A concentration-dependent endocytic trap and sink mechanism converts Bmper from an activator to an inhibitor of Bmp signaling

Bone morphogenetic proteins 4 and 7 (BMP4 and BMP7) are morphogens that signal as either homodimers or heterodimers to regulate embryonic development and adult homeostasis. BMP4/7 heterodimers exhibit markedly higher signaling activity than either homodimer, but the mechanism underlying the enhanced activity is unknown. BMPs are synthesized as inactive precursors that dimerize and are then cleaved to generate both the bioactive ligand and prodomain fragments, which lack signaling activity. Our study reveals a previously unknown requirement for the BMP4 prodomain in promoting heterodimer activity. We show that BMP4 and BMP7 precursor proteins preferentially or exclusively form heterodimers when coexpressed in vivo. In addition, we show that the BMP4 prodomain is both necessary and sufficient for generation of stable heterodimeric ligands with enhanced activity and can enable homodimers to signal in a context in which they normally lack activity. Our results suggest that intrinsic properties of the BMP4 prodomain contribute to the relative bioactivities of homodimers versus heterodimers in vivo. These findings have clinical implications for the use of BMPs as regenerative agents for the treatment of bone injury and disease.B one morphogenetic proteins (BMPs) are members of the TGFβ superfamily that were originally isolated as boneinducing morphogens and were subsequently found to play central roles during embryogenesis and in adult homeostasis (1). BMPs are clinically important therapeutic agents that are used to reverse bone loss caused by trauma, disease, and tumor resection (2). Their use as regenerative agents is limited, however, by their short half-life and low specific activity when implanted in vivo. Understanding how BMP activity is regulated is important for the development of more effective therapeutic agents for the treatment of bone injury and disease.BMPs bind to and activate a receptor complex consisting of type I and type II transmembrane serine/threonine kinases. Following ligand binding, activated receptors propagate their signal by phosphorylating one of the SMADs that is specific for the BMP pathway (SMAD1, -5, or -8). The phosphorylated Smads then form heterooligomers with the common Smad, Smad4, and this complex translocates into the nucleus where it binds to BMP response elements and activates transcription of target genes (1).BMPs are classified into subfamilies based on sequence homology. They signal as either homodimers, or as heterodimers from different subfamilies. For example, class I BMPs, which consist of BMP2 and BMP4, can heterodimerize with class II BMPs, consisting of BMP5-8 (3). Heterodimers composed of distinct BMP family members show a higher specific activity than do homodimers of either subunit. Homodimers of BMP2, -4, or -7, for example, can all induce bone formation, but heterodimers of BMP2 plus BMP7, or BMP4 plus BMP7 are significantly more potent (5-to 20-fold) than any of the homodimers in osteogenic differentiation assays (4-6). BMP2/7 and BMP4/7 heterodimers also sho...

Section: Discussionmentioning

confidence: 99%

The prodomain of BMP4 is necessary and sufficient to generate stable BMP4/7 heterodimers with enhanced bioactivity in vivo

Neugebauer¹,

Kwon

Kim³

et al. 2015

“…sfrp5 has been described as a Wnt inhibitor in several systems (27,28), an integrator of Wnt-BMP signaling in the zebrafish gut (29), and a regulator of mouse incisor renewal (30). bmper has been reported as both a positive and negative mediator of BMP signaling in other organs of the mouse, frog, and fly (31)(32)(33), and as an antagonist of BMP in mouse incisor ameloblasts (34). We assayed expression of these candidates in species with divergent densities of teeth/taste buds: C. afra, a species with few teeth and taste buds and Labeotropheus fueleborni, a species with many of both (SI Appendix, Fig.…”

Section: Cichlid Tooth and Taste Bud Fields Are Specified From A Commonmentioning

confidence: 99%

Coevolutionary patterning of teeth and taste buds

Bloomquist

Parnell

Phillips

et al. 2015

Teeth and taste buds are iteratively patterned structures that line the oro-pharynx of vertebrates. Biologists do not fully understand how teeth and taste buds develop from undifferentiated epithelium or how variation in organ density is regulated. These organs are typically studied independently because of their separate anatomical location in mammals: teeth on the jaw margin and taste buds on the tongue. However, in many aquatic animals like bony fishes, teeth and taste buds are colocalized one next to the other. Using genetic mapping in cichlid fishes, we identified shared loci controlling a positive correlation between tooth and taste bud densities. Genome intervals contained candidate genes expressed in tooth and taste bud fields. sfrp5 and bmper, notable for roles in Wingless (Wnt) and bone morphogenetic protein (BMP) signaling, were differentially expressed across cichlid species with divergent tooth and taste bud density, and were expressed in the development of both organs in mice. Synexpression analysis and chemical manipulation of Wnt, BMP, and Hedgehog (Hh) pathways suggest that a common cichlid oral lamina is competent to form teeth or taste buds. Wnt signaling couples tooth and taste bud density and BMP and Hh mediate distinct organ identity. Synthesizing data from fish and mouse, we suggest that the Wnt-BMP-Hh regulatory hierarchy that configures teeth and taste buds on mammalian jaws and tongues may be an evolutionary remnant inherited from ancestors wherein these organs were copatterned from common epithelium.quantitative trait loci | tooth/taste bud development | placode patterning | bipotency | plasticity

“…In zebrafish, chordin becomes a significantly more effective BMP inhibitor in vivo following cleavage of the C-terminal domain (16). However, there is significant evidence that chordin fragments have reduced biological activity in Xenopus (1,17) and are subject to more rapid endocytotic turnover (18). Chordin interacts with cell surface proteins including crossveinless-2 (CV2) (19) and collagen IV (20).…”

mentioning

confidence: 99%

Nanoscale structure of the BMP antagonist chordin supports cooperative BMP binding

Troilo

Zuk²,

Tunnicliffe

et al. 2014

Bone morphogenetic proteins (BMPs) orchestrate key cellular events, such as proliferation and differentiation, in development and homeostasis. Extracellular antagonists, such as chordin, are essential regulators of BMP signaling. Chordin binds to BMPs blocking interaction with receptors, and cleavage by tolloid proteinases is thought to relieve this inhibition. A model has been previously proposed where chordin adopts a horseshoe-like arrangement enabling BMP binding cooperatively by terminal domains (1). Here, we present the nanoscale structure of human chordin using electron microscopy, small angle X-ray scattering, and solution-based biophysical techniques, which together show that chordin indeed has a compact horseshoe-shaped structure. Chordin variants were used to map domain locations within the chordin molecule. The terminal BMP-binding domains protrude as prongs from the main body of the chordin structure, where they are well positioned to interact with the growth factor. The spacing provided by the chordin domains supports the principle of a cooperative BMP-binding arrangement that the original model implied in which growth factors bind to both an N-and C-terminal von Willebrand factor C domain of chordin. Using binding and bioactivity assays, we compared full-length chordin with two truncated chordin variants, such as those produced by partial tolloid cleavage. Cleavage of either terminal domain has little effect on the affinity of chordin for BMP-4 and BMP-7 but C-terminal cleavage increases the efficacy of chordin as a BMP-4 inhibitor. Together these data suggest that partial tolloid cleavage is insufficient to ablate BMP inhibition and the C-terminal chordin domains play an important role in BMP regulation.