Tissue-specific requirements for the proprotein convertase furin/SPC1 during embryonic turning and heart looping

Proprotein convertase furina is required for heart development in zebrafish

Cardiac looping and trabeculation are key processes during cardiac chamber maturation. However, the underlying mechanisms remain incompletely understood. Here, we report the isolation, cloning and characterization of the proprotein convertase furina from the cardiovascular mutant loft in zebrafish. loft is an ethylnitrosourea-induced mutant and has evident defects in the cardiac outflow tract, heart looping and trabeculation, the craniofacial region and pharyngeal arch arteries. Positional cloning revealed that furina mRNA was barely detectable in loft mutants, and loft failed to complement the TALEN-induced furina mutant pku338, confirming that furina is responsible for the loft mutant phenotypes. Mechanistic studies demonstrated that Notch reporter Tg(tp1:mCherry) signals were largely eliminated in mutant hearts, and overexpression of the Notch intracellular domain partially rescued the mutant phenotypes, probably due to the lack of Furina-mediated cleavage processing of Notch1b proteins, the only Notch receptor expressed in the heart. Together, our data suggest a potential post-translational modification of Notch1b proteins via the proprotein convertase Furina in the heart, and unveil the function of the Furina-Notch1b axis in cardiac looping and trabeculation in zebrafish, and possibly in other organisms.

Furin, a transcriptional target of NKX2-5, has an essential role in heart development and function

The homeodomain transcription factor NKX2-5 is known to be essential for both normal heart development and for heart function. But little is yet known about the identities of its downstream effectors or their function during differentiation of cardiac progenitor cells (CPCs). We have used transgenic analysis and CRISPR-mediated ablation to identify a cardiac enhancer of the Furin gene. The Furin gene, encoding a proprotein convertase, is directly repressed by NKX2-5. Deletion of Furin in CPCs is embryonic lethal, with mutant hearts showing a range of abnormalities in the outflow tract. Those defects are associated with a reduction in proliferation and premature differentiation of the CPCs. Deletion of Furin in differentiated cardiomyocytes results in viable adult mutant mice showing an elongation of the PR interval, a phenotype that is consistent with the phenotype of mice and human mutant for Nkx2-5. Our results show that Furin mediate some aspects of Nkx2-5 function in the heart.

Mutations in Bcl9 and Pygo genes cause congenital heart defects by tissue-specific perturbation of Wnt/β-catenin signaling

Bcl9 and Pygopus (Pygo) are obligate Wnt/β-catenin cofactors in Drosophila, yet their contribution to Wnt signaling during vertebrate development remains unresolved. Combining zebrafish and mouse genetics, we document a conserved, β-catenin-associated function for BCL9 and Pygo proteins during vertebrate heart development. Disrupting the β-catenin-BCL9-Pygo complex results in a broadly maintained canonical Wnt response yet perturbs heart development and proper expression of key cardiac regulators. Our work highlights BCL9 and Pygo as selective β-catenin cofactors in a subset of canonical Wnt responses during vertebrate development. Moreover, our results implicate alterations in BCL9 and BCL9L in human congenital heart defects.

Cleavage activates dispatched for Sonic Hedgehog ligand release

Hedgehog ligands activate an evolutionarily conserved signaling pathway that provides instructional cues during tissue morphogenesis, and when corrupted, contributes to developmental disorders and cancer. The transmembrane protein Dispatched is an essential component of the machinery that deploys Hedgehog family ligands from producing cells, and is absolutely required for signaling to long-range targets. Despite this crucial role, regulatory mechanisms controlling Dispatched activity remain largely undefined. Herein, we reveal vertebrate Dispatched is activated by proprotein convertase-mediated cleavage at a conserved processing site in its first extracellular loop. Dispatched processing occurs at the cell surface to instruct its membrane re-localization in polarized epithelial cells. Cleavage site mutation alters Dispatched membrane trafficking and reduces ligand release, leading to compromised pathway activity in vivo. As such, convertase-mediated cleavage is required for Dispatched maturation and functional competency in Hedgehog ligand-producing cells.

Distinct SoxB1 networks are required for naïve and primed pluripotency

Deletion of Sox2 from mouse embryonic stem cells (ESCs) causes trophectodermal differentiation. While this can be prevented by enforced expression of the related SOXB1 proteins, SOX1 or SOX3, the roles of SOXB1 proteins in epiblast stem cell (EpiSC) pluripotency are unknown. Here, we show that Sox2 can be deleted from EpiSCs with impunity. This is due to a shift in the balance of SoxB1 expression in EpiSCs, which have decreased Sox2 and increased Sox3 compared to ESCs. Consistent with functional redundancy, Sox3 can also be deleted from EpiSCs without eliminating self-renewal. However, deletion of both Sox2 and Sox3 prevents self-renewal. The overall SOXB1 levels in ESCs affect differentiation choices: neural differentiation of Sox2 heterozygous ESCs is compromised, while increased SOXB1 levels divert the ESC to EpiSC transition towards neural differentiation. Therefore, optimal SOXB1 levels are critical for each pluripotent state and for cell fate decisions during exit from naïve pluripotency.

Morphogenesis and maturation of the embryonic and postnatal intestine

The intestine is a vital organ responsible for nutrient absorption, bile and waste excretion, and a major site of host immunity. In order to keep up with daily demands, the intestine has evolved a mechanism to expand the absorptive surface area by undergoing a morphogenetic process to generate finger-like units called villi. These villi house specialized cell types critical for both absorbing nutrients from food, and for protecting the host from commensal and pathogenic microbes present in the adult gut. In this review, we will discuss mechanisms that coordinate intestinal development, growth, and maturation of the small intestine, starting from the formation of the early gut tube, through villus morphogenesis and into early postnatal life when the intestine must adapt to the acquisition of nutrients through food intake, and to interactions with microbes.

