Gdf3 is required for robust Nodal signaling during germ layer formation and left-right patterning

Optogenetic control of Nodal signaling patterns

A crucial step in early embryogenesis is the establishment of spatial patterns of signaling activity. Tools to perturb morphogen signals with high resolution in space and time can help reveal how embryonic cells decode these signals to make appropriate fate decisions. Here, we present new optogenetic reagents and an experimental pipeline for creating designer Nodal signaling patterns in live zebrafish embryos. Nodal receptors were fused to the light-sensitive heterodimerizing pair Cry2/CIB1N, and the type II receptor was sequestered to the cytosol. The improved optoNodal2 reagents eliminate dark activity and improve response kinetics, without sacrificing dynamic range. We adapted an ultra-widefield microscopy platform for parallel light patterning in up to 36 embryos, and demonstrated precise spatial control over Nodal signaling activity and downstream gene expression. Patterned Nodal activation drove precisely controlled internalization of endodermal precursors. Furthermore, we used patterned illumination to generate synthetic signaling patterns in Nodal signaling mutants, rescuing several characteristic developmental defects. This study establishes an experimental toolkit for systematic exploration of Nodal signaling patterns in live embryos.

Towards an integrated view and understanding of embryonic signalling during murine gastrulation

At the onset of mammalian gastrulation, secreted signalling molecules belonging to the Bmp, Wnt, Nodal and Fgf signalling pathways induce and pattern the primitive streak, marking the start for the cellular rearrangements that generate the body plan. Our current understanding of how signalling specifies and organises the germ layers in three dimensions, was mainly derived from genetic experimentation using mouse embryos performed over many decades. However, the exact spatiotemporal sequence of events is still poorly understood, both because of a lack of tractable models that allow for real time visualisation of signalling and differentiation and because of the molecular and cellular complexity of these early developmental events. In recent years, a new wave of in vitro embryo models has begun to shed light on the dynamics of signalling during primitive streak formation. Here we discuss the similarities and differences between a widely adopted mouse embryo model, termed gastruloids, and real embryos from a signalling perspective. We focus on the gene regulatory networks that underlie signalling pathway interactions and outline some of the challenges ahead. Finally, we provide a perspective on how embryo models may be used to advance our understanding of signalling dynamics through computational modelling.

The Interaction Between the asb5a and asb5b Subtypes Jointly Regulates the L-R Asymmetrical Development of the Heart in Zebrafish

The asb5 gene, a member of the Asb protein subfamily characterized by six ankyrin repeat domains, is highly conserved and comprises two subtypes, asb5a and asb5b, in zebrafish. Our previous research has demonstrated that a deficiency of the asb5 gene significantly impairs early cardiac contractile function, highlighting its close relationship with heart development. Zebrafish asb5 expression was disrupted by both morpholino (MO) antisense oligomer-mediated knockdown and a CRISPR-Cas9 system. A high-throughput RNA-Seq analysis was used to analyze the possible molecular regulatory mechanism of asb5 gene deletion leading to left-right (L-R) asymmetry defects in the heart. Whole-mount in situ hybridization (WISH) was conducted to evaluate gene expression patterns of Nodal signaling components and the positions of heart organs. Heart looping was defective in zebrafish asb5 morphants. Rescue experiments in the asb5-deficiency group (inactivating both asb5a and asb5b) demonstrated that the injection of either asb5a-mRNA or asb5b-mRNA alone was insufficient to rectify the abnormal L-R asymmetry of the heart. In contrast, the simultaneous injection of both asb5a-mRNA and asb5b-mRNA successfully rescued the morphological phenotype. A high-throughput RNA-Seq analysis of embryos at 48 h post fertilization (hpf) revealed that numerous genes associated with L-R asymmetry exhibited expression imbalances in the asb5-deficiency group. WISH further confirmed that the expression of genes such as fli1a, acta1b, hand2, has2, prrx1a, notch1b, and foxa3 were upregulated, while the expression of mei2a and tal1 was downregulated. These results indicated that loss of the asb5 gene in zebrafish led to the disordered development of L-R asymmetry in the heart, resulting in an imbalance in the expression of genes associated with the regulation of L-R asymmetry. Subsequently, we examined the expression patterns of classical Nodal signaling pathway-related genes using WISH. The results showed that the midline barrier factor gene lefty1 was downregulated at early stages in the asb5-deficiency group, and the expression of spaw and lefty2, which are specific to the left lateral plate mesoderm (LPM), was disrupted. This study reveals that the two subtypes of the asb5 gene in zebrafish, asb5a and asb5b, interact and jointly regulate the establishment of early cardiac L-R asymmetry through the Nodal-spaw-lefty signaling pathway.

The temporal control and activity of maternal zsquildlike-A/hnrnpaba during zebrafish embryogenesis indicate a role in early pattern formation

During embryogenesis in Danio rerio (zebrafish), the earliest morphological patterning events are dependent on the precise temporal translation and/or localization of specific maternal mRNAs/proteins. Dorsoventral patterning in particular requires the translocation of maternal factors that are present in the Balbiani Body from the vegetal region of the unfertilized egg to the future dorsal side of the embryo (Fuentes et al., 2020), leading to the localized activation of the β-catenin pathway in the cells in that region. Since zebrafish are chordates, this dorsoventral patterning then leads to the formation of neural tissue on the dorsal side of the embryo. What is not yet clear is the identity of all maternal and zygotic factors that first establish dorsoventral patterning, and which factors lead to the establishment of neural versus non-neural tissue. Taking an evolutionary approach to this question, we investigated a gene in zebrafish, zsquidlike-A (hnrnpaba), that is homologous to a key dorsoventral patterning gene in fruit flies (Drosophila melanogaster) called squid (Kelley, 1993). While dorsoventral patterning in flies and fish looks quite different both morphologically and at the molecular level, we demonstrate that not only has a key dorsoventral patterning gene in flies been conserved in fish, maternal fish zsquidlike-A protein is synthesized precisely as dorsoventral patterning is unfolding in fish embryos, and in its absence, dorsoventral patterning is severely disrupted.

Myosin1G promotes Nodal signaling to control zebrafish left-right asymmetry

Myosin1D (Myo1D) has recently emerged as a conserved regulator of animal Left-Right (LR) asymmetry that governs the morphogenesis of the vertebrate central LR Organizer (LRO). In addition to Myo1D, the zebrafish genome encodes the closely related Myo1G. Here we show that while Myo1G also controls LR asymmetry, it does so through an entirely different mechanism. Myo1G promotes the Nodal-mediated transfer of laterality information from the LRO to target tissues. At the cellular level, Myo1G is associated with endosomes positive for the TGFβ signaling adapter SARA. myo1g mutants have fewer SARA-positive Activin receptor endosomes and a reduced responsiveness to Nodal ligands that results in a delay of left-sided Nodal propagation and tissue-specific laterality defects in organs that are most distant from the LRO. Additionally, Myo1G promotes signaling by different Nodal ligands in specific biological contexts. Our findings therefore identify Myo1G as a context-dependent regulator of the Nodal signaling pathway.

