BMP/SMAD1 signaling sets a threshold for the left/right pathway in lateral plate mesoderm and limits availability of SMAD4

Stem cell-based embryo models: a tool to study early human development

How a mammalian fertilized egg acquires totipotency and develops into a full-term offspring is a fundamental scientific question. Human embryonic development is difficult to study due to limited resources, technical challenges and ethics. Moreover, the precise regulatory mechanism underlying early human embryonic development remains unknown. In recent years, the emergence of stem cell-based embryo models (SCBEM) provides the opportunity to reconstitute pre- to post-implantation development in vitro. These models to some extent mimic the embryo morphologically and transcriptionally, and thus may be used to study key events in mammalian pre- and post-implantation development. Many groups have successfully generated SCBEM of the mouse and human. Here, we provide a comparative review of the mouse and human SCBEM, discuss the capability of these models to mimic natural embryos and give a perspective on their potential future applications.

Notch3 is an asymmetric gene and a modifier of heart looping defects in Nodal mouse mutants

The TGFβ secreted factor NODAL is a major left determinant required for the asymmetric morphogenesis of visceral organs, including the heart. Yet, when this signaling is absent, shape asymmetry, for example of the embryonic heart loop, is not fully abrogated, indicating that there are other factors regulating left-right patterning. Here, we used a tailored transcriptomic approach to screen for genes asymmetrically expressed in the field of heart progenitors. We thus identify Notch3 as a novel left-enriched gene and validate, by quantitative in situ hybridization, its transient asymmetry in the lateral plate mesoderm and node crown, overlapping with Nodal. In mutant embryos, we analyzed the regulatory hierarchy and demonstrate that Nodal in the lateral plate mesoderm amplifies Notch3 asymmetric expression. The function of Notch3 was uncovered in an allelic series of mutants. In single neonate mutants, we observe that Notch3 is required with partial penetrance for ventricle thickness, septation and aortic valve, in addition to its known role in coronary arteries. In compound mutants, we reveal that Notch3 acts as a genetic modifier of heart looping direction and shape defects in Nodal mutants. Whereas Notch3 was previously mainly associated with the CADASIL syndrome, our observations in the mouse and a human cohort support a novel role in congenital heart defects and laterality defects.

Initiation and maintenance of the pluripotent epiblast in pre-implantation human development is independent of NODAL signaling

The human blastocyst contains the pluripotent epiblast from which human embryonic stem cells (hESCs) can be derived. ACTIVIN/NODAL signaling maintains expression of the transcription factor NANOG and in vitro propagation of hESCs. It is unknown whether this reflects a functional requirement for epiblast development in human embryos. Here, we characterized NODAL signaling activity during pre-implantation human development. We showed that NANOG is an early molecular marker restricted to the nascent human pluripotent epiblast and was initiated prior to the onset of NODAL signaling. We further demonstrated that expression of pluripotency-associated transcription factors NANOG, SOX2, OCT4, and KLF17 were maintained in the epiblast in the absence of NODAL signaling activity. Genome-wide transcriptional analysis showed that NODAL signaling inhibition did not decrease NANOG transcription or impact the wider pluripotency-associated gene regulatory network. These data suggest differences in the signaling requirements regulating pluripotency in the pre-implantation human epiblast compared with existing hESC culture.

Control of epiblast cell fate by mechanical cues

In amniotes, embryonic tissues originate from multipotent epiblast cells, arranged in a mosaic of presumptive territories. How these domains fated to specific lineages become segregated during body formation remains poorly understood. Using single cell RNA sequencing analysis and lineage tracing in the chicken embryo, we identify epiblast cells contributing descendants to the neural tube, somites and lateral plate after completion of gastrulation. We show that intercalation after cell division generates important movements of epiblast cells which lead to their relocation to different presumptive territories, explaining this broad spectrum of fates. This tissue remodeling phase is transient, being soon followed by the establishment of boundaries restricting cell movements therefore defining the presumptive territories of the epiblast. Finally, we find that the epiblast faces distinct mechanical constraints along the antero-posterior axis, leading to cell fate alterations when challenged. Together, we demonstrate the critical role of mechanical cues in epiblast fate determination.

R-Spondin 2 governs Xenopus left-right body axis formation by establishing an FGF signaling gradient

Establishment of the left-right (LR, sinistral, dextral) body axis in many vertebrate embryos relies on cilia-driven leftward fluid flow within an LR organizer (LRO). A cardinal question is how leftward flow triggers symmetry breakage. The chemosensation model posits that ciliary flow enriches a signaling molecule on the left side of the LRO that promotes sinistral cell fate. However, the nature of this sinistralizing signal has remained elusive. In the Xenopus LRO, we identified the stem cell growth factor R-Spondin 2 (Rspo2) as a symmetrically expressed, sinistralizing signal. As predicted for a flow-mediated signal, Rspo2 operates downstream of leftward flow but upstream of the asymmetrically expressed gene dand5. Unexpectedly, in LR patterning, Rspo2 acts as an FGF receptor antagonist: Rspo2 via its TSP1 domain binds Fgfr4 and promotes its membrane clearance by Znrf3-mediated endocytosis. Concordantly, we find that at flow-stage, FGF signaling is dextralizing and forms a gradient across the LRO, high on the dextral- and low on the sinistral side. Rspo2 gain- and loss-of function equalize this FGF signaling gradient and sinistralize and dextralize development, respectively. We propose that leftward flow of Rspo2 produces an FGF signaling gradient that governs LR-symmetry breakage.

Serum-Induced Proliferation of Human Cardiac Stem Cells Is Modulated via TGFβRI/II and SMAD2/3

The ageing phenotype is strongly driven by the exhaustion of adult stem cells (ASCs) and the accumulation of senescent cells. Cardiovascular diseases (CVDs) and heart failure (HF) are strongly linked to the ageing phenotype and are the leading cause of death. As the human heart is considered as an organ with low regenerative capacity, treatments targeting the rejuvenation of human cardiac stem cells (hCSCs) are of great interest. In this study, the beneficial effects of human blood serum on proliferation and senescence of hCSCs have been investigated at the molecular level. We show the induction of a proliferation-related gene expression response by human blood serum at the mRNA level. The concurrent differential expression of the TGFβ target and inhibitor genes indicates the participation of TGFβ signalling in this context. Surprisingly, the application of TGFβ1 as well as the inhibition of TGFβ type I and type II receptor (TGFβRI/II) signalling strongly increased the proliferation of hCSCs. Likewise, both human blood serum and TGFβ1 reduced the senescence in hCSCs. The protective effect of serum on senescence in hCSCs was enhanced by simultaneous TGFβRI/II inhibition. These results strongly indicate a dual role of TGFβ signalling in terms of the serum-mediated effects on hCSCs. Further analysis via RNA sequencing (RNA-Seq) revealed the participation of Ras-inactivating genes wherefore a prevention of hyperproliferation upon serum-treatment in hCSCs via TGFβ signalling and Ras-induced senescence is suggested. These insights may improve treatments of heart failure in the future.

