Glycosylation network mapping and site-specific glycan maturation in vivo.

Glycoprotein processing along a complex highly compartmentalized pathway is a hallmark of eukaryotic cells. We followed the kinetics of intracellular, site-specific glycan processing of a model protein with five distinct N-glycosylation sites and deduced a mathematical model of the secretory pathway that describes a complex set of processing reactions localized in defined intracellular compartments such as the endoplasmic reticulum the Golgi, or the lysosome. The model was able to accommodate site-specific N-glycan processing and we identified phosphorylated glycan structures of the mannose-6-phosphate pathway responsible for the lysosomal sorting of the glycoprotein. Importantly, our model protein can take different routes of the cellular secretory pathway, resulting in an increased glycan complexity of the secreted protein.

N-Glycosylation Enhances Conformational Flexibility of Protein Disulfide Isomerase Revealed by Microsecond Molecular Dynamics and Markov State Modeling.

Secreted proteins of eukaryotes are decorated with branched carbohydrate oligomers called glycans. This fact is only starting to be considered for in silico investigations of protein dynamics. Using all-atom molecular dynamics (MD) simulations and Markov state modeling (MSM), we unveil the influence of glycans on the conformational flexibility of the multidomain protein disulfide isomerase (PDI), which is a ubiquitous chaperone in the endoplasmic reticulum (ER). Yeast PDI (yPDI) from Saccharomyces cerevisiae is glycosylated at asparagine side chains and the knowledge of its five modified sites enables a realistic computational modeling. We compare simulations of glycosylated and unglycosylated yPDI and find that the presence of glycan-glycan and glycan-protein interactions influences the flexibility of PDI in different ways. For example, glycosylation reduces interdomain interactions, shifting the conformational ensemble toward more open, extended structures. In addition, we compare our results on yPDI with structural information of homologous proteins such as human PDI (hPDI), which is natively unglycosylated. Interestingly, hPDI lacks a surface recess that is present in yPDI. We find that glycosylation of yPDI facilitates its catalytic site to reach close to this surface recess. Hence, this might point to a possible functional relevance of glycosylation in yeast to act on substrates, while glycosylation seems redundant for the human homologous protein. We conclude that glycosylation is fundamental for protein dynamics, making it a necessity for a truthful representation of the flexibility and function in in silico studies of glycoproteins.

Glycan-protein interactions determine kinetics of N-glycan remodeling.

A hallmark of N-linked glycosylation in the secretory compartments of eukaryotic cells is the sequential remodeling of an initially uniform oligosaccharide to a site-specific, heterogeneous ensemble of glycostructures on mature proteins. To understand site-specific processing, we used protein disulfide isomerase (PDI), a model protein with five glycosylation sites, for molecular dynamics (MD) simulations and compared the result to a biochemical in vitro analysis with four different glycan processing enzymes. As predicted by an analysis of the accessibility of the N-glycans for their processing enzymes derived from the MD simulations, N-glycans at different glycosylation sites showed different kinetic properties for the processing enzymes. In addition, altering the tertiary structure of the glycoprotein PDI affected its N-glycan remodeling in a site-specific way. We propose that the observed differential N-glycan reactivities depend on the surrounding protein tertiary structure and lead to different glycan structures in the same protein through kinetically controlled processing pathways.

Mutations in the translocon-associated protein complex subunit SSR3 cause a novel congenital disorder of glycosylation.

The translocon-associated protein (TRAP) complex facilitates the translocation of proteins across the endoplasmic reticulum membrane and associates with the oligosaccharyl transferase (OST) complex to maintain proper glycosylation of nascent polypeptides. Pathogenic variants in either complex cause a group of rare genetic disorders termed, congenital disorders of glycosylation (CDG). We report an individual who presented with severe intellectual and developmental disabilities and sensorineural deafness with an unsolved type I CDG, and sought to identify the underlying genetic basis. Exome sequencing identified a novel homozygous variant c.278_281delAGGA [p.Glu93Valfs*7] in the signal sequence receptor 3 (SSR3) subunit of the TRAP complex. Biochemical studies in patient fibroblasts showed the variant destabilized the TRAP complex with a complete loss of SSR3 protein and partial loss of SSR1 and SSR4. Importantly, all subunit levels were corrected by expression of wild-type SSR3. Abnormal glycosylation status in fibroblasts was confirmed using two markers proteins, GP130 and ICAM1. Our findings confirm mutations in SSR3 cause a novel CDG. A novel frameshift variant in the translocon associated protein, SSR3, disrupts the stability of the TRAP complex and causes a novel Congenital Disorder of Glycosylation.

