Two nodal-responsive enhancers control left-right asymmetric expression ofNodal

A change of heart: new roles for cilia in cardiac development and disease

Although cardiac abnormalities have been observed in a growing class of human disorders caused by defective primary cilia, the function of cilia in the heart remains an underexplored area. The primary function of cilia in the heart was long thought to be restricted to left-right axis patterning during embryogenesis. However, new findings have revealed broad roles for cilia in congenital heart disease, valvulogenesis, myocardial fibrosis and regeneration, and mechanosensation. In this Review, we describe advances in our understanding of the mechanisms by which cilia function contributes to cardiac left-right axis development and discuss the latest findings that highlight a broader role for cilia in cardiac development. Specifically, we examine the growing line of evidence connecting cilia function to the pathogenesis of congenital heart disease. Furthermore, we also highlight research from the past 10 years demonstrating the role of cilia function in common cardiac valve disorders, including mitral valve prolapse and aortic valve disease, and describe findings that implicate cardiac cilia in mechanosensation potentially linking haemodynamic and contractile forces with genetic regulation of cardiac development and function. Finally, given the presence of cilia on cardiac fibroblasts, we also explore the potential role of cilia in fibrotic growth and summarize the evidence implicating cardiac cilia in heart regeneration.

Understanding embryonic development for cell-based therapies of intervertebral disc degeneration: Toward an effort to treat disc degeneration subphenotypes

Chronic low back and neck pain are associated with intervertebral disc degeneration and are major contributors to the global burden of disability. New evidence now suggests that disc degeneration comprises a spectrum of subphenotypes influenced by genetic background, age, and environmental factors, which may be contributing to the mixed outcomes seen in clinical trials of cell-based therapies that aim to treat disc degeneration. This problem is further compounded by the fact that disc degeneration and aging coincide with an exhaustion of endogenous progenitor cells, imposing limitations on the regenerative capacity of the disc. At the bench-side, current work is focused on applying our knowledge of embryonic disc development to direct and refine differentiation of adult and human-induced pluripotent stem cells into notochord-like and nucleus pulposus-like cells for use in novel cell-based therapies. Accordingly, this review presents the salient features of intervertebral disc development, post-natal maintenance, and regeneration, with emphasis on recent advancements. We also discuss how a stratified approach can be undertaken for the development of future cell-based therapies to bring emerging subphenotypes into consideration.

MNK1/NODAL Signaling Promotes Invasive Progression of Breast Ductal Carcinoma In Situ

The mechanisms by which breast cancers progress from relatively indolent ductal carcinoma in situ (DCIS) to invasive ductal carcinoma (IDC) are not well understood. However, this process is critical to the acquisition of metastatic potential. MAPK-interacting serine/threonine-protein kinase 1 (MNK1) signaling can promote cell invasion. NODAL, a morphogen essential for embryogenic patterning, is often reexpressed in breast cancer. Here we describe a MNK1/NODAL signaling axis that promotes DCIS progression to IDC. We generated MNK1 knockout (KO) or constitutively active MNK1 (caMNK1)-expressing human MCF-10A-derived DCIS cell lines, which were orthotopically injected into the mammary glands of mice. Loss of MNK1 repressed NODAL expression, inhibited DCIS to IDC conversion, and decreased tumor relapse and metastasis. Conversely, caMNK1 induced NODAL expression and promoted IDC. The MNK1/NODAL axis promoted cancer stem cell properties and invasion in vitro. The MNK1/2 inhibitor SEL201 blocked DCIS progression to invasive disease in vivo. In clinical samples, IDC and DCIS with microinvasion expressed higher levels of phospho-MNK1 and NODAL versus low-grade (invasion-free) DCIS. Cumulatively, our data support further development of MNK1 inhibitors as therapeutics for preventing invasive disease. SIGNIFICANCE: These findings provide new mechanistic insight into progression of ductal carcinoma and support clinical application of MNK1 inhibitors to delay progression of indolent ductal carcinoma in situ to invasive ductal carcinoma.

