Mechanical control of global cell behaviour during dorsal closure in Drosophila

JNK signaling and integrins cooperate to maintain cell adhesion during epithelial fusion in Drosophila

The fusion of epithelial sheets is an essential and conserved morphogenetic event that requires the maintenance of tissue continuity. This is secured by membrane-bound or diffusible signals that instruct the epithelial cells, in a coordinated fashion, to change shapes and adhesive properties and when, how and where to move. Here we show that during Dorsal Closure (DC) in Drosophila, the Jun kinase (JNK) signaling pathway modulates integrins expression and ensures tissue endurance. An excess of JNK activity, as an outcome of a failure in the negative feedback implemented by the dual-specificity phosphatase Puckered (Puc), promotes the loss of integrins [the ß-subunit Myospheroid (Mys)] and amnioserosa detachment. Likewise, integrins signal back to the pathway to regulate the duration and strength of JNK activity. Mys is necessary for the regulation of JNK activity levels and in its absence, puc expression is downregulated and JNK activity increases.

Biomechanical models and mechanisms of cellular morphogenesis and cerebral cortical expansion and folding

Morphogenesis of the nervous system involves a highly complex spatio-temporal pattern of physical forces (mainly tension and pressure) acting on cells and tissues that are pliable but have an intricately organized cytoskeletal infrastructure. This review begins by covering basic principles of biomechanics and the core cytoskeletal toolkit used to regulate the shapes of cells and tissues during embryogenesis and neural development. It illustrates how the principle of 'tensegrity' provides a useful conceptual framework for understanding how cells dynamically respond to forces that are generated internally or applied externally. The latter part of the review builds on this foundation in considering the development of mammalian cerebral cortex. The main focus is on cortical expansion and folding - processes that take place over an extended period of prenatal and postnatal development. Cortical expansion and folding are likely to involve many complementary mechanisms, some related to regulating cell proliferation and migration and others related to specific types and patterns of mechanical tension and pressure. Three distinct multi-mechanism models are evaluated in relation to a set of 18 key experimental observations and findings. The Composite Tension Plus (CT+) model is introduced as an updated version of a previous multi-component Differential Expansion Sandwich Plus (DES+) model (Van Essen, 2020); the new CT+ model includes 10 distinct mechanisms and has the greatest explanatory power among published models to date. Much needs to be done in order to validate specific mechanistic components and to assess their relative importance in different species, and important directions for future research are suggested.

The extended analogy of extraembryonic development in insects and amniotes

It is fascinating that the amnion and serosa/chorion, two extraembryonic (EE) tissues that are characteristic of the amniote vertebrates (mammals, birds and reptiles), have also independently evolved in insects. In this review, we offer the first detailed, macroevolutionary comparison of EE development and tissue biology across these animal groups. Some commonalities represent independent solutions to shared challenges for protecting the embryo (environmental assaults, risk of pathogens) and supporting its development, including clear links between cellular properties (e.g. polyploidy) and physiological function. Further parallels encompass developmental features such as the early segregation of the serosa/chorion compared to later, progressive differentiation of the amnion and formation of the amniotic cavity from serosal-amniotic folds as a widespread morphogenetic mode across species. We also discuss common developmental roles for orthologous transcription factors and BMP signalling in EE tissues of amniotes and insects, and between EE and cardiac tissues, supported by our exploration of new resources for global and tissue-specific gene expression. This highlights the degree to which general developmental principles and protective tissue features can be deduced from each of these animal groups, emphasizing the value of broad comparative studies to reveal subtle developmental strategies and answer questions that are common across species. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

How two extraembryonic epithelia became one: serosa and amnion features and functions of Drosophila's amnioserosa

The conservation of gene networks that specify and differentiate distinct tissues has long been a subject of great interest to evolutionary developmental biologists, but the question of how pre-existing tissue-specific developmental trajectories merge is rarely asked. During the radiation of flies, two extraembryonic epithelia, known as serosa and amnion, evolved into one, called amnioserosa. This unique extraembryonic epithelium is found in fly species of the group Schizophora, including the genetic model organism Drosophila melanogaster, and has been studied in depth. Close relatives of this group develop a serosa and a rudimentary amnion. The scuttle fly Megaselia abdita has emerged as an excellent model organism to study this extraembryonic tissue organization. In this review, development and functions of the extraembryonic tissue complements of Drosophila and Megaselia are compared. It is concluded that the amnioserosa combines cells, genetic pathway components and functions that were previously associated either with serosa development or amnion development. The composite developmental trajectory of the amnioserosa raises the question of whether merging tissue-specific gene networks is a common evolutionary process. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

Focal adhesion-mediated cell anchoring and migration: from in vitro to in vivo

Cell-extracellular matrix interactions have been studied extensively using cells cultured in vitro. These studies indicate that focal adhesion (FA)-based cell-extracellular matrix interactions are essential for cell anchoring and cell migration. Whether FAs play a similarly important role in vivo is less clear. Here, we summarize the formation and function of FAs in cultured cells and review how FAs transmit and sense force in vitro. Using examples from animal studies, we also describe the role of FAs in cell anchoring during morphogenetic movements and cell migration in vivo. Finally, we conclude by discussing similarities and differences in how FAs function in vitro and in vivo.

Modulating RhoA effectors induces transitions to oscillatory and more wavelike RhoA dynamics in C. elegans zygotes

Pulsatile RhoA dynamics underlie a wide range of cell and tissue behaviors. The circuits that produce these dynamics in different cells share common architectures based on fast positive and delayed negative feedback through F-actin, but they can produce very different spatiotemporal patterns of RhoA activity. However, the underlying causes of this variation remain poorly understood. Here we asked how this variation could arise through modulation of actin network dynamics downstream of active RhoA in early C. elegans embryos. We find that perturbing two RhoA effectors - formin and anillin - induce transitions from non-recurrent focal pulses to either large noisy oscillatory pulses (formin depletion) or noisy oscillatory waves (anillin depletion). In both cases these transitions could be explained by changes in local F-actin levels and depletion dynamics, leading to changes in spatial and temporal patterns of RhoA inhibition. However, the underlying mechanisms for F-actin depletion are distinct, with different dependencies on myosin II activity. Thus, modulating actomyosin network dynamics could shape the spatiotemporal dynamics of RhoA activity for different physiological or morphogenetic functions. [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text].

Differential cell adhesion implemented by Drosophila Toll corrects local distortions of the anterior-posterior compartment boundary

Maintaining lineage restriction boundaries in proliferating tissues is vital to animal development. A long-standing thermodynamics theory, the differential adhesion hypothesis, attributes cell sorting phenomena to differentially expressed adhesion molecules. However, the contribution of the differential adhesion system during tissue morphogenesis has been unsubstantiated despite substantial theoretical support. Here, we report that Toll-1, a transmembrane receptor protein, acts as a differentially expressed adhesion molecule that straightens the fluctuating anteroposterior compartment boundary in the abdominal epidermal epithelium of the Drosophila pupa. Toll-1 is expressed across the entire posterior compartment under the control of the selector gene engrailed and displays a sharp expression boundary that coincides with the compartment boundary. Toll-1 corrects local distortions of the boundary in the absence of cable-like Myosin II enrichment along the boundary. The reinforced adhesion of homotypic cell contacts, together with pulsed cell contraction, achieves a biased vertex sliding action by resisting the separation of homotypic cell contacts in boundary cells. This work reveals a self-organizing system that integrates a differential adhesion system with pulsed contraction of cells to maintain lineage restriction boundaries.

