Conformational equilibria of the L-iduronate residue in non-sulphated di-, tetra- and hexa-saccharides and their alditols derived from dermatan sulphate

Constructing 3-Dimensional Atomic-Resolution Models of Nonsulfated Glycosaminoglycans with Arbitrary Lengths Using Conformations from Molecular Dynamics

Glycosaminoglycans (GAGs) are the linear carbohydrate components of proteoglycans (PGs) and are key mediators in the bioactivity of PGs in animal tissue. GAGs are heterogeneous, conformationally complex, and polydisperse, containing up to 200 monosaccharide units. These complexities make studying GAG conformation a challenge for existing experimental and computational methods. We previously described an algorithm we developed that applies conformational parameters (i.e., all bond lengths, bond angles, and dihedral angles) from molecular dynamics (MD) simulations of nonsulfated chondroitin GAG 20-mers to construct 3-D atomic-resolution models of nonsulfated chondroitin GAGs of arbitrary length. In the current study, we applied our algorithm to other GAGs, including hyaluronan and nonsulfated forms of dermatan, keratan, and heparan and expanded our database of MD-generated GAG conformations. Here, we show that individual glycosidic linkages and monosaccharide rings in 10- and 20-mers of hyaluronan and nonsulfated dermatan, keratan, and heparan behave randomly and independently in MD simulation and, therefore, using a database of MD-generated 20-mer conformations, that our algorithm can construct conformational ensembles of 10- and 20-mers of various GAG types that accurately represent the backbone flexibility seen in MD simulations. Furthermore, our algorithm efficiently constructs conformational ensembles of GAG 200-mers that we would reasonably expect from MD simulations.

The conformation of the idopyranose ring revisited: How subtle O-substituent induced changes can be deduced from vicinal 1H-NMR coupling constants

The idopyranose ring plays a pivotal role in the conformational, dynamical, and intermolecular binding aspects of glycosaminoglycans like heparin and dermatan sulfate and it was early on assigned a role in the Sugar Code governing biological recognition processes. There is consensus that next to the two canonical ¹C₄ and ⁴C₁ chair conformations, the conformational space accessible to the idopyranose ring entails a ²S_O skew-boat conformation, but the equilibrium between these three ring puckers has evaded satisfactory quantification. In this study a meta-analysis of X-ray solid-state data and vicinal NMR coupling constants is presented, based on the Truncated Fourier Puckering (TFP) formalism and the generalized Karplus (CAGPLUS) equation. This approach yields a model-free, granular and consistent reckoning of 159 idopyranose solution puckering equilibria studied by NMR and allows us to reproduce the involved 636 NMR vicinal couplings with an overall residual RMS(J_obs-J_calc) of 0.184 Hz. Our analyses show that for all ring systems examined, the idopyranosyl chair conformations take up the same ring pucker irrespective of the ring substituent pattern or a vast variety in experimental conditions. Instead, it is the (skew-)boat conformation that adapts to the substitution pattern of the idopyranose ring or a specific sulfation pattern of neighboring saccharides. All idopyranose rings are involved in conformational equilibria that subsume the aforementioned conformers which turn out to differ only a few kJ/mole in conformational energy. Thus, the plasticity and flexibility of idopyranose remains intact under practically all circumstances and, as the glycosidic linkages in heparin are considered to be relatively stiff, the iduronic moiety functions as the linchpin of heparin flexibility thereby being rather a "space(r)" than a "letter" in the alleged Sugar Code alphabet.

A decorin-deficient matrix affects skin chondroitin/dermatan sulfate levels and keratinocyte function

