ICAM-1 nanoclusters regulate hepatic epithelial cell polarity by leukocyte adhesion-independent control of apical actomyosin

Epithelial intercellular adhesion molecule (ICAM)-1 is apically polarized, interacts with, and guides leukocytes across epithelial barriers. Polarized hepatic epithelia organize their apical membrane domain into bile canaliculi and ducts, which are not accessible to circulating immune cells but that nevertheless confine most of ICAM-1. Here, by analyzing ICAM-1_KO human hepatic cells, liver organoids from ICAM-1_KO mice and rescue-of-function experiments, we show that ICAM-1 regulates epithelial apicobasal polarity in a leukocyte adhesion-independent manner. ICAM-1 signals to an actomyosin network at the base of canalicular microvilli, thereby controlling the dynamics and size of bile canalicular-like structures. We identified the scaffolding protein EBP50/NHERF1/SLC9A3R1, which connects membrane proteins with the underlying actin cytoskeleton, in the proximity interactome of ICAM-1. EBP50 and ICAM-1 form nano-scale domains that overlap in microvilli, from which ICAM-1 regulates EBP50 nano-organization. Indeed, EBP50 expression is required for ICAM-1-mediated control of BC morphogenesis and actomyosin. Our findings indicate that ICAM-1 regulates the dynamics of epithelial apical membrane domains beyond its role as a heterotypic cell-cell adhesion molecule and reveal potential therapeutic strategies for preserving epithelial architecture during inflammatory stress.

ROCK and the actomyosin network control biomineral growth and morphology during sea urchin skeletogenesis

Biomineralization had apparently evolved independently in different phyla, using distinct minerals, organic scaffolds, and gene regulatory networks (GRNs). However, diverse eukaryotes from unicellular organisms, through echinoderms to vertebrates, use the actomyosin network during biomineralization. Specifically, the actomyosin remodeling protein, Rho-associated coiled-coil kinase (ROCK) regulates cell differentiation and gene expression in vertebrates' biomineralizing cells, yet, little is known on ROCK's role in invertebrates' biomineralization. Here, we reveal that ROCK controls the formation, growth, and morphology of the calcite spicules in the sea urchin larva. ROCK expression is elevated in the sea urchin skeletogenic cells downstream of the Vascular Endothelial Growth Factor (VEGF) signaling. ROCK inhibition leads to skeletal loss and disrupts skeletogenic gene expression. ROCK inhibition after spicule formation reduces the spicule elongation rate and induces ectopic spicule branching. Similar skeletogenic phenotypes are observed when ROCK is inhibited in a skeletogenic cell culture, indicating that these phenotypes are due to ROCK activity specifically in the skeletogenic cells. Reduced skeletal growth and enhanced branching are also observed under direct perturbations of the actomyosin network. We propose that ROCK and the actomyosin machinery were employed independently, downstream of distinct GRNs, to regulate biomineral growth and morphology in Eukaryotes.

YAP mediates apoptosis through failed integrin adhesion reinforcement

Extracellular matrix (ECM) rigidity is a major effector of cell fate decisions. Whereas cell proliferation on stiff matrices, wherein Yes-associated protein (YAP) plays a pivotal role, is well documented, activation of apoptosis in response to soft matrices is poorly understood. Here, we show that YAP drives the apoptotic decision as well. We find that in cells on soft matrices, YAP is recruited to small adhesions, phosphorylated at the Y357 residue, and translocated into the nucleus, ultimately leading to apoptosis. In contrast, Y357 phosphorylation levels are dramatically low in large adhesions on stiff matrices. Furthermore, mild attenuation of actomyosin contractility allows adhesion growth on soft matrices, leading to reduced Y357 phosphorylation levels and resulting in cell growth. These findings indicate that failed adhesion reinforcement drives rigidity-dependent apoptosis through YAP and that this decision is not determined solely by ECM rigidity but rather by the balance between cellular forces and ECM rigidity.

Maternal vgll4a regulates zebrafish epiboly through Yap1 activity

Gastrulation in zebrafish embryos commences with the morphogenetic rearrangement of blastodermal cells, which undergo a coordinated spreading from the animal pole to wrap around the egg at the vegetal pole. This rearrangement, known as epiboly, relies on the orchestrated activity of maternal transcripts present in the egg, compensating for the gradual activation of the zygotic genome. Epiboly involves the mechano-transducer activity of yap1 but what are the regulators of yap1 activity and whether these are maternally or zygotically derived remain elusive. Our study reveals the crucial role of maternal vgll4a, a proposed Yap1 competitor, during zebrafish epiboly. In embryos lacking maternal/zygotic vgll4a (MZvgll4a), the progression of epiboly and blastopore closure is delayed. This delay is associated with the ruffled appearance of the sliding epithelial cells, decreased expression of yap1-downstream targets and transient impairment of the actomyosin ring at the syncytial layer. Our study also shows that, rather than competing with yap1, vgll4a modulates the levels of the E-cadherin/β-catenin adhesion complex at the blastomeres' plasma membrane and hence their actin cortex distribution. Taking these results together, we propose that maternal vgll4a acts at epiboly initiation upstream of yap1 and the E-cadherin/β-catenin adhesion complex, contributing to a proper balance between tissue tension/cohesion and contractility, thereby promoting a timely epiboly progression.

Pulses of RhoA signaling stimulate actin polymerization and flow in protrusions to drive collective cell migration

In animals, cells often move as collectives to shape organs, close wounds, or-in the case of disease-metastasize. To accomplish this, cells need to generate force to propel themselves forward. The motility of singly migrating cells is driven largely by an interplay between Rho GTPase signaling and the actin network. Whether cells migrating as collectives use the same machinery for motility is unclear. Using the zebrafish posterior lateral line primordium as a model for collective cell migration, we find that active RhoA and myosin II cluster on the basal sides of the primordium cells and are required for primordium motility. Positive and negative feedbacks cause RhoA and myosin II activities to pulse. These pulses of RhoA signaling stimulate actin polymerization at the tip of the protrusions and myosin-II-dependent actin flow and protrusion retraction at the base of the protrusions and deform the basement membrane underneath the migrating primordium. This suggests that RhoA-induced actin flow on the basal sides of the cells constitutes the motor that pulls the primordium forward, a scenario that likely underlies collective migration in other contexts.

Adhesion-induced cortical flows pattern E-cadherin-mediated cell contacts

Metazoan development relies on the formation and remodeling of cell-cell contacts. Dynamic reorganization of adhesion receptors and the actomyosin cell cortex in space and time plays a central role in cell-cell contact formation and maturation. Nevertheless, how this process is mechanistically achieved when new contacts are formed remains unclear. Here, by building a biomimetic assay composed of progenitor cells adhering to supported lipid bilayers functionalized with E-cadherin ectodomains, we show that cortical F-actin flows, driven by the depletion of myosin-2 at the cell contact center, mediate the dynamic reorganization of adhesion receptors and cell cortex at the contact. E-cadherin-dependent downregulation of the small GTPase RhoA at the forming contact leads to both a depletion of myosin-2 and a decrease of F-actin at the contact center. At the contact rim, in contrast, myosin-2 becomes enriched by the retraction of bleb-like protrusions, resulting in a cortical tension gradient from the contact rim to its center. This tension gradient, in turn, triggers centrifugal F-actin flows, leading to further accumulation of F-actin at the contact rim and the progressive redistribution of E-cadherin from the contact center to the rim. Eventually, this combination of actomyosin downregulation and flows at the contact determines the characteristic molecular organization, with E-cadherin and F-actin accumulating at the contact rim, where they are needed to mechanically link the contractile cortices of the adhering cells.