Time space and single-cell resolved tissue lineage trajectories and laterality of body plan at gastrulation

Understanding of the molecular drivers of lineage diversification and tissue patterning during primary germ layer development requires in-depth knowledge of the dynamic molecular trajectories of cell lineages across a series of developmental stages of gastrulation. Through computational modeling, we constructed at single-cell resolution, a spatio-temporal transcriptome of cell populations in the germ-layers of gastrula-stage mouse embryos. This molecular atlas enables the inference of molecular network activity underpinning the specification and differentiation of the germ-layer tissue lineages. Heterogeneity analysis of cellular composition at defined positions in the epiblast revealed progressive diversification of cell types. The single-cell transcriptome revealed an enhanced BMP signaling activity in the right-side mesoderm of late-gastrulation embryo. Perturbation of asymmetric BMP signaling activity at late gastrulation led to randomization of left-right molecular asymmetry in the lateral mesoderm of early-somite-stage embryo. These findings indicate the asymmetric BMP activity during gastrulation may be critical for the symmetry breaking process.

The RNA helicase DDX6 controls early mouse embryogenesis by repressing aberrant inhibition of BMP signaling through miRNA-mediated gene silencing

The evolutionarily conserved RNA helicase DDX6 is a central player in post-transcriptional regulation, but its role during embryogenesis remains elusive. We here show that DDX6 enables proper cell lineage specification from pluripotent cells by analyzing Ddx6 knockout (KO) mouse embryos and employing an in vitro epiblast-like cell (EpiLC) induction system. Our study unveils that DDX6 is an important BMP signaling regulator. Deletion of Ddx6 causes the aberrant upregulation of the negative regulators of BMP signaling, which is accompanied by enhanced expression of Nodal and related genes. Ddx6 KO pluripotent cells acquire higher pluripotency with a strong inclination toward neural lineage commitment. During gastrulation, abnormally expanded Nodal and Eomes expression in the primitive streak likely promotes endoderm cell fate specification while inhibiting mesoderm differentiation. We also genetically dissected major DDX6 pathways by generating Dgcr8, Dcp2, and Eif4enif1 KO models in addition to Ddx6 KO. We found that the miRNA pathway mutant Dgcr8 KO phenocopies Ddx6 KO, indicating that DDX6 mostly works along with the miRNA pathway during early development, whereas its P-body-related functions are dispensable. Therefore, we conclude that DDX6 prevents aberrant upregulation of BMP signaling inhibitors by participating in miRNA-mediated gene silencing processes. Overall, this study delineates how DDX6 affects the development of the three primary germ layers during early mouse embryogenesis and the underlying mechanism of DDX6 function.

Dissecting signaling hierarchies in the patterning of the mouse primitive streak using micropatterned EpiLC colonies

Embryo studies have established that the patterning of the mouse gastrula depends on a regulatory network in which the WNT, BMP, and NODAL signaling pathways cooperate, but aspects of their respective contributions remain unclear. Studying their impact on the spatial organization and developmental trajectories of micropatterned epiblast-like cell (EpiLC) colonies, we show that NODAL is required prior to BMP action to establish the mesoderm and endoderm lineages. The presence of BMP then forces NODAL and WNT to support the formation of posterior primitive streak (PS) derivatives, while its absence allows them to promote that of anterior PS derivatives. Also, a Nodal mutation elicits more severe patterning defects in vitro than in the embryo, suggesting that ligands of extra-embryonic origin can rescue them. These results support the implication of a combinatorial process in PS patterning and illustrate how the study of micropatterned EpiLC colonies can complement that of embryos.

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BMP or WNT cannot rescue the impact a Nodal KO has on primitive streak formation

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BMP exposure results in Nodal promoting posterior rather than anterior PS formation

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The maintenance of posterior mesodermal identities is dependent on Nodal expression

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Low Nodal expression does not prevent the emergence of anterior PS derivatives

Establishment and interpretation of NODAL and BMP signaling gradients in early vertebrate development

Transforming growth factor β (TGF-β) family ligands play crucial roles in orchestrating early embryonic development. Most significantly, two family members, NODAL and BMP form signaling gradients and indeed in fish, frogs and sea urchins these two opposing gradients are sufficient to organize a complete embryonic axis. This review focuses on how these gradients are established and interpreted during early vertebrate development. The review highlights key principles that are emerging, in particular the importance of signaling duration as well as ligand concentration in both gradient generation and their interpretation. Feedforward and feedback loops involving other signaling pathways are also essential for providing spatial and temporal information downstream of the NODAL and BMP signaling pathways. Finally, new data suggest the existence of buffering mechanisms, whereby early signaling defects can be readily corrected downstream later in development, suggesting that signaling gradients do not have to be as precise as previously thought.

A genotype and phenotype analysis of SMAD6 mutant patients with radioulnar synostosis

Background

SMAD6 variants have been reported in patients with radioulnar synostosis (RUS). This study aimed to investigate the genotypes and phenotypes for a large cohort of patients with RUS having mutant SMAD6.

Methods

Genomic DNA samples were isolated from 251 RUS sporadic patients (with their parents) and 27 RUS pedigrees. Sanger sequencing was performed for the SMAD6 coding regions. For positive probands, co‐segregation and parental‐origin analysis of SMAD6 variants and phenotypic re‐evaluation were performed for their family members.

Results

We identified 50 RUS probands with SMAD6 variants (13 co‐segregated with RUS in pedigrees and 37 in RUS‐sporadic patients). Based on the new and previous data, we identified SMAD6 mutated in 16/38 RUS pedigrees and 61/393 RUS sporadic patients, respectively. Overall, 93 SMAD6 mutant patients with RUS were identified, among which 29 patients had unilateral RUS, where the left side was more involved than the right side (left:right = 20:9). Female protective effects and non‐full penetrance were observed, in which only 6.90% mothers (vs. ~50% fathers) of SMAD6 mutant RUS probands had RUS. Pleiotropy was observed as a re‐evaluation of SMAD6 mutant families identified: (a) three families had axial skeletal malformations; (b) two families had polydactyly; and (c) eight families had other known malformations.

Conclusion

SMAD6 was mutated in 42.11% RUS pedigrees and 15.52% RUS sporadic patients.

The RUS patients with SMAD6 variants exhibit both non‐full‐penetrance, variable expressivity, pleiotropy, female protective effects, and the left side is more susceptible than the right side.

Right, left and cilia: How asymmetry is established

The initial breaking of left-right (L-R) symmetry in the embryo is controlled by a motile-cilia-driven leftward fluid flow in the left-right organiser (LRO), resulting in L-R asymmetric gene expression flanking the LRO. Ultimately this results in left- but not right-sided activation of the Nodal-Pitx2 pathway in more lateral tissues. While aspects of the initial breaking event clearly vary between vertebrates, events in the Lateral Plate Mesoderm (LPM) are conserved through the vertebrate lineage. Evidence from model systems and humans highlights the role of cilia both in the initial symmetry breaking and in the ability of more lateral tissues to exhibit asymmetric gene expression. In this review we concentrate on the process of L-R determination in mouse and humans.

