Cerberus is a feedback inhibitor of Nodal asymmetric signaling in the chick embryo

Leveraging chicken embryos for studying human enhancers

The dynamic activity of complex gene regulatory networks stands at the core of all cellular functions that define cell identity and behaviour. Gene regulatory networks comprise transcriptional enhancers, acted upon by cell-specific transcription factors to control gene expression in a spatial and temporal specific manner. Enhancers are found in the non-coding genome; pathogenic variants can disrupt enhancer activity and lead to disease. Correlating non-coding variants with aberrant enhancer activity remains a significant challenge. Due to their clinical significance, there is a longstanding interest in understanding enhancer function during early embryogenesis. With the onset of the omics era, it is now feasible to identify putative tissue-specific enhancers from epigenome data. However, such predictions in vivo require validation. The early stages of chick embryogenesis closely parallel those of human, offering an accessible in vivo model in which to assess the activity of putative human enhancer sequences. This review explores the unique advantages and recent advancements in employing chicken embryos to elucidate the activity of human transcriptional enhancers and the potential implications of these findings in human disease.

Morphological changes and two Nodal paralogs drive left-right asymmetry in the squamate veiled chameleon (C. calyptratus)

The ancestral mode of left-right (L-R) patterning involves cilia in the L-R organizer. However, the mechanisms regulating L-R patterning in non-avian reptiles remains an enigma, since most squamate embryos are undergoing organogenesis at oviposition. In contrast, veiled chameleon (Chamaeleo calyptratus) embryos are pre-gastrula at oviposition, making them an excellent organism for studying L-R patterning evolution. Here we show that veiled chameleon embryos lack motile cilia at the time of L-R asymmetry establishment. Thus, the loss of motile cilia in the L-R organizers is a synapomorphy of all reptiles. Furthermore, in contrast to avians, geckos and turtles, which have one Nodal gene, veiled chameleon exhibits expression of two paralogs of Nodal in the left lateral plate mesoderm, albeit in non-identical patterns. Using live imaging, we observed asymmetric morphological changes that precede, and likely trigger, asymmetric expression of the Nodal cascade. Thus, veiled chameleons are a new and unique model for studying the evolution of L-R patterning.

A novel biallelic loss-of-function variant in DAND5 causes heterotaxy syndrome

The majority of heterotaxy cases do not obtain a molecular diagnosis, although pathogenic variants in more than 50 genes are known to cause heterotaxy. A heterozygous missense variant in DAND5, a nodal inhibitor, which functions in early development for establishment of right-left patterning, has been implicated in heterotaxy. Recently, the first case was reported of a DAND5 biallelic loss-of-function (LoF) variant in an individual with heterotaxy. Here, we describe a second unrelated individual with heterotaxy syndrome and a homozygous frameshift variant in DAND5 (NM_152654.2:c.197del [p.Leu66ArgfsTer22]). Using an in vitro assay, we demonstrate that the DAND5 c.197del variant is unable to inhibit nodal signaling when compared with the wild-type expression construct. This work strengthens the genetic and functional evidence for biallelic LoF variants in DAND5 causing an autosomal recessive heterotaxy syndrome.

Hand2 delineates mesothelium progenitors and is reactivated in mesothelioma

The mesothelium lines body cavities and surrounds internal organs, widely contributing to homeostasis and regeneration. Mesothelium disruptions cause visceral anomalies and mesothelioma tumors. Nonetheless, the embryonic emergence of mesothelia remains incompletely understood. Here, we track mesothelial origins in the lateral plate mesoderm (LPM) using zebrafish. Single-cell transcriptomics uncovers a post-gastrulation gene expression signature centered on hand2 in distinct LPM progenitor cells. We map mesothelial progenitors to lateral-most, hand2-expressing LPM and confirm conservation in mouse. Time-lapse imaging of zebrafish hand2 reporter embryos captures mesothelium formation including pericardium, visceral, and parietal peritoneum. We find primordial germ cells migrate with the forming mesothelium as ventral migration boundary. Functionally, hand2 loss disrupts mesothelium formation with reduced progenitor cells and perturbed migration. In mouse and human mesothelioma, we document expression of LPM-associated transcription factors including Hand2, suggesting re-initiation of a developmental program. Our data connects mesothelium development to Hand2, expanding our understanding of mesothelial pathologies.

