Wnt/PCP signaling controls intracellular position of MTOCs during gastrulation convergence and extension movements

FOXO1 promotes the expression of canonical WNT target genes in examined basal-like breast and glioblastoma multiforme cancer cells

Basal-like breast cancer (BBC) and glioblastoma multiforme (GBM) are aggressive cancers associated with poor prognosis. BBC and GBM have stem cell-like gene expression signatures, which are in part driven by forkhead box O (FOXO) transcription factors. To gain further insight into the impact of FOXO1 in BBC, we treated BT549 cells with AS1842856 and performed RNA sequencing. AS1842856 binds to unphosphorylated FOXO1 and inhibits its ability to directly bind to DNA. Gene Set Enrichment Analysis indicated that a set of WNT pathway target genes, including lymphoid enhancer-binding factor 1 (LEF1) and transcription factor 7 (TCF7), were robustly induced after AS1842856 treatment. These same genes were also induced in GBM cell lines U87MG, LN18, LN229, A172, and DBTRG upon AS1842856 treatment. By contrast, follow-up RNA interference (RNAi) targeting of FOXO1 led to reduced LEF1 and TCF7 gene expression in BT549 and U87MG cells. In agreement with RNAi experiments, CRISPR Cas9-mediated FOXO1 disruption reduced the expression of canonical WNT genes LEF1 and TCF7 in U87MG cells. The loss of TCF7 gene expression in FOXO1 disruption mutants was restored by exogenous expression of the DNA-binding-deficient FOXO1-H215R. Therefore, FOXO1 induces TCF7 in a DNA-binding-independent manner, similar to other published FOXO1-activated genes such as TCF4 and hes family bHLH transcription factor 1. Our work demonstrates that FOXO1 promotes canonical WNT gene expression in examined BBC and GBM cells, similar to results found in Drosophila melanogaster, T-cell development, and murine acute myeloid leukemia models.

Cyclic stretching combined with cell-cell adhesion is sufficient for inducing cell intercalation

Although the important role of cell intercalation within a collective has long been recognized particularly for morphogenesis, the underlying mechanism remains poorly understood. Here we investigate the possibility that cellular responses to cyclic stretching play a major role in this process. By applying synchronized imaging and cyclic stretching to epithelial cells cultured on micropatterned polyacrylamide (PAA) substrates, we discovered that uniaxial cyclic stretching induces cell intercalation along with cell shape change and cell-cell interfacial remodeling. The process involved intermediate steps as previously reported for cell intercalation during embryonic morphogenesis, including the appearance of cell vertices, anisotropic vertex resolution, and directional expansion of cell-cell interface. Using mathematical modeling, we further found that cell shape change in conjunction with dynamic cell-cell adhesions was sufficient to account for the observations. Further investigation with small-molecule inhibitors indicated that disruption of myosin II activities suppressed cyclic stretching-induced intercalation while inhibiting the appearance of oriented vertices. Inhibition of Wnt signaling did not suppress stretch-induced cell shape change but disrupted cell intercalation and vertex resolution. Our results suggest that cyclic stretching, by inducing cell shape change and reorientation in the presence of dynamic cell-cell adhesions, can induce at least some aspects of cell intercalation and that this process is dependent in distinct ways on myosin II activities and Wnt signaling.

Wnt/planar cell polarity signaling controls morphogenetic movements of gastrulation and neural tube closure

Gastrulation and neurulation are successive morphogenetic processes that play key roles in shaping the basic embryonic body plan. Importantly, they operate through common cellular and molecular mechanisms to set up the three spatially organized germ layers and to close the neural tube. During gastrulation and neurulation, convergent extension movements driven by cell intercalation and oriented cell division generate major forces to narrow the germ layers along the mediolateral axis and elongate the embryo in the anteroposterior direction. Apical constriction also makes an important contribution to promote the formation of the blastopore and the bending of the neural plate. Planar cell polarity proteins are major regulators of asymmetric cell behaviors and critically involved in a wide variety of developmental processes, from gastrulation and neurulation to organogenesis. Mutations of planar cell polarity genes can lead to general defects in the morphogenesis of different organs and the co-existence of distinct congenital diseases, such as spina bifida, hearing deficits, kidney diseases, and limb elongation defects. This review outlines our current understanding of non-canonical Wnt signaling, commonly known as Wnt/planar cell polarity signaling, in regulating morphogenetic movements of gastrulation and neural tube closure during development and disease. It also attempts to identify unanswered questions that deserve further investigations.

Superresolution microscopy localizes endogenous Dvl2 to Wnt signaling-responsive biomolecular condensates

Wnt signaling governs cell fate and tissue polarity across species. The Dishevelled proteins are central to Wnt signaling cascades. Wnt-mediated multiprotein complexes such as the “signalosome” and the “destruction complex” have been proposed to represent biomolecular condensates. These nonmembranous, specialized compartments have been suggested to form through liquid–liquid phase separation and ensure correctly proceeding physiological reactions. Although biomolecular condensates have increasingly been studied, key questions remain regarding, for example, their architecture and physiological regulation. Here, superresolution microscopy after endogenous labeling of Dishevelled-2 gives insights into protein functions and Wnt signaling at physiological levels. It reveals the distinct molecular architecture of endogenous Wnt condensates at single-molecule resolution and illustrates close interactions at the centrosome.

During organismal development, homeostasis, and disease, Dishevelled (Dvl) proteins act as key signaling factors in beta-catenin–dependent and beta-catenin–independent Wnt pathways. While their importance for signal transmission has been genetically demonstrated in many organisms, our mechanistic understanding is still limited. Previous studies using overexpressed proteins showed Dvl localization to large, punctate-like cytoplasmic structures that are dependent on its DIX domain. To study Dvl’s role in Wnt signaling, we genome engineered an endogenously expressed Dvl2 protein tagged with an mEos3.2 fluorescent protein for superresolution imaging. First, we demonstrate the functionality and specificity of the fusion protein in beta-catenin–dependent and beta-catenin–independent signaling using multiple independent assays. We performed live-cell imaging of Dvl2 to analyze the dynamic formation of the supramolecular cytoplasmic Dvl2_mEos3.2 condensates. While overexpression of Dvl2_mEos3.2 mimics the previously reported formation of abundant large “puncta,” supramolecular condensate formation at physiological protein levels is only observed in a subset of cells with approximately one per cell. We show that, in these condensates, Dvl2 colocalizes with Wnt pathway components at gamma-tubulin and CEP164-positive centrosomal structures and that the localization of Dvl2 to these condensates is Wnt dependent. Single-molecule localization microscopy using photoactivated localization microscopy (PALM) of mEos3.2 in combination with DNA-PAINT demonstrates the organization and repetitive patterns of these condensates in a cell cycle–dependent manner. Our results indicate that the localization of Dvl2 in supramolecular condensates is coordinated dynamically and dependent on cell state and Wnt signaling levels. Our study highlights the formation of endogenous and physiologically regulated biomolecular condensates in the Wnt pathways at single-molecule resolution.

Alignment of the cell long axis by unidirectional tension acts cooperatively with Wnt signalling to establish planar cell polarity

Planar cell polarity (PCP) is the aligned cell polarity within a tissue plane. Mechanical signals are known to act as a global cue for PCP, yet their exact role is still unclear. In this study, we focused on PCP in the posterior neuroectoderm of Xenopus laevis and investigated how mechanical signals regulate polarity. We reveal that the neuroectoderm is under a greater tension in the anterior-posterior direction and that perturbation of this tension causes PCP disappearance. We show that application of uniaxial stretch to explant tissues can control the orientation of PCP and that cells sense the tissue stretch indirectly through a change in their shape, rather than directly through detection of anisotropic tension. Furthermore, we reveal that PCP is most strongly established when the orientation of tissue stretch coincides with that of diffusion of locally expressed Wnt ligands, suggesting a cooperative relationship between these two PCP regulators.