Reactivation of BMP signaling by suboptimal concentrations of MEK inhibitor and FK506 reduces organ-specific breast cancer metastasis

TGFβ-SMAD3 signaling is a major driving force for cancer metastasis, while BMP-SMAD1/5 signaling can counteract this response. Analysis of gene expression profiles revealed that an increased TGFβ-SMAD3 and a reduced BMP-SMAD1/5 targeted gene expression signature correlated with shortened distant metastasis free survival and overall survival of patients. At molecular levels, we discovered that TGFβ abolished BMP-induced SMAD1/5 activation in the highly-invasive breast cancer MDA-MB-231 cells, but to a less extent in the non-invasive cancer and normal breast cells. This suggests an inverse correlation between BMP signaling and invasiveness of tumor cells and TGFβ signaling acts in a double whammy fashion in driving cancer invasion and metastasis. Sustained ERK activation by TGFβ was specifically observed in MDA-MB-231 cells, and MEK inhibitor (MEKi) treatment restored BMP-SMAD1/5 signaling while not affecting SMAD2/3 activation. FK506 potently activated BMP, but not TGFβ signaling in breast cancer cells. MEKi or FK506 alone inhibited MDA-MB-231 extravasation in a zebrafish xenograft cancer model. Importantly, when administrated at suboptimal concentrations MEKi and FK506 strongly synergized in promoting BMP-SMAD1/5 signaling and inhibiting cancer cell extravasation. Furthermore, this combination of suboptimal concentrations treatment in a mouse tumor model resulted in real-time reduction of BMP-SMAD1/5 signaling in live tumors, and consequently potently inhibited tumor self-seeding, liver and bone metastasis, but not lung and brain metastasis. Mechanistically, it is the first time to identify BMP-SMAD1/5 signaling as an underlying molecular driver for organ-specific metastasis. Combining of MEKi and FK506, or their analogues, may be explored for clinical development of breast cancer.

Spatial and temporal deletion reveals a latent effect of Megf8 on the left-right patterning and heart development

Laterality disease is frequently associated with congenital heart disease (CHD). However, it is unclear what is behind this association, a pleiotropic effect of common genetic causes of laterality diseases or the impact of abnormal left-right patterning on the downstream cardiovascular development. MEGF8 is a disease gene of Carpenter syndrome characterized by defective lateralization and CHD. Here we performed spatial and temporal deletion to dissect the tissue and time requirements of Megf8 on cardiovascular development. None of conditional deletions in cardiomyocytes, endothelium/endocardium, epicardium, cardiac mesoderm or neural crest cells led to cardiovascular defects. More surprisingly, temporal deletion with a ubiquitous Cre driver at embryonic day 7.5 (E7.5), a time point before symmetry break and cardiogenesis, causes preaxial polydactyly (PPD) and exencephaly, but not laterality and cardiovascular defects. These data suggested that Megf8 was dispensable for cardiac organogenesis. Only with E6.5 deletion, we observed aortic arch artery defects including right aortic arch, an indicator of reversed left-right patterning. The concurrence of laterality and cardiovascular defects in pre-streak stage deletion rather than cardiac organogenesis stage deletion indicates that the laterality defect may directly impact heart development. Interestingly, the latent effect of Megf8 on the left-right patterning suggests that the regulation of laterality may be much earlier than we previously thought.

Genetic dissection of Nodal and Bmp signalling requirements during primordial germ cell development in mouse

The essential roles played by Nodal and Bmp signalling during early mouse development have been extensively documented. Here we use conditional deletion strategies to investigate functional contributions made by Nodal, Bmp and Smad downstream effectors during primordial germ cell (PGC) development. We demonstrate that Nodal and its target gene Eomes provide early instructions during formation of the PGC lineage. We discover that Smad2 inactivation in the visceral endoderm results in increased numbers of PGCs due to an expansion of the PGC niche. Smad1 is required for specification, whereas in contrast Smad4 controls the maintenance and migration of PGCs. Additionally we find that beside Blimp1, down-regulated phospho-Smad159 levels also distinguishes PGCs from their somatic neighbours so that emerging PGCs become refractory to Bmp signalling that otherwise promotes mesodermal development in the posterior epiblast. Thus balanced Nodal/Bmp signalling cues regulate germ cell versus somatic cell fate decisions in the early posterior epiblast.

Bone morphogenetic protein signaling in inflammation

IMPACT STATEMENT

By compiling findings from recent studies, this review will garner novel insight on the dynamic and complex role of BMP signaling in diseases of inflammation, highlighting the specific roles played by both individual ligands and endogenous antagonists. Ultimately, this summary will help inform the high therapeutic value of targeting this pathway for modulating diseases of inflammation.

Left-right asymmetry in heart development and disease: forming the right loop

Extensive studies have shown how bilateral symmetry of the vertebrate embryo is broken during early development, resulting in a molecular left-right bias in the mesoderm. However, how this early asymmetry drives the asymmetric morphogenesis of visceral organs remains poorly understood. The heart provides a striking model of left-right asymmetric morphogenesis, undergoing rightward looping to shape an initially linear heart tube and align cardiac chambers. Importantly, abnormal left-right patterning is associated with severe congenital heart defects, as exemplified in heterotaxy syndrome. Here, we compare the mechanisms underlying the rightward looping of the heart tube in fish, chick and mouse embryos. We propose that heart looping is not only a question of direction, but also one of fine-tuning shape. This is discussed in the context of evolutionary and clinical perspectives.

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.

Live imaging of heart tube development in mouse reveals alternating phases of cardiac differentiation and morphogenesis

During vertebrate heart development, two progenitor populations, first and second heart fields (FHF, SHF), sequentially contribute to longitudinal subdivisions of the heart tube (HT), with the FHF contributing the left ventricle and part of the atria, and the SHF the rest of the heart. Here, we study the dynamics of cardiac differentiation and morphogenesis by tracking individual cells in live analysis of mouse embryos. We report that during an initial phase, FHF precursors differentiate rapidly to form a cardiac crescent, while limited morphogenesis takes place. In a second phase, no differentiation occurs while extensive morphogenesis, including splanchnic mesoderm sliding over the endoderm, results in HT formation. In a third phase, cardiac precursor differentiation resumes and contributes to SHF-derived regions and the dorsal closure of the HT. These results reveal tissue-level coordination between morphogenesis and differentiation during HT formation and provide a new framework to understand heart development.

We all start life as a single cell, which – over the course of nine months – multiplies to generate the billions of cells that can be found in a newborn. As an embryo develops, the cells need to achieve two major tasks: they need to diversify into different types of cells, such as blood cells or muscle cells, and they need to organize themselves in space to form tissues and organs.

The heart of an embryo, for example, first forms a simple structure called the heart tube that can pump blood and later develops into the four chambers that we see in adults. The tube is made up of cells from two different origins, known as the first and second heart fields. Unlike other organs, the heart has to start beating while it is still developing, and until now, it was unclear how the heart manages this difficult task.

Here, Ivanovich et al. studied mouse embryos grown outside the womb by using a combination of advanced microscopy and genetic labeling to track how single cells turn into beating cells and move while the heart forms. The results showed that specializing into beating cells and forming the heart tube shape happened during alternating phases. The first heart-field cells turned into beating cells and began to contract at an early stage before the heart tube was formed. Next, the cells of the second heart field did not instantly develop into beating cells, but instead, helped the first heart-field cells to acquire the shape of a heart tube. Once this was completed, the second heart-field cells started to specialize into beating cells and created the additional parts of the more complex adult heart.

This research shows that the second heart field plays an active role in helping the heart tube form. The alternating phases of cell specialization and tissue formation allow the heart to become active whilst it is still developing. A better insight into how the heart forms may help us to create new treatments for various genetic heart conditions. The methods used here could also help to study how cells build other organs.