Role of miR-24, Furin, and Transforming Growth Factor-β1 Signal Pathway in Fibrosis After Cardiac Infarction

BACKGROUND Cardiac fibrosis after primary infarction is a type of pathological phenomena as shown by increased collagen in myocardial cells. Transforming growth factor (TGF)-β1 is a critical factor participating in myocardial fibrosis. A previous study has shown the inhibitory role on TGF-β1 by microRNA-24 (miR-24) via targeting Furin. This study thus investigated the role of miR-24 and Furin/TGF-β1 in rat myocardial fibrosis. MATERIAL AND METHODS A total of 40 adult SD rats (both males and females) were prepared for myocardial infarction model by ligating the descending branch of left coronary artery after anesthesia. HE staining was performed to observe myocardial fibrosis after 1, 2, and 4 weeks. Tissue RNA was extracted to detect mRNA levels of Furin, TGF-β1, and miR-24 by real-time PCR. Western blotting was used to quantify protein expression of Furin and TGF-β1 in myocardial tissues. RESULTS Increased connective tissues were observed in myocardial tissues at 4 weeks after infarction by HE staining, which also revealed widening of the intra-myocardial cleft, along with more inflammatory cells and fibroblast hypertrophy. miR-24 expression was significantly depressed at 2 and 4 weeks after cardiac infarction (p<0.05). mRNA levels of Furin and TGF-β1 were elevated after infarction (p<0.05). With prolonged time periods of myocardial infarction, protein levels of Furin and TGF-β1 were further increased. The level of miR-24 was positively correlated with left ventricular end-diastolic diameter, left ventricular systolic diameter, and left ventricular ejection fraction. However, the level of Furin or TGF-β1 was negatively correlated with the above parameters. CONCLUSIONS This study demonstrated the important role of abnormal expression of miR-24 in myocardial fibrosis after infarction, and may provide drug targets for treating myocardial fibrosis.

Tissue morphodynamics shaping the early mouse embryo

Generation of the elongated vertebrate body plan from the initially radially symmetrical embryo requires comprehensive changes to tissue form. These shape changes are generated by specific underlying cell behaviors, coordinated in time and space. Major principles and also specifics are emerging, from studies in many model systems, of the cell and physical biology of how region-specific cell behaviors produce regional tissue morphogenesis, and how these, in turn, are integrated at the level of the embryo. New technical approaches have made it possible more recently, to examine the morphogenesis of the mouse embryo in depth, and to elucidate the underlying cellular mechanisms. This review focuses on recent advances in understanding the cellular basis for the early fundamental events that establish the basic form of the embryo.

ProNodal acts via FGFR3 to govern duration of Shh expression in the prechordal mesoderm

The secreted glycoprotein sonic hedgehog (Shh) is expressed in the prechordal mesoderm, where it plays a crucial role in induction and patterning of the ventral forebrain. Currently little is known about how Shh is regulated in prechordal tissue. Here we show that in the embryonic chick, Shh is expressed transiently in prechordal mesoderm, and is governed by unprocessed Nodal. Exposure of prechordal mesoderm microcultures to Nodal-conditioned medium, the Nodal inhibitor CerS, or to an ALK4/5/7 inhibitor reveals that Nodal is required to maintain both Shh and Gsc expression, but whereas Gsc is largely maintained through canonical signalling, Nodal signals through a non-canonical route to maintain Shh. Further, Shh expression can be maintained by a recombinant Nodal cleavage mutant, proNodal, but not by purified mature Nodal. A number of lines of evidence suggest that proNodal acts via FGFR3. ProNodal and FGFR3 co-immunoprecipitate and proNodal increases FGFR3 tyrosine phosphorylation. In microcultures, soluble FGFR3 abolishes Shh without affecting Gsc expression. Further, prechordal mesoderm cells in which Fgfr3 expression is reduced by Fgfr3 siRNA fail to bind to proNodal. Finally, targeted electroporation of Fgfr3 siRNA to prechordal mesoderm in vivo results in premature Shh downregulation without affecting Gsc. We report an inverse correlation between proNodal-FGFR3 signalling and pSmad1/5/8, and show that proNodal-FGFR3 signalling antagonises BMP-mediated pSmad1/5/8 signalling, which is poised to downregulate Shh. Our studies suggest that proNodal/FGFR3 signalling governs Shh duration by repressing canonical BMP signalling, and that local BMPs rapidly silence Shh once endogenous Nodal-FGFR3 signalling is downregulated.

Highlighted article: In the chick prechordal mesoderm, the Nodal precursor proNodal acts via a non-canonical route to inhibit BMP signalling and thus maintain Shh expression

Activin/Nodal signalling in stem cells

Activin/Nodal growth factors control a broad range of biological processes, including early cell fate decisions, organogenesis and adult tissue homeostasis. Here, we provide an overview of the mechanisms by which the Activin/Nodal signalling pathway governs stem cell function in these different stages of development. We describe recent findings that associate Activin/Nodal signalling to pathological conditions, focusing on cancer stem cells in tumorigenesis and its potential as a target for therapies. Moreover, we will discuss future directions and questions that currently remain unanswered on the role of Activin/Nodal signalling in stem cell self-renewal, differentiation and proliferation.

Distinct mechanisms determine organ left-right asymmetry patterning in an uncoupled way

Disruption of Nodal in the lateral plate mesoderm (LPM) usually leads to left-right (LR) patterning defects in multiple organs. However, whether the LR patterning of organs is always regulated in a coupled way has largely not yet been elucidated. In addition, whether other crucial regulators exist in the LPM that coordinate with Nodal in regulating organ LR patterning is also undetermined. In this paper, after briefly summarizing the common process of LR patterning, the most puzzling question regarding the initiation of asymmetry is considered and the divergent mechanisms underlying the uncoupled LR patterning in different organs are discussed. On the basis of cases in which different organ LR patterning is determined in an uncoupled way via an independent mechanism or at a different time, we propose that there are other critical factors in the LPM that coordinate with Nodal to regulate heart LR asymmetry patterning during early LR patterning.

Genetic dissection of ventral folding morphogenesis in mouse: embryonic visceral endoderm-supplied BMP2 positions head and heart

Ventral folding morphogenesis (VFM), a vital morphogenetic process in amniotes, mediates gut endoderm internalization, linear heart tube formation, ventral body wall closure and encasement of the fetus in extraembryonic membranes. Aberrant VFM underlies a number of birth defects such as gastroschisis and ectopia cordis in human and misplacement of head and heart in mouse. Recent cell lineage-specific mouse mutant analyses identified the Bone Morphogenetic Protein (BMP) pathway and Anterior Visceral Endoderm (AVE) as key regulators of anterior VFM. Loss of BMP2 expression solely from embryonic visceral endoderm (EmVE) and the AVE blocks formation of foregut invagination, and simultaneously, aberrantly positions the heart anterior/dorsal to the head, suggesting a mechanistic link between foregut and head/heart morphogenesis.

