Abstract

Heparan sulfate proteoglycans are found at the cell open and in the extracellular matrix, where they interact with a overplus of ligands. Over the last decade, new insights have emerged regarding the mechanism and biological meaning of these interactions. here, we discuss changing views on the specificity of protein–heparan sulfate bind and the action of HSPGs as receptors and coreceptors. Although few in number, heparan sulfate proteoglycans have profound effects at the cellular, tissue, and organismal level .

A BIRD’S-EYE VIEW OF STRUCTURE AND ASSEMBLY

An idealize video of a HSPG is shown in. Each proteoglycan consists of a protein and one or more covalently attached HS chains. comprehensive reviews have appeared on the assembly procedure and structural word picture of the chains, and consequently these subjects will not be discussed foster here ( Esko and Selleck 2002 ; Sugahara and Kitagawa 2002 ; Sasisekharan et aluminum. 2006 ; Ori et aluminum. 2008 ; Laremore et aluminum. 2009 ). however, several features are authoritative to consider in the context of their biological activities. ( 1 ) HSPGs are polyanionic and have strange hydrodynamic volume because of the presence of long HS chains ( 40–300 carbohydrate residues, ∼20–150 nanometer ), sulfate groups, and uronic acids. thus, different HSPGs often copurify by techniques that rely just on the anionic detergent characteristics of the chains or gel filtration. HS and other sulfated GAGs are amongst the most highly negatively charged biopolymers in nature and variation in the count and duration of the chains gives arise to enormous polydispersity. ( 2 ) Some proteoglycans contain only one GAG chain ( for example, CD44v3 and betaglycan ), whereas others have three to five chains ( for example, syndecan ). Furthermore, the stoichiometry of GAG chain substitution can vary depending on reference and growth conditions. “ part-time ” proteoglycans can exist with or without a GAG range ( ). ( 3 ) Some proteoglycans contain other types of glycans ( for example, asparagine-linked [ N-linked ] and serine/threonine-linked [ O-linked ] mucin-type chains ). Some proteoglycans, such as syndecan-1, contain both HS and chondroitin/dermatan sulfate, another type of GAG. early types of posttranslational modifications can occur ( for example, phosphorylation on cytoplasmic domains of trans-membrane proteoglycans ). ( 4 ) The number and sulfation department of state of the chains can vary according to growth conditions and in response to growth factors. ( 5 ) The musical arrangement of negatively charged sulfate groups and the orientation of the carboxyl groups specify the localization of ligand-binding sites. furthermore, the sulfate residues are clustered in regions along the range containing mixtures of iduronic acidic and glucuronic acerb ( NS domains, ) and are separated by nonsulfated domains rich in glucuronic acid ( NA domains ). ( 6 ) The traffic pattern of sulfation, extent of uronic acidic epimerization, and organization of the modify residue is broadly thought to depend on the cell type in which HS is expressed rather than on the nature of the effect protein ( Kato et alabama. 1994 ). thus, the overall composition of HS on unlike core proteins expressed by the same cell appears to be like, but great mutant occurs between cell types. This concept might be an oversimplification, as some pas seul has been suggested to occur in ligand-binding properties and constitution dependent on the effect protein ( Shworak et alabama. 1993 ; Tveit et alabama. 2005 ).

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GENERALIZATIONS ABOUT THE INTERACTION OF PROTEIN LIGANDS WITH HS PROTEOGLYCANS

HSPGs tie to many ligands, normally via the HS chains. In fact heparin, a highly sulfate form of HS, is much used as an “ affinity ” matrix for purifying proteins, and many of the increase factors in practice today were purified by heparin affinity chromatography. Heparin-binding ligands include growth factors, cytokines, chemokines, enzymes, enzyme inhibitors, and extracellular matrix proteins. In early areas of glycobiology, glycan-binding proteins are referred to as “ lectins, ” a appointment based on the presence of a carbohydrate recognition knowledge domain defined by characteristic protein folds or sequence motifs indicating their membership in an evolutionarily conserved gene family ( Varki et aluminum. 2009 ). In contrast, proteins that bind to HS appear to have evolved by convergent development ; that is, they do not possess a particular fold or recognizable amino acid sequence practice ( Esko and Linhardt 2009 ). Heparin-binding sites often occur on the external surface of proteins or in shallow grooves lined with positively charged amino acids. Attempts have been made to define “ consensus ” sequences in heparin-binding proteins based on subject and spacing of positively charged amino acid residues within linear sequences ( Cardin and Weintraub 1989 ; Hileman et alabama. 1998 ; Capila and Linhardt 2002 ). however, the binding web site for HS is frequently defined by incontrovertible residues contributed by noncontiguous segments of the protein. Although electrostatic interactions contribute much of the binding energy, hydrogen-bonding, van five hundred Waal interactions, and hydrophobic effects besides participate ( Conrad 1998 ; Capila and Linhardt 2002 ). The adhere of a ligand to HS follows the lapp principles that underlie the interaction of other macromolecules, but the take after considerations are crucial .

