Position-specific N- and O-glycosylation of the reactive center loop impacts neutrophil elastase-mediated proteolysis of corticosteroid-binding globulin.

Corticosteroid-binding globulin (CBG) delivers anti-inflammatory cortisol to inflamed tissues through proteolysis of an exposed reactive center loop (RCL) by neutrophil elastase (NE). We previously demonstrated that RCL-localized Asn347-linked N-glycans impact NE proteolysis, but a comprehensive structure-function characterization of the RCL glycosylation is still required to better understand CBG glycobiology. Herein, we first performed RCL-centric glycoprofiling of serum-derived CBG to elucidate the Asn347-glycans and then used molecular dynamics simulations to study their impact on NE proteolysis. Importantly, we also identified O-glycosylation (di/sialyl T) across four RCL sites (Thr338/Thr342/Thr345/Ser350) of serum CBG close to the NE-targeted Val344-Thr345 cleavage site. A restricted N- and O-glycan co-occurrence pattern on the RCL involving exclusively Asn347 and Thr338 glycosylation was experimentally observed and supported in silico by modeling of a CBG-GalNAc-transferase (GalNAc-T) complex with various RCL glycans. GalNAc-T2 and GalNAc-T3 abundantly expressed by liver and gall bladder, respectively, showed in vitro a capacity to transfer GalNAc (Tn) to multiple RCL sites suggesting their involvement in RCL O-glycosylation. Recombinant CBG was then used to determine roles of RCL O-glycosylation through longitudinal NE-centric proteolysis experiments, which demonstrated that both sialoglycans (disialyl T) and asialoglycans (T) decorating Thr345 inhibit NE proteolysis. Synthetic RCL O-glycopeptides expanded on these findings by showing that Thr345-Tn and Thr342-Tn confer strong and moderate protection against NE cleavage, respectively. Molecular dynamics substantiated that short Thr345-linked O-glycans abrogate NE interactions. In conclusion, we report on biologically relevant CBG RCL glycosylation events, which improve our understanding of mechanisms governing cortisol delivery to inflamed tissues.

Glycoproteome remodeling and organelle-specific N-glycosylation accompany neutrophil granulopoiesis.

Neutrophils store microbicidal glycoproteins in cytosolic granules to fight intruding pathogens, but their granule distribution and formation mechanism(s) during granulopoiesis remain unmapped. Herein, we comprehensively profile the neutrophil N-glycoproteome with spatiotemporal resolution by analyzing four key types of intracellular organelles isolated from blood-derived neutrophils and during their maturation from bone marrow-derived progenitors using a glycomics-guided glycoproteomics approach. Interestingly, the organelles of resting neutrophils exhibited distinctive glycophenotypes including, most strikingly, highly truncated N-glycans low in α2,6-sialylation and Lewis fucosylation decorating a diverse set of microbicidal proteins (e.g., myeloperoxidase, azurocidin, neutrophil elastase) in the azurophilic granules. Excitingly, proteomics and transcriptomics data from discrete myeloid progenitor stages revealed that profound glycoproteome remodeling underpins the promyelocytic-to-metamyelocyte transition and that the glycophenotypic differences are driven primarily by dynamic changes in protein expression and less by changes within the glycosylation machinery. Notable exceptions were the oligosaccharyltransferase subunits responsible for initiation of N-glycoprotein biosynthesis that were strongly expressed in early myeloid progenitors correlating with relatively high levels of glycosylation of the microbicidal proteins in the azurophilic granules. Our study provides spatiotemporal insights into the complex neutrophil N-glycoproteome featuring intriguing organelle-specific N-glycosylation patterns formed by dynamic glycoproteome remodeling during the early maturation stages of the myeloid progenitors.

Membrane glycome is impacted by the cell culturing mode of neuroblastoma cells with differing migration and invasion potential.

