Integrin-ECM interactions regulate cadherin-dependent cell adhesion and are required for convergent extension in Xenopus

Mesodermal fibronectin controls cell shape, polarity, and mechanotransduction in the second heart field during cardiac outflow tract development

Failure in the elongation of the cardiac outflow tract (OFT) results in congenital heart disease due to the misalignment of the great arteries with the left and right ventricles. The OFT lengthens via the accretion of progenitors from the second heart field (SHF). SHF cells are exquisitely regionalized and organized into an epithelial-like layer, forming the dorsal pericardial wall (DPW). Tissue tension, cell polarity, and proliferation within the DPW are important for the addition of SHF-derived cells to the heart and OFT elongation. However, the genes controlling these processes are not completely characterized. Using conditional mutagenesis in the mouse, we show that fibronectin (FN1) synthesized by the mesoderm coordinates multiple cellular behaviors in the anterior DPW. FN1 is enriched in the anterior DPW and plays a role in OFT elongation by maintaining a balance between pro- and anti-adhesive cell-extracellular matrix (ECM) interactions and controlling DPW cell shape, polarity, cohesion, proliferation, and mechanotransduction.

N-cadherin and β1 integrin coordinately regulate growth plate cartilage architecture

Spatial and temporal regulation of chondrocyte maturation in the growth plate drives growth of many bones. One essential event to generate the ordered cell array characterizing growth plate cartilage is the formation of chondrocyte columns in the proliferative zone via 90-degree rotation of daughter cells to align with the long axis of the bone. Previous studies have suggested crucial roles for cadherins and integrin β1 in column formation. The purpose of this study was to determine the relative contributions of cadherin- and integrin-mediated cell adhesion in column formation. Here we present new mechanistic insights generated by application of live time-lapse confocal microscopy of cranial base explant cultures, robust genetic mouse models, and new quantitative methods to analyze cell behavior. We show that conditional deletion of either the cell-cell adhesion molecule Cdh2 or the cell-matrix adhesion molecule Itgb1 disrupts column formation. Compound mutants were used to determine a potential reciprocal regulatory interaction between the two adhesion surfaces and identified that defective chondrocyte rotation in a N-cadherin mutant was restored by a heterozygous loss of integrin β1. Our results support a model for which integrin β1, and not N-cadherin, drives chondrocyte rotation and for which N-cadherin is a potential negative regulator of integrin β1 function.

A morphogenetic wave in the chick embryo lateral mesoderm generates mesenchymal-epithelial transition through a 3D-rosette intermediate

Formation of epithelia through mesenchymal-epithelial transition (MET) is essential for embryonic development and for many physiological and pathological processes. This study investigates MET in vivo in the chick embryo lateral mesoderm, where a multilayered mesenchyme transforms into two parallel epithelial sheets that constitute the coelomic lining of the embryonic body cavity. Prior to MET initiation, mesenchymal cells exhibit non-polarized distribution of multiple polarity markers, albeit not aPKC. We identified an epithelializing wave that sweeps across the lateral mesoderm, the wavefront of which is characterized by the accumulation of basal fibronectin and a network of 3D rosettes composed of polarized, wedge-shaped cells surrounding a central focus of apical markers, now including aPKC. Initiation of the MET process is dependent on extracellular matrix-integrin signaling acting through focal adhesion kinase and talin, whereas progression through the rosette phase requires aPKC function. We present a stepwise model for MET, comprising polarization, 3D-rosette, and epithelialization stages.

Adherens junctions stimulate and spatially guide integrin activation and extracellular matrix deposition

Cadherins and integrins are intrinsically linked through the actin cytoskeleton and are largely responsible for the mechanical integrity and organization of tissues. We show that cadherin clustering stimulates and spatially guides integrin activation. Adherens junction (AJ)-associated integrin activation depends on locally generated tension and does not require extracellular matrix ligands. It leads to the creation of primed integrin clusters, which spatially determine where focal adhesions will form if ligands are present and where ligands will be deposited. AJs that display integrin activation are targeted by microtubules facilitating their disassembly via caveolin-based endocytosis, showing that integrin activation impacts the stability of the core cadherin complex. Thus, the interplay between cadherins and integrins is more intimate than what was once believed and is rooted in the capacity of active integrins to be stabilized via AJ-generated tension. Altogether, our data establish a mechanism of cross-regulation between cadherins and integrins.

Mechanical disruption of E-cadherin complexes with epidermal growth factor receptor actuates growth factor-dependent signaling

Increased intercellular tension is associated with enhanced cell proliferation and tissue growth. Here, we present evidence for a force-transduction mechanism that links mechanical perturbations of epithelial (E)-cadherin (CDH1) receptors to the force-dependent activation of epidermal growth factor receptor (EGFR, ERBB1)-a key regulator of cell proliferation. Here, coimmunoprecipitation studies first show that E-cadherin and EGFR form complexes at the plasma membrane that are disrupted by either epidermal growth factor (EGF) or increased tension on homophilic E-cadherin bonds. Although force on E-cadherin bonds disrupts the complex in the absence of EGF, soluble EGF is required to mechanically activate EGFR at cadherin adhesions. Fully quantified spectral imaging fluorescence resonance energy transfer further revealed that E-cadherin and EGFR directly associate to form a heterotrimeric complex of two cadherins and one EGFR protein. Together, these results support a model in which the tugging forces on homophilic E-cadherin bonds trigger force-activated signaling by releasing EGFR monomers to dimerize, bind EGF ligand, and signal. These findings reveal the initial steps in E-cadherin-mediated force transduction that directly link intercellular force fluctuations to the activation of growth regulatory signaling cascades.

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.

