Force Feedback Controls Motor Activity and Mechanical Properties of Self-Assembling Branched Actin Networks

Molecular mechanism of actin filament elongation by formins

Formins control the assembly of actin filaments (F-actin) that drive cell morphogenesis and motility in eukaryotes. However, their molecular interaction with F-actin and their mechanism of action remain unclear. In this work, we present high-resolution cryo-electron microscopy structures of F-actin barbed ends bound by three distinct formins, revealing a common asymmetric formin conformation imposed by the filament. Formation of new intersubunit contacts during actin polymerization sterically displaces formin and triggers its translocation. This "undock-and-lock" mechanism explains how actin-filament growth is coordinated with formin movement. Filament elongation speeds are controlled by the positioning and stability of actin-formin interfaces, which distinguish fast and slow formins. Furthermore, we provide a structure of the actin-formin-profilin ring complex, which resolves how profilin is rapidly released from the barbed end during filament elongation.

Local monomer levels and established filaments potentiate non-muscle myosin 2 assembly

The ability to dynamically assemble contractile networks is required throughout cell physiology, yet direct biophysical mechanisms regulating non-muscle myosin 2 filament assembly in living cells are lacking. Here, we use a suite of dynamic, quantitative imaging approaches to identify deterministic factors that drive myosin filament appearance and amplification. We find that actin dynamics regulate myosin assembly, but that the static actin architecture plays a less clear role. Instead, remodeling of actin networks modulates the local myosin monomer levels and facilitates assembly through myosin:myosin-driven interactions. Using optogenetically controlled myosin, we demonstrate that locally concentrating myosin is sufficient to both form filaments and jump-start filament amplification and partitioning. By counting myosin monomers within filaments, we demonstrate a myosin-facilitated assembly process that establishes filament stacks prior to partitioning into clusters that feed higher-order networks. Together, these findings establish the biophysical mechanisms regulating the assembly of non-muscle contractile structures that are ubiquitous throughout cell biology.

Release of extracellular vesicles triggered by low-intensity pulsed ultrasound: immediate and delayed reactions

Previous studies have shown that ultrasound may stimulate the release of extracellular vesicles, improving the efficiency of tumor detection. However, it is unclear whether ultrasonic stimulation affects the distribution of extracellular vesicles, and the duration of such stimulation release has not been extensively studied. In this study, we stimulated cells with low-intensity pulsed ultrasound and used liposomes containing black hole quenchers to simulate natural extracellular vesicles, confirming that ultrasound has a destructive effect on vesicles and thus affects particle size distribution. Furthermore, we used proteomics technology to examine the protein expression profile of small vesicles and discovered that the expression of proteins involved in exosome biogenesis was down-regulated. We then looked into the regulation of the actin cytoskeleton and endocytosis pathways, which are required for intracellular vesicle transport, and discovered that ultrasound might induce F-actin depolymerization. The intracellular transport of the cation-independent mannose-6-phosphate receptor (CI-MPR) in the trans-Golgi network (TGN) and the amount of Rab7a protein were proportional to the culture time after LIPUS treatment.

A Hierarchical Mechanotransduction System: From Macro to Micro

Mechanotransduction is a strictly regulated process whereby mechanical stimuli, including mechanical forces and properties, are sensed and translated into biochemical signals. Increasing data demonstrate that mechanotransduction is crucial for regulating macroscopic and microscopic dynamics and functionalities. However, the actions and mechanisms of mechanotransduction across multiple hierarchies, from molecules, subcellular structures, cells, tissues/organs, to the whole-body level, have not been yet comprehensively documented. Herein, the biological roles and operational mechanisms of mechanotransduction from macro to micro are revisited, with a focus on the orchestrations across diverse hierarchies. The implications, applications, and challenges of mechanotransduction in human diseases are also summarized and discussed. Together, this knowledge from a hierarchical perspective has the potential to refresh insights into mechanotransduction regulation and disease pathogenesis and therapy, and ultimately revolutionize the prevention, diagnosis, and treatment of human diseases.

The stabilization of Arp2/3 complex generated actin filaments

The Arp2/3 complex, which generates both branched but also linear actin filaments via activation of SPIN90, is evolutionarily conserved in eukaryotes. Several factors regulate the stability of filaments generated by the Arp2/3 complex to maintain the dynamics and architecture of actin networks. In this review, we summarise recent studies on the molecular mechanisms governing the tuning of Arp2/3 complex nucleated actin filaments, which includes investigations using microfluidics and single-molecule imaging to reveal the mechanosensitivity, dissociation and regeneration of actin branches. We also discuss the high-resolution cryo-EM structure of cortactin bound to actin branches, as well as the differences and similarities between the stability of Arp2/3 complex nucleated branches and linear filaments. These new studies provide a clearer picture of the stabilisation of Arp2/3 nucleated filaments at the molecular level. We also identified gaps in our understanding of how different factors collectively contribute to the stabilisation of Arp2/3 complex-generated actin networks.

Adaptive nonequilibrium design of actin-based metamaterials: Fundamental and practical limits of control

The adaptive and surprising emergent properties of biological materials self-assembled in far-from-equilibrium environments serve as an inspiration for efforts to design nanomaterials. In particular, controlling the conditions of self-assembly can modulate material properties, but there is no systematic understanding of either how to parameterize external control or how controllable a given material can be. Here, we demonstrate that branched actin networks can be encoded with metamaterial properties by dynamically controlling the applied force under which they grow and that the protocols can be selected using multi-task reinforcement learning. These actin networks have tunable responses over a large dynamic range depending on the chosen external protocol, providing a pathway to encoding "memory" within these structures. Interestingly, we obtain a bound that relates the dissipation rate and the rate of "encoding" that gives insight into the constraints on control-both physical and information theoretical. Taken together, these results emphasize the utility and necessity of nonequilibrium control for designing self-assembled nanostructures.

Controlling Physical and Biochemical Parameters of Actin Nucleation Using a Patterned Model Lipid Membrane

Self-assembly of nanoscale actin cytoskeletal proteins into filamentous networks requires organizing actin nucleation areas on the plasma membrane through recruiting actin nucleators and nucleation-promoting factors (NPFs) to the areas. To investigate impacts of the nucleation geometry on actin network assembly, we localized NPF or nucleator on defined micropatterns of laterally mobile lipid bilayers confined in a framework of a polymerized lipid bilayer. We demonstrated that actin network assembly in purified protein mixtures was confined on NPF- or nucleator-localized fluid bilayers. By controlling the shape and size of nucleation areas as well as the density and types of localized NPFs and nucleators, we showed that these parameters regulate actin network architectures. Actin network assembly in Xenopus egg extracts was also spatially controlled by patterning bilayers containing phosphatidylinositol 4,5-bisphoshate (PI(4,5)P2), an essential lipid signaling mediator. Therefore, the system provides a promising platform to investigate the physical and biochemical principles for actin network assembly.

