E-cadherin is required for gastrulation cell movements in zebrafish

Mechano-gradients drive morphogen-noise correction to ensure robust patterning

Morphogen gradients instruct cells to pattern tissues. Although the mechanisms by which morphogens transduce chemical signals have been extensively studied, the roles and regulation of the physical communication between morphogen-receiver cells remain unclear. Here, we show that the Wnt/β-catenin-morphogen gradient, which patterns the embryonic anterior-posterior (AP) axis, generates intercellular tension gradients along the AP axis by controlling membrane cadherin levels in zebrafish embryos. This "mechano-gradient" is used for the cell competition-driven correction of noisy morphogen gradients. Naturally and artificially generated unfit cells, producing noisy Wnt/β-catenin gradients, induce local deformation of the mechano-gradients that activate mechanosensitive calcium channels in the neighboring fit cells, which then secrete annexin A1a to kill unfit cells. Thus, chemo-mechanical interconversion-mediated competitive communication between the morphogen-receiver cells ensures precise tissue patterning.

Stiffness-dependent active wetting enables optimal collective cell durotaxis

The directed migration of cellular clusters enables morphogenesis, wound healing, and collective cancer invasion. Gradients of substrate stiffness are known to direct the migration of cellular clusters in a process called collective durotaxis, but underlying mechanisms remain unclear. Here, we unveil a connection between collective durotaxis and the wetting properties of cellular clusters. We show that clusters of cancer cells dewet soft substrates and wet stiff ones. At intermediate stiffness, at the crossover from low to high wettability, clusters on uniform-stiffness substrates become maximally motile, and clusters on stiffness gradients exhibit optimal durotaxis. Durotactic velocity increases with cluster size, stiffness gradient, and actomyosin activity. We demonstrate this behavior on substrates coated with the cell-cell adhesion protein E-cadherin and then establish its generality on substrates coated with extracellular matrix. We develop a physical model of three-dimensional active wetting that explains this mode of collective durotaxis in terms of a balance between in-plane active traction and tissue contractility, and out-of-plane surface tension. Finally, we show that the distribution of cluster displacements has a heavy tail, with infrequent but large cellular hops that contribute to durotactic migration. Our study demonstrates a physical mechanism of collective durotaxis, through both cell-cell and cell-substrate adhesion ligands, based on the wetting properties of active droplets.

Origin, form and function of extraembryonic structures in teleost fishes

Teleost eggs have evolved a highly derived early developmental pattern within vertebrates as a result of the meroblastic cleavage pattern, giving rise to a polar stratified architecture containing a large acellular yolk and a small cellular blastoderm on top. Besides the acellular yolk, the teleost-specific yolk syncytial layer (YSL) and the superficial epithelial enveloping layer are recognized as extraembryonic structures that play critical roles throughout embryonic development. They provide enriched microenvironments in which molecular feedback loops, cellular interactions and mechanical signals emerge to sculpt, among other things, embryonic patterning along the dorsoventral and left-right axes, mesendodermal specification and the execution of morphogenetic movements in the early embryo and during organogenesis. An emerging concept points to a critical role of extraembryonic structures in reinforcing early genetic and morphogenetic programmes in reciprocal coordination with the embryonic blastoderm, providing the necessary boundary conditions for development to proceed. In addition, the role of the enveloping cell layer in providing mechanical, osmotic and immunological protection during early stages of development, and the autonomous nutritional support provided by the yolk and YSL, have probably been key aspects that have enabled the massive radiation of teleosts to colonize every ecological niche on the Earth. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

Dephosphorylation of Y228 and Y217 and phosphorylation of Y335 in p120 catenin activate convergent extension during zebrafish gastrulation

BACKGROUND

The cadherin-associated protein p120 catenin regulates convergent extension through interactions with cadherin proteins, Cdc42, and Rac1, as we previously showed in zebrafish (Danio rerio). Phosphorylation of p120 catenin changes the nature of its activity in vitro but is virtually unexplored in embryos. We used our previously developed antisense RNA splice-site morpholino targeted to endogenous p120 catenin-δ1 to cause defects in axis elongation probing the functions of three p120 catenin tyrosine-phosphorylation sites in gastrulating zebrafish embryos.

RESULTS

The morpholino-induced defects were rescued by co-injections with mouse p120 catenin-δ1-3A mRNAs mutated at residues Y228 and Y217 to a non-phosphorylatable phenylalanine (F) or mutated at residue Y335 to a phosphomimetic glutamic acid (E). Co-injection of the complementary mutations Y228E, Y217E, or Y335F mRNAs partially rescued embryos whereas dual mutation to Y228E-Y217E blocked rescue. Immunopurification showed Y228F mutant proteins preferentially interacted with Rac1, potentially promoting cell migration. In contrast, the phosphomimetic Y228E preferentially interacted with E-cadherin increasing adhesion. Both Y228F and Y335F strongly bind VAV2.

CONCLUSIONS

p120 catenin serves dual roles during gastrulation of zebrafish. Phosphorylation and dephosphorylation of tyrosine residues Y217, Y228, and Y335 precisely balance cell adhesion and cell migration to facilitate somite compaction and axis elongation.

Phosphorylation of serine residues S252, S268/S269, and S879 in p120 catenin activates migration of presomitic mesoderm in gastrulating zebrafish embryos

BACKGROUND

Cadherin-associated protein p120 catenin regulates cell adhesion and migration in cell cultures and is required for axial elongation in embryos. Its roles in adhesion and cell migration are regulated by phosphorylation. We determined the effects of phosphorylation of six serine and three threonine residues in p120 catenin during zebrafish (Danio rerio) embryogenesis.

RESULTS

We knocked down endogenous p120 catenin-δ1 with an antisense RNA-splice-site morpholino (Sp-MO) causing defects in axis elongation. These defects were rescued by co-injections of mRNAs for wildtype mouse p120 catenin-δ1-3A or various mutated forms. Several mRNAs containing serine or threonine codons singly or doubly mutated to phosphomimetic glutamic acid rescued, and some nonphosphorylatable mutants did not.

CONCLUSIONS

We discovered that phosphorylation of serine residue S252 or S879 is required for convergent extension of zebrafish embryos, since rescue occurred only when these residues were mutated to glutamic acid. In addition, the phosphorylation of either S268 or S269 is required, not both, consistent with the presence of only a single one of these residues in two isoforms of zebrafish and Xenopus laevis. In summary, phosphorylation of multiple serine and threonine residues of p120 catenin activates migration of presomitic mesoderm of zebrafish embryos facilitating elongation of the dorsal axis.