A zebrafish embryo screen utilizing gastrulation identifies the HTR2C inhibitor pizotifen as a suppressor of EMT-mediated metastasis

Metastasis is responsible for approximately 90% of cancer-associated mortality but few models exist that allow for rapid and effective screening of anti-metastasis drugs. Current mouse models of metastasis are too expensive and time consuming to use for rapid and high-throughput screening. Therefore, we created a unique screening concept utilizing conserved mechanisms between zebrafish gastrulation and cancer metastasis for identification of potential anti-metastatic drugs. We hypothesized that small chemicals that interrupt zebrafish gastrulation might also suppress metastatic progression of cancer cells and developed a phenotype-based chemical screen to test the hypothesis. The screen used epiboly, the first morphogenetic movement in gastrulation, as a marker and enabled 100 chemicals to be tested in 5 hr. The screen tested 1280 FDA-approved drugs and identified pizotifen, an antagonist for serotonin receptor 2C (HTR2C) as an epiboly-interrupting drug. Pharmacological and genetic inhibition of HTR2C suppressed metastatic progression in a mouse model. Blocking HTR2C with pizotifen restored epithelial properties to metastatic cells through inhibition of Wnt signaling. In contrast, HTR2C induced epithelial-to-mesenchymal transition through activation of Wnt signaling and promoted metastatic dissemination of human cancer cells in a zebrafish xenotransplantation model. Taken together, our concept offers a novel platform for discovery of anti-metastasis drugs.

Scribble mutation disrupts convergent extension and apical constriction during mammalian neural tube closure

Morphogenesis of the vertebrate neural tube occurs by elongation and bending of the neural plate, tissue shape changes that are driven at the cellular level by polarized cell intercalation and cell shape changes, notably apical constriction and cell wedging. Coordinated cell intercalation, apical constriction, and wedging undoubtedly require complex underlying cytoskeletal dynamics and remodeling of adhesions. Mutations of the gene encoding Scribble result in neural tube defects in mice, however the cellular and molecular mechanisms by which Scrib regulates neural cell behavior remain unknown. Analysis of Scribble mutants revealed defects in neural tissue shape changes, and live cell imaging of mouse embryos showed that the Scrib mutation results in defects in polarized cell intercalation, particularly in rosette resolution, and failure of both cell apical constriction and cell wedging. Scrib mutant embryos displayed aberrant expression of the junctional proteins ZO-1, Par3, Par6, E- and N-cadherins, and the cytoskeletal proteins actin and myosin. These findings show that Scribble has a central role in organizing the molecular complexes regulating the morphomechanical neural cell behaviors underlying vertebrate neurulation, and they advance our understanding of the molecular mechanisms involved in mammalian neural tube closure.

β1 Integrin regulates convergent extension in mouse notogenesis, ensures notochord integrity and the morphogenesis of vertebrae and intervertebral discs

The notochord drives longitudinal growth of the body axis by convergent extension, a highly conserved developmental process that depends on non-canonical Wnt/planar cell polarity (PCP) signaling. However, the role of cell-matrix interactions mediated by integrins in the development of the notochord is unclear. We developed transgenic Cre mice, in which the β1 integrin gene (Itgb1) is ablated at E8.0 in the notochord only or in the notochord and tail bud. These Itgb1 conditional mutants display misaligned, malformed vertebral bodies, hemi-vertebrae and truncated tails. From early somite stages, the notochord was interrupted and displaced in these mutants. Convergent extension of the notochord was impaired with defective cell movement. Treatment of E7.25 wild-type embryos with anti-β1 integrin blocking antibodies, to target node pit cells, disrupted asymmetric localization of VANGL2. Our study implicates pivotal roles of β1 integrin for the establishment of PCP and convergent extension of the developing notochord, its structural integrity and positioning, thereby ensuring development of the nucleus pulposus and the proper alignment of vertebral bodies and intervertebral discs. Failure of this control may contribute to human congenital spine malformations.

Polarity Establishment and Maintenance in Ascidian Notochord

Cell and tissue polarity due to the extracellular signaling and intracellular gene cascades, in turn, signals the directed cell behaviors and asymmetric tissue architectures that play a crucial role in organogenesis and embryogenesis. The notochord is a characteristic midline organ in chordate embryos that supports the body structure and produces positioning signaling. This review summarizes cellular and tissue-level polarities during notochord development in ascidians. At the early stage, planar cell polarity (PCP) is initialized, which drives cell convergence extension and migration to form a rod-like structure. Subsequently, the notochord undergoes a mesenchymal-epithelial transition, becoming an unusual epithelium in which cells have two opposing apical domains facing the extracellular lumen deposited between adjacent notochord cells controlled by apical-basal (AB) polarity. Cytoskeleton distribution is one of the main downstream events of cell polarity. Some cytoskeleton polarity patterns are a consequence of PCP: however, an additional polarized cytoskeleton, together with Rho signaling, might serve as a guide for correct AB polarity initiation in the notochord. In addition, the notochord's mechanical properties are associated with polarity establishment and transformation, which bridge signaling regulation and tissue mechanical properties that enable the coordinated organogenesis during embryo development.

Collagen, stiffness, and adhesion: the evolutionary basis of vertebrate mechanobiology

The emergence of collagen I in vertebrates resulted in a dramatic increase in the stiffness of the extracellular environment, supporting long-range force propagation and the development of low-compliant tissues necessary for the development of vertebrate traits including pressurized circulation and renal filtration. Vertebrates have also evolved integrins that can bind to collagens, resulting in the generation of higher tension and more efficient force transmission in the extracellular matrix. The stiffer environment provides an opportunity for the vertebrates to create new structures such as the stress fibers, new cell types such as endothelial cells, new developmental processes such as neural crest delamination, and new tissue organizations such as the blood-brain barrier. Molecular players found only in vertebrates allow the modification of conserved mechanisms as well as the design of novel strategies that can better serve the physiological needs of the vertebrates. These innovations collectively contribute to novel morphogenetic behaviors and unprecedented increases in the complexities of tissue mechanics and functions.