Heterogeneity of Melanoma with Stem Cell Properties

Metastatic melanoma continues to present a significant challenge-with a cure rate of less than 10% and a median survival of 6-9 months. Despite noteworthy advances in the field, the heterogeneity of melanoma tumors, comprised of cell subpopulations expressing a cancer stem cell (CSC) phenotype concomitant with drug resistance markers presents a formidable challenge in the design of current therapies. Particularly vexing is the ability of distinct subpopulations of melanoma cells to resist standard-of-care treatments, resulting in relapse and progression to metastasis. Recent studies have provided new information and insights into the expression and function of CSC markers associated with the aggressive melanoma phenotype, such as the embryonic morphogen Nodal and CD133, together with a drug resistance marker ABCA1. This chapter highlights major findings that demonstrate the promise of targeting Nodal as a viable option to pursue in combination with standard-of-care therapy. In recognizing that aggressive melanoma tumors utilize multiple mechanisms to survive, we must consider a more strategic approach to effectively target heterogeneity, tumor cell plasticity, and functional adaptation and resistance to current therapies-to eliminate relapse, disease progression, and metastasis.

TGF-β Family Signaling in Early Vertebrate Development

TGF-β family ligands function in inducing and patterning many tissues of the early vertebrate embryonic body plan. Nodal signaling is essential for the specification of mesendodermal tissues and the concurrent cellular movements of gastrulation. Bone morphogenetic protein (BMP) signaling patterns tissues along the dorsal-ventral axis and simultaneously directs the cell movements of convergence and extension. After gastrulation, a second wave of Nodal signaling breaks the symmetry between the left and right sides of the embryo. During these processes, elaborate regulatory feedback between TGF-β ligands and their antagonists direct the proper specification and patterning of embryonic tissues. In this review, we summarize the current knowledge of the function and regulation of TGF-β family signaling in these processes. Although we cover principles that are involved in the development of all vertebrate embryos, we focus specifically on three popular model organisms: the mouse Mus musculus, the African clawed frog of the genus Xenopus, and the zebrafish Danio rerio, highlighting the similarities and differences between these species.

Asymmetric pitx2 expression in medaka epithalamus is regulated by nodal signaling through an intronic enhancer

The epithalamic region of fishes shows prominent left-right asymmetries that are executed by nodal signaling upstream of the asymmetry-determining transcription factor pitx2. Previous reports have identified that nodal controls the left-sided pitx2 expression in the lateral plate mesoderm through an enhancer present in the last intron of this gene. However, whether similar regulation occurs also in the case of epithalamic asymmetry is currently unresolved. Here, we address some of the cis-regulatory information that control asymmetric pitx2 expression in epithalamus by presenting a Tg(pitx2:EGFP) 116-17 transgenic medaka model, which expresses enhanced green fluorescent protein (EGFP) under control of an intronic enhancer. We show that this transgene recapitulates epithalamic expression of the endogenous pitx2 and that it responds to nodal signaling inhibition. Further, we identify that three foxh1-binding sites present in this enhancer modulate expression of the transgene and that the second site is absolutely necessary for the left-sided epithalamic expression while the other two sites may have subtler regulative roles. We provide evidence that left-sided epithalamic pitx2 expression is controlled through an enhancer present in the last intron of this gene and that the regulatory logic underlying asymmetric pitx2 expression is shared between epithalamic and lateral plate mesoderm regions.

Nodal signaling from the visceral endoderm is required to maintain Nodal gene expression in the epiblast and drive DVE/AVE migration

In the early mouse embryo, a specialized population of extraembryonic visceral endoderm (VE) cells called the distal VE (DVE) arises at the tip of the egg cylinder stage embryo and then asymmetrically migrates to the prospective anterior, recruiting additional distal cells. Upon migration these cells, called the anterior VE (AVE), establish the anterior posterior (AP) axis by restricting gastrulation-inducing signals to the opposite pole. The Nodal-signaling pathway has been shown to have a critical role in the generation and migration of the DVE/AVE. The Nodal gene is expressed in both the VE and in the pluripotent epiblast, which gives rise to the germ layers. Previous findings have provided conflicting evidence as to the relative importance of Nodal signaling from the epiblast vs. VE for AP patterning. Here we show that conditional mutagenesis of the Nodal gene specifically within the VE leads to reduced Nodal expression levels in the epiblast and incomplete or failed DVE/AVE migration. These results support a required role for VE Nodal to maintain normal levels of expression in the epiblast, and suggest signaling from both VE and epiblast is important for DVE/AVE migration.

