New insights into the mechanisms of activin action and inhibition

Like other members of the transforming growth factor-β (TGF-β) superfamily, activins are synthesised as precursor molecules comprising an N-terminal prodomain and C-terminal mature region. During synthesis, the prodomain interacts non-covalently with mature activin, maintaining the molecule in a conformation competent for dimerisation. Dimeric precursors are cleaved by proprotein convertases and activin is secreted from the cell non-covalently associated with its propeptide. Extracellularly, the propeptide interacts with heparan sulfate proteoglycans to regulate activin localization within tissues. The mature activin dimer exhibits the classic 'open-hand' structure of TGF-β ligands with 'finger-like' domains projecting outward from the cysteine knot core of the molecule. These finger domains form the binding epitopes for type I and II serine/threonine kinase receptors. Activins ability to access its signalling receptors is regulated by the extracellular binding proteins, follistatin, follistatin-like-3, and by inhibins, which, in the presence of betaglycan, sequester type II receptors.

Identification and analysis of type II TGF-β receptors in BMP-9-induced osteogenic differentiation of C3H10T1/2 mesenchymal stem cells

Our previous studies have demonstrated that bone morphogenetic protein 9 (BMP-9) is one of the most efficacious BMPs to induce osteoblast differentiation of mesenchymal stem cells (MSCs). However, the molecular mechanism underlying the BMP-9-induced osteogenic differentiation of MSCs remains to be fully elucidated. In this study, dominant negative (DN) type II TGF-β receptors were constructed and introduced into C3H10T1/2 stem cells, then in vitro and in vivo assays were carried out to analyze and identify the type II TGF-β receptors required for BMP-9-induced osteogenesis. We found that three DN type II TGF-β receptors, DN-BMPRII, DN-ActRII, and DN-ActRIIB, diminished BMP-9-induced alkaline phosphatase (ALP) activity, led to a decrease in BMP-9-induced Smad binding element (SBE)-controled reporter activity, reduced BMP-9-induced expressions of Smad6 and Smad7, and decreased BMP-9-induced mineralization in vitro and ectopic bone formation in vivo, finally resulted in decreased bone masses and immature osteogenesis. These findings strongly suggested that three wild-type II TGF-β receptors, BMPRII, ActRII and ActRIIB, may play a functional role in BMP-9-induced osteogenic differentiation of C3H10T1/2 cells. However, C3H10T1/2 stem cells can express BMPRII and ActRII, but not ActRIIB. Using RNA interference (RNAi), we found that luciferase reporter activity and ALP activity induced by BMP-9 were accordingly inhibited along with the knockdown of BMPRII and ActRII. Taken together, our results demonstrated that BMPRII and ActRII are the functional type II TGF-β receptors in BMP-9-induced osteogenic differentiation of C3H10T1/2 cells.

Repulsive Guidance Molecule RGMa Alters Utilization of Bone Morphogenetic Protein (BMP) Type II Receptors by BMP2 and BMP4