Local Wnt signalling in the asymmetric migrating vertebrate cells

Wnt signalling is known to generate cellular asymmetry via Wnt/planar cell polarity pathway (Wnt/PCP). Wnt/PCP acts locally (i) to orient membrane polarity and asymmetric establishment of intercellular junctions via conserved set of PCP proteins most specifically represented by Vangl and Prickle, and (ii) to asymmetrically rearrange cytoskeletal structures via downstream effectors of Dishevelled (Dvl). This process is best described on stable phenotypes of epithelial cells. Here, however, we review the activity of Wnt signalling in migratory cells which experience the extensive rearrangements of cytoskeleton and consequently dynamic asymmetry, making the localised effects of Wnt signalling easier to distinguish. Firstly, we focused on migration of neuronal axons, which allows to study how the pre-existent cellular asymmetry can influence Wnt signalling outcome. Then, we reviewed the role of Wnt signalling in models of mesenchymal migration including neural crest, melanoma, and breast cancer cells. Last, we collected evidence for local Wnt signalling in amoeboid cells, especially lymphocytes. As the outcome of this review, we identify blank spots in our current understanding of this topic, propose models that synthesise the current observations and allow formulation of testable hypotheses for the future research.

Glypican 4 regulates planar cell polarity of endoderm cells by controlling the localization of Cadherin 2

Noncanonical Wnt/planar cell polarity (Wnt/PCP) signaling has been implicated in endoderm morphogenesis. However, the underlying cellular and molecular mechanisms of this process are unclear. We found that, during convergence and extension (C&E) in zebrafish, gut endodermal cells are polarized mediolaterally, with GFP-Vangl2 enriched at the anterior edges. Endoderm cell polarity is lost and intercalation is impaired in the absence of glypican 4 (gpc4), a heparan-sulfate proteoglycan that promotes Wnt/PCP signaling, suggesting that this signaling is required for endodermal cell polarity. Live imaging revealed that endoderm C&E is accomplished by polarized cell protrusions and junction remodeling, which are impaired in gpc4-deficient endodermal cells. Furthermore, in the absence of gpc4, Cadherin 2 expression on the endodermal cell surface is increased as a result of impaired Rab5c-mediated endocytosis, which partially accounts for the endodermal defects in these mutants. These findings indicate that Gpc4 regulates endodermal planar cell polarity during endoderm C&E by influencing the localization of Cadherin 2. Thus, our study uncovers a new mechanism by which Gpc4 regulates planar cell polarity and reveals the role of Wnt/PCP signaling in endoderm morphogenesis.

Planar polarization of cilia in the zebrafish floor-plate involves Par3-mediated posterior localization of highly motile basal bodies

Epithelial cilia, whether motile or primary, often display an off-center planar localization within the apical cell surface. This form of planar cell polarity (PCP) involves the asymmetric positioning of the ciliary basal body (BB). Using the monociliated epithelium of the embryonic zebrafish floor-plate, we investigated the dynamics and mechanisms of BB polarization by live imaging. BBs were highly motile, making back-and-forth movements along the antero-posterior (AP) axis and contacting both the anterior and posterior membranes. Contacts exclusively occurred at junctional Par3 patches and were often preceded by membrane digitations extending towards the BB, suggesting focused cortical pulling forces. Accordingly, BBs and Par3 patches were linked by dynamic microtubules. Later, BBs became less motile and eventually settled at posterior apical junctions enriched in Par3. BB posterior positioning followed Par3 posterior enrichment and was impaired upon Par3 depletion or disorganization of Par3 patches. In the PCP mutant vangl2, BBs were still motile but displayed poorly oriented membrane contacts that correlated with Par3 patch fragmentation and lateral spreading. Thus, we propose an unexpected function for posterior Par3 enrichment in controlling BB positioning downstream of the PCP pathway.

The recycling endosome protein Rab25 coordinates collective cell movements in the zebrafish surface epithelium

In emerging epithelial tissues, cells undergo dramatic rearrangements to promote tissue shape changes. Dividing cells remain interconnected via transient cytokinetic bridges. Bridges are cleaved during abscission and currently, the consequences of disrupting abscission in developing epithelia are not well understood. We show that the Rab GTPase Rab25 localizes near cytokinetic midbodies and likely coordinates abscission through endomembrane trafficking in the epithelium of the zebrafish gastrula during epiboly. In maternal-zygotic Rab25a and Rab25b mutant embryos, morphogenic activity tears open persistent apical cytokinetic bridges that failed to undergo timely abscission. Cytokinesis defects result in anisotropic cell morphologies that are associated with a reduction of contractile actomyosin networks. This slows cell rearrangements and alters the viscoelastic responses of the tissue, all of which likely contribute to delayed epiboly. We present a model in which Rab25 trafficking coordinates cytokinetic bridge abscission and cortical actin density, impacting local cell shape changes and tissue-scale forces.

PCP and Wnt pathway components act in parallel during zebrafish mechanosensory hair cell orientation

Planar cell polarity (PCP) plays crucial roles in developmental processes such as gastrulation, neural tube closure and hearing. Wnt pathway mutants are often classified as PCP mutants due to similarities between their phenotypes. Here, we show that in the zebrafish lateral line, disruptions of the PCP and Wnt pathways have differential effects on hair cell orientations. While mutations in the PCP genes vangl2 and scrib cause random orientations of hair cells, mutations in wnt11f1, gpc4 and fzd7a/b induce hair cells to adopt a concentric pattern. This concentric pattern is not caused by defects in PCP but is due to misaligned support cells. The molecular basis of the support cell defect is unknown but we demonstrate that the PCP and Wnt pathways work in parallel to establish proper hair cell orientation. Consequently, hair cell orientation defects are not solely explained by defects in PCP signaling, and some hair cell phenotypes warrant re-evaluation.

Planar cell polarity (PCP) regulates hair cell orientation in the zebrafish lateral line. Here, the authors show that mutating Wnt pathway genes (wnt11f1, fzd7a/b, and gpc4) causes concentric hair cell patterns not regulated by PCP, thus showing PCP/Wnt pathway genes have different consequences on hair cell orientation.

Spatiotemporal characterization of dynamic epithelial filopodia during zebrafish epiboly

BACKGROUND

During zebrafish epiboly, the embryonic cell mass, or blastoderm, spreads to enclose the yolk cell. The blastoderm consists of an outer epithelial sheet, the enveloping layer (EVL), and the underlying deep cell layer (DEL). Studies have provided insights into the mechanisms of EVL and deep cell epiboly, but little is known about the interactions between the two cell layers and what role they may play during epiboly.

RESULTS

We used live imaging to examine EVL basal protrusions. We identified them as filopodia based on f-actin content and localization of fluorescently tagged filopodial markers. A spatiotemporal analysis revealed that the largest number of EVL filopodia were present during early epiboly at the animal pole. In functional studies, expression of a constitutively active actin-bundling protein resulted in increased filopodial length and delayed gastrulation.

CONCLUSIONS

We identified protrusions on the basal surface of EVL cells as filopodia and showed that they are present throughout the EVL during epiboly. The largest number of filopodia was at the animal pole during early epiboly, which is when and where deep cell radial intercalations occur to the greatest extent. These findings suggest that EVL filopodia may function during epiboly to promote deep cell rearrangements during epiboly initiation.

Vangl2 coordinates cell rearrangements during gut elongation

BACKGROUND

The embryonic gut tube undergoes extensive lengthening to generate the surface area required for nutrient absorption across the digestive epithelium. In Xenopus, narrowing and elongation of the tube is driven by radial rearrangements of its core of endoderm cells, a process that concomitantly opens the gut lumen and facilitates epithelial morphogenesis. How endoderm rearrangements are properly oriented and coordinated to achieve this complex morphogenetic outcome is unknown.

