The actin cortex at a glance

Finding MEMO-Emerging Evidence for MEMO1's Function in Development and Disease

Although conserved throughout animal kingdoms, the protein encoded by the gene Mediator of ERBB2 Driven Cell Motility 1 or MEMO1, has only recently come into focus. True to its namesake, MEMO1 first emerged from a proteomic screen of molecules bound to the ERBB2 receptor and was found to be necessary for efficient cell migration upon receptor activation. While initially placed within the context of breast cancer metastasis—a pathological state that has provided tremendous insight into MEMO1′s cellular roles—MEMO1′s function has since expanded to encompass additional cancer cell types, developmental processes during embryogenesis and homeostatic regulation of adult organ systems. Owing to MEMO1′s deep conservation, a variety of model organisms have been amenable to uncovering biological facets of this multipurpose protein; facets ranging from the cellular (e.g., receptor signaling, cytoskeletal regulation, redox flux) to the organismal (e.g., mineralization and mineral homeostasis, neuro/gliogenesis, vasculogenesis) level. Although these facets emerge at the intersection of numerous biological and human disease processes, how and if they are interconnected remains to be resolved. Here, we review our current understanding of this ‘enigmatic’ molecule, its role in development and disease and open questions emerging from these previous studies.

Effects of Netarsudil on Actin-Driven Cellular Functions in Normal and Glaucomatous Trabecular Meshwork Cells: A Live Imaging Study

The actin cytoskeleton of trabecular meshwork (TM) cells is a therapeutic target for lowering intraocular pressure (IOP) in glaucoma patients. Netarsudil (the active ingredient in Rhopressa^TM) is a Rho-associated protein kinase inhibitor that induces disassembly of actin stress fibers. Here, we used live cell imaging of SiR-actin-labeled normal (NTM) and glaucomatous TM (GTM) cells to investigate actin dynamics during actin-driven biological processes with and without netarsudil treatment. Actin stress fibers were thicker in GTM than NTM cells and took longer (>120 min) to disassemble following addition of 1 µM netarsudil. Actin-rich extracellular vesicles (EVs) were derived by two mechanisms: exocytosis of intracellular-derived vesicles, and cleavage of filopodial tips, which detached the filopodia from the substratum, allowing them to retract to the cell body. While some phagocytosis was noted in untreated TM cells, netarsudil potently stimulated phagocytic uptake of EVs. Netarsudil treatment induced lateral fusion of tunneling nanotubes (TNTs) that connected adjacent TM cells; TNTs are important for TM cellular communication. Together, our results suggest that netarsudil may clear outflow channels in TM tissue by inducing phagocytosis and/or by modulating TM communication via EVs and TNTs. These cellular functions likely work together to regulate IOP in normal and glaucomatous TM.

Differential Ser phosphorylation of vasodilator-stimulated phosphoprotein regulates colon tumor formation and growth

AIMS

Vasodilator-stimulated phosphoprotein (VASP) controls actin dynamics associated with the malignant phenotype of colorectal tumors. Oncogenic VASP function, in turn, is finely regulated by cyclic nucleotide-dependent phosphorylation of serine (Ser) residues 157 and 239, whose differential expression determines cell survival behavior in colon cancer. However, the role of differential VASP Ser phosphorylation in colorectal carcinogenesis remains unclear.

MAIN METHODS

Specific VASP phosphomutant constructs were employed to selectively silence Ser157 or Ser239 phosphorylation in human colon carcinoma cells. Cyclic nucleotide-dependent manipulation of VASP Ser phosphorylation was performed with 8-bromoadenosine 3',5'-cyclic adenosine monophosphate (8-Br-cAMP) or 8-chlorophenylthio 3',5'-cyclic guanosine monophosphate (8-CPT-cGMP). Tumorigenic and locomotory phenotypes were examined in vitro with clonogenic and wound healing assays, respectively. Finally, tumor formation and growth were investigated in vivo employing two distinct xenograft models of colorectal cancer.

KEY FINDINGS

Disruption of VASP Ser157 phosphorylation weakened the clonogenic and migratory abilities of human colon cancer cells, effects mimicked by 8-CPT-cGMP-dependent regulation of VASP Ser239. In contrast, inhibition of VASP Ser239 phosphorylation enhanced cell clonogenicity and migration and was phenocopied by 8-Br-cAMP-dependent regulation of VASP Ser157. Importantly, cancer cells bearing the phosphomutant construct targeting VASP Ser157 decreased, while those with the phosphomutation at Ser239 improved their abilities to establish productive tumor colonies and grow in the peritoneal cavity or subcutaneous tissues of nude mice.

SIGNIFICANCE

Together, present observations suggest differential VASP Ser phosphorylation is a relevant, targetable molecular event underlying tumor formation and progression in colon cancer.

Elasticity spectra as a tool to investigate actin cortex mechanics

Background

The mechanical properties of single living cells have proven to be a powerful marker of the cell physiological state. The use of nanoindentation-based single cell force spectroscopy provided a wealth of information on the elasticity of cells, which is still largely to be exploited. The simplest model to describe cell mechanics is to treat them as a homogeneous elastic material and describe it in terms of the Young’s modulus. Beside its simplicity, this approach proved to be extremely informative, allowing to assess the potential of this physical indicator towards high throughput phenotyping in diagnostic and prognostic applications.

Results

Here we propose an extension of this analysis to explicitly account for the properties of the actin cortex. We present a method, the Elasticity Spectra, to calculate the apparent stiffness of the cell as a function of the indentation depth and we suggest a simple phenomenological approach to measure the thickness and stiffness of the actin cortex, in addition to the standard Young’s modulus.

Conclusions

The Elasticity Spectra approach is tested and validated on a set of cells treated with cytoskeleton-affecting drugs, showing the potential to extend the current representation of cell mechanics, without introducing a detailed and complex description of the intracellular structure.

Going with the flow: insights from Caenorhabditis elegans zygote polarization

Cell polarity is the asymmetric distribution of cellular components along a defined axis. Polarity relies on complex signalling networks between conserved patterning proteins, including the PAR (partitioning defective) proteins, which become segregated in response to upstream symmetry breaking cues. Although the mechanisms that drive the asymmetric localization of these proteins are dependent upon cell type and context, in many cases the regulation of actomyosin cytoskeleton dynamics is central to the transport, recruitment and/or stabilization of these polarity effectors into defined subcellular domains. The transport or advection of PAR proteins by an actomyosin flow was first observed in the Caenorhabditis elegans zygote more than a decade ago. Since then a multifaceted approach, using molecular methods, high-throughput screens, and biophysical and computational models, has revealed further aspects of this flow and how polarity regulators respond to and modulate it. Here, we review recent findings on the interplay between actomyosin flow and the PAR patterning networks in the polarization of the C. elegans zygote. We also discuss how these discoveries and developed methods are shaping our understanding of other flow-dependent polarizing systems. This article is part of a discussion meeting issue 'Contemporary morphogenesis'.

