Lefty Blocks a Subset of TGFβ Signals by Antagonizing EGF-CFC Coreceptors

1The analysis of whole-genome sequences has revealed that most signaling systems consist of multiple ligands that converge on a relatively small set of receptors and pathway-specific transcription factors. In the case of human transforming growth factor-β (TGFβ) signaling, 42 TGFβs converge on seven type I receptors, five type II receptors, and two classes of Smad signal transducers (reviewed in ). This convergence has raised the question of how ligand diversity and signaling specificity among different signals can be achieved. If different TGFβs activate the same receptors, it is unclear how these ligands can vary in their function (diversity) or how a given signal can have a unique role (specificity). Biochemical studies have suggested that ligand diversity can be attained by differential stability and receptor affinity, leading to differences in signaling strength (reviewed in; ). An additional source of ligand variability stems from differential ligand movement through a field of cells, rendering related signals either short- or long-range. Finally, specificity and diversity can also be determined by ligand-specific cofactors or inhibitors. A prominent example involves epidermal growth factor–Cripto/FRL-1/Cryptic (EGF-CFC) coreceptors and the TGFβs Activin, Nodal, and Vg1/GDF1 (growth and differentiation factor-1). In this case, differential dependence on a coreceptor leads to ligand diversity and signaling specificity (reviewed in ).

2Members of the Nodal, Activin, and Vg1/GDF1 subfamilies display similar activities and are potent mesendoderm inducers in vertebrates (reviewed in ). Genetic and biochemical studies have shown that EGF-CFC proteins are essential for signaling by Nodal and Vg1/GDF1. EGF-CFC proteins are extracellular glycosylphosphatidylinositol (GPI)-linked factors and include One-eyed pinhead (Oep) in zebrafish and mammalian Cripto and Cryptic (reviewed in;; ). Genetic studies in zebrafish and mouse have shown that EGF-CFC proteins and Nodal are required for mesoderm and endoderm induction (, 1994;;;; ). For example, zebrafish embryos lacking both the maternal and zygotic contribution of Oep (MZoep) lack all endoderm and most mesoderm, similar to the double mutants for the zebrafish Nodal-related genes cyclops and squint (sqt). Moreover, Nodal and Vg1/GDF1 are inactive in MZoep mutants. During later stages of development, Oep, Cryptic, Nodal, and GDF1 are required for proper left–right axis formation.

3The EGF-CFC protein Cripto is highly overexpressed in human epithelial cancers, such as breast and colon carcinomas (reviewed in ), and has been implicated in tumorigenesis (, 1994;;;; ). The mechanism by which Cripto mediates tumorigenesis is not well understood. Several possibilities include mediating Nodal/GDF1 signaling, antagonizing Activin signaling, or activating Akt and mitogen-activated protein kinase (MAPK) pathways independently of the TGFβ signals and Activin receptors (1999;, 2003). Whatever the molecular mechanism of Cripto activity, inhibition of Cripto by antisense or antibody blockade can inhibit tumor cell proliferation in vitro and in vivo.

4Biochemically, EGF-CFC proteins can act as coreceptors for Nodal and Vg1/GDF1 to bind and activate the type I Activin receptor Alk4 and the type II Activin receptor ActRIIB. In the absence of EGF-CFC proteins, these TGFβs cannot form a complex with Activin receptors. Strikingly, Activin utilizes the same receptors as Nodal and Vg1/GDF1, but does not require EGF-CFC coreceptors (;, 1996;;;; ). For instance, Activin can signal in MZoep mutants. This ligand diversity between Activin and Nodal or Vg1/GDF1 raises the question of which sequences confer coreceptor dependence or independence. Activin, Nodal, and Vg1/GDF1 are highly related and are thought to acquire very similar folds. Like other TGFβ ligands, Activin has four major structural features: a β-stranded Finger 1, an α-helical Heel, a β-stranded Finger 2, and three conserved disulfide bonds that form a cysteine knot. Sequence comparisons indicate that the highest divergence among Activin, Nodal, and Vg1/GDF1 is in the N-terminal segment of Finger 1, the central α-helix, and the loop of Finger 2 with approximately 10%, approximately 15%, and approximately 25% sequence identity, respectively. These regions are potential candidates to determine the specificity of receptor–coreceptor–ligand interactions.

