Actin Rings of Power

Role of WAVE3 as an actin binding protein in the pathology of triple negative breast cancer

Breast cancer, a prevalent global health concern, has sparked extensive research efforts, particularly focusing on triple negative breast cancer (TNBC), a subtype lacking estrogen receptor (ER), progesterone receptor, and epidermal growth factor receptor. TNBC's aggressive nature and resistance to hormone-based therapies heightens the risk of tumor progression and recurrence. Actin-binding proteins, specifically WAVE3 from the Wiskott-Aldrich syndrome protein (WASP) family, have emerged as major drivers in understanding TNBC biology. This review delves into the intricate molecular makeup of TNBC, shedding light on actin's fundamental role in cellular processes. Actin, a structural element in the cytoskeleton, regulates various cellular pathways essential for homeostasis. Its dynamic nature enables functions such as cell migration, motility, intracellular transport, cell division, and signal transduction. Actin-binding proteins, including WAVE3, play pivotal roles in these processes. WAVE3, a member of the WASP family, remains the focus of this review due to its potential involvement in TNBC progression. While actin-binding proteins are studied for their roles in healthy cellular cycles, their significance in TNBC remains underexplored. This review aims to discuss WAVE3's impact on TNBC, exploring its molecular makeup, functions, and significance in tumor progression. The intricate structure of WAVE3, featuring elements like the verprolin-cofilin-acidic domain and regulatory elements, plays a crucial role in regulating actin dynamics. Dysregulation of WAVE3 in TNBC has been linked to enhanced cell migration, invasion, extracellular matrix remodeling, epithelial-mesenchymal transition, tumor proliferation, and therapeutic resistance. Understanding the role of actin-binding proteins in cancer biology has potential clinical implications, making them potential prognostic biomarkers and promising therapeutic targets. The review emphasizes the need for further research into actin-binding proteins' clinical applications, diagnostic value, and therapeutic interventions. In conclusion, this comprehensive review explores the complex interplay between actin and actin-binding proteins, with special emphasis on WAVE3, in the context of TNBC. By unraveling the molecular intricacies, structural characteristics, and functional significance, the review paves the way for future research directions, clinical applications, and potential therapeutic strategies in the challenging landscape of TNBC.

Subcellular context-specific tuning of actomyosin ring contractility within a common cytoplasm

The non-muscle actomyosin cytoskeleton generates contractile force through the dynamic rearrangement of its constituent parts. Actomyosin rings are a specialization of the non-muscle actomyosin cytoskeleton that drive cell shape changes during division, wound healing, and other events. Contractile rings throughout phylogeny and in a range of cellular contexts are built from conserved components including non-muscle myosin II (NMMII), actin filaments (F-actin), and crosslinking proteins. However, it is unknown whether diverse actomyosin rings close via a single unifying mechanism. To explore how contractile forces are generated by actomyosin rings, we studied three instances of ring closure within the common cytoplasm of the C. elegans oogenic germline: mitotic cytokinesis of germline stem cells (GSCs), apoptosis of meiotic compartments, and cellularization of oocytes. We found that each ring type closed with unique kinetics, protein density and abundance dynamics. These measurements suggested that the mechanism of contractile force generation varied across the subcellular contexts. Next, we formulated a physical model that related the forces generated by filament-filament interactions to the material properties of these rings that dictate the kinetics of their closure. Using this framework, we related the density of conserved cytoskeletal proteins anillin and NMMII to the kinematics of ring closure. We fitted model rings to in situ measurements to estimate parameters that are currently experimentally inaccessible, such as the asymmetric distribution of protein along the length of F-actin, which occurs naturally due to differences in the dimensions of the crosslinker and NMMII filaments. Our work predicted that the role of NMMII varies across these ring types, due in part to its distribution along F-actin and motoring. Our model also predicted that the degree of contractility and the impact of ring material properties on contractility differs among ring types.

Pharmacological control of angiogenesis by regulating phosphorylation of myosin light chain 2

BACKGROUND

Control of angiogenesis is widely considered a therapeutic strategy, but reliable control methods are still under development. Phosphorylation of myosin light chain 2 (MLC2), which regulates actin-myosin interaction, is critical to the behavior of vascular endothelial cells (ECs) during angiogenesis. MLC2 is phosphorylated by MLC kinase (MLCK) and dephosphorylated by MLC phosphatase (MLCP) containing a catalytic subunit PP1. We investigated the potential role of MLC2 in the pharmacological control of angiogenesis.

METHODS AND RESULTS

We exposed transgenic zebrafish Tg(fli1a:Myr-mCherry)ncv1 embryos to chemical inhibitors and observed vascular development. PP1 inhibition by tautomycetin increased length of intersegmental vessels (ISVs), whereas MLCK inhibition by ML7 decreased it; these effects were not accompanied by structural dysplasia. ROCK inhibition by Y-27632 also decreased vessel length. An in vitro angiogenesis model of human umbilical vein endothelial cells (HUVECs) showed that tautomycetin increased vascular cord formation, whereas ML7 and Y-27632 decreased it. These effects appear to be influenced by regulation of cell morphology rather than cell viability or motility. Actin co-localized with phosphorylated MLC2 (pMLC2) was abundant in vascular-like elongated-shaped ECs, but poor in non-elongated ECs. pMLC2 was associated with tightly arranged actin, but not with loosely arranged actin. Moreover, knockdown of MYL9 gene encoding MLC2 reduced total MLC2 and pMLC2 protein and inhibited angiogenesis in HUVECs.

CONCLUSION

The present study found that MLC2 is a pivotal regulator of angiogenesis. MLC2 phosphorylation may be involved in the regulation of of cell morphogenesis and cell elongation. The functionally opposite inhibitors positively or negatively control angiogenesis, probably through the regulating EC morphology. These findings may provide a unique therapeutic target for angiogenesis.

Lumen expansion is initially driven by apical actin polymerization followed by osmotic pressure in a human epiblast model

Post-implantation, the pluripotent epiblast in a human embryo forms a central lumen, paving the way for gastrulation. Osmotic pressure gradients are considered the drivers of lumen expansion across development, but their role in human epiblasts is unknown. Here, we study lumenogenesis in a pluripotent-stem-cell-based epiblast model using engineered hydrogels. We find that leaky junctions prevent osmotic pressure gradients in early epiblasts and, instead, forces from apical actin polymerization drive lumen expansion. Once the lumen reaches a radius of ∼12 μm, tight junctions mature, and osmotic pressure gradients develop to drive further growth. Computational modeling indicates that apical actin polymerization into a stiff network mediates initial lumen expansion and predicts a transition to pressure-driven growth in larger epiblasts to avoid buckling. Human epiblasts show transcriptional signatures consistent with these mechanisms. Thus, actin polymerization drives lumen expansion in the human epiblast and may serve as a general mechanism of early lumenogenesis.

