TLNRD1 is a CCM complex component and regulates endothelial barrier integrity.

We previously identified talin rod domain-containing protein 1 (TLNRD1) as a potent actin-bundling protein in vitro. Here, we report that TLNRD1 is expressed in the vasculature in vivo. Its depletion leads to vascular abnormalities in vivo and modulation of endothelial cell monolayer integrity in vitro. We demonstrate that TLNRD1 is a component of the cerebral cavernous malformations (CCM) complex through its direct interaction with CCM2, which is mediated by a hydrophobic C-terminal helix in CCM2 that attaches to a hydrophobic groove on the four-helix domain of TLNRD1. Disruption of this binding interface leads to CCM2 and TLNRD1 accumulation in the nucleus and actin fibers. Our findings indicate that CCM2 controls TLNRD1 localization to the cytoplasm and inhibits its actin-bundling activity and that the CCM2-TLNRD1 interaction impacts endothelial actin stress fiber and focal adhesion formation. Based on these results, we propose a new pathway by which the CCM complex modulates the actin cytoskeleton and vascular integrity.

Mechanically induced topological transition of spectrin regulates its distribution in the mammalian cell cortex.

The cell cortex is a dynamic assembly formed by the plasma membrane and underlying cytoskeleton. As the main determinant of cell shape, the cortex ensures its integrity during passive and active deformations by adapting cytoskeleton topologies through yet poorly understood mechanisms. The spectrin meshwork ensures such adaptation in erythrocytes and neurons by adopting different organizations. Erythrocytes rely on triangular-like lattices of spectrin tetramers, whereas in neurons they are organized in parallel, periodic arrays. Since spectrin is ubiquitously expressed, we exploited Expansion Microscopy to discover that, in fibroblasts, distinct meshwork densities co-exist. Through biophysical measurements and computational modeling, we show that the non-polarized spectrin meshwork, with the intervention of actomyosin, can dynamically transition into polarized clusters fenced by actin stress fibers that resemble periodic arrays as found in neurons. Clusters experience lower mechanical stress and turnover, despite displaying an extension close to the tetramer contour length. Our study sheds light on the adaptive properties of spectrin, which participates in the protection of the cell cortex by varying its densities in response to key mechanical features.

Plakophilin 4 controls the spatio-temporal activity of RhoA at adherens junctions to promote cortical actin ring formation and tissue tension.

Plakophilin 4 (PKP4) is a component of cell-cell junctions that regulates intercellular adhesion and Rho-signaling during cytokinesis with an unknown function during epidermal differentiation. Here we show that keratinocytes lacking PKP4 fail to develop a cortical actin ring, preventing adherens junction maturation and generation of tissue tension. Instead, PKP4-depleted cells display increased stress fibers. PKP4-dependent RhoA localization at AJs was required to activate a RhoA-ROCK2-MLCK-MLC2 axis and organize actin into a cortical ring. AJ-associated PKP4 provided a scaffold for the Rho activator ARHGEF2 and the RhoA effectors MLCK and MLC2, facilitating the spatio-temporal activation of RhoA signaling at cell junctions to allow cortical ring formation and actomyosin contraction. In contrast, association of PKP4 with the Rho suppressor ARHGAP23 reduced ARHGAP23 binding to RhoA which prevented RhoA activation in the cytoplasm and stress fiber formation. These data identify PKP4 as an AJ component that transduces mechanical signals into cytoskeletal organization.

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.

Targeting F-actin stress fibers to suppress the dedifferentiated phenotype in chondrocytes.

