Generation of stress fibers through myosin-driven reorganization of the actin cortex

Tailored Physicochemical Cues Direct Human Mesenchymal Stem Cell Differentiation through Epigenetic Regulation Using Colloidal Self-Assembled Patterns

The extracellular matrix (ECM) shapes the stem cell fate during differentiation by exerting relevant biophysical cues. However, the mechanism of stem cell fate decisions in response to ECM-backed complex biophysical cues has not been fully understood due to the lack of versatile ECMs. Here, we designed two versatile ECMs using colloidal self-assembly technology to probe the mechanisms of their effects on mechanotransduction and stem cell fate regulation. Binary colloidal crystals (BCC) with a hexagonally close-packed structure, composed of silica (5 μm) and polystyrene (0.4 μm) particles as well as a polydimethylsiloxane-embedded BCC (BCCP), were fabricated. They have defined surface chemistry, roughness, stiffness, ion release, and protein adsorption properties, which can modulate the cell adhesion, proliferation, and differentiation of human adipose-derived stem cells (hASCs). On the BCC, hASCs preferred osteogenesis at an early stage but showed a higher tendency toward adipogenesis at later stages. In contrast, the results of BCCP diverged from those of BCC, suggesting a unique regulation of ECM-dependent mechanotransduction. The BCC-mediated cell adhesion reduced the size of the focal adhesion complex, accompanying an ordered spatial organization and cytoskeletal rearrangement. This morphological restriction led to the modulation of mechanosensitive transcription factors, such as c-FOS, the enrichment of transcripts in specific signaling pathways such as PI3K/AKT, and the activation of the Hippo signaling pathway. Epigenetic analyses showed changes in histone modifications across different substrates, suggesting that chromatin remodeling participated in BCC-mediated mechanotransduction. This study demonstrates that BCCs are versatile artificial ECMs that can regulate human stem cells' fate through unique biological signaling, which is beneficial in biomaterial design and stem cell engineering.

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.

Arp2/3 mediated dynamic lamellipodia of the hPSC colony edges promote liposome-based DNA delivery

Cationic liposome-mediated delivery of drugs, DNA, or RNA plays a pivotal role in small molecule therapy, gene editing, and immunization. However, our current knowledge regarding the cellular structures that facilitate this process remains limited. Here, we used human pluripotent stem cells (hPSCs), which form compact colonies consisting of dynamically active cells at the periphery and epithelial-like cells at the core. We discovered that cells at the colony edges selectively got transfected by cationic liposomes through actin-related protein 2/3 (Arp2/3) dependent dynamic lamellipodia, which is augmented by myosin II inhibition. Conversely, cells at the core establish tight junctions at their apical surfaces, impeding liposomal access to the basal lamellipodia and thereby inhibiting transfection. In contrast, liposomes incorporating mannosylated lipids are internalized throughout the entire colony via receptor-mediated endocytosis. These findings contribute a novel mechanistic insight into enhancing therapeutic delivery via liposomes, particularly in cell types characterized by dynamic lamellipodia, such as immune cells or those comprising the epithelial layer.

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 third dimension of the actin cortex

The actin cortex, commonly described as a thin 2-dimensional layer of actin filaments beneath the plasma membrane, is beginning to be recognized as part of a more dynamic and three-dimensional composite material. In this review, we focus on the elements that contribute to the three-dimensional architecture of the actin cortex. We also argue that actin-rich structures such as filopodia and stress fibers can be viewed as specialized integral parts of the 3D actin cortex. This broadens our definition of the cortex, shifting from its simplified characterization as a thin, two-dimensional layer of actin filaments.

The Role of the Host Cytoskeleton in the Formation and Dynamics of Rotavirus Viroplasms

Rotavirus (RV) replicates within viroplasms, membraneless electron-dense globular cytosolic inclusions with liquid-liquid phase properties. In these structures occur the virus transcription, replication, and packaging of the virus genome in newly assembled double-layered particles. The viroplasms are composed of virus proteins (NSP2, NSP5, NSP4, VP1, VP2, VP3, and VP6), single- and double-stranded virus RNAs, and host components such as microtubules, perilipin-1, and chaperonins. The formation, coalescence, maintenance, and perinuclear localization of viroplasms rely on their association with the cytoskeleton. A stabilized microtubule network involving microtubules and kinesin Eg5 and dynein molecular motors is associated with NSP5, NSP2, and VP2, facilitating dynamic processes such as viroplasm coalescence and perinuclear localization. Key post-translation modifications, particularly phosphorylation events of RV proteins NSP5 and NSP2, play pivotal roles in orchestrating these interactions. Actin filaments also contribute, triggering the formation of the viroplasms through the association of soluble cytosolic VP4 with actin and the molecular motor myosin. This review explores the evolving understanding of RV replication, emphasizing the host requirements essential for viroplasm formation and highlighting their dynamic interplay within the host cell.

Kinetic trapping organizes actin filaments within liquid-like protein droplets

Several actin-binding proteins (ABPs) phase separate to form condensates capable of curating the actin network shapes. Here, we use computational modeling to understand the principles of actin network organization within VASP condensate droplets. Our simulations reveal that the different actin shapes, namely shells, rings, and mixture states are highly dependent on the kinetics of VASP-actin interactions, suggesting that they arise from kinetic trapping. Specifically, we show that reducing the residence time of VASP on actin filaments reduces degree of bundling, thereby promoting assembly of shells rather than rings. We validate the model predictions experimentally using a VASP-mutant with decreased bundling capability. Finally, we investigate the ring opening within deformed droplets and found that the sphere-to-ellipsoid transition is favored under a wide range of filament lengths while the ellipsoid-to-rod transition is only permitted when filaments have a specific range of lengths. Our findings highlight key mechanisms of actin organization within phase-separated ABPs.

Patterning of the cell cortex by Rho GTPases

The Rho GTPases - RHOA, RAC1 and CDC42 - are small GTP binding proteins that regulate basic biological processes such as cell locomotion, cell division and morphogenesis by promoting cytoskeleton-based changes in the cell cortex. This regulation results from active (GTP-bound) Rho GTPases stimulating target proteins that, in turn, promote actin assembly and myosin 2-based contraction to organize the cortex. This basic regulatory scheme, well supported by in vitro studies, led to the natural assumption that Rho GTPases function in vivo in an essentially linear matter, with a given process being initiated by GTPase activation and terminated by GTPase inactivation. However, a growing body of evidence based on live cell imaging, modelling and experimental manipulation indicates that Rho GTPase activation and inactivation are often tightly coupled in space and time via signalling circuits and networks based on positive and negative feedback. In this Review, we present and discuss this evidence, and we address one of the fundamental consequences of coupled activation and inactivation: the ability of the Rho GTPases to self-organize, that is, direct their own transition from states of low order to states of high order. We discuss how Rho GTPase self-organization results in the formation of diverse spatiotemporal cortical patterns such as static clusters, oscillatory pulses, travelling wave trains and ring-like waves. Finally, we discuss the advantages of Rho GTPase self-organization and pattern formation for cell function.