Identifying Key Genetic Regions for Cell Sheet Morphogenesis on Chromosome 2L Using a Drosophila Deficiency Screen in Dorsal Closure

Cell sheet morphogenesis is essential for metazoan development and homeostasis of animal form - it contributes to developmental milestones including gastrulation, neural tube closure, heart and palate formation and to tissue maintenance during wound healing. Dorsal closure, a well-characterized stage in Drosophila embryogenesis and a model for cell sheet morphogenesis, is a remarkably robust process during which coordination of conserved gene expression patterns and signaling cascades regulate the cellular shape changes and movements. New 'dorsal closure genes' continue to be discovered due to advances in imaging and genetics. Here, we extend our previous study of the right arm of the 2^nd chromosome to the left arm of the 2^nd chromosome using the Bloomington deficiency kit's set of large deletions, which collectively remove 98.9% of the genes on the left arm of chromosome two (2L) to identify 'dorsal closure deficiencies'. We successfully screened 87.2% of the genes and identified diverse dorsal closure defects in embryos homozygous for 49 deficiencies, 27 of which delete no known dorsal closure gene. These homozygous deficiencies cause defects in cell shape, canthus formation and tissue dynamics. Within these deficiencies, we have identified pimples, odd-skipped, paired, and sloppy-paired 1 as dorsal closure genes on 2L that affect lateral epidermal cells. We will continue to identify novel 'dorsal closure genes' with further analysis. These forward genetic screens are expected to identify new processes and pathways that contribute to closure and links between pathways and structures already known to coordinate various aspects of closure.

Tethering, evagination, and vesiculation via cell-cell interactions in microvascular flow

Vesiculation is a ubiquitous process undergone by most cell types and serves a variety of vital cell functions; vesiculation from erythrocytes, in particular, is a well-known example and constitutes a self-protection mechanism against premature clearance, inter alia. Herein, we explore a paradigm that red blood cell derived vesicles may form within the microvascular, in intense shear flow, where cells become adhered to either other cells or the extracellular matrix, by forming tethers or an evagination. Adherence may be enhanced, or caused, by diseased states or chemical anomalies as are discussed herein. The mechanisms for such processes are detailed via numerical simulations that are patterned directly from video-recorded cell microflow within the splenic venous sinus (MacDonald et al. 1987), as included, e.g., as Supplementary Material. The mechanisms uncovered highlight the necessity of accounting for remodeling of the erythrocyte's membrane skeleton and, specifically, for the time scales associated with that process that is an integral part of cell deformation. In this way, the analysis provides pointed, and vital, insights into the notion of what the, often used phrase, cell deformability actually entails in a more holistic manner. The analysis also details what data are required to make further quantitative descriptions possible and suggests experimental pathways for acquiring such.

The multifarious regulation of the apical junctional complex

Epithelial cells form highly organized polarized sheets with characteristic cell morphologies and tissue architecture. Cell–cell adhesion and intercellular communication are prerequisites of such cohesive sheets of cells, and cell connectivity is mediated through several junctional assemblies, namely desmosomes, adherens, tight and gap junctions. These cell–cell junctions form signalling hubs that not only mediate cell–cell adhesion but impact on multiple aspects of cell behaviour, helping to coordinate epithelial cell shape, polarity and function. This review will focus on the tight and adherens junctions, constituents of the apical junctional complex, and aims to provide a comprehensive overview of the complex signalling that underlies junction assembly, integrity and plasticity.

Integrin-Mediated Focal Anchorage Drives Epithelial Zippering during Mouse Neural Tube Closure

Epithelial fusion is a key process of morphogenesis by which tissue connectivity is established between adjacent epithelial sheets. A striking and poorly understood feature of this process is "zippering," whereby a fusion point moves directionally along an organ rudiment. Here, we uncover the molecular mechanism underlying zippering during mouse spinal neural tube closure. Fusion is initiated via local activation of integrin β1 and focal anchorage of surface ectoderm cells to a shared point of fibronectin-rich basement membrane, where the neural folds first contact each other. Surface ectoderm cells undergo proximal junction shortening, establishing a transitory semi-rosette-like structure at the zippering point that promotes juxtaposition of cells across the midline enabling fusion propagation. Tissue-specific ablation of integrin β1 abolishes the semi-rosette formation, preventing zippering and causing spina bifida. We propose integrin-mediated anchorage as an evolutionarily conserved mechanism of general relevance for zippering closure of epithelial gaps whose disturbance can produce clinically important birth defects.

Mechanosensitivity Occurs along the Adhesome's Force Train and Affects Traction Stress

Herein, we consider the process of force development along the adhesome within cell focal adhesions. Our model adhesome consists of the actin cytoskeleton-vinculin-talin-integrin-ligand-extracellular matrix-substrate force train. We specifically consider the effects of substrate stiffness on the force levels expected along the train and on the traction stresses they create at the substrate. We find that significant effects of substrate stiffness are manifest within each constitutive component of the force train and on the density and distribution of integrin/ligand anchorage points with the substrate. By following each component of the force train, we are able to delineate specific gaps in the quantitative descriptions of bond survival that must be addressed so that improved quantitative forecasts become possible. Our analysis provides, however, a rational description for the various levels of traction stresses that have been reported and of the effect of substrate stiffness. Our approach has the advantage of being quite clear as to how each constituent contributes to the net development of force and traction stress. We demonstrate that to provide truly quantitative forecasts for traction stress, a far more detailed description of integrin/ligand density and distribution is required. Although integrin density is already a well-recognized important feature of adhesion, our analysis places a finer point on it in the manner of how we evaluate the magnitude of traction stress. We provide mechanistic insight into how understanding of this vital element of the adhesion process may proceed by addressing mechanistic causes of integrin clustering that may lead to patterning.

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.