Actin is a central mediator of the chondrocyte phenotype. Monolayer expansion of articular chondrocytes on tissue culture polystyrene, for cell-based repair therapies, leads to chondrocyte dedifferentiation. During dedifferentiation, chondrocytes spread and filamentous (F-)actin reorganizes from a cortical to a stress fiber arrangement causing a reduction in cartilage matrix expression and an increase in fibroblastic matrix and contractile molecule expression. While the downstream mechanisms regulating chondrocyte molecular expression by alterations in F-actin organization have become elucidated, the critical upstream regulators of F-actin networks in chondrocytes are not completely known. Tropomyosin (TPM) and the RhoGTPases are known regulators of F-actin networks. The main purpose of this study is to elucidate the regulation of passaged chondrocyte F-actin stress fiber networks and cell phenotype by the specific TPM, TPM3.1, and the RhoGTPase, CDC42. Our results demonstrated that TPM3.1 associates with cortical F-actin and stress fiber F-actin in primary and passaged chondrocytes, respectively. In passaged cells, we found that pharmacological TPM3.1 inhibition or siRNA knockdown causes F-actin reorganization from stress fibers back to cortical F-actin and causes an increase in G/F-actin. CDC42 inhibition also causes formation of cortical F-actin. However, pharmacological CDC42 inhibition, but not TPM3.1 inhibition, leads to the re-association of TPM3.1 with cortical F-actin. Both TPM3.1 and CDC42 inhibition, as well as TPM3.1 knockdown, reduces nuclear localization of myocardin related transcription factor, which suppresses dedifferentiated molecule expression. We confirmed that TPM3.1 or CDC42 inhibition partially redifferentiates passaged cells by reducing fibroblast matrix and contractile expression, and increasing chondrogenic SOX9 expression. A further understanding on the regulation of F-actin in passaged cells may lead into new insights to stimulate cartilage matrix expression in cells for regenerative therapies.

The concerted action of SEPT9 and EPLIN modulates the adhesion and migration of human fibroblasts.

Septins are cytoskeletal proteins that participate in cell adhesion, migration, and polarity establishment. The septin subunit SEPT9 directly interacts with the single LIM domain of epithelial protein lost in neoplasm (EPLIN), an actin-bundling protein. Using a human SEPT9 KO fibroblast cell line, we show that cell adhesion and migration are regulated by the interplay between both proteins. The low motility of SEPT9-depleted cells could be partly rescued by increased levels of EPLIN. The normal organization of actin-related filopodia and stress fibers was directly dependent on the expression level of SEPT9 and EPLIN. Increased levels of SEPT9 and EPLIN enhanced the size of focal adhesions in cell protrusions, correlating with stabilization of actin bundles. Conversely, decreased levels had the opposite effect. Our work thus establishes the interaction between SEPT9 and EPLIN as an important link between the septin and the actin cytoskeleton, influencing cell adhesion, motility, and migration.

A novel strategy to facilitate uniform epithelial cell maturation using liquid-liquid interfaces.

Epithelial tissue forms and maintains a critical barrier function in the body. A novel culture design aimed at promoting uniform maturation of epithelial cells using liquid materials is described. Culturing Madin-Darby canine kidney (MDCK) cells at the liquid-liquid interface yielded reduced migration and stimulated active cell growth. Similar to solid-liquid interfaces, cells cultured on a fibronectin-coated liquid-liquid interface exhibited active migration and growth, ultimately reaching a confluent state. These cells exhibited reduced stress fiber formation and adopted a cobblestone-like shape, which led to their even distribution in the culture vessel. To inhibit stress fiber formation and apoptosis, the exposure of cells on liquid-liquid interfaces to Y27632, a specific inhibitor of the Rho-associated protein kinase (ROCK), facilitated tight junction formation (frequency of ZO-2-positive cells, FZ = 0.73). In Y27632-exposed cells on the liquid-liquid interface, the value obtained by subtracting the standard deviation of the ratio of nucleus densities in each region that compartmentalized a culture vessel from 1, denoted as HLN, was 0.93 ± 0.01, indicated even cell distribution in the culture vessel at t = 72 h. The behavior of epithelial cells on liquid-liquid interfaces contributes to the promotion of their uniform maturation.

Acetylenic tricyclic bis-(cyano enone) interacts with Cys 374 of actin, a residue necessary for stress fiber formation and cell migration.

