Dual roles of Cripto as a ligand and coreceptor in the nodal signaling pathway

Regulation of extra-embryonic endoderm stem cell differentiation by Nodal and Cripto signaling

The signaling pathway for Nodal, a ligand of the TGFβ superfamily, plays a central role in regulating the differentiation and/or maintenance of stem cell types that can be derived from the peri-implantation mouse embryo. Extra-embryonic endoderm stem (XEN) cells resemble the primitive endoderm of the blastocyst, which normally gives rise to the parietal and the visceral endoderm in vivo, but XEN cells do not contribute efficiently to the visceral endoderm in chimeric embryos. We have found that XEN cells treated with Nodal or Cripto (Tdgf1), an EGF-CFC co-receptor for Nodal, display upregulation of markers for visceral endoderm as well as anterior visceral endoderm (AVE), and can contribute to visceral endoderm and AVE in chimeric embryos. In culture, XEN cells do not express Cripto, but do express the related EGF-CFC co-receptor Cryptic (Cfc1), and require Cryptic for Nodal signaling. Notably, the response to Nodal is inhibited by the Alk4/Alk5/Alk7 inhibitor SB431542, but the response to Cripto is unaffected, suggesting that the activity of Cripto is at least partially independent of type I receptor kinase activity. Gene set enrichment analysis of genome-wide expression signatures generated from XEN cells under these treatment conditions confirmed the differing responses of Nodal- and Cripto-treated XEN cells to SB431542. Our findings define distinct pathways for Nodal and Cripto in the differentiation of visceral endoderm and AVE from XEN cells and provide new insights into the specification of these cell types in vivo.

Reactivation of embryonic nodal signaling is associated with tumor progression and promotes the growth of prostate cancer cells

BACKGROUND

Nodal is a member of the transforming growth factor β (TGFβ) superfamily that directs embryonic patterning and promotes the plasticity and tumorigenicity of tumor cells, but its role in the prostate is unknown. The goal of this study was to characterize the expression and function of Nodal in prostate cancer and determine whether, like other TGFβ ligands, it modulates androgen receptor (AR) activity.

METHODS

Nodal expression was investigated using immunohistochemistry of tissue microarrays and Western blots of prostate cell lines. The functional role of Nodal was examined using Matrigel and soft agar growth assays. Cross-talk between Nodal and AR signaling was assessed with luciferase reporter assays and expression of endogenous androgen regulated genes.

RESULTS

Significantly increased Nodal expression was observed in cancer compared with benign prostate specimens. Nodal was only expressed by DU145 and PC3 cells. All cell lines expressed Nodal's co-receptor, Cripto-1, but lacked Lefty, a critical negative regulator of Nodal signaling. Recombinant human Nodal triggered downstream Smad2 phosphorylation in DU145 and LNCaP cells, and stable transfection of pre-pro-Nodal enhanced the growth of LNCaP cells in Matrigel and soft agar. Finally, Nodal attenuated AR signaling, reducing the activity of a PSA promoter construct in luciferase assays and down-regulating the endogenous expression of androgen regulated genes.

CONCLUSIONS

An aberrant Nodal signaling pathway is re-expressed and functionally active in prostate cancer cells.

Molecular evolution of the EGF-CFC protein family

The epidermal growth factor-Cripto-1/FRL-1/Cryptic (EGF-CFC) proteins, characterized by the highly conserved EGF and CFC domains, are extracellular membrane associated growth factor-like glycoproteins. These proteins are essential components of the Nodal signaling pathway during early vertebrate embryogenesis. Homologs of the EGF-CFC family have also been implicated in tumorigenesis in humans. Yet, little is known about the mode of molecular evolution in this family. Here we investigate the origin, extent of conservation and evolutionary relationships of EGF-CFC proteins across the metazoa. The results suggest that the first appearance of the EGF-CFC gene occurred in the ancestor of the deuterostomes. Phylogenetic analysis supports the classification of the family into distinct subfamilies that appear to have evolved through lineage-specific duplication and divergence. Site-specific analyses of evolutionary rate shifts between the two major mammalian paralogous subfamilies, Cripto and Cryptic, reveal critical amino acid sites that may account for the observed functional divergence. Furthermore, estimates of functional divergence suggest that rapid change of evolutionary rates at sites located mainly in the CFC domain may contribute towards distinct functional properties of the two paralogs.

Intercellular transfer regulation of the paracrine activity of GPI-anchored Cripto-1 as a Nodal co-receptor

Cripto-1 (CR-1) is a glycosylphosphatidylinositol-anchored glycoprotein which acts as an obligate co-receptor of a TGFβ family ligand, Nodal. Previous studies have demonstrated that CR-1 functions in a paracrine fashion by a cellular mechanism which has not been fully described. This paracrine activity was observed only when CR-1 was expressed as a membrane-bound form and was abolished when CR-1 was expressed in a soluble form. In the current study, we found that there were few biochemical differences in post-translational modifications between membrane-anchored and soluble forms of CR-1. Flow cytometric analysis revealed an intercellular transfer of the membrane-bound form of CR-1 between cells. CR-1-expressing cells formed unique membrane extensions, generated more membrane fragments than control cells, and exhibited enhanced cellular adhesion. Thus, expression of CR-1 may alter the physiochemical properties of the plasma membrane resulting in an enhancement of intercellular transfer of cellular signaling components which may account for the paracrine activity of CR-1.

Nodal morphogens

Nodal signals belong to the TGF-beta superfamily and are essential for the induction of mesoderm and endoderm and the determination of the left-right axis. Nodal signals can act as morphogens-they have concentration-dependent effects and can act at a distance from their source of production. Nodal and its feedback inhibitor Lefty form an activator/inhibitor pair that behaves similarly to postulated reaction-diffusion models of tissue patterning. Nodal morphogen activity is also regulated by microRNAs, convertases, TGF-beta signals, coreceptors, and trafficking factors. This article describes how Nodal morphogens pattern embryonic fields and discusses how Nodal morphogen signaling is modulated.

Imaging proprotein convertase activities and their regulation in the implanting mouse blastocyst

The CLIP biosensor reveals the spatiotemporal activity of the Nodal proprotein convertases Furin and Pace4 during embryonic development.