Myosin-I Synergizes with Arp2/3 Complex to Enhance Pushing Forces of Branched Actin Networks

Myosin-Is colocalize with Arp2/3 complex-nucleated actin networks at sites of membrane protrusion and invagination, but the mechanisms by which myosin-I motor activity coordinates with branched actin assembly to generate force are unknown. We mimicked the interplay of these proteins using the "comet tail" bead motility assay, where branched actin networks are nucleated by Arp2/3 complex on the surface of beads coated with myosin-I and the WCA domain of N-WASP. We observed that myosin-I increased bead movement efficiency by thinning actin networks without affecting growth rates. Remarkably, myosin-I triggered symmetry breaking and comet-tail formation in dense networks resistant to spontaneous fracturing. Even with arrested actin assembly, myosin-I alone could break the network. Computational modeling recapitulated these observations suggesting myosin-I acts as a repulsive force shaping the network's architecture and boosting its force-generating capacity. We propose that myosin-I leverages its power stroke to amplify the forces generated by Arp2/3 complex-nucleated actin networks.

Rearrangement of GUV-confined actin networks in response to micropipette aspiration

Although diverse actin network architectures found inside the cell have been individually reconstituted outside of the cell, how different types of actin architectures reorganize under applied forces is not entirely understood. Recently, bottom-up reconstitution has enabled studies where dynamic and phenotypic characteristics of various actin networks can be recreated in an isolated cell-like environment. Here, by creating a giant unilamellar vesicle (GUV)-based cell model encapsulating actin networks, we investigate how actin networks rearrange in response to localized stresses applied by micropipette aspiration. We reconstitute actin bundles and branched bundles in GUVs separately and mechanically perturb them. Interestingly, we find that, when aspirated, protrusive actin bundles that are otherwise randomly oriented in the GUV lumen collapse and align along the axis of the micropipette. However, when branched bundles are aspirated, the network remains intact and outside of the pipette while the GUV membrane is aspirated into the micropipette. These results reveal distinct responses in the rearrangement of actin networks in a network architecture-dependent manner when subjected to physical forces.

Regeneration of actin filament branches from the same Arp2/3 complex

Branched actin filaments are found in many key cellular structures. Branches are nucleated by the Arp2/3 complex activated by nucleation-promoting factor (NPF) proteins and bound to the side of preexisting "mother" filaments. Over time, branches dissociate from their mother filament, leading to network reorganization and turnover, but this mechanism is less understood. Here, using microfluidics and purified proteins, we examined the dissociation of individual branches under controlled biochemical and mechanical conditions. We observe that the Arp2/3 complex remains bound to the mother filament after most debranching events, even when accelerated by force. Strikingly, this surviving Arp2/3 complex readily nucleates a new actin filament branch, without being activated anew by an NPF: It simply needs to exchange its nucleotide and bind an actin monomer. The protein glia maturation factor (GMF), which accelerates debranching, prevents branch renucleation. Our results suggest that actin filament renucleation can provide a self-repair mechanism, helping branched networks to sustain mechanical stress in cells over extended periods of time.

Myosin-independent stiffness sensing by fibroblasts is regulated by the viscoelasticity of flowing actin

The stiffness of the extracellular matrix induces differential tension within integrin-based adhesions, triggering differential mechanoresponses. However, it has been unclear if the stiffness-dependent differential tension is induced solely by myosin activity. Here, we report that in the absence of myosin contractility, 3T3 fibroblasts still transmit stiffness-dependent differential levels of traction. This myosin-independent differential traction is regulated by polymerizing actin assisted by actin nucleators Arp2/3 and formin where formin has a stronger contribution than Arp2/3 to both traction and actin flow. Intriguingly, despite only slight changes in F-actin flow speed observed in cells with the combined inhibition of Arp2/3 and myosin compared to cells with sole myosin inhibition, they show a 4-times reduction in traction than cells with myosin-only inhibition. Our analyses indicate that traditional models based on rigid F-actin are inadequate for capturing such dramatic force reduction with similar actin flow. Instead, incorporating the F-actin network's viscoelastic properties is crucial. Our new model including the F-actin viscoelasticity reveals that Arp2/3 and formin enhance stiffness sensitivity by mechanically reinforcing the F-actin network, thereby facilitating more effective transmission of flow-induced forces. This model is validated by cell stiffness measurement with atomic force microscopy and experimental observation of model-predicted stiffness-dependent actin flow fluctuation.

Drive-specific selection in multistable mechanical networks

Systems with many stable configurations abound in nature, both in living and inanimate matter, encoding a rich variety of behaviors. In equilibrium, a multistable system is more likely to be found in configurations with lower energy, but the presence of an external drive can alter the relative stability of different configurations in unexpected ways. Living systems are examples par excellence of metastable nonequilibrium attractors whose structure and stability are highly dependent on the specific form and pattern of the energy flow sustaining them. Taking this distinctively lifelike behavior as inspiration, we sought to investigate the more general physical phenomenon of drive-specific selection in nonequilibrium dynamics. To do so, we numerically studied driven disordered mechanical networks of bistable springs possessing a vast number of stable configurations arising from the two stable rest lengths of each spring, thereby capturing the essential physical properties of a broad class of multistable systems. We found that there exists a range of forcing amplitudes for which the attractor states of driven disordered multistable mechanical networks are fine-tuned with respect to the pattern of external forcing to have low energy absorption from it. Additionally, we found that these drive-specific attractor states are further stabilized by precise matching between the multidimensional shape of their orbit and that of the potential energy well they inhabit. Lastly, we showed evidence of drive-specific selection in an experimental system and proposed a general method to estimate the range of drive amplitudes for drive-specific selection.

