Regulation of Xenopus gastrulation by ErbB signaling

During Xenopus gastrulation, mesendodermal cells are internalized and display different movements. Head mesoderm migrates along the blastocoel roof, while trunk mesoderm undergoes convergent extension (C&E). Different signals are implicated in these processes. Our previous studies reveal that signals through ErbB receptor tyrosine kinases modulate Xenopus gastrulation, but the mechanisms employed are not understood. Here we report that ErbB signals control both C&E and head mesoderm migration. Inhibition of ErbB pathway blocks elongation of dorsal marginal zone explants and activin-treated animal caps without removing mesodermal gene expression. Bipolar cell shape and cell mixing in the dorsal region are impaired. Inhibition of ErbB signaling also interferes with migration of prechordal mesoderm on fibronectin. Cell-cell and cell-matrix interaction and cell spreading are reduced when ErbB signaling is blocked. Using antisense morpholino oligonucleotides, we show that ErbB4 is involved in Xenopus gastrulation morphogenesis, and it partially regulates cell movements through modulation of cell adhesion and membrane protrusions. Our results reveal for the first time that vertebrate ErbB signaling modulates gastrulation movements, thus providing a novel pathway, in addition to non-canonical Wnt and FGF signals, that controls gastrulation. We further demonstrate that regulation of cell adhesive properties and cell morphology may underlie the functions of ErbBs in gastrulation.     

Wnt/Frizzled signaling controls C. elegans gastrulation by activating actomyosin contractility

BACKGROUND: Embryonic patterning mechanisms regulate the cytoskeletal machinery that drives morphogenesis, but there are few cases where links between patterning mechanisms and morphogenesis are well understood. We have used a combination of genetics, in vivo imaging, and cell manipulations to identify such links in C. elegans gastrulation. Gastrulation in C. elegans begins with the internalization of endodermal precursor cells in a process that depends on apical constriction of ingressing cells. RESULTS: We show that ingression of the endodermal precursor cells is regulated by pathways, including a Wnt-Frizzled signaling pathway, that specify endodermal cell fate. We find that Wnt signaling has a role in gastrulation in addition to its earlier roles in regulating endodermal cell fate and cell-cycle timing. In the absence of Wnt signaling, endodermal precursor cells polarize and enrich myosin II apically but fail to contract their apical surfaces. We show that a regulatory myosin light chain normally becomes phosphorylated on the apical side of ingressing cells at a conserved site that can lead to myosin-filament formation and contraction of actomyosin networks and that this phosphorylation depends on Wnt signaling. CONCLUSIONS: We conclude that Wnt signaling regulates C. elegans gastrulation through regulatory myosin light-chain phosphorylation, which results in the contraction of the apical surface of ingressing cells. These findings forge new links between cell-fate specification and morphogenesis, and they represent a novel mechanism by which Wnt signaling can regulate morphogenesis.     

PI3K signaling in the regulation of branching morphogenesis

Branching morphogenesis drives the formation of epithelial organs including the mammary gland, lung, kidney, salivary gland and prostate. Branching at the cellular level also drives development of the nervous and vascular systems. A variety of signaling pathways are orchestrated together to establish the pattern of these branched organs. The phosphoinositide 3-kinase (PI3K) signaling network is of particular interest because of the diverse outcomes it generates, including proliferation, motility, growth, survival and cell death. Here, we focus on the role of the PI3K pathway in the development of branched tissues. Cultured cells, explants and transgenic mice have revealed that the PI3K pathway is critical for the regulation of cell proliferation, apoptosis and motility during branching of tissues.     

Dynamic decapentaplegic signaling regulates patterning and adhesion in the Drosophila pupal retina

The correct organization of cells within an epithelium is essential for proper tissue and organ morphogenesis. The role of Decapentaplegic/Bone morphogenetic protein (Dpp/BMP) signaling in cellular morphogenesis during epithelial development is poorly understood. In this paper, we used the developing Drosophila pupal retina--looking specifically at the reorganization of glial-like support cells that lie between the retinal ommatidia--to better understand the role of Dpp signaling during epithelial patterning. Our results indicate that Dpp pathway activity is tightly regulated across time in the pupal retina and that epithelial cells in this tissue require Dpp signaling to achieve their correct shape and position within the ommatidial hexagon. These results point to the Dpp pathway as a third component and functional link between two adhesion systems, Hibris-Roughest and DE-cadherin. A balanced interplay between these three systems is essential for epithelial patterning during morphogenesis of the pupal retina. Importantly, we identify a similar functional connection between Dpp activity and DE-cadherin and Rho1 during cell fate determination in the wing, suggesting a broader link between Dpp function and junctional integrity during epithelial development.     