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.

Aligned Nanofibers Promote Myoblast Polarization and Myogenesis through Activating Rac-Related Signaling Pathways

The extracellular matrix (ECM) plays a crucial role in regulating cellular behaviors and functions. However, the impact of ECM topography on muscle cell adhesion and differentiation remains poorly understood from a mechanosensing perspective. In this study, we fabricated aligned and random electrospun polycaprolactone (PCL) nanofibers to mimic the structural characteristics of ECM. Mechanism investigations revealed that the orientation of nanofibers promoted C2C12 polarization and myogenesis through Rac-related signaling pathways. Conversely, cells cultured on random fibers exhibited spreading behavior mediated by RhoA/ROCK pathways, resulting in enhanced stress fiber formation but reduced capacity for myogenic differentiation. Our findings highlight the critical role of an ECM structure in muscle regeneration and damage repair, providing novel insights into mechanosensing mechanisms underlying muscle injury diseases.

Effects of Temperature Adaptation on the Metabolism and Physiological Properties of Sturgeon Fish Larvae Cell Line

This study investigated how Atlantic sturgeon cells respond to elevated temperatures, shedding light on the potential impacts of climate change on fish. Atlantic sturgeon (Acipenser oxyrinchus), an IUCN (International Union for Conservation of Nature) Red List species and evolutionarily related to paleonisiform species, may have considerable physiological adaptability, suggesting that this species may be able to cope with changing climatic conditions and higher temperatures. To test this hypothesis, the AOXlar7y cell line was examined at 20 °C (control) and at elevated temperatures of 25 °C and 28 °C. Parameters including proliferation, vitality, morphology, and gene expressions related to proliferation, stemness, and stress were evaluated. Additionally, to achieve a comprehensive understanding of cellular changes, mitochondrial and metabolic activities were assessed using Seahorse XF96. AOXlar7y cells adapted to 28 °C exhibited enhanced mitochondrial adaptability, plasticity, heightened cell proliferation, and increased hsp70 expression. Increased baseline respiration indicated elevated ATP demand, which is potentially linked to higher cell proliferation and heat stress defense. Cells at 28 °C also displayed elevated reserve respiration capacity, suggesting adaptation to energy demands. At 25 °C, AOXlar7y cells showed no changes in basal respiration or mitochondrial capacity, suggesting unchanged ATP demand compared to cells cultivated at 20 °C. Proliferation and glycolytic response to energy requirements were diminished, implying a connection between glycolysis inhibition and proliferation suppression. These research results indicate sturgeon cells are capable of withstanding and adapting to an 8 °C temperature increase. This cellular analysis lays a foundation for future studies aimed at a deeper understanding of fish cell physiological adaptations, which will contribute to a better knowledge of environmental threats facing Atlantic sturgeon and fish populations amid climate change.

Quantum-enhanced diamond molecular tension microscopy for quantifying cellular forces

The constant interplay and information exchange between cells and the microenvironment are essential to their survival and ability to execute biological functions. To date, a few leading technologies such as traction force microscopy, optical/magnetic tweezers, and molecular tension-based fluorescence microscopy are broadly used in measuring cellular forces. However, the considerable limitations, regarding the sensitivity and ambiguities in data interpretation, are hindering our thorough understanding of mechanobiology. Here, we propose an innovative approach, namely, quantum-enhanced diamond molecular tension microscopy (QDMTM), to precisely quantify the integrin-based cell adhesive forces. Specifically, we construct a force-sensing platform by conjugating the magnetic nanotags labeled, force-responsive polymer to the surface of a diamond membrane containing nitrogen-vacancy centers. Notably, the cellular forces will be converted into detectable magnetic variations in QDMTM. After careful validation, we achieved the quantitative cellular force mapping by correlating measurement with the established theoretical model. We anticipate our method can be routinely used in studies like cell-cell or cell-material interactions and mechanotransduction.

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.

Septins provide microenvironment sensing and cortical actomyosin partitioning in motile amoeboid T lymphocytes

The all-terrain motility of lymphocytes in tissues and tissue-like gels is best described as amoeboid motility. For amoeboid motility, lymphocytes do not require specific biochemical or structural modifications to the surrounding extracellular matrix. Instead, they rely on changing shape and steric interactions with the microenvironment. However, the exact mechanism of amoeboid motility remains elusive. Here, we report that septins participate in amoeboid motility of T cells, enabling the formation of F-actin and α-actinin-rich cortical rings at the sites of cell cortex-indenting collisions with the extracellular matrix. Cortical rings compartmentalize cells into chains of spherical segments that are spatially conformed to the available lumens, forming transient "hourglass"-shaped steric locks onto the surrounding collagen fibers. The steric lock facilitates pressure-driven peristaltic propulsion of cytosolic content by individually contracting cell segments. Our results suggest that septins provide microenvironment-guided partitioning of actomyosin contractility and steric pivots required for amoeboid motility of T cells in tissue-like microenvironments.

Quantifying cytoskeletal organization from optical microscopy data

The actin cytoskeleton plays a pivotal role in a broad range of physiological processes including directing cell shape and subcellular organization, determining cell mechanical properties, and sensing and transducing mechanical forces. The versatility of the actin cytoskeleton arises from the ability of actin filaments to assemble into higher order structures through their interaction with a vast set of regulatory proteins. Actin filaments assemble into bundles, meshes, and networks, where different combinations of these structures fulfill specific functional roles. Analyzing the organization and abundance of different actin structures from optical microscopy data provides a valuable metric for assessing cell physiological function and changes associated with disease. However, quantitative measurements of the size, abundance, orientation, and distribution of different types of actin structure remains challenging both from an experimental and image analysis perspective. In this review, we summarize image analysis methods for extracting quantitative values that can be used for characterizing the organization of actin structures and provide selected examples. We summarize the potential sample types and metric reported with different approaches as a guide for selecting an image analysis strategy.