Engineering ssRNA tile filaments for (dis)assembly and membrane binding

Cytoskeletal protein filaments such as actin and microtubules confer mechanical support to cells and facilitate many cellular functions such as motility and division. Recent years have witnessed the development of a variety of molecular scaffolds that mimic such filaments. Indeed, filaments that are programmable and compatible with biological systems may prove useful in studying or substituting such proteins. Here, we explore the use of ssRNA tiles to build and modify filaments in vitro. We engineer a number of functionalities that are crucial to the function of natural proteins filaments into the ssRNA tiles, including the abilities to assemble or disassemble filaments, to tune the filament stiffness, to induce membrane binding, and to bind proteins. This work paves the way for building dynamic cytoskeleton-mimicking systems made out of rationally designed ssRNA tiles that can be transcribed in natural or synthetic cells.

Mechanoresponse of Curved Epithelial Monolayers Lining Bowl-Shaped 3D Microwells

The optimal functioning of many organs relies on the curved architecture of their epithelial tissues. However, the mechanoresponse of epithelia to changes in curvature remains misunderstood. Here, bowl-shaped microwells in hydrogels are designed via photopolymerization to faithfully replicate the shape and dimensions of lobular structures. Leveraging these hydrogel-based microwells, curved epithelial monolayers are engineered, and how in-plane and Gaussian curvatures at the microwell entrance influence epithelial behavior is investigated. Cells and nuclei around the microwell edge display a more pronounced centripetal orientation as the in-plane curvature decreases, and enhanced cell straightness and speed. Moreover, cells reorganize their actin cytoskeleton by forming a supracellular actin cable at the microwell edge, with its size becoming more pronounced as the in-plane curvature decreases. The Gaussian curvature at the microwell entrance enhances the maturation of the supracellular actin cable architecture and leads to a vertical orientation of nuclei toward the bottom of the microwell. Increasing Gaussian curvature results in flattened and elongated nuclear morphologies characterized by highly compacted chromatin states. This approach provides better understanding of the mechanoresponse of curved epithelial monolayers curvatures lining lobular structures. In addition, bowl-shaped microwells offer a powerful platform to study curvature-dependent mechanotransduction pathways in anatomically relevant 3D structures.

Methionine redox regulation of actin-interacting proteins primarily governs antioxidative signaling and response to the salvianolic acid B treatment in EA.hy926 cells

Actin-interacting proteins are important molecules for filament assembly and cytoskeletal signaling within vascular endothelium. Disruption in their interactions causes endothelial pathogenesis through redox imbalance. Actin filament redox regulation remains largely unexplored, in the context of pharmacological treatment. This work focused on the peptidyl methionine (M) redox regulation of actin-interacting proteins, aiming at elucidating its role on governing antioxidative signaling and response. Endothelial EA.hy926 cells were subjected to treatment with salvianolic acid B (Sal B) and tert-butyl-hydroperoxide (tBHP) stimulation. Mass spectrometry was employed to characterize redox status of proteins, including actin, myosin-9, kelch-like erythroid-derived cap-n-collar homology-associated protein 1 (Keap1), plastin-3, prelamin-A/C and vimentin. The protein redox landscape revealed distinct stoichiometric ratios or reaction site transitions mediated by M sulfoxide reductase and reactive oxygen species. In comparison with effects of tBHP stimulation, Sal B treatment prevented oxidation at actin M325, myosin-9 M1489/1565, Keap1 M120, plastin-3 M592, prelamin-A/C M187/371/540 and vimentin M344. For Keap1, reaction site was transitioned within its scaffolding region to the actin ring. These protein M oxidation regulations contributed to the Sal B cytoprotective effects on actin filament. Additionally, regarding the Keap1 homo-dimerization region, Sal B preventive roles against M120 oxidation acted as a primary signal driver to activate nuclear factor erythroid 2-related factor 2 (Nrf2). Transcriptional splicing of non-POU domain-containing octamer-binding protein was validated during the Sal B-mediated overexpression of NAD(P)H dehydrogenase [quinone] 1. This molecular redox regulation of actin-interacting proteins provided valuable insights into the phenolic structures of Sal B analogs, showing potential antioxidative effects on vascular endothelium.

Cortex-driven cytoplasmic flows in elongated cells: fluid mechanics and application to nuclear transport in Drosophila embryos

The Drosophila melanogaster embryo, an elongated multi-nucleated cell, is a classical model system for eukaryotic development and morphogenesis. Recent work has shown that bulk cytoplasmic flows, driven by cortical contractions along the walls of the embryo, enable the uniform spreading of nuclei along the anterior-posterior axis necessary for proper embryonic development. Here, we propose two mathematical models to characterize cytoplasmic flows driven by tangential cortical contractions in elongated cells. Assuming Newtonian fluid flow at low Reynolds number in a spheroidal cell, we first compute the flow field exactly, thereby bypassing the need for numerical computations. We then apply our results to recent experiments on nuclear transport in cell cycles 4-6 of Drosophila embryo development. By fitting the cortical contractions in our model to measurements, we reveal that experimental cortical flows enable near-optimal axial spreading of nuclei. A second mathematical approach, applicable to general elongated cell geometries, exploits a long-wavelength approximation to produce an even simpler solution, with errors below [Formula: see text] compared with the full model. An application of this long-wavelength result to transport leads to fully analytical solutions for the nuclear concentration that capture the essential physics of the system, including optimal axial spreading of nuclei.

Convergence of Calcium Channel Regulation and Mechanotransduction in Skeletal Regenerative Biomaterial Design

Cells are known to perceive their microenvironment through extracellular and intracellular mechanical signals. Upon sensing mechanical stimuli, cells can initiate various downstream signaling pathways that are vital to regulating proliferation, growth, and homeostasis. One such physiologic activity modulated by mechanical stimuli is osteogenic differentiation. The process of osteogenic mechanotransduction is regulated by numerous calcium ion channels-including channels coupled to cilia, mechanosensitive and voltage-sensitive channels, and channels associated with the endoplasmic reticulum. Evidence suggests these channels are implicated in osteogenic pathways such as the YAP/TAZ and canonical Wnt pathways. This review aims to describe the involvement of calcium channels in regulating osteogenic differentiation in response to mechanical loading and characterize the fashion in which those channels directly or indirectly mediate this process. The mechanotransduction pathway is a promising target for the development of regenerative materials for clinical applications due to its independence from exogenous growth factor supplementation. As such, also described are examples of osteogenic biomaterial strategies that involve the discussed calcium ion channels, calcium-dependent cellular structures, or calcium ion-regulating cellular features. Understanding the distinct ways calcium channels and signaling regulate these processes may uncover potential targets for advancing biomaterials with regenerative osteogenic capabilities.