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor-beta superfamily of multifunctional ligands that transduce their signals through type I and II serine/threonine kinase receptors and intracellular Smad proteins. Recently, we identified the glycosylphosphatidylinositol-anchored repulsive guidance molecules RGMa, DRAGON (RGMb), and hemojuvelin (RGMc) as coreceptors for BMP signaling (Babbit, J. L., Huang, F. W., Wrighting, D. W., Xia, Y., Sidis, Y., Samad, T. A., Campagna, J. A., Chung, R., Schneyer, A., Woolf, C. J., Andrews, N. C., and Lin, H. Y. (2006) Nat. Genet. 38, 531-539; Babbit, J. L., Zhang, Y., Samad, T. A., Xia, Y., Tang, J., Schneyer, A., Woolf, C. J., and Lin, H. Y. (2005) J. Biol. Chem. 280, 29820-29827; Samad, T. A., Rebbapragada, A., Bell, E., Zhang, Y., Sidis, Y., Jeong, S. J., Campagna, J. A., Perusini, S., Fabrizio, D. A., Schneyer, A. L., Lin, H. Y., Brivanlou, A. H., Attisano, L., and Woolf, C. J. (2005) J. Biol. Chem. 280, 14122-14129). However, the mechanism by which RGM family members enhance BMP signaling remains unknown. Here, we report that RGMa bound to radiolabeled BMP2 and BMP4 with Kd values of 2.4+/-0.2 and 1.4+/-0.1 nm, respectively. In KGN human ovarian granulosa cells and mouse pulmonary artery smooth muscle cells, BMP2 and BMP4 signaling required BMP receptor type II (BMPRII), but not activin receptor type IIA (ActRIIA) or ActRIIB, based on changes in BMP signaling by small interfering RNA inhibition of receptor expression. In contrast, cells transfected with RGMa utilized both BMPRII and ActRIIA for BMP2 or BMP4 signaling. Furthermore, in BmpRII-null pulmonary artery smooth muscle cells, BMP2 and BMP4 signaling was reduced by inhibition of endogenous RGMa expression, and RGMa-mediated BMP signaling required ActRIIA expression. These findings suggest that RGMa facilitates the use of ActRIIA by endogenous BMP2 and BMP4 ligands that otherwise prefer signaling via BMPRII and that increased utilization of ActRIIA leads to generation of an enhanced BMP signal.

BMP-9 signals via ALK1 and inhibits bFGF-induced endothelial cell proliferation and VEGF-stimulated angiogenesis

Genetic studies in mice and humans have shown that the transforming growth factor-beta (TGF-beta) type-I receptor activin receptor-like kinase 1 (ALK1) and its co-receptor endoglin play an important role in vascular development and angiogenesis. Here, we demonstrate that ALK1 is a signalling receptor for bone morphogenetic protein-9 (BMP-9) in endothelial cells (ECs). BMP-9 bound with high affinity to ALK1 and endoglin, and weakly to the type-I receptor ALK2 and to the BMP type-II receptor (BMPR-II) and activin type-II receptor (ActR-II) in transfected COS cells. Binding of BMP-9 to ALK2 was greatly facilitated when BMPR-II or ActR-II were co-expressed. Whereas BMP-9 predominantly bound to ALK1 and BMPR-II in ECs, it bound to ALK2 and BMPR-II in myoblasts. In addition, we observed binding of BMP-9 to ALK1 and endoglin in glioblastoma cells. BMP-9 activated Smad1 and/or Smad5, and induced ID1 protein and endoglin mRNA expression in ECs. Furthermore, BMP-9 was found to inhibit basic fibroblast growth factor (bFGF)-stimulated proliferation and migration of bovine aortic ECs (BAECs) and to block vascular endothelial growth factor (VEGF)-induced angiogenesis. Taken together, these results suggest that BMP-9 is a physiological ALK1 ligand that plays an important role in the regulation of angiogenesis.

Generation of activin receptor type IIB isoform-specific hypomorphic alleles

Activin receptor type IIB (Acvr2b) mediates multiple signals for transforming growth factor-beta (TGF-beta) family members, including Activin, Nodal, Bmp7, Gdf1, Gdf3, Myostatin (Gdf8), and Gdf11. Mouse Acvr2b gene generates four transcriptional isoforms (Acvr2b(1-4)) via alternative splicing of two sequence domains located at the juxtaposition of the transmembrane domain. To investigate whether these splicing domains are essential for signal transduction of the Acvr2b receptor in vivo, we have generated a strain of mutant mice (Acvr2b(4/4)) which produce only the Acvr2b(4) isoform, which lacks both splicing domains. Most homozygous Acvr2b(4(neo)/4(neo)) mice, in which a neomycin-resistant cassette was inserted in Intron 4 displayed a mild form of anterior vertebral transformations. However, the penetrance of the vertebral defect was dramatically decreased when the neomycin-resistant cassette was deleted. These results suggest that the Acvr2b(4) isoform is capable of compensating for the deficiency of the other three isoforms. In the absence of its subfamily receptor Acvr2a, however, the development of Acvr2b(4/4) mice was arrested at the gastrulation stage, recapitulating the Acvr2a(-/-); Acvr2b(+/-) mutant phenotype. In this study, we demonstrate that this phenomenon is most likely due to the reduction in the expressed Acvr2b(4) levels rather than to the functional deficiency of the Acvr2b(4) isoform itself.