From cytoskeletal dynamics to organ asymmetry: a nonlinear, regulative pathway underlies left-right patterning

Consistent left-right (LR) asymmetry is a fundamental aspect of the bodyplan across phyla, and errors of laterality form an important class of human birth defects. Its molecular underpinning was first discovered as a sequential pathway of left- and right-sided gene expression that controlled positioning of the heart and visceral organs. Recent data have revised this picture in two important ways. First, the physical origin of chirality has been identified; cytoskeletal dynamics underlie the asymmetry of single-cell behaviour and patterning of the LR axis. Second, the pathway is not linear: early disruptions that alter the normal sidedness of upstream asymmetric genes do not necessarily induce defects in the laterality of the downstream genes or in organ situs Thus, the LR pathway is a unique example of two fascinating aspects of biology: the interplay of physics and genetics in establishing large-scale anatomy, and regulative (shape-homeostatic) pathways that correct molecular and anatomical errors over time. Here, we review aspects of asymmetry from its intracellular, cytoplasmic origins to the recently uncovered ability of the LR control circuitry to achieve correct gene expression and morphology despite reversals of key 'determinant' genes. We provide novel functional data, in Xenopus laevis, on conserved elements of the cytoskeleton that drive asymmetry, and comparatively analyse it together with previously published results in the field. Our new observations and meta-analysis demonstrate that despite aberrant expression of upstream regulatory genes, embryos can progressively normalize transcriptional cascades and anatomical outcomes. LR patterning can thus serve as a paradigm of how subcellular physics and gene expression cooperate to achieve developmental robustness of a body axis.This article is part of the themed issue 'Provocative questions in left-right asymmetry'.

A predictive model of asymmetric morphogenesis from 3D reconstructions of mouse heart looping dynamics

How left-right patterning drives asymmetric morphogenesis is unclear. Here, we have quantified shape changes during mouse heart looping, from 3D reconstructions by HREM. In combination with cell labelling and computer simulations, we propose a novel model of heart looping. Buckling, when the cardiac tube grows between fixed poles, is modulated by the progressive breakdown of the dorsal mesocardium. We have identified sequential left-right asymmetries at the poles, which bias the buckling in opposite directions, thus leading to a helical shape. Our predictive model is useful to explore the parameter space generating shape variations. The role of the dorsal mesocardium was validated in Shh^-/- mutants, which recapitulate heart shape changes expected from a persistent dorsal mesocardium. Our computer and quantitative tools provide novel insight into the mechanism of heart looping and the contribution of different factors, beyond the simple description of looping direction. This is relevant to congenital heart defects.

Mutations in the Katnb1 gene cause left-right asymmetry and heart defects

BACKGROUND

The microtubule-severing protein complex katanin is composed two subunits, the ATPase subunit, KATNA1, and the noncatalytic regulatory subunit, KATNB1. Recently, the Katnb1 gene has been linked to infertility, regulation of centriole and cilia formation in fish and mammals, as well as neocortical brain development. KATNB1 protein is expressed in germ cells in humans and mouse, mitotic/meiotic spindles and cilia, although the full expression pattern of the Katnb1 gene has not been described.

RESULTS

Using a knockin-knockout mouse model of Katnb1 dysfunction we demonstrate that Katnb1 is ubiquitously expressed during embryonic development, although a stronger expression is seen in the crown cells of the gastrulation organizer, the murine node. Furthermore, null and hypomorphic Katnb1 gene mutations show a novel correlation between Katnb1 dysregulation and the development of impaired left-right signaling, including cardiac malformations.

CONCLUSIONS

Katanin function is a critical regulator of heart development in mice. These findings are potentially relevant to human cardiac development. Developmental Dynamics 246:1027-1035, 2017. © 2017 Wiley Periodicals, Inc.

Dysregulated endocardial TGFβ signaling and mesenchymal transformation result in heart outflow tract septation failure

Heart outflow tract septation in mouse embryos carrying mutations in retinoic acid receptor genes fails with complete penetrance. In this mutant background, ectopic TGFβ signaling in the distal outflow tract is responsible for septation failure, but it was uncertain what tissue was responsive to ectopic TGFβ and why this response interfered with septation. By combining RAR gene mutation with tissue-specific Cre drivers and a conditional type II TGFβ receptor (Tgfbr2) allele, we determined that ectopic activation of TGFβ signaling in the endocardium is responsible for septation defects. Ectopic TGFβ signaling results in ectopic mesenchymal transformation of the endocardium and thereby in improperly constituted distal OFT cushions. Our analysis highlights the interactions between myocardium, endocardium, and neural crest cells in outflow tract morphogenesis, and demonstrates the requirement for proper TGFβ signaling in outflow tract cushion organization and septation.

The Maternal Maverick/GDF15-like TGF-β Ligand Panda Directs Dorsal-Ventral Axis Formation by Restricting Nodal Expression in the Sea Urchin Embryo

Specification of the dorsal-ventral axis in the highly regulative sea urchin embryo critically relies on the zygotic expression of nodal, but whether maternal factors provide the initial spatial cue to orient this axis is not known. Although redox gradients have been proposed to entrain the dorsal-ventral axis by acting upstream of nodal, manipulating the activity of redox gradients only has modest consequences, suggesting that other factors are responsible for orienting nodal expression and defining the dorsal-ventral axis. Here we uncover the function of Panda, a maternally provided transforming growth factor beta (TGF-β) ligand that requires the activin receptor-like kinases (Alk) Alk3/6 and Alk1/2 receptors to break the radial symmetry of the embryo and orient the dorsal-ventral axis by restricting nodal expression. We found that the double inhibition of the bone morphogenetic protein (BMP) type I receptors Alk3/6 and Alk1/2 causes a phenotype dramatically more severe than the BMP2/4 loss-of-function phenotype, leading to extreme ventralization of the embryo through massive ectopic expression of nodal, suggesting that an unidentified signal acting through BMP type I receptors cooperates with BMP2/4 to restrict nodal expression. We identified this ligand as the product of maternal Panda mRNA. Double inactivation of panda and bmp2/4 led to extreme ventralization, mimicking the phenotype caused by inactivation of the two BMP receptors. Inhibition of maternal panda mRNA translation disrupted the early spatial restriction of nodal, leading to persistent massive ectopic expression of nodal on the dorsal side despite the presence of Lefty. Phylogenetic analysis indicates that Panda is not a prototypical BMP ligand but a member of a subfamily of TGF-β distantly related to Inhibins, Lefty, and TGF-β that includes Maverick from Drosophila and GDF15 from vertebrates. Indeed, overexpression of Panda does not appear to directly or strongly activate phosphoSmad1/5/8 signaling, suggesting that although this TGF-β may require Alk1/2 and/or Alk3/6 to antagonize nodal expression, it may do so by sequestering a factor essential for Nodal signaling, by activating a non-Smad pathway downstream of the type I receptors, or by activating extremely low levels of pSmad1/5/8. We provide evidence that, although panda mRNA is broadly distributed in the early embryo, local expression of panda mRNA efficiently orients the dorsal-ventral axis and that Panda activity is required locally in the early embryo to specify this axis. Taken together, these findings demonstrate that maternal panda mRNA is both necessary and sufficient to orient the dorsal-ventral axis. These results therefore provide evidence that in the highly regulative sea urchin embryo, the activity of spatially restricted maternal factors regulates patterning along the dorsal-ventral axis.