Disrupting Foxh1-Groucho interaction reveals robustness of nodal-based embryonic patterning

The winged-helix transcription factor Foxh1 is an essential regulator of Nodal signaling during the key developmental processes of gastrulation, anterior-posterior (A-P) patterning, and the derivation of left-right (L-R) asymmetry. Current models have Foxh1 bound to phospho-Smad2/3 (pSmad2/3) as a central transcriptional activator for genes targeted by Nodal signaling including Nodal itself, the feedback inhibitor Lefty2, and the positive transcriptional effector Pitx2. However, the conserved Engrailed homology-1 (EH1) motif present in Foxh1 suggests that modulated interaction with Groucho (Grg) co-repressors would allow Foxh1 to function as a transcriptional switch, toggling between transcriptional on and off states via pSmad2-Grg protein-switching, to ensure the properly timed initiation and suppression, and/or amplitude, of expression of Nodal and its target genes. We minimally mutated the Foxh1 EH1 motif, creating a novel Foxh1(mEH1) allele to test directly the contribution of Foxh1-Grg-mediated repression on the transient, dynamic pattern of Nodal signaling in mice. All aspects of Nodal and its target gene expression in Foxh1(mEH1/mEH1) embryos were equivalent to wild type. A-P patterning and organ situs in homozygous embryos and adult mice were also unaffected. The finding that Foxh1-Grg-mediated repression is not essential for Nodal expression during mouse embryogenesis suggests that other regulators compensate for the loss of repressive regulatory input that is mediated by Grg interactions. We suggest that the pervasive inductive properties of Nodal signaling exist within the context of a strongly buffered regulatory system that contributes to resilience and accuracy of its dynamic expression pattern.

A novel nodal enhancer dependent on pluripotency factors and smad2/3 signaling conditions a regulatory switch during epiblast maturation

HBE, a newly discovered enhancer element, mediates the influence of pluripotency factors and Activin/Nodal signaling on early Nodal expression in the mouse embryo, and controls the activation of later-acting Nodal enhancers.

During early development, modulations in the expression of Nodal, a TGFβ family member, determine the specification of embryonic and extra-embryonic cell identities. Nodal has been extensively studied in the mouse, but aspects of its early expression remain unaccounted for. We identified a conserved hotspot for the binding of pluripotency factors at the Nodal locus and called this sequence “highly bound element” (HBE). Luciferase-based assays, the analysis of fluorescent HBE reporter transgenes, and a conditional mutation of HBE allowed us to establish that HBE behaves as an enhancer, is activated ahead of other Nodal enhancers in the epiblast, and is essential to Nodal expression in embryonic stem cells (ESCs) and in the mouse embryo. We also showed that HBE enhancer activity is critically dependent on its interaction with the pluripotency factor Oct4 and on Activin/Nodal signaling. Use of an in vitro model of epiblast maturation, relying on the differentiation of ESCs into epiblast stem cells (EpiSCs), revealed that this process entails a shift in the regulation of Nodal expression from an HBE-driven phase to an ASE-driven phase, ASE being another autoregulatory Nodal enhancer. Deletion of HBE in ESCs or in EpiSCs allowed us to show that HBE, although not necessary for Nodal expression in EpiSCs, is required in differentiating ESCs to activate the differentiation-promoting ASE and therefore controls this regulatory shift. Our findings clarify how early Nodal expression is regulated and suggest how this regulation can promote the specification of extra-embryonic precusors without inducing premature differentiation of epiblast cells. More generally, they open new perspectives on how pluripotency factors achieve their function.

In the early mouse embryo, Nodal, a member of the TGFbeta superfamily of signalling proteins, promotes the differentiation of extra-embryonic tissues, as well as tissues within the developing embryo itself. Characterising the regulation of Nodal gene expression is essential to understand how Nodal signals in diverse tissue types and at different stages of embryonic development. Four distinct enhancer sequences have been shown to regulate Nodal expression, although none could account for it in the preimplantation epiblast or in embryonic stem cells. We identified a novel enhancer, HBE, responsible for the earliest aspects of Nodal expression. We show that activation of HBE depends on its interaction with a well-known pluripotency factor called Oct4. HBE itself also controls the activation of at least one other Nodal enhancer. Our findings clarify how early Nodal expression is regulated and reveal how pluripotency factors may control the onset of differentiation in embryonic tissues.