Coordinated efforts of different actin filament populations are needed for optimal cell wound repair

Cells are subjected to a barrage of daily insults that often lead to their cortices being ripped open and requiring immediate repair. An important component of the cell's repair response is the formation of an actomyosin ring at the wound periphery to mediate its closure. Here we show that inhibition of myosin or the linear actin nucleation factors Diaphanous and/or dishevelled associated activator of morphogenesis results in a disrupted contractile apparatus and delayed wound closure. We also show that the branched actin nucleators WASp and SCAR function nonredundantly as scaffolds to assemble and maintain this contractile actomyosin cable. Removing branched actin leads to the formation of smaller circular actin-myosin structures at the cell cortex and to slow wound closure. Removing linear and branched actin simultaneously results in failed wound closure. Surprisingly, removal of branched actin and myosin results in the formation of parallel linear F-actin filaments that undergo a chiral swirling movement to close the wound, uncovering a new mechanism of cell wound closure. Taken together, we demonstrate the roles of different actin substructures that are required for optimal actomyosin ring formation and the extraordinary resilience of the cell to undergo wound repair when it is unable to form different subsets of these substructures.

Characterizing emerging features in cell dynamics using topological data analysis methods

Filament-motor interactions inside cells play essential roles in many developmental as well as other biological processes. For instance, actin-myosin interactions drive the emergence or closure of ring channel structures during wound healing or dorsal closure. These dynamic protein interactions and the resulting protein organization lead to rich time-series data generated by using fluorescence imaging experiments or by simulating realistic stochastic models. We propose methods based on topological data analysis to track topological features through time in cell biology data consisting of point clouds or binary images. The framework proposed here is based on computing the persistent homology of the data at each time point and on connecting topological features through time using established distance metrics between topological summaries. The methods retain aspects of monomer identity when analyzing significant features in filamentous structure data, and capture the overall closure dynamics when assessing the organization of multiple ring structures through time. Using applications of these techniques to experimental data, we show that the proposed methods can describe features of the emergent dynamics and quantitatively distinguish between control and perturbation experiments.

Actomyosin-Driven Division of a Synthetic Cell

One of the major challenges of bottom-up synthetic biology is rebuilding a minimal cell division machinery. From a reconstitution perspective, the animal cell division apparatus is mechanically the simplest and therefore attractive to rebuild. An actin-based ring produces contractile force to constrict the membrane. By contrast, microbes and plant cells have a cell wall, so division requires concerted membrane constriction and cell wall synthesis. Furthermore, reconstitution of the actin division machinery helps in understanding the physical and molecular mechanisms of cytokinesis in animal cells and thus our own cells. In this review, we describe the state-of-the-art research on reconstitution of minimal actin-mediated cytokinetic machineries. Based on the conceptual requirements that we obtained from the physics of the shape changes involved in cell division, we propose two major routes for building a minimal actin apparatus capable of division. Importantly, we acknowledge both the passive and active roles that the confining lipid membrane can play in synthetic cytokinesis. We conclude this review by identifying the most pressing challenges for future reconstitution work, thereby laying out a roadmap for building a synthetic cell equipped with a minimal actin division machinery.

Wasl is crucial to maintain microglial core activities during glioblastoma initiation stages

Microglia actively promotes the growth of high-grade gliomas. Within the glioma microenvironment an amoeboid microglial morphology has been observed, however the underlying causes and the related impact on microglia functions and their tumor promoting activities is unclear. Using the advantages of the larval zebrafish model, we identified the underlying mechanism and show that microglial morphology and functions are already impaired during glioma initiation stages. The presence of pre-neoplastic HRasV12 expressing cells induces an amoeboid morphology of microglia, increases microglial numbers and decreases their motility and phagocytic activity. RNA sequencing analysis revealed lower expression levels of the actin nucleation promoting factor wasla in microglia. Importantly, a microglia specific rescue of wasla expression restores microglial morphology and functions. This results in increased phagocytosis of pre-neoplastic cells and slows down tumor progression. In conclusion, we identified a mechanism that de-activates core microglial functions within the emerging glioma microenvironment. Restoration of this mechanism might provide a way to impair glioma growth.

Lmo7 recruits myosin II heavy chain to regulate actomyosin contractility and apical domain size in Xenopus ectoderm

Apical constriction, or a reduction in size of the apical domain, underlies many morphogenetic events during development. Actomyosin complexes play an essential role in apical constriction; however, the detailed analysis of molecular mechanisms is still pending. Here, we show that Lim domain only protein 7 (Lmo7), a multidomain adaptor at apical junctions, promotes apical constriction in the Xenopus superficial ectoderm, whereas apical domain size increases in Lmo7-depleted cells. Lmo7 is primarily localized at apical junctions and promotes the formation of the dense circumferential actomyosin belt. Strikingly, Lmo7 binds non-muscle myosin II (NMII) and recruits it to apical junctions and the apical cortex. This NMII recruitment is essential for Lmo7-mediated apical constriction. Lmo7 knockdown decreases NMIIA localization at apical junctions and delays neural tube closure in Xenopus embryos. Our findings suggest that Lmo7 serves as a scaffold that regulates actomyosin contractility and apical domain size.

Plasmodium parasitophorous vacuole membrane-resident protein UIS4 manipulates host cell actin to avoid parasite elimination

Parasite-derived PVM-resident proteins are critical for complete parasite development inside hepatocytes, although the function of most of these proteins remains unknown. Here, we show that the upregulated in infectious sporozoites 4 (UIS4) protein, resident at the PVM, interacts with the host cell actin. By suppressing filamentous actin formation, UIS4 avoids parasite elimination. Host cell actin dynamics increases around UIS4-deficient parasites, which is associated with subsequent parasite elimination. Notably, parasite elimination is impaired significantly by the inhibition of host myosin-II, possibly through relieving the compression generated by actomyosin complexes at the host-parasite interface. Together, these data reveal that UIS4 has a critical role in the evasion of host defensive mechanisms, enabling hence EEF survival and development.

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Plasmodium PVM-resident protein UIS4 interacts with host cell actin

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Host actin dynamics is altered around exoerythocytic forms (EEFs) lacking UIS4

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Actin activity around EEFs lacking UIS4 is associated with parasite elimination

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Parasite elimination depends on actomyosin complexes formed around the PVM

Simulated actin reorganization mediated by motor proteins

Cortical actin networks are highly dynamic and play critical roles in shaping the mechanical properties of cells. The actin cytoskeleton undergoes significant reorganization in many different contexts, including during directed cell migration and over the course of the cell cycle, when cortical actin can transition between different configurations such as open patched meshworks, homogeneous distributions, and aligned bundles. Several types of myosin motor proteins, characterized by different kinetic parameters, have been involved in this reorganization of actin filaments. Given the limitations in studying the interactions of actin with myosin in vivo, we propose stochastic agent-based models and develop a set of data analysis measures to assess how myosin motor proteins mediate various actin organizations. In particular, we identify individual motor parameters, such as motor binding rate and step size, that generate actin networks with different levels of contractility and different patterns of myosin motor localization, which have previously been observed experimentally. In simulations where two motor populations with distinct kinetic parameters interact with the same actin network, we find that motors may act in a complementary way, by tuning the actin network organization, or in an antagonistic way, where one motor emerges as dominant. This modeling and data analysis framework also uncovers parameter regimes where spatial segregation between motor populations is achieved. By allowing for changes in kinetic rates during the actin-myosin dynamic simulations, our work suggests that certain actin-myosin organizations may require additional regulation beyond mediation by motor proteins in order to reconfigure the cytoskeleton network on experimentally-observed timescales.