The migratory and invasive potential of tumour cells relies on the actin cytoskeleton. We previously demonstrated that the tricyclic compound, TBE-31, inhibits actin polymerization and here we further examine the precise interaction between TBE-31 and actin. We demonstrate that iodoacetamide, a cysteine (Cys) alkylating agent, interferes with the ability of TBE-31 to interact with actin. In addition, in silico analysis identified Cys 217, Cys 272, Cys 285 and Cys 374 as potential binding sites for TBE-31. Using mass spectrometry analysis, we determined that TBE-31 associates with actin with a stoichiometric ratio of 1:1. We mutated the identified cysteines of actin to alanine and performed a pull-down analysis with a biotin labeled TBE-31 and demonstrated that by mutating Cys 374 to alanine the association between TBE-31 and actin was significantly reduced, suggesting that TBE-31 binds to Cys 374. A characterization of the NIH3T3 cells overexpressing eGFP-actin-C374A showed reduced stress fiber formation, suggesting Cys 374 is necessary for efficient incorporation into filamentous actin. Furthermore, migration of eGFP-Actin-WT expressing cells were observed to be inhibited by TBE-31, however fewer eGFP-Actin-C374A expressing cells were observed to migrate compared to the cells expressing eGFP-Actin-WT in the presence or absence of TBE-31. Taken together, our results suggest that TBE-31 binds to Cys 374 of actin to inhibit actin stress fiber formation and may potentially be a mechanism through which TBE-31 inhibits cell migration.

The Snail signaling branch downstream of the TGF-ß/Smad3 pathway mediates Rho activation and subsequent stress fiber formation.

Cancer cells acquire malignant phenotypes through an epithelial-mesenchymal transition, which is induced by environmental factors or extracellular signaling molecules, including transforming growth factor-ß (TGF-ß). Among epithelial-mesenchymal transition-associated cell responses, cell morphological changes and cell motility are closely associated with remodeling of the actin stress fibers. Here, we examined the TGF-ß signaling pathways leading to these cell responses. Through knockdown experiments in A549 lung adenocarcinoma cells, we found that Smad3-mediated induction of Snail, but not that of Slug, is indispensable for morphological changes, stress fiber formation, and enhanced motility in cells stimulated with TGF-ß. Ectopic expression of Snail in SMAD3-knockout cells rescued the defect in morphological changes and stress fiber formation by TGF-ß, indicating that the role of Smad3 in these responses is to upregulate Snail expression. Mechanistically, Snail is required for TGF-ß-induced upregulation of Wnt5b, which in turn activates RhoA and subsequent stress fiber formation in cooperation with phosphoinositide 3-kinase. However, ectopic expression of Snail in SMAD3-knockout cells failed to rescue the defect in cell motility enhancement by TGF-ß, indicating that activation of the Smad3/Snail/Wnt5b axis is indispensable but not sufficient for enhancing cell motility; a Smad3-dependent but Snail-independent pathway to activate Rac1 is additionally required. Therefore, the Smad3-dependent pathway leading to enhanced cell motility has two branches: a Snail-dependent branch to activate RhoA and a Snail-independent branch to activate Rac1. Coordinated activation of these branches, together with activation of non-Smad signaling pathways, mediates enhanced cell motility induced by TGF-ß.

Tuftelin1 drives experimental pulmonary fibrosis progression by facilitating stress fiber assembly.

BACKGROUND: Idiopathic pulmonary fibrosis (IPF) is a progressive interstitial lung disease (ILD) with unknown etiology, characterized by sustained damage repair of epithelial cells and abnormal activation of fibroblasts, the underlying mechanism of the disease remains elusive. METHODS: To evaluate the role of Tuftelin1 (TUFT1) in IPF and elucidate its molecular mechanism. We investigated the level of TUFT1 in the IPF and bleomycin-induced mouse models and explored the influence of TUFT1 deficiency on pulmonary fibrosis. Additionally, we explored the effect of TUFT1 on the cytoskeleton and illustrated the relationship between stress fiber and pulmonary fibrosis. RESULTS: Our results demonstrated a significant upregulation of TUFT1 in IPF and the bleomycin (BLM)-induced fibrosis model. Disruption of TUFT1 exerted inhibitory effects on pulmonary fibrosis in both in vivo and in vitro. TUFT1 facilitated the assembly of microfilaments in A549 and MRC-5 cells, with a pronounced association between TUFT1 and Neuronal Wiskott-Aldrich syndrome protein (N-WASP) observed during microfilament formation. TUFT1 can promote the phosphorylation of tyrosine residue 256 (Y256) of the N-WASP (pY256N-WASP). Furthermore, TUFT1 promoted transforming growth factor-ß1 (TGF-ß1) induced fibroblast activation by increasing nuclear translocation of pY256N-WASP in fibroblasts, while wiskostatin (Wis), an N-WASP inhibitor, suppressed these processes. CONCLUSIONS: Our findings suggested that TUFT1 plays a critical role in pulmonary fibrosis via its influence on stress fiber, and blockade of TUFT1 effectively reduces pro-fibrotic phenotypes. Pharmacological targeting of the TUFT1-N-WASP axis may represent a promising therapeutic approach for pulmonary fibrosis.