Planar Cell Polarity Signaling in Collective Cell Movements During Morphogenesis and Disease

Collective and directed cell movements are crucial for diverse developmental processes in the animal kingdom, but they are also involved in wound repair and disease. During these processes groups of cells are oriented within the tissue plane, which is referred to as planar cell polarity (PCP). This requires a tight regulation that is in part conducted by the PCP pathway. Although this pathway was initially characterized in flies, subsequent studies in vertebrates revealed a set of conserved core factors but also effector molecules and signal modulators, which build the fundamental PCP machinery. The PCP pathway in Drosophila regulates several developmental processes involving collective cell movements such as border cell migration during oogenesis, ommatidial rotation during eye development, and embryonic dorsal closure. During vertebrate embryogenesis, PCP signaling also controls collective and directed cell movements including convergent extension during gastrulation, neural tube closure, neural crest cell migration, or heart morphogenesis. Similarly, PCP signaling is linked to processes such as wound repair, and cancer invasion and metastasis in adults. As a consequence, disruption of PCP signaling leads to pathological conditions. In this review, we will summarize recent findings about the role of PCP signaling in collective cell movements in flies and vertebrates. In addition, we will focus on how studies in Drosophila have been relevant to our understanding of the PCP molecular machinery and will describe several developmental defects and human disorders in which PCP signaling is compromised. Therefore, new discoveries about the contribution of this pathway to collective cell movements could provide new potential diagnostic and therapeutic targets for these disorders.     

Wnt signaling in adult intestinal stem cells and cancer

Signaling initiated by secreted glycoproteins of the Wnt family regulates many aspects of embryonic development and it is involved in homeostasis of adult tissues. In the gastrointestinal (GI) tract the Wnt pathway maintains the self-renewal capacity of epithelial stem cells. The stem cell attributes are conferred by mutual interactions of the stem cell with its local microenvironment, the stem cell niche. The niche ensures that the threshold of Wnt signaling in the stem cell is kept in physiological range. In addition, the Wnt pathway involves various feedback loops that balance the opposing processes of cell proliferation and differentiation. Today, we have compelling evidence that mutations causing aberrant activation of the Wnt pathway promote expansion of undifferentiated progenitors and lead to cancer.The review summarizes recent advances in characterization of adult epithelial stem cells in the gut. We mainly focus on discoveries related to molecular mechanisms regulating the output of the Wnt pathway. Moreover, we present novel experimental approaches utilized to investigate the epithelial cell signaling circuitry in vivo and in vitro. Pivotal aspects of tissue homeostasis are often deduced from studies of tumor cells; therefore, we also discuss some latest results gleaned from the deep genome sequencing studies of human carcinomas of the colon and rectum.     

FAK is required for tension-dependent organization of collective cell movements in Xenopus mesendoderm

Collective cell movements are integral to biological processes such as embryonic development and wound healing and also have a prominent role in some metastatic cancers. In migrating Xenopus mesendoderm, traction forces are generated by cells through integrin-based adhesions and tension transmitted across cadherin adhesions. This is accompanied by assembly of a mechanoresponsive cadherin adhesion complex containing keratin intermediate filaments and the catenin-family member plakoglobin. We demonstrate that focal adhesion kinase (FAK), a major component of integrin adhesion complexes, is required for normal morphogenesis at gastrulation, closure of the anterior neural tube, axial elongation and somitogenesis. Depletion of zygotically expressed FAK results in disruption of mesendoderm tissue polarity similar to that observed when expression of keratin or plakoglobin is inhibited. Both individual and collective migrations of mesendoderm cells from FAK depleted embryos are slowed, cell protrusions are disordered, and cell spreading and traction forces are decreased. Additionally, keratin filaments fail to organize at the rear of cells in the tissue and association of plakoglobin with cadherin is diminished. These findings suggest that FAK is required for the tension-dependent assembly of the cadherin adhesion complex that guides collective mesendoderm migration, perhaps by modulating the dynamic balance of substrate traction forces and cell cohesion needed to establish cell polarity.     