5In addition to coreceptors, antagonists represent another class of extracellular factors that control ligand access to receptors (reviewed in;;; ). For example, the divergent TGFβ class of Lefty proteins antagonizes Nodal signaling (reviewed in; ). Unlike other TGFβs, Lefty proteins may function as monomers due to the lack of a cysteine residue involved in dimerization. Lefty overexpression in zebrafish induces phenotypes identical to cyclops;sqt double mutants and MZoep mutants. Furthermore, the loss of Lefty activity leads to enhanced Nodal signaling during mesoderm induction and left–right axis determination (, 2001;;;; ). Although it has not been determined whether Lefty directly blocks Vg1/GDF1 signaling, it has been proposed that Lefty inhibits signaling by Activin. Misexpression of Activin or ActRIIB can overcome the inhibitory effects of Lefty. Hence, some members of the Lefty family have been called Antivins for their anti-Activin properties. However, it has been elusive how Lefty functions at the molecular level.

6Here we present genetic and biochemical studies in zebrafish and Xenopus that indicate that Lefty is an in vivo antagonist of EGF-CFC coreceptors. We find that Lefty can antagonize signaling by the coreceptor-dependent ligands Nodal and Vg1/GDF1, but not Activin. Lefty genetically interacts with Cripto and Oep and competes with Nodal for binding to Cripto, representing a novel mechanism for antagonizing TGFβ signaling. We identify a short region in Finger 2 of Activin, Nodal, and Vg1 that determines EGF-CFC coreceptor-dependent or coreceptor-independent signaling and governs susceptibility to Lefty. These results indicate that subtle sequence variations in TGFβ ligands can dramatically expand signaling diversity by determining interactions with coreceptors and their antagonists.

7TGFβ ligands that activate Activin receptors can be categorized into two classes. The Activin class activates Activin receptors in an EGF-CFC coreceptor-independent manner, whereas the Nodal and Vg1/GDF1 classes require EGF-CFC proteins for receptor activation. To determine whether these classes are also differentially susceptible to inhibition by the TGFβ antagonist Lefty, we coexpressed zebrafish Lefty1 with Xenopus ActivinβB, Xenopus ActivinβA, Sqt (a zebrafish Nodal-related protein), or zebrafish Vg1 (a chimeric molecule containing the Xenopus ActivinβB prodomain fused to the mature domain of zebrafish Vg1) in zebrafish embryos (;;;; Erter et al. 1998;; ). As a readout for active signaling, we analyzed the ectopic induction of the Nodal downstream gene goosecoid (gsc). ActivinβB, ActivinβA, Sqt, and Vg1-induced ectopic gsc expression in wild-type embryos (Figure 1D, 1G, 1J, and 1M;; ). Coexpression of Lefty1 efficiently inhibited gsc induction by Sqt (Figure 1K and 1L;;; ) and Vg1 (Figure 1N and 1O), but not ActivinβB or ActivinβA (Figure 1E, 1F, 1H, and 1I). To examine whether Lefty1 can antagonize the induction of a gene that responds to very low levels of Activin signaling, we titrated ActivinβB levels so that no tail (ntl; also known as brachyury/T) expression was only weakly induced (see arrowhead in Figure 1A). The coexpression of Lefty1 did not inhibit ntl induction by ActivinβB (Figure 1B and 1C), but inhibited the dorsal margin expression of ntl that is dependent on endogenous Nodal signaling (see asterisks in Figure 1B and 1C; ). In a more quantitative assay, we overexpressed Lefty1, ActivinβB, Sqt, and Vg1 in zebrafish embryos in the presence of the luciferase reporter A3-luc, which contains FoxH1/P-Smad2 response elements. Consistent with the gsc and ntl induction assays, Sqt and Vg1 signaling, but not ActivinβB signaling, is inhibited by Lefty1 (Figure 1P). These results indicate that Lefty1 efficiently antagonizes Nodal and Vg1/GDF1 signaling, but not Activin signaling.