An Inverse Modeling Approach to Estimate Three-Dimensional Aortic Valve Interstitial Cell Stress Fiber Force Levels.

Within the aortic valve (AV) leaflet exists a population of interstitial cells (AVICs) that maintain the constituent tissues by extracellular matrix (ECM) secretion, degradation, and remodeling. AVICs can transition from a quiescent, fibroblast-like phenotype to an activated, myofibroblast phenotype in response to growth or disease. AVIC dysfunction has been implicated in AV disease processes, yet our understanding of AVIC function remains quite limited. A major characteristic of the AVIC phenotype is its contractile state, driven by contractile forces generated by the underlying stress fibers (SF). However, direct assessment of the AVIC SF contractile state and structure within physiologically mimicking three-dimensional environments remains technically challenging, as the size of single SFs are below the resolution of light microscopy. Therefore, in the present study, we developed a three-dimensional (3D) computational approach of AVICs embedded in 3D hydrogels to estimate their SF local orientations and contractile forces. One challenge with this approach is that AVICs will remodel the hydrogel, so that the gel moduli will vary spatially. We thus utilized our previous approach (Khang et al. 2023, "Estimation of Aortic Valve Interstitial Cell-Induced 3D Remodeling of Poly (Ethylene Glycol) Hydrogel Environments Using an Inverse Finite Element Approach," Acta Biomater., 160, pp. 123-133) to define local hydrogel mechanical properties. The AVIC SF model incorporated known cytosol and nucleus mechanical behaviors, with the cell membrane assumed to be perfectly bonded to the surrounding hydrogel. The AVIC SFs were first modeled as locally unidirectional hyperelastic fibers with a contractile force component. An adjoint-based inverse modeling approach was developed to estimate local SF orientation and contractile force. Substantial heterogeneity in SF force and orientations were observed, with the greatest levels of SF alignment and contractile forces occurring in AVIC protrusions. The addition of a dispersed SF orientation to the modeling approach did not substantially alter these findings. To the best of our knowledge, we report the first fully 3D computational contractile cell models which can predict locally varying stress fiber orientation and contractile force levels.

Modelling Cell Orientation Under Stretch: The Effect of Substrate Elasticity.

When cells are seeded on a cyclically deformed substrate like silicon, they tend to reorient their major axis in two ways: either perpendicular to the main stretching direction, or forming an oblique angle with it. However, when the substrate is very soft such as a collagen gel, the oblique orientation is no longer observed, and the cells align either along the stretching direction, or perpendicularly to it. To explain this switch, we propose a simplified model of the cell, consisting of two elastic elements representing the stress fiber/focal adhesion complexes in the main and transverse directions. These elements are connected by a torsional spring that mimics the effect of crosslinking molecules among the stress fibers, which resist shear forces. Our model, consistent with experimental observations, predicts that there is a switch in the asymptotic behaviour of the orientation of the cell determined by the stiffness of the substratum, related to a change from a supercritical bifurcation scenario, whereby the oblique configuration is stable for a sufficiently large stiffness, to a subcritical bifurcation scenario at a lower stiffness. Furthermore, we investigate the effect of cell elongation and find that the region of the parameter space leading to an oblique orientation decreases as the cell becomes more elongated. This implies that elongated cells, such as fibroblasts and smooth muscle cells, are more likely to maintain an oblique orientation with respect to the main stretching direction. Conversely, rounder cells, such as those of epithelial or endothelial origin, are more likely to switch to a perpendicular or parallel orientation on soft substrates.

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.

Quantification of basal stem cell elongation and stress fiber accumulation in the pseudostratified airway epithelium during the unjamming transition.