ALK1 signalling analysis identifies angiogenesis related genes and reveals disparity between TGF-beta and constitutively active receptor induced gene expression

Background

TGF-β1 is an important angiogenic factor involved in the different aspects of angiogenesis and vessel maintenance. TGF-β signalling is mediated by the TβRII/ALK5 receptor complex activating the Smad2/Smad3 pathway. In endothelial cells TGF-β utilizes a second type I receptor, ALK1, activating the Smad1/Smad5 pathway. Consequently, a perturbance of ALK1, ALK5 or TβRII activity leads to vascular defects. Mutations in ALK1 cause the vascular disorder hereditary hemorrhagic telangiectasia (HHT).

Methods

The identification of ALK1 and not ALK5 regulated genes in endothelial cells, might help to better understand the development of HHT. Therefore, the human microvascular endothelial cell line HMEC-1 was infected with a recombinant constitutively active ALK1 adenovirus, and gene expression was studied by using gene arrays and quantitative real-time PCR analysis.

Results

After 24 hours, 34 genes were identified to be up-regulated by ALK1 signalling. Analysing ALK1 regulated gene expression after 4 hours revealed 13 genes to be up- and 2 to be down-regulated. Several of these genes, including IL-8, ET-1, ID1, HPTPη and TEAD4 are reported to be involved in angiogenesis. Evaluation of ALK1 regulated gene expression in different human endothelial cell types was not in complete agreement. Further on, disparity between constitutively active ALK1 and TGF-β1 induced gene expression in HMEC-1 cells and primary HUVECs was observed.

Conclusion

Gene array analysis identified 49 genes to be regulated by ALK1 signalling and at least 14 genes are reported to be involved in angiogenesis. There was substantial agreement between the gene array and quantitative real-time PCR data. The angiogenesis related genes might be potential HHT modifier genes. In addition, the results suggest endothelial cell type specific ALK1 and TGF-β signalling.

Smad7 and protein phosphatase 1alpha are critical determinants in the duration of TGF-beta/ALK1 signaling in endothelial cells

BACKGROUND

In endothelial cells (EC), transforming growth factor-beta (TGF-beta) can bind to and transduce signals through ALK1 and ALK5. The TGF-beta/ALK5 and TGF-beta/ALK1 pathways have opposite effects on EC behaviour. Besides differential receptor binding, the duration of TGF-beta signaling is an important specificity determinant for signaling responses. TGF-beta/ALK1-induced Smad1/5 phosphorylation in ECs occurs transiently.

RESULTS

The temporal activation of TGF-beta-induced Smad1/5 phosphorylation in ECs was found to be affected by de novo protein synthesis, and ALK1 and Smad5 expression levels determined signal strength of TGF-beta/ALK1 signaling pathway. Smad7 and protein phosphatase 1alpha (PP1alpha) mRNA expression levels were found to be specifically upregulated by TGF-beta/ALK1. Ectopic expression of Smad7 or PP1alpha potently inhibited TGF-beta/ALK1-induced Smad1/5 phosphorylation in ECs. Conversely, siRNA-mediated knockdown of Smad7 or PP1alpha enhanced TGF-beta/ALK1-induced signaling responses. PP1alpha interacted with ALK1 and this association was further potentiated by Smad7. Dephosphorylation of the ALK1, immunoprecipitated from cell lysates, was attenuated by a specific PP1 inhibitor.