The biology and therapeutic targeting of the proprotein convertases

The mammalian proprotein convertases constitute a family of nine secretory serine proteases that are related to bacterial subtilisin and yeast kexin. Seven of these (proprotein convertase 1 (PC1), PC2, furin, PC4, PC5, paired basic amino acid cleaving enzyme 4 (PACE4) and PC7) activate cellular and pathogenic precursor proteins by cleavage at single or paired basic residues, whereas subtilisin kexin isozyme 1 (SKI-1) and proprotein convertase subtilisin kexin 9 (PCSK9) regulate cholesterol and/or lipid homeostasis via cleavage at non-basic residues or through induced degradation of receptors. Proprotein convertases are now considered to be attractive targets for the development of powerful novel therapeutics. In this Review, we summarize the physiological functions and pathological implications of the proprotein convertases, and discuss proposed strategies to control some of their activities, including their therapeutic application and validation in selected disease states.

Pten regulates collective cell migration during specification of the anterior-posterior axis of the mouse embryo

Pten, the potent tumor suppressor, is a lipid phosphatase that is best known as a regulator of cell proliferation and cell survival. Here we show that mouse embryos that lack Pten have a striking set of morphogenetic defects, including the failure to correctly specify the anterior-posterior body axis, that are not caused by changes in proliferation or cell death. The majority of Pten null embryos express markers of the primitive streak at ectopic locations around the embryonic circumference, rather than at a single site at the posterior of the embryo. Epiblast-specific deletion shows that Pten is not required in the cells of the primitive streak; instead, Pten is required for normal migration of cells of the Anterior Visceral Endoderm (AVE), an extraembryonic organizer that controls the position of the streak. Cells of the wild-type AVE migrate within the visceral endoderm epithelium from the distal tip of the embryo to a position adjacent to the extraembryonic region. In all Pten null mutants, AVE cells move a reduced distance and disperse in random directions, instead of moving as a coordinated group to the anterior of the embryo. Aberrant AVE migration is associated with the formation of ectopic F-actin foci, which indicates that absence of Pten disrupts the actin-based migration of these cells. After the initiation of gastrulation, embryos that lack Pten in the epiblast show defects in the migration of mesoderm and/or endoderm. The findings suggest that Pten has an essential and general role in the control of mammalian collective cell migration.

Nerve growth factor regulates axial rotation during early stages of chick embryo development

Nerve growth factor (NGF) was discovered because of its neurotrophic actions on sympathetic and sensory neurons in the developing chicken embryo. NGF was subsequently found to influence and regulate the function of many neuronal and non neuronal cells in adult organisms. Little is known, however, about the possible actions of NGF during early embryonic stages. However, mRNAs encoding for NGF and its receptors TrkA and p75(NTR) are expressed at very early stages of avian embryo development, before the nervous system is formed. The question, therefore, arises as to what might be the functions of NGF in early chicken embryo development, before its well-established actions on the developing sympathetic and sensory neurons. To investigate possible roles of NGF in the earliest stages of development, stage HH 11-12 chicken embryos were injected with an anti-NGF antibody (mAb αD11) that binds mature NGF with high affinity. Treatment with anti-NGF, but not with a control antibody, led to a dose-dependent inversion of the direction of axial rotation. This effect of altered rotation after anti NGF injection was associated with an increased cell death in somites. Concurrently, a microarray mRNA expression analysis revealed that NGF neutralization affects the expression of genes linked to the regulation of development or cell proliferation. These results reveal a role for NGF in early chicken embryo development and, in particular, in the regulation of somite survival and axial rotation, a crucial developmental process linked to left-right asymmetry specification.

Anterior visceral endoderm directs ventral morphogenesis and placement of head and heart via BMP2 expression

In amniotes, ventral folding morphogenesis achieves gut internalization, linear heart tube formation, ventral body wall closure, and encasement of the fetus in extraembryonic membranes. Impairment of ventral morphogenesis results in human birth defects involving body wall, gut, and heart malformations and in mouse misplacement of head and heart. Absence of knowledge about genetic pathways and cell populations directing ventral folding in mammals has precluded systematic study of cellular mechanisms driving this vital morphogenetic process. We report tissue-specific mouse mutant analyses identifying the bone morphogenetic protein (BMP) pathway as a key regulator of ventral morphogenesis. BMP2 expressed in anterior visceral endoderm (AVE) signals to epiblast derivatives during gastrulation to orchestrate initial stages of ventral morphogenesis, including foregut development and positioning of head and heart. These findings identify unanticipated functions for the AVE in organizing the gastrulating embryo and indicate that visceral endoderm-expressed BMP2 coordinates morphogenetic cell behaviors in multiple epiblast lineages.

Targets of the tumor suppressor miR-200 in regulation of the epithelial-mesenchymal transition in cancer

The microRNA-200 (miR-200) family restricts epithelial-mesenchymal transition (EMT) and metastasis in tumor cell lines derived from mice that develop metastatic lung adenocarcinoma. To determine the mechanisms responsible for EMT and metastasis regulated by this microRNA, we conducted a global liquid chromatography/tandem mass spectrometry analysis to compare metastatic and nonmetastatic murine lung adenocarcinoma cells which had undergone EMT because of loss of miR-200. An analysis of syngeneic tumors generated by these cells identified multiple novel proteins linked to metastasis. In particular, the analysis of conditioned media, cell surface proteins, and whole-cell lysates from metastatic and nonmetastatic cells revealed large-scale modifications in the tumor microenvironment. Specific increases were documented in extracellular matrix (ECM) proteins, peptidases, and changes in distribution of cell adhesion proteins in the metastatic cell lines. Integrating proteomic data from three subproteomes, we defined constituents of a multilayer protein network that both regulated and mediated the effects of TGFβ. Lastly, we identified ECM proteins and peptidases that were directly regulated by miR-200. Taken together, our results reveal how expression of miR-200 alters the tumor microenvironment to inhibit the processes of EMT and metastasis.

The multiple roles of Notch signaling during left-right patterning

The establishment of left-right (LR) asymmetry is regulated by intricate signaling mechanisms during embryogenesis and this asymmetry is critical for morphogenesis as well as the positioning of internal organs within the organism. Recent progress including elucidation of ion transporters, leftward nodal flow, and regulation of asymmetric gene expression contributes to our understanding of how the breaking of the symmetry is initiated and how this laterality information is subsequently transmitted to the organ primordium. A number of developmental signaling pathways have been implicated in this complex process. In this review, we will focus on the roles of the Notch signaling pathway during development of LR asymmetry. The Notch signaling pathway is a short-range communication system between neighboring cells. While Notch signaling plays essential roles in regulating the morphogenesis of the node and left-specific expression of Nodal in the lateral plate mesoderm, a hallmark gene in LR patterning, Notch signaling also suppresses the expression of Pitx2 that is a direct downstream target of Nodal during later stages of development. This negative activity of Notch signaling towards left-specific activity was recently shown to be inhibited by the B cell lymphoma 6 (BCL6)/BCL6 co-repressor (BcoR) transcriptional repressor complex in a target-specific manner. The complex regulation of Notch-dependent gene expression for LR asymmetry will be highlighted in this review.