Nodal as a biomarker for melanoma progression and a new therapeutic target for clinical intervention

Nodal, an embryonic morphogen belonging to the TGF-β superfamily, is an important regulator of embryonic stem cell fate. We have recently demonstrated that Nodal is expressed significantly in aggressive melanoma. Surprisingly, expression of the Nodal coreceptor, Cripto-1, was detected in only a small fraction of the melanoma tumor cell population, indicating a primary role for Cripto-1-independent signaling of Nodal in melanoma. In this review, we discuss how regulatory factors present in an embryonic environment, such as Lefty, can downregulate Nodal expression and inhibit tumorigenicity and plasticity of melanoma cells. Our translational studies show that antibodies against Nodal are capable of repressing melanoma vasculogenic mimicry and of inducing apoptosis in melanoma tumors in an in vivo lung-colonization assay. Our previous work and ongoing studies suggest that Nodal may represent a novel diagnostic marker and therapeutic target in melanoma.

Roles of cilia, fluid flow, and Ca2+ signaling in breaking of left-right symmetry

The emergence of left-right (L-R) asymmetry during embryogenesis is a classic problem in developmental biology. It is only since the 1990s, however, that substantial insight into this problem has been achieved by molecular and genetic approaches. Various genes required for L-R asymmetric morphogenesis in vertebrates have now been identified, and many of these genes are required for the formation and motility of cilia. Breaking of L-R symmetry in the mouse embryo occurs in the ventral node, where two types of cilia are present. Whereas centrally located motile cilia generate a leftward fluid flow, peripherally located immotile cilia sense a flow-dependent signal, which is either chemical or mechanical in nature. Although Ca2+ signaling is implicated in flow sensing, the precise mechanism remains unknown. Here we summarize current knowledge of L-R symmetry breaking in vertebrates (focusing on the mouse), with a special emphasis on the roles of cilia, fluid flow, and Ca2+ signaling.

Left-right patterning: conserved and divergent mechanisms

The left-right (LR) asymmetry of visceral organs is fundamental to their function and position within the body. Over the past decade or so, the molecular mechanisms underlying the establishment of such LR asymmetry have been revealed in many vertebrate and invertebrate model organisms. These studies have identified a gene network that contributes to this process and is highly conserved from sea urchin to mouse. By contrast, some specific steps of the process, such as the symmetry-breaking event and situs-specific organogenesis, appear to have diverged during evolution. Here, we summarize the common and divergent mechanisms by which LR asymmetry is established in vertebrates.

Low oxygen levels induce the expression of the embryonic morphogen Nodal

Low oxygen (O(2)) levels characterize the microenvironment of both stem cells and rapidly growing tumors. Moreover, hypoxia is associated with the maintenance of stem cell-like phenotypes and increased invasion, angiogenesis and metastasis in cancer patients. Metastatic cancers, such as breast cancer and melanoma, aberrantly express the embryonic morphogen Nodal, and the presence of this protein is correlated with metastatic disease. In this paper, we demonstrate that hypoxia induces Nodal expression in melanoma and breast cancer cells concomitant with increased cellular invasion and angiogenic phenotypes. Of note, Nodal expression remains up-regulated up to 48 h following reoxygenation. The oxygen-mediated regulation of Nodal expression occurs via a combinatorial mechanism. Within the first 24 h of exposure to low O(2), there is an increase in protein stability. This increase in stability is accompanied by an induction of transcription, mediated by the HIF-1α-dependent activation of Notch-responsive elements in the node-specific enhancer of the Nodal gene locus. Finally, Nodal expression is maintained upon reoxygenation by a canonical SMAD-dependent feed-forward mechanism. This work provides insight into the O(2)-mediated regulation of Nodal, a key stem cell-associated factor, and reveals that Nodal may be a target for the treatment and prevention of hypoxia-induced tumor progression.

