Cell mechanics and feedback regulation of actomyosin networks

Septins provide microenvironment sensing and cortical actomyosin partitioning in motile amoeboid T lymphocytes

The all-terrain motility of lymphocytes in tissues and tissue-like gels is best described as amoeboid motility. For amoeboid motility, lymphocytes do not require specific biochemical or structural modifications to the surrounding extracellular matrix. Instead, they rely on changing shape and steric interactions with the microenvironment. However, the exact mechanism of amoeboid motility remains elusive. Here, we report that septins participate in amoeboid motility of T cells, enabling the formation of F-actin and α-actinin-rich cortical rings at the sites of cell cortex-indenting collisions with the extracellular matrix. Cortical rings compartmentalize cells into chains of spherical segments that are spatially conformed to the available lumens, forming transient "hourglass"-shaped steric locks onto the surrounding collagen fibers. The steric lock facilitates pressure-driven peristaltic propulsion of cytosolic content by individually contracting cell segments. Our results suggest that septins provide microenvironment-guided partitioning of actomyosin contractility and steric pivots required for amoeboid motility of T cells in tissue-like microenvironments.

Mechanical compression creates a quiescent muscle stem cell niche

Tissue stem cell niches are regulated by their mechanical environment, notably the extracellular matrix (ECM). Skeletal muscles consist of bundled myofibers for force transmission. Within this macroscopic architecture, quiescent Pax7-expressing (Pax7⁺) muscle stem cells (MuSCs) are compressed between ECM basally and myofiber apically. Muscle injury causes MuSCs to lose apical compression from the myofiber and re-enter the cell cycle for regeneration. While ECM elasticities have been shown to affect MuSC's renewal, the significance of apical compression remains unknown. To investigate the role of apical compression, we simulate the MuSCs' in vivo mechanical environment by applying physical compression to MuSCs' apical surface. We demonstrate that compression drives activated MuSCs back to a quiescent stem cell state, regardless of basal elasticities and chemistries. By mathematical modeling and cell tension manipulation, we conclude that low overall tension combined with high axial tension generated by compression leads to MuSCs' stemness and quiescence. Unexpectedly, we discovered that apical compression results in up-regulation of Notch downstream genes, accompanied by the increased levels of nuclear Notch1&3 in a Delta ligand (Dll) and ADAM10/17 independent manner. Our results fill a knowledge gap on the role of apical compression for MuSC fate and have implications to stem cells in other tissues.

The Mechanical Interplay Between Differentiating Mesenchymal Stem Cells and Gelatin-Based Substrates Measured by Atomic Force Microscopy

Traditional methods to assess hMSCs differentiation typically require long-term culture until cells show marked expression of histological markers such as lipid accumulation inside the cytoplasm or mineral deposition onto the surrounding matrix. In parallel, stem cell differentiation has been shown to involve the reorganization of the cell’s cytoskeleton shortly after differentiation induced by soluble factors. Given the cytoskeleton’s role in determining the mechanical properties of adherent cells, the mechanical characterization of stem cells could thus be a potential tool to assess cellular commitment at much earlier time points. In this study, we measured the mechanical properties of hMSCs cultured on soft gelatin-based hydrogels at multiple time points after differentiation induction toward adipogenic or osteogenic lineages. Our results show that the mechanical properties of cells (stiffness and viscosity) and the organization of the actin cytoskeleton are highly correlated with lineage commitment. Most importantly, we also found that the mechanical properties and the topography of the gelatin substrate in the vicinity of the cells are also altered as differentiation progresses toward the osteogenic lineage, but not on the adipogenic case. Together, these results confirm the biophysical changes associated with stem cell differentiation and suggest a mechanical interplay between the differentiating stem cells and their surrounding extracellular matrix.