Activin/Nodal signalling before implantation: setting the stage for embryo patterning

Activins and Nodal are members of the transforming growth factor beta (TGF-β) family of growth factors. Their Smad2/3-dependent signalling pathway is well known for its implication in the patterning of the embryo after implantation. Although this pathway is active early on at preimplantation stages, embryonic phenotypes for loss-of-function mutations of prominent components of the pathway are not detected before implantation. It is only fairly recently that an understanding of the role of the Activin/Nodal signalling pathway at these stages has started to emerge, notably from studies detailing how it controls the expression of target genes in embryonic stem cells. We review here what is currently known of the TGF-β-related ligands that determine the activity of Activin/Nodal signalling at preimplantation stages, and recent advances in the elucidation of the Smad2/3-dependent mechanisms underlying developmental progression.

Signaling Pathways and Gene Regulatory Networks in Cardiomyocyte Differentiation

Strategies for harnessing stem cells as a source to treat cell loss in heart disease are the subject of intense research. Human pluripotent stem cells (hPSCs) can be expanded extensively in vitro and therefore can potentially provide sufficient quantities of patient-specific differentiated cardiomyocytes. Although multiple stimuli direct heart development, the differentiation process is driven in large part by signaling activity. The engineering of hPSCs to heart cell progeny has extensively relied on establishing proper combinations of soluble signals, which target genetic programs thereby inducing cardiomyocyte specification. Pertinent differentiation strategies have relied as a template on the development of embryonic heart in multiple model organisms. Here, information on the regulation of cardiomyocyte development from in vivo genetic and embryological studies is critically reviewed. A fresh interpretation is provided of in vivo and in vitro data on signaling pathways and gene regulatory networks (GRNs) underlying cardiopoiesis. The state-of-the-art understanding of signaling pathways and GRNs presented here can inform the design and optimization of methods for the engineering of tissues for heart therapies.

Mechanisms of left-right asymmetry and patterning: driver, mediator and responder

The establishment of a left-right (LR) organizer in the form of the ventral node is an absolute prerequisite for patterning the tissues on contralateral sides of the body of the mouse embryo. The experimental findings to date are consistent with a mechanistic paradigm that the laterality information, which is generated in the ventral node, elicits asymmetric molecular activity and cellular behaviour in the perinodal tissues. This information is then relayed to the cells in the lateral plate mesoderm (LPM) when the left-specific signal is processed and translated into LR body asymmetry. Here, we reflect on our current knowledge and speculate on the following: (a) what are the requisite anatomical and functional attributes of an LR organizer, (b) what asymmetric information is emanated from this organizer, and (c) how this information is transferred across the paraxial tissue compartment and elicits a molecular response specifically in the LPM.

An atypical canonical bone morphogenetic protein (BMP) signaling pathway regulates Msh homeobox 1 (Msx1) expression during odontogenesis

Bone morphogenetic protein (BMP) signaling plays an essential role in early tooth development, evidenced by disruption of BMP signaling leading to an early arrested tooth development. Despite being a central mediator of BMP canonical signaling pathway, inactivation of Smad4 in dental mesenchyme does not result in early developmental defects. In the current study, we investigated the mechanism of receptor-activated Smads (R-Smads) and Smad4 in the regulation of the odontogenic gene Msx1 expression in the dental mesenchyme. We showed that the canonical BMP signaling is not operating in the early developing tooth, as assessed by failed activation of the BRE-Gal transgenic allele and the absence of phospho-(p)Smad1/5/8-Smad4 complexes. The absence of pSmad1/5/8-Smad4 complex appeared to be the consequence of saturation of Smad4 by pSmad2/3 in the dental mesenchyme as knockdown of Smad2/3 or overexpression of Smad4 led to the formation of pSmad1/5/8-Smad4 complexes and activation of canonical BMP signaling in dental mesenchymal cells. We showed that Smad1/5 but not Smad4 are required for BMP-induced expression of Msx1 in dental mesenchymal cells. We further presented evidence that in the absence of Smad4, BMPs are still able to induce pSmad1/5/8 nuclear translocation and their binding to the Msx1 promoter directly in dental mesenchymal cells. Our results demonstrate the functional operation of an atypical canonical BMP signaling (Smad4-independent and Smad1/5/8-dependent) pathway in the dental mesenchyme during early odontogenesis, which may have general implication in the development of other organs.

Gtpbp2 is required for BMP signaling and mesoderm patterning in Xenopus embryos

Smad proteins convey canonical intracellular signals for activated receptors in the TGFβ superfamily, but the activity of Smads and their impact on target genes are further regulated by a wide variety of cofactors and partner proteins. We have identified a new Smad1 partner, a GTPase named Gtpbp2 that is a distant relative of the translation factor eEf1a. Gtpbp2 affects canonical signaling in the BMP branch of the TGFβ superfamily, as morpholino knockdown of Gtpbp2 decreases, and overexpression of Gtpbp2 enhances, animal cap responses to BMP4. During Xenopus development, gtpbp2 transcripts are maternally expressed and localized to the egg animal pole, and partitioned into the nascent ectodermal and mesodermal cells during cleavage and early gastrulation stages. Subsequently, gtpbp2 is expressed in the neural folds, and in early tadpoles undergoing organogenesis gtpbp2 is expressed prominently in the brain, eyes, somites, ventral blood island and branchial arches. Consistent with its expression, morpholino knockdown of Gtpbp2 causes defects in ventral-posterior germ layer patterning, gastrulation and tadpole morphology. Overexpressed Gtpbp2 can induce ventral-posterior marker genes and localize to cell nuclei in Xenopus animal caps, highlighting its role in regulating BMP signaling in the early embryo. Here, we introduce this large GTPase as a novel factor in BMP signaling and ventral-posterior patterning.

Oct4 is required ~E7.5 for proliferation in the primitive streak

Oct4 is a widely recognized pluripotency factor as it maintains Embryonic Stem (ES) cells in a pluripotent state, and, in vivo, prevents the inner cell mass (ICM) in murine embryos from differentiating into trophectoderm. However, its function in somatic tissue after this developmental stage is not well characterized. Using a tamoxifen-inducible Cre recombinase and floxed alleles of Oct4, we investigated the effect of depleting Oct4 in mouse embryos between the pre-streak and headfold stages, ∼E6.0–E8.0, when Oct4 is found in dynamic patterns throughout the embryonic compartment of the mouse egg cylinder. We found that depletion of Oct4 ∼E7.5 resulted in a severe phenotype, comprised of craniorachischisis, random heart tube orientation, failed turning, defective somitogenesis and posterior truncation. Unlike in ES cells, depletion of the pluripotency factors Sox2 and Oct4 after E7.0 does not phenocopy, suggesting that ∼E7.5 Oct4 is required within a network that is altered relative to the pluripotency network. Oct4 is not required in extraembryonic tissue for these processes, but is required to maintain cell viability in the embryo and normal proliferation within the primitive streak. Impaired expansion of the primitive streak occurs coincident with Oct4 depletion ∼E7.5 and precedes deficient convergent extension which contributes to several aspects of the phenotype.

Embryogenesis is an intricate process requiring that division, differentiation and position of cells are coordinated. During mammalian development early pluripotent populations are canalized or restricted in potency during embryogenesis. Due to considerable interest in how this fundamental state of pluripotency is maintained, and the requirement of the transcription factor Oct4 to maintain pluripotency, Oct4 has been intensively studied in culture. However, it is not clear what role Oct4 has during lineage specification of pluripotent cells. Oct4 removal during lineage specification indicates that it is required in the primitive streak of mouse embryos to maintain proliferation. The consequences of Oct4 removal diverge from the consequences of removing another factor required for pluripotency between preimplantation development and early cell fate specification suggesting that the network Oct4 acts within is altered between these stages.