Identification of Novel Hemangioblast Genes in the Early Chick Embryo

During early vertebrate embryogenesis, both hematopoietic and endothelial lineages derive from a common progenitor known as the hemangioblast. Hemangioblasts derive from mesodermal cells that migrate from the posterior primitive streak into the extraembryonic yolk sac. In addition to primitive hematopoietic cells, recent evidence revealed that yolk sac hemangioblasts also give rise to tissue-resident macrophages and to definitive hematopoietic stem/progenitor cells. In our previous work, we used a novel hemangioblast-specific reporter to isolate the population of chick yolk sac hemangioblasts and characterize its gene expression profile using microarrays. Here we report the microarray profile analysis and the identification of upregulated genes not yet described in hemangioblasts. These include the solute carrier transporters SLC15A1 and SCL32A1, the cytoskeletal protein RhoGap6, the serine protease CTSG, the transmembrane receptor MRC1, the transcription factors LHX8, CITED4 and PITX1, and the previously uncharacterized gene DIA1R. Expression analysis by in situ hybridization showed that chick DIA1R is expressed not only in yolk sac hemangioblasts but also in particular intraembryonic populations of hemogenic endothelial cells, suggesting a potential role in the hemangioblast-derived hemogenic lineage. Future research into the function of these newly identified genes may reveal novel important regulators of hemangioblast development.

Agonists and Antagonists of TGF-β Family Ligands

The discovery of the transforming growth factor β (TGF-β) family ligands and the realization that their bioactivities need to be tightly controlled temporally and spatially led to intensive research that has identified a multitude of extracellular modulators of TGF-β family ligands, uncovered their functions in developmental and pathophysiological processes, defined the mechanisms of their activities, and explored potential modulator-based therapeutic applications in treating human diseases. These studies revealed a diverse repertoire of extracellular and membrane-associated molecules that are capable of modulating TGF-β family signals via control of ligand availability, processing, ligand-receptor interaction, and receptor activation. These molecules include not only soluble ligand-binding proteins that were conventionally considered as agonists and antagonists of TGF-β family of growth factors, but also extracellular matrix (ECM) proteins and proteoglycans that can serve as "sink" and control storage and release of both the TGF-β family ligands and their regulators. This extensive network of soluble and ECM modulators helps to ensure dynamic and cell-specific control of TGF-β family signals. This article reviews our knowledge of extracellular modulation of TGF-β growth factors by diverse proteins and their molecular mechanisms to regulate TGF-β family signaling.

The Bone Morphogenetic Proteins and Their Antagonists

The bone morphogenetic proteins (BMPs) and the growth and differentiation factors comprise a single family of some 20 homologous, dimeric cytokines which share the cystine-knot domain typical of the TGF-β superfamily. They control the differentiation and activity of a range of cell types, including many outside bone and cartilage. They serve as developmental morphogens, but are also important in chronic pathologies, including tissue fibrosis and cancer. One mechanism for enabling tight spatiotemporal control of their activities is through a number of antagonist proteins, including Noggin, Follistatin, Chordin, Twisted gastrulation (TSG), and the seven members of the Cerberus and Dan family. These antagonists are secreted proteins that bind selectively to particular BMPs with high affinity, thereby blocking receptor engagement and signaling. Most of these antagonists also possess a TGF-β cystine-knot domain. Here, we discuss current knowledge and understanding of the structures and activities of the BMPs and their antagonists, with a particular focus on the latter proteins. Recent advances in structural biology of BMP antagonists have begun the process of elucidating the molecular basis of their activity, displaying a surprising variety between the modes of action of these closely related proteins. We also discuss the interactions of the antagonists with the glycosaminoglycan heparan sulfate, which is found ubiquitously on cell surfaces and in the extracellular matrix.

