The N- and O-linked carbohydrate chains of human, bovine and porcine plasminogen. Species specificity in relation to sialylation and fucosylation patterns

Proteoform-Resolved Profiling of Plasminogen Activation Reveals Novel Abundant Phosphorylation Site and Primary N-Terminal Cleavage Site

Plasminogen (Plg), the zymogen of plasmin (Plm), is a glycoprotein involved in fibrinolysis and a wide variety of other physiological processes. Plg dysregulation has been implicated in a range of diseases. Classically, human Plg is categorized into two types, supposedly having different functional features, based on the presence (type I) or absence (type II) of a single N-linked glycan. Using high-resolution native mass spectrometry, we uncovered that the proteoform profiles of human Plg (and Plm) are substantially more extensive than this simple binary classification. In samples derived from human plasma, we identified up to 14 distinct proteoforms of Plg, including a novel highly stoichiometric phosphorylation site at Ser339. To elucidate the potential functional effects of these post-translational modifications, we performed proteoform-resolved kinetic analyses of the Plg-to-Plm conversion using several canonical activators. This conversion is thought to involve at least two independent cleavage events: one to remove the N-terminal peptide and another to release the active catalytic site. Our analyses reveal that these processes are not independent but are instead tightly regulated and occur in a step-wise manner. Notably, N-terminal cleavage at the canonical site (Lys77) does not occur directly from intact Plg. Instead, an activation intermediate corresponding to cleavage at Arg68 is initially produced, which only then is further processed to the canonical Lys77 product. Based on our results, we propose a refined categorization for human Plg proteoforms. In addition, we reveal that the proteoform profile of human Plg is more extensive than that of rat Plg, which lacks, for instance, the here-described phosphorylation at Ser339.

Key Matrix Remodeling Enzymes: Functions and Targeting in Cancer

Tissue functionality and integrity demand continuous changes in distribution of major components in the extracellular matrices (ECMs) under normal conditions aiming tissue homeostasis. Major matrix degrading proteolytic enzymes are matrix metalloproteinases (MMPs), plasminogen activators, atypical proteases such as intracellular cathepsins and glycolytic enzymes including heparanase and hyaluronidases. Matrix proteases evoke epithelial-to-mesenchymal transition (EMT) and regulate ECM turnover under normal procedures as well as cancer cell phenotype, motility, invasion, autophagy, angiogenesis and exosome formation through vital signaling cascades. ECM remodeling is also achieved by glycolytic enzymes that are essential for cancer cell survival, proliferation and tumor progression. In this article, the types of major matrix remodeling enzymes, their effects in cancer initiation, propagation and progression as well as their pharmacological targeting and ongoing clinical trials are presented and critically discussed.

Effects of Glycosylation on the Enzymatic Activity and Mechanisms of Proteases

Posttranslational modifications are an important feature of most proteases in higher organisms, such as the conversion of inactive zymogens into active proteases. To date, little information is available on the role of glycosylation and functional implications for secreted proteases. Besides a stabilizing effect and protection against proteolysis, several proteases show a significant influence of glycosylation on the catalytic activity. Glycans can alter the substrate recognition, the specificity and binding affinity, as well as the turnover rates. However, there is currently no known general pattern, since glycosylation can have both stimulating and inhibiting effects on activity. Thus, a comparative analysis of individual cases with sufficient enzyme kinetic and structural data is a first approach to describe mechanistic principles that govern the effects of glycosylation on the function of proteases. The understanding of glycan functions becomes highly significant in proteomic and glycomic studies, which demonstrated that cancer-associated proteases, such as kallikrein-related peptidase 3, exhibit strongly altered glycosylation patterns in pathological cases. Such findings can contribute to a variety of future biomedical applications.

