Cations modulate polysaccharide structure to determine FGF-FGFR signaling: a comparison of signaling and inhibitory polysaccharide interactions with FGF-1 in solution

Interactions of proteins with heparan sulfate

Heparan sulfate (HS) is a glycosaminoglycan, polysaccharides that are considered to have arisen in the last common unicellular ancestor of multicellular animals. In this light, the large interactome of HS and its myriad functions in relation to the regulation of cell communication are not surprising. The binding of proteins to HS determines their localisation and diffusion, essential for embryonic development and homeostasis. Following the biosynthesis of the initial heparosan polymer, the subsequent modifications comprise an established canonical pathway and a minor pathway. The more frequent former starts with N-deacetylation and N-sulfation of GlcNAc residues, the latter with C-5 epimerisation of a GlcA residue adjacent to a GlcNAc. The binding of proteins to HS is driven by ionic interactions. The multivalent effect arising from the many individual ionic bonds between a single protein and a polysaccharide chain results in a far stronger interaction than would be expected from an ion-exchange process. In many instances, upon binding, both parties undergo substantial conformational change, the resulting hydrogen and van der Waal bonds contributing significant free energy to the binding reaction. Nevertheless, ionic bonds dominate the protein-polysaccharide interaction kinetically. Together with the multivalent effect, this provides an explanation for the observed trapping of HS-binding proteins in extracellular matrix. Importantly, individual ionic bonds have been observed to be dynamic; breaking and reforming, while the protein remains bound to the polysaccharide. These considerations lead to a model for 1D diffusion of proteins in extracellular matrix on HS, involving mechanisms such as sliding, chain switching and rolling.

Introduction to the Molecules Special Edition Entitled 'Heparan Sulfate and Heparin: Challenges and Controversies': Some Outstanding Questions in Heparan Sulfate and Heparin Research

The scope of this article is to provide a brief general introduction to heparan sulfate (HS) and heparin, and attempt to identify some of the central challenges regarding research into the chemistry and biology of glycosaminoglycans (GAGs), some of which are the subject of contributions to the special issue of Molecules (published in volume 23, 2018) entitled 'Heparan Sulfate and Heparin: Challenges and Controversies' [...].

The nature of the conserved basic amino acid sequences found among 437 heparin binding proteins determined by network analysis

In multicellular organisms, a large number of proteins interact with the polyanionic polysaccharides heparan sulphate (HS) and heparin. These interactions are usually assumed to be dominated by charge-charge interactions between the anionic carboxylate and/or sulfate groups of the polysaccharide and cationic amino acids of the protein. A major question is whether there exist conserved amino acid sequences for HS/heparin binding among these diverse proteins. Potentially conserved HS/heparin binding sequences were sought amongst 437 HS/heparin binding proteins. Amino acid sequences were extracted and compared using a Levenshtein distance metric. The resultant similarity matrices were visualised as graphs, enabling extraction of strongly conserved sequences from highly variable primary sequences while excluding short, core regions. This approach did not reveal extensive, conserved HS/heparin binding sequences, rather a number of shorter, more widely spaced sequences that may work in unison to form heparin-binding sites on protein surfaces, arguing for convergent evolution. Thus, it is the three-dimensional arrangement of these conserved motifs on the protein surface, rather than the primary sequence per se, which are the evolutionary elements.

Antiangiogenic platinum through glycan targeting

Heparan sulfate is identified as a ligand receptor for polynuclear platinum anti-cancer agents through sulfate cluster binding. We present a new biological role for platinum and coordination compounds and a new target for metal-based drugs while presenting a new chemotype for heparanase and growth factor inhibition through modulation (metalloshielding) of their interactions. Masking of extracellular (ECM)-resident heparan sulfate (HS) through metalloshielding results in very effective inhibition of physiologically critical HS functions including enzyme (heparanase, HPSE) and protein growth factor recognition. The interaction of the highly cationic polynuclear platinum complexes (PPCs) with the highly sulfated pentasaccharide Fondaparinux (FPX, in this case as a model HS-like substrate) results in inhibition of its cleavage by the HS-related enzyme heparanase. Binding of the fibroblast growth factor FGF-2 to HS is also inhibited with consequences for downstream signalling events as measured by a reduction in accumulation of phospho-S6 ribosomal protein in human colon tumor HCT-116 cells. The end-point of inhibition of HPSE activity and growth factor growth factor signaling is the prevention of cell invasion and angiogenesis. Finally these events culminate in inhibition of HCT-116 cell invasion at sub-cytotoxic concentrations and the process of angiogenesis. A competition assay shows that Fondaparinux can sequester the 8+ TriplatinNC from bound DNA, emphasising the strength of PPC-HS interactions. Altering the profile of platinum agents from cytotoxic to anti-metastatic has profound implications for future directions in the development of platinum-based chemotherapeutics.