CONCLUSION

Our results suggest that upon its induction by the TGF-beta/ALK1 pathway, Smad7 may recruit PP1alpha to ALK1, and thereby control TGF-beta/ALK1-induced Smad1/5 phosphorylation.

Hereditary hemorrhagic telangiectasia, a vascular dysplasia affecting the TGF-beta signaling pathway

Hereditary hemorrhagic telangiectasia (HHT) is caused by mutations in endoglin (ENG; HHT1) or ACVRL1/ALK1 (HHT2) genes and is an autosomal dominant vascular dysplasia. Clinically, HHT is characterized by epistaxis, telangiectases and arteriovenous malformations in some internal organs such as the lung, brain or liver. Endoglin and ALK1 proteins are specific endothelial receptors of the transforming growth factor (TGF)-beta superfamily that are essential for vascular integrity. Genetic studies in mice and humans have revealed the pivotal role of TGF-beta signaling during angiogenesis. Through binding to the TGF-beta type II receptor, TGF-beta can activate two distinct type I receptors (ALK1 and ALK5) in endothelial cells, each one leading to opposite effects on endothelial cell proliferation and migration. The recent isolation and characterization of circulating endothelial cells from HHT patients has revealed a decreased endoglin expression, impaired ALK1- and ALK5-dependent TGF-beta signaling, disorganized cytoskeleton and the failure to form cord-like structures which may lead to the fragility of small vessels with bleeding characteristic of HHT vascular dysplasia or to disrupted and abnormal angiogenesis after injuries and may explain the clinical symptoms associated with this disease.

TGFbeta inhibition of yolk-sac-like differentiation of human embryonic stem-cell-derived embryoid bodies illustrates differences between early mouse and human development

Transforming growth factor beta (TGFbeta) plays an important role in development and maintenance of murine yolk sac vascular development. Targeted deletions of Tgfb1 and other components of this signaling pathway, such as Acvrl1, Tgfbr1 and Tgfbr2, result in abnormal vascular development especially of the yolk sac, leading to embryonic lethality. There are significant differences between murine and primate development that limit interpretation of studies from mouse models. Thus, to examine the role of TGFbeta in early human vascular development we used the model of differentiating human embryonic stem cell-derived embryoid bodies to recapitulate early stages of embryonic development. TGFbeta was applied for different time frames after initiation of embryoid body cultures to assess its effect on differentiation. TGFbeta inhibited the expression of endodermal, endothelial and hematopoietic markers, which contrasts with findings in the mouse in which TGFbeta reduced the level of endodermal markers but increased endothelial marker expression. The inhibition observed was not due to changes in proliferation or apoptosis. This marked contrast between the two species may reflect the different origins of the yolk sac hemangiogenic lineages in mouse and human. TGFbeta effects on the hypoblast, from which these cell lineages are derived in human, would decrease subsequent differentiation of hematopoietic, endothelial and endodermal cells. By contrast, TGFbeta action on murine hypoblast, while affecting endoderm would not affect the hemangiogenic lineages that are epiblast-derived in the mouse. This study highlights important differences between early human and mouse embryonic development and suggests a role of TGFbeta in human hypoblast differentiation.

Cloning of a second form of activin-betaA cDNA and regulation of activin-betaA subunits and activin type II receptor mRNA expression by gonadotropin in the zebrafish ovary