Guidance by followers ensures long-range coordination of cell migration through α-catenin mechanoperception

Morphogenesis, wound healing, and some cancer metastases depend upon the migration of cell collectives that need to be guided to their destination as well as coordinated with other cell movements. During zebrafish gastrulation, the extension of the embryonic axis is led by the mesendodermal polster that migrates toward the animal pole, followed by the axial mesoderm that undergoes convergence and extension. Here, we investigate how polster cells are guided toward the animal pole. Using a combination of precise laser ablations, advanced transplants, and functional as well as in silico approaches, we establish that each polster cell is oriented by its immediate follower cells. Each cell perceives the migration of followers, through E-cadherin/α-catenin mechanotransduction, and aligns with them. Therefore, directional information propagates from cell to cell over the whole tissue. Such guidance of migrating cells by followers ensures long-range coordination of movements and developmental robustness.

Wnt-frizzled planar cell polarity signaling in the regulation of cell motility

The molecular complexes underlying planar cell polarity (PCP) were first identified in Drosophila through analysis of mutant phenotypes in the adult cuticle and the orientation of associated polarized protrusions such as wing hairs and sensory bristles. The same molecules are conserved in vertebrates and are required for the localization of polarized protrusions such as primary or sensory cilia and the orientation of hair follicles. Not only is PCP signaling required to align cellular structures across a tissue, it is also required to coordinate movement during embryonic development and adult homeostasis. PCP signaling allows cells to interpret positional cues within a tissue to move in the appropriate direction and to coordinate this movement with their neighbors. In this review we outline the molecular basis of the core Wnt-Frizzled/PCP pathway, and describe how this signaling orchestrates collective motility in Drosophila and vertebrates. Here we cover the paradigms of ommatidial rotation and border cell migration in Drosophila, and convergent extension in vertebrates. The downstream cell biological processes that underlie polarized motility include cytoskeletal reorganization, and adherens junctional and extracellular matrix remodeling. We discuss the contributions of these processes in the respective cell motility contexts. Finally, we address examples of individual cell motility guided by PCP factors during nervous system development and in cancer disease contexts.

CARMIL3 is important for cell migration and morphogenesis during early development in zebrafish

Cell migration is important during early animal embryogenesis. Cell migration and cell shape are controlled by actin assembly and dynamics, which depend on capping proteins, including the barbed-end heterodimeric actin capping protein (CP). CP activity can be regulated by capping-protein-interacting (CPI) motif proteins, including CARMIL (capping protein Arp2/3 myosin-I linker) family proteins. Previous studies of CARMIL3, one of the three highly conserved CARMIL genes in vertebrates, have largely been limited to cells in culture. Towards understanding CARMIL function during embryogenesis in vivo, we analyzed zebrafish lines carrying mutations of carmil3. Maternal-zygotic mutants showed impaired endodermal migration during gastrulation, along with defects in dorsal forerunner cell (DFC) cluster formation, which affected the morphogenesis of Kupffer's vesicle (KV). Mutant KVs were smaller, contained fewer cells and displayed decreased numbers of cilia, leading to defects in left/right (L/R) patterning with variable penetrance and expressivity. The penetrance and expressivity of the KV phenotype in carmil3 mutants correlated well with the L/R heart positioning defect at the end of embryogenesis. This in vivo animal study of CARMIL3 reveals its new role during morphogenesis of the vertebrate embryo. This role involves migration of endodermal cells and DFCs, along with subsequent morphogenesis of the KV and L/R asymmetry.

The Adaptor Protein Lurap1 Is Required for Cell Cohesion during Epiboly Movement in Zebrafish

Simple Summary

Cell adhesion and active cell shape changes play an important role in morphogenetic movements during embryonic development. Zebrafish is an attractive model for the study of cellular and molecular mechanisms underlying these processes. Epiboly is a conserved gastrulation cell movement, which describes the thinning and spreading of an external sheet of cells to cover other groups of cells in the embryo. It involves differential cellular adhesive properties and dynamic cytoskeletal organization across the embryo, but how these are regulated remains elusive. We found that the adaptor protein Lurap1, which interacts with other proteins required for cell migration, plays a role in cell adhesion during epiboly. In zebrafish mutants with loss of Lurap1 function, there is a reduced cellular cohesion in the epithelial blastoderm cells and a delayed epiboly movement. Our observations suggest that Lurap1 is implicated in the regulation of cellular behavior changes for coordinated morphogenetic movements in vertebrate embryos.

Abstract

Cell adhesion and polarized cellular behaviors play critical roles in a wide variety of morphogenetic events. In the zebrafish embryo, epiboly represents an important process of epithelial morphogenesis that involves differential cell adhesion and dynamic cell shape changes for coordinated movements of different cell populations, but the underlying mechanism remains poorly understood. The adaptor protein Lurap1 functions to link myotonic dystrophy kinase-related Rac/Cdc42-binding kinase with MYO18A for actomyosin retrograde flow in cell migration. We previously reported that it interacts with Dishevelled in convergence and extension movements during gastrulation. Here, we show that it regulates blastoderm cell adhesion and radial cell intercalation during epiboly. In zebrafish mutant embryos with loss of both maternal and zygotic Lurap1 function, deep cell multilayer of the blastoderm exhibit delayed epiboly with respect to the superficial layer. Time-lapse imaging reveals that these deep cells undergo unstable intercalation, which impedes their expansion over the yolk cell. Cell sorting and adhesion assays indicate reduced cellular cohesion of the blastoderm. These defects are correlated with disrupted cytoskeletal organization in the cortex of blastoderm cells. Thus, the present results extend our previous works by demonstrating that Lurap1 is required for cell adhesion and cell behavior changes to coordinate cell movements during epithelial morphogenesis. They provide insights for a further understanding of the regulation of cytoskeletal organization during gastrulation cell movements.

