Wnt signaling and PKA control Nodal expression and left-right determination in the chick embryo

Gene expression profiling of skeletal myogenesis in human embryonic stem cells reveals a potential cascade of transcription factors regulating stages of myogenesis, including quiescent/activated satellite cell-like gene expression

Human embryonic stem cell (hESC)-derived skeletal muscle progenitors (SMP)—defined as PAX7-expressing cells with myogenic potential—can provide an abundant source of donor material for muscle stem cell therapy. As in vitro myogenesis is decoupled from in vivo timing and 3D-embryo structure, it is important to characterize what stage or type of muscle is modeled in culture. Here, gene expression profiling is analyzed in hESCs over a 50 day skeletal myogenesis protocol and compared to datasets of other hESC-derived skeletal muscle and adult murine satellite cells. Furthermore, day 2 cultures differentiated with high or lower concentrations of CHIR99021, a GSK3A/GSK3B inhibitor, were contrasted. Expression profiling of the 50 day time course identified successively expressed gene subsets involved in mesoderm/paraxial mesoderm induction, somitogenesis, and skeletal muscle commitment/formation which could be regulated by a putative cascade of transcription factors. Initiating differentiation with higher CHIR99021 concentrations significantly increased expression of MSGN1 and TGFB-superfamily genes, notably NODAL, resulting in enhanced paraxial mesoderm and reduced ectoderm/neuronal gene expression. Comparison to adult satellite cells revealed that genes expressed in 50-day cultures correlated better with those expressed by quiescent or early activated satellite cells, which have the greatest therapeutic potential. Day 50 cultures were similar to other hESC-derived skeletal muscle and both expressed known and novel SMP surface proteins. Overall, a putative cascade of transcription factors has been identified which regulates four stages of myogenesis. Subsets of these factors were upregulated by high CHIR99021 or their binding sites were significantly over-represented during SMP activation, ranging from quiescent to late-activated stages. This analysis serves as a resource to further study the progression of in vitro skeletal myogenesis and could be mined to identify novel markers of pluripotent-derived SMPs or regulatory transcription/growth factors. Finally, 50-day hESC-derived SMPs appear similar to quiescent/early activated satellite cells, suggesting they possess therapeutic potential.

Divergent early mesoderm specification underlies distinct head and trunk muscle programmes in vertebrates

Head and trunk muscles have discrete embryological origins and are governed by distinct regulatory programmes. Whereas the developmental route of trunk muscles from mesoderm is well studied, that of head muscles is ill defined. Here, we show that, unlike the myogenic trunk paraxial mesoderm, head mesoderm development is independent of the T/Tbx6 network in mouse. We reveal that, in contrast to Wnt and FGF-driven trunk mesoderm, dual inhibition of Wnt/β-catenin and Nodal specifies head mesoderm. Remarkably, the progenitors derived from embryonic stem cells by dual inhibition efficiently differentiate into cardiac and skeletal muscle cells. This twin potential is the defining feature of cardiopharyngeal mesoderm: the head subtype giving rise to heart and branchiomeric head muscles. Therefore, our findings provide compelling evidence that dual inhibition specifies head mesoderm and unravel the mechanism that diversifies head and trunk muscle programmes during early mesoderm fate commitment. Significantly, this is the first report of directed differentiation of pluripotent stem cells, without transgenes, into progenitors with muscle/heart dual potential. Ability to generate branchiomeric muscle in vitro could catalyse efforts in modelling myopathies that selectively involve head muscles.