Quantitative Profiling of N-linked Glycosylation Machinery in Yeast Saccharomyces cerevisiae.

Asparagine-linked glycosylation is a common posttranslational protein modification regulating the structure, stability and function of many proteins. The N-linked glycosylation machinery involves enzymes responsible for the assembly of the lipid-linked oligosaccharide (LLO), which is then transferred to the asparagine residues on the polypeptides by the enzyme oligosaccharyltransferase (OST). A major goal in the study of protein glycosylation is to establish quantitative methods for the analysis of site-specific extent of glycosylation. We developed a sensitive approach to examine glycosylation site occupancy in Saccharomyces cerevisiae by coupling stable isotope labeling (SILAC) approach to parallel reaction monitoring (PRM) mass spectrometry (MS). We combined the method with genetic tools and validated the approach with the identification of novel glycosylation sites dependent on the Ost3p and Ost6p regulatory subunits of OST. Based on the observations that alternations in LLO substrate structure and OST subunits activity differentially alter the systemic output of OST, we conclude that sequon recognition is a direct property of the catalytic subunit Stt3p, auxiliary subunits such as Ost3p and Ost6p extend the OST substrate range by modulating interfering pathways such as protein folding. In addition, our proteomics approach revealed a novel regulatory network that connects isoprenoid lipid biosynthesis and LLO substrate assembly.

Influence of protein/glycan interaction on site-specific glycan heterogeneity.

To study how the interaction between N-linked glycans and the surrounding amino acids influences oligosaccharide processing, we used protein disulfide isomerase (PDI), a glycoprotein bearing 5 N-glycosylation sites, as a model system and expressed it transiently in a Chinese hamster ovary (CHO)-S cell line. PDI was produced as both secreted Sec-PDI and endoplasmic reticulum-retained glycoprotein (ER)-PDI, to study glycan processing by ER and Golgi resident enzymes. Quantitative site-specific glycosylation profiles were obtained, and flux analysis enabled modeling site-specific glycan processing. By altering the primary sequence of PDI, we changed the glycan/protein interaction and thus the site-specific glycoprofile because of the improved enzymatic fluxes at enzymatic bottlenecks. Our results highlight the importance of direct interactions between N-glycans and surface-exposed amino acids of glycoproteins on processing in the ER and the Golgi and the possibility of changing a site-specific N-glycan profile by modulating such interactions and thus the associated enzymatic fluxes. Altering the primary protein sequence can therefore be used to glycoengineer recombinant proteins.-Losfeld, M.-E., Scibona, E., Lin, C.-W., Villiger, T. K., Gauss, R., Morbidelli, M., Aebi, M. Influence of protein/glycan interaction on site-specific glycan heterogeneity.

Multiple phenotypes in phosphoglucomutase 1 deficiency.

BACKGROUND: Congenital disorders of glycosylation are genetic syndromes that result in impaired glycoprotein production. We evaluated patients who had a novel recessive disorder of glycosylation, with a range of clinical manifestations that included hepatopathy, bifid uvula, malignant hyperthermia, hypogonadotropic hypogonadism, growth retardation, hypoglycemia, myopathy, dilated cardiomyopathy, and cardiac arrest. METHODS: Homozygosity mapping followed by whole-exome sequencing was used to identify a mutation in the gene for phosphoglucomutase 1 (PGM1) in two siblings. Sequencing identified additional mutations in 15 other families. Phosphoglucomutase 1 enzyme activity was assayed on cell extracts. Analyses of glycosylation efficiency and quantitative studies of sugar metabolites were performed. Galactose supplementation in fibroblast cultures and dietary supplementation in the patients were studied to determine the effect on glycosylation. RESULTS: Phosphoglucomutase 1 enzyme activity was markedly diminished in all patients. Mass spectrometry of transferrin showed a loss of complete N-glycans and the presence of truncated glycans lacking galactose. Fibroblasts supplemented with galactose showed restoration of protein glycosylation and no evidence of glycogen accumulation. Dietary supplementation with galactose in six patients resulted in changes suggestive of clinical improvement. A new screening test showed good discrimination between patients and controls. CONCLUSIONS: Phosphoglucomutase 1 deficiency, previously identified as a glycogenosis, is also a congenital disorder of glycosylation. Supplementation with galactose leads to biochemical improvement in indexes of glycosylation in cells and patients, and supplementation with complex carbohydrates stabilizes blood glucose. A new screening test has been developed but has not yet been validated. (Funded by the Netherlands Organization for Scientific Research and others.).