T cell migration requires ion and water influx to regulate actin polymerization

Migration of T cells is essential for their ability to mount immune responses. Chemokine-induced T cell migration requires WNK1, a kinase that regulates ion influx into the cell. However, it is not known why ion entry is necessary for T cell movement. Here we show that signaling from the chemokine receptor CCR7 leads to activation of WNK1 and its downstream pathway at the leading edge of migrating CD4+ T cells, resulting in ion influx and water entry by osmosis. We propose that WNK1-induced water entry is required to swell the membrane at the leading edge, generating space into which actin filaments can polymerize, thereby facilitating forward movement of the cell. Given the broad expression of WNK1 pathway proteins, our study suggests that ion and water influx are likely to be essential for migration in many cell types, including leukocytes and metastatic tumor cells.

Reconstitution of actin-based cellular processes: Why encapsulation changes the rules

While in vitro reconstitution of cellular processes is progressing rapidly, the encapsulation of biomimetic systems to reproduce the cellular environment is a major challenge. Here we review the difficulties, using reconstitution of processes dependent on actin polymerization as an example. Some of the problems are purely technical, due to the need for engineering strategies to encapsulate concentrated solutions in micrometer-sized compartments. However, other significant issues arise from the reduction of experimental volumes, which alters the chemical evolution of these non-equilibrium systems. Important parameters to consider for successful reconstitutions are the amount of each component, their consumption and renewal rates to guarantee their continuous availability.

Palmitate-induced insulin resistance causes actin filament stiffness and GLUT4 mis-sorting without altered Akt signalling

Skeletal muscle insulin resistance, a major contributor to type 2 diabetes, is linked to the consumption of saturated fats. This insulin resistance arises from failure of insulin-induced translocation of glucose transporter type 4 (GLUT4; also known as SLC2A4) to the plasma membrane to facilitate glucose uptake into muscle. The mechanisms of defective GLUT4 translocation are poorly understood, limiting development of insulin-sensitizing therapies targeting muscle glucose uptake. Although many studies have identified early insulin signalling defects and suggest that they are responsible for insulin resistance, their cause-effect has been debated. Here, we find that the saturated fat palmitate (PA) causes insulin resistance owing to failure of GLUT4 translocation in skeletal muscle myoblasts and myotubes without impairing signalling to Akt2 or AS160 (also known as TBC1D4). Instead, PA altered two basal-state events: (1) the intracellular localization of GLUT4 and its sorting towards a perinuclear storage compartment, and (2) actin filament stiffness, which prevents Rac1-dependent actin remodelling. These defects were triggered by distinct mechanisms, respectively protein palmitoylation and endoplasmic reticulum (ER) stress. Our findings highlight that saturated fats elicit muscle cell-autonomous dysregulation of the basal-state machinery required for GLUT4 translocation, which 'primes' cells for insulin resistance.

The evolution and diversity of actin-dependent cell migration

Many eukaryotic cells, including animal cells and unicellular amoebae, use dynamic-actin networks to crawl across solid surfaces. Recent discoveries of actin-dependent crawling in additional lineages have sparked interest in understanding how and when this type of motility evolved. Tracing the evolution of cell crawling requires understanding the molecular mechanisms underlying motility. Here we outline what is known about the diversity and evolution of the molecular mechanisms that drive cell motility, with a focus on actin-dependent crawling. Classic studies and recent work have revealed a surprising number of distinct mechanical modes of actin-dependent crawling used by different cell types and species to navigate different environments. The overlap in actin network regulators driving multiple types of actin-dependent crawling, along with cortical-actin networks that support the plasma membrane in these cells, suggest that actin motility and cortical actin networks might have a common evolutionary origin. The rapid development of additional evolutionarily diverse model systems, advanced imaging technologies, and CRISPR-based genetic tools, is opening the door to testing these and other new ideas about the evolution of actin-dependent cell crawling.

ActuAtor, a Listeria-inspired molecular tool for physical manipulation of intracellular organizations through de novo actin polymerization

Form and function are often interdependent throughout biology. Inside cells, mitochondria have particularly attracted attention since both their morphology and functionality are altered under pathophysiological conditions. However, directly assessing their causal relationship has been beyond reach due to the limitations of manipulating mitochondrial morphology in a physiologically relevant manner. By engineering a bacterial actin regulator, ActA, we developed tools termed "ActuAtor" that inducibly trigger actin polymerization at arbitrary subcellular locations. The ActuAtor-mediated actin polymerization drives striking deformation and/or movement of target organelles, including mitochondria, Golgi apparatus, and nucleus. Notably, ActuAtor operation also disperses non-membrane-bound entities such as stress granules. We then implemented ActuAtor in functional assays, uncovering the physically fragmented mitochondria being slightly more susceptible to degradation, while none of the organelle functions tested are morphology dependent. The modular and genetically encoded features of ActuAtor should enable its application in studies of the form-function interplay in various intracellular contexts.

Endocytic myosin-1 is a force-insensitive, power-generating motor

Myosins are required for clathrin-mediated endocytosis, but their precise molecular roles in this process are not known. This is, in part, because the biophysical properties of the relevant motors have not been investigated. Myosins have diverse mechanochemical activities, ranging from powerful contractility against mechanical loads to force-sensitive anchoring. To better understand the essential molecular contribution of myosin to endocytosis, we studied the in vitro force-dependent kinetics of the Saccharomyces cerevisiae endocytic type I myosin called Myo5, a motor whose role in clathrin-mediated endocytosis has been meticulously studied in vivo. We report that Myo5 is a low-duty-ratio motor that is activated ∼10-fold by phosphorylation and that its working stroke and actin-detachment kinetics are relatively force-insensitive. Strikingly, the in vitro mechanochemistry of Myo5 is more like that of cardiac myosin than that of slow anchoring myosin-1s found on endosomal membranes. We, therefore, propose that Myo5 generates power to augment actin assembly-based forces during endocytosis in cells.

A mechanosensing mechanism controls plasma membrane shape homeostasis at the nanoscale

As cells migrate and experience forces from their surroundings, they constantly undergo mechanical deformations which reshape their plasma membrane (PM). To maintain homeostasis, cells need to detect and restore such changes, not only in terms of overall PM area and tension as previously described, but also in terms of local, nanoscale topography. Here, we describe a novel phenomenon, by which cells sense and restore mechanically induced PM nanoscale deformations. We show that cell stretch and subsequent compression reshape the PM in a way that generates local membrane evaginations in the 100 nm scale. These evaginations are recognized by I-BAR proteins, which triggers a burst of actin polymerization mediated by Rac1 and Arp2/3. The actin polymerization burst subsequently re-flattens the evagination, completing the mechanochemical feedback loop. Our results demonstrate a new mechanosensing mechanism for PM shape homeostasis, with potential applicability in different physiological scenarios.