Quantitative Imaging of pN Intercellular Force and Energetic Costs during Collective Cell Migration in Epithelial Wound Healing

Collective cell migration plays a key role in tissue repair, metastasis, and development. Cellular tension is a vital mechanical regulator during the force-driven cell movements. However, the contribution and mechanism of cell-cell force interaction and energetic costs during cell migration are yet to be understood. Here, we attempted to unfold the mechanism of collective cell movement through quantification of the intercellular tension and energetic costs. The measurement of pN intercellular force is based on a "spring-like" DNA-probe and a molecular tension fluorescence microscopy. During the process of wound healing, the intercellular force along with the cell monolayer mainly originates from actin polymerization, which is strongly related to the cellular energy metabolism level. Intracellular force at different spatial regions of wound and the energetic costs of leader and follower cells were measured. The maximum force and energy consumption are mainly concentrated at the wound edge and dynamically changed along with different stages of wound healing. These results indicated the domination of leader cells other than follower cells during the collective cell migration.

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.

Regulation of Cell Behavior by Hydrostatic Pressure

Hydrostatic pressure (HP) regulates diverse cell behaviors including differentiation, migration, apoptosis, and proliferation. Abnormal HP is associated with pathologies including glaucoma and hypertensive fibrotic remodeling. In this review, recent advances in quantifying and predicting how cells respond to HP across several tissue systems are presented, including tissues of the brain, eye, vasculature and bladder, as well as articular cartilage. Finally, some promising directions on the study of cell behaviors regulated by HP are proposed.

The Drosophila Afadin and ZO-1 homologues Canoe and Polychaetoid act in parallel to maintain epithelial integrity when challenged by adherens junction remodeling

During morphogenesis, cells must change shape and move without disrupting tissue integrity. This requires cell-cell junctions to allow dynamic remodeling while resisting forces generated by the actomyosin cytoskeleton. Multiple proteins play roles in junctional-cytoskeletal linkage, but the mechanisms by which they act remain unclear. Drosophila Canoe maintains adherens junction-cytoskeletal linkage during gastrulation. Canoe's mammalian homologue Afadin plays similar roles in cultured cells, working in parallel with ZO-1 proteins, particularly at multicellular junctions. We take these insights back to the fly embryo, exploring how cells maintain epithelial integrity when challenged by adherens junction remodeling during germband extension and dorsal closure. We found that Canoe helps cells maintain junctional-cytoskeletal linkage when challenged by the junctional remodeling inherent in mitosis, cell intercalation, and neuroblast invagination or by forces generated by the actomyosin cable at the leading edge. However, even in the absence of Canoe, many cells retain epithelial integrity. This is explained by a parallel role played by the ZO-1 homologue Polychaetoid. In embryos lacking both Canoe and Polychaetoid, cell junctions fail early, with multicellular junctions especially sensitive, leading to widespread loss of epithelial integrity. Our data suggest that Canoe and Polychaetoid stabilize Bazooka/Par3 at cell-cell junctions, helping maintain balanced apical contractility and tissue integrity.

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.

Role of extracellular matrix and microenvironment in regulation of tumor growth and LAR-mediated invasion in glioblastoma

