title: Cell-surface heparan sulfate proteoglycans potentiate chordin antagonism of bone morphogenetic protein signaling and are necessary for cellular uptake of chordin.
abstract: Signaling by bone morphogenetic proteins (BMPs) plays a central role in early embryonic patterning, organogenesis, and homeostasis in a broad range of species. Chordin, an extracellular antagonist of BMP signaling, is thought to readily diffuse in tissues, thus forming gradients of BMP inhibition that result in reciprocal gradients of BMP signaling. The latter determine cell fates along the embryonic dorsoventral axis. The secreted protein Twisted Gastrulation (TSG) is thought to help shape BMP signaling gradients by acting as a cofactor that enhances Chordin inhibition of BMP signaling. Here, we demonstrate that mammalian Chordin binds heparin with an affinity similar to that of factors known to functionally interact with heparan sulfate proteoglycans (HSPGs) in tissues. We further demonstrate that Chordin binding in mouse embryonic tissues was dependent upon its interaction with cell-surface HSPGs and that Chordin bound to cell-surface HSPGs (e.g. syndecans), but not to basement membranes containing the HSPG perlecan. Surprisingly, mammalian TSG did not bind heparin unless prebound to Chordin and/or BMP-4, although Drosophila TSG has been reported to bind heparin on its own. Results are also presented that indicate that Chordin-HSPG interactions strongly potentiate the antagonism of BMP signaling by Chordin and are necessary for the retention and uptake of Chordin by cells. These data and others regarding Chordin diffusion have implications for the paradigm of how Chordin is thought to regulate BMP signaling in the extracellular space and how gradients of BMP signaling are formed.
authors.affiliation: Program in Molecular and Cellular Pharmacology, University of Wisconsin, Madison, Wisconsin 53706, USA.
authors.forename: Reema
authors.initials: R
authors.lastname: Jasuja
authors.forename: Benjamin L
authors.initials: BL
authors.lastname: Allen
authors.forename: William N
authors.initials: WN
authors.lastname: Pappano
authors.forename: Alan C
authors.initials: AC
authors.lastname: Rapraeger
authors.forename: Daniel S
authors.initials: DS
authors.lastname: Greenspan
mesh_terms: D000818:Animals; D015153:Blotting, Western; D055415:Bone Morphogenetic Protein 4; D019485:Bone Morphogenetic Proteins / Q000378:metabolism*; D019070:Cell Lineage; D002462:Cell Membrane / Q000378:metabolism*; D002478:Cells, Cultured; D018920:Coculture Techniques; D003545:Cysteine / Q000737:chemistry; D004330:Drosophila; D000939:Epitopes / Q000737:chemistry; D005109:Extracellular Matrix / Q000378:metabolism; D005347:Fibroblasts / Q000378:metabolism; D006023:Glycoproteins / Q000037:antagonists & inhibitors* / Q000378:metabolism; D019812:Heparan Sulfate Proteoglycans / Q000737:chemistry*; D006493:Heparin / Q000737:chemistry; D036341:Intercellular Signaling Peptides and Proteins / Q000378:metabolism; D008168:Lung / Q000378:metabolism; D051379:Mice; D008856:Microscopy, Fluorescence; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D015398:Signal Transduction; D014018:Tissue Distribution; D014162:Transfection
chemical_list: C525810:Bmp4 protein, mouse; D055415:Bone Morphogenetic Protein 4; D019485:Bone Morphogenetic Proteins; D000939:Epitopes; D006023:Glycoproteins; D019812:Heparan Sulfate Proteoglycans; D036341:Intercellular Signaling Peptides and Proteins; D006493:Heparin; C054234:chordin; D003545:Cysteine