Expression and function of Ccbe1 in the chick early cardiogenic regions are required for correct heart development

During the course of a differential screen to identify transcripts specific for chick heart/hemangioblast precursor cells, we have identified Ccbe1 (Collagen and calcium-binding EGF-like domain 1). While the importance of Ccbe1 for the development of the lymphatic system is now well demonstrated, its role in cardiac formation remained unknown. Here we show by whole-mount in situ hybridization analysis that cCcbe1 mRNA is initially detected in early cardiac progenitors of the two bilateral cardiogenic fields (HH4), and at later stages on the second heart field (HH9-18). Furthermore, cCcbe1 is expressed in multipotent and highly proliferative cardiac progenitors. We characterized the role of cCcbe1 during early cardiogenesis by performing functional studies. Upon morpholino-induced cCcbe1 knockdown, the chick embryos displayed heart malformations, which include aberrant fusion of the heart fields, leading to incomplete terminal differentiation of the cardiomyocytes. cCcbe1 overexpression also resulted in severe heart defects, including cardia bifida. Altogether, our data demonstrate that although cardiac progenitors cells are specified in cCcbe1 morphants, the migration and proliferation of cardiac precursors cells are impaired, suggesting that cCcbe1 is a key gene during early heart development.

Left Right Patterning, Evolution and Cardiac Development

Many aspects of heart development are determined by the left right axis and as a result several congenital diseases have their origins in aberrant left-right patterning. Establishment of this axis occurs early in embryogenesis before formation of the linear heart tube yet impacts upon much later morphogenetic events. In this review I discuss the differing mechanisms by which left-right polarity is achieved in the mouse and chick embryos and comment on the evolution of this system. I then discus three major classes of cardiovascular defect associated with aberrant left-right patterning seen in mouse mutants and human disease. I describe phenotypes associated with the determination of atrial identity and venous connections, looping morphogenesis of the heart tube and finally the asymmetric remodelling of the embryonic branchial arch arterial system to form the leftward looped arch of aorta and associated great arteries. Where appropriate, I consider left right patterning defects from an evolutionary perspective, demonstrating how developmental processes have been modified in species over time and illustrating how comparative embryology can aide in our understanding of congenital heart disease.

A role for Vg1/Nodal signaling in specification of the intermediate mesoderm

The intermediate mesoderm (IM) is the embryonic source of all kidney tissue in vertebrates. The factors that regulate the formation of the IM are not yet well understood. Through investigations in the chick embryo, the current study identifies and characterizes Vg1/Nodal signaling (henceforth referred to as 'Nodal-like signaling') as a novel regulator of IM formation. Excess Nodal-like signaling at gastrulation stages resulted in expansion of the IM at the expense of the adjacent paraxial mesoderm, whereas inhibition of Nodal-like signaling caused repression of IM gene expression. IM formation was sensitive to levels of the Nodal-like pathway co-receptor Cripto and was inhibited by a truncated form of the secreted molecule cerberus, which specifically blocks Nodal, indicating that the observed effects are specific to the Nodal-like branch of the TGFβ signaling pathway. The IM-promoting effects of Nodal-like signaling were distinct from the known effects of this pathway on mesoderm formation and left-right patterning, a finding that can be attributed to specific time windows for the activities of these Nodal-like functions. Finally, a link was observed between Nodal-like and BMP signaling in the induction of IM. Activation of IM genes by Nodal-like signaling required an active BMP signaling pathway, and Nodal-like signals induced phosphorylation of Smad1/5/8, which is normally associated with activation of BMP signaling pathways. We postulate that Nodal-like signaling regulates IM formation by modulating the IM-inducing effects of BMP signaling.