The cellular dispersion and therapeutic control of glioblastoma, the most aggressive type of primary brain cancer, depends critically on the migration patterns after surgery and intracellular responses of the individual cancer cells in response to external biochemical cues in the microenvironment. Recent studies have shown that miR-451 regulates downstream molecules including AMPK/CAB39/MARK and mTOR to determine the balance between rapid proliferation and invasion in response to metabolic stress in the harsh tumor microenvironment. Surgical removal of the main tumor is inevitably followed by recurrence of the tumor due to inaccessibility of dispersed tumor cells in normal brain tissue. In order to address this complex process of cell proliferation and invasion and its response to conventional treatment, we propose a mathematical model that analyzes the intracellular dynamics of the miR-451-AMPK- mTOR-cell cycle signaling pathway within a cell. The model identifies a key mechanism underlying the molecular switches between proliferative phase and migratory phase in response to metabolic stress in response to fluctuating glucose levels. We show how up- or down-regulation of components in these pathways affects the key cellular decision to infiltrate or proliferate in a complex microenvironment in the absence and presence of time delays and stochastic noise. Glycosylated chondroitin sulfate proteoglycans (CSPGs), a major component of the extracellular matrix (ECM) in the brain, contribute to the physical structure of the local brain microenvironment but also induce or inhibit glioma invasion by regulating the dynamics of the CSPG receptor LAR as well as the spatiotemporal activation status of resident astrocytes and tumor-associated microglia. Using a multi-scale mathematical model, we investigate a CSPG-induced switch between invasive and non-invasive tumors through the coordination of ECM-cell adhesion and dynamic changes in stromal cells. We show that the CSPG-rich microenvironment is associated with non-invasive tumor lesions through LAR-CSGAG binding while the absence of glycosylated CSPGs induce the critical glioma invasion. We illustrate how high molecular weight CSPGs can regulate the exodus of local reactive astrocytes from the main tumor lesion, leading to encapsulation of non-invasive tumor and inhibition of tumor invasion. These different CSPG conditions also change the spatial profiles of ramified and activated microglia. The complex distribution of CSPGs in the tumor microenvironment can determine the nonlinear invasion behaviors of glioma cells, which suggests the need for careful therapeutic strategies.

The scaffolding protein ZO-1 coordinates actomyosin and epithelial apical specializations in vitro and in vivo

Polarized epithelia assemble into sheets that compartmentalize organs and generate tissue barriers by integrating apical surfaces into a single, unified structure. This tissue organization is shared across organs, species, and developmental stages. The processes that regulate development and maintenance of apical epithelial surfaces are, however, undefined. Here, using an intestinal epithelial-specific knockout (KO) mouse and cultured epithelial cells, we show that the tight junction scaffolding protein zonula occludens-1 (ZO-1) is essential for development of unified apical surfaces in vivo and in vitro We found that U5 and GuK domains of ZO-1 are necessary for proper apical surface assembly, including organization of microvilli and cortical F-actin; however, direct interactions with F-actin through the ZO-1 actin-binding region (ABR) are not required. ZO-1 lacking the PDZ1 domain, which binds claudins, rescued apical structure in ZO-1-deficient epithelia, but not in cells lacking both ZO-1 and ZO-2, suggesting that heterodimerization with ZO-2 restores PDZ1-dependent ZO-1 interactions that are vital to apical surface organization. Pharmacologic F-actin disruption, myosin II motor inhibition, or dynamin inactivation restored apical epithelial structure in vitro and in vivo, indicating that ZO-1 directs epithelial organization by regulating actomyosin contraction and membrane traffic. We conclude that multiple ZO-1-mediated interactions contribute to coordination of epithelial actomyosin function and genesis of unified apical surfaces.

Quantitative analysis of cell shape and the cytoskeleton in developmental biology

Computational approaches that enable quantification of microscopy data have revolutionized the field of developmental biology. Due to its inherent complexity, elucidating mechanisms of development requires sophisticated analysis of the structure, shape, and kinetics of cellular processes. This need has prompted the creation of numerous techniques to visualize, quantify, and merge microscopy data. These approaches have defined the order and structure of developmental events, thus, providing insight into the mechanisms that drive them. This review describes current computational approaches that are being used to answer developmental questions related to morphogenesis and describe how these approaches have impacted the field. Our intent is not to comprehensively review techniques, but to highlight examples of how different approaches have impacted our understanding of development. Specifically, we focus on methods to quantify cell shape and cytoskeleton structure and dynamics in developing tissues. Finally, we speculate on where the future of computational analysis in developmental biology might be headed. This article is categorized under: Technologies > Analysis of Cell, Tissue, and Animal Phenotypes Early Embryonic Development > Gastrulation and Neurulation Early Embryonic Development > Development to the Basic Body Plan.

