The assembly and function of perinuclear actin cap in migrating cells

Perception and response of skeleton to mechanical stress

Mechanical stress stands as a fundamental factor in the intricate processes governing the growth, development, morphological shaping, and maintenance of skeletal mass. The profound influence of stress in shaping the skeletal framework prompts the assertion that stress essentially births the skeleton. Despite this acknowledgment, the mechanisms by which the skeleton perceives and responds to mechanical stress remain enigmatic. In this comprehensive review, our scrutiny focuses on the structural composition and characteristics of sclerotin, leading us to posit that it serves as the primary structure within the skeleton responsible for bearing and perceiving mechanical stress. Furthermore, we propose that osteocytes within the sclerotin emerge as the principal mechanical-sensitive cells, finely attuned to perceive mechanical stress. And a detailed analysis was conducted on the possible transmission pathways of mechanical stress from the extracellular matrix to the nucleus.

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

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

Characterization of RACK1-depleted mammalian cells by a palette of microscopy approaches reveals defects in cell cycle progression and polarity establishment

The Receptor for Activated C Kinase 1 (RACK1) is an evolutionarily conserved scaffold protein involved in the regulation of numerous cellular processes. Here, we used CRISPR/Cas9 and siRNA to reduce the expression of RACK1 in Madin-Darby Canine Kidney (MDCK) epithelial cells and Rat2 fibroblasts, respectively. RACK1-depleted cells were examined using coherence-controlled holographic microscopy, immunofluorescence, and electron microscopy. RACK1 depletion resulted in decreased cell proliferation, increased cell area and perimeter, and in the appearance of large binucleated cells suggesting a defect in the cell cycle progression. Our results show that the depletion of RACK1 has a pleiotropic effect on both epithelial and mesenchymal cell lines and support its essential role in mammalian cells.

Gravitational effects on fibroblasts' function in relation to wound healing

The spaceflight environment imposes risks for maintaining a healthy skin function as the observed delayed wound healing can contribute to increased risks of infection. To counteract delayed wound healing in space, a better understanding of the fibroblasts' reaction to altered gravity levels is needed. In this paper, we describe experiments that were carried out at the Large Diameter Centrifuge located in ESA-ESTEC as part of the ESA Academy 2021 Spin Your Thesis! Campaign. We exposed dermal fibroblasts to a set of altered gravity levels, including transitions between simulated microgravity and hypergravity. The addition of the stress hormone cortisol to the cell culture medium was done to account for possible interaction effects of gravity and cortisol exposure. Results show a main impact of cortisol on the secretion of pro-inflammatory cytokines as well as extracellular matrix proteins. Altered gravity mostly induced a delay in cellular migration and changes in mechanosensitive cell structures. Furthermore, 20 × g hypergravity transitions induced changes in nuclear morphology. These findings provide insights into the effect of gravity transitions on the fibroblasts' function related to wound healing, which may be useful for the development of countermeasures.

Biochemical Pathways of Cellular Mechanosensing/Mechanotransduction and Their Role in Neurodegenerative Diseases Pathogenesis

In this review, we shed light on recent advances regarding the characterization of biochemical pathways of cellular mechanosensing and mechanotransduction with particular attention to their role in neurodegenerative disease pathogenesis. While the mechanistic components of these pathways are mostly uncovered today, the crosstalk between mechanical forces and soluble intracellular signaling is still not fully elucidated. Here, we recapitulate the general concepts of mechanobiology and the mechanisms that govern the mechanosensing and mechanotransduction processes, and we examine the crosstalk between mechanical stimuli and intracellular biochemical response, highlighting their effect on cellular organelles' homeostasis and dysfunction. In particular, we discuss the current knowledge about the translation of mechanosignaling into biochemical signaling, focusing on those diseases that encompass metabolic accumulation of mutant proteins and have as primary characteristics the formation of pathological intracellular aggregates, such as Alzheimer's Disease, Huntington's Disease, Amyotrophic Lateral Sclerosis and Parkinson's Disease. Overall, recent findings elucidate how mechanosensing and mechanotransduction pathways may be crucial to understand the pathogenic mechanisms underlying neurodegenerative diseases and emphasize the importance of these pathways for identifying potential therapeutic targets.

Reciprocal regulation of actin filaments and cellular metabolism

For cells to adhere, migrate and proliferate, remodeling of the actin cytoskeleton is required. This process consumes a large amount of ATP while having an intimate connection with cellular metabolism. Signaling pathways that regulate energy homeostasis can also affect actin dynamics, whereas a variety of actin binding proteins directly or indirectly interact with the anabolic and catabolic regulators in cells. Here, we discuss the inter-regulation between actin filaments and cellular metabolism, reviewing recent discoveries on key metabolic enzymes that respond to actin remodeling as well as historical findings on metabolic stress-induced cytoskeletal reorganization. We also address emerging techniques that would benefit the study of cytoskeletal dynamics and cellular metabolism in high spatial-temporal resolution.

