryotic OST complexes has been identified as its catalytic centre, yet although many other subunits are equally well conserved their functions are unknown. We used RNA interference to investigate the function of ribophorin I, an ER-translocon-associated subunit of the OST complex previously shown to associate with newly synthesised membrane proteins. We show that ribophorin I dramatically enhances the N-glycosylation of selected membrane proteins and provide evidence that it is not essential for N-glycosylation per se. Parallel studies confirm that STT3 is essential for transferase activity of the complex, but reveal that the two mammalian isoforms are not functionally equivalent when modifying bona fide polypeptide substrates. We propose a new model for OST function where ribophorin I acts as a chaperone or escort to promote the N-glycosylation of selected substrates by the catalytic STT3 subunits.

Metabolic oligosaccharide engineering: perspectives, applications, and future directions

Many adhesion and signaling molecules critical for development, as well as surface markers implicated in diseases ranging from cancer to influenza, contain oligosaccharides that modify their functions. Inside a cell, complex glycosylation pathways assemble these oligosaccharides and attach them to proteins and lipids as they traffic to the cell surface. Until recently, practical technologies to manipulate glycosylation have lagged unlike the molecular biologic and genetic methods available to intervene in nucleic acid and protein biochemistry; now, metabolic oligosaccharide engineering shows promise for manipulating glycosylation. In this methodology, exogenously-supplied non-natural sugars intercept biosynthetic pathways and exploit the remarkable ability of many of the enzymes involved in glycosylation to process metabolites with slightly altered chemical structures. To date, non-natural forms of sialic acid, GalNAc, GlcNAc, and fucose have been incorporated into glycoconjugates that appear on the cell surface; in addition O-GlcNAc protein modification involved in intracellular signaling has been tagged with modified forms of this sugar. Reactive functional groups, including ketones, azides, and thiols, have been incorporated into glycoconjugates and thereby provide chemical 'tags' that can be used for diverse purposes ranging from drug delivery to new modes of carbohydrate-based cell adhesion that can be used to control stem cell destiny. Finally, strategies for further engineering non-natural sugars to improve their pharmacological properties and provide complementary biological activities, such as addition of short chain fatty acids, are discussed in this article.

Complex oligosaccharides are N-linked to Kv3 voltage-gated K+ channels in rat brain

Neuronal Kv3 voltage-gated K(+) channels have two absolutely conserved N-glycosylation sites. Here, it is shown that Kv3.1, 3.3, and 3.4 channels are N-glycosylated in rat brain. Digestion of total brain membranes with peptide N glycosidase F (PNGase F) produced faster migrating immunobands than those of undigested membranes. Additionally, partial PNGase F digests showed that both sites are occupied by oligosaccharides. Neuraminidase treatment produced a smaller immunoband shift relative to PNGase F treatment. These results indicate that both sites are highly available and occupied by N-linked oligosaccharides for Kv3.1, 3.3, and 3.4 in rat brain, and furthermore that at least one oligosaccharide is of complex type. Additionally, these results point to an extracytoplasmic S1-S2 linker in Kv3 proteins expressed in native membranes. We suggest that N-glycosylation processing of Kv3 channels is critical for the expression of K(+) currents at the surface of neurons, and perhaps contributes to the pathophysiology of congenital disorders of glycosylation.

Respiratory distress and neonatal lethality in mice lacking Golgi alpha1,2-mannosidase IB involved in N-glycan maturation

There are three mammalian Golgi alpha1,2-mannosidases, encoded by different genes, that form Man5GlcNAc2 from Man(8-9)GlcNAc2 for the biosynthesis of hybrid and complex N-glycans. Northern blot analysis and in situ hybridization indicate that the three paralogs display distinct developmental and tissue-specific expression. The physiological role of Golgi alpha1,2-mannosidase IB was investigated by targeted gene ablation. The null mice have normal gross appearance at birth, but they display respiratory distress and die within a few hours. Histology of fetal lungs the day before birth indicate some delay in development, whereas neonatal lungs show extensive pulmonary hemorrhage in the alveolar region. No significant histopathological changes occur in other tissues. No remarkable ultrastructural differences are detected between wild type and null lungs. The membranes of a subset of bronchiolar epithelial cells are stained with lectins from Phaseolus vulgaris (leukoagglutinin and erythroagglutinin) and Datura stramonium in wild type lungs, but this staining disappears in lungs from null mice. Mass spectrometry of N-glycans from different tissues shows no significant changes in global N-glycans of null mice. Therefore, only a few glycoproteins required for normal lung function depend on alpha1,2-mannosidase IB for maturation. There are no apparent differences in the expression of several lung epithelial cell and endothelial cell markers between null and wild type mice. The alpha1,2-mannosidase IB null phenotype differs from phenotypes caused by ablation of other enzymes in N-glycan biosynthesis and from other mouse gene disruptions that affect pulmonary development and function.