8The molecular mechanism of Lefty action has been unresolved. Lefty seems to act upstream of the Activin receptors, as Lefty cannot block signaling from ligand-independent constitutively activated receptors. Our finding that Lefty blocks Nodal and Vg1 signaling, but not Activin signaling, suggests that Lefty blocks extracellular components specific to the Nodal and Vg1 pathways. The only such factors identified to date are the EGF-CFC coreceptors. We therefore examined whether the EGF-CFC genes zebrafish oep and mouse Cripto genetically interact with Lefty1. Overexpression of Lefty1 in wild-type zebrafish resulted in embryos lacking head and trunk mesendoderm due to inhibition of endogenous Nodal signaling (Figure 2A1 and 2A′;;; ). Coexpression of Cripto or Oep partially suppressed Lefty-induced defects (Figure 2B1–2B3; data not shown), restoring trunk and head mesoderm, including the notochord, and resulting in the separation of the eye field into two eyes. These results indicate an antagonistic relationship between EGF-CFC coreceptors and Leftys.

9Because Lefty and EGF-CFC proteins interact genetically, we examined whether Lefty interacts biochemically with Cripto and/or ActRIIB/Alk4 receptor complexes. We expressed and immunoprecipitated epitope-tagged ligands (zebrafish Lefty1/HA [hemagglutinin], zebrafish Lefty1/Glu, or zebrafish Sqt/HA), receptors (mouse ActRIIB[KR]/Myc and human Alk4[KR]/Flag), and a coreceptor (mouse Cripto/Flag) in Xenopus embryos. Similar to other Nodal-related proteins, Sqt formed a complex with the type II receptor ActRIIB, type I receptor Alk4, and Cripto (Figure 3A). In contrast, Lefty1 coimmunoprecipitated Cripto, but not ActRIIB or Alk4 (Figure 3A). Since Cripto is bound to Alk4 even in the absence of ligand, Lefty seemed to disrupt the Cripto–Alk4 interaction. In reverse experiments, Cripto efficiently coimmunoprecipitated Lefty1 (Figure 3B). Since Sqt and Lefty1 can both bind to Cripto (Figure 3C;;;;; ), these two ligands might compete for binding to Cripto. Indeed, the coexpression of Lefty1 led to decreased interactions between Cripto and Sqt (Figure 3C). To determine whether Cripto can directly interact with Lefty, we immunoprecipitated purified mouse Lefty1 protein (mLefty1) in the presence of either purified mouse Cripto protein or a control cysteine-rich protein, mouse vascular endothelial growth factor-D (VEGF-D). mLefty1 protein directly interacted with Cripto, but not with VEGF-D. Together, these results suggest that Lefty inhibits Nodal signaling by associating with Cripto and blocking it from interacting with Nodal.

10The finding that TGFβ ligands that activate Activin receptors can be grouped into a EGF-CFC coreceptor-dependent class that is susceptible to inhibition by Lefty (Nodal and Vg1) and a class that is independent of EGF-CFC proteins and resistant to Lefty (Activin) prompted us to examine which sequences underlie this ligand diversity. We therefore generated chimeras between Activins (EGF-CFC-independent) and Sqt or Vg1 (EGF-CFC-dependent) (Figures 4 and 5). As a readout for active signaling, we injected mRNAs encoding these chimeric ligands into wild-type and MZoep zebrafish embryos and analyzed the ectopic induction of the downstream genes ntl and gsc. Sqt, Vg1, and Activins induced these genes in wild-type embryos, allowing us to determine which chimeric ligands were active. Activins, but not Sqt or Vg1, were active in MZoep mutants, allowing us to test which sequences conferred EGF-CFC coreceptor dependence or independence.

11Initially, swaps of the Finger 1, Heel, or Finger 2 domains of Sqt and ActivinβB were generated. As shown in Figure 4, the Finger 2 region of ActivinβB contains sequence determinants that conferred EGF-CFC-independent activity on chimeric ligands. Chimeric SSA, ASA, and SAA that contain the Finger 2 region of ActivinβB were active in both wild-type and MZoep embryos (Figure 4). To further delineate this region, we generated additional chimeras (Figure 5B). Short stretches of full-length Sqt were replaced by the corresponding region of ActivinβB, including the β6β7, loop, β8, or β9 subregions (Figure 5A and 5B; data not shown). Analysis of these chimeras revealed that the 14 amino acids encoding the loop and β8 region of ActivinβB (SqtAct β B[loop β 8]; the bracketed region in superscript denotes substituted domains) were sufficient to confer EGF-CFC independence. Further dissection of this region into loop alone (SqtAct β B[loop]) or β8 alone (SqtAct β B[ β 8]) yielded no or much weaker activity in MZoep mutants as compared with wild-type embryos (Figure 5B and 5C). These results suggest that a 14 amino acid region in Activin is sufficient to confer EGF-CFC independence when placed into Sqt.

