The structure of the heparan sulphate-carrying proteoglycan glypican is briefly reviewed. The mode of membrane-attachment provides means of apical sorting in polarized cells and potential concentration to caveolae, as well as several different cleavage points resulting in release from its lipid anchor. The heparan sulphate side-chains are degraded in steps involving endoglycosidic cleavage to form large oligosaccharides followed by terminal exo-degradation. Data supporting internalization and partial degradation in endosomes, followed by recycling of the intact core protein back to the cell-surface are specifically reviewed. The means of degradation (enzymatic vs. non-enzymatic) and re-processing by re-synthesis of heparan sulphate on remaining core-protein stubs are discussed. Several investigations have provided evidence that cell-surface heparan sulphate proteoglycans play a role in the internalization of growth factors, polyamine-DNA complexes, viruses and microbes. In polarized cells, recycling heparan sulphate proteoglycans may be the transport vehicle in transcytosis.
Heparan sulfate is a structurally complex biopolymer that has been the subject of active investigation due to it's role in a variety of important biological processes. The activity of heparan sulfate primarily resides in it's glycosaminoglycan chains. This glycosaminoglycan is comprised of a number of diverse structurally disaccharide residues occuring in a variety of different chain lengths. The resulting, highly complex, structure has not been amenable to complete characterization despite the application of a variety of different approaches. Recent structural studies have focused on examining smaller structural domains of this macromolecule, corresponding to those responsible for many of its important biological activities. This mini review examines the preparation, purification and complete characterization of heparan sulfate-derived oligosaccharides. The emphasis of this review is on high resolution separation technology such as high performance liquid chromatography, polyacrylamide gel and capillary electrophoresis as well as unambiguous characterization relying on mass spectrometry and nuclear magnetic resonance spectroscopy.
The Glypican-related integral membrane proteoglycans (GRIPs) or Glypicans compose a distinctive, emerging family of heparan sulfate proteoglycans. The five known vertebrate glypicans include glypican (glypican-1), Cerebroglycan (glypican-2), OCI-5 (glypican-3), K-glypican (glypican-4) and glypican-5. The members of this family have similar core protein sizes (about 60kDa), share a unique and very conserved cysteine spacing, and are linked to the cell membrane by a glycosyl phosphatidyl inositol (GPI)-anchor. All the structural features of the vertebrate glypicans are also represented in the product of dally (division abnormally delayed), a locus identified in Drosophila melanogaster. The dally mutants, the well established co-receptor activities of the cell surface proteoglycans for various ligands that are known to mediate developmental instructions, and the tissue and stage-specific expressions of the glypicans, all implicate the Glypican group of integral membrane proteoglycans in the control of cell division and patterning during development. This contention has recently been corroborated by identifying mutations in GPC3, the gene coding for the human homologue of OCI-5, that cause the Simpson-Golabi-Behmel syndrome, which is characterized by foetal and post-natal overgrowth, visceral and skeletal anomalies and a predisposition to the development of embryonal tumors.
Cytokines are diffusible and soluble factors with pleiotropic actions. However, heparan sulfate proteoglycan (HS-PG) on either the cell surface or matrices provides the following advantages to the function of heparin-binding cytokines such as chemokines, by immobilizing them and presents cytokines to their receptors on target cells: 1) HS-PG promotes the accumulation of cytokines at high concentration by binding them on the appropriate location where they encounter their target cells; 2) HS-PG protects cytokines from both chemical and physiological stimuli; 3) HS-PG induces conformation-dependent association of the cell surface molecules by binding them. Furthermore, interactions between HS-PG and cytokines promotes the assembly of the appropriate molecular complex to initiate signal transduction, because many of them are highly specific, and variation in the HS-PG determines the specificity of binding of cytokines. HS-PG thereby appears to play a pivotal role in the promotion and regulation of the multicrine regulatory mechanisms of particular cytokine functions, which allows for the regulated paracrine, autocrine, juxtacrine and matricrine stimulation of heparin-binding cytokines. Such multicrine-type cytokine functions mediated by HS-PG-binding efficiently facilitates regulated cellular interactions through cytokines as well as adhesion molecules, which is relevant not only to the process of leukemic cell infiltration and tumor metastasis, but also to the regulation of inflammation and leukocyte trafficking.