Nodal cis-regulatory elements reveal epiblast and primitive endoderm heterogeneity in the peri-implantation mouse embryo

Nodal, a secreted factor known for its conserved functions in cell-fate specification and the establishment of embryonic axes, is also required in mammals to maintain the pluripotency of the epiblast, the tissue that gives rise to all fetal lineages. Although Nodal is expressed as early as E3.5 in the mouse embryo, its regulation and functions at pre- and peri-implantation stages are currently unknown. Sensitive reporter transgenes for two Nodal cis-regulatory regions, the PEE and the ASE, exhibit specific expression profiles before implantation. Mutant and inhibitor studies find them respectively regulated by Wnt/β-catenin signaling and Activin/Nodal signaling, and provide evidence for localized and heterogeneous activities of these pathways in the inner cell mass, the epiblast and the primitive endoderm. These studies also show that Nodal and its prime effector, FoxH1, are not essential to preimplantation Activin/Nodal signaling. Finally, a strong upregulation of the ASE reporter in implanting blastocysts correlates with a downregulation of the pluripotency factor Nanog in the maturing epiblast. This study uncovers conservation in the mouse blastocyst of Wnt/β-catenin and Activin/Nodal-dependent activities known to govern Nodal expression and the establishment of polarity in the blastula of other deuterostomes. Our results indicate that these pathways act early on to initiate distinct cell-specification processes in the ICM derivatives. Our data also suggest that the activity of the Activin/Nodal pathway is dampened by interactions with the molecular machinery of pluripotency until just before implantation, possibly delaying cell-fate decisions in the mouse embryo.

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.

Nodal signaling recruits the histone demethylase Jmjd3 to counteract polycomb-mediated repression at target genes

Both intercellular signaling and epigenetic mechanisms regulate embryonic development, but it is unclear how they are integrated to establish and maintain lineage-specific gene expression programs. Here, we show that a key function of the developmentally essential Nodal-Smads2/3 (Smad2 and Smad3) signaling pathway is to recruit the histone demethylase Jmjd3 to target genes, thereby counteracting repression by Polycomb. Smads2/3 bound to Jmjd3 and recruited it to chromatin in a manner that was dependent on active Nodal signaling. Knockdown of Jmjd3 alone substantially reduced Nodal target gene expression, whereas in the absence of Polycomb, target loci were expressed independently of Nodal signaling. These data establish a role for Polycomb in imposing a dependency on Nodal signaling for the expression of target genes and reveal how developmental signaling integrates with epigenetic processes to control gene expression.

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.

Role of nodal signaling and the microenvironment underlying melanoma plasticity

The incidence of melanoma has increased dramatically over the last 50 yr, and although melanoma accounts for only 10% of all skin cancers, it is responsible for over 80% of skin cancer deaths. Recent studies have uncovered critical molecular events underlying melanocytic transformation and melanomagenesis. Among these noteworthy observations are the acquisition of stem cell-associated proteins, such as the Notch receptors and Nodal, which have also been implicated in melanoma progression. For example, we have demonstrated that Nodal expression is limited to invasive vertical growth phase and metastatic melanoma lesions, and that inhibition of Nodal signaling promotes the reversion of metastatic melanoma cells toward a more differentiated, less invasive non-tumorigenic phenotype. In addition, molecular cross-talk exists between the Notch and Nodal signaling pathways. Interestingly, the acquisition of stem cell-associated plasticity is often acquired via epigenetic mechanisms, and is therefore receptive to reprogramming in response to embryonic microenvironments. Here, we review the concept of melanoma plasticity, with an emphasis on the emerging role of Nodal as a regulator of melanoma tumorigenesis and progression, and present findings related to epigenetic reprogramming.

Long-range action of Nodal requires interaction with GDF1

GDF1 (growth/differentiation factor 1), a Vg1-related member of the transforming growth factor-beta superfamily, is required for left-right patterning in the mouse, but the precise function of GDF1 has remained largely unknown. In contrast to previous observations, we now show that GDF1 itself is not an effective ligand but rather functions as a coligand for Nodal. GDF1 directly interacts with Nodal and thereby greatly increases its specific activity. Gdf1 expression in the node was found necessary and sufficient for initiation of asymmetric Nodal expression in the lateral plate of mouse embryos. Coexpression of GDF1 with Nodal in frog embryos increased the range of the Nodal signal. Introduction of Nodal alone into the lateral plate of Gdf1 knockout mouse embryos did not induce Lefty1 expression at the midline, whereas introduction of both Nodal and GDF1 did, showing that GDF1 is required for long-range Nodal signaling from the lateral plate to the midline. These results suggest that GDF1 regulates the activity and signaling range of Nodal through direct interaction.