Under homeostatic conditions, epithelial cells remain non-migratory. However, during embryonic development and pathological conditions, they become migratory. The mechanism underlying the transition of the epithelial layer between non-migratory and migratory phases is a fundamental question in biology. Using well-differentiated primary human bronchial epithelial cells that form a pseudostratified epithelium, we have previously identified that a confluent epithelial layer can transition from a non-migratory to migratory phase through an unjamming transition (UJT). We previously defined collective cellular migration and apical cell elongation as hallmarks of UJT. However, other cell-type-specific changes have not been previously studied in the pseudostratified airway epithelium, which consists of multiple cell types. Here, we focused on the quantifying morphological changes in basal stem cells during the UJT. Our data demonstrate that during the UJT, airway basal stem cells elongated and enlarged, and their stress fibers elongated and aligned. These morphological changes observed in basal stem cells correlated to the previously defined hallmarks of the UJT. Moreover, basal cell and stress fiber elongation were observed prior to apical cell elongation. Together, these morphological changes indicate that basal stem cells in pseudostratified airway epithelium are actively remodeling, presumably through accumulation of stress fibers during the UJT.

BAG6 supports stress fiber formation by preventing the ubiquitin-mediated degradation of RhoA.

The Rho family of small GTPases is a key regulator of cytoskeletal actin polymerization. Although the ubiquitination of Rho proteins is reported to control their activity, the mechanisms by which the ubiquitination of Rho family proteins is controlled by ubiquitin ligases have yet to be elucidated. In this study, we identified BAG6 as the first factor needed to prevent the ubiquitination of RhoA, a critical Rho family protein in F-actin polymerization. We found that BAG6 is necessary for stress fiber formation by stabilizing endogenous RhoA. BAG6 deficiency enhanced the association between RhoA and Cullin-3-based ubiquitin ligases, thus promoting its polyubiquitination and subsequent degradation, leading to the abrogation of actin polymerization. In contrast, the restoration of RhoA expression through transient overexpression rescued the stress fiber formation defects induced by BAG6 depletion. BAG6 was also necessary for the appropriate assembly of focal adhesions as well as cell migration events. These findings reveal a novel role for BAG6 in maintaining the integrity of actin fiber polymerization and establish BAG6 as a RhoA-stabilizing holdase, which binds to and supports the function of RhoA.

Emergence of multiple set-points of cellular homeostatic tension.

Stress fibers (SFs), a contractile actin bundle in nonmuscle mesenchymal cells, are known to intrinsically sustain a constant level of tension or tensional stress, a process called cellular tensional homeostasis. Malfunction in this homeostatic process has been implicated in many diseases such atherosclerosis, but its mechanisms remain incompletely understood. Interestingly, the homeostatic stress in individual SFs is altered upon recruitment of α-smooth muscle actin in particular cellular contexts to reinforce the preexisting SFs. While this transition of the set-point stress is somewhat a universal process observed across different cell types, no clear explanation has been provided as to why cells end up possessing different stable stresses. To address the underlying physics, here we describe that imposing a realistic assumption on the nature of SFs yields the presence of multiple set-points of the homeostatic stress, which transition among them depending on the magnitude of the cellular tension. We analytically derive non-dimensional parameters that characterize the extent of the transition and predict that SFs tend to acquire secondary stable stresses if they are subject to as large a change in stiffness as possible or to as immediate a transition as possible upon increasing the tension. This is a minimal and simple explanation, but given the frequent emergence of force-dependent transformation of various subcellular structures in addition to that of SFs, the theoretical concept presented here would offer an essential guide to addressing potential common mechanisms governing complicated cellular mechanobiological responses.

On the role of myosin-induced actin depolymerization during cell migration.

Mammalian cell migration in open spaces requires F-actin polymerization and myosin contraction. While many studies have focused on myosin's coupling to focal adhesion and stress fibers, the indirect effect of myosin contraction on cell migration through actin depolymerization is not well studied. In this work, we quantified how cell velocity and effective power output are influenced by the rate of actin depolymerization, which is affected by myosin contraction. In addition, we derived scaling laws to provide physical insights into cell migration. Model analysis shows that the cell migration velocity displays a biphasic dependence on the rate of actin depolymerization and myosin contraction. Our model further predicts that the effective cell energy output depends not only on the cell velocity but also on myosin contractility. The work has implications on in vivo processes such as immune response and cancer metastasis, where cells overcome barriers imposed by the physical environment.