Neospora caninum Infection Triggers S-phase Arrest and Alters Nuclear Characteristics in Primary Bovine Endothelial Host Cells

Neospora caninum represents a major cause of abortive disease in bovines and small ruminants worldwide. As a typical obligate intracellular apicomplexan parasite, N. caninum needs to modulate its host cell for successful replication. In the current study, we focused on parasite-driven interference with host cell cycle progression. By performing DNA content-based cell cycle phase analyses in N. caninum-infected primary bovine umbilical vein endothelial cells (BUVEC), a parasite-driven S-phase arrest was detected at both 24 and 32 h p. i., being paralleled by fewer host cells experiencing the G0/G1 cell cycle phase. When analyzing S-subphases, proliferation cell nuclear antigen (per PCNA)-based experiments showed a reduced population of BUVEC in the late S-phase. Analyses on key molecules of cell cycle regulation documented a significant alteration of cyclin A2 and cyclin B1 abundance in N. caninum-infected host endothelial cells, thereby confirming irregularities in the S-phase and S-to-G2/M-phase transition. In line with cell cycle alterations, general nuclear parameters revealed smaller nuclear sizes and morphological abnormalities of BUVEC nuclei within the N. caninum-infected host cell layer. The latter observations were also confirmed by transmission electron microscopy (TEM) and by analyses of lamin B1 as a marker of nuclear lamina, which illustrated an inhomogeneous nuclear lamin B1 distribution, nuclear foldings, and invaginations, thereby reflecting nuclear misshaping. Interestingly, the latter finding applied to both non-infected and infected host cells within parasitized BUVEC layer. Additionally, actin detection indicated alterations in the perinuclear actin cap formation since typical nucleo-transversal filaments were consistently lacking in N. caninum-infected BUVEC, as also documented by significantly decreased actin-related intensities in the perinuclear region. These data indicate that N. caninum indeed alters host cell cycle progression and severely affects the host cell nuclear phenotype in primary bovine endothelial host cells. In summary, these findings add novel data on the complex N. caninum-specific modulation of host cell and nucleus, thereby demonstrating clear differences in cell cycle progression modulation driven by other closely related apicomplexans like Toxoplasma gondii and Besnotia besnoiti.

Septin-microtubule association via a motif unique to the isoform 1 of septin 9 tunes stress fibers

Septins, a family of GTP-binding proteins assembling into higher order structures, interface with the membrane, actin filaments and microtubules, which positions them as important regulators of cytoarchitecture. Septin 9 (SEPT9), which is frequently overexpressed in tumors and mutated in hereditary neuralgic amyotrophy (HNA), mediates the binding of septins to microtubules, but the molecular determinants of this interaction remained uncertain. We demonstrate that a short MAP-like motif unique to SEPT9 isoform 1 (SEPT9_i1) drives septin octamer-microtubule interaction in cells and in vitro reconstitutions. Septin-microtubule association requires polymerizable septin octamers harboring SEPT9_i1. Although outside of the MAP-like motif, HNA mutations abrogates this association, identifying a putative regulatory domain. Removal of this domain from SEPT9_i1 sequesters septins on microtubules, promotes microtubule stability and alters actomyosin fiber distribution and tension. Thus, we identify key molecular determinants and potential regulatory roles of septin-microtubule interaction, paving the way to deciphering the mechanisms underlying septin-associated pathologies.

A polarized nucleus-cytoskeleton-ECM connection in migrating cardioblasts controls heart tube formation in Drosophila

The formation of the cardiac tube is a remarkable example of complex morphogenetic processes conserved from invertebrates to humans. It involves coordinated collective migration of contralateral rows of cardiac cells. The molecular processes underlying the specification of cardioblasts (CBs) prior to migration are well established and significant advances have been made in understanding the process of lumen formation. However, the mechanisms of collective cardiac cells migration remain elusive. Here, we have identified CAP and MSP300 as novel actors involved during CB migration. They both exhibit highly similar temporal and spatial expression patterns in Drosophila migrating cardiac cells, and are necessary for the correct number and alignment of CBs, a prerequisite for the coordination of their collective migration. Our data suggest that CAP and MSP300 are part of a protein complex linking focal adhesion sites to nuclei via the actin cytoskeleton that maintains post-mitotic state and correct alignment of CBs.