Enhanced Wnt Signalling in Hepatocytes is Associated with Schistosoma japonicum Infection and Contributes to Liver Fibrosis

Liver fibrosis is the most serious pathology caused by Schistosoma japonicum infection, which arises when schistosome eggs are deposited in the liver. Eosinophils, macrophages and hepatic stellate cells (HSCs) have been identified as major cellular contributors to the development of granulomas and fibrosis, but little is known about the effects of hepatocytes on granuloma formation. Here, we found that the levels of Wnt signalling-related molecules, transforming growth factor β (TGF-β) and connective tissue growth factor (CTGF) in hepatocytes were markedly elevated after S. japonicum infection. Liver fibrosis was exacerbated when exogenous Wnt3a was introduced, but was alleviated when Wnt signalling was suppressed by DKK1, accompanied by the reduced expression of TGF-β and CTGF in hepatocytes. These results indicate that the hepatocytic expression of TGF-β and CTGF is mediated by Wnt signalling. Additionally, the hepatocytes isolated from infected mice promoted the activation of primary HSCs in vitro, however, this effect was not observed when hepatocytes from DKK1 treated S. japonicum-infected mice was employed in the co-culture system. Our findings identify a novel pro-fibrogenic role of hepatocytes in schistosomiasis-induced liver fibrosis that is dependent on Wnt signalling, which may serve as a potential target for ameliorating hepatic fibrosis caused by helminths.

Wnts talking with the TGF-β superfamily: WISPers about modulation of osteoarthritis

The Wnt signalling pathway is gaining increasing attention in the field of joint pathologies, attributable to its role in the development and homeostasis of the tissues found in the joint, including bone and cartilage. Imbalance in this pathway has been implicated in the development and progression of OA, and interference with the pathway might therefore depict an effective treatment strategy. Though offering multiple opportunities, it is yet to be decided which starting point will bring forth the most promising results. The complexity of the pathway and its interaction with other pathways (such as the TGF-β signalling pathway, which also has a central role in the maintenance of joint homeostasis) means that acting directly on proteins in this signalling cascade entails a high risk of undesired side effects. Therefore, interference with Wnt-induced proteins, such as WISP1, might be an overall more effective and safer therapeutic approach to inhibit the pathological events that take place during OA.

Wnt11b is involved in cilia-mediated symmetry breakage during Xenopus left-right development

Breakage of bilateral symmetry in amphibian embryos depends on the development of a ciliated epithelium at the gastrocoel roof during early neurulation. Motile cilia at the gastrocoel roof plate (GRP) give rise to leftward flow of extracellular fluids. Flow is required for asymmetric gene expression and organ morphogenesis. Wnt signaling has previously been involved in two steps, Wnt/ß-catenin mediated induction of Foxj1, a regulator of motile cilia, and Wnt/planar cell polarity (PCP) dependent cilia polarization to the posterior pole of cells. We have studied Wnt11b in the context of laterality determination, as this ligand was reported to activate canonical and non-canonical Wnt signaling. Wnt11b was found to be expressed in the so-called superficial mesoderm (SM), from which the GRP derives. Surprisingly, Foxj1 was only marginally affected in loss-of-function experiments, indicating that another ligand acts in this early step of laterality specification. Wnt11b was required, however, for polarization of GRP cilia and GRP morphogenesis, in line with the known function of Wnt/PCP in cilia-driven leftward flow. In addition Xnr1 and Coco expression in the lateral-most GRP cells, which sense flow and generate the first asymmetric signal, was attenuated in morphants, involving Wnt signaling in yet another process related to symmetry breakage in Xenopus.

The protocadherin 11X/Y (PCDH11X/Y) gene pair as determinant of cerebral asymmetry in modern Homo sapiens

Annett's right-shift theory proposes that human cerebral dominance (the functional and anatomical asymmetry or torque along the antero-posterior axis) and handedness are determined by a single “right-shift” gene. Familial transmission of handedness and specific deviations of cerebral dominance in sex chromosome aneuploidies implicate a locus within an X–Y homologous region of the sex chromosomes. The Xq21.3/Yp11.2 human-specific region of homology includes the protocadherin 11X/Y (PCDH11X/Y) gene pair, which encode cell adhesion molecules subject to accelerated evolution following the separation of the human and chimpanzee lineages six million years ago. PCDH11X and PCDH11Y, differentially regulated by retinoic acid, are highly expressed in the ventricular zone, subplate, and cortical plate of the developing cerebral cortex. Both proteins interact with β-catenin, a protein that plays a role in determining axis formation and regulating cortical size. In this way, the PCDH11X/Y gene pair determines cerebral asymmetry by initiating the right shift in Homo sapiens.