Elongated Cells Drive Morphogenesis in a Surface-Wrapped Finite-Element Model of Germband Retraction

During Drosophila embryogenesis, the germband first extends to curl around the posterior end of the embryo and then retracts back; however, retraction is not simply the reversal of extension. At a tissue level, extension is coincident with ventral furrow formation, and at a cellular level, extension occurs via convergent cell neighbor exchanges in the germband, whereas retraction involves only changes in cell shape. To understand how cell shapes, tissue organization, and cellular forces drive germband retraction, we investigate this process using a whole-embryo, surface-wrapped cellular finite-element model. This model represents two key epithelial tissues-amnioserosa and germband-as adjacent sheets of two-dimensional cellular finite elements that are wrapped around an ellipsoidal three-dimensional approximation of an embryo. The model reproduces the detailed kinematics of in vivo retraction by fitting just one free model parameter, the tension along germband cell interfaces; all other cellular forces are constrained to follow ratios inferred from experimental observations. With no additional parameter adjustments, the model also reproduces quantitative assessments of mechanical stress using laser dissection and failures of retraction when amnioserosa cells are removed via mutations or microsurgery. Surprisingly, retraction in the model is robust to changes in cellular force values but is critically dependent on starting from a configuration with highly elongated amnioserosa cells. Their extreme cellular elongation is established during the prior process of germband extension and is then used to drive retraction. The amnioserosa is the one tissue whose cellular morphogenesis is reversed from germband extension to retraction, and this reversal coordinates the forces needed to retract the germband back to its pre-extension position and shape. In this case, cellular force strengths are less important than the carefully established cell shapes that direct them. VIDEO ABSTRACT.

Adherens Junction Length during Tissue Contraction Is Controlled by the Mechanosensitive Activity of Actomyosin and Junctional Recycling

During epithelial contraction, cells generate forces to constrict their surface and, concurrently, fine-tune the length of their adherens junctions to ensure force transmission. While many studies have focused on understanding force generation, little is known on how junctional length is controlled. Here, we show that, during amnioserosa contraction in Drosophila dorsal closure, adherens junctions reduce their length in coordination with the shrinkage of apical cell area, maintaining a nearly constant junctional straightness. We reveal that junctional straightness and integrity depend on the endocytic machinery and on the mechanosensitive activity of the actomyosin cytoskeleton. On one hand, upon junctional stretch and decrease in E-cadherin density, actomyosin relocalizes from the medial area to the junctions, thus maintaining junctional integrity. On the other hand, when junctions have excess material and ruffles, junction removal is enhanced, and high junctional straightness and tension are restored. These two mechanisms control junctional length and integrity during morphogenesis.

Decoupling from yolk sac is required for extraembryonic tissue spreading in the scuttle fly Megaselia abdita

Extraembryonic tissues contribute to animal development, which often entails spreading over embryo or yolk. Apart from changes in cell shape, the requirements for this tissue spreading are not well understood. Here, we analyze spreading of the extraembryonic serosa in the scuttle fly Megaselia abdita. The serosa forms from a columnar blastoderm anlage, becomes a squamous epithelium, and eventually spreads over the embryo proper. We describe the dynamics of this process in long-term, whole-embryo time-lapse recordings, demonstrating that free serosa spreading is preceded by a prolonged pause in tissue expansion. Closer examination of this pause reveals mechanical coupling to the underlying yolk sac, which is later released. We find mechanical coupling prolonged and serosa spreading impaired after knockdown of M. abdita Matrix metalloprotease 1. We conclude that tissue–tissue interactions provide a critical functional element to constrain spreading epithelia.

Dynamics of PAR Proteins Explain the Oscillation and Ratcheting Mechanisms in Dorsal Closure

We present a vertex-based model for Drosophila dorsal closure that predicts the mechanics of cell oscillation and contraction from the dynamics of the PAR proteins. Based on experimental observations of how aPKC, Par-6, and Bazooka translocate from the circumference of the apical surface to the medial domain, and how they interact with each other and ultimately regulate the apicomedial actomyosin, we formulate a system of differential equations that captures the key features of dorsal closure, including distinctive behaviors in its early, slow, and fast phases. The oscillation in cell area in the early phase of dorsal closure results from an intracellular negative feedback loop that involves myosin, an actomyosin regulator, aPKC, and Bazooka. In the slow phase, gradual sequestration of apicomedial aPKC by Bazooka clusters causes incomplete disassembly of the actomyosin network over each cycle of oscillation, thus producing a so-called ratchet. The fast phase of rapid cell and tissue contraction arises when medial myosin, no longer antagonized by aPKC, builds up in time and produces sustained contraction. Thus, a minimal set of rules governing the dynamics of the PAR proteins, extracted from experimental observations, can account for all major mechanical outcomes of dorsal closure, including the transitions between its three distinct phases.

Mathematical models of dorsal closure

Dorsal closure is a model cell sheet movement that occurs midway through Drosophila embryogenesis. A dorsal hole, filled with amnioserosa, closes through the dorsalward elongation of lateral epidermal cell sheets. Closure requires contributions from 5 distinct tissues and well over 140 genes (see Mortensen et al., 2018, reviewed in Kiehart et al., 2017 and Hayes and Solon, 2017). In spite of this biological complexity, the movements (kinematics) of closure are geometrically simple at tissue, and in certain cases, at cellular scales. This simplicity has made closure the target of a number of mathematical models that seek to explain and quantify the processes that underlie closure's kinematics. The first (purely kinematic) modeling approach recapitulated well the time-evolving geometry of closure even though the underlying physical principles were not known. Almost all subsequent models delve into the forces of closure (i.e. the dynamics of closure). Models assign elastic, contractile and viscous forces which impact tissue and/or cell mechanics. They write rate equations which relate the forces to one another and to other variables, including those which represent geometric, kinematic, and or signaling characteristics. The time evolution of the variables is obtained by computing the solution of the model's system of equations, with optimized model parameters. The basis of the equations range from the phenomenological to biophysical first principles. We review various models and present their contribution to our understanding of the molecular mechanisms and biophysics of closure. Models of closure will contribute to our understanding of similar movements that characterize vertebrate morphogenesis.

Cbt modulates Foxo activation by positively regulating insulin signaling in Drosophila embryos

In late Drosophila embryos, the epidermis exhibits a dorsal hole as a consequence of germ band retraction. It is sealed during dorsal closure (DC), a morphogenetic process in which the two lateral epidermal layers converge towards the dorsal midline and fuse. We previously demonstrated the involvement of the Cbt transcription factor in Drosophila DC. However its molecular role in the process remained obscure. In this study, we used genomic approaches to identify genes regulated by Cbt as well as its direct targets during late embryogenesis. Our results reveal a complex transcriptional circuit downstream of Cbt and evidence that it is functionally related with the Insulin/insulin-like growth factor signaling pathway. In this context, Cbt may act as a positive regulator of the pathway, leading to the repression of Foxo activity. Our results also suggest that the DC defects observed in cbt embryos could be partially due to Foxo overactivation and that a regulatory feedback loop between Foxo and Cbt may be operating in the DC context.

Radially patterned cell behaviours during tube budding from an epithelium

The budding of tubular organs from flat epithelial sheets is a vital morphogenetic process. Cell behaviours that drive such processes are only starting to be unraveled. Using live-imaging and novel morphometric methods, we show that in addition to apical constriction, radially oriented directional intercalation of cells plays a major contribution to early stages of invagination of the salivary gland tube in the Drosophila embryo. Extending analyses in 3D, we find that near the pit of invagination, isotropic apical constriction leads to strong cell-wedging. Further from the pit cells interleave circumferentially, suggesting apically driven behaviours. Supporting this, junctional myosin is enriched in, and neighbour exchanges are biased towards the circumferential orientation. In a mutant failing pit specification, neither are biased due to an inactive pit. Thus, tube budding involves radially patterned pools of apical myosin, medial as well as junctional, and radially patterned 3D-cell behaviours, with a close mechanical interplay between invagination and intercalation.