3D-Printed Submicron Patterns Reveal the Interrelation between Cell Adhesion, Cell Mechanics, and Osteogenesis

The surface topography of implantable devices is of crucial importance for guiding the cascade of events that starts from the initial contact of the cells with the surface and continues until the complete integration of the device in its immediate environment. There is, however, limited quantitative information available regarding the relationships between the different stages of such cascade(s) and how the design of surface topography influences them. We, therefore, used direct laser writing to 3D-print submicron pillars with precisely controlled dimensions and spatial arrangements to perform a systematic study of such relationships. Using single-cell force spectroscopy, we measured the adhesion force and the work of adhesion of the preosteoblast cells residing on the different types of surfaces. Not only the adhesion parameters (after 2-60 s) but also the formation of focal adhesions was strongly dependent on the geometry and arrangement of the pillars: sufficiently tall and dense pillars enhanced both adhesion parameters and the formation of focal adhesions. Our morphological study of the cells (after 24 h) showed that those enhancements were associated with a specific way of cell settlement onto the surface (i.e., "top state"). The cells interacting with tall and dense pillars were also characterized by numerous thick actin stress fibers in the perinuclear region and possibly high internal stresses. Furthermore, living cells with highly organized cytoskeletal networks exhibited greater values of the elastic modulus. The early responses of the cells predicted their late response including matrix mineralization: tall and dense submicron pillars significantly upregulated the expression of osteopontin after 21 days of culture under both osteogenic and nonosteogenic conditions. Our findings paint a detailed picture of at least one possible cascade of events that starts from initial cell adhesion and continues to subsequent cellular functions and eventual matrix mineralization. These observations could inform the future developments of instructive surfaces for medical devices based on physical surface cues and early markers.

Epigallocatechin-3-gallate plays more predominant roles than caffeine for inducing actin-crosslinking, ubiquitin/proteasome activity and glycolysis, and suppressing angiogenesis features of human endothelial cells

A recent expression proteomics study has reported changes in cellular proteome (set of proteins) of human endothelial cells (ECs) induced by caffeine and epigallocatechin-3-gallate (EGCG), the most abundant bioactive compounds in coffee and green tea, respectively. Although both common and differential changes were highlighted by bioinformatics prediction, no experimental validation was performed. Herein, we reanalyzed these proteome datasets and performed protein-protein interactions network analysis followed by functional investigations using various assays to address the relevance of such proteome changes in human ECs functions. Protein-protein interactions network analysis revealed actin-crosslink formation, ubiquitin-proteasome activity and glycolysis as the three main networks among those significantly altered proteins induced by caffeine and EGCG. The experimental data showed predominant increases of actin-crosslink formation, ubiquitin-proteasome activity, and glycolysis (as reflected by increased F-actin and β-actin, declined ubiquitinated proteins and increased intracellular ATP, respectively) in the EGCG-treated cells. Investigations on angiogenesis features revealed that EGCG predominantly reduced ECs proliferation, migration/invasion, endothelial tube formation (as determined by numbers of nodes/junctions and meshes), barrier function (as determined by levels of VE-cadherin, zonula occludens-1 (ZO-1) and transendothelial resistance (TER)), and angiopoietin-2 secretion. However, both caffeine and EGCG had no effects on matrix metalloproteinase-2 (MMP-2) secretion. These data indicate that EGCG exhibits more potent effects on human ECs functions to induce actin-crosslink, ubiquitin-proteasome activity and glycolysis, and to suppress angiogenesis processes that commonly occur in various diseases, particularly cancers.

Lovastatin Inhibits EMT and Metastasis of Triple-Negative Breast Cancer Stem Cells Through Dysregulation of Cytoskeleton-Associated Proteins