Axis formation and allocation of pluripotent progenitor cells to the germ layers are governed by the TGF-β–related Nodal precursor and its secreted proprotein convertases (PCs) Furin and Pace4. However, when and where Furin and Pace4 first become active have not been determined. To study the distribution of PCs, we developed a novel cell surface–targeted fluorescent biosensor (cell surface–linked indicator of proteolysis [CLIP]). Live imaging of CLIP in wild-type and Furin- and Pace4-deficient embryonic stem cells and embryos revealed that Furin and Pace4 are already active at the blastocyst stage in the inner cell mass and can cleave membrane-bound substrate both cell autonomously and nonautonomously. CLIP was also cleaved in the epiblast of implanted embryos, in part by a novel activity in the uterus that is independent of zygotic Furin and Pace4, suggesting a role for maternal PCs during embryonic development. The unprecedented sensitivity and spatial resolution of CLIP opens exciting new possibilities to elucidate PC functions in vivo.

APOBEC2, a selective inhibitor of TGFβ signaling, regulates left-right axis specification during early embryogenesis

The specification of left-right asymmetry is an evolutionarily conserved developmental process in vertebrates. The interplay between two TGFβ ligands, Derrière/GDF1 and Xnr1/Nodal, together with inhibitors such as Lefty and Coco/Cerl2, have been shown to provide the signals that lead to the establishment of laterality. However, molecular events leading to and following these signals remain mostly unknown. We find that APOBEC2, a member of the cytidine deaminase family of DNA/RNA editing enzymes, is induced by TGFβ signaling, and that its activity is necessary to specify the left-right axis in Xenopus and zebrafish embryos. Surprisingly, we find that APOBEC2 selectively inhibits Derrière, but not Xnr1, signaling. The inhibitory effect is conserved, as APOBEC2 blocks TGFβ signaling, and promotes muscle differentiation, in a mammalian myoblastic cell line. This demonstrates for the first time that a putative RNA/DNA editing enzyme regulates TGFβ signaling and plays a major role in development.

Expression of the embryonic morphogen Nodal in cutaneous melanocytic lesions

Nodal, a potent embryonic morphogen in the transforming growth factor-beta family, is a proposed key regulator of melanoma tumorigenicity. However, there has been no systematic study of Nodal expression in melanocytic lesions. We investigated Nodal expression by immunohistochemistry in 269 melanocytic lesions, including compound nevi, dysplastic nevi, congenital nevi, Spitz nevi, melanoma in situ, malignant melanoma including the variant desmoplastic melanoma, and metastatic melanoma. We found that the Nodal expression was significantly increased in malignant lesions (including melanoma in situ, malignant melanoma, and metastatic melanoma) compared with compound nevi, Spitz nevi, and dysplastic nevi. Surprisingly, congenital nevi expressed a level of Nodal comparable with malignant lesions, whereas desmoplastic melanoma showed lower expression than nondesmoplastic malignant melanoma (P<0.05). Deep melanoma (Breslow depth >1 mm) displayed a higher percentage of Nodal-positive tumor cells than did superficial melanoma (Breslow depth < or =1 mm), although there was no statistical difference in the overall staining intensity (P=0.18). Melanomas in situ showed a lower level of Nodal expression than did deep melanomas and metastatic melanomas (P<0.05). The low expression of Nodal in normal and dysplastic nevi, and its increasing expression with the progression of malignant lesions, are suggestive of a role for Nodal in melanoma progression.

O-fucose modulates Notch-controlled blood lineage commitment

Notch receptors are cell surface molecules essential for cell fate determination. Notch signaling is subject to tight regulation at multiple levels, including the posttranslational modification of Notch receptors by O-linked fucosylation, a reaction that is catalyzed by protein O-fucosyltransferase-1 (Pofut1). Our previous studies identified a myeloproliferative phenotype in mice conditionally deficient in cellular fucosylation that is attributable to a loss of Notch-dependent suppression of myelopoiesis. Here, we report that hematopoietic stem cells deficient in cellular fucosylation display decreased frequency and defective repopulating ability as well as decreased lymphoid but increased myeloid developmental potential. This phenotype may be attributed to suppressed Notch ligand binding and reduced downstream signaling of Notch activity in hematopoietic stem cells. Consistent with this finding, we further demonstrate that mouse embryonic stem cells deficient in Notch1 (Notch1(-/-)) or Pofut1 (Pofut1(-/-)) fail to generate T lymphocytes but differentiate into myeloid cells while coculturing with Notch ligand-expressing bone marrow stromal cells in vitro. Moreover, in vivo hematopoietic reconstitution of CD34(+) progenitor cells derived from either Notch1(-/-) or Pofut1(-/-) embryonic stem cells show enhanced granulopoiesis with depressed lymphoid lineage development. Together, these results indicate that Notch signaling maintains hematopoietic lineage homeostasis by promoting lymphoid development and suppressing overt myelopoiesis, in part through processes controlled by O-linked fucosylation of Notch receptors.

Functional redundancy of EGF-CFC genes in epiblast and extraembryonic patterning during early mouse embryogenesis

During early mouse embryogenesis, multiple patterning and differentiation events require the activity of Nodal, a ligand of the transforming growth factor-beta (TGFbeta) family. Although Nodal signaling is known to require activity of EGF-CFC co-receptors in many contexts, it has been unclear whether all Nodal signaling in the early mouse embryo is EGF-CFC dependent. We have investigated the double null mutant phenotypes for the EGF-CFC genes Cripto and Cryptic, which encode co-receptors for Nodal, and have found that they have partially redundant functions in early mouse development. Expression of Cripto and Cryptic is non-overlapping prior to gastrulation, since Cripto is expressed solely in the epiblast whereas Cryptic is expressed in the primitive endoderm of the late blastocyst and the visceral endoderm after implantation. Despite these non-overlapping expression patterns, Cripto; Cryptic double mutants display severe defects in epiblast, extraembryonic ectoderm, and anterior visceral endoderm (AVE), resulting in phenotypes that are highly similar to those of Nodal null mutants. Our results indicate that both Cripto and Cryptic function non-cell-autonomously during normal development, and that most if not all Nodal activity in early mouse embryogenesis is EGF-CFC-dependent.

Two distinct regions of latency-associated peptide coordinate stability of the latent transforming growth factor-beta1 complex

Transforming growth factor-beta1 (TGF-beta1) is secreted as part of an inactive complex consisting of the mature dimer, the TGF-beta1 propeptide (latency-associated peptide (LAP)), and latent TGF-beta-binding proteins. Using in vitro mutagenesis, we identified the regions of LAP that govern the cooperative assembly and stability of the latent TGF-beta1 complex. Initially, hydrophobic LAP residues (Ile(53), Leu(54), Leu(57), and Leu(59)), which form a contiguous epitope on one surface of an amphipathic alpha-helix, interact with mature TGF-beta1 to form the small latent complex. TGF-beta1 binding is predicted to alter LAP conformation, exposing ionic residues (Arg(45), Arg(50), Lys(56), and Arg(58)) on the other side of the alpha-helix, which form the binding site for latent TGF-beta-binding proteins. The stability of the resultant large latent complex is dependent upon covalent dimerization of LAP, which is facilitated by key residues (Phe(198), Asp(199), Val(200), Leu(208), Phe(217), and Leu(219)) at the dimer interface. Significantly, genetic mutations in LAP (e.g. R218H) that cause the rare bone disorder Camurati-Engelmann disease disrupted dimerization and reduced the stability of the latent TGF-beta1 complex.