De Novo Assembly, Characterization, and Comparative Transcriptome Analysis of Mature Male and Female Gonads of Rabbitfish (Siganus oramin) (Bloch & Schneider, 1801)

The rabbitfish, Siganus oramin, is a commercially important table fish in southeastern China. However, there have been few studies on its gonad development and reproduction regulation. Comparative transcriptome analysis was first performed on adult male and female gonads of S. oramin. In total, 47,070 unigenes were successfully assembled and 22,737 unigenes were successfully annotated. Through comparative transcriptome analysis of male and female gonads, a total of 6722 differentially expressed genes were successfully identified, with 3528 upregulated genes and 3154 downregulated genes in the testes. In addition, 39 differentially expressed reproduction-related genes were identified. Finally, quantitative real-time PCR was used to validate the expression levels of several differentially expressed genes. These results provide important data for further studying the function of reproduction-related genes and the molecular mechanism regulating gonad development and reproduction in S. oramin.

Optogenetic control of Nodal signaling patterns

A crucial step in early embryogenesis is the establishment of spatial patterns of signaling activity. Tools to perturb morphogen signals with high resolution in space and time can help reveal how embryonic cells decode these signals to make appropriate fate decisions. Here, we present new optogenetic reagents and an experimental pipeline for creaHng designer Nodal signaling patterns in live zebrafish embryos. Nodal receptors were fused to the light-sensitive heterodimerizing pair Cry2/CIB1N, and the Type II receptor was sequestered to the cytosol. The improved optoNodal2 reagents eliminate dark activity and improve response kinetics, without sacrificing dynamic range. We adapted an ultra-widefield microscopy platform for parallel light patterning in up to 36 embryos and demonstrated precise spatial control over Nodal signaling activity and downstream gene expression. Patterned Nodal activation drove precisely controlled internalization of endodermal precursors. Further, we used patterned illumination to generate synthetic signaling patterns in Nodal signaling mutants, rescuing several characteristic developmental defects. This study establishes an experimental toolkit for systematic exploration of Nodal signaling patterns in live embryos.

Evolution of the gene regulatory network of body axis by enhancer hijacking in amphioxus

A central goal of evolutionary developmental biology is to decipher the evolutionary pattern of gene regulatory networks (GRNs) that control embryonic development, and the mechanism underlying GRNs evolution. The Nodal signaling that governs the body axes of deuterostomes exhibits a conserved GRN orchestrated principally by Nodal, Gdf1/3, and Lefty. Here we show that this GRN has been rewired in cephalochordate amphioxus. We found that while the amphioxus Gdf1/3 ortholog exhibited nearly no embryonic expression, its duplicate Gdf1/3-like, linked to Lefty, was zygotically expressed in a similar pattern as Lefty. Consistent with this, while Gdf1/3-like mutants showed defects in axial development, Gdf1/3 mutants did not. Further transgenic analyses showed that the intergenic region between Gdf1/3-like and Lefty could drive reporter gene expression as that of the two genes. These results indicated that Gdf1/3-like has taken over the axial development role of Gdf1/3 in amphioxus, possibly through hijacking Lefty enhancers. We finally demonstrated that, to compensate for the loss of maternal Gdf1/3 expression, Nodal has become an indispensable maternal factor in amphioxus and its maternal mutants caused axial defects as Gdf1/3-like mutants. We therefore demonstrated a case that the evolution of GRNs could be triggered by enhancer hijacking events. This pivotal event has allowed the emergence of a new GRN in extant amphioxus, presumably through a stepwise process. In addition, the co-expression of Gdf1/3-like and Lefty achieved by a shared regulatory region may have provided robustness during body axis formation, which provides a selection-based hypothesis for the phenomena called developmental system drift.

Molecular mechanisms controlling the biogenesis of the TGF-β signal Vg1

The TGF-beta signals Vg1 (Dvr1/Gdf3) and Nodal form heterodimers to induce vertebrate mesendoderm. The Vg1 proprotein is a monomer retained in the endoplasmic reticulum (ER) and is processed and secreted upon heterodimerization with Nodal, but the mechanisms underlying Vg1 biogenesis are largely elusive. Here, we clarify the mechanisms underlying Vg1 retention, processing, secretion, and signaling and introduce a Synthetic Processing (SynPro) system that enables the programmed cleavage of ER-resident and extracellular proteins. First, we find that Vg1 can be processed by intra- or extracellular proteases. Second, Vg1 can be processed without Nodal but requires Nodal for secretion and signaling. Third, Vg1-Nodal signaling activity requires Vg1 processing, whereas Nodal can remain unprocessed. Fourth, Vg1 employs exposed cysteines, glycosylated asparagines, and BiP chaperone-binding motifs for monomer retention in the ER. These observations suggest two mechanisms for rapid mesendoderm induction: Chaperone-binding motifs help store Vg1 as an inactive but ready-to-heterodimerize monomer in the ER, and the flexibility of Vg1 processing location allows efficient generation of active heterodimers both intra- and extracellularly. These results establish SynPro as an in vivo processing system and define molecular mechanisms and motifs that facilitate the generation of active TGF-beta heterodimers.

Heterodimerization-dependent secretion of bone morphogenetic proteins in Drosophila

Combinatorial signaling is key to instruct context-dependent cell behaviors. During embryonic development, adult homeostasis, and disease, bone morphogenetic proteins (BMPs) act as dimers to instruct specific cellular responses. BMP ligands can form both homodimers or heterodimers; however, obtaining direct evidence of the endogenous localization and function of each form has proven challenging. Here, we make use of precise genome editing and direct protein manipulation via protein binders to dissect the existence and functional relevance of BMP homodimers and heterodimers in the Drosophila wing imaginal disc. This approach identified in situ the existence of Dpp (BMP2/4)/Gbb (BMP5/6/7/8) heterodimers. We found that Gbb is secreted in a Dpp-dependent manner in the wing imaginal disc. Dpp and Gbb form a gradient of heterodimers, whereas neither Dpp nor Gbb homodimers are evident under endogenous physiological conditions. We find that the formation of heterodimers is critical for obtaining optimal signaling and long-range BMP distribution.

Novel Cardiokine GDF3 Predicts Adverse Fibrotic Remodeling After Myocardial Infarction

BACKGROUND

Myocardial infarction (MI) induces a repair response that ultimately generates a stable fibrotic scar. Although the scar prevents cardiac rupture, an excessive profibrotic response impairs optimal recovery by promoting the development of noncontractile fibrotic areas. The mechanisms that lead to cardiac fibrosis are diverse and incompletely characterized. We explored whether the expansion of cardiac fibroblasts after MI can be regulated through a paracrine action of cardiac stromal cells.