Investigating the relationship between temperature, conformation and calcium binding in heparin model oligosaccharides

Glycosaminoglycans such as heparan sulfate (HS) are major components of the cell surface and extracellular matrix (ECM) of all multicellular animals, connecting cells to each other as well as to their environment. The ECM must, therefore, both sense and accommodate changes to external conditions. Heparin, a model compound for HS, responds to increased temperatures, involving changes in the populations of conformational states with implications for the binding of HS to proteins, cations and, potentially, for its activity. A fully ¹³C and ¹⁵N labelled model octasasccharide; D-GlcNS6S α(1-4) L-IdoA2S [α(1-4) D-GlcNS6S α(1-4) L-IdoA2S]₂ α(1-4) D-GlcNS6S α(1-4) L-IdoA1,6an, was studied by ¹H, ¹³C and ¹⁵N NMR, revealing complex changes in chemical shifts and conformation, over temperatures (280-305 K), comfortably within the range relevant to terrestrial biology. These complex conformational changes indicated an interaction between the carboxylate group of L-iduronate and D-glucosamine residues that was susceptible to temperature changes in this range, while the well-documented hydrogen bond between the N-sulfamido group of glucosamine and the hydroxyl group at position-3 of iduronate remained intact. Unexpectedly, despite the presence of similar thermally-induced conformational changes in a heparin octasaccharide fraction in the sodium ion form, its subsequent binding to calcium ions and their resulting conformation was stringently maintained, as judged by comparisons of ¹H NMR chemical shifts.

Heparan sulfate and heparin interactions with proteins

Heparan sulfate (HS) polysaccharides are ubiquitous components of the cell surface and extracellular matrix of all multicellular animals, whereas heparin is present within mast cells and can be viewed as a more sulfated, tissue-specific, HS variant. HS and heparin regulate biological processes through interactions with a large repertoire of proteins. Owing to these interactions and diverse effects observed during in vitro, ex vivo and in vivo experiments, manifold biological/pharmacological activities have been attributed to them. The properties that have been thought to bestow protein binding and biological activity upon HS and heparin vary from high levels of sequence specificity to a dependence on charge. In contrast to these opposing opinions, we will argue that the evidence supports both a level of redundancy and a degree of selectivity in the structure-activity relationship. The relationship between this apparent redundancy, the multi-dentate nature of heparin and HS polysaccharide chains, their involvement in protein networks and the multiple binding sites on proteins, each possessing different properties, will also be considered. Finally, the role of cations in modulating HS/heparin activity will be reviewed and some of the implications for structure-activity relationships and regulation will be discussed.

Heparan sulfate phage display antibodies recognise epitopes defined by a combination of sugar sequence and cation binding

Phage display antibodies are widely used to follow heparan sulfate (HS) expression in tissues and cells. We demonstrate by ELISA, that cations alter phage display antibody binding profiles to HS and this is mediated by changes in polysaccharide conformation, demonstrated by circular dichroism spectroscopy. Native HS structures, expressed on the cell surfaces of neuroblastoma and fibroblast cells, also exhibited altered antibody binding profiles following exposure to low mM concentrations of these cations. Phage display antibodies recognise conformationally-defined HS epitopes, rather than sequence alone, as has been assumed, and resemble proteins in being sensitive to changes in both charge distribution and conformation following binding of cations to HS polysaccharides.

On the catalytic mechanism of polysaccharide lyases: evidence of His and Tyr involvement in heparin lysis by heparinase I and the role of Ca2+

The structurally diverse polysaccharide lyase enzymes are distributed from plants to animals but share common catalytic mechanisms. One, heparinase I (F. heparinum), is employed in the production of the major anticoagulant drug, low molecular weight heparin, and is a mainstay of cell surface proteoglycan analysis. We demonstrate that heparinase I specificity and efficiency depend on the cationic form of the substrate. Ca(2+)-heparin, in which α-L-iduronate-2-O-sulfate residues adopt (1)C4 conformation preferentially, is a substrate, while Na(+)-heparin is an inhibitor. His and Tyr residues are identified in the catalytic step and a model based on molecular dynamics and docking is proposed, in which deprotonated His203 initiates β-elimination by abstracting the C5 proton of the α-L-iduonate-2-O-sulfate residue in the substrate, and protonated Tyr357 provides the donor to the hexosamine leaving group.