The metabolic origins of mannose in glycoproteins.

Mannose in N-glycans is derived from glucose through phosphomannose isomerase (MPI, Fru-6-P ↔ Man-6-P) whose deficiency causes a congenital disorder of glycosylation (CDG)-Ib (MPI-CDG). Mannose supplements improve patients' symptoms because exogenous mannose can also directly contribute to N-glycan synthesis through Man-6-P. However, the quantitative contributions of these and other potential pathways to glycosylation are still unknown. We developed a sensitive GC-MS-based method using [1,2-(13)C]glucose and [4-(13)C]mannose to measure their contribution to N-glycans synthesized under physiological conditions (5 mm glucose and 50 µm mannose). Mannose directly provides ∼10-45% of the mannose found in N-glycans, showing up to a 100-fold preference for mannose over exogenous glucose based on their exogenous concentrations. Normal human fibroblasts normally derive 25-30% of their mannose directly from exogenous mannose, whereas MPI-deficient CDG fibroblasts with reduced glucose flux secure 80% of their mannose directly. Thus, both MPI activity and exogenous mannose concentration determine the metabolic flux into the N-glycosylation pathway. Using various stable isotopes, we found that gluconeogenesis, glycogen, and mannose salvaged from glycoprotein degradation do not contribute mannose to N-glycans in fibroblasts under physiological conditions. This quantitative assessment of mannose contribution and its metabolic fate provides information that can help bolster therapeutic strategies for treating glycosylation disorders with exogenous mannose.

A new congenital disorder of glycosylation caused by a mutation in SSR4, the signal sequence receptor 4 protein of the TRAP complex.

Nearly 50 congenital disorders of glycosylation (CDG) are known, but many patients biochemically diagnosed with CDG do not have mutations in known genes. Here, we describe a 16-year-old male who was born with microcephaly, developed intellectual disability, gastroesophageal reflux and a seizure disorder. We identified a de novo variant in the X-linked SSR4 gene which encodes a protein of the heterotetrameric translocon-associated protein (TRAP) complex. The c.316delT causes a p.F106Sfs*53 in SSR4 and also reduces expression of other TRAP complex proteins. The glycosylation marker Glyc-ER-GFP was used to confirm the underglycosylation in fibroblasts from the patient. Overexpression of the wild-type SSR4 allele partially restores glycosylation of the marker and of the other members of the TRAP complex. This is the first evidence that the TRAP complex, which binds to the oligosaccharyltransferase complex, is directly involved in N-glycosylation.

Mutations in STT3A and STT3B cause two congenital disorders of glycosylation.

We describe two unreported types of congenital disorders of glycosylation (CDG) which are caused by mutations in different isoforms of the catalytic subunit of the oligosaccharyltransferase (OST). Each isoform is encoded by a different gene (STT3A or STT3B), resides in a different OST complex and has distinct donor and acceptor substrate specificities with partially overlapping functions in N-glycosylation. The two cases from unrelated consanguineous families both show neurologic abnormalities, hypotonia, intellectual disability, failure to thrive and feeding problems. A homozygous mutation (c.1877T > C) in STT3A causes a p.Val626Ala change and a homozygous intronic mutation (c.1539 + 20G > T) in STT3B causes the other disorder. Both mutations impair glycosylation of a GFP biomarker and are rescued with the corresponding cDNA. Glycosylation of STT3A- and STT3B-specific acceptors is decreased in fibroblasts carrying the corresponding mutated gene and expression of the STT3A (p.Val626Ala) allele in STT3A-deficient HeLa cells does not rescue glycosylation. No additional cases were found in our collection or in reviewing various databases. The STT3A mutation significantly impairs glycosylation of the biomarker transferrin, but the STT3B mutation only slightly affects its glycosylation. Additional cases of STT3B-CDG may be missed by transferrin analysis and will require exome or genome sequencing.

A sensitive green fluorescent protein biomarker of N-glycosylation site occupancy.

