A Vertebrate Homolog of the Cell Cycle Regulator Dbf4 Is an Inhibitor of Wnt Signaling Required for Heart Development

The Role of DACT Family Members in Tumorigenesis and Tumor Progression

Disheveled-associated antagonist of β-catenin (DACT), which ubiquitously expressed in human tissue, is critical for regulating cell proliferation and several developmental processes in different cellular contexts. In addition, DACT is essential for some other cellular processes, such as cell apoptosis, migration and differentiation. Given the importance of DACT in these cellular processes, many scientists are gradually interested in studying the role of DACT in tumorigenesis and cancer progression. This review article focuses on the latest research regarding the essential functions and potential DACT mechanisms in the occurrence and progression of tumors. Our study indicates that DACT may act as a tumor biomarker for cancer diagnosis and prognosis, as well as a promising therapeutic target in cancers.

Dact1 is expressed during chicken and mouse skeletal myogenesis and modulated in human muscle diseases

Vertebrate skeletal muscle development and repair relies on the precise control of Wnt signaling. Dact1 (Dapper/Frodo) is an important modulator of Wnt signaling, interacting with key components of the various Wnt transduction pathways. Here, we characterized Dact1 mRNA and protein expression in chicken and mouse fetal muscles in vivo and during the differentiation of chick primary and mouse C2C12 myoblasts in vitro. We also performed in silico analysis to investigate Dact1 gene expression in human myopathies, and evaluated the Dact1 protein structure to seek an explanation for the accumulation of Dact1 protein aggregates in the nuclei of myogenic cells. Our results show for the first time that in both chicken and mouse, Dact1 is expressed during myogenesis, with a strong upregulation as cells engage in terminal differentiation, cell cycle withdrawal and cell fusion. In humans, Dact1 expression was found to be altered in specific muscle pathologies, including muscular dystrophies. Our bioinformatic analyses of Dact1 proteins revealed long intrinsically disordered regions, which may underpin the ability of Dact1 to interact with its many partners in the various Wnt pathways. In addition, we found that Dact1 has strong propensity for liquid-liquid phase separation, a feature that explains its ability to form nuclear aggregates and points to a possible role as a molecular 'on'-'off' switch. Taken together, our data suggest Dact1 as a candidate, multi-faceted regulator of amniote myogenesis with a possible pathophysiological role in human muscular diseases.

Chk1 Inhibition of the Replication Factor Drf1 Guarantees Cell-Cycle Elongation at the Xenopus laevis Mid-blastula Transition

The early cell divisions of many metazoan embryos are rapid and occur in the near absence of transcription. At the mid-blastula transition (MBT), the cell cycle elongates and several processes become established including the onset of bulk transcription and cell-cycle checkpoints. How these events are timed and coordinated is poorly understood. Here we show in Xenopus laevis that developmental activation of the checkpoint kinase Chk1 at the MBT results in the SCF^β-TRCP-dependent degradation of a limiting replication initiation factor Drf1. Inhibition of Drf1 is the primary mechanism by which Chk1 blocks cell-cycle progression in the early embryo and is an essential function of Chk1 at the blastula-to-gastrula stage of development. This study defines the downregulation of Drf1 as an important mechanism to coordinate the lengthening of the cell cycle and subsequent developmental processes.

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Activation of Chk1 at the Xenopus MBT results in the degradation of Drf1

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Drf1 degradation is SCF^β-TRCP dependent

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Chk1 blocks the cell cycle in the early embryo through inhibition of Drf1

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Inhibition of Drf1 is an essential function of Chk1 during gastrulation

Conserved signaling mechanisms in Drosophila heart development

Signal transduction through multiple distinct pathways regulates and orchestrates the numerous biological processes comprising heart development. This review outlines the roles of the FGFR, EGFR, Wnt, BMP, Notch, Hedgehog, Slit/Robo, and other signaling pathways during four sequential phases of Drosophila cardiogenesis-mesoderm migration, cardiac mesoderm establishment, differentiation of the cardiac mesoderm into distinct cardiac cell types, and morphogenesis of the heart and its lumen based on the proper positioning and cell shape changes of these differentiated cardiac cells-and illustrates how these same cardiogenic roles are conserved in vertebrates. Mechanisms bringing about the regulation and combinatorial integration of these diverse signaling pathways in Drosophila are also described. This synopsis of our present state of knowledge of conserved signaling pathways in Drosophila cardiogenesis and the means by which it was acquired should facilitate our understanding of and investigations into related processes in vertebrates. Developmental Dynamics 246:641-656, 2017. © 2017 Wiley Periodicals, Inc.