RESULTS

We find that, prior to gut elongation, the core Wnt/PCP component Vangl2 becomes enriched at both the anterior and apical aspects of individual endoderm cells. In Vangl2-depleted guts, the cells remain unpolarized, down-regulate cell-cell adhesion proteins, and, consequently, fail to rearrange, leading to a short gut with an occluded lumen and undifferentiated epithelium. In contrast, endoderm cells with ectopic Vangl2 protein acquire abnormal polarity and adhesive contacts. As a result, endoderm cells also fail to rearrange properly and undergo ectopic differentiation, resulting in guts with multiple torturous lumens, irregular epithelial architecture, and variable intestinal topologies.

CONCLUSIONS

Asymmetrical enrichment of Vangl2 in individual gut endoderm cells orients polarity and adhesion during radial rearrangements, coordinating digestive epithelial morphogenesis and lumen formation with gut tube elongation.

Microtubules are required for the maintenance of planar cell polarity in monociliated floorplate cells

The asymmetric localization of planar cell polarity (PCP) proteins is essential for the establishment of many planar polarized cellular processes, but the mechanisms that maintain these asymmetric distributions remain poorly understood. A body of evidence has tied oriented subapical microtubules (MTs) to the establishment of PCP protein polarity, yet recent studies have suggested that the MT cytoskeleton is later dispensable for the maintenance of this asymmetry. As MTs underlie the vesicular trafficking of membrane-bound proteins within cells, the requirement for MTs in the maintenance of PCP merited further investigation. We investigated the complex interactions between PCP proteins and the MT cytoskeleton in the polarized context of the floorplate of the zebrafish neural tube. We demonstrated that the progressive posterior polarization of the primary cilia of floorplate cells requires not only Vangl2 but also Fzd3a. We determined that GFP-Vangl2 asymmetrically localizes to anterior membranes whereas Fzd3a-GFP does not polarize on anterior or posterior membranes but maintains a cytosolic enrichment at the base of the primary cilium. Vesicular Fzd3a-GFP is rapidly trafficked along MTs primarily toward the apical membrane during a period of PCP maintenance, whereas vesicular GFP-Vangl2 is less frequently observed. Nocodazole-induced loss of MT polymerization disrupts basal body positioning as well as GFP-Vangl2 localization and reduces cytosolic Fzd3a-GFP movements. Removal of nocodazole after MT disruption restores MT polymerization but does not restore basal body polarity. Interestingly, GFP-Vangl2 repolarizes to anterior membranes and vesicular Fzd3a-GFP dynamics recover after multiple hours of recovery, even in the context of unpolarized basal bodies. Together our findings challenge previous work by revealing an ongoing role for MT-dependent transport of PCP proteins in maintaining both cellular and PCP protein asymmetry during development.

Biophysics in oviduct: Planar cell polarity, cilia, epithelial fold and tube morphogenesis, egg dynamics

Organs and tissues in multi-cellular organisms exhibit various morphologies. Tubular organs have multi-scale morphological features which are closely related to their functions. Here we discuss morphogenesis and the mechanical functions of the vertebrate oviduct in the female reproductive tract, also known as the fallopian tube. The oviduct functions to convey eggs from the ovary to the uterus. In the luminal side of the oviduct, the epithelium forms multiple folds (or ridges) well-aligned along the longitudinal direction of the tube. In the epithelial cells, cilia are formed orienting toward the downstream of the oviduct. The cilia and the folds are supposed to be involved in egg transportation. Planar cell polarity (PCP) is developed in the epithelium, and the disruption of the Celsr1 gene, a PCP related-gene, causes randomization of both cilia and fold orientations, discontinuity of the tube, inefficient egg transportation, and infertility. In this review article, we briefly introduce various biophysical and biomechanical issues in the oviduct, including physical mechanisms of formation of PCP and organized cilia orientation, epithelial cell shape regulation, fold pattern formation generated by mechanical buckling, tubulogenesis, and egg transportation regulated by fluid flow. We also mention about possible roles of the oviducts in egg shape formation and embryogenesis, sinuous patterns of tubes, and fold and tube patterns observed in other tubular organs such as the gut, airways, etc.

Prickle1 is required for EMT and migration of zebrafish cranial neural crest

The neural crest-a key innovation of the vertebrates-gives rise to diverse cell types including melanocytes, neurons and glia of the peripheral nervous system, and chondrocytes of the jaw and skull. Proper development of the cephalic region is dependent on the tightly-regulated specification and migration of cranial neural crest cells (NCCs). The core PCP proteins Frizzled and Disheveled have previously been implicated in NCC migration. Here we investigate the functions of the core PCP proteins Prickle1a and Prickle1b in zebrafish cranial NCC development. Using analysis of pk1a and pk1b mutant embryos, we uncover similar roles for both genes in facilitating cranial NCC migration. Disruption of either gene causes pre-migratory NCCs to cluster together at the dorsal aspect of the neural tube, where they adopt aberrant polarity and movement. Critically, in investigating Pk1-deficient cells that fail to migrate ventrolaterally, we have also uncovered roles for pk1a and pk1b in the epithelial-to-mesenchymal transition (EMT) of pre-migratory NCCs that precedes their collective migration to the periphery. Normally, during EMT, pre-migratory NCCs transition from a neuroepithelial to a bleb-based and subsequently, mesenchymal morphology capable of directed migration. When either Pk1a or Pk1b is disrupted, NCCs continue to perform blebbing behaviors characteristic of pre-migratory cells over extended time periods, indicating a block in a key transition during EMT. Although some Pk1-deficient NCCs transition successfully to mesenchymal, migratory morphologies, they fail to separate from neighboring NCCs. Additionally, Pk1b-deficient NCCs show elevated levels of E-Cadherin and reduced levels of N-Cadherin, suggesting that Prickle1 molecules regulate Cadherin levels to ensure the completion of EMT and the commencement of cranial NCC migration. We conclude that Pk1 plays crucial roles in cranial NCCs both during EMT and migration. These roles are dependent on the regulation of E-Cad and N-Cad.

A cargo model of yolk syncytial nuclear migration during zebrafish epiboly

In teleost fish, the multinucleate yolk syncytial layer functions as an extra-embryonic signaling center to pattern mesendoderm, coordinate morphogenesis and supply nutrients to the embryo. External yolk syncytial nuclei (e-YSN) undergo microtubule-dependent movements that distribute the nuclei over the large yolk mass. How e-YSN migration proceeds, and the role of the yolk microtubules, is not understood, but it is proposed that e-YSN are pulled vegetally as the microtubule network shortens from the vegetal pole. Live imaging revealed that nuclei migrate along microtubules, consistent with a cargo model in which e-YSN are moved down the microtubules by direct association with motor proteins. We found that blocking the plus-end directed microtubule motor kinesin significantly attenuated yolk nuclear movement. Blocking the outer nuclear membrane LINC complex protein Syne2a also slowed e-YSN movement. We propose that e-YSN movement is mediated by the LINC complex, which functions as the adaptor between yolk nuclei and motor proteins. Our work provides new insights into the role of microtubules in morphogenesis of an extra-embryonic tissue and further contributes to the understanding of nuclear migration mechanisms during development.

Inhibition of microRNA suppression of Dishevelled results in Wnt pathway-associated developmental defects in sea urchin

MicroRNAs (miRNAs) are highly conserved, small non-coding RNAs that regulate gene expressions by binding to the 3' untranslated region of target mRNAs thereby silencing translation. Some miRNAs are key regulators of the Wnt signaling pathways, which impact developmental processes. This study investigates miRNA regulation of different isoforms of Dishevelled (Dvl/Dsh), which encode a key component in the Wnt signaling pathway. The sea urchin Dvl mRNA isoforms have similar spatial distribution in early development, but one isoform is distinctively expressed in the larval ciliary band. We demonstrated that Dvl isoforms are directly suppressed by miRNAs. By blocking miRNA suppression of Dvl isoforms, we observed dose-dependent defects in spicule length, patterning of the primary mesenchyme cells, gut morphology, and cilia. These defects likely result from increased Dvl protein levels, leading to perturbation of Wnt-dependent signaling pathways and additional Dvl-mediated processes. We further demonstrated that overexpression of Dvl isoforms recapitulated some of the Dvl miRNATP-induced phenotypes. Overall, our results indicate that miRNA suppression of Dvl isoforms plays an important role in ensuring proper development and function of primary mesenchyme cells and cilia.