Furin is the major processing enzyme of the cardiac-specific growth factor bone morphogenetic protein 10

Bone morphogenetic protein 10 (BMP10) is a member of the TGF-β superfamily and plays a critical role in heart development. In the postnatal heart, BMP10 is restricted to the right atrium. The inactive pro-BMP10 (∼60 kDa) is processed into active BMP10 (∼14 kDa) by an unknown protease. Proteolytic cleavage occurs at the RIRR(316)↓ site (human), suggesting the involvement of proprotein convertase(s) (PCs). In vitro digestion of a 12-mer peptide encompassing the predicted cleavage site with furin, PACE4, PC5/6, and PC7, showed that furin cleaves the best, whereas PC7 is inactive on this peptide. Ex vivo studies in COS-1 cells, a cell line lacking PC5/6, revealed efficient processing of pro-BMP10 by endogenous PCs other than PC5/6. The lack of processing of overexpressed pro-BMP10 in the furin- and PACE4-deficient cell line, CHO-FD11, and in furin-deficient LoVo cells, was restored by stable (CHO-FD11/Fur cells) or transient (LoVo cells) expression of furin. Use of cell-permeable and cell surface inhibitors suggested that endogenous PCs process pro-BMP10 mostly intracellularly, but also at the cell surface. Ex vivo experiments in mouse primary hepatocytes (wild type, PC5/6 knock-out, and furin knock-out) corroborated the above findings that pro-BMP10 is a substrate for endogenous furin. Western blot analyses of heart right atria extracts from wild type and PACE4 knock-out adult mice showed no significant difference in the processing of pro-BMP10, implying no in vivo role of PACE4. Overall, our in vitro, ex vivo, and in vivo data suggest that furin is the major convertase responsible for the generation of BMP10.

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

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

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

BCL6 canalizes Notch-dependent transcription, excluding Mastermind-like1 from selected target genes during left-right patterning

Although the Notch signaling pathway is one of the most intensely studied intracellular signaling pathways, the mechanisms by which Notch signaling regulates transcription remain incompletely understood. Here, we report that B cell leukemia/lymphoma 6 (BCL6), a transcriptional repressor, is a Notch-associated factor. BCL6 is necessary to maintain the expression of Pitx2 in the left lateral plate mesoderm during the patterning of left-right asymmetry in Xenopus embryos. For this process, BCL6 forms a complex with BCL6 corepressor (BCoR) on the promoters of selected Notch target genes such as enhancer of split related 1. BCL6 also inhibits the transcription of these genes by competing for the Notch1 intracellular domain, preventing the coactivator Mastermind-like1 (MAM1) from binding. These results define a mechanism restricting Notch-activated transcription to cell-type-appropriate subsets of target genes, and elucidate its relevance in vivo during left-right asymmetric development.

Formation of the murine endoderm: lessons from the mouse, frog, fish, and chick

The mammalian definitive endoderm arises as a simple epithelial sheet. This sheet of cells will eventually produce the innermost tube that comprises the entire digestive tract from the esophagus to the colon as well as the epithelial component of the digestive and respiratory organs including the thymus, thyroid, lung, liver, gallbladder, and pancreas. Thus a wide array of tissue types are derived from the early endodermal sheet, and understanding the morphological and molecular mechanisms used to produce this tissue is integral to understanding the development of all these organs. The goal of this chapter is to summarize what is known about the morphological and molecular mechanisms used to produce this embryonic germ layer. Although this chapter mainly focuses on the mechanisms used to generate the murine endoderm, supportive or suggestive data from other species, including chick, frog (Xenopus laevis), and the Zebrafish (Danio rerio) are also examined.

Expression and function of mouse Sox17 gene in the specification of gallbladder/bile-duct progenitors during early foregut morphogenesis

In early-organogenesis-stage mouse embryos, the posteroventral foregut endoderm adjacent to the heart tube gives rise to liver, ventral pancreas and gallbladder. Hepatic and pancreatic primordia become specified in the posterior segment of the ventral foregut endoderm at early somite stages. The mechanisms for demarcating gallbladder and bile duct primordium, however, are poorly understood. Here, we demonstrate that the gallbladder and bile duct progenitors are specified in the paired lateral endoderm domains outside the heart field at almost the same timing as hepatic and pancreatic induction. In the anterior definitive endoderm, Sox17 reactivation occurs in a certain population within the most lateral domains posterolateral to the anterior intestinal portal (AIP) lip on both the left and right sides. During foregut formation, the paired Sox17-positive domains expand ventromedially to merge in the midline of the AIP lip and become localized between the liver and pancreatic primordia. In Sox17-null embryos, these lateral domains are missing, resulting in a complete loss of the gallbladder/bile-duct structure. Chimera analyses revealed that Sox17-null endoderm cells in the posteroventral foregut do not display any gallbladder/bile-duct molecular characters. Our findings show that Sox17 functions cell-autonomously to specify gallbladder/bile-duct in the mouse embryo.

Activation of the epithelial sodium channel (ENaC) by serine proteases

The study of human monogenic diseases [pseudohypoaldosteronism type 1 (PHA-1) and Liddle's syndrome] as well as mouse models mimicking the salt-losing syndrome (PHA-1) or salt-sensitive hypertension (Liddle's syndrome) have established the epithelial sodium channel ENaC as a limiting factor in vivo in the control of ionic composition of the extracellular fluid, regulation of blood volume and blood pressure, lung alveolar clearance, and airway mucociliary clearance. In this review, we discuss more specifically the activation of ENaC by serine proteases. Recent in vitro and in vivo experiments indicate that membrane-bound serine proteases are of critical importance in the activation of ENaC in different organs, such as the kidney, the lung, or the cochlea. Progress in understanding the basic mechanism of proteolytic activation of ENaC is accelerating, but uncertainty about the most fundamental aspects persists, leaving numerous still-unanswered questions.