A novel mechanoeffector role of fibroblast S100A4 in myofibroblast transdifferentiation and fibrosis

Fibroblast to myofibroblast transdifferentiation mediates numerous fibrotic disorders, such as idiopathic pulmonary fibrosis (IPF). We have previously demonstrated that non-muscle myosin II (NMII) is activated in response to fibrotic lung extracellular matrix, thereby mediating myofibroblast transdifferentiation. NMII-A is known to interact with the calcium-binding protein S100A4, but the mechanism by which S100A4 regulates fibrotic disorders is unclear. In this study, we show that fibroblast S100A4 is a calcium-dependent, mechanoeffector protein that is uniquely sensitive to pathophysiologic-range lung stiffness (8-25 kPa) and thereby mediates myofibroblast transdifferentiation. Re-expression of endogenous fibroblast S100A4 rescues the myofibroblastic phenotype in S100A4 KO fibroblasts. Analysis of NMII-A/actin dynamics reveals that S100A4 mediates the unraveling and redistribution of peripheral actomyosin to a central location, resulting in a contractile myofibroblast. Furthermore, S100A4 loss protects against murine in vivo pulmonary fibrosis, and S100A4 expression is dysregulated in IPF. Our data reveal a novel mechanosensor/effector role for endogenous fibroblast S100A4 in inducing cytoskeletal redistribution in fibrotic disorders such as IPF.

Direct investigation of cell contraction signal networks by light-based perturbation methods

Cell contraction plays an important role in many physiological and pathophysiological processes. This includes functions in skeletal, heart, and smooth muscle cells, which lead to highly coordinated contractions of multicellular assemblies, and functions in non-muscle cells, which are often highly localized in subcellular regions and transient in time. While the regulatory processes that control cell contraction in muscle cells are well understood, much less is known about cell contraction in non-muscle cells. In this review, we focus on the mechanisms that control cell contraction in space and time in non-muscle cells, and how they can be investigated by light-based methods. The review particularly focusses on signal networks and cytoskeletal components that together control subcellular contraction patterns to perform functions on the level of cells and tissues, such as directional migration and multicellular rearrangements during development. Key features of light-based methods that enable highly local and fast perturbations are highlighted, and how experimental strategies can capitalize on these features to uncover causal relationships in the complex signal networks that control cell contraction.

Nuclear Actin Polymerization Regulates Cell Epithelial-Mesenchymal Transition

Current studies on actin function primarily rely on cytoplasmic actin due to the absence of cellular models specifically expressing nuclear actin. Here, cell models capable of expressing varying levels of nuclear F/G-actin are generated and a significant role of nuclear actin in the regulation of epithelial-mesenchymal transition (EMT) is uncovered. Through immunoprecipitation and mass spectrometry analyses, distinct binding partners for nuclear F-actin (β-catenin, SMAD2, and SMAD3) and nuclear G-actin (MYBBP1A, NKRF, and MYPOP) are investigated, which respectively modulate EMT-promoting and EMT-repressing transcriptional events. While nuclear F-actin promotes EMT with enhanced cell migration, survival, and elongated mesenchymal morphology, nuclear G-actin represses EMT and related cell activities. Mechanistically, nuclear F-actin enhances β-catenin, SMAD2, and SMAD3 expression and stability in the nuclei, while nuclear G-actin increases MYBBP1A, NKRF, and MYPOP expression and stability in the nuclei. The association between nuclear F/G-actin and N-cadherin/E-cadherin in the cell lines (in vitro), and increased nuclear actin polymerization in the wound healing cells (in vivo) affirm a significant role of nuclear actin in EMT regulation. With evidence of nuclear actin polymerization and EMT during development, and irregularities in disease states such as cancer and fibrosis, targeting nuclear actin dynamics to trigger dysregulated EMT warrants ongoing study.

Friction patterns guide actin network contraction

The shape of cells is the outcome of the balance of inner forces produced by the actomyosin network and the resistive forces produced by cell adhesion to their environment. The specific contributions of contractile, anchoring and friction forces to network deformation rate and orientation are difficult to disentangle in living cells where they influence each other. Here, we reconstituted contractile actomyosin networks in vitro to study specifically the role of the friction forces between the network and its anchoring substrate. To modulate the magnitude and spatial distribution of friction forces, we used glass or lipids surface micropatterning to control the initial shape of the network. We adapted the concentration of Nucleating Promoting Factor on each surface to induce the assembly of actin networks of similar densities and compare the deformation of the network toward the centroid of the pattern shape upon myosin-induced contraction. We found that actin network deformation was faster and more coordinated on lipid bilayers than on glass, showing the resistance of friction to network contraction. To further study the role of the spatial distribution of these friction forces, we designed heterogeneous micropatterns made of glass and lipids. The deformation upon contraction was no longer symmetric but biased toward the region of higher friction. Furthermore, we showed that the pattern of friction could robustly drive network contraction and dominate the contribution of asymmetric distributions of myosins. Therefore, we demonstrate that during contraction, both the active and resistive forces are essential to direct the actin network deformation.

Yes-associated protein regulates cortical actin architecture and dynamics through intracellular translocation of Rho GTPase-activating protein 18

Yes-associated protein (YAP) is a transcriptional co-activator that controls the transcription of target genes and modulates the structures of various cytoskeletal architecture as mechanical responses. Although it has been known that YAP regulates actin-regulatory proteins, the detailed molecular mechanism of how they control and coordinate intracellular actin architecture remains elusive. Herein, we aimed to examine the structure and dynamics of intracellular actin architecture from molecular to cellular scales in normal and YAP-knockout (YAP-KO) cells utilizing high-speed atomic force microscopy (HS-AFM) for live-cell imaging and other microscope-based mechanical manipulation and measurement techniques. YAP-KO Madin-Darby canine kidney cells had a higher density and turnover of actin filaments in the cell cortex and a higher elastic modulus. Laser aberration assay demonstrated that YAP-KO cells were more resistant to damage than normal cells. We also found that Rho GTPase-activating protein 18 (ARHGAP18), a downstream factor of YAP, translocated from the cortex to the edge of sparsely cultured YAP-KO cells. It resulted in high RhoA activity and promotion of actin polymerization in the cell cortex and their reductions at the edge. HS-AFM imaging of live cell edge and a cell-migration assay demonstrated lower membrane dynamics and motility of YAP-KO cells than those of normal cells, suggesting lower actin dynamics at the edge. Together, these results demonstrate that a YAP-dependent pathway changes the intracellular distribution of RhoGAP and modulates actin dynamics in different parts of the cell, providing a mechanistic insight into how a mechano-sensitive transcription cofactor regulates multiple intracellular actin architecture and coordinates mechano-responses.

Cooperative cell-cell actin network remodeling to perform Gap junction endocytosis

BACKGROUND

The endocytosis of Gap junction plaques (GJP) requires cytoskeletal forces to internalize such large membranous structures. Actin, which partners the connexin proteins constituting Gap junctions and is located close to Annular Gap Junctions (AGJ), could be actively involved in this physiological process.