Identifying Genetic Players in Cell Sheet Morphogenesis Using a Drosophila Deficiency Screen for Genes on Chromosome 2R Involved in Dorsal Closure

Cell sheet morphogenesis characterizes key developmental transitions and homeostasis, in vertebrates and throughout phylogeny, including gastrulation, neural tube formation and wound healing. Dorsal closure, a process during Drosophila embryogenesis, has emerged as a model for cell sheet morphogenesis. ∼140 genes are currently known to affect dorsal closure and new genes are identified each year. Many of these genes were identified in screens that resulted in arrested development. Dorsal closure is remarkably robust and many questions regarding the molecular mechanisms involved in this complex biological process remain. Thus, it is important to identify all genes that contribute to the kinematics and dynamics of closure. Here, we used a set of large deletions (deficiencies), which collectively remove 98.5% of the genes on the right arm of Drosophila melanogaster’s 2^nd chromosome to identify “dorsal closure deficiencies”. Through two crosses, we unambiguously identified embryos homozygous for each deficiency and time-lapse imaged them for the duration of closure. Images were analyzed for defects in cell shapes and tissue movements. Embryos homozygous for 47 deficiencies have notable, diverse defects in closure, demonstrating that a number of discrete processes comprise closure and are susceptible to mutational disruption. Further analysis of these deficiencies will lead to the identification of at least 30 novel “dorsal closure genes”. We expect that many of these novel genes will identify links to pathways and structures already known to coordinate various aspects of closure. We also expect to identify new processes and pathways that contribute to closure.

Cell Sheet Morphogenesis: Dorsal Closure in Drosophila melanogaster as a Model System

Dorsal closure is a key process during Drosophila morphogenesis that models cell sheet movements in chordates, including neural tube closure, palate formation, and wound healing. Closure occurs midway through embryogenesis and entails circumferential elongation of lateral epidermal cell sheets that close a dorsal hole filled with amnioserosa cells. Signaling pathways regulate the function of cellular structures and processes, including Actomyosin and microtubule cytoskeletons, cell-cell/cell-matrix adhesion complexes, and endocytosis/vesicle trafficking. These orchestrate complex shape changes and movements that entail interactions between five distinct cell types. Genetic and laser perturbation studies establish that closure is robust, resilient, and the consequence of redundancy that contributes to four distinct biophysical processes: contraction of the amnioserosa, contraction of supracellular Actomyosin cables, elongation (stretching?) of the lateral epidermis, and zipping together of two converging cell sheets. What triggers closure and what the emergent properties are that give rise to its extraordinary resilience and fidelity remain key, extant questions.

The elmo-mbc complex and rhogap19d couple Rho family GTPases during mesenchymal-to-epithelial-like transitions

Many metazoan developmental processes require cells to transition between migratory mesenchymal- and adherent epithelial-like states. These transitions require Rho GTPase-mediated actin rearrangements downstream of integrin and cadherin pathways. A regulatory toolbox of GEF and GAP proteins precisely coordinates Rho protein activities, yet defining the involvement of specific regulators within a cellular context remains a challenge due to overlapping and coupled activities. Here we demonstrate that Drosophila dorsal closure is a powerful model for Rho GTPase regulation during transitions from leading edges to cadherin contacts. During these transitions a Rac GEF elmo-mbc complex regulates both lamellipodia and Rho1-dependent, actomyosin-mediated tension at initial cadherin contacts. Moreover, the Rho GAP Rhogap19d controls Rac and Rho GTPases during the same processes and genetically regulates the elmo-mbc complex. This study presents a fresh framework to understand the inter-relationship between GEF and GAP proteins that tether Rac and Rho cycles during developmental processes.

Predicting the Functional Impact of CDH1 Missense Mutations in Hereditary Diffuse Gastric Cancer

The role of E-cadherin in Hereditary Diffuse Gastric Cancer (HDGC) is unequivocal. Germline alterations in its encoding gene (CDH1) are causative of HDGC and occur in about 40% of patients. Importantly, while in most cases CDH1 alterations result in the complete loss of E-cadherin associated with a well-established clinical impact, in about 20% of cases the mutations are of the missense type. The latter are of particular concern in terms of genetic counselling and clinical management, as the effect of the sequence variants in E-cadherin function is not predictable. If a deleterious variant is identified, prophylactic surgery could be recommended. Therefore, over the last few years, intensive research has focused on evaluating the functional consequences of CDH1 missense variants and in assessing E-cadherin pathogenicity. In that context, our group has contributed to better characterize CDH1 germline missense variants and is now considered a worldwide reference centre. In this review, we highlight the state of the art methodologies to categorize CDH1 variants, as neutral or deleterious. This information is subsequently integrated with clinical data for genetic counseling and management of CDH1 variant carriers.

Taking the strain: quantifying the contributions of all cell behaviours to changes in epithelial shape

Computer-assisted tracking of the shapes of many cells over long periods of development has driven the exploration of novel ways to quantify the contributions of different cell behaviours to morphogenesis. A handful of similar methods have now been published that are used to calculate tissue deformations (strain rates) in epithelia. These methods are further used to quantify strain rates attributable to each of the cell behaviours in the tissue, such as cell shape change, cell rearrangement and cell division, that together sum to the tissue strain rates. In this review, aimed at developmental biologists, I will introduce the general approach, characterize differences in current approaches and highlight extensions of these methods that remain to be fully explored. The methods will make a major contribution to the emerging field of tissue mechanics. Precisely quantified strain rates are an essential first step towards exploring constitutive equations relating stress to strain via tissue mechanical properties.This article is part of the themed issue 'Systems morphodynamics: understanding the development of tissue hardware'.

Challenging FRET-based E-Cadherin force measurements in Drosophila

Mechanical forces play a critical role during embryonic development. Cellular and tissue wide forces direct cell migration, drive tissue morphogenesis and regulate organ growth. Despite the relevance of mechanics for these processes, our knowledge of the dynamics of mechanical forces in living tissues remains scarce. Recent studies have tried to address this problem with the development of tension sensors based on Förster resonance energy transfer (FRET). These sensors are integrated into force bearing proteins and allow the measurement of mechanical tensions on subcellular structures. Here, we developed such a FRET-based sensor to measure E-Cadherin tensions in different Drosophila tissues in and ex vivo. Similar to previous studies, we integrated the sensor module into E-cadherin. We assessed the sensitivity of the sensor by measuring dynamic, developmental processes and mechanical modifications in three Drosophila tissues: the wing imaginal disc, the amnioserosa cells and the migrating border cells. However, these assays revealed that the sensor is not functional to measure the magnitude of tensions occurring in any of the three tissues. Moreover, we encountered technical problems with the measurement of FRET, which might represent more general pitfalls with FRET sensors in living tissues. These insights will help future studies to better design and control mechano-sensing experiments.

