Quantification of N-glycosylation site occupancy status based on labeling/label-free strategies with LC-MS/MS

Inclusion of deuterated glycopeptides provides increased sequence coverage in hydrogen/deuterium exchange mass spectrometry analysis of SARS-CoV-2 spike glycoprotein

RATIONALE

Hydrogen/deuterium exchange mass spectrometry (HDX-MS) can provide precise analysis of a protein's conformational dynamics across varied states, such as heat-denatured versus native protein structures, localizing regions that are specifically affected by such conditional changes. Maximizing protein sequence coverage provides high confidence that regions of interest were located by HDX-MS, but one challenge for complete sequence coverage is N-glycosylation sites. The deuteration of peptides post-translationally modified by asparagine-bound glycans (glycopeptides) has not always been identified in previous reports of HDX-MS analyses, causing significant sequence coverage gaps in heavily glycosylated proteins and uncertainty in structural dynamics in many regions throughout a glycoprotein.

METHODS

We detected deuterated glycopeptides with a Tribrid Orbitrap Eclipse mass spectrometer performing data-dependent acquisition. An MS scan was used to identify precursor ions; if high-energy collision-induced dissociation MS/MS of the precursor indicated oxonium ions diagnostic for complex glycans, then electron transfer low-energy collision-induced dissociation MS/MS scans of the precursor identified the modified asparagine residue and the glycan's mass. As in traditional HDX-MS, the identified glycopeptides were then analyzed at the MS level in samples labeled with D2 O.

RESULTS

We report HDX-MS analysis of the SARS-CoV-2 spike protein ectodomain in its trimeric prefusion form, which has 22 predicted N-glycosylation sites per monomer, with and without heat treatment. We identified glycopeptides and calculated their average isotopic mass shifts from deuteration. Inclusion of the deuterated glycopeptides increased sequence coverage of spike ectodomain from 76% to 84%, demonstrated that glycopeptides had been deuterated, and improved confidence in results localizing structural rearrangements.

CONCLUSION

Inclusion of deuterated glycopeptides improves the analysis of the conformational dynamics of glycoproteins such as viral surface antigens and cellular receptors.

Innovative Metrics for Reporting and Comparing the Glycan Structural Profile in Biotherapeutics

Glycosylation is a critical quality attribute in biotherapeutics, impacting properties such as protein stability, solubility, clearance rate, efficacy, immunogenicity, and safety. Due to the heterogenic and complex nature of protein glycosylation, comprehensive characterization is demanding. Moreover, the lack of standardized metrics for evaluating and comparing glycosylation profiles hinders comparability studies and the establishment of manufacturing control strategies. To address both challenges, we propose a standardized approach based on novel metrics for a comprehensive glycosylation fingerprint which greatly facilitates the reporting and objective comparison of glycosylation profiles. The analytical workflow is based on a liquid chromatography-mass spectrometry-based multi-attribute method. Based on the analytical data, a matrix of glycosylation-related quality attributes, both at site-specific and whole molecule level, are computed, which provide metrics for a comprehensive product glycosylation fingerprint. Two case studies illustrate the applicability of the proposed indices as a standardized and versatile approach for reporting all dimensions of the glycosylation profile. The proposed approach further facilitates the assessments of risks associated with changes in the glycosylation profile that may affect efficacy, clearance, and immunogenicity.

Glucose-mediated N-glycosylation of RPTPα affects its subcellular localization and Src activation

Receptor-type protein tyrosine phosphatase α (RPTPα) is one of the typical PTPs that play indispensable roles in many cellular processes associated with cancers. It has been considered as the most powerful regulatory oncogene for Src activation, however it is unclear how its biological function is regulated by post-translational modifications. Here, we show that the extracellular segment of RPTPα is highly N-glycosylated precisely at N21, N36, N68, N80, N86, N104 and N124 sites. Such N-glycosylation modifications mediated by glucose concentration alter the subcellular localization of RPTPα from Golgi apparatus to plasma membrane, enhance the interaction of RPTPα with Src, which in turn enhances the activation of Src and ultimately promotes tumor development. Our results identified the N-glycosylation modifications of RPTPα, and linked it to glucose starvation and Src activation for promoting tumor development, which provides new evidence for the potential antitumor therapy.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2017-2018