Mechanics of the cell: Interaction mechanisms and mechanobiological models

The importance of cell mechanics has long been recognized for the cell development and function. Biomechanics plays an important role in cell metabolism, regulation of mechanotransduction pathways and also modulation of nuclear response. The mechanical properties of the cell are likely determined by, among many others, the cytoskeleton elasticity, membrane tension and cell-substrate adhesion. This coordinated but complex mechanical interplay is required however, for the cell to respond to and influence in a reciprocal manner the chemical and mechanical signals from the extracellular matrix (ECM). In an effort to better and more fully understand the cell mechanics, the role of nuclear mechanics has emerged as an important contributor to the overall cellular mechanics. It is not too difficult to appreciate the physical connection between the nucleus and the cytoskeleton network that may be connected to the ECM through the cell membrane. Transmission of forces from ECM through this connection is essential for a wide range of cellular behaviors and functions such as cytoskeletal reorganization, nuclear movement, cell migration and differentiation. Unlike the cellular mechanics that can be measured using a number of biophysical techniques that were developed in the past few decades, it still remains a daunting challenge to probe the nuclear mechanics directly. In this paper, we therefore aim to provide informative description of the cell membrane and cytoskeleton mechanics, followed by unique computational modeling efforts to elucidate the nucleus-cytoskeleton coupling. Advances in our knowledge of complete cellular biomechanics and mechanotransduction may lead to clinical relevance and applications in mechano-diseases such as atherosclerosis, stem cell-based therapies, and the development of tissue engineered products.

FoxM1 insufficiency hyperactivates Ect2-RhoA-mDia1 signaling to drive cancer

FoxM1 activates genes that regulate S-G2-M cell-cycle progression and, when overexpressed, is associated with poor clinical outcome in multiple cancers. Here we identify FoxM1 as a tumor suppressor in mice that, through its N-terminal domain, binds to and inhibits Ect2 to limit the activity of RhoA GTPase and its effector mDia1, a catalyst of cortical actin nucleation. FoxM1 insufficiency impedes centrosome movement through excessive cortical actin polymerization, thereby causing the formation of non-perpendicular mitotic spindles that missegregate chromosomes and drive tumorigenesis in mice. Importantly, low FOXM1 expression correlates with RhoA GTPase hyperactivity in multiple human cancer types, indicating that suppression of the newly discovered Ect2-RhoAmDia1 oncogenic axis by FoxM1 is clinically relevant. Furthermore, by dissecting the domain requirements through which FoxM1 inhibits Ect2 GEF activity, we provide mechanistic insight for the development of pharmacological approaches that target protumorigenic RhoA activity.

Viscoelasticity and Volume of Cortical Neurons under Glutamate Excitotoxicity and Osmotic Challenges

Neural activity depends on the maintenance of ionic and osmotic homeostasis. Under these conditions, the cell volume must be regulated to maintain optimal neural function. A disturbance in the neuronal volume regulation often occurs in pathological conditions such as glutamate excitotoxicity. The cell volume, mechanical properties, and actin cytoskeleton structure are tightly connected to achieve the cell homeostasis. Here, we studied the effects of glutamate-induced excitotoxicity, external osmotic pressure, and inhibition of actin polymerization on the viscoelastic properties and volume of neurons. Atomic force microscopy was used to map the viscoelastic properties of neurons in time-series experiments to observe the dynamical changes and a possible recovery. The data obtained on cultured rat cortical neurons were compared with the data obtained on rat fibroblasts. The neurons were found to be more responsive to the osmotic challenges but less sensitive to the inhibition of actin polymerization than fibroblasts. The alterations of the viscoelastic properties caused by glutamate excitotoxicity were similar to those induced by the hypoosmotic stress, but, in contrast to the latter, they did not recover after the glutamate removal. These data were consistent with the dynamic volume changes estimated using ratiometric fluorescent dyes. The recovery after the glutamate-induced excitotoxicity was slow or absent because of a steady increase in intracellular calcium and sodium concentrations. The viscoelastic parameters and their changes were related to such parameters as the actin cortex stiffness, tension, and cytoplasmic viscosity.

PIG-1 MELK-dependent phosphorylation of nonmuscle myosin II promotes apoptosis through CES-1 Snail partitioning

The mechanism(s) through which mammalian kinase MELK promotes tumorigenesis is not understood. We find that the C. elegans orthologue of MELK, PIG-1, promotes apoptosis by partitioning an anti-apoptotic factor. The C. elegans NSM neuroblast divides to produce a larger cell that differentiates into a neuron and a smaller cell that dies. We find that in this context, PIG-1 MELK is required for partitioning of CES-1 Snail, a transcriptional repressor of the pro-apoptotic gene egl-1 BH3-only. pig-1 MELK is controlled by both a ces-1 Snail- and par-4 LKB1-dependent pathway, and may act through phosphorylation and cortical enrichment of nonmuscle myosin II prior to neuroblast division. We propose that pig-1 MELK-induced local contractility of the actomyosin network plays a conserved role in the acquisition of the apoptotic fate. Our work also uncovers an auto-regulatory loop through which ces-1 Snail controls its own activity through the formation of a gradient of CES-1 Snail protein.

Apoptosis is critical for the elimination of ‘unwanted’ cells. What distinguishes wanted from unwanted cells in developing animals is poorly understood. We report that in the C. elegans NSM neuroblast lineage, the level of CES-1, a Snail-family member and transcriptional repressor of the pro-apoptotic gene egl-1, contributes to this process. In addition, we demonstrate that C. elegans PIG-1, the orthologue of mammalian proto-oncoprotein MELK, plays a critical role in controlling CES-1^Snail levels. Specifically, during NSM neuroblast division, PIG-1^MELK controls partitioning of CES-1^Snail into one but not the other daughter cell thereby promoting the making of one wanted and one unwanted cell. Furthermore, we present evidence that PIG-1^MELK acts prior to NSM neuroblast division by locally activating the actomyosin network.

Heterogeneity and Actin Cytoskeleton in Osteoclast and Macrophage Multinucleation

Osteoclast signatures are determined by two transcriptional programs, the lineage-determining transcription pathway and the receptor activator of nuclear factor kappa-B ligand (RANKL)-dependent differentiation pathways. During differentiation, mononuclear precursors become multinucleated by cell fusion. Recently, live-cell imaging has revealed a high level of heterogeneity in osteoclast multinucleation. This heterogeneity includes the difference in the differentiation states and the mobility of the fusion precursors, as well as the mode of fusion among the fusion precursors with different numbers of nuclei. In particular, fusion partners often form morphologically distinct actin-based linkages that allow two cells to exchange lipids and proteins before membrane fusion. However, the origin of this heterogeneity remains elusive. On the other hand, osteoclast multinucleation is sensitive to the environmental cues. Such cues promote the reorganization of the actin cytoskeleton, especially the formation and transformation of the podosome, an actin-rich punctate adhesion. This review covers the heterogeneity of osteoclast multinucleation at the pre-fusion stage with reference to the environment-dependent signaling pathway responsible for reorganizing the actin cytoskeleton. Furthermore, we compare osteoclast multinucleation with macrophage fusion, which results in multinucleated giant macrophages.