RESULTS

Electron Microscopy and Light Microscopy images, associated with time-lapse analysis and 3D reconstruction, used at high resolution and enhanced using ImageJ based software analysis, revealed that: i) actin cables, originating from Donor cells, insert on the edge of GJP and contribute to their invagination, giving rise to AGJ, whereas actin cables on the Acceptor cell side of the plaque are not modified; ii) actin cables from the Donor cell are continuous with the actin network present over the entire GJP surface. These actin cables fuse at a single point distant from the plaque, which then detaches itself from the membrane, condensing to form an actin mass during the final internalization process; iii) the Acceptor cell participates in the last step of the endocytic invagination process by forming an annular actin structure known as an actin ring.

CONCLUSIONS

Together, these data suggest that the endocytosis of GJP is an example of a unique cooperative mechanism between the Donor (the traction of its actin cables) and the Acceptor cells (forming the actin ring).

Range and strength of mechanical interactions of force dipoles in elastic fiber networks

Mechanical forces generated by myosin II molecular motors drive diverse cellular processes, most notably shape change, division and locomotion. These forces may be transmitted over long range through the cytoskeletal medium - a disordered, viscoelastic network of biopolymers. The resulting cell size scale force chains can in principle mediate mechanical interactions between distant actomyosin units, leading to self-organized structural order in the cell cytoskeleton. Inspired by such force transmission through elastic structures in the cytoskeleton, we consider a percolated fiber lattice network, where fibers are represented as linear elastic elements that can both bend and stretch, and the contractile activity of myosin motors is represented by force dipoles. Then, by using a variety of metrics, we show how two such contractile force dipoles interact with each other through their mutual mechanical deformations of the elastic fiber network. As a prelude to two-dipole interactions, we quantify how forces propagate through the network from a single anisotropic force dipole by analyzing clusters of nodes connected by highly strained bonds, as well as through the decay rate of strain energy with distance from a force dipole. We show that predominant fiber bending screens out force propagation, resulting in reduced and strongly network configuration-dependent dipole interactions. On the other hand, stretching-dominated networks support longer-ranged inter-dipole interactions that recapitulate the predictions of linear elasticity theory. By characterizing the differences between tensile and compressive force propagation in the fiber network, we show how inter-dipole interaction depends on the dipoles' mutual separation and orientation. The resulting elastic interaction energy may mediate a force between multiple distant dipoles, leading to their self-organization into ordered configurations. This provides a potential pathway for active mechanical force-driven structural order in elastic biopolymer networks.

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.

Visualization of Cell Membrane Tension Regulated by the Microfilaments as a "Shock Absorber" in Micropatterned Cells

The extracellular stress signal transmits along the cell membrane-cytoskeleton-focal adhesions (FAs) complex, regulating the cell function through membrane tension. However, the mechanism of the complex regulating membrane tension is still unclear. This study designed polydimethylsiloxane stamps with specific shapes to change the actin filaments' arrangement and FAs' distribution artificially in live cells, visualized the membrane tension in real time, and introduced the concept of information entropy to describe the order degree of the actin filaments and plasma membrane tension. The results showed that the actin filaments' arrangement and FAs' distribution in the patterned cells were changed significantly. The hypertonic solution resulted in the plasma membrane tension of the pattern cell changing more evenly and slowly in the zone rich in cytoskeletal filaments than in the zone lacking filaments. In addition, the membrane tension changed less in the adhesive area than in the non-adhesive area when destroying the cytoskeletal microfilaments. This suggested that patterned cells accumulated more actin filaments in the zone where FAs were difficult to generate to maintain the stability of the overall membrane tension. The actin filaments act as shock absorbers to cushion the alternation in membrane tension without changing the final value of membrane tension.

Myosin and [Formula: see text]-actinin regulation of stress fiber contractility under tensile stress

Stress fibers are actomyosin bundles that regulate cellular mechanosensation and force transduction. Interacting with the extracellular matrix through focal adhesion complexes, stress fibers are highly dynamic structures regulated by myosin motors and crosslinking proteins. Under external mechanical stimuli such as tensile forces, the stress fiber remodels its architecture to adapt to external cues, displaying properties of viscoelastic materials. How the structural remodeling of stress fibers is related to the generation of contractile force is not well understood. In this work, we simulate mechanochemical dynamics and force generation of stress fibers using the molecular simulation platform MEDYAN. We model stress fiber as two connecting bipolar bundles attached at the ends to focal adhesion complexes. The simulated stress fibers generate contractile force that is regulated by myosin motors and [Formula: see text]-actinin crosslinkers. We find that stress fibers enhance contractility by reducing the distance between actin filaments to increase crosslinker binding, and this structural remodeling ability depends on the crosslinker turnover rate. Under tensile pulling force, the stress fiber shows an instantaneous increase of the contractile forces followed by a slow relaxation into a new steady state. While the new steady state contractility after pulling depends only on the overlap between actin bundles, the short-term contractility enhancement is sensitive to the tensile pulling distance. We further show that this mechanical response is also sensitive to the crosslinker turnover rate. Our results provide new insights into the stress fiber mechanics that have significant implications for understanding cellular adaptation to mechanical signaling.

The Migration and the Fate of Dental Pulp Stem Cells

Human dental pulp stem cells (hDPSCs) are adult mesenchymal stem cells (MSCs) obtained from dental pulp and derived from the neural crest. They can differentiate into odontoblasts, osteoblasts, chondrocytes, adipocytes and nerve cells, and they play a role in tissue repair and regeneration. In fact, DPSCs, depending on the microenvironmental signals, can differentiate into odontoblasts and regenerate dentin or, when transplanted, replace/repair damaged neurons. Cell homing depends on recruitment and migration, and it is more effective and safer than cell transplantation. However, the main limitations of cell homing are the poor cell migration of MSCs and the limited information we have on the regulatory mechanism of the direct differentiation of MSCs. Different isolation methods used to recover DPSCs can yield different cell types. To date, most studies on DPSCs use the enzymatic isolation method, which prevents direct observation of cell migration. Instead, the explant method allows for the observation of single cells that can migrate at two different times and, therefore, could have different fates, for example, differentiation and self-renewal. DPSCs use mesenchymal and amoeboid migration modes with the formation of lamellipodia, filopodia and blebs, depending on the biochemical and biophysical signals of the microenvironment. Here, we present current knowledge on the possible intriguing role of cell migration, with particular attention to microenvironmental cues and mechanosensing properties, in the fate of DPSCs.