Decorin is a small leucine-rich proteoglycan harboring a single glycosaminoglycan chain, which, in skin, is mainly composed of dermatan sulfate (DS). Mutant mice with targeted disruption of the decorin gene (Dcn(-/-)) exhibit an abnormal collagen architecture in the dermis and reduced tensile strength, collectively leading to a skin fragility phenotype. Notably, Ehlers-Danlos patients with mutations in enzymes involved in the biosynthesis of DS display a similar phenotype, and recent studies indicate that DS is involved in growth factor binding and signaling. To determine the impact of the loss of DS-decorin in the dermis, we analyzed the glycosaminoglycan content of Dcn(-/-) and wild-type mouse skin. The total amount of chondroitin/dermatan sulfate (CS/DS) was increased in the Dcn(-/-) skin, but was overall less sulfated with a significant reduction in bisulfated ΔDiS2,X (X=4 or 6) disaccharide units, due to the reduced expression of uronyl 2-O sulfotransferase (Ust). With increasing age, sulfation declined; however, Dcn(-/-) CS/DS was constantly undersulfated vis-à-vis wild-type. Functionally, we found altered fibroblast growth factor (Fgf)-7 and -2 binding due to changes in the micro-heterogeneity of skin Dcn(-/-) CS/DS. To better delineate the role of decorin, we used a 3D Dcn(-/-) fibroblast cell culture model. We found that the CS/DS extracts of wild-type and Dcn(-/-) fibroblasts were similar to the skin sugars, and this correlated with the lack of uronyl 2-O sulfotransferase in the Dcn(-/-) fibroblasts. Moreover, Ffg7 binding to total CS/DS was attenuated in the Dcn(-/-) samples. Surprisingly, wild-type CS/DS significantly reduced the binding of Fgf7 to keratinocytes in a concentration dependent manner unlike the Dcn(-/-) CS/DS that only affected the binding at higher concentrations. Although binding to cell-surfaces was quite similar at higher concentrations, keratinocyte proliferation was differentially affected. Higher concentration of Dcn(-/-) CS/DS induced proliferation in contrast to wild-type CS/DS. 3D co-cultures of fibroblasts and keratinocytes showed that, unlike Dcn(-/-) CS/DS, wild-type CS/DS promoted differentiation of keratinocytes. Collectively, our results provide novel mechanistic explanations for the reported defects in wound healing in Dcn(-/-) mice and possibly Ehlers-Danlos patients. Moreover, the lack of decorin-derived DS and an altered CS/DS composition differentially influence keratinocyte behavior.

Desulfated galactosaminoglycans are potential ligands for galectins: evidence from frontal affinity chromatography

Galectins, a group of beta-galactoside-binding lectins, are involved in multiple functions through specific binding to their oligosaccharide ligands. No previous work has focused on their interaction with glycosaminoglycans (GAGs). In the present work, affinities of established members of human galectins toward a series of GAGs were investigated, using frontal affinity chromatography. Structurally-defined keratan sulfate (KS) oligosaccharides showed significant affinity to a wide range of galectins if Gal residue(s) remained unsulfated, while GlcNAc sulfation had relatively little effect. Consistently, galectins showed much higher affinity to corneal type I than cartilageous type II KS. Unexpectedly, galectin-3, -7, and -9 also exerted significant affinity to desulfated, GalNAc-containing GAGs, i.e., chondroitin and dermatan, but not at all to hyaluronan and N-acetylheparosan. These observations revealed that the integrity of 6-OH of betaGalNAc is important for galectin recognition of these galactosaminoglycans, which were shown, for the first time, to be implicated as potential ligands of galectins.

The human D-glucuronyl C5-epimerase gene is transcriptionally activated through the beta-catenin-TCF4 pathway

Heparan sulphate (HS) is a ubiquitous constituent of the extracellular matrix that is required for the biological activity of circulating soluble and insoluble extracellular ligands. GLCE (D-glucuronyl C5-epimerase), an enzyme responsible for the epimerization of D-glucuronic acid into L-iduronic acid of HS, endows the nascent polysaccharide chain with the ability to bind to growth factors and cytokines. In order to examine the mechanism of regulation of GLCE expression, the functional organization of the human GLCE gene promoter has been investigated. Studies utilizing stepwise deleted and site-directed mutagenized promoter constructs have shed light on the functional relevance of two cis-acting binding elements for the beta-catenin-TCF4 complex (where TCF4 stands for T-cell factor 4) that are located in the enhancer region of the promoter. The ability of the putative binding sequences to bind the beta-catenin-TCF4 complex has been confirmed through electrophoretic mobility-shift and supershift analyses. We have found that, in a set of human colon carcinoma cell lines, the expression of GLCE correlates with the degree of activation of the beta-catenin-TCF4 transactivation complex. Furthermore, the ectopic expression of beta-catenin-TCF4 in cells that constitutively express low levels of the transactivation complex produces a significant increase of GLCE transcript level and, at the same time, enhances the rate of D-glucuronic acid epimerization in HS. The data obtained are consistent with the idea that the beta-catenin-TCF4 transactivation pathway plays a major role in modulating GLCE expression, thus contributing to the regulation of HS biosynthesis and its structural organization.

D-glucuronyl C5-epimerase acts in dorso-ventral axis formation in zebrafish

BACKGROUND

Heparan sulfate (HS) is an ubiquitous component of the extracellular matrix that binds and modulates the activity of growth factors, cytokines and proteases. Animals with defective HS biosynthesis display major developmental abnormalities however the processes that are affected remain to be defined. D-glucuronyl-C5-epimerase (Glce) is a key HS chain modifying enzyme that catalyses the conversion of glucuronic acid into iduronic acid, a biosynthetic step that enhances HS biological activity. In this study the role of Glce during early zebrafish development has been investigated.