Wnt/β-catenin signaling is hyperactivated in systemic sclerosis and induces Smad-dependent fibrotic responses in mesenchymal cells

OBJECTIVE

Fibrosis in human diseases and animal models is associated with aberrant Wnt/β-catenin pathway activation. The aim of this study was to characterize the regulation, activity, mechanism of action, and significance of Wnt/β-catenin signaling in the context of systemic sclerosis (SSc).

METHODS

The expression of Wnt signaling pathway components in SSc skin biopsy specimens was analyzed. The regulation of profibrotic responses by canonical Wnt/β-catenin was examined in explanted human mesenchymal cells. Fibrotic responses were studied using proliferation, migration, and gel contraction assays. The cell fate specification of subcutaneous preadipocytes by canonical Wnt signaling was evaluated.

RESULTS

Analysis of published genome-wide expression data revealed elevated expression of the Wnt receptor FZD2 and the Wnt target LEF1 and decreased expression of Wnt antagonists DKK2 and WIF1 in skin biopsy specimens from subsets of patients with diffuse cutaneous SSc compared to the other distinct subsets. Immunohistochemical analysis showed increased nuclear β-catenin expression in these biopsy specimens. In vitro, Wnt-3a induced β-catenin activation, stimulated fibroblast proliferation and migration, collagen gel contraction, and myofibroblast differentiation, and enhanced profibrotic gene expression. Genetic and pharmacologic approaches were used to demonstrate that these profibrotic responses involved autocrine transforming growth factor β signaling via Smads. In contrast, in explanted subcutaneous preadipocytes, Wnt-3a repressed adipogenesis and promoted myofibroblast differentiation.

CONCLUSION

Canonical Wnt signaling was hyperactivated in SSc skin biopsy specimens. In explanted mesenchymal cells, Wnt-3a stimulated fibrogenic responses while suppressing adipogenesis. Taken together, these results indicate that Wnts have potent profibrotic effects, and that canonical Wnt signaling plays an important role in the pathogenesis of fibrosis and lipoatrophy in SSc.

Bicc1 links the regulation of cAMP signaling in polycystic kidneys to microRNA-induced gene silencing

Genetic defects in autosomal-dominant polycystic kidney disease (ADPKD) promote cystic growth of renal tubules, at least in part by stimulating the accumulation of cAMP. How renal cAMP levels are regulated is incompletely understood. We show that cAMP and the expression of its synthetic enzyme adenylate cyclase-6 (AC6) are up-regulated in cystic kidneys of Bicc1(-)(/-) knockout mice. Bicc1, a protein comprising three K homology (KH) domains and a sterile alpha motif (SAM), is expressed in proximal tubules. The KH domains independently bind AC6 mRNA and recruit the miR-125a from Dicer, whereas the SAM domain enables silencing by Argonaute and TNRC6A/GW182. Bicc1 similarly induces silencing of the protein kinase inhibitor PKIα by miR-27a. Thus, Bicc1 is needed on these target mRNAs for silencing by specific miRNAs. The repression of AC6 by Bicc1 might explain why cysts in ADPKD patients preferentially arise from distal tubules.