Competition and Synergy of Arp2/3 and Formins in Nucleating Actin Waves

The assembly and disassembly of actin filaments and their regulatory proteins are crucial for maintaining cell structure or changing physiological state. However, because of the tremendous global impact of actin on diverse cellular processes, dissecting the specific role of actin regulatory proteins remains challenging. In this study, we employ actin waves that propagate on the cortex of mast cell to investigate the interplay between formins and the Arp2/3 complex in the nucleating and turnover of cortical actin. Our findings reveal that the recruitment of FMNL1 and mDia3 precedes the Arp2/3 complex in cortical actin waves. Membrane and GTPase-interaction can drive oscillations of FMNL1 in an actin-dependent manner, but active Cdc42 waves or constitutively-active FMNL1 mutant can form without actin waves. In addition to the apparent coordinated assembly of formins and Arp2/3, we further reveal their antagonism, where inhibition of Arp2/3 complex by CK-666 led to a transient increase in the recruitment of formins and actin polymerization. Our analysis suggest that the antagonism could not be explained for the competition between FMNL1 and Arp2/3 for monomeric actin. Rather, it is regulated by a limited pool of their common upstream regulator, Cdc42, whose level is negatively regulated by Arp2/3. Collectively, our study highlights the multifaceted interactions, cooperative or competitive, between formins and Arp2/3 complex, in the intricate and dynamic control of actin cytoskeletal network.

Actin polymerization and depolymerization in developing vertebrates

Development is a complex process that occurs throughout the life cycle. F-actin, a major component of the cytoskeleton, is essential for the morphogenesis of tissues and organs during development. F-actin is formed by the polymerization of G-actin, and the dynamic balance of polymerization and depolymerization ensures proper cellular function. Disruption of this balance results in various abnormalities and defects or even embryonic lethality. Here, we reviewed recent findings on the structure of G-actin and F-actin and the polymerization of G-actin to F-actin. We also focused on the functions of actin isoforms and the underlying mechanisms of actin polymerization/depolymerization in cellular and organic morphogenesis during development. This information will extend our understanding of the role of actin polymerization in the physiologic or pathologic processes during development and may open new avenues for developing therapeutics for embryonic developmental abnormalities or tissue regeneration.

Polymerization force-regulated actin filament-Arp2/3 complex interaction dominates self-adaptive cell migrations

Cells migrate by adapting their leading-edge behaviors to heterogeneous extracellular microenvironments (ECMs) during cancer invasions and immune responses. Yet it remains poorly understood how such complicated dynamic behaviors emerge from millisecond-scale assembling activities of protein molecules, which are hard to probe experimentally. To address this gap, we establish a spatiotemporal "resistance-adaptive propulsion" theory based on the interactions between Arp2/3 complexes and polymerizing actin filaments and a multiscale dynamic modeling system spanning from molecular proteins to the cell. We quantitatively find that cells can accurately self-adapt propulsive forces to overcome heterogeneous ECMs via a resistance-triggered positive feedback mechanism, dominated by polymerization-induced actin filament bending and the bending-regulated actin-Arp2/3 binding. However, for high resistance regions, resistance triggers a negative feedback, hindering branched filament assembly, which adapts cellular morphologies to circumnavigate the obstacles. Strikingly, the synergy of the two opposite feedbacks not only empowers the cell with both powerful and flexible migratory capabilities to deal with complex ECMs but also enables efficient utilization of intracellular proteins by the cell. In addition, we identify that the nature of cell migration velocity depending on ECM history stems from the inherent temporal hysteresis of cytoskeleton remodeling. We also show that directional cell migration is dictated by the competition between the local stiffness of ECMs and the local polymerizing rate of actin network caused by chemotactic cues. Our results reveal that it is the polymerization force-regulated actin filament-Arp2/3 complex binding interaction that dominates self-adaptive cell migrations in complex ECMs, and we provide a predictive theory and a spatiotemporal multiscale modeling system at the protein level.

A platform for dissecting force sensitivity and multivalency in actin networks

The physical structure and dynamics of cells are supported by micron-scale actin networks with diverse geometries, protein compositions, and mechanical properties. These networks are composed of actin filaments and numerous actin binding proteins (ABPs), many of which engage multiple filaments simultaneously to crosslink them into specific functional architectures. Mechanical force has been shown to modulate the interactions between several ABPs and individual actin filaments, but it is unclear how this phenomenon contributes to the emergent force-responsive functional dynamics of actin networks. Here, we engineer filament linker complexes and combine them with photo-micropatterning of myosin motor proteins to produce an in vitro reconstitution platform for examining how force impacts the behavior of ABPs within multi-filament assemblies. Our system enables the monitoring of dozens of actin networks with varying architectures simultaneously using total internal reflection fluorescence microscopy, facilitating detailed dissection of the interplay between force-modulated ABP binding and network geometry. We apply our system to study a dimeric form of the critical cell-cell adhesion protein α-catenin, a model force-sensitive ABP. We find that myosin forces increase α-catenin's engagement of small filament bundles embedded within networks. This activity is absent in a force-sensing deficient mutant, whose binding scales linearly with bundle size in both the presence and absence of force. These data are consistent with filaments in smaller bundles bearing greater per-filament loads that enhance α-catenin binding, a mechanism that could equalize α-catenin's distribution across actin-myosin networks of varying sizes in cells to regularize their stability and composition.

Local Monomer Levels and Established Filaments Potentiate Non-Muscle Myosin 2 Assembly

The ability to dynamically assemble contractile networks is required throughout cell physiology, yet the biophysical mechanisms regulating non-muscle myosin 2 filament assembly in living cells are lacking. Here we use a suite of dynamic, quantitative imaging approaches to identify deterministic factors that drive myosin filament appearance and amplification. We find that actin dynamics regulate myosin assembly, but that the actin architecture plays a minimal direct role. Instead, remodeling of actin networks modulates the local myosin monomer levels and facilitates assembly through myosin:myosin driven interactions. Using optogenetically controlled myosin, we demonstrate that locally concentrating myosin is sufficient to both form filaments and jump-start filament amplification and partitioning. By counting myosin monomers within filaments, we demonstrate a myosin-facilitated assembly process that establishes sub-resolution filament stacks prior to partitioning into clusters that feed higher-order networks. Together these findings establish the biophysical mechanisms regulating the assembly of non-muscle contractile structures that are ubiquitous throughout cell biology.