Neuroblastoma is a highly metastatic childhood cancer for which studies indicate an association between protein glycosylation and tumor behavior. However, there is a lack of detailed glycome analysis on neuroblastoma cells that have varying metastatic potential. Furthermore, the impact of the cell culturing mode, i.e. 2-dimensional (2D) versus 3-dimensional (3D) spheroids, on the membrane protein glycome is unknown. To address these gaps in knowledge, we mapped membrane protein N- and O-glycosylation of neuroblastoma cells that have lower invasive and metastatic potential (Stathmin shRNA-expressing cells, StmnSeq2SH, and StmnSeq3SH) compared with control cells (control shRNA-expressing cells, CtrlSH). We showed that the neuroblastoma cells with different migratory and invasive potential underwent drastic changes in their membrane protein N-glycosylation exclusively when cultured in 3D spheroids. We also investigated the impact of 2D and 3D cell culture methods on cellular glycosylation using the neuroblastoma cells and found the cell N-glycome was markedly impacted by the culture method, with the 2D grown cells showing an abundance of oligomannosidic glycans, whereas 3D spheroids expressed more complex type glycans on their membrane proteins. In summary, this study provides the first comprehensive protein glycome profiling of neuroblastoma cells that have varying invasiveness and migratory potential and unravels the distinct membrane glycan features of cells that are grown under 2D versus 3D culture conditions.

N-acetyl-ß-D-hexosaminidases mediate the generation of paucimannosidic proteins via a putative noncanonical truncation pathway in human neutrophils.

We recently discovered that human neutrophils express immunomodulatory glycoproteins carrying unusual and highly truncated paucimannosidic N-glycans (Man1-3GlcNAc2Fuc0-1), but their biosynthesis remains elusive. Guided by the well-characterized truncation pathway in invertebrates and plants in which the N-acetyl-ß-D-hexosaminidase (Hex) isoenzymes catalyze paucimannosidic protein (PMP) formation, we here set out to test if the homologous human Hex α and ß subunits encoded by HEXA and HEXB drive a similar truncation pathway in human neutrophils. To this end, we performed quantitative glycomics and glycoproteomics of several CRISPR-Cas9-edited Hex-disrupted neutrophil-like HL-60 mutants (HEXA-KO and HEXB-KO) and matching unedited cell lines. Hex disruption was validated using next-generation sequencing, enzyme-linked immunosorbent assay (ELISA), quantitative proteomics and Hex activity assays. Excitingly, all Hex-disrupted mutants displayed significantly reduced levels of paucimannosylation, particularly Man2-3GlcNAc2Fuc1, relative to unedited HL-60 suggesting that both HEXA and HEXB contribute to PMP formation via a hitherto unexplored truncation pathway in neutrophils. Quantitative N-glycomics indeed demonstrated reduced utilization of a putative noncanonical truncation pathway in favor of the canonical elongation pathway in all Hex-disrupted mutants relative to unedited controls. Quantitative glycoproteomics recapitulated the truncation-to-elongation switch in all Hex-disrupted mutants and showed a greater switch for N-glycoproteins cotrafficking with Hex to the azurophilic granules of neutrophils such as myeloperoxidase. Finally, we supported the Hex-PMP relationship by documenting that primary neutrophils isolated from an early-onset Sandhoff disease patient (HEXB-/-) displayed dramatically reduced paucimannosylation relative to neutrophils from an age-matched unaffected donor. We conclude that both human Hex α and ß mediate PMP formation via a putative noncanonical truncation pathway in neutrophils.

Occurrence of free N-glycans with a single GlcNAc at the reducing termini in animal sera.

Recent studies demonstrated the occurrence of sialyl free N-glycans (FNGs) in sera from a variety of animals. Unlike the intracellular FNGs that mainly carry a single N-acetylglucosamine at their reducing termini (Gn1-type), these extracellular FNGs have an N,N'-diacetylchitobiose at their reducing termini (Gn2-type). The detailed mechanism for how they are formed, however, remains unclarified. In this study, we report on an improved method for isolating FNGs from sera and found that, not only sialyl FNGs, but also neutral FNGs are present in animal sera. Most of the neutral oligomannose-type FNGs were found to be Gn1-type. We also found that a small portion of sialyl FNGs were Gn1-type. The ratio of Gn1-type sialyl FNGs varies between species, and appears to be partially correlated with the distribution of lysosomal chitobiase activity. We also identified small sialylated glycans similar to milk oligosaccharides, such as sialyl lactose or sialyl N-acetyllactosamine in sera. Our results indicate that there are varieties of free oligosaccharides in sera and the mechanism responsible for their formation is more complicated than currently envisaged.