Triple-negative breast cancer (TNBC) is more aggressive and has poorer prognosis compared to other subtypes of breast cancer. Epithelial-to-mesenchymal transition (EMT) is a process in which epithelial cells transform into mesenchymal-like cells capable of migration, invasion, and metastasis. Recently, we have demonstrated that lovastatin, a 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitor and a lipid-lowering drug, could inhibit stemness properties of cancer stem cells (CSCs) derived from TNBC cell in vitro and in vivo. This study is aimed at investigating whether lovastatin inhibits TNBC CSCs by inhibiting EMT and suppressing metastasis and the mechanism involved. In the present study, we found that lovastatin dysregulated lysine succinylation of cytoskeleton-associated proteins in CSCs derived from TNBC MDA-MB-231 cell. Lovastatin inhibited EMT as demonstrated by down-regulation of the protein levels of Vimentin and Twist in MDA-MB-231 CSCs in vitro and vivo and by reversal of TGF-β1-induced morphological change in MCF10A cells. Lovastatin also inhibited the migration of MDA-MB-231 CSCs. The disruption of cytoskeleton in TNBC CSCs by lovastatin was demonstrated by the reduction of the number of pseudopodia and the relocation of F-actin cytoskeleton. Combination of lovastatin with doxorubicin synergistically inhibited liver metastasis of MDA-MB-231 CSCs. Bioinformatics analysis revealed that higher expression levels of cytoskeleton-associated genes were characteristic of TNBC and predicted survival outcomes in breast cancer patients. These data suggested that lovastatin could inhibit the EMT and metastasis of TNBC CSCs in vitro and in vivo through dysregulation of cytoskeleton-associated proteins.

Articular Chondrocyte Phenotype Regulation through the Cytoskeleton and the Signaling Processes That Originate from or Converge on the Cytoskeleton: Towards a Novel Understanding of the Intersection between Actin Dynamics and Chondrogenic Function

Numerous studies have assembled a complex picture, in which extracellular stimuli and intracellular signaling pathways modulate the chondrocyte phenotype. Because many diseases are mechanobiology-related, this review asked to what extent phenotype regulators control chondrocyte function through the cytoskeleton and cytoskeleton-regulating signaling processes. Such information would generate leverage for advanced articular cartilage repair. Serial passaging, pro-inflammatory cytokine signaling (TNF-α, IL-1α, IL-1β, IL-6, and IL-8), growth factors (TGF-α), and osteoarthritis not only induce dedifferentiation but also converge on RhoA/ROCK/Rac1/mDia1/mDia2/Cdc42 to promote actin polymerization/crosslinking for stress fiber (SF) formation. SF formation takes center stage in phenotype control, as both SF formation and SOX9 phosphorylation for COL2 expression are ROCK activity-dependent. Explaining how it is molecularly possible that dedifferentiation induces low COL2 expression but high SF formation, this review theorized that, in chondrocyte SOX9, phosphorylation by ROCK might effectively be sidelined in favor of other SF-promoting ROCK substrates, based on a differential ROCK affinity. In turn, actin depolymerization for redifferentiation would “free-up” ROCK to increase COL2 expression. Moreover, the actin cytoskeleton regulates COL1 expression, modulates COL2/aggrecan fragment generation, and mediates a fibrogenic/catabolic expression profile, highlighting that actin dynamics-regulating processes decisively control the chondrocyte phenotype. This suggests modulating the balance between actin polymerization/depolymerization for therapeutically controlling the chondrocyte phenotype.

Nuclear Morphological Remodeling in Human Granulocytes Is Linked to Prenylation Independently from Cytoskeleton

Nuclear shape modulates cell behavior and function, while aberrant nuclear morphologies correlate with pathological phenotype severity. Nevertheless, functions of specific nuclear morphological features and underlying molecular mechanisms remain poorly understood. Here, we investigate a nucleus-intrinsic mechanism driving nuclear lobulation and segmentation concurrent with granulocyte specification, independently from extracellular forces and cytosolic cytoskeleton contributions. Transcriptomic regulation of cholesterol biosynthesis is equally concurrent with nuclear remodeling. Its putative role as a regulatory element is supported by morphological aberrations observed upon pharmacological impairment of several enzymatic steps of the pathway, most prominently the sterol ∆14-reductase activity of laminB-receptor and protein prenylation. Thus, we support the hypothesis of a nuclear-intrinsic mechanism for nuclear shape control with the putative involvement of the recently discovered GGTase III complex. Such process could be independent from or complementary to the better studied cytoskeleton-based nuclear remodeling essential for cell migration in both physiological and pathological contexts such as immune system function and cancer metastasis.

Spatial organization and crosstalk of vimentin and actin stress fibers regulate the osteogenic differentiation of human adipose-derived stem cells