An activin/furin regulatory loop modulates the processing and secretion of inhibin alpha- and betaB-subunit dimers in pituitary gonadotrope cells

Of all ligands of the transforming growth factor beta superfamily, inhibins and activins are a physiologically relevant pair that are functional antagonists of each other. Activin stimulates whereas inhibin blocks follicle-stimulating hormone biosynthesis and secretion from pituitary gonadotrope cells, and together, inhibin and activin control the pituitary gonadal axis essential for normal reproductive function. Sharing a similar beta-subunit, the secretion of inhibin heterodimers (alpha/beta) or activin homodimers (beta/beta) as mature bioactive ligands depends, in part, on the proteolytic processing of precursor proteins. A short loop regulatory pathway controlling precursor processing and dimer secretion was discovered. Activin stimulates endogenous inhibin alpha- and betaB-subunit mRNA, protein, and proteolytic processing. Simultaneously, activin stimulated the proconvertase furin through a Smad2/3-dependent process. The data provide a mechanism where the regulation of furin and inhibin subunits cooperates in an important positive short feedback loop. This regulatory loop augments the secretion of bioactive mature activin B, as well as inhibin B dimers, necessary for local follicle-stimulating hormone beta regulation.

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

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

In vivo functions of the proprotein convertase PC5/6 during mouse development: Gdf11 is a likely substrate

The proprotein convertase PC5/6 cleaves protein precursors after basic amino acids and is essential for implantation in CD1/129/Sv/C57BL/6 mixed-background mice. Conditional inactivation of Pcsk5 in the epiblast but not in the extraembryonic tissue bypassed early embryonic lethality but resulted in death at birth. PC5/6-deficient embryos exhibited Gdf11-related phenotypes such as altered anteroposterior patterning with extra vertebrae and lack of tail and kidney agenesis. They also exhibited Gdf11-independent phenotypes, such as a smaller size, multiple hemorrhages, collapsed alveoli, and retarded ossification. In situ hybridization revealed overlapping PC5/6 and Gdf11 mRNA expression patterns. In vitro and ex vivo analyses showed that the selectivity of PC5/6 for Gdf11 essentially resides in the presence of a P1' Asn in the RSRR downward arrowN cleavage motif. This work identifies Gdf11 as a likely in vivo specific substrate of PC5/6 and opens the way to the identification of other key substrates of this convertase.

Temporospatially regulated expression of subtilisin-like proprotein convertase PACE4 (SPC4) during development of the rat submandibular gland

The temporospatial expression of PACE4, a member of the mammalian subtilisin-like proprotein convertase family involved in the activation of growth/differentiation factors, was investigated by in situ hybridization during the development of the rat submandibular gland (SMG). At the initiation stage (day 15.5 of gestation; E15), PACE4 was intensely expressed in the submandibular epithelium, but weakly expressed in the mesenchymal cells. At E16 when the branching morphogenesis becomes obvious, the expression of PACE4 in the mesenchyme was further decreased, although its level in the submandibular epithelium had not changed remarkably from that at E15. During the next stage of embryonic development (E17-E20), PACE4 was expressed in the cells derived from the submandibular epithelium, which include the proacinar, terminal tubular, and presumptive ductal cells. In the perinatal SMG, PACE4 was still expressed intensely in the terminal portion of the SMG containing the proacinar and terminal tubular cells, whereas its expression in the ductal cells was obviously decreased at the second postnatal day (P2) and at P6. Acinar cells expressing no PACE4 appeared, and their numbers increased following their development (P9-P20). At P30 when the PACE4 expression in the acinar cells was completely suppressed, its expression in the ductal cells became intense again. This temporospatially regulated expression of PACE4 suggests its apparent association with the proliferation, differentiation, and establishment of functional acinar and ductal cells of the SMG.

Anteriorward shifting of asymmetric Xnr1 expression and contralateral communication in left-right specification in Xenopus

Transient asymmetric Nodal signaling in the left lateral plate mesoderm (L LPM) during tailbud/early somitogenesis stages is associated in all vertebrates examined with the development of stereotypical left-right (L-R) organ asymmetry. In Xenopus, asymmetric expression of Nodal-related 1 (Xnr1) begins in the posterior L LPM shortly after the initiation of bilateral perinotochordal expression in the posterior tailbud. The L LPM expression domain rapidly shifts forward to cover much of the flank of the embryo before being progressively downregulated, also in a posterior-to-anterior direction. The mechanisms underlying the initiation and propagation of Nodal/Xnr1 expression in the L LPM, and its transient nature, are not well understood. Removing the posterior tailbud domain prevents Xnr1 expression in the L LPM, consistent with the idea that normal embryos respond to a posteriorly derived asymmetrically acting positive inductive signal. The forward propagation of asymmetric Xnr1 expression occurs LPM-autonomously via planar tissue communication. The shifting is prevented by Nodal signaling inhibitors, implicating an underlying requirement for Xnr1-to-Xnr1 induction. It is also unclear how asymmetric Nodal signals are modulated during L-R patterning. Small LPM grafts overexpressing Xnr1 placed into the R LPM of tailbud embryos induced the expression of the normally L-sided genes Xnr1, Xlefty, and XPitx2, and inverted body situs, demonstrating the late-stage plasticity of the LPM. Orthogonal Xnr1 signaling from the LPM strongly induced Xlefty expression in the midline, consistent with recent findings in the mouse and demonstrating for the first time in another species conservation in the mechanism that induces and maintains the midline barrier. Our findings suggest that there is long-range contralateral communication between L and R LPM, involving Xlefty in the midline, over a substantial period of tailbud embryogenesis, and therefore lend further insight into how, and for how long, the midline maintains a L versus R status in the LPM.

RNAi-mediated silencing of prohormone convertase (PC) 5/6 expression leads to impairment in processing of cocaine- and amphetamine-regulated transcript (CART) precursor

Understanding the functions of the widely expressed PCs (prohormone/proprotein convertases), including PC5/6, furin and PACE4 (paired basic amino acid cleaving enzyme 4), in animal models is difficult since individual knockouts of these PCs in mice exhibit early embryonic lethality. To investigate the roles of PC5/6 in processing pro-CART (pro-cocaine- and amphetamine-regulated transcript), an important anorexigenic peptide precursor, we have generated GH3 cells silenced for PC5/6 expression by RNAi (RNA interference). We show, following transient knockdown of PC5/6 in these neuroendocrine cells, that generation of the two bioactive forms, CART I (amino acids 42-89/55-102) and CART II (amino acids 49-89/62-102), from pro-CART is impaired due to a lack particularly of the A isoform of PC5/6. The results indicate that PC5/6A shares specificities primarily with PC2 (PC5/6A<PC2) in cleaving the pairs of basic residues, KR (40, 41 //53, 54/) and KK (47, 48//60, 61/), within the pro-CART isoforms [see Dey, Zhu, Carroll, Turck, Stein and Steiner (2003) J. Biol. Chem. 278, 15007-15014]. We do not find any significant role of PC5/6A in processing the RXXR (29-32/) site for production of intermediate CART (amino acids 33-102) from long pro-CART. The findings taken altogether indicate that PC5/6 participates in normal processing of pro-CART.