Glycan Profile Analysis of Engineered Trastuzumab with Rationally Added Glycosylation Sequons Presents Significantly Increased Glycan Complexity.

Protein aggregation constitutes a recurring complication in the manufacture and clinical use of therapeutic monoclonal antibodies (mAb) and mAb derivatives. Antibody aggregates can reduce production yield, cause immunogenic reactions, decrease the shelf-life of the pharmaceutical product and impair the capacity of the antibody monomer to bind to its cognate antigen. A common strategy to tackle protein aggregation involves the identification of surface-exposed aggregation-prone regions (APR) for replacement through protein engineering. It was shown that the insertion of N-glycosylation sequons on amino acids proximal to an aggregation-prone region can increase the physical stability of the protein by shielding the APR, thus preventing self-association of antibody monomers. We recently implemented this approach in the Fab region of full-size adalimumab and demonstrated that the thermodynamic stability of the Fab domain increases upon N-glycosite addition. Previous experimental data reported for this technique have lacked appropriate confirmation of glycan occupancy and structural characterization of the ensuing glycan profile. Herein, we mutated previously identified candidate positions on the Fab domain of Trastuzumab and employed tandem mass spectrometry to confirm attachment and obtain a detailed N-glycosylation profile of the mutants. The Trastuzumab glycomutants displayed a glycan profile with significantly higher structural heterogeneity compared to the HEK Trastuzumab antibody, which contains a single N-glycosylation site per heavy chain located in the CH2 domain of the Fc region. These findings suggest that Fab N-glycosites have higher accessibility to enzymes responsible for glycan maturation. Further, we have studied effects on additional glycosylation on protein stability via accelerated studies by following protein folding and aggregation propensities and observed that additional glycosylation indeed enhances physical stability and prevent protein aggregation. Our findings shed light into mAb glycobiology and potential implications in the application of this technique for the development of "biobetter" antibodies.

Proteomics of Campylobacter jejuni Growth in Deoxycholate Reveals Cj0025c as a Cystine Transport Protein Required for Wild-type Human Infection Phenotypes.

Campylobacter jejuni is a major cause of food-borne gastroenteritis. Proteomics by label-based two-dimensional liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) identified proteins associated with growth in 0.1% sodium deoxycholate (DOC, a component of gut bile salts), and system-wide validation was performed by data-independent acquisition (DIA-SWATH-MS). LC-MS/MS quantified 1326 proteins (∼82% of the predicted C. jejuni proteome), of which 1104 were validated in additional biological replicates by DIA-SWATH-MS. DOC resulted in a profound proteome shift with 512 proteins showing significantly altered abundance. Induced proteins were associated with flagellar motility and antibiotic resistance; and these correlated with increased DOC motility and resistance to polymyxin B and ciprofloxacin. DOC also increased human Caco-2 cell adherence and invasion. Abundances of proteins involved in nutrient transport were altered by DOC and aligned with intracellular changes to their respective carbon sources. DOC increased intracellular levels of sulfur-containing amino acids (cysteine and methionine) and the dipeptide cystine (Cys-Cys), which also correlated with reduced resistance to oxidative stress. A DOC induced transport protein was Cj0025c, which has sequence similarity to bacterial Cys-Cys transporters. Deletion of cj0025c (Δcj0025c) resulted in proteome changes consistent with sulfur starvation, as well as attenuated invasion, reduced motility, atypical morphology, increased antimicrobial susceptibility and poor biofilm formation. Targeted metabolomics showed Δcj0025c could use known C. jejuni amino and organic acid substrates commensurate with wild-type. Medium Cys-Cys levels however, were maintained in Δcj0025c relative to wild-type. A toxic Cys-Cys mimic (selenocystine) inhibited wild-type growth, but not Δcj0025c Provision of an alternate sulfur source (2 mm thiosulfate) restored Δcj0025c motility. Our data confirm that Cj0025c is a Cys-Cys transporter that we have named TcyP consistent with the nomenclature of homologous proteins in other species.