The dynamic right-to-left translocation of Cerl2 is involved in the regulation and termination of Nodal activity in the mouse node

The determination of left-right body asymmetry in mouse embryos depends on the interplay of molecules in a highly sensitive structure, the node. Here, we show that the localization of Cerl2 protein does not correlate to its mRNA expression pattern, from 3-somite stage onwards. Instead, Cerl2 protein displays a nodal flow-dependent dynamic behavior that controls the activity of Nodal in the node, and the transmission of the laterality information to the left lateral plate mesoderm (LPM). Our results indicate that Cerl2 initially localizes and prevents the activation of Nodal genetic circuitry on the right side of the embryo, and later its right-to-left translocation shutdowns Nodal activity in the node. The consequent prolonged Nodal activity in the node by the absence of Cerl2 affects local Nodal expression and prolongs its expression in the LPM. Simultaneous genetic removal of both Nodal node inhibitors, Cerl2 and Lefty1, sustains even longer and bilateral this LPM expression.

Secreted and transmembrane wnt inhibitors and activators

Signaling by the Wnt family of secreted glycoproteins plays important roles in embryonic development and adult homeostasis. Wnt signaling is modulated by a number of evolutionarily conserved inhibitors and activators. Wnt inhibitors belong to small protein families, including sFRP, Dkk, WIF, Wise/SOST, Cerberus, IGFBP, Shisa, Waif1, APCDD1, and Tiki1. Their common feature is to antagonize Wnt signaling by preventing ligand-receptor interactions or Wnt receptor maturation. Conversely, the Wnt activators, R-spondin and Norrin, promote Wnt signaling by binding to Wnt receptors or releasing a Wnt-inhibitory step. With few exceptions, these antagonists and agonists are not pure Wnt modulators, but also affect additional signaling pathways, such as TGF-β and FGF signaling. Here we discuss their interactions with Wnt ligands and Wnt receptors, their role in developmental processes, as well as their implication in disease.

Collective epithelial and mesenchymal cell migration during gastrulation

Gastrulation, the process that puts the three major germlayers, the ectoderm, mesoderm and endoderm in their correct topological position in the developing embryo, is characterised by extensive highly organised collective cell migration of epithelial and mesenchymal cells. We discuss current knowledge and insights in the mechanisms controlling these cell behaviours during gastrulation in the chick embryo. We discuss several ideas that have been proposed to explain the observed large scale vortex movements of epithelial cells in the epiblast during formation of the primitive streak. We review current insights in the control and execution of the epithelial to mesenchymal transition (EMT) underlying the formation of the hypoblast and the ingression of the mesendoderm cells through the streak. We discuss the mechanisms by which the mesendoderm cells move, the nature and dynamics of the signals that guide these movements, as well as the interplay between signalling and movement that result in tissue patterning and morphogenesis. We argue that instructive cell-cell signaling and directed chemotactic movement responses to these signals are instrumental in the execution of all phases of gastrulation.

Multi-modal effects of BMP signaling on Nodal expression in the lateral plate mesoderm during left-right axis formation in the chick embryo