N-glycosylation mediates many biological functions. Genetic defects in the N-glycosylation pathway cause >35 inherited human disorders called congenital disorders of glycosylation (CDGs). As a result, some N-glycosylation sites are unoccupied. Serum transferrin is a diagnostic marker for these patients, but there are no corresponding cellular markers to assess glycosylation competence. Therefore, we engineered a green fluorescent protein (GFP) construct to measure N-glycosylation site occupancy. We designed an endoplasmic reticulum-retained GFP biomarker whose fluorescence is lost when it is N-glycosylated due to steric hindrance by the glycan. This marker is a highly sensitive indicator of N-glycosylation site occupancy. In CDG cells carrying the GFP construct, a 25% decrease of glycosylation efficiency induces a 5-fold increase in fluorescence, while cDNA complementation of the genetic defect results in a 5-fold decrease in fluorescence. This engineered GFP detects impaired N-glycosylation in multiple cell lines, including CHO cells, HeLa cells, normal and patient fibroblasts, induced pluripotent stem cells (iPSCs), and human embryonic stem cells (hESCs). This marker is a highly sensitive tool to study N-glycosylation site occupancy. It can be used to screen for compounds that reverse poor N-glycosylation site occupancy.

Identification of intercellular cell adhesion molecule 1 (ICAM-1) as a hypoglycosylation marker in congenital disorders of glycosylation cells.

Many human inherited disorders cause protein N-glycosylation defects, but there are few cellular markers to test gene complementation for such defects. Plasma membrane glycoproteins are potential biomarkers because they may be reduced or even absent in plasma membranes of glycosylation-deficient cells. We combined stable isotope labeling by amino acids in cell culture (SILAC) with linear ion trap mass spectrometry (LTQ Orbitrap(TM)) to identify and quantify membrane proteins from wild-type CHO and glycosylation-deficient CHO (Lec9) cells. We identified 165 underrepresented proteins from 1447 unique quantified proteins, including 18 N-glycosylated plasma membrane proteins. Using various methods, we found that intercellular cell adhesion molecule 1 (ICAM-1) was reduced in Lec9 cells and in fibroblasts from 31 congenital disorder of glycosylation (CDG) patients compared with normal controls. Mannose supplementation of phosphomannose isomerase-deficient CDG-Ib (MPI-CDG) cells and complementation with PMM2 in PMM2-deficient CDG-Ia (PMM2-CDG) cells partially corrected hypoglycosylation based on increased ICAM-1 presence on the plasma membrane. These data indicate that ICAM-1 could be a useful hypoglycosylation biomarker to assess gene complementation of CDG-I patient cells and to monitor improved glycosylation in response to therapeutic drugs.

DDOST mutations identified by whole-exome sequencing are implicated in congenital disorders of glycosylation.

Congenital disorders of glycosylation (CDG) are inherited autosomal-recessive diseases that impair N-glycosylation. Approximately 20% of patients do not survive beyond the age of 5 years old as a result of widespread organ dysfunction. Although most patients receive a CDG diagnosis based on abnormal glycosylation of transferrin, this test cannot provide a genetic diagnosis; indeed, many patients with abnormal transferrin do not have mutations in any known CDG genes. Here, we combined biochemical analysis with whole-exome sequencing (WES) to identify the genetic defect in an untyped CDG patient, and we found a 22 bp deletion and a missense mutation in DDOST, whose product is a component of the oligosaccharyltransferase complex that transfers the glycan chain from a lipid carrier to nascent proteins in the endoplasmic reticulum lumen. Biochemical analysis with three biomarkers revealed that N-glycosylation was decreased in the patient's fibroblasts. Complementation with wild-type-DDOST cDNA in patient fibroblasts restored glycosylation, indicating that the mutations were pathological. Our results highlight the power of combining WES and biochemical studies, including a glyco-complementation system, for identifying and confirming the defective gene in an untyped CDG patient. This approach will be very useful for uncovering other types of CDG as well.

N-Glycosylation influences the structure and self-association abilities of recombinant nucleolin.