METHODS

We performed a bioinformatic secretome analysis of cardiac stromal PW1⁺ cells isolated from normal and post-MI mouse hearts to identify novel secreted proteins. Functional assays were used to screen secreted proteins that promote fibroblast proliferation. The expressions of candidates were subsequently analyzed in mouse and human hearts and plasmas. The relationship between levels of circulating protein candidates and adverse post-MI cardiac remodeling was examined in a cohort of 80 patients with a first ST-segment-elevation MI and serial cardiac magnetic resonance imaging evaluations.

RESULTS

Cardiac stromal PW1⁺ cells undergo a change in paracrine behavior after MI, and the conditioned media from these cells induced a significant increase in the proliferation of fibroblasts. We identified a total of 12 candidates as secreted proteins overexpressed by cardiac PW1⁺ cells after MI. Among these factors, GDF3 (growth differentiation factor 3), a member of the TGF-β (transforming growth factor-β) family, was markedly upregulated in the ischemic hearts. Conditioned media specifically enriched with GDF3 induced fibroblast proliferation at a high level by stimulation of activin-receptor-like kinases. In line with the secretory nature of this protein, we next found that GDF3 can be detected in mice and human plasma samples, with a significant increase in the days after MI. In humans, higher GDF3 circulating levels (measured in the plasma at day 4 after MI) were significantly associated with an increased risk of adverse remodeling 6 months after MI (adjusted odds ratio, 1.76 [1.03-3.00]; P=0.037), including lower left ventricular ejection fraction and a higher proportion of akinetic segments.

CONCLUSIONS

Our findings define a mechanism for the profibrotic action of cardiac stromal cells through secreted cardiokines, such as GDF3, a candidate marker of adverse fibrotic remodeling after MI.

REGISTRATION

URL: https://www.

CLINICALTRIALS

gov; Unique identifier: NCT01113268.

Understanding laterality disorders and the left-right organizer: Insights from zebrafish

Vital internal organs display a left-right (LR) asymmetric arrangement that is established during embryonic development. Disruption of this LR asymmetry-or laterality-can result in congenital organ malformations. Situs inversus totalis (SIT) is a complete concordant reversal of internal organs that results in a low occurrence of clinical consequences. Situs ambiguous, which gives rise to Heterotaxy syndrome (HTX), is characterized by discordant development and arrangement of organs that is associated with a wide range of birth defects. The leading cause of health problems in HTX patients is a congenital heart malformation. Mutations identified in patients with laterality disorders implicate motile cilia in establishing LR asymmetry. However, the cellular and molecular mechanisms underlying SIT and HTX are not fully understood. In several vertebrates, including mouse, frog and zebrafish, motile cilia located in a "left-right organizer" (LRO) trigger conserved signaling pathways that guide asymmetric organ development. Perturbation of LRO formation and/or function in animal models recapitulates organ malformations observed in SIT and HTX patients. This provides an opportunity to use these models to investigate the embryological origins of laterality disorders. The zebrafish embryo has emerged as an important model for investigating the earliest steps of LRO development. Here, we discuss clinical characteristics of human laterality disorders, and highlight experimental results from zebrafish that provide insights into LRO biology and advance our understanding of human laterality disorders.

Molecular Characterization of TGF-Beta Gene Family in Buffalo to Identify Gene Duplication and Functional Mutations

The TGF-β superfamily is ubiquitously distributed from invertebrates to vertebrates with diverse cellular functioning such as cell adhesion, motility, proliferation, apoptosis, and differentiation. The present study aimed to characterize the TGF-β gene superfamily in buffalo through evolutionary, structural, and single nucleotide polymorphism (SNPs) analyses to find the functional effect of SNPs in selected genes. We detected 32 TGF-β genes in buffalo genome and all TGF-β proteins exhibited basic nature except INHA, INHBC, MSTN, BMP10, and GDF2, which showed acidic properties. According to aliphatic index, TGF-β proteins were thermostable but unstable in nature. Except for GDF1 and AMH, TGF-β proteins depicted hydrophilic nature. Moreover, all the detected buffalo TGF-β genes showed evolutionary conserved nature. We also identified eight segmental and one tandem duplication event TGF-β gene family in buffalo, and the ratio of Ka/Ks demonstrated that all the duplicated gene pairs were under selective pressure. Comparative amino acid analysis demonstrated higher variation in buffalo TGF-β gene family, as a total of 160 amino acid variations in all the buffalo TGF-β proteins were detected. Mutation analysis revealed that 13 mutations had an overall damaging effect that might have functional consequences on buffalo growth, folliculogenesis, or embryogenesis.

dmrt2 and myf5 Link Early Somitogenesis to Left-Right Axis Determination in Xenopus laevis

The vertebrate left-right axis is specified during neurulation by events occurring in a transient ciliated epithelium termed left-right organizer (LRO), which is made up of two distinct cell types. In the axial midline, central LRO (cLRO) cells project motile monocilia and generate a leftward fluid flow, which represents the mechanism of symmetry breakage. This directional fluid flow is perceived by laterally positioned sensory LRO (sLRO) cells, which harbor non-motile cilia. In sLRO cells on the left side, flow-induced signaling triggers post-transcriptional repression of the multi-pathway antagonist dand5. Subsequently, the co-expressed Tgf-β growth factor Nodal1 is released from Dand5-mediated repression to induce left-sided gene expression. Interestingly, Xenopus sLRO cells have somitic fate, suggesting a connection between LR determination and somitogenesis. Here, we show that doublesex and mab3-related transcription factor 2 (Dmrt2), known to be involved in vertebrate somitogenesis, is required for LRO ciliogenesis and sLRO specification. In dmrt2 morphants, misexpression of the myogenic transcription factors tbx6 and myf5 at early gastrula stages preceded the misspecification of sLRO cells at neurula stages. myf5 morphant tadpoles also showed LR defects due to a failure of sLRO development. The gain of myf5 function reintroduced sLRO cells in dmrt2 morphants, demonstrating that paraxial patterning and somitogenesis are functionally linked to LR axis formation in Xenopus.

Genetic and Clinical Features of Heterotaxy in a Prenatal Cohort

Objectives: Some genetic causes of heterotaxy have been identified in a small number of heterotaxy familial cases or animal models. However, knowledge on the genetic causes of heterotaxy in the fetal population remains scarce. Here, we aimed to investigate the clinical characteristics and genetic spectrum of a fetal cohort with heterotaxy.

Methods: We retrospectively investigated all fetuses with a prenatal diagnosis of heterotaxy at a single center between October 2015 and November 2020. These cases were studied using the genetic testing data acquired from a combination of copy number variation sequencing (CNV-seq) and whole-exome sequencing (WES), and their clinical phenotypes were also reviewed.