Amyloid precursor protein-b facilitates cell adhesion during early development in zebrafish

Understanding the biological function of amyloid beta (Aβ) precursor protein (APP) beyond its role in Alzheimer's disease is emerging. Yet, its function during embryonic development is poorly understood. The zebrafish APP orthologue, Appb, is strongly expressed during early development but thus far has only been studied via morpholino-mediated knockdown. Zebrafish enables analysis of cellular processes in an ontogenic context, which is limited in many other vertebrates. We characterized zebrafish carrying a homozygous mutation that introduces a premature stop in exon 2 of the appb gene. We report that appb mutants are significantly smaller until 2 dpf and display perturbed enveloping layer (EVL) integrity and cell protrusions at the blastula stage. Moreover, appb mutants surviving beyond 48 hpf exhibited no behavioral defects at 6 dpf and developed into healthy and fertile adults. The expression of the app family member, appa, was also found to be altered in appb mutants. Taken together, we show that appb is involved in the initial development of zebrafish by supporting the integrity of the EVL, likely by mediating cell adhesion properties. The loss of Appb might then be compensated for by other app family members to maintain normal development.

Hatching gland development and hatching in zebrafish embryos: A role for zinc and its transporters Zip10 and Znt1a

Zinc transporters of the ZIP (Slc39, importers) and ZnT (Slc30, exporters) protein families have evolutionary conserved roles in biology. The aim of the present study was to explore the role of zinc, and zinc transporters Zip10 and Znt1a in zebrafish hatching gland development and larval hatching. In the study, knockdown of genes for Zip10 and Znt1a in zebrafish embryos was achieved using morpholino-modified oligonucleotides. A partial loss-of-function Znt1a mutant (Znt1a^sa17) allowed comparison with the Znt1a morphant. Free Zn²⁺ in embryos and apoptosis were investigated using fluorescent dyes whereas gene expression was investigated by whole-mount in situ hybridization (WISH). The results showed high levels of free Zn²⁺ in the hatching gland cells (HGC) along with abundant expression of zip10 and znt1a in normal embryo. Knockdown of zip10 reduced free Zn²⁺ in HGC, ceased their normal developmental apoptosis, and resulted in displacement and later disappearance of hatching glands and hatching enzymes he1a and catL1b, and inability to hatch. Conversely, knockdown of znt1a or the Znt1a^sa17 mutation accelerated hatching and coincided with high expression of hatching enzymes and free Zn²⁺ in the HGC. Thus, Zip10 and free Zn²⁺ in the HGC are required both for their development and function. This study also demonstrated the opposite functions of the two zinc transporters, ZIP10 and ZnT1 as well as shedding light on the role of Zn²⁺ in regulation of the human hatching enzyme homologue, ovastacin, which is activated by zinc and cleaves the zona pellucida protein, ZP2, to prevent polyspermy.

Maternal contributions to gastrulation in zebrafish

Gastrulation is a critical early morphogenetic process of animal development, during which the three germ layers; mesoderm, endoderm and ectoderm, are rearranged by internalization movements. Concurrent epiboly movements spread and thin the germ layers while convergence and extension movements shape them into an anteroposteriorly elongated body with head, trunk, tail and organ rudiments. In zebrafish, gastrulation follows the proliferative and inductive events that establish the embryonic and extraembryonic tissues and the embryonic axis. Specification of these tissues and embryonic axes are controlled by the maternal gene products deposited in the egg. These early maternally controlled processes need to generate sufficient cell numbers and establish the embryonic polarity to ensure normal gastrulation. Subsequently, after activation of the zygotic genome, the zygotic gene products govern mesoderm and endoderm induction and germ layer patterning. Gastrulation is initiated during the maternal-to-zygotic transition, a process that entails both activation of the zygotic genome and downregulation of the maternal transcripts. Genomic studies indicate that gastrulation is largely controlled by the zygotic genome. Nonetheless, genetic studies that investigate the relative contributions of maternal and zygotic gene function by comparing zygotic, maternal and maternal zygotic mutant phenotypes, reveal significant contribution of maternal gene products, transcripts and/or proteins, that persist through gastrulation, to the control of gastrulation movements. Therefore, in zebrafish, the maternally expressed gene products not only set the stage for, but they also actively participate in gastrulation morphogenesis.

Stepwise polarisation of developing bilayered epidermis is mediated by aPKC and E-cadherin in zebrafish

The epidermis, a multilayered epithelium, surrounds and protects the vertebrate body. It develops from a bilayered epithelium formed of the outer periderm and underlying basal epidermis. How apicobasal polarity is established in the developing epidermis has remained poorly understood. We show that both the periderm and the basal epidermis exhibit polarised distribution of adherens junctions in zebrafish. aPKC, an apical polarity regulator, maintains the robustness of polarisation of E-cadherin- an adherens junction component- in the periderm. E-cadherin in one layer controls the localisation of E-cadherin in the second layer in a layer non-autonomous manner. Importantly, E-cadherin controls the localisation and levels of Lgl, a basolateral polarity regulator, in a layer autonomous as well non-autonomous manner. Since periderm formation from the enveloping layer precedes the formation of the basal epidermis, our analyses suggest that peridermal polarity, initiated by aPKC, is transduced in a stepwise manner by E-cadherin to the basal layer.

Maternally regulated gastrulation as a source of variation contributing to cavefish forebrain evolution

Sequential developmental events, starting from the moment of fertilization, are crucial for the acquisition of animal body plan. Subtle modifications in such early events are likely to have major impacts in later morphogenesis, bringing along morphological diversification. Here, comparing the blind cave and the surface morphotypes of Astyanax mexicanus fish, we found heterochronies during gastrulation that produce organizer and axial mesoderm tissues with different properties (including differences in the expression of dkk1b) that may have contributed to cavefish brain evolution. These variations observed during gastrulation depend fully on maternal factors. The developmental evolution of retinal morphogenesis and hypothalamic patterning are among those traits that retained significant maternal influence at larval stages. Transcriptomic analysis of fertilized eggs from both morphotypes and reciprocal F₁ hybrids showed a strong and specific maternal signature. Our work strongly suggests that maternal effect genes and developmental heterochronies that occur during gastrulation have impacted morphological brain change during cavefish evolution.