Heart defects in X-linked heterotaxy: evidence for a genetic interaction of Zic3 with the nodal signaling pathway

The complex cardiac defects that occur in heterotaxy result from abnormal left-right patterning. Mutations in the zinc finger transcription factor ZIC3 cause X-linked heterotaxy, HTX1. We previously have generated a targeted deletion of the murine Zic3 locus and demonstrated that these knockout mice correctly model HTX1. Fifty percent of Zic3 null embryos have cardiac looping anomalies at embryonic day 10.5 to 14.5, with ventral looping and sinistral looping as the predominant phenotypes. The penetrance of these phenotypes is increased in mice that are also haploinsufficient for Nodal. Zic3(+/-); Nodal (+/-) compound heterozygous mice are born in significantly reduced numbers (P=0.0001), indicating a genetic interaction between the loci. Furthermore, an upstream Nodal enhancer is responsive to Zic3 in both Xenopus and mouse. These studies provide evidence that Zic3 interacts genetically with Nodal in left-right patterning and subsequent cardiac development and delineate a critical Zic3-responsive enhancer required for mediating Nodal expression at the node.

Sulfated glycosaminoglycans are necessary for Nodal signal transmission from the node to the left lateral plate in the mouse embryo

Situs-specific organogenesis in the mouse results from leftward fluid flow in the node cavity and subsequent left-sided expression of Nodal in the lateral plate mesoderm (LPM). Nodal expression at the node is essential for the subsequent asymmetric Nodal expression in the left LPM, but the precise role of Nodal produced at the node has remained unknown. We have now investigated how the Nodal signal is transferred from the node to the LPM. Externally supplied Nodal protein failed to signal to the LPM,suggesting that the Nodal signal is transferred to the LPM via an internal route rather than an external one. Transgenic rescue experiments showed that the Nodal co-receptor Cryptic (Cfc1) is required only in the LPM, not at the node, for asymmetric Nodal expression in the LPM, indicating that the Nodal signal is not relayed indirectly between the node and LPM. Nodal interacts in vitro with sulfated glycosaminoglycans (GAGs), which are specifically localized to the basement membrane-like structure between the node and LPM in the mouse embryo. Inhibition of sulfated GAG biosynthesis prevents Nodal expression in the LPM. These data suggest that Nodal produced at the node might travel directly to the LPM via interaction with sulfated GAGs.

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.

Generation of robust left-right asymmetry in the mouse embryo requires a self-enhancement and lateral-inhibition system

The bilateral symmetry of the mouse embryo is broken by leftward fluid flow in the node. However, it is unclear how this directional flow is then translated into the robust, left side-specific Nodal gene expression that determines and coordinates left-right situs throughout the embryo. While manipulating Nodal and Lefty gene expression, we have observed phenomena that are indicative of the involvement of a self-enhancement and lateral-inhibition (SELI) system. We constructed a mathematical SELI model that not only simulates, but also predicts, experimental data. As predicted by the model, Nodal expression initiates even on the right side. These results indicate that directional flow represents an initial small difference between the left and right sides of the embryo, but is insufficient to determine embryonic situs. Nodal and Lefty are deployed as a SELI system required to amplify this initial bias and convert it into robust asymmetry.

The left-right axis in the mouse: from origin to morphology

The past decade or so has seen rapid progress in our understanding of how left-right (LR) asymmetry is generated in vertebrate embryos. However, many important questions about this process remain unanswered. Although a leftward flow of extra-embryonic fluid in the node cavity (nodal flow) is likely to be the symmetry-breaking event, at least in the mouse embryo, it is not yet known how this flow functions or how the asymmetric signal generated in the node is transferred to the lateral plate. The final step in left-right patterning - translation of the asymmetric signal into morphology - is also little understood.