Hedgehog signaling regulates cell motility and optic fissure and stalk formation during vertebrate eye morphogenesis

Establishment of precise three-dimensional tissue structure is vital for organ function. In the visual system, optic fissure and stalk morphogenesis is a crucial yet poorly understood process, disruptions of which can lead to coloboma, a birth defect causing visual impairment. Here, we use four-dimensional imaging, cell tracking, and molecular genetics in zebrafish to define the cell movements underlying normal optic fissure and stalk formation. We determine how these events are disrupted in a coloboma model in which the Hedgehog (Hh) receptor ptch2 is lost, resulting in overactive Hh signaling. In the ptch2 mutant, cells exhibit defective motile behaviors and morphology. Cells that should contribute to the fissure do not arrive at their correct position, and instead contribute to an ectopically large optic stalk. Our results suggest that overactive Hh signaling, through overexpression of downstream transcriptional targets, impairs cell motility underlying optic fissure and stalk formation, via non-cell-autonomous and cell-autonomous mechanisms. More broadly, our cell motility and morphology analyses provide a new framework for studying other coloboma-causing mutations that disrupt optic fissure or stalk formation.

Summary: Multidimensional imaging of ptch2 mutant zebrafish uncovers a role for the Hh signaling pathway in regulating the cell and tissue dynamics underlying early eye morphogenesis.

Basal epithelial tissue folding is mediated by differential regulation of microtubules

The folding of epithelial tissues is crucial for development of three-dimensional structure and function. Understanding this process can assist in determining the etiology of developmental disease and engineering of tissues for the future of regenerative medicine. Folding of epithelial tissues towards the apical surface has long been studied, but the molecular mechanisms that mediate epithelial folding towards the basal surface are just emerging. Here, we utilize zebrafish neuroepithelium to identify mechanisms that mediate basal tissue folding to form the highly conserved embryonic midbrain-hindbrain boundary. Live imaging revealed Wnt5b as a mediator of anisotropic epithelial cell shape, both apically and basally. In addition, we uncovered a Wnt5b-mediated mechanism for specific regulation of basal anisotropic cell shape that is microtubule dependent and likely to involve JNK signaling. We propose a model in which a single morphogen can differentially regulate apical versus basal cell shape during tissue morphogenesis.

Summary: Examination of cell shape changes during zebrafish neuroepithelium tissue folding reveals that Wnt5b specifically regulates basal anisotropic cell shape via a microtubule-dependent mechanism, likely involving JNK signaling.

Glypican 4 and Mmp14 interact in regulating the migration of anterior endodermal cells by limiting extracellular matrix deposition

During embryogenesis, the germ layers, including the endoderm, undergo convergence and extension movements to narrow and elongate the body plan. In zebrafish, the dorsal migration of endodermal cells during gastrulation is controlled by chemokine signaling, but little is known about how they migrate during segmentation. Here, we show that glypican 4 (Gpc4), a member of the heparin sulfate proteoglycan family, is required for efficient migration of anterior endodermal cells during early segmentation, regulating Rac activation to maintain polarized actin-rich lamellipodia. An endoderm transplantation assay showed that Gpc4 regulates endoderm migration in a non-cell-autonomous fashion. Further analyses revealed that the impaired endoderm migration in gpc4 mutants results from increases in the expression and assembly of fibronectin and laminin, major components of the extracellular matrix (ECM). Notably, we found that matrix metalloproteinase 14 (Mmp14a/b) is required for the control of ECM expression during endoderm migration, with Gpc4 acting through Mmp14a/b to limit ECM expression. Our results suggest that Gpc4 is crucial for generating the environment required for efficient migration of endodermal cells, uncovering a novel function of Gpc4 during development.

Leucine repeat adaptor protein 1 interacts with Dishevelled to regulate gastrulation cell movements in zebrafish

Gastrulation is a fundamental morphogenetic event that requires polarised cell behaviours for coordinated asymmetric cell movements. Wnt/PCP signalling plays a critical role in this process. Dishevelled is an important conserved scaffold protein that relays Wnt/PCP signals from membrane receptors to the modulation of cytoskeleton organisation. However, it remains unclear how its activity is regulated for the activation of downstream effectors. Here, we report that Lurap1 is a Dishevelled-interacting protein that regulates Wnt/PCP signalling in convergence and extension movements during vertebrate gastrulation. Its loss-of-function leads to enhanced Dishevelled membrane localisation and increased JNK activity. In maternal-zygotic lurap1 mutant zebrafish embryos, cell polarity and directional movement are disrupted. Time-lapse analyses indicate that Lurap1, Dishevelled, and JNK functionally interact to orchestrate polarised cellular protrusive activity, and Lurap1 is required for coordinated centriole/MTOC positioning in movement cells. These findings demonstrate that Lurap1 functions to regulate cellular polarisation and motile behaviours during gastrulation movements.

Gastrulation is an early morphogenic event driven by coordinated asymmetric/polarised cell movements. Here, the authors show in zebrafish that Lurap1, a protein that interacts with Dishevelled, regulates Wnt and planar cell polarity, coordinating centriole positioning during convergence and extension.

Interstitial fluid osmolarity modulates the action of differential tissue surface tension in progenitor cell segregation during gastrulation

The segregation of different cell types into distinct tissues is a fundamental process in metazoan development. Differences in cell adhesion and cortex tension are commonly thought to drive cell sorting by regulating tissue surface tension (TST). However, the role that differential TST plays in cell segregation within the developing embryo is as yet unclear. Here, we have analyzed the role of differential TST for germ layer progenitor cell segregation during zebrafish gastrulation. Contrary to previous observations that differential TST drives germ layer progenitor cell segregation in vitro, we show that germ layers display indistinguishable TST within the gastrulating embryo, arguing against differential TST driving germ layer progenitor cell segregation in vivo. We further show that the osmolarity of the interstitial fluid (IF) is an important factor that influences germ layer TST in vivo, and that lower osmolarity of the IF compared with standard cell culture medium can explain why germ layers display differential TST in culture but not in vivo. Finally, we show that directed migration of mesendoderm progenitors is required for germ layer progenitor cell segregation and germ layer formation.

Highlighted Article: Segregation of the germ layer progenitors in the zebrafish gastrula is driven by directed cell migration, rather than differential tissue surface tension.

hmmr mediates anterior neural tube closure and morphogenesis in the frog Xenopus

Development of the central nervous system requires orchestration of morphogenetic processes which drive elevation and apposition of the neural folds and their fusion into a neural tube. The newly formed tube gives rise to the brain in anterior regions and continues to develop into the spinal cord posteriorly. Conspicuous differences between the anterior and posterior neural tube become visible already during neural tube closure (NTC). Planar cell polarity (PCP)-mediated convergent extension (CE) movements are restricted to the posterior neural plate, i.e. hindbrain and spinal cord, where they propagate neural fold apposition. The lack of CE in the anterior neural plate correlates with a much slower mode of neural fold apposition anteriorly. The morphogenetic processes driving anterior NTC have not been addressed in detail. Here, we report a novel role for the breast cancer susceptibility gene and microtubule (MT) binding protein Hmmr (Hyaluronan-mediated motility receptor, RHAMM) in anterior neurulation and forebrain development in Xenopus laevis. Loss of hmmr function resulted in a lack of telencephalic hemisphere separation, arising from defective roof plate formation, which in turn was caused by impaired neural tissue narrowing. hmmr regulated polarization of neural cells, a function which was dependent on the MT binding domains. hmmr cooperated with the core PCP component vangl2 in regulating cell polarity and neural morphogenesis. Disrupted cell polarization and elongation in hmmr and vangl2 morphants prevented radial intercalation (RI), a cell behavior essential for neural morphogenesis. Our results pinpoint a novel role of hmmr in anterior neural development and support the notion that RI is a major driving force for anterior neurulation and forebrain morphogenesis.