Activins are dimeric proteins consisting of two inhibin beta subunits. Homo- and hetero-dimerizations of two isoforms of beta subunits, betaA and betaB, produce three forms of activins, activin-A, -B, and -AB. Recent studies have suggested that activin-A mediates gonadotropin-induced oocyte maturation in the zebrafish. To further understand the physiological role of activin-A in the zebrafish ovary, we have cloned cDNAs for a second isoform of the activin-betaA subunit and the activin type IIA (ActRIIA) receptor and determined their regulation by gonadotropin. Two sequences were obtained during the cloning of activin-betaA subunit, both of which showed high identity to betaA subunits of other species, and were therefore designated as isoform 1 and 2. Real-time PCR quantification was used to measure mRNA levels of activin-betaA1 and -betaA2, as well as two type II receptors, ActRIIA and ActRIIB, in the zebrafish ovary. Activin-betaA1 mRNA levels in stages III and IV follicles were similar and higher than those in stage II while high activin-betaA2 mRNA levels were only found in stage IV follicles. Highest levels of mRNA expression were detected in small and large stage III follicles for ActRIIA and ActRIIB, respectively. Treatment with human chorionic gonadotropin induced dose- and time-dependent increases in mRNA levels of activin-betaA1 and -betaA, as well as ActRIIA and ActRIIB. These findings further support the involvement of the activin signaling cascade in gonadotropin-regulated gonadal activities.

Bone Morphogenetic Protein (BMP) Type II Receptor Deletion Reveals BMP Ligand-specific Gain of Signaling in Pulmonary Artery Smooth Muscle Cells

Bone morphogenetic protein (BMP) ligands signal by binding the BMP type II receptor (BMPR2) or the activin type II receptors (ActRIIa and ActRIIb) in conjunction with type I receptors to activate SMADs 1, 5, and 8, as well as members of the mitogen-activated protein kinase family. Loss-of-function mutations in Bmpr2 have been implicated in tumorigenesis and in the etiology of primary pulmonary hypertension. Because several different type II receptors are known to recognize BMP ligands, the specific contribution of BMPR2 to BMP signaling is not defined. Here we report that the ablation of Bmpr2 in pulmonary artery smooth muscle cells, using an ex vivo conditional knock-out (Cre-lox) approach, as well as small interfering RNA specific for Bmpr2, does not abolish BMP signaling. Disruption of Bmpr2 leads to diminished signaling by BMP2 and BMP4 and augmented signaling by BMP6 and BMP7. Using small interfering RNAs to inhibit the expression of other BMP receptors, we found that wild-type cells transduce BMP signals via BMPR2, whereas BMPR2-deficient cells transduce BMP signals via ActRIIa in conjunction with a set of type I receptors distinct from those utilized by BMPR2. These findings suggest that disruption of Bmpr2 leads to the net gain of signaling by some, but not all, BMP ligands via the activation of ActRIIa.

Zebrafishacvr2a andacvr2b exhibit distinct roles in craniofacial development

To examine the roles of activin type II receptor signaling in craniofacial development, full-length zebrafish acvr2a and acvr2b clones were isolated. Although ubiquitously expressed as maternal mRNAs and in early embryogenesis, by 24 hr postfertilization (hpf), acvr2a and acvr2b exhibit restricted expression in neural, hindbrain, and neural crest cells (NCCs). A morpholino-based targeted protein depletion approach was used to reveal discrete functions for each acvr2 gene product. The acvr2a morphants exhibited defects in the development of most cranial NCC-derived cartilage, bone, and pharyngeal tooth structures, whereas acvr2b morphant defects were largely restricted to posterior arch structures and included the absence and/or aberrant migration of posterior NCC streams, defects in NCC-derived posterior arch cartilages, and dysmorphic pharyngeal tooth development. These studies revealed previously uncharacterized roles for acvr2a and acvr2b in hindbrain and NCC patterning, in NCC derived pharyngeal arch cartilage and joint formation, and in tooth development.

Activin type II receptor restoration in ACVR2-deficient colon cancer cells induces transforming growth factor-beta response pathway genes