Cell shape is dictated by a scaffolding network called the cytoskeleton. Actin filaments, a main component of the cytoskeleton, are found predominantly at the periphery of the cell, where they organize into different patterns in response to various stimuli, such as progression through the cell cycle. The actin filament reorganizations are mediated by motor proteins from the myosin superfamily. Using a realistic stochastic model that simulates actin filament and motor protein dynamics and interactions, we systematically vary motor protein kinetics and investigate their effect on actin filament organization. Using novel measures of spatial organization, we quantify conditions under which motor proteins, either alone or in combination, can produce the different actin filament organizations observed in vitro and in vivo. These results yield new insights into the role of motor proteins, as well as into how multiple types of motors can work collectively to produce specific actomyosin network patterns.

The mechanical forces that shape our senses

Developing organs are shaped, in part, by physical interaction with their environment in the embryo. In recent years, technical advances in live-cell imaging and material science have greatly expanded our understanding of the mechanical forces driving organ formation. Here, we provide a broad overview of the types of forces generated during embryonic development and then focus on a subset of organs underlying our senses: the eyes, inner ears, nose and skin. The epithelia in these organs emerge from a common origin: the ectoderm germ layer; yet, they arrive at unique and complex forms over developmental time. We discuss exciting recent animal studies that show a crucial role for mechanical forces in, for example, the thickening of sensory placodes, the coiling of the cochlea and the lengthening of hair. Finally, we discuss how microfabricated organoid systems can now provide unprecedented insights into the physical principles of human development.

Mechanical regulation of early vertebrate embryogenesis

Embryonic cells grow in environments that provide a plethora of physical cues, including mechanical forces that shape the development of the entire embryo. Despite their prevalence, the role of these forces in embryonic development and their integration with chemical signals have been mostly neglected, and scrutiny in modern molecular embryology tilted, instead, towards the dissection of molecular pathways involved in cell fate determination and patterning. It is now possible to investigate how mechanical signals induce downstream genetic regulatory networks to regulate key developmental processes in the embryo. Here, we review the insights into mechanical control of early vertebrate development, including the role of forces in tissue patterning and embryonic axis formation. We also highlight recent in vitro approaches using individual embryonic stem cells and self-organizing multicellular models of human embryos, which have been instrumental in expanding our understanding of how mechanics tune cell fate and cellular rearrangements during human embryonic development.

Transient mechanical interactions between cells and viscoelastic extracellular matrix

During various physiological processes, such as wound healing and cell migration, cells continuously interact mechanically with a surrounding extracellular matrix (ECM). Contractile forces generated by the actin cytoskeleton are transmitted to a surrounding ECM, resulting in structural remodeling of the ECM. To better understand how matrix remodeling takes place, a myriad of in vitro experiments and simulations have been performed during recent decades. However, physiological ECMs are viscoelastic, exhibiting stress relaxation or creep over time. The time-dependent nature of matrix remodeling induced by cells remains poorly understood. Here, we employed a discrete model to investigate how the viscoelastic nature of ECMs affects matrix remodeling and stress profiles. In particular, we used explicit transient cross-linkers with varied density and unbinding kinetics to capture viscoelasticity unlike most of the previous models. Using this model, we quantified the time evolution of generation, propagation, and relaxation of stresses induced by a contracting cell in an ECM. It was found that matrix connectivity, regulated by fiber concentration and cross-linking density, significantly affects the magnitude and propagation of stress and subsequent matrix remodeling, as characterized by fiber displacements and local net deformation. In addition, we demonstrated how the base rate and force sensitivity of cross-linker unbinding regulate stress profiles and matrix remodeling. We verified simulation results using in vitro experiments performed with fibroblasts encapsulated in a three-dimensional collagen matrix. Our study provides key insights into the dynamics of physiologically relevant mechanical interactions between cells and a viscoelastic ECM.

Actin Cytoskeletal Dynamics in Single-Cell Wound Repair

The plasma membrane protects the eukaryotic cell from its surroundings and is essential for cell viability; thus, it is crucial that membrane disruptions are repaired quickly to prevent immediate dyshomeostasis and cell death. Accordingly, cells have developed efficient repair mechanisms to rapidly reseal ruptures and reestablish membrane integrity. The cortical actin cytoskeleton plays an instrumental role in both plasma membrane resealing and restructuring in response to damage. Actin directly aids membrane repair or indirectly assists auxiliary repair mechanisms. Studies investigating single-cell wound repair have often focused on the recruitment and activation of specialized repair machinery, despite the undeniable need for rapid and dynamic cortical actin modulation; thus, the role of the cortical actin cytoskeleton during wound repair has received limited attention. This review aims to provide a comprehensive overview of membrane repair mechanisms directly or indirectly involving cortical actin cytoskeletal remodeling.

Emergence of a smooth interface from growth of a dendritic network against a mechanosensitive contractile material

Structures and machines require smoothening of raw materials. Self-organized smoothening guides cell and tissue morphogenesis and is relevant to advanced manufacturing. Across the syncytial Drosophila embryo surface, smooth interfaces form between expanding Arp2/3-based actin caps and surrounding actomyosin networks, demarcating the circumferences of nascent dome-like compartments used for pseudocleavage. We found that forming a smooth and circular boundary of the surrounding actomyosin domain requires Arp2/3 in vivo. To dissect the physical basis of this requirement, we reconstituted the interacting networks using node-based models. In simulations of actomyosin networks with local clearances in place of Arp2/3 domains, rough boundaries persisted when myosin contractility was low. With addition of expanding Arp2/3 network domains, myosin domain boundaries failed to smoothen, but accumulated myosin nodes and tension. After incorporating actomyosin mechanosensitivity, Arp2/3 network growth locally induced a surrounding contractile actomyosin ring that smoothened the interface between the cytoskeletal domains, an effect also evident in vivo. In this way, a smooth structure can emerge from the lateral interaction of irregular active materials.

Cytoskeletal control of early mammalian development

The cytoskeleton - comprising actin filaments, microtubules and intermediate filaments - serves instructive roles in regulating cell function and behaviour during development. However, a key challenge in cell and developmental biology is to dissect how these different structures function and interact in vivo to build complex tissues, with the ultimate aim to understand these processes in a mammalian organism. The preimplantation mouse embryo has emerged as a primary model system for tackling this challenge. Not only does the mouse embryo share many morphological similarities with the human embryo during its initial stages of life, it also permits the combination of genetic manipulations with live-imaging approaches to study cytoskeletal dynamics directly within an intact embryonic system. These advantages have led to the discovery of novel cytoskeletal structures and mechanisms controlling lineage specification, cell-cell communication and the establishment of the first forms of tissue architecture during development. Here we highlight the diverse organization and functions of each of the three cytoskeletal filaments during the key events that shape the early mammalian embryo, and discuss how they work together to perform key developmental tasks, including cell fate specification and morphogenesis of the blastocyst. Collectively, these findings are unveiling a new picture of how cells in the early embryo dynamically remodel their cytoskeleton with unique spatial and temporal precision to drive developmental processes in the rapidly changing in vivo environment.