Result: A total of 72 fetuses diagnosed with heterotaxy and complete clinical and genetic results were enrolled in our research. Of the 72 fetuses, 18 (25%) and 54 (75%) had left and right isomerism, respectively. Consistent with the results of a previous study, intracardiac anomalies were more severe in patients with right atrial isomerism than in those with left atrial isomerism (LAI) and mainly manifested as atrial situs inversus, bilateral right atrial appendages, abnormal pulmonary venous connection, single ventricles or single atria, and pulmonary stenosis or atresia. In 18 fetuses diagnosed with LAI, the main intracardiac anomalies were bilateral left atrial appendages. Of the 72 fetuses that underwent CNV-seq and WES, 11 (15.3%) had positive genetic results, eight had definitive pathogenic variants, and three had likely pathogenic variants. The diagnostic genetic variant rate identified using WES was 11.1% (8/72), in which primary ciliary dyskinesia (PCD)-associated gene mutations (CCDC40, CCDC114, DNAH5, DNAH11, and ARMC4) accounted for the vast majority (n = 5). Other diagnostic genetic variants, such as KMT2D and FOXC1, have been rarely reported in heterotaxy cases, although they have been verified to play roles in congenital heart disease.

Conclusion: Thus, diagnostic genetic variants contributed to a substantial fraction in the etiology of fetal heterotaxy. PCD mutations accounted for approximately 6.9% of heterotaxy cases in our fetal cohort. WES was identified as an effective tool to detect genetic causes prenatally in heterotaxy patients.

Hnrnpul1 controls transcription, splicing, and modulates skeletal and limb development in vivo

Mutations in RNA-binding proteins can lead to pleiotropic phenotypes including craniofacial, skeletal, limb, and neurological symptoms. Heterogeneous nuclear ribonucleoproteins (hnRNPs) are involved in nucleic acid binding, transcription, and splicing through direct binding to DNA and RNA, or through interaction with other proteins in the spliceosome. We show a developmental role for Hnrnpul1 in zebrafish, resulting in reduced body and fin growth and missing bones. Defects in craniofacial tendon growth and adult-onset caudal scoliosis are also seen. We demonstrate a role for Hnrnpul1 in alternative splicing and transcriptional regulation using RNA-sequencing, particularly of genes involved in translation, ubiquitination, and DNA damage. Given its cross-species conservation and role in splicing, it would not be surprising if it had a role in human development. Whole-exome sequencing detected a homozygous frameshift variant in HNRNPUL1 in 2 siblings with congenital limb malformations, which is a candidate gene for their limb malformations. Zebrafish Hnrnpul1 mutants suggest an important developmental role of hnRNPUL1 and provide motivation for exploring the potential conservation of ancient regulatory circuits involving hnRNPUL1 in human development.

Evolution and loss of ß-catenin and TCF-dependent axis specification in insects

Mechanisms and evolution of primary axis specification in insects are discussed in the context of the roles of ß-catenin and TCF in polarizing metazoan embryos. Three hypotheses are presented. First, insects with sequential segmentation and posterior growth use cell-autonomous mechanisms for establishing embryo polarity via the nuclear ratio of ß-catenin and TCF. Second, TCF homologs establish competence for anterior specification. Third, the evolution of simultaneous segmentation mechanisms, also known as long-germ development, resulted in primary axis specification mechanisms that are independent of ß-catenin but reliant on TCF, a condition that preceded the frequent replacement of anterior determinants in long germ insects.

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.

Defining the Pluripotent Marker Genes for Identification of Teleost Fish Cell Pluripotency During Reprogramming

Pluripotency is a transient state in early embryos, which is regulated by an interconnected network of pluripotency-related genes. The pluripotent state itself seems to be highly dynamic, which leads to significant differences in the description of induced pluripotent stem cells from different species at the molecular level. With the application of cell reprogramming technology in fish, the establishment of a set of molecular standards for defining pluripotency will be important for the research and potential application of induced pluripotent stem cells in fish. In this study, by BLAST search and expression pattern analysis, we screen out four pluripotent genes (Oct4, Nanog, Tdgf1, and Gdf3) in zebrafish (Danio rerio) and crucian carp (Carassius). These genes were highly expressed in the short period of early embryonic development, but significantly down-regulated after differentiation. Moreover, three genes (Oct4, Nanog and Tdgf1) have been verified that are suitable for identifying the pluripotency of induced pluripotent stem cells in zebrafish and crucian carp. Our study expands the understanding of the pluripotent markers of induced pluripotent stem cells in fish.

Nodal is a short-range morphogen with activity that spreads through a relay mechanism in human gastruloids

Morphogens are signaling molecules that convey positional information and dictate cell fates during development. Although ectopic expression in model organisms suggests that morphogen gradients form through diffusion, little is known about how morphogen gradients are created and interpreted during mammalian embryogenesis due to the combined difficulties of measuring endogenous morphogen levels and observing development in utero. Here we take advantage of a human gastruloid model to visualize endogenous Nodal protein in living cells, during specification of germ layers. We show that Nodal is extremely short range so that Nodal protein is limited to the immediate neighborhood of source cells. Nodal activity spreads through a relay mechanism in which Nodal production induces neighboring cells to transcribe Nodal. We further show that the Nodal inhibitor Lefty, while biochemically capable of long-range diffusion, also acts locally to control the timing of Nodal spread and therefore of mesoderm differentiation during patterning. Our study establishes a paradigm for tissue patterning by an activator-inhibitor pair.

Fish Pluripotent Stem-Like Cell Line Induced by Small-Molecule Compounds From Caudal Fin and its Developmental Potentiality

The technique of induced pluripotent stem cells has significant application value in breeding and preserving the genetic integrity of fish species. However, it is still unclear whether the chemically induced pluripotent stem cells can be induced from non-mammalian cells or not. In this article, we first verify that fibroblasts of fish can be chemically reprogrammed into pluripotent stem cells. These induced pluripotent stem-like cells possess features of colony morphology, expression of pluripotent marker genes, formation of embryoid bodies, teratoma formation, and the potential to differentiate into germ cell-like cells in vitro. Our findings will offer a new way to generate induced pluripotent stem cells in teleost fish and a unique opportunity to breed commercial fish and even save endangered fish species.

Bicc1 and Dicer regulate left-right patterning through post-transcriptional control of the Nodal inhibitor Dand5

Rotating cilia at the vertebrate left-right organizer (LRO) generate an asymmetric leftward flow, which is sensed by cells at the left LRO margin. Ciliary activity of the calcium channel Pkd2 is crucial for flow sensing. How this flow signal is further processed and relayed to the laterality-determining Nodal cascade in the left lateral plate mesoderm (LPM) is largely unknown. We previously showed that flow down-regulates mRNA expression of the Nodal inhibitor Dand5 in left sensory cells. De-repression of the co-expressed Nodal, complexed with the TGFß growth factor Gdf3, drives LPM Nodal cascade induction. Here, we show that post-transcriptional repression of dand5 is a central process in symmetry breaking of Xenopus, zebrafish and mouse. The RNA binding protein Bicc1 was identified as a post-transcriptional regulator of dand5 and gdf3 via their 3'-UTRs. Two distinct Bicc1 functions on dand5 mRNA were observed at pre- and post-flow stages, affecting mRNA stability or flow induced translational inhibition, respectively. To repress dand5, Bicc1 co-operates with Dicer1, placing both proteins in the process of flow sensing. Intriguingly, Bicc1 mediated translational repression of a dand5 3'-UTR mRNA reporter was responsive to pkd2, suggesting that a flow induced Pkd2 signal triggers Bicc1 mediated dand5 inhibition during symmetry breakage.