Hedgehog signaling regulates expansion of pancreatic epithelial cells

Embryonic Hedgehog signaling is essential for proper tissue morphogenesis and organ formation along the developing gastrointestinal tract. Hedgehog ligands are expressed throughout the endodermal epithelium at early embryonic stages but excluded from the region that will form the pancreas. Ectopic activation of Hedgehog signaling at the onset of pancreas development has been shown to inhibit organ morphogenesis. In contrast, Hedgehog signaling components are found within pancreatic tissue during subsequent stages of development as well as in the mature organ, indicating that a certain level of pathway activation is required for normal organ development and function. Here, we ectopically activate the Hedgehog pathway midway through pancreas development via expression of either Sonic (Shh) or Indian Hedgehog (Ihh) under control of the human Pax4-promoter. Similar pancreatic defects are observed in both Pax4-Shh and Pax4-Ihh transgenic lines, suggesting that regulation of the overall level of Hedgehog activity is critical for proper pancreas development. We also show that Hedgehog signaling controls mesenchymal vs. epithelial tissue differentiation and that pathway activation impairs formation of epithelial progenitors. Thus, tight control of Hedgehog pathway activity throughout embryonic development ensures proper pancreas organogenesis.     

Planar cell polarity signaling in vertebrates

Planar cell polarity (PCP) refers to the polarization of a field of cells within the plane of a cell sheet. This form of polarization is required for diverse cellular processes in vertebrates, including convergent extension (CE), the establishment of PCP in epithelial tissues and ciliogenesis. Perhaps the most distinct example of vertebrate PCP is the uniform orientation of stereociliary bundles at the apices of sensory hair cells in the mammalian auditory sensory organ. The establishment of PCP in the mammalian cochlea occurs concurrently with CE in this ciliated epithelium, therefore linking three cellular processes regulated by the vertebrate PCP pathway in the same tissue and emerging as a model system for dissecting PCP signaling. This review summarizes the morphogenesis of this model system to assist the interpretation of the emerging data and proposes molecular mechanisms underlying PCP signaling in vertebrates.     

Collective Epithelial and Mesenchymal Cell Migration During Gastrulation

Gastrulation, the process that puts the three major germlayers, the ectoderm, mesoderm and endoderm in their correct topological position in the developing embryo, is characterised by extensive highly organised collective cell migration of epithelial and mesenchymal cells. We discuss current knowledge and insights in the mechanisms controlling these cell behaviours during gastrulation in the chick embryo. We discuss several ideas that have been proposed to explain the observed large scale vortex movements of epithelial cells in the epiblast during formation of the primitive streak. We review current insights in the control and execution of the epithelial to mesenchymal transition (EMT) underlying the formation of the hypoblast and the ingression of the mesendoderm cells through the streak. We discuss the mechanisms by which the mesendoderm cells move, the nature and dynamics of the signals that guide these movements, as well as the interplay between signalling and movement that result in tissue patterning and morphogenesis. We argue that instructive cell-cell signaling and directed chemotactic movement responses to these signals are instrumental in the execution of all phases of gastrulation.     

The Rap GTPase activator Drosophila PDZ-GEF regulates cell shape in epithelial migration and morphogenesis

Epithelial morphogenesis is characterized by an exquisite control of cell shape and position. Progression through dorsal closure in Drosophila gastrulation depends on the ability of Rap1 GTPase to signal through the adherens junctional multidomain protein Canoe. Here, we provide genetic evidence that epithelial Rap activation and Canoe effector usage are conferred by the Drosophila PDZ-GEF (dPDZ-GEF) exchange factor. We demonstrate that dPDZ-GEF/Rap/Canoe signaling modulates cell shape and apicolateral cell constriction in embryonic and wing disc epithelia. In dPDZ-GEF mutant embryos with strong dorsal closure defects, cells in the lateral ectoderm fail to properly elongate. Postembryonic dPDZ-GEF mutant cells generated in mosaic tissue display a striking extension of lateral cell perimeters in the proximity of junctional complexes, suggesting a loss of normal cell contractility. Furthermore, our data indicate that dPDZ-GEF signaling is linked to myosin II function. Both dPDZ-GEF and cno show strong genetic interactions with the myosin II-encoding gene, and myosin II distribution is severely perturbed in epithelia of both mutants. These findings provide the first insight into the molecular machinery targeted by Rap signaling to modulate epithelial plasticity. We propose that dPDZ-GEF-dependent signaling functions as a rheostat linking Rap activity to the regulation of cell shape in epithelial morphogenesis at different developmental stages.     