Cell tension and mechanical regulation of cell volume

Animal cells use an unknown mechanism to control their growth and physical size. Here, using the fluorescence exclusion method, we measure cell volume for adherent cells on substrates of varying stiffness. We discover that the cell volume has a complex dependence on substrate stiffness and is positively correlated with the size of the cell adhesion to the substrate. From a mechanical force–balance condition that determines the geometry of the cell surface, we find that the observed cell volume variation can be predicted quantitatively from the distribution of active myosin through the cell cortex. To connect cell mechanical tension with cell size homeostasis, we quantified the nuclear localization of YAP/TAZ, a transcription factor involved in cell growth and proliferation. We find that the level of nuclear YAP/TAZ is positively correlated with the average cell volume. Moreover, the level of nuclear YAP/TAZ is also connected to cell tension, as measured by the amount of phosphorylated myosin. Cells with greater apical tension tend to have higher levels of nuclear YAP/TAZ and a larger cell volume. These results point to a size-sensing mechanism based on mechanical tension: the cell tension increases as the cell grows, and increasing tension feeds back biochemically to growth and proliferation control.

Apical and Basal Matrix Remodeling Control Epithelial Morphogenesis

Epithelial tissues can elongate in two dimensions by polarized cell intercalation, oriented cell division, or cell shape change, owing to local or global actomyosin contractile forces acting in the plane of the tissue. In addition, epithelia can undergo morphogenetic change in three dimensions. We show that elongation of the wings and legs of Drosophila involves a columnar-to-cuboidal cell shape change that reduces cell height and expands cell width. Remodeling of the apical extracellular matrix by the Stubble protease and basal matrix by MMP1/2 proteases induces wing and leg elongation. Matrix remodeling does not occur in the haltere, a limb that fails to elongate. Limb elongation is made anisotropic by planar polarized Myosin-II, which drives convergent extension along the proximal-distal axis. Subsequently, Myosin-II relocalizes to lateral membranes to accelerate columnar-to-cuboidal transition and isotropic tissue expansion. Thus, matrix remodeling induces dynamic changes in actomyosin contractility to drive epithelial morphogenesis in three dimensions.

Notch-dependent epithelial fold determines boundary formation between developmental fields in the Drosophila antenna

Compartment boundary formation plays an important role in development by separating adjacent developmental fields. Drosophila imaginal discs have proven valuable for studying the mechanisms of boundary formation. We studied the boundary separating the proximal A1 segment and the distal segments, defined respectively by Lim1 and Dll expression in the eye-antenna disc. Sharp segregation of the Lim1 and Dll expression domains precedes activation of Notch at the Dll/Lim1 interface. By repressing bantam miRNA and elevating the actin regulator Enable, Notch signaling then induces actomyosin-dependent apical constriction and epithelial fold. Disruption of Notch signaling or the actomyosin network reduces apical constriction and epithelial fold, so that Dll and Lim1 cells become intermingled. Our results demonstrate a new mechanism of boundary formation by actomyosin-dependent tissue folding, which provides a physical barrier to prevent mixing of cells from adjacent developmental fields.

During development, boundary formation between adjacent developmental fields is important to maintain the integrity of complex organs and tissues. We examined how boundaries become established between adjacent developmental fields—which are defined by expression of distinct selector genes and developmental fates—using the Drosophila eye-antennal disc as a model. We show that boundary formation is a progressive process. We focused our analysis on the antennal A1 fold that separates the A1 and A2-Ar segments, corresponding to the evolutionarily conserved segregation between coxopodite and telopodite segments of arthropod appendages. We describe a clear temporal and causal sequence of events from selector gene expression to establishment of a lineage-restricting boundary. We found that Notch activation at the boundary between adjacent fields of selector gene expression triggers actomyosin-mediated cell apical constriction, which induces the formation of an epithelial fold and prevents intermixing of cells from adjacent fields. Our findings describe a novel mechanism by which epithelial fold provides a physical barrier for cell segregation.