Cell competition corrects noisy Wnt morphogen gradients to achieve robust patterning in the zebrafish embryo

Morphogen signalling forms an activity gradient and instructs cell identities in a signalling strength-dependent manner to pattern developing tissues. However, developing tissues also undergo dynamic morphogenesis, which may produce cells with unfit morphogen signalling and consequent noisy morphogen gradients. Here we show that a cell competition-related system corrects such noisy morphogen gradients. Zebrafish imaging analyses of the Wnt/β-catenin signalling gradient, which acts as a morphogen to establish embryonic anterior-posterior patterning, identify that unfit cells with abnormal Wnt/β-catenin activity spontaneously appear and produce noise in the gradient. Communication between unfit and neighbouring fit cells via cadherin proteins stimulates apoptosis of the unfit cells by activating Smad signalling and reactive oxygen species production. This unfit cell elimination is required for proper Wnt/β-catenin gradient formation and consequent anterior-posterior patterning. Because this gradient controls patterning not only in the embryo but also in adult tissues, this system may support tissue robustness and disease prevention.

Gradients of morphogens such as Wnt provide instructive cues for cell identities during development. Here, the authors report that in the developing zebrafish embryo, cell competition and elimination of unfit cells are required for proper Wnt gradient formation.

A cadherin switch marks germ layer formation in the diploblastic sea anemone Nematostella vectensis

Morphogenesis is a shape-building process during development of multicellular organisms. During this process, the establishment and modulation of cell-cell contacts play an important role. Cadherins, the major cell adhesion molecules, form adherens junctions connecting epithelial cells. Numerous studies of Bilateria have shown that cadherins are associated with the regulation of cell differentiation, cell shape changes, cell migration and tissue morphogenesis. To date, the role of cadherins in non-bilaterians is unknown. Here, we study the expression and function of two paralogous classical cadherins, Cadherin 1 and Cadherin 3, in a diploblastic animal, the sea anemone Nematostella vectensis We show that a cadherin switch accompanies the formation of germ layers. Using specific antibodies, we show that both cadherins are localized to adherens junctions at apical and basal positions in ectoderm and endoderm. During gastrulation, partial epithelial-to-mesenchymal transition of endodermal cells is marked by stepwise downregulation of Cadherin 3 and upregulation of Cadherin 1. Knockdown experiments show that both cadherins are required for maintenance of tissue integrity and tissue morphogenesis. Thus, both sea anemones and bilaterians use independently duplicated cadherins combinatorially for tissue morphogenesis and germ layer differentiation.

Developmental effects in fish embryos exposed to oil dispersions - The impact of crude oil micro-droplets

During accidental crude oil spills and permitted discharges of produced water into the marine environment, a large fraction of naturally occurring oil components will be contained in micron-sized oil droplets. Toxicity is assumed to be associated with the dissolved fraction of oil components, however the potential contribution of oil droplets to toxicity is currently not well known. In the present work we wanted to evaluate the contribution of oil droplets to effects on normal development of Atlantic cod (Gadus morhua) through exposing embryos for 96 h to un-filtered (dispersions containing droplets) and filtered (water soluble fractions) dispersions in a flow-through system at dispersion concentrations ranging from 0.14 to 4.34 mg oil/L. After exposure, the embryos were kept in clean seawater until hatch when survival, development and morphology were assessed. The experiment was performed at two different stages of embryonic development to cover two potentially sensitive stages (gastrulation and organogenesis). Exposure of cod embryos to crude oil dispersions caused acute and delayed toxicity, including manifestation of morphological deformations in hatched larvae. Oil droplets appear to contribute to some of the observed effects including mortality, larvae condition (standard length, body surface, and yolk sac size), spinal deformations as well as alterations in craniofacial and jaw development. The timing of exposure may be essential for the development of effects as higher acute mortality was observed when embryos were exposed from the start of gastrulation (Experiment 1) than when exposed during organogenesis (Experiment 2). Even though low mortality was observed when exposed during organogenesis, concentration-dependent mortality was observed during recovery.

Mechanisms of zebrafish epiboly: A current view

Epiboly is a conserved gastrulation movement describing the thinning and spreading of a sheet or multi-layer of cells. The zebrafish embryo has emerged as a vital model system to address the cellular and molecular mechanisms that drive epiboly. In the zebrafish embryo, the blastoderm, consisting of a simple squamous epithelium (the enveloping layer) and an underlying mass of deep cells, as well as a yolk nuclear syncytium (the yolk syncytial layer) undergo epiboly to internalize the yolk cell during gastrulation. The major events during zebrafish epiboly are: expansion of the enveloping layer and the internal yolk syncytial layer, reduction and removal of the yolk membrane ahead of the advancing blastoderm margin and deep cell rearrangements between the enveloping layer and yolk syncytial layer to thin the blastoderm. Here, work addressing the cellular and molecular mechanisms as well as the sources of the mechanical forces that underlie these events is reviewed. The contribution of recent findings to the current model of epiboly as well as open questions and future prospects are also discussed.

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.

Fluidization-mediated tissue spreading by mitotic cell rounding and non-canonical Wnt signalling

Tissue morphogenesis is driven by mechanical forces that elicit changes in cell size, shape and motion. The extent by which forces deform tissues critically depends on the rheological properties of the recipient tissue. Yet, whether and how dynamic changes in tissue rheology affect tissue morphogenesis and how they are regulated within the developing organism remain unclear. Here, we show that blastoderm spreading at the onset of zebrafish morphogenesis relies on a rapid, pronounced and spatially patterned tissue fluidization. Blastoderm fluidization is temporally controlled by mitotic cell rounding-dependent cell-cell contact disassembly during the last rounds of cell cleavages. Moreover, fluidization is spatially restricted to the central blastoderm by local activation of non-canonical Wnt signalling within the blastoderm margin, increasing cell cohesion and thereby counteracting the effect of mitotic rounding on contact disassembly. Overall, our results identify a fluidity transition mediated by loss of cell cohesion as a critical regulator of embryo morphogenesis.

NADPH-Oxidase-derived reactive oxygen species are required for cytoskeletal organization, proper localization of E-cadherin and cell motility during zebrafish epiboly

Cell movements are essential for morphogenesis during animal development. Epiboly is the first morphogenetic process in zebrafish in which cells move en masse to thin and spread the deep and enveloping cell layers of the blastoderm over the yolk cell. While epiboly has been shown to be controlled by complex molecular networks, the contribution of reactive oxygen species (ROS) to this process has not previously been studied. Here, we show that ROS are required for epiboly in zebrafish. Visualization of ROS in whole embryos revealed dynamic patterns during epiboly progression. Significantly, inhibition of NADPH oxidase activity leads to a decrease in ROS formation, delays epiboly, alters E-cadherin and cytoskeleton patterns and, by 24 h post-fertilization, decreases embryo survival, effects that are rescued by hydrogen peroxide treatment. Our findings suggest that a delicate ROS balance is required during early development and that disruption of that balance interferes with cell adhesion, leading to defective cell motility and epiboly progression.