Viscoelasticity of Native and Artificial Actin Cortices Assessed by Nanoindentation Experiments

Cell cortices are responsible for the resilience and morphological dynamics of cells. Measuring their mechanical properties is impeded by contributions from other filament types, organelles, and the crowded cytoplasm. We established a versatile concept for the precise assessment of cortical viscoelasticity based on force cycle experiments paired with continuum mechanics. Apical cell membranes of confluent MDCK II cells were deposited on porous substrates and locally deformed. Force cycles could be described with a time-dependent area compressibility modulus obeying the same power law as employed for whole cells. The reduced fluidity of apical cell membranes compared to living cells could partially be restored by reactivating myosin motors. A comparison with artificial minimal actin cortices (MACs) reveals lower stiffness and higher fluidity attributed to missing cross-links in MACs.

Cellular Membranes, a Versatile Adaptive Composite Material

Cellular membranes belong to the most vital yet least understood biomaterials of live matter. For instance, its biomechanical requirements substantially vary across species and subcellular sites, raising the question how membranes manage to adjust to such dramatic changes. Central to its adaptability at the cell surface is the interplay between the plasma membrane and the adjacent cell cortex, forming an adaptive composite material that dynamically adjusts its mechanical properties. Using a hypothetical composite material, we identify core challenges, and discuss how cellular membranes solved these tasks. We further muse how pathological changes in material properties affect membrane mechanics and cell function, before closing with open questions and future challenges arising when studying cellular membranes.

Interplay between actomyosin and E-cadherin dynamics regulates cell shape in the Drosophila embryonic epidermis

Precise regulation of cell shape is vital for building functional tissues. Here, we study the mechanisms that lead to the formation of highly elongated anisotropic epithelial cells in the Drosophila epidermis. We demonstrate that this cell shape is the result of two counteracting mechanisms at the cell surface that regulate the degree of elongation: actomyosin, which inhibits cell elongation downstream of RhoA (Rho1 in Drosophila) and intercellular adhesion, modulated via clathrin-mediated endocytosis of E-cadherin (encoded by shotgun in flies), which promotes cell elongation downstream of the GTPase Arf1 (Arf79F in Drosophila). We show that these two mechanisms do not act independently but are interconnected, with RhoA signalling reducing Arf1 recruitment to the plasma membrane. Additionally, cell adhesion itself regulates both mechanisms - p120-catenin, a regulator of intercellular adhesion, promotes the activity of both Arf1 and RhoA. Altogether, we uncover a complex network of interactions between cell-cell adhesion, the endocytic machinery and the actomyosin cortex, and demonstrate how this network regulates cell shape in an epithelial tissue in vivo.

Flow-accelerated platelet biogenesis is due to an elasto-hydrodynamic instability

Blood platelets are formed by fragmentation of long membrane extensions from bone marrow megakaryocytes in the blood flow. Using lattice-Boltzmann/immersed boundary simulations we propose a biological Rayleigh-Plateau instability as the biophysical mechanism behind this fragmentation process. This instability is akin to the surface tension-induced breakup of a liquid jet but is driven by active cortical processes including actomyosin contractility and microtubule sliding. Our fully three-dimensional simulations highlight the crucial role of actomyosin contractility, which is required to trigger the instability, and illustrate how the wavelength of the instability determines the size of the final platelets. The elasto-hydrodynamic origin of the fragmentation explains the strong acceleration of platelet biogenesis in the presence of an external flow, which we observe in agreement with experiments. Our simulations then allow us to disentangle the influence of specific flow conditions: While a homogeneous flow with uniform velocity leads to the strongest acceleration, a shear flow with a linear velocity gradient can cause fusion events of two developing platelet-sized swellings during fragmentation. A fusion event may lead to the release of larger structures which are observable as preplatelets in experiments. Together, our findings strongly indicate a mainly physical origin of fragmentation and regulation of platelet size in flow-accelerated platelet biogenesis.

Prestress and Area Compressibility of Actin Cortices Determine the Viscoelastic Response of Living Cells

Shape, dynamics, and viscoelastic properties of eukaryotic cells are primarily governed by a thin, reversibly cross-linked actomyosin cortex located directly beneath the plasma membrane. We obtain time-dependent rheological responses of fibroblasts and MDCK II cells from deformation-relaxation curves using an atomic force microscope to access the dependence of cortex fluidity on prestress. We introduce a viscoelastic model that treats the cell as a composite shell and assumes that relaxation of the cortex follows a power law giving access to cortical prestress, area-compressibility modulus, and the power law exponent (fluidity). Cortex fluidity is modulated by interfering with myosin activity. We find that the power law exponent of the cell cortex decreases with increasing intrinsic prestress and area-compressibility modulus, in accordance with previous finding for isolated actin networks subject to external stress. Extrapolation to zero tension returns the theoretically predicted power law exponent for transiently cross-linked polymer networks. In contrast to the widely used Hertzian mechanics, our model provides viscoelastic parameters independent of indenter geometry and compression velocity.

The Mechanics of Mitotic Cell Rounding

When animal cells enter mitosis, they round up to become spherical. This shape change is accompanied by changes in mechanical properties. Multiple studies using different measurement methods have revealed that cell surface tension, intracellular pressure and cortical stiffness increase upon entry into mitosis. These cell-scale, biophysical changes are driven by alterations in the composition and architecture of the contractile acto-myosin cortex together with osmotic swelling and enable a mitotic cell to exert force against the environment. When the ability of cells to round is limited, for example by physical confinement, cells suffer severe defects in spindle assembly and cell division. The requirement to push against the environment to create space for spindle formation is especially important for cells dividing in tissues. Here we summarize the evidence and the tools used to show that cells exert rounding forces in mitosis in vitro and in vivo, review the molecular basis for this force generation and discuss its function for ensuring successful cell division in single cells and for cells dividing in normal or diseased tissues.