Capsule self-oscillating gels showing cell-like nonthermal membrane/shape fluctuations

A primary interest in cell membrane and shape fluctuations is establishing experimental models reflecting only nonthermal active contributions. Here we report a millimeter-scaled capsule self-oscillating gel model mirroring the active contribution effect on cell fluctuations. In the capsule self-oscillating gels, the propagating chemical signals during a Belousov-Zhabotinsky (BZ) reaction induce simultaneous local deformations in the various regions, showing cell-like shape fluctuations. The capsule self-oscillating gels do not fluctuate without the BZ reaction, implying that only the active chemical parameter induces the gel fluctuations. The period and amplitude depend on the gel layer thickness and the concentration of the chemical substrate for the BZ reaction. Our results allow for a solid experimental platform showing actively driven cell-like fluctuations, which can potentially contribute to investigating the active parameter effect on cell fluctuations.

Membrane tension induces F-actin reorganization and flow in a biomimetic model cortex

The accumulation and transmission of mechanical stresses in the cell cortex and membrane determines the mechanics of cell shape and coordinates essential physical behaviors, from cell polarization to cell migration. However, the extent that the membrane and cytoskeleton each contribute to the transmission of mechanical stresses to coordinate diverse behaviors is unclear. Here, we reconstitute a minimal model of the actomyosin cortex within liposomes that adheres, spreads and ultimately ruptures on a surface. During spreading, accumulated adhesion-induced (passive) stresses within the membrane drive changes in the spatial assembly of actin. By contrast, during rupture, accumulated myosin-induced (active) stresses within the cortex determine the rate of pore opening. Thus, in the same system, devoid of biochemical regulation, the membrane and cortex can each play a passive or active role in the generation and transmission of mechanical stress, and their relative roles drive diverse biomimetic physical behaviors.

Recycling of the actin monomer pool limits the lifetime of network turnover

Intracellular organization is largely mediated by actin turnover. Cellular actin networks continuously assemble and disassemble, while maintaining their overall appearance. This behavior, called "dynamic steady state," allows cells to sense and adapt to their environment. However, how structural stability can be maintained during the constant turnover of a limited actin monomer pool is poorly understood. To answer this question, we developed an experimental system where polystyrene beads are propelled by an actin comet in a microwell containing a limited amount of components. We used the speed and the size of the actin comet tails to evaluate the system's monomer consumption and its lifetime. We established the relative contribution of actin assembly, disassembly, and recycling for a bead movement over tens of hours. Recycling mediated by cyclase-associated protein (CAP) is the key step in allowing the reuse of monomers for multiple assembly cycles. ATP supply and protein aging are also factors that limit the lifetime of actin turnover. This work reveals the balancing mechanism for long-term network assembly with a limited amount of building blocks.

On multistability and constitutive relations of cell motion on fibronectin lanes

Cell motility on flat substrates exhibits coexisting steady and oscillatory morphodynamics, the biphasic adhesion-velocity relation, and the universal correlation between speed and persistence (UCSP) as simultaneous observations common to many cell types. Their universality and concurrency suggest a unifying mechanism causing all three of them. Stick-slip models for cells on one-dimensional lanes suggest multistability to arise from the nonlinear friction of retrograde flow. This study suggests a mechanical mechanism controlled by integrin signaling on the basis of a biophysical model and analysis of trajectories of MDA-MB-231 cells on fibronectin lanes, which additionally explains the constitutive relations. The experiments exhibit cells with steady or oscillatory morphodynamics and either spread or moving with spontaneous transitions between the dynamic regimes, spread and moving, and spontaneous direction reversals. Our biophysical model is based on the force balance at the protrusion edge, the noisy clutch of retrograde flow, and a response function of friction and membrane drag to integrin signaling. The theory reproduces the experimentally observed cell states, characteristics of oscillations, and state probabilities. Analysis of experiments with the biophysical model establishes a stick-slip oscillation mechanism, and explains multistability of cell states and the statistics of state transitions. It suggests protrusion competition to cause direction reversal events, the statistics of which explain the UCSP. The effect of integrin signaling on drag and friction explains the adhesion-velocity relation and cell behavior at fibronectin density steps. The dynamics of our mechanism are nonlinear flow mechanics driven by F-actin polymerization and shaped by the noisy clutch of retrograde flow friction, protrusion competition via membrane tension, and drag forces. Integrin signaling controls the parameters of the mechanical system.

Novel imaging methods and force probes for molecular mechanobiology of cytoskeleton and adhesion

Detection and conversion of mechanical forces into biochemical signals is known as mechanotransduction. From cells to tissues, mechanotransduction regulates migration, proliferation, and differentiation in processes such as immune responses, development, and cancer progression. Mechanosensitive structures such as integrin adhesions, the actin cortex, ion channels, caveolae, and the nucleus sense and transmit forces. In vitro approaches showed that mechanosensing is based on force-dependent protein deformations and reorganizations. However, the mechanisms in cells remained unclear since cell imaging techniques lacked molecular resolution. Thanks to recent developments in super-resolution microscopy (SRM) and molecular force sensors, it is possible to obtain molecular insight of mechanosensing in live cells. We discuss how understanding of molecular mechanotransduction was revolutionized by these innovative approaches, focusing on integrin adhesions, actin structures, and the plasma membrane.

Deciphering the molecular mechanisms of actin cytoskeleton regulation in cell migration using cryo-EM

The actin cytoskeleton plays a key role in cell migration and cellular morphodynamics in most eukaryotes. The ability of the actin cytoskeleton to assemble and disassemble in a spatiotemporally controlled manner allows it to form higher-order structures, which can generate forces required for a cell to explore and navigate through its environment. It is regulated not only via a complex synergistic and competitive interplay between actin-binding proteins (ABP), but also by filament biochemistry and filament geometry. The lack of structural insights into how geometry and ABPs regulate the actin cytoskeleton limits our understanding of the molecular mechanisms that define actin cytoskeleton remodeling and, in turn, impact emerging cell migration characteristics. With the advent of cryo-electron microscopy (cryo-EM) and advanced computational methods, it is now possible to define these molecular mechanisms involving actin and its interactors at both atomic and ultra-structural levels in vitro and in cellulo. In this review, we will provide an overview of the available cryo-EM methods, applicable to further our understanding of the actin cytoskeleton, specifically in the context of cell migration. We will discuss how these methods have been employed to elucidate ABP- and geometry-defined regulatory mechanisms in initiating, maintaining, and disassembling cellular actin networks in migratory protrusions.