The Ringleaders: Understanding the Apicomplexan Basal Complex Through Comparison to Established Contractile Ring Systems

The actomyosin contractile ring is a key feature of eukaryotic cytokinesis, conserved across many eukaryotic kingdoms. Recent research into the cell biology of the divergent eukaryotic clade Apicomplexa has revealed a contractile ring structure required for asexual division in the medically relevant genera Toxoplasma and Plasmodium; however, the structure of the contractile ring, known as the basal complex in these parasites, remains poorly characterized and in the absence of a myosin II homolog, it is unclear how the force required of a cytokinetic contractile ring is generated. Here, we review the literature on the basal complex in Apicomplexans, summarizing what is known about its formation and function, and attempt to provide possible answers to this question and suggest new avenues of study by comparing the Apicomplexan basal complex to well-studied, established cytokinetic contractile rings and their mechanisms in organisms such as S. cerevisiae and D. melanogaster. We also compare the basal complex to structures formed during mitochondrial and plastid division and cytokinetic mechanisms of organisms beyond the Opisthokonts, considering Apicomplexan diversity and divergence.

Reconstitution of contractile actomyosin rings in vesicles

One of the grand challenges of bottom-up synthetic biology is the development of minimal machineries for cell division. The mechanical transformation of large-scale compartments, such as Giant Unilamellar Vesicles (GUVs), requires the geometry-specific coordination of active elements, several orders of magnitude larger than the molecular scale. Of all cytoskeletal structures, large-scale actomyosin rings appear to be the most promising cellular elements to accomplish this task. Here, we have adopted advanced encapsulation methods to study bundled actin filaments in GUVs and compare our results with theoretical modeling. By changing few key parameters, actin polymerization can be differentiated to resemble various types of networks in living cells. Importantly, we find membrane binding to be crucial for the robust condensation into a single actin ring in spherical vesicles, as predicted by theoretical considerations. Upon force generation by ATP-driven myosin motors, these ring-like actin structures contract and locally constrict the vesicle, forming furrow-like deformations. On the other hand, cortex-like actin networks are shown to induce and stabilize deformations from spherical shapes.

Cytoskeletal networks support and direct cell shape and guide intercellular transport, but relatively little is understood about the self-organization of cytoskeletal components on the scale of an entire cell. Here, authors use an in vitro system and observe the assembly of different types of actin networks and the condensation of membrane-bound actin into single rings.

An actomyosin clamp assembled by the Amphiphysin-Rho1-Dia/DAAM-Rok pathway reinforces somatic cell membrane folded around spermatid heads

Membrane curvature recruits Bin-Amphiphysin-Rvs (BAR)-domain proteins and induces local F-actin assembly, which further modifies the membrane curvature and dynamics. The downstream molecular pathway in vivo is still unclear. Here, we show that a tubular endomembrane scaffold supported by contractile actomyosin stabilizes the somatic cyst cell membrane folded around rigid spermatid heads during the final stages of sperm maturation in Drosophila testis. The structure resembles an actin "basket" covering the bundle of spermatid heads. Genetic analyses suggest that the actomyosin organization is nucleated exclusively by the formins - Diaphanous and Dishevelled Associated Activator of Morphogenesis (DAAM) - downstream of Rho1, which is recruited by the BAR-domain protein Amphiphysin. Actomyosin activity at the actin basket gathers the spermatid heads into a compact bundle and resists the somatic cell invasion by intruding spermatids. These observations reveal a distinct response mechanism of actin-membrane interactions, which generates a cell-adhesion-like strategy through active clamping.

Non-cell-autonomous migration of RasV12-transformed cells towards the basal side of surrounding normal cells

Oncogenic transformation enables cells to behave differently from their neighboring normal cells. Both cancer and normal cells recognize each other, often promoting the extrusion of the former from the epithelial cell layer. Here, we show that RasV12-transformed normal rat kidney 52E (NRK-52E) cells are extruded towards the basal side of the surrounding normal cells, which is concomitant with enhanced motility. The active migration of the basally extruded RasV12 cells is observed when surrounded by normal cells, indicating a non-cell-autonomous mechanism. Furthermore, specific inhibitor treatment and knockdown experiments elucidate the roles of PI3K and myosin IIA in the basal extrusion of Ras cells. Our findings reveal a new aspect of cancer cell invasion mediated by functional interactions with surrounding non-transformed cells.

Topological Data Analysis Approaches to Uncovering the Timing of Ring Structure Onset in Filamentous Networks

In developmental biology as well as in other biological systems, emerging structure and organization can be captured using time-series data of protein locations. In analyzing this time-dependent data, it is a common challenge not only to determine whether topological features emerge, but also to identify the timing of their formation. For instance, in most cells, actin filaments interact with myosin motor proteins and organize into polymer networks and higher-order structures. Ring channels are examples of such structures that maintain constant diameters over time and play key roles in processes such as cell division, development, and wound healing. Given the limitations in studying interactions of actin with myosin in vivo, we generate time-series data of protein polymer interactions in cells using complex agent-based models. Since the data has a filamentous structure, we propose sampling along the actin filaments and analyzing the topological structure of the resulting point cloud at each time. Building on existing tools from persistent homology, we develop a topological data analysis (TDA) method that assesses effective ring generation in this dynamic data. This method connects topological features through time in a path that corresponds to emergence of organization in the data. In this work, we also propose methods for assessing whether the topological features of interest are significant and thus whether they contribute to the formation of an emerging hole (ring channel) in the simulated protein interactions. In particular, we use the MEDYAN simulation platform to show that this technique can distinguish between the actin cytoskeleton organization resulting from distinct motor protein binding parameters.

Adhesion-mediated heterogeneous actin organization governs apoptotic cell extrusion

Apoptotic extrusion is crucial in maintaining epithelial homeostasis. Current literature supports that epithelia respond to extrusion by forming a supracellular actomyosin purse-string in the neighbors. However, whether other actin structures could contribute to extrusion and how forces generated by these structures can be integrated are unknown. Here, we found that during extrusion, a heterogeneous actin network composed of lamellipodia protrusions and discontinuous actomyosin cables, was reorganized in the neighboring cells. The early presence of basal lamellipodia protrusion participated in both basal sealing of the extrusion site and orienting the actomyosin purse-string. The co-existence of these two mechanisms is determined by the interplay between the cell-cell and cell-substrate adhesions. A theoretical model integrates these cellular mechanosensitive components to explain why a dual-mode mechanism, which combines lamellipodia protrusion and purse-string contractility, leads to more efficient extrusion than a single-mode mechanism. In this work, we provide mechanistic insight into extrusion, an essential epithelial homeostasis process.