Use of a 2,5-Dihydroxybenzoic Acid/Aniline MALDI Matrix for Improved Detection and On-Target Derivatization of Glycans: A Preliminary Report

N-Linked glycans derived from human and bovine alpha1-acid glycoprotein, as well as chicken egg white albumin, were analyzed by MALDI-TOF mass spectrometry using a novel MALDI matrix consisting of 2,5-dihydroxybenzoic acid (DHB) and aniline. A significant increase in signal was observed for these oligosaccharides relative to the signal obtained when unmodified DHB was used as a matrix for the same set of samples. The use of aniline/DHB matrix also led to facile on-target derivatization of the glycans via nonreductive amination, as aniline was found to form a stable Schiff base with the reducing end GlcNAc residue without the need for prolonged incubation periods and elevated temperatures. Both native and derivatized glycans ionized as sodium adducts and had similar MS/MS fragmentation patterns consisting mainly of Y/B-cleavage ions. In our experiments, we obtained evidence for persistence of the derivatization reaction in the solid phase; i.e., the reaction appeared to be taking place even after the sample-matrix spot had dried. This is the first report on such solid-phase on-target derivatization of carbohydrates for subsequent analysis by MALDI mass spectrometry.

The use of mass spectrometry for the proteomic analysis of glycosylation

Of all protein PTMs, glycosylation is by far the most common, and is a target for proteomic research. Glycosylation plays key roles in controlling various cellular processes and the modifications of the glycan structures in diseases highlight the clinical importance of this PTM. Glycosylation analysis remains a difficult task. MS, in combination with modern separation methodologies, is one of the most powerful and versatile techniques for the structural analysis of glycoconjugates. This review describes methodologies based on MS for detailed characterization of glycoconjugates in complex biological samples at the sensitivity required for proteomic work.

Fucosylation in prokaryotes and eukaryotes

Fucosylated carbohydrate structures are involved in a variety of biological and pathological processes in eukaryotic organisms including tissue development, angiogenesis, fertilization, cell adhesion, inflammation, and tumor metastasis. In contrast, fucosylation appears less common in prokaryotic organisms and has been suggested to be involved in molecular mimicry, adhesion, colonization, and modulating the host immune response. Fucosyltransferases (FucTs), present in both eukaryotic and prokaryotic organisms, are the enzymes responsible for the catalysis of fucose transfer from donor guanosine-diphosphate fucose to various acceptor molecules including oligosaccharides, glycoproteins, and glycolipids. To date, several subfamilies of mammalian FucTs have been well characterized; these enzymes are therefore delineated and used as models. Non-mammalian FucTs that possess different domain construction or display distinctive acceptor substrate specificity are highlighted. It is noteworthy that the glycoconjugates from plants and schistosomes contain some unusual fucose linkages, suggesting the presence of novel FucT subfamilies as yet to be characterized. Despite the very low sequence homology, striking functional similarity is exhibited between mammalian and Helicobacter pylori alpha1,3/4 FucTs, implying that these enzymes likely share a conserved mechanistic and structural basis for fucose transfer; such conserved functional features might also exist when comparing other FucT subfamilies from different origins. Fucosyltranferases are promising tools used in synthesis of fucosylated oligosaccharides and glycoconjugates, which show great potential in the treatment of infectious and inflammatory diseases and tumor metastasis.

Human dolichol kinase, a polytopic endoplasmic reticulum membrane protein with a cytoplasmically oriented CTP-binding site