RESULTS

Two Glce-like proteins (Glce-A and -B) are expressed in zebrafish at all times. They are the products of two distinct genes that, based on chromosomal mapping, are both orthologues of the same single human gene. Transcripts for both proteins were detected in fertilized zebrafish embryos prior to the onset of zygotic transcription indicating their maternal origin. At later developmental stages the epimerases are expressed widely throughout gastrulation and then become restricted to the hindbrain at 24 h post-fertilization. By monitoring the expression of well characterized marker genes during gastrulation, we have found that misexpression of Glce causes a dose-dependent expansion of the ventral structures, whereas protein knockdown using targeted antisense morpholino oligonucleotides promotes axis dorsalization. The ventralizing activity of Bmp2b is enhanced by Glce overexpression whereas Glce knockdown impairs Bmp2b activity.

CONCLUSION

Glce activity is an important determinant of of dorso-ventral axis formation and patterning in zebrafish. In particular Glce acts during gastrulation by affecting Bmp-mediated cell specification. The results obtained further corroborate the concept that HS encodes information that affect morphogenesis during early vertebrate development.

Core protein dependence of epimerization of glucuronosyl residues in galactosaminoglycans

Chondroitin sulfate and dermatan sulfate proteoglycans are distinguished by differences in their proportion of d-glucuronosyl and l-iduronosyl residues, the latter being formed by chondroitin-glucuronate 5-epimerase during or after glycosaminoglycan chain polymerization. To investigate the influence of the core protein on the extent of epimerization, we expressed chimeric proteins in 293 HEK cells constructed from intact or modified Met(1)-Gln(153) of decorin (DCN), which normally has a single dermatan sulfate chain at Ser(34), in combination with intact or modified Leu(241)-Ser(353) of CSF-1, which has a chondroitin sulfate attachment site at Ser(309). Transfected DCN(M1-Q153), like full-length DCN, contained approximately 20% l-iduronate. Conversely, transfected CSF-1(L241-S353), attached C-terminally on the DCN prepropeptide, contained almost exclusively d-glucuronate. Transfected intact chimeric DCN(M1-Q153)-CSF-1(L241-S353), with two glycosaminoglycan chains, also contained almost exclusively d-glucuronate in chains at both sites, as did chimeras in which alanine was substituted for serine at either of the glycosaminoglycan attachment sites. Nevertheless, undersulfated intact chimeric proteoglycan was an effective substrate for epimerization of glucuronate to iduronate residues when incubated with microsomal proteins and 3'-phosphoadenylylphosphosulfate. C-terminal truncation constructs were prepared from the full-length chimera with an alanine substitution at the CSF-1 glycosaminoglycan attachment site. Transfected truncations retaining the alanine-blocked site contained chains with essentially only glucuronate, whereas those further truncated by 49 or more amino acids and missing the modified attachment site contained chains with approximately 15% iduronate. This 49-amino acid region contains a 7-amino acid motif that appears to be conserved in several chondroitin sulfate proteoglycans. The results are consistent with a model in which the core protein, possibly via this motif, is responsible for routing to subcellular compartments with or without sufficient access to chondroitin-glucuronate 5-epimerase for the addition of chains with or without iduronate residues, respectively.

Synthesis of conformationally locked L-iduronic acid derivatives: direct evidence for a critical role of the skew-boat 2S0 conformer in the activation of antithrombin by heparin

We have used organic synthesis to understand the role of L-iduronic acid conformational flexibility in the activation of antithrombin by heparin. Among known synthetic analogues of the genuine pentasaccharidic sequence representing the antithrombin binding site of heparin, we have selected as a reference compound the methylated anti-factor Xa pentasaccharide 1. As in the genuine original fragment, the single L-iduronic acid moiety of this molecule exists in water solution as an equilibrium between three conformers 1C4, 4C1 and 2S0. We have thus synthesized three analogues of 1, in which the L-iduronic acid unit is locked in one of these three fixed conformations. A covalent two atom bridge between carbon atoms two and five of L-iduronic acid was first introduced to lock the pseudorotational itinerary of the pyranoid ring around the 2S0 form. A key compound to achieve this connection was the D-glucose derivative 5 in which the H-5 hydrogen atom has been replaced by a vinyl group, which is a progenitor of the carboxylic acid. Selective manipulations of this molecule resulted in the 2S0-type pentasaccharide 23. Starting from the D-glucose derivative 28, a covalent two atom bridge was now built up between carbon atoms three and five to lock the L-iduronic acid moiety around the 1C4 chair form conformation, and the 1C4-type pentasaccharide 43 was synthesized. Finally the L-iduronic acid containing disaccharide 58 which, due to the presence of the methoxymethyl substituent at position five adopts a 4C1 conformation, was directly used to synthesize the 4C1-type pentasaccharide 61. The locked pentasaccharide 23 showed about the same activity as the reference compound 1 in an antithrombin-mediated anti-Xa assay, whereas the two pentasaccharides 43 and 61 displayed very low activity. These results clearly establish the critical importance of the 2S0 conformation of L-iduronic acid in the activation of antithrombin by heparin.

