Cell shape change and invagination of the cephalic furrow involves reorganization of F-actin

Endogenous OptoRhoGEFs reveal biophysical principles of epithelial tissue furrowing

During development, epithelia function as malleable substrates that undergo extensive remodeling to shape developing embryos. Optogenetic control of Rho signaling provides an avenue to investigate the mechanisms of epithelial morphogenesis, but transgenic optogenetic tools can be limited by variability in tool expression levels and deleterious effects of transgenic overexpression on development. Here, we use CRISPR/Cas9 to tag Drosophila RhoGEF2 and Cysts/Dp114RhoGEF with components of the iLID/SspB optogenetic heterodimer, permitting light-dependent control over endogenous protein activities. Using quantitative optogenetic perturbations, we uncover a dose-dependence of tissue furrow depth and bending behavior on RhoGEF recruitment, revealing mechanisms by which developing embryos can shape tissues into particular morphologies. We show that at the onset of gastrulation, furrows formed by cell lateral contraction are oriented and size-constrained by a stiff basal actomyosin layer. Our findings demonstrate the use of quantitative, 3D-patterned perturbations of cell contractility to precisely shape tissue structures and interrogate developmental mechanics.

Morphogenesis: Setting the pace of embryo folding

Tissue folding is a key process for shape generation during embryonic development. A new study reports how a fold in the Drosophila embryo forms by a propagating trigger wave.

A mechanical wave travels along a genetic guide to drive the formation of an epithelial furrow during Drosophila gastrulation

Epithelial furrowing is a fundamental morphogenetic process during gastrulation, neurulation, and body shaping. A furrow often results from a fold that propagates along a line. How fold formation and propagation are controlled and driven is poorly understood. To shed light on this, we study the formation of the cephalic furrow, a fold that runs along the embryo dorsal-ventral axis during Drosophila gastrulation and the developmental role of which is still unknown. We provide evidence of its function and show that epithelial furrowing is initiated by a group of cells. This cellular cluster works as a pacemaker, triggering a bidirectional morphogenetic wave powered by actomyosin contractions and sustained by de novo medial apex-to-apex cell adhesion. The pacemaker's Cartesian position is under the crossed control of the anterior-posterior and dorsal-ventral gene patterning systems. Thus, furrow formation is driven by a mechanical trigger wave that travels under the control of a multidimensional genetic guide.

The mechanics of cephalic furrow formation in the Drosophila embryo

Cephalic furrow formation (CFF) is a major morphogenetic movement during gastrulation in Drosophila melanogaster embryos that gives rise to a deep, transitory epithelial invagination. Recent studies have identified the individual cell shape changes that drive the initiation and progression phases of CFF; however, the underlying mechanics are not yet well understood. During the progression phase, the furrow deepens as columnar cells from both the anterior and posterior directions fold inwards rotating by 90°. To analyze the mechanics of this process, we have developed an advanced two-dimensional lateral vertex model that includes multinode representation of cellular membranes and allows us to capture the membrane curvature associated with pressure variation. Our investigations reveal some key potential mechanical features of CFF, as follows. When cells begin to roll over the cephalic furrow cleft, they become wedge shaped as their apical cortices and overlying membranes expand, lateral cortices and overlying membranes release tension, internal pressures drop, and basal cortices and membranes contract. Then, cells reverse this process by shortening apical cortices and membranes, increasing lateral tension, and causing internal pressures to rise. Since the basal membranes expand, the cells recover their rotated columnar shape once in the furrow. Interestingly, our findings indicate that the basal membranes may be passively reactive throughout the progression phase. We also find that the smooth rolling of cells over the cephalic furrow cleft necessitates that internalized cells provide a solid base through high levels of membrane tension and internal pressure, which allows the transmission of tensile force that pulls new cells into the furrow. These results lead us to suggest that CFF helps to establish a baseline tension across the apical surface of the embryo to facilitate cellular coordination of other morphogenetic movements via mechanical stress feedback mechanisms.