Physical confinement signals regulate the organization of stem cells in three dimensions

During embryogenesis, the spherical inner cell mass (ICM) proliferates in the confined environment of a blastocyst. Embryonic stem cells (ESCs) are derived from the ICM, and mimicking embryogenesis in vitro, mouse ESCs (mESCs) are often cultured in hanging droplets. This promotes the formation of a spheroid as the cells sediment and aggregate owing to increased physical confinement and cell-cell interactions. In contrast, mESCs form two-dimensional monolayers on flat substrates and it remains unclear if the difference in organization is owing to a lack of physical confinement or increased cell-substrate versus cell-cell interactions. Employing microfabricated substrates, we demonstrate that a single geometric degree of physical confinement on a surface can also initiate spherogenesis. Experiment and computation reveal that a balance between cell-cell and cell-substrate interactions finely controls the morphology and organization of mESC aggregates. Physical confinement is thus an important regulatory cue in the three-dimensional organization and morphogenesis of developing cells.

Coronin 1B supports RhoA signaling at cell-cell junctions through Myosin II

Non-muscle myosin II (NMII) motor proteins are responsible for generating contractile forces inside eukaryotic cells. There is also a growing interest in the capacity for these motor proteins to influence cell signaling through scaffolding, especially in the context of RhoA GTPase signaling. We previously showed that NMIIA accumulation and stability within specific regions of the cell cortex, such as the zonula adherens (ZA), allows the formation of a stable RhoA signaling zone. Now we demonstrate a key role for Coronin 1B in maintaining this junctional pool of NMIIA, as depletion of Coronin 1B significantly compromised myosin accumulation and stability at junctions. The loss of junctional NMIIA, upon Coronin 1B knockdown, perturbed RhoA signaling due to enhanced junctional recruitment of the RhoA antagonist, p190B Rho GAP. This effect was blocked by the expression of phosphomimetic MRLC-DD, thus reinforcing the central role of NMII in regulating RhoA signaling.

Active Biochemical Regulation of Cell Volume and a Simple Model of Cell Tension Response

Active contractile forces exerted by eukaryotic cells play significant roles during embryonic development, tissue formation, and cell motility. At the molecular level, small GTPases in signaling pathways can regulate active cell contraction. Here, starting with mechanical force balance at the cell cortex, and the recent discovery that tension-sensitive membrane channels can catalyze the conversion of the inactive form of Rho to the active form, we show mathematically that this active regulation of cellular contractility together with osmotic regulation can robustly control the cell size and membrane tension against external mechanical or osmotic shocks. We find that the magnitude of active contraction depends on the rate of mechanical pulling, but the cell tension can recover. The model also predicts that the cell exerts stronger contractile forces against a stiffer external environment, and therefore exhibits features of mechanosensation. These results suggest that a simple system for maintaining homeostatic values of cell volume and membrane tension could explain cell tension response and mechanosensation in different environments.

Willing to Be Involved in Cancer

Genome sequencing is now a common procedure, but prior to this, screening experiments using protein baits was one of the routinely used methods that, occasionally, allowed the identification of new gene products. One such experiment uncovered the gene product called willin/human Expanded/FRMD6. Initial characterization studies found that willin bound phospholipids and was strongly co-localised with actin. However, subsequently, willin was found to be the closest human sequence homologue of the Drosophila protein Expanded (Ex), sharing 60% homology with the Ex FERM domain. This in turn suggested, and then was proven that willin could activate the Hippo signalling pathway. This review describes the increasing body of knowledge about the actions of willin in a number of cellular functions related to cancer. However, like many gene products involved in aspects of cell signalling, a convincing direct role for willin in cancer remains tantalisingly elusive, at present.

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.