  • dissociation constants for HS-dependent ligands range from millimolar to nanomolar values. many growth factors bind with high affinity ( for example, fibroblast growth factors, FGFs ), whereas many matrix proteins bind with depleted affinity ( for example, fibronectin ). however, affinity does not dictate selectivity ; some abject affinity ligands achieve high eagerness through dimerization ( for example, chemokines ) or by clustering ( for example, fibronectin fibrils ) .
  • The HS chains can facilitate diffusion of ligands by allowing them to bind and slide or dissociate/reassociate through adjacent ski binding sites ( mass action ) .
  • Binding can lead to a conformational change in the protein. The best-studied exemplar is the allosteric impression of heparin on antithrombin .
  • HS can act as a template to approximate two proteins next to each other. Antithrombin inactivation of thrombin serves as the paradigm for this type of interaction, but other examples include the association of some emergence factors with their receptor tyrosine kinases ( for example, FGF with FGF receptors ) .

In accession to the HS chains, the protein core of HSPGs can besides bind ligands. For exemplar, the Drosophila glypican ortholog, Dally, can immediately interact with a number of morphogens, such as decapentaplegic ( Dpp ) and bone morphogenetic gene 4, in the absence of HS chains ( Kirkpatrick et aluminum. 2006 ). expression of HS-deficient Dally can rescue several mutant phenotypes in Dally-deficient flies, indicating that a number of biologically relevant functions are mediated by the glypican protein core independently of HS. In mammals, the glypican-3 core protein interacts with porcupine ( Hh ) independently of HS, and Gpc3-null embryo display increased Hh sign, which might explain the giantism phenotype observed in patients lacking glypican-3 ( Simpson–Golabi–Behmel syndrome ) ( Capurro et alabama. 2008 ). As discourse below, a peptide succession in the kernel protein of syndecan-1 interacts with αVβ3 and αVβ5 integrins and modulates cell attachment ( Beauvais et aluminum. 2004 ; McQuade et aluminum. 2006 ). ultimately, perlecan, collagen XVIII, and agrin are large proteins composed of multiple, functionally autonomous domains that can bind to other matrix components and growth factors ( Iozzo et aluminum. 2009 ).

PROTEOGLYCANS REGULATE GROWTH FACTOR BINDING TO ECM AND CELL MIGRATION

The ECM provides a morphologic net for mediate and regulating cellular movement ( for example, during development and wound haunt ). One of the ways the ECM regulates cell migration is to directly bind growth factors, such as platelet-derived growth factor ( PDGF ), providing directional and stimulatory cues for moving cells ( Smith et alabama. 2009 ). interestingly, the association of PDGF with ECM appears to be pendent on HS, but does not involve lead bind to HS ( Symes et alabama. 2010 ). similarly, HS-dependent interactions between fibronectin and VEGF have been reported ( Mitsi et aluminum. 2006 ). The mechanism by which HS regulates the bind of growth factors to fibronectin appears to stem from its ability to induce the passage of fibronectin from a ball-shaped form to a more stable widen form, revealing growth factor–binding sites ( Mitsi et alabama. 2008 ). This activity depends on the size and typography of the chains, as shown by studies in which merely heparin chains longer than 22 saccharides and with sulfation at the 6-O- and N-positions of glucosamine units retained the ability to modify fibronectin structure and allow VEGF tie down ( Mitsi et aluminum. 2006 ). Tissue-specific construction of unlike proteoglycans during zebrafish embryogenesis besides has been shown to play a role in determining the structure and function of the extracellular matrix. Syndecan-2 expression in the extraembryonic egg yolk syncytial layer induces fibronectin and laminin matrix assembly throughout the embryo and directs aboriginal cellular telephone migration ( Arrington and Yost 2009 ). interestingly, overexpression of syndecan-2 in the embryo does not rescue embryonic defects resulting from egg yolk syncytial layer insufficiency, suggesting that proteoglycans in specific cellular telephone types can act in a unique manner because of either positional or structural differences. other studies have besides shown that the loss of particular proteoglycans such as syndecan-4 in Xenopus laevis can have adverse effects on neural crest cell migration ( Matthews et aluminum. 2008 ) and convergent extension movements ( Munoz et alabama. 2006 ). These model systems provide a potent empiric approach for determining the engagement of HSPGs in matrix deposition and cell migration .