Site-specific O-Glycosylation Analysis of Human Blood Plasma Proteins

Site-specific glycosylation analysis is key to investigate structure-function relationships of glycoproteins, e.g. in the context of antigenicity and disease progression. The analysis, though, is quite challenging and time consuming, in particular for O-glycosylated proteins. In consequence, despite their clinical and biopharmaceutical importance, many human blood plasma glycoproteins have not been characterized comprehensively with respect to their O-glycosylation. Here, we report on the site-specific O-glycosylation analysis of human blood plasma glycoproteins. To this end pooled human blood plasma of healthy donors was proteolytically digested using a broad-specific enzyme (Proteinase K), followed by a precipitation step, as well as a glycopeptide enrichment and fractionation step via hydrophilic interaction liquid chromatography, the latter being optimized for intact O-glycopeptides carrying short mucin-type core-1 and -2 O-glycans, which represent the vast majority of O-glycans on human blood plasma proteins. Enriched O-glycopeptide fractions were subjected to mass spectrometric analysis using reversed-phase liquid chromatography coupled online to an ion trap mass spectrometer operated in positive-ion mode. Peptide identity and glycan composition were derived from low-energy collision-induced dissociation fragment spectra acquired in multistage mode. To pinpoint the O-glycosylation sites glycopeptides were fragmented using electron transfer dissociation. Spectra were annotated by database searches as well as manually. Overall, 31 O-glycosylation sites and regions belonging to 22 proteins were identified, the majority being acute-phase proteins. Strikingly, also 11 novel O-glycosylation sites and regions were identified. In total 23 O-glycosylation sites could be pinpointed. Interestingly, the use of Proteinase K proved to be particularly beneficial in this context. The identified O-glycan compositions most probably correspond to mono- and disialylated core-1 mucin-type O-glycans (T-antigen). The developed workflow allows the identification and characterization of the major population of the human blood plasma O-glycoproteome and our results provide new insights, which can help to unravel structure-function relationships. The data were deposited to ProteomeXchange PXD003270.

Microfluidic Chip-LC/MS-based Glycomic Analysis Revealed Distinct N-glycan Profile of Rat Serum

The rat is an important alternative for studying human pathology owing to certain similarities to humans. Glycomic studies on rat serum have revealed that variations in the N-glycans of glycoproteins correlated with disease progression, which is consistent with the findings in human serum. Therefore, we comprehensively characterized the rat serum N-glycome using microfluidic chip-LC-ESI-QTOF MS and MS/MS techniques. In total, 282 N-glycans, including isomers, were identified. This study is the first to present comprehensive profiling of N-glycans containing O-acetylated sialic acid, among which 27 N-glycans are novel. In addition, the co-existence of N-acetylneuraminic acid (NeuAc) and N-glycolylneuraminic acid (NeuGc) in a single N-glycan ('mixed' N-glycan) was detected and represents a new type of N-glycan in rat serum. The existence of O-acetylated sialic acid is the characteristic feature of rat serum that distinguishes it from mouse and human sera. Comparisons between the rat, mouse, and human serum glycomes revealed that the rat glycome is more similar to that of human sera than to that of mouse sera. Our findings highlight the similarities between the glycomic profile of rat and human sera and provided important selection criteria for choosing an appropriate animal model for pathological and pharmacological studies.

The plasmin-antiplasmin system: structural and functional aspects

The plasmin-antiplasmin system plays a key role in blood coagulation and fibrinolysis. Plasmin and α(2)-antiplasmin are primarily responsible for a controlled and regulated dissolution of the fibrin polymers into soluble fragments. However, besides plasmin(ogen) and α(2)-antiplasmin the system contains a series of specific activators and inhibitors. The main physiological activators of plasminogen are tissue-type plasminogen activator, which is mainly involved in the dissolution of the fibrin polymers by plasmin, and urokinase-type plasminogen activator, which is primarily responsible for the generation of plasmin activity in the intercellular space. Both activators are multidomain serine proteases. Besides the main physiological inhibitor α(2)-antiplasmin, the plasmin-antiplasmin system is also regulated by the general protease inhibitor α(2)-macroglobulin, a member of the protease inhibitor I39 family. The activity of the plasminogen activators is primarily regulated by the plasminogen activator inhibitors 1 and 2, members of the serine protease inhibitor superfamily.

