The extracellular regulation of bone morphogenetic protein signaling

Abstract

In many cases, the level, positioning and timing of signaling through the bone morphogenetic protein (BMP) pathway are regulated by molecules that bind BMP ligands in the extracellular space. Whereas many BMP-binding proteins inhibit signaling by sequestering BMPs from their receptors, other BMP-binding proteins cause remarkably context-specific gains or losses in signaling. Here, we review recent findings and hypotheses on the complex mechanisms that lead to these effects, with data from developing systems, biochemical analyses and mathematical modeling.

Fig. 1.

Sog-mediated shuttling of BMPs and the role of membrane-localized reactions. (A) Schematic cross-section of the perivitelline (PV) space in the early Drosophila embryo. The BMP heterodimer Dpp-Scw is expressed broadly along the dorsal side (up), whereas Sog is expressed ventrolaterally (side). Binding of Sog to Dpp-Scw, and the subsequent flow of this complex from ventrolateral to dorsal cells, shuttles the BMPs from ventrolateral to the dorsal-most cells. This detailed view of the shuttling model includes Tsg in the Dpp-Scw-Sog complex, and dorsal cleavage of the complex by the Tld protease, freeing BMPs for signaling. (B) A detailed view of binding reactions between Dpp-Scw and Sog in the extracellular space, including ligand binding and release from membrane-bound type IV collagen, binding and release from signaling receptors, and binding and release from the Dpp-Scw-Sog complex. (C,D) The membrane binding and release processes taking place on type IV collagen. x, the position along the dorsal ventral axis; h₁, the overall height of the PV space; h₂, the height of collagen into the PV space. (E) Early Drosophila embryos, showing high (red) to medium (yellow, green) to low (blue) levels of Sog (left) and of BMP signaling as indicated by pMad (center and right), along the dorsoventral axis; dorsal is up and ventral is down. Initially, dorsal BMP signaling is broad and low, but ventrolateral Sog inhibits ventrolateral signaling and increases signaling in the dorsal-most cells. Dpp, Decapentaplegic; pMad, phosphorylated Mothers against decapentaplegic; Scw, Screw; Sog, Short gastrulation; Tsg, Twisted gastrulation; Tld, Tolloid.

Fig. 2.

Chordin-mediated BMP shuttling and its possible role in rescaling the dorsoventral axis of ligated Xenopus embryos. A schematic of a Xenopus embryo. BMPs, the Xlr protease and the Xlr inhibitor Szl are high ventrally (V, blue), whereas Chordin and the BMP ADMP are high dorsally (D, red). Following ligation, one model (Ben-Zvi et al., 2008) proposes that Chordin shuttles ADMP ventrally, where subsequent ADMP-mediated BMP signaling stimulates expression of BMPs; these then further reinforce their own expression to reform the gradient of signaling. Models that do not rely on shuttling have also been proposed (Kimelman and Pyati, 2005). ADMP, Anti-dorsalizing morphogenetic protein; BMP, bone morphogenetic protein; Chd, Chordin; Szl, Sizzled; Xlr, Xolloid-like metalloprotease.

Fig. 3.

Structure and activity of Cv2. (A) Structure of the uncleaved and cleaved versions of secreted Cv2. CR, cysteine-rich domain; vWFD, von Willebrand Factor D domain; HSPG, heparan sulfate proteoglycan. (B) Model of posterior crossvein (PCV) development in the pupal Drosophila wing. BMPs - probably a mixture of Dpp, Gbb and Dpp-Gbb heterodimers - are transported from the adjacent longitudinal veins (red) into the PCV (yellow) by a complex of Sog and a second Drosophila Tsg protein called Crossveinless (Tsg2), where it is released for signaling by the Tolloid-related protease (not shown). Drosophila Cv-2 is expressed in the PCV region, where it locally increases BMP signaling, leading to PCV formation. (C) Changes in BMP signaling in response to increasing concentrations of Cv-2. Increasing Cv-2 concentration can cause either a biphasic response (left), in which BMP signaling initially increases before being inhibited with increasing Cv-2 concentration, or a purely inhibitory response (right). BMP, bone morphogenetic protein; Cv2, Cv-2 (Crossveinless 2; BMPER); Sog, Short gastrulation; Tsg, Twisted gastrulation.

Fig. 4.

Models of Cv2 activity. (A) The exchange model of Cv2 activity, in which binding between Cv2 and type I BMP receptors either increases the flow of BMPs to the receptors or sequesters BMPs in inactive complexes. The model also hypothesizes a similar exchange of BMPs between Cv2 and Chordin/Sog, mediated by binding between Cv2 and a Chordin/Sog-Tsg complex, which could increase the release of BMPs from Chordin/Sog, increasing the levels of Cv2-bound BMPs available for exchange with the BMP receptors. (B) The sink model of Cv2 activity. (Left) Cv2 is expressed at a distance from the region of BMP, Chordin/Sog and Tsg production. By binding Chordin/Sog, Tsg or a Chordin-Tsg-BMP complex (as shown), Cv2 locally reduces the concentration of free Chordin/Sog, Tsg or Chordin-Tsg-BMP complex. This will also result in an increase in the flow of the Cv2-binding proteins towards Cv2-expressing cells. (Right) In the absence of Cv2, the levels of free Cv2-binding proteins rise in both nearby and distant tissues, and, after levels reach an equilibrium, the net flow of the Cv2-binding proteins away from their region of production is reduced. BMP, bone morphogenetic protein; Cv2 (Crossveinless 2; BMPER); Sog, Short gastrulation; Tsg, Twisted gastrulation.

Fig. 5.

Putative roles for proteoglycans in BMP signaling. (A) The exchange model applied to the modulation of BMP signal reception by a membrane-bound glypican HSPG. (B) Reduced movement of tagged Dpp through a Drosophila wing disc clone that lacks heparan sulfate synthesis or the glypicans Dally and Dally-like (Dlp). (C-F) Models of how HSPGs promote BMP (Dpp) movement in Drosophila wing discs. (C) HSPGs prevent the loss of BMPs via either receptor-mediated endocytosis or diffusion out of the epithelium, increasing the levels available for the next cell. In the absence of HSPGs, less BMP is available for diffusion to the next cell. (D) HSPGs increase BMP diffusion across a single cell by moving along the membrane of that cell. (E) Diffusible HSPGs shuttle BMPs to adjacent cells. (F) HSPGs help mediate BMP transcytosis, increasing the levels of BMPs available for the next cell. BMP, bone morphogenetic protein; Dpp, Decapentaplegic; HSPG, heparan sulfate proteoglycan.