Regulation of metabolic homeostasis by the TGF-β superfamily receptor ALK7

ALK7 (Activin receptor-like kinase 7) is a member of the TGF-β receptor superfamily predominantly expressed by cells and tissues involved in endocrine functions, such as neurons of the hypothalamus and pituitary, pancreatic β-cells and adipocytes. Recent studies have begun to delineate the processes regulated by ALK7 in these tissues and how these become integrated with the homeostatic regulation of mammalian metabolism. The picture emerging indicates that ALK7's primary function in metabolic regulation is to limit catabolic activities and preserve energy. Aside of the hypothalamic arcuate nucleus, the function of ALK7 elsewhere in the brain, particularly in the cerebellum, where it is abundantly expressed, remains to be elucidated. Although our understanding of the basic molecular events underlying ALK7 signaling has benefited from the vast knowledge available on TGF-β receptor mechanisms, how these connect to the physiological functions regulated by ALK7 in different cell types is still incompletely understood. Findings of missense and nonsense variants in the Acvr1c gene, encoding ALK7, of some mouse strains and human subjects indicate a tolerance to ALK7 loss of function. Recent discoveries suggest that specific inhibitors of ALK7 may have therapeutic applications in obesity and metabolic syndrome without overt adverse effects.

The pattern of nodal morphogen signaling is shaped by co-receptor expression

Embryos must communicate instructions to their constituent cells over long distances. These instructions are often encoded in the concentration of signals called morphogens. In the textbook view, morphogen molecules diffuse from a localized source to form a concentration gradient, and target cells adopt fates by measuring the local morphogen concentration. However, natural patterning systems often incorporate numerous co-factors and extensive signaling feedback, suggesting that embryos require additional mechanisms to generate signaling patterns. Here, we examine the mechanisms of signaling pattern formation for the mesendoderm inducer Nodal during zebrafish embryogenesis. We find that Nodal signaling activity spans a normal range in the absence of signaling feedback and relay, suggesting that diffusion is sufficient for Nodal gradient formation. We further show that the range of endogenous Nodal ligands is set by the EGF-CFC co-receptor Oep: in the absence of Oep, Nodal activity spreads to form a nearly uniform distribution throughout the embryo. In turn, increasing Oep levels sensitizes cells to Nodal ligands. We recapitulate these experimental results with a computational model in which Oep regulates the diffusive spread of Nodal ligands by setting the rate of capture by target cells. This model predicts, and we confirm in vivo, the surprising observation that a failure to replenish Oep transforms the Nodal signaling gradient into a travelling wave. These results reveal that patterns of Nodal morphogen signaling are shaped by co-receptor-mediated restriction of ligand spread and sensitization of responding cells.

A case for revisiting Nodal signaling in human pluripotent stem cells

Nodal is a transforming growth factor-β (TGF-β) superfamily member that plays a number of critical roles in mammalian embryonic development. Nodal is essential for the support of the peri-implantation epiblast in the mouse embryo and subsequently acts to specify mesendodermal fate at the time of gastrulation and, later, left-right asymmetry. Maintenance of human pluripotent stem cells (hPSCs) in vitro is dependent on Nodal signaling. Because it has proven difficult to prepare a biologically active form of recombinant Nodal protein, Activin or TGFB1 are widely used as surrogates for NODAL in hPSC culture. Nonetheless, the expression of the components of an endogenous Nodal signaling pathway in hPSC provides a potential autocrine pathway for the regulation of self-renewal in this system. Here we review recent studies that have clarified the role of Nodal signaling in pluripotent stem cell populations, highlighted spatial restrictions on Nodal signaling, and shown that Nodal functions in vivo as a heterodimer with GDF3, another TGF-β superfamily member expressed by hPSC. We discuss the role of this pathway in the maintenance of the epiblast and hPSC in light of these new advances.

Integration of Nodal and BMP Signaling by Mutual Signaling Effector Antagonism

Opposing sources of bone morphogenetic protein (BMP) and Nodal signaling molecules are sufficient to induce the formation of a full axis in zebrafish embryos. To address how these signals orchestrate patterning, we transplant sources of fluorescently tagged Nodal and BMP into zebrafish embryos, robustly inducing the formation of secondary axes. Nodal and BMP signal non-cell-autonomously and form similar protein gradients in this context, but the signaling range of Nodal (pSmad2) is shorter than the BMP range (pSmad5). This yields a localized region of pSmad2 activity around the Nodal source, overlapping with a broad domain of pSmad5 activity across the embryo. Cell fates induced in various regions stereotypically correlate with pSmad2-to-pSmad5 ratios and can even be induced BMP- and Nodal-independently with different ratios of constitutively active Smad2 and Smad5. Strikingly, we find that Smad2 and Smad5 antagonize each other for specific cell fates, providing a mechanism for how cells integrate and discriminate between overlapping signals during development.

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Nodal induces pSmad at a shorter range than BMP due to slower activation kinetics

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Different ratios of active Smad2 and Smad5 can induce different embryonic structures

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Smad2 and Smad5 inhibit each other or act synergistically to induce specific cell fates

BMP heterodimers signal via distinct type I receptor class functions

TGF-β family heterodimeric ligands show increased or exclusive signaling compared to homodimeric ligands in both vertebrate and insect development as well as in therapeutically relevant processes, like osteogenesis. However, the mechanisms that differentiate heterodimer and homodimer signaling remain uncharacterized. We show that BMP antagonists do not account for the exclusive signaling of Bmp2/7 heterodimers in zebrafish development. We found that overexpressed homodimers can signal but surprisingly require two distinct type I receptors, like heterodimers, indicating a required activity of the heteromeric type I receptor complex. We further demonstrate that a canonical type I receptor function has been delegated to only one of these receptors, Acvr1. Our findings should inform both basic and translational research in multiple TGF-β family signaling contexts.