12To determine whether the loop-β8 region has a wider role in conferring coreceptor independence, we generated additional chimeras using ActivinβA (another EGF-CFC-independent ligand) and Vg1. SqtAct β A[loop β 8] (full-length Sqt with an ActivinβA loop-β8 region) and Vg1Act β B[loop β 8] (Vg1 with an ActivinβB loop-β8 region) both induced gsc expression in MZoep mutants with similar efficiencies as in wild-type embryos (Figure 5D). These results were also corroborated using the A3-luc reporter assay (Figure 5E) and suggest that the loop-β8 region has a general role in conferring EGF-CFC coreceptor independence.

13SqtAct β B[loop β 8] can signal in an EGF-CFC-independent manner in vivo, suggesting that this chimeric protein might bind to ActRIIB and Alk4 receptors in the absence of EGF-CFC coreceptors. To test this idea, we coexpressed and immunoprecipitated epitope-tagged ligands (ActivinβB/HA, Sqt/HA, SqtAct β B[loop β 8]/HA), receptors (ActRIIB[KR]/Myc and Alk4[KR]/Flag), and a coreceptor (Cripto/Flag) in Xenopus embryos. We found that Sqt binding to the ActRIIB and Alk4 receptor complex required Cripto (Figure 5F). In contrast, ActivinβB and SqtAct β B[loop β 8] can bind to Activin receptors in the absence of Cripto. Moreover, Cripto did not significantly increase SqtAct β B[loop β 8] ligand–receptor complex formation. These results indicate that the loop-β8 region is a determinant of TGFβ ligand binding to Activin receptors independent of EGF-CFC coreceptors.

14An alignment of the loop-β8 region of EGF-CFC-dependent and EGF-CFC-independent TGFβs (Figure 5A) reveals the presence of several residues unique to Activins. These include (i) a Lys102–X–Asp104 motif (numbering from ActivinβA) that forms a significant binding interface with the type II receptor ActRII; (ii) Gln/Pro106 and Asn107, which contribute to the dimerization interface responsible for conformational arrangement; and (iii) an Asn insertion at position 99. We therefore mutated the corresponding residues in Sqt, individually or in combination, to the ActivinβB sequence and tested them in the gsc/ntl induction assay (Figure 6). All constructs were active in wild-type embryos. The Sqt3 and Sqt5 constructs containing the Lys102–X–Asp104 motif and Asn99 insertion showed weak expansion of ntl expression animally and dorsally in MZoep mutants. The incorporation of Pro106–Asn107 (Sqt2, Sqt4, and Sqt5) in Sqt did not enhance activity in MZoep mutants. These results suggest that multiple residues contribute to coreceptor independence, with the type II receptor-binding interface being an essential determinant.

15The results described above identified the loop-β8 region of Activin as a region that confers coreceptor-independent signaling to ligands that are normally EGF-CFC-dependent. In a reverse set of experiments, we asked which regions confer dependence on EGF-CFC coreceptors. To identify domains that confer EGF-CFC dependence, chimeras between ActivinβB and Sqt (see Figures 4 and 7) were analyzed for their inability to signal in MZoep mutants. Chimeras containing the Sqt Finger 2 domain (AAS and ASS; see Figure 4) or only the Sqt loop-β8 region (ActSqt[loop β 8]; Figure 7) were inactive in both wild-type and MZoep embryos. Western blot analysis demonstrated that these chimeras generate stable ligands (data not shown). The addition of Finger 1 in SAS or ActSqt[Finger1-loop β 8] relieved the inhibitory effect of the loop-β8 region of Sqt in wild-type embryos (Figure 7). These chimeras were inactive in MZoep mutants. These results indicate that the loop-β8 region in Sqt acts as an inhibitory domain and that Finger 1 relieves this inhibition by conferring dependence on EGF-CFC coreceptors.