Grp94 Regulates the Recruitment of Aneural AChR Clusters for the Assembly of Postsynaptic Specializations by Modulating ADF/Cofilin Activity and Turnover

Temperature is a physiological factor that affects neuronal growth and synaptic homeostasis at the invertebrate neuromuscular junctions (NMJs); however, whether temperature stress could also regulate the structure and function of the vertebrate NMJs remains unclear. In this study, we use Xenopus laevis primary cultures as a vertebrate model system for investigating the involvement of heat shock protein 90 (HSP90) family of stress proteins in NMJ development. First, cold temperature treatment or HSP90 inhibition attenuates the formation of aneural acetylcholine receptor (AChR) clusters, but increases their stability after they are formed, in cultured muscles. HSP90 inhibition specifically affects the stability of aneural AChR clusters and their associated intracellular scaffolding protein rapsyn, instead of causing a global change in cell metabolism and protein expression in Xenopus muscle cultures. Upon synaptogenic stimulation, a specific HSP90 family member, glucose-regulated protein 94 (Grp94), modulates the phosphorylation and dynamic turnover of actin depolymerizing factor (ADF)/cofilin at aneural AChR clusters, leading to the recruitment of AChR molecules from aneural clusters to the assembly of agrin-induced postsynaptic specializations. Finally, postsynaptic Grp94 knock-down significantly inhibits nerve-induced AChR clustering and postsynaptic activity in nerve-muscle co-cultures as demonstrated by live-cell imaging and electrophysiological recording, respectively. Collectively, this study suggests that temperature-dependent alteration in Grp94 expression and activity inhibits the assembly of postsynaptic specializations through modulating ADF/cofilin phosphorylation and activity at aneural AChR clusters, which prevents AChR molecules from being recruited to the postsynaptic sites via actin-dependent vesicular trafficking, at developing vertebrate NMJs.

PSGL-1 inhibits HIV-1 infection by restricting actin dynamics and sequestering HIV envelope proteins

PSGL-1 has recently been identified as an HIV restriction factor that inhibits HIV DNA synthesis and more potently, virion infectivity. But the underlying mechanisms of these inhibitions are unknown. Here we show that PSGL-1 directly binds to cellular actin filaments (F-actin) to restrict actin dynamics, which leads to inhibition of HIV DNA synthesis. PSGL-1 is incorporated into nascent virions and restricts actin dynamics in the virions, which partially accounts for the inhibition of virion infectivity. More potently, PSGL-1 inhibits incorporation of Env proteins into nascent virions, causing a loss of envelope spikes on the virions as shown by Cryo-electron microscopy and super-resolution imaging. This loss is associated with a profound defect in viral entry. Mechanistically, PSGL-1 binds gp41 and sequesters gp41 at the plasma membrane, explaining the inhibition of Env incorporation in nascent virions. PSGL-1’s dual anti-HIV mechanisms represent novel strategies of human cells to defend against HIV infection.

Symmetry Breaking during Cell Movement in the Context of Excitability, Kinetic Fine-Tuning and Memory of Pseudopod Formation

The path of moving eukaryotic cells depends on the kinetics and direction of extending pseudopods. Amoeboid cells constantly change their shape with pseudopods extending in different directions. Detailed analysis has revealed that time, place and direction of pseudopod extension are not random, but highly ordered with strong prevalence for only one extending pseudopod, with defined life-times, and with reoccurring events in time and space indicative of memory. Important components are Ras activation and the formation of branched F-actin in the extending pseudopod and inhibition of pseudopod formation in the contractile cortex of parallel F-actin/myosin. In biology, order very often comes with symmetry. In this essay, I discuss cell movement and the dynamics of pseudopod extension from the perspective of symmetry and symmetry changes of Ras activation and the formation of branched F-actin in the extending pseudopod. Combining symmetry of Ras activation with kinetics and memory of pseudopod extension results in a refined model of amoeboid movement that appears to be largely conserved in the fast moving Dictyostelium and neutrophils, the slow moving mesenchymal stem cells and the fungus B.d. chytrid.

The Janus Role of Adhesion in Chondrogenesis

Tackling the first stages of the chondrogenic commitment is essential to drive chondrogenic differentiation to healthy hyaline cartilage and minimize hypertrophy. During chondrogenesis, the extracellular matrix continuously evolves, adapting to the tissue adhesive requirements at each stage. Here, we take advantage of previously developed nanopatterns, in which local surface adhesiveness can be precisely tuned, to investigate its effects on prechondrogenic condensation. Fluorescence live cell imaging, immunostaining, confocal microscopy and PCR analysis are used to follow the condensation process on the nanopatterns. Cell tracking parameters, condensate morphology, cell-cell interactions, mechanotransduction and chondrogenic commitment are evaluated in response to local surface adhesiveness. Results show that only condensates on the nanopatterns of high local surface adhesiveness are stable in culture and able to enter the chondrogenic pathway, thus highlighting the importance of controlling cell-substrate adhesion in the tissue engineering strategies for cartilage repair.

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.

Actin cytoskeleton in mesenchymal-to-amoeboid transition of cancer cells

During development of metastasis, tumor cells migrate through different tissues and encounter different extracellular matrices. An ability of cells to adapt mechanisms of their migration to these diverse environmental conditions, called migration plasticity, gives tumor cells an advantage over normal cells for long distant dissemination. Different modes of individual cell motility-mesenchymal and amoeboid-are driven by different molecular mechanisms, which largely depend on functions of the actin cytoskeleton that can be modulated in a wide range by cellular signaling mechanisms in response to environmental conditions. Various triggers can switch one motility mode to another, but regulations of these transitions are incompletely understood. However, understanding of the mechanisms driving migration plasticity is instrumental for finding anti-cancer treatment capable to stop cancer metastasis. In this review, we discuss cytoskeletal features, which allow the individually migrating cells to switch between mesenchymal and amoeboid migrating modes, called mesenchymal-to-amoeboid transition (MAT). We briefly describe main characteristics of different cell migration modes, and then discuss the triggering factors that initiate MAT with special attention to cytoskeletal features essential for migration plasticity.

Fast and synchronized fluctuations of cortical actin negatively correlate with nucleoli liquid-liquid phase separation in T cells

Liquid-liquid phase separation is an important mechanism by which eukaryotic cells functionally organize their intracellular content and has been related to cell malignancy and neurodegenerative diseases. These cells also undergo ATP-driven mechanical fluctuations, yet the effect of these fluctuations on the liquid-liquid phase separation remains poorly understood. Here, we employ high-resolution microscopy and atomic force microscopy of live Jurkat T cells to characterize the spectrum of their mechanical fluctuations, and to relate these fluctuations to the extent of nucleoli liquid-liquid phase separation (LLPS). We find distinct fluctuation of the cytoskeleton and of the cell diameter around 110 Hz, which depend on ATP and on myosin activity. Importantly, these fluctuations negatively correlate to nucleoli LLPS. According to a model of cell viscoelasticity, we propose that these fluctuations generate mechanical work that increases intracellular homogeneity by inhibiting LLPS. Thus, active mechanical fluctuations serve as an intracellular regulatory mechanism that could affect multiple pathophysiological conditions.