Dolichol kinase (DK) catalyzes the CTP-dependent phosphorylation of dolichol in the biosynthesis de novo and possibly the recycling of dolichyl monophosphate in yeast and mammals. A cDNA clone from human brain encoding the mammalian homologue, hDKp, of the yeast enzyme has recently been identified. In this study hDK has been overexpressed in Chinese hamster ovary cells and shown to be a polytopic membrane protein localized in the endoplasmic reticulum with an N terminus extended into the lumen and a cytoplasmically oriented C terminus. A conserved sequence, DXXAXXXGXXXGX(8)KKTXEG, found in several enzymes utilizing CTP as substrate including DKs, phytol kinases, and several CDP-diacylglycerol synthetases has been identified, and the possibility that it is part of the CTP-binding domain of hDKp has been investigated. Topological studies indicate that the loop between transmembrane domains (TMD) 11 and TMD12 of hDKp, containing the putative CTP binding domain, faces the cytoplasm. Deletion of the loop between TMD11-12, hDK(Delta459-474), or mutation of selected conserved residues within the cytoplasmic loop results in either a partial or total loss of activity and significant reductions in the affinity for CTP. In addition, the SEC59 gene in the yeast DK mutant was sequenced, and a G420D substitution was found. Conversion of the corresponding residue Gly-443 in hDKp to aspartic acid resulted in inactivation of the mammalian enzyme. These results extend the information on the topological arrangement of hDKp and indicate that the cytoplasmic loop between TMDs 11-12, containing the critical conserved residues, lysine 470 and lysine 471 in the (470)KKTXEG(475) motif, is part of the CTP-binding site in hDK.

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.

Genetic defects in the human glycome

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

A new intronic mutation in the DPM1 gene is associated with a milder form of CDG Ie in two French siblings

Congenital disorders of glycosylation (CDG) type I (CDG I) are rare autosomal recessive diseases caused by deficiencies in the assembly of the dolichol-linked oligosaccharide (DLO) that is required for N-glycoprotein biosynthesis. CDG Ie is due to a defect in the synthesis of dolichyl-phosphoryl-mannose (Dol-P-Man), which is needed for DLO biosynthesis as well as for other glycosylation pathways. Human Dol-P-Man synthase is a heterotrimeric complex composed of DPM1p, DPM2p, and DPM3p, with DPM1p being the catalytic subunit. Here, we report two new CDG Ie patients who present milder symptoms than the five other CDG Ie patients described to date. The clinical pictures of the patients MS and his sister MT are dominated by major ataxia, with no notable hepatic involvement. MS cells accumulate the immature DLO species Dol-PP-GlcNAc2Man5 and possess only residual Dol-P-Man synthase activity. One homozygous intronic mutation, g.IVS4-5T>A, was found in the DPM1 gene, leading to exon skipping and transcription of a shortened transcript. Moreover, DPM1 expression was reduced by more than 90% in MS cells, in a nonsense-mediated mRNA decay (NMD)-independent manner. Full analysis of the DPM2 and DPM3 genes revealed a decrease in DPM2 expression and normal expression of DPM3. This description emphasizes the large spectrum of symptoms characterizing CDG I patients.

The congenital disorders of glycosylation: a multifaceted group of syndromes

The congenital disorders of glycosylation (CDG) are a rapidly expanding group of metabolic syndromes with a wide symptomatology and severity. They all stem from deficient N-glycosylation of proteins. To date the group contains 18 different subtypes: 12 of Type I (disrupted synthesis of the lipid-linked oligosaccharide precursor) and 6 of Type II (malfunctioning trimming/processing of the protein-bound oligosaccharide). Main features of CDG involve psychomotor retardation; ataxia; seizures; retinopathy; liver fibrosis; coagulopathies; failure to thrive; dysmorphic features, including inverted nipples and subcutaneous fat pads; and strabismus. No treatment currently is available for the vast majority of these syndromes (CDG-Ib and CDG-IIc are exceptions), even though attempts to synthesize drugs for the most common subtype, CDG-Ia, have been made. In this review we will discuss the individual syndromes, with focus on their neuronal involvement, available and possible treatments, and future directions.

Applied glycoproteomics—approaches to study genetic-environmental collisions causing protein-losing enteropathy

Protein-losing enteropathy (PLE), the loss of plasma proteins through the intestine, is a life-threatening symptom associated with seemingly unrelated conditions including Crohn's disease, congenital disorder of glycosylation, or Fontan surgery to correct univentricular hearts. Emerging commonalities between these and other disorders led us to hypothesize that PLE develops when genetic insufficiencies collide with simultaneous or sequential environmental insults. Most intriguing is the loss of heparan sulfate (HS) proteoglycans (HSPG) specifically from the basolateral surface of intestinal epithelial cells only during PLE episodes suggesting a direct link to protein leakage. Reasons for HSPG loss are unknown, but genetic insufficiencies affecting HSPG biosynthesis, trafficking, or degradation may be involved. Here, we describe cell-based assays we devised to identify key players contributing to protein leakage. Results from these assays confirm that HS loss directly causes protein leakage, but more importantly, it amplifies the effects of other factors, e.g., cytokines and increased pressure. Thus, HS loss appears to play a central role for PLE. To transfer our in vitro results back to the in vivo situation, we established methods to assess enteric protein leakage in mice and present several genetically deficient strains mimicking intestinal HS loss observed in PLE patients. Preliminary results indicate that mice with haploinsufficient genes involved in HS biosynthesis or HSPG trafficking develop intestinal protein leakage upon additional environmental stress. Our goal is to model PLE in vitro and in vivo to unravel the pathomechanisms underlying PLE, identify patients at risk, and provide them with a safe and effective therapy.

