A Linkage-specific Sialic Acid Labeling Strategy Reveals Different Site-specific Glycosylation Patterns in SARS-CoV-2 Spike Protein Produced in CHO and HEK Cell Substrates

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2021-2022

The use of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry for the analysis of carbohydrates and glycoconjugates is a well-established technique and this review is the 12th update of the original article published in 1999 and brings coverage of the literature to the end of 2022. As with previous review, this review also includes a few papers that describe methods appropriate to analysis by MALDI, such as sample preparation, even though the ionization method is not MALDI. The review follows the same format as previous reviews. It is divided into three sections: (1) general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation, quantification and the use of computer software for structural identification. (2) Applications to various structural types such as oligo- and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals, and (3) other general areas such as medicine, industrial processes, natural products and glycan synthesis where MALDI is extensively used. Much of the material relating to applications is presented in tabular form. MALDI is still an ideal technique for carbohydrate analysis, particularly in its ability to produce single ions from each analyte and advancements in the technique and range of applications show little sign of diminishing.

Examining sialic acid derivatives as potential inhibitors of SARS-CoV-2 spike protein receptor binding domain

Severe acute respiratory syndrome coronavirus 2 (SARS CoV-2) has been the primary reason behind the COVID-19 global pandemic which has affected millions of lives worldwide. The fundamental cause of the infection is the molecular binding of the viral spike protein receptor binding domain (SP-RBD) with the human cell angiotensin-converting enzyme 2 (ACE2) receptor. The infection can be prevented if the binding of RBD-ACE2 is resisted by utilizing certain inhibitors or drugs that demonstrate strong binding affinity towards the SP RBD. Sialic acid based glycans found widely in human cells and tissues have notable propensity of binding to viral proteins of the coronaviridae family. Recent experimental literature have used N-acetyl neuraminic acid (Sialic acid) to create diagnostic sensors for SARS-CoV-2, but a detailed interrogation of the underlying molecular mechanisms is warranted. Here, we perform all atom molecular dynamics (MD) simulations for the complexes of certain Sialic acid-based molecules with that of SP RBD of SARS CoV-2. Our results indicate that Sialic acid not only reproduces a binding affinity comparable to the RBD-ACE2 interactions, it also assumes the longest time to dissociate completely from the protein binding pocket of SP RBD. Our predictions corroborate that a combination of electrostatic and van der Waals energies as well the polar hydrogen bond interactions between the RBD residues and the inhibitors influence free energy of binding.Communicated by Ramaswamy H. Sarma.

Deep mutational scanning of H5 hemagglutinin to inform influenza virus surveillance

H5 influenza is a potential pandemic threat. Previous studies have identified molecular phenotypes of the viral hemagglutinin (HA) protein that contribute to pandemic risk, including cell entry, receptor preference, HA stability, and reduced neutralization by polyclonal sera. Here we use pseudovirus deep mutational scanning to measure how all mutations to a clade 2.3.4.4b H5 HA affect each phenotype. We identify mutations that allow HA to better bind a2-6-linked sialic acids, and show that some viruses already carry mutations that stabilize HA. We also identify recent viral strains with reduced neutralization to sera elicited by candidate vaccine virus. Overall, the systematic nature of deep mutational scanning combined with the safety of pseudoviruses enables comprehensive characterization of mutations to inform surveillance of H5 influenza.

Chemical inhibitors of hexokinase-2 enzyme reduce lactate accumulation, alter glycosylation processing, and produce altered glycoforms in CHO cell cultures

Chinese hamster ovary (CHO) cells, predominant hosts for recombinant biotherapeutics production, generate lactate as a major glycolysis by-product. High lactate levels adversely impact cell growth and productivity. The goal of this study was to reduce lactate in CHO cell cultures by adding chemical inhibitors to hexokinase-2 (HK2), the enzyme catalyzing the conversion of glucose to glucose 6-phosphate, and examine their impact on lactate accumulation, cell growth, protein titers, and N-glycosylation. Five inhibitors of HK2 enzyme at different concentrations were evaluated, of which 2-deoxy- d-glucose (2DG) and 5-thio- d-glucose (5TG) successfully reduced lactate accumulation with only limited impacts on CHO cell growth. Individual 2DG and 5TG supplementation led to a 35%-45% decrease in peak lactate, while their combined supplementation resulted in a 60% decrease in peak lactate. Inhibitor supplementation led to at least 50% decrease in moles of lactate produced per mol of glucose consumed. Recombinant EPO-Fc titers peaked earlier relative to the end of culture duration in supplemented cultures leading to at least 11% and as high as 32% increase in final EPO-Fc titers. Asparagine, pyruvate, and serine consumption rates also increased in the exponential growth phase in 2DG and 5TG treated cultures, thus, rewiring central carbon metabolism due to low glycolytic fluxes. N-glycan analysis of EPO-Fc revealed an increase in high mannose glycans from 5% in control cultures to 25% and 37% in 2DG and 5TG-supplemented cultures, respectively. Inhibitor supplementation also led to a decrease in bi-, tri-, and tetra-antennary structures and up to 50% lower EPO-Fc sialylation. Interestingly, addition of 2DG led to the incorporation of 2-deoxy-hexose (2DH) on EPO-Fc N-glycans and addition of 5TG resulted in the first-ever observed N-glycan incorporation of 5-thio-hexose (5TH). Six percent to 23% of N-glycans included 5TH moieties, most likely 5-thio-mannose and/or 5-thio-galactose and/or possibly 5-thio-N-acetylglucosamine, and 14%-33% of N-glycans included 2DH moieties, most likely 2-deoxy-mannose and/or 2-deoxy-galactose, for cultures treated with different concentrations of 5TG and 2DG, respectively. Our study is the first to evaluate the impact of these glucose analogs on CHO cell growth, protein production, cell metabolism, N-glycosylation processing, and formation of alternative glycoforms.

