Stress fibers are generated by two distinct actin assembly mechanisms in motile cells

Actomyosin forces in cell migration: Moving beyond cell body retraction

In textbook illustrations of migrating cells, actomyosin contractility is typically depicted as the contraction force necessary for cell body retraction. This dogma has been transformed by the molecular clutch model, which acknowledges that actomyosin traction forces also generate and transmit biomechanical signals at the leading edge, enabling cells to sense and shape their migratory path in mechanically complex environments. To fulfill these complementary functions, the actomyosin system assembles a gradient of contractile energy along the front-rear axis of migratory cells. Here, we highlight the hierarchic assembly and self-regulatory network structure of the actomyosin system and explain how the kinetics of different nonmuscle myosin II (NM II) paralogs synergize during contractile force generation. Our aim is to emphasize how protrusion formation, cell adhesion, contraction, and retraction are spatiotemporally integrated during different modes of migration, including chemotaxis and durotaxis. Finally, we hypothesize how different NM II paralogs might tune aspects of migration in vivo, highlighting future research directions.

Diffusion model predicts the geometry of actin cytoskeleton from cell morphology

Cells exhibit various morphological characteristics due to their physiological activities, and changes in cell morphology are inherently accompanied by the assembly and disassembly of the actin cytoskeleton. Stress fibers are a prominent component of the actin-based intracellular structure and are highly involved in numerous physiological processes, e.g., mechanotransduction and maintenance of cell morphology. Although it is widely accepted that variations in cell morphology interact with the distribution and localization of stress fibers, it remains unclear if there are underlying geometric principles between the cell morphology and actin cytoskeleton. Here, we present a machine learning system that uses the diffusion model to convert the cell shape to the distribution and alignment of stress fibers. By training with corresponding cell shape and stress fibers datasets, our system learns the conversion to generate the stress fiber images from its corresponding cell shape. The predicted stress fiber distribution agrees well with the experimental data. With this conversion relation, our system allows for performing virtual experiments that provide a visual map showing the probability of stress fiber distribution from the virtual cell shape. Our system potentially provides a powerful approach to seek further hidden geometric principles regarding how the configuration of subcellular structures is determined by the boundary of the cell structure; for example, we found that the stress fibers of cells with small aspect ratios tend to localize at the cell edge while cells with large aspect ratios have homogenous distributions.

The Toxoplasma secreted effector TgWIP modulates dendritic cell motility by activating host tyrosine phosphatases Shp1 and Shp2

The obligate intracellular parasite Toxoplasma gondii causes life-threatening toxoplasmosis to immunocompromised individuals. The pathogenesis of Toxoplasma relies on its swift dissemination to the central nervous system through a 'Trojan Horse' mechanism using infected leukocytes as carriers. Previous work found TgWIP, a protein secreted from Toxoplasma, played a role in altering the actin cytoskeleton and promoting cell migration in infected dendritic cells (DCs). However, the mechanism behind these changes was unknown. Here, we report that TgWIP harbors two SH2-binding motifs that interact with tyrosine phosphatases Shp1 and Shp2, leading to phosphatase activation. DCs infected with Toxoplasma exhibited hypermigration, accompanying enhanced F-actin stress fibers and increased membrane protrusions such as filopodia and pseudopodia. By contrast, these phenotypes were abrogated in DCs infected with Toxoplasma expressing a mutant TgWIP lacking the SH2-binding motifs. We further demonstrated that the Rho-associated kinase (Rock) is involved in the induction of these phenotypes, in a TgWIP-Shp1/2 dependent manner. Collectively, the data uncover a molecular mechanism by which TgWIP modulates the migration dynamics of infected DCs in vitro.

The WAVE complex forms linear arrays at negative membrane curvature to instruct lamellipodia formation

Cells generate a wide range of actin-based membrane protrusions for various cell behaviors. These protrusions are organized by different actin nucleation promoting factors. For example, N-WASP controls finger-like filopodia, whereas the WAVE complex controls sheet-like lamellipodia. These different membrane morphologies likely reflect different patterns of nucleator self-organization. N-WASP phase separation has been successfully studied through biochemical reconstitutions, but how the WAVE complex self-organizes to instruct lamellipodia is unknown. Because WAVE complex self-organization has proven refractory to cell-free studies, we leverage in vivo biochemical approaches to investigate WAVE complex organization within its native cellular context. With single molecule tracking and molecular counting, we show that the WAVE complex forms highly regular multilayered linear arrays at the plasma membrane that are reminiscent of a microtubule-like organization. Similar to the organization of microtubule protofilaments in a curved array, membrane curvature is both necessary and sufficient for formation of these WAVE complex linear arrays, though actin polymerization is not. This dependency on negative membrane curvature could explain both the templating of lamellipodia and their emergent behaviors, including barrier avoidance. Our data uncover the key biophysical properties of mesoscale WAVE complex patterning and highlight an integral relationship between NPF self-organization and cell morphogenesis.

Focal adhesion and actin orientation regulated by cellular geometry determine stem cell differentiation via mechanotransduction

Tuning cell adhesion geometry can affect cytoskeleton organization and the distribution of cytoskeleton forces, which play critical roles in controlling cell functions. To elucidate the geometrical relationship with cytoskeleton force distribution, it is necessary to control cell morphology. In this study, a series of dextral vortex micropatterns were prepared to precisely control cell morphology for investigating the influence of the curvature degree of adhesion curves on intracellular force distribution and stem cell differentiation at a sub-cellular level. Peripherial actin filaments of micropatterned cells were assembled along the adhesion curves and showed different orientations, filament thicknesses and densities. Focal adhesion and cytoskeleton force distribution were dependent on the curvature degree. Intracellular force distribution was also regulated by adhesion curves. The cytoskeleton and force distribution affected the osteogenic differentiation of mesenchymal stem cells through a YAP/TAZ-mediated mechanotransduction process. Thus, regulation of cell adhesion curvature, especially at cytoskeletal filament level, is critical for cell function manipulation. STATEMENT OF SIGNIFICANCE: In this study, a series of dextral micro-vortexes were prepared and used for the culture of human mesenchymal stem cells (hMSCs) to precisely control adhesive curvatures (0°, 30°, 60°, and 90°). The single MSCs on the micropatterns had the same size and shape but showed distinct focal adhesion (FA) and cytoskeleton orientations. Cellular nanomechanics were observed to be correlated with the curvature degrees, subsequently influencing nuclear morphological features. As a consequence, the localization of the mechanotransduction sensor and activator-YAP/TAZ was affected, influencing osteogenic differentiation. The results revealed the pivotal role of adhesive curvatures in the manipulation of stem cell differentiation via the machanotransduction process, which has rarely been investigated.

The Toxoplasma secreted effector TgWIP modulates dendritic cell motility by activating host tyrosine phosphatases Shp1 and Shp2

The obligate intracellular parasite Toxoplasma gondii causes life-threatening toxoplasmosis to immunocompromised individuals. The pathogenesis of Toxoplasma relies on its swift dissemination to the central nervous system through a 'Trojan Horse' mechanism using infected leukocytes as carriers. Previous work found TgWIP, a protein secreted from Toxoplasma, played a role in altering the actin cytoskeleton and promoting cell migration in infected dendritic cells (DCs). However, the mechanism behind these changes was unknown. Here, we report that TgWIP harbors two SH2-binding motifs that interact with tyrosine phosphatases Shp1 and Shp2, leading to phosphatase activation. DCs infected with Toxoplasma exhibited hypermigration, accompanying enhanced F-actin stress fibers and increased membrane protrusions such as filopodia and pseudopodia. By contrast, these phenotypes were abrogated in DCs infected with Toxoplasma expressing a mutant TgWIP lacking the SH2-binding motifs. We further demonstrated that the Rho-associated kinase (Rock) is involved in the induction of these phenotypes, in a TgWIP-Shp1/2 dependent manner. Collectively, the data uncover a molecular mechanism by which TgWIP modulates the migration dynamics of infected DCs in vitro.