β-Catenin 1 and β-catenin 2 play similar and distinct roles in left-right asymmetric development of zebrafish embryos

β-Catenin-mediated canonical Wnt signaling has been found to be required for left-right (LR) asymmetric development. However, the implication of endogenous β-catenin in LR development has not been demonstrated by loss-of-function studies. In zebrafish embryos, two β-catenin genes, β-catenin 1 (ctnnb1) and β-catenin 2 (ctnnb2) are maternally expressed and their zygotic expression occurs in almost all types of tissues, including Kupffer's vesicle (KV), an essential organ that initiates LR development in teleost fish. We demonstrate here that morpholino-mediated knockdown of ctnnb1, ctnnb2, or both, in the whole embryo or specifically in dorsal forerunner cells (DFCs) interrupts normal asymmetry of the heart, liver and pancreas. Global knockdown of ctnnb2 destroys the midline physical and molecular barrier, while global knockdown of ctnnb1 impairs the formation of the midline molecular barrier. Depletion of either gene or both in DFCs/KV leads to poor KV cell proliferation, abnormal cilia formation and disordered KV fluid flow with downregulation of ntl and tbx16 expression. ctnnb1 and ctnnb2 in DFCs/KV differentially regulate the expression of charon, a Nodal antagonist, and spaw, a key Nodal gene for laterality development in zebrafish. Loss of ctnnb1 in DFCs/KV inhibits the expression of charon around KV and of spaw in the posterior lateral plate mesoderm, while ctnnb2 knockdown results in loss of spaw expression in the anterior lateral plate mesoderm with little alteration of charon expression. Taken together, our findings suggest that ctnnb1 and ctnnb2 regulate multiple processes of laterality development in zebrafish embryos through similar and distinct mechanisms.

Two mutations in human BICC1 resulting in Wnt pathway hyperactivity associated with cystic renal dysplasia

Bicaudal C homologue 1 (Bicc1) knockout in mice causes polycystic kidney disease and pancreas development defects, including a reduction in insulin-producing β-cells and ensuing diabetes. We therefore screened 137 patients with renal abnormalities or association of early-onset diabetes and renal disease for genetic alterations in BICC1. We identified two heterozygous mutations, one nonsense in the first K Homology (KH) domain and one missense in the sterile alpha motif (SAM) domain. In mice, Bicc1 blocks canonical Wnt signaling, mostly via its SAM domain. We show that the human BICC1, similar to its mouse counterpart, blocks canonical Wnt signaling. The nonsense mutation identified results in a complete loss of Wnt inhibitory activity. The point mutation in the SAM domain has a similar effect to a complete SAM domain deletion, resulting in a 22% loss of activity.

Role of Cripto-1 in stem cell maintenance and malignant progression

Cripto-1 is critical for early embryonic development and, together with its ligand Nodal, has been found to be associated with the undifferentiated status of mouse and human embryonic stem cells. Like other embryonic genes, Cripto-1 performs important roles in the formation and progression of several types of human tumors, stimulating cell proliferation, migration, epithelial to mesenchymal transition, and tumor angiogenesis. Several studies have demonstrated that cell fate regulation during embryonic development and cell transformation during oncogenesis share common signaling pathways, suggesting that uncontrolled activation of embryonic signaling pathways might drive cell transformation and tumor progression in adult tissues. Here we review our current understanding of how Cripto-1 controls stem cell biology and how it integrates with other major embryonic signaling pathways. Because many cancers are thought to derive from a subpopulation of cancer stem-like cells, which may re-express embryonic genes, Cripto-1 signaling may drive tumor growth through the generation or expansion of tumor initiating cells bearing stem-like characteristics. Therefore, the Cripto-1/Nodal signaling may represent an attractive target for treatment in cancer, leading to the elimination of undifferentiated stem-like tumor initiating cells.

Smad signaling in skeletal development and regeneration

Smad proteins are intracellular molecules that mediate the canonical signaling cascade of TGFbeta superfamily growth factors. The TGFbeta superfamily comprises two groups of growth factors, BMPs and TGFbetas. Both groups can be further divided into several sub-groups based on sequence homologies and functional similarities. Ligands of the TGFbeta superfamily bind to cell surface receptors to activate Smad proteins in the cytoplasm; then the activated Smad proteins translocate into the nucleus to activate or repress specific target gene transcription. Both groups of growth factors play important roles in skeletal development and regeneration. However, whether these effects reflect signaling through canonical Smad pathways, or other non-canonical signaling pathways in vivo remains a mystery. Moreover, the mechanisms utilized by Smad proteins to initiate nuclear events and their interactions with cytoplasmic proteins are still under intensive investigation. This review will discuss the most recent progress understanding Smad signaling in the context of skeletal development and regeneration.