Formulation development and comparability studies with an aluminum-salt adjuvanted SARS-CoV-2 Spike ferritin nanoparticle vaccine antigen produced from two different cell lines

The development of safe and effective second-generation COVID-19 vaccines to improve affordability and storage stability requirements remains a high priority to expand global coverage. In this report, we describe formulation development and comparability studies with a self-assembled SARS-CoV-2 spike ferritin nanoparticle vaccine antigen (called DCFHP), when produced in two different cell lines and formulated with an aluminum-salt adjuvant (Alhydrogel, AH). Varying levels of phosphate buffer altered the extent and strength of antigen-adjuvant interactions, and these formulations were evaluated for their (1) in vivo performance in mice and (2) in vitro stability profiles. Unadjuvanted DCFHP produced minimal immune responses while AH-adjuvanted formulations elicited greatly enhanced pseudovirus neutralization titers independent of ∼100%, ∼40% or ∼10% of the DCFHP antigen adsorbed to AH. These formulations differed, however, in their in vitro stability properties as determined by biophysical studies and a competitive ELISA for measuring ACE2 receptor binding of AH-bound antigen. Interestingly, after one month of 4°C storage, small increases in antigenicity with concomitant decreases in the ability to desorb the antigen from the AH were observed. Finally, we performed a comparability assessment of DCFHP antigen produced in Expi293 and CHO cells, which displayed expected differences in their N-linked oligosaccharide profiles. Despite consisting of different DCFHP glycoforms, these two preparations were highly similar in their key quality attributes including molecular size, structural integrity, conformational stability, binding to ACE2 receptor and mouse immunogenicity profiles. Taken together, these studies support future preclinical and clinical development of an AH-adjuvanted DCFHP vaccine candidate produced in CHO cells.

Assign-MALDI - A free software for assignment of MALDI-TOF MS spectra of glycans derivatized using common and novel labeling strategies

Matrix Assisted Laser Desorption/Ionization Time-of-flight (MALDI-ToF) MS is a popular method to analyze glycans released from proteins, cell lines, and tissue samples. Chemical modification of glycans (derivatization) can enhance ionization, enable semi-quantitation, and assist in linkage identification. However, the mass changes incurred by novel and more recently developed derivatizations are not accommodated by most spectral assignment programs, necessitating manual assignment which increases both the difficultly and the likelihood of error. AssignMALDI is a software tool designed to create glycan databases with customized derivatizations (labels) and automatically assign glycan masses in MALDI-TOF spectra using the new database. It can also average peak intensities across multiple spectra and prepare publication-ready assignment tables. To make it easy to use with different platforms, all input files and most output files are in text format. An interactive display enables users to inspect and edit peak assignments prior to producing charts and tables for publication. The program is freely available through GitHUB and Python-savvy users can add or adjust features as needed.