LUZP1 regulates the maturation of contractile actomyosin bundles

Contractile actomyosin bundles play crucial roles in various physiological processes, including cell migration, morphogenesis, and muscle contraction. The intricate assembly of actomyosin bundles involves the precise alignment and fusion of myosin II filaments, yet the underlying mechanisms and factors involved in these processes remain elusive. Our study reveals that LUZP1 plays a central role in orchestrating the maturation of thick actomyosin bundles. Loss of LUZP1 caused abnormal cell morphogenesis, migration, and the ability to exert forces on the environment. Importantly, knockout of LUZP1 results in significant defects in the concatenation and persistent association of myosin II filaments, severely impairing the assembly of myosin II stacks. The disruption of these processes in LUZP1 knockout cells provides mechanistic insights into the defective assembly of thick ventral stress fibers and the associated cellular contractility abnormalities. Overall, these results significantly contribute to our understanding of the molecular mechanism involved in actomyosin bundle formation and highlight the essential role of LUZP1 in this process.

A modular framework for multi-scale tissue imaging and neuronal segmentation

The development of robust tools for segmenting cellular and sub-cellular neuronal structures lags behind the massive production of high-resolution 3D images of neurons in brain tissue. The challenges are principally related to high neuronal density and low signal-to-noise characteristics in thick samples, as well as the heterogeneity of data acquired with different imaging methods. To address this issue, we design a framework which includes sample preparation for high resolution imaging and image analysis. Specifically, we set up a method for labeling thick samples and develop SENPAI, a scalable algorithm for segmenting neurons at cellular and sub-cellular scales in conventional and super-resolution STimulated Emission Depletion (STED) microscopy images of brain tissues. Further, we propose a validation paradigm for testing segmentation performance when a manual ground-truth may not exhaustively describe neuronal arborization. We show that SENPAI provides accurate multi-scale segmentation, from entire neurons down to spines, outperforming state-of-the-art tools. The framework will empower image processing of complex neuronal circuitries.

Force-activated zyxin assemblies coordinate actin nucleation and crosslinking to orchestrate stress fiber repair

As the cytoskeleton sustains cell and tissue forces, it incurs physical damage that must be repaired to maintain mechanical homeostasis. The LIM-domain protein zyxin detects force-induced ruptures in actin-myosin stress fibers, coordinating downstream repair factors to restore stress fiber integrity through unclear mechanisms. Here, we reconstitute stress fiber repair with purified proteins, uncovering detailed links between zyxin's force-regulated binding interactions and cytoskeletal dynamics. In addition to binding individual tensed actin filaments (F-actin), zyxin's LIM domains form force-dependent assemblies that bridge broken filament fragments. Zyxin assemblies engage repair factors through multi-valent interactions, coordinating nucleation of new F-actin by VASP and its crosslinking into aligned bundles by ɑ-actinin. Through these combined activities, stress fiber repair initiates within the cores of micron-scale damage sites in cells, explaining how these F-actin depleted regions are rapidly restored. Thus, zyxin's force-dependent organization of actin repair machinery inherently operates at the network scale to maintain cytoskeletal integrity.

Pathological mechanisms of cold and mechanical stress in modulating cancer progression

Environmental temperature and cellular mechanical force are the inherent factors that participate in various biological processes and regulate cancer progress, which have been hot topics worldwide. They occupy a dominant part in the cancer tissues through different approaches. However, extensive investigation regarding pathological mechanisms in the carcinogenic field. After research, we found cold stress via two means to manipulate tumors: neuroscience and mechanically sensitive ion channels (MICHs) such as TRP families to regulate the physiological and pathological activities. Excessive cold stimulation mediated neuroscience acting on every cancer stage through the hypothalamus-pituitary-adrenocorticoid (HPA) to reach the target organs. Comparatively speaking, mechanical force via Piezo of MICHs controls cancer development. The progression of cancer depends on the internal activation of proto-oncogenes and the external tumorigenic factors; the above two means eventually lead to genetic disorders at the molecular level. This review summarizes the interaction of bidirectional communication between them and the tumor. It covers the main processes from cytoplasm to nucleus related to metastasis cascade and tumor immune escape.

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.

Designed Ankyrin Repeat Proteins as Actin Labels of Distinct Cytoskeletal Structures in Living Cells

The orchestrated assembly of actin and actin-binding proteins into cytoskeletal structures coordinates cell morphology changes during migration, cytokinesis, and adaptation to external stimuli. The accurate and unbiased visualization of the diverse actin assemblies within cells is an ongoing challenge. We describe here the identification and use of designed ankyrin repeat proteins (DARPins) as synthetic actin binders. Actin-binding DARPins were identified through ribosome display and validated biochemically. When introduced or expressed inside living cells, fluorescently labeled DARPins accumulated at actin filaments, validated through phalloidin colocalization on fixed cells. Nevertheless, different DARPins displayed different actin labeling patterns: some DARPins labeled efficiently dynamic structures, such as filopodia, lamellipodia, and blebs, while others accumulated primarily in stress fibers. This differential intracellular distribution correlated with DARPin-actin binding kinetics, as measured by fluorescence recovery after photobleaching experiments. Moreover, the rapid arrest of actin dynamics induced by pharmacological treatment led to the fast relocalization of DARPins. Our data support the hypothesis that the localization of actin probes depends on the inherent dynamic movement of the actin cytoskeleton. Compared to the widely used LifeAct probe, one DARPin exhibited enhanced signal-to-background ratio while retaining a similar ability to label stress fibers. In summary, we propose DARPins as promising actin-binding proteins for labeling or manipulation in living cells.

Focal adhesions contain three specialized actin nanoscale layers

Focal adhesions (FAs) connect inner workings of cell to the extracellular matrix to control cell adhesion, migration and mechanosensing. Previous studies demonstrated that FAs contain three vertical layers, which connect extracellular matrix to the cytoskeleton. By using super-resolution iPALM microscopy, we identify two additional nanoscale layers within FAs, specified by actin filaments bound to tropomyosin isoforms Tpm1.6 and Tpm3.2. The Tpm1.6-actin filaments, beneath the previously identified α-actinin cross-linked actin filaments, appear critical for adhesion maturation and controlled cell motility, whereas the adjacent Tpm3.2-actin filament layer beneath seems to facilitate adhesion disassembly. Mechanistically, Tpm3.2 stabilizes ACF-7/MACF1 and KANK-family proteins at adhesions, and hence targets microtubule plus-ends to FAs to catalyse their disassembly. Tpm3.2 depletion leads to disorganized microtubule network, abnormally stable FAs, and defects in tail retraction during migration. Thus, FAs are composed of distinct actin filament layers, and each may have specific roles in coupling adhesions to the cytoskeleton, or in controlling adhesion dynamics.

