Organ-specific heparan sulfate structural phenotypes

Glycomics analysis of mammalian heparan sulfates modified by the human extracellular sulfatase HSulf2

Background

The Sulfs are a family of endosulfatases that selectively modify the 6O-sulfation state of cell-surface heparan sulfate (HS) molecules. Sulfs serve as modulators of cell-signaling events because the changes they induce alter the cell surface co-receptor functions of HS chains. A variety of studies have been aimed at understanding how Sulfs modify HS structure, and many of these studies utilize Sulf knockout cell lines as the source for the HS used in the experiments. However, genetic manipulation of Sulfs has been shown to alter the expression levels of HS biosynthetic enzymes, and in these cases an assessment of the fine structural changes induced solely by Sulf enzymatic activity is not possible. Therefore, the present work aims to extend the understanding of substrate specificities of HSulf2 using in vitro experiments to compare HSulf2 activities on HS from different organ tissues.

Methodology/Principal Findings

To further the understanding of Sulf enzymatic activity, we conducted in vitro experiments where a variety of mammalian HS substrates were modified by recombinant human Sulf2 (HSulf2). Subsequent to treatment with HSulf2, the HS samples were exhaustively depolymerized and analyzed using size-exclusion liquid chromatography-mass spectrometry (SEC-LC/MS). We found that HSulf2 activity was highly dependent on the structural features of the HS substrate. Additionally, we characterized, for the first time, the activity of HSulf2 on the non-reducing end (NRE) of HS chains. The results indicate that the action pattern of HSulf2 at the NRE is different compared to internally within the HS chain.

Conclusions/Significance

The results of the present study indicate that the activity of Sulfs is dependent on the unique structural features of the HS populations that they edit. The activity of HSulf2 at HS NREs implicates the Sulfs as key regulators of this region of the chains, and concomitantly, the protein-binding events that occur there.

Analysis of Glycosaminoglycans Using Mass Spectrometry

The glycosaminoglycans (GAGs) are linear polysaccharides expressed on animal cell surfaces and in extracellular matrices. Their biosynthesis is under complex control and confers a domain structure that is essential to their ability to bind to protein partners. Key to understanding the functions of GAGs are methods to determine accurately and rapidly patterns of sulfation, acetylation and uronic acid epimerization that correlate with protein binding or other biological activities. Mass spectrometry (MS) is particularly suitable for the analysis of GAGs for biomedical purposes. Using modern ionization techniques it is possible to accurately determine molecular weights of GAG oligosaccharides and their distributions within a mixture. Methods for direct interfacing with liquid chromatography have been developed to permit online mass spectrometric analysis of GAGs. New tandem mass spectrometric methods for fine structure determination of GAGs are emerging. This review summarizes MS-based approaches for analysis of GAGs, including tissue extraction and chromatographic methods compatible with LC/MS and tandem MS.

Glycosaminoglycans of the porcine central nervous system

Glycosaminoglycans (GAGs) are known to participate in central nervous system processes such as development, cell migration, and neurite outgrowth. In this paper, we report an initial glycomics study of GAGs from the porcine central nervous system. GAGs of the porcine central nervous system, brain and spinal cord were isolated and purified by defatting, proteolysis, anion-exchange chromatography, and methanol precipitation. The isolated GAG content in brain was 5 times higher than in spinal cord (0.35 mg/g of dry sample, compared to 0.07 mg/g of dry sample). In both tissues, chondroitin sulfate (CS) and heparan sulfate (HS) were the major and the minor GAG, respectively. The average molecular masses of CS from brain and spinal cord were 35.5 and 47.1 kDa, respectively, and those for HS from brain and spinal cord were 56.9 and 34 kDa, respectively. The disaccharide analysis showed that the compositions of CS from brain and spinal cords are similar, with uronic acid (1→3) 4-O-sulfo-N-acetylgalactosamine residue corresponding to the major disaccharide unit (CS type A) along with five minor disaccharide units. The major disaccharides of both brain and spinal cord HS were uronic acid (1→4) N-acetylglucosamine and uronic acid (1→4) 6-O-sulfo-N-sulfoglucosamine, but their composition of minor disaccharides differed. Analysis by (1)H and two-dimensional NMR spectroscopy confirmed these disaccharide analyses and provided the glucuronic/iduronic acid ratio. Finally, both purified CS and HS were biotinylated and immobilized on BIAcore SA biochips. Interactions between these GAGs and fibroblast growth factors (FGF1 and FGF2) and sonic hedgehog (Shh) were investigated by surface plasmon resonance.