The activin type II receptor (ACVR2) gene is a putative tumor suppressor gene that is frequently mutated in microsatellite-unstable colon cancers (MSI-H colon cancers). ACVR2 is a member of the transforming growth factor (TGF)-beta type II receptor (TGFBR2) family and controls cell growth and differentiation. SMAD proteins are major intracellular effectors shared by ACVR2 and TGFBR2 signaling; however, additional shared effector mechanisms remain to be explored. To discover novel mechanisms transmitting the ACVR2 signal, we restored ACVR2 function by transfecting wild-type ACVR2 (wt-ACVR2) into a MSI-H colon cancer cell line carrying an ACVR2 frameshift mutation. The effect of ACVR2 restoration on cell growth, SMAD phosphorylation, and global molecular phenotype was then evaluated. Decreased cell growth was observed in wt-ACVR2 transfectants relative to ACVR2-deficient vector-transfected controls. Western blotting revealed higher expression of phosphorylated SMAD2 in wt-ACVR2 transfectants versus controls, suggesting cells deficient in ACVR2 had impaired SMAD signaling. Microarray-based differential expression analysis revealed substantial ACVR2-induced overexpression of genes implicated in the control of cell growth and tumorigenesis, including the activator protein (AP)-1 complex genes JUND, JUN, and FOSB, as well as the small GTPase signal transduction family members, RHOB, ARHE, and ARHGDIA. Overexpression of these genes is shared with TGFBR2 activation. This observed similarity between the activin and TGF-beta signaling systems suggests that activin may serve as an alternative activator of TGF-beta effectors, including SMADs, and that frameshift mutation of ACVR2 may contribute to MSI-H colon tumorigenesis via disruption of alternate TGF-beta effector pathways.

Controlling cell fate by bone morphogenetic protein receptors

Bone morphogenetic proteins (BMPs) are multifunctional proteins that regulate the fate of different cell types, including mesenchymal and endothelial cells. BMPs inhibit myogenic differentiation, but promote the differentiation of mesenchymal cells into osteoblasts. Furthermore, endothelial migration and tube formation are stimulated by BMPs. Like other members of the transforming growth factor-beta (TGF-beta) superfamily, BMPs elicit their cellular effects via specific types I and II serine/threonine receptors. The activated BMP type I receptor phosphorylates specific receptor-regulated (R)-Smad proteins, which assemble into heteromeric complexes with common partner (Co)-Smad4. Heteromeric Smad complexes efficiently translocate into the nucleus, where they regulate the transcription of target genes. Inhibitors of differentiation (Id) are genes that are specifically induced by BMPs in tissues of different origin. Promoter analysis of Id1 indicates three distinct sequence elements that are sufficient and essential for efficient BMP-induced activation. Furthermore, recent studies reveal an important effector function for Id1 in various BMP-induced biological responses.

Inhibin is an antagonist of bone morphogenetic protein signaling

Inhibins are endogenous antagonists of activin signaling, long recognized as important regulators of gonadal function and pituitary FSH release. Inhibin, in concert with its co-receptor, betaglycan, can compete with activin for binding to type II activin receptors and, thus, prevent activin signaling. Because bone morphogenetic proteins (BMPs) also utilize type II activin receptors, we hypothesized that BMP signaling might also be sensitive to inhibin blockade. Here we show that inhibin blocks cellular responses to diverse BMP family members in a variety of BMP-responsive cell types. Inhibin abrogates BMP-induced Smad signaling and transcription responses. Inhibin competes with BMPs for type II activin receptors, and this competition is facilitated by betaglycan. Betaglycan also enables inhibin to bind to and compete with BMPs for binding to the BMP-specific type II receptor BMPRII, which does not bind inhibin in the absence of betaglycan. Betaglycan can confer inhibin responsiveness on cells that are otherwise insensitive to inhibin. These findings demonstrate that inhibin, acting through betaglycan, can function as an antagonist of BMP responses, suggesting a broader role for inhibin and betaglycan in restricting and refining a wide spectrum of transforming growth factor beta superfamily signals.