Definitive endoderm of the mouse embryo: formation, cell fates, and morphogenetic function

The endoderm is one of the primary germ layers but, in comparison to ectoderm and mesoderm, has received less attention. The definitive endoderm forms during gastrulation and replaces the extraembryonic visceral endoderm. It participates in the complex morphogenesis of the gut tube and contributes to the associated visceral organs. This review highlights the role of the definitive endoderm as a source of patterning cues for the morphogenesis of other germ-layer tissues, such as the anterior neurectoderm and the pharyngeal region, and also emphasizes the intricate patterning that the endoderm itself undergoes enabling the acquisition of regionalized cell fates.

Processing of cocaine- and amphetamine-regulated transcript (CART) precursor proteins by prohormone convertases (PCs) and its implications

Cocaine- and amphetamine-regulated transcript (CART) peptides are expressed in several neuroendocrine tissues, including hypothalamus, pituitary, gut, adrenal and pancreas, and are involved in regulating important biological processes including feeding/appetite, drug reward and stress. CART is synthesized as larger, inactive peptide precursors (pro-CART) that require endoproteolytic processing to generate smaller, active forms. Prohormone/proprotein convertases (PCs), a family of calcium-dependent, serine endoproteases, have been shown to cleave many protein precursors in the regulated/constitutive secretory pathway to generate smaller fragments. In our previous studies, we have demonstrated the important roles of the two neuroendocrine-specific PCs, PC2 and PC1/3, in processing the two pro-CART isoforms, long (102aa) and short (89aa), to generate the bioactive CART peptides, I (55-102/42-89) and II (62-102/49-89) as well as the intermediate fragments, 10-89 and 33-102. Our subsequent studies have revealed the participation of another PC family member, PC5/6A (the soluble isoform of a widely expressed PC, PC5/6), in cleaving both precursor isoforms. We conclude that PC5/6A contributes to the normal efficient processing of pro-CART and is functionally more redundant with PC2 than PC1/3 in generating both CART I and II.

Subtilisin-like proprotein convertase activity is necessary for left–right axis determination in Xenopus neurula embryos

Signaling by members of TGF-beta superfamily requires the activity of a family of site-specific endopeptidases, known as Subtilisin-like proprotein convertases (SPCs), which cleave these ligands into mature, active forms. To explore the role of SPCs in lateral plate mesoderm (LPM) differentiation in Xenopus, two SPC inhibitors, decanoyl-Arg-Val-Lys-Arg-chloromethylketone (Dec-RVKR-CMK) and hexa-arginine, were injected into the left and right LPM of Xenopus neurulae. Left-side injection caused heart-specific left-right reversal, and this phenotype was rescued by co-injection of mature Nodal protein. In contrast, right-side injection caused left-right reversal of both the heart and gut. Tailbud embryos were less sensitive to SPC inhibitors than neurula embryos. Injection of inhibitors into either side of neurula embryos completely abolished expression of the left-LPM-specific genes, Xnr-1, antivin, and pitx2. SPC1 enzyme (Furin) was injected into the left or right LPM of mid-neurula embryos to determine the effect of enhancing SPC activity. Left-side injection of SPC1 did not cause a significant left-right reversal of the internal organs. However, right-side injection of SPC1 strongly induced the expression of Xnr-1 and pitx2 in the right LPM, and caused 100% left-right reversal of both the heart and gut. These results suggest that moderate level of SPC activity in the right LPM of the neurulae is necessary for proper left-right specification. Taken together, SPC enzymatic activity must be present in both LPMs for expression of the left-handed genes and left-right axis determination of the heart and gut in Xenopus embryos.

Left-right asymmetry in the vertebrate embryo: from early information to higher-level integration

Although vertebrates seem to be essentially bilaterally symmetrical on the exterior, there are numerous interior left-right asymmetries in the disposition and placement of internal organs. These asymmetries are established during embryogenesis by complex epigenetic and genetic cascades. Recent studies in a range of model organisms have made important progress in understanding how this laterality information is generated and conveyed to large regions of the embryo. Both commonalities and divergences are emerging in the mechanisms that different vertebrates use in left-right axis specification. Recent evidence also provides intriguing links between the establishment of left-right asymmetries and the symmetrical elongation of the anterior-posterior axis.

Cardiac morphogenesis: matrix metalloproteinase coordination of cellular mechanisms underlying heart tube formation and directionality of looping

During heart organogenesis, the spatiotemporal organization of the extracellular matrix (ECM) undergoes significant remodeling. Because matrix metalloproteinases (MMPs) are known to be key regulators of cell-matrix interactions, we analyzed the role(s) of MMPs, and specifically MMP-2, in early heart development. Both MMP-2 neutralizing antibody and the broad-spectrum MMP inhibitor Ilomastat in a temporal manner, when applied between chick embryonic stages 5 (primitive streak stage) to stage 12 ( approximately 16-somites), produced severe heart tube defects. Exposure to the MMP inhibitor at stage 5 produced various degrees of cardia bifida. At the seven-somite stage, MMP-2/Ilomastat inhibition caused a shift in normal left-right patterning of cell proliferation within the dorsal mesocardium and mesoderm of the anterior heart field that correlated with a change in looping direction. MMP inhibition at the 10- to 12-somite stage resulted in an arrest of heart tube bending by inhibiting the breakdown of the dorsal mesocardial ECM. The experimental observations suggest that MMP activity regulates the coordination of early heart organogenesis by affecting ventral closure of the heart and gut tubes, asymmetric cell proliferation in the dorsal mesocardium to drive looping direction, and ECM degradation within the dorsal mesocardium allowing looping to proceed toward completion.

Proprotein convertase genes in Xenopus development

Proprotein convertases (PCs) are a family of serine endoproteases that proteolytically activate many precursor proteins within various secretory pathway compartments. Loss-of-function studies have demonstrated a critical role for these proteases in embryonic patterning and adult homeostasis, yet little is known about how substrate selectivity is achieved. We have identified Xenopus orthologs of three PCs: furin, PC6, and PC4. In addition to previously described isoforms of PC6 and furin, four novel splice isoforms of PC6, which are predicted to encode constitutively secreted proteases, and a putative transmembrane isoform of PC4 were identified. Furin and PC6 are expressed in dynamic, tissue-specific patterns throughout embryogenesis, whereas PC4 transcripts are restricted primarily to germ cells and brain in adult frogs.