Mechanical feedback and robustness of apical constrictions in Drosophila embryo ventral furrow formation

Formation of the ventral furrow in the Drosophila embryo relies on the apical constriction of cells in the ventral region to produce bending forces that drive tissue invagination. In our recent paper we observed that apical constrictions during the initial phase of ventral furrow formation produce elongated patterns of cellular constriction chains prior to invagination and argued that these are indicative of tensile stress feedback. Here, we quantitatively analyze the constriction patterns preceding ventral furrow formation and find that they are consistent with the predictions of our active-granular-fluid model of a monolayer of mechanically coupled stress-sensitive constricting particles. Our model shows that tensile feedback causes constriction chains to develop along underlying precursor tensile stress chains that gradually strengthen with subsequent cellular constrictions. As seen in both our model and available optogenetic experiments, this mechanism allows constriction chains to penetrate or circumvent zones of reduced cell contractility, thus increasing the robustness of ventral furrow formation to spatial variation of cell contractility by rescuing cellular constrictions in the disrupted regions.

Small GTPases modulate intrinsic and extrinsic forces that control epithelial folding in Drosophila embryos

Epithelial folding is a common means to execute morphogenetic movements. The gastrulating Drosophila embryo offers many examples of epithelial folding events, including the ventral, cephalic, and dorsal furrows. Each of these folding events is associated with changes in intracellular contractility and/or cytoskeleton structures that autonomously promote epithelial folding. Here, we review accumulating evidence that suggests the progression and final form of ventral, cephalic, and dorsal furrows are also influenced by the behaviour of cells neighbouring these folds. We further discuss the prevalence and importance of junctional rearrangements during epithelial folding events, suggesting adherens junction components are prime candidates to modulate the transmission of the intercellular forces that influence folding events. Finally, we discuss how recently developed methods that enable precise spatial and/or temporal control of protein activity allow direct testing of molecular models of morphogenesis in vivo.

The origin and the mechanism of mechanical polarity during epithelial folding

Epithelial tissues are sheet-like tissue structures that line the inner and outer surfaces of animal bodies and organs. Their remarkable ability to actively produce, or passively adapt to, complex surface geometries has fascinated physicists and biologists alike for centuries. The most simple and yet versatile process of epithelial deformation is epithelial folding, through which curved shapes, tissue convolutions and internal structures are produced. The advent of quantitative live imaging, combined with experimental manipulation and computational modeling, has rapidly advanced our understanding of epithelial folding. In particular, a set of mechanical principles has emerged to illustrate how forces are generated and dissipated to instigate curvature transitions in a variety of developmental contexts. Folding a tissue requires that mechanical loads or geometric changes be non-uniform. Given that polarity is the most distinct and fundamental feature of epithelia, understanding epithelial folding mechanics hinges crucially on how forces become polarized and how polarized differential deformation arises, for which I coin the term 'mechanical polarity'. In this review, five typical modules of mechanical processes are distilled from a diverse array of epithelial folding events. Their mechanical underpinnings with regard to how forces and polarity intersect are analyzed to accentuate the importance of mechanical polarity in the understanding of epithelial folding.

When the Community Silences Disruptive Elements: Multiscale Mechanical Coupling Buffers Morphogenetic Imprecisions

How the high degree of shape reproducibility is conferred between individuals remains unclear. In this issue of Developmental Cell, Eritano et al. show that tissue-scale mechanical coupling corrects the intrinsic noise of gene expression pattern and tissue mechanics to ensure the high degree of precision of Drosophila cephalic furrow formation.

Increased lateral tension is sufficient for epithelial folding in Drosophila

The folding of epithelial sheets is important for tissues, organs and embryos to attain their proper shapes. Epithelial folding requires subcellular modulations of mechanical forces in cells. Fold formation has mainly been attributed to mechanical force generation at apical cell sides, but several studies indicate a role of mechanical tension at lateral cell sides in this process. However, whether lateral tension increase is sufficient to drive epithelial folding remains unclear. Here, we have used optogenetics to locally increase mechanical force generation at apical, lateral or basal sides of epithelial Drosophila wing disc cells, an important model for studying morphogenesis. We show that optogenetic recruitment of RhoGEF2 to apical, lateral or basal cell sides leads to local accumulation of F-actin and increase in mechanical tension. Increased lateral tension, but not increased apical or basal tension, results in sizeable fold formation. Our results stress the diversification of folding mechanisms between different tissues and highlight the importance of lateral tension increase for epithelial folding.