Multivalent proteoglycan modulation of FGF mitogenic responses in perivascular cells

Sprouting of angiogenic perivascular cells is thought to be highly dependent upon autocrine and paracrine growth factor stimulation. Accordingly, we report that corneal angiogenesis induced by ectopic FGF implantation is strongly impaired in NG2/CSPG4 proteoglycan (PG) null mice known to harbour a putative deficit in pericyte proliferation/mobilization. Conversely, no significant differences were seen between wild type and knockout corneas when VEGF was used as an angiocrine factor. Perturbed responsiveness of NG2-deficient pericytes to paracrine and autocrine stimulation by several FGFs could be confirmed in cells isolated from NG2 null mice, while proliferation induced by other growth factors was equivalent in wild type and knockout cells. Identical results were obtained after siRNA-mediated knock-down of NG2 in human smooth muscle-like cell lines, as also demonstrated by the decreased levels of FGF receptor phosphorylation detected in these NG2 deprived cells. Binding assays with recombinant proteins and molecular interactions examined on live cells asserted that FGF-2 bound to NG2 in a glycosaminoglycan-independent, core protein-mediated manner and that the PG was alone capable of retaining FGF-2 on the cell membrane for subsequent receptor presentation. The use of dominant-negative mutant cells, engineered by combined transduction of NG2 deletion constructs and siRNA knock-down of the endogenous PG, allowed us to establish that the FGF co-receptor activity of NG2 is entirely mediated by its extracellular portion. In fact, forced overexpression of the NG2 ectodomain in human smooth muscle-like cells increased their FGF-2-induced mitosis and compensated for low levels of FGF receptor surface expression, in a manner equivalent to that produced by overexpression of the full-length NG2. Upon FGF binding, the cytoplasmic domain of NG2 is phosphorylated, but there is no evidence that this event elicits signal transductions that could bypass the FGFR-mediated ones. Pull-down experiments, protein-protein binding assays and flow cytometry FRET coherently revealed an elective ligand-independent association of NG2 with FGFR1 and FGFR3. The NG2 cooperation with these receptors was also corroborated functionally by the outcome of FGF-2 treatments of cells engineered to express diverse NG2/FGFR combinations. Comprehensively, the findings suggest that perivascular NG2 may serve as a dual modulator of the availability/accessibility of FGF at the cell membrane, as well as the resulting FGFR transducing activity.

A zinc complex of heparan sulfate destabilises lysozyme and alters its conformation

The naturally occurring anionic cell surface polysaccharide heparan sulfate is involved in key biological activities and is implicated in amyloid formation. Following addition of Zn-heparan sulfate, hen lysozyme, a model amyloid forming protein, resembled β-rich amyloid by far UV circular dichroism (increased β-sheet: +25%), with a significantly reduced melting temperature (from 68 to 58 °C) by fluorescence shift assay. Secondary structure stability of the Zn-heparan sulfate complex with lysozyme was also distinct from that with heparan sulfate, under stronger denaturation conditions using synchrotron radiation circular dichroism. Changing the cation associated with heparan sulfate is sufficient to alter the conformation and stability of complexes formed between heparan sulfate and lysozyme, substantially reducing the stability of the protein. Complexes of heparan sulfate and cations, such as Zn, which are abundant in the brain, may provide alternative folding routes for proteins.

Heparan sulfate glycomics: towards systems biology strategies

HSs (heparan sulfates) are a complex family of cell-surface and matrix polysaccharides that have diverse biological functions, underpinned by structurally diverse patterns of backbone chain modification, especially by sulfate groups. These variant structures represent a molecular code, the 'heparanome', that confers the ability to interact selectively with a wide interactome of proteins, the 'heparactome', and thereby influence a network of cellular events. It is becoming increasingly apparent that understanding the structure-activity relationships of these enigmatic molecules requires the development of a holistic systems biology view of their structure and interactions. In the present paper, I describe some of the new tools available to realize this strategy, and discuss the future potential for the combined application of glycomics and other '-omics' approaches to define the molecular code of the heparanome.

Conformational degeneracy restricts the effective information content of heparan sulfate

The linear, sulfated polysaccharide heparan sulfate occupies a pivotal position in intercellular signalling events, interacting with numerous proteins on the cell surface and in the extracellular matrix. Its complex sequences suggest high potential information content but, despite extensive efforts, a clear relationship between its substitution pattern and biological activity remains elusive. This results from technical limitations, compounded by attempts to correlate substitution pattern directly with activity without considering other conformational factors. For a series of systematically modified analogues of heparan sulfate, the relationship between substitution pattern and experimental (13)C NMR chemical shifts, which act as reporters of the presence of conformational change, particularly around the glycosidic linkages, was explored through chemometric analysis. From analysis of the experimental data it was evident that wide linkage variation arose from O-sulfation in iduronate and N-sulfation in glucosamine residues but, their effects were distinct, while 6-O-sulfation had much less impact. Models of saccharide sequences showed that the maximum spread of variation in glycosidic linkages occurred before maximum sequence diversity and revealed a highly degenerate system: a fraction of possible sequences is sufficient to provide diverse backbone conformations to satisfy particular protein binding requirements. The unique information content potentially available in HS sequences, defined ultimately by conformation, is vastly inferior to the potential sequence diversity.