Human adipose-derived stem cells (hASCs) are ideal seed cells for tissue engineering due to their multidirectional differentiation potential. Microfilaments, microtubules, and intermediate filaments are responsible for supporting the intracellular space. Vimentin, a type III intermediate filament protein that is specifically expressed in cells of mesenchymal origin, can function as a scaffold and endow cells with tension and shear stress resistance. Actin stress fibers (ASF) act as an important physical device in stress signal transduction, providing stiffness for cells, and promoting osteogenesis. Through direct physical contact, cross-linkers, and spatial interactions, vimentin and actin networks exist as intersecting entities. Spatial interactions occur in the overlapping area of cytoskeleton subsystems, which could affect cell morphology, cell mechanics, and cell fate. However, how does the spatial organization between the cytoskeletal subsystems changed during osteogenesis, especially between vimentin and ASF, is still not understood, and its mechanism effect on cell fate remains unclear. In our study, WB experiment was used to detect the expression changes in Vimentin, ASF, and other proteins. Cells were reconstructed by three-dimensional scanning with fluorescence microscope, and the spatial thickness of vimentin and ASF cytoskeletons and the thickness of the overlapping area between them were calculated, respectively, so as to observe the spatial reorganization of vimentin and ASF in cells. Cytochalasin D (an inhibitor of actin polymerization) and vimentin upregulated/downregulated cells were used to verify the change in the spatial organization between vimentin and ASF and its influence on osteogenesis. Then, heat shock protein 27 (HSP27) was downregulated to illuminate the regulatory mechanisms of spatial organization between vimentin and ASF during osteogenesis. The amounts and the spatial positions of vimentin and actin stress fiber exhibited opposite trends during osteogenesis. Through controlling the anchor sites on the nucleus, intermediate filaments vimentin can reduce the spatial proportion of actin stress fibers, which can be regulated by HSP27. In addition, depolymerization of actin stress fibers lead to lower osteogenic differentiation ability, resulting in osteogenesis and lipogenesis existed simultaneously, that can be resisted by vimentin. Our data indicate that the spatial reorganization of vimentin and actin stress fibers is a key factor in the regulation of the differentiation state of hASCs. And their spatial overlapping area is detrimental to hASCs osteogenesis, providing a new perspective for further exploring the mechanism underlying hASCs osteogenesis.

Mechanoadaptive organization of stress fiber subtypes in epithelial cells under cyclic stretches and stretch release

Cyclic stretch applied to cells induces the reorganization of stress fibers. However, the correlation between the reorganization of stress fiber subtypes and strain-dependent responses of the cytoplasm and nucleus has remained unclear. Here, we investigated the dynamic involvement of stress fiber subtypes in the orientation and elongation of cyclically stretched epithelial cells. We applied uniaxial cyclic stretches at 5%, 10%, and 15% strains to cells followed by the release of the mechanical stretch. Dorsal, transverse arcs, and peripheral stress fibers were mainly involved in the cytoplasm responses whereas perinuclear cap fibers were associated with the reorientation and elongation of the nucleus. Dorsal stress fibers and transverse arcs rapidly responded within 15 min regardless of the strain magnitude to facilitate the subsequent changes in the orientation and elongation of the cytoplasm. The cyclic stretches induced the additional formation of perinuclear cap fibers and their increased number was almost maintained with a slight decline after 2-h-long stretch release. The slow formation and high stability of perinuclear cap fibers were linked to the slow reorientation kinetics and partial morphology recovery of nucleus in the presence or absence of cyclic stretches. The reorganization of stress fiber subtypes occurred in accordance with the reversible distribution of myosin II. These findings allowed us to propose a model for stretch-induced responses of the cytoplasm and nucleus in epithelial cells based on different mechanoadaptive properties of stress fiber subtypes.

Increased Lamin B1 Levels Promote Cell Migration by Altering Perinuclear Actin Organization

Cell migration requires reposition and reshaping of the cell nucleus. The nuclear lamina is highly important for migration of both primary and cancer cells. B-type lamins are important for proper migration of epicardial cells and neurons and increased lamin B to lamin A ratio accelerates cancer cell migration through confined spaces. Moreover, a positive association between lamin B1 levels and tumor formation and progression is found in various cancer types. Still, the molecular mechanism by which B-type lamins promote cell migration is not fully understood. To better understand this mechanism, we tested the effects of lamin B1 on perinuclear actin organization. Here we show that induction of melanoma cell migration leads to the formation of a cytosolic Linker of Nucleoskeleton and Cytoskeleton (LINC) complex-independent perinuclear actin rim, which has not been detected in migrating cells, yet. Significantly, increasing the levels of lamin B1 but not the levels of lamin A prevented perinuclear actin rim formation while accelerated the cellular migration rate. To interfere with the perinuclear actin rim, we generated a chimeric protein that is localized to the outer nuclear membrane and cleaves perinuclear actin filaments in a specific manner without disrupting other cytosolic actin filaments. Using this tool, we found that disruption of the perinuclear actin rim accelerated the cellular migration rate in a similar manner to lamin B1 over-expression. Taken together, our results suggest that increased lamin B1 levels can accelerate cell migration by inhibiting the association of the nuclear envelope with actin filaments that may reduce nuclear movement and deformability.