Enhancing the stability of adalimumab by engineering additional glycosylation motifs.

Monoclonal antibodies (mAbs) are of high value in the diagnostic and treatment of many debilitating diseases such as cancers, auto-immune disorders and infections. Unfortunately, protein aggregation is one of the ongoing challenges, limiting the development and application of mAbs as therapeutic products by decreasing half-life, increasing immunogenicity and reducing activity. We engineered an aggregation-prone region of adalimumab, the top selling mAb product worldwide - with additional glycosylation sites to enhance its resistance to aggregation by steric hindrance as a next generation biologic. We found that the addition of N-glycans in the Fab domain significantly enhanced its conformational stability, with some variants increasing the melting temperature of the Fab domain by >6 °C. The mutations tested had minimal impact on antigen binding affinity, or affinity to Fcγ receptors responsible for effector function. Our findings highlight the significant utility of this rational engineering approach for enhancing the conformational stability of therapeutic mAbs and other next-generation antibody formats.

Identifying the targets and functions of N-linked protein glycosylation in Campylobacter jejuni.

Campylobacter jejuni is a major cause of bacterial gastroenteritis in humans that is primarily associated with the consumption of inadequately prepared poultry products, since the organism is generally thought to be asymptomatic in avian species. Unlike many other microorganisms, C. jejuni is capable of performing extensive post-translational modification (PTM) of proteins by N- and O-linked glycosylation, both of which are required for optimal chicken colonization and human virulence. The biosynthesis and attachment of N-glycans to C. jejuni proteins is encoded by the pgl (protein glycosylation) locus, with the PglB oligosaccharyltransferase (OST) enabling en bloc transfer of a heptasaccharide N-glycan from a lipid carrier in the inner membrane to proteins exposed within the periplasm. Seventy-eight C. jejuni glycoproteins (represented by 134 sites of experimentally verified N-glycosylation) have now been identified, and include inner and outer membrane proteins, periplasmic proteins and lipoproteins, which are generally of poorly defined or unknown function. Despite our extensive knowledge of the targets of this apparently widespread process, we still do not fully understand the role N-glycosylation plays biologically, although several phenotypes, including wild-type stress resistance, biofilm formation, motility and chemotaxis have been related to a functional pgl system. Recent work has described enzymatic processes (nitrate reductase NapAB) and antibiotic efflux (CmeABC) as major targets requiring N-glycan attachment for optimal function, and experimental evidence also points to roles in cell binding via glycan-glycan interactions, protein complex formation and protein stability by conferring protection against host and bacterial proteolytic activity. Here we examine the biochemistry of the N-linked glycosylation system, define its currently known protein targets and discuss evidence for the structural and functional roles of this PTM in individual proteins and globally in C. jejuni pathogenesis.

Protein Paucimannosylation Is an Enriched N-Glycosylation Signature of Human Cancers.

While aberrant protein glycosylation is a recognized characteristic of human cancers, advances in glycoanalytics continue to discover new associations between glycoproteins and tumorigenesis. This glycomics-centric study investigates a possible link between protein paucimannosylation, an under-studied class of human N-glycosylation [Man1-3 GlcNAc2 Fuc0-1 ], and cancer. The paucimannosidic glycans (PMGs) of 34 cancer cell lines and 133 tissue samples spanning 11 cancer types and matching non-cancerous specimens are profiled from 467 published and unpublished PGC-LC-MS/MS N-glycome datasets collected over a decade. PMGs, particularly Man2-3 GlcNAc2 Fuc1 , are prominent features of 29 cancer cell lines, but the PMG level varies dramatically across and within the cancer types (1.0-50.2%). Analyses of paired (tumor/non-tumor) and stage-stratified tissues demonstrate that PMGs are significantly enriched in tumor tissues from several cancer types including liver cancer (p = 0.0033) and colorectal cancer (p = 0.0017) and is elevated as a result of prostate cancer and chronic lymphocytic leukaemia progression (p < 0.05). Surface expression of paucimannosidic epitopes is demonstrated on human glioblastoma cells using immunofluorescence while biosynthetic involvement of N-acetyl-ß-hexosaminidase is indicated by quantitative proteomics. This intriguing association between protein paucimannosylation and human cancers warrants further exploration to detail the biosynthesis, cellular location(s), protein carriers, and functions of paucimannosylation in tumorigenesis and metastasis.