Integrins Modulate Extracellular Matrix Organization to Control Cell Signaling during Hematopoiesis

During hematopoiesis, progenitor cells receive and interpret a diverse array of regulatory signals from their environment. These signals control the maintenance of the progenitors and regulate the production of mature blood cells. Integrins are well known in vertebrates for their roles in hematopoiesis, particularly in assisting in the migration to, as well as the physical attachment of, progenitors to the niche. However, whether and how integrins are also involved in the signaling mechanisms that control hematopoiesis remains to be resolved. Here, we show that integrins play a key role during fly hematopoiesis in regulating cell signals that control the behavior of hematopoietic progenitors. Integrins can regulate hematopoiesis directly, via focal adhesion kinase (FAK) signaling, and indirectly, by directing extracellular matrix (ECM) assembly and/or maintenance. ECM organization and density controls blood progenitor behavior by modulating multiple signaling pathways, including bone morphogenetic protein (BMP) and Hedgehog (Hh). Furthermore, we show that integrins and the ECM are reduced following infection, which may assist in activating the immune response. Our results provide mechanistic insight into how integrins can shape the signaling environment around hematopoietic progenitors.

Convergent extension in the amphibian, Xenopus laevis

This review is a comprehensive analysis of the cell biology and biomechanics of Convergent Extension in Xenopus.

Proteomic analysis of decellularized pancreatic matrix identifies collagen V as a critical regulator for islet organogenesis from human pluripotent stem cells

In pancreatic tissue engineering, generating human pancreatic islet organoids from stem cells has been challenging due mainly to a poor understanding of niches required for multicellular tissue self-assembly in vitro. In this study, we aimed to identify bioactive, chemically defined niches from natural, biological materials for islet development in vitro. We investigated the proteomics of decellularized rat pancreatic extracellular matrix (dpECM) hydrogel using advanced bioinformatics analysis, and identified that type V collagen (ColV) is constantly and abundantly present in dpECM hydrogel. Niches provided to human pluripotent stem cells (iPSCs) by presenting ColV in matrix coating substrates permitted stem cells progression into islet-like organoids that consist of all major pancreatic endocrine cell types, i.e. α, β, δ, and pancreatic polypeptide cells. In the presence of ColV niches, gene expressions of all key pancreatic transcription factors and major hormone genes significantly increased in iPSC-derived organoids. Most importantly, ColV-containing microenvironment resulted in enhanced glucose responsive secretions of both insulin and glucagon hormone from organoids. The study demonstrates that ColV is a critical regulator that augments islet self-assembly from iPSCs, and it is feasible to utilize natural biomaterials to build tissue cues essential for multicellular tissue production in vitro.

Integrins are required for synchronous ommatidial rotation in the Drosophila eye linking planar cell polarity signalling to the extracellular matrix

Integrins mediate the anchorage between cells and their environment, the extracellular matrix (ECM), and form transmembrane links between the ECM and the cytoskeleton, a conserved feature throughout development and morphogenesis of epithelial organs. Here, we demonstrate that integrins and components of the ECM are required during the planar cell polarity (PCP) signalling-regulated cell movement of ommatidial rotation in the Drosophila eye. The loss-of-function mutations of integrins or ECM components cause defects in rotation, with mutant clusters rotating asynchronously compared to wild-type clusters. Initially, mutant clusters tend to rotate faster, and at later stages they fail to be synchronous with their neighbours, leading to aberrant rotation angles and resulting in a disorganized ommatidial arrangement in adult eyes. We further demonstrate that integrin localization changes dynamically during the rotation process. Our data suggest that core Frizzled/PCP factors, acting through RhoA and Rho kinase, regulate the function/activity of integrins and that integrins thus contribute to the complex interaction network of PCP signalling, cell adhesion and cytoskeletal elements required for a precise and synchronous 90° rotation movement.

Extracellular Matrix (ECM) and the Sculpting of Embryonic Tissues

Extracellular matrices (ECMs) are structurally and compositionally diverse networks of collagenous and noncollagenous glycoproteins, glycosaminoglycans, proteoglycans, and associated molecules that together comprise the metazoan matrisome. Proper deposition and assembly of ECM is of profound importance to cell proliferation, survival, and differentiation, and the morphogenesis of tissues and organ systems that define sequential steps in the development of all animals. Importantly, it is now clear that the instructive influence of a particular ECM at various points in development reflects more than a simple summing of component parts; cellular responses also reflect the dynamic assembly and changing topology of embryonic ECM, which in turn affect its biomechanical properties. This review highlights recent advances in understanding how biophysical features attributed to ECM, such as stiffness and viscoelasticity, play important roles in the sculpting of embryonic tissues and the regulation of cell fates. Forces generated within cells and tissues are transmitted both through integrin-based adhesions to ECM, and through cadherin-dependent cell-cell adhesions; the resulting short- and long-range deformations of embryonic tissues drive morphogenesis. This coordinate regulation of cell-ECM and cell-cell adhesive machinery has emerged as a common theme in a variety of developmental processes. In this review we consider select examples in the embryo where ECM is implicated in setting up tissue barriers and boundaries, in resisting pushing or pulling forces, or in constraining or promoting cell and tissue movement. We reflect on how each of these processes contribute to morphogenesis.

Local and global dynamics in collective movements of embryonic cells

Several important morphogenetic processes belong to the category of collective cell movements (CCM), by which we mean coordinated rearrangements of many neighboring cells. The causes of the dynamic order established during CCM are still unclear. We performed statistical studies of rates and angular orientations of cell rearrangements in two kinds of embryonic tissues, which we categorized as "committed" (in the sense of being capable of autonomous CCM) as opposed to "naïve" tissues, which are those that require external forces in order to exhibit full scale CCM. In addition, we distinguished two types of cell rearrangements: first, those in which mutual cell-cell shifts characterizing the local dynamics (LD); and, second, those which moved in reference to common external coordinates (global dynamics, GD). We observed that in most cases LD rates deviated from normal distributions and do so by creating excesses of extensively converging and moderately diverging cells. In contrast, GD was characterized by nearly random behavior of slowly moving cells, combined with increased angular focusing of the fast cells trajectories as well as bimodal distribution of cell rates. When committed tissues were opposed by external mechanical forces, then they tended to preserve the inherent CCM patterns. On the other hand, the naïve ones reacted by creating two orthogonal cells flows, one of these coinciding with the force direction. We consider CCM as a self-organizing process based on feedbacks between converging and diverging cell shifts, which is able to focus the trajectories imposed by external forces.