Cell mechanics regulate the migration and invasion of hepatocellular carcinoma cells via JNK signaling

Hepatocellular carcinoma (HCC) cells, especially those with metastatic competence, show reduced stiffness compared to the non-malignant counterparts. However, it is still unclear whether and how the mechanics of HCC cells influence their migration and invasion. This study reports that HCC cells with enhanced motility show reduced mechanical stiffness and cytoskeleton, suggesting the inverse correlation between cellular stiffness and motility. Through pharmacologic and genetic approaches, inhibiting actomyosin activity reduces HCC cellular stiffness but promotes their migration and invasion, while activating it increases cell stiffness but impairs cell motility. Actomyosin regulates cell motility through the influence on cellular stiffness. Mechanistically, weakening/strengthening cells inhibits/promotes c-Jun N terminal kinase (JNK) phosphorylation, activation/inhibition of which rescues the effects of cell mechanics on their migration and invasion. Further, HCC cancer stem cells (CSCs) exhibit higher motility but lower stiffness than control cells. Increasing CSC stiffness weakens migration and invasion through the activation of JNK signaling. In conclusion, our findings unveil a new regulatory role of actomyosin-mediated cellular mechanics in tumor cell motility and present new evidence to support that tumor cell softening may be one driving force for HCC metastasis. STATEMENT OF SIGNIFICANCE: Tumor cells progressively become softened during metastasis and low cell stiffness is associated with high metastatic potential. However, it remains unclear whether tumor cell softening is a by-product of or a driving force for tumor progression. This work reports that the stiffness of hepatocellular carcinoma cells is linked to their migration and invasion. Importantly, tumor cell softening promotes migration and invasion, while cell stiffening impairs the mobility. Weakening/strengthening cells inhibits/promotes JNK phosphorylation, activation/inhibition of which rescues the effects of cell mechanics on their migration and invasion ability. Further, stiffening liver cancer stem cells attenuates their motility through activating JNK signaling. In summary, our study uncovers a previously unappreciated role of tumor cell mechanics in migration and invasion and implicates the therapeutic potential of cell mechanics in the mechanotargeting of metastasis.

Cooperative actin filament nucleation by the Arp2/3 complex and formins maintains the homeostatic cortical array in Arabidopsis epidermal cells

Precise control over how and where actin filaments are created leads to the construction of unique cytoskeletal arrays within a common cytoplasm. Actin filament nucleators are key players in this activity and include the conserved actin-related protein 2/3 (Arp2/3) complex as well as a large family of formins. In some eukaryotic cells, these nucleators compete for a common pool of actin monomers and loss of one favors the activity of the other. To test whether this mechanism is conserved, we combined the ability to image single filament dynamics in the homeostatic cortical actin array of living Arabidopsis (Arabidopsis thaliana) epidermal cells with genetic and/or small molecule inhibitor approaches to stably or acutely disrupt nucleator activity. We found that Arp2/3 mutants or acute CK-666 treatment markedly reduced the frequency of side-branched nucleation events as well as overall actin filament abundance. We also confirmed that plant formins contribute to side-branched filament nucleation in vivo. Surprisingly, simultaneous inhibition of both classes of nucleator increased overall actin filament abundance and enhanced the frequency of de novo nucleation events by an unknown mechanism. Collectively, our findings suggest that multiple actin nucleation mechanisms cooperate to generate and maintain the homeostatic cortical array of plant epidermal cells.

Asymmetric response emerges between creation and disintegration of force-bearing subcellular structures as revealed by percolation analysis

Cells dynamically remodel their internal structures by modulating the arrangement of actin filaments (AFs). In this process, individual AFs exhibit stochastic behavior without knowing the macroscopic higher-order structures they are meant to create or disintegrate, but the mechanism allowing for such stochastic process-driven remodeling of subcellular structures remains incompletely understood. Here we employ percolation theory to explore how AFs interacting only with neighboring ones without recognizing the overall configuration can nonetheless create a substantial structure referred to as stress fibers (SFs) at particular locations. We determined the interaction probabilities of AFs undergoing cellular tensional homeostasis, a fundamental property maintaining intracellular tension. We showed that the duration required for the creation of SFs is shortened by the increased amount of preexisting actin meshwork, while the disintegration occurs independently of the presence of actin meshwork, suggesting that the coexistence of tension-bearing and non-bearing elements allows cells to promptly transition to new states in accordance with transient environmental changes. The origin of this asymmetry between creation and disintegration, consistently observed in actual cells, is elucidated through a minimal model analysis by examining the intrinsic nature of mechano-signal transmission. Specifically, unlike the symmetric case involving biochemical communication, physical communication to sense environmental changes is facilitated via AFs under tension, while other free AFs dissociated from tension-bearing structures exhibit stochastic behavior. Thus, both the numerical and minimal models demonstrate the essence of intracellular percolation, in which macroscopic asymmetry observed at the cellular level emerges not from microscopic asymmetry in the interaction probabilities of individual molecules, but rather only as a consequence of the manner of the mechano-signal transmission. These results provide novel insights into the role of the mutual interplay between distinct subcellular structures with and without tension-bearing capability. Insight: Cells continuously remodel their internal elements or structural proteins in response to environmental changes. Despite the stochastic behavior of individual structural proteins, which lack awareness of the larger subcellular structures they are meant to create or disintegrate, this self-assembly process somehow occurs to enable adaptation to the environment. Here we demonstrated through percolation simulations and minimal model analyses that there is an asymmetry in the response between the creation and disintegration of subcellular structures, which can aid environmental adaptation. This asymmetry inherently arises from the nature of mechano-signal transmission through structural proteins, namely tension-mediated information exchange within cells, despite the stochastic behavior of individual proteins lacking asymmetric characters in themselves.

Limiting pool and actin architecture controls myosin cluster sizes in adherent cells

The actomyosin cytoskeleton generates mechanical forces that power important cellular processes, such as cell migration, cell division, and mechanosensing. Actomyosin self-assembles into contractile networks and bundles that underlie force generation and transmission in cells. A central step is the assembly of the myosin II filament from myosin monomers, regulation of which has been extensively studied. However, myosin filaments are almost always found as clusters within the cell cortex. While recent studies characterized cluster nucleation dynamics at the cell periphery, how myosin clusters grow on stress fibers remains poorly characterized. Here, we utilize a U2OS osteosarcoma cell line with endogenously tagged myosin II to measure the myosin cluster size distribution in the lamella of adherent cells. We find that myosin clusters can grow with Rho-kinase (ROCK) activity alone in the absence of myosin motor activity. Time-lapse imaging reveals that myosin clusters grow via increased myosin association to existing clusters, which is potentiated by ROCK-dependent myosin filament assembly. Enabling myosin motor activity allows further myosin cluster growth through myosin association that is dependent on F-actin architecture. Using a toy model, we show that myosin self-affinity is sufficient to recapitulate the experimentally observed myosin cluster size distribution, and that myosin cluster sizes are determined by the pool of myosin available for cluster growth. Together, our findings provide new insights into the regulation of myosin cluster sizes within the lamellar actomyosin cytoskeleton.

S100A11 promotes focal adhesion disassembly via myosin II-driven contractility and Piezo1-mediated Ca2+ entry

S100A11 is a small Ca2+-activatable protein known to localize along stress fibers (SFs). Analyzing S100A11 localization in HeLa and U2OS cells further revealed S100A11 enrichment at focal adhesions (FAs). Strikingly, S100A11 levels at FAs increased sharply, yet transiently, just before FA disassembly. Elevating intracellular Ca2+ levels with ionomycin stimulated both S100A11 recruitment and subsequent FA disassembly. However, pre-incubation with the non-muscle myosin II (NMII) inhibitor blebbistatin or with an inhibitor of the stretch-activatable Ca2+ channel Piezo1 suppressed S100A11 recruitment, implicating S100A11 in an actomyosin-driven FA recruitment mechanism involving Piezo1-dependent Ca2+ influx. Applying external forces on peripheral FAs likewise recruited S100A11 to FAs even if NMII activity was inhibited, corroborating the mechanosensitive recruitment mechanism of S100A11. However, extracellular Ca2+ and Piezo1 function were indispensable, indicating that NMII contraction forces act upstream of Piezo1-mediated Ca2+ influx, in turn leading to S100A11 activation and FA recruitment. S100A11-knockout cells display enlarged FAs and had delayed FA disassembly during cell membrane retraction, consistent with impaired FA turnover in these cells. Our results thus demonstrate a novel function for S100A11 in promoting actomyosin contractility-driven FA disassembly.