A stereochemical approach to pyranose ring flexibility: its implications for the conformation of dermatan sulfate

Glycosaminoglycans, such as heparin, heparan sulfate, and dermatan sulfate, are characterized by a disaccharide repeating unit of a uronate and a hexosamine and are increasingly understood to be important physiologically as soluble components of the extracellular matrix. The secondary structure of this class of acidic polysaccharides is believed to play a key role in determining the wide range of biological specificities. Central to the structural diversity of the glycosaminoglycans is the experimentally documented conformational flexibility of the iduronate residue. Here, we outline an approach to explore the iduronate conformational flexibility by imposing stereochemical criteria of nonbonded contact distances. By performing a complete search of all possible torsions that define the iduronate ring geometry, we eliminate any prior bias with regard to minimum energy conformers. The approach led to alternative feasible conformers for the iduronate ring that are stereochemically satisfactory and are consistent with the available physico-chemical data.

Small proteoglycans

In this review the structure and functions of two non-related proteoglycan families are discussed. One family represents a group of extracellular matrix macromolecules characterized by core proteins with leucine-rich repeat motifs. Within this family special attention is given to those members which carry chondroitin or dermatan sulfate glycosaminoglycan chains. The second family is characterized by repeat sequences of serine and glycine. Their members are products of a single core protein gene and are characteristic constituents of secondary vesicles in cells of the haematopoietic lineage.

Small proteoglycans

In this review the structure and functions of two non-related proteoglycan families are discussed. One family represents a group of extracellular matrix macromolecules characterized by core proteins with leucine-rich repeat motifs. Within this family special attention is given to those members which carry chondroitin or dermatan sulfate glycosaminoglycan chains. The second family is characterized by repeat sequences of serine and glycine. Their members are products of a single core protein gene and are characteristic constituents of secretory vesicles in cells of the haematopoietic lineage.

Heavy metal binding to heparin disaccharides. II. First evidence for zinc chelation

To map out the heavy metal binding sites of iduronic acid containing oligosaccharides isolated from human kidneys, we studied Zn(II) binding by nuclear magnetic resonance (NMR) and molecular modeling to two disaccharides isolated after nitrous acid depolymerization of heparin and two synthetic disaccharides representative of the heparin structure, namely, IdopA2S (alpha 1,4)AnManOH, 1 alpha, IdopA2S (alpha 1,4)AnManOH6S, 1b, IdopA2S-(alpha 1,4)GlcNS alpha Me, 2a, and IdopA2S (alpha 1,4)GlcNS6S alpha Me, 2b (see previous article in this series). A conformational analysis of the metal free and metal bound solutions was made by comparing calculated [(NOE)]s, [T1]s, and [J]s to experimental values. The 1C4, 4C1, and 2S0 conformations of the L-idopyranosiduronate ring and the 4E and 4T3 of the anhydro-D-mannitol ring are evaluated as are rotations about the C5-C6 hydroxymethylene of the AnManOH(6S) or GlcNS (6S) residues. The NOE between IdopA2S H1 and H3 and the known NOE between H2 and H5, as well as the T1 of IdopA2S H3, are introduced as NMR observables sensitive to the IdopA2S ring conformation. Similarly, a NOE between IdopA2S H5 and AnManOH(6S) or GlcNS(6S) H3 was observed that directly restricts the allowed interglycosidic conformational space. For all disaccharides, the Zn(II) bound spectral data are consistent with models in which these motions are partially "frozen" such that the 1C4 conformation of the IdopA2S is stabilized along with the 4T3 conformation of the AnManOH(6S) ring. The interglycosidic conformation is also stabilized in one of two minima. Electrostatic potential energy calculations gave the best overall agreement with experiment and suggest metal binding conformations with the carboxylate and ring oxygen of the IdopA2S residues (1C4 conformation) and either O3 of the GlcNS(6S) residues or the sulfate oxygens of the 6-sulphate for 2b providing additional chelating sites. These chelation models concur with the observation of marked 13C and 1H NMR chemical shifts for the IdopA2S resonances and of GlcNS H3 for 2 alpha and GlcNS6S C6 for 2b. This study of model compounds implicates the IdopA2S(alpha 1,4)GlcNS6S group as part of the heavy metal binding site in biologically important acidic oligosaccharides such as heparin.