Molecular basis of clot retraction and its role in wound healing

Clot retraction is important for the prevention of bleeding, in the manifestations of thrombosis and for tissue repair. The molecular mechanisms behind clot formation are complex. Platelet involvement begins with adhesion at sites of vessel injury followed by platelet aggregation, thrombin generation and fibrin production. Other blood cells incorporate into a fibrin mesh that is consolidated by FXIIIa-mediated crosslinking and platelet contractile activity. The latter results in the asymmetric redistribution of erythrocytes into a tighter central mass providing the clot with stability and resistance to fibrinolysis. Integrin αIIbβ3 on platelets is the key player in these events, bridging fibrin and the platelet cytoskeleton. Glycoprotein VI participates in thrombus formation but not in the retraction. Rheological and environmental factors influence clot construction with retraction driven by the platelet cytoskeleton with actomyosin acting as the motor. Activated platelets provide procoagulant activity stimulating thrombin generation together with the release of a plethora of biologically active proteins and substances from storage pools; many form chemotactic gradients within the fibrin or the underlying matrix. Also released are newly synthesized metabolites and lipid-rich vesicles that circulate within the vasculature and mimic platelet functions. Platelets and their released elements play key roles in wound healing. This includes promoting stem cell and mesenchymal stromal cell recruitment, fibroblast and endothelial cell migration, angiogenesis and matrix formation. These properties have led to the use of autologous clots in therapies designed to accelerate tissue repair while offering the potential for genetic manipulation in both inherited and acquired diseases.

Actomyosin Activity and Piezo1 Activity Synergistically Drive Urinary System Fibroblast Activation

Mechanical cues play a crucial role in activating myofibroblasts from quiescent fibroblasts during fibrosis, and the stiffness of the extracellular matrix is of significant importance in this process. While intracellular force mediated by myosin II and calcium influx regulated by Piezo1 are the primary mechanisms by which cells sense and respond to mechanical forces, their intercellular mechanical interaction remains to be elucidated. Here, hydrogels with tunable substrate are used to systematically investigate the crosstalk of myosin II and Piezo1 in fibroblast to myofibroblast transition (FMT). The findings reveal that the two distinct signaling pathways are integrated to convert mechanical stiffness signals into biochemical signals during bladder-specific FMT. Moreover, it is demonstrated that the crosstalk between myosin II and Piezo1 sensing mechanisms synergistically establishes a sustained feed-forward loop that contributes to chromatin remodeling, induces the expression of downstream target genes, and ultimately exacerbates FMT, in which the intracellular force activates Piezo1 by PI3K/PIP3 pathway-mediated membrane tension and the Piezo1-regulated calcium influx enhances intracellular force by the classical FAK/RhoA/ROCK pathway. Finally, the multifunctional Piezo1 in the complex feedback circuit of FMT drives to further identify that targeting Piezo1 as a therapeutic option for ameliorating bladder fibrosis and dysfunction.

Cleavage furrow-directed cortical flows bias PAR polarization pathways to link cell polarity to cell division

During development, the conserved PAR polarity network is continuously redeployed, requiring that it adapt to changing cellular contexts and environmental cues. In the early C. elegans embryo, polarity shifts from being a cell-autonomous process in the zygote to one that must be coordinated between neighbors as the embryo becomes multicellular. Here, we sought to explore how the PAR network adapts to this shift in the highly tractable C. elegans germline P lineage. We find that although P lineage blastomeres exhibit a distinct pattern of polarity emergence compared with the zygote, the underlying mechanochemical processes that drive polarity are largely conserved. However, changes in the symmetry-breaking cues of P lineage blastomeres ensure coordination of their polarity axis with neighboring cells. Specifically, we show that furrow-directed cortical flows associated with cytokinesis of the zygote induce symmetry breaking in the germline blastomere P1 by transporting PAR-3 into the nascent cell contact. This pool of PAR-3 then biases downstream PAR polarization pathways to establish the polarity axis of P1 with respect to the position of its anterior sister, AB. Thus, our data suggest that cytokinesis itself induces symmetry breaking through the advection of polarity proteins by furrow-directed flows. By directly linking cell polarity to cell division, furrow-directed cortical flows could be a general mechanism to ensure proper organization of cell polarity within actively dividing systems.

Apical polarity and actomyosin dynamics control Kibra subcellular localization and function in Drosophila Hippo signaling

The Hippo pathway is an evolutionarily conserved regulator of tissue growth that integrates inputs from both polarity and actomyosin networks. An upstream activator of the Hippo pathway, Kibra, localizes at the junctional and medial regions of the apical cortex in epithelial cells, and medial accumulation promotes Kibra activity. Here, we demonstrate that cortical Kibra distribution is controlled by a tug-of-war between apical polarity and actomyosin dynamics. We show that while the apical polarity network, in part via atypical protein kinase C (aPKC), tethers Kibra at the junctional cortex to silence its activity, medial actomyosin flows promote Kibra-mediated Hippo complex formation at the medial cortex, thereby activating the Hippo pathway. This study provides a mechanistic understanding of the relationship between the Hippo pathway, polarity, and actomyosin cytoskeleton, and it offers novel insights into how fundamental features of epithelial tissue architecture can serve as inputs into signaling cascades that control tissue growth, patterning, and morphogenesis.

Length limitation of astral microtubules orients cell divisions in murine intestinal crypts

Planar spindle orientation is critical for epithelial tissue organization and is generally instructed by the long cell-shape axis or cortical polarity domains. We introduced mouse intestinal organoids in order to study spindle orientation in a monolayered mammalian epithelium. Although spindles were planar, mitotic cells remained elongated along the apico-basal (A-B) axis, and polarity complexes were segregated to basal poles, so that spindles oriented in an unconventional manner, orthogonal to both polarity and geometric cues. Using high-resolution 3D imaging, simulations, and cell-shape and cytoskeleton manipulations, we show that planar divisions resulted from a length limitation in astral microtubules (MTs) which precludes them from interacting with basal polarity, and orient spindles from the local geometry of apical domains. Accordingly, lengthening MTs affected spindle planarity, cell positioning, and crypt arrangement. We conclude that MT length regulation may serve as a key mechanism for spindles to sense local cell shapes and tissue forces to preserve mammalian epithelial architecture.