From WRC to Arp2/3: Collective molecular mechanisms of branched actin network assembly

Branched actin networks have emerged as major force-generating structures driving the protrusions in various distinct cell types and processes, ranging from lamellipodia operating in mesenchymal and epithelial cell migration or tails pushing intracellular pathogens and vesicles to developing spine heads on neurons. Many key molecular features are conserved among all those Arp2/3 complex-containing, branched actin networks. Here, we will review recent progress in our molecular understanding of the core biochemical machinery driving branched actin nucleation, from the generation of filament primers to Arp2/3 activator recruitment, regulation and turnover. Due to the wealth of information on distinct, Arp2/3 network-containing structures, we are largely focusing-in an exemplary fashion-on canonical lamellipodia of mesenchymal cells, which are regulated by Rac GTPases, their downstream effector WAVE Regulatory Complex and its target Arp2/3 complex. Novel insight additionally confirms that WAVE and Arp2/3 complexes regulate or are themselves tuned by additional prominent actin regulatory factors, including Ena/VASP family members and heterodimeric capping protein. Finally, we are considering recent insights into effects exerted by mechanical force, both at the branched network and individual actin regulator level.

Twist response of actin filaments

Actin cytoskeleton force generation, sensing, and adaptation are dictated by the bending and twisting mechanics of filaments. Here, we use magnetic tweezers and microfluidics to twist and pull individual actin filaments and evaluate their response to applied loads. Twisted filaments bend and dissipate torsional strain by adopting a supercoiled plectoneme. Pulling prevents plectoneme formation, which causes twisted filaments to sever. Analysis over a range of twisting and pulling forces and direct visualization of filament and single subunit twisting fluctuations yield an actin filament torsional persistence length of ~10 µm, similar to the bending persistence length. Filament severing by cofilin is driven by local twist strain at boundaries between bare and decorated segments and is accelerated by low pN pulling forces. This work explains how contractile forces generated by myosin motors accelerate filament severing by cofilin and establishes a role for filament twisting in the regulation of actin filament stability and assembly dynamics.

Biomechanical, biophysical and biochemical modulators of cytoskeletal remodelling and emergent stem cell lineage commitment

Across complex, multi-time and -length scale biological systems, redundancy confers robustness and resilience, enabling adaptation and increasing survival under dynamic environmental conditions; this review addresses ubiquitous effects of cytoskeletal remodelling, triggered by biomechanical, biophysical and biochemical cues, on stem cell mechanoadaptation and emergent lineage commitment. The cytoskeleton provides an adaptive structural scaffold to the cell, regulating the emergence of stem cell structure-function relationships during tissue neogenesis, both in prenatal development as well as postnatal healing. Identification and mapping of the mechanical cues conducive to cytoskeletal remodelling and cell adaptation may help to establish environmental contexts that can be used prospectively as translational design specifications to target tissue neogenesis for regenerative medicine. In this review, we summarize findings on cytoskeletal remodelling in the context of tissue neogenesis during early development and postnatal healing, and its relevance in guiding lineage commitment for targeted tissue regeneration. We highlight how cytoskeleton-targeting chemical agents modulate stem cell differentiation and govern responses to mechanical cues in stem cells' emerging form and function. We further review methods for spatiotemporal visualization and measurement of cytoskeletal remodelling, as well as its effects on the mechanical properties of cells, as a function of adaptation. Research in these areas may facilitate translation of stem cells' own healing potential and improve the design of materials, therapies, and devices for regenerative medicine.

Moving through a changing world: Single cell migration in 2D vs. 3D

Migration of single adherent cells is frequently observed in the developing and adult organism and has been the subject of many studies. Yet, while elegant work has elucidated molecular and mechanical cues affecting motion dynamics on a flat surface, it remains less clear how cells migrate in a 3D setting. In this review, we explore the changing parameters encountered by cells navigating through a 3D microenvironment compared to cells crawling on top of a 2D surface, and how these differences alter subcellular structures required for propulsion. We further discuss how such changes at the micro-scale impact motion pattern at the macro-scale.

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.

Biochemical and mechanical regulation of actin dynamics

Polymerization of actin filaments against membranes produces force for numerous cellular processes, such as migration, morphogenesis, endocytosis, phagocytosis and organelle dynamics. Consequently, aberrant actin cytoskeleton dynamics are linked to various diseases, including cancer, as well as immunological and neurological disorders. Understanding how actin filaments generate forces in cells, how force production is regulated by the interplay between actin-binding proteins and how the actin-regulatory machinery responds to mechanical load are at the heart of many cellular, developmental and pathological processes. During the past few years, our understanding of the mechanisms controlling actin filament assembly and disassembly has evolved substantially. It has also become evident that the activities of key actin-binding proteins are not regulated solely by biochemical signalling pathways, as mechanical regulation is critical for these proteins. Indeed, the architecture and dynamics of the actin cytoskeleton are directly tuned by mechanical load. Here we discuss the general mechanisms by which key actin regulators, often in synergy with each other, control actin filament assembly, disassembly, and monomer recycling. By using an updated view of actin dynamics as a framework, we discuss how the mechanics and geometry of actin networks control actin-binding proteins, and how this translates into force production in endocytosis and mesenchymal cell migration.

Bending forces and nucleotide state jointly regulate F-actin structure

ATP-hydrolysis-coupled actin polymerization is a fundamental mechanism of cellular force generation1-3. In turn, force4,5 and actin filament (F-actin) nucleotide state6 regulate actin dynamics by tuning F-actin's engagement of actin-binding proteins through mechanisms that are unclear. Here we show that the nucleotide state of actin modulates F-actin structural transitions evoked by bending forces. Cryo-electron microscopy structures of ADP-F-actin and ADP-Pi-F-actin with sufficient resolution to visualize bound solvent reveal intersubunit interfaces bridged by water molecules that could mediate filament lattice flexibility. Despite extensive ordered solvent differences in the nucleotide cleft, these structures feature nearly identical lattices and essentially indistinguishable protein backbone conformations that are unlikely to be discriminable by actin-binding proteins. We next introduce a machine-learning-enabled pipeline for reconstructing bent filaments, enabling us to visualize both continuous structural variability and side-chain-level detail. Bent F-actin structures reveal rearrangements at intersubunit interfaces characterized by substantial alterations of helical twist and deformations in individual protomers, transitions that are distinct in ADP-F-actin and ADP-Pi-F-actin. This suggests that phosphate rigidifies actin subunits to alter the bending structural landscape of F-actin. As bending forces evoke nucleotide-state dependent conformational transitions of sufficient magnitude to be detected by actin-binding proteins, we propose that actin nucleotide state can serve as a co-regulator of F-actin mechanical regulation.