Basal Cell-Extracellular Matrix Adhesion Regulates Force Transmission during Tissue Morphogenesis

Tissue morphogenesis requires force-generating mechanisms to organize cells into complex structures. Although many such mechanisms have been characterized, we know little about how forces are integrated across developing tissues. We provide evidence that integrin-mediated cell-extracellular matrix (ECM) adhesion modulates the transmission of apically generated tension during dorsal closure (DC) in Drosophila. Integrin-containing adhesive structures resembling focal adhesions were identified on the basal surface of the amnioserosa (AS), an extraembryonic epithelium essential for DC. Genetic modulation of integrin-mediated adhesion results in defective DC. Quantitative image analysis and laser ablation experiments reveal that basal cell-ECM adhesions provide resistance to apical cell displacements and force transmission between neighboring cells in the AS. Finally, we provide evidence for integrin-dependent force transmission to the AS substrate. Overall, we find that integrins regulate force transmission within and between cells, thereby playing an essential role in transmitting tension in developing tissues.

Cell-cell and cell-extracellular matrix adhesions cooperate to organize actomyosin networks and maintain force transmission during dorsal closure

Cell–extracellular matrix (ECM) and cell–cell adhesion are interdependent during dorsal closure in the fly. Cell–ECM adhesion is required for normal myosin dynamics and organization of both cell–cell adhesions and actin networks during dorsal closure. Loss of cell–cell adhesion affects cell–ECM adhesion and tissue biomechanics.

Tissue morphogenesis relies on the coordinated action of actin networks, cell–cell adhesions, and cell–extracellular matrix (ECM) adhesions. Such coordination can be achieved through cross-talk between cell–cell and cell–ECM adhesions. Drosophila dorsal closure (DC), a morphogenetic process in which an extraembryonic tissue called the amnioserosa contracts and ingresses to close a discontinuity in the dorsal epidermis of the embryo, requires both cell–cell and cell–ECM adhesions. However, whether the functions of these two types of adhesions are coordinated during DC is not known. Here we analyzed possible interdependence between cell–cell and cell–ECM adhesions during DC and its effect on the actomyosin network. We find that loss of cell–ECM adhesion results in aberrant distributions of cadherin-mediated adhesions and actin networks in the amnioserosa and subsequent disruption of myosin recruitment and dynamics. Moreover, loss of cell–cell adhesion caused up-regulation of cell–ECM adhesion, leading to reduced cell deformation and force transmission across amnioserosa cells. Our results show how interdependence between cell–cell and cell–ECM adhesions is important in regulating cell behaviors, force generation, and force transmission critical for tissue morphogenesis.

The interplay of stiffness and force anisotropies drives embryo elongation

The morphogenesis of tissues, like the deformation of an object, results from the interplay between their material properties and the mechanical forces exerted on them. The importance of mechanical forces in influencing cell behaviour is widely recognized, whereas the importance of tissue material properties, in particular stiffness, has received much less attention. Using Caenorhabditis elegans as a model, we examine how both aspects contribute to embryonic elongation. Measuring the opening shape of the epidermal actin cortex after laser nano-ablation, we assess the spatiotemporal changes of actomyosin-dependent force and stiffness along the antero-posterior and dorso-ventral axis. Experimental data and analytical modelling show that myosin-II-dependent force anisotropy within the lateral epidermis, and stiffness anisotropy within the fiber-reinforced dorso-ventral epidermis are critical in driving embryonic elongation. Together, our results establish a quantitative link between cortical tension, material properties and morphogenesis of an entire embryo.

DOI:http://dx.doi.org/10.7554/eLife.23866.001

Animals come in all shapes and size, from ants to elephants. In all cases, the tissues and organs in the animal’s body acquire their shape as the animal develops. Cells in developing tissues squeeze themselves or push and pull on one another, and the resulting forces generate the final shape. This process is called morphogenesis and it is often studied in a worm called Caenorhabditis elegans. This worm’s simplicity makes it easy to work with in the laboratory. Yet processes that occur in C. elegans also take place in other animals, including humans, and so the discoveries made using this worm can have far-reaching implications.

As they develop, the embryos of C. elegans transform from a bean-shaped cluster of cells into the characteristic long shape of a worm, with the head at one end and the tail at the other. The force required to power this elongation is provided by the outer layer of cells of the embryo, known as the epidermis. In these cells, motor-like proteins called myosins pull against a mesh-like scaffold within the cell called the actin cytoskeleton; this pulling is thought to squeeze the embryo all around and cause it to grow longer.

Six strips of cells, running from the head to the tail, make up the epidermis of a C. elegans embryo. Myosin is mostly active in two strips of cells that run along the two sides of the embryo. In the strips above and below these strips (in other words, those on the upper and lower sides of the worm), the myosins are much less active. However, it is not fully understood how this distribution of myosin causes worms to elongate only along the head-to-tail axis.

Vuong-Brender et al. have now mapped the forces exerted in the cells of the worm’s epidermis. The experiments show that, in the strips of cells on the sides of the embryo, myosin’s activity causes the epidermis to constrict around the embryo, akin to a boa constrictor tightening around its prey. At the same time, the actin filaments in the other strips form rigid bundles oriented along the circumference that stiffen the cells in these strips. This prevents the constriction from causing the embryo to inflate at the top and bottom strips. As such, the only direction the embryo can expand is along the axis that runs from its head to its tail.

Together, these findings suggest that a combination of oriented force and stiffness ensure that the embryo only elongates along the head-to-tail axis. The next step is to understand how this orientation and the coordination between cells are controlled at the molecular level.

DOI:http://dx.doi.org/10.7554/eLife.23866.002

Optogenetic inhibition of apical constriction during Drosophila embryonic development

Morphogenesis of multicellular organisms is driven by changes in cell behavior, which happen at precise locations and defined developmental stages. Therefore, the studying of morphogenetic events would greatly benefit from tools that allow the perturbation of cell activity with spatial and temporal precision. We recently developed an optogenetic approach to modulate cell contractility with cellular precision and on fast (seconds) timescales during Drosophila embryogenesis. We present here a protocol to handle genetically engineered photosensitive Drosophila embryos and achieve light-mediated inhibition of apical constriction during tissue invagination. The possibility to modulate the levels of optogenetic activation at different laser powers makes this method suited also for studying how mechanical stresses are sensed and interpreted in vivo. Given the conserved function of cell contractility during animal development, the application of this method to other morphogenetic processes will facilitate our understanding of tissue mechanics and cell-cell interaction during morphogenesis.