New paradigms in actomyosin energy transduction: Critical evaluation of non-traditional models for orthophosphate release

Release of the ATP hydrolysis product ortophosphate (Pi) from the active site of myosin is central in chemo-mechanical energy transduction and closely associated with the main force-generating structural change, the power-stroke. Despite intense investigations, the relative timing between Pi-release and the power-stroke remains poorly understood. This hampers in depth understanding of force production by myosin in health and disease and our understanding of myosin-active drugs. Since the 1990s and up to today, models that incorporate the Pi-release either distinctly before or after the power-stroke, in unbranched kinetic schemes, have dominated the literature. However, in recent years, alternative models have emerged to explain apparently contradictory findings. Here, we first compare and critically analyze three influential alternative models proposed previously. These are either characterized by a branched kinetic scheme or by partial uncoupling of Pi-release and the power-stroke. Finally, we suggest critical tests of the models aiming for a unified picture.

Unveiling the physics underlying symmetry breaking of the actin cytoskeleton: An artificial cell-based approach

Single-cell behaviors cover many biological functions, such as cell division during morphogenesis and tissue metastasis, and cell migration during cancer cell invasion and immune cell responses. Symmetry breaking of the positioning of organelles and the cell shape are often associated with these biological functions. One of the main players in symmetry breaking at the cellular scale is the actin cytoskeleton, comprising actin filaments and myosin motors that generate contractile forces. However, because the self-organization of the actomyosin network is regulated by the biochemical signaling in cells, how the mechanical contraction of the actin cytoskeleton induces diverse self-organized behaviors and drives the cell-scale symmetry breaking remains unclear. In recent times, to understand the physical underpinnings of the symmetry breaking exhibited in the actin cytoskeleton, artificial cell models encapsulating the cytoplasmic actomyosin networks covered with lipid monolayers have been developed. By decoupling the actomyosin mechanics from the complex biochemical signaling within living cells, this system allows one to study the self-organization of actomyosin networks confined in cell-sized spaces. We review the recent developments in the physics of confined actomyosin networks and provide future perspectives on the artificial cell-based approach. This review article is an extended version of the Japanese article, The Physical Principle of Cell Migration Under Confinement: Artificial Cell-based Bottom-up Approach, published in SEIBUTSU BUTSURI Vol. 63, p. 163-164 (2023).

Consumption of a polarized membrane reservoir drives asymmetric membrane expansion during the unequal divisions of neural stem cells

The asymmetric divisions of Drosophila neural stem cells (NSCs) produce unequally sized siblings, with most volume directed into the sibling that retains the NSC fate. Sibling size asymmetry results from the preferential expansion of the NSC sibling surface during division. Here, we show that a polarized membrane reservoir constructed by the NSC in early mitosis provides the source for expansion. The reservoir is formed from membrane domains that contain folds and microvilli that become polarized by apically directed cortical flows of actomyosin early in mitosis. When furrow ingression begins and internal pressure increases, the stores of membrane within the apical reservoir are rapidly consumed. Expansion is substantially diminished in NSCs that lack a reservoir, and membrane expansion equalizes when the reservoir is not polarized. Our results suggest that the cortical flows that remodel the plasma membrane during asymmetric cell division function to satisfy the dynamic surface area requirements of unequally dividing cells.

Elastomeric Pillar Cages Modulate Actomyosin Contractility of Epithelial Microtissues by Substrate Stiffness and Topography

Cell contractility regulates epithelial tissue geometry development and homeostasis. The underlying mechanobiological regulation circuits are poorly understood and experimentally challenging. We developed an elastomeric pillar cage (EPC) array to quantify cell contractility as a mechanoresponse of epithelial microtissues to substrate stiffness and topography. The spatially confined EPC geometry consisted of 24 circularly arranged slender pillars (1.2 MPa, height: 50 µm; diameter: 10 µm, distance: 5 µm). These high-aspect-ratio pillars were confined at both ends by planar substrates with different stiffness (0.15-1.2 MPa). Analytical modeling and finite elements simulation retrieved cell forces from pillar displacements. For evaluation, highly contractile myofibroblasts and cardiomyocytes were assessed to demonstrate that the EPC device can resolve static and dynamic cellular force modes. Human breast (MCF10A) and skin (HaCaT) cells grew as adherence junction-stabilized 3D microtissues within the EPC geometry. Planar substrate areas triggered the spread of monolayered clusters with substrate stiffness-dependent actin stress fiber (SF)-formation and substantial single-cell actomyosin contractility (150-200 nN). Within the same continuous microtissues, the pillar-ring topography induced the growth of bilayered cell tubes. The low effective pillar stiffness overwrote cellular sensing of the high substrate stiffness and induced SF-lacking roundish cell shapes with extremely low cortical actin tension (11-15 nN). This work introduced a versatile biophysical tool to explore mechanobiological regulation circuits driving low- and high-tensional states during microtissue development and homeostasis. EPC arrays facilitate simultaneously analyzing the impact of planar substrate stiffness and topography on microtissue contractility, hence microtissue geometry and function.

Actomyosin contractility in olfactory placode neurons opens the skin epithelium to form the zebrafish nostril

Despite their barrier function, epithelia can locally lose their integrity to create physiological openings during morphogenesis. The mechanisms driving the formation of these epithelial breaks are only starting to be investigated. Here, we study the formation of the zebrafish nostril (the olfactory orifice), which opens in the skin epithelium to expose the olfactory neurons to external odorant cues. Combining live imaging, drug treatments, laser ablation, and tissue-specific functional perturbations, we characterize a mechanical interplay between olfactory placode neurons and the skin, which plays a crucial role in the formation of the orifice: the neurons pull on the overlying skin cells in an actomyosin-dependent manner which, in combination with a local reorganization of the skin epithelium, triggers the opening of the orifice. This work identifies an original mechanism to break an epithelial sheet, in which an adjacent group of cells mechanically assists the epithelium to induce its local rupture.

Coordination of non-professional efferocytosis and actomyosin contractility during epithelial tissue morphogenesis

In developing embryos, specific cell populations are often removed to remodel tissue architecture for organogenesis. During urinary tract development, an epithelial duct called the common nephric duct (CND) gets shortened and eventually eliminated to remodel the entry point of the ureter into the bladder. Here we show that non-professional efferocytosis (the process in which epithelial cells engulf apoptotic bodies) is the main mechanism that contributes to CND shortening. Combining biological metrics and computational modeling, we show that efferocytosis with actomyosin contractility are essential factors that drive the CND shortening without compromising the ureter-bladder structural connection. The disruption of either apoptosis, non-professional efferocytosis, or actomyosin results in contractile tension reduction and deficient CND shortening. Actomyosin activity helps to maintain tissue architecture while non-professional efferocytosis removes cellular volume. Together our results demonstrate that non-professional efferocytosis with actomyosin contractility are important morphogenetic factors controlling CND morphogenesis.

Integrating comparative modeling and accelerated simulations reveals conformational and energetic basis of actomyosin force generation

Muscle contraction is performed by arrays of contractile proteins in the sarcomere. Serious heart diseases, such as cardiomyopathy, can often be results of mutations in myosin and actin. Direct characterization of how small changes in the myosin-actin complex impact its force production remains challenging. Molecular dynamics (MD) simulations, although capable of studying protein structure-function relationships, are limited owing to the slow timescale of the myosin cycle as well as a lack of various intermediate structures for the actomyosin complex. Here, employing comparative modeling and enhanced sampling MD simulations, we show how the human cardiac myosin generates force during the mechanochemical cycle. Initial conformational ensembles for different myosin-actin states are learned from multiple structural templates with Rosetta. This enables us to efficiently sample the energy landscape of the system using Gaussian accelerated MD. Key myosin loop residues, whose substitutions are related to cardiomyopathy, are identified to form stable or metastable interactions with the actin surface. We find that the actin-binding cleft closure is allosterically coupled to the myosin motor core transitions and ATP-hydrolysis product release from the active site. Furthermore, a gate between switch I and switch II is suggested to control phosphate release at the prepowerstroke state. Our approach demonstrates the ability to link sequence and structural information to motor functions.