Glycoproteomic Analyses of Influenza A Viruses Using timsTOF Pro MS

N-Linked glycosylation in hemagglutinin and neuraminidase glycoproteins of influenza viruses affects antigenic and receptor binding properties, and precise analyses of site-specific glycoforms in these proteins are critical in understanding the antigenic and immunogenic properties of influenza viruses. In this study, we developed a glycoproteomic approach by using a timsTOF Pro mass spectrometer (MS) to determine the abundance and heterogeneity of site-specific glycosylation for influenza glycoproteins. Compared with a Q Exactive HF MS, the timsTOF Pro MS method without the hydrophilic interaction liquid chromatography column enrichment achieved similar glycopeptide coverage and quantities but was more effective in identifying low-abundance glycopeptides. We quantified the distributions of intact site-specific glycopeptides in hemagglutinin of A/chicken/Wuxi/0405005/2013 (H7N9) and A/mute swan/Rhode Island/A00325125/2008 (H7N3). Results showed that hemagglutinin for both viruses had complex N-glycans at N22, N38, N240, and N483 but only high-mannose glycans at N411 and, however, that the type and quantities of glycans were distinct between these viruses. Collisional cross section (CCS) provided by the ion mobility spectrometry from the timsTOF Pro MS data differentiated sialylation linkages of the glycopeptides. In summary, timsTOF Pro MS method can quantify intact site-specific glycans for influenza glycoproteins without enrichment and thus facilitate influenza vaccine development and production.

Comprehensive N- and O-Glycoproteomic Analysis of Multiple Chinese Hamster Ovary Host Cell Lines

Glycoproteomic analysis of three Chinese hamster ovary (CHO) suspension host cell lines (CHO-K1, CHO-S, and CHO-Pro5) commonly utilized in biopharmaceutical settings for recombinant protein production is reported. Intracellular and secreted glycoproteins were examined. We utilized an immobilization and chemoenzymatic strategy in our analysis. Glycoproteins or glycopeptides were first immobilized through reductive amination, and the sialyl moieties were amidated for protection. The desired N- or O-glycans and glycopeptides were released from the immobilization resin by enzymatic or chemical digestion. Glycopeptides were studied by Orbitrap Liquid chromatography-mass spectrometry (LC/MS), and the released glycans were analyzed by Matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF). Differences were detected in the relative abundances of N- and O-glycopeptide types, their resident and released glycans, and their glycoprotein complexity. Ontogeny analysis revealed key differences in features, such as general metabolic and biosynthetic pathways, including glycosylation systems, as well as distributions in cellular compartments. Host cell lines and subfraction differences were observed in both N- and O-glycan and glycoprotein pools. Differences were observed in sialyl and fucosyl glycan distributions. Key differences were also observed among glycoproteins that are problematic contaminants in recombinant antibody production. The differences revealed in this study should inform the choice of cell lines best suited for a particular bioproduction application.

Structural- and Site-Specific N-Glycosylation Characterization of COVID-19 Virus Spike with StrucGP

The spike (S) protein plays a key role in COVID-19 (SARS-CoV-2) infection and host-cell entry. Previous studies have systematically analyzed site-specific glycan compositions as well as many important structural motifs of the S protein. Here, we further provide structural-clear N-glycosylation of the S protein at a site-specific level by using our recently developed structural- and site-specific N-glycoproteomics sequencing algorithm, StrucGP. In addition to the common N-glycans as detected in previous studies, many uncommon glycosylation structures such as LacdiNAc structures, Lewis structures, Mannose 6-phosphate (M6P) residues, and bisected core structures were unambiguously mapped at a total of 20 glycosites in the S protein trimer and protomer. These data further support the glycosylation structural-functional investigations of the COVID-19 virus spike.

Clinical and Diagnostic Significance of Sialic Acids Determination in Biological Material

Sialic acids (SA) are neuraminic acid derivatives, located at the terminal position in the chains of monosaccharide residues of various glycoconjugates. SA play a dual role: they either mask recognition sites, or, on the contrary, represent biological targets that can be recognized by receptor proteins and serve as ligands. The desialylation/sialylation processes can be considered as a dynamic modification regulated by sialyltransferases and sialidases in response to external or internal stimuli. This review describes the structural and functional diversity and the potential use of SA fractions as biomarkers for various pathological conditions. Almost any extreme impact on the body and inflammatory processes are accompanied by an increase in the level of both total and free SA in the blood and tissues. Possible reasons for the increase of sialoglycoconjugate metabolism indicators in biological material include: (i) activation of the hepatocyte synthesis and secretion of various acute-phase proteins, many of which are sialoglycoproteins, (ii) impaired membrane integrity and destruction of body cells, (iii) high activity of sialidases (neurominidases) and sialyltransferases. Most acute and chronic liver diseases are characterized by the decrease in the total level of SA in the blood serum (because many plasma proteins are synthesized and glycosylated in hepatocytes). Aberrant sialylation results in changes of sialoglycoconjugate structure, its ability to perform biological functions and sialoglycoconjugate half-life. Glycosylation is the most common post-translational modification of proteins in the virus, which not only promotes the formation of specific conformation of viral proteins, but also modulates their interaction with receptors and affects host cell recognition, viral replication and infectivity. Serum total SA concentration increases in some benign and inflammatory conditions, which indicates a lack of specificity and limits their use for early detection and screening of neoplastic diseases. Clinical and diagnostic value of determining the sialoglycoconjugate metabolic indicators, including changes in the content of both SA fractions and specific proteins in various biological fluids and tissues, consists in establishing the causes and mechanisms of biochemical changes in the body in certain diseases. In combination with the measurement of existing markers, they can be used to improve diagnosis, staging and monitoring of therapeutic response in some pathological conditions where the need for specificity is less than for specific diagnostics.