Left-right asymmetry in embryonic development: a comprehensive review

Embryonic morphogenesis occurs along three orthogonal axes. While the patterning of the anterior-posterior and dorsal-ventral axes has been increasingly well characterized, the left-right (LR) axis has only recently begun to be understood at the molecular level. The mechanisms which ensure invariant LR asymmetry of the heart, viscera, and brain represent a thread connecting biomolecular chirality to human cognition, along the way involving fundamental aspects of cell biology, biophysics, and evolutionary biology. An understanding of LR asymmetry is important not only for basic science, but also for the biomedicine of a wide range of birth defects and human genetic syndromes. This review summarizes the current knowledge regarding LR patterning in a number of vertebrate and invertebrate species, discusses several poorly understood but important phenomena, and highlights some important open questions about the evolutionary origin and conservation of mechanisms underlying embryonic asymmetry.

Nodal stability determines signaling range

Secreted TGFbeta proteins of the Nodal family pattern the vertebrate body axes and induce mesoderm and endoderm . Nodal proteins can act as morphogens , but the mechanisms regulating their activity and signaling range are poorly understood. In particular, it has been unclear how inefficient processing or rapid turnover of the Nodal protein influences autocrine and paracrine signaling properties . Here, we evaluate the role of Nodal processing and stability in tissue culture and zebrafish embryos. Removal of the pro domain potentiates autocrine signaling but reduces Nodal stability and signaling range. Insertion of an N-glycosylation site present in several related TGFbeta proteins increases the stability of mature Nodal. The stabilized form of Nodal acts at a longer range than the wild-type form. These results suggest that increased proteolytic maturation of Nodal potentiates autocrine signaling, whereas increased Nodal stability extends paracrine signaling.

Developmental mechanism and evolutionary origin of vertebrate left/right asymmetries

The systematically 'handed', or directionally asymmetrical way in which the major viscera are packed within the vertebrate body is known as situs. Other less obvious vertebrate lateralisations concern cognitive neural function, and include the human phenomena of hand-use preference and language-associated cognitive partitioning. An overview, rather than an exhaustive scholarly review, is given of recent advances in molecular understanding of the mechanism that ensures normal development of 'correct' situs. While the asymmetry itself and its left/right direction are clearly vertebrate-conserved characters, data available from various embryo types are compared in order to assess the likelihood that the developmental mechanism is evolutionarily conserved in its entirety. A conserved post-gastrular 'phylotypic' stage, with left- and right-specific cascades of key, orthologous gene expressions, clearly exists. It now seems probable that earlier steps, in which symmetry-breaking information is reliably transduced to trigger these cascades on the correct sides, are also conserved at depth although it remains unclear exactly how these steps operate. Earlier data indicated that the initiation of symmetry-breaking had been transformed, among the different vertebrate classes, as drastically as has the anatomy of pre-gastrular development itself, but it now seems more likely that this apparent diversity is deceptive. Ideas concerning the functional advantages to the vertebrate lifestyle of a systematically asymmetrical visceral packing arrangement, while untestable, are accepted because they form a plausible adaptationist 'just-so' story. Nevertheless, two contrasting beliefs are possible about the evolutionary origins of situs. Major recent advances in analysis of its developmental mechanism are largely due not to zoologists, comparative anatomists or evolutionary systematists, but to molecular geneticists, and these workers have generally assumed that the asymmetry is an evolutionary novelty imposed on a true bilateral symmetry, at or close to the origin of the vertebrate clade. A major purpose of this review is to advocate an alternative view, on the grounds of comparative anatomy and molecular systematics together with the comparative study of expressions of orthologous genes in different forms. This view is that situs represents a co-optation of a pre-existing, evolutionarily ancient non-bilaterality of the adult form in a vertebrate ancestor. Viewed this way, vertebrate or chordate origins are best understood as the novel imposition of an adaptively bilateral locomotory-skeletal-neural system, around a retained non-symmetrical 'visceral' animal. One component of neuro-anatomical asymmetry, the habenular/parapineal one that originates in the diencephalon, has recently been found (in teleosts) to be initiated from the same 'phylotypic' gene cascade that controls situs development. But the function of this particular diencephalic asymmetry is currently unclear. Other left-right partitionings of brain function, including the much more recently evolved, cerebral cortically located one associated with human language and hand-use, may be controlled entirely separately from situs even though their directionality has a particular relation to it in a majority of individuals. Finally, possible relationships are discussed between the vertebrate directional asymmetries and those that occur sporadically among protostome bilaterian forms. These may have very different evolutionary and molecular bases, such that there may have been constraints, in protostome evolution, upon any exploitation of left and right for complex organismic, and particularly cognitive neural function.

Plasticity of Extended Subsites Facilitates Divergent Substrate Recognition by Kex2 and Furin

Yeast Kex2 and human furin are subtilisin-related proprotein convertases that function in the late secretory pathway and exhibit similar though distinguishable patterns of substrate recognition. Although both enzymes prefer Arg at P(1) and basic residues at P(2), the two differ in recognition of P(4) and P(6) residues. To probe P(4) and P(6) recognition by Kex2p, furin-like substitutions were made in the putative S(4) and S(6) subsites of Kex2. T252D and Q283E mutations were introduced to increase the preference for Arg at P(4) and P(6), respectively. Glu(255) was replaced with Ile to limit recognition of P(4) Arg. The effects of putative S(4) and S(6) mutations were determined by examining the cleavage by purified mutant enzymes of a series of fluorogenic substrates with systematic changes in P(4) and/or P(6). Whereas wild Kex2 exhibited little preference type for Arg at P(6), the T252D mutant and T252D/Q283E double mutant exhibited clear interactions with P(6) Arg. Moreover, the T252D and T252D/Q283E substitutions altered the influence of the P(6) residue on P(4) recognition. We infer that cross-talk between S(4) and S(6), not seen in furin, allows wild type and mutant forms of Kex2 to adapt their subsites for altered modes of recognition. This apparent plasticity may allow the subsites to rearrange their local environment to interact with different substrates in a productive manner. E255I-Kex2 exhibited significantly decreased recognition of P(4) Arg in a tetrapeptide substrate with Lys at P(1), although the general pattern of selectivity for aliphatic residues at P(4) remained unchanged.