Probing Cadherin Interactions in Zebrafish with E- and N-Cadherin Missense Mutants

Cadherins are cell adhesion molecules that regulate numerous adhesive interactions during embryonic development and adult life. Consistent with these functions, when their expression goes astray cells lose their normal adhesive properties resulting in defective morphogenesis, disease, and even metastatic cancer. In general, classical cadherins exert their effect by homophilic interactions via their five characteristic extracellular (EC) repeats. The EC1 repeat provides the mechanism for cadherins to dimerize with each other whereas the EC2 repeat may facilitate dimerization. Less is known about the other EC repeats. Here, we show that a zebrafish missense mutation in the EC5 repeat of N-cadherin is a dominant gain-of-function mutation and demonstrate that this mutation alters cell adhesion almost to the same degree as a zebrafish missense mutation in the EC1 repeat of N-cadherin. We also show that zebrafish E- and N-cadherin dominant gain-of-function missense mutations genetically interact. Perturbation of cell adhesion in embryos that are heterozygous mutant at both loci is similar to that observed in single homozygous mutants. Introducing an E-cadherin EC5 missense allele into the homozygous N-cadherin EC1 missense mutant more radically affects morphogenesis, causing synergistic phenotypes consistent with interdependent functions being disrupted. Our studies indicate that a functional EC5 repeat is critical for cadherin-mediated cell affinity, suggesting that its role may be more important than previously thought. These results also suggest the possibility that E- and N-cadherin have heterophilic interactions during early morphogenesis of the embryo; interactions that might help balance the variety of cell affinities needed during embryonic development.

Endocytic trafficking factor VPS45 is essential for spatial regulation of lens fiber differentiation in zebrafish

In vertebrate lens, lens epithelial cells cover the anterior half of the lens fiber core. Lens epithelial cells proliferate, move posteriorly and start to differentiate into lens fiber cells at the lens equator. Although FGF signaling promotes this equatorial commencement of lens fiber differentiation, the underlying mechanism is not fully understood. Here, we show that lens epithelial cells abnormally enter lens fiber differentiation without passing through the equator in zebrafish vps45 mutants. VPS45 belongs to the Sec1/Munc18-like protein family and promotes endosome trafficking, which differentially modulates signal transduction. Ectopic lens fiber differentiation in vps45 mutants does not depend on FGF, but is mediated through activation of TGFβ signaling and inhibition of canonical Wnt signaling. Thus, VPS45 normally suppresses lens fiber differentiation in the anterior region of lens epithelium by modulating TGFβ and canonical Wnt signaling pathways. These data indicate a novel role of endosome trafficking to ensure equator-dependent commencement of lens fiber differentiation.

Summary: The endocytic regulator VPS45 suppresses FGF-independent lens fiber differentiation and ensures the spatial pattern of lens development.

Glucose-6-phosphate dehydrogenase is indispensable in embryonic development by modulation of epithelial-mesenchymal transition via the NOX/Smad3/miR-200b axis

Glucose-6-phosphate dehydrogenase (G6PD) is a housekeeping enzyme involved in the pentose phosphate shunt for producing nicotinamide adenine dinucleotide phosphate (NADPH). Severe G6PD deficiency leads to embryonic lethality, but the underlying mechanism is unclear. In the current study, the effects of G6PD on epithelial-mesenchymal transition (EMT), especially during embryonic development, were investigated. The knockdown of G6PD induced morphological changes, accompanied by the suppression of epithelial markers, E-cadherin and β-catenin, in A549 and MDCK cells. Such modulation of EMT was corroborated by the enhancement of migration ability in G6PD-knockdown A549 cells. Zebrafish embryos with g6pd knockdown exhibited downregulation of the E-cadherin/β-catenin adhesion molecules and impaired embryonic development through reduction in epiboly rate and increase in cell shedding at the embryo surface. The dysregulation in zebrafish embryonic development caused by g6pd knockdown could be rescued through human G6PD or CDH1 (E-cadherin gene) cRNA coinjection. The Smad3/miR-200b axis was dysregulated upon G6PD knockdown, and the reconstitution of SMAD3 in G6PD-knockdown A549 cells restored the expression of E-cadherin/β-catenin. The inhibition of NADPH oxidase (NOX) activation through the loss of p22_phox signaling was involved in the dysregulation of the Smad3/miR-200b axis upon G6PD knockdown. The reconstitution of G6PD led to the recovery of the regulation of NOX/Smad3/miR-200b signaling and increased the expression of E-cadherin/β-catenin in G6PD-knockdown cells. Thus, these results suggest that in the EMT process, G6PD plays an important regulatory role as an integral component of the NOX/Smad3/miR-200b axis.

Mis-expression of grainyhead-like transcription factors in zebrafish leads to defects in enveloping layer (EVL) integrity, cellular morphogenesis and axial extension

The grainyhead-like (grhl) transcription factors play crucial roles in craniofacial development, epithelial morphogenesis, neural tube closure, and dorso-ventral patterning. By utilising the zebrafish to differentially regulate expression of family members grhl2b and grhl3, we show that both genes regulate epithelial migration, particularly convergence-extension (CE) type movements, during embryogenesis. Genetic deletion of grhl3 via CRISPR/Cas9 results in failure to complete epiboly and pre-gastrulation embryonic rupture, whereas morpholino (MO)-mediated knockdown of grhl3 signalling leads to aberrant neural tube morphogenesis at the midbrain-hindbrain boundary (MHB), a phenotype likely due to a compromised overlying enveloping layer (EVL). Further disruptions of grhl3-dependent pathways (through co-knockdown of grhl3 with target genes spec1 and arhgef19) confirm significant MHB morphogenesis and neural tube closure defects. Concomitant MO-mediated disruption of both grhl2b and grhl3 results in further extensive CE-like defects in body patterning, notochord and somite morphogenesis. Interestingly, over-expression of either grhl2b or grhl3 also leads to numerous phenotypes consistent with disrupted cellular migration during gastrulation, including embryo dorsalisation, axial duplication and impaired neural tube migration leading to cyclopia. Taken together, our study ascribes novel roles to the Grhl family in the context of embryonic development and morphogenesis.