Mutant glycosyltransferases assist in the development of a targeted drug delivery system and contrast agents for MRI

The availability of structural information on glycosyltransferases is beginning to make structure-based reengineering of these enzymes possible. Mutant glycosyltransferases have been generated that can transfer a sugar residue with a chemically reactive unique functional group to a sugar moiety of glycoproteins, glycolipids, and proteoglycans (glycoconjugates). The presence of modified sugar moiety on a glycoprotein makes it possible to link bioactive molecules via modified glycan chains, thereby assisting in the assembly of bionanoparticles that are useful for developing the targeted drug delivery system and contrast agents for magnetic resonance imaging. The reengineered recombinant glycosyltransferases also make it possible to (1) remodel the oligosaccharide chains of glycoprotein drugs, and (2) synthesize oligosaccharides for vaccine development.

Mass spectrometry for congenital disorders of glycosylation, CDG

Congenital disorders of glycosylation (CDG) constitute a group of diseases affecting N-linked glycosylation pathways. The classical type of CDG, now called CDG-I, results from deficiencies in the early glycosylation pathway for biosynthesis of lipid-linked oligosaccharide and its transfer to proteins in endoplasmic reticulum, while the CDG-II diseases are caused by defects in the subsequent processing steps. Mass spectrometry (MS) produced a milestone in CDG research, by localizing the CDG-I defect to the early glycosylation pathway in 1992. Currently, MS of transferrin, either by electrospray ionization or matrix-assisted laser desorption/ionization, plays the central role in laboratory screening of CDG-I. On the other hand, the glycopeptide analysis recently developed for site-specific glycans of glycoproteins allows detailed glycan analysis in a high throughput manner and will solve problems in CDG-II diagnosis. These techniques will facilitate studying CDG, a field now expanding to O-linked glycosylation and to acquired as well as inherited conditions that can affect protein glycosylation.

Galactosemia: The good, the bad, and the unknown

Alpha-D-galactose is metabolized in species ranging from E. coli to mammals predominantly via a series of sequential reactions collectively known as the Leloir pathway. Deficiency of any one of these enzymes in humans results in a form of the inherited metabolic disorder, galactosemia, although the symptoms and severity depend upon the enzyme impaired, and the degree of functional deficiency (Tyfield and Walter, 2002, The Metabolic and Molecular Bases of Inherited Disease. New York: McGraw Hill.). Studies of these enzymes, and the disorders associated with their loss, have led to a much deeper appreciation of the intricate and interwoven levels of regulation that govern their normal function. These insights have further identified likely mediators of outcome severity in patients, and have enabled a rational approach to the development of novel strategies of intervention.

An evolving view of the eukaryotic oligosaccharyltransferase

Asparagine-linked glycosylation (ALG) is one of the most common protein modification reactions in eukaryotic cells, as many proteins that are translocated across or integrated into the rough endoplasmic reticulum (RER) carry N-linked oligosaccharides. Although the primary focus of this review will be the structure and function of the eukaryotic oligosaccharyltransferase (OST), key findings provided by the analysis of the archaebacterial and eubacterial OST homologues will be reviewed, particularly those that provide insight into the recognition of donor and acceptor substrates. Selection of the fully assembled donor substrate will be considered in the context of the family of human diseases known as congenital disorders of glycosylation (CDG). The yeast and vertebrate OST are surprisingly complex hetero-oligomeric proteins consisting of seven or eight subunits (Ost1p, Ost2p, Ost3p/Ost6p, Ost4p, Ost5p, Stt3p, Wbp1p, and Swp1p in yeast; ribophorin I, DAD1, N33/IAP, OST4, STT3A/STT3B, Ost48, and ribophorin II in mammals). Recent findings from several laboratories have provided overwhelming evidence that the STT3 subunit is critical for catalytic activity. Here, we will consider the evolution and assembly of the eukaryotic OST in light of recent genomic evidence concerning the subunit composition of the enzyme in diverse eukaryotes.