Small heat shock proteins are necessary for heart migration and laterality determination in zebrafish

Small heat shock proteins (sHsps) regulate cellular functions not only under stress, but also during normal development, when they are expressed in organ-specific patterns. Here we demonstrate that two small heat shock proteins expressed in embryonic zebrafish heart, hspb7 and hspb12, have roles in the development of left-right asymmetry. In zebrafish, laterality is determined by the motility of cilia in Kupffer's vesicle (KV), where hspb7 is expressed; knockdown of hspb7 causes laterality defects by disrupting the motility of these cilia. In embryos with reduced hspb7, the axonemes of KV cilia have a 9+0 structure, while control embyros have a predominately 9+2 structure. Reduction of either hspb7 or hspb12 alters the expression pattern of genes that propagate the signals that establish left-right asymmetry: the nodal-related gene southpaw (spaw) in the lateral plate mesoderm, and its downstream targets pitx2, lefty1 and lefty2. Partial depletion of hspb7 causes concordant heart, brain and visceral laterality defects, indicating that loss of KV cilia motility leads to coordinated but randomized laterality. Reducing hspb12 leads to similar alterations in the expression of downstream laterality genes, but at a lower penetrance. Simultaneous reduction of hspb7 and hspb12 randomizes heart, brain and visceral laterality, suggesting that these two genes have partially redundant functions in the establishment of left-right asymmetry. In addition, both hspb7 and hspb12 are expressed in the precardiac mesoderm and in the yolk syncytial layer, which supports the migration and fusion of mesodermal cardiac precursors. In embryos in which the reduction of hspb7 or hspb12 was limited to the yolk, migration defects predominated, suggesting that the yolk expression of these genes rather than heart expression is responsible for the migration defects.

Smad2 is essential for maintenance of the human and mouse primed pluripotent stem cell state

Human embryonic stem cells and mouse epiblast stem cells represent a primed pluripotent stem cell state that requires TGF-β/activin signaling. TGF-β and/or activin are commonly thought to regulate transcription through both Smad2 and Smad3. However, the different contributions of these two Smads to primed pluripotency and the downstream events that they may regulate remain poorly understood. We addressed the individual roles of Smad2 and Smad3 in the maintenance of primed pluripotency. We found that Smad2, but not Smad3, is required to maintain the undifferentiated pluripotent state. We defined a Smad2 regulatory circuit in human embryonic stem cells and mouse epiblast stem cells, in which Smad2 acts through binding to regulatory promoter sequences to activate Nanog expression while in parallel repressing autocrine bone morphogenetic protein signaling. Increased autocrine bone morphogenetic protein signaling caused by Smad2 down-regulation leads to cell differentiation toward the trophectoderm, mesoderm, and germ cell lineages. Additionally, induction of Cdx2 expression, as a result of decreased Smad2 expression, leads to repression of Oct4 expression, which, together with the decreased Nanog expression, accelerates the loss of pluripotency. These findings reveal that Smad2 is a unique integrator of transcription and signaling events and is essential for the maintenance of the mouse and human primed pluripotent stem cell state.

Integration of nodal and BMP signals in the heart requires FoxH1 to create left-right differences in cell migration rates that direct cardiac asymmetry

Failure to properly establish the left–right (L/R) axis is a major cause of congenital heart defects in humans, but how L/R patterning of the embryo leads to asymmetric cardiac morphogenesis is still unclear. We find that asymmetric Nodal signaling on the left and Bmp signaling act in parallel to establish zebrafish cardiac laterality by modulating cell migration velocities across the L/R axis. Moreover, we demonstrate that Nodal plays the crucial role in generating asymmetry in the heart and that Bmp signaling via Bmp4 is dispensable in the presence of asymmetric Nodal signaling. In addition, we identify a previously unappreciated role for the Nodal-transcription factor FoxH1 in mediating cell responsiveness to Bmp, further linking the control of these two pathways in the heart. The interplay between these TGFβ pathways is complex, with Nodal signaling potentially acting to limit the response to Bmp pathway activation and the dosage of Bmp signals being critical to limit migration rates. These findings have implications for understanding the complex genetic interactions that lead to congenital heart disease in humans.

Defects in left–right (L/R) patterning can lead to severe defects in the formation of the heart. In fact, three of the most common forms of congenital heart disease, transposition of the great arteries, chamber septation defects, and chamber isomerisms, can be caused by earlier defects in L/R asymmetry. The Nodal and Bmp signaling pathways influence the development of cardiac asymmetry, but how these signals function in this process is not well understood. In this report, we have clarified the specific roles for the Nodal versus Bmp pathways in the heart. We find that Nodal signals increase the rate of cardiac cell migration, while Bmp signals decrease cardiac cell velocities. We demonstrate that asymmetric Nodal signaling plays a critical role in directing asymmetry in the heart in contrast to reports suggesting that signaling via Bmp4 is the more critical pathway. In fact, we find that Bmp4 signaling is dispensable for correct asymmetry in the heart in the presence of asymmetric Nodal signals. In addition, we have identified a novel integration between these two pathways at the level of the transcription factor FoxH1, which is required for cardiac cell responsiveness to both Nodal and Bmp signals. Taken together, this work significantly increases our understanding of how the signals regulating cardiac asymmetry function and integrate to consistently establish cardiac laterality. These results also suggest that human congenital heart defects that have not been found to result from single mutations within individual genes may develop due to combinations of mutations within components of these two separate pathways.

Foxc1 controls the growth of the murine frontal bone rudiment by direct regulation of a Bmp response threshold of Msx2

The mammalian skull vault consists of several intricately patterned bones that grow in close coordination. The growth of these bones depends on the precise regulation of the migration and differentiation of osteogenic cells from undifferentiated precursor cells located above the eye. Here, we demonstrate a role for Foxc1 in modulating the influence of Bmp signaling on the expression of Msx2 and the specification of these cells. Inactivation of Foxc1 results in a dramatic reduction in skull vault growth and causes an expansion of Msx2 expression and Bmp signaling into the area occupied by undifferentiated precursor cells. Foxc1 interacts directly with a Bmp responsive element in an enhancer upstream of Msx2, and acts to reduce the occupancy of P-Smad1/5/8. We propose that Foxc1 sets a threshold for the Bmp-dependent activation of Msx2, thus controlling the differentiation of osteogenic precursor cells and the rate and pattern of calvarial bone development.

Reciprocal signaling between the ectoderm and a mesendodermal left-right organizer directs left-right determination in the sea urchin embryo

During echinoderm development, expression of nodal on the right side plays a crucial role in positioning of the rudiment on the left side, but the mechanisms that restrict nodal expression to the right side are not known. Here we show that establishment of left-right asymmetry in the sea urchin embryo relies on reciprocal signaling between the ectoderm and a left-right organizer located in the endomesoderm. FGF/ERK and BMP2/4 signaling are required to initiate nodal expression in this organizer, while Delta/Notch signaling is required to suppress formation of this organizer on the left side of the archenteron. Furthermore, we report that the H(+)/K(+)-ATPase is critically required in the Notch signaling pathway upstream of the S3 cleavage of Notch. Our results identify several novel players and key early steps responsible for initiation, restriction, and propagation of left-right asymmetry during embryogenesis of a non-chordate deuterostome and uncover a functional link between the H(+)/K(+)-ATPase and the Notch signaling pathway.