Integration of actomyosin contractility with cell-cell adhesion during dorsal closure

In this work, we combine genetic perturbation, time-lapse imaging and quantitative image analysis to investigate how pulsatile actomyosin contractility drives cell oscillations, apical cell contraction and tissue closure during morphogenesis of the amnioserosa, the main force-generating tissue during the dorsal closure in Drosophila We show that Myosin activity determines the oscillatory and contractile behaviour of amnioserosa cells. Reducing Myosin activity prevents cell shape oscillations and reduces cell contractility. By contrast, increasing Myosin activity increases the amplitude of cell shape oscillations and the time cells spend in the contracted phase relative to the expanded phase during an oscillatory cycle, promoting cell contractility and tissue closure. Furthermore, we show that in AS cells, Rok controls Myosin foci formation and Mbs regulates not only Myosin phosphorylation but also adhesion dynamics through control of Moesin phosphorylation, showing that Mbs coordinates actomyosin contractility with cell-cell adhesion during amnioserosa morphogenesis.

α-Catenin stabilises Cadherin-Catenin complexes and modulates actomyosin dynamics to allow pulsatile apical contraction

We have investigated how cell contractility and adhesion are functionally integrated during epithelial morphogenesis. To this end, we have analysed the role of α-Catenin, a key molecule linking E-Cadherin-based adhesion and the actomyosin cytoskeleton, during Drosophila embryonic dorsal closure, by studying a newly developed allelic series. We find that α-Catenin regulates pulsatile apical contraction in the amnioserosa, the main force-generating tissue driving closure of the embryonic epidermis. α-Catenin controls actomyosin dynamics by stabilising and promoting the formation of actomyosin foci, and also stabilises DE-Cadherin (Drosophila E-Cadherin, also known as Shotgun) at the cell membrane, suggesting that medioapical actomyosin contractility regulates junction stability. Furthermore, we uncover a genetic interaction between α-Catenin and Vinculin, and a tension-dependent recruitment of Vinculin to amniosersoa apical cell membranes, suggesting the existence of a mechano-sensitive module operating in this tissue.

Amnioserosa cell constriction but not epidermal actin cable tension autonomously drives dorsal closure

Tissue morphogenesis requires coordination of multiple force-producing components. During dorsal closure in fly embryogenesis, an epidermis opening closes. A tensioned epidermal actin/MyosinII cable, which surrounds the opening, produces a force that is thought to combine with another MyosinII force mediating apical constriction of the amnioserosa cells that fill the opening. A model proposing that each force could autonomously drive dorsal closure was recently challenged by a model in which the two forces combine in a ratchet mechanism. Acute force elimination via selective MyosinII depletion in one or the other tissue shows that the amnioserosa tissue autonomously drives dorsal closure while the actin/MyosinII cable cannot. These findings exclude both previous models, although a contribution of the ratchet mechanism at dorsal closure onset remains likely. This shifts the current view of dorsal closure being a combinatorial force-component system to a single tissue-driven closure event.

A Chemomechanical Model of Matrix and Nuclear Rigidity Regulation of Focal Adhesion Size

In this work, a chemomechanical model describing the growth dynamics of cell-matrix adhesion structures (i.e., focal adhesions (FAs)) is developed. We show that there are three regimes for FA evolution depending on their size. Specifically, nascent adhesions with initial lengths below a critical value that are yet to engage in actin fibers will dissolve, whereas bigger ones will grow into mature FAs with a steady state size. In adhesions where growth surpasses the steady state size, disassembly will occur until their sizes are reduced to the equilibrium state. This finding arises from the fact that polymerization of adhesion proteins is force-dependent. Under actomyosin contraction, individual integrin bonds within small FAs (i.e., nascent adhesions or focal complexes) must transmit higher loads while the phenomenon of stress concentration occurs at the edge of large adhesion patches. As such, an effective stiffness of the FA-extracellular matrix complex that is either too small or too large will be relatively low, resulting in a limited actomyosin pulling force developed at the edge that is insufficient to prevent disassembly. Furthermore, it is found that a stiffer extracellular matrix and/or nucleus, as well as a stronger chemomechanical feedback, will induce larger adhesions along with a higher level of contraction force. Interestingly, switching the extracellular side from an elastic half-space, corresponding to some widely used in vitro gel substrates, to a one-dimensional fiber (as in the case of cells anchoring to a fibrous scaffold in vivo) does not qualitative change these conclusions. Our model predictions are in good agreement with a variety of experimental observations obtained in this study as well as those reported in the literature. Furthermore, this new model, to our knowledge, provides a framework with which to understand how both intracellular and extracellular perturbations lead to changes in adhesion structure number and size.

Amnioserosa development and function in Drosophila embryogenesis: Critical mechanical roles for an extraembryonic tissue

Despite being a short-lived, extraembryonic tissue, the amnioserosa plays critical roles in the major morphogenetic events of Drosophila embryogenesis. These roles involve both cellular mechanics and biochemical signaling. Its best-known role is in dorsal closure-well studied by both developmental biologists and biophysicists-but the amnioserosa is also important during earlier developmental stages. Here, we provide an overview of amnioserosa specification and its role in several key developmental stages: germ band extension, germ band retraction, and dorsal closure. We also compare embryonic development in Drosophila and its relative Megaselia to highlight how the amnioserosa and its roles have evolved. Placed in context, the amnioserosa provides a fascinating example of how signaling, mechanics, and morphogen patterns govern cell-type specification and subsequent morphogenetic changes in cell shape, orientation, and movement. Developmental Dynamics 245:558-568, 2016. © 2016 Wiley Periodicals, Inc.

Knockdown of zebrafish YY1a can downregulate the phosphatidylserine (PS) receptor expression, leading to induce the abnormal brain and heart development

Background

Yin Yang 1 (YY1) is a ubiquitously expressed GLI-Kruppel zinc finger-containing transcriptional regulator. YY1 plays a fundamental role in normal biologic processes such as embryogenesis, differentiation, and cellular proliferation. YY1 effects on the genes involved in these processes are mediated via initiation, activation, or repression of transcription depending upon the context in which it binds. The role of the multifunctional transcription factor Yin Yang 1 (YY1) in tissue development is poorly understood. In the present, we investigated YY1a role in developing zebrafish on PSR-mediated apoptotic cell engulfment during organic morphogenesis.

Results

YY1a is first expressed 0.5 h post-fertilization (hpf), in the whole embryo 12 hpf, and in brain, eyes, and heart 72 hpf by in situ hybridization assay. The nucleotide sequence of zebrafish YY1a transcription factor (clone zfYY1a; HQ 166834) was found to be similar to that of zebrafish YY1a (99 % sequence identity; NM 212617). With the loss-of-function assay, YY1a knockdown by a morpholino oligonucleotide led to downregulation of the phosphatidylserine engulfing receptor zfPSR during embryonic segmentation and to the accumulation of a large number of dead apoptotic cells throughout the entire early embryo, especially in the posterior area. Up to 24 hpf, these cells interfered with embryonic cell migration and cell-cell interactions that normally occur in the brain, heart, eye, and notochord. Finally, with gain-of-function assay, defective morphants could be rescued by injecting both YY1a mRNA and PSR mRNA and trigger resumption of normal development.

Conclusions

Taken together, our results suggest that YY1a regulates PS receptor expression that linked to function of PSR-phagocyte mediated apoptotic cell engulfment during development, especially the development of organs such as the brain and heart. YY1a/PSR-mediated engulfing system may involve in diseases.