During development of left-right asymmetry in the vertebrate embryo, Nodal plays a central role for determination of left-handedness. Bone morphogenetic protein (BMP) signaling has an important role for regulation of Nodal expression, although there is controversy over whether BMP signaling has a positive or negative effect on Nodal expression in the chick embryo. As BMP is a morphogen, we speculated that different concentrations might induce different responses in the cells of the lateral plate mesoderm (LPM). To test this hypothesis, we analyzed the effects of various concentrations of BMP4 and NOGGIN on Nodal expression in the LPM. We found that the effect on Nodal expression varied in a complex fashion with the concentration of BMP. In agreement with previous reports, we found that a high level of BMP signaling induced Nodal expression in the LPM, whereas a low level inhibited expression. However, a high intermediate level of BMP signaling was found to suppress Nodal expression in the left LPM, whereas a low intermediate level induced Nodal expression in the right LPM. Thus, the high and the low intermediate levels of BMP signaling up-regulated Nodal expression, but the high intermediate and low levels of BMP signaling down-regulated Nodal expression. Next, we sought to identify the mechanisms of this complex regulation of Nodal expression by BMP signaling. At the low intermediate level of BMP signaling, regulation depended on a NODAL positive-feedback loop suggesting the possibility of crosstalk between BMP and NODAL signaling. Overexpression of a constitutively active BMP receptor, a constitutively active ACTIVIN/NODAL receptor and SMAD4 indicated that SMAD1 and SMAD2 competed for binding to SMAD4 in the cells of the LPM. Nodal regulation by the high and low levels of BMP signaling was dependent on Cfc up-regulation or down-regulation, respectively. We propose a model for the variable effects of BMP signaling on Nodal expression in which different levels of BMP signaling regulate Nodal expression by a balance between BMP-pSMAD1/4 signaling and NODAL-pSMAD2/4 signaling.

Paraxial left-sided nodal expression and the start of left-right patterning in the early chick embryo

A common element during early left-right patterning of the vertebrate body is left-sided nodal expression in the early-somite stage lateral plate mesoderm. Leftward cell movements near the node of the gastrulating chick embryo recently offered a plausible mechanism for breaking the presomite-stage molecular symmetry in those vertebrates which lack rotating cilia on the notochord or equivalent tissues. However, the temporal and functional relationships between generation of the known morphological node asymmetry, onset of leftward cell movements and establishment of stable molecular asymmetry in the chick remain unresolved. This study uses high-resolution light microscopy and in situ gene expression analysis to show that intranodal cell rearrangement during the phase of counter-clockwise node torsion at stage 4+ is immediately followed by symmetry loss and rearrangement of shh and fgf8 expression in node epiblast between stages 5- and 5+. Surprisingly, left-sided nodal expression starts at stage 5-, too, but lies in the paraxial mesoderm next to the forming notochordal plate, and can be rendered symmetrical by minimal mechanical disturbance of distant tissue integrity at stage 4. The "premature" paraxial nodal expression together with morphological and molecular asymmetries in, and near, midline compartments occurring at defined substages of early gastrulation help to identify a new narrow time window for early steps in left-right patterning in the chick and support the concept of a causal relationship between a-still enigmatic-chiral (motor) protein, cell movements and incipient left-right asymmetry in the amniote embryo.

Targeting the hemangioblast with a novel cell type-specific enhancer

BACKGROUND

Hemangioblasts are known as the common precursors for primitive hematopoietic and endothelial lineages. Their existence has been supported mainly by the observation that both cell types develop in close proximity and by in vitro differentiation and genetic studies. However, more compelling evidence will arise from tracking their cell fates using a lineage-specific marker.

RESULTS

We report the identification of a hemangioblast-specific enhancer (Hb) located in the cis-regulatory region of chick Cerberus gene (cCer) that is able to direct the expression of enhanced green fluorescent protein (eGFP) to the precursors of yolk sac blood and endothelial cells in electroporated chick embryos. Moreover, we present the Hb-eGFP reporter as a powerful live imaging tool for visualizing hemangioblast cell fate and blood island morphogenesis.

CONCLUSIONS

We hereby introduce the Hb enhancer as a valuable resource for genetically targeting the hemangioblast population as well as for studying the dynamics of vascular and blood cell development.