Wnt4 is required for ostia development in the Drosophila heart

The Drosophila ostia are valve-like structures in the heart with functional similarity to vertebrate cardiac valves. The Wnt/β-catenin signaling pathway is critical for valve development in zebrafish and mouse, but the key ligand(s) for valve induction remains unclear. We observed high levels of Wnt4 gene expression in Drosophila ostia progenitor cells, immediately prior to morphological differentiation of these cells associated with ostia formation. This differentiation was blocked in Wnt4 mutants and in flies expressing canonical Wnt signaling pathway inhibitors but not inhibitors of the planar cell polarity pathway. High levels of Wnt4 dependent activation of a canonical Wnt signaling reporter was observed specifically in ostia progenitor cells. In vertebrate valve formation Wnt signaling is active in cells undergoing early endothelial-mesenchymal transition (EMT) and the Wnt9 homolog of Drosophila Wnt4 is expressed in valve progenitors. In demonstrating an essential role for Wnt4 in ostia development we have identified similarities between molecular and cellular events associated with early EMT during vertebrate valve development and the differentiation and partial delamination of ostia progenitor cells in the process of ostia formation.

The involvement of PCP proteins in radial cell intercalations during Xenopus embryonic development

The planar cell polarity (PCP) pathway orients cells in diverse epithelial tissues in Drosophila and vertebrate embryos and has been implicated in many human congenital defects and diseases, such as ciliopathies, polycystic kidney disease and malignant cancers. During vertebrate gastrulation and neurulation, PCP signaling is required for convergent extension movements, which are primarily driven by mediolateral cell intercalations, whereas the role for PCP signaling in radial cell intercalations has been unclear. In this study, we examine the function of the core PCP proteins Vangl2, Prickle3 (Pk3) and Disheveled in the ectodermal cells, which undergo radial intercalations during Xenopus gastrulation and neurulation. In the epidermis, multiciliated cell (MCC) progenitors originate in the inner layer, but subsequently migrate to the embryo surface during neurulation. We find that the Vangl2/Pk protein complexes are enriched at the apical domain of intercalating MCCs and are essential for the MCC intercalatory behavior. Addressing the underlying mechanism, we identified KIF13B, as a motor protein that binds Disheveled. KIF13B is required for MCC intercalation and acts synergistically with Vangl2 and Disheveled, indicating that it may mediate microtubule-dependent trafficking of PCP proteins necessary for cell shape regulation. In the neural plate, the Vangl2/Pk complexes were also concentrated near the outermost surface of deep layer cells, suggesting a general role for PCP in radial intercalation. Consistent with this hypothesis, the ectodermal tissues deficient in Vangl2 or Disheveled functions contained more cell layers than normal tissues. We propose that PCP signaling is essential for both mediolateral and radial cell intercalations during vertebrate morphogenesis. These expanded roles underscore the significance of vertebrate PCP proteins as factors contributing to a number of diseases, including neural tube defects, tumor metastases, and various genetic syndromes characterized by abnormal migratory cell behaviors.

Dact genes are chordate specific regulators at the intersection of Wnt and Tgf-β signaling pathways

Background

Dacts are multi-domain adaptor proteins. They have been implicated in Wnt and Tgfβ signaling and serve as a nodal point in regulating many cellular activities. Dact genes have so far only been identified in bony vertebrates. Also, the number of Dact genes in a given species, the number and roles of protein motifs and functional domains, and the overlap of gene expression domains are all not clear. To address these problems, we have taken an evolutionary approach, screening for Dact genes in the animal kingdom and establishing their phylogeny and the synteny of Dact loci. Furthermore, we performed a deep analysis of the various Dact protein motifs and compared the expression patterns of different Dacts.

Results

Our study identified previously not recognized dact genes and showed that they evolved late in the deuterostome lineage. In gnathostomes, four Dact genes were generated by the two rounds of whole genome duplication in the vertebrate ancestor, with Dact1/3 and Dact2/4, respectively, arising from the two genes generated during the first genome duplication. In actinopterygians, a further dact4r gene arose from retrotranscription. The third genome duplication in the teleost ancestor, and subsequent gene loss in most gnathostome lineages left extant species with a subset of Dact genes. The distribution of functional domains suggests that the ancestral Dact function lied with Wnt signaling, and a role in Tgfβ signaling may have emerged with the Dact2/4 ancestor. Motif reduction, in particular in Dact4, suggests that this protein may counteract the function of the other Dacts. Dact genes were expressed in both distinct and overlapping domains, suggesting possible combinatorial function.