The plasminogen-like molecule apically secreted by epithelial thyroid cells is sulfated

Plasminogen (Pl), a circulating protease synthesized in the liver, is also present in several tissues. In the thyroid gland a Pl-like protease was found in the apical lumen where it is involved, through its proteolytic activity, in luminal degradation of thyroglobulin (Tg). Here, we showed for the first time that the Pl-like protease apically secreted by epithelial thyroid cells is sulfated, both on tyrosine residue(s) and on oligosaccharide side chains. The Pl molecule is composed of a large N-terminal moiety made of five distinct Kringle domains (K1-K5) separated by small peptidic fragments, and of a C-terminal domain with serine protease activity. Using a software tool able to predict tyrosine sulfation sites in protein sequences we localized the potential tyrosine sulfation sites of Pl. Then, we became aware that, whatever the species considered, at least three of the four potential tyrosine sulfation sites of Pl were located on Kringle sites, and more precisely, for K1, on the highly conserved binding domain of K1. We determined with the same software tool which potential sulfation sites were the most likely to be really sulfated. We hypothesize that the sulfation of these sites modulates the binding properties of Pl.

Evidence for a site-specific fucosylation of N-linked oligosaccharide of immunoglobulin A1 from normal human serum

Glycopeptides containing the N-linked oligosaccharide from human serum IgA1 were analyzed by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOFMS). Two glycopeptides, GP1 and GP2, prepared from the endoproteinase Asp-N digest of the IgA1 heavy chain, were derived from the CH2 domain (N-glycan site at Asn263) and the tailpiece portion (N-glycan site at Asn459), respectively. The structure of the attached sugar chain was deduced from the mass number of the glycopeptide and confirmed by a two-dimensional mapping technique for a pyridylaminated oligosaccharide. GP1 was composed of two major components having a fully galactosylated bianntena sugar chain with or without a bisecting N-acetylglucosamine (GlcNAc) residue. On the other hand, the GP2 fraction corresponded to the glycopeptides having a fully galactosylated and fucosylated bianntena sugar chain partly bearing a bisecting GlcNAc residue. Thus, the site-specific fucosylation of the N-linked oligosaccharide on the tailpiece of the alpha1 chain became evident for normal human serum IgA1.

Jacalin: a jackfruit (Artocarpus heterophyllus) seed-derived lectin of versatile applications in immunobiological research

Jacalin, the major protein from the jackfruit (Artocarpus heterophyllus) seeds, is a tetrameric two-chain lectin (molecular mass 65 kDa) combining a heavy alpha chain of 133 amino acid residues with a light beta chain of 20-21 amino acid residues. It is highly specific for the alpha-O-glycoside of the disaccharide Thomsen-Friedenreich antigen (Gal beta1-3GalNAc), even in its sialylated form. This property has made jacalin suitable for studying various O-linked glycoproteins, particularly human IgA1. Jacalin's uniqueness in being strongly mitogenic for human CD4+ T lymphocytes has made it a useful tool for the evaluation of the immune status of patients infected with human immunodeficiency virus (HIV)-1. The abundance of source material for the production of jacalin, its ease of purification, yield and stability have made it an attractive cost-effective lectin. It has found applications in diverse areas such as the isolation of human plasma glycoproteins (IgA1, C1-inhibitor, hemopexin, alpha2-HSG), the investigation of IgA-nephropathy, the analysis of O-linked glycoproteins and the detection of tumours.