Protrusive activity guides changes in cell-cell tension during epithelial cell scattering

Knowing how epithelial cells regulate cell-matrix and cell-cell adhesions is essential to understand key events in morphogenesis as well as pathological events such as metastasis. During epithelial cell scattering, epithelial cell islands rupture their cell-cell contacts and migrate away as single cells on the extracellular matrix (ECM) within hours of growth factor stimulation, even as adhesion molecules such as E-cadherin are present at the cell-cell contact. How the stability of cell-cell contacts is modulated to effect such morphological transitions is still unclear. Here, we report that in the absence of ECM, E-cadherin adhesions continue to sustain substantial cell-generated forces upon hepatocyte growth factor (HGF) stimulation, consistent with undiminished adhesion strength. In the presence of focal adhesions, constraints that preclude the spreading and movement of cells at free island edges also prevent HGF-mediated contact rupture. To explore the role of cell motion and cell-cell contact rupture, we examine the biophysical changes that occur during the scattering of cell pairs. We show that the direction of cell movement with respect to the cell-cell contact is correlated with changes in the average intercellular force as well as the initial direction of cell-cell contact rupture. Our results suggest an important role for protrusive activity resulting in cell displacement and force redistribution in guiding cell-cell contact rupture during scattering.

Why is cytoskeletal contraction required for cardiac fusion before but not after looping begins?

Cytoskeletal contraction is crucial to numerous morphogenetic processes, but its role in early heart development is poorly understood. Studies in chick embryos have shown that inhibiting myosin-II-based contraction prior to Hamburger-Hamilton (HH) stage 10 (33 h incubation) impedes fusion of the mesodermal heart fields that create the primitive heart tube (HT), as well as the ensuing process of cardiac looping. If contraction is inhibited at or after looping begins at HH10, however, fusion and looping proceed relatively normally. To explore the mechanisms behind this seemingly fundamental change in behavior, we measured spatiotemporal distributions of tissue stiffness, stress, and strain around the anterior intestinal portal (AIP), the opening to the foregut where contraction and cardiac fusion occur. The results indicate that stiffness and tangential tension decreased bilaterally along the AIP with distance from the embryonic midline. The gradients in stiffness and tension, as well as strain rate, increased to peaks at HH9 (30 h) and decreased afterward. Exposure to the myosin II inhibitor blebbistatin reduced these effects, suggesting that they are mainly generated by active cytoskeletal contraction, and finite-element modeling indicates that the measured mechanical gradients are consistent with a relatively uniform contraction of the endodermal layer in conjunction with constraints imposed by the attached mesoderm. Taken together, our results suggest that, before HH10, endodermal contraction pulls the bilateral heart fields toward the midline where they fuse to create the HT. By HH10, however, the fusion process is far enough along to enable apposing cardiac progenitor cells to keep 'zipping' together during looping without the need for continued high contractile forces. These findings should shed new light on a perplexing question in early heart development.

Sensing the local environment: actin architecture and Hippo signalling

The Hippo network is a major conserved growth suppressor pathway that participates in organ size control during development and prevents tumour formation during adult homeostasis. Recent evidence has implicated the actin cytoskeleton as a link between tissue architecture and Hippo signalling. In this review, we will consider the evidence and models proposed for the regulation of Hippo signalling by actin dynamics and structure. We cover aspects of signalling regulation by mechanotransduction, cytoskeletal tethering and the spatial reorganization of signalling components. We also examine the physiological and pathological contexts in which these mechanisms are relevant.

Computational analysis of three-dimensional epithelial morphogenesis using vertex models

The folding of epithelial sheets, accompanied by cell shape changes and rearrangements, gives rise to three-dimensional structures during development. Recently, some aspects of epithelial morphogenesis have been modeled using vertex models, in which each cell is approximated by a polygon; however, these models have been largely confined to two dimensions. Here, we describe an adaptation of these models in which the classical two-dimensional vertex model is embedded in three dimensions. This modification allows for the construction of complex three-dimensional shapes from simple sheets of cells. We describe algorithmic, computational, and biophysical aspects of our model, with the view that it may be useful for formulating and testing hypotheses regarding the mechanical forces underlying a wide range of morphogenetic processes.