Tools of the trade: studying actin in zebrafish

Actin is a conserved cytoskeletal protein with essential functions. Here, we review the state-of-the-art reagents, tools and methods used to probe actin biology and functions in zebrafish embryo and larvae. We also discuss specific cell types and tissues where the study of actin in zebrafish has provided new insights into its functions.

Pattern regulation in a regenerating jellyfish

Jellyfish, with their tetraradial symmetry, offer a novel paradigm for addressing patterning mechanisms during regeneration. Here we show that an interplay between mechanical forces, cell migration and proliferation allows jellyfish fragments to regain shape and functionality rapidly, notably by efficient restoration of the central feeding organ (manubrium). Fragmentation first triggers actomyosin-powered remodeling that restores body umbrella shape, causing radial smooth muscle fibers to converge around 'hubs' which serve as positional landmarks. Stabilization of these hubs, and associated expression of Wnt6, depends on the configuration of the adjoining muscle fiber 'spokes'. Stabilized hubs presage the site of the manubrium blastema, whose growth is Wnt/β-catenin dependent and fueled by both cell proliferation and long-range cell recruitment. Manubrium morphogenesis is modulated by its connections with the gastrovascular canal system. We conclude that body patterning in regenerating jellyfish emerges mainly from local interactions, triggered and directed by the remodeling process.

Injured Axons Instruct Schwann Cells to Build Constricting Actin Spheres to Accelerate Axonal Disintegration

After a peripheral nerve lesion, distal ends of injured axons disintegrate into small fragments that are subsequently cleared by Schwann cells and later by macrophages. Axonal debris clearing is an early step of the repair process that facilitates regeneration. We show here that Schwann cells promote distal cut axon disintegration for timely clearing. By combining cell-based and in vivo models of nerve lesion with mouse genetics, we show that this mechanism is induced by distal cut axons, which signal to Schwann cells through PlGF mediating the activation and upregulation of VEGFR1 in Schwann cells. In turn, VEGFR1 activates Pak1, leading to the formation of constricting actomyosin spheres along unfragmented distal cut axons to mediate their disintegration. Interestingly, oligodendrocytes can acquire a similar behavior as Schwann cells by enforced expression of VEGFR1. These results thus identify controllable molecular cues of a neuron-glia crosstalk essential for timely clearing of damaged axons.

A mechanical perspective on phagocytic cup formation

Phagocytosis is a widespread and evolutionarily conserved process with diverse biological functions, ranging from engulfment of invading microbes during infection to clearance of apoptotic debris in tissue homeostasis. Along with differences in biochemical composition, phagocytic targets greatly differ in physical attributes, such as size, shape, and rigidity, which are now recognized as important regulators of this process. Force exertion at the cell-target interface and cellular mechanical changes during phagocytosis are emerging as crucial factors underlying sensing of such target properties. With technological developments, mechanical aspects of phagocytosis are increasingly accessible experimentally, revealing remarkable organizational complexity of force exertion. An increasingly high-resolution picture is emerging of how target physical cues and cellular mechanical properties jointly govern important steps throughout phagocytic engulfment.

Alternative pathways control actomyosin contractility in epitheliomuscle cells during morphogenesis and body contraction

In adult Hydra, epitheliomuscle cells form the monolayered ecto- and endodermal epithelia. Their basal myonemes function as a longitudinal and circular muscle, respectively. Based on the observation that a Rho/Rock pathway, controlling the cell shape changes during detachment of Hydra buds, is not involved in body movement, at least two actomyosin compartments must exist in these cells: a basal one for body movement and a cortical one for cell shape changes. We therefore analyzed the regional and subcellular localization of the Ser19-phosphorylated myosin regulatory light chain (pMLC20). Along the body column, pMLC20 was detected strongly in the basal myonemes and weakly in the apical cell compartments of ectodermal epitheliomuscle cells. In cells of the bud base undergoing morphogenesis, pMLC20 was localized to intracellular stress fibers as well as to the apical and additionally to the lateral cortical compartment. Pharmacological inhibition revealed that pMLC20 is induced in these compartments by at least two independent pathways. In myonemes, MLC is phosphorylated mainly by myosin light chain kinase (MLCK). In contrast, the cortical apical and lateral MLC phosphorylation in constricting ectodermal cells of the bud base is stimulated via the Rho/ROCK pathway.

Long-Range and Directional Allostery of Actin Filaments Plays Important Roles in Various Cellular Activities

A wide variety of uniquely localized actin-binding proteins (ABPs) are involved in various cellular activities, such as cytokinesis, migration, adhesion, morphogenesis, and intracellular transport. In a micrometer-scale space such as the inside of cells, protein molecules diffuse throughout the cell interior within seconds. In this condition, how can ABPs selectively bind to particular actin filaments when there is an abundance of actin filaments in the cytoplasm? In recent years, several ABPs have been reported to induce cooperative conformational changes to actin filaments allowing structural changes to propagate along the filament cables uni- or bidirectionally, thereby regulating the subsequent binding of ABPs. Such propagation of ABP-induced cooperative conformational changes in actin filaments may be advantageous for the elaborate regulation of cellular activities driven by actin-based machineries in the intracellular space, which is dominated by diffusion. In this review, we focus on long-range allosteric regulation driven by cooperative conformational changes of actin filaments that are evoked by binding of ABPs, and discuss roles of allostery of actin filaments in narrow intracellular spaces.

A tensile ring drives tissue flows to shape the gastrulating amniote embryo

Tissue morphogenesis is driven by local cellular deformations that are powered by contractile actomyosin networks. How localized forces are transmitted across tissues to shape them at a mesoscopic scale is still unclear. Analyzing gastrulation in entire avian embryos, we show that it is driven by the graded contraction of a large-scale supracellular actomyosin ring at the margin between the embryonic and extraembryonic territories. The propagation of these forces is enabled by a fluid-like response of the epithelial embryonic disk, which depends on cell division. A simple model of fluid motion entrained by a tensile ring quantitatively captures the vortex-like "polonaise" movements that accompany the formation of the primitive streak. The geometry of the early embryo thus arises from the transmission of active forces generated along its boundary.

Dynamic assembly of a higher-order septin structure during appressorium morphogenesis by the rice blast fungus

The rice blast fungus Magnaporthe oryzae differentiates a specialized infection structure called an appressorium, which is used to break into plant cells by directed application of enormous turgor force. Appressorium-mediated plant infection requires timely assembly of a higher-order septin ring structure at the base of the appressorium, which is needed to spatially orchestrate appressorium repolarization. Here we use quantitative 4D widefield fluorescence imaging to gain new insight into the spatiotemporal dynamics of septin ring formation, and septin-mediated actin re-organization, during appressorium morphogenesis by M. oryzae. We anticipate that the new knowledge will provide a quantitative framework for dissecting the molecular mechanisms of higher-order septin ring assembly in this devastating plant pathogenic fungus.