This review is the tenth update of the original article published in 1999 on the application of matrix-assisted laser desorption/ionization mass spectrometry (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2018. Also included are papers that describe methods appropriate to glycan and glycoprotein analysis by MALDI, such as sample preparation techniques, even though the ionization method is not MALDI. Topics covered in the first part of the review include general aspects such as theory of the MALDI process, new methods, matrices, derivatization, MALDI imaging, fragmentation and the use of arrays. The second part of the review is devoted to applications to various structural types such as oligo- and poly-saccharides, glycoproteins, glycolipids, glycosides, and biopharmaceuticals. Most of the applications are presented in tabular form. The third part of the review covers medical and industrial applications of the technique, studies of enzyme reactions, and applications to chemical synthesis. The reported work shows increasing use of combined new techniques such as ion mobility and highlights the impact that MALDI imaging is having across a range of diciplines. MALDI is still an ideal technique for carbohydrate analysis and advancements in the technique and the range of applications continue steady progress.

DeGlyPHER: Highly sensitive site-specific analysis of N-linked glycans on proteins

Traditional mass spectrometry-based glycoproteomic approaches have been widely used for site-specific N-glycoform analysis, but a large amount of starting material is needed to obtain sampling that is representative of the vast diversity of N-glycans on glycoproteins. These methods also often include a complicated workflow and very challenging data analysis. These limitations have prevented glycoproteomics from being adapted to high-throughput platforms, and the sensitivity of the analysis is currently inadequate for elucidating N-glycan heterogeneity in clinical samples. Heavily glycosylated spike proteins of enveloped viruses, recombinantly expressed as potential vaccines, are prime targets for glycoproteomic analysis. Since the immunogenicity of spike proteins may be impacted by their glycosylation patterns, site-specific analysis of N-glycoforms provides critical information for vaccine design. Using recombinantly expressed soluble HIV Env trimer, we describe DeGlyPHER, a modification of our previously reported sequential deglycosylation strategy to yield a "single-pot" process. DeGlyPHER is an ultrasensitive, simple, rapid, robust, and efficient approach for site-specific analysis of protein N-glycoforms, that we developed for analysis of limited quantities of glycoproteins.

Significantly Enhanced Thermostability of Aspergillus niger Xylanase by Modifying Its Highly Flexible Regions

In this study, the thermostability of an acid-resistant GH11 xylanase (xynA) from Aspergillus niger AG11 was enhanced through systematic modification of its four highly flexible regions (HFRs) predicted using MD simulations. Among them, HFR I (residues 92-100) and HFR II (residues 121-130) were modified by iterative saturation mutagenesis (ISM), yielding mutants G92F/G97S/G100K and T121V/A124P/I126V/T129L/A130N, respectively. For HFR III, the N-(residues 1-37) and C-termini (residues 179-188) were, respectively, substituted with the corresponding sequences from thermophilic EvXyn11^TS and Nesterenkonia xinjiangensis xylanase. N-Glycosylation was introduced into HFR IV (residues 50-70) through site-directed mutation (A55N/D57S/S61N) and the recombinant expression in A. niger AG11. Combining these positive mutations from each HFR yielded the variant xynAm1 with 137.6- and 1.3-fold increases in half-life at 50 °C and specific activity compared to the wild-type xynA, respectively. With the highest thermostability at 80 and 90 °C in reports, xynAm1 could be a robust candidate for industrial applications in functional foods, feed products, and bioethanol production.

Structural Characterization of RC28-E, a Recombinant Fusion Protein With Dual Targets on VEGF and FGF2

Vascular endothelial growth factor (VEGF) and fibroblast growthfactor (FGF) play important roles in angiogenesis-related diseases. RC28-E is a soluble fusion protein composed of the human VEGF receptor 1 (VEGFR1) extracellular domain 2 (ECD 2), VEGFR2 ECD 3, FGFR1 ECDs 2 and 3, and the Fc regions of human immunoglobulin G1. By targeting both VEGF and FGF2, RC28-E may represent a useful antiangiogenetic agent, but structural and functional characterizations of this fusion protein are needed. Liquid chromatography–tandem mass spectrometry, size exclusion high-performance liquid chromatography, capillary electrophoresis-sodium dodecyl sulfate, imaged capillary isoelectric focusing, and bio-layer interferometry were used to characterize the properties of RC28-E. The purity of RC28-E was confirmed to be 98% or greater. The glycosylation modification of RC28-E was found to be very complicated, with 11 potential N-linked glycosylation points and 23 types of N-glycans, causing high heterogeneity of the protein. The primary modifications of the amino acid sequence of RC28-E protein included C-terminal K truncation, N-deamidation, and M-oxidation modification. Notably, RC28-E demonstrated a higher affinity for both VEGF and FGF2 than VEGF trap or FGF trap for their respective targets.