Cadherins function during the collective cell migration of Xenopus Cranial Neural Crest cells: revisiting the role of E-cadherin

Collective cell migration is a process whereby cells move while keeping contact with other cells. The Xenopus Cranial Neural Crest (CNC) is a population of cells that emerge during early embryogenesis and undergo extensive migration from the dorsal to ventral part of the embryo's head. These cells migrate collectively and require cadherin mediated cell-cell contact. In this review, we will describe the key features of Xenopus CNC migration including the key molecules driving their migration. We will also review the role of the various cadherins during Xenopus CNC emergence and migration. Lastly, we will discuss the recent and seemingly controversial findings showing that E-cadherin presence is essential for CNC migration.

Cell division and cadherin-mediated adhesion regulate lens epithelial cell movement in zebrafish

In vertebrates, lens epithelial cells cover the anterior half of the lens fiber core. During development, lens epithelial cells proliferate, move posteriorly and differentiate into lens fiber cells after passing through the equator. To elucidate the mechanisms underlying lens epithelial cell movement, we conducted time-lapse imaging of zebrafish lens epithelium. Lens epithelial cells do not intermingle but maintain their relative positions during development. Cell division induces epithelial rearrangement, which subsequently promotes cell movement towards the equator. These data suggest that cell division is the major driving force for cell movement. In zebrafish, E-cadherin is expressed in lens epithelium, whereas N-cadherin is required for lens fiber growth. E-cadherin reduced lens epithelial cell movement, whereas N-cadherin enhanced it. Laser ablation experiments revealed that lens epithelium is governed by pulling tension, which is modulated by these cadherins. Thus, cell division and cadherin-mediated adhesion regulate lens epithelial cell movement via modulation of epithelial tension.

The Molecular Basis of Radial Intercalation during Tissue Spreading in Early Development

Radial intercalation is a fundamental process responsible for the thinning of multilayered tissues during large-scale morphogenesis; however, its molecular mechanism has remained elusive. Using amphibian epiboly, the thinning and spreading of the animal hemisphere during gastrulation, here we provide evidence that radial intercalation is driven by chemotaxis of cells toward the external layer of the tissue. This role of chemotaxis in tissue spreading and thinning is unlike its typical role associated with large-distance directional movement of cells. We identify the chemoattractant as the complement component C3a, a factor normally linked with the immune system. The mechanism is explored by computational modeling and tested in vivo, ex vivo, and in vitro. This mechanism is robust against fluctuations of chemoattractant levels and expression patterns and explains expansion during epiboly. This study provides insight into the fundamental process of radial intercalation and could be applied to a wide range of morphogenetic events.

Extra-embryonic tissue spreading directs early embryo morphogenesis in killifish

The spreading of mesenchymal-like cell layers is critical for embryo morphogenesis and tissue repair, yet we know little of this process in vivo. Here we take advantage of unique developmental features of the non-conventional annual killifish embryo to study the principles underlying tissue spreading in a simple cellular environment, devoid of patterning signals and major morphogenetic cell movements. Using in vivo experimentation and physical modelling we reveal that the extra-embryonic epithelial enveloping cell layer, thought mainly to provide protection to the embryo, directs cell migration and the spreading of embryonic tissue during early development. This function relies on the ability of embryonic cells to couple their autonomous random motility to non-autonomous signals arising from the expansion of the extra-embryonic epithelium, mediated by cell membrane adhesion and tension. Thus, we present a mechanism of extra-embryonic control of embryo morphogenesis that couples the mechanical properties of adjacent tissues in the early killifish embryo.

Cell reintegration: Stray epithelial cells make their way home

Ongoing work shows that misplaced epithelial cells have the capacity to reintegrate back into tissue layers. This movement appears to underlie tissue stability and may also control aspects of tissue structure. A recent study reveals that cell reintegration in at least one tissue, the Drosophila follicular epithelium, is based on adhesion molecules that line lateral cell surfaces. In this article we will review these observations, discuss their implications for epithelial tissue development and maintenance, and identify future directions for study.

Friction forces position the neural anlage

During embryonic development, mechanical forces are essential for cellular rearrangements driving tissue morphogenesis. Here, we show that in the early zebrafish embryo, friction forces are generated at the interface between anterior axial mesoderm (prechordal plate, ppl) progenitors migrating towards the animal pole and neurectoderm progenitors moving in the opposite direction towards the vegetal pole of the embryo. These friction forces lead to global rearrangement of cells within the neurectoderm and determine the position of the neural anlage. Using a combination of experiments and simulations, we show that this process depends on hydrodynamic coupling between neurectoderm and ppl as a result of E-cadherin-mediated adhesion between those tissues. Our data thus establish the emergence of friction forces at the interface between moving tissues as a critical force-generating process shaping the embryo.

The Physical Basis of Coordinated Tissue Spreading in Zebrafish Gastrulation

Embryo morphogenesis relies on highly coordinated movements of different tissues. However, remarkably little is known about how tissues coordinate their movements to shape the embryo. In zebrafish embryogenesis, coordinated tissue movements first become apparent during "doming," when the blastoderm begins to spread over the yolk sac, a process involving coordinated epithelial surface cell layer expansion and mesenchymal deep cell intercalations. Here, we find that active surface cell expansion represents the key process coordinating tissue movements during doming. By using a combination of theory and experiments, we show that epithelial surface cells not only trigger blastoderm expansion by reducing tissue surface tension, but also drive blastoderm thinning by inducing tissue contraction through radial deep cell intercalations. Thus, coordinated tissue expansion and thinning during doming relies on surface cells simultaneously controlling tissue surface tension and radial tissue contraction.