BMP inhibition by DAN in Hensen's node is a critical step for the establishment of left-right asymmetry in the chick embryo

During left-right (L-R) axis formation, Nodal is expressed in the node and has a central role in the transfer of L-R information in the vertebrate embryo. Bone morphogenetic protein (BMP) signaling also has an important role for maintenance of gene expression around the node. Several members of the Cerberus/Dan family act on L-R patterning by regulating activity of the transforming growth factor-β (TGF-β) family. We demonstrate here that chicken Dan plays a critical role in L-R axis formation. Chicken Dan is expressed in the left side of the node shortly after left-handed Shh expression and before the appearance of asymmetrically expressed genes in the lateral plate mesoderm (LPM). In vitro experiments revealed that DAN inhibited BMP signaling but not NODAL signaling. SHH had a positive regulatory effect on Dan expression while BMP4 had a negative effect. Using overexpression and RNA interference-mediated knockdown strategies, we demonstrate that Dan is indispensable for Nodal expression in the LPM and for Lefty-1 expression in the notochord. In the perinodal region, expression of Dan and Nodal was independent of each other. Nodal up-regulation by DAN required NODAL signaling, suggesting that DAN might act synergistically with NODAL. Our data indicate that Dan plays an essential role in the establishment of the L-R axis by inhibiting BMP signaling around the node.

Identification of differentially expressed genes in the heart precursor cells of the chick embryo

Genetic evidence has implicated several genes as being critical for heart development. However, the inducers of these genes as well as their targets and pathways they are involved with, remain largely unknown. Previous studies in the avian embryo showed that at HH4 Cerberus (cCer) transcripts are detected in the anterior endomesoderm including the heart precursor cells and later in the left lateral plate mesoderm. We have identified a promoter element of chick cCer able to drive EGFP expression in a population of cells that consistently exit from the anterior primitive streak region, from as early as stage HH3+, and that later will populate the heart. Using this promoter element as a tool allowed us to identify novel genes previously not known to potentially play a role in heart development. In order to identify and study genes expressed and involved in the correct development and differentiation of the vertebrate heart precursor cell (HPC) lineages, a differential screening using Affymetrix GeneChip system technologies was performed. Remarkably, this screening led to the identification of more than 700 transcripts differentially expressed in the heart forming regions (HFR). Bioinformatic tools allowed us to filter the large amount of data generated from this approach and to select a few transcripts for in vivo validation. Whole-mount in situ hybridization and sectioning of selected genes showed heart and vascular expression patterns for these transcripts during early chick development. We have developed an effective strategy to specifically identify genes that are differentially expressed in the HPC lineages. Within this set we have identified several genes that are expressed in the heart, blood and vascular lineages, which are likely to play a role in their development. These genes are potential candidates for future functional studies on early embryonic patterning.

Characterization of Cer-1 cis-regulatory region during early Xenopus development

Cerberus-related molecules are well-known Wnt, Nodal, and BMP inhibitors that have been implicated in different processes including anterior–posterior patterning and left–right asymmetry. In both mouse and frog, two Cerberus-related genes have been isolated, mCer-1 and mCer-2, and Xcer and Xcoco, respectively. Until now, little is known about the mechanisms involved in their transcriptional regulation. Here, we report a heterologous analysis of the mouse Cerberus-1 gene upstream regulatory regions, responsible for its expression in the visceral endodermal cells. Our analysis showed that the consensus sequences for a TATA, CAAT, or GC boxes were absent but a TGTGG sequence was present at position -172 to -168 bp, relative to the ATG. Using a series of deletion constructs and transient expression in Xenopus embryos, we found that a fragment of 1.4 kb of Cer-1 promoter sequence could reproduce the endogenous expression pattern of Xenopus cerberus. A 0.7-kb mcer-1 upstream region was able to drive reporter expression to the involuting mesendodermal cells, while further deletions abolished reporter gene expression. Our results suggest that although no sequence similarity was found between mouse and Xenopus cerberus cis-regulatory regions, the signaling cascades regulating cerberus expression, during gastrulation, is conserved.

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.