The developing murine kidney actively negotiates geometric packing conflicts to avoid defects

The physiological functions of several organs rely on branched epithelial tubule networks bearing specialized structures for secretion, gas exchange, or filtration. Little is known about conflicts in development between building enough tubules for adequate function and geometric constraints imposed by organ size. We show that the mouse embryonic kidney epithelium negotiates a physical packing conflict between increasing tubule tip numbers through branching and limited organ surface area. Through imaging of whole kidney explants, combined with computational and soft material modeling of tubule families, we identify six possible geometric packing phases, including two defective ones. Experiments in explants show that a radially oriented tension on tubule families is necessary and sufficient for them to switch to a vertical packing arrangement that increases surface tip density while avoiding defects. These results reveal developmental contingencies in response to physical limitations and create a framework for classifying congenital kidney defects.

L-histidine inhibits the heat-induced gelation of actomyosin in a low ionic strength solution

The heat-induced gelation of actomyosin plays a key role in meat processing. Our previous study showed that L-histidine could affect the characteristics of a heat-induced gel of myosin on a low ionic strength. To apply the specific effect of L-histidine to meat processing, the heat-induced gel properties of actomyosin in the presence of L-histidine were investigated. Actomyosin in a low ionic strength solution containing L-histidine did not form a gel upon heating. The dynamic rheological properties of actomyosin in low ionic strength solutions were distinct depending on the presence or absence of L-histidine. Electron microscopy showed that, heated at 50°C, actomyosin in a low ionic strength solution containing L-histidine remained a filamentous structure. The surface hydrophobicity of actomyosin was stable up to 50°C in a low ionic strength solution containing L-histidine. In conclusion, L-histidine might suppress the aggregation of actomyosin and inhibit heat-induced gelation in a low ionic strength solution.

How dynamic prestress governs the shape of living systems, from the subcellular to tissue scale

Cells and tissues change shape both to carry out their function and during pathology. In most cases, these deformations are driven from within the systems themselves. This is permitted by a range of molecular actors, such as active crosslinkers and ion pumps, whose activity is biologically controlled in space and time. The resulting stresses are propagated within complex and dynamical architectures like networks or cell aggregates. From a mechanical point of view, these effects can be seen as the generation of prestress or prestrain, resulting from either a contractile or growth activity. In this review, we present this concept of prestress and the theoretical tools available to conceptualize the statics and dynamics of living systems. We then describe a range of phenomena where prestress controls shape changes in biopolymer networks (especially the actomyosin cytoskeleton and fibrous tissues) and cellularized tissues. Despite the diversity of scale and organization, we demonstrate that these phenomena stem from a limited number of spatial distributions of prestress, which can be categorized as heterogeneous, anisotropic or differential. We suggest that in addition to growth and contraction, a third type of prestress-topological prestress-can result from active processes altering the microstructure of tissue.

Cellular mechanisms of reverse epithelial curvature in tissue morphogenesis

Epithelial bending plays an essential role during the multiple stages of organogenesis and can be classified into two types: invagination and evagination. The early stages of invaginating and evaginating organs are often depicted as simple concave and convex curves respectively, but in fact majority of the epithelial organs develop through a more complex pattern of curvature: concave flanked by convex and vice versa respectively. At the cellular level, this is far from a geometrical truism: locally cells must passively adapt to, or actively create such an epithelial structure that is typically composed of opposite and connected folds that form at least one s-shaped curve that we here, based on its appearance, term as "reverse curves." In recent years, invagination and evagination have been studied in increasing cellular detail. A diversity of mechanisms, including apical/basal constriction, vertical telescoping and extrinsic factors, all orchestrate epithelial bending to give different organs their final shape. However, how cells behave collectively to generate reverse curves remains less well-known. Here we review experimental models that characteristically form reverse curves during organogenesis. These include the circumvallate papillae in the tongue, crypt-villus structure in the intestine, and early tooth germ and describe how, in each case, reverse curves form to connect an invaginated or evaginated placode or opposite epithelial folds. Furthermore, by referring to the multicellular system that occur in the invagination and evagination, we attempt to provide a summary of mechanisms thought to be involved in reverse curvature consisting of apical/basal constriction, and extrinsic factors. Finally, we describe the emerging techniques in the current investigations, such as organoid culture, computational modelling and live imaging technologies that have been utilized to improve our understanding of the cellular mechanisms in early tissue morphogenesis.

Diversity is the spice of life: An overview of how cytokinesis regulation varies with cell type

Cytokinesis is required to physically cleave a cell into two daughters at the end of mitosis. Decades of research have led to a comprehensive understanding of the core cytokinesis machinery and how it is regulated in animal cells, however this knowledge was generated using single cells cultured in vitro, or in early embryos before tissues develop. This raises the question of how cytokinesis is regulated in diverse animal cell types and developmental contexts. Recent studies of distinct cell types in the same organism or in similar cell types from different organisms have revealed striking differences in how cytokinesis is regulated, which includes different threshold requirements for the structural components and the mechanisms that regulate them. In this review, we highlight these differences with an emphasis on pathways that are independent of the mitotic spindle, and operate through signals associated with the cortex, kinetochores, or chromatin.

Cytokinetic diversity in mammalian cells is revealed by the characterization of endogenous anillin, Ect2 and RhoA

Cytokinesis is required to physically separate the daughter cells at the end of mitosis. This crucial process requires the assembly and ingression of an actomyosin ring, which must occur with high fidelity to avoid aneuploidy and cell fate changes. Most of our knowledge of mammalian cytokinesis was generated using over-expressed transgenes in HeLa cells. Over-expression can introduce artefacts, while HeLa are cancerous human cells that have lost their epithelial identity, and the mechanisms controlling cytokinesis in these cells could be vastly different from other cell types. Here, we tagged endogenous anillin, Ect2 and RhoA with mNeonGreen and characterized their localization during cytokinesis for the first time in live human cells. Comparing anillin localization in multiple cell types revealed cytokinetic diversity with differences in the duration and symmetry of ring closure, and the timing of cortical recruitment. Our findings show that the breadth of anillin correlates with the rate of ring closure, and support models where cell size or ploidy affects the cortical organization, and intrinsic mechanisms control the symmetry of ring closure. This work highlights the need to study cytokinesis in more diverse cell types, which will be facilitated by the reagents generated for this study.