Mechanobiological Adaptation to Hyperosmolarity Enhances Barrier Function in Human Vascular Microphysiological System

In infectious disease such as sepsis and COVID-19, blood vessel leakage treatment is critical to prevent fatal progression into multi-organ failure and ultimately death, but the existing effective therapeutic modalities that improve vascular barrier function are limited. Here, this study reports that osmolarity modulation can significantly improve vascular barrier function, even in an inflammatory condition. 3D human vascular microphysiological systems and automated permeability quantification processes for high-throughput analysis of vascular barrier function are utilized. Vascular barrier function is enhanced by >7-folds with 24-48 h hyperosmotic exposure (time window of emergency care; >500 mOsm L^-1 ) but is disrupted after hypo-osmotic exposure (<200 mOsm L^-1 ). By integrating genetic and protein level analysis, it is shown that hyperosmolarity upregulates vascular endothelial-cadherin, cortical F-actin, and cell-cell junction tension, indicating that hyperosmotic adaptation mechanically stabilizes the vascular barrier. Importantly, improved vascular barrier function following hyperosmotic exposure is maintained even after chronic exposure to proinflammatory cytokines and iso-osmotic recovery via Yes-associated protein signaling pathways. This study suggests that osmolarity modulation may be a unique therapeutic strategy to proactively prevent infectious disease progression into severe stages via vascular barrier function protection.

Vitronectin acts as a key regulator of adhesion and migration in human umbilical cord-derived MSCs under different stress conditions

To improve mesenchymal stem cell (MSC)-based therapy efficacy, it is critical to identify factors involved in regulating migration and adhesion of MSCs under microenvironmental stress conditions. We observed that human Wharton's jelly-derived MSCs (WJ-MSCs) exhibited increase in cell spread area and adhesion, with reduction in cellular migration under serum starvation stress. The changes in adhesion and migration characteristics were accompanied by formation of large number of super mature focal adhesions along with extensive stress fibres and altered ECM gene expression with notable induction in vitronectin (VTN) expression. NF-κβ was found to be a positive regulator of VTN expression while ERK pathway regulated it negatively. Inhibition of these signalling pathways or knocking down of VTN under serum starvation established the correlation between increase in VTN expression and increased cellular adhesion with corresponding reduction in cell migration. VTN knockdown also resulted in reduction of super mature focal adhesions and extensive stress fibres, formed under serum starvation stress. Additionally, VTN induction was not detected in hypoxia-treated WJ-MSCs, and the MSCs showed no significant change in the adhesion or migration properties under hypoxia. VTN is established as a key player which possibly regulates the adhesion and migration properties of WJ-MSCs via focal adhesion signalling.

Cell shape and tension alter focal adhesion structure

Cells are not only anchored to the extracellular matrix via the focal adhesion complex, the focal adhesion complex also serves as a sensor for force transduction. How tension influences the structure of focal adhesions is not well understood. Here, we analyse the effect of tension on the location of key focal adhesion proteins, namely vinculin, paxillin and actin. We use micropatterning on gold surfaces to manipulate the cell shape, to create focal adhesions at specific cell areas, and to perform metal-induced energy transfer (MIET) measurements on the patterned cells. MIET resolves the different protein locations with respect to the gold surface with nanometer accuracy. Further, we use drugs influencing the cellular motor protein myosin or mechanosensitive ion channels to get deeper insight into focal adhesions at different tension states. We show here that in particular actin is affected by the rationally tuned force balance. Blocking mechanosensitive ion channels has a particularly high influence on the actin and focal adhesion architecture, resulting in larger focal adhesions with elevated paxillin and vinculin and strongly lowered actin stress fibres. Our results can be explained by a balance of adhesion tension with cellular tension together with ion channel-controlled focal adhesion homeostasis, where high cellular tension leads to an elevation of vinculin and actin, while high adhesion tension lowers these proteins.

The metastatic capacity of high-grade serous ovarian cancer cells changes along disease progression: inhibition by mifepristone

BACKGROUND

Simplistic two-dimensional (2D) in vitro assays have long been the standard for studying the metastatic abilities of cancer cells. However, tri-dimensional (3D) organotypic models provide a more complex environment, closer to that seen in patients, and thereby provide a more accurate representation of their true capabilities. Our laboratory has previously shown that the antiprogestin and antiglucocorticoid mifepristone can reduce the growth, adhesion, migration, and invasion of various aggressive cancer cells assessed using 2D assays. In this study, we characterize the metastatic capabilities of high-grade serous ovarian cancer cells generated along disease progression, in both 2D and 3D assays, and the ability of cytostatic doses of mifepristone to inhibit them.

METHODS

High-grade serous ovarian cancer cells collected from two separate patients at different stages of their disease were used throughout the study. The 2D wound healing and Boyden chamber assays were used to study migration, while a layer of extracellular matrix was added to the Boyden chamber to study invasion. A 3D organotypic model, composed of fibroblasts embedded in collagen I and topped with a monolayer of mesothelial cells was used to further study cancer cell adhesion and mesothelial displacement. All assays were studied in cells, which were originally harvested from two patients at different stages of disease progression, in the absence or presence of cytostatic doses of mifepristone.

RESULTS

2D in vitro assays demonstrated that the migration and invasive rates of the cells isolated from both patients decreased along disease progression. Conversely, in both patients, cells representing late-stage disease demonstrated a higher adhesion capacity to the 3D organotypic model than those representing an early-stage disease. This adhesive behavior is associated with the in vivo tumor capacity of the cells. Regardless of these differences in adhesive, migratory, and invasive behavior among the experimental protocols used, cytostatic doses of mifepristone were able to inhibit the adhesion, migration, and invasion rates of all cells studied, regardless of their basal capabilities over simplistic or organotypic metastatic in vitro model systems. Finally, we demonstrate that when cells acquire the capacity to grow spontaneously as spheroids, they do attach to a 3D organotypic model system when pre-incubated with conditioned media. Of relevance, mifepristone was able to cause dissociation of these multicellular structures.

CONCLUSION

Differences in cellular behaviours were observed between 2 and 3D assays when studying the metastatic capabilities of high-grade serous ovarian cancer cells representing disease progression. Mifepristone inhibited these metastatic capabilities in all assays studied.

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.

Transforming growth factor-β in tumour development

Transforming growth factor-β (TGFβ) is a ubiquitous cytokine essential for embryonic development and postnatal tissue homeostasis. TGFβ signalling regulates several biological processes including cell growth, proliferation, apoptosis, immune function, and tissue repair following injury. Aberrant TGFβ signalling has been implicated in tumour progression and metastasis. Tumour cells, in conjunction with their microenvironment, may augment tumourigenesis using TGFβ to induce epithelial-mesenchymal transition, angiogenesis, lymphangiogenesis, immune suppression, and autophagy. Therapies that target TGFβ synthesis, TGFβ-TGFβ receptor complexes or TGFβ receptor kinase activity have proven successful in tissue culture and in animal models, yet, due to limited understanding of TGFβ biology, the outcomes of clinical trials are poor. Here, we review TGFβ signalling pathways, the biology of TGFβ during tumourigenesis, and how protein quality control pathways contribute to the tumour-promoting outcomes of TGFβ signalling.