Cerberus functions as a BMP agonist to synergistically induce nodal expression during left-right axis determination in the chick embryo

Left-sided expression of Nodal in the lateral plate mesoderm (LPM) during early embryogenesis is a crucial step in establishing the left-right (L-R) axis in vertebrates. In the chick, it was suggested that chick Cerberus (cCer), a Cerberus/Dan family member, induces Nodal expression by antagonizing bone morphogenetic protein (BMP) activity in the left LPM. In contrast, it has also been shown that BMPs positively regulate Nodal expression in the left LPM in the chick embryo. Thus, it is still unclear how the bilaterally expressed BMPs induce Nodal expression only in the left LPM. In this study, we demonstrate that BMP signaling is necessary and sufficient for the induction of Nodal expression in the chick LPM where the type I BMP receptor-IB (BMPR-IB) likely mediates this induction. Tissue grafting experiments indicate the existence of a Nodal inductive factor in the left LPM rather than the presence of a Nodal inhibitory factor in the right LPM. We demonstrate that cCer functions as a BMP agonist instead of antagonist, being able to enhance BMP signaling in cell culture. This conclusion is further supported by the immunoprecipitation assays that provide convincing biochemical evidence for a direct interaction between cCer and BMP receptor. Because cCer is expressed restrictedly in the left LPM, BMPs and cCer appear to act synergistically to activate Nodal expression in the left LPM in the chick.

Signaling cross-talk between TGF-beta/BMP and other pathways

Transforming growth factor-beta (TGF-beta)/bone morphogenic protein (BMP) signaling is involved in the vast majority of cellular processes and is fundamentally important during the entire life of all metazoans. Deregulation of TGF-beta/BMP activity almost invariably leads to developmental defects and/or diseases, including cancer. The proper functioning of the TGF-beta/BMP pathway depends on its constitutive and extensive communication with other signaling pathways, leading to synergistic or antagonistic effects and eventually desirable biological outcomes. The nature of such signaling cross-talk is overwhelmingly complex and highly context-dependent. Here we review the different modes of cross-talk between TGF-beta/BMP and the signaling pathways of Mitogen-activated protein kinase, phosphatidylinositol-3 kinase/Akt, Wnt, Hedgehog, Notch, and the interleukin/interferon-gamma/tumor necrosis factor-alpha cytokines, with an emphasis on the underlying molecular mechanisms.

Balancing segmentation and laterality during vertebrate development

Somites are the mesodermal segments of vertebrate embryos that become the vertebral column, skeletal muscle and dermis. Somites arise within the paraxial mesoderm by the periodic, bilaterally symmetric process of somitogenesis. However, specification of left-right asymmetry occurs in close spatial and temporal proximity to somitogenesis and involves some of the same cell signaling pathways that govern segmentation. Here, we review recent evidence that identifies cross-talk between these processes and that demonstrates a role for retinoic acid in maintaining symmetrical somitogenesis by preventing impingement of left-right patterning signals upon the paraxial mesoderm.

Left-right axis development: examples of similar and divergent strategies to generate asymmetric morphogenesis in chick and mouse embryos

Left-right asymmetry of internal organs is widely distributed in the animal kingdom. The chick and mouse embryos have served as important model organisms to analyze the mechanisms underlying the establishment of the left-right axis. In the chick embryo many genes have been found to be asymmetrically expressed in and around the node, while the same genes in the mouse show symmetric expression patterns. In the mouse there is strong evidence for an establishment of left-right asymmetry through nodal cilia. In contrast, in the chick and in many other organisms left-right asymmetry is probably generated by an early-acting event involving membrane depolarization. In both birds and mammals a conserved Nodal-Lefty-Pitx2 module exists that controls many aspects of asymmetric morphogenesis. This review also gives examples of divergent mechanisms of establishing asymmetric organ formation. Thus there is ample evidence for conserved and non-conserved strategies to generate asymmetry in birds and mammals.