Imaging and Analysis of the Dynamics of Filamentous Actin Structures in Live Endothelial Cells

The ability to view and record the movements of subcellular structures is a powerful tool that has accelerated the discovery and understanding of signaling mechanisms that control microvascular functions such as the control of endothelial permeability. Advances in molecular biology over the past few decades have facilitated the generation of fusion proteins in which fluorescent reporters based upon the structure of green fluorescent protein can be linked to proteins found in human endothelial cells, such as VE-cadherin or β-actin. These fusion proteins have been found to incorporate into structures alongside their native protein counterparts, allowing the dynamic visualization of how these subcellular structures are modified when cells are challenged with stimuli such as inflammatory mediators. The result of such studies has been a much more advanced view of the complex mechanisms by which endothelial cells maintain barrier properties than previously obtained by only viewing fixed cells labeled by immunofluorescence. Here, we describe our protocols that we have used to view the dynamics of actin filaments using time-lapse microscopy to record endothelial cells expressing GFP-actin and the analysis tools available to quantify dynamics of subcellular structures.

Cell stretching activates an ATM mechano-transduction pathway that remodels cytoskeleton and chromatin

Ataxia telangiectasia mutated (ATM) and ataxia telangiectasia and Rad3-related (ATR) DNA damage response (DDR) kinases contain elastic domains. ATM also responds to reactive oxygen species (ROS) and ATR to nuclear mechanical stress. Mre11 mediates ATM activation following DNA damage; ATM mutations cause ataxia telangiectasia (A-T). Here, using in vivo imaging, electron microscopy, proteomic, and mechano-biology approaches, we study how ATM responds to mechanical stress. We report that cytoskeleton and ROS, but not Mre11, mediate ATM activation following cell deformation. ATM deficiency causes hyper-stiffness, stress fiber accumulation, Yes-associated protein (YAP) nuclear enrichment, plasma and nuclear membrane alterations during interstitial migration, and H3 hyper-methylation. ATM locates to the actin cytoskeleton and, following cytoskeleton stress, promotes phosphorylation of key cytoskeleton and chromatin regulators. Our data contribute to explain some clinical features of patients with A-T and pinpoint the existence of an integrated mechano-response in which ATM and ATR have distinct roles unrelated to their canonical DDR functions.

RACK1 promotes Shigella flexneri actin-mediated invasion, motility, and cell-to-cell spreading

Shigella flexneri is an intracellular bacterium that hijacks the host actin cytoskeleton to invade and disseminate within the colonic epithelium. Shigella's virulence factors induce actin polymerization, leading to bacterial uptake, actin tail formation, actin-mediated motility, and cell-to-cell spreading. Many host factors involved in the Shigella-prompted actin rearrangements remain elusive. Here, we studied the role of a host protein receptor for activated C kinase 1 (RACK1) in actin cytoskeleton dynamics and Shigella infection. We used time-lapse imaging to demonstrate that RACK1 facilitates Shigella-induced actin cytoskeleton remodeling at multiple levels during infection of epithelial cells. Silencing RACK1 expression impaired Shigella-induced rapid polymerizing structures, reducing host cell invasion, bacterial motility, and cell-to-cell spreading. In uninfected cells, RACK1 silencing reduced jasplakinolide-mediated filamentous actin aggregate formation and negatively affected actin turnover in fast polymerizing structures, such as membrane ruffles. Our findings provide a role of RACK1 in actin cytoskeleton dynamics and Shigella infection.

Stratifin (SFN) Regulates Cervical Cancer Cell Proliferation, Apoptosis, and Cytoskeletal Remodeling and Metastasis Progression Through LIMK2/Cofilin Signaling

The aberrant expression of Stratifin (SFN) is intricately associated with the initiation and progression of numerous tumors. This study aims to investigate whether SFN regulates the metastasis of cervical cancer cells through the LIMK2/Cofilin signaling pathway. In this study, we compared the expression of SFN in normal cervical tissues and cervical carcinoma tissues. We established SFN overexpression and SFN silencing cellular models to assess the invasive and migratory capabilities of cervical cancer cells using transwell and scratch assays. YO-PRO-1/PI and EdU staining were employed to evaluate apoptotic and proliferative capacities, while Actin-Tracker Green-488 was utilized to investigate cytoskeletal remodeling. The expression levels of SFN, LIMK2, p-LIMK2, Cofilin, and p-Cofilin were examined through Western blotting and immunofluorescence. Our findings revealed elevated expression of SFN in cervical squamous cell carcinoma tissues. SFN overexpression was observed to enhance invasion and migration of cervical cancer cells, induce cytoskeletal remodeling, facilitate cell proliferation, and suppress apoptosis. Furthermore, SFN overexpression upregulated the expression levels of LIMK2, p-LIMK2, Cofilin, and p-Cofilin. Conversely, silencing SFN exerted opposite effects. SFN plays an important role in the diagnosis of cervical cancer. SFN can regulate cervical cancer cell proliferation, apoptosis, cytoskeletal remodeling and metastasis through LIMK2/Cofilin signaling.

Dynamic remodeling of actin networks by cyclase-associated protein and CAP-Abp1 complexes

How actin filaments are spatially organized and remodeled into diverse higher-order networks in vivo is still not well understood. Here, we report an unexpected F-actin "coalescence" activity driven by cyclase-associated protein (CAP) and enhanced by its interactions with actin-binding protein 1 (Abp1). We directly observe S. cerevisiae CAP and Abp1 rapidly transforming branched or linear actin networks by bundling and sliding filaments past each other, maximizing filament overlap, and promoting compaction into bundles. This activity does not require ATP and is conserved, as similar behaviors are observed for the mammalian homologs of CAP and Abp1. Coalescence depends on the CAP oligomerization domain but not the helical folded domain (HFD) that mediates its functions in F-actin severing and depolymerization. Coalescence by CAP-Abp1 further depends on interactions between CAP and Abp1 and interactions between Abp1 and F-actin. Our results are consistent with a mechanism in which the formation of energetically favorable sliding CAP and CAP-Abp1 crosslinks drives F-actin bundle compaction. Roles for CAP and CAP-Abp1 in actin remodeling in vivo are supported by strong phenotypes arising from deletion of the CAP oligomerization domain and by genetic interactions between sac6Δ and an srv2-301 mutant that does not bind Abp1. Together, these observations identify a new actin filament remodeling function for CAP, which is further enhanced by its direct interactions with Abp1.

Measuring response functions of active materials from data

From flocks of birds to biomolecular assemblies, systems in which many individual components independently consume energy to perform mechanical work exhibit a wide array of striking behaviors. Methods to quantify the dynamics of these so-called active systems generally aim to extract important length or time scales from experimental fields. Because such methods focus on extracting scalar values, they do not wring maximal information from experimental data. We introduce a method to overcome these limitations. We extend the framework of correlation functions by taking into account the internal headings of displacement fields. The functions we construct represent the material response to specific types of active perturbation within the system. Utilizing these response functions we query the material response of disparate active systems composed of actin filaments and myosin motors, from model fluids to living cells. We show we can extract critical length scales from the turbulent flows of an active nematic, anticipate contractility in an active gel, distinguish viscous from viscoelastic dissipation, and even differentiate modes of contractility in living cells. These examples underscore the vast utility of this method which measures response functions from experimental observations of complex active systems.