Nucleolin is a major nucleolar protein involved in fundamental processes of ribosome biogenesis, regulation of cell proliferation and growth. Nucleolin is known to shuttle between nucleus, cytoplasm and cell surface. We have previously found that nucleolin undergoes complex N- and O-glycosylations in extra-nuclear isoforms. We found that surface nucleolin is exclusively glycosylated and that N-glycosylation is required for its expression on the cells. Interestingly, the two N-glycans are located in the RNA-binding domains (RBDs) which participate in the self-association properties of nucleolin. We hypothesized that the occupancy of RBDs by N-glycans plays a role in these self-association properties. Here, owing to the inability to quantitatively produce full-size nucleolin, we expressed four N-glycosylation nucleolin variants lacking the N-terminal acidic domain in a baculovirus/insect cell system. As assessed by heptafluorobutyrate derivatization and mass spectrometry, this strategy allowed the production of proteins bearing or not paucimannosidic-type glycans on either one or two of the potential N-glycosylation sites. Their structure was investigated by circular dichroism and fluorimetry, and their ability to self-interact was analyzed by electrophoresis and surface plasmon resonance. Our results demonstrate that all nucleolin-derived variants are able to self-interact and that N-glycosylation on both RBD1 and RBD3, or RBD3 alone, but not RBD1 alone, modifies the structure of the N-terminally truncated nucleolin and enhances its self-association properties. In contrast, N-glycosylation does not modify interaction with lactoferrin, a ligand of cell surface nucleolin. Our results suggest that the occupancy of the N-glycosylation sites may contribute to expression and functions of surface nucleolin.

The cell surface expressed nucleolin is a glycoprotein that triggers calcium entry into mammalian cells.

Nucleolin is an ubiquitous nucleolar phosphoprotein involved in fundamental aspects of transcription regulation, cell proliferation and growth. It has also been described as a shuttling molecule between nucleus, cytosol and the cell surface. Several studies have demonstrated that surface nucleolin serves as a receptor for various extracellular ligands implicated in cell proliferation, differentiation, adhesion, mitogenesis and angiogenesis. Previously, we reported that nucleolin in the extranuclear cell compartment is a glycoprotein containing N- and O-glycans. In the present study, we show that glycosylation is an essential requirement for surface nucleolin expression, since it is prevented when cells are cultured in the presence of tunicamycin, an inhibitor of N-glycosylation. Accordingly, surface but not nuclear nucleolin is radioactively labeled upon metabolic labeling of cells with [(3)H]glucosamine. Besides its well-demonstrated role in the internalization of specific ligands, here we show that ligand binding to surface nucleolin could also induce Ca(2+) entry into cells. Indeed, by flow cytometry, microscopy and patch-clamp experiments, we show that the HB-19 pseudopeptide, which binds specifically surface nucleolin, triggers rapid and intense membrane Ca(2+) fluxes in various types of cells. The use of several drugs then indicated that Store-Operated Ca(2+) Entry (SOCE)-like channels are involved in the generation of these fluxes. Taken together, our findings suggest that binding of an extracellular ligand to surface nucleolin could be involved in the activation of signaling pathways by promoting Ca(2+) entry into cells.

Modulation of HSP70 GlcNAc-directed lectin activity by glucose availability and utilization.

It is well-accepted that protein quality control (occurring either after protein synthesis or after cell damage) is mainly ensured by HSP, but the mechanism by which HSP decides whether the protein will be degraded or not is poorly understood. Within this framework, it has been hypothesized that O-GlcNAc, a cytosolic and nuclear-specific glycosylation whose functions remain unclear, could take a part in the protection of proteins against degradation by modifying both the proteins themselves and the proteasome. Because the synthesis of O-GlcNAc is tightly correlated to glucose metabolism and Hsp70 was endowed with GlcNAc-binding property, we studied the relationship between GlcNAc-binding activity of both Hsp70 and Hsc70 (the nucleocytoplasmic forms of HSP70 family) and glucose availability and utilization. We thus demonstrated that low glucose concentration, inhibition of glucose utilization with 2DG, or inhibition of glucose transport with CytB led to an increase of Hsp70 and Hsc70 lectin activities. Interestingly, the response of Hsp70 and Hsc70 lectin activities toward variations of glucose concentration appeared different: Hsp70 lost its lectin activity when glucose concentration was >5 mM (i.e., physiological glucose concentration) in contrast to Hsc70 that exhibited a maximal lectin activity for glucose concentration approximately 5 mM and at high glucose concentrations. This work also demonstrates that HSP70 does not regulate its GlcNAc-binding properties through its own O-GlcNAc glycosylation.