Conclusions

The gnathostome Dact gene family comprises four members, derived from a chordate-specific ancestor. The ability to control Wnt signaling seems to be part of the ancestral repertoire of Dact functions, while the ability to inhibit Tgfβ signaling and to carry out specialized, ortholog-specific roles may have evolved later. The complement of Dact genes coexpressed in a tissue provides a complex way to fine-tune Wnt and Tgfβ signaling. Our work provides the basis for future structural and functional studies aimed at unraveling intracellular regulatory networks.

SEC14 and spectrin domains 1 (Sestd1) and Dapper antagonist of catenin 1 (Dact1) scaffold proteins cooperatively regulate the Van Gogh-like 2 (Vangl2) four-pass transmembrane protein and planar cell polarity (PCP) pathway during embryonic development in mice

The planar cell polarity (PCP) pathway is a conserved non-canonical (β-catenin-independent) branch of Wnt signaling crucial to embryogenesis, during which it regulates cell polarity and polarized cell movements. Disruption of PCP components in mice, including Vangl2 and Dact1, results in defective neural tube closure and other developmental defects. Here, we show that Sestd1 is a novel binding partner of Vangl2 and Dact1. The Sestd1-Dact1 interface is formed by circumscribed regions of Sestd1 (the carboxyl-terminal region) and Dact1 (the amino-terminal region). Remarkably, we show that loss of Sestd1 precisely phenocopies loss of Dact1 during embryogenesis in mice, leading to a spectrum of birth malformations, including neural tube defects, a shortened and/or curly tail, no genital tubercle, blind-ended colons, hydronephrotic kidneys, and no bladder. Moreover, as with Dact1, a knock-out mutation at the Sestd1 locus exhibits reciprocal genetic rescue interactions during development with a semidominant mutation at the Vangl2 locus. Consistent with this, examination of Wnt pathway activities in Sestd1 mutant mouse embryonic tissue reveals disrupted PCP pathway biochemistry similar to that characterized in Dact1 mutant embryos. The Sestd1 protein is a divergent member of the Trio family of GTPase regulatory proteins that lacks a guanine nucleotide exchange factor domain. Nonetheless, in cell-based assays the Sestd1-Dact1 interaction can induce Rho GTPase activation. Together, our data indicate that Sestd1 cooperates with Dact1 in Vangl2 regulation and in the PCP pathway during mammalian embryonic development.

All Dact (Dapper/Frodo) scaffold proteins dimerize and exhibit conserved interactions with Vangl, Dvl, and serine/threonine kinases

Background

The Dact family of scaffold proteins was discovered by virtue of binding to Dvl proteins central to Wnt and Planar Cell Polarity (PCP) signaling. Subsequently Dact proteins have been linked to a growing list of potential partners implicated in β-catenin-dependent and β-catenin-independent forms of Wnt and other signaling. To clarify conserved and non-conserved roles for this protein family, we systematically compared molecular interactions of all three murine Dact paralogs by co-immunoprecipitation of proteins recombinantly expressed in cultured human embryonic kidney cells.

Results

Every Dact paralog readily formed complexes with the Vangl, Dvl, and CK1δ/ε proteins of species ranging from fruit flies to humans, as well as with PKA and PKC. Dact proteins also formed complexes with themselves and with each other; their conserved N-terminal leucine-zipper domains, which have no known binding partners, were necessary and sufficient for this interaction, suggesting that it reflects leucine-zipper-mediated homo- and hetero-dimerization. We also found weaker, though conserved, interactions of all three Dact paralogs with the catenin superfamily member p120ctn. Complex formation with other previously proposed partners including most other catenins, GSK3, LEF/TCF, HDAC1, and TGFβ receptors was paralog-specific, comparatively weak, and/or more sensitive to empirical conditions.

Conclusions

Combined with published functional evidence from targeted knock-out mice, these data support a conserved role for Dact proteins in kinase-regulated biochemistry involving Vangl and Dvl. This strongly suggests that a principal role for all Dact family members is in the PCP pathway or a molecularly related signaling cascade in vertebrates.