A Rab-bit hole: Rab40 GTPases as new regulators of the actin cytoskeleton and cell migration

The regulation of machinery involved in cell migration is vital to the maintenance of proper organism function. When migration is dysregulated, a variety of phenotypes ranging from developmental disorders to cancer metastasis can occur. One of the primary structures involved in cell migration is the actin cytoskeleton. Actin assembly and disassembly form a variety of dynamic structures which provide the pushing and contractile forces necessary for cells to properly migrate. As such, actin dynamics are tightly regulated. Classically, the Rho family of GTPases are considered the major regulators of the actin cytoskeleton during cell migration. Together, this family establishes polarity in the migrating cell by stimulating the formation of various actin structures in specific cellular locations. However, while the Rho GTPases are acknowledged as the core machinery regulating actin dynamics and cell migration, a variety of other proteins have become established as modulators of actin structures and cell migration. One such group of proteins is the Rab40 family of GTPases, an evolutionarily and functionally unique family of Rabs. Rab40 originated as a single protein in the bilaterians and, through multiple duplication events, expanded to a four-protein family in higher primates. Furthermore, unlike other members of the Rab family, Rab40 proteins contain a C-terminally located suppressor of cytokine signaling (SOCS) box domain. Through the SOCS box, Rab40 proteins interact with Cullin5 to form an E3 ubiquitin ligase complex. As a member of this complex, Rab40 ubiquitinates its effectors, controlling their degradation, localization, and activation. Because substrates of the Rab40/Cullin5 complex can play a role in regulating actin structures and cell migration, the Rab40 family of proteins has recently emerged as unique modulators of cell migration machinery.

MAP4K4 regulates forces at cell-cell and cell-matrix adhesions to promote collective cell migration

Collective cell migration is not only important for development and tissue homeostasis but can also promote cancer metastasis. To migrate collectively, cells need to coordinate cellular extensions and retractions, adhesion sites dynamics, and forces generation and transmission. Nevertheless, the regulatory mechanisms coordinating these processes remain elusive. Using A431 carcinoma cells, we identify the kinase MAP4K4 as a central regulator of collective migration. We show that MAP4K4 inactivation blocks the migration of clusters, whereas its overexpression decreases cluster cohesion. MAP4K4 regulates protrusion and retraction dynamics, remodels the actomyosin cytoskeleton, and controls the stability of both cell-cell and cell-substrate adhesion. MAP4K4 promotes focal adhesion disassembly through the phosphorylation of the actin and plasma membrane crosslinker moesin but disassembles adherens junctions through a moesin-independent mechanism. By analyzing traction and intercellular forces, we found that MAP4K4 loss of function leads to a tensional disequilibrium throughout the cell cluster, increasing the traction forces and the tension loading at the cell-cell adhesions. Together, our results indicate that MAP4K4 activity is a key regulator of biomechanical forces at adhesion sites, promoting collective migration.

Targeting CaMKK2 Inhibits Actin Cytoskeletal Assembly to Suppress Cancer Metastasis

Triple-negative breast cancers (TNBC) tend to become invasive and metastatic at early stages in their development. Despite some treatment successes in early-stage localized TNBC, the rate of distant recurrence remains high, and long-term survival outcomes remain poor. In a search for new therapeutic targets for this disease, we observed that elevated expression of the serine/threonine kinase calcium/calmodulin (CaM)-dependent protein kinase kinase 2 (CaMKK2) is highly correlated with tumor invasiveness. In validation studies, genetic disruption of CaMKK2 expression or inhibition of its activity with small molecule inhibitors disrupted spontaneous metastatic outgrowth from primary tumors in murine xenograft models of TNBC. High-grade serous ovarian cancer (HGSOC), a high-risk, poor prognosis ovarian cancer subtype, shares many features with TNBC, and CaMKK2 inhibition effectively blocked metastatic progression in a validated xenograft model of this disease. Mechanistically, CaMKK2 increased the expression of the phosphodiesterase PDE1A, which hydrolyzed cyclic guanosine monophosphate (cGMP) to decrease the cGMP-dependent activity of protein kinase G1 (PKG1). Inhibition of PKG1 resulted in decreased phosphorylation of vasodilator-stimulated phosphoprotein (VASP), which in its hypophosphorylated state binds to and regulates F-actin assembly to facilitate cell movement. Together, these findings establish a targetable CaMKK2-PDE1A-PKG1-VASP signaling pathway that controls cancer cell motility and metastasis by impacting the actin cytoskeleton. Furthermore, it identifies CaMKK2 as a potential therapeutic target that can be exploited to restrict tumor invasiveness in patients diagnosed with early-stage TNBC or localized HGSOC.

SIGNIFICANCE

CaMKK2 regulates actin cytoskeletal dynamics to promote tumor invasiveness and can be inhibited to suppress metastasis of breast and ovarian cancer, indicating CaMKK2 inhibition as a therapeutic strategy to arrest disease progression.

Small lipid droplets are rigid enough to indent a nucleus, dilute the lamina, and cause rupture

The nucleus in many cell types is a stiff organelle, but fat-filled lipid droplets (FDs) in cytoplasm are seen to indent and displace the nucleus. FDs are phase-separated liquids with a poorly understood interfacial tension γ that determines how FDs interact with other organelles. Here, micron-sized FDs remain spherical as they indent peri-nuclear actomyosin and the nucleus, while causing local dilution of Lamin-B1 independent of Lamin-A,C and sometimes triggering nuclear rupture. Focal accumulation of the cytosolic DNA sensor cGAS at the rupture site is accompanied by sustained mislocalization of DNA repair factors to cytoplasm, increased DNA damage, and delayed cell cycle. Macrophages show FDs and engulfed rigid beads cause similar indentation dilution. Spherical shapes of small FDs indicate a high γ, which we measure for FDs mechanically isolated from fresh adipose tissue as ∼40 mN/m. This value is far higher than that of protein condensates, but typical of oils in water and sufficiently rigid to perturb cell structures including nuclei.

Spatiotemporal regulation of Rho GTPase signaling during endothelial barrier remodeling

The vasculature is characterized by a thin cell layer that comprises the inner wall of all blood vessels, the continuous endothelium. Endothelial cells can also be found in the eye's cornea. And even though cornea and vascular endothelial (VE) cells differ from each other in structure, they both function as barriers and express similar junctional proteins such as the adherens junction VE-cadherin and tight-junction member claudin-5. How these barriers are controlled to maintain the barrier and thereby its integrity is of major interest in the development of potential therapeutic targets. An important target of endothelial barrier remodeling is the actin cytoskeleton, which is centrally coordinated by Rho GTPases that are in turn regulated by Rho-regulatory proteins. In this review, we give a brief overview of how Rho-regulatory proteins themselves are spatiotemporally regulated during the process of endothelial barrier remodeling. Additionally, we propose a roadmap for the comprehensive dissection of the Rho GTPase signaling network in its entirety.

Rationally Designed Enzyme-Resistant Peptidic Assemblies for Plasma Membrane Targeting in Cancer Treatment

Peptides are being increasingly important for subcellular targeted cancer treatment to improve specificity and reverse multidrug resistance. However, there has been yet any report on targeting plasma membrane (PM) through self-assembling peptides. A simple synthetic peptidic molecule (tF4) is developed. It is revealed that tF4 is carboxyl esterase-resistant and self-assembles into vesical nanostructures. Importantly, tF4 assemblies interact with PM through orthogonal hydrogen bonding and hydrophobic interaction to regulate cancer cellular functions. Mechanistically, tF4 assemblies induce stress fiber formation, cytoskeleton reconstruction, and death receptor 4/5 (DR4/5) expression in cancer cells. DR4/5 triggers extrinsic caspase-8 signaling cascade, resulting in cell death. The results provide a new strategy for developing enzyme-resistant and PM-targeting peptidic molecules against cancer.