Regulation of membrane scission in yeast endocytosisDepartment of Biochemistry and National Centre of Competence in Research, Chemical Biology, University of Geneva, Geneva, Switzerland

During clathrin-mediated endocytosis, a flat plasma membrane is shaped into an invagination that undergoes scission to form a vesicle. In mammalian cells, the force that drives the transition from invagination to vesicle is primarily provided by the GTPase dynamin that acts in concert with crescent-shaped BAR domain proteins. In yeast cells, the mechanism of endocytic scission is unclear. The yeast BAR domain protein complex Rvs161/167 (Rvs) nevertheless plays an important role in this process: deletion of Rvs dramatically reduces scission efficiency. A mechanistic understanding of the influence of Rvs on scission however, remains incomplete. We used quantitative live-cell imaging and genetic manipulation to understand the recruitment and function of Rvs and other late-stage proteins at yeast endocytic sites. We found that arrival of Rvs at endocytic sites is timed by interaction of its BAR domain with specific membrane curvature. A second domain of Rvs167 - the SH3 domain - affects localization efficiency of Rvs. We show that Myo3, one of the two type-I myosins in Saccharomyces cerevisiae, has a role in recruiting Rvs167 via the SH3 domain. Removal of the SH3 domain also affects assembly and disassembly of actin and impedes membrane invagination. Our results indicate that both BAR and SH3 domains are important for the role of Rvs as a regulator of scission. We tested other proteins implicated in vesicle formation in Saccharomyces cerevisiae, and found that neither synaptojanins nor dynamin contribute directly to membrane scission. We propose that recruitment of Rvs BAR domains delays scission and allows invaginations to grow by stabilizing them. We also propose that vesicle formation is dependent on the force exerted by the actin network.

Intrinsic cell rheology drives junction maturation

A fundamental property of higher eukaryotes that underpins their evolutionary success is stable cell-cell cohesion. Yet, how intrinsic cell rheology and stiffness contributes to junction stabilization and maturation is poorly understood. We demonstrate that localized modulation of cell rheology governs the transition of a slack, undulated cell-cell contact (weak adhesion) to a mature, straight junction (optimal adhesion). Cell pairs confined on different geometries have heterogeneous elasticity maps and control their own intrinsic rheology co-ordinately. More compliant cell pairs grown on circles have slack contacts, while stiffer triangular cell pairs favour straight junctions with flanking contractile thin bundles. Counter-intuitively, straighter cell-cell contacts have reduced receptor density and less dynamic junctional actin, suggesting an unusual adaptive mechano-response to stabilize cell-cell adhesion. Our modelling informs that slack junctions arise from failure of circular cell pairs to increase their own intrinsic stiffness and resist the pressures from the neighbouring cell. The inability to form a straight junction can be reversed by increasing mechanical stress artificially on stiffer substrates. Our data inform on the minimal intrinsic rheology to generate a mature junction and provide a springboard towards understanding elements governing tissue-level mechanics.

Under the hood of a moving cell

Experiments using purified proteins reveal how the network of filaments that underlie cell movement becomes denser when pushing against a stronger mechanical force.

Single-molecule analysis of actin filament debranching by cofilin and GMF

Eukaryotic cells contain branched actin networks that are essential for endocytosis, motility, and other key cellular processes. These networks, which are formed by filamentous actin and the Arp2/3 complex, must subsequently be debranched to allow network remodeling and to recycle the Arp2/3 complex. Debranching appears to be catalyzed by two different members of the actin depolymerizing factor homology protein family: cofilin and glial maturation factor (GMF). However, their mechanisms of debranching are only partially understood. Here, we used single-molecule fluorescence imaging of Arp2/3 complex and actin filaments under physiological ionic conditions to observe debranching by GMF and cofilin. We demonstrate that cofilin, like GMF, is an authentic debrancher independent of its filament-severing activity and that the debranching activities of the two proteins are additive. While GMF binds directly to the Arp2/3 complex, cofilin selectively accumulates on branch-junction daughter filaments in tropomyosin-decorated networks just prior to debranching events. Quantitative comparison of debranching rates with the known kinetics of cofilin-actin binding suggests that cofilin occupancy of a particular single actin site at the branch junction is sufficient to trigger debranching. In rare cases in which the order of departure could be resolved during GMF- or cofilin-induced debranching, the Arp2/3 complex left the branch junction bound to the pointed end of the daughter filament, suggesting that both GMF and cofilin can work by destabilizing the mother filament-Arp2/3 complex interface. Taken together, these observations suggest that GMF and cofilin promote debranching by distinct yet complementary mechanisms.

Building the phagocytic cup on an actin scaffold

Cells ingest large particles, such as bacteria, viruses, or apoptotic cells, via the process of phagocytosis, which involves formation of an actin-rich structure known as the phagocytic cup. Phagocytic cup assembly and closure results from a concerted action of phagocytic receptors, regulators of actin polymerization, and myosin motors. Recent studies using advanced imaging approaches and biophysical techniques have revealed new information regarding phagocytic cup architecture, regulation of actin assembly, and the distribution, direction, and magnitude of the forces produced by the cytoskeletal elements that form the cup. These findings provide insights into the mechanisms leading to the assembly, expansion, and closure of phagocytic cups. The new data show that engulfment and internalization of phagocytic targets rely on several distinct yet complementary mechanisms that support the robust uptake of foreign objects and may be precisely tailored to the demands of specific phagocytic pathways.