Signaling pathways in early cardiac development

Cardiomyocyte differentiation is a complex multistep process requiring the proper temporal and spatial integration of multiple signaling pathways. Previous embryological and genetic studies have identified a number of signaling pathways that are critical to mediate the initial formation of the mesoderm and its allocation to the cardiomyocyte lineage. It has become clear that some of these signaling networks work autonomously, in differentiating myocardial cells whereas others work non-autonomously, in neighboring tissues, to regulate cardiac differentiation indirectly. Here, we provide an overview of three signaling networks that mediate cardiomyocyte specification and review recent insights into their specific roles in heart development. In addition, we demonstrate how systems level, 'omic approaches' and other high-throughput techniques such as small molecules screens are beginning to impact our understanding of cardiomyocyte specification and, to identify novel signaling pathways involved in this process. In particular, it now seems clear that at least one chemokine receptor CXCR4 is an important marker for cardiomyocyte progenitors and may play a functional role in their differentiation. Finally, we discuss some gaps in our current understanding of early lineage selection that could be addressed by various types of omic analysis.

The multiple phases and faces of wnt signaling during cardiac differentiation and development

Understanding heart development on a molecular level is a prerequisite for uncovering the causes of congenital heart diseases. Therapeutic approaches that try to enhance cardiac regeneration or that involve the differentiation of resident cardiac progenitor cells or patient-specific induced pluripotent stem cells will also benefit tremendously from this knowledge. Wnt proteins have been shown to play multiple roles during cardiac differentiation and development. They are extracellular growth factors that activate different intracellular signaling branches. Here, we summarize our current understanding of how these factors affect different aspects of cardiogenesis, starting from early specification of cardiac progenitors and continuing on to later developmental steps, such as morphogenetic processes, valve formation, and establishment of the conduction system.

Dact1 is a postsynaptic protein required for dendrite, spine, and excitatory synapse development in the mouse forebrain

Dact1 (Dapper/Frodo), an intracellular phosphoprotein that binds Dishevelled, catenins, and other signaling proteins, is expressed in the developing and mature mammalian CNS, but its function there is unknown. Dact1 colocalized with synaptic markers and partitioned to postsynaptic fractions from cultured mouse forebrain neurons. Hippocampal neurons from Dact1 knock-out mice had simpler dendritic arbors and fewer spines than hippocampal neurons from wild-type littermates. This correlated with reductions in excitatory synapses and miniature EPSCs, whereas inhibitory synapses were not affected. Loss of Dact1 resulted in a decrease in activated Rac, and recombinant expression of either Dact1 or constitutively active Rac, but not Rho or Cdc42, rescued dendrite and spine phenotypes in Dact1 mutant neurons. Our findings suggest that, during neuronal differentiation, Dact1 plays a critical role in a molecular pathway promoting Rac activity underlying the elaboration of dendrites and the establishment of spines and excitatory synapses.

Loss of Dact1 disrupts planar cell polarity signaling by altering dishevelled activity and leads to posterior malformation in mice

Wnt signaling plays a key role in embryogenesis and cancer development. Dvl (Dishevelled) is a central mediator for both the canonical and noncanonical Wnt pathways. Dact1 (Dapper1, Dpr1), a Dvl interactor, has been shown to negatively modulate Wnt signaling by promoting lysosomal degradation of Dvl. Here we report that Dact1-deficient mice have multiple physiological defects that resemble the human neonate disease congenital caudal regression syndrome, including caudal vertebrae agenesis, anorectal malformation, renal agenesis/dysplasia, fused kidneys, and loss of bladder. These urogenital defects can be traced to impaired hindgut formation starting at embryonic day 8.25. Examination of morphological changes and Wnt target gene expression revealed that the planar cell polarity (PCP) signaling is deregulated, whereas the canonical Wnt/beta-catenin pathway is largely unaffected in mutant embryos. Consistently, the activity of the PCP signal mediators Rho GTPase and c-Jun N-terminal kinase is altered in Dact1(-/-) mouse embryonic fibroblasts. We further observed alterations in the protein level and the cellular distribution of Dvl in the primitive streak of mutant embryos. An increased amount of Dvl2 tends to be accumulated in the cortical regions of the cells, especially at the primitive streak ectoderm close to the posterior endoderm that lately forms the hindgut diverticulum. Together, these data suggest that Dact1 may regulate vertebrate PCP by controlling the level and the cellular localization of Dvl protein.