Exploring the interdependence between self-organization and functional morphology in cellular systems

All living matter is subject to continuous adaptation and functional optimization via natural selection. Consequentially, structures with close morphological resemblance repeatedly appear across the phylogenetic tree. How these designs emerge at the cellular level is not fully understood. Here, we explore core concepts of functional morphology and discuss its cause and consequences, with a specific focus on emerging properties of self-organizing systems as the potential driving force. We conclude with open questions and limitations that are present when studying shape-function interdependence in single cells and cellular ensembles.

Actin Cell Cortex: Structure and Molecular Organization

The actin cytoskeleton consists of structurally and biochemically different actin filament arrays. Among them, the actin cortex is thought to have key roles in cell mechanics, but remains a poorly characterized part of the actin cytoskeleton. The cell cortex is typically defined as a thin layer of actin meshwork that uniformly underlies the plasma membrane of the entire cell. However, this definition applies only to specific cases. In general, the cortex structure and subcellular distribution vary significantly across cell types and physiological states of the cell. In this review, I focus on our current knowledge of the structure and molecular composition of the cell cortex.

Listeria monocytogenes Interferes with Host Cell Mitosis through Its Virulence Factors InlC and ActA

Listeria monocytogenes is among the best-characterized intracellular pathogens. Its virulence factors, and the way they interfere with host cells to hijack host functions and promote the establishment and dissemination of the infection, have been the focus of multiple studies over the last 30 years. During cellular infection, L. monocytogenes was shown to induce host DNA damage and delay the host cell cycle to its own benefit. However, whether the cell cycle stage would interfere with the capacity of Listeria to infect human cultured cell lines was never assessed. We found here that L. monocytogenes preferentially infects cultured cells in G2/M phases. Inside G2/M cells, the bacteria lead to an increase in the overall mitosis duration by delaying the mitotic exit. We showed that L. monocytogenes infection causes a sustained activation of the spindle assembly checkpoint, which we correlated with the increase in the percentage of misaligned chromosomes detected in infected cells. Moreover, we demonstrated that chromosome misalignment in Listeria-infected cells required the function of two Listeria virulence factors, ActA and InlC. Our findings show the pleiotropic role of Listeria virulence factors and their cooperative action in successfully establishing the cellular infection.

Coupling of melanocyte signaling and mechanics by caveolae is required for human skin pigmentation

Tissue homeostasis requires regulation of cell-cell communication, which relies on signaling molecules and cell contacts. In skin epidermis, keratinocytes secrete factors transduced by melanocytes into signaling cues promoting their pigmentation and dendrite outgrowth, while melanocytes transfer melanin pigments to keratinocytes to convey skin photoprotection. How epidermal cells integrate these functions remains poorly characterized. Here, we show that caveolae are asymmetrically distributed in melanocytes and particularly abundant at the melanocyte-keratinocyte interface in epidermis. Caveolae in melanocytes are modulated by ultraviolet radiations and keratinocytes-released factors, like miRNAs. Preventing caveolae formation in melanocytes increases melanin pigment synthesis through upregulation of cAMP signaling and decreases cell protrusions, cell-cell contacts, pigment transfer and epidermis pigmentation. Altogether, we identify that caveolae serve as molecular hubs that couple signaling outputs from keratinocytes to mechanical plasticity of pigment cells. The coordination of intercellular communication and contacts by caveolae is thus crucial to skin pigmentation and tissue homeostasis.

Actin and Myosin in Non-Neuronal Exocytosis

Cellular secretion depends on exocytosis of secretory vesicles and discharge of vesicle contents. Actin and myosin are essential for pre-fusion and post-fusion stages of exocytosis. Secretory vesicles depend on actin for transport to and attachment at the cell cortex during the pre-fusion phase. Actin coats on fused vesicles contribute to stabilization of large vesicles, active vesicle contraction and/or retrieval of excess membrane during the post-fusion phase. Myosin molecular motors complement the role of actin. Myosin V is required for vesicle trafficking and attachment to cortical actin. Myosin I and II members engage in local remodeling of cortical actin to allow vesicles to get access to the plasma membrane for membrane fusion. Myosins stabilize open fusion pores and contribute to anchoring and contraction of actin coats to facilitate vesicle content release. Actin and myosin function in secretion is regulated by a plethora of interacting regulatory lipids and proteins. Some of these processes have been first described in non-neuronal cells and reflect adaptations to exocytosis of large secretory vesicles and/or secretion of bulky vesicle cargoes. Here we collate the current knowledge and highlight the role of actomyosin during distinct phases of exocytosis in an attempt to identify unifying molecular mechanisms in non-neuronal secretory cells.

A biophysical perspective on receptor-mediated virus entry with a focus on HIV

As part of their entry and infection strategy, viruses interact with specific receptor molecules expressed on the surface of target cells. The efficiency and kinetics of the virus-receptor interactions required for a virus to productively infect a cell is determined by the biophysical properties of the receptors, which are in turn influenced by the receptors' plasma membrane (PM) environments. Currently, little is known about the biophysical properties of these receptor molecules or their engagement during virus binding and entry. Here we review virus-receptor interactions focusing on the human immunodeficiency virus type 1 (HIV), the etiological agent of acquired immunodeficiency syndrome (AIDS), as a model system. HIV is one of the best characterised enveloped viruses, with the identity, roles and structure of the key molecules required for infection well established. We review current knowledge of receptor-mediated HIV entry, addressing the properties of the HIV cell-surface receptors, the techniques used to measure these properties, and the macromolecular interactions and events required for virus entry. We discuss some of the key biophysical principles underlying receptor-mediated virus entry and attempt to interpret the available data in the context of biophysical mechanisms. We also highlight crucial outstanding questions and consider how new tools might be applied to advance understanding of the biophysical properties of viral receptors and the dynamic events leading to virus entry.

Sterol biosensor reveals LAM-family Ltc1-dependent sterol flow to endosomes upon Arp2/3 inhibition

Sterols are crucial components of biological membranes, which are synthetized in the ER and accumulate in the plasma membrane (PM). Here, by applying a genetically encoded sterol biosensor (D4H), we visualize a sterol flow between PM and endosomes in the fission yeast Schizosaccharomyces pombe. Using time-lapse and correlative light-electron microscopy, we found that inhibition of Arp2/3-dependent F-actin assembly promotes the reversible relocalization of D4H from the PM to internal sterol-rich compartments (STRIC) labeled by synaptobrevin Syb1. Retrograde sterol internalization to STRIC is independent of endocytosis or an intact Golgi, but depends on Ltc1, a LAM/StARkin-family protein localized to ER-PM contact sites. The PM in ltc1Δ cells over-accumulates sterols and upon Arp2/3 inhibition forms extended ER-interacting invaginations, indicating that sterol transfer contributes to PM size homeostasis. Anterograde sterol movement from STRIC is independent of canonical vesicular trafficking but requires Arp2/3, suggesting a novel role for this complex. Thus, transfer routes orthogonal to vesicular trafficking govern the flow of sterols in the cell.