O-GlcNAc modification of the extracellular domain of Notch receptors

Epidermal growth factor (EGF) domains are posttranslationally modified with unique O-linked glycans. The classical types of O-glycans on EGF domains are O-fucose and O-glucose glycans, found on many plasma glycoproteins and signaling molecules, whose biological functions have been demonstrated especially in the context of the Notch signaling pathway. We recently discovered O-GlcNAc modification as a new modification of the EGF domain that occurs on the conserved Ser/Thr residue located between the fifth and sixth cysteine residues within the EGF domain of Notch receptors in Drosophila. Here, we describe the methods employed to detect the O-GlcNAc modification of EGF repeats of Notch receptors. These methods include mass spectrometric analysis, galactosyltransferase labeling, immunoblotting with a specific antibody, and beta-N-acetyl-hexosaminidase digestion experiments. We also describe a method to detect O-GlcNAc transferase activity from crude membrane fraction proteins prepared from cultured S2 cells.

The RGM/DRAGON family of BMP co-receptors

The BMP signaling pathway controls a number of cell processes during development and in adult tissues. At the cellular level, ligands of the BMP family act by binding a hetero-tetrameric signaling complex, composed of two type I and two type II receptors. BMP ligands make use of a limited number of receptors, which in turn activate a common signal transduction cascade at the intracellular level. A complex regulatory network is required in order to activate the signaling cascade at proper times and locations, and to generate specific downstream effects in the appropriate cellular context. One such regulatory mechanism is the repulsive guidance molecule (RGM) family of BMP co-receptors. This article reviews the current knowledge regarding the structure, regulation, and function of RGMs, focusing on known and potential roles of RGMs in physiology and pathophysiology.

Blockade of Cripto binding to cell surface GRP78 inhibits oncogenic Cripto signaling via MAPK/PI3K and Smad2/3 pathways

Cripto is a developmental oncoprotein that signals via mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK), phosphatidylinositol 3-kinase (PI3K)/Akt and Smad2/3 pathways. However, the molecular basis for Cripto coupling to these pathways during embryogenesis and tumorigenesis is not fully understood. In this regard, we recently demonstrated that Cripto forms a cell surface complex with the HSP70 family member glucose-regulated protein-78 (GRP78). Here, we provide novel functional evidence demonstrating that cell surface GRP78 is a necessary mediator of Cripto signaling in human tumor, mammary epithelial and embryonic stem cells. We show that targeted disruption of the cell surface Cripto/GRP78 complex using shRNAs or GRP78 immunoneutralization precludes Cripto activation of MAPK/PI3K pathways and modulation of activin-A, activin-B, Nodal and transforming growth factor-beta1 signaling. We further demonstrate that blockade of Cripto binding to cell surface GRP78 prevents Cripto from increasing cellular proliferation, downregulating E-Cadherin, decreasing cell adhesion and promoting pro-proliferative responses to activin-A and Nodal. Thus, disrupting the Cripto/GRP78 binding interface blocks oncogenic Cripto signaling and may have important therapeutic value in the treatment of cancer.

Emerging roles of nodal and Cripto-1: from embryogenesis to breast cancer progression

Breast carcinoma cells and embryonic progenitors similarly implement stem cell-associated signaling pathways to sustain continued growth and plasticity. Indeed, recent studies have implicated signaling pathways, including those associated with the Notch, and Transforming Growth Factor-Beta (TGF-beta) superfamilies, as instrumental to both embryological development and breast cancer progression. In particular, Nodal, an embryonic morphogen belonging to the TGF-beta superfamily, and its co-receptor, Cripto-1, are requisite to both embryogenesis and mammary gland maturation. Moreover, these developmental proteins have been shown to promote breast cancer progression. Here, we review the role of Nodal and its co-receptor Cripto-1 during development and we describe how this signaling pathway may be involved in breast cancer tumorigenesis. Moreover, we emphasize the potential utility of this signaling pathway as a novel target for the treatment and diagnosis of breast cancer.

Identification and functional characterization of NODAL rare variants in heterotaxy and isolated cardiovascular malformations

NODAL and its signaling pathway are known to play a key role in specification and patterning of vertebrate embryos. Mutations in several genes encoding components of the NODAL signaling pathway have previously been implicated in the pathogenesis of human left-right (LR) patterning defects. Therefore, NODAL, a member of TGF-beta superfamily of developmental regulators, is a strong candidate to be functionally involved in congenital LR axis patterning defects or heterotaxy. Here we have investigated whether variants in NODAL are present in patients with heterotaxy and/or isolated cardiovascular malformations (CVM) thought to be caused by abnormal heart tube looping. Analysis of a large cohort of cases (n = 269) affected with either classic heterotaxy or looping CVM revealed four different missense variants, one in-frame insertion/deletion and two conserved splice site variants in 14 unrelated subjects (14/269, 5.2%). Although similar with regard to other associated defects, individuals with the NODAL mutations had a significantly higher occurrence of pulmonary valve atresia (P = 0.001) compared with cases without a detectable NODAL mutation. Functional analyses demonstrate that the missense variant forms of NODAL exhibit significant impairment of signaling as measured by decreased Cripto (TDGF-1) co-receptor-mediated activation of artificial reporters. Expression of these NODAL proteins also led to reduced induction of Smad2 phosphorylation and impaired Smad2 nuclear import. Taken together, these results support a role for mutations and rare deleterious variants in NODAL as a cause for sporadic human LR patterning defects.

Characterization of the glycosylphosphatidylinositol-anchor signal sequence of human Cryptic with a hydrophilic extension

Epidermal Growth Factor-Cripto-1/FRL-1/Cryptic (EGF-CFC) proteins, including human Cripto-1 (hCFC2/hCR-1) and human Cryptic (hCFC1), are membrane-associated Nodal co-receptors, which have critical roles in vertebrate development. Most of the EGF-CFC proteins have been experimentally proven or predicted to be glycosylphosphatidylinositol (GPI)-anchored proteins. However, unlike other EGF-CFC proteins, hCFC1 does not exhibit a typical GPI-signal sequence, containing a 32-amino acid hydrophilic extension in its COOH-terminal end. Here we experimentally demonstrate that the COOH-terminal sequence of hCFC1 functions as a GPI-anchoring signal. Moreover, addition of a hydrophilic epitope tag of 55-amino acids (V5-His) after the GPI signal of hCR-1 interfered with generation of a GPI-anchored form of hCR-1. In contrast, addition of the same epitope tag to the end of GPI signal of hCFC1 did not affect the GPI-attachment of hCFC1. The COOH-terminal signal of hCFC1 could produce two different forms of the protein; a GPI-anchored form and an unprocessed form which was more prone to be secreted into the conditioned medium. The hydrophilic extension of hCFC1 negatively regulates the activity of hCFC1 as a Nodal co-receptor. These results demonstrate the presence of endogenous GPI-signal sequence with a hydrophilic extension, which can generate both GPI-anchored and soluble forms of the protein.