Characterization of Anticancer Effects of the Analogs of DJ4, a Novel Selective Inhibitor of ROCK and MRCK Kinases

The Rho associated coiled-coil containing protein kinase (ROCK1 and ROCK2) and myotonic dystrophy-related Cdc-42 binding kinases (MRCKα and MRCKβ) are critical regulators of cell proliferation and cell plasticity, a process intimately involved in cancer cell migration and invasion. Previously, we reported the discovery of a novel small molecule (DJ4) selective multi-kinase inhibitor of ROCK1/2 and MRCKα/β. Herein, we further characterized the anti-proliferative and apoptotic effects of DJ4 in non-small cell lung cancer and triple-negative breast cancer cells. To further optimize the ROCK/MRCK inhibitory potency of DJ4, we generated a library of 27 analogs. Among the various structural modifications, we identified four additional active analogs with enhanced ROCK/MRCK inhibitory potency. The anti-proliferative and cell cycle inhibitory effects of the active analogs were examined in non-small cell lung cancer, breast cancer, and melanoma cell lines. The anti-proliferative effectiveness of DJ4 and the active analogs was further demonstrated against a wide array of cancer cell types using the NCI-60 human cancer cell line panel. Lastly, these new analogs were tested for anti-migratory effects in highly invasive MDA-MB-231 breast cancer cells. Together, our results demonstrate that selective inhibitors of ROCK1/2 (DJE4, DJ-Allyl) inhibited cell proliferation and induced cell cycle arrest at G2/M but were less effective in cell death induction compared with dual ROCK1/2 and MRCKα/β (DJ4 and DJ110).

Src-Dependent NM2A Tyrosine Phosphorylation Regulates Actomyosin Remodeling

Non-muscle myosin 2A (NM2A) is a key cytoskeletal enzyme that, along with actin, assembles into actomyosin filaments inside cells. NM2A is fundamental for cell adhesion and motility, playing important functions in different stages of development and during the progression of viral and bacterial infections. Phosphorylation events regulate the activity and the cellular localization of NM2A. We previously identified the tyrosine phosphorylation of residue 158 (pTyr¹⁵⁸) in the motor domain of the NM2A heavy chain. This phosphorylation can be promoted by Listeria monocytogenes infection of epithelial cells and is dependent on Src kinase; however, its molecular role is unknown. Here, we show that the status of pTyr¹⁵⁸ defines cytoskeletal organization, affects the assembly/disassembly of focal adhesions, and interferes with cell migration. Cells overexpressing a non-phosphorylatable NM2A variant or expressing reduced levels of Src kinase display increased stress fibers and larger focal adhesions, suggesting an altered contraction status consistent with the increased NM2A activity that we also observed. We propose NM2A pTyr¹⁵⁸ as a novel layer of regulation of actomyosin cytoskeleton organization.

BORG family proteins in physiology and human disease

The binder of rho GTPases (BORG)/Cdc42 effector proteins (Cdc42EP) family is composed of five Rho GTPase binding proteins whose functions and mechanism of actions are of emerging interest. Here, we review recent findings pertaining to the family as a whole and consider how these change our understanding of cellular organization. Recent studies have implicated BORGs in both fundamental physiology and in human diseases, mainly cancers. An emerging pattern suggests that BORG family members cancer-promoting properties are related to their ability to regulate the cytoskeleton, with many impacting the organization of acto-myosin stress fibers. This is consistent with the broader literature indicating that BORG family members are regulators of both the septin and actin cytoskeleton networks. The exact mechanism through which BORGs modify the cytoskeleton is not clear, but we consider here a few data-supported and speculative possibilities. Finally, we delve into how the Rho GTPase Cdc42 modifies BORG function in cells. This remains open-ended as Cdc42's effects on BORGs appear cell type- and cell state-dependent. Collectively, these data point to the importance of the BORG family and suggest broader themes in their function and regulation.

Limiting Pool and Actin Architecture Controls Myosin Cluster Sizes in Adherent Cells

The actomyosin cytoskeleton generates mechanical forces that power important cellular processes, such as cell migration, cell division, and mechanosensing. Actomyosin self-assembles into contractile networks and bundles that underlie force generation and transmission in cells. A central step is the assembly of the myosin II filament from myosin monomers, regulation of which has been extensively studied. However, myosin filaments are almost always found as clusters within the cell cortex. While recent studies characterized cluster nucleation dynamics at the cell periphery, how myosin clusters grow on stress fibers remains poorly characterized. Here, we utilize a U2OS osteosarcoma cell line with endogenously tagged myosin II to measure the myosin cluster size distribution in the lamella of adherent cells. We find that myosin clusters can grow with Rho-kinase (ROCK) activity alone in the absence of myosin motor activity. Time-lapse imaging reveals that myosin clusters grow via increased myosin association to existing clusters, which is potentiated by ROCK-dependent myosin filament assembly. Enabling myosin motor activity allows further myosin cluster growth through myosin association that is dependent on F-actin architecture. Using a toy model, we show that myosin self-affinity is sufficient to recapitulate the experimentally observed myosin cluster size distribution, and that myosin cluster sizes are determined by the pool of myosin available for cluster growth. Together, our findings provide new insights into the regulation of myosin cluster sizes within the lamellar actomyosin cytoskeleton.

Early committed polarization of intracellular tension in response to cell shape determines the osteogenic differentiation of mesenchymal stromal cells

Within the heterogeneous tissue architecture, a comprehensive understanding of how cell shapes regulate cytoskeletal mechanics by adjusting focal adhesions (FAs) signals to correlate with the lineage commitment of mesenchymal stromal cells (MSCs) remains obscure. Here, via engineered extracellular matrices, we observed that the development of mature FAs, coupled with a symmetrical pattern of radial fiber bundles, appeared at the right-angle vertices in cells with square shape. While circular cells aligned the transverse fibers parallel to the cell edge, and moved them centripetally in a counter-clockwise direction, symmetrical bundles of radial fibers at the vertices of square cells disrupted the counter-clockwise swirling and bridged the transverse fibers to move centripetally. In square cells, the contractile force, generated by the myosin IIA-enriched transverse fibers, were concentrated and transmitted outwards along the symmetrical bundles of radial fibers, to the extracellular matrix through FAs, and thereby driving FA organization and maturation. The symmetrical radial fiber bundles concentrated the transverse fibers contractility inward to the linkage between the actin cytoskeleton and the nuclear envelope. The tauter cytoskeletal network adjusted the nuclear-actomyosin force balance to cause nuclear deformability and to increase nuclear translocation of the transcription co-activator YAP, which in turn modulated the switch in MSC commitment. Thus, FAs dynamically respond to geometric cues and remodel actin cytoskeletal network to re-distribute intracelluar tension towards the cell nucleus, and thereby controlling YAP mechanotransduction signaling in regulating MSC fate decision. STATEMENT OF SIGNIFICANCE: We decipher how cellular mechanics is self-organized depending on extracellular geometric features to correlate with mesenchymal stromal cell lineage commitment. In response to geometry constrains on cell morphology, symmetrical radial fiber bundles are assembled and clustered depending on the maturation state of focal adhesions and bridge with the transverse fibers, and thereby establishing the dynamic cytoskeletal network. Contractile force, generated by the myosin-IIA-enriched transverse fibers, is transmitted and dynamically drives the retrograde movement of the actin cytoskeletal network, which appropriately adjusts the nuclear-actomyosin force balance and deforms the cell nucleus for YAP mechano-transduction signaling in regulating mesenchymal stromal cell fate decision.