Membrane Elastic Properties During Neural Precursor Cell Differentiation

Neural precursor cells differentiate into several cell types that display distinct functions. However, little is known about how cell surface mechanics vary during the differentiation process. Here, by precisely measuring membrane tension and bending modulus, we map their variations and correlate them with changes in neural precursor cell morphology along their distinct differentiation fates. Both cells maintained in culture as neural precursors as well as those plated in neurobasal medium reveal a decrease in membrane tension over the first hours of culture followed by stabilization, with no change in bending modulus. During astrocyte differentiation, membrane tension initially decreases and then increases after 72 h, accompanied by consolidation of glial fibrillary acidic protein expression and striking actin reorganization, while bending modulus increases following observed alterations. For oligodendrocytes, the changes in membrane tension are less abrupt over the first hours, but their values subsequently decrease, correlating with a shift from oligodendrocyte marker O4 to myelin basic protein expressions and a remarkable actin reorganization, while bending modulus remains constant. Oligodendrocytes at later differentiation stages show membrane vesicles with similar membrane tension but higher bending modulus as compared to the cell surface. Altogether, our results display an entire spectrum of how membrane elastic properties are varying, thus contributing to a better understanding of neural differentiation from a mechanobiological perspective.

Homodimerization of coronin A through the C-terminal coiled-coil domain is essential for multicellular differentiation of Dictyostelium discoideum

Coronin proteins are widely expressed among eukaryotic organisms. Most coronins consist of a WD-repeat domain followed by a C-terminal coiled coil. Dictyostelium discoideum expresses a single short coronin coronin A, which has been implicated in both actin modulation and multicellular differentiation. Whether coronin A's coiled coil is important for functionality, as well as the oligomeric state of coronin A is not known. Here, we show that the coiled-coil domain in Dictyostelium coronin A functions in homodimerization, is dispensable for coronin A stability and localization but essential for multicellular differentiation. These results allow a better understanding of the role for the coiled-coil domain of coronin A in oligomerization and demonstrate that its presence is essential for multicellular differentiation.

The Lateral Organization and Mobility of Plasma Membrane Components

Over the last several decades, an impressive array of advanced microscopic and analytical tools, such as single-particle tracking and nanoscopic fluorescence correlation spectroscopy, has been applied to characterize the lateral organization and mobility of components in the plasma membrane. Such analysis can tell researchers about the local dynamic composition and structure of membranes and is important for predicting the outcome of membrane-based reactions. However, owing to the unresolved complexity of the membrane and the structures peripheral to it, identification of the detailed molecular origin of the interactions that regulate the organization and mobility of the membrane has not proceeded quickly. This Perspective presents an overview of how cell-surface structure may give rise to the types of lateral mobility that are observed and some potentially fruitful future directions to elucidate the architecture of these structures in more molecular detail.

The crosstalk between microtubules, actin and membranes shapes cell division

Mitotic progression is orchestrated by morphological and mechanical changes promoted by the coordinated activities of the microtubule (MT) cytoskeleton, the actin cytoskeleton and the plasma membrane (PM). MTs assemble the mitotic spindle, which assists sister chromatid separation, and contact the rigid and tensile actomyosin cortex rounded-up underneath the PM. Here, we highlight the dynamic crosstalk between MTs, actin and cell membranes during mitosis, and discuss the molecular connections between them. We also summarize recent views on how MT traction forces, the actomyosin cortex and membrane trafficking contribute to spindle positioning in isolated cells in culture and in epithelial sheets. Finally, we describe the emerging role of membrane trafficking in synchronizing actomyosin tension and cell shape changes with cell-substrate adhesion, cell-cell contacts and extracellular signalling events regulating proliferation.

Cytoskeleton Force Exertion in Bulk Cytoplasm

The microtubule and actin cytoskeletons generate forces essential to position centrosomes, nuclei, and spindles for division plane specification. While the largest body of work has documented force exertion at, or close to the cell surface, mounting evidence suggests that cytoskeletal polymers can also produce significant forces directly from within the cytoplasm. Molecular motors such as kinesin or dynein may for instance displace cargos and endomembranes in the viscous cytoplasm yielding friction forces that pull or push microtubules. Similarly, the dynamics of bulk actin assembly/disassembly or myosin-dependent contractions produce cytoplasmic forces which influence the spatial organization of cells in a variety of processes. We here review the molecular and physical mechanisms supporting bulk cytoplasmic force generation by the cytoskeleton, their limits and relevance to organelle positioning, with a particular focus on cell division.

Phenotypic and Functional Alterations in Tunneling Nanotubes Formed by Glaucomatous Trabecular Meshwork Cells

Purpose

Trabecular meshwork (TM) cells detect and coordinate responses to intraocular pressure (IOP) in the eye. TM cells become dysfunctional in glaucoma where IOP is often elevated. Recently, we showed that normal TM (NTM) cells communicate by forming tubular connections called tunneling nanotubes (TNTs). Here, we investigated TNTs in glaucomatous TM (GTM) cells.

Methods

Primary GTM and NTM cells were established from cadaver eyes. Transfer of Vybrant DiO and DiD-labeled vesicles via TNT connections was measured. Imaris software measured the number and length of cell protrusions from immunofluorescent confocal images. Live-cell imaging of the actin cytoskeleton was performed. The distribution of myosin-X, a regulator of TNTs/filopodia, was investigated in TM cells and tissue.

Results

GTM cells contained significantly more transferred fluorescent vesicles than NTM cells (49.6% vs. 35%). Although NTM cells had more protrusions at the cell surface than GTM cells (7.61 vs. 4.65 protrusions/cell), GTM protrusions were significantly longer (12.1 μm vs. 9.76 μm). Live-cell imaging demonstrated that the GTM actin cytoskeleton was less dynamic, and vesicle transfer between cells was significantly slower than NTM cells. Furthermore, rearrangement of the actin cortex adjacent to the TNT may influence TNT formation. Myosin-X immunostaining was punctate and disorganized in GTM cells and tissue compared to age-matched NTM controls.

Conclusions

Together, our data demonstrate that GTM cells have phenotypic and functional differences in their TNTs. Significantly slower vesicle transfer via TNTs in GTM cells may delay the timely propagation of cellular signals when pressures become elevated in glaucoma.