Wnt/Axin1/beta-catenin signaling regulates asymmetric nodal activation, elaboration, and concordance of CNS asymmetries

Nodal activity in the left lateral plate mesoderm (LPM) is required to activate left-sided Nodal signaling in the epithalamic region of the zebrafish forebrain. Epithalamic Nodal signaling subsequently determines the laterality of neuroanatomical asymmetries. We show that overactivation of Wnt/Axin1/beta-catenin signaling during late gastrulation leads to bilateral epithalamic expression of Nodal pathway genes independently of LPM Nodal signaling. This is consistent with a model whereby epithalamic Nodal signaling is normally bilaterally repressed, with Nodal signaling from the LPM unilaterally alleviating repression. We suggest that Wnt signaling regulates the establishment of the bilateral repression. We identify a second role for the Wnt pathway in the left/right regulation of LPM Nodal pathway gene expression, and finally, we show that at later stages Axin1 is required for the elaboration of concordant neuroanatomical asymmetries.

Manipulations of PKA in chick limb development reveal roles in digit patterning including a positive role in Sonic Hedgehog signaling

Sonic Hedgehog (Shh) signaling by the polarizing region, at the posterior of the vertebrate limb bud, is pivotal in determining digit number and identity. Shh establishes a gradient of the bifunctional transcriptional effector, Gli3, with high levels of full-length activator (Gli3A) in the posterior bud, where digits form, and high levels of shorter repressor (Gli3R) in the anterior. Repressor formation depends on protein kinase A (PKA), but in Drosophila, PKA also plays a role in activator function. Increasing PKA levels in chick limb development using Forskolin had no effect on posterior polarizing activity but weak polarizing activity, based on ligand-independent Shh signaling, was induced in anterior limb bud cells resulting in extra digits. Manipulating PKA activity levels directly with a retrovirus expressing activated PKA induced extra digits similar to those induced by Forskolin treatment suggesting that PKA may have a previously unrecognized positive role in Shh signaling in vertebrate limbs. Expressing dominant negative PKA also induced extra, sometimes multiple digits, from anterior limb bud demonstrating the negative role in Shh signaling. PKA levels in the limb bud are high posteriorly and low anteriorly, suggesting that PKA activity may influence the outcome of Shh signaling in normal development.

Cripto-1: a multifunctional modulator during embryogenesis and oncogenesis

It is increasingly evident that genes known to perform critical roles during early embryogenesis, particularly during stem cell renewal, pluripotentiality and survival, are also expressed during the development of cancer. In this regard, oncogenesis may be considered as the recapitulation of embryogenesis in an inappropriate temporal and spatial manner. The epidermal growth factor-Cripto-1/FRL1/cryptic family of proteins consists of extracellular and cell-associated proteins that have been identified in several vertebrate species. During early embryogenesis, epidermal growth factor-Cripto-1/FRL1/cryptic proteins perform an obligatory role as coreceptors for the transforming growth factor-beta subfamily of proteins, which includes Nodal. Cripto-1 has also been shown to function as a ligand through a Nodal/Alk4-independent signaling pathway that involves binding to glypican-1 and the subsequent activation through src of phosphoinositol-3 kinase/Akt and ras/mitogen-activated protein kinase intracellular pathways. Expression of Cripto-1 is increased in several human cancers and its overexpression is associated with the development of mammary tumors in mice. Here, we review the role of Cripto-1 during embryogenesis, cell migration, invasion and angiogenesis and how these activities may relate to cellular transformation and tumorigenesis. We also briefly discuss evidence suggesting that Cripto-1 may be involved in stem cell maintenance.

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.