Dishevelled Paralogs in Vertebrate Development: Redundant or Distinct?

Dishevelled (DVL) proteins are highly conserved in the animal kingdom and are important key players in β-Catenin-dependent and -independent Wnt signaling pathways. Vertebrate genomes typically comprise three DVL genes, DVL1, DVL2, and DVL3. Expression patterns and developmental functions of the three vertebrate DVL proteins however, are only partially redundant in any given species. Moreover, expression and function of DVL isoforms have diverged between different vertebrate species. All DVL proteins share basic functionality in Wnt signal transduction. Additional, paralog-specific interactions and functions combined with context-dependent availability of DVL isoforms may play a central role in defining Wnt signaling specificity and add selectivity toward distinct downstream pathways. In this review, we recapitulate briefly cellular functions of DVL paralogs, their role in vertebrate embryonic development and congenital disease.

A role for planar cell polarity during early endoderm morphogenesis

The zebrafish endoderm begins to develop at gastrulation stages as a monolayer of cells. The behaviour of the endoderm during gastrulation stages is well understood. However, knowledge of the morphogenic movements of the endoderm during somitogenesis stages, as it forms a mesenchymal rod, is lacking. Here we characterise endodermal development during somitogenesis stages, and describe the morphogenic movements as the endoderm transitions from a monolayer of cells into a mesenchymal endodermal rod. We demonstrate that, unlike the overlying mesoderm, endodermal cells are not polarised during their migration to the midline at early somitogenesis stages. Specifically, we describe the stage at which endodermal cells begin to leave the monolayer, a process we have termed 'midline aggregation'. The planar cell polarity (PCP) signalling pathway is known to regulate mesodermal and ectodermal cell convergence towards the dorsal midline. However, a role for PCP signalling in endoderm migration to the midline during somitogenesis stages has not been established. In this report, we investigate the role for PCP signalling in multiple phases of endoderm development during somitogenesis stages. Our data exclude involvement of PCP signalling in endodermal cells as they leave the monolayer.

Sinup is essential for the integrity of centrosomes and mitotic spindles in zebrafish embryos

Fish lineage-specific gene, sinup [Siaz-interacting nuclear protein], modulates neural plate formation in embryogenesis and shares homology with human TPX2 protein, a member of the vertebrate mitogen-activating protein family. In spite of the presence of the TPX2 domain in Sinup, its cellular function has been unknown. As an initial approach to this question, we expressed Sinup by injecting sinup-EGFP mRNAs into zebrafish embryos at the one- to two-cell stage. First of all, Sinup-EGFP was associated with centrosomes and mitotic spindles. In particular, Sinup was localized to the spindle poles and midbody microtubules during the period between anaphase and cytokinesis. Second, various deleted mutants of Sinup-EGFP failed to be associated with the centrosomes and mitotic spindles. Third, a Sinup mutant, where the 144th Serine residue was converted to alanine, not only disturbed the mitotic spindle organization, such as multipolar spindles, fragmented spindle poles, and flattened spindles, but also arrested the cell cycle at metaphase and cell movement. Finally, Sinup is phosphorylated by Aurora A and the 144th Serine mutant of Sinup is partially phosphorylated by Aurora A kinase. We thus propose that Sinup is an essential element for the integrity of centrosomes and mitotic spindle fibers as well as for the normal process of cell cycle and cellular movement in vertebrate embryos.

Zebrafish models of non-canonical Wnt/planar cell polarity signalling: fishing for valuable insight into vertebrate polarized cell behavior

Planar cell polarity (PCP) coordinates the uniform orientation, structure and movement of cells within the plane of a tissue or organ system. It is beautifully illustrated in the polarized arrangement of bristles and hairs that project from specialized cell surfaces of the insect abdomen and wings, and pioneering genetic studies using the fruit fly, Drosophila melanogaster, have defined a core signalling network underlying PCP. This core PCP/non-canonical Wnt signalling pathway is evolutionarily conserved, and studies in zebrafish have helped transform our understanding of PCP from a peculiarity of polarized epithelia to a more universal cellular property that orchestrates a diverse suite of polarized cell behaviors that are required for normal vertebrate development. Furthermore, application of powerful genetics, embryonic cell-transplantation, and live-imaging capabilities afforded by the zebrafish model have yielded novel insights into the establishment and maintenance of vertebrate PCP, over the course of complex and dynamic morphogenetic events like gastrulation and neural tube morphogenesis. Although key questions regarding vertebrate PCP remain, with the emergence of new genome-editing technologies and the promise of endogenous labeling and Cre/LoxP conditional targeting strategies, zebrafish remains poised to deliver fundamental new insights into the function and molecular dynamic regulation of PCP signalling from embryonic development through to late-onset phenotypes and adult disease states. WIREs Dev Biol 2017, 6:e267. doi: 10.1002/wdev.267 For further resources related to this article, please visit the WIREs website.

Planar cell polarity in development and disease

Planar cell polarity (PCP) is an essential feature of animal tissues, whereby distinct polarity is established within the plane of a cell sheet. Tissue-wide establishment of PCP is driven by multiple global cues, including gradients of gene expression, gradients of secreted WNT ligands and anisotropic tissue strain. These cues guide the dynamic, subcellular enrichment of PCP proteins, which can self-assemble into mutually exclusive complexes at opposite sides of a cell. Endocytosis, endosomal trafficking and degradation dynamics of PCP components further regulate planar tissue patterning. This polarization propagates throughout the whole tissue, providing a polarity axis that governs collective morphogenetic events such as the orientation of subcellular structures and cell rearrangements. Reflecting the necessity of polarized cellular behaviours for proper development and function of diverse organs, defects in PCP have been implicated in human pathologies, most notably in severe birth defects.

Distinct requirements of wls, wnt9a, wnt5b and gpc4 in regulating chondrocyte maturation and timing of endochondral ossification

Formation of the mandible requires progressive morphologic change, proliferation, differentiation and organization of chondrocytes preceding osteogenesis. The Wnt signaling pathway is involved in regulating bone development and maintenance. Chondrocytes that are fated to become bone require Wnt to polarize and orientate appropriately to initiate the endochondral ossification program. Although the canonical Wnt signaling has been well studied in the context of bone development, the effects of non-canonical Wnt signaling in regulating the timing of cartilage maturation and subsequent bone formation in shaping ventral craniofacial structure is not fully understood.. Here we examined the role of the non-canonical Wnt signaling pathway (wls, gpc4, wnt5b and wnt9a) in regulating zebrafish Meckel's cartilage maturation to the onset of osteogenic differentiation. We found that disruption of wls resulted in a significant loss of craniofacial bone, whereas lack of gpc4, wnt5b and wnt9a resulted in severely delayed endochondral ossification. This study demonstrates the importance of the non-canonical Wnt pathway in regulating coordinated ventral cartilage morphogenesis and ossification.

Wnt-Frizzled/planar cell polarity signaling: cellular orientation by facing the wind (Wnt)

The establishment of planar cell polarity (PCP) in epithelial and mesenchymal cells is a critical, evolutionarily conserved process during development and organogenesis. Analyses in Drosophila and several vertebrate model organisms have contributed a wealth of information on the regulation of PCP. A key conserved pathway regulating PCP, the so-called core Wnt-Frizzled PCP (Fz/PCP) signaling pathway, was initially identified through genetic studies of Drosophila. PCP studies in vertebrates, most notably mouse and zebrafish, have identified novel factors in PCP signaling and have also defined cellular features requiring PCP signaling input. These studies have shifted focus to the role of Van Gogh (Vang)/Vangl genes in this molecular system. This review focuses on new insights into the core Fz/Vangl/PCP pathway and recent advances in Drosophila and vertebrate PCP studies. We attempt to integrate these within the existing core Fz/Vangl/PCP signaling framework.