The same but different: cell intercalation as a driver of tissue deformation and fluidity

The ability of cells to exchange neighbours, termed intercalation, is a key feature of epithelial tissues. Intercalation is predominantly associated with tissue deformations that drive morphogenesis. More recently, however, intercalation that is not associated with large-scale tissue deformations has been described both during animal development and in mature epithelial tissues. This latter form of intercalation appears to contribute to an emerging phenomenon that we refer to as tissue fluidity-the ability of cells to exchange neighbours without changing the overall dimensions of the tissue. Here, we discuss the contribution of junctional dynamics to intercalation governing both morphogenesis and tissue fluidity. In particular, we focus on the relative roles of junctional contractility and cell-cell adhesion as the driving forces behind intercalation. These two contributors to junctional mechanics can be used to simulate cellular intercalation in mechanical computational models, to test how junctional cell behaviours might regulate tissue fluidity and contribute to the maintenance of tissue integrity and the onset of disease.This article is part of the Theo Murphy meeting issue 'Mechanics of development'.

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.

Intracellular calcium signal at the leading edge regulates mesodermal sheet migration during Xenopus gastrulation

During the gastrulation stage in animal embryogenesis, the cells leading the axial mesoderm migrate toward the anterior side of the embryo, vigorously extending cell protrusions such as lamellipodia. It is thought that the leading cells sense gradients of chemoattractants emanating from the ectodermal cells and translate them to initiate and maintain the cell movements necessary for gastrulation. However, it is unclear how the extracellular information is converted to the intracellular chemical reactions that lead to motion. Here we demonstrated that intracellular Ca2+ levels in the protrusion-forming leading cells are markedly higher than those of the following cells and the axial mesoderm cells. We also showed that inhibiting the intracellular Ca2+ significantly retarded the gastrulation cell movements, while increasing the intracellular Ca2+ with an ionophore enhanced the migration. We further found that the ionophore treatment increased the active form of the small GTPase Rac1 in these cells. Our results suggest that transient intracellular Ca2+ signals play an essential role in the active cell migration during gastrulation.

Roles for Xenopus aquaporin-3b (aqp3.L) during gastrulation: Fibrillar fibronectin and tissue boundary establishment in the dorsal margin

Aquaporins and aquaglyceroporins are a large family of membrane channel proteins that allow rapid movement of water and small, uncharged solutes into and out of cells along concentration gradients. Recently, aquaporins have been gaining recognition for more complex biological roles than the regulation of cellular osmotic homeostasis. We have identified a specific expression pattern for Xenopus aqp3b (also called aqp3.L) during gastrulation, where it is localized to the sensorial (deep) layer of the blastocoel roof and dorsal margin. Interference with aqp3b expression resulted in loss of fibrillar fibronectin matrix in Brachet's cleft at the dorsal marginal zone, but not on the free surface of the blastocoel. Detailed observation showed that the absence of fibronectin matrix correlated with compromised border integrities between involuted mesendoderm and noninvoluted ectoderm in the marginal zone. Knockdown of aqp3b also led to delayed closure of the blastopore, suggesting defects in gastrulation movements. Radial intercalation was not affected in aqp3b morphants, while the data presented are consistent with impeded convergent extension movements of the dorsal mesoderm in response to loss of aqp3b. Our emerging model suggests that aqp3b is part of a mechanism that promotes proper interaction between cells and the extracellular matrix, thereby playing a critical role in gastrulation.

Models of convergent extension during morphogenesis

Convergent extension (CE) is a fundamental and conserved collective cell movement that forms elongated tissues during embryonic development. Thus far, studies have demonstrated two different mechanistic models of collective cell movements during CE. The first, termed the crawling mode, was discovered in the process of notochord formation in Xenopus laevis embryos, and has been the established model of CE for decades. The second model, known as the contraction mode, was originally reported in studies of germband extension in Drosophila melanogaster embryos and was recently demonstrated to be a conserved mechanism of CE among tissues and stages of development across species. This review summarizes the two modes of CE by focusing on the differences in cytoskeletal behaviors and relative expression of cell adhesion molecules. The upstream molecules regulating these machineries are also discussed. There are abundant studies of notochord formation in X. laevis embryos, as this was one of the pioneering model systems in this field. Therefore, the present review discusses these findings as an approach to the fundamental biological question of collective cell regulation. WIREs Dev Biol 2018, 7:e293. doi: 10.1002/wdev.293

This article is categorized under: 1

Early Embryonic Development > Gastrulation and Neurulation

2

Comparative Development and Evolution > Model Systems

Mechanisms of collective cell movement lacking a leading or free front edge in vivo

Collective cell movement is one of the strategies for achieving the complex shapes of tissues and organs. In this process, multiple cells within a group held together by cell-cell adhesion acquire mobility and move together in the same direction. In some well-studied models of collective cell movement, the mobility depends strongly on traction generated at the leading edge by cells located at the front. However, recent advances in live-imaging techniques have led to the discovery of other types of collective cell movement lacking a leading edge or even a free edge at the front, in a diverse array of morphological events, including tubule elongation, epithelial sheet extension, and tissue rotation. We herein review some of the developmental events that are organized by collective cell movement and attempt to elucidate the underlying cellular and molecular mechanisms, which include membrane protrusions, guidance cues, cell intercalation, and planer cell polarity, or chirality pathways.

Single and collective cell migration: the mechanics of adhesions

Chemical and physical properties of the environment control cell proliferation, differentiation, or apoptosis in the long term. However, to be able to move and migrate through a complex three-dimensional environment, cells must quickly adapt in the short term to the physical properties of their surroundings. Interactions with the extracellular matrix (ECM) occur through focal adhesions or hemidesmosomes via the engagement of integrins with fibrillar ECM proteins. Cells also interact with their neighbors, and this involves various types of intercellular adhesive structures such as tight junctions, cadherin-based adherens junctions, and desmosomes. Mechanobiology studies have shown that cell-ECM and cell-cell adhesions participate in mechanosensing to transduce mechanical cues into biochemical signals and conversely are responsible for the transmission of intracellular forces to the extracellular environment. As they migrate, cells use these adhesive structures to probe their surroundings, adapt their mechanical properties, and exert the appropriate forces required for their movements. The focus of this review is to give an overview of recent developments showing the bidirectional relationship between the physical properties of the environment and the cell mechanical responses during single and collective cell migration.