Gastrulation in Drosophila melanogaster: Genetic control, cellular basis and biomechanics

Gastrulation is generally understood as the morphogenetic processes that result in the spatial organization of the blastomere into the three germ layers, ectoderm, mesoderm and endoderm. This review summarizes our current knowledge of the morphogenetic mechanisms in Drosophila gastrulation. In addition to the events that drive mesoderm invagination and germband elongation, we pay particular attention to other, less well-known mechanisms including midgut invagination, cephalic furrow formation, dorsal fold formation, and mesoderm layer formation. This review covers topics ranging from the identification and functional characterization of developmental and morphogenetic control genes to the analysis of the physical properties of cells and tissues and the control of cell and tissue mechanics of the morphogenetic movements in the gastrula.

Tissue-Scale Mechanical Coupling Reduces Morphogenetic Noise to Ensure Precision during Epithelial Folding

Morphological constancy is universal in developing systems. It is unclear whether precise morphogenesis stems from faithful mechanical interpretation of gene expression patterns. We investigate the formation of the cephalic furrow, an epithelial fold that is precisely positioned with a linear morphology. Fold initiation is specified by a precise genetic code with single-cell row resolution. This positional code activates and spatially confines lateral myosin contractility to induce folding. However, 20% of initiating cells are mis-specified because of fluctuating myosin intensities at the cellular level. Nevertheless, the furrow remains linearly aligned. We find that lateral myosin is planar polarized, integrating contractile membrane interfaces into supracellular "ribbons." Local reduction of mechanical coupling at the "ribbons" using optogenetics decreases furrow linearity. Furthermore, 3D vertex modeling indicates that polarized, interconnected contractility confers morphological robustness against noise. Thus, tissue-scale mechanical coupling functions as a denoising mechanism to ensure morphogenetic precision despite noisy decoding of positional information.

Setting up for gastrulation: D. melanogaster

Drosophila melanogaster embryos develop initially as a syncytium of totipotent nuclei and subsequently, once cellularized, undergo morphogenetic movements associated with gastrulation to generate the three somatic germ layers of the embryo: mesoderm, ectoderm, and endoderm. In this chapter, we focus on the first phase of gastrulation in Drosophila involving patterning of early embryos when cells differentiate their gene expression programs. This patterning process requires coordination of multiple developmental processes including genome reprogramming at the maternal-to-zygotic transition, combinatorial action of transcription factors to support distinct gene expression, and dynamic feedback between this genetic patterning by transcription factors and changes in cell morphology. We discuss the gene regulatory programs acting during patterning to specify the three germ layers, which involve the regulation of spatiotemporal gene expression coupled to physical tissue morphogenesis.

Yorkie controls tube length and apical barrier integrity during airway development

Skouloudaki et al. identify an alternative role of the transcriptional coactivator Yorkie (Yki) in controlling water impermeability and tube size of developing Drosophila airways. Tracheal impermeability is triggered by Yki-mediated transcriptional regulation of δ-aminolevulinate synthase (Alas), whereas tube elongation is controlled by binding of Yki to the actin-severing factor Twinstar.

Epithelial organ size and shape depend on cell shape changes, cell–matrix communication, and apical membrane growth. The Drosophila melanogaster embryonic tracheal network is an excellent model to study these processes. Here, we show that the transcriptional coactivator of the Hippo pathway, Yorkie (YAP/TAZ in vertebrates), plays distinct roles in the developing Drosophila airways. Yorkie exerts a cytoplasmic function by binding Drosophila Twinstar, the orthologue of the vertebrate actin-severing protein Cofilin, to regulate F-actin levels and apical cell membrane size, which are required for proper tracheal tube elongation. Second, Yorkie controls water tightness of tracheal tubes by transcriptional regulation of the δ-aminolevulinate synthase gene (Alas). We conclude that Yorkie has a dual role in tracheal development to ensure proper tracheal growth and functionality.

