Yanlan Mao

Planar polarization of the atypical myosin Dachs orients cell divisions in Drosophila

Tissues can grow in a particular direction by controlling the orientation of cell divisions. This phenomenon is evident in the developing Drosophila wing epithelium, where the tissue becomes elongated along the proximal-distal axis. We show that orientation of cell divisions in the wing requires planar polarization of an atypical myosin, Dachs. Our evidence suggests that Dachs constricts cell-cell junctions to alter the geometry of cell shapes at the apical surface, and that cell shape then determines the orientation of the mitotic spindle. Using a computational model of a growing epithelium, we show that polarized cell tension is sufficient to orient cell shapes, cell divisions, and tissue growth. Planar polarization of Dachs is ultimately oriented by long-range gradients emanating from compartment boundaries, and is therefore a mechanism linking these gradients with the control of tissue shape.

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.

EpiTools: An Open-Source Image Analysis Toolkit for Quantifying Epithelial Growth Dynamics

Summary Epithelia grow and undergo extensive rearrangements to achieve their final size and shape. Imaging the dynamics of tissue growth and morphogenesis is now possible with advances in time-lapse microscopy, but a true understanding of their complexities is limited by automated image analysis tools to extract quantitative data. To overcome such limitations, we have designed a new open-source image analysis toolkit called EpiTools. It provides user-friendly graphical user interfaces for accurately segmenting and tracking the contours of cell membrane signals obtained from 4D confocal imaging. It is designed for a broad audience, especially biologists with no computer-science background. Quantitative data extraction is integrated into a larger bioimaging platform, Icy, to increase the visibility and usability of our tools. We demonstrate the usefulness of EpiTools by analyzing Drosophila wing imaginal disc growth, revealing previously overlooked properties of this dynamic tissue, such as the patterns of cellular rearrangements.

Fundamental physical cellular constraints drive self‐organization of tissues

Morphogenesis is driven by small cell shape changes that modulate tissue organization. Apical surfaces of proliferating epithelial sheets have been particularly well studied. Currently, it is accepted that a stereotyped distribution of cellular polygons is conserved in proliferating tissues among metazoans. In this work, we challenge these previous findings showing that diverse natural packed tissues have very different polygon distributions. We use Voronoi tessellations as a mathematical framework that predicts this diversity. We demonstrate that Voronoi tessellations and the very different tissues analysed share an overriding restriction: the frequency of polygon types correlates with the distribution of cell areas. By altering the balance of tensions and pressures within the packed tissues using disease, genetic or computer model perturbations, we show that as long as packed cells present a balance of forces within tissue, they will be under a physical constraint that limits its organization. Our discoveries establish a new framework to understand tissue architecture in development and disease.

Tug of war--the influence of opposing physical forces on epithelial cell morphology.

The shape of a single animal cell is determined both by its internal cytoskeleton and through physical interactions with its environment. In a tissue context, this extracellular environment is made up largely of other cells and the extracellular matrix. As a result, the shape of cells residing within an epithelium will be determined both by forces actively generated within the cells themselves and by their deformation in response to forces generated elsewhere in the tissue as they propagate through cell-cell junctions. Together these complex patterns of forces combine to drive epithelial tissue morphogenesis during both development and homeostasis. Here we review the role of both active and passive cell shape changes and mechanical feedback control in tissue morphogenesis in different systems.

Cost-benefit analysis of the mechanisms that enable migrating cells to sustain motility upon changes in matrix environments

Cells can move through extracellular environments with varying geometries and adhesive properties. Adaptation to these differences is achieved by switching between different modes of motility, including lamellipod-driven and blebbing motility. Further, cells can modulate their level of adhesion to the extracellular matrix (ECM) depending on both the level of force applied to the adhesions and cell intrinsic biochemical properties. We have constructed a computational model of cell motility to investigate how motile cells transition between extracellular environments with varying surface continuity, confinement and adhesion. Changes in migration strategy are an emergent property of cells as the ECM geometry and adhesion changes. The transition into confined environments with discontinuous ECM fibres is sufficient to induce shifts from lamellipod-based to blebbing motility, while changes in confinement alone within a continuous geometry are not. The geometry of the ECM facilitates plasticity, by inducing shifts where the cell has high marginal gain from a mode change, and conserving persistency where the cell can continue movement regardless of the motility mode. This regulation of cell motility is independent of global changes in cytoskeletal properties, but requires locally higher linkage between the actin network and the plasma membrane at the cell rear, and changes in internal cell pressure. In addition to matrix geometry, we consider how cells might transition between ECM of different adhesiveness. We find that this requires positive feedback between the forces cells apply on the adhesion points, and the strength of the cell–ECM adhesions on those sites. This positive feedback leads to the emergence of a small number of highly adhesive cores, similar to focal adhesions. While the range of ECM adhesion levels the cell can invade is expanded with this feedback mechanism; the velocities are lowered for conditions where the positive feedback is not vital. Thus, plasticity of cell motility sacrifices the benefits of specialization, for robustness.