Emergent collective organization of bone cells in complex curvature fields.

Individual cells and multicellular systems respond to cell-scale curvatures in their environments, guiding migration, orientation, and tissue formation. However, it remains largely unclear how cells collectively explore and pattern complex landscapes with curvature gradients across the Euclidean and non-Euclidean spectra. Here, we show that mathematically designed substrates with controlled curvature variations induce multicellular spatiotemporal organization of preosteoblasts. We quantify curvature-induced patterning and find that cells generally prefer regions with at least one negative principal curvature. However, we also show that the developing tissue can eventually cover unfavorably curved territories, can bridge large portions of the substrates, and is often characterized by collectively aligned stress fibers. We demonstrate that this is partly regulated by cellular contractility and extracellular matrix development, underscoring the mechanical nature of curvature guidance. Our findings offer a geometric perspective on cell-environment interactions that could be harnessed in tissue engineering and regenerative medicine applications.

Size-dependent response of cells in epithelial tissue modulated by contractile stress fibers.

Although cells with distinct apical areas have been widely observed in epithelial tissues, how the size of cells affects their behavior during tissue deformation and morphogenesis as well as key physical factors modulating such influence remains elusive. Here, we showed that the elongation of cells within the monolayer under anisotropic biaxial stretching increases with their size because the strain released by local cell rearrangement (i.e., T1 transition) is more significant for small cells that possess higher contractility. On the other hand, by incorporating the nucleation, peeling, merging, and breakage dynamics of subcellular stress fibers into classical vertex formulation, we found that stress fibers with orientations predominantly aligned with the main stretching direction will be formed at tricellular junctions, in good agreement with recent experiments. The contractile forces generated by stress fibers help cells to resist imposed stretching, reduce the occurrence of T1 transitions, and, consequently, modulate their size-dependent elongation. Our findings demonstrate that epithelial cells could utilize their size and internal structure to regulate their physical and related biological behaviors. The theoretical framework proposed here can also be extended to investigate the roles of cell geometry and intracellular contraction in processes such as collective cell migration and embryo development.

Glypican-4 regulated actin cytoskeletal reorganization in glucocorticoid treated trabecular meshwork cells and involvement of Wnt/PCP signaling.

A common adverse response to the clinical use of glucocorticoids (GCs) is elevated intraocular pressure (IOP) which is a major risk factor for glaucoma. Elevated IOP arises due to impaired outflow of aqueous humor (AH) through the trabecular meshwork (TM). Although GC-induced changes in actin cytoskeletal dynamics, contractile characteristics, and cell adhesive interactions of TM cells are believed to influence AH outflow and IOP, the molecular mechanisms mediating changes in these cellular characteristics are poorly understood. Our studies focused on evaluating changes in the cytoskeletal and cytoskeletal-associated protein (cytoskeletome) profile of human TM cells treated with dexamethasone (Dex) using label-free mass spectrometric quantification, identified elevated levels of specific proteins known to regulate actin stress fiber formation, contraction, actin networks crosslinking, cell adhesion, and Wnt signaling, including LIMCH1, ArgBP2, CNN3, ITGBL1, CTGF, palladin, FAT1, DIAPH2, EPHA4, SIPA1L1, and GPC4. Several of these proteins colocalized with the actin cytoskeleton and underwent alterations in distribution profile in TM cells treated with Dex, and an inhibitor of Abl/Src kinases. Wnt/Planar Cell Polarity (PCP) signaling agonists-Wnt5a and 5b were detected prominently in the cytoskeletome fraction of TM cells, and studies using siRNA to suppress expression of glypican-4 (GPC4), a known modulator of the Wnt/PCP pathway revealed that GPC4 deficiency impairs Dex induced actin stress fiber formation, and activation of c-Jun N-terminal Kinase (JNK) and Rho kinase. Additionally, while Dex augmented, GPC4 deficiency suppressed the formation of actin stress fibers in TM cells in the presence of Dex and Wnt5a. Taken together, these results identify the GPC4-dependent Wnt/PCP signaling pathway as one of the crucial upstream regulators of Dex induced actin cytoskeletal reorganization and cell adhesion in TM cells, opening an opportunity to target the GPC4/Wnt/PCP pathway for treatment of ocular hypertension in glaucoma.