16Our genetic and biochemical studies suggested that Lefty blocks Nodal and Vg1 signaling via EGF-CFC coreceptors. In contrast, the coreceptor-independent signaling by Activins cannot be blocked by Lefty. This finding predicts that the EGF-CFC-independent chimeric ligands SqtAct β B[loop β 8], SqtAct β A[loop β 8], and Vg1Act β B[loop β 8] should also be resistant to Lefty. Conversely, the coreceptor-dependent chimera ActSqt[Finger1-loop β 8] should be suspectible to inhibition by Lefty. To test this hypothesis, we coexpressed chimeric ligands and Lefty1 and analyzed gsc expression and A3-luc reporter induction (Figure 8A–8M). As predicted, Lefty1 did not inhibit signaling by SqtAct β B[loop β 8] (Figure 8B and 8C), SqtAct β A[loop β 8] (Figure 8E and 8F), or Vg1Act β B[loop β 8] (Figure 8H and 8I), but antagonized ActSqt[Finger1-loop β 8] (Figure 8K and 8L). These results indicate that the incorporation of the Activin loop-β8 region into Nodal and Vg1 can render these ligands EGF-CFC-independent and therefore resistant to Lefty.

17Lefty molecules are key regulators of mesendoderm development and left–right axis determination, but the molecular basis of Lefty-mediated antagonism of Activin-like pathways has been elusive. Our genetic and biochemical studies provide three lines of evidence that Lefty blocks EGF-CFC coreceptors. First, Lefty only inhibits EGF-CFC-dependent TGFβ ligands such as Nodal and Vg1, but not EGF-CFC-independent ligands such as Activins. A striking example of this coreceptor-specific interaction is the finding that changing only 14 amino acids in Nodal or Vg1 to the corresponding residues in Activins renders the resulting TGFβ ligands independent of EGF-CFC coreceptors and resistant to Lefty. Second, the EGF-CFC proteins mouse Cripto and zebrafish Oep can partially suppress the effects of Lefty overexpression in zebrafish. Third, Leftys can bind to EGF-CFC coreceptors and block the coreceptors from interacting with Nodal. Furthermore, Lefty/EGF-CFC complexes seem to exclude interactions with type I and type II Activin receptors. Taken together, these results indicate that Lefty blocks a subset of TGFβ signals by the novel mechanism of blocking pathway-specific coreceptors (Figure 9A–9D).

18The observation that Lefty does not block signaling by Activin seems in apparent contrast to previous studies that led to naming some Lefty family members Antivins, for their anti-Activin properties. In particular, it has been found that misexpression of Activin can suppress the defects caused by Lefty misexpression in vivo. Our results do not undermine this finding, but suggest an alternative explanation. Previous studies have shown that Activin can suppress the loss of EGF-CFC activity in MZoep mutants. Analogously, we suggest that the blocking of EGF-CFC activity by Lefty can be bypassed by Activin, because this ligand can activate Activin receptors independent of co-receptors. A similar scenario can also account for the suppression of Lefty gain-of-function phenotypes by misexpression and activation of Activin receptors. Hence, Activin and Activin receptors bypass the loss of EGF-CFC coreceptor function that is caused either by mutations in oep or by overexpression of Lefty. Conversely, Lefty cannot block Activin signals and Activin receptors because of their independence from EGF-CFC coreceptors.

19Is the block of EGF-CFC coreceptors by Lefty a general and conserved mechanism? Although we have only analyzed a representative subset of these protein families (zebrafish and mouse Lefty1; zebrafish Oep; mouse Cripto; zebrafish Sqt), previous studies have suggested that heterologous Nodal, Lefty, and EGF-CFC proteins have similar activities in zebrafish. For example, mouse Nodal, mouse Lefty2, and mouse Cripto are active in zebrafish, despite less than 30% overall sequence conservation with their zebrafish counterparts. These studies suggest that the molecular mechanisms described here apply to most, if not all, Nodal/Lefty/EGF-CFC interactions. This does not exclude the possibility that Lefty has additional means of blocking TGFβ signaling. First, Leftys might block the processing of Nodals. However, Sqt is processed normally at levels of Lefty that block Nodal signaling (unpublished data). Second, Leftys might bind Nodal signals. This could result in blocking receptor interactions or antagonizing TGFβ dimerization. However, Sqt is not bound by Lefty at Lefty levels that block Nodal signaling and lead to complex formation with Cripto (unpublished data). Moreover, a Sqt protein containing the loop-β8 region of Activin is resistant to Lefty, whereas changing only the dimerization residues in this region does not confer resistance. Third, Leftys might interact with additional extracellular factors. Indeed, the overexpression of the extracellular domain of the type II receptor ActRIIB has been shown to suppress Lefty activity. Although zebrafish Lefty1 does not appear to bind to ActRIIB, it is conceivable that overexpression of soluble ActRIIB might protect EGF-CFC coreceptors or another yet-to-be identified protein from antagonism by Lefty. In addition, overexpression of EGF-CFC proteins in zebrafish does not induce dominant phenotypes. It is therefore possible that an additional factor would be required to completely block Lefty in these experiments. Alternatively, overexpression levels of EGF-CFC coreceptors might not be high enough to block Lefty at blastula stages. It is also possible that coreceptor overexpression might block Nodal signals, because it has been shown that EGF-CFC proteins and Nodals can directly interact. The complex feedback interactions between Lefty and Nodal might also overcome an initial reduction of Lefty activity by increasing Lefty transcription. These considerations and the data presented here therefore suggest that a major, but perhaps not exclusive, role of Leftys is to block a subset of TGFβ signals by interaction with EGF-CFC coreceptors.