Polarization of Myosin II refines tissue material properties to buffer mechanical stress.

As tissues develop, they are subjected to a variety of mechanical forces. Some of these forces, such as those required for morphogenetic movements, are instrumental to the development and sculpting of tissues. However, mechanical forces can also lead to accumulation of substantial tensile stress, which if maintained, can result in tissue damage and impair tissue function. Despite our extensive understanding of force-guided morphogenesis, we have only a limited understanding of how tissues prevent further morphogenesis, once shape is determined after development. Buffering forces to prevent cellular changes in response to fluctuations of mechanical stress is critical during the lifetime of an adult organism. Here, through the development of a novel tissue-stretching device, we uncover a mechanosensitive pathway that regulates tissue responses to mechanical stress through the polarization of Myosin II across the tissue. Mechanistically, this process is independent of conserved Rho-kinase signaling but is mediated by force-induced linear actin polymerization and depolymerization via the formin Diaphanous and actin severing protein Cofilin, respectively. Importantly, these stretch-induced actomyosin cables stiffen the tissue to limit changes in cell shape and to protect the tissue from further mechanical damage prior to stress dissipation. This tissue rigidification prevents fractures in the tissue from propagating by confining the damage locally to the injured cells. Overall this mechanism of force-induced changes in tissue mechanical properties provides a general model of force buffering that rapidly protects tissues from physical damage to preserve tissue shape.

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

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

Isotropic myosin-generated tissue tension is required for the dynamic orientation of the mitotic spindle

The ability of epithelial cells to divide along their long cell axis, known as “Hertwig’s rule”, has been proposed to play an important and wide-ranging role in homogenising epithelial cell packing during tissue development and homeostasis. Since the position of the anaphase spindle defines the division plane, how divisions are oriented requires an understanding of the mechanisms that position the mitotic spindle. While many of the molecules required to orient the mitotic spindle have been identified in genetic screens, the mechanisms by which spindles read and align with the long cell axis remain poorly understood. Here, in exploring the dynamics of spindle orientation in mechanically distinct regions of the fly notum, we find that the ability of cells to properly orient their divisions depends both on cortical cues and on local tissue tension. Thus, spindles align with the long cell axis in tissues in which isotropic tension is elevated, but fail to do so in elongated cells within the crowded midline, where tension is low. Importantly, these region-specific differences in spindle behaviour can be reversed by decreasing or increasing the activity of non-muscle Myosin II. In addition, spindles in a tissue experiencing isotropic stress fail to align with the long cell axis if cells are mechanically isolated from their neighbours. These data lead us to propose that isotropic tension is required within an epithelium to provide cells with a mechanically stable substrate upon which localised cortical Dynein can pull on astral microtubules to orient the spindle.

Tissue Fluidity Promotes Epithelial Wound Healing

Epithelial tissues are inevitably damaged from time to time and must therefore have robust repair mechanisms. The behaviour of tissues depends on their mechanical properties and those of the surrounding environment1. However, it remains poorly understood how tissue mechanics regulates wound healing, particularly in in vivo animal tissues. Here we show that by tuning epithelial cell junctional tension, we can alter the rate of wound healing. We observe cells moving past each other at the wound edge by intercalating, like molecules in a fluid, resulting in seamless wound closure. Using a computational model, we counterintuitively predict that an increase in tissue fluidity, via a reduction in junctional tension, can accelerate the rate of wound healing. This is contrary to previous evidence that actomyosin tensile structures are important for wound closure2–6. When we experimentally reduce tissue tension, cells intercalate faster and wounds close in less time. The role we describe for tissue fluidity in wound healing, in addition to its known roles in developing7,8 and mature tissues9, reinforces the importance of the fluid state of a tissue.