Ca 2+ /Calmodulin Dependent Protein Kinase Kinase-2 (CaMKK2) promotes Protein Kinase G (PKG)-dependent actin cytoskeletal assembly to increase tumor metastasis

Triple-negative breast cancers (TNBCs) tend to become highly invasive early during cancer development. Despite some successes in the initial treatment of patients diagnosed with early-stage localized TNBC, the rate of metastatic recurrence remains high with poor long-term survival outcomes. Here we show that elevated expression of the serine/threonine-kinase, Calcium/Calmodulin (CaM)-dependent protein kinase kinase-2 (CaMKK2), is highly correlated with tumor invasiveness. We determined that genetic disruption of CaMKK2 expression, or inhibition of its activity, disrupted spontaneous metastatic outgrowth from primary tumors in murine xenograft models of TNBC. High-grade serous ovarian cancer (HGSOC), a high-risk, poor-prognosis ovarian cancer subtype, shares many genetic features with TNBC, and importantly, CaMKK2 inhibition effectively blocked metastatic progression in a validated xenograft model of this disease. Probing the mechanistic links between CaMKK2 and metastasis we defined the elements of a new signaling pathway that impacts actin cytoskeletal dynamics in a manner which increases cell migration/invasion and metastasis. Notably, CaMKK2 increases the expression of the phosphodiesterase PDE1A which decreases the cGMP-dependent activity of protein kinase G1 (PKG1). This inhibition of PKG1 results in decreased phosphorylation of Vasodilator-Stimulated Phosphoprotein (VASP), which in its hypophosphorylated state binds to and regulates F-actin assembly to facilitate contraction/cell movement. Together, these data establish a targetable CaMKK2-PDE1A-PKG1-VASP signaling pathway that controls cancer cell motility and metastasis. Further, it credentials CaMKK2 as a therapeutic target that can be exploited in the discovery of agents for use in the neoadjuvant/adjuvant setting to restrict tumor invasiveness in patients diagnosed with early-stage TNBC or localized HGSOC.

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.

Traction Force and Mechanosensing can be Functionally Distinguished Through the Use of Specific Domains of the Calpain Small Subunit

Cell migration is a fundamental process pertaining to many critical physiological events. The ability to form and release adhesion structures is necessary for cell migration. The Calpain family of cysteine proteases are known to target adhesion proteins as their substrates and modulate adhesion dynamics. The two best studied Calpains, Calpain 1 and Calpain 2 form catalytically active holoenzymes through heterodimerization with a common non-catalytic regulatory small subunit known as Calpain 4. In previous studies, we determined that calpains are important in the production of traction forces and in the sensing of localized mechanical stimulation from the external environment. We found that perturbation of either Calpain 1 or 2 had no effect on the generation of traction forces. However, traction forces were weak when Calpain 4 was silenced. On the other hand, silencing of Calpain 1, 2, or 4 resulted in deficient sensing of external mechanical stimuli. These results together suggest that Calpain 4 functions independent of the catalytic large subunits in the generation of traction forces but functions together with either catalytic subunit in sensing external mechanical stimuli. The small subunit Calpain 4 contains 268 a.a. and is composed of 2 domains, the N-terminal domain V and C-terminal domain VI. Domain VI is a calmodulinlike domain containing five consecutive EF-hand motifs, of which the fifth one heterodimerizes with a large subunit. Moreover, domain V contains the common sequence GTAMRILGGVI that suggests cell membrane interactions. Given these attributes of domain V and VI of Calpain 4, we speculated that an individual domain might provide the functional properties for either traction or sensing. Therefore, each domain was cloned and expressed individually in Capn4-/- cells and assayed for traction and sensing. Results revealed that over-expression of domain V was sufficient to rescue the traction forces defect in Capn4-/- cells while overexpression of domain VI did not rescue the traction force. Consistent with our hypothesis, overexpression of domain VI rescued the sensing defect in Capn4-/- cells while overexpression of domain V had no effect. These results suggest that individual domains of Calpain 4 do indeed function independently to regulate either traction force or the sensing of external stimuli. We speculate that membrane association of Calpain 4 is required for the regulation of traction force and its association with a catalytic subunit is necessary for mechanosensing.

Actin Filaments Couple the Protrusive Tips to the Nucleus through the I-BAR Domain Protein IRSp53 during the Migration of Cells on 1D Fibers

The cell migration cycle, well-established in 2D, proceeds with forming new protrusive structures at the cell membrane and subsequent redistribution of contractile machinery. Three-dimensional (3D) environments are complex and composed of 1D fibers, and 1D fibers are shown to recapitulate essential features of 3D migration. However, the establishment of protrusive activity at the cell membrane and contractility in 1D fibrous environments remains partially understood. Here the role of membrane curvature regulator IRSp53 is examined as a coupler between actin filaments and plasma membrane during cell migration on single, suspended 1D fibers. IRSp53 depletion reduced cell-length spanning actin stress fibers that originate from the cell periphery, protrusive activity, and contractility, leading to uncoupling of the nucleus from cellular movements. A theoretical model capable of predicting the observed transition of IRSp53-depleted cells from rapid stick-slip migration to smooth and slower migration due to reduced actin polymerization at the cell edges is developed, which is verified by direct measurements of retrograde actin flow using speckle microscopy. Overall, it is found that IRSp53 mediates actin recruitment at the cellular tips leading to the establishment of cell-length spanning fibers, thus demonstrating a unique role of IRSp53 in controlling cell migration in 3D.

Amyloid-Based Albumin Hydrogels

Amyloid fibrils may serve as building blocks for the preparation of novel hydrogel materials from abundant, low-cost, and biocompatible polypeptides. This work presents the formation of physically cross-linked, self-healing hydrogels based on bovine serum albumin at room temperature through a straightforward disulfide reduction step induced by tris (2-carboxyethyl) phosphine hydrochloride. The structure and surface charge of the amyloid-like fibrils is determined by the pH of the solution during self-assembly, giving rise to hydrogels with distinct physicochemical properties. The hydrogel surface can be readily functionalized with the extracellular matrix protein fibronectin and supports cell adhesion, spreading, and long-term culture. This study offers a simple, versatile, and inexpensive method to prepare amyloid-based albumin hydrogels with potential applications in the biomedical field.

Actin Bundles Dynamics and Architecture

Cells use the actin cytoskeleton for many of their functions, including their division, adhesion, mechanosensing, endo- and phagocytosis, migration, and invasion. Actin bundles are the main constituent of actin-rich structures involved in these processes. An ever-increasing number of proteins that crosslink actin into bundles or regulate their morphology is being identified in cells. With recent advances in high-resolution microscopy and imaging techniques, the complex process of bundles formation and the multiple forms of physiological bundles are beginning to be better understood. Here, we review the physiochemical and biological properties of four families of highly conserved and abundant actin-bundling proteins, namely, α-actinin, fimbrin/plastin, fascin, and espin. We describe the similarities and differences between these proteins, their role in the formation of physiological actin bundles, and their properties-both related and unrelated to their bundling abilities. We also review some aspects of the general mechanism of actin bundles formation, which are known from the available information on the activity of the key actin partners involved in this process.

The compass to follow: Focal adhesion turnover

How cells move is a fundamental biological question. The directionality of adherent migrating cells depends on the assembly and disassembly (turnover) of focal adhesions (FAs). FAs are micron-sized actin-based structures that link cells to the extracellular matrix. Traditionally, microtubules have been considered key to triggering FA turnover. Through the years, advancements in biochemistry, biophysics, and bioimaging tools have been invaluable for many research groups to unravel a variety of mechanisms and molecular players that contribute to FA turnover, beyond microtubules. Here, we discuss recent discoveries of key molecular players that affect the dynamics and organization of the actin cytoskeleton to enable timely FA turnover and consequently proper directed cell migration.