A right-sided pathway involving FGF8/Snai1 controls asymmetric development of the proepicardium in the chick embryo

The proepicardium (PE) is a transient structure that forms at the venous pole of the embryonic vertebrate heart. This cardiac progenitor cell population gives rise to the epicardium, coronary vasculature, and fibroblasts. In the chicken embryo, the PE displays left-right (L-R) asymmetry and develops only on the right side, while on the left only a vestigial PE is formed, which subsequently gets lost by apoptosis. In this study, we analyzed how the L-R asymmetry pathway affects PE formation. Experimental manipulation of left-side determinants such as Shh, Nodal, and Cfc as well as forced expression of Pitx2 had no effect on the sidedness of PE development. In contrast, inhibition of early-acting regulators of L-R axis formation such as H(+)/K(+)-ATPase or primitive streak apoptosis affected the sidedness of PE development. Experimental interference with the right-side determinants Fgf8 or Snai1 prevented PE formation, whereas ectopic left-sided expression of Fgf8 or Snai1 resulted in bilateral PE development. These data provide novel insight into the molecular control of asymmetric morphogenesis suggesting that also the right side harbors an instructive signaling pathway that is involved in the control of PE development. This pathway might be of general relevance for setting up L-R asymmetries at the venous pole of the heart.

BMP modulators regulate the function of BMP during body patterning and disease progression

Bone morphogenetic proteins (BMPs) are phylogenetically conserved signaling molecules that belong to the transforming growth factor (TGF)-beta superfamily and are involved in the cascades of body patterning and morphogenesis. The activities of BMPs are precisely regulated at various stages, and extracellulary, mainly regulated by certain classes of molecules termed as BMP antagonists and pro-BMP factors. BMP antagonists inhibit BMP function by prohibiting them from binding their cognate receptors, whereas pro-BMP factors stimulate BMP function. In this review, the functions of these BMP regulators will be discussed. (c) 2009 International Union of Biochemistry and Molecular Biology, Inc.

Cerberus functions as a BMP agonist to synergistically induce nodal expression during left-right axis determination in the chick embryo

Left-sided expression of Nodal in the lateral plate mesoderm (LPM) during early embryogenesis is a crucial step in establishing the left-right (L-R) axis in vertebrates. In the chick, it was suggested that chick Cerberus (cCer), a Cerberus/Dan family member, induces Nodal expression by antagonizing bone morphogenetic protein (BMP) activity in the left LPM. In contrast, it has also been shown that BMPs positively regulate Nodal expression in the left LPM in the chick embryo. Thus, it is still unclear how the bilaterally expressed BMPs induce Nodal expression only in the left LPM. In this study, we demonstrate that BMP signaling is necessary and sufficient for the induction of Nodal expression in the chick LPM where the type I BMP receptor-IB (BMPR-IB) likely mediates this induction. Tissue grafting experiments indicate the existence of a Nodal inductive factor in the left LPM rather than the presence of a Nodal inhibitory factor in the right LPM. We demonstrate that cCer functions as a BMP agonist instead of antagonist, being able to enhance BMP signaling in cell culture. This conclusion is further supported by the immunoprecipitation assays that provide convincing biochemical evidence for a direct interaction between cCer and BMP receptor. Because cCer is expressed restrictedly in the left LPM, BMPs and cCer appear to act synergistically to activate Nodal expression in the left LPM in the chick.

Modified whole-mount in situ hybridization protocol for the detection of transgene expression in electroporated chick embryos

Background

In vivo electroporation has been extensively used as an effective means of DNA transfer for analyzing gene function as well as gene regulation in developmental systems. In any of these two types of studies, the correct spatial and temporal expression of the electroporated transgene can only be accurately assessed by in situ hybridization.

Methodology/Principal Findings

While analyzing transgene expression in electroporated chicken embryos, we verified that transgene riboprobes cross-hybridized with the exogenous plasmid DNA when embryos were processed by conventional whole-mount in situ hybridization (WISH).

Conclusions/Significance

Here we describe a modification to the WISH protocol that is essential to prevent DNA cross-hybridization and to specifically detect transgene mRNA transcripts in electroporated embryos. Our optimized WISH procedure can be applied not only to electroporated chick embryos but also to other embryos or adult tissues that have been transfected with large amounts of reporter- or expression construct DNA.