Intracellular Golgi Complex organization reveals tissue specific polarity during zebrafish embryogenesis

BACKGROUND

Cell polarity is essential for directed migration of mesenchymal cells and morphogenesis of epithelial tissues. Studies in cultured cells indicate that a condensed Golgi Complex (GC) is essential for directed protein trafficking to establish cell polarity underlying directed cell migration. Dynamic changes of the GC intracellular organization during early vertebrate development remain to be investigated.

RESULTS

We used antibody labeling and fusion proteins in vivo to study the organization and intracellular placement of the GC during early zebrafish embryogenesis. We found that the GC was dispersed into several puncta containing cis- and trans-Golgi Complex proteins, presumably ministacks, until the end of the gastrula period. By early segmentation stages, the GC condensed in cells of the notochord, adaxial mesoderm, and neural plate, and its intracellular position became markedly polarized away from borders between these tissues.

CONCLUSIONS

We find that GC is dispersed in early zebrafish cells, even when cells are engaged in massive gastrulation movements. The GC accumulates into patches in a stage and cell-type specific manner, and becomes polarized away from borders between the embryonic tissues. With respect to tissue borders, intracellular GC polarity in notochord is independent of mature apical/basal polarity, Wnt/PCP, or signals from adaxial mesoderm. Developmental Dynamics 245:678-691, 2016. © 2016 Wiley Periodicals, Inc.

Symmetry Breaking in an Edgeless Epithelium by Fat2-Regulated Microtubule Polarity

Planar cell polarity (PCP) information is a critical determinant of organ morphogenesis. While PCP in bounded epithelial sheets is increasingly well understood, how PCP is organized in tubular and acinar tissues is not. Drosophila egg chambers (follicles) are an acinus-like "edgeless epithelium" and exhibit a continuous, circumferential PCP that does not depend on pathways active in bounded epithelia; this follicle PCP directs formation of an ellipsoid rather than a spherical egg. Here, we apply an imaging algorithm to "unroll" the entire 3D tissue surface and comprehensively analyze PCP onset. This approach traces chiral symmetry breaking to plus-end polarity of microtubules in the germarium, well before follicles form and rotate. PCP germarial microtubules provide chiral information that predicts the direction of whole-tissue rotation as soon as independent follicles form. Concordant microtubule polarity, but not microtubule alignment, requires the atypical cadherin Fat2, which acts at an early stage to translate plus-end bias into coordinated actin-mediated collective cell migration. Because microtubules are not required for PCP or migration after follicle rotation initiates, while dynamic actin and extracellular matrix are, polarized microtubules lie at the beginning of a handoff mechanism that passes early chiral PCP of the cytoskeleton to a supracellular planar polarized extracellular matrix and elongates the organ.

Positioning of centrioles is a conserved readout of Frizzled planar cell polarity signalling

Planar cell polarity (PCP) signalling is a well-conserved developmental pathway regulating cellular orientation during development. An evolutionarily conserved pathway readout is not established and, moreover, it is thought that PCP mediated cellular responses are tissue-specific. A key PCP function in vertebrates is to regulate coordinated centriole/cilia positioning, a function that has not been associated with PCP in Drosophila. Here we report instructive input of Frizzled-PCP (Fz/PCP) signalling into polarized centriole positioning in Drosophila wings. We show that centrioles are polarized in pupal wing cells as a readout of PCP signalling, with both gain and loss-of-function Fz/PCP signalling affecting centriole polarization. Importantly, loss or gain of centrioles does not affect Fz/PCP establishment, implicating centriolar positioning as a conserved PCP-readout, likely downstream of PCP-regulated actin polymerization. Together with vertebrate data, these results suggest a unifying model of centriole/cilia positioning as a common downstream effect of PCP signalling from flies to mammals.

Planar cell polarity (PCP) contributes to cellular orientation during development but how this is regulated in Drosophila is unclear. Here, the authors identify Frizzled-PCP signalling as regulating polarised centriole positioning in the wing disc.

Microtubule-associated protein 1b is required for shaping the neural tube

BACKGROUND

Shaping of the neural tube, the precursor of the brain and spinal cord, involves narrowing and elongation of the neural tissue, concomitantly with other morphogenetic changes that contribue to this process. In zebrafish, medial displacement of neural cells (neural convergence or NC), which drives the infolding and narrowing of the neural ectoderm, is mediated by polarized migration and cell elongation towards the dorsal midline. Failure to undergo proper NC results in severe neural tube defects, yet the molecular underpinnings of this process remain poorly understood.

RESULTS

We investigated here the role of the microtubule (MT) cytoskeleton in mediating NC in zebrafish embryos using the MT destabilizing and hyperstabilizing drugs nocodazole and paclitaxel respectively. We found that MTs undergo major changes in organization and stability during neurulation and are required for the timely completion of NC by promoting cell elongation and polarity. We next examined the role of Microtubule-associated protein 1B (Map1b), previously shown to promote MT dynamicity in axons. map1b is expressed earlier than previously reported, in the developing neural tube and underlying mesoderm. Loss of Map1b function using morpholinos (MOs) or δMap1b (encoding a truncated Map1b protein product) resulted in delayed NC and duplication of the neural tube, a defect associated with impaired NC. We observed a loss of stable MTs in these embryos that is likely to contribute to the NC defect. Lastly, we found that Map1b mediates cell elongation in a cell autonomous manner and polarized protrusive activity, two cell behaviors that underlie NC and are MT-dependent.

CONCLUSIONS

Together, these data highlight the importance of MTs in the early morphogenetic movements that shape the neural tube and reveal a novel role for the MT regulator Map1b in mediating cell elongation and polarized cell movement in neural progenitor cells.

Spatial and temporal aspects of Wnt signaling and planar cell polarity during vertebrate embryonic development

Wnt signaling pathways act at multiple locations and developmental stages to specify cell fate and polarity in vertebrate embryos. A long-standing question is how the same molecular machinery can be reused to produce different outcomes. The canonical Wnt/β-catenin branch modulates target gene transcription to specify cell fates along the dorsoventral and anteroposterior embryonic axes. By contrast, the Wnt/planar cell polarity (PCP) branch is responsible for cell polarization along main body axes, which coordinates morphogenetic cell behaviors during gastrulation and neurulation. Whereas both cell fate and cell polarity are modulated by spatially- and temporally-restricted Wnt activity, the downstream signaling mechanisms are very diverse. This review highlights recent progress in the understanding of Wnt-dependent molecular events leading to the establishment of PCP and linking it to early morphogenetic processes.

Planar polarization of Vangl2 in the vertebrate neural plate is controlled by Wnt and Myosin II signaling

The vertebrate neural tube forms as a result of complex morphogenetic movements, which require the functions of several core planar cell polarity (PCP) proteins, including Vangl2 and Prickle. Despite the importance of these proteins for neurulation, their subcellular localization and the mode of action have remained largely unknown. Here we describe the anteroposterior planar cell polarity (AP-PCP) of the cells in the Xenopus neural plate. At the neural midline, the Vangl2 protein is enriched at anterior cell edges and that this localization is directed by Prickle, a Vangl2-interacting protein. Our further analysis is consistent with the model, in which Vangl2 AP-PCP is established in the neural plate as a consequence of Wnt-dependent phosphorylation. Additionally, we uncover feedback regulation of Vangl2 polarity by Myosin II, reiterating a role for mechanical forces in PCP. These observations indicate that both Wnt signaling and Myosin II activity regulate cell polarity and cell behaviors during vertebrate neurulation.

