Sizzled controls dorso-ventral polarity by repressing cleavage of the Chordin protein

Abstract

The Bone morphogenetic protein (Bmp) signalling gradient has a major function in the formation of the dorso-ventral axis. The zebrafish ventralized mutant, ogon, encodes Secreted Frizzled (Sizzled). sizzled is ventrally expressed in a Bmp-dependent manner and is required for the suppression of Bmp signalling on the ventral side of zebrafish embryos. However, it remains unclear how Sizzled inhibits Bmp signalling and controls ventro-lateral cell fate. We found that Sizzled stabilizes Chordin, a Bmp antagonist, by binding and inhibiting the Tolloid-family metalloproteinase, Bmp1a, which cleaves and inactivates Chordin. The cysteine-rich domain of Sizzled is required for inhibition of Bmp1a activity. Loss of both Bmp1a and Tolloid-like1 (Tll1; another Tolloid-family metalloproteinase) function leads to a complete suppression and reversal of the ogon mutant phenotype. These results indicate that Sizzled represses the activities of Tolloid-family proteins, thereby creating the Chordin-Bmp activity gradient along the dorso-ventral axis. Here, we describe a previously unrecognized role for a secreted Frizzled-related protein.

SFRPs act as negative modulators of ADAM10 to regulate retinal neurogenesis

“…In vertebrates, BMP activity is in part controlled by the BMP antagonist Chordin which, in turn, is inactivated through cleavage by Tolloid. Sizzled, but apparently not Sfrp2 13 binds to Tolloid and behaves as a TIMP acting as competitive inhibitor of the enzymatic activity, thus preventing Chordin cleavage 15 ; 16 . Sfrp2 instead interacts through its CRD to the non-protease domain of Tolloid proteases and enhances their procollagen-C proteinase activity 13 or inhibits it depending of its concentration 14 .…”

“…Sfrp2 has also been described to interact with an integrinfibronectin complex modulating apoptosis 8 . Furthermore, Sizzled a member of the family not present in mammals, acts as a negative feedback regulator of BMP signaling by binding to BMP1/Tolloid, a metalloprotease that normally degrades the Bmp antagonist chordin 15 ; 16 .…”

The bone morphogenetic protein 1/Tolloid-like metalloproteinases

“…mTLL2 may provide such activity in vivo, since it has Chordinase activity in vitro in the presence of the modulator protein twisted gastrulation (TSG) ; also see below). In contrast to mammals, knockdown of the expression of the bmp1a and tll1 genes in zebrafish is sufficient to severely dorsalize embryos (Jasuja et al, 2006;Muraoka et al, 2006), indicating less redundancy for Chordin cleavage in teleosts than in mammals. In addition to their roles in patterning, BMPs 2 and 4 are potently osteoinductive .…”

“…Apparently due to partial genome duplication in teleosts, two genes exist for BMP1 in zebrafish (bmp1a and bmp1b) (Muraoka et al, 2006;Jasuja et al, 2006), although only sequences for longer mTLD-like proteinases have been described for these genes, and it has yet to be determined whether they also encode alternatively spliced mRNAs for shorter BMP1 forms. In contrast, cDNA encoding the short form of BMP1 (suBMP), has been described in the sea urchin S. purpuratus (Hwang et al, 1994), with a longer alternatively spliced form yet to be described.…”

“…Importantly, the multiple roles of BMP1/TLD-like proteinases in collagen fibrillogenesis make them attractive targets for anti-fibrotic interventions. Approaches towards therapeutic inhibition of BMP1/TLD-like proteinases may take advantage of the manipulation of recently reported endogenous inhibitors (Lee et al, 2006;Muraoka et al, 2006; and/or development of small molecule inhibitors (Veitch et al, 2003). Processing of the N-and C-propeptides of type I procollagen by ADAMTS-2 and BMP-1, respectively.…”