[Clinical and diagnostic significance of sialic acids determination in biological material]

Sialic acids (SA) are derivatives of neuraminic acid; they are located at the terminal position in the chains of monosaccharide residues of various glycoconjugates. SA play a dual role, they either mask recognition sites, or, on the contrary, represent biological targets that can be recognized by receptor proteins and serve as ligands. The desialylation/sialylation processes can be viewed as a dynamic modification regulated by sialyltransferases and sialidases in response to external or internal stimuli. This review describes the structural and functional diversity and the potential use of SA fractions as biomarkers for various pathological conditions. Almost any extreme effects on the body and inflammatory processes lead to an increase in the level of both total and free SA in the blood and tissues. Possible reasons for the increase of sialoglycoconjugate metabolism indicators in biological material include activation of the hepatocyte synthesis and secretion of various acute-phase proteins, many of which are sialoglycoproteins, violation of the membrane integrity and destruction of body cells, and also high activity of sialidases (neurominidases) and sialyltransferases. Most acute and chronic liver diseases are characterized by the decrease in the total level of SA in the blood serum (because many plasma proteins are synthesized and glycosylated in hepatocytes). Aberrant sialylation results in changes of sialoglycoconjugate structure, its ability to perform biological functions and half-life. Glycosylation is the most common post-translational modification of proteins in the virus, which not only promotes the formation of specific conformation of viral proteins, but also modulates their interaction with receptors and affects host cell recognition, viral replication and infectivity. Serum total SA concentration increases in some benign and inflammatory conditions, which indicates a lack of specificity and limits their use for early detection and screening of neoplastic diseases. Nevertheless, determining blood SA level and measuring concentration of existing biomarkers can be used to improve diagnostic indicators, to stage and monitor therapeutic response in some types of cancer, when the need for specificity is less than for diagnosis. Clinical and diagnostic value of determining the sialoglycoconjugate metabolic indicators, including changes in the content of both SA fractions and specific proteins in various biological fluids and tissues, lies in establishing the causes and mechanisms of biochemical changes in the body in certain diseases.

Site-Specific Glycosylation Patterns of the SARS-CoV-2 Spike Protein Derived From Recombinant Protein and Viral WA1 and D614G Strains

The SARS-CoV-2 spike protein is heavily glycosylated, having 22 predicted N-glycosylation sites per monomer. It is also O-glycosylated, although the number of O-glycosites is less defined. Recent studies show that spike protein glycans play critical roles in viral entry and infection. The spike monomer has two subdomains, S1 and S2, and a receptor-binding domain (RBD) within the S1 domain. In this study, we have characterized the site-specific glycosylation patterns of the HEK293 recombinant spike RBD and S1 domains as well as the intact spike derived from the whole virus produced in Vero cells. The Vero cell-derived spike from the WA1 strain and a D614G variant was analyzed. All spike proteins, S1, and RBDs were analyzed using hydrophilic interaction chromatography (HILIC) and LC-MS/MS on an Orbitrap Eclipse Tribrid mass spectrometer. N-glycans identified in HEK293-derived S1 were structurally diverse. Those found in the HEK293-derived RBD were highly similar to those in HEK293 S1 where N-glycosites were shared. Comparison of the whole cell-derived WA1 and D614G spike proteins revealed that N-glycosites local to the mutation site appeared to be more readily detected, hinting that these sites are more exposed to glycosylation machinery. Moreover, recombinant HEK293-derived S1 was occupied almost completely with complex glycan, while both WA1 and D614G derived from the Vero E6 cell whole virus were predominantly high-mannose glycans. This stands in stark contrast to glycosylation patterns seen in both CHO- and HEK cell-derived recombinant S1, S2, and the whole spike previously reported. Concerning O-glycosylation, our analyses revealed that HEK293 recombinant proteins possessed a range of O-glycosites with compositions consistent with Core type 1 and 2 glycans. The O-glycosites shared between the S1 and RBD constructs, sites T323 and T523, were occupied by a similar range of Core 1 and 2 type O-glycans. Overall, this study reveals that the sample nature and cell substrate used for production of these proteins can have a dramatic impact on the glycosylation profile. SARS-CoV-2 spike glycans are associated with host ACE2 receptor interaction efficiency. Therefore, understanding such differences will serve to better understand these host–pathogen interactions and inform the choice of cell substrates to suite downstream investigations.