Retinoic acid signalling links left–right asymmetric patterning and bilaterally symmetric somitogenesis in the zebrafish embryo

During embryogenesis, cells are spatially patterned as a result of highly coordinated and stereotyped morphogenetic events. In the vertebrate embryo, information on laterality is conveyed to the node, and subsequently to the lateral plate mesoderm, by a complex cascade of epigenetic and genetic events, eventually leading to a left-right asymmetric body plan. At the same time, the paraxial mesoderm is patterned along the anterior-posterior axis in metameric units, or somites, in a bilaterally symmetric fashion. Here we characterize a cascade of laterality information in the zebrafish embryo and show that blocking the early steps of this cascade (before it reaches the lateral plate mesoderm) results in random left-right asymmetric somitogenesis. We also uncover a mechanism mediated by retinoic acid signalling that is crucial in buffering the influence of the flow of laterality information on the left-right progression of somite formation, and thus in ensuring bilaterally symmetric somitogenesis.

Unveiling the establishment of left-right asymmetry in the chick embryo

Vertebrates display striking left-right asymmetries in the placement of internal organs, which are concealed by a seemingly bilaterally symmetric body plan. The establishment of asymmetries about the left-right axis occurs early during embryo development and requires the concerted and sequential action of several epigenetic, genetic and cellular mechanisms. Experiments in the chick embryo model have contributed crucially to our current understanding of such mechanisms and are reviewed here. Particular emphasis is given to the elucidation of a genetic network that conveys left-right information from Hensen's node to the organ primordia, characterized to a significant degree of detail in the chick embryo. We also point out a number of early and late events in the determination of left-right asymmetries that are currently poorly understood and for whose study the chick embryo model presents several advantages. We anticipate that the availability of the chick genome sequence will be combined with multidisciplinary approaches from experimental embryology, biophysics, live-cell imaging, and mathematical modeling to boost up our knowledge of left-right organ asymmetry in the near future.

Cross-regulation of Wnt signaling and cell adhesion

The complex cross-regulation between Wnt signaling, cell-cell adhesion, and cell-matrix adhesion has revealed a number of regulatory components important in development and cancer progression. In the following, we would like to highlight and summarize some of the steps where pathways converge or diverge in regulating Wnt activity, matrix-induced pathways, and cell adhesion. We would like to focus on the involvement of heparan sulfate proteoglycan-rich proteins (HSPGs), integrin-mediated outside-in signaling, and cadherin-mediated cell-cell adhesion on Wnt pathways and the transcriptional regulation of extracellular matrix components and cell adhesion molecules by Wnt signaling.

The Opitz syndrome gene MID1 is essential for establishing asymmetric gene expression in Hensen's node

Patterning the avian left-right (L/R) body axis involves the establishment of asymmetric molecular signals on the left and right sides of Hensen's node. We have examined the role of the chick Midline 1 gene, cMid1, in generating asymmetric gene expression in the node. cMid1 is initially expressed bilaterally, but its expression is then confined to the right side of the node. We show that this restriction of cMid1 expression is a result of repression by Shh on the left side of the node. Misexpression of cMid1 on the left side of the node results in bilateral Bmp4 expression and a loss of Shh expression. Correspondingly, downstream left pathway genes are repressed while right pathway genes are ectopically activated. Conversely, knocking down endogenous right-sided cMid1 results in a loss of Bmp4 expression and bilateral Shh expression. This results in an absence of right pathway genes and the ectopic activation of the left pathway on the right. Here, we present a revised model for the establishment of asymmetric gene expression in Hensen's node based on the epistatic interactions observed between Shh, cMid1, and Bmp4.