Identification of AGR2 Gene-Specific Expression Patterns Associated with Epithelial-Mesenchymal Transition

The TGF-β signaling pathway is involved in numerous cellular processes, and its deregulation may result in cancer development. One of the key processes in tumor progression and metastasis is epithelial to mesenchymal transition (EMT), in which TGF-β signaling plays important roles. Recently, AGR2 was identified as a crucial component of the cellular machinery responsible for maintaining the epithelial phenotype, thereby interfering with the induction of mesenchymal phenotype cells by TGF-β effects in cancer. Here, we performed transcriptomic profiling of A549 lung cancer cells with CRISPR-Cas9 mediated AGR2 knockout with and without TGF-β treatment. We identified significant changes in transcripts associated with focal adhesion and eicosanoid production, in particular arachidonic acid metabolism. Changes in transcripts associated with the focal adhesion pathway were validated by RT-qPCR of COL4A1, COL4A2, FLNA, VAV3, VEGFA, and VINC mRNAs. In addition, immunofluorescence showed the formation of stress fibers and vinculin foci in cells without AGR2 and in response to TGF-β treatment, with synergistic effects observed. These findings imply that both AGR2 downregulation and TGF-β have a role in focal adhesion formation and cancer cell migration and invasion. Transcripts associated with arachidonic acid metabolism were downregulated after both AGR2 knockout and TGF-β treatment and were validated by RT-qPCR of GPX2, PTGS2, and PLA2G4A. Since PGE₂ is a product of arachidonic acid metabolism, its lowered concentration in media from AGR2-knockout cells was confirmed by ELISA. Together, our results demonstrate that AGR2 downregulation and TGF-β have an essential role in focal adhesion formation; moreover, we have identified AGR2 as an important component of the arachidonic acid metabolic pathway.

Inhibiting DNA methylation as a strategy to enhance adipose-derived stem cells differentiation: Focus on the role of Akt/mTOR and Wnt/β-catenin pathways on adipogenesis

Adipose-derived mesenchymal stem cells (ASCs) represent a valid therapeutic option for clinical application in several diseases, due to their ability to repair damaged tissues and to mitigate the inflammatory/immune response. A better understanding of the underlying mechanisms regulating ASC biology might represent the chance to modulate their in vitro characteristics and differentiation potential for regenerative medicine purposes. Herein, we investigated the effects of the demethylating agent 5-azacytidine (5-aza) on proliferation, clonogenicity, migration, adipogenic differentiation and senescence of ASCs, to identify the molecular pathways involved. Through functional assays, we observed a detrimental effect of 5-aza on ASC self-renewal capacity and migration, accompanied by actin cytoskeleton reorganization, with decreased stress fibers. Conversely, 5-aza treatment enhanced ASC adipogenic differentiation, as assessed by lipid accumulation and expression of lineage-specific markers. We analyzed the involvement of the Akt/mTOR, MAPK and Wnt/β-catenin pathways in these processes. Our results indicated impairment of Akt and ERK phosphorylation, potentially explaining the reduced cell proliferation and migration. We observed a 5-aza-mediated inhibition of the Wnt signaling pathway, this potentially explaining the pro-adipogenic effect of the drug. Finally, 5-aza treatment significantly induced ASC senescence, through upregulation of the p53/p21 axis. Our data may have important translational implications, by helping in clarifying the potential risks and advantages of using epigenetic treatment to improve ASC characteristics for cell-based clinical approaches.

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.

Coro1B and Coro1C regulate lamellipodia dynamics and cell motility by tuning branched actin turnover

Actin filament dynamics must be precisely controlled in cells to execute behaviors such as vesicular trafficking, cytokinesis, and migration. Coronins are conserved actin-binding proteins that regulate several actin-dependent subcellular processes. Here, we describe a new conditional knockout cell line for two ubiquitous coronins, Coro1B and Coro1C. These coronins, which strongly co-localize with Arp2/3-branched actin, require Arp2/3 activity for proper subcellular localization. Coronin null cells have altered lamellipodial protrusion dynamics due to increased branched actin density and reduced actin turnover within lamellipodia, leading to defective haptotaxis. Surprisingly, excessive cofilin accumulates in coronin null lamellipodia, a result that is inconsistent with the current models of coronin-cofilin functional interaction. However, consistent with coronins playing a pro-cofilin role, coronin null cells have increased F-actin levels. Lastly, we demonstrate that the loss of coronins increases accompanied by an increase in cellular contractility. Together, our observations reveal that coronins are critical for proper turnover of branched actin networks and that decreased actin turnover leads to increased cellular contractility.

Myosin motors on the pathway of viral infections

Molecular motors are microscopic machines that use energy from adenosine triphosphate (ATP) hydrolysis to generate movement. While kinesins and dynein are molecular motors associated with microtubule tracks, myosins bind to and move on actin filaments. Mammalian cells express several myosin motors. They power cellular processes such as endo- and exocytosis, intracellular trafficking, transcription, migration, and cytokinesis. As viruses navigate through cells, they may take advantage or be hindered by host components and machinery, including the cytoskeleton. This review delves into myosins' cell roles and compares them to their reported functions in viral infections. In most cases, the previously described myosin functions align with their reported role in viral infections, although not in all cases. This opens the possibility that knowledge obtained from studying myosins in viral infections might shed light on new physiological roles for myosins in cells. However, given the high number of myosins expressed and the variety of viruses investigated in the different studies, it is challenging to infer whether the interactions found are specific to a single virus or can be applied to other viruses with the same characteristics. We conclude that the participation of myosins in viral cycles is still a largely unexplored area, especially concerning unconventional myosins.