Heterodimeric TGF-β ligands outperform homodimers in a variety of developmental, cell culture, and therapeutic contexts; however, the mechanisms underlying this increased potency remain uncharacterized. Here, we use dorsal–ventral axial patterning of the zebrafish embryo to interrogate the BMP2/7 heterodimer signaling mechanism. We demonstrate that differential interactions with BMP antagonists do not account for the reduced signaling ability of homodimers. Instead, we find that while overexpressed BMP2 homodimers can signal, they require two nonredundant type I receptors, one from the Acvr1 subfamily and one from the Bmpr1 subfamily. This implies that all BMP signaling within the zebrafish gastrula, even BMP2 homodimer signaling, requires Acvr1. This is particularly surprising as BMP2 homodimers do not bind Acvr1 in vitro. Furthermore, we find that the roles of the two type I receptors are subfunctionalized within the heterodimer signaling complex, with the kinase activity of Acvr1 being essential, while that of Bmpr1 is not. These results suggest that the potency of the Bmp2/7 heterodimer arises from the ability to recruit both Acvr1 and Bmpr1 into the same signaling complex.

TGFβ family signaling and development

The transforming growth factor β (TGFβ) signaling family is evolutionarily conserved in metazoans. The signal transduction mechanisms of TGFβ family members have been expansively investigated and are well understood. During development and homeostasis, numerous TGFβ family members are expressed in various cell types with temporally changing levels, playing diverse roles in embryonic development, adult tissue homeostasis and human diseases by regulating cell proliferation, differentiation, adhesion, migration and apoptosis. Here, we discuss the molecular mechanisms underlying signal transduction and regulation of the TGFβ subfamily pathways, and then highlight their key functions in mesendoderm induction, dorsoventral patterning and laterality development, as well as in the formation of several representative tissues/organs.

Zebrafish Cdx1b modulates epithalamic asymmetry by regulating ndr2 and lft1 expression

Nodal signaling is essential for mesoderm and endoderm formation, as well as neural plate induction and establishment of left-right asymmetry. However, the mechanisms controlling expression of Nodal pathway genes in these contexts are not fully known. Previously, we showed that Cdx1b induces expression of downstream Nodal signaling factors during early endoderm formation. In this study, we show that Cdx1b also regulates epithalamic asymmetry in zebrafish embryos by modulating expression of ndr2 and lft1. We first knocked down cdx1b with translation-blocking and splicing-blocking morpholinos (MOs). Most embryos injected with translation-blocking MOs showed absent ndr2, lft1 and pitx2c expression in the left dorsal diencephalon during segmentation and pharyngula stages accompanied by aberrant parapineal migration and habenular laterality at 72 ​h post fertilization (hpf). These defects were less frequent in embryos injected with splicing-blocking MO. To confirm the morphant phenotype, we next generated both zygotic (Z)cdx1b^-/- and maternal zygotic (MZ)cdx1b^-/- mutants by CRISPR-Cas9 mutagenesis. Expression of ndr2, lft1 and pitx2c was absent in the left dorsal diencephalon of a high proportion of MZcdx1b^-/- mutants; however, aberrant dorsal diencephalic pitx2c expression patterns were observed at low frequency in Zcdx1b^-/- mutant embryos. Correspondingly, dysregulated parapineal migration and habenular laterality were also observed in MZcdx1b^-/- mutant embryos at 72 hpf. On the other hand, Kupffer's vesicle cilia length and number, expression pattern of spaw in the lateral plate mesoderm and pitx2c in the gut as well as left-right patterning of various visceral organs were not altered in MZcdx1b^-/- mutants compared to wild-type embryos. Chromatin immunoprecipitation revealed that Cdx1b directly regulates ndr2 and lft1 expression. Furthermore, injection of cdx1b-vivo MO1 but not cdx1b-vivo 4 ​mm MO1 in the forebrain ventricle at 18 hpf significantly downregulated lft1 expression in the left dorsal diencephalon at 23-24 ​s stages. Together, our results suggest that Cdx1b regulates transcription of ndr2 and lft1 to maintain proper Nodal activity in the dorsal diencephalon and epithalamic asymmetry in zebrafish embryos.

Fibrodysplasia ossificans progressiva mutant ACVR1 signals by multiple modalities in the developing zebrafish

Fibrodysplasia ossificans progressiva (FOP) is a rare human genetic disorder characterized by altered skeletal development and extraskeletal ossification. All cases of FOP are caused by activating mutations in the type I BMP/TGFβ cell surface receptor ACVR1, which over-activates signaling through phospho-Smad1/5 (pSmad1/5). To investigate the mechanism by which FOP-ACVR1 enhances pSmad1/5 activation, we used zebrafish embryonic dorsoventral (DV) patterning as an assay for BMP signaling. We determined that the FOP mutants ACVR1-R206H and -G328R do not require their ligand binding domain to over-activate BMP signaling in DV patterning. However, intact ACVR1-R206H has the ability to respond to both Bmp7 and Activin A ligands. Additionally, BMPR1, a type I BMP receptor normally required for BMP-mediated patterning of the embryo, is dispensable for both ligand-independent signaling pathway activation and ligand-responsive signaling hyperactivation by ACVR1-R206H. These results demonstrate that FOP-ACVR1 is not constrained by the same receptor/ligand partner requirements as WT-ACVR1.

The maternal coordinate system: Molecular-genetics of embryonic axis formation and patterning in the zebrafish

Axis specification of the zebrafish embryo begins during oogenesis and relies on proper formation of well-defined cytoplasmic domains within the oocyte. Upon fertilization, maternally-regulated cytoplasmic flow and repositioning of dorsal determinants establish the coordinate system that will build the structure and developmental body plan of the embryo. Failure of specific genes that regulate the embryonic coordinate system leads to catastrophic loss of body structures. Here, we review the genetic principles of axis formation and discuss how maternal factors orchestrate axis patterning during zebrafish early embryogenesis. We focus on the molecular identity and functional contribution of genes controlling critical aspects of oogenesis, egg activation, blastula, and gastrula stages. We examine how polarized cytoplasmic domains form in the oocyte, which set off downstream events such as animal-vegetal polarity and germ line development. After gametes interact and form the zygote, cytoplasmic segregation drives the animal-directed reorganization of maternal determinants through calcium- and cell cycle-dependent signals. We also summarize how maternal genes control dorsoventral, anterior-posterior, mesendodermal, and left-right cell fate specification and how signaling pathways pattern these axes and tissues during early development to instruct the three-dimensional body plan. Advances in reverse genetics and phenotyping approaches in the zebrafish model are revealing positional patterning signatures at the single-cell level, thus enhancing our understanding of genotype-phenotype interactions in axis formation. Our emphasis is on the genetic interrogation of novel and specific maternal regulatory mechanisms of axis specification in the zebrafish.