Activation of a Nodal-independent signaling pathway by Cripto-1 mutants with impaired activation of a Nodal-dependent signaling pathway

Cripto-1, a co-receptor for Nodal, can activate Nodal-dependent and Nodal-independent signaling pathways. In this study we have investigated whether Cripto-1 mutants, that fail to activate a Nodal-dependent signaling pathway, are capable to activate a Nodal-independent signaling pathway in mammary epithelial cells. Cripto-1 mutants expressed in EpH4 mouse mammary epithelial cells are fully functional in regard to activation of a Nodal-independent signaling pathway, leading to phosphorylation of mitogen-activated protein kinase (MAPK) and Akt and to enhanced proliferation and motility of these cells, suggesting that Cripto-1 mutants with impaired Nodal signaling are still active in a Nodal-independent signaling pathway.

Cripto recruits Furin and PACE4 and controls Nodal trafficking during proteolytic maturation

The glycosylphosphatidylinositol (GPI)-anchored proteoglycan Cripto binds Nodal and its type I receptor Alk4 to activate Smad2,3 transcription factors, but a role during Nodal precursor processing has not been described. We show that Cripto also binds the proprotein convertases Furin and PACE4 and localizes Nodal processing at the cell surface. When coexpressed as in early embryonic cells, Cripto and uncleaved Nodal already associated during secretion, and a Cripto-interacting region in the Nodal propeptide potentiated the effect of proteolytic maturation on Nodal signalling. Disruption of the trans-Golgi network (TGN) by brefeldin A blocked secretion, but export of Cripto and Nodal to the cell surface was not inhibited, indicating that Nodal is exposed to extracellular convertases before entering the TGN/endosomal system. Density fractionation and antibody uptake experiments showed that Cripto guides the Nodal precursor in detergent-resistant membranes to endocytic microdomains marked by GFP-Flotillin. We conclude that Nodal processing and endocytosis are coupled in signal-receiving cells.

Identification of DPPA4 and other genes as putative Sox2:Oct-3/4 target genes using a combination of in silico analysis and transcription-based assays

Sox2 and Oct-3/4 function as master regulators during mammalian embryogenesis, where they are believed to regulate a critical gene regulatory network by cooperatively binding to DNA regulatory regions composed of adjacent HMG and POU motifs (HMG/POU cassettes). Previous studies have identified seven genes that contain highly active HMG/POU cassettes (referred to as Sox2:Oct-3/4 target genes). Importantly, nearly all known Sox2:Oct-3/4 target genes appear to be essential for embryogenesis. Recent genome-wide ChIP-chip studies identified over 300 genes that are co-occupied by Sox2 and Oct-3/4, which suggests that most Sox2:Oct-3/4 target genes remain to be identified. The work described here used a 3-step strategy for identifying additional Sox2:Oct-3/4 target genes. First, we employed in silico analysis to search for putative HMG/POU cassettes in 50 genes reported to be co-occupied by Sox2 and Oct-3/4 in embryonic stem cells. We identified 39 genes that contain putative HMG/POU cassettes. Next, we tested the activity of seven of the putative HMG/POU cassettes in a transcription-based assay and determined that nearly all are functional. Finally, as a proof-of-principle, we tested one of the seven cassettes (DPPA4) in the context of its endogenous promoter using a promoter/reporter gene construct. DPPA4 was tested in part because it was shown recently to play an important role in ES cell self-renewal. We determined that the 5' flanking region of the DPPA4 gene contains a functional HMG/POU cassette and behaves as a Sox2:Oct-3/4 target gene. Finally, we used a transcription-based assay to help develop a refined consensus sequence for HMG/POU cassettes.

VACTERL/caudal regression/Currarino syndrome-like malformations in mice with mutation in the proprotein convertase Pcsk5

We have identified an ethylnitrosourea (ENU)-induced recessive mouse mutation (Vcc) with a pleiotropic phenotype that includes cardiac, tracheoesophageal, anorectal, anteroposterior patterning defects, exomphalos, hindlimb hypoplasia, a presacral mass, renal and palatal agenesis, and pulmonary hypoplasia. It results from a C470R mutation in the proprotein convertase PCSK5 (PC5/6). Compound mutants (Pcsk5(Vcc/null)) completely recapitulate the Pcsk5(Vcc/Vcc) phenotype, as does an epiblast-specific conditional deletion of Pcsk5. The C470R mutation ablates a disulfide bond in the P domain, and blocks export from the endoplasmic reticulum and proprotein convertase activity. We show that GDF11 is cleaved and activated by PCSK5A, but not by PCSK5A-C470R, and that Gdf11-deficient embryos, in addition to having anteroposterior patterning defects and renal and palatal agenesis, also have a presacral mass, anorectal malformation, and exomphalos. Pcsk5 mutation results in abnormal expression of several paralogous Hox genes (Hoxa, Hoxc, and Hoxd), and of Mnx1 (Hlxb9). These include known Gdf11 targets, and are necessary for caudal embryo development. We identified nonsynonymous mutations in PCSK5 in patients with VACTERL (vertebral, anorectal, cardiac, tracheoesophageal, renal, limb malformation OMIM 192350) and caudal regression syndrome, the phenotypic features of which resemble the mouse mutation. We propose that Pcsk5, at least in part via GDF11, coordinately regulates caudal Hox paralogs, to control anteroposterior patterning, nephrogenesis, skeletal, and anorectal development.