N-Glycosylation influences human corticosteroid-binding globulin measurements.

OBJECTIVE: Discrepancies in ELISA measurements of human corticosteroid-binding globulin (CBG) using detection monoclonal antibodies that recognize an epitope (9G12) within its reactive center loop (RCL), versus an epitope (12G2) in a different location, have suggested that CBG with a proteolytically cleaved RCL exists in blood samples. We have previously been unable to verify this biochemically, and sought to determine if N-glycosylation differences account for discrepancies in ELISA measurements of CBG. METHODS AND SUBJECTS: Molecular biological, biochemical and glycopeptide analyses were used to examine how N-glycosylation at specific sites, including at N347 within the RCL, affect CBG ELISA or steroid-binding capacity assay (BCA) measurements. Plasma from patients with congenital disorders of glycosylation (CDG) was also examined in these assays as examples of N-glycosylation defects. RESULTS: We demonstrate that an N-glycan at N347 within the CBG RCL limits the 9G12 antibody from recognizing its epitope, whereas the 12G2 antibody reactivity is unaffected, thereby contributing to discrepancies in ELISA measurements using these two antibodies. Qualitative differences in N-glycosylation at N238 also negatively affect the steroid-binding of CBG in the absence of an N-glycan at N347 caused by a T349A substitution. Desialylation increased both ELISA measurements relative to BCA values. Similarly, plasma CBG levels in both ELISAs were much higher than BCA values in several CDG patients. CONCLUSIONS: Plasma CBG measurements are influenced by variations in N-glycosylation. This is important given the increasing number of CDG defects identified recently and because N-glycosylation abnormalities are common in patients with metabolic and liver diseases.

Asn347 Glycosylation of Corticosteroid-binding Globulin Fine-tunes the Host Immune Response by Modulating Proteolysis by Pseudomonas aeruginosa and Neutrophil Elastase.

Corticosteroid-binding globulin (CBG) delivers anti-inflammatory cortisol to inflamed tissues upon elastase-based proteolysis of the exposed reactive center loop (RCL). However, the molecular mechanisms that regulate the RCL proteolysis by co-existing host and bacterial elastases in inflamed/infected tissues remain unknown. We document that RCL-localized Asn(347) glycosylation fine-tunes the RCL cleavage rate by human neutrophil elastase (NE) and Pseudomonas aeruginosa elastase (PAE) by different mechanisms. NE- and PAE-generated fragments of native and exoglycosidase-treated blood-derived CBG of healthy individuals were monitored by gel electrophoresis and LC-MS/MS to determine the cleavage site(s) and Asn(347) glycosylation as a function of digestion time. The site-specific (Val(344)-Thr(345)) and rapid (seconds to minutes) NE-based RCL proteolysis was significantly antagonized by several volume-enhancing Asn(347) glycan features (i.e. occupancy, triantennary GlcNAc branching, and α1,6-fucosylation) and augmented by Asn(347) NeuAc-type sialylation (all p < 0.05). In contrast, the inefficient (minutes to hours) PAE-based RCL cleavage, which occurred equally well at Thr(345)-Leu(346) and Asn(347)-Leu(348), was abolished by the presence of Asn(347) glycosylation but was enhanced by sialoglycans on neighboring CBG N-sites. Molecular dynamics simulations of various Asn(347) glycoforms of uncleaved CBG indicated that multiple Asn(347) glycan features are modulating the RCL digestion efficiencies by NE/PAE. Finally, high concentrations of cortisol showed weak bacteriostatic effects toward virulent P. aeruginosa, which may explain the low RCL potency of the abundantly secreted PAE during host infection. In conclusion, site-specific CBG N-glycosylation regulates the bioavailability of cortisol in inflamed environments by fine-tuning the RCL proteolysis by endogenous and exogenous elastases. This study offers new molecular insight into host- and pathogen-based manipulation of the human immune system.