Maternal contributions to gastrulation in zebrafish

Gastrulation is a critical early morphogenetic process of animal development, during which the three germ layers; mesoderm, endoderm and ectoderm, are rearranged by internalization movements. Concurrent epiboly movements spread and thin the germ layers while convergence and extension movements shape them into an anteroposteriorly elongated body with head, trunk, tail and organ rudiments. In zebrafish, gastrulation follows the proliferative and inductive events that establish the embryonic and extraembryonic tissues and the embryonic axis. Specification of these tissues and embryonic axes are controlled by the maternal gene products deposited in the egg. These early maternally controlled processes need to generate sufficient cell numbers and establish the embryonic polarity to ensure normal gastrulation. Subsequently, after activation of the zygotic genome, the zygotic gene products govern mesoderm and endoderm induction and germ layer patterning. Gastrulation is initiated during the maternal-to-zygotic transition, a process that entails both activation of the zygotic genome and downregulation of the maternal transcripts. Genomic studies indicate that gastrulation is largely controlled by the zygotic genome. Nonetheless, genetic studies that investigate the relative contributions of maternal and zygotic gene function by comparing zygotic, maternal and maternal zygotic mutant phenotypes, reveal significant contribution of maternal gene products, transcripts and/or proteins, that persist through gastrulation, to the control of gastrulation movements. Therefore, in zebrafish, the maternally expressed gene products not only set the stage for, but they also actively participate in gastrulation morphogenesis.

Nodal and planar cell polarity signaling cooperate to regulate zebrafish convergence and extension gastrulation movements

During vertebrate gastrulation, convergence and extension (C and E) of the primary anteroposterior (AP) embryonic axis is driven by polarized mediolateral (ML) cell intercalations and is influenced by AP axial patterning. Nodal signaling is essential for patterning of the AP axis while planar cell polarity (PCP) signaling polarizes cells with respect to this axis, but how these two signaling systems interact during C and E is unclear. We find that the neuroectoderm of Nodal-deficient zebrafish gastrulae exhibits reduced C and E cell behaviors, which require Nodal signaling in both cell- and non-autonomous fashions. PCP signaling is partially active in Nodal-deficient embryos and its inhibition exacerbates their C and E defects. Within otherwise naïve zebrafish blastoderm explants, however, Nodal induces C and E in a largely PCP-dependent manner, arguing that Nodal acts both upstream of and in parallel with PCP during gastrulation to regulate embryonic axis extension cooperatively.

Failed Progenitor Specification Underlies the Cardiopharyngeal Phenotypes in a Zebrafish Model of 22q11.2 Deletion Syndrome

Microdeletions involving TBX1 result in variable congenital malformations known collectively as 22q11.2 deletion syndrome (22q11.2DS). Tbx1-deficient mice and zebrafish recapitulate several disease phenotypes, including pharyngeal arch artery (PAA), head muscle (HM), and cardiac outflow tract (OFT) deficiencies. In zebrafish, these structures arise from nkx2.5⁺ progenitors in pharyngeal arches 2-6. Because pharyngeal arch morphogenesis is compromised in Tbx1-deficient animals, the malformations were considered secondary. Here, we report that the PAA, HM, and OFT phenotypes in tbx1 mutant zebrafish are primary and arise prior to pharyngeal arch morphogenesis from failed specification of the nkx2.5⁺ pharyngeal lineage. Through in situ analysis and lineage tracing, we reveal that nkx2.5 and tbx1 are co-expressed in this progenitor population. Furthermore, we present evidence suggesting that gdf3-ALK4 signaling is a downstream mediator of nkx2.5⁺ pharyngeal lineage specification. Collectively, these studies support a cellular mechanism potentially underlying the cardiovascular and craniofacial defects observed in the 22q11.2DS population.

Temporal dynamics in the formation and interpretation of Nodal and BMP morphogen gradients

One of the most powerful ideas in developmental biology has been that of the morphogen gradient. In the classical view, a signaling molecule is produced at a local source from where it diffuses, resulting in graded levels across the tissue. This gradient provides positional information, with thresholds in the level of the morphogen determining the position of different cell fates. While experimental studies have uncovered numerous potential morphogens in biological systems, it is becoming increasingly apparent that one important feature, not captured in the simple model, is the role of time in both the formation and interpretation of morphogen gradients. We will focus on two members of the transforming growth factor-β family that are known to play a vital role as morphogens in early vertebrate development: the Nodals and the bone morphogenetic proteins (BMPs). Primarily drawing on the early zebrafish embryo, we will show how recent studies have demonstrated the importance of feedback and other interactions that evolve through time, in shaping morphogen gradients. We will further show how rather than simply reading out levels of a morphogen, the duration of ligand exposure can be a crucial determinant of how cells interpret morphogens, in particular through the unfolding of downstream transcriptional events and in their interactions with other pathways.

Left-right asymmetric heart jogging increases the robustness of dextral heart looping in zebrafish

Building a left-right (L-R) asymmetric organ requires asymmetric information. This comes from various sources, including asymmetries in embryo-scale genetic cascades (including the left-sided Nodal cascade), organ-intrinsic mechanical forces, and cell-level chirality, but the relative influence of these sources and how they collaborate to drive asymmetric morphogenesis is not understood. During zebrafish heart development, the linear heart tube extends to the left of the midline in a process known as jogging. The jogged heart then undergoes dextral (i.e. rightward) looping to correctly position the heart chambers relative to one another. Left lateralized jogging is governed by the left-sided expression of Nodal in mesoderm tissue, while looping laterality is mainly controlled by heart-intrinsic cell-level asymmetries in the actomyosin cytoskeleton. The purpose of lateralized jogging is not known. Moreover, after jogging, the heart tube returns to an almost midline position and so it is not clear whether or how jogging may impact the dextral loop. Here, we characterize a novel loss-of-function mutant in the zebrafish Nodal homolog southpaw (spaw) that appears to be a true null. We then assess the relationship between jogging and looping laterality in embryos lacking asymmetric Spaw signals. We found that the probability of a dextral loop occurring, does not depend on asymmetric Spaw signals per se, but does depend on the laterality of jogging. Thus, we conclude that the role of leftward jogging is to spatially position the heart tube in a manner that promotes robust dextral looping. When jogging laterality is abnormal, the robustness of dextral looping decreases. This establishes a cooperation between embryo-scale Nodal-dependent L-R asymmetries and organ-intrinsic cellular chirality in the control of asymmetric heart morphogenesis and shows that the transient laterality of the early heart tube has consequences for later heart morphogenetic events.

Signaling events regulating embryonic polarity and formation of the primitive streak in the chick embryo

The avian embryo is a key experimental model system for early development of amniotes. One key difference with invertebrates and "lower" vertebrates like fish and amphibians is that amniotes do not rely so heavily on maternal messages because the zygotic genome is activated very early. Early development also involves considerable growth in volume and mass of the embryo, with cell cycles that include G1 and G2 phases from very early cleavage. The very early maternal to zygotic transition also allows the embryo to establish its own polarity without relying heavily on maternal determinants. In many amniotes including avians and non-rodent mammals, this enables an ability of the embryo to "regulate": a single multicellular embryo can give rise to more than one individual-monozygotic twins. Here we discuss the embryological, cellular, molecular and evolutionary underpinnings of gastrulation in avian embryos as a model amniote embryo. Many of these properties are shared by human embryos.