Morphogenesis can be driven by properly parametrised mechanical feedback

A fundamental problem of morphogenesis is whether it presents itself as a succession of links that are each driven by its own specific cause-effect relationship, or whether all of the links can be embraced by a common law that is possible to formulate in physical terms. We suggest that a common biophysical background for most, if not all, morphogenetic processes is based upon feedback between mechanical stresses (MS) that are imposed to a given part of a developing embryo by its other parts and MS that are actively generated within that part. The latter are directed toward hyper-restoration (restoration with an overshoot) of the initial MS values. We show that under mechanical constraints imposed by other parts, these tendencies drive forth development. To provide specificity for morphogenetic reactions, this feedback should be modulated by long-term parameters and/or initial conditions that are set up by genetic factors. The experimental and model data related to this concept are reviewed.

Differential proliferation rates generate patterns of mechanical tension that orient tissue growth

Orientation of cell divisions is a key mechanism of tissue morphogenesis. In the growing Drosophila wing imaginal disc epithelium, most of the cell divisions in the central wing pouch are oriented along the proximal-distal (P-D) axis by the Dachsous-Fat-Dachs planar polarity pathway. However, cells at the periphery of the wing pouch instead tend to orient their divisions perpendicular to the P-D axis despite strong Dachs polarization. Here, we show that these circumferential divisions are oriented by circumferential mechanical forces that influence cell shapes and thus orient the mitotic spindle. We propose that this circumferential pattern of force is not generated locally by polarized constriction of individual epithelial cells. Instead, these forces emerge as a global tension pattern that appears to originate from differential rates of cell proliferation within the wing pouch. Accordingly, we show that localized overgrowth is sufficient to induce neighbouring cell stretching and reorientation of cell division. Our results suggest that patterned rates of cell proliferation can influence tissue mechanics and thus determine the orientation of cell divisions and tissue shape.

The mechanics behind cell polarity

The generation of cell polarity is one of the most intriguing symmetry-breaking events in biology. It is involved in almost all physiological and developmental processes and, despite the differences between plant and animal cell structures, cell polarity is generated by a similar core mechanism that comprises the extracellular matrix (ECM), Rho GTPase, the cytoskeleton, and the membranes. Several recent articles show that mechanical factors also contribute to the establishment and robustness of cell polarity, and the different molecular actors of cell polarity are now viewed as integrators of both biochemical and mechanical signals. Although cell polarity remains a complex process, some level of functional convergence between plants and animals is revealed. Following comparative presentation of cell polarity in plants and animals, we will discuss the theoretical background behind the role of mechanics in polarity and the relevant experimental tests, focusing on ECM anchorage, cytoskeleton behavior, and membrane tension.

Mechanotransduction is enhanced by the synergistic action of heterotypic cell interactions and TGF-β1

With the use of planar substrates and collagen gels, the field of mechanotransduction has focused on the role of extracellular matrix stiffness, mechanical tension, and TGF-β1 in generating a more contractile fibroblast. However, little is known about the role of cell-cell interactions in inducing cellular contraction. We used 3-dimensional self-assembled microtissues, in which cell-cell interactions dominate, and a recently developed cell power assay (an assay for mechanotransduction) to quantify the effects of TGF-β1 vs. the heterotypic cell interface on the power exerted by pure normal human fibroblast (NHF) and pure rat hepatocyte 35 (H35) microtissues and their mixes. As a control, we found that TGF-β1 only doubled the power output of pure NHF and pure H35 microtissues, whereas the heterotypic environment resulted in a 5-fold increase in cell power (0.24±0.05 to 1.17±0.13 fJ/h). Seeding TGF-β1-treated NHFs with untreated H35 cells demonstrated that the heterotypic environment and TGF-β1 synergistically increase cell power by 22× by maximizing heterotypic cell interactions. Using a mathematical simulation of stress generation, we showed that tensile forces can be enhanced by heterotypic cell interactions. These data render a new understanding of how heterotypic cell interactions may increase cellular force generation during wound healing.