The pro-domain of the zebrafish Nodal-related protein Cyclops regulates its signaling activities

Nodal proteins are secreted signaling factors of the transforming growth factor β (TGFβ) family with essential roles in embryonic development in vertebrates. Mutations affecting the Nodal factors have severe consequences in mammals and fish. Furthermore, increased Nodal levels have been associated with melanoma tumor progression. Like other TGFβ-related proteins, Nodal factors consist of a pro-domain and a mature domain. The pro-domain of mouse Nodal protein stabilizes its precursor. However, the mechanisms by which the pro-domains exert their activities are unknown. Here, we characterize the zebrafish Nodal-related factor Cyclops (Cyc) and find unexpected functions for the pro-domain in regulating Cyc activity. We identified a lysosome-targeting region in the Cyc pro-domain that destabilizes the precursor and restricts Cyc activity, revealing the molecular basis for the short-range signaling activities of Cyc. We show that both the pro- and mature-domains of Cyc regulate its stability. We also characterize a mutation in the pro-domain of human NODAL (hNODAL) that underlies congenital heterotaxia. Heterologous expression of mutant hNODAL increases expression of Nodal-response genes. Our studies reveal unexpected roles for the pro-domain of the Nodal factors and provide a possible mechanism for familial heterotaxia.

Reduced NODAL signaling strength via mutation of several pathway members including FOXH1 is linked to human heart defects and holoprosencephaly

Abnormalities of embryonic patterning are hypothesized to underlie many common congenital malformations in humans including congenital heart defects (CHDs), left-right disturbances (L-R) or laterality, and holoprosencephaly (HPE). Studies in model organisms suggest that Nodal-like factors provide instructions for key aspects of body axis and germ layer patterning; however, the complex genetics of pathogenic gene variant(s) in humans are poorly understood. Here we report our studies of FOXH1, CFC1, and SMAD2 and summarize our mutational analysis of three additional components in the human NODAL-signaling pathway: NODAL, GDF1, and TDGF1. We identify functionally abnormal gene products throughout the pathway that are clearly associated with CHD, laterality, and HPE. Abnormal gene products are most commonly detected in patients within a narrow spectrum of isolated conotruncal heart defects (minimum 5%-10% of subjects), and far less commonly in isolated laterality or HPE patients (approximately 1% for each). The difference in the mutation incidence between these groups is highly significant. We show that apparent gene dosage discrepancies between humans and model organisms can be reconciled by considering a broader combination of sequence variants. Our studies confirm that (1) the genetic vulnerabilities inferred from model organisms with defects in Nodal signaling are indeed analogous to humans; (2) the molecular analysis of an entire signaling pathway is more complete and robust than that of individual genes and presages future studies by whole-genome analysis; and (3) a functional genomics approach is essential to fully appreciate the complex genetic interactions necessary to produce these effects in humans.

Regulation of TGF-(beta) signalling by N-acetylgalactosaminyltransferase-like 1

The TGF-beta superfamily of secreted signalling molecules plays a pivotal role in the regulation of early embryogenesis, organogenesis and adult tissue homeostasis. Here we report the identification of Xenopus N-acetylgalactosaminyltransferase-like 1 (xGalntl-1) as a novel important regulator of TGF-beta signalling. N-acetylgalactosaminyltransferases mediate the first step of mucin-type glycosylation, adding N-acetylgalactose to serine or threonine side chains. xGalntl-1 is expressed in the anterior mesoderm and neural crest territory at neurula stage, and in the anterior neural crest, notochord and the mediolateral spinal cord at tailbud stage. Inhibition of endogenous xGalntl-1 protein synthesis, using specific morpholino oligomers, interfered with the formation of anterior neural crest, anterior notochord and the spinal cord. Xenopus and mammalian Galntl-1 inhibited Activin as well as BMP signalling in the early Xenopus embryo and in human HEK 293T cells. Gain- and loss-of-function experiments showed that xGalntl-1 interferes with the activity of the common TGF-beta type II receptor ActR-IIB in vivo. In addition, our biochemical data demonstrated that xGalntl-1 specifically interferes with the binding of ActR-IIB to Activin- and BMP-specific type I receptors. This inhibitory activity of xGalntl-1 was dependent on mucin-type glycosylation, as it was sensitive to the chemical inhibitor benzyl-GalNAc. These studies reveal an important role of a N-acetylgalactosaminyltransferase in the regulation of TGF-beta signalling. This novel regulatory mechanism is evolutionarily conserved and, thus, might provide a new paradigm for the regulation of TGF-beta signalling in vertebrates.

Cripto promotes A-P axis specification independently of its stimulatory effect on Nodal autoinduction

The EGF-CFC gene cripto governs anterior-posterior (A-P) axis specification in the vertebrate embryo. Existing models suggest that Cripto facilitates binding of Nodal to an ActRII-activin-like kinase (ALK) 4 receptor complex. Cripto also has a crucial function in cellular transformation that is independent of Nodal and ALK4. However, how ALK4-independent Cripto pathways function in vivo has remained unclear. We have generated cripto mutants carrying the amino acid substitution F78A, which blocks the Nodal-ALK4-Smad2 signaling both in embryonic stem cells and cell-based assays. In cripto(F78A/F78A) mouse embryos, Nodal fails to expand its own expression domain and that of cripto, indicating that F78 is essential in vivo to stimulate Smad-dependent Nodal autoinduction. In sharp contrast to cripto-null mutants, cripto(F78A/F78A) embryos establish an A-P axis and initiate gastrulation movements. Our findings provide in vivo evidence that Cripto is required in the Nodal-Smad2 pathway to activate an autoinductive feedback loop, whereas it can promote A-P axis formation and initiate gastrulation movements independently of its stimulatory effect on the canonical Nodal-ALK4-Smad2 signaling pathway.

Biological functions of glycosyltransferase genes involved in O-fucose glycan synthesis

Rare types of glycosylation often occur in a domain-specific manner and are involved in specific biological processes. Well-known examples of such modification are O-linked fucose (O-fucose) and O-linked glucose (O-glucose) glycans on epidermal growth factor (EGF) domains. In particular, O-fucose glycans are reported to regulate the functions of EGF domain-containing proteins such as urinary-type plasminogen activator and Notch receptors. Two glycosyltransferases catalyze the initiation and elongation of O-fucose glycans. The initiation process is catalyzed by O-fucosyltransferase 1, which is essential for Notch signalling in both Drosophila and mice. O-fucosyltransferase 1 can affect the folding, ligand interaction and endocytosis of Notch receptors, and both the glycosyltransferase and non-catalytic activities of O-fucosyltransferase 1 have been reported. The elongation of O-fucose monosaccharide is catalyzed by Fringe-related genes, which differentially modulate the interaction between Notch and two classes of ligands, namely, Delta and Serrate/Jagged. In this article, we have reviewed the recent reports addressing the distinctive features of the glycosyltransferases and O-glycans present on the EGF domains.