Strabismus regulates asymmetric cell divisions and cell fate determination in the mouse brain

The planar cell polarity (PCP) pathway organizes the cytoskeleton and polarizes cells within embryonic tissue. We investigate the relationship between PCP signaling and cell fate determination during asymmetric division of neural progenitors (NPs) in mouse embryos. The cortex of Lp/Lp (Loop-tail) mice deficient in the essential PCP mediator Vangl2, homologue of Drosophila melanogaster Strabismus (Stbm), revealed precocious differentiation of neural progenitors into early-born neurons at the expense of late-born neurons and glia. Although Lp/Lp NPs were easily maintained in vitro, they showed premature differentiation and loss of asymmetric distribution of Leu-Gly-Asn–enriched protein (LGN)/partner of inscuteable (Pins), a regulator of mitotic spindle orientation. Furthermore, we observed a decreased frequency in asymmetric distribution of the LGN target nuclear mitotic apparatus protein (NuMa) in Lp/Lp cortical progenitors in vivo. This was accompanied by an increase in the number of vertical cleavage planes typically associated with equal daughter cell identities. These findings suggest that Stbm/Vangl2 functions to maintain cortical progenitors and regulates mitotic spindle orientation during asymmetric divisions in the vertebrate brain.

Dapper1 is a nucleocytoplasmic shuttling protein that negatively modulates Wnt signaling in the nucleus

Wnt signaling, via the activation of the canonical beta-catenin and lymphoid enhancer factor (LEF)/T-cell factor pathway, plays an important role in embryogenesis and cancer development by regulating the expression of genes involved in cell proliferation, differentiation, and survival. Dapper (Dpr), as a Dishevelled interactor, has been suggested to modulate Wnt signaling by promoting Dishevelled degradation. Here, we provide evidence that Dpr1 shuttles between the cytoplasm and the nucleus. Although overexpressed Dpr1 was mainly found in the cytoplasm, endogenous Dpr1 was localized over the cell, and Wnt1 induced its nuclear export. Treatment with leptomycin B induced nuclear accumulation of both endogenous and overexpressed Dpr1. We further identified the nuclear localization signal and the nuclear export signal within Dpr1. Using reporter assay and in vivo zebrafish embryo assay, we demonstrated that the forced nuclearly localized Dpr1 possessed the ability to antagonize Wnt signaling. Dpr1 interacted with beta-catenin and LEF1 and disrupted their complex formation. Furthermore, Dpr1 could associate with histone deacetylase 1 (HDAC1) and enhance the LEF1-HDAC1 interaction. Together, our findings suggest that Dpr1 negatively modulates the basal activity of Wnt/beta-catenin signaling in the nucleus by keeping LEF1 in the repressive state. Thus, Dpr1 controls Wnt/beta-catenin signaling in both the cytoplasm and the nucleus.

Genetic factors in congenital heart malformation

Congenital heart disease is the commonest malformation in humans and contributes greatly to the burden of disease in infancy. Increasingly, developmental origins are also implicated in heart disease in adults. Significant advances have been made over the past decade in elucidating morphogenetic events of heart formation and their underlying molecular cascades, mostly in animal models. Clinical studies are increasingly successful in quantifying and unraveling genetic factors. This review focuses on recent progress made in understanding the genetic underpinnings of normal and abnormal heart formation and highlights the importance of understanding these mechanisms to improve patient management.

The contribution of Islet1-expressing splanchnic mesoderm cells to distinct branchiomeric muscles reveals significant heterogeneity in head muscle development

During embryogenesis, paraxial mesoderm cells contribute skeletal muscle progenitors, whereas cardiac progenitors originate in the lateral splanchnic mesoderm (SpM). Here we focus on a subset of the SpM that contributes to the anterior or secondary heart field (AHF/SHF), and lies adjacent to the cranial paraxial mesoderm (CPM), the precursors for the head musculature. Molecular analyses in chick embryos delineated the boundaries between the CPM, undifferentiated SpM progenitors of the AHF/SHF, and differentiating cardiac cells. We then revealed the regionalization of branchial arch mesoderm: CPM cells contribute to the proximal region of the myogenic core, which gives rise to the mandibular adductor muscle. SpM cells contribute to the myogenic cells in the distal region of the branchial arch that later form the intermandibular muscle. Gene expression analyses of these branchiomeric muscles in chick uncovered a distinct molecular signature for both CPM- and SpM-derived muscles. Islet1 (Isl1) is expressed in the SpM/AHF and branchial arch in both chick and mouse embryos. Lineage studies using Isl1-Cre mice revealed the significant contribution of Isl1(+) cells to ventral/distal branchiomeric (stylohyoid, mylohyoid and digastric) and laryngeal muscles. By contrast, the Isl1 lineage contributes to mastication muscles (masseter, pterygoid and temporalis) to a lesser extent, with virtually no contribution to intrinsic and extrinsic tongue muscles or extraocular muscles. In addition, in vivo activation of the Wnt/beta-catenin pathway in chick embryos resulted in marked inhibition of Isl1, whereas inhibition of this pathway increased Isl1 expression. Our findings demonstrate, for the first time, the contribution of Isl1(+) SpM cells to a subset of branchiomeric skeletal muscles.