Phosphoinositides in cell architecture

Inositol phospholipids have been implicated in almost all aspects of cellular physiology including spatiotemporal regulation of cellular signaling, acquisition of cellular polarity, specification of membrane identity, cytoskeletal dynamics, and regulation of cellular adhesion, motility, and cytokinesis. In this review, we examine the critical role phosphoinositides play in these processes to execute the establishment and maintenance of cellular architecture. Epithelial tissues perform essential barrier and transport functions in almost all major organs. Key to their development and function is the establishment of epithelial cell polarity. We place a special emphasis on highlighting recent studies demonstrating phosphoinositide regulation of epithelial cell polarity and how individual cells use phosphoinositides to further organize into epithelial tissues.

Tension and epithelial morphogenesis in Drosophila early embryos

During morphogenesis, tissues are shaped by cell behaviors such as apical cell constriction and cell intercalation, which are the result of cell intrinsic forces, but are also shaped passively by forces acting on the cells. The latter extrinsic forces can be produced either within the deforming tissue by the tissue-scale integration of intrinsic forces, or outside the tissue by other tissue movements or by fluid flows. Here we review the intrinsic and extrinsic forces that sculpt the epithelium of early Drosophila embryos, focusing on three conserved morphogenetic processes: tissue internalization, axis extension, and segment boundary formation. Finally, we look at how the actomyosin cytoskeleton forms force-generating structures that power these three morphogenetic events at the cell and the tissue scales.

Mechanical stress as a regulator of cytoskeletal contractility and nuclear shape in embryonic epithelia

The mechano-sensitive responses of the heart and brain were examined in the chick embryo during Hamburger and Hamilton stages 10-12. During these early stages of development, cells in these structures are organized into epithelia. Isolated hearts and brains were compressed by controlled amounts of surface tension (ST) at the surface of the sample, and microindentation was used to measure tissue stiffness following several hours of culture. The response of both organs was qualitatively similar, as they stiffened under reduced loading. With increased loading, however, the brain softened while heart stiffness was similar to controls. In the brain, changes in nuclear shape and morphology correlated with these responses, as nuclei became more elliptical with decreased loading and rounder with increased loading. Exposure to the myosin inhibitor blebbistatin indicated that these changes in stiffness and nuclear shape are likely caused by altered cytoskeletal contraction. Computational modeling suggests that this behavior tends to return peak tissue stress back toward the levels it has in the intact heart and brain. These results suggest that developing cardiac and neural epithelia respond similarly to changes in applied loads by altering contractility in ways that tend to restore the original mechanical stress state. Hence, this study supports the view that stress-based mechanical feedback plays a role in regulating epithelial development.

Mechanotransduction in development

Biochemical patterning and morphogenetic movements coordinate the design of embryonic development. The molecular processes that pattern and closely control morphogenetic movements are today becoming well understood. Recent experimental evidence demonstrates that mechanical cues generated by morphogenesis activate mechanotransduction pathways, which in turn regulate cytoskeleton remodeling, cell proliferation, tissue differentiation. From Drosophila oocytes and embryos to Xenopus and mouse embryos and Arabidopsis meristem, here we review the developmental processes known to be activated in vivo by the mechanical strains associated to embryonic multicellular tissue morphogenesis. We describe the genetic, mechanical, and magnetic tools that have allowed the testing of mechanical induction in development by a step-by-step uncoupling of genetic inputs from mechanical inputs in embryogenesis. We discuss the known underlying molecular mechanisms involved in such mechanotransduction processes, including the Armadillo/β-catenin activation of Twist and the Fog-dependent stabilization of Myosin-II. These mechanotransduction processes are associated with a variety of physiological functions, such as mid-gut differentiation, mesoderm invagination and skeletal joint differentiation in embryogenesis, cell migration and internal pressure regulation during oogenesis, and meristem morphogenesis. We describe how the conservation of associated mechanosensitive pathways in embryonic and adult tissues opens new perspectives on mechanical involvement, potentially in evolution, and in cancer progression.