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 cellsurface 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. Transforming growth factor-␤ (TGF-␤)1 -like bone morphogenetic proteins (BMPs) were first isolated from osteogenic extracts of bone, and some BMPs are capable of inducing ectopic bone formation (1-3). In addition to roles in bone and cartilage formation, BMPs are involved in morphogenesis of various soft tissues and organs during development and play central roles in dorsoventral patterning in early embryogenesis (4). Dorsoventral patterning in Xenopus requires BMP-2, -4, and -7 for specification of ventral mesodermal and ectodermal cell fates (5-7), whereas specification of dorsal mesoderm (Spemann's organizer) and neuroectoderm requires inhibition of BMP signaling. Dorsal inhibition of BMP signaling is achieved by secreted BMP antagonists such as Chordin and Noggin, which are expressed in Spemann's organizer (for a review, see Ref. 8).In Drosophila, dorsoventral patterning of the embryonic axis is mediated by the BMP-2/4 homolog Decapentaplegic (DPP) (9) and the BMP-7 homolog Screw (10), whereas Chordin is represented by the homolog Short Gastrulation (SOG) (11,12). Drosophila homologs do not exist for the other vertebrate extracellular antagonists of BMP signaling, such as Noggin and the Dan family of BMP inhibitors (13). In Drosophila, DPP is expressed dorsally, whereas SOG is expressed ventrolaterally. Secreted SOG is thought to inhibit signaling by binding DPP in an extracellular latent complex (14), and it appears that SOG diff...

von Willebrand Factor Type C Domain-containing Proteins Regulate Bone Morphogenetic Protein Signaling through Different Recognition Mechanisms

“…Rather, it may be that the adjacent sequence interacts with other proteins involved in BMP/chordin regulation (e.g. heparan sulfate proteoglycan or integrins) (41,42). How the specificity of VWCs for BMPs contributes to the regulation of BMP function by chordin in vivo remains to be investigated in future studies.…”

Chordin forms a self-organizing morphogen gradient in the extracellular space between ectoderm and mesoderm in the Xenopus embryo

“…Thus, the long-range diffusion of morphogens along the ECM separating cell layers might be a general phenomenon in growth-factor signaling in embryonic development. Syndecan-1, a transmembrane HSPG that has been shown to bind Chordin (48), is expressed in the inner layer of the ectoderm immediately adjacent to Brachet's cleft at the gastrula stage in Xenopus (49) and could regulate diffusion.…”

Cell biology of osteochondromas: Bone morphogenic protein signalling and heparan sulphates

“…Membrane bound HSPGs bind to chordin, affect the retention of chordin at cell surfaces and are necessary for its cellular uptake [13]. Membrane bound HSPGs intensify chordin antagonism of BMP signalling by concentrating it at the cell surface [13]. Thus, HSPGs contribute to shape and stabilise the fields of BMP inhibition while at the same time shaping and stabilising reciprocal fields of BMP signalling [13].…”

“…Chordin expression is increased by PTHLH (parathyroid hormone-like hormone) and decreased by BMP-2 [42]. Membrane bound HSPGs bind to chordin, affect the retention of chordin at cell surfaces and are necessary for its cellular uptake [13]. Membrane bound HSPGs intensify chordin antagonism of BMP signalling by concentrating it at the cell surface [13].…”

“…Membrane bound HSPGs intensify chordin antagonism of BMP signalling by concentrating it at the cell surface [13]. Thus, HSPGs contribute to shape and stabilise the fields of BMP inhibition while at the same time shaping and stabilising reciprocal fields of BMP signalling [13].…”