Genetic engineering to study testicular tumorigenesis

In humans, Sertoli cell tumors account for approximately 4% of all testicular tumors, and 20% of these are malignant. The mechanisms underlying Sertoli cell tumorigenesis remain largely unknown. Using gene knockout technology, we previously generated mutant mice lacking the alpha subunit of inhibin dimers. The inhibin alpha-null male mice develop testicular Sertoli cell tumors with 100% penetrance. These tumors develop as early as 4 weeks of age and cause a cachexia-like wasting syndrome. Castrated inhibin alpha knockout mice develop sex steroidogenic adrenal cortical tumors. These studies have identified inhibins as secreted tumor suppressors with specificity for the gonads and adrenal glands. It had been suggested that endocrine factors play roles in Sertoli cell tumorigenesis by altering cell cycle machinery of the Sertoli cells. To test the potential of these factors to function as modifiers of Sertoli cell tumorigenesis, we have employed a genetic intercross strategy, breeding inhibin a mutant mice with mutant mice deficient in endocrine signaling factors including gonadotropin releasing hormone (hypogonadal, hpg mice), follicle stimulating hormone, anti-Miillerian hormone (AMH), activin receptor type II, or androgen receptor (testicular feminization, tfm mice), or mice overexpressing follistatin. We are also investigating the effects of loss of critical cell cycle regulators, such as cyclin dependent kinase inhibitor p27, on Sertoli cell tumorigenesis in inhibin alpha knockout males. These studies clearly demonstrate the roles of these factors as modifiers of the Sertoli cell tumorigenesis. Activin signaling through activin receptor type II is responsible for the cachexia-like syndrome observed in the inhibin a knockout mice with tumors. The gonadotropin hormones are essential for testicular tumor development, but elevated FSH levels are not sufficient to cause Sertoli cell tumors. Absence of FSH, lack of androgen receptor, or overexpression of follistatin slows the tumor growth and minimizes the cachexia symptoms, thus prolonging the life span of these double mutant mice. In contrast, absence of AMH or p27 causes earlier onset and more aggressive development of testicular tumor, with an earlier death of double mutant mice. We are currently investigating roles of estrogen signaling pathways, and other cell cycle regulators, in tumor development in the inhibin alpha knockout mice by generating mice with double or triple mutations. Genetic engineering in mouse models provides a powerful tool to study the mechanisms of testicular tumorigenesis and define the important genetic modifiers in vivo.

EGF-CFC proteins are essential coreceptors for the TGF-beta signals Vg1 and GDF1

The TGF-beta signals Nodal, Activin, GDF1, and Vg1 have been implicated in mesoderm induction and left-right patterning. Nodal and Activin both activate Activin receptors, but only Nodal requires EGF-CFC coreceptors for signaling. We report that Vg1 and GDF1 signaling in zebrafish also depends on EGF-CFC proteins, but not on Nodal signals. Correspondingly, we find that in Xenopus Vg1 and GDF1 bind to and signal through Activin receptors only in the presence of EGF-CFC proteins. These results establish that multiple TGF-beta signals converge on Activin receptor/EGF-CFC complexes and suggest a more widespread requirement for coreceptors in TGF-beta signaling than anticipated previously.

Activin type IIA and IIB receptors mediate Gdf11 signaling in axial vertebral patterning

Vertebral bodies are segmented along the anteroposterior (AP) body axis, and the segmental identity of the vertebrae is determined by the unique expression pattern of multiple Hox genes. Recent studies have demonstrated that a transforming growth factor beta (TGF-beta) family protein, Gdf11 (growth and differentiation factor 11), and the activin type II receptor, ActRIIB, are involved in controlling the spatiotemporal expression of multiple Hox genes along the AP axis, and that the disruption of each of these genes causes anterior transformation of the vertebrae. Skeletal defects are more severe in Gdf11-null mice than in ActRIIB-null mice, however, leaving it uncertain whether Gdf11 signals via ActRIIB. Here we demonstrate using genetic and biochemical studies that ActRIIB and its subfamily receptor, ActRIIA, cooperatively mediate the Gdf11 signal in patterning the axial vertebrae, and that Gdf11 binds to both ActRIIA and ActRIIB, and induces phosphorylation of Smad2. In addition, we also show that these two receptors can functionally compensate for one another to mediate signaling of another TGF-beta ligand, nodal, during left-right patterning and the development of anterior head structure.