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Here we report an unexpected role for the secreted Frizzled-related protein (sFRP) Sizzled/Ogon as an inhibitor of the extracellular proteolytic reaction that controls BMP signaling during Xenopus gastrulation. Microinjection experiments suggest that the Frizzled domain of Sizzled regulates the activity of Xolloid-related (Xlr), a metalloproteinase that degrades Chordin, through the following molecular pathway: Szl -| Xlr -| Chd -| BMP --> P-Smad1 --> Szl. In biochemical assays, the Xlr proteinase has similar affinities for its endogenous substrate Chordin and for its competitive inhibitor Sizzled, which is resistant to enzyme digestion. Extracellular levels of Sizzled and Chordin in the gastrulaembryo and enzyme reaction constants were all in the 10(-8) M range, consistent with a physiological role in the regulation of dorsal-ventral patterning. Sizzled is also a natural inhibitor of BMP1, a Tolloid metalloproteinase of medical interest. Furthermore, mouse sFRP2 inhibited Xlr, suggesting a wider role for this molecular mechanism.
Figure 1. Sizzled Is Expressed Ventrally Yet Requires a Dorsal Component to Function(A–C) Szl-depleted embryo is ventralized, as marked by the accumulation of Szl ventro-posterior transcripts and phenocopies knockdown of Chd.(D) Chd and Szl are expressed at opposite poles of the Xenopus gastrula.(E) DV patterning is regulated via proteins secreted by the dorsal and ventral signaling centers.(F) Embryo bisected at blastula across the DV axis can self-regulate to form a well-proportioned dorsal-half embryo, while the ventral half forms a belly-piece. Sox2 marks the CNStissue. Uninjected wild-type embryo shown on top.(G) Injection of Szl mRNA increases Sox2 staining in intact (top) and dorsal-half embryos but is without phenotypic effect in the ventral-half embryo (n = 40).(H) A Spemann organizer transplant on the ventral side of a wild-type embryo forms a Y-shaped Siamese embryo with two heads and a single tailbud (n = 10).(I) Wild-type Spemann organizer grafted into a Szl-depleted host forms an H-shaped embryo with CNS induction and two distinct tailbuds (n = 11).
Figure 2. Sizzled Is a Feedback Inhibitor of BMP Signaling that Requires Xlr Activity(A) Endogenous Smad1 phosphorylation at early gastrula is increased by Szl MO.(B) An autoregulatory negative feedback loop is interrupted in Szl MO embryos, resulting in ventro-posterior accumulation of Szl transcripts.(C) Szl upregulation is mediated by BMP4 signaling.(D) Chd protein, but not Chd mRNA levels, are decreased by Szl knockdown (compare lanes 1 and 2). BMP2/4/7 MO injection serves as negative control.(E and F) Xlr mRNA overexpression or Chd knockdown increase ventralSzl transcripts, indicating increased BMP signaling.(G) Szl-Fc protein injection into the blastula cavity suppresses the pro-BMP effects of Xlr mRNA.(H) Szl-Fc protein has no effect in Chd-depleted embryos. Inset shows phenotype of Szl-Fc injection.(I) The role of Xlr in the regulation of Chordin anti-BMP activity. Tsg (Twisted gastrulation) is a cofactor of Chordin.(J) Microinjection of DN-Xlr mRNA expands Otx2 expression, a sign of dorsalization.(K) Szl MO feedback loop requires endogenous Xlr function since it is inhibited by DN-Xlr mRNA; compare with inset showing sibling embryo injected with Szl MO alone.All microinjection results were performed at least three times and minimum of 15 embryos were analyzed by hybridization in each sample.(L) Schematic diagram of the proposed BMP negative feedback loop via Sizzled, in which Szl inhibits Xlr activity.
Figure 4. The SzlD92N Point Mutation Mimicking Zebrafish ogon Inhibits Xlr Binding and Biological Activity(A) Diagram of Szl-Fc and SzlD92N/Ogon-Fc constructs.(B) BIAcore measurements of binding of 10 μg/ml Xlr in the flow to wild-type Szl-Fc or SzlD92N/Ogon-Fc immobilized on the chip. Note the ten-fold decrease in binding affinity to Xlr.(C–H) Microinjection of 40 nl of 25 μM wild-type Szl-Fc protein into the blastula cavity dorsalizes the Xenopus embryo (E) and rescues the Szl knockdown phenotype (F). Injection of same concentration of SzlD92N/Ogon-Fc protein had no obvious phenotypic effect on wild-type (G) or Szl MO (H) embryos.
Figure 5. Sizzled, but not SzlD92N Mutant, Inhibits the Proteolytic Activity of Xlr on Its Natural Substrate Chordin(A) Diagram of Chordin protein structure and antibody recognition sites.(B) Specific proteolysis of Chd (20 nM) by Xlr enzyme can by inhibited by increasing concentrations of Szl-Fc (lanes 3–8, 4–100 nM).(C) Increasing concentrations of SzlD92N/Ogon-Fc do not inhibit Xlr activity.(D) Dorsalizing biological activity of Szl is caused by its frizzled domain, not the netrin domain.(E) Sizzled inhibits Xlr proteolysis of Chordin via its frizzled domain.(F) Sizzled is not digested by Xlr after 2 hr digestion with 2 nM Xlr enzyme.
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