Rasl11b knock down in zebrafish suppresses one-eyed-pinhead mutant phenotype

The EGF-CFC factor Oep/Cripto1/Frl1 has been implicated in embryogenesis and several human cancers. During vertebrate development, Oep/Cripto1/Frl1 has been shown to act as an essential coreceptor in the TGFβ/Nodal pathway, which is crucial for germ layer formation. Although studies in cell cultures suggest that Oep/Cripto1/Frl1 is also implicated in other pathways, in vivo it is solely regarded as a Nodal coreceptor. We have found that Rasl11b, a small GTPase belonging to a Ras subfamily of putative tumor suppressor genes, modulates Oep function in zebrafish independently of the Nodal pathway. rasl11b down regulation partially rescues endodermal and prechordal plate defects of zygotic oep^−/− mutants (Zoep). Rasl11b inhibitory action was only observed in oep-deficient backgrounds, suggesting that normal oep expression prevents Rasl11b function. Surprisingly, rasl11b down regulation does not rescue mesendodermal defects in other Nodal pathway mutants, nor does it influence the phosphorylation state of the downstream effector Smad2. Thus, Rasl11b modifies the effect of Oep on mesendoderm development independently of the main known Oep output: the Nodal signaling pathway. This data suggests a new branch of Oep signaling that has implications for germ layer development, as well as for studies of Oep/Frl1/Cripto1 dysfunction, such as that found in tumors.

Cripto is a noncompetitive activin antagonist that forms analogous signaling complexes with activin and nodal

Cripto plays critical roles during embryogenesis and has been implicated in promoting the growth and spread of tumors. Cripto is required for signaling by certain transforming growth factor-beta superfamily members, such as Nodal, but also antagonizes others, such as activin. The opposing effects of Cripto on Nodal and activin signaling seem contradictory, however, because these closely related ligands utilize the same type I (ALK4) and type II (ActRII/IIB) receptors. Here, we have addressed this apparent paradox by demonstrating that Cripto forms analogous receptor complexes with Nodal and activin and functions as a noncompetitive activin antagonist. Our results show that activin-A and Nodal elicit similar maximal signaling responses in the presence of Cripto that are substantially lower than that of activin-A in the absence of Cripto. In addition, we provide biochemical evidence for complexes containing activin-A, Cripto, and both receptor types and show that the assembly of such complexes is competitively inhibited by Nodal. We further demonstrate that Nodal and activin-A share the same binding site on ActRII and that ALK4 has distinct and separable binding sites for activin-A and Cripto. Finally, we show that ALK4 mutants with disrupted activin-A binding retain Cripto binding and prevent the effects of Cripto on both activin-A and Nodal signaling. Together, our data indicate that Cripto facilitates Nodal signaling and inhibits activin signaling by forming receptor complexes with these ligands that are structurally and functionally similar.

Requirement of glycosylphosphatidylinositol anchor of Cripto-1 for trans activity as a Nodal co-receptor

Cripto-1 (CR-1) has an indispensable role as a Nodal co-receptor for patterning of body axis in embryonic development. CR-1 is reported to have a paracrine activity as a Nodal co-receptor, although CR-1 is primarily produced as a glycosylphosphatidylinositol (GPI)-anchored membrane protein. Regulation of cis and trans function of CR-1 should be important to establish the precise body patterning. However, the mechanism by which GPI-anchored CR-1 can act in trans is not well known. Here we confirmed the paracrine activity of CR-1 by fluorescent cell-labeling and immunofluorescent staining. We generated COOH-terminal-truncated soluble forms of CR-1 based on the attachment site for the GPI moiety (omega-site), which we identified in the present study. GPI-anchored CR-1 has a significantly higher activity than COOH-terminal-truncated soluble forms to induce Nodal signal in trans as well as in cis. Moreover, transmembrane forms of CR-1 partially retained their ability to induce Nodal signaling only when type I receptor Activin-like kinase 4 was overexpressed. NTERA2/D1 cells, which express endogenous CR-1, lost the cell-surface expression of CR-1 after phosphatidylinositol-phospholipase C treatment and became refractory to stimulation of Nodal. These observations suggest that GPI attachment of CR-1 is required for the paracrine activity as a Nodal co-receptor.

Growth factor induction of Cripto-1 shedding by glycosylphosphatidylinositol-phospholipase D and enhancement of endothelial cell migration

Cripto-1 (CR-1) is a glycosylphosphatidylinositol (GPI)-anchored membrane glycoprotein that has been shown to play an important role in embryogenesis and cellular transformation. CR-1 is reported to function as a membrane-bound co-receptor and as a soluble ligand. Although a number of studies implicate the role of CR-1 as a soluble ligand in tumor progression, it is unclear how transition from the membrane-bound to the soluble form is physiologically regulated and whether differences in biological activity exist between these forms. Here, we demonstrate that CR-1 protein is secreted from tumor cells into the conditioned medium after treatment with serum, epidermal growth factor, or lysophosphatidic acid, and this soluble form of CR-1 exhibits the ability to promote endothelial cell migration as a paracrine chemoattractant. On the other hand, membrane-bound CR-1 can stimulate endothelial cell sprouting through direct cell-cell interaction. Shedding of CR-1 occurs at the GPI-anchorage site by the activity of GPI-phospholipase D (GPI-PLD), because CR-1 shedding was suppressed by siRNA knockdown of GPI-PLD and enhanced by overexpression of GPI-PLD. These findings describe a novel molecular mechanism of CR-1 shedding, which may contribute to endothelial cell migration and possibly tumor angiogenesis.

Role of RGM coreceptors in bone morphogenetic protein signaling

Background

The repulsive guidance molecule (RGM) proteins, originally discovered for their roles in neuronal development, have been recently identified as co-receptors in the bone morphogenetic protein (BMP) signaling pathway. BMPs are members of the TGFβ superfamily of signaling cytokines, and serve to regulate many aspects of cellular growth and differentiation.

Results

Here, we investigate whether RGMa, RGMb, and RGMc play required roles in BMP and TGFβ signaling in the mouse myoblast C2C12 cell line. These cells are responsive to BMPs and are frequently used to study BMP/TGFβ signaling pathways. Using siRNA reagents to specifically knock down each RGM protein, we show that the RGM co-receptors are required for significant BMP signaling as reported by two cell-based BMP activity assays: endogenous alkaline phosphatase activity and a luciferase-based BMP reporter assay. Similar cell-based assays using a TGFβ-induced luciferase reporter show that the RGM co-receptors are not required for TGFβ signaling. The binding interaction of each RGM co-receptor to each of BMP2 and BMP12 is observed and quantified, and equilibrium dissociation constants in the low nanomolar range are reported.

Conclusion

Our results demonstrate that the RGMs play a significant role in BMP signaling and reveal that these molecules cannot functionally compensate for one another.