Towards a structural understanding of the remodeling of the actin cytoskeleton

Actin filaments (F-actin) are a key component of eukaryotic cells. Whether serving as a scaffold for myosin or using their polymerization to push onto cellular components, their function is always related to force generation. To control and fine-tune force production, cells have a large array of actin-binding proteins (ABPs) dedicated to control every aspect of actin polymerization, filament localization, and their overall mechanical properties. Although great advances have been made in our biochemical understanding of the remodeling of the actin cytoskeleton, the structural basis of this process is still being deciphered. In this review, we summarize our current understanding of this process. We outline how ABPs control the nucleation and disassembly, and how these processes are affected by the nucleotide state of the filaments. In addition, we highlight recent advances in the understanding of actomyosin force generation, and describe recent advances brought forward by the developments of electron cryomicroscopy.

Dynamic polyhedral actomyosin lattices remodel micron-scale curved membranes during exocytosis in live mice

Actomyosin networks, the cell's major force production machineries, remodel cellular membranes during myriad dynamic processes^1,2 by assembling into various architectures with distinct force generation properties^3,4. While linear and branched actomyosin architectures are well characterized in cell-culture and cell-free systems³, it is not known how actin and myosin networks form and function to remodel membranes in complex three-dimensional mammalian tissues. Here, we use four-dimensional spinning-disc confocal microscopy with image deconvolution to acquire macromolecular-scale detail of dynamic actomyosin networks in exocrine glands of live mice. We address how actin and myosin organize around large membrane-bound secretory vesicles and generate the forces required to complete exocytosis^5-7. We find that actin and non-muscle myosin II (NMII) assemble into previously undescribed polyhedral-like lattices around the vesicle membrane. The NMII lattice comprises bipolar minifilaments^8-10 as well as non-canonical three-legged configurations. Using photobleaching and pharmacological perturbations in vivo, we show that actomyosin contractility and actin polymerization together push on the underlying vesicle membrane to overcome the energy barrier and complete exocytosis⁷. Our imaging approach thus unveils a force-generating actomyosin lattice that regulates secretion in the exocrine organs of live animals.

Cell Adhesion Structures in Epithelial Cells Are Formed in Dynamic and Cooperative Ways

There are many morphologically distinct membrane structures with different functions at the surface of epithelial cells. Among these, adherens junctions (AJ) and tight junctions (TJ) are responsible for the mechanical linkage of epithelial cells and epithelial barrier function, respectively. In the process of new cell-cell adhesion formation between two epithelial cells, such as after wounding, AJ form first and then TJ form on the apical side of AJ. This process is very complicated because AJ formation triggers drastic changes in the organization of actin cytoskeleton, the activity of Rho family of small GTPases, and the lipid composition of the plasma membrane, all of which are required for subsequent TJ formation. In this review, the authors focus on the relationship between AJ and TJ as a representative example of specialization of plasma membrane regions and introduce recent findings on how AJ formation promotes the subsequent formation of TJ.

Expansion and contraction of the umbrella cell apical junctional ring in response to bladder filling and voiding

The epithelial junctional complex, composed of tight junctions, adherens junctions, desmosomes, and an associated actomyosin cytoskeleton, forms the apical junctional ring (AJR), which must maintain its continuity in the face of external mechanical forces that accompany normal physiological functions. The AJR of umbrella cells, which line the luminal surface of the bladder, expands during bladder filling and contracts upon voiding; however, the mechanisms that drive these events are unknown. Using native umbrella cells as a model, we observed that the umbrella cell's AJR assumed a nonsarcomeric organization in which filamentous actin and ACTN4 formed unbroken continuous rings, while nonmuscle myosin II (NMMII) formed linear tracts along the actin ring. Expansion of the umbrella cell AJR required formin-dependent actin assembly, but was independent of NMMII ATPase function. AJR expansion also required membrane traffic, RAB13-dependent exocytosis, specifically, but not trafficking events regulated by RAB8A or RAB11A. In contrast, the voiding-induced contraction of the AJR depended on NMMII and actin dynamics, RHOA, and dynamin-dependent endocytosis. Taken together, our studies indicate that a mechanism by which the umbrella cells retain continuity during cyclical changes in volume is the expansion and contraction of their AJR, processes regulated by the actomyosin cytoskeleton and membrane trafficking events.

Regulation of the apical extension morphogenesis tunes the mechanosensory response of microvilliated neurons

Multiple types of microvilliated sensory cells exhibit an apical extension thought to be instrumental in the detection of sensory cues. The investigation of the mechanisms underlying morphogenesis of sensory apparatus is critical to understand the biology of sensation. Most of what we currently know comes from the study of the hair bundle of the inner ear sensory cells, but morphogenesis and function of other sensory microvilliated apical extensions remain poorly understood. We focused on spinal sensory neurons that contact the cerebrospinal fluid (CSF) through the projection of a microvilliated apical process in the central canal, referred to as cerebrospinal fluid-contacting neurons (CSF-cNs). CSF-cNs respond to pH and osmolarity changes as well as mechanical stimuli associated with changes of flow and tail bending. In vivo time-lapse imaging in zebrafish embryos revealed that CSF-cNs are atypical neurons that do not lose their apical attachment and form a ring of actin at the apical junctional complexes (AJCs) that they retain during differentiation. We show that the actin-based protrusions constituting the microvilliated apical extension arise and elongate from this ring of actin, and we identify candidate molecular factors underlying every step of CSF-cN morphogenesis. We demonstrate that Crumbs 1 (Crb1), Myosin 3b (Myo3b), and Espin orchestrate the morphogenesis of CSF-cN apical extension. Using calcium imaging in crb1 and espin mutants, we further show that the size of the apical extension modulates the amplitude of CSF-cN sensory response to bending of the spinal cord. Based on our results, we propose that the apical actin ring could be a common site of initiation of actin-based protrusions in microvilliated sensory cells. Furthermore, our work provides a set of actors underlying actin-based protrusion elongation shared by different sensory cell types and highlights the critical role of the apical extension shape in sensory detection.

A study of differentiating spinal sensory neurons in vivo uncovers critical factors required for the morphogenesis of sensory microvilli and reveals fine modulation of mechanosensory responses by microvillus length.

Src-transformed cells hijack mitosis to extrude from the epithelium

At the initial stage of carcinogenesis single mutated cells appear within an epithelium. Mammalian in vitro experiments show that potentially cancerous cells undergo live apical extrusion from normal monolayers. However, the mechanism underlying this process in vivo remains poorly understood. Mosaic expression of the oncogene vSrc in a simple epithelium of the early zebrafish embryo results in extrusion of transformed cells. Here we find that during extrusion components of the cytokinetic ring are recruited to adherens junctions of transformed cells, forming a misoriented pseudo-cytokinetic ring. As the ring constricts, it separates the basal from the apical part of the cell releasing both from the epithelium. This process requires cell cycle progression and occurs immediately after vSrc-transformed cell enters mitosis. To achieve extrusion, vSrc coordinates cell cycle progression, junctional integrity, cell survival and apicobasal polarity. Without vSrc, modulating these cellular processes reconstitutes vSrc-like extrusion, confirming their sufficiency for this process.