Dynamic motions of molecular motors in the actin cytoskeleton

During intracellular transport, cellular cargos, such as organelles, vesicles, and proteins, are transported within cells. Intracellular transport plays an important role in diverse cellular functions. Molecular motors walking on the cytoskeleton facilitate active intracellular transport, which is more efficient than diffusion-based passive transport. Active transport driven by kinesin and dynein walking on microtubules has been studied well during recent decades. However, mechanisms of active transport occurring in disorganized actin networks via myosin motors remain elusive. To provide physiologically relevant insights, we probed motions of myosin motors in actin networks under various conditions using our well-established computational model that rigorously accounts for the mechanical and dynamical behaviors of the actin cytoskeleton. We demonstrated that myosin motions can be confined due to three different reasons in the absence of F-actin turnover. We verified mechanisms of motor stalling using in vitro reconstituted actomyosin networks. We also found that with F-actin turnover, motors consistently move for a long time without significant confinement. Our study sheds light on the importance of F-actin turnover for effective active transport in the actin cytoskeleton.

Actin shells control buckling and wrinkling of biomembranes

Global changes of cell shape under mechanical or osmotic external stresses are mostly controlled by the mechanics of the cortical actin cytoskeleton underlying the cell membrane. Some aspects of this process can be recapitulated in vitro on reconstituted actin-and-membrane systems. In this paper, we investigate how the mechanical properties of a branched actin network shell, polymerized at the surface of a liposome, control membrane shape when the volume is reduced. We observe a variety of membrane shapes depending on the actin thickness. Thin shells undergo buckling, characterized by a cup-shape deformation of the membrane that coincides with the one of the actin network. Thick shells produce membrane wrinkles, but do not deform their outer layer. For intermediate micrometer-thick shells, wrinkling of the membrane is observed, and the actin layer is slightly deformed. Confronting our experimental results with a theoretical description, we determine the transition between buckling and wrinkling, which depends on the thickness of the actin shell and the size of the liposome. We thus unveil the generic mechanism by which biomembranes are able to accommodate their shape against mechanical compression, through thickness adaptation of their cortical cytoskeleton.

Spatial EGFR Dynamics and Metastatic Phenotypes Modulated by Upregulated EphB2 and Src Pathways in Advanced Prostate Cancer

Advanced prostate cancer is a very heterogeneous disease reflecting in diverse regulations of oncogenic signaling pathways. Aberrant spatial dynamics of epidermal growth factor receptor (EGFR) promote their dimerization and clustering, leading to constitutive activation in oncogenesis. The EphB2 and Src signaling pathways are associated with the reorganization of the cytoskeleton leading to malignancy, but their roles in regulating EGFR dynamics and activation are scarcely reported. Using single-particle tracking techniques, we found that highly phosphorylated EGFR in the advanced prostate cancer cell line, PC3, was associated with higher EGFR diffusivity, as compared with LNCaP and less aggressive DU145. The increased EGFR activation and biophysical dynamics were consistent with high proliferation, migration, and invasion. After performing single-cell RNA-seq on prostate cancer cell lines and circulating tumor cells from patients, we identified that upregulated gene expression in the EphB2 and Src pathways are associated with advanced malignancy. After dasatinib treatment or siRNA knockdowns of EphB2 or Src, the PC3 cells exhibited significantly lower EGFR dynamics, cell motility, and invasion. Partial inhibitory effects were also found in DU145 cells. The upregulation of parts of the EphB2 and Src pathways also predicts poor prognosis in the prostate cancer patient cohort of The Cancer Genome Atlas. Our results provide evidence that overexpression of the EphB2 and Src signaling pathways regulate EGFR dynamics and cellular aggressiveness in some advanced prostate cancer cells.

Cardiomyocyte mechanodynamics under conditions of actin remodelling

The mechanical performance of cardiomyocytes (CMs) is an important indicator of their maturation state and of primary importance for the development of therapies based on cardiac stem cells. As the mechanical analysis of adherent cells at high-throughput remains challenging, we explore the applicability of real-time deformability cytometry (RT-DC) to probe cardiomyocytes in suspension. RT-DC is a microfluidic technology allowing for real-time mechanical analysis of thousands of cells with a throughput exceeding 1000 cells per second. For CMs derived from human-induced pluripotent stem cells, we determined a Young's modulus of 1.25 ± 0.08 kPa which is in close range to previous reports. Upon challenging the cytoskeleton with cytochalasin D (CytoD) to induce filamentous actin depolymerization, we distinguish three different regimes in cellular elasticity. Transitions are observed below 10 nM and above 103 nM and are characterized by a decrease in Young's modulus. These regimes can be linked to cytoskeletal and sarcomeric actin contributions by CM contractility measurements at varying CytoD concentrations, where we observe a significant reduction in pulse duration only above 103 nM while no change is found for compound exposure at lower concentrations. Comparing our results to mechanical cell measurements using atomic force microscopy, we demonstrate for the first time to our knowledge, the feasibility of using a microfluidic technique to measure mechanical properties of large samples of adherent cells while linking our results to the composition of the cytoskeletal network. This article is part of a discussion meeting issue 'Single cell ecology'.

Functional Ultrastructure of the Excretory Gland Cell in Zoonotic Anisakids (Anisakidae, Nematoda)

Excretory and secretory products are crucial for parasite infectivity and host immunomodulation, but the functioning and ultrastructure of the excretory gland cell (EC) that produces these products are still scarcely understood and described. In light of growing reports on anisakiasis cases in Europe, we aimed to characterise the EC of larval Anisakispegreffii and adult Pseudoterranovaazarasi. In the latter, EC starts 0.85 mm from the head tip, measuring 1.936 × 0.564 mm. Larval EC shows a long nucleus with thorn-like extravaginations toward the cytoplasm, numerous electron-dense and -lucent secretory granules spanning from the perinuclear to subplasmalemmal space, an elevated number of free ribosomes, small, spherical mitochondria with few cristae and a laminated matrix, small and few Golgi apparatuses, and few endoplasmic reticula, with wide cisternae complexes. Ultrastructure suggests that anaerobic glycolysis is the main metabolic pathway, obtained through nutrient endocytosis across the pseudocoelomic surface of the EC plasmalemma and its endocytic canaliculi. Thorn-like extravaginations of EC karyotheca likely mediate specific processes (Ca2+ signaling, gene expression, transport, nuclear lipid metabolism) into the extremely wide EC cytosol, enabling focal delivery of a signal to specific sites in a short time. These functional annotations of parasitic EC should help to clarify anisakiasis pathogenesis.