Acetylcholinesterase plays a non-neuronal, non-esterase role in organogenesis

Acetylcholinesterase (AChE) is crucial for degrading acetylcholine at cholinergic synapses. In vitro studies suggest that, in addition to its role in nervous system signaling, AChE can also modulate non-neuronal cell properties, although it remains controversial whether AChE functions in this capacity in vivo Here, we show that AChE plays an essential non-classical role in vertebrate gut morphogenesis. Exposure of Xenopus embryos to AChE-inhibiting chemicals results in severe defects in intestinal development. Tissue-targeted loss-of-function assays (via microinjection of antisense morpholino or CRISPR-Cas9) confirm that AChE is specifically required in the gut endoderm tissue, a non-neuronal cell population, where it mediates adhesion to fibronectin and regulates cell rearrangement events that drive gut lengthening and digestive epithelial morphogenesis. Notably, the classical esterase activity of AChE is dispensable for this activity. As AChE is deeply conserved, widely expressed outside of the nervous system, and the target of many environmental chemicals, these results have wide-reaching implications for development and toxicology.

Microfluidic Technology for the Generation of Cell Spheroids and Their Applications

A three-dimensional (3D) tissue model has significant advantages over the conventional two-dimensional (2D) model. A 3D model mimics the relevant in-vivo physiological conditions, allowing a cell culture to serve as an effective tool for drug discovery, tissue engineering, and the investigation of disease pathology. The present reviews highlight the recent advances and the development of microfluidics based methods for the generation of cell spheroids. The paper emphasizes on the application of microfluidic technology for tissue engineering including the formation of multicellular spheroids (MCS). Further, the paper discusses the recent technical advances in the integration of microfluidic devices for MCS-based high-throughput drug screening. The review compares the various microfluidic techniques and finally provides a perspective for the future opportunities in this research area.

Integrins in synapse regulation

Integrins are a large family of extracellular matrix (ECM) receptors. In the developing and adult brain, many integrins are present at high levels at synapses. The tetrapartite structure of synapses - which comprises presynaptic and postsynaptic neurons, the ECM and glial processes - places synaptic integrins in an excellent position to sense dynamic changes in the synaptic environment and use this information to coordinate further changes in synapse structure and function that will shape neural circuit properties. Recent developments in our understanding of the cellular and physiological roles of integrins, which range from control of neural process outgrowth and synapse formation to regulation of synaptic plasticity and memory, enable us to attempt a synthesis of synaptic integrin function.

Genomic Basis of Adaptive Evolution: The Survival of Amur Ide (Leuciscus waleckii) in an Extremely Alkaline Environment

The Amur ide (Leuciscus waleckii) is a cyprinid fish that is widely distributed in Northeast Asia. The Lake Dali Nur population inhabits one of the most extreme aquatic environments on Earth, with an alkalinity up to 50 mmol/L (pH 9.6), thus providing an exceptional model with which to characterize the mechanisms of genomic evolution underlying adaptation to extreme environments. Here, we developed the reference genome assembly for L. waleckii from Lake Dali Nur. Intriguingly, we identified unusual expanded long terminal repeats (LTRs) with higher nucleotide substitution rates than in many other teleosts, suggesting their more recent insertion into the L. waleckii genome. We also identified expansions in genes encoding egg coat proteins and natriuretic peptide receptors, possibly underlying the adaptation to extreme environmental stress. We further sequenced the genomes of 10 additional individuals from freshwater and 18 from Lake Dali Nur populations, and we detected a total of 7.6 million SNPs from both populations. In a genome scan and comparison of these two populations, we identified a set of genomic regions under selective sweeps that harbor genes involved in ion homoeostasis, acid-base regulation, unfolded protein response, reactive oxygen species elimination, and urea excretion. Our findings provide comprehensive insight into the genomic mechanisms of teleost fish that underlie their adaptation to extreme alkaline environments.

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.

Fibronectin contributes to notochord intercalation in the invertebrate chordate, Ciona intestinalis

BACKGROUND

Genomic analysis has upended chordate phylogeny, placing the tunicates as the sister group to the vertebrates. This taxonomic rearrangement raises questions about the emergence of a tunicate/vertebrate ancestor.

RESULTS

Characterization of developmental genes uniquely shared by tunicates and vertebrates is one promising approach for deciphering developmental shifts underlying acquisition of novel, ancestral traits. The matrix glycoprotein Fibronectin (FN) has long been considered a vertebrate-specific gene, playing a major instructive role in vertebrate embryonic development. However, the recent computational prediction of an orthologous "vertebrate-like" Fn gene in the genome of a tunicate, Ciona savignyi, challenges this viewpoint suggesting that Fn may have arisen in the shared tunicate/vertebrate ancestor. Here we verify the presence of a tunicate Fn ortholog. Transgenic reporter analysis was used to characterize a Ciona Fn enhancer driving expression in the notochord. Targeted knockdown in the notochord lineage indicates that FN is required for proper convergent extension.

CONCLUSIONS

These findings suggest that acquisition of Fn was associated with altered notochord morphogenesis in the vertebrate/tunicate ancestor.