Regulated Nodal signaling promotes differentiation of the definitive endoderm and mesoderm from ES cells

Nodal signaling induces the formation of the endoderm and mesoderm during gastrulation. Nodal expression persists until the definitive endoderm progenitor has completely formed, and disappears thereafter. A tightly regulated Nodal expression system is essential for the differentiation of embryonic stem (ES) cells into distinct tissue lineages. On this basis, we established an ES cell differentiation system with the tetracycline-regulated expression of Nodal. The upregulated Nodal signaling pathway and its downstream transcriptional targets induced the specification of ES cells into definitive endoderm and mesoderm derivatives, and the subsequent downregulation of Nodal signaling promoted further maturation of the gut tube both in vitro and in vivo. Sustained expression of the Nodal gene inhibited the maturation of the definitive endoderm owing to persistent Oct3 and/or Oct4 expression and teratoma formation. Furthermore, quantitative single cell analysis by flow cytometry using CXCR4, VEGFR2 and PDGFR-α indicated that this protocol for definitive endoderm and mesoderm differentiation is superior to any other available protocol. Our findings also indicated that the Nodal or Nodal-related molecules secreted from Nodal-expressing ES cells could cause genetically unmanipulated ES cells to induce the expression of the Nodal signaling pathway and its downstream targets, which consequently leads to the differentiation of the ES cells into definitive endoderm and mesoderm. Our differentiation system, using tightly regulated Nodal expression, enabled us to investigate the mechanism of ES cell differentiation into definitive endoderm or mesoderm derivatives. Our findings also demonstrate that Nodal-expressing ES cells might be a source of highly active proteins that could be used for developing endoderm or mesoderm tissues in regenerative medicine.

Environmental and genetic modifiers of squint penetrance during zebrafish embryogenesis

The Nodal-related subgroup of the TGFbeta superfamily of secreted cytokines regulates the specification of the mesodermal and endodermal germ layers during gastrulation. Two Nodal-related proteins - Squint (Sqt) and Cyclops (Cyc) - are expressed during germ-layer specification in zebrafish. Genetic sqt mutant phenotypes have defined a variable requirement for zygotic Sqt, but not for maternal Sqt, in midline mesendoderm development. However a comparison of phenotypes arising from oocytes or zygotes injected with Sqt antisense morpholinos has suggested a novel requirement for maternal Sqt in dorsal specification. In this study we examined maternal-zygotic mutants for each of two sqt alleles and we also compared phenotypes of closely related zygotic and maternal-zygotic sqt mutants. Each of these approaches indicated there is no general requirement for maternal Sqt. To better understand the dispensability of maternal and zygotic Sqt, we sought out developmental contexts that more rigorously demand intact Sqt signalling. We found that sqt penetrance is influenced by genetic modifiers, by environmental temperature, by levels of residual Activin-like activity and by Heat-Shock Protein 90 (HSP90) activity. Therefore, Sqt may confer an evolutionary advantage by protecting early-stage embryos against detrimental interacting alleles and environmental challenges.

The threonine that carries fucose, but not fucose, is required for Cripto to facilitate Nodal signaling

Cripto is a membrane-bound co-receptor for Nodal, a member of the transforming growth factor-beta superfamily. Mouse embryos lacking either Cripto or Nodal have the same lethal phenotype at embryonic day 7.5. Previous studies suggest that O-fucosylation of the epidermal growth factor-like (EGF) repeat in Cripto is essential for the facilitation of Nodal signaling. Substitution of Ala for the Thr to which O-fucose is attached led to functional inactivation of both human and mouse Cripto. However, embryos null for protein O-fucosyltransferase 1, the enzyme that adds O-fucose to EGF repeats, do not exhibit a Cripto null phenotype and die at about embryonic day 9.5. This suggested that the loss of O-fucose from the EGF repeat may not have led to the inactivation of Cripto in previous studies. Here we investigate this hypothesis and show the following: 1) protein O-fucosyltransferase 1 is indeed the enzyme that adds O-fucose to Cripto; 2) Pofut1(-/-) embryonic stem cells behave the same as Pofut1(+/+) embryonic stem cells in a Nodal signaling assay; 3) Pofut1(-/-) and Pofut1(+/+) embryoid bodies are indistinguishable in their ability to differentiate into cardiomyocytes; and 4) none of 10 amino acid substitutions at Thr(72), including Ser which acquires O-fucose, rescues the activity of mouse Cripto in Nodal signaling assays. Therefore, the Thr to which O-fucose is linked in Cripto plays a key functional role, but O-fucose at Thr(72) is not required for Cripto to function in cell-based signaling assays or in vivo. By contrast, we show that O-fucose, and not the Thr to which it is attached, is required in the ligand-binding domain of Notch1 for Notch1 signaling.

Activin-Nodal signaling is involved in propagation of mouse embryonic stem cells

Embryonic stem (ES) cells are self-renewing cells that maintain pluripotency to differentiate into all types of cells. Because of their potential to provide a variety of tissues for use in regenerative medicine, there is great interest in the identification of growth factors that govern these unique properties of ES cells. However, the signaling pathways controlling ES cell proliferation remain largely unknown. Since transforming growth factor beta (TGFbeta) superfamily members have been implicated in the processes of early embryogenesis, we investigated their roles in ES cell self-renewal. Inhibition of activin-Nodal-TGFbeta signaling by Smad7 or SB-431542 dramatically decreased ES cell proliferation without decreasing ES pluripotency. By contrast, inhibition of bone morphogenetic protein (BMP) signaling by Smad6 did not exhibit such effects, suggesting that activin-Nodal-TGFbeta signaling, but not BMP signaling, is indispensable for ES cell propagation. In serum-free culture, supplementation of recombinant activin or Nodal, but not TGFbeta or BMP, significantly enhanced ES cell propagation without affecting pluripotency. We also found that activin-Nodal signaling was constitutively activated in an autocrine fashion in serum-free cultured ES cells, and that inhibition of such endogenous signaling by SB-431542 decreased ES cell propagation in serum-free conditions. These findings suggest that endogenously activated autocrine loops of activin-Nodal signaling promote ES cell self-renewal.

Nodal signaling: developmental roles and regulation

Nodal-related ligands of the transforming growth factor-beta (TGFbeta) superfamily play central roles in patterning the early embryo during the induction of mesoderm and endoderm and the specification of left-right asymmetry. Additional roles for this pathway in the maintenance of embryonic stem cell pluripotency and in carcinogenesis have been uncovered more recently. Consistent with its crucial developmental functions, Nodal signaling is tightly regulated by diverse mechanisms including the control of ligand processing, utilization of co-receptors, expression of soluble antagonists, as well as positive- and negative-feedback activities.