Expression and downregulation of WNT signaling pathway genes in rhesus monkey oocytes and embryos

Mammalian WNT genes encode secreted glycoproteins that are conserved homologues of the Drosophila Wingless gene, which plays a crucial role in Drosophila development. Recently, WNT pathway signaling has been implicated in ovarian development, oogenesis, and early development. We sought to evaluate whether these genes may contribute to the formation of healthy human oocytes or embryos, and whether the expression of these genes could provide informative markers of human oocyte and embryo quality. To do this, we employed the primate embryo gene expression resource (PREGER; www.preger.org) to examine expression of mRNAs encoding 38 components of the WNT signaling pathway in rhesus monkey oocytes and embryos as a nonhuman primate model. We observed considerable conservation between rhesus monkey and mouse of expression of WNT, FZD, and effector gene mRNAs, and a generalized downregulation of genes encoding key components of the WNT signaling pathway during preimplantation development. Our results support a role for WNT signaling during oocyte growth or maturation, but not during preimplantation development. Additionally, we observed differences between in vitro cultured and in vivo developing blastocysts, indicating possible effects of culture on WNT signaling during the peri-implantation period.

The evolutionally conserved activity of Dapper2 in antagonizing TGF-beta signaling

Dapper1 and Dapper2, two divergent members of the Dapper family, have been suggested to modulate Wnt and TGF-beta/Nodal signaling in Xenopus and zebrafish. To get a better understanding of Dapper function in mammals, we have cloned the mouse ortholog of zebrafish Dapper2, mDpr2 and investigated its function in regulating TGF-beta signaling activity. Here, we showed that, like zebrafish Dapper2, overexpression of mDpr2 inhibited the TGF-beta-induced expression of the Smad-responsive reporters and targeted TGF-beta type I receptor ALK5 for degradation in mammalian cells. Overexpression of mDpr2 in the zebrafish embryos led to a decrease in expression of the mesoderm marker no tail and goosecoid at the shield stage and eye fusion later, implying that mDpr2 may have an intrinsic in vivo activity similar to fish Dapper2 activity. The expression of mDpr2 was detected throughout the epiblast around the onset of gastrulation and in somites, the neural tube and gut at later stages in mouse embryos, implying a role in early embryonic development. Our data indicate that the function of Dpr2 as a negative regulator of the TGF-beta/Nodal signal pathway is evolutionally conserved, at least in part, in fish and mammals.

Xenopus as a model system for vertebrate heart development

The African clawed frog, Xenopus laevis, is a valuable model system for studies of vertebrate heart development. In the following review, we describe a range of embryological and molecular methodologies that are used in Xenopus research and discuss key discoveries relating to heart development that have been made using this model system. We also discuss how the sequence of the Xenopus tropicalis genome provides a valuable tool for identification of orthologous genes and for identification of evolutionarily conserved promoter elements. Finally, both forward and reverse genetic approaches are currently being applied to Xenopus for the study of vertebrate heart development.

Three Dact gene family members are expressed during embryonic development and in the adult brains of mice

Members of the Dact protein family initially were identified through binding to Dishevelled (Dvl), a cytoplasmic protein central to Wnt signaling. During mouse development, Dact1 is detected in the presomitic mesoderm and somites during segmentation, in the limb bud mesenchyme and other mesoderm-derived tissues, and in the central nervous system (CNS). Dact2 expression is most prominent during organogenesis of the thymus, kidneys, and salivary glands, with much lower levels in the somites and in the developing CNS. Dact3, not previously described in any organism, is expressed in the ventral region of maturing somites, limb bud and branchial arch mesenchyme, and in the embryonic CNS; of the three paralogs, it is the most highly expressed in the adult cerebral cortex. These data are consistent with studies in other vertebrates showing that Dact paralogs have distinct signaling and developmental roles and suggest they may differentially contribute to postnatal brain physiology.