Quantitative Phase Imaging of Spreading Fibroblasts Identifies the Role of Focal Adhesion Kinase in the Stabilization of the Cell Rear

Cells attaching to the extracellular matrix spontaneously acquire front-rear polarity. This self-organization process comprises spatial activation of polarity signaling networks and the establishment of a protruding cell front and a non-protruding cell rear. Cell polarization also involves the reorganization of cell mass, notably the nucleus that is positioned at the cell rear. It remains unclear, however, how these processes are regulated. Here, using coherence-controlled holographic microscopy (CCHM) for non-invasive live-cell quantitative phase imaging (QPI), we examined the role of the focal adhesion kinase (FAK) and its interacting partner Rack1 in dry mass distribution in spreading Rat2 fibroblasts. We found that FAK-depleted cells adopt an elongated, bipolar phenotype with a high central body mass that gradually decreases toward the ends of the elongated processes. Further characterization of spreading cells showed that FAK-depleted cells are incapable of forming a stable rear; rather, they form two distally positioned protruding regions. Continuous protrusions at opposite sides results in an elongated cell shape. In contrast, Rack1-depleted cells are round and large with the cell mass sharply dropping from the nuclear area towards the basal side. We propose that FAK and Rack1 act differently yet coordinately to establish front-rear polarity in spreading cells.

Formin 2 Regulates Lysosomal Degradation of Wnt-Associated β-Catenin in Neural Progenitors

Although neural progenitor proliferation along the ventricular zone is regulated by β-catenin through Wnt signaling, the cytoskeletal mechanisms that regulate expression and localization of these proteins are not well understood. Our prior studies have shown that loss of the actin-binding Filamin A (FlnA) and actin-nucleating protein Formin 2 (Fmn2) impairs endocytosis of low-density-lipoprotein-receptor-related protein 6 (Lrp6), thereby disrupting β-catenin activation, resulting in decreased brain size. Here, we report that activated RhoA-GTPase disengages Fmn2 N- to C-terminal binding to promote Fmn2 activation and redistribution into lysosomal vesicles. Fmn2 colocalizes with β-catenin in lysosomes and promotes its degradation. Further, Fmn2 binds the E3 ligase Smurf2, enhances Smurf2-dependent ubiquitination, and degradation of Dishevelled-2 (Dvl2), thereby initiates β-catenin degradation. Finally, Fmn2 overexpression disrupts neuroepithelial integrity, neuronal migration, and proliferation-phenotypes in E13 mouse embryos, as seen with loss of Fmn2+FlnA function. Conversely, co-expression of Dvl2 with Fmn2 rescues the proliferation defect due to Fmn2 overexpression in mouse embryos. These findings suggest that there is a homeostatic feedback mechanism in the cytoskeletal-dependent regulation of neural proliferation within the cerebral cortex. Upstream, Fmn2 promotes proliferation by stabilizing the Lrp6 receptor, leading to β-catenin activation. Downstream, RhoA-activated Fmn2 promotes lysosomal degradation of Dvl2, leading to β-catenin degradation.

The role of the cell nucleus in mechanotransduction

Mechanical forces are known to influence cellular processes with consequences at the cellular and physiological level. The cell nucleus is the largest and stiffest organelle, and it is connected to the cytoskeleton for proper cellular function. The connection between the nucleus and the cytoskeleton is in most cases mediated by the linker of nucleoskeleton and cytoskeleton (LINC) complex. Not surprisingly, the nucleus and the associated cytoskeleton are implicated in multiple mechanotransduction pathways important for cellular activities. Herein, we review recent advances describing how the LINC complex, the nuclear lamina, and nuclear pore complexes are involved in nuclear mechanotransduction. We will also discuss how the perinuclear actin cytoskeleton is important for the regulation of nuclear mechanotransduction. Additionally, we discuss the relevance of nuclear mechanotransduction for cell migration, development, and how nuclear mechanotransduction impairment leads to multiple disorders.

ILK supports RhoA/ROCK-mediated contractility of human intestinal epithelial crypt cells by inducing the fibrillogenesis of endogenous soluble fibronectin during the spreading process

BACKGROUND

Fibronectin (FN) assembly into an insoluble fibrillar matrix is a crucial step in many cell responses to extracellular matrix (ECM) properties, especially with regards to the integrin-related mechanosensitive signaling pathway. We have previously reported that the silencing of expression of integrin-linked kinase (ILK) in human intestinal epithelial crypt (HIEC) cells causes significant reductions in proliferation and spreading through concomitantly acquired impairment of soluble FN deposition. These defects in ILK-depleted cells are rescued by growth on exogenous FN. In the present study we investigated the contribution of ILK in the fibrillogenesis of FN and its relation to integrin-actin axis signaling and organization.