αE-catenin-dependent mechanotransduction is essential for proper convergent extension in zebrafish

Cadherin complexes mediate cell-cell adhesion and are crucial for embryonic development. Besides their structural function, cadherin complexes also transduce tension across the junction-actomyosin axis into proportional biochemical responses. Central to this mechanotransduction is the stretching of the cadherin-F-actin-linker α-catenin, which opens its central domain for binding to effectors such as vinculin. Mechanical unfolding of α-catenin leads to force-dependent reinforcement of cadherin-based junctions as studied in cell culture. The importance of cadherin mechanotransduction for embryonic development has not been studied yet. Here we used TALEN-mediated gene disruption to perturb endogenous αE-catenin in zebrafish development. Zygotic α-catenin mutants fail to maintain their epithelial barrier, resulting in tissue rupturing. We then specifically disrupted mechanotransduction, while maintaining cadherin adhesion, by expressing an αE-catenin construct in which the mechanosensitive domain was perturbed. Expression of either wild-type or mechano-defective α-catenin fully rescues barrier function in α-catenin mutants; however, expression of mechano-defective α-catenin also induces convergence and extension defects. Specifically, the polarization of cadherin-dependent, lamellipodia-driven cell migration of the lateral mesoderm was lost. These results indicate that cadherin mechanotransduction is crucial for proper zebrafish morphogenesis, and uncover one of the essential processes affected by its perturbation.

Summary: Cadherin adhesions transduce tension across the junction-actomyosin axis into proportional biochemical responses via α-catenin. Here we show for the first time how this function of the cadherin complex is important during zebrafish morphogenesis.

Zinc transporter ZIP10 forms a heteromer with ZIP6 which regulates embryonic development and cell migration

Zinc is involved in cell migration during embryo development and in cancer. We show that a zinc transporter consisting of two proteins, ZIP6 and ZIP10, stimulates both cell migration and division in mammalian cells and in the zebrafish embryo.

There is growing evidence that zinc and its transporters are involved in cell migration during development and in cancer. In the present study, we show that zinc transporter ZIP10 (SLC39A10) stimulates cell motility and proliferation, both in mammalian cells and in the zebrafish embryo. This is associated with inactivation of GSK (glycogen synthase kinase)-3α and -3β and down-regulation of E-cadherin (CDH1). Morpholino-mediated knockdown of zip10 causes delayed epiboly and deformities of the head, eye, heart and tail. Furthermore, zip10 deficiency results in overexpression of cdh1, zip6 and stat3, the latter gene product driving transcription of both zip6 and zip10. The non-redundant requirement of Zip6 and Zip10 for epithelial to mesenchymal transition (EMT) is consistent with our finding that they exist as a heteromer. We postulate that a subset of ZIPs carrying prion protein (PrP)-like ectodomains, including ZIP6 and ZIP10, are integral to cellular pathways and plasticity programmes, such as EMT.

Impaired cytoskeletal arrangements and failure of ventral body wall closure in chick embryos treated with rock inhibitor (Y-27632)

AIM

Rho-associated kinase (ROCK) signaling regulates numerous fundamental developmental processes during embryogenesis, primarily by controlling actin-cytoskeleton assembly and cell contractility. ROCK knockout mice exhibit a ventral body wall defect (VBWD) phenotype due to disorganization of actin filaments at the umbilical ring. However, the exact molecular mechanisms leading to VBWD still remain unclear. Improper somitogenesis has been hypothesized to contribute to failure of VBW closure. We designed this study to investigate the hypothesis that administration of ROCK inhibitor (Y-27632) disrupts cytoskeletal arrangements in morphology during early chick embryogenesis, which may contribute to the development of VBWD.

METHODS

At 60 h incubation, chick embryos were explanted into shell-less culture and treated with 50 µL of vehicle for controls (n = 33) or 50 µL of 500 µM of Y-27632 for the experimental group (Y-27, n = 56). At 8 h post-treatment, RT-PCR was performed to evaluate mRNA levels of N-cadherin, E-cadherin and connexin43. Immunofluorescence confocal microscopy was performed to analyze the expression and distribution of actin, vinculin and microtubules in the neural tube and somites. A further cohort of embryos was treated in ovo by dropping 50 µL of vehicle or 50 µL of different concentrations of Y-27632 onto the embryo and allowing development to 12 and 14 days for further assessment.

RESULTS

Gene expression levels of N-cadherin, E-cadherin and connexin43 were significantly decreased in treated embryos compared with controls (p < 0.05). Thickened actin filament bundles were recorded in the neural tube of Y-27 embryos. In somites, cells were dissociated with reduced actin distribution in affected embryos. Clumping of vinculin expression was found in the neural tube and somites, whereas reduced expression of microtubules was observed in Y-27 embryos compared with controls. At 12 and 14 days of development, affected embryos presented with an enlarged umbilical ring and herniation of abdominal contents through the defect.

CONCLUSION

ROCK inhibition alters cytoskeletal arrangement during early chick embryogenesis, which may contribute to failure of anterior body wall closure causing VBWD at later stages of development.

Type IV Collagen Controls the Axogenesis of Cerebellar Granule Cells by Regulating Basement Membrane Integrity in Zebrafish

Granule cells (GCs) are the major glutamatergic neurons in the cerebellum, and GC axon formation is an initial step in establishing functional cerebellar circuits. In the zebrafish cerebellum, GCs can be classified into rostromedial and caudolateral groups, according to the locations of their somata in the corresponding cerebellar lobes. The axons of the GCs in the caudolateral lobes terminate on crest cells in the dorsal hindbrain, as well as forming en passant synapses with Purkinje cells in the cerebellum. In the zebrafish mutant shiomaneki, the caudolateral GCs extend aberrant axons. Positional cloning revealed that the shiomaneki (sio) gene locus encodes Col4a6, a subunit of type IV collagen, which, in a complex with Col4a5, is a basement membrane (BM) component. Both col4a5 and col4a6 mutants displayed similar abnormalities in the axogenesis of GCs and retinal ganglion cells (RGCs). Although type IV collagen is reported to control axon targeting by regulating the concentration gradient of an axonal guidance molecule Slit, Slit overexpression did not affect the GC axons. The structure of the BM surrounding the tectum and dorsal hindbrain was disorganized in the col4a5 and col4a6 mutants. Moreover, the abnormal axogenesis of the caudolateral GCs and the RGCs was coupled with aberrant BM structures in the type IV collagen mutants. The regrowth of GC axons after experimental ablation revealed that the original and newly formed axons displayed similar branching and extension abnormalities in the col4a6 mutants. These results collectively suggest that type IV collagen controls GC axon formation by regulating the integrity of the BM, which provides axons with the correct path to their targets.