Identification and classification of epithelial cells in nephron segments by actin cytoskeleton patterns

The basic functional unit in a kidney is the nephron, which is a long and morphologically segmented tubule. The nephron begins with a cluster of capillaries called glomerulus through which the blood is filtered into the Bowman's space. The filtrate flows through the nephron segments. During this flow, electrolytes and solutes are reabsorbed by channels and transport systems into the capillaries wrapped around the nephron. Many questions related to renal function focus on identifying the sites of expression of these systems. In this study, we mapped whole kidney sections by confocal microscopic imaging of fluorescent phalloidin, which binds to actin filaments. In tile scans (composed of hundreds of images) of these sections, the cortex and the medullary regions (outer and inner stripes of the outer medulla, and inner medulla) could be easily identified by their cytoskeletal patterns. At a higher resolution, we identified distinct features of the actin cytoskeleton in the apical, basal, and lateral borders of the cells. These features could be used to identify segments of a nephron (the proximal tubule, thin and thick segments of Henle's loop, and distal tubule), the collecting duct system, the papillary ducts in the papilla, and the urothelium that covers the pelvis. To verify our findings, we used additional markers, including aquaporin isoforms, cytokeratin 8‐18, and WGA lectin. This study highlights the power of high‐resolution confocal microscopy for identifying specific cell types using the simple probe of F‐actin‐binding phalloidin.

Par-1 controls the composition and growth of cortical actin caps during Drosophila embryo cleavage

The cell cortex is populated by various proteins, but it is unclear how they interact to change cell shape. Jiang and Harris find that the kinase Par-1 is required for Diaphanous-based actin bundles, and that these bundles intersperse with separately induced Arp2/3 networks to form an actin cap that grows into a metaphase compartment of the syncytial Drosophila embryo.

Cell structure depends on the cortex, a thin network of actin polymers and additional proteins underlying the plasma membrane. The cell polarity kinase Par-1 is required for cells to form following syncytial Drosophila embryo development. This requirement stems from Par-1 promoting cortical actin caps that grow into dome-like metaphase compartments for dividing syncytial nuclei. We find the actin caps to be a composite material of Diaphanous (Dia)-based actin bundles interspersed with independently formed, Arp2/3-based actin puncta. Par-1 and Dia colocalize along extended regions of the bundles, and both are required for the bundles and for each other’s bundle-like localization, consistent with an actin-dependent self-reinforcement mechanism. Par-1 helps establish or maintain these bundles in a cortical domain with relatively low levels of the canonical formin activator Rho1-GTP. Arp2/3 is required for displacing the bundles away from each other and toward the cap circumference, suggesting interactions between these cytoskeletal components could contribute to the growth of the cap into a metaphase compartment.

Mechanical Point Loading Induces Cortex Stiffening and Actin Reorganization

Global cytoskeleton reorganization is well-recognized when cells are exposed to distinct mechanical stimuli, but the localized responses at a specified region of a cell are still unclear. In this work, we mapped the cell-surface mechanical property of single cells in situ before and after static point loading these cells using atomic force microscopy in PeakForce-Quantitative Nano Mechanics mode. Cell-surface stiffness was elevated at a maximum of 1.35-fold at the vicinity of loading site, indicating an enhanced structural protection of the cortex to the cell. Mechanical modeling also elucidated the structural protection from the stiffened cell cortex, in which 9-15% and 10-19% decrease of maximum stress and strain of the nucleus were obtained. Furthermore, the flat-ended atomic force microscopy probes were used to capture cytoskeleton reorganization after point loading quantitatively, revealing that the larger the applied force and the longer the loading time are, the more pronounced cytoskeleton reorganization is. Also, point loading using a microneedle combined with real-time confocal microscopy uncovered the fast dynamics of actin cytoskeleton reorganization for actin-stained live cells after point loading (<10 s). These results furthered the understandings in the transmission of localized mechanical forces into an adherent cell.

How the mechanobiome drives cell behavior, viewed through the lens of control theory

Cells have evolved sophisticated systems that integrate internal and external inputs to coordinate cell shape changes during processes, such as development, cell identity determination, and cell and tissue homeostasis. Cellular shape-change events are driven by the mechanobiome, the network of macromolecules that allows cells to generate, sense and respond to externally imposed and internally generated forces. Together, these components build the cellular contractility network, which is governed by a control system. Proteins, such as non-muscle myosin II, function as both sensors and actuators, which then link to scaffolding proteins, transcription factors and metabolic proteins to create feedback loops that generate the foundational mechanical properties of the cell and modulate cellular behaviors. In this Review, we highlight proteins that establish and maintain the setpoint, or baseline, for the control system and explore the feedback loops that integrate different cellular processes with cell mechanics. Uncovering the genetic, biophysical and biochemical interactions between these molecular components allows us to apply concepts from control theory to provide a systems-level understanding of cellular processes. Importantly, the actomyosin network has emerged as more than simply a 'downstream' effector of linear signaling pathways. Instead, it is also a significant driver of cellular processes traditionally considered to be 'upstream'.

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.

Intracellular protozoan parasites: living probes of the host cell surface molecular repertoire

Intracellular protozoans co-evolved with their mammalian host cells a range of strategies to cope with the composite and dynamic cell surface features they encounter during migration and infection. Therefore, these single-celled eukaryotic parasites represent a fascinating source of living probes for precisely capturing the dynamic coupling between the membrane and contractile cortex components of the cell surface. Such biomechanical changes drive a constant re-sculpting of the host cell surface, enabling rapid adjustments that contribute to cellular homeostasis. As emphasized in this review, through the design of specific molecular devices and stratagems to interfere with the biomechanics of the mammalian cell surface these parasitic microbes escape from dangerous or unfavourable microenvironments by breaching host cell membranes, directing the membrane repair machinery to wounded membrane areas, or minimizing membrane assault using discretion and speed when invading host cells for sustained residence.

3D Microenvironment Stiffness Regulates Tumor Spheroid Growth and Mechanics via p21 and ROCK

The mechanical properties of cancer cells and their microenvironment contribute to breast cancer progression. While mechanosensing has been extensively studied using 2D substrates, much less is known about it in a physiologically more relevant 3D context. Here it is demonstrated that breast cancer tumor spheroids, growing in 3D polyethylene glycol-heparin hydrogels, are sensitive to their environment stiffness. During tumor spheroid growth, compressive stresses of up to 2 kPa build up, as quantitated using elastic polymer beads as stress sensors. Atomic force microscopy reveals that tumor spheroid stiffness increases with hydrogel stiffness. Also, constituent cell stiffness increases in a Rho associated kinase (ROCK)- and F-actin-dependent manner. Increased hydrogel stiffness correlated with attenuated tumor spheroid growth, a higher proportion of cells in G0/G1 phase, and elevated levels of the cyclin-dependent kinase inhibitor p21. Drug-mediated ROCK inhibition not only reverses cell stiffening upon culture in stiff hydrogels but also increases tumor spheroid growth. Taken together, a mechanism by which the growth of a tumor spheroid can be regulated via cytoskeleton rearrangements in response to its mechanoenvironment is revealed here. Thus, the findings contribute to a better understanding of how cancer cells react to compressive stress when growing under confinement in stiff environments.