The cell adhesion-associated protein Git2 regulates morphogenetic movements during zebrafish embryonic development

Signaling through cell adhesion complexes plays a critical role in coordinating cytoskeletal remodeling necessary for efficient cell migration. During embryonic development, normal morphogenesis depends on a series of concerted cell movements; but the roles of cell adhesion signaling during these movements are poorly understood. The transparent zebrafish embryo provides an excellent system to study cell migration during development. Here, we have identified zebrafish git2a and git2b, two new members of the GIT family of genes that encode ArfGAP proteins associated with cell adhesions. Loss-of-function studies revealed an essential role for Git2a in zebrafish cell movements during gastrulation. Time-lapse microscopy analysis demonstrated that antisense depletion of Git2a greatly reduced or arrested cell migration towards the vegetal pole of the embryo. These defects were rescued by expression of chicken GIT2, indicating a specific and conserved role for Git2 in controlling embryonic cell movements. Git2a knockdown embryos showed defects in cell morphology that were associated with reduced cell contractility. We show that Git2a is required for phosphorylation of myosin light chain (MLC), which regulates myosin II-mediated cell contractility. Consistent with this, embryos treated with Blebbistatin-a small molecule inhibitor for myosin II activity-exhibited cell movement defects similar to git2a knockdown embryos. These observations provide in vivo evidence of a physiologic role for Git2a in regulating cell morphogenesis and directed cell migration via myosin II activation during zebrafish embryonic development.     

Apical Accumulation of the Sevenless Receptor Tyrosine Kinase During Drosophila Eye Development Is Promoted by the Small GTPase Rap1

The Ras/MAPK-signaling pathway plays pivotal roles during development of metazoans by controlling cell proliferation and cell differentiation elicited, in several instances, by receptor tyrosine kinases (RTKs). While the internal mechanism of RTK-driven Ras/MAPK signaling is well understood, far less is known regarding its interplay with other corequired signaling events involved in developmental decisions. In a genetic screen designed to identify new regulators of RTK/Ras/MAPK signaling during Drosophila eye development, we identified the small GTPase Rap1, PDZ-GEF, and Canoe as components contributing to Ras/MAPK-mediated R7 cell differentiation. Rap1 signaling has recently been found to participate in assembling cadherin-based adherens junctions in various fly epithelial tissues. Here, we show that Rap1 activity is required for the integrity of the apical domains of developing photoreceptor cells and that reduced Rap1 signaling hampers the apical accumulation of the Sevenless RTK in presumptive R7 cells. It thus appears that, in addition to its role in cell–cell adhesion, Rap1 signaling controls the partitioning of the epithelial cell membrane, which in turn influences signaling events that rely on apico-basal cell polarity.     

The regulation of Notch signaling in muscle stem cell activation and postnatal myogenesis

The Notch signaling pathway is an evolutionarily conserved pathway that is critical for tissue morphogenesis during development, but is also involved in tissue maintenance and repair in the adult. In skeletal muscle, regulation of Notch signaling is involved in somitogenesis, muscle development, and the proliferation and cell fate determination of muscle stems cells during regeneration. During each of these processes, the spatial and temporal control of Notch signaling is essential for proper tissue formation. That control is mediated by a series of regulatory proteins and protein complexes that enhance or inhibit Notch signaling by regulating protein processing, localization, activity, and stability. In this review, we focus on the regulation of Notch signaling during postnatal muscle regeneration when muscle stem cells ("satellite cells") must activate, proliferate, progress along a myogenic lineage pathway, and ultimately differentiate to form new muscle. We review the regulators of Notch signaling, such as Numb and Deltex, that have documented roles in myogenesis as well as other regulators that may play a role in modulating Notch signaling during satellite cell activation and postnatal myogenesis.     

ROP GTPase-mediated auxin signaling regulates pavement cell interdigitation in Arabidopsis thaliana

In multicellular plant organs, cell shape formation depends on molecular switches to transduce developmental or environmental signals and to coordinate cell‐to‐cell communication. Plants have a specific subfamily of the Rho GT Pase family, usually called Rho of Plants(ROP), which serve as a critical signal transducer involved in many cellular processes. In the last decade, important advances in the ROP‐mediated regulation of plant cell morphogenesis have been made by using Arabidopsis thaliana leaf and cotyledon pavement cells.Especially, the auxin‐ROP signaling networks have been demonstrated to control interdigitated growth of pavement cells to form jigsaw‐puzzle shapes. Here, we review findings related to the discovery of this novel auxin‐signaling mechanism at the cell surface. This signaling pathway is to a large extent independent of the well‐known Transport Inhibitor Response(TIR)–Auxin Signaling F‐Box(AFB) pathway, and instead requires Auxin Binding Protein 1(ABP1) interaction with the plasma membrane‐localized, transmembrane kinase(TMK) receptor‐like kinase to regulate ROP proteins. Once activated, ROP influences cytoskeletal organization and inhibits endocytosis of the auxin transporter PIN1. The present review focuses on ROP signaling and its self‐organizing feature allowing ROP proteins to serve as a bustling signal decoder and integrator for plant cell morphogenesis.     