Donor substrate specificities of Gal beta 1,4GlcNAc alpha 2,6-sialyltransferase and Gal beta 1,3GalNAc alpha 2,3-sialyltransferase: comparison of N-acetyl and N-glycolylneuraminic acids

Using cloned sialyltransferases, Gal beta 1,3GalNAc alpha 2,3-sialyltransferase (ST3Gal I) and Gal beta 1,4GlcNAc alpha 2,6-sialyltransferase (ST6Gal I) from both chicken and mouse, CMP-NeuAc and CMP-NeuGc were compared as donor substrates with pyridylamino-oligo-saccharides as acceptors. ST6Gal I showed 4-7-times higher activity toward CMP-NeuGc than CMP-NeuAc, while for ST3Gal I there was no significant difference between them, irrespective of the origin of the enzymes. Also, the difference in donor substrate (i.e., NeuAc and NeuGc) had little effect on the preference to acceptor substrates of these enzymes. Thus, the results showed that the cloned sialyltransferases can utilize both CMP-NeuAc and CMP-NeuGc as donor substrates, and that the preference difference between the sialyltransferases to CMP-NeuGc and CMP-NeuAc could, at least partly, explain the discrepancy in the ratio of NeuAc and NeuGc in glycolipids and glycoproteins in individual tissues.

Analysis of sialic acid-containing mucin oligosaccharides from porcine small intestine by high-temperature gas chromatography-mass spectrometry of their dimethylamides

Acetylation of sialic acid-containing oligosaccharides lactonises the sialic acid residue quantitatively for all oligosaccharides studied except for 6'-sialyl-lactose. The modified, unsulphated, sialylated and sulphated oligosaccharides can then be fractionated by anion-exchange chromatography. Ammonolysis of the lactones followed by methylation yielded the dimethylamides, which are amenable to g.l.c.-m.s. and give intense and informative mass spectra. This approach has been used to characterise the sialic acid-containing O-linked oligosaccharides obtained from the mucin glycopeptides of the small intestine of the pig. At least 28 structures were found, having NeuAc or NeuGc 6-linked to the HexNAc attached to the peptide core or to a Hex 3-linked to HexNAc. Four different disialylated oligosaccharides were found having NeuAc or NeuGc on the Hex residue 3-linked to HexNAc and 6-linked to HexNAc.

The Oligosaccharides of Glycoproteins: Bioprocess Factors Affecting Oligosaccharide Structure and their Effect on Glycoprotein Properties

In this review, we organize the recent data concerning the effects of bioprocess factors on the oligosaccharide structure of human therapeutic glycoproteins, with particular emphasis on the influence of the host cell. We also discuss the effect of oligosaccharide structure on glycoprotein properties, including antigenicity, immunogenicity and plasma clearance rate.

Isolation of glycopeptides containing O-linked oligosaccharides by lectin affinity chromatography on jacalin-agarose

Jacalin is a lectin which has high specificity and affinity for the core disaccharide, 1-beta-galactopyranosyl-3-(alpha-2-acetamido-2-deoxygalactopyranoside ), in O-linked oligosaccharides. Here, it is shown that this lectin can be used for isolation of glycopeptides bearing O-linked oligosaccharides. Peptides produced by digestion of reduced and carboxamidomethylated human plasminogen or of bovine protein Z were chromatographed on a column of jacalin-agarose. Reverse-phase high-performance liquid chromatography revealed that two peptides from plasminogen and one from protein Z were eluted from the jacalin-agarose column by alpha-methylgalactopyranoside. Amino acid sequence and compositional analysis showed that both of the peptides from plasminogen consisted of residues 330-357 and that the single peptide from protein Z represented residues 385-396. These sequences contain the single known site of attachment of O-linked oligosaccharides to these proteins. The present analysis suggested that there may be a fraction of plasminogen with two sites of O-linked glycosylation. The two tryptic peptides isolated from plasminogen represented the same segment of the protein but sequence analysis showed that one peptide was modified only at Thr346, the known site of glycosylation, and the other peptide contained a modification of Ser339 as well. Results of the present study indicate that lectin affinity chromatography using jacalin-agarose can be a useful technique for isolating glycopeptides containing O-linked oligosaccharides and thereby localizing sites of attachment of these oligosaccharides.