Elasticity of podosome actin networks produces nanonewton protrusive forces

Actin filaments assemble into force-generating systems involved in diverse cellular functions, including cell motility, adhesion, contractility and division. It remains unclear how networks of actin filaments, which individually generate piconewton forces, can produce forces reaching tens of nanonewtons. Here we use in situ cryo-electron tomography to unveil how the nanoscale architecture of macrophage podosomes enables basal membrane protrusion. We show that the sum of the actin polymerization forces at the membrane is not sufficient to explain podosome protrusive forces. Quantitative analysis of podosome organization demonstrates that the core is composed of a dense network of bent actin filaments storing elastic energy. Theoretical modelling of the network as a spring-loaded elastic material reveals that it exerts forces of a few tens of nanonewtons, in a range similar to that evaluated experimentally. Thus, taking into account not only the interface with the membrane but also the bulk of the network, is crucial to understand force generation by actin machineries. Our integrative approach sheds light on the elastic behavior of dense actin networks and opens new avenues to understand force production inside cells.

Actin filaments generate force in diverse contexts, although how they can produce nanonewtons of force is unclear. Here, the authors apply cryo-electron tomography, quantitative analysis, and modelling to reveal the podosome core is a dense, spring-loaded, actin network storing elastic energy.

Cellular force-sensing through actin filaments

The actin cytoskeleton orchestrates cell mechanics and facilitates the physical integration of cells into tissues, while tissue-scale forces and extracellular rigidity in turn govern cell behaviour. Here, we discuss recent evidence that actin filaments (F-actin), the core building blocks of the actin cytoskeleton, also serve as molecular force sensors. We delineate two classes of proteins, which interpret forces applied to F-actin through enhanced binding interactions: 'mechanically tuned' canonical actin-binding proteins, whose constitutive F-actin affinity is increased by force, and 'mechanically switched' proteins, which bind F-actin only in the presence of force. We speculate mechanically tuned and mechanically switched actin-binding proteins are biophysically suitable for coordinating cytoskeletal force-feedback and mechanical signalling processes, respectively. Finally, we discuss potential mechanisms mediating force-activated actin binding, which likely occurs both through the structural remodelling of F-actin itself and geometric rearrangements of higher-order actin networks. Understanding the interplay of these mechanisms will enable the dissection of force-activated actin binding's specific biological functions.

The molecular mechanism of load adaptation by branched actin networks

Branched actin networks are self-assembling molecular motors that move biological membranes and drive many important cellular processes, including phagocytosis, endocytosis, and pseudopod protrusion. When confronted with opposing forces, the growth rate of these networks slows and their density increases, but the stoichiometry of key components does not change. The molecular mechanisms governing this force response are not well understood, so we used single-molecule imaging and AFM cantilever deflection to measure how applied forces affect each step in branched actin network assembly. Although load forces are observed to increase the density of growing filaments, we find that they actually decrease the rate of filament nucleation due to inhibitory interactions between actin filament ends and nucleation promoting factors. The force-induced increase in network density turns out to result from an exponential drop in the rate constant that governs filament capping. The force dependence of filament capping matches that of filament elongation and can be explained by expanding Brownian Ratchet theory to cover both processes. We tested a key prediction of this expanded theory by measuring the force-dependent activity of engineered capping protein variants and found that increasing the size of the capping protein increases its sensitivity to applied forces. In summary, we find that Brownian Ratchets underlie not only the ability of growing actin filaments to generate force but also the ability of branched actin networks to adapt their architecture to changing loads.

Anillin governs mitotic rounding during early epidermal development

Background

The establishment of tissue architecture requires coordination between distinct processes including basement membrane assembly, cell adhesion, and polarity; however, the underlying mechanisms remain poorly understood. The actin cytoskeleton is ideally situated to orchestrate tissue morphogenesis due to its roles in mechanical, structural, and regulatory processes. However, the function of many pivotal actin-binding proteins in mammalian development is poorly understood.

Results

Here, we identify a crucial role for anillin (ANLN), an actin-binding protein, in orchestrating epidermal morphogenesis. In utero RNAi-mediated silencing of Anln in mouse embryos disrupted epidermal architecture marked by adhesion, polarity, and basement membrane defects. Unexpectedly, these defects cannot explain the profoundly perturbed epidermis of Anln-depleted embryos. Indeed, even before these defects emerge, Anln-depleted epidermis exhibits abnormalities in mitotic rounding and its associated processes: chromosome segregation, spindle orientation, and mitotic progression, though not in cytokinesis that was disrupted only in Anln-depleted cultured keratinocytes. We further show that ANLN localizes to the cell cortex during mitotic rounding, where it regulates the distribution of active RhoA and the levels, activity, and structural organization of the cortical actomyosin proteins.

Conclusions

Our results demonstrate that ANLN is a major regulator of epidermal morphogenesis and identify a novel role for ANLN in mitotic rounding, a near-universal process that governs cell shape, fate, and tissue morphogenesis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01345-9.

Granger-causal inference of the lamellipodial actin regulator hierarchy by live cell imaging without perturbation

Many cell regulatory systems implicate nonlinearity and redundancy among components. The regulatory network governing lamellipodial and lamellar actin structures is prototypical of such a system, containing tens of actin-nucleating and -modulating molecules with functional overlap and feedback loops. Due to instantaneous and long-term compensation, phenotyping the system response to perturbation provides limited information on the roles the targeted component plays in the unperturbed system. Accordingly, how individual actin regulators contribute to lamellipodial dynamics remains ambiguous. Here, we present a perturbation-free reconstruction of cause-effect relations among actin regulators by applying Granger-causal inference to constitutive image fluctuations that indicate regulator recruitment as a proxy for activity. Our analysis identifies distinct zones of actin regulator activation and of causal effects on filament assembly and delineates actin-dependent and actin-independent regulator roles in controlling edge motion. We propose that edge motion is driven by assembly of two independently operating actin filament systems.

An actin mechanostat ensures hyphal tip sharpness in Phytophthora infestans to achieve host penetration

Filamentous plant pathogens apply mechanical forces to pierce their hosts surface and penetrate its tissues. Devastating Phytophthora pathogens harness a specialized form of invasive tip growth to slice through the plant surface, wielding their hypha as a microscopic knife. Slicing requires a sharp hyphal tip that is not blunted at the site of the mechanical interaction. How tip shape is controlled, however, is unknown. We uncover an actin-based mechanostat in Phytophthora infestans that controls tip sharpness during penetration. Mechanical stimulation of the hypha leads to the emergence of an aster-like actin configuration, which shows fast, local, and quantitative feedback to the local stress. We evidence that this functions as an adaptive mechanical scaffold that sharpens the invasive weapon and prevents it from blunting. The hyphal tip mechanostat enables the efficient conversion of turgor into localized invasive pressures that are required to achieve host penetration.