FGF signaling regulates cytoskeletal remodeling during epithelial morphogenesis

Changes in the cytoskeletal architecture underpin the dynamic changes in tissue shape that occur during development. It is clear that such changes must be coordinated so that individual cell behaviors are synchronized; however, the mechanisms by which morphogenesis is instructed and coordinated are unknown. After its induction in non-neural ectoderm, the inner ear undergoes morphogenesis, being transformed from a flat ectodermal disk on the surface of the embryo to a hollowed sphere embedded in the head. We provide evidence that this shape change relies on extrinsic signals subsequent to genetic specification. By using specific inhibitors, we find that local fibroblast growth factor (FGF) signaling triggers a phosphorylation cascade that activates basal myosin II through the activation of phospholipase Cgamma. Myosin II exhibits a noncanonical activity that results in the local depletion of actin filaments. Significantly, the resulting apical actin enrichment drives morphogenesis of the inner ear. Thus, FGF signaling directly exerts profound cytoskeletal effects on otic cells, coordinating the morphogenesis of the inner ear. The iteration of this morphogenetic signaling system suggests that it is a more generally applicable mechanism in other epithelial tissues undergoing shape change.     

Xenopus paraxial protocadherin has signaling functions and is involved in tissue separation

Protocadherins have homophilic adhesion properties and mediate selective cell-cell adhesion and cell sorting. Knockdown of paraxial protocadherin (PAPC) function in the Xenopus embryo impairs tissue separation, a process that regulates separation of cells of ectodermal and mesodermal origin during gastrulation. We show that PAPC can modulate the activity of the Rho GTPase and c-jun N-terminal kinase, two regulators of the cytoskeletal architecture and effectors of the planar cell polarity pathway. This novel signaling function of PAPC is essential for the regulation of tissue separation. In addition, PAPC can interact with the Xenopus Frizzled 7 receptor, and both proteins contribute to the development of separation behavior by activating Rho and protein kinase Calpha.     

Hedgehog signaling: progenitor phenotype in small-cell lung cancer

Recently, we have shown that small cell lung cancer (SCLC) is dependent on activation of Hedgehog signaling, an embryonic pathway implicated in development, morphogenesis and the regulation of stem cell fates. These findings form the framework for an emerging view of cancer as a process of aberrant organogenesis in which progenitor/ stem cells escape dependence on niche signaling through mutation in genes such as Ptch, or through persistent activation of progenitor cell pathways. Interestingly, the normally quiescent airway epithelial compartment uses the Hh pathway to repopulate itself when challenged by injury. How Hh signaling works to promote the malignant phenotype promises to be as important biologically as the promise of Hh pathway inhibitors are clinically.     

The mitochondrial-apoptotic pathway is triggered in Xenopus mesoderm cells deprived of PDGF receptor signaling during gastrulation

Platelet-derived growth factor receptor (PDGFR) signaling is required for normal gastrulation in Xenopus laevis. Embryos deprived of PDGFR signaling develop with a range of gastrulation-specific defects including spina bifida, shortened anteroposterior axis, and reduced anterior structures. These defects arise because the involuting mesoderm fails to move appropriately. In this study, we determine that inhibition of PDGFR signaling causes prospective head mesoderm cells to appear in the blastocoel cavity at the onset of gastrulation, stage 10. These aberrant cells undergo apoptosis via the caspase 3 pathway at an embryonic checkpoint called the early gastrula transition (EGT). They are TUNEL-positive and have increased levels of caspase 3 activity compared to control embryos. Apoptotic death of these mesoderm cells can be prevented by co-injection of mRNA encoding Bcl-2 or by injection of either a general caspase inhibitor or a caspase 3-specific inhibitor. Prevention of cell death, however, is not sufficient to rescue gastrulation defects in these embryos. Based on these data, we propose that PDGFR signaling is necessary for survival of prospective head mesoderm cells, and also plays an essential role in the control of their cell movement during gastrulation.