Functional modeling in zebrafish demonstrates that the atrial-fibrillation-associated gene GREM2 regulates cardiac laterality, cardiomyocyte differentiation and atrial rhythm

Atrial fibrillation (AF) is the most common cardiac arrhythmia and carries a significant risk of stroke and heart failure. The molecular etiologies of AF are poorly understood, leaving patients with limited therapeutic options. AF has been recognized as an inherited disease in almost 30% of patient cases. However, few genetic loci have been identified and the mechanisms linking genetic variants to AF susceptibility remain unclear. By sequencing 193 probands with lone AF, we identified a Q76E variant within the coding sequence of the bone morphogenetic protein (BMP) antagonist gremlin-2 (GREM2) that increases its inhibitory activity. Functional modeling in zebrafish revealed that, through regulation of BMP signaling, GREM2 is required for cardiac laterality and atrial differentiation during embryonic development. GREM2 overactivity results in slower cardiac contraction rates in zebrafish, and induction of previously identified AF candidate genes encoding connexin-40, sarcolipin and atrial natriuretic peptide in differentiated mouse embryonic stem cells. By live heart imaging in zebrafish overexpressing wild-type or variant GREM2, we found abnormal contraction velocity specifically in atrial cardiomyocytes. These results implicate, for the first time, regulators of BMP signaling in human AF, providing mechanistic insights into the pathogenesis of the disease and identifying potential new therapeutic targets.

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.

Early cardiac development: a view from stem cells to embryos

From the 1920s, early cardiac development has been studied in chick and, later, in mouse embryos in order to understand the first cell fate decisions that drive specification and determination of the endocardium, myocardium, and epicardium. More recently, mouse and human embryonic stem cells (ESCs) have demonstrated faithful recapitulation of early cardiogenesis and have contributed significantly to this research over the past few decades. Derived almost 15 years ago, human ESCs have provided a unique developmental model for understanding the genetic and epigenetic regulation of early human cardiogenesis. Here, we review the biological concepts underlying cell fate decisions during early cardiogenesis in model organisms and ESCs. We draw upon both pioneering and recent studies and highlight the continued role for in vitro stem cells in cardiac developmental biology.

Antagonism of Nodal signaling by BMP/Smad5 prevents ectopic primitive streak formation in the mouse amnion

The strength and spatiotemporal activity of Nodal signaling is tightly controlled in early implantation mouse embryos, including by autoregulation and feedback loops, and involves secreted and intracellular antagonists. These control mechanisms, which are established at the extra-embryonic/embryonic interfaces, are essential for anterior-posterior patterning of the epiblast and correct positioning of the primitive streak. Formation of an ectopic primitive streak, or streak expansion, has previously been reported in mutants lacking antagonists that target Nodal signaling. Here, we demonstrate that loss-of-function of a major bone morphogenetic protein (BMP) effector, Smad5, results in formation of an ectopic primitive streak-like structure in mutant amnion accompanied by ectopic Nodal expression. This suggests that BMP/Smad5 signaling contributes to negative regulation of Nodal. In cultured cells, we find that BMP-activated Smad5 antagonizes Nodal signaling by interfering with the Nodal-Smad2/4-Foxh1 autoregulatory pathway through the formation of an unusual BMP4-induced Smad complex containing Smad2 and Smad5. Quantitative expression analysis supports that ectopic Nodal expression in the Smad5 mutant amnion is induced by the Nodal autoregulatory loop and a slow positive-feedback loop. The latter involves BMP4 signaling and also induction of ectopic Wnt3. Ectopic activation of these Nodal feedback loops in the Smad5 mutant amnion results in the eventual formation of an ectopic primitive streak-like structure. We conclude that antagonism of Nodal signaling by BMP/Smad5 signaling prevents primitive streak formation in the amnion of normal mouse embryos.

Opposing nodal and BMP signals regulate left-right asymmetry in the sea urchin larva

Nodal and BMP signals are important for establishing left-right (LR) asymmetry in vertebrates. In sea urchins, Nodal signaling prevents the formation of the rudiment on the right side. However, the opposing pathway to Nodal signaling during LR axis establishment is not clear. Here, we revealed that BMP signaling is activated in the left coelomic pouch, specifically in the veg2 lineage, but not in the small micromeres. By perturbing BMP activities, we demonstrated that BMP signaling is required for activating the expression of the left-sided genes and the formation of the left-sided structures. On the other hand, Nodal signals on the right side inhibit BMP signaling and control LR asymmetric separation and apoptosis of the small micromeres. Our findings show that BMP signaling is the positive signal for left-sided development in sea urchins, suggesting that the opposing roles of Nodal and BMP signals in establishing LR asymmetry are conserved in deuterostomes.

Differential diffusivity of Nodal and Lefty underlies a reaction-diffusion patterning system

Biological systems involving short-range activators and long-range inhibitors can generate complex patterns. Reaction-diffusion models postulate that differences in signaling range are caused by differential diffusivity of inhibitor and activator. Other models suggest that differential clearance underlies different signaling ranges. To test these models, we measured the biophysical properties of the Nodal/Lefty activator/inhibitor system during zebrafish embryogenesis. Analysis of Nodal and Lefty gradients revealed that Nodals have a shorter range than Lefty proteins. Pulse-labeling analysis indicated that Nodals and Leftys have similar clearance kinetics, whereas fluorescence recovery assays revealed that Leftys have a higher effective diffusion coefficient than Nodals. These results indicate that differential diffusivity is the major determinant of the differences in Nodal/Lefty range and provide biophysical support for reaction-diffusion models of activator/inhibitor-mediated patterning.

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.

Two additional midline barriers function with midline lefty1 expression to maintain asymmetric Nodal signaling during left-right axis specification in zebrafish

Left-right (L/R) patterning is crucial for the proper development of all vertebrates and requires asymmetric expression of nodal in the lateral plate mesoderm (LPM). The mechanisms governing asymmetric initiation of nodal have been studied extensively, but because Nodal is a potent activator of its own transcription, it is also crucial to understand the regulation required to maintain this asymmetry once it is established. The 'midline barrier', consisting of lefty1 expression, is a conserved mechanism for restricting Nodal activity to the left. However, the anterior and posterior extremes of the LPM are competent to respond to Nodal signals yet are not adjacent to this barrier, suggesting that lefty1 is not the only mechanism preventing ectopic Nodal activation. Here, we demonstrate the existence of two additional midline barriers. The first is a 'posterior barrier' mediated by Bmp signaling that prevents nodal propagation through the posterior LPM. In contrast to previous reports, we find that Bmp represses Nodal signaling independently of lefty1 expression and through the activity of a ligand other than Bmp4. The 'anterior barrier' is mediated by lefty2 expression in the left cardiac field and prevents Nodal activation from traveling across the anterior limit of the notochord and propagating down the right LPM. Both barriers appear to be conserved across model systems and are thus likely to be present in all vertebrates.