Nodal signaling activates differentiation genes during zebrafish gastrulation

Nodal signals induce mesodermal and endodermal progenitors during vertebrate development. To determine the role of Nodal signaling at a genomic level, we isolated Nodal-regulated genes by expression profiling using macroarrays and gene expression databases. Putative Nodal-regulated genes were validated by in situ hybridization screening in wild type and Nodal signaling mutants. 46 genes were identified, raising the currently known number of Nodal-regulated genes to 72. Based on their expression patterns along the dorsoventral axis, most of these genes can be classified into two groups. One group is expressed in the dorsal margin, whereas the other group is expressed throughout the margin. In addition to transcription factors and signaling components, the screens identified several new functional classes of Nodal-regulated genes, including cytoskeletal components and molecules involved in protein secretion or endoplasmic reticulum stress. We found that x-box binding protein-1 (xbp1) is a direct target of Nodal signaling and required for the terminal differentiation of the hatching gland, a specialized secretory organ whose specification is also dependent on Nodal signaling. These results indicate that Nodal signaling regulates not only specification genes but also differentiation genes.

All in the family: TGF-beta family action in testis development

To achieve and maintain fertility, the adult mammalian testis produces many generations of sperm. While testicular integrity is established in the fetus and develops further in juvenile life, sperm production does not ensue until much later in life, following the onset of puberty. Signals from the transforming growth factor-beta superfamily of proteins are vital for governance of testis development and spermatogenesis, and this review discusses our current understanding of the mechanisms and processes in which they have been implicated with a focus on the fetal and juvenile testis.

Cripto binds transforming growth factor beta (TGF-beta) and inhibits TGF-beta signaling

Cripto is a developmental oncoprotein and a member of the epidermal growth factor-Cripto, FRL-1, Cryptic family of extracellular signaling molecules. In addition to having essential functions during embryogenesis, Cripto is highly expressed in tumors and promotes tumorigenesis. During development, Cripto acts as an obligate coreceptor for transforming growth factor beta (TGF-beta) ligands, including nodals, growth and differentiation factor 1 (GDF1), and GDF3. As an oncogene, Cripto is thought to promote tumor growth via mechanisms including activation of mitogenic signaling pathways and antagonism of activin signaling. Here, we provide evidence supporting a novel mechanism in which Cripto inhibits the tumor suppressor function of TGF-beta. Cripto bound TGF-beta and reduced the association of TGF-beta with its type I receptor, TbetaRI. Consistent with its ability to block receptor assembly, Cripto suppressed TGF-beta signaling in multiple cell types and diminished the cytostatic effects of TGF-beta in mammary epithelial cells. Furthermore, targeted disruption of Cripto expression by use of small inhibitory RNA enhanced TGF-beta signaling, indicating that endogenous Cripto plays a role in restraining TGF-beta responses.

The structural basis of TGF-β, bone morphogenetic protein, and activin ligand binding

The transforming growth factor-β (TGF-β) superfamily is a large group of structurally related growth factors that play prominent roles in a variety of cellular processes. The importance and prevalence of TGF-β signaling are also reflected by the complex network of check points that exist along the signaling pathway, including a number of extracellular antagonists and membrane-level signaling modulators. Recently, a number of important TGF-β crystal structures have emerged and given us an unprecedented clarity on several aspects of the signal transduction process. This review will highlight these latest advances and present our current understanding on the mechanisms of specificity and regulation on TGF-β signaling outside the cell.

Activin signaling and its role in regulation of cell proliferation, apoptosis, and carcinogenesis

Activins, cytokine members of the transforming growth factor-beta superfamily, have various effects on many physiological processes, including cell proliferation, cell death, metabolism, homeostasis, differentiation, immune responses endocrine function, etc. Activins interact with two structurally related serine/threonine kinase receptors, type I and type II, and initiate downstream signaling via Smads to regulate gene expression. Understanding how activin signaling is controlled extracellularly and intracellularly would not only lead to more complete understanding of cell growth and apoptosis, but would also provide the basis for therapeutic strategies to treat cancer and other related diseases. This review focuses on the recent progress on activin-receptor interactions, regulations of activin signaling by ligand-binding proteins, receptor-binding proteins, and nucleocytoplasmic shuttling of Smad proteins.

Chemical synthesis of mouse cripto CFC variants

We report for the first time the chemical synthesis of refolded CFC domain of mouse Cripto (mCFC) and of two variants bearing mutations on residues W107 and H104 involved in Alk4 binding. The domains undergo spontaneous and quantitative refolding in about 4 h, yet with very different kinetics. Disulfide linkages have been assessed by enzyme digestion and mass spectrometry analysis of resulting fragments, and the first experimental studies on structural organization have been conducted by circular dichroism spectroscopy under different pH conditions. Upon refolding, the domains considerably change their conformations, although they do not assume canonical structures, and become highly resistant to enzyme degradation. A comparative study of receptor binding shows that the CFC domain can bind Alk4 and confirms the importance of W107 and H104 for receptor recognition.

BMP is an important regulator of proepicardial identity in the chick embryo

The proepicardium (PE) is a transient structure formed by pericardial coelomic mesothelium at the venous pole of the embryonic heart and gives rise to several cell types of the mature heart. In order to study PE development in chick embryos, we have analyzed the expression pattern of the marker genes Tbx18, Wt1, and Cfc. During PE induction, the three marker genes displayed a left-right asymmetric expression pattern. In each case, expression on the right side was stronger than on the left side. The left-right asymmetric gene expression observed here is in accord with the asymmetric formation of the proepicardium in the chick embryo. While initially the marker genes were expressed in the primitive sinus horn, subsequently, expression became confined to the PE mesothelium. In order to search for signaling factors involved in PE development, we studied Bmp2 and Bmp4 expression. Bmp2 was bilaterally expressed in the sinus venosus. In contrast, Bmp4 expression was initially expressed unilaterally in the right sinus horn and subsequently in the PE. In order to assess its functional role, BMP signaling was experimentally modulated by supplying exogenous BMP2 and by inhibiting endogenous BMP signaling through the addition of Noggin. Both supplying BMP and blocking BMP signaling resulted in a loss of PE marker gene expression. Surprisingly, both experimental situations lead to cardiac myocyte formation in the PE cultures. Careful titration experiments with exogenously added BMP2 or Noggin revealed that PE-specific marker gene expression depends on a low level of BMP signaling. Implantation of BMP2-secreting cells or beads filled with Noggin protein into the right sinus horn of HH stage 11 embryos resulted in downregulation of Tbx18 expression, corresponding to the results of the explant assay. Thus, a distinct level of BMP signaling is required for PE formation in the chick embryo.