The cell biology of planar cell polarity

Planar cell polarity (PCP) refers to the coordinated alignment of cell polarity across the tissue plane. Key to the establishment of PCP is asymmetric partitioning of cortical PCP components and intercellular communication to coordinate polarity between neighboring cells. Recent progress has been made toward understanding how protein transport, endocytosis, and intercellular interactions contribute to asymmetric PCP protein localization. Additionally, the functions of gradients and mechanical forces as global cues that bias PCP orientation are beginning to be elucidated. Together, these findings are shedding light on how global cues integrate with local cell interactions to organize cellular polarity at the tissue level.

Fat-Dachsous signaling coordinates cartilage differentiation and polarity during craniofacial development

Organogenesis requires coordinated regulation of cellular differentiation and morphogenesis. Cartilage cells in the vertebrate skeleton form polarized stacks, which drive the elongation and shaping of skeletal primordia. Here we show that an atypical cadherin, Fat3, and its partner Dachsous-2 (Dchs2), control polarized cell-cell intercalation of cartilage precursors during craniofacial development. In zebrafish embryos deficient in Fat3 or Dchs2, chondrocytes fail to stack and misregulate expression of sox9a. Similar morphogenetic defects occur in rerea/atr2a^−/− mutants, and Fat3 binds REREa, consistent with a model in which Fat3, Dchs2 and REREa interact to control polarized cell-cell intercalation and simultaneously control differentiation through Sox9. Chimaeric analyses support such a model, and reveal long-range influences of all three factors, consistent with the activation of a secondary signal that regulates polarized cell-cell intercalation. This coordinates the spatial and temporal morphogenesis of chondrocytes to shape skeletal primordia and defects in these processes underlie human skeletal malformations. Similar links between cell polarity and differentiation mechanisms are also likely to control organ formation in other contexts.

Little is known about the mechanisms of cell-cell communication necessary to assemble skeletal elements of appropriate size and shape. In this study, we investigate the roles of genetic factors belonging to a developmental pathway that affects skeletal progenitor behavior: the atypical cadherins Fat3 and Dachsous2 (Dchs2), and REREa/Atr2a. We show that cartilage precursors fail to rearrange into linear stacks and at the same time misregulate expression of sox9a, a key regulator of cartilage differentiation, in zebrafish embryos deficient in Fat3 or its partner Dchs2. Similar cartilage defects are observed in rerea^−/− mutants, and Fat3 interacts physically and genetically with REREa. Our results suggest that Fat3, Dchs2 and REREa interact to control polarized cell-cell intercalation and simultaneously control skeletal differentiation through Sox9. By transplanting cartilage precursors between wild-type and Fat3, Dchs2 or REREa deficient embryos we demonstrate that all three factors exert long-range influences on neighboring cells, most likely mediated by another polarizing signal. We propose a model in which this coordinates the polarity and differentiation of chondrocytes to shape skeletal primordia, and that defects in these processes underlie human skeletal malformations.

Understanding epithelial homeostasis in the intestine: An old battlefield of ideas, recent breakthroughs and remaining controversies

The intestinal epithelium constitutes the barrier between the gut lumen and the rest of the body and a very actively renewing cell population. The crypt/villus and crypt/cuff units of the mouse small intestine and colon are its basic functional units. The field is confronted with competing concepts with regard to the nature of the cells that are responsible for all the day-to day cell replacement and those that act to regenerate the tissue upon injury and with two diametrically opposed models for lineage specification. The review revisits groundbreaking pioneering studies to provide non expert readers and crypt watchers with a factual analysis of the origins of the current models deduced from the latest spectacular advances. It also discusses recent progress made by addressing these issues in the crypts of the colon, which need to be better understood, since they are the preferred sites of major pathologies.

Atmin mediates kidney morphogenesis by modulating Wnt signaling

The DNA damage protein and transcription factor Atmin (Asciz) is required for both lung tubulogenesis and ciliogenesis. Like the lungs, kidneys contain a tubular network that is critical for their function and in addition, renal ciliary dysfunction has been implicated in the pathogenesis of cystic kidney disease. Using the Atmin mouse mutant Gasping6 (Gpg6), we investigated kidney development and found it severely disrupted with reduced branching morphogenesis, resulting in fewer epithelial structures being formed. Unexpectedly, transcriptional levels of key cilia associated genes were not altered in Atmin^Gpg6/Gpg6 kidneys. Instead, Gpg6 homozygous kidneys exhibited altered cytoskeletal organization and modulation of Wnt signaling pathway molecules, including β-catenin and non-canonical Wnt/planar cell polarity (PCP) pathway factors, such as Daam2 and Vangl2. Wnt signaling is important for kidney development and perturbation of Wnt signaling pathways can result in cystic, and other, renal abnormalities. In common with other PCP pathway mutants, Atmin^Gpg6/Gpg6 mice displayed a shortened rostral-caudal axis and mis-oriented cell division. Moreover, intercrosses between Atmin^Gpg6/+ and Vangl2^Lp/+ mice revealed a genetic interaction between Atmin and Vangl2. Thus we show for the first time that Atmin is critical for normal kidney development and we present evidence that mechanistically, Atmin modifies Wnt signaling pathways, specifically placing it as a novel effector molecule in the non-canonical Wnt/PCP pathway. The identification of a novel modulator of Wnt signaling has important implications for understanding the pathobiology of renal disease.

Bcl-wav and the mitochondrial calcium uniporter drive gastrula morphogenesis in zebrafish

Bcl-2 proteins are acknowledged as key regulators of programmed cell death. However, increasing data suggest additional roles, including regulation of the cell cycle, metabolism and cytoskeletal dynamics. Here we report the discovery and characterization of a new Bcl-2-related multidomain apoptosis accelerator, Bcl-wav, found in fish and frogs. Genetic and molecular studies in zebrafish indicate that Bcl-wav and the recently identified mitochondrial calcium uniporter (MCU) contribute to the formation of the notochord axis by controlling blastomere convergence and extension movements during gastrulation. Furthermore, we found that Bcl-wav controls intracellular Ca(2+) trafficking by acting on the mitochondrial voltage-dependent anion channel, and possibly on MCU, with direct consequences on actin microfilament dynamics and blastomere migration guidance. Thus, from an evolutionary point of view, the original function of Bcl-2 proteins might have been to contribute in controlling the global positioning system of blastomeres during gastrulation, a critical step in metazoan development.

Excess PLAC8 promotes an unconventional ERK2-dependent EMT in colon cancer

The epithelial-to-mesenchymal transition (EMT) transcriptional program is characterized by repression of E-cadherin (CDH1) and induction of N-cadherin (CDH2), and mesenchymal genes like vimentin (VIM). Placenta-specific 8 (PLAC8) has been implicated in colon cancer; however, how PLAC8 contributes to disease is unknown, and endogenous PLAC8 protein has not been studied. We analyzed zebrafish and human tissues and found that endogenous PLAC8 localizes to the apical domain of differentiated intestinal epithelium. Colon cancer cells with elevated PLAC8 levels exhibited EMT features, including increased expression of VIM and zinc finger E-box binding homeobox 1 (ZEB1), aberrant cell motility, and increased invasiveness. In contrast to classical EMT, PLAC8 overexpression reduced cell surface CDH1 and upregulated P-cadherin (CDH3) without affecting CDH2 expression. PLAC8-induced EMT was linked to increased phosphorylated ERK2 (p-ERK2), and ERK2 knockdown restored cell surface CDH1 and suppressed CDH3, VIM, and ZEB1 upregulation. In vitro, PLAC8 directly bound and inactivated the ERK2 phosphatase DUSP6, thereby increasing p-ERK2. In a murine xenograft model, knockdown of endogenous PLAC8 in colon cancer cells resulted in smaller tumors, reduced local invasion, and decreased p-ERK2. Using MultiOmyx, a multiplex immunofluorescence-based methodology, we observed coexpression of cytosolic PLAC8, CDH3, and VIM at the leading edge of a human colorectal tumor, supporting a role for PLAC8 in cancer invasion in vivo.