Nuclear pore complexes concentrate on Actin/LINC/Lamin nuclear lines in response to mechanical stress in a SUN1 dependent manner

Formation of robust actomyosin stress fibers (SF) in response to cell stretch plays a key role in the transfer of information from the cytoplasm into the nucleus. Actin/LINC/Lamin (ALL) nuclear lines provide mechanical linkage between the actin cytoskeleton and the lamin nucleoskeleton across the nuclear envelope. To understand the establishment of ALL lines, we used live cell imaging of cells exposed to cyclic stretch. We discovered that nuclear pore complexes (NPCs) concentrate along ALL lines that are generated in response to uniaxial cyclic stretch. The ALL-associated NPCs display increased fluorescence intensity of nucleoporins Pom121, TPR and Nup153 relative to nucleoporins that are distal to the ALL lines. Here we test the hypothesis that a LINC complex component of ALL lines, SUN1 is involved in the integration of NPCs with ALL lines. We generated CRISPR SUN1 knockdown and knockout cell lines and show that SUN1 is essential for normal integration of NPCs to ALL lines. Loss or elimination of SUN1 significantly diminishes NPC/ALL line integration, demonstrating a key role for SUN1 in the recruitment or stabilization of NPCs to a discrete subdomain of the nuclear envelope at ALL lines. This work provides new insight into the mechanism by which cells respond to mechanical force through nuclear envelope remodeling.

Architectural control of mesenchymal stem cell phenotype through nuclear actin

There is growing appreciation that architectural components of the nucleus regulate gene accessibility by altering chromatin organization. While nuclear membrane connector proteins link the mechanosensitive actin cytoskeleton to the nucleoskeleton, actin's contribution to the inner architecture of the nucleus remains enigmatic. Control of actin transport into the nucleus, plus the presence of proteins that control actin structure (the actin tool-box) within the nucleus, suggests that nuclear actin may support biomechanical regulation of gene expression. Cellular actin structure is mechanoresponsive: actin cables generated through forces experienced at the plasma membrane transmit force into the nucleus. We posit that dynamic actin remodeling in response to such biomechanical cues provides a novel level of structural control over the epigenetic landscape. We here propose to bring awareness to the fact that mechanical forces can promote actin transfer into the nucleus and control structural arrangements as illustrated in mesenchymal stem cells, thereby modulating lineage commitment.

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.

Pointed-end processive elongation of actin filaments by Vibrio effectors VopF and VopL

According to the cellular actin dynamics paradigm, filaments grow at their barbed ends and depolymerize predominantly from their pointed ends to form polar structures and do productive work. We show that actin can elongate at the pointed end when assisted by Vibrio VopF/L toxins, which act as processive polymerases. In cells, processively moving VopF/L speckles are inhibited by factors blocking the pointed but not barbed ends. Multispectral single-molecule imaging confirmed that VopF molecules associate with the pointed end, actively promoting its elongation even in the presence of profilin. Consequently, VopF/L can break the actin cytoskeleton's polarity by compromising actin-based cellular processes. Therefore, actin filament design allows processive growth at both ends, which suggests unforeseen possibilities for cellular actin organization, particularly in specialized cells and compartments.

Long-term molecular turnover of actin stress fibers revealed by advection-reaction analysis in fluorescence recovery after photobleaching

Fluorescence recovery after photobleaching (FRAP) is a versatile technique to evaluate the intracellular molecular exchange called turnover. Mechanochemical models of FRAP typically consider the molecular diffusion and chemical reaction that simultaneously occur on a time scale of seconds to minutes. Particularly for long-term measurements, however, a mechanical advection effect can no longer be ignored, which transports the proteins in specific directions within the cells and accordingly shifts the spatial distribution of the local chemical equilibrium. Nevertheless, existing FRAP models have not considered the spatial shift, and as such, the turnover rate is often analyzed without considering the spatiotemporally updated chemical equilibrium. Here we develop a new FRAP model aimed at long-term measurements to quantitatively determine the two distinct effects of the advection and chemical reaction, i.e., the different major sources of the change in fluorescence intensity. To validate this approach, we carried out FRAP experiments on actin in stress fibers over a time period of more than 900 s, and the advection rate was shown to be comparable in magnitude to the chemical dissociation rate. We further found that the actin–myosin interaction and actin polymerization differently affect the advection and chemical dissociation. Our results suggest that the distinction between the two effects is indispensable to extract the intrinsic chemical properties of the actin cytoskeleton from the observations of complicated turnover in cells.

Caldesmon controls stress fiber force-balance through dynamic cross-linking of myosin II and actin-tropomyosin filaments

Contractile actomyosin bundles are key force-producing and mechanosensing elements in muscle and non-muscle tissues. Whereas the organization of muscle myofibrils and mechanism regulating their contractility are relatively well-established, the principles by which myosin-II activity and force-balance are regulated in non-muscle cells have remained elusive. We show that Caldesmon, an important component of smooth muscle and non-muscle cell actomyosin bundles, is an elongated protein that functions as a dynamic cross-linker between myosin-II and tropomyosin-actin filaments. Depletion of Caldesmon results in aberrant lateral movement of myosin-II filaments along actin bundles, leading to irregular myosin distribution within stress fibers. This manifests as defects in stress fiber network organization and contractility, and accompanied problems in cell morphogenesis, migration, invasion, and mechanosensing. These results identify Caldesmon as critical factor that ensures regular myosin-II spacing within non-muscle cell actomyosin bundles, and reveal how stress fiber networks are controlled through dynamic cross-linking of tropomyosin-actin and myosin filaments.

Reforming the Barrier: The Role of Formins in Wound Repair

The restoration of an intact epidermal barrier after wound injury is the culmination of a highly complex and exquisitely regulated physiological process involving multiple cells and tissues, overlapping dynamic events and protein synthesis and regulation. Central to this process is the cytoskeleton, a system of intracellular proteins that are instrumental in regulating important processes involved in wound repair including chemotaxis, cytokinesis, proliferation, migration, and phagocytosis. One highly conserved family of cytoskeletal proteins that are emerging as major regulators of actin and microtubule nucleation, polymerization, and stabilization are the formins. The formin family includes 15 different proteins categorized into seven subfamilies based on three formin homology domains (FH1, FH2, and FH3). The formins themselves are regulated in different ways including autoinhibition, activation, and localization by a range of proteins, including Rho GTPases. Herein, we describe the roles and effects of the formin family of cytoskeletal proteins on the fundamental process of wound healing and highlight recent advances relating to their important functions, mechanisms, and regulation at the molecular and cellular levels.

Merging of ventral fibers at adhesions drives the remodeling of cellular contractile systems in fibroblasts

Ventral stress fibers (VSFs) are contractile actin fibers dynamically attached to cell-matrix focal adhesions. VSFs are critical in cellular traction force production and migration. VSFs vary from randomly oriented short, thinner fibers to long, thick fibers that span along the whole long axis of a cell. De novo VSF formation was shown to occur by cortical actin mesh condensation or by crosslinking of dorsal stress fibers and transverse arcs at the cell front. However, the formation of long VSFs that extend across the whole cell axis is not well understood. Here, we report a novel phenomenon of VSF merging in migratory fibroblast cells, which is guided by mechanical force balance and contributes to VSF alignment along the long cell axis. The mechanism of VSF merging involves two steps: connection of two ventral fibers by an emerging myosin II bridge at an intervening adhesion and intervening adhesion dissolution. Our data indicate that these two steps are interdependent: slow adhesion disassembly leads to the slowing of the myosin bridge formation. Cellular data and computational modeling show that the contact angle between merging fibers decides successful merging, with shallow angles leading to merge failure. Our data and modeling further show that merging increases the share of uniformly aligned long VSFs, likely contributing to directional traction force production. Thus, we characterize merging as a process for dynamic reorganization of VSFs with functional significance for directional cell migration.