Inappropriate cadherin switching in the mouse epiblast compromises proper signaling between the epiblast and the extraembryonic ectoderm during gastrulation

Cadherin switching from E-cadherin (E-cad) to N-cadherin (N-cad) is a key step of the epithelial-mesenchymal transition (EMT) processes that occurs during gastrulation and cancer progression. We investigate whether cadherins actively participate in progression of EMT by crosstalk to signaling pathways. We apply ectopic cadherin switching before the onset of mouse gastrulation. Mutants with an induced E-cad to N-cad switch (Ncadki) die around E8.5. Severe morphological changes including a small epiblast, a rounded shape, an enlarged extra-embryonic compartment and lack of the amnion, combined with a massive cell detachment from the ectodermal layer are detected. In contrast to epiblast-specific E-cad depletion, gastrulation is initiated in Ncadki embryos, but patterning of the germ-layers is abnormal. An overall reduction in BMP signaling, expansion of Nodal and Eomes domains, combined with reduced Wnt3a expression at the primitive streak is observed. Our results show that in addition to cadherin-dependent adhesion, proper embryonic development requires E-cad mediated signaling function to facilitate a feedback loop that stabilizes Bmp4 and Bmp2 expression in the extraembryonic ectoderm and sustained downstream activity in the epiblast. Moreover, for proper morphogenesis a fine-tuned spatio-temporal control of cadherin switching is required during EMT at gastrulation to avoid premature cell detachment and migration.

A force balance can explain local and global cell movements during early zebrafish development

Embryonic morphogenesis takes place via a series of dramatic collective cell movements. The mechanisms that coordinate these intricate structural transformations across an entire organism are not well understood. In this study, we used gentle mechanical deformation of developing zebrafish embryos to probe the role of physical forces in generating long-range intercellular coordination during epiboly, the process in which the blastoderm spreads over the yolk cell. Geometric distortion of the embryo resulted in nonuniform blastoderm migration and realignment of the anterior-posterior (AP) axis, as defined by the locations at which the head and tail form, toward the new long axis of the embryo and away from the initial animal-vegetal axis defined by the starting location of the blastoderm. We found that local alterations in the rate of blastoderm migration correlated with the local geometry of the embryo. Chemical disruption of the contractile ring of actin and myosin immediately vegetal to the blastoderm margin via Ca²⁺ reduction or treatment with blebbistatin restored uniform migration and eliminated AP axis reorientation in mechanically deformed embryos; it also resulted in cellular disorganization at the blastoderm margin. Our results support a model in which tension generated by the contractile actomyosin ring coordinates epiboly on both the organismal and cellular scales. Our observations likewise suggest that the AP axis is distinct from the initial animal-vegetal axis in zebrafish.

A ligand-independent integrin β1 mechanosensory complex guides spindle orientation

Control of spindle orientation is a fundamental process for embryonic development, morphogenesis and tissue homeostasis, while defects are associated with tumorigenesis and other diseases. Force sensing is one of the mechanisms through which division orientation is determined. Here we show that integrin β1 plays a critical role in this process, becoming activated at the lateral regions of the cell cortex in a ligand-independent manner. This activation is force dependent and polar, correlating with the spindle capture sites. Inhibition of integrin β1 activation on the cortex and disruption of its asymmetric distribution leads to spindle misorientation, even when cell adhesion is β1 independent. Examining downstream targets reveals that a cortical mechanosensory complex forms on active β1, and regulates spindle orientation irrespective of cell context. We propose that ligand-independent integrin β1 activation is a conserved mechanism that allows cell responses to external stimuli.

Qualitative research of alternatively splice variants of fibronectin in different development stage of mice heart

BACKGROUND

Fibronectin (FN) plays vital roles in cell adhesion, differentiation, proliferation and migration. It is involved in the process of embryonic development and is highly conserved during evolution. The EIIIA and EIIIB of FN show a very high degree of homology among vertebrates. Embryos deleting both EIIIA and EIIIB displayed multiple embryonic cardiovascular defects, implying their crucial role during embryogenesis. The correlation of spliced EIIIB, EIIIA, and IIICS of FN to heart development was studied by observing their chronological expression in mice heart.

METHODS

C57 mice embryos at E11.5, E12.5, E13.5, E14.5, E15.5, E16.5, E17.5, E18.5, E19.5 days, postnatal day 1 (P1d), and adult male mice (3 months) were used. For each alternatively spliced FN1 domain (EIIIB, EIIIA and IIICS), primer pairs were designed for specific amplification. Total RNA was extracted from the heart tissue, reverse transcripted to cDNA, followed by RT-PCR with specific primers. The PCR amplification was verified by agarose gel electrophoresis, showing specific fragments of the expected sizes.

RESULTS

In adult mice heart, only alternatively splice variants of EIIIA-, EIIIB-, IIICS+ were expressed. While in embryonic mice, spliced variant of EIIIA+/-, EIIIB+/-, IIICS+ were observed. The expression of EIIIA and EIIIB changed during heart development.

CONCLUSIONS

FN is crucial for the normal development of the embryonic heart by modulating cardiac neural crest (CNC) proliferation and survival, and maintenance of CNC cells. FN1 gene seems to play a significant role by expression of highly conserved EIIIA and EIIIB in embryonic heart development.

p70 S6 kinase drives ovarian cancer metastasis through multicellular spheroid-peritoneum interaction and P-cadherin/b1 integrin signaling activation

Peritoneal dissemination as a manifestation of ovarian cancer is an adverse prognostic factor associated with poor clinical outcome, and is thus a potentially promising target for improved treatment. Sphere forming cells (multicellular spheroids) present in malignant ascites of patients with ovarian cancer represent a major impediment to effective treatment. p70 S6 kinase (p70^S6K), which is a downstream effector of mammalian target of rapamycin, is frequently hyperactivated in human ovarian cancer. Here, we identified p70^S6K as an important regulator for the seeding and successful colonization of ovarian cancer spheroids on the peritoneum. Furthermore, we provided evidence for the existence of a novel crosstalk between P-cadherin and β1 integrin, which was crucial for the high degree of specificity in cell adhesion. In particular, we demonstrated that the upregulation of mature β1 integrin occurred as a consequence of P-cadherin expression through the induction of the Golgi glycosyltransferase, ST6Gal-I, which mediated β1 integrin hypersialylation. Loss of p70^S6K or targeting the P-cadherin/β1-integrin interplay could significantly attenuate the metastatic spread onto the peritoneum in vivo. These findings establish a new role for p70^S6K in tumor spheroid-mesothelium communication in ovarian cancer and provide a preclinical rationale for targeting p70^S6K as a new avenue for microenvironment-based therapeutic strategy.