Potential Biomarkers for Liver Cancer Diagnosis Based on Multi-Omics Strategy

Liver cancer is the fourth leading cause of cancer-related death worldwide. Hepatocellular carcinoma (HCC) accounts for about 85%-90% of all primary liver malignancies. However, only 20-30% of HCC patients are eligible for curative therapy mainly due to the lack of early-detection strategies, highlighting the significance of reliable and accurate biomarkers. The integration of multi-omics became an important tool for biomarker screening and unique alterations in tumor-associated genes, transcripts, proteins, post-translational modifications and metabolites have been observed. We here summarized the novel biomarkers for HCC diagnosis based on multi-omics technology as well as the clinical significance of these potential biomarkers in the early detection of HCC.

Glycosylation Heterogeneity of Hyperglycosylated Recombinant Human Interferon-β (rhIFN-β)

We previously developed a biobetter version of rhIFN-β (R27T) that possesses an additional glycosylation site compared with rhIFN-β 1a. Herein, we characterized N-glycosylation heterogeneity of R27T, which includes both N-glycan site occupancy heterogeneity (macro-heterogeneity) and complexity of carbohydrate moieties (micro-heterogeneity). N-glycan site occupancy manifested as distinct differences in size and isoelectric point. The analysis of complex carbohydrate moieties of R27T involved the common biopharmaceutical glycosylation critical quality attributes such as core fucosylation, antennary composition, sialylation, N-acetyllactosamine extensions, linkages, and overall glycan profiles using weak anion-exchange and hydrophilic interaction high-performance liquid chromatography with 2-aminobenzoic acid-labeled N-glycans. The double-glycosylated form accounted for approx. 94% R27T, while the single-glycosylated form accounted for 6% R27T. N-glycans consisted of a mixture of bi-, tri-, and tetra-antennary glycans, some with N-acetyllactosamine extensions, but neither outer arm fucose nor α-galactose was detected. Sialic acid major variants, N-acetyl- and N-glycolyl-neuraminic acid, were more abundant in R27T than in Rebif. The major N-glycan, accounting for ∼42% of total N-glycans, had a di-sialylated, core-fucosylated bi-antennary structure.

Monitoring the glycosylation of α-amino-3-hydroxy-5-methyl-4-isoxazole-propionate-type glutamate receptors using specific antibodies reveals a novel regulatory mechanism of N-glycosylation occupancy by molecular chaperones in mice

In the mammalian nervous system, protein N-glycosylation plays an important role in neuronal physiology. In this study, we performed a comprehensive N-glycosylation analysis of mouse GluA1, one of the major subunits of α-amino-3-hydroxy-5-methyl-4-isoxazole-propionate type glutamate receptor, which possesses six potential N-glycosylation sites in the N-terminal domain. By mass spectrometry-based analysis, we identified the N-glycoforms and semiquantitatively determined the site-specific N-glycosylation occupancy of GluA1. In addition, only the N401-glycosylation site demonstrated incomplete N-glycosylation occupancy. Therefore, we generated a peptide antibody that specifically detects the N401-glycan-free form to precisely quantify N401-glycosylation occupancy. Using this antibody, we clarified that N401 occupancy varies between cell types and increases in an age-dependent manner in mouse forebrains. To address the regulatory mechanism of N401-glycosylation, binding proteins of GluA1 around the N401 site were screened. HSP70 family proteins, including Bip, were identified as candidates. Bip has been known as a molecular chaperone that plays a key role in protein folding in the ER (endoplasmic reticulum). To examine the involvement of Bip in N401-glycosylation, the effect of Bip over-expression on N401 occupancy was evaluated in HEK293T cells, and the results demonstrated Bip increases the N401 glycan-free form by mediating selective prolongation of its protein half-life. Taken together, we propose that the N401-glycosite of GluA1 receives a unique control of modification, and we also propose a novel N-glycosylation occupancy regulatory mechanism by Bip that might be associated with α-amino-3-hydroxy-5-methyl-4-isoxazole-propionate receptors function in the brain.