No haploinsufficiency but loss of heterozygosity for EXT in multiple osteochondromas

Multiple osteochondromas (MO) is an autosomal dominant disorder caused by germline mutations in EXT1 and/or EXT2. In contrast, solitary osteochondroma (SO) is nonhereditary. Products of the EXT gene are involved in heparan sulfate (HS) biosynthesis. In this study, we investigated whether osteochondromas arise via either loss of heterozygosity (2 hits) or haploinsufficiency. An in vitro three-dimensional chondrogenic pellet model was used to compare heterozygous bone marrow-derived mesenchymal stem cells (MSCs EXT(wt/-)) of MO patients with normal MSCs and the corresponding tumor specimens (presumed EXT(-/-)). We demonstrated a second hit in EXT in five of eight osteochondromas. HS chain length and structure, in vitro chondrogenesis, and EXT expression levels were identical in both EXT(wt/-) and normal MSCs. Immunohistochemistry for HS, HS proteoglycans, and HS-dependent signaling pathways (eg, TGF-β/BMP, Wnt, and PTHLH) also showed no differences. The cartilaginous cap of osteochondroma contained a mixture of HS-positive and HS-negative cells. Because a heterozygous EXT mutation does not affect chondrogenesis, EXT, HS, or downstream signaling pathways in MSCs, our results refute the haploinsufficiency theory. We found a second hit in 63% of analyzed osteochondromas, supporting the hypothesis that osteochondromas arise via loss of heterozygosity. The detection of the second hit may depend on the ratio of HS-positive (normal) versus HS-negative (mutated) cells in the cartilaginous cap of the osteochondroma.

Specific sides to multifaceted glycosaminoglycans are observed in embryonic development

Ubiquitously found in the extracellular matrix and attached to the surface of most cells, glycosaminoglycans (GAGs) mediate many intercellular interactions. Originally described in 1889 as the primary carbohydrate in cartilage and then in 1916 as a coagulation inhibitor from liver, various GAGs have since been identified as key regulators of normal physiology. GAGs are critical mediators of differentiation, migration, tissue morphogenesis, and organogenesis during embryonic development. While GAGs are simple polysaccharide chains, many GAGs acquire a considerable degree of complexity by extensive modifications involving sulfation and epimerization. Embryos that lack specific GAG modifying enzymes have distinct developmental defects, illuminating the importance of GAG complexity. Revealing how these complex molecules specifically function in the embryo has often required additional approaches, the results of which suggest that GAG modifications might instructively mediate embryonic development.

Screening for anticoagulant heparan sulfate octasaccharides and fine structure characterization using tandem mass spectrometry

Heparan sulfate (HS) is a sulfated glycosaminoglycan located on the surface and extracellular matrix of mammalian cells. HS is constituted of highly N-sulfated domains (NS domains) interrupted by lower sulfation domains. The arrangement of these domains dictates the function of HS which is mainly involved in binding proteins and regulating their biological activities. Heparin, a heparan sulfate analogue present in mast cells, resembles the NS domains of HS but lacks the alternating high and low sulfation architecture. Because the NS domains that range up to hexadecasaccharide in size are the main protein binders, heparin has been used as a model for HS in protein binding studies. Heparan sulfate, however, is the more physiologically relevant modulator of growth factor-receptor interactions. In this work, liquid chromatography and mass spectrometry (LC-MS) were used to compare the compositions of affinity-purified heparin and HS octasaccharides with anticoagulant activities versus library octasaccharides. The fine structures of the biologically active HS compositions were then compared against those of library octasaccharides using low-energy collision-induced dissociation tandem mass spectrometry. This approach confirmed isomeric enrichment of these compositions and, most importantly, produces ions diagnostic of their biological activity.