Integration of cell-cell and cell-ECM adhesion in vertebrate morphogenesis

In this review, we highlight recent re-evaluations of the classical cell sorting models and their application to understanding embryonic morphogenesis. Modern genetic and biophysical techniques reveal that tissue self-assembly is not solely a result of differential adhesion, but rather incorporates dynamic cytoskeletal tension and extracellular matrix assembly. There is growing evidence that these biomechanical modules cooperate to organize developing tissues. We describe the contributions of Cadherins and Integrins to tissue assembly and propose a model in which these very different adhesive regimes affect the same outcome through separate but convergent mechanisms.

Changes in organelle position and epithelial architecture associated with loss of CrebA

Drosophila CrebA facilitates high-level secretion by transcriptional upregulation of the protein components of the core secretory machinery. In CrebA mutant embryos, both salivary gland (SG) morphology and epidermal cuticle secretion are abnormal, phenotypes similar to those observed with mutations in core secretory pathway component genes. Here, we examine the cellular defects associated with CrebA loss in the SG epithelium. Apically localized secretory vesicles are smaller and less abundant, consistent with overall reductions in secretion. Unexpectedly, global mislocalization of cellular organelles and excess membrane accumulation in the septate junctions (SJs) are also observed. Whereas mutations in core secretory pathway genes lead to organelle localization defects similar to those of CrebA mutants, they have no effect on SJ-associated membrane. Mutations in tetraspanin genes, which are normally repressed by CrebA, have mild defects in SJ morphology that are rescued by simultaneous CrebA loss. Correspondingly, removal of several tetraspanins gives partial rescue of the CrebA SJ phenotype, supporting a role for tetraspanins in SJ organization.

Epithelial-mesenchymal transitions: from cell plasticity to concept elasticity

Epithelial-mesenchymal transition (EMT) is a developmental cellular process occurring during early embryo development, including gastrulation and neural crest cell migration. It can be broken down in distinct functional steps: (1) loss of baso-apical polarization characterized by cytoskeleton, tight junctions, and hemidesmosomes remodeling; (2) individualization of cells, including a decrease in cell-cell adhesion forces, (3) emergence of motility, and (4) invasive properties, including passing through the subepithelial basement membrane. These phases occur in an uninterrupted process, without requiring mitosis, in an order and with a degree of completion dictated by the microenvironment. The whole process reflects the activation of specific transcription factor families, called EMT transcription factors. Several mechanisms can combine to induce EMT. Some are reversible, involving growth factors and cytokines and/or environmental signals including extracellular matrix and local physical conditions. Others are irreversible, such as genomic alterations during carcinoma progression, along a selective and irreversible clonal drift. In carcinomas, these signals can converge to initiate a metastable phenotype. In this state, similarly to activated keratinocytes during re-epithelialization, cells can initiate a cohort migration and engage into a transient and reversible EMT controlled by the local environment prior to efficient intravasation and metastasis. EMT transcription factors also participate in cancer progression by inducing apoptosis resistance and maintaining stem-like properties exposed in tumor recurrences. These properties, very important on a clinical point of view, are not intrinsically linked to EMT, but can share common pathways.

Anchors and signals: the diverse roles of integrins in development

Integrins mediate cell adhesion by providing a link between the actin cytoskeleton and the extracellular matrix. As well as acting to anchor cells, integrin adhesions provide sensory input via mechanotransduction and synergism with signaling pathways, and provide the cell with the conditions necessary for differentiation in a permissive manner. In this review, we explore how integrins contribute to development, and what this tells us about how they work. From a signaling perspective, the influence of integrins on cell viability and fate is muted in a developmental context as compared to cell culture. Integrin phenotypes tend to arise from a failure of normally specified cells to create tissues properly, due to defective adhesion. The diversity of integrin functions in development shows how cell adhesion is continuously adjusted, both within and between animals, to fit developmental purpose.

Surrounding tissues canalize motile cardiopharyngeal progenitors towards collective polarity and directed migration

Collectively migrating cells maintain group polarity and interpret external cues to reach their destination. The cardiogenic progenitors (also known as trunk ventral cells, TVCs) of the ascidian Ciona intestinalis provide a simple chordate model with which to study collective migration. Bilateral pairs of associated TVCs undergo a stereotyped polarized migration away from the tail towards the ventral trunk, arguably constituting the simplest possible example of directed collective migration. To identify tissues contributing to TVC polarity and migration, we quantified the contact between TVCs and surrounding tissues, and blocked the secretory pathway in a tissue-specific manner. Even though TVCs normally migrate as an invariably determined leader-trailer polarized pair of adherent cells, they are capable of migrating individually, albeit a shorter distance and with altered morphology. The mesenchyme contacts newborn TVCs and contributes to robust specification of the trailer but appears to have only minor effects on directed migration. The notochord does not contact the TVCs but contributes to the onset of migration. The trunk endoderm first contacts the leader TVC, then 'encases' both migrating cells and provides the inputs maintaining leader-trailer polarity. Migrating TVCs adhere to the epidermis and need this contact for their cohesion. These phenomenological studies reveal that inherently motile cardiopharyngeal progenitors are channeled into stereotyped behaviors by interactions with surrounding tissues.

Abnormal arrangement of a collagen/apatite extracellular matrix orthogonal to osteoblast alignment is constructed by a nanoscale periodic surface structure

Morphological and directional alteration of cells is essential for structurally appropriate construction of tissues and organs. In particular, osteoblast alignment is crucial for the realization of anisotropic bone tissue microstructure. In this article, the orientation of a collagen/apatite extracellular matrix (ECM) was established by controlling osteoblast alignment using a surface geometry with nanometer-sized periodicity induced by laser ablation. Laser irradiation induced self-organized periodic structures (laser-induced periodic surface structures; LIPSS) with a spatial period equal to the wavelength of the incident laser on the surface of biomedical alloys of Ti-6Al-4V and Co-Cr-Mo. Osteoblast orientation was successfully induced parallel to the grating structure. Notably, both the fibrous orientation of the secreted collagen matrix and the c-axis of the produced apatite crystals were orientated orthogonal to the cell direction. To the best of our knowledge, this is the first report demonstrating that bone tissue anisotropy is controllable, including the characteristic organization of a collagen/apatite composite orthogonal to the osteoblast orientation, by controlling the cell alignment using periodic surface geometry.