Enhanced glycosylation of an S-layer protein enables a psychrophilic methanogenic archaeon to adapt to elevated temperatures in abundant substrates

Adaptation to higher temperatures would increase the environmental competitiveness of psychrophiles, organisms that thrive in low-temperature environments. Methanolobus psychrophilus, a cold wetland methanogen, 'evolved' as a mesophile, growing optimally at 30 °C after subculturings, and cells grown with ample substrates exhibited higher integrity. Here, we investigated N-glycosylation of S-layer proteins, the major archaeal envelope component, with respect to mesophilic adaptation. Lectin affinity enriched a glycoprotein in cells grown at 30 °C under ample substrate availability, which was identified as the S-layer protein Mpsy_1486. Four N-glycosylation sites were identified on Mpsy_1486, which exhibited different glycosylation profiles, with N94 only found in cells cultured at 30 °C. An N-linked glycosylation inhibitor, tunicamycin, reduced glycosylation levels of Mpsy_1486 and growth at 30 °C, thus establishing a link between S-layer protein glycosylation and higher temperature adaptation of the psychrophilic archaeon M. psychrophilus.

Mapping the Secretome and Its N-Linked Glycosylation of Pleurotus eryngii and Pleurotus ostreatus Grown on Hemp Stalks

Our previous research showed that Pleurotus eryngii and Pleurotus ostreatus were effective fungi for pretreatment of industrial hemp stalks to improve enzymatic saccharification. The secretomes of these two fungi were analyzed to search for the effective enzyme cocktails degrading hemp lignin during the pretreatment process. In total, 169 and 155 proteins were identified in Pleurotus eryngii and Pleurotus ostreatus, respectively, and 50% of the proteins involved in lignocellulose degradation were CAZymes. Because most of the extracellular proteins secreted by fungi are glycosylated proteins, the N-linked glycosylation of enzymes could be mapped. In total, 27 and 24 N-glycosylated peptides were detected in Pleurotus eryngii and Pleurotus ostreatus secretomes, respectively. N-Glycosylated peptides of laccase, GH92, exoglucanase, phenol oxidase, α-galactosidase, carboxylic ester hydrolase, and pectin lyase were identified. Deglycosylation could decrease enzymatic saccharification of hemp stalks. The activities of laccase, α-galactosidase, and phenol oxidase and the thermal stability of laccase were reduced after deglycosylation.

Development a hydrazide-functionalized thermosensitive polymer based homogeneous system for highly efficient N-glycoprotein/glycopeptide enrichment from human plasma exosome

As one of the most common post-translational modifications, protein N-glycosylation precipitates in many important biological processes and has closely correlations with the occurrence and progression of multiple diseases. Plasma exosomes secreted by cells contain various bioactive N-glycoproteins which may serve as potential biomarkers for early disease diagnosis and treatment. However, the protein N-glycosylation profile in human plasma exosome is largely unknown, due to the technical challenges in glycoprotein identification. Signals of the rare N-glycoproteins/N-glycopeptides are severely suppressed by the abundant coexisting non-glycosylated counterparts in mass spectrometry analysis. Therefore, specific enrichment of N-glycoprotein/glycopeptide is a prerequisite for large scale N-glycosylation profiling. In this work, we developed a hydrazide functionalized thermosensitive polymer for efficient enrichment and in-depth identification of protein N-glycosylation in human plasma exosome by mass spectrometry. The polymer chains completely dissolve in the enrichment system to form a homogeneous solution. Therefore, efficient covalent coupling between the N-glycoprotein/glycopeptide and the polymer chain is achieved, due to the reduced interfacial mass transfer resistance and the densely packed accessible functional groups on the polymer chains. Furthermore, the thermosensitive polymer can be easily precipitated and recovered by simply rising the system temperature to above 34 °C. As a result, 329 N-glycosylation sites corresponding to 180 N-glycoproteins were enriched and identified from plasma exosomes of glioma patients and healthy subjects using the thermosensitive polymer. By quantitative comparison, we found 26 N-glycoproteins significantly changed between the glioma patients and the healthy subjects, demonstrating the potential of this new strategy for N-glycoproteome research of plasma exosome and biomarker discovery.