Heart development: molecular insights into cardiac specification and early morphogenesis

The heart develops from two bilateral heart fields that are formed during early gastrulation. In recent years, signaling pathways that specify cardiac mesoderm have been extensively analyzed. In addition, a battery of transcription factors that regulate different aspects of cardiac morphogenesis and cytodifferentiation have been identified and characterized in model organisms. At the anterior pole, a secondary heart field is formed, which in its molecular make-up, appears to be similar to the primary heart field. The cardiac outflow tract and the right ventricle to a large extent are derivatives of this anterior heart field. Cardiac mesoderm receives positional information by which it is patterned along the three body axes. The molecular control of left-right axis development has received particular attention, and the underlying regulatory network begins to emerge. Cardiac chamber development involves the activation of a transcription program that is different from the one present in the primary heart field and regulates cardiac morphogenesis in a region-specific manner. This review also attempts to identify areas in which additional research is needed to fully understand early cardiac development.

Embryonic and larval staging of summer flounder (Paralichthys dentatus)

Early development of flatfishes such as the summer flounder Paralichthys dentatus (Pleuronectiformes) has not been extensively documented, largely because of a dearth of material; however, the recent expansion of flatfish aquaculture has made embryos of P. dentatus readily available for developmental studies. We divide development of P. dentatus embryos and larvae into two main periods, pre- and posthatching, and assign stages within each of those primary divisions. Stages from fertilization to hatching loosely follow the general teleost staging scheme suggested by Shardo ([1995] J Morphol 225:125-167); stages from hatching through metamorphosis are aligned with the series used for Japanese flounder, P. olivaceus (Minami [1982] Nippon Suisan Gakkaishi 48:1581-1588; Fukuhara [1986] Nippon Suisan Gakkaishi 52:81-91). Although length, width, and age may serve as approximate indicators of developmental progression in summer flounder, these characteristics are too variable to form the sole basis of a staging table. Therefore, we define stages by morphological criteria drawn from the development of the jaw apparatus and digestive system, eye migration, and notochord tip flexion. Examination of these morphological features in hatched larvae allows accurate and consistent assessment of developmental stage despite variation in timing and size. The staging scheme for flounder embryonic and larval development presented here should facilitate both experimental and comparative research on summer flounder and other flatfish species.

Inversin forms a complex with catenins and N-cadherin in polarized epithelial cells

Nephrogenesis starts with the reciprocal induction of two embryonically distinct analages, metanephric mesenchyme and ureteric bud. This complex process requires the refined and coordinated expression of numerous developmental genes, such as inv. Mice that are homozygous for a mutation in the inv gene (inv/inv) develop renal cysts resembling autosomal-recessive polycystic kidney disease. The gene locus containing inv has been proposed to serve as a common modifier for some human and rodent polycystic kidney disease phenotypes. We generated polyclonal antibodies to inversin to study its subcellular distribution, potential binding partners, and functional aspects in cultured murine proximal tubule cells. A 125-kDa inversin protein isoform was found at cell-cell junctions. Two inversin isoforms, 140- and 90-kDa, were identified in the nuclear and perinuclear compartments. Plasma membrane allocation of inversin is dependent upon cell-cell contacts and was redistributed when cell adhesion was disrupted after incubation of the cell monolayer with low-calcium/EGTA medium. We further show that the membrane-associated 125-kDa inversin forms a complex with N-cadherin and the catenins. The 90-kDa nuclear inversin complexes with beta-catenin. These findings indicate that the inv gene product functions in several cellular compartments, including the nucleus and cell-cell adhesion sites.

The other side of TGF-beta superfamily signal regulation: thinking outside the cell

The transforming growth factor beta (TGF-beta) superfamily of paracrine and autocrine signaling molecules regulates a vast array of developmental and homeostatic processes and is itself exquisitely regulated. The misregulation of these molecules often results in cancer and other diseases. Here, we focus on new research that explores how TGF-beta superfamily signaling is controlled between the secreting cell and the target cell. Regulation can occur upon ligand secretion (in a latent protein complex) and in the creation of signaling gradients. Proteins in the extracellular milieu sequester ligand away from or facilitate ligand binding to receptor serine kinases. Ligands even positively regulate their own negative regulators. Studies of how TGF-beta signaling is regulated extracellularly have broadened our understanding of TGF-beta pathways, and could provide clues to our understanding and treatment of diseases resulting from misregulation of these pathways.