BMP receptor IA is required in the mammalian embryo for endodermal morphogenesis and ectodermal patterning

BMPRIA is a receptor for bone morphogenetic proteins with high affinity for BMP2 and BMP4. Mouse embryos lacking Bmpr1a fail to gastrulate, complicating studies on the requirements for BMP signaling in germ layer development. Recent work shows that BMP4 produced in extraembryonic tissues initiates gastrulation. Here we use a conditional allele of Bmpr1a to remove BMPRIA only in the epiblast, which gives rise to all embryonic tissues. Resulting embryos are mosaics composed primarily of cells homozygous null for Bmpr1a, interspersed with heterozygous cells. Although mesoderm and endoderm do not form in Bmpr1a null embryos, these tissues are present in the mosaics and are populated with mutant cells. Thus, BMPRIA signaling in the epiblast does not restrict cells to or from any of the germ layers. Cells lacking Bmpr1a also contribute to surface ectoderm; however, from the hindbrain forward, little surface ectoderm forms and the forebrain is enlarged and convoluted. Prechordal plate, early definitive endoderm, and anterior visceral endoderm appear to be expanded, likely due to defective morphogenesis. These data suggest that the enlarged forebrain is caused in part by increased exposure of the ectoderm to signaling sources that promote anterior neural fate. Our results reveal critical roles for BMP signaling in endodermal morphogenesis and ectodermal patterning.

Breaking symmetry: a clinical overview of left-right patterning

It is increasingly recognized that mutations in genes and pathways critical for left-right (L-R) patterning are involved in common isolated congenital malformations such as congenital heart disease, biliary tract anomalies, renal polycystic disease, and malrotation of the intestine, indicating that disorders of L-R development are far more common than a 1 in 10,000 incidence of heterotaxia might suggest. Understanding L-R patterning disorders requires knowledge of molecular biology, embryology, pediatrics, and internal medicine and is relevant to day-to-day clinical genetics practice. We have reviewed data from mammalian (human and mouse) L-R patterning disorders to provide a clinically oriented perspective that might afford the clinician or researcher additional insights into this diagnostically challenging area.

Nodal signaling in vertebrate development

TGFss signals belonging to the Nodal family set up the embryonic axes, induce mesoderm and endoderm, pattern the nervous system, and determine left-right asymmetry in vertebrates. Nodal signaling activates a canonical TGFss pathway involving activin receptors, Smad2 transcription factors, and FoxH1 coactivators. In addition, Nodal signaling is dependent on coreceptors of the EGF-CFC family and antagonized by the Lefty and Cerberus families of secreted factors. Additional modulators of Nodal signaling include convertases that regulate the generation of the mature signal, and factors such as Arkadia and DRAP1 that regulate the cellular responses to the signal. Complex regulatory cascades and autoregulatory loops coordinate Nodal signaling during early development. Nodals have concentration-dependent roles and can act both locally and at a distance. These studies demonstrate that Nodal signaling is modulated at almost every level to precisely orchestrate tissue patterning during vertebrate embryogenesis.

Genetics of human laterality disorders: insights from vertebrate model systems

Many internal organs in the vertebrate body are asymmetrically oriented along the left-right (L-R) body axis. Organ asymmetry and some components of the molecular signaling pathways that direct L-R development are highly conserved among vertebrate species. Although individuals with full reversal of organ L-R asymmetry (situs inversus totalis) are healthy, significant morbidity and mortality is associated with perturbations in laterality that result in discordant orientation of organ systems and complex congenital heart defects. In humans and other vertebrates, genetic alterations of L-R signaling pathways can result in a wide spectrum of laterality defects. In this review we categorize laterality defects in humans, mice, and zebrafish into specific classes based on altered patterns of asymmetric gene expression, organ situs defects, and midline phenotypes. We suggest that this classification system provides a conceptual framework to help consolidate the disparate laterality phenotypes reported in humans and vertebrate model organisms, thereby refining our understanding of the genetics of L-R development. This approach helps suggest candidate genes and genetic pathways that might be perturbed in human laterality disorders and improves diagnostic criteria.

Notch activity induces Nodal expression and mediates the establishment of left-right asymmetry in vertebrate embryos

Left-sided expression of Nodal in the lateral plate mesoderm is a conserved feature necessary for the establishment of normal left-right asymmetry during vertebrate embryogenesis. By using gain- and loss-of-function experiments in zebrafish and mouse, we show that the activity of the Notch pathway is necessary and sufficient for Nodal expression around the node, and for proper left-right determination. We identify Notch-responsive elements in the Nodal promoter, and unveil a direct relationship between Notch activity and Nodal expression around the node. Our findings provide evidence for a mechanism involving Notch activity that translates an initial symmetry-breaking event into asymmetric gene expression.

Notch signaling regulates left-right asymmetry determination by inducing Nodal expression

Generation of left-right asymmetry is an integral part of the establishment of the vertebrate body plan. Here we show that the Notch signaling pathway plays a primary role in the establishment of left-right asymmetry in mice by directly regulating expression of the Nodal gene. Embryos mutant for the Notch ligand Dll1 or doubly mutant for the Notch1 and Notch2 receptors exhibit multiple defects in left-right asymmetry. Analysis of the enhancer regulating node-specific Nodal expression revealed the presence of binding sites for the RBP-J protein, the primary transcriptional mediator of Notch signaling. Mutation of these sites destroyed the ability of this enhancer to direct node-specific gene expression in transgenic mice. Our results demonstrate that Dll1-mediated Notch signaling is essential for generation of left-right asymmetry, and that the Notch pathway acts upstream of Nodal expression during left-right asymmetry determination in mice.

Angiomotin regulates visceral endoderm movements during mouse embryogenesis

In pregastrula stage mouse embryos, visceral endoderm (VE) migrates from a distal to anterior position to initiate anterior identity in the adjacent epiblast. This anterior visceral endoderm (AVE) is then displaced away from the epiblast by the definitive endoderm to become associated with the extra-embryonic ectoderm and subsequently contributes to the yolk sac. Little is known about the molecules that regulate this proximal displacement. Here we describe a role for mouse angiomotin (amot) in VE movements. amot expression is initially detected in the AVE and subsequently in the VE associated with the extra-embryonic ectoderm. Most amot mutant mice die soon after gastrulation with distinct furrows of VE located at the junction of the embryonic and extra-embryonic regions. Mutant analysis suggests that VE accumulation in these furrows is caused by defects in cell migration into proximal extra-embryonic regions, although distal-to-anterior movements associated with the epiblast, definitive endoderm formation, and anterior specification of the epiblast appear to be normal. These results suggest that amot acts within subregions of the VE to regulate morphogenetic movements that are required for embryo viability.