A study on the regulation of N-glycoloylneuraminic acid biosynthesis and utilization in rat and mouse liver

The relative contribution of N-glycoloyl-beta-D-neuraminic acid (Neu5Gc) to total sialic acids expressed in mouse and rat liver glycoconjugates was found to be 95% and 11%, respectively. This considerable difference in sialic acid composition made these two tissues suitable models for a comparative investigation into the regulation of Neu5Gc biosynthesis and utilization. An examination of the CMP-glycoside specificity of Golgi-associated sialyltransferases using CMP-N-acetyl-beta-D-neuraminic acid (CMP-Neu5Ac) and CMP-Neu5Gc revealed no significant tissue-dependent differences. The Golgi membrane CMP-sialic acid transport system from rat liver did, however, exhibit a slightly higher internalisation rate for CMP-Neu5Ac, though no preferential affinity for this sugar nucleotide over CMP-Neu5Gc was observed. In experiments, where Golgi membrane preparations were incubated with an equimolar mixture of labelled CMP-Neu5Ac and CMP-Neu5Gc, no significant tissue-dependent differences in [14C]sialic acid composition were observed, either in the luminal soluble sialic acid fraction or in the precipitable sialic acid fraction, results which are consistent with the above observations. From this experiment, evidence was also obtained for the presence of a Golgi-lumen-associated CMP--sialic acid hydrolase which exhibited no apparent specificity for either CMP-Neu5Ac or CMP-Neu5Gc. The specific activity of the CMP-Neu5Ac hydroxylase, the enzyme responsible for the biosynthesis of Neu5Gc, was found to be 28-fold greater in high-speed supernatants of mouse liver than of rat liver. No hydroxylase activity was detected in the Golgi membrane preparations. It is therefore proposed that the cytoplasmic ratio of CMP-Neu5Ac and CMP-Neu5Gc produced by the hydroxylase, remains largely unmodified after CMP-glycoside uptake into the Golgi apparatus and transfer on to growing glycoconjugate glycan chains. The close relationship between the total sialic acid composition and the sialic acid pattern in the CMP-glycoside pools of the tissues lends considerable weight to this hypothesis.

Lectin affinity chromatography of proteins bearing O-linked oligosaccharides: application of jacalin-agarose

The lectin jacalin immobilized on agarose was found to bind a variety of glycoproteins known to contain typical O-linked oligosaccharides, including human IgA, C1 inhibitor, chorionic gonadotropin, plasminogen, bovine protein Z, bovine coagulation factor X, and fetuin. These proteins were eluted from columns of jacalin-agarose specifically by alpha-galactopyranosides such as melibiose and alpha-methylgalactopyranoside but not by lactose or other sugars. Treatment of asialofetuin with endo--alpha--N--acetylgalactosaminidase eliminated its affinity for the lectin column, and other proteins known to contain only N-linked oligosaccharides such as ovalbumin, transferrin, and alpha 1-acid glycoprotein were not retained by the lectin. Binding of proteins with O-linked oligosaccharides to the lectin column did not require divalent cations and was affected little by changes in pH and ionic strength over a wide range. Virtually all of the glycosidically linked oligosaccharides of fetuin, chorionic gonadotropin, and plasminogen are known to be sialated. Thus, binding of these glycoproteins to jacalin, which is known to have affinity for the core disaccharide, 1-beta-galactopyranosyl-3-(alpha-2-acetamido-2-deoxygalactopyranoside ), in O-linked oligosaccharides of these proteins, was not prevented by the presence of sialic acids. Affinity of oligosaccharides for jacalin did appear to be reduced by occurrence of sialic acids as it was found that higher concentrations of melibiose were required to elute asialofetuin than fetuin from jacalin-agarose. Results of the present study indicate that affinity chromatography using this lectin is a widely applicable technique for identifying and purifying proteins bearing O-linked oligosaccharides.