Mechanical forces drive neuroblast morphogenesis and are required for epidermal closure

Tissue morphogenesis requires myosin-dependent events such as cell shape changes and migration to be coordinated between cells within a tissue, and/or with cells from other tissues. However, few studies have investigated the simultaneous morphogenesis of multiple tissues in vivo. We found that during Caenorhabditis elegans ventral enclosure, when epidermal cells collectively migrate to cover the ventral surface of the embryo, the underlying neuroblasts (neuronal precursor cells) also undergo morphogenesis. We found that myosin accumulates as foci along the junction-free edges of the ventral epidermal cells to form a ring, whose closure is myosin-dependent. We also observed the accumulation of myosin foci and the adhesion junction proteins E-cadherin and α-catenin in the underlying neuroblasts. Myosin may help to reorganize a subset of neuroblasts into a rosette-like pattern, and decrease their surface area as the overlying epidermal cells constrict. Since myosin is required in the neuroblasts for ventral enclosure, we propose that mechanical forces in the neuroblasts influence constriction of the overlying epidermal cells. In support of this model, disrupting neuroblast cell division or altering their fate influences myosin localization in the overlying epidermal cells. The coordination of myosin-dependent events and forces between cells in different tissues could be a common theme for coordinating morphogenetic events during metazoan development.

Spreading the word: non-autonomous effects of apoptosis during development, regeneration and disease

Apoptosis, in contrast to other forms of cell death such as necrosis, was originally regarded as a 'silent' mechanism of cell elimination designed to degrade the contents of doomed cells. However, during the past decade it has become clear that apoptotic cells can produce diverse signals that have a profound impact on neighboring cells and tissues. For example, apoptotic cells can release factors that influence the proliferation and survival of adjacent tissues. Apoptosis can also affect tissue movement and morphogenesis by modifying tissue tension in surrounding cells. As we review here, these findings reveal unexpected roles for apoptosis in tissue remodeling during development, as well as in regeneration and cancer.

Morphogenetic functions of extraembryonic membranes in insects

Morphogenetic functions of the amnioserosa, the serosa, the amnion, and the yolk sac are reviewed on the basis of recent studies in flies (Drosophila, Megaselia), beetles (Tribolium), and hemipteran bugs (Oncopeltus). Three hypotheses are presented. First, it is suggested that the amnioserosa of Drosophila and the dorsal amnion of other fly species function in a similar manner. Second, it is proposed that in many species with an amniotic cavity, the amnion determines the site of serosa rupture, which, through interactions between the serosa and the amnion, enables the embryo to break free from the amniotic cavity and to close its backside. Finally, it is concluded that the yolk sac is likely an important player in insect morphogenesis.

Capabilities and Limitations of Tissue Size Control through Passive Mechanical Forces

Embryogenesis is an extraordinarily robust process, exhibiting the ability to control tissue size and repair patterning defects in the face of environmental and genetic perturbations. The size and shape of a developing tissue is a function of the number and size of its constituent cells as well as their geometric packing. How these cellular properties are coordinated at the tissue level to ensure developmental robustness remains a mystery; understanding this process requires studying multiple concurrent processes that make up morphogenesis, including the spatial patterning of cell fates and apoptosis, as well as cell intercalations. In this work, we develop a computational model that aims to understand aspects of the robust pattern repair mechanisms of the Drosophila embryonic epidermal tissues. Size control in this system has previously been shown to rely on the regulation of apoptosis rather than proliferation; however, to date little work has been done to understand the role of cellular mechanics in this process. We employ a vertex model of an embryonic segment to test hypotheses about the emergence of this size control. Comparing the model to previously published data across wild type and genetic perturbations, we show that passive mechanical forces suffice to explain the observed size control in the posterior (P) compartment of a segment. However, observed asymmetries in cell death frequencies across the segment are demonstrated to require patterning of cellular properties in the model. Finally, we show that distinct forms of mechanical regulation in the model may be distinguished by differences in cell shapes in the P compartment, as quantified through experimentally accessible summary statistics, as well as by the tissue recoil after laser ablation experiments.

Emergent material properties of developing epithelial tissues

BACKGROUND

Force generation and the material properties of cells and tissues are central to morphogenesis but remain difficult to measure in vivo. Insight is often limited to the ratios of mechanical properties obtained through disruptive manipulation, and the appropriate models relating stress and strain are unknown. The Drosophila amnioserosa epithelium progressively contracts over 3 hours of dorsal closure, during which cell apices exhibit area fluctuations driven by medial myosin pulses with periods of 1.5-6 min. Linking these two timescales and understanding how pulsatile contractions drive morphogenetic movements is an urgent challenge.

RESULTS

We present a novel framework to measure in a continuous manner the mechanical properties of epithelial cells in the natural context of a tissue undergoing morphogenesis. We show that the relationship between apicomedial myosin fluorescence intensity and strain during fluctuations is consistent with a linear behaviour, although with a lag. We thus used myosin fluorescence intensity as a proxy for active force generation and treated cells as natural experiments of mechanical response under cyclic loading, revealing unambiguous mechanical properties from the hysteresis loop relating stress to strain. Amnioserosa cells can be described as a contractile viscoelastic fluid. We show that their emergent mechanical behaviour can be described by a linear viscoelastic rheology at timescales relevant for tissue morphogenesis. For the first time, we establish relative changes in separate effective mechanical properties in vivo. Over the course of dorsal closure, the tissue solidifies and effective stiffness doubles as net contraction of the tissue commences. Combining our findings with those from previous laser ablation experiments, we show that both apicomedial and junctional stress also increase over time, with the relative increase in apicomedial stress approximately twice that of other obtained measures.

CONCLUSIONS

Our results show that in an epithelial tissue undergoing net contraction, stiffness and stress are coupled. Dorsal closure cell apical contraction is driven by the medial region where the relative increase in stress is greater than that of stiffness. At junctions, by contrast, the relative increase in the mechanical properties is the same, so the junctional contribution to tissue deformation is constant over time. An increase in myosin activity is likely to underlie, at least in part, the change in medioapical properties and we suggest that its greater effect on stress relative to stiffness is fundamental to actomyosin systems and confers on tissues the ability to regulate contraction rates in response to changes in external mechanics.

Crumbs is an essential regulator of cytoskeletal dynamics and cell-cell adhesion during dorsal closure in Drosophila

The evolutionarily conserved Crumbs protein is required for epithelial polarity and morphogenesis. Here we identify a novel role of Crumbs as a negative regulator of actomyosin dynamics during dorsal closure in the Drosophila embryo. Embryos carrying a mutation in the FERM (protein 4.1/ezrin/radixin/moesin) domain-binding motif of Crumbs die due to an overactive actomyosin network associated with disrupted adherens junctions. This phenotype is restricted to the amnioserosa and does not affect other embryonic epithelia. This function of Crumbs requires DMoesin, the Rho1-GTPase, class-I p21-activated kinases and the Arp2/3 complex. Data presented here point to a critical role of Crumbs in regulating actomyosin dynamics, cell junctions and morphogenesis.