Myosin turnover controls actomyosin contractile instability

Contractile force produced by myosin II that binds and pulls constrained filamentous actin is harnessed by cells for diverse processes such as cell division. However, contractile actomyosin systems are vulnerable to an intrinsic aggregation instability that destroys actomyosin architecture if unchecked. Punctate myosin distributions are widely observed, but how cells prevent more advanced aggregation remains unclear. Here, we studied cytokinetic contractile rings in fission yeast cell ghosts lacking component turnover, when myosin aggregated hierarchically. Simulations reproduced the severe organizational disruption and a dead-end state with isolated aggregates and ring tension loss. We conclude that in normal cells, myosin turnover regulates actomyosin contractile instability by continuous injection of homogeneously distributed myosin, permitting functional aggregates to develop but intercepting catastrophic runaway aggregation.

Actomyosin contractile force produced by myosin II molecules that bind and pull actin filaments is harnessed for diverse functions, from cell division by the cytokinetic contractile ring to morphogenesis driven by supracellular actomyosin networks during development. However, actomyosin contractility is intrinsically unstable to self-reinforcing spatial variations that may destroy the actomyosin architecture if unopposed. How cells control this threat is not established, and while large myosin fluctuations and punctateness are widely reported, the full course of the instability in cells has not been observed. Here, we observed the instability run its full course in isolated cytokinetic contractile rings in cell ghosts where component turnover processes are absent. Unprotected by turnover, myosin II merged hierarchically into aggregates with increasing amounts of myosin and increasing separation, up to a maximum separation. Molecularly explicit simulations reproduced the hierarchical aggregation which precipitated tension loss and ring fracture and identified the maximum separation as the length of actin filaments mediating mechanical communication between aggregates. In the final simulated dead-end state, aggregates were morphologically quiescent, including asters with polarity-sorted actin, similar to the dead-end state observed in actomyosin systems in vitro. Our results suggest the myosin II turnover time controls actomyosin contractile instability in normal cells, long enough for aggregation to build robust aggregates but sufficiently short to intercept catastrophic hierarchical aggregation and fracture.

Bisphenol Exposure Disrupts Cytoskeletal Organization and Development of Pre-Implantation Embryos

The endocrine disrupting activity of bisphenol compounds is well documented, but less is known regarding their impact on cell division and early embryo formation. Here, we tested the effects of acute in vitro exposure to bisphenol A (BPA) and its common substitute, bisphenol F (BPF), during critical stages of mouse pre-implantation embryo development, including the first mitotic division, cell polarization, as well as morula and blastocyst formation. Timing of initial cleavage was determined by live-cell imaging, while subsequent divisions, cytoskeletal organization and lineage marker labeling were assessed by high-resolution fluorescence microscopy. Our analysis reveals that brief culture with BPA or BPF impeded cell division and disrupted embryo development at all stages tested. Surprisingly, BPF was more detrimental to the early embryo than BPA. Notably, poor embryo development was associated with cytoskeletal disruptions of the actomyosin network, apical domain formation during cell polarization, actin ring zippering for embryo sealing and altered cell lineage marker profiles. These results underscore that bisphenols can disrupt cytoskeletal integrity and remodeling that is vital for early embryo development and raise concerns regarding the use of BPF as a ‘safe’ BPA substitute.

T cell cytoskeletal forces shape synapse topography for targeted lysis via membrane curvature bias of perforin

Cytotoxic T lymphocytes (CTLs) lyse target cells by delivering lytic granules that contain the pore former perforin to the cytotoxic immunological synapse. Here, we establish that opposing cytoskeletal forces drive lytic granule polarization and simultaneously shape T cell synapse topography to enhance target perforation. At the cell rear, actomyosin contractility drives the anterograde movement of lytic granules toward the nucleus. At the synapse, dynein-derived forces induce negatively curved membrane pockets to which granules are transported around the nucleus. These highly concave degranulation pockets are located directly opposite positively curved bulges on the target cell membrane. We identify a curvature bias in the action of perforin, which preferentially perforates positively curved tumor cell membrane. Together, these findings demonstrate murine and human T cell-mediated cytotoxicity to be a highly tuned mechano-biochemical system, in which the forces that polarize lytic granules locally bend the synaptic membrane to favor the unidirectional perforation of the target cell.

Discovery of the regulatory role of calcium ion in muscle contraction and relaxation: Setsuro Ebashi and the international emergence of Japanese muscle research

In the early 1950s Setsuro Ebashi was a graduate student at Tokyo University studying the biochemical models of muscle contraction. The muscle components in these models contracted in the presence of ATP, but what caught his attention was that the components did not relax when ATP was exhausted. Ebashi decided in 1952 to attempt to elucidate the mechanism of muscle relaxation using these models. This decision started a journey that would lead him to be the first to propose the calcium concept of muscle contraction and relaxation in 1961. It was an unpopular theory with biochemists who refused to accept that anything as simple as an inorganic ion, Ca²⁺, could control anything as important as muscle contraction. Ebashi was convinced that he was correct. He proceeded to show that micromolar concentrations of Ca²⁺ activated contraction. In 1961 he discovered the particulate nature of the ATP-dependent relaxing factor (the sarcoplasmic reticulum) and determined that it acted by binding Ca²⁺. Most notably, in 1966 he discovered troponin, the Ca²⁺ receptor in muscle, which mediated Ca²⁺ control of contraction. Ebashi's discoveries were considered the most important in the muscle field since the 1950s. Ebashi had to overcome the doubt of the scientific community. This story is one of great scientific achievement against great odds that marked the emergence of Japanese muscle research onto the international scientific stage.NEW & NOTEWORTHY Setsuro Ebashi proposed the calcium concept of muscle contraction and relaxation in 1961. It was a very unpopular theory. He showed that Ca²⁺ activated contraction and that the sarcoplasmic reticulum caused relaxation by binding Ca²⁺ in an ATP-dependent manner. Most notably, he discovered the receptor that mediated Ca²⁺ control of contraction and named it "troponin." Ebashi's discoveries are considered to be the most important in the muscle field since the 1950s.

Rapid assembly of a polar network architecture drives efficient actomyosin contractility

Actin network architecture and dynamics play a central role in cell contractility and tissue morphogenesis. RhoA-driven pulsed contractions are a generic mode of actomyosin contractility, but the mechanisms underlying how their specific architecture emerges and how this architecture supports the contractile function of the network remain unclear. Here we show that, during pulsed contractions, the actin network is assembled by two subpopulations of formins: a functionally inactive population (recruited) and formins actively participating in actin filament elongation (elongating). We then show that elongating formins assemble a polar actin network, with barbed ends pointing out of the pulse. Numerical simulations demonstrate that this geometry favors rapid network contraction. Our results show that formins convert a local RhoA activity gradient into a polar network architecture, causing efficient network contractility, underlying the key function of kinetic controls in the assembly and mechanics of cortical network architectures.

RhoA-driven actomyosin contractility plays a key role in driving cell and tissue contractility during morphogenesis. Tracking individual formins, Costache et al. show that the network assembled downstream of RhoA displays a polar architecture, barbed ends pointing outward, a feature that supports efficient contractility and force transmission during pulsed contractions.