Dact1presomitic mesoderm expression oscillates in phase withAxin2in the somitogenesis clock of mice

During segmentation (somitogenesis) in vertebrate embryos, somites form in a rostral-to-caudal sequence according to a species-specific rhythm called the somitogenesis clock. The expression of genes participating in somitogenesis oscillates in the presomitic mesoderm (PSM) in time with this clock. We previously reported that the Dact1 gene (aka Dpr1/Frd1/ThyEx3), which encodes a Dishevelled-binding intracellular regulator of Wnt signaling, is prominently expressed in the PSM as well as in a caudal-rostral gradient across the somites of mouse embryos. This observation led us to examine whether Dact1 expression oscillates in the PSM. We have found that Dact1 PSM expression does indeed oscillate in time with the somitogenesis clock. Consistent with its known signaling functions and with the "clock and wavefront" model of signal regulation during somitogenesis, the oscillation of Dact1 occurs in phase with the Wnt signaling component Axin2, and out of phase with the Notch signaling component Lfng.

Mesoderm progenitor cells of common origin contribute to the head musculature and the cardiac outflow tract

During early embryogenesis, heart and skeletal muscle progenitor cells are thought to derive from distinct regions of the mesoderm (i.e. the lateral plate mesoderm and paraxial mesoderm, respectively). In the present study, we have employed both in vitro and in vivo experimental systems in the avian embryo to explore how mesoderm progenitors in the head differentiate into both heart and skeletal muscles. Using fate-mapping studies, gene expression analyses, and manipulation of signaling pathways in the chick embryo, we demonstrate that cells from the cranial paraxial mesoderm contribute to both myocardial and endocardial cell populations within the cardiac outflow tract. We further show that Bmp signaling affects the specification of mesoderm cells in the head: application of Bmp4, both in vitro and in vivo, induces cardiac differentiation in the cranial paraxial mesoderm and blocks the differentiation of skeletal muscle precursors in these cells. Our results demonstrate that cells within the cranial paraxial mesoderm play a vital role in cardiogenesis, as a new source of cardiac progenitors that populate the cardiac outflow tract in vivo. A deeper understanding of mesodermal lineage specification in the vertebrate head is expected to provide insights into the normal, as well as pathological, aspects of heart and craniofacial development.

Wnt signal transduction and the formation of the myocardium

Soon after fertilization, vertebrate embryos grow very rapidly. Thus, early in gestation, a sizeable yet underdeveloped organism requires circulating blood. This need dictates the early appearance of a contractile heart, which is the first functional organ in both the avian and mammalian embryo. The heart arises from paired mesodermal regions within the anterior half of the embryo. As development proceeds, these bilateral precardiac fields merge at the midline to give rise to the primary heart tube. How specific areas of nondifferentiated mesoderm organize into myocardial tissue has been a question that has long intrigued developmental biologists. In recent years, the regulation of Wnt signal transduction has been implicated as an important event that initiates cardiac development. While initial reports in Drosophila and the bird had implicated Wnt proteins as promoters of cardiac tissue formation, subsequent findings that the WNT inhibitors Dkk1 and crescent possess cardiac-inducing activities led to the contrary hypothesis that WNTs actively inhibit cardiogenesis. This seeming contradiction has been resolved, in part, by more recent information indicating that Wnts stimulate multiple signal transduction pathways. In this review, we will examine what is presently known about the importance of regulated Wnt activity for the formation of the heart and the development of the myocardium and discuss this information in context of the emerging complexity of Wnt signal transduction.

Xenopus CDC7/DRF1 complex is required for the initiation of DNA replication

The Cdc7 kinase is essential for the initiation of DNA replication in eukaryotes. Two regulatory subunits of the Xenopus Cdc7 kinase have been identified: XDbf4 and XDrf1. In this study we determined the expression pattern of XDbf4 and XDrf1 and examined their involvement in DNA replication. We show that XDrf1 expression is restricted to oogenesis and early embryos, whereas XDbf4 is expressed throughout development. Immunodepletion from Xenopus egg extracts indicated that both proteins are only found in complexes with XCdc7 and there is a 5-fold molar excess of the XCdc7/Drf1 over SCdc7/Dbf4 complexes. Both complexes exhibit kinase activity and are differentially phosphorylated during the cell cycle. Depletion of the XCdc7/Drf1 from egg extracts inhibited DNA replication, whereas depletion of XCdc7/Dbf4 had little effect. Chromatin binding studies indicated that XCdc7/Drf1 is required for pre-replication complex activation but not their assembly. XCdc7/Dbf4 complexes bound to the chromatin in two steps: the first step was independent of pre-replication complex assembly and the second step was dependent on pre-replication complex activation. By contrast, binding of XCdc7/Drf1 complexes was entirely dependent on pre-replication complex assembly. Finally, we present evidence that the association of the two complexes on the chromatin is not regulated by ATR checkpoint pathways that result from DNA replication blocks. These data suggest that Cdc7/Drf1 but not Cdc7/Dbf4 complexes support the initiation of DNA replication in Xenopus egg extracts and during early embryonic development.