RESULTS

We show that de novo fibrillogenesis of endogenous soluble FN is ILK-dependent. This function seemingly induces the assembly of an ECM that supports increased cytoskeletal tension and the development of a fully spread contractile cell phenotype. We observed that HIEC cell adhesion to exogenous FN or collagen-I (Col-I) is sufficient to restore fibrillogenesis of endogenous FN in ILK-depleted cells. We also found that optimal engagement of the Ras homolog gene family member A (RhoA) GTPase/Rho-associated kinase (ROCK-1, ROCK-2)/myosin light chain (MLC) pathway, actin ventral stress fiber formation, and integrin adhesion complex (IAC) maturation rely primarily upon the cell's capacity to execute FN fibrillogenesis, independent of any significant ILK input. Lastly, we confirm the integrin α5β1 as the main integrin responsible for FN assembly, although in ILK-depleted cells αV-class integrins expression is needed to allow the rescue of FN fibrillogenesis on exogenous substrate.

CONCLUSION

Our study demonstrates that ILK specifically induces the initiation of FN fibrillogenesis during cell spreading, which promotes RhoA/ROCK-dependent cell contractility and maturation of the integrin-actin axis structures. However, the fibrillogenesis process and its downstream effect on RhoA signaling, cell contractility and spreading are ILK-independent in human intestinal epithelial crypt cells.

SUN1/2 Are Essential for RhoA/ROCK-Regulated Actomyosin Activity in Isolated Vascular Smooth Muscle Cells

Vascular smooth muscle cells (VSMCs) are the predominant cell type in the blood vessel wall. Changes in VSMC actomyosin activity and morphology are prevalent in cardiovascular disease. The actin cytoskeleton actively defines cellular shape and the LInker of Nucleoskeleton and Cytoskeleton (LINC) complex, comprised of nesprin and the Sad1p, UNC-84 (SUN)-domain family members SUN1/2, has emerged as a key regulator of actin cytoskeletal organisation. Although SUN1 and SUN2 function is partially redundant, they possess specific functions and LINC complex composition is tailored for cell-type-specific functions. We investigated the importance of SUN1 and SUN2 in regulating actomyosin activity and cell morphology in VSMCs. We demonstrate that siRNA-mediated depletion of either SUN1 or SUN2 altered VSMC spreading and impaired actomyosin activity and RhoA activity. Importantly, these findings were recapitulated using aortic VSMCs isolated from wild-type and SUN2 knockout (SUN2 KO) mice. Inhibition of actomyosin activity, using the rho-associated, coiled-coil-containing protein kinase1/2 (ROCK1/2) inhibitor Y27632 or blebbistatin, reduced SUN2 mobility in the nuclear envelope and decreased the association between SUN2 and lamin A, confirming that SUN2 dynamics and interactions are influenced by actomyosin activity. We propose that the LINC complex exists in a mechanical feedback circuit with RhoA to regulate VSMC actomyosin activity and morphology.

Nonmuscle myosin IIA and IIB differentially modulate migration and alter gene expression in primary mouse tumorigenic cells

Though many cancers are known to show up-regulation of nonmuscle myosin (NM) IIA and IIB, the mechanism by which NMIIs aid in cancer development remains unexplored. Here we demonstrate that tumor-generating, fibroblast-like cells isolated from 3-methylcholanthrene (3MC)-induced murine tumor exhibit distinct phospho-dependent localization of NMIIA and NMIIB at the perinuclear area and tip of the filopodia and affect cell migration differentially. While NMIIA-KD affects protrusion dynamics and increases cell directionality, NMIIB-KD lowers migration speed and increases filopodial branching. Strategically located NMIIs at the perinuclear area colocalize with the linker of nucleoskeleton and cytoskeleton (LINC) protein Nesprin2 and maintain the integrity of the nuclear-actin cap. Interestingly, knockdown of NMIIs results in altered expression of genes involved in epithelial-to-mesenchymal transition, angiogenesis, and cellular senescence. NMIIB-KD cells display down-regulation of Gsc and Serpinb2, which is strikingly similar to Nesprin2-KD cells as assessed by quantitative PCR analysis. Further gene network analysis predicts that NMIIA and NMIIB may act on similar pathways but through different regulators. Concomitantly, knockdown of NMIIA or NMIIB lowers the growth rate and tumor volume of 3MC-induced tumor in vivo. Altogether, these results open a new window to further investigate the effect of LINC-associated perinuclear actomyosin complex on mechanoresponsive gene expression in the growing tumor.