DOI:http://dx.doi.org/10.7554/eLife.07398.001

A layer of epithelial cells covers the body surface of animals. Epithelial cells have a property known as polarity; this means that they have two different poles, one of which is in contact with the environment. Midway through embryonic development, the Drosophila embryo is covered by two kinds of epithelial sheets; the epidermis on the front, the belly and the sides of the embryo, and the amnioserosa on the back. In the second half of embryonic development, the amnioserosa is brought into the embryo in a process called dorsal closure, while the epidermis expands around the back of the embryo to encompass it.

One of the major activities driving dorsal closure is the contraction of amnioserosa cells. This contraction depends on the highly dynamic activity of the protein network that helps give cells their shape, known as the actomyosin cytoskeleton. One major question in the field is how changes in the actomyosin cytoskeleton are controlled as tissues take shape (a process known as “morphogenesis”) and how the integrity of epithelial tissues is maintained during these processes.

A key regulator of epidermal and amnioserosa polarity is an evolutionarily conserved protein called Crumbs. The epithelial tissues of mutant embryos that do not produce Crumbs lose polarity and integrity, and the embryos fail to develop properly.

Flores-Benitez and Knust have now studied the role of Crumbs in the morphogenesis of the amnioserosa during dorsal closure. This revealed that fly embryos that produce a mutant Crumbs protein that cannot interact with a protein called Moesin (which links the cell membrane and the actomyosin cytoskeleton) are unable to complete dorsal closure. Detailed analyses showed that this failure of dorsal closure is due to the over-activity of the actomyosin cytoskeleton in the amnioserosa. This results in increased and uncoordinated contractions of the cells, and is accompanied by defects in cell-cell adhesion that ultimately cause the amnioserosa to lose integrity. Flores-Benitez and Knust’s genetic analyses further showed that several different signalling systems participate in this process.

Flores-Benitez and Knust’s results reveal an unexpected role of Crumbs in coordinating polarity, actomyosin activity and cell-cell adhesion. Further work is now needed to understand the molecular mechanisms and interactions that enable Crumbs to coordinate these processes; in particular, to unravel how Crumbs influences the periodic contractions that drive changes in cell shape. It will also be important to investigate whether Crumbs is involved in similar mechanisms that operate in other developmental events in which actomyosin oscillations have been linked to tissue morphogenesis.

DOI:http://dx.doi.org/10.7554/eLife.07398.002

The receptor tyrosine kinase Pvr promotes tissue closure by coordinating corpse removal and epidermal zippering

A leading cause of human birth defects is the incomplete fusion of tissues, often manifested in the palate, heart or neural tube. To investigate the molecular control of tissue fusion, embryonic dorsal closure and pupal thorax closure in Drosophila are useful experimental models. We find that Pvr mutants have defects in dorsal midline closure with incomplete amnioserosa internalization and epidermal zippering, as well as cardia bifida. These defects are relatively mild in comparison to those seen with other signaling mutants, such as in the JNK pathway, and we demonstrate that JNK signaling is not perturbed by altering Pvr receptor tyrosine kinase activity. Rather, modulation of Pvr levels in the ectoderm has an impact on PIP3 membrane accumulation, consistent with a link to PI3K signal transduction. Polarized PI3K activity influences protrusive activity from the epidermal leading edge and the protrusion area changes in accord with Pvr signaling intensity, providing a possible mechanism to explain Pvr mutant phenotypes. Tissue-specific rescue experiments indicate a partial requirement in epithelial tissue, but confirm the essential role of Pvr in hemocytes for embryonic survival. Taken together, we argue that inefficient removal of the internalizing amnioserosa tissue by mutant hemocytes coupled with impaired midline zippering of mutant epithelium creates a situation in some embryos whereby dorsal midline closure is incomplete. Based on these observations, we suggest that efferocytosis (corpse clearance) could contribute to proper tissue closure and thus might underlie some congenital birth defects.

The interdependence of the Rho GTPases and apicobasal cell polarity

Signaling via the Rho GTPases provides crucial regulation of numerous cell polarization events, including apicobasal (AB) polarity, polarized cell migration, polarized cell division and neuronal polarity. Here we review the relationships between the Rho family GTPases and epithelial AB polarization events, focusing on the 3 best-characterized members: Rho, Rac and Cdc42. We discuss a multitude of processes that are important for AB polarization, including lumen formation, apical membrane specification, cell-cell junction assembly and maintenance, as well as tissue polarity. Our discussions aim to highlight the immensely complex regulatory mechanisms that encompass Rho GTPase signaling during AB polarization. More specifically, in this review we discuss several emerging common themes, that include: 1) the need for Rho GTPase activities to be carefully balanced in both a spatial and temporal manner through a multitude of mechanisms; 2) the existence of signaling feedback loops and crosstalk to create robust cellular responses; and 3) the frequent multifunctionality that exists among AB polarity regulators. Regarding this latter theme, we provide further discussion of the potential plasticity of the cell polarity machinery and as a result the possible implications for human disease.

Mechanical force sensing in tissues

Tissue size, shape, and organization reflect individual cell behaviors such as proliferation, shape change, and movement. Evidence suggests that mechanical signals operate in tandem with biochemical cues to properly coordinate cell behavior and pattern tissues. The objective of this chapter is to present recent evidence demonstrating that forces transmitted between cells act as signals that coordinate cell behavior across tissues. We first briefly summarize molecular and cellular mechanisms by which forces are sensed by cells with an emphasis on forces generated and transmitted by cytoskeletal networks. We then discuss evidence for these mechanisms operating in multicellular contexts to coordinate complex cell and tissue behaviors that occur during embryonic development: specifically growth and morphogenesis.

Complete canthi removal reveals that forces from the amnioserosa alone are sufficient to drive dorsal closure in Drosophila

Drosophila's dorsal closure provides an excellent model system with which to analyze biomechanical processes during morphogenesis. During native closure, the amnioserosa, flanked by two lateral epidermal sheets, forms an eye-shaped opening with canthi at each corner. The dynamics of amnioserosa cells and actomyosin purse strings in the leading edges of epidermal cells promote closure, whereas the bulk of the lateral epidermis opposes closure. Canthi maintain purse string curvature (necessary for their dorsalward forces), and zipping at the canthi shortens leading edges, ensuring a continuous epithelium at closure completion. We investigated the requirement for intact canthi during closure with laser dissection approaches. Dissection of one or both canthi resulted in tissue recoil and flattening of each purse string. After recoil and a temporary pause, closure resumed at approximately native rates until slowing near the completion of closure. Thus the amnioserosa alone can drive closure after dissection of one or both canthi, requiring neither substantial purse string curvature nor zipping during the bulk of closure. How the embryo coordinates multiple, large forces (each of which is orders of magnitude greater than the net force) during native closure and is also resilient to multiple perturbations are key extant questions.