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

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

Forces generated by cell intercalation tow epidermal sheets in mammalian tissue morphogenesis

While gastrulation movements offer mechanistic paradigms for how collective cellular movements shape developing embryos, far less is known about coordinated cellular movements that occur later in development. Studying eyelid closure, we explore a case where an epithelium locally reshapes, expands, and moves over another epithelium. Live imaging, gene targeting, and cell-cycle inhibitors reveal that closure does not require overlying periderm, proliferation, or supracellular actin cable assembly. Laser ablation and quantitative analyses of tissue deformations further distinguish the mechanism from wound repair and dorsal closure. Rather, cell intercalations parallel to the tissue front locally compress it perpendicularly, pulling the surrounding epidermis along the closure axis. Functional analyses in vivo show that the mechanism requires localized myosin-IIA- and α5β1 integrin/fibronectin-mediated migration and E-cadherin downregulation likely stimulated by Wnt signaling. These studies uncover a mode of epithelial closure in which forces generated by cell intercalation are leveraged to tow the surrounding tissue.

Quantitative cell polarity imaging defines leader-to-follower transitions during collective migration and the key role of microtubule-dependent adherens junction formation

The directed migration of cell collectives drives the formation of complex organ systems. A characteristic feature of many migrating collectives is a ‘tissue-scale’ polarity, whereby ‘leader’ cells at the edge of the tissue guide trailing ‘followers’ that become assembled into polarised epithelial tissues en route. Here, we combine quantitative imaging and perturbation approaches to investigate epithelial cell state transitions during collective migration and organogenesis, using the zebrafish lateral line primordium as an in vivo model. A readout of three-dimensional cell polarity, based on centrosomal-nucleus axes, allows the transition from migrating leaders to assembled followers to be quantitatively resolved for the first time in vivo. Using live reporters and a novel fluorescent protein timer approach, we investigate changes in cell-cell adhesion underlying this transition by monitoring cadherin receptor localisation and stability. This reveals that while cadherin 2 is expressed across the entire tissue, functional apical junctions are first assembled in the transition zone and become progressively more stable across the leader-follower axis of the tissue. Perturbation experiments demonstrate that the formation of these apical adherens junctions requires dynamic microtubules. However, once stabilised, adherens junction maintenance is microtubule independent. Combined, these data identify a mechanism for regulating leader-to-follower transitions within migrating collectives, based on the relocation and stabilisation of cadherins, and reveal a key role for dynamic microtubules in this process.

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.

PI3K/Akt1 signalling specifies foregut precursors by generating regionalized extra-cellular matrix

During embryonic development signalling pathways act repeatedly in different contexts to pattern the emerging germ layers. Understanding how these different responses are regulated is a central question for developmental biology. In this study, we used mouse embryonic stem cell (mESC) differentiation to uncover a new mechanism for PI3K signalling that is required for endoderm specification. We found that PI3K signalling promotes the transition from naïve endoderm precursors into committed anterior endoderm. PI3K promoted commitment via an atypical activity that delimited epithelial-to-mesenchymal transition (EMT). Akt1 transduced this activity via modifications to the extracellular matrix (ECM) and appropriate ECM could itself induce anterior endodermal identity in the absence of PI3K signalling. PI3K/Akt1-modified ECM contained low levels of Fibronectin (Fn1) and we found that Fn1 dose was key to specifying anterior endodermal identity in vivo and in vitro. Thus, localized PI3K activity affects ECM composition and ECM in turn patterns the endoderm.

DOI:http://dx.doi.org/10.7554/eLife.00806.001

From conception to birth, a single fertilised egg will multiply into trillions of cells, with each cell becoming one of the 200 or so different types of cell that are found in the human body. The development of an embryo is complex and dynamic, with cells giving up their ability to become any cell type and committing to becoming a specific cell type within a given tissue. At the same time, different groups of cells migrate to the appropriate locations within the developing embryo. Although it is challenging to decipher the roles of the individual signalling pathways that control an embryo’s development, several important components have been found.

Fibroblast growth factor (FGF) is a protein that regulates the formation of the endoderm: this is the innermost of the three layers of cells that form in the early embryo, and it gives rise to internal organs such as the gut, liver and pancreas. As well as ‘telling’ cells to become the front part, or anterior, of the endoderm, FGF also controls the migration of these cells within the embryo. However, uncoupling these two roles has been a major challenge, and the molecular mechanisms behind them are unclear.

Now, Villegas et al. have discovered that FGF activates a signalling cascade involving two enzymes called PI3K and Akt1. In lab-grown embryonic stem cells—cells that can be coaxed to become any of the cell types formed during development—this signalling cascade is essential for FGF to trigger differentiation of the cell types found in the anterior endoderm. The PI3K/Akt1 signalling cascade achieves this by reducing the level of a protein called fibronectin in the ‘extracellular matrix’ that surrounds the cells. This low level of fibronectin will in turn induce cells to stick together in an organized layer; and this rearrangement of cell-cell and cell-matrix interactions appears linked to triggering the differentiation of anterior endoderm cell types.

Villegas et al. showed that the PI3K/Akt1 pathway was also essential for endoderm formation in living mouse embryos. As a normal embryo develops, the anterior endoderm cells move into a ‘groove’ at the front the embryo, where the level of fibronectin is lower than it is at the posterior end of the embryo.

These findings highlight the importance of the extracellular matrix in the regulation of embryonic development, and should assist in the effort to turn lab-grown stem cells into the useful cell types found in internal organs.

DOI:http://dx.doi.org/10.7554/eLife.00806.002