Cardiac ventricular myosin and slow skeletal myosin exhibit dissimilar chemomechanical properties despite bearing the same myosin heavy chain isoform

The myosin II motors are ATP-powered force-generating machines driving cardiac and muscle contraction. Myosin II heavy chain isoform-beta (β-MyHC) is primarily expressed in the ventricular myocardium and in slow-twitch muscle fibers, such as M. soleus. M. soleus–derived myosin II (SolM-II) is often used as an alternative to the ventricular β-cardiac myosin (βM-II); however, the direct assessment of biochemical and mechanical features of the native myosins is limited. By employing optical trapping, we examined the mechanochemical properties of native myosins isolated from the rabbit heart ventricle and soleus muscles at the single-molecule level. We found purified motors from the two tissue sources, despite expressing the same MyHC isoform, displayed distinct motile and ATPase kinetic properties. We demonstrate βM-II was approximately threefold faster in the actin filament–gliding assay than SolM-II. The maximum actomyosin (AM) detachment rate derived in single-molecule assays was also approximately threefold higher in βM-II, while the power stroke size and stiffness of the “AM rigor” crossbridge for both myosins were comparable. Our analysis revealed a higher AM detachment rate for βM-II, corresponding to the enhanced ADP release rates from the crossbridge, likely responsible for the observed differences in the motility driven by these myosins. Finally, we observed a distinct myosin light chain 1 isoform (MLC1sa) that associates with SolM-II, which might contribute to the observed kinetics differences between βM-II and SolM-II. These results have important implications for the choice of tissue sources and justify prerequisites for the correct myosin heavy and light chains to study cardiomyopathies.

A particle size threshold governs diffusion and segregation of PAR-3 during cell polarization

The actomyosin cortex regulates the localization and function of proteins at the plasma membrane. Here, we study how membrane binding, cortical movements, and diffusion determine membrane protein distribution. In Caenorhabditis elegans zygotes, actomyosin flows transport PAR polarity proteins to establish the anterior-posterior axis. Oligomerization of a key scaffold protein, PAR-3, is required for polarization. PAR-3 oligomers are a heterogeneous population of many different sizes, and it remains unclear how oligomer size affects PAR-3 segregation. To address this question, we engineered PAR-3 to defined sizes. We report that PAR-3 trimers are necessary and sufficient for PAR-3 function during polarization and later embryo development. Quantitative analysis of PAR-3 diffusion shows that a threshold size of three subunits allows PAR-3 clusters to stably bind the membrane, where they are corralled and transported by the actomyosin cortex. Our study provides a quantitative model for size-dependent protein transportation of peripheral membrane proteins by cortical flow.

The actomyosin cytoskeleton is a major regulator of cellular organization. Chang and Dickinson develop protein-engineering and particle-tracking tools to study how clustered membrane-bound proteins are transported by actomyosin contractions in vivo. Data-driven modeling reveals how membrane binding, diffusion, and collisions with F-actin contribute to protein movement.

Condensation of the Drosophila nerve cord is oscillatory and depends on coordinated mechanical interactions

During development, organs reach precise shapes and sizes. Organ morphology is not always obtained through growth; a classic counterexample is the condensation of the nervous system during Drosophila embryogenesis. The mechanics underlying such condensation remain poorly understood. Here, we characterize the condensation of the embryonic ventral nerve cord (VNC) at both subcellular and tissue scales. This analysis reveals that condensation is not a unidirectional continuous process but instead occurs through oscillatory contractions. The VNC mechanical properties spatially and temporally vary, and forces along its longitudinal axis are spatially heterogeneous. We demonstrate that the process of VNC condensation is dependent on the coordinated mechanical activities of neurons and glia. These outcomes are consistent with a viscoelastic model of condensation, which incorporates time delays and effective frictional interactions. In summary, we have defined the progressive mechanics driving VNC condensation, providing insights into how a highly viscous tissue can autonomously change shape and size.

Extracellular mechanical forces drive endocardial cell volume decrease during zebrafish cardiac valve morphogenesis

Organ morphogenesis involves dynamic changes of tissue properties while cells adapt to their mechanical environment through mechanosensitive pathways. How mechanical cues influence cell behaviors during morphogenesis remains unclear. Here, we studied the formation of the zebrafish atrioventricular canal (AVC) where cardiac valves develop. We show that the AVC forms within a zone of tissue convergence associated with the increased activation of the actomyosin meshwork and cell-orientation changes. We demonstrate that tissue convergence occurs with a reduction of cell volume triggered by mechanical forces and the mechanosensitive channel TRPP2/TRPV4. Finally, we show that the extracellular matrix component hyaluronic acid controls cell volume changes. Together, our data suggest that multiple force-sensitive signaling pathways converge to modulate cell volume. We conclude that cell volume reduction is a key cellular feature activated by mechanotransduction during cardiovascular morphogenesis. This work further identifies how mechanical forces and extracellular matrix influence tissue remodeling in developing organs.

Extracellular hyaluronate pressure shaped by cellular tethers drives tissue morphogenesis

How tissues acquire complex shapes is a fundamental question in biology and regenerative medicine. Zebrafish semicircular canals form from invaginations in the otic epithelium (buds) that extend and fuse to form the hubs of each canal. We find that conventional actomyosin-driven behaviors are not required. Instead, local secretion of hyaluronan, made by the enzymes uridine 5'-diphosphate dehydrogenase (ugdh) and hyaluronan synthase 3 (has3), drives canal morphogenesis. Charged hyaluronate polymers osmotically swell with water and generate isotropic extracellular pressure to deform the overlying epithelium into buds. The mechanical anisotropy needed to shape buds into tubes is conferred by a polarized distribution of actomyosin and E-cadherin-rich membrane tethers, which we term cytocinches. Most work on tissue morphogenesis ascribes actomyosin contractility as the driving force, while the extracellular matrix shapes tissues through differential stiffness. Our work inverts this expectation. Hyaluronate pressure shaped by anisotropic tissue stiffness may be a widespread mechanism for powering morphological change in organogenesis and tissue engineering.

Anti-Malignant Effect of Tensile Loading to Adherens Junctions in Cutaneous Squamous Cell Carcinoma Cells

Actomyosin contractility regulates various cellular processes including proliferation and differentiation while dysregulation of actomyosin activity contributes to cancer development and progression. Previously, we have reported that actomyosin-generated tension at adherens junctions is required for cell density-dependent inhibition of proliferation of normal skin keratinocytes. However, it remains unclear how actomyosin contractility affects the hyperproliferation ability of cutaneous squamous cell carcinoma (cSCC) cells. In this study, we find that actomyosin activity is impaired in cSCC cells both in vitro and in vivo. External application of tensile loads to adherens junctions by sustained mechanical stretch attenuates the proliferation of cSCC cells, which depends on intact adherens junctions. Forced activation of actomyosin of cSCC cells also inhibits their proliferation in a cell-cell contact-dependent manner. Furthermore, the cell cycle arrest induced by tensile loading to adherens junctions is accompanied by epidermal differentiation in cSCC cells. Our results show that the degree of malignant properties of cSCC cells can be reduced by applying tensile loads to adherens junctions, which implies that the mechanical status of adherens junctions may serve as a novel therapeutic target for cSCC.