STEM CELL NICHE

The genesis, care and haunt of unlike tissues during growth is regulated by stem turn cell populations that reside in defined cellular microenvironments known as stem cell niches. These niches are essential for determining the ability of stem cells to retain a self-perpetuating pluripotent state or to differentiate into commit tissue particular progenitors ( Nurcombe and Cool 2007 ). Interestingly, many of the signaling molecules involved in stem cell maintenance, such as Wnts and FGFs, are regulated by HSPGs ( Sato et aluminum. 2004 ; Xu et alabama. 2005a, speed of light ). Furthermore, embryonic bow cells change the structure of their HS as they differentiate into specific lineages ( Johnson et alabama. 2007 ; Baldwin et alabama. 2008 ). To immediately address the character of HSPGs in shank cell differentiation, shiner embryonic root cells with mutations in HS biosynthesis have been studied. embryonic stem cells that lack HS because of Ext1 gene lack are incapable of differentiation on removal of leukemia inhibitory agent, apparently caused by a defective response to FGF ( Kraushaar et alabama. 2010 ). These findings were corroborated by studies in embryonic stem cells lacking Ndst1/2, which besides can not differentiate in answer to FGF because of shrink sulfation ( Lanner et aluminum. 2010 ). In addition, shiner embryonic bow cells deficient in Ndst1/2 were found to be ineffective to respond to VEGF, preventing their specialization into lineage capillary structures ( Jakobsson et alabama. 2006 ). Taken together, these studies substantiate the importance of HS in stem turn cell specialization at least antique vivo. To address how HSPGs might regulate stem cells in their native cellular environments, Nakato and colleagues examined whether glypican participated in the sustenance of stem cells in the Drosophila germline stem turn cell recess. interestingly, this function appears to be related to the ability of glypicans to restrict the localization of function and natural process of the morphogen Dpp to the out boundary of the niche ( Hayashi et alabama. 2009 ). Stem cells directly adjacent to this Dpp-rich pocket were shown to be activated in trans by this morphogen and remained pluripotent. Daughter cells that were not able to physically associate with this region remained tolerant to Dpp signaling and subsequently undergo differentiation. These findings will probable have authoritative implications for stem-cell-based treatments of disease and for the design of synthetic matrices for stem-cell-based weave mastermind .

CONCLUDING REMARKS

The function of this article was to provide an overview of HSPGs and their biological roles in the ECM. As described above, HSPGs bind many ligands, modulate numerous cellular activities, and aid in weave architecture and physiology. The examples selected for presentation map alone a subset of activities associated with HSPGs. however, it is striking that sol many essential activities appear to be regulated by such a minor family of macromolecules. Understanding how cells regulate the construction and constitution of HSPGs to achieve these divers activities in a coordinated fashion is a major biological problem to solve. The problem may be angstrom complex as unraveling the genetic code, given the enormous complexity of heparan sulfate .

ACKNOWLEDGMENTS

The authors acknowledge grants GM33063 and HL57345 ( to J.D.E ) and F32DK085905 ( to W.C.L ) from the National Institutes of Health and a grant from Fondation pour louisiana Recherche Medicale ( to S.S. ) .

Footnotes

Editors : Richard Hynes and Kenneth Yamada extra Perspectives on Extracellular Matrix Biology available at www.cshperspectives.org