The cerebellum is involved in motor coordination and motor learning. Granule cells are the major excitatory neurons in the cerebellum. It is largely unknown how the formation of cerebellar neural circuits, including the elaboration of granule cell axons, is controlled. We investigated a zebrafish mutant shiomaneki, in which some of the granule cells have abnormal axons. We identified collagen (col) 4a6 as the gene responsible for the mutant phenotype. Col4a6 forms a complex with Col4a5, which is a component of the basement membrane. We found that mutants of both col4a5 and col4a6 showed similar axonal abnormalities in both the granule cells and the retinal ganglion cells, and that the basement membrane structure surrounding the central nervous system was disrupted in these mutants. Furthermore, the abnormalities in granule cell axon formation were coupled with aberrant basement membrane structures in the col4a6 mutants. These data suggest that type IV collagen controls the axon formation of some types of neurons by establishing and/or maintaining the integrity of the basement membrane, which provides axons with the correct path to their targets. These findings may explain some aspects of a human disorder, Alport syndrome, which is caused by mutations in type IV collagen genes.

Spatial pattern of cell geometry and cell-division orientation in zebrafish lens epithelium

Cell proliferation is a key regulator of tissue morphogenesis. We examined cell proliferation and cell division in zebrafish lens epithelium by visualizing cell-cycle phases and nuclear positions, using fluorescent-labeled geminin and histone proteins. Proliferation was low in the anterior region of lens epithelium and higher in the marginal zone anterior to the equator, suggesting that the proliferation zone, called the germinative zone, is formed in zebrafish lens. Interestingly, cell-division orientation was biased longitudinally in the anterior region, shifted from longitudinal to circumferential along the anterior-posterior axis of lens sphere, and was biased circumferentially in the peripheral region. These data suggest that cell-division orientation is spatially regulated in zebrafish lens epithelium. The Hertwig rule indicates that cells tend to divide along their long axes. Orientation of long axes and cell division were biased similarly in zebrafish lens epithelium, suggesting that cell geometry correlates with cell-division orientation. A cell adhesion molecule, E-cadherin, is expressed in lens epithelium. In a zebrafish e-cadherin mutant, the long axes and cell-division orientation were shifted more longitudinally. These data suggest that E-cadherin is required for the spatial pattern of cell geometry and cell-division orientation in zebrafish lens epithelium.

Spatio-temporally restricted expression of cell adhesion molecules during chicken embryonic development

Differential cell adhesive properties are known to regulate important developmental events like cell sorting and cell migration. Cadherins and protocadherins are known to mediate these cellular properties. Though a large number of such molecules have been predicted, their characterization in terms of interactive properties and cellular roles is far from being comprehensive. To narrow down the tissue context and collect correlative evidence for tissue specific roles of these molecules, we have carried out whole-mount in situ hybridization based RNA expression study for seven cadherins and four protocadherins. In developing chicken embryos (HH stages 18, 22, 26 and 28) cadherins and protocadherins are expressed in tissue restricted manner. This expression study elucidates precise expression domains of cell adhesion molecules in the context of developing embryos. These expression domains provide spatio-temporal context in which the function of these genes can be further explored.

Cell adhesion in zebrafish embryos is modulated by March 8

March8 is a member of a family of transmembrane E3 ubiquitin ligases that have been studied mostly for their role in the immune system. We find that March8 is expressed in the zebrafish egg and early embryo, suggesting a role in development. Both knock-down and overexpression of March8 leads to abnormal development. The phenotype of zebrafish embryos and Xenopus animal explants overexpressing March8 implicates impairment of cell adhesion as a cause of the effect. In zebrafish embryos and in cultured cells, overexpression of March8 leads to a reduction in the surface levels of E-cadherin, a major cell-cell adhesion molecule. Experiments in cell culture further show that E-cadherin can be ubiquitinated by March8. On the basis of these observations we suggest that March8 functions in the embryo to modulate the strength of cell adhesion by regulating the localization of E-cadherin.

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.

Cell intercalation from top to bottom

Animal development requires a carefully orchestrated cascade of cell fate specification events and cellular movements. A surprisingly small number of choreographed cellular behaviours are used repeatedly to shape the animal body plan. Among these, cell intercalation lengthens or spreads a tissue at the expense of narrowing along an orthogonal axis. Key steps in the polarization of both mediolaterally and radially intercalating cells have now been clarified. In these different contexts, intercalation seems to require a distinct combination of mechanisms, including adhesive changes that allow cells to rearrange, cytoskeletal events through which cells exert the forces needed for cell neighbour exchange, and in some cases the regulation of these processes through planar cell polarity.

Apolipoprotein C1 regulates epiboly during gastrulation in zebrafish

Apolipoprotein C1 (Apoc1) is associated with lipoprotein metabolism, but its physiological role during embryogenesis is largely unknown. We reveal a new function of Apoc1b, a transcript isoform of Apoc1, in epiboly during zebrafish gastrulation. Apoc1b is expressed in yolk syncytial layers and in deep cells of the ventral and lateral region of the embryos. It displays a radial gradient with high levels in the interior layer and low levels in the superficial layer. Knockdown of Apoc1b by injecting antisense morpholino (MO) caused the epiboly arrest in deep cells. Moreover, we show that the radial intercalation and the radial gradient distribution of E-cadherin are disrupted both in Apoc1b knockdown and overexpressed embryos. Therefore, Apoc1b controls epiboly via E-cadherin-mediated radial intercalation in a gradient-dependent manner.

Nodal promotes mir206 expression to control convergence and extension movements during zebrafish gastrulation

Nodal, a member of the transforming growth factor β (TGF-β) superfamily, has been shown to play a role in mesendoderm induction and gastrulation movements. The activity of Nodal signaling can be modulated by microRNAs (miRNAs) as previously reported, but little is known about which miRNAs are regulated by Nodal during gastrulation. In the present study, we found that the expression of mir206, one of the most abundant miRNAs during zebrafish early embryo development, is regulated by Nodal signaling. Abrogation of Nodal signal activity results in defective convergence and extension (CE) movements, and these cell migration defects can be rescued by supplying an excess of mir206, suggesting that mir206 acts downstream of Nodal signaling to regulate CE movements. Furthermore, in mir206 morphants, the expression of cell adhesion molecule E-cadherin is significantly increased, while the key transcriptional repressor of E-cadherin, snail1a, is depressed. Our study uncovers a novel mechanism by which Nodal-regulated mir206 modulates gastrulation movements in connection with the Snail/E-cadherin pathway.