Septin Remodeling During Mammalian Cytokinesis

Cytokinesis mediates the final separation of a mother cell into two daughter cells. Septins are recruited to the cleavage furrow at an early stage. During cytokinetic progression the septin cytoskeleton is constantly rearranged, ultimately leading to a concentration of septins within the intercellular bridge (ICB), and to the formation of two rings adjacent to the midbody that aid ESCRT-dependent abscission. The molecular mechanisms underlying this behavior are poorly understood. Based on observations that septins can associate with actin, microtubules and associated motors, we review here established roles of septins in mammalian cytokinesis, and discuss, how septins may support cytokinetic progression by exerting their functions at particular sites. Finally, we discuss how this might be assisted by phosphoinositide-metabolizing enzymes.

The amoeboid state as part of the epithelial-to-mesenchymal transition programme

Cell migration is essential for many biological processes, while abnormal cell migration is characteristic of cancer cells. Epithelial cells become motile by undergoing epithelial-to-mesenchymal transition (EMT), and mesenchymal cells increase migration speed by adopting amoeboid features. This review highlights how amoeboid behaviour is not merely a migration mode but rather a cellular state - within the EMT spectra - by which cancer cells survive, invade and colonise challenging microenvironments. Molecular biomarkers and physicochemical triggers associated with amoeboid behaviour are discussed, including an amoeboid associated tumour microenvironment. We reflect on how amoeboid characteristics support metastasis and how their liabilities could turn into therapeutic opportunities.

High-resolution structures of the actomyosin-V complex in three nucleotide states provide insights into the force generation mechanism

The molecular motor myosin undergoes a series of major structural transitions during its force-producing motor cycle. The underlying mechanism and its coupling to ATP hydrolysis and actin binding are only partially understood, mostly due to sparse structural data on actin-bound states of myosin. Here, we report 26 high-resolution cryo-EM structures of the actomyosin-V complex in the strong-ADP, rigor, and a previously unseen post-rigor transition state that binds the ATP analog AppNHp. The structures reveal a high flexibility of myosin in each state and provide valuable insights into the structural transitions of myosin-V upon ADP release and binding of AppNHp, as well as the actomyosin interface. In addition, they show how myosin is able to specifically alter the structure of F-actin.

Phases of cortical actomyosin dynamics coupled to the neuroblast polarity cycle

The Par complex dynamically polarizes to the apical cortex of asymmetrically dividing Drosophila neuroblasts where it directs fate determinant segregation. Previously, we showed that apically directed cortical movements that polarize the Par complex require F-actin (Oon and Prehoda, 2019). Here, we report the discovery of cortical actomyosin dynamics that begin in interphase when the Par complex is cytoplasmic but ultimately become tightly coupled to cortical Par dynamics. Interphase cortical actomyosin dynamics are unoriented and pulsatile but rapidly become sustained and apically-directed in early mitosis when the Par protein aPKC accumulates on the cortex. Apical actomyosin flows drive the coalescence of aPKC into an apical cap that depolarizes in anaphase when the flow reverses direction. Together with the previously characterized role of anaphase flows in specifying daughter cell size asymmetry, our results indicate that multiple phases of cortical actomyosin dynamics regulate asymmetric cell division.

Orchestration of Force Generation and Nuclear Collapse in Apoptotic Cells

Apoptosis, or programmed cell death, is a form of cell suicide that is extremely important for ridding the body of cells that are no longer required, to protect the body against hazardous cells, such as cancerous ones, and to promote tissue morphogenesis during animal development. Upon reception of a death stimulus, the doomed cell activates biochemical pathways that eventually converge on the activation of dedicated enzymes, caspases. Numerous pieces of information on the biochemical control of the process have been gathered, from the successive events of caspase activation to the identification of their targets, such as lamins, which constitute the nuclear skeleton. Yet, evidence from multiple systems now shows that apoptosis is also a mechanical process, which may even ultimately impinge on the morphogenesis of the surrounding tissues. This mechanical role relies on dramatic actomyosin cytoskeleton remodelling, and on its coupling with the nucleus before nucleus fragmentation. Here, we provide an overview of apoptosis before describing how apoptotic forces could combine with selective caspase-dependent proteolysis to orchestrate nucleus destruction.

Targeted disruption of GAK stagnates autophagic flux by disturbing lysosomal dynamics

The autophagy-lysosome system allows cells to adapt to environmental changes by regulating the degradation and recycling of cellular components, and to maintain homeostasis by removing aggregated proteins and defective organelles. Cyclin G-associated kinase (GAK) is involved in the regulation of clathrin-dependent endocytosis and cell cycle progression. In addition, a single nucleotide polymorphism at the GAK locus has been reported as a risk factor for Parkinson's disease. However, the roles of GAK in the autophagy-lysosome system are not completely understood, thus the present study aimed to clarify this. In the present study, under genetic disruption or chemical inhibition of GAK, analyzing autophagic flux and observing morphological changes of autophagosomes and autolysosomes revealed that GAK controlled lysosomal dynamics via actomyosin regulation, resulting in a steady progression of autophagy. GAK knockout (KO) in A549 cells impaired autophagosome-lysosome fusion and autophagic lysosome reformation, which resulted in the accumulation of enlarged autophagosomes and autolysosomes during prolonged starvation. The stagnation of autophagic flux accompanied by these phenomena was also observed with the addition of a GAK inhibitor. Furthermore, the addition of Rho-associated protein kinase (ROCK) inhibitor or ROCK1 knockdown mitigated GAK KO-mediated effects. The results suggested a vital role of GAK in controlling lysosomal dynamics via maintaining lysosomal homeostasis during autophagy.

The amoebae of Idionectes vortex (Cutosea, Amoebozoa): Motility, cytoskeleton architecture and extracellular scales

The Cutosea represent a deep-branching lineage within the phylum Amoebozoa that is still relatively poorly explored. Currently, there are four cutosean representatives known - the monotypic genera Armaparvus, Idionectes, Sapocribrum, and Squamamoeba - with marked genetic distances. Idionectes vortex is the deepest-branching species and differs markedly from the other Cutosea in ecology, life history, and most importantly, in its ability to form a flagellated swarmer with an exceptional swimming mechanism. As far as we know, the other Cutosea lack flagella and rather represent small, marine amoebae with a characteristic cell coat. The present paper focuses on the amoeboid life history stage of the algivorous amoeboflagellate Idionectes vortex to provide data for a first in-depth comparison with other Cutosea and to document structural specialties. The amoeboid stage of Idionectes is mainly associated with the specific feeding process, that is, the interaction with algal prey cells and phagocytosis of protoplast material. Yet, the present data from time-lapse microscopy, cytochemical stainings, and electron microscopy demonstrate clear similarities with the other cutosean species concerning amoeboid locomotion and cell coat ultrastructure. Furthermore, Idionectes amoebae exhibit a well-developed microtubular cytoskeleton, and an unusual basal apparatus that seems to undergo marked changes during the life history of this exceptional amoebozoan.

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.