Extended N-sulfated domains reside at the nonreducing end of heparan sulfate chains

Heparan sulfate (HS) serves as a cell-surface co-receptor for growth factors, morphogens, and chemokines. These HS and protein binding events depend on the fine structure and distribution of domains along an HS chain. A given domain can vary in terms of uronic acid epimer, N- and O-sulfate, and N-acetate content. The most highly sulfated regions of HS chains, N-sulfated (NS) domains, play prominent roles in HS and protein binding. We have analyzed HS oligosaccharides from various mammalian sources and provide evidence that NS domains residing at the nonreducing end (NRE) are, on average, longer than those residing in the internal regions of the chain. Additionally, they are more highly sulfated than their internal counterparts. These features are independent of the sulfation pattern of the bulk HS chains. From disaccharide analysis, it is clear that NS domains do not always occupy HS NREs. However, when they do, they tend to terminate in a subset of N-sulfated disaccharides. Our observations are consistent with a significant role of NRE NS domains in HS-growth factor interactions.

Improved hydrophilic interaction chromatography LC/MS of heparinoids using a chip with postcolumn makeup flow

Heparan sulfate (HS) and heparin are linear, heterogeneous carbohydrates of the glycosaminoglycan (GAG) family that are modified by N-acetylation, N-sulfation, O-sulfation, and uronic acid epimerization. HS interacts with growth factors in the extracellular matrix, thereby modulating signaling pathways that govern cell growth, development, differentiation, proliferation, and adhesion. High-performance liquid chromatography (HPLC)-chip-based hydrophilic interaction liquid chromatography/mass spectrometry has emerged as a method for analyzing the domain structure of GAGs. However, analysis of highly sulfated GAG structures decasaccharide or larger in size has been limited by spray instability in the negative-ion mode. This report demonstrates that addition of postcolumn makeup flow to the amide-HPLC-chip configuration permits robust and reproducible analysis of extended GAG domains (up to degree of polymerization 18) from HS and heparin. This platform provides quantitative information regarding the oligosaccharide profile, degree of sulfation, and nonreducing chain termini. It is expected that this technology will enable quantitative, comparative glycomics profiling of extended GAG oligosaccharide domains of functional interest.

Fluorescent-tagged heparan sulfate precursor oligosaccharides to probe the enzymatic action of heparitinase I

Heparitinase I, a key lyase enzyme essential for structural analysis of heparan sulfate (HS), degrades HS domains that are undersulfated at glucuronyl residues through an elimination mechanism. Earlier studies employed viscosimetric measurements and electrophoresis to deduce the mechanism of action of heparitinase I and two other related lyases, heparitinase II and heparitinase III. However, these findings lack molecular evidence for the intermediates formed and could not distinguish whether the cleavage occurred from the reducing end or the nonreducing end. In the current study, 2-aminoacridone (2-AMAC)-labeled HS precursor oligosaccharides of various sizes were prepared to investigate the mechanism of heparitinase I-mediated depolymerization using sensitive and quantitative methodologies. Furthermore, fluorescent (2-AMAC) tagging of HS precursor oligosaccharides allowed us to distinguish fragments that result from cleavage of the substrates at various time intervals and sites farther away from the reducing and nonreducing ends of oligosaccharide substrates. This study provides the first direct molecular evidence for a predominantly random endolytic mechanism of cleavage of HS precursor oligosaccharides by heparitinase I. This robust strategy can be adapted to deduce the mechanism of action of other heparitinases and also to deduce structural information of complex HS oligosaccharides of biological importance.