In silico approach to quantify nucleus self-deformation on micropillared substrates

Considering the major role of confined cell migration in biological processes and diseases, such as embryogenesis or metastatic cancer, it has become increasingly important to design relevant experimental set-ups for in vitro studies. Microfluidic devices have recently presented great opportunities in their respect since they offer the possibility to study all the steps from a suspended to a spread, and eventually crawling cell or a cell with highly deformed nucleus. Here, we focus on the nucleus self-deformation over a micropillared substrate. Actin networks have been observed at two locations in this set-up: above the nucleus, forming the perinuclear actin cap (PAC), and below the nucleus, surrounding the pillars. We can then wonder which of these contractile networks is responsible for nuclear deformation. The cytoplasm and the nucleus are represented through the superposition of a viscous and a hyperelastic material and follow a series of processes. First, the suspended cell settles on the pillars due to gravity. Second, an adhesive spreading force comes into play, and then, active deformations contract one or both actin domains and consequently the nucleus. Our model is first tested on a flat substrate to validate its global behaviour before being confronted to a micropillared substrate. Overall, the nucleus appears to be mostly pulled towards the pillars, while the mechanical action of the PAC is weak. Eventually, we test the influence of gravity and prove that the gravitational force does not play a role in the final deformation of the nucleus.

Gaussian Curvature Directs Stress Fiber Orientation and Cell Migration

We show that substrates with nonzero Gaussian curvature influence the organization of stress fibers and direct the migration of cells. To study the role of Gaussian curvature, we developed a sphere-with-skirt surface in which a positive Gaussian curvature spherical cap is seamlessly surrounded by a negative Gaussian curvature draping skirt, both with principal radii similar to cell-length scales. We find significant reconfiguration of two subpopulations of stress fibers when fibroblasts are exposed to these curvatures. Apical stress fibers in cells on skirts align in the radial direction and avoid bending by forming chords across the concave gap, whereas basal stress fibers bend along the convex direction. Cell migration is also strongly influenced by the Gaussian curvature. Real-time imaging shows that cells migrating on skirts repolarize to establish a leading edge in the azimuthal direction. Thereafter, they migrate in that direction. This behavior is notably different from migration on planar surfaces, in which cells typically migrate in the same direction as the apical stress fiber orientation. Thus, this platform reveals that nonzero Gaussian curvature not only affects the positioning of cells and alignment of stress fiber subpopulations but also directs migration in a manner fundamentally distinct from that of migration on planar surfaces.

Prestressed cells are prone to cytoskeleton failures under localized shear strain: an experimental demonstration on muscle precursor cells

We report on a wavelet based space-scale decomposition method for analyzing the response of living muscle precursor cells (C2C12 myoblasts and myotubes) upon sharp indentation with an AFM cantilever and quantifying their aptitude to sustain such a local shear strain. Beyond global mechanical parameters which are currently used as markers of cell contractility, we emphasize the necessity of characterizing more closely the local fluctuations of the shear relaxation modulus as they carry important clues about the mechanisms of cytoskeleton strain release. Rupture events encountered during fixed velocity shear strain are interpreted as local disruptions of the actin cytoskeleton structures, the strongest (brittle) ones being produced by the tighter and stiffer stress fibers or actin agglomerates. These local strain induced failures are important characteristics of the resilience of these cells, and their aptitude to maintain their shape via a quick recovery from local strains. This study focuses on the perinuclear region because it can be considered as a master mechanical organizing center of these muscle precursor cells. Using this wavelet-based method, we combine the global and local approaches for a comparative analysis of the mechanical parameters of normal myoblasts, myotubes and myoblasts treated with actomyosin cytoskeleton disruptive agents (ATP depletion, blebbistatin).

Role of the Nucleus as a Sensor of Cell Environment Topography

The proper integration of biophysical cues from the cell vicinity is crucial for cells to maintain homeostasis, cooperate with other cells within the tissues, and properly fulfill their biological function. It is therefore crucial to fully understand how cells integrate these extracellular signals for tissue engineering and regenerative medicine. Topography has emerged as a prominent component of the cellular microenvironment that has pleiotropic effects on cell behavior. This progress report focuses on the recent advances in the understanding of the topography sensing mechanism with a special emphasis on the role of the nucleus. Here, recent techniques developed for monitoring the nuclear mechanics are reviewed and the impact of various topographies and their consequences on nuclear organization, gene regulation, and stem cell fate is summarized. The role of the cell nucleus as a sensor of cell-scale topography is further discussed.