Germ layers to organs: Using Xenopus to study “later” development

The amphibian embryo is a highly successful model system with great promise for organogenesis research. Since the late 1800s, amphibians have been employed to understand vertebrate development and since the 1950s, the African clawed frog Xenopus laevis has been the amphibian of choice. In the past two decades, Xenopus has led the way forward in, among other things, identifying transcription factors, gene regulatory networks and inter- and intracellular signaling pathways that control early development (from fertilization through gastrulation and neurulation). Perhaps the best measure of how successful Xenopus has been as a model for early mammalian development is the observation that much of the knowledge gleaned from Xenopus studies has subsequently directly translated to discoveries of similar mechanisms operating in mouse development. Despite this great success in early development, research on organogenesis in Xenopus has lagged behind the mouse. However, recent technical advances now make Xenopus amenable for studies on later development, including organogenesis. Here, we discuss why Xenopus is well suited for such research and, we believe, permits addressing questions that have been difficult to approach using other model systems. We also highlight how Xenopus researchers have already begun studying a number of major organs, pancreas, liver, kidney and heart, and suggest how Xenopus might contribute more to these areas in the near future.

Dual role of the Cdc7-regulatory protein Dbf4 during yeast meiosis

The Dbf4-dependent Cdc7 kinase (DDK) is essential for chromosome duplication in all eukaryotes, but was proposed to be dispensable for yeast pre-meiotic DNA replication. This discrepancy led us to investigate the role of the unstable Cdc7-regulatory protein Dbf4 in meiosis. We show that, when Dbf4 is depleted at the time of meiotic induction, cells enter the meiotic program but do not replicate their chromosomes. Surprisingly when Dbf4 is depleted after the initiation of DNA synthesis, S phase goes to completion, but most cells arrest before anaphase I. Deletion of the cohesin Rec8 suppresses this phenotype, suggesting a distinct role of DDK for meiotic chromosome segregation. As after Cdc5 depletion, a fraction of cells undergo a single equational division suggesting a failure to mono-orient sister kinetochores. Our results demonstrate that Dbf4 is essential for DNA replication during meiosis like in vegetative cells and provide evidence for an additional role in setting up the reductional division of meiosis I.

Regulation of Lethal giant larvae by Dishevelled

The establishment of polarity in many cell types depends on Lgl, the tumour suppressor product of lethal giant larvae, which is involved in basolateral protein targeting. The conserved complex of Par3, Par6 and atypical protein kinase C phosphorylates and inactivates Lgl at the apical surface; however, the signalling mechanisms that coordinate cell polarization in development are not well defined. Here we show that a vertebrate homologue of Lgl associates with Dishevelled, an essential mediator of Wnt signalling, and that Dishevelled regulates the localization of Lgl in Xenopus ectoderm and Drosophila follicular epithelium. We show that both Lgl and Dsh are required for normal apical-basal polarity of Xenopus ectodermal cells. In addition, we show that the Wnt receptor Frizzled 8, but not Frizzled 7, causes Lgl to dissociate from the cortex with the concomitant loss of its activity in vivo. These findings suggest a molecular basis for the regulation of cell polarity by Frizzled and Dishevelled.

Frodo proteins: modulators of Wnt signaling in vertebrate development

The Frodo/dapper (Frd) proteins are recently discovered signaling adaptors, which functionally and physically interact with Wnt and Nodal signaling pathways during vertebrate development. The Frd1 and Frd2 genes are expressed in dynamic patterns in early embryos, frequently in cells undergoing epithelial-mesenchymal transition. The Frd proteins function in multiple developmental processes, including mesoderm and neural tissue specification, early morphogenetic cell movements, and organogenesis. Loss-of-function studies using morpholino antisense oligonucleotides demonstrate that the Frd proteins regulate Wnt signal transduction in a context-dependent manner and may be involved in Nodal signaling. The identification of Frd-associated factors and cellular targets of the Frd proteins should shed light on the molecular mechanisms underlying Frd functions in embryonic development and in cancer.