Plectin-Mediated Intermediate Filament Functions: Why Isoforms Matter

This essay focuses on the role of plectin and its various isoforms in mediating intermediate filament (IF) network functions. It is based on previous studies that provided comprehensive evidence for a concept where plectin acts as an IF recruiter, and plectin-mediated IF networking and anchoring are key elements in IF function execution. Here, plectin’s global role as modulator of IF functionality is viewed from different perspectives, including the mechanical stabilization of IF networks and their docking platforms, contribution to cellular viscoelasticity and mechanotransduction, compartmentalization and control of the actomyosin machinery, connections to the microtubule system, and mechanisms and specificity of isoform targeting. Arguments for IF networks and plectin acting as mutually dependent partners are also given. Lastly, a working model is presented that describes a unifying mechanism underlying how plectin–IF networks mechanically control and propagate actomyosin-generated forces, affect microtubule dynamics, and contribute to mechanotransduction.

Distinct roles of nonmuscle myosin II isoforms for establishing tension and elasticity during cell morphodynamics

Nonmuscle myosin II (NM II) is an integral part of essential cellular processes, including adhesion and migration. Mammalian cells express up to three isoforms termed NM IIA, B, and C. We used U2OS cells to create CRISPR/Cas9-based knockouts of all three isoforms and analyzed the phenotypes on homogenously coated surfaces, in collagen gels, and on micropatterned substrates. In contrast to homogenously coated surfaces, a structured environment supports a cellular phenotype with invaginated actin arcs even in the absence of NM IIA-induced contractility. A quantitative shape analysis of cells on micropatterns combined with a scale-bridging mathematical model reveals that NM IIA is essential to build up cellular tension during initial stages of force generation, while NM IIB is necessary to elastically stabilize NM IIA-generated tension. A dynamic cell stretch/release experiment in a three-dimensional scaffold confirms these conclusions and in addition reveals a novel role for NM IIC, namely the ability to establish tensional homeostasis.

From Microspikes to Stress Fibers: Actin Remodeling in Breast Acini Drives Myosin II-Mediated Basement Membrane Invasion

The cellular mechanisms of basement membrane (BM) invasion remain poorly understood. We investigated the invasion-promoting mechanisms of actin cytoskeleton reorganization in BM-covered MCF10A breast acini. High-resolution confocal microscopy has characterized actin cell protrusion formation and function in response to tumor-resembling ECM stiffness and soluble EGF stimulation. Traction force microscopy quantified the mechanical BM stresses that invasion-triggered acini exerted on the BM-ECM interface. We demonstrate that acini use non-proteolytic actin microspikes as functional precursors of elongated protrusions to initiate BM penetration and ECM probing. Further, these microspikes mechanically widened the collagen IV pores to anchor within the BM scaffold via force-transmitting focal adhesions. Pre-invasive basal cells located at the BM-ECM interface exhibited predominantly cortical actin networks and actin microspikes. In response to pro-invasive conditions, these microspikes accumulated and converted subsequently into highly contractile stress fibers. The phenotypical switch to stress fiber cells matched spatiotemporally with emerging high BM stresses that were driven by actomyosin II contractility. The activation of proteolytic invadopodia with MT1-MMP occurred at later BM invasion stages and only in cells already disseminating into the ECM. Our study demonstrates that BM pore-widening filopodia bridge mechanical ECM probing function and contractility-driven BM weakening. Finally, these EMT-related cytoskeletal adaptations are critical mechanisms inducing the invasive transition of benign breast acini.

Morphofunctional programming of microglia requires distinct roles of type II myosins

The ramified morphology of microglia and the dynamics of their membrane protrusions are essential for their functions in central nervous system development, homeostasis, and disease. Although their ability to change and control shape critically depends on the actin and actomyosin cytoskeleton, the underlying regulatory mechanisms remain largely unknown. In this study, we systematically analyzed the actomyosin cytoskeleton and regulators downstream of the small GTPase RhoA in the control of microglia shape and function. Our results reveal that (i) Myh9 controls cortical tension levels and affects microglia protrusion formation, (ii) cofilin-mediated maintenance of actin turnover regulates microglia protrusion extension, and (iii) Myh10 influences microglia inflammatory activation. Overall we uncover molecular pathways that regulate microglia morphology and identify type-II myosins as important regulators of microglia biology with differential roles in the control of cell shape (Myh9) and functions (Myh10).

Comparison of the Mechanical Properties Between the Convex and Concave Inner/Apical Surfaces of the Developing Cerebrum

The inner/apical surface of the embryonic brain wall is important as a major site for cell production by neural progenitor cells (NPCs). We compared the mechanical properties of the apical surfaces of two neighboring but morphologically distinct cerebral wall regions in mice from embryonic day (E) E12–E14. Through indentation measurement using atomic force microscopy (AFM), we first found that Young’s modulus was higher at a concave-shaped apical surface of the pallium than at a convex-shaped apical surface of the ganglionic eminence (GE). Further AFM analysis suggested that contribution of actomyosin as revealed with apical surface softening by blebbistatin and stiffness of dissociated NPCs were both comparable between pallium and GE, not accounting for the differential apical surface stiffness. We then found that the density of apices of NPCs was greater, with denser F-actin meshwork, in the apically stiffer pallium than in GE. A similar correlation was found between the decreasing density between E12 and E14 of NPC apices and the declining apical surface stiffness in the same period in both the pallium and the GE. Thus, one plausible explanation for the observed difference (pallium > GE) in apical surface stiffness may be differential densification of NPC apices. In laser ablation onto the apical surface, the convex-shaped GE apical surface showed quicker recoils of edges than the pallial apical surface did, with a milder inhibition of recoiling by blebbistatin than in pallium. This greater pre-stress in GE may provide an indication of how the initially apically concave wall then becomes an apically convex “eminence.”

Direct measurement of near-nanoNewton forces developed by self-organizing actomyosin fibres bound α-catenin

Actin cytoskeleton contractility plays a critical role in morphogenetic processes by generating forces that are then transmitted to cell-cell and cell-ECM adhesion complexes. In turn, mechanical properties of the environment are sensed and transmitted to the cytoskeleton at cell adhesion sites, influencing cellular processes such as cell migration, differentiation and survival. Anchoring of the actomyosin cytoskeleton to adhesion sites is mediated by adaptor proteins such as talin or α-catenin that link F-actin to transmembrane cell adhesion receptors, thereby allowing mechanical coupling between the intracellular and extracellular compartments. Thus, a key issue is to be able to measure the forces generated by actomyosin and transmitted to the adhesion complexes. Approaches developed in cells and those probing single molecule mechanical properties of α-catenin molecules allowed to identify α-catenin, an F-actin binding protein which binds to the cadherin complexes as a major player in cadherin-based mechanotransduction. However, it is still very difficult to bridge intercellular forces measured at cellular levels and those measured at the single-molecule level. Here, we applied an intermediate approach allowing reconstruction of the actomyosin-α-catenin complex in acellular conditions to probe directly the transmitted forces. For this, we combined micropatterning of purified α-catenin and spontaneous actomyosin network assembly in the presence of G-actin and Myosin II with microforce sensor arrays used so far to measure cell-generated forces. Using this method, we show that self-organizing actomyosin bundles bound to micrometric α-catenin patches can apply near-nanoNewton forces, paving the way for future studies on molecular/cellular mechanotarnsduction and mechanosensing. This article is protected by copyright. All rights reserved.