20Our finding that Leftys can block Vg1 signaling also has important implications for the developmental control of TGFβ signaling. Based on previous studies revealing that Lefty proteins inhibit Nodal signaling, the mouse Lefty mutant phenotypes have been interpreted as a consequence of increased or sustained Nodal signaling. For example, loss of mouse Lefty2 has been thought to increase Nodal signaling, resulting in an enlarged primitive streak. Similarly, the left–right defects observed in mouse Lefty1 and left-side specific Lefty2 (Lefty2ΔASE) mutants have been thought to be caused by inappropriate spread of Nodal signaling (, 2001). Our finding that Vg1/GDF1 signaling can also be blocked by Lefty suggests a more complex scenario. In particular, GDF1 (the mouse homologue of Vg1) is required for proper left–right axis determination. GDF1 mutants appear to have the opposite phenotypes as Lefty1 and Lefty2ΔASE mutants. While GDF1 promotes the expression of left-side-specific genes such as Pitx2 on the left, Leftys appear to block Pitx2 expression on the right (, 2001; ). In light of our findings, we suggest that during left–right axis formation, Leftys act as antagonists of not only Nodal, but also GDF1. In this scenario, loss of Lefty1 or Lefty2 would lead to ectopic and sustained GDF1 signaling. This model is particularly attractive when one considers the expression patterns of Lefty1, Lefty2, the EGF-CFC gene Cryptic, Nodal, and GDF1. Lefty 1 and GDF1 are coexpressed in the developing midline (, 1997; ), whereas Lefty2 and Nodal are coexpressed in left-lateral plate mesoderm. Cryptic is expressed in both the lateral plate and midline. It is therefore conceivable that GDF1 signaling is restricted by Lefty1-mediated inhibition of Cryptic in the midline and its progenitors, whereas Nodal signaling is antagonized by Lefty2-mediated block of Cryptic in the lateral plate.

21Our results might also have implications for the role of Cripto in tumorigenesis. Cripto is highly overexpressed in human epithelial cancers, such as breast and colon carcinomas, and has been implicated in tumor formation (, 1994;;;; ). Although the mechanisms by which Cripto acts in these circumstances are unclear, inhibition of Cripto by antisense or antibody blockade can inhibit tumor cell proliferation (;;;; reviewed by ). Since Lefty is an in vivo antagonist of EGF-CFC activity, it might also serve as a therapeutic agent to block Cripto.

22The finding that the highly related ligands Activin, Nodal, and Vg1/GDF1 activate the same receptors but differ in their dependence on coreceptors allowed us to determine how ligand diversity and signaling specificity can be achieved. We have identified the loop-β8 region as a 14 amino acid domain, a mere 4% of the entire TGFβ signal, that contributes to coreceptor dependence or independence. Sqt and Vg1 incorporating the loop-β8 region of Activin can bind to the Activin receptors in the absence of EGF-CFC proteins. Conversely, Activin incorporating the loop-β8 region of Sqt is inactive, suggesting that the Nodal/Vg1 loop-β8 region might be inhibitory. This inhibition can be relieved by the Finger 1 domain of Sqt, which results in the dependence on EGF-CFC coreceptors (Figure 9D). These results indicate that rather subtle sequence variations can lead to striking changes in ligand diversity.