Embryonic development involves the generation of progenitor cells from proliferating stem cells, followed by migration of progenitor cells to specific sites in tissues and their differentiation into specific cell types. In the nervous system, this stereotypic developmental process proceeds more exquisitely than in any other tissues because of the highly complicated connectivities of neurons required for mature nervous system. Recent studies have revealed that heparan sulfate proteoglycans may play crucial roles in various developmental processes in the nervous system.
We review the role of HSPGs in mediating the actions of three critical effectors of the cardiovascular system-the fibroblast growth factors (FGFs), lipoprotein lipase and antithrombin (AT). HSPGs modulate the transduction of signals by FGFs through FGF receptors, serve to transport and localize lipoprotein lipase to key sites of action, and directly stimulate the anticoagulant activity of AT. These diverse mechanisms of action are all made possible by highly specific interactions between each effector molecule and distinct sequences of monosaccharides of the HS chain. These sequences are predominantly determined by the nonrandom arrangement of N-, 2-O-, 6-O-, and 3-O-sulfate groups along the heparan sulfate chain (HS); thus, the biologic functions of HSPGs are controlled by biosynthetic events which define HS fine structure. In particular, endothelial cell production of the AT binding site is regulated by the kinetically limiting activity of 3-O-sulfotrans-ferase-1. A comparison of the structural and functional properties of the known HS sulfotransferases reveals that multiple isoforms, with clear cut precursor/product relationships, can occur and function within an at least partially ordered biochemical pathway. On the basis of these observations, we propose a model for the regulated synthesis of defined HS sequences.
Aberrant vascular smooth muscle cell (VSMC) proliferation is the hallmark of both atherosclerosis and restenosis seen after vascular surgery. Heparin and heparan sulfate proteoglycans have been shown to inhibit VSMC proliferation in vitro and in vivo. Although the precise molecular mechanism of action of the atiproliferative effect of heparin is not yet understood, a number of studies have focused on: 1) binding of heparin to specific cell surface receptors and its subsequent internalization; 2) effects on cell cycle control machinery; 3) alteration of mitogenic signaling pathways; 4) regulation of gene expression, especially genes which encode proteins required for proliferation and genes encoding extracellular matrix proteins. In this review, we present an overview of the major contributions to understanding the antiproliferative mechanism of action of heparin.
Syndecans constitute a family of transmembrane heparan sulfate proteoglycans, in which four members have been identified in vertebrates, and Drosophila and C. elegance homologues have been recently cloned. Syndecans have highly conserved amino acid sequences of transmembrane and cytoplasmic domains of the core proteins and a similar exon-intron organization of genomic DNA, suggesting that they share a common ancestral gene. Syndecans mediate the interaction of cells with the microenvironment. Iy is through their heparan sulfate side chains taht these proteoglycans interact with a wide variety of ligands including humoral growth factors such as FGFs, HGF, and midkine, and the constituents of solid extracellular matrix such as fibronectin, tenascin, laminin and collagens. Although these interactions are considered to be involved in regulation of cell proliferation and control of cell behavior in association with the cytoskeletal organization, the cascade for signal transduction through syndecans is still unclear. However, in this regard, recent works have demonstrated dimerization/multimerization of syndecan-3 and -4, phosphorylation of the cytoplasmic domain of syndecan-2, and activation of protein kinase C through its binding to syndecan-4. On the basis of these results, we report here our recent work with murine Lewis lung carcinoma-derived different metastatic clones, showing that cell attachment to fibronectin via both integrin α5β1 and syndecan-2 or only integrin α5β1 results in the distinctly different cytoskeletal organization, i.e., stress fibers or ruffling membrane.
The various activities of heparin and heparan sulfate depend mainly on interactions of the polysaccharides with various functional proteins, mediated by specific domains with distinct saccharide sequences. For example, interaction of members of the FGF family with heparin and heparan sulfate requires different combinations of sulfate groups, hence different saccharide sequences. The sulfate groups mentioned above can be modified or substituted to introduce structural changes to the polysaccharides. Several such changes have been already performed on heparin in order to modulate the biological activities of FGF.