Emergence and maintenance of variable-length actin filaments in a limiting pool of building blocks

Actin is one of the key structural components of the eukaryotic cytoskeleton that regulates cellular architecture and mechanical properties. Dynamic regulation of actin filament length and organization is essential for the control of many physiological processes including cell adhesion, motility and division. While previous studies have mostly focused on the mechanisms controlling the mean length of individual actin filaments, it remains poorly understood how distinct actin filament populations in cells maintain different lengths using the same set of molecular building blocks. Here we develop a theoretical model for the length regulation of multiple actin filaments by nucleation and growth rate modulation by actin binding proteins in a limiting pool of monomers. We first show that spontaneous nucleation of actin filaments naturally leads to heterogeneities in filament length distribution. We then investigate the effects of filament growth inhibition by capping proteins and growth promotion by formin proteins on filament length distribution. We find that filament length heterogeneity can be increased by growth inhibition, whereas growth promoters do not significantly affect length heterogeneity. Interestingly, a competition between filament growth inhibitors and growth promoters can give rise to bimodal filament length distribution as well as a highly heterogeneous length distribution with large statistical dispersion. We quantitatively predict how heterogeneity in actin filament length can be modulated by tuning F-actin nucleation and growth rates in order to create distinct filament subpopulations with different lengths. SIGNIFICANCE: Actin filaments organize into different functional network architectures within eukaryotic cells. To maintain distinct actin network architectures, it is essential to regulate the lengths of actin filaments. While the mechanisms controlling the lengths of individual actin filaments have been extensively studied, the regulation of length heterogeneity in actin filament populations is not well understood. Here we show that the modulation of actin filament growth and nucleation rates by actin binding proteins can regulate actin length distribution and create distinct sub-populations with different lengths. In particular, by tuning concentrations of formin, profilin and capping proteins, various aspects of actin filament length distribution can be controlled. Insights gained from our results may have significant implications for the regulation of actin filament length heterogeneity and architecture within a cell.

Precise Tuning and Characterization of Viscoelastic Interfaces for the Study of Early Epithelial-Mesenchymal Transition Behaviors

There is growing evidence that cellular functions are regulated by the viscoelastic nature of surrounding matrices. This study aimed to investigate the impact of interfacial viscoelasticity on adhesion and epithelial-mesenchymal transition (EMT) behaviors of epithelial cells. The interfacial viscoelasticity was manipulated using spin-coated thin films composed of copolymers of ε-caprolactone and d,l-lactide photo-cross-linked with benzophenone, whose mechanical properties were characterized using atomic force microscopy and a rheometer. The critical range for the morphological transition of epithelial Madin-Darby canine kidney (MDCK) cells was of the order of 10² ms relaxation time, which was 1-2 orders of magnitude smaller than the relaxation times reported (10-10² s). An analysis of strain rate-dependent viscoelastic properties revealed that the difference was caused by the different strain rate/frequency used for the mechanical characterization of the interface and bulk. Furthermore, decoupling of the interfacial viscous and elastic terms demonstrated that E/N-cadherin expression levels were regulated differently by interfacial relaxation and elasticity. These results confirm the significance of precise manipulation and characterization of interfacial viscoelasticity in mechanobiology studies on EMT progression.

α-Parvin Defines a Specific Integrin Adhesome to Maintain the Glomerular Filtration Barrier

BACKGROUND

The cell-matrix adhesion between podocytes and the glomerular basement membrane is essential for the integrity of the kidney's filtration barrier. Despite increasing knowledge about the complexity of integrin adhesion complexes, an understanding of the regulation of these protein complexes in glomerular disease remains elusive.

METHODS

We mapped the in vivo composition of the podocyte integrin adhesome. In addition, we analyzed conditional knockout mice targeting a gene (Parva) that encodes an actin-binding protein (α-parvin), and murine disease models. To evaluate podocytes in vivo, we used super-resolution microscopy, electron microscopy, multiplex immunofluorescence microscopy, and RNA sequencing. We performed functional analysis of CRISPR/Cas9-generated PARVA single knockout podocytes and PARVA and PARVB double knockout podocytes in three- and two-dimensional cultures using specific extracellular matrix ligands and micropatterns.

RESULTS

We found that PARVA is essential to prevent podocyte foot process effacement, detachment from the glomerular basement membrane, and the development of FSGS. Through the use of in vitro and in vivo models, we identified an inherent PARVB-dependent compensatory module at podocyte integrin adhesion complexes, sustaining efficient mechanical linkage at the filtration barrier. Sequential genetic deletion of PARVA and PARVB induces a switch in structure and composition of integrin adhesion complexes. This redistribution of these complexes translates into a loss of the ventral actin cytoskeleton, decreased adhesion capacity, impaired mechanical resistance, and dysfunctional extracellular matrix assembly.

CONCLUSIONS

The findings reveal adaptive mechanisms of podocyte integrin adhesion complexes, providing a conceptual framework for therapeutic strategies to prevent podocyte detachment in glomerular disease.

Influence of ROCK Pathway Manipulation on the Actin Cytoskeleton Height

The actin cytoskeleton with its dynamic properties serves as the driving force for the movement and division of cells and gives the cell shape and structure. Disorders in the actin cytoskeleton occur in many diseases. Deeper understanding of its regulation is essential in order to better understand these biochemical processes. In our study, we use metal-induced energy transfer (MIET) as a tool to quantitatively examine the rarely considered third dimension of the actin cytoskeleton with nanometer accuracy. In particular, we investigate the influence of different drugs acting on the ROCK pathway on the three-dimensional actin organization. We find that cells treated with inhibitors have a lower actin height to the substrate while treatment with a stimulator for the ROCK pathway increases the actin height to the substrate, while the height of the membrane remains unchanged. This reveals the precise tuning of adhesion and cytoskeleton tension, which leads to a rich three-dimensional structural behaviour of the actin cytoskeleton. This finetuning is differentially affected by either inhibition or stimulation. The high axial resolution shows the importance of the precise finetuning of the actin cytoskeleton and the disturbed regulation of the ROCK pathway has a significant impact on the actin behavior in the z dimension.

Integrating Soft Hydrogel with Nanostructures Reinforces Stem Cell Adhesion and Differentiation

Biophysical cues can regulate stem cell behaviours and have been considered as critical parameters of synthetic biomaterials for tissue engineering. In particular, hydrogels have been utilized as promising biomimetic and biocompatible materials to emulate the microenvironment. Therefore, well-defined mechanical properties of a hydrogel are important to direct desirable phenotypes of cells. Yet, limited research pays attention to engineering soft hydrogel with improved cell adhesive property, which is crucial for stem cell differentiation. Herein, we introduce silica nanoparticles (SiO2 NPs) onto the surface of methacrylated hyaluronic (MeHA) hydrogel to manipulate the presentation of cell adhesive ligands (RGD) clusters, while remaining similar bulk mechanical properties (2.79 ± 0.31 kPa) to that of MeHA hydrogel (3.08 ± 0.68 kPa). RGD peptides are either randomly decorated in the MeHA hydrogel network or on the immobilized SiO2 NPs (forming MeHA–SiO2). Our results showed that human mesenchymal stem cells exhibited a ~1.3-fold increase in the percentage of initial cell attachment, a ~2-fold increase in cell spreading area, and enhanced expressions of early-stage osteogenic markers (RUNX2 and alkaline phosphatase) for cells undergoing osteogenic differentiation with the osteogenic medium on MeHA–SiO2 hydrogel, compared to those cultured on MeHA hydrogel. Importantly, the cells cultivated on MeHA–SiO2 expressed a ~5-fold increase in nuclear localization ratio of the yes-associated protein, which is known to be mechanosensory in stem cells, compared to the cells cultured on MeHA hydrogel, thereby promoting osteogenic differentiation of stem cells. These findings demonstrate the potential use of nanomaterials into a soft polymeric matrix for enhanced cell adhesion and provide valuable guidance for the rational design of biomaterials for implantation.