Protein phosphatase 1 β paralogs encode the zebrafish myosin phosphatase catalytic subunit

Background

The myosin phosphatase is a highly conserved regulator of actomyosin contractility. Zebrafish has emerged as an ideal model system to study the invivo role of myosin phosphatase in controlling cell contractility, cell movement and epithelial biology. Most work in zebrafish has focused on the regulatory subunit of the myosin phosphatase called Mypt1. In this work, we examined the critical role of Protein Phosphatase 1, PP1, the catalytic subunit of the myosin phosphatase.

Methodology/Principal Findings

We observed that in zebrafish two paralogous genes encoding PP1β, called ppp1cba and ppp1cbb, are both broadly expressed during early development. Furthermore, we found that both gene products interact with Mypt1 and assemble an active myosin phosphatase complex. In addition, expression of this complex results in dephosphorylation of the myosin regulatory light chain and large scale rearrangements of the actin cytoskeleton. Morpholino knock-down of ppp1cba and ppp1cbb results in severe defects in morphogenetic cell movements during gastrulation through loss of myosin phosphatase function.

Conclusions/Significance

Our work demonstrates that zebrafish have two genes encoding PP1β, both of which can interact with Mypt1 and assemble an active myosin phosphatase. In addition, both genes are required for convergence and extension during gastrulation and correct dosage of the protein products is required.

The nucleus of endothelial cell as a sensor of blood flow direction

Hemodynamic shear stresses cause endothelial cells (ECs) to polarize in the plane of the flow. Paradoxically, under strong shear flows, ECs disassemble their primary cilia, common sensors of shear, and thus must use an alternative mechanism of sensing the strength and direction of flow. In our experiments in microfluidic perfusion chambers, confluent ECs developed planar cell polarity at a rate proportional to the shear stress. The location of Golgi apparatus and microtubule organizing center was biased to the upstream side of the nucleus, i.e. the ECs polarized against the flow. These in vitro results agreed with observations in murine blood vessels, where EC polarization against the flow was stronger in high flow arteries than in veins. Once established, flow-induced polarization persisted over long time intervals without external shear. Transient destabilization of acto-myosin cytoskeleton by inhibition of myosin II or depolymerization of actin promoted polarization of EC against the flow, indicating that an intact acto-myosin cytoskeleton resists flow-induced polarization. These results suggested that polarization was induced by mechanical displacement of EC nuclei downstream under the hydrodynamic drag. This hypothesis was confirmed by the observation that acute application of a large hydrodynamic force to ECs resulted in an immediate downstream displacement of nuclei and was sufficient to induce persistent polarization. Taken together, our data indicate that ECs can sense the direction and strength of blood flow through the hydrodynamic drag applied to their nuclei.

Shaping the nervous system: role of the core planar cell polarity genes

Planar cell polarity (PCP) is complementary to the intrinsic polarization of single cells and refers to the global coordination of cell behaviour in the plane of a tissue and, by extension, to the signalling pathways that control it. PCP is most evident in cell sheets, and research into PCP was for years confined to studies in Drosophila melanogaster. However, PCP has more recently emerged as an important phenomenon in vertebrates, in which it regulates various developmental processes and is associated with multiple disorders. In particular, core PCP genes are crucial for the development and function of the nervous system. They are involved in neural tube closure, ependymal polarity, neuronal migration, dendritic growth and axon guidance.

Centrosome positioning in vertebrate development

The centrosome, a major organizer of microtubules, has important functions in regulating cell shape, polarity, cilia formation and intracellular transport as well as the position of cellular structures, including the mitotic spindle. By means of these activities, centrosomes have important roles during animal development by regulating polarized cell behaviors, such as cell migration or neurite outgrowth, as well as mitotic spindle orientation. In recent years, the pace of discovery regarding the structure and composition of centrosomes has continuously accelerated. At the same time, functional studies have revealed the importance of centrosomes in controlling both morphogenesis and cell fate decision during tissue and organ development. Here, we review examples of centrosome and centriole positioning with a particular emphasis on vertebrate developmental systems, and discuss the roles of centrosome positioning, the cues that determine positioning and the mechanisms by which centrosomes respond to these cues. The studies reviewed here suggest that centrosome functions extend to the development of tissues and organs in vertebrates.

Jun N-terminal kinase maintains tissue integrity during cell rearrangement in the gut

Tissue elongation is a fundamental morphogenetic process that generates the proper anatomical topology of the body plan and vital organs. In many elongating embryonic structures, tissue lengthening is driven by Rho family GTPase-mediated cell rearrangement. During this dynamic process, the mechanisms that modulate intercellular adhesion to allow individual cells to change position without compromising structural integrity are not well understood. In vertebrates, Jun N-terminal kinase (JNK) is also required for tissue elongation, but the precise cellular role of JNK in this context has remained elusive. Here, we show that JNK activity is indispensable for the rearrangement of endoderm cells that underlies the elongation of the Xenopus gut tube. Whereas Rho kinase is necessary to induce cell intercalation and remodel adhesive contacts, we have found that JNK is required to maintain cell-cell adhesion and establish parallel microtubule arrays; without JNK activity, the reorganizing endoderm dissociates. Depleting polymerized microtubules phenocopies this effect of JNK inhibition on endoderm morphogenesis, consistent with a model in which JNK regulates microtubule architecture to preserve adhesive contacts between rearranging gut cells. Thus, in contrast to Rho kinase, which generates actomyosin-based tension and cell movement, JNK signaling is required to establish microtubule stability and maintain tissue cohesion; both factors are required to achieve proper cell rearrangement and gut extension. This model of gut elongation has implications not only for the etiology of digestive tract defects, but sheds new light on the means by which intra- and intercellular forces are balanced to promote topological change, while preserving structural integrity, in numerous morphogenetic contexts.

Quantitative analysis of polarity in 3D reveals local cell coordination in the embryonic mouse heart

Anisotropies that underlie organ morphogenesis have been quantified in 2D, taking advantage of a reference axis. However, morphogenesis is a 3D process and it remains a challenge to analyze cell polarities in 3D. Here, we have designed a novel procedure that integrates multidisciplinary tools, including image segmentation, statistical analyses, axial clustering and correlation analysis. The result is a sensitive and unbiased assessment of the significant alignment of cell orientations in 3D, compared with a random axial distribution. Taking the mouse heart as a model, we validate the procedure at the fetal stage, when cardiomyocytes are known to be aligned. At the embryonic stage, our study reveals that ventricular cells are already coordinated locally. The centrosome-nucleus axes and the cell division axes are biased in a plane parallel to the outer surface of the heart, with a minor transmural component. We show further alignment of these axes locally in the plane of the heart surface. Our method is generally applicable to other sets of vectors or axes in 3D tissues to map the regions where they show significant alignment.

In vivo cell and tissue dynamics underlying zebrafish fin fold regeneration

Background

Zebrafish (Danio rerio) has a remarkable capacity to regenerate many organs and tissues. During larval stages the fin fold allows the possibility of performing long time-lapse imaging making this system very appealing to study the relationships between tissue movements, cell migration and proliferation necessary for the regeneration process.

Results

Through the combined use of transgenic fluorescently-labeled animals and confocal microscopy imaging, we characterized in vivo the complete fin fold regeneration process. We show, for the first time, that there is an increase in the global rate of epidermal growth as a response to tissue loss. Also enhanced significantly is cell proliferation, which upon amputation happens in a broad area concerning the amputation level and not in a blastema-restricted way. This reveals a striking difference with regard to the adult fin regeneration system. Finally, an accumulation of migratory, shape-changing fibroblasts occurs proximally to the wound area, resembling a blastemal-like structure, which may act as a signaling center for the regeneration process to proceed.

Conclusions

These findings provide a novel in vivo description of fundamental mechanisms occurring during the fin fold regeneration process, thereby contributing to a better knowledge of this regenerative system and to reveal variations in the epimorphic regeneration field.