Setting up for gastrulation in zebrafish

Soon after fertilization the zebrafish embryo generates the pool of cells that will give rise to the germline and the three somatic germ layers of the embryo (ectoderm, mesoderm and endoderm). As the basic body plan of the vertebrate embryo emerges, evolutionarily conserved developmental signaling pathways, including Bmp, Nodal, Wnt, and Fgf, direct the nearly totipotent cells of the early embryo to adopt gene expression profiles and patterns of cell behavior specific to their eventual fates. Several decades of molecular genetics research in zebrafish has yielded significant insight into the maternal and zygotic contributions and mechanisms that pattern this vertebrate embryo. This new understanding is the product of advances in genetic manipulations and imaging technologies that have allowed the field to probe the cellular, molecular and biophysical aspects underlying early patterning. The current state of the field indicates that patterning is governed by the integration of key signaling pathways and physical interactions between cells, rather than a patterning system in which distinct pathways are deployed to specify a particular cell fate. This chapter focuses on recent advances in our understanding of the genetic and molecular control of the events that impart cell identity and initiate the patterning of tissues that are prerequisites for or concurrent with movements of gastrulation.

Evolution of nodal and nodal-related genes and the putative composition of the heterodimers that trigger the nodal pathway in vertebrates

Nodal is a signaling molecule that belongs to the transforming growth factor-β superfamily that plays key roles during the early stages of development of animals. In vertebrates Nodal forms an heterodimer with a GDF1/3 protein to activate the Nodal pathway. Vertebrates have a paralog of nodal in their genomes labeled Nodal-related, but the evolutionary history of these genes is a matter of debate, mainly because of the presence of a variable numbers of genes in the vertebrate genomes sequenced so far. Thus, the goal of this study was to investigate the evolutionary history of the Nodal and Nodal-related genes with an emphasis in tracking changes in the number of genes among vertebrates. Our results show the presence of two gene lineages (Nodal and Nodal-related) that can be traced back to the ancestor of jawed vertebrates. These lineages have undergone processes of differential retention and lineage-specific expansions. Our results imply that Nodal and Nodal-related duplicated at the latest in the ancestor of gnathostomes, and they still retain a significant level of functional redundancy. By comparing the evolution of the Nodal/Nodal-related with GDF1/3 gene family, it is possible to infer that there are several types of heterodimers that can trigger the Nodal pathway among vertebrates.

ISM1 regulates NODAL signaling and asymmetric organ morphogenesis during development

Isthmin1 (ISM1) was originally identified as a fibroblast group factor expressed in Xenopus laevis embryonic brain, but its biological functions remain unclear. The spatiotemporal distribution of ISM1, with high expression in the anterior primitive streak of the chick embryo and the anterior mesendoderm of the mouse embryo, suggested that ISM1 may regulate signaling by the NODAL subfamily of TGB-β cytokines that control embryo patterning. We report that ISM1 is an inhibitor of NODAL signaling. ISM1 has little effect on TGF-β1, ACTIVIN-A, or BMP4 signaling but specifically inhibits NODAL-induced phosphorylation of SMAD2. In line with this observation, ectopic ISM1 causes defective left-right asymmetry and abnormal heart positioning in chick embryos. Mechanistically, ISM1 interacts with NODAL ligand and type I receptor ACVR1B through its AMOP domain, which compromises the NODAL-ACVR1B interaction and down-regulates phosphorylation of SMAD2. Therefore, we identify ISM1 as an extracellular antagonist of NODAL and reveal a negative regulatory mechanism that provides greater plasticity for the fine-tuning of NODAL signaling.

Conserved regulation of Nodal-mediated left-right patterning in zebrafish and mouse

Nodal is the major effector of left-right axis development. In mice, Nodal forms heterodimers with Gdf1 and is inhibited by Cerl2/Dand5 at the node, and by Lefty1 in the lateral plate mesoderm (LPM). Studies in zebrafish have suggested some parallels, but also differences, between left-right patterning in mouse and zebrafish. To address these discrepancies, we generated single and double zebrafish mutants for southpaw (spaw, the Nodal ortholog), dand5 and lefty1, and performed biochemical and activity assays with Spaw and Vg1/Gdf3 (the Gdf1 ortholog). Contrary to previous findings, spaw mutants failed to initiate spaw expression in the LPM, and asymmetric heart looping was absent, similar to mouse Nodal mutants. In blastoderm assays, Vg1 and Spaw were interdependent for target gene induction, and contrary to previous results, formed heterodimers. Loss of Dand5 or Lefty1 caused bilateral spaw expression, similar to mouse mutants, and Lefty1 was replaceable with a uniform Nodal signaling inhibitor. Collectively, these results indicate that Dand5 activity biases Spaw-Vg1 heterodimer activity to the left, Spaw around Kupffer's vesicle induces the expression of spaw in the LPM and global Nodal inhibition maintains the left bias of Spaw activity, demonstrating conservation between zebrafish and mouse mechanisms of left-right patterning.

Phylogenetic evidence for independent origins of GDF1 and GDF3 genes in anurans and mammals

Growth differentiation factors 1 (GDF1) and 3 (GDF3) are members of the transforming growth factor superfamily (TGF-β) that is involved in fundamental early-developmental processes that are conserved across vertebrates. The evolutionary history of these genes is still under debate due to ambiguous definitions of homologous relationships among vertebrates. Thus, the goal of this study was to unravel the evolution of the GDF1 and GDF3 genes of vertebrates, emphasizing the understanding of homologous relationships and their evolutionary origin. Our results revealed that the GDF1 and GDF3 genes found in anurans and mammals are the products of independent duplication events of an ancestral gene in the ancestor of each of these lineages. The main implication of this result is that the GDF1 and GDF3 genes of anurans and mammals are not 1:1 orthologs. In other words, genes that participate in fundamental processes during early development have been reinvented two independent times during the evolutionary history of tetrapods.

Contextual determinants of TGFβ action in development, immunity and cancer

Few cell signals match the impact of the transforming growth factor-β (TGFβ) family in metazoan biology. TGFβ cytokines regulate cell fate decisions during development, tissue homeostasis and regeneration, and are major players in tumorigenesis, fibrotic disorders, immune malfunctions and various congenital diseases. The effects of the TGFβ family are mediated by a combinatorial set of ligands and receptors and by a common set of receptor-activated mothers against decapentaplegic homologue (SMAD) transcription factors, yet the effects can differ dramatically depending on the cell type and the conditions. Recent progress has illuminated a model of TGFβ action in which SMADs bind genome-wide in partnership with lineage-determining transcription factors and additionally integrate inputs from other pathways and the chromatin to trigger specific cellular responses. These new insights clarify the operating logic of the TGFβ pathway in physiology and disease.

Heterodimers reign in the embryo

Experiments by three independent groups on zebrafish have clarified the role of two signaling factors, Nodal and Gdf3, during the early stages of development