Micro- and macroheterogeneity of N-glycosylation yields size and charge isoforms of human sex hormone binding globulin circulating in serum.

Human sex hormone binding globulin (hSHBG) is a serum glycoprotein central to the transport and targeted delivery of sex hormones to steroid-sensitive tissues. Several molecular mechanisms of action of hSHBG, including the function of its attached glycans remain unknown. Here, we perform a detailed site-specific characterization of the N- and O-linked glycosylation of serum-derived hSHBG. MS-driven glycoproteomics and glycomics combined with exoglycosidase treatment were used in a bottom-up and top-down manner to determine glycosylation sites, site-specific occupancies and monosaccharide compositions, detailed glycan structures, and the higher level arrangement of glycans on intact hSHBG. It was found that serum-derived hSHBG is N-glycosylated at Asn(351) and Asn(367) with average molar occupancies of 85.1 and 95.3%, respectively. Both sites are occupied by the same six sialylated and partly core fucosylated bi- and triantennary N-Glycoforms with lactosamine-type antennas of the form (±NeuAcα6)Galß4GlcNAc. N-Glycoforms of Asn(367) were slightly more branched and core fucosylated than Asn(351) N-glycoforms due probably to a more surface-exposed glycosylation site. The N-terminal Thr(7) was fully occupied by the two O-linked glycans NeuAcα3Galß3(NeuAcα6)GalNAc (where NeuAc is N-acetylneuraminic acid and GalNAc is N-acetylgalactosamine) and NeuAcα3Galß3GalNAc in a 1:6 molar ratio. Electrophoretic analysis of intact hSHBG revealed size and charge heterogeneity of the isoforms circulating in blood serum. Interestingly, the size and charge heterogeneity were shown to originate predominantly from differential Asn(351) glycan occupancies and N-glycan sialylation that may modulate the hSHBG activity. To date, this work represents the most detailed structural map of the heterogeneous hSHBG glycosylation, which is a prerequisite for investigating the functional aspects of the hSHBG glycans.

N-glycans modulate the function of human corticosteroid-binding globulin.

Human corticosteroid-binding globulin (CBG), a heavily glycosylated protein containing six N-linked glycosylation sites, transports cortisol and other corticosteroids in blood circulation. Here, we investigate the biological importance of the N-glycans of CBG derived from human serum by performing a structural and functional characterization of CBG N-glycosylation. Liquid chromatography-tandem MS-based glycoproteomics and glycomics combined with exoglycosidase treatment revealed 26 complex type N-glycoforms, all of which were terminated with α2,3-linked neuraminic acid (NeuAc) residues. The CBG N-glycans showed predominantly bi- and tri-antennary branching, but higher branching was also observed. N-glycans from all six N-glycosylation sites were identified with high site occupancies (70.5-99.5%) and glycoforms from all sites contained a relatively low degree of core-fucosylation (0-34.9%). CBG showed site-specific glycosylation and the site-to-site differences in core-fucosylation and branching could be in silico correlated with the accessibility to the individual glycosylation sites on the maturely folded protein. Deglycosylated and desialylated CBG analogs were generated to investigate the biological importance of CBG N-glycans. As a functional assay, MCF-7 cells were challenged with native and glycan-modified CBG and the amount of cAMP, which is produced as a quantitative response upon CBG binding to its cell surface receptor, was used to evaluate the CBG:receptor interaction. The removal of both CBG N-glycans and NeuAc residues increased the production of cAMP significantly. This confirms that N-glycans are involved in the CBG:receptor interaction and indicates that the modulation is performed by steric and/or electrostatic means through the terminal NeuAc residues.