23Structural considerations suggest that the loop-β8 region determines coreceptor independence or dependence at least in part by its interactions with type II receptors. The conserved Lys102–X–Asp104 motif in the Activin loop-β8 region has been shown to be important for high-affinity binding to the ActRII receptor. In the crystal structure of the ActivinβA-ActRIIB complex, Lys102–X–Asp104 forms an intramolecular salt bridge that interacts with a hydrophobic interface on ActRIIB. Mutational analysis has shown that substituting Lys102 with a neutral charge (Ala) significantly reduces receptor binding affinity and signaling. In contrast to Activin, EGF-CFC-dependent ligands such as Nodals and Vg1/GDF1 have the differentially charged residues Met/Leu102 and His104 at the corresponding positions (numbering according to ActivinβA). Similarly, in BMP7 the corresponding residues are Leu102 and Lys104. It has been shown that modeling Lys onto the aligned 102 residue in BMP7 positions it within hydrogen-bonding distance to Glu29 of ActRII and may allow for greater hydrophobic packing at the interface. Analogously, we propose that SqtAct β A[loop β 8], SqtAct β B[loop β 8],Vg1Act β B[loop β 8], Sqt3, and Sqt5 are coreceptor-independent because of their favorable binding to ActRIIB receptors. Conversely, the corresponding region in Sqt and Vg1 might be inhibitory because of inefficient interaction with ActRIIB receptors. Detailed structural studies should reveal whether EGF-CFC proteins overcome this inhibition by changing the conformation of Nodal and Vg1 or by providing an additional interaction surface that allows the assembly of receptor complexes.

24In summary, our results lead to two major conclusions. First, Lefty inhibits a subset of TGFβ signals by using the novel mechanism of blocking pathway-specific coreceptors belonging to the EGF-CFC family. Second, subtle sequence changes in TGFβs determine their signaling specificity and dependence on coreceptors. Although Drosophila has an Activin signaling pathway, Nodals, Leftys, and EGF-CFC proteins seem to be restricted to chordates. The evolution of Activin-like signaling pathways therefore represents a remarkable example of how a simple signaling pathway consisting of ligand and receptors can be diversified by subtle sequence changes that modulate the interaction with coreceptors and their inhibitors.

25Adult homozygous fish for oeptz57 were generated as described previously. Xenopus embryos were obtained as described in ).

26Epitope-tagged and chimeric constructs were made using PCR-based methods and confirmed by sequencing. pCS2-zebrafish Lefty1/HA and Lefty1/Glu constructs were generated by inserting three tandem copies of HA-epitope or Glu-epitope, respectively, after Val145. The initial three-lettered (XXX) Sqt/ActivinβB chimeras were generated by subcloning the prodomain of Xenopus ActivinβB (codons Met1 to Gly256) fused to an HA-epitope/XhoI fragment (YPYDVPDYALE) and followed by the mature chimeric ligand into pcDNA3 vector. S denotes Sqt; A denotes ActivinβB. The boundaries for Sqt mature ligand domains are as indicated: Finger 1 (Asn263 to Cys325), Heel (Pro326 to Cys358), and Finger 2 (Val359 to His392). The boundaries for Xenopus ActivinβB mature ligand domains are as indicated: Finger 1 (Cys215 to Cys299), Heel (Pro300 to Cys335), and Finger 2 (Ile336 to Ala370). Full-length chimeras were generated by incorporating the indicated regions into Sqt, ActivinβB, or zebrafish Vg1, which were then subcloned into the pT7TS vector. The boundaries for Sqt Finger 2 structural subregions are as indicated: β6β7 (Val359 to Try370), loop (Tyr371 to Met376), β8 (Val377 to Gly383), and β9 (Met384 to His392). The boundaries for Xenopus ActivinβB Finger 2 structural subregions are as indicated: β6β7 (Ile336 to Try347), loop (Phe348 to Ile354), β8 (Val355 to Asn356), and β9 (Met357 to Ala370). The Xenopus ActivinβA loop-β8 region sequence is FDRNNNVLKTDIAD (also identical in Xenopus ActivinβD). The zebrafish Vg1 loop-β8 region is from Try332 to Asp345. pcDNA3-zebrafish Vg1/HA, pcDNA3-Squint/HA, pCS2-Alk4(KR)/Flag (a kinase-defective mutant of human Alk4 with Lys234 to Arg234 substitution), pCS2-ActRIIB(KR)/Myc (a kinase-defective mutant of mouse ActRIIB with Lys217 to Arg217 substitution), and pCS2-Cripto/Flag have been described elsewhere.