Developing biomaterials to mediate the spatial distribution of integrins

Innovation in material design to regulate cell behavior and function is one of the primary tasks in materials science. Integrins, a family of cell surface-adhesion receptors that mechanically connect the extracellular matrix (ECM) to the intracellular cytoskeleton, have long served as primary targets for the design of biomaterials because their activity is not only critical to a wide range of cell and tissue functions but also subject to very tight and complex regulations from the outside environment. To review the recent progress of material innovations targeting the spatial distribution of integrins, we first introduce the interaction mechanisms between cells and the ECM by highlighting integrin-based cell adhesions, describing how integrins respond to environmental stimuli, including variations in ligand presentation, mechanical cues, and topographical variations. Then, we overview the current development of soft materials in guiding cell behaviors and functions via spatial regulation of integrins. Finally, we discuss the current limitations of these technologies and the advances that may be achieved in the future. Undoubtedly, synthetic soft materials that mediate the spatial distribution of integrins play an important role in biomaterial innovations for advancing biomedical applications and addressing fundamental biological questions.

DAAM mediates the assembly of long-lived, treadmilling stress fibers in collectively migrating epithelial cells in Drosophila

Stress fibers (SFs) are actomyosin bundles commonly found in individually migrating cells in culture. However, whether and how cells use SFs to migrate in vivo or collectively is largely unknown. Studying the collective migration of the follicular epithelial cells in Drosophila, we found that the SFs in these cells show a novel treadmilling behavior that allows them to persist as the cells migrate over multiple cell lengths. Treadmilling SFs grow at their fronts by adding new integrin-based adhesions and actomyosin segments over time. This causes the SFs to have many internal adhesions along their lengths, instead of adhesions only at the ends. The front-forming adhesions remain stationary relative to the substrate and typically disassemble as the cell rear approaches. By contrast, a different type of adhesion forms at the SF's terminus that slides with the cell's trailing edge as the actomyosin ahead of it shortens. We further show that SF treadmilling depends on cell movement and identify a developmental switch in the formins that mediate SF assembly, with Dishevelled-associated activator of morphogenesis acting during migratory stages and Diaphanous acting during postmigratory stages. We propose that treadmilling SFs keep each cell on a linear trajectory, thereby promoting the collective motility required for epithelial migration.

A Layered View on Focal Adhesions

Simple Summary

The cytoskeleton is a network of protein fibres within cells that provide structure and support intracellular transport. Focal adhesions are protein complexes associated with the outer cell membrane that are found at the ends of specialised actin fibres of this cytoskeleton. They mediate cell adhesion by connecting the cytoskeleton to the extracellular matrix, a protein and sugar network that surrounds cells in tissues. Focal adhesions also translate forces on actin fibres into forces contributing to cell migration. Cell adhesion and migration are crucial to diverse biological processes such as embryonic development, proper functioning of the immune system or the metastasis of cancer cells. Advances in fluorescence microscopy and data analysis methods provided a more detailed understanding of the dynamic ways in which proteins bind and dissociate from focal adhesions and how they are organised within these protein complexes. In this review, we provide an overview of the advances in the current scientific understanding of focal adhesions and summarize relevant imaging techniques. One of the key insights is that focal adhesion proteins are organised into three layers parallel to the cell membrane. We discuss the relevance of this layered nature for the functioning of focal adhesion.

Abstract

The cytoskeleton provides structure to cells and supports intracellular transport. Actin fibres are crucial to both functions. Focal Adhesions (FAs) are large macromolecular multiprotein assemblies at the ends of specialised actin fibres linking these to the extracellular matrix. FAs translate forces on actin fibres into forces contributing to cell migration. This review will discuss recent insights into FA protein dynamics and their organisation within FAs, made possible by advances in fluorescence imaging techniques and data analysis methods. Over the last decade, evidence has accumulated that FAs are composed of three layers parallel to the plasma membrane. We focus on some of the most frequently investigated proteins, two from each layer, paxillin and FAK (bottom, integrin signalling layer), vinculin and talin (middle, force transduction layer) and zyxin and VASP (top, actin regulatory layer). Finally, we discuss the potential impact of this layered nature on different aspects of FA behaviour.

Physics of liquid crystals in cell biology

The past decade has witnessed a rapid growth in understanding of the pivotal roles of mechanical stresses and physical forces in cell biology. As a result, an integrated view of cell biology is evolving, where genetic and molecular features are scrutinised hand in hand with physical and mechanical characteristics of cells. Physics of liquid crystals has emerged as a burgeoning new frontier in cell biology over the past few years, fuelled by an increasing identification of orientational order and topological defects in cell biology, spanning scales from subcellular filaments to individual cells and multicellular tissues. Here, we provide an account of the most recent findings and developments, together with future promises and challenges in this rapidly evolving interdisciplinary research direction.

Discrete mechanical model of lamellipodial actin network implements molecular clutch mechanism and generates arcs and microspikes

Mechanical forces, actin filament turnover, and adhesion to the extracellular environment regulate lamellipodial protrusions. Computational and mathematical models at the continuum level have been used to investigate the molecular clutch mechanism, calculating the stress profile through the lamellipodium and around focal adhesions. However, the forces and deformations of individual actin filaments have not been considered while interactions between actin networks and actin bundles is not easily accounted with such methods. We develop a filament-level model of a lamellipodial actin network undergoing retrograde flow using 3D Brownian dynamics. Retrograde flow is promoted in simulations by pushing forces from the leading edge (due to actin polymerization), pulling forces (due to molecular motors), and opposed by viscous drag in cytoplasm and focal adhesions. Simulated networks have densities similar to measurements in prior electron micrographs. Connectivity between individual actin segments is maintained by permanent and dynamic crosslinkers. Remodeling of the network occurs via the addition of single actin filaments near the leading edge and via filament bond severing. We investigated how several parameters affect the stress distribution, network deformation and retrograde flow speed. The model captures the decrease in retrograde flow upon increase of focal adhesion strength. The stress profile changes from compression to extension across the leading edge, with regions of filament bending around focal adhesions. The model reproduces the observed reduction in retrograde flow speed upon exposure to cytochalasin D, which halts actin polymerization. Changes in crosslinker concentration and dynamics, as well as in the orientation pattern of newly added filaments demonstrate the model's ability to generate bundles of filaments perpendicular (actin arcs) or parallel (microspikes) to the protruding direction.