Geometry can provide long-range mechanical guidance for embryogenesis

Downstream of gene expression, effectors such as the actomyosin contractile machinery drive embryo morphogenesis. During Drosophila embryonic axis extension, actomyosin has a specific planar-polarised organisation, which is responsible for oriented cell intercalation. In addition to these cell rearrangements, cell shape changes also contribute to tissue deformation. While cell-autonomous dynamics are well described, understanding the tissue-scale behaviour challenges us to solve the corresponding mechanical problem at the scale of the whole embryo, since mechanical resistance of all neighbouring epithelia will feedback on individual cells. Here we propose a novel numerical approach to compute the whole-embryo dynamics of the actomyosin-rich apical epithelial surface. We input in the model specific patterns of actomyosin contractility, such as the planar-polarisation of actomyosin in defined ventro-lateral regions of the embryo. Tissue strain rates and displacements are then predicted over the whole embryo surface according to the global balance of stresses and the material behaviour of the epithelium. Epithelia are modelled using a rheological law that relates the rate of deformation to the local stresses and actomyosin anisotropic contractility. Predicted flow patterns are consistent with the cell flows observed when imaging Drosophila axis extension in toto, using light sheet microscopy. The agreement between model and experimental data indicates that the anisotropic contractility of planar-polarised actomyosin in the ventro-lateral germband tissue can directly cause the tissue-scale deformations of the whole embryo. The three-dimensional mechanical balance is dependent on the geometry of the embryo, whose curved surface is taken into account in the simulations. Importantly, we find that to reproduce experimental flows, the model requires the presence of the cephalic furrow, a fold located anteriorly of the extending tissues. The presence of this geometric feature, through the global mechanical balance, guides the flow and orients extension towards the posterior end.

Forces directing germ-band extension in Drosophila embryos

Body axis elongation by convergent extension is a conserved developmental process found in all metazoans. Drosophila embryonic germ-band extension is an important morphogenetic process during embryogenesis, by which the length of the germ-band is more than doubled along the anterior-posterior axis. This lengthening is achieved by typical convergent extension, i.e. narrowing the lateral epidermis along the dorsal-ventral axis and simultaneous extension along the anterior-posterior axis. Germ-band extension is largely driven by cell intercalation, whose directionality is determined by the planar polarity of the tissue and ultimately by the anterior-posterior patterning system. In addition, extrinsic tensile forces originating from the invaginating endoderm induce cell shape changes, which transiently contribute to germ-band extension. Here, we review recent progress in understanding of the role of mechanical forces in germ-band extension.

Embryo as an active granular fluid: stress-coordinated cellular constriction chains

Mechanical stress plays an intricate role in gene expression in individual cells and sculpting of developing tissues. However, systematic methods of studying how mechanical stress and feedback help to harmonize cellular activities within a tissue have yet to be developed. Motivated by our observation of the cellular constriction chains (CCCs) during the initial phase of ventral furrow formation in the Drosophila melanogaster embryo, we propose an active granular fluid (AGF) model that provides valuable insights into cellular coordination in the apical constriction process. In our model, cells are treated as circular particles connected by a predefined force network, and they undergo a random constriction process in which the particle constriction probability P is a function of the stress exerted on the particle by its neighbors. We find that when P favors tensile stress, constricted particles tend to form chain-like structures. In contrast, constricted particles tend to form compact clusters when P favors compression. A remarkable similarity of constricted-particle chains and CCCs observed in vivo provides indirect evidence that tensile-stress feedback coordinates the apical constriction activity. Our particle-based AGF model will be useful in analyzing mechanical feedback effects in a wide variety of morphogenesis and organogenesis phenomena.

Stable Force Balance between Epithelial Cells Arises from F-Actin Turnover

The propagation of force in epithelial tissues requires that the contractile cytoskeletal machinery be stably connected between cells through E-cadherin-containing adherens junctions. In many epithelial tissues, the cells' contractile network is positioned at a distance from the junction. However, the mechanism or mechanisms that connect the contractile networks to the adherens junctions, and thus mechanically connect neighboring cells, are poorly understood. Here, we identified the role for F-actin turnover in regulating the contractile cytoskeletal network's attachment to adherens junctions. Perturbing F-actin turnover via gene depletion or acute drug treatments that slow F-actin turnover destabilized the attachment between the contractile actomyosin network and adherens junctions. Our work identifies a critical role for F-actin turnover in connecting actomyosin to intercellular junctions, defining a dynamic process required for the stability of force balance across intercellular contacts in tissues.