Few Smad proteins and many Smad-interacting proteins yield multiple functions and action modes in TGFβ/BMP signaling in vivo

Signaling by the many ligands of the TGFβ family strongly converges towards only five receptor-activated, intracellular Smad proteins, which fall into two classes i.e. Smad2/3 and Smad1/5/8, respectively. These Smads bind to a surprisingly high number of Smad-interacting proteins (SIPs), many of which are transcription factors (TFs) that co-operate in Smad-controlled target gene transcription in a cell type and context specific manner. A combination of functional analyses in vivo as well as in cell cultures and biochemical studies has revealed the enormous versatility of the Smad proteins. Smads and their SIPs regulate diverse molecular and cellular processes and are also directly relevant to development and disease. In this survey, we selected appropriate examples on the BMP-Smads, with emphasis on Smad1 and Smad5, and on a number of SIPs, i.e. the CPSF subunit Smicl, Ttrap (Tdp2) and Sip1 (Zeb2, Zfhx1b) from our own research carried out in three different vertebrate models.

Systematic analysis of gene expression differences between left and right atria in different mouse strains and in human atrial tissue

BACKGROUND

Normal development of the atria requires left-right differentiation during embryonic development. Reduced expression of Pitx2c (paired-like homeodomain transcription factor 2, isoform c), a key regulator of left-right asymmetry, has recently been linked to atrial fibrillation. We therefore systematically studied the molecular composition of left and right atrial tissue in adult murine and human atria.

METHODS

We compared left and right atrial gene expression in healthy, adult mice of different strains and ages by employing whole genome array analyses on freshly frozen atrial tissue. Selected genes with enriched expression in either atrium were validated by RT-qPCR and Western blot in further animals and in shock-frozen left and right atrial appendages of patients undergoing open heart surgery.

RESULTS

We identified 77 genes with preferential expression in one atrium that were common in all strains and age groups analysed. Independent of strain and age, Pitx2c was the gene with the highest enrichment in left atrium, while Bmp10, a member of the TGFβ family, showed highest enrichment in right atrium. These differences were validated by RT-qPCR in murine and human tissue. Western blot showed a 2-fold left-right concentration gradient in PITX2 protein in adult human atria. Several of the genes and gene groups enriched in left atria have a known biological role for maintenance of healthy physiology, specifically the prevention of atrial pathologies involved in atrial fibrillation, including membrane electrophysiology, metabolic cellular function, and regulation of inflammatory processes. Comparison of the array datasets with published array analyses in heterozygous Pitx2c(+/-) atria suggested that approximately half of the genes with left-sided enrichment are regulated by Pitx2c.

CONCLUSIONS

Our study reveals systematic differences between left and right atrial gene expression and supports the hypothesis that Pitx2c has a functional role in maintaining "leftness" in the atrium in adult murine and human hearts.

Bmp and nodal independently regulate lefty1 expression to maintain unilateral nodal activity during left-right axis specification in zebrafish

In vertebrates, left-right (LR) axis specification is determined by a ciliated structure in the posterior region of the embryo. Fluid flow in this ciliated structure is responsible for the induction of unilateral left-sided Nodal activity in the lateral plate mesoderm, which in turn regulates organ laterality. Bmp signalling activity has been implied in repressing Nodal expression on the right side, however its mechanism of action has been controversial. In a forward genetic screen for mutations that affect LR patterning, we identified the zebrafish linkspoot (lin) mutant, characterized by cardiac laterality and mild dorsoventral patterning defects. Mapping of the lin mutation revealed an inactivating missense mutation in the Bmp receptor 1aa (bmpr1aa) gene. Embryos with a mutation in lin/bmpr1aa and a novel mutation in its paralogue, bmpr1ab, displayed a variety of dorsoventral and LR patterning defects with increasing severity corresponding with a decrease in bmpr1a dosage. In Bmpr1a-deficient embryos we observed bilateral expression of the Nodal-related gene, spaw, coupled with reduced expression of the Nodal-antagonist lefty1 in the midline. Using genetic models to induce or repress Bmp activity in combination with Nodal inhibition or activation, we found that Bmp and Nodal regulate lefty1 expression in the midline independently of each other. Furthermore, we observed that the regulation of lefty1 by Bmp signalling is required for its observed downregulation of Nodal activity in the LPM providing a novel explanation for this phenomenon. From these results we propose a two-step model in which Bmp regulates LR patterning. Prior to the onset of nodal flow and Nodal activation, Bmp is required to induce lefty1 expression in the midline. When nodal flow has been established and Nodal activity is apparent, both Nodal and Bmp independently are required for lefty1 expression to assure unilateral Nodal activation and correct LR patterning.

Although vertebrates are bilaterally symmetric when observed from the outside, inside the body cavity the organs are positioned asymmetrically with respect to the left and right sides. Cases where all the organs are mirror imaged, known as situs inversus, are not associated with any medical defects. Severe medical problems occur however in infants with a partial organ reversal (situs ambigious or heterotaxia), which arises during embryonic development. Left-right asymmetry in the embryo is established by unilateral expression of Nodal, a member of the Tgf-ß superfamily of secreted growth factors, a role that has been conserved from human to snails. By performing a genetic screen in zebrafish for laterality mutants, we have identified the linkspoot mutant, which displayed partial defects in asymmetric left-right positioning of the internal organs. The gene disrupted in the linkspoot mutant encodes a receptor for bone morphogenetic proteins (Bmp), another member of the Tgf-ß superfamily of secreted growth factors. Further analysis of Bmp over-expression or knock-down models demonstrate that Bmp signalling is required for unilateral Nodal expression, through the initiation and maintenance of an embryonic midline barrier. Our results demonstrate a novel and important mechanism by which left-right asymmetry in the vertebrate embryo is established and regulated.

Wdr18 is required for Kupffer's vesicle formation and regulation of body asymmetry in zebrafish

Correct specification of the left-right (L-R) axis is important for organ morphogenesis. Conserved mechanisms involving cilia rotation inside node-like structures and asymmetric Nodal signaling in the lateral plate mesoderm (LPM), which are important symmetry-breaking events, have been intensively studied. In zebrafish, the clustering and migration of dorsal forerunner cells (DFCs) is critical for the formation of the Kuppfer's vesicle (KV). However, molecular events underlying DFC clustering and migration are less understood. The WD-repeat proteins function in a variety of biological processes, including cytoskeleton assembly, intracellular trafficking, mRNA splicing, transcriptional regulation and cell migration. However, little is known about the function of WD-repeat proteins in L-R asymmetry determination. Here, we report the identification and functional analyses of zebrafish wdr18, a novel gene that encodes a WD-repeat protein that is highly conserved among vertebrate species. wdr18 was identified from a Tol2 transposon-mediated enhancer trap screen. Follow-up analysis of wdr18 mRNA expression showed that it was detected in DFCs or the KV progenitor cells and later in the KV at early somitogenesis stages. Morpholino knockdown of wdr18 resulted in laterality defects in the visceral organs, which were preceded by the mis-expression of Nodal-related genes, including spaw and pitx2. Examination of morphants at earlier stages revealed that the KV had fewer and shorter cilia which are immotile and a smaller cavity. We further investigated the organization of DFCs in wdr18 morphant embryos using ntl and sox17 as specific markers and found that the clustering and migration of DFC was altered, leading to a disorganized KV. Finally, through a combination of wdr18 and itgb1b morpholino injections, we provided evidence that wdr18 and itgb1b genetically interact in the laterality determination process. Thus, we reveal a new and essential role for WD-repeat proteins in the determination and regulation of L-R asymmetry and propose a potential mechanism for wdr18 in the regulation of DFC clustering and migration and KV formation.