Potentially cancerous cells undergo live apical extrusion from normal monolayers and vSrc expression induces this in zebrafish epithelia. Here, the authors show that vSrc coordinates cytokinetic ring formation, cell cycle progression, junctional integrity, cell survival and apicobasal polarity to induce extrusion of transformed cells.

Active bundles of polar and bipolar filaments

Bundles of actin filaments and molecular motors of the myosin family are a common subcellular organizational motif. Typically, such bundles are under contractile stress resulting from interactions between the filaments and the motors. This holds in particular for contractile rings that appear in the late stages of cell division in animal cells and that cleave the mother into two daughter cells. It was recently shown that myosin organizes into regularly spaced clusters along rings in mammalian cells, whereas myosin clusters in fission yeast travel along the perimeter of actomyosin rings [Wollrab et al., Nat. Commun. 7, 11860 (2016)2041-172310.1038/ncomms11860]. A mechanism based on the association of the structurally polar actin filaments into bipolar structures was shown to provide a common explanation for both observations. Here, we analyze the dynamics of this mechanism in detail. We find a rich phase diagram depending on the actomyosin interaction strength and the stability of the bipolar structures. The system can notably organize into traveling waves. Furthermore, we identify the nature of the bifurcations connecting the various patterns as parameters are changed. Finally, we report experimental patterns observed in cytokinetic rings in fission yeast and link them to solutions of our dynamic equations. Our analysis highlights the possible role played by local polarity sorting of actin filaments for the dynamics and functionality of actomyosin networks.

Principles of Actomyosin Regulation In Vivo

The actomyosin cytoskeleton is responsible for most force-driven processes in cells and tissues. How it assembles into the necessary structures at the right time and place is an important question. Here, we focus on molecular mechanisms of actomyosin regulation recently elucidated in animal models, and highlight several common principles that emerge. The architecture of the actomyosin network - an important determinant of its function - results from actin polymerization, crosslinking and turnover, localized myosin activation, and contractility-driven self-organization. Spatiotemporal regulation is achieved by tissue-specific expression and subcellular localization of Rho GTPase regulators. Subcellular anchor points of actomyosin structures control the outcome of their contraction, and molecular feedback mechanisms dictate whether they are transient, cyclic, or persistent.

Into the breach: how cells cope with wounds

Repair of wounds to individual cells is crucial for organisms to survive daily physiological or environmental stresses, as well as pathogen assaults, which disrupt the plasma membrane. Sensing wounds, resealing membranes, closing wounds and remodelling plasma membrane/cortical cytoskeleton are four major steps that are essential to return cells to their pre-wounded states. This process relies on dynamic changes of the membrane/cytoskeleton that are indispensable for carrying out the repairs within tens of minutes. Studies from different cell wound repair models over the last two decades have revealed that the molecular mechanisms of single cell wound repair are very diverse and dependent on wound type, size, and/or species. Interestingly, different repair models have been shown to use similar proteins to achieve the same end result, albeit sometimes by distinctive mechanisms. Recent studies using cutting edge microscopy and molecular techniques are shedding new light on the molecular mechanisms during cellular wound repair. Here, we describe what is currently known about the mechanisms underlying this repair process. In addition, we discuss how the study of cellular wound repair—a powerful and inducible model—can contribute to our understanding of other fundamental biological processes such as cytokinesis, cell migration, cancer metastasis and human diseases.

The first line of defence: insights into mechanisms and relevance of phagocytosis in epithelial cells

Epithelial tissues cover most of the external and internal surfaces of the body and its organs. Inevitably, these tissues serve as first line of defence against inorganic, organic, and microbial intruders. Epithelial cells are the main cell type of these tissues. Besides their function as cellular barrier, there is growing evidence that epithelial cells are of particular relevance as initial sensors of danger and also as executers of adequate defence responses. These cells feature various essential functions to maintain tissue integrity in health and disease. In this review, we survey some of the different innate immune functions of epithelial cells in mucosal tissues being constantly exposed to a plethora of harmless contaminants but also of pathogens. We discuss how epithelial cells avoid inadequate immune responses in such conditions. In particular, we will focus on the diverse types and mechanisms of phagocytosis used by epithelial cells to not only maintain homeostasis but to also harness the host response against invading pathogens.

The actin cortex at a glance

Precisely controlled cell deformations are key to cell migration, division and tissue morphogenesis, and have been implicated in cell differentiation during development, as well as cancer progression. In animal cells, shape changes are primarily driven by the cellular cortex, a thin actomyosin network that lies directly underneath the plasma membrane. Myosin-generated forces create tension in the cortical network, and gradients in tension lead to cellular deformations. Recent studies have provided important insight into the molecular control of cortical tension by progressively unveiling cortex composition and organization. In this Cell Science at a Glance article and the accompanying poster, we review our current understanding of cortex composition and architecture. We then discuss how the microscopic properties of the cortex control cortical tension. While many open questions remain, it is now clear that cortical tension can be modulated through both cortex composition and organization, providing multiple levels of regulation for this key cellular property during cell and tissue morphogenesis.

Summary: A summary of the composition, architecture, mechanics and function of the cellular actin cortex, which determines the shape of animal cells, and, thus, provides the foundation for cell and tissue morphogenesis.

Theoretical Analysis of Stress Distribution and Cell Polarization Surrounding a Model Wound

A growing amount of experimental evidence shows that the local elastic field acting on cells governs their spatial organization and polarity in a tissue. Interestingly, experiments on wound healing reveal a universal formation of thick actomyosin bundles around the margins of epithelial gaps. Although the forces involved in this process have been measured, the mechanisms governing cellular alignment and contractile ring formation are still not fully understood. To theoretically investigate this process, we have carried out a self-consistent calculation of the elastic field that is actively generated around a circular gap in a contractile cell monolayer that is adhered to an elastic substrate, taking into account the responsiveness of actomyosin activity to the locally generated stress. We model actomyosin contractility by a radial distribution of point force dipoles that may alter in magnitude and orientation in response to the local elastic stress. In addition, the model takes into account the forces exerted by leader cells on the margins of the cell monolayer. Our model suggests that the presence of a hole in the center of a contractile cell monolayer creates a mechanical tendency for actomyosin forces to polarize tangentially around the hole margin. In addition, it predicts that this tendency optimizes with substrate rigidity, thickness, and strength of cell adhesion to the substrate. Our calculations support the view that the universal formation of a peripheral contractile ring is a consequence of actomyosin contractility in the bulk and its inherent responsiveness to the local stress.