27Plasmids were linearized and sense strand-capped mRNA was synthesized using the mMESSAGE mMACHINE system (Ambion, Austin, Texas, United States). Zebrafish embryos were dechorinated by pronase treatment and injected between the one- and four-cell stage. Xenopus embryos at the one- to two-cell stage were used for injections into the animal pole.

28Zebrafish embryos at 24 h were mounted in 2% methylcellulose and photographed using a Zeiss (Oberkochen, Germany) M2Bio dissecting microscope. In situ hybrization was performed as described previously, using RNA probes to gsc and ntl.

29Luciferase assays were performed with three to six samples and five embryos in each sample. Results are representative of three independent experiments. The injection mixtures were equalized with respect to total mRNA amount with LacZ mRNA. The A3-luc reporter DNA construct (25 pg) was also coinjected. Whole zebrafish embryos were harvested at shield stage. Luciferase activity was analyzed using the Luciferase Reporter Assay system (Promega, Madison, Wisconsin, United States) according to the manufacturer's instruction in a Lumat LB9501 (Berthold Technologies, Bad Wildbad, Germany). Owing to the technical aspects of microinjections, in rare circumstances, a single outlier was statistically removed from a population using Grubbs' test/extreme studentized deviate method. Inclusion of outliers into the populations does not change the statistical significance of the p values; that is, p remains <0.05, where indicated.

30Xenopus embryos were harvested at stage 10. For chemical cross-linking of proteins, animal halves were incubated in PBS with 10 mM 3,3′-dithiobis(sulfo-succinimidyl propionate) (DTSSP) (Pierce Biotechnology, Rockford, Illinois, United States) and incubated for 2 h on ice. Coimmunoprecipitation was performed as described previously. Purified processed mouse Lefty1, soluble mouse Cripto, and mouse VEGF-D proteins were obtained from R&D Systems (Minneapolis, Minnesota, United States). Activity assays were performed by R&D Systems. The proteins were incubated in PBS with 1 mM DTSSP for 1 h on ice. Coimmunoprecipitation was performed as described previously. Samples were treated with 100 mM DTT to cleave DTSSP prior to SDS-PAGE analysis. The following antibodies were used for immunoprecipitation and Western blot analysis: anti-Flag mouse monoclonal antibody (clone M2; Sigma, St. Louis, Missouri, United States), anti-HA mouse monoclonal antibody (clone 16B12; Covance, Princeton, New Jersey, United States), anti-HA rabbit polyclonal antibody (Y-11; Santa Cruz Biotechnology, Santa Cruz, California, United States), anti-c-Myc rabbit polyclonal antibody (A-14; Santa Cruz Biotechnology), anti-c-Myc mouse monoclonal antibody (clone Ab-1; Oncogene Science, Tarrytown, New York, United States), anti-His mouse monoclonal antibody (clone 6-His; Covance), anti-mLefty1 goat polyclonal antibody (R&D Systems), and anti-Glu mouse monoclonal antibody (clone Glu-Glu; Covance). Proteins were visualized using the Super Signal West Pico/Femto Chemiluminescent Substrate system (Pierce).

31The GenBank (http://www.ncbi.nlm.nih.gov/entrez/query.fc-gi?db=Nucleotide) accession numbers for the sequences discussed in this paper are ActivinβA (Q9W6I6), ActivinβB (Q91350), human ActRIIB (P08476), mouse ActRIIB (P27040), rat ActRIIB (P38445), Alk4 (Z22536), BMP7 (P23359), Cripto (P51865), Cryptic (P97766), cyclops (P87358), GDF1 (P20863), goosecoid (P53544), Lefty1 (Q9W6I6), Lefty2 (P57785), mLefty1 (Q64280), Nodal (P43021), no tail (Q07998), one-eyed pinhead (O57516), Pitx2 (P97474), squint (O13144), VEGF-D (P97946), and Vg1 (P09534).