Actin cap associated focal adhesions and their distinct role in cellular mechanosensing

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.

Dual role of the nucleus in cell migration on planar substrates

Cell migration is essential to sustain life. There have been significant advances in the understanding of the mechanisms that control cell crawling, but the role of the nucleus remains poorly understood. The nucleus exerts a tight control of cell migration in 3D environments, but its influence in 2D migration on planar substrates remains unclear. Here, we study the role of the cell nucleus in 2D cell migration using a computational model of fish keratocytes. Our results indicate that the apparently minor role played by the nucleus emerges from two antagonist effects: While the nucleus modifies the spatial distributions of actin and myosin in a way that reduces cell velocity (e.g., the nucleus displaces myosin to the sides and front of the cell), its mechanical connection with the cytoskeleton alters the intracellular stresses promoting cell migration. Overall, the favorable effect of the nucleus-cytoskeleton connection prevails, which may explain why regular cells usually move faster than enucleated cells.

Cell engineering: Biophysical regulation of the nucleus

Cells live in a complex and dynamic microenvironment, and a variety of microenvironmental cues can regulate cell behavior. In addition to biochemical signals, biophysical cues can induce not only immediate intracellular responses, but also long-term effects on phenotypic changes such as stem cell differentiation, immune cell activation and somatic cell reprogramming. Cells respond to mechanical stimuli via an outside-in and inside-out feedback loop, and the cell nucleus plays an important role in this process. The mechanical properties of the nucleus can directly or indirectly modulate mechanotransduction, and the physical coupling of the cell nucleus with the cytoskeleton can affect chromatin structure and regulate the epigenetic state, gene expression and cell function. In this review, we will highlight the recent progress in nuclear biomechanics and mechanobiology in the context of cell engineering, tissue remodeling and disease development.

Mechanoresponse of stem cells for vascular repair

Stem cells have shown great potential in vascular repair. Numerous evidence indicates that mechanical forces such as shear stress and cyclic strain can regulate the adhesion, proliferation, migration, and differentiation of stem cells via serious signaling pathways. The enrichment and differentiation of stem cells play an important role in the angiogenesis and maintenance of vascular homeostasis. In normal tissues, blood flow directly affects the microenvironment of vascular endothelial cells (ECs); in pathological status, the abnormal interactions between blood flow and vessels contribute to the injury of vessels. Next, the altered mechanical forces are transduced into cells by mechanosensors to trigger the reformation of vessels. This process occurs when signaling pathways related to EC differentiation are initiated. Hence, a deep understanding of the responses of stem cells to mechanical stresses and the underlying mechanisms involved in this process is essential for clinical translation. In this the review, we provide an overview of the role of stem cells in vascular repair, outline the performance of stem cells under the mechanical stress stimulation, and describe the related signaling pathways.

Force Production by a Bundle of Growing Actin Filaments Is Limited by Its Mechanical Properties

Bundles of actin filaments are central to a large variety of cellular structures such as filopodia, stress fibers, cytokinetic rings, and focal adhesions. The mechanical properties of these bundles are critical for proper force transmission and force bearing. Previous mathematical modeling efforts have focused on bundles' rigidity and shape. However, it remains unknown how bundle length and buckling are controlled by external physical factors. In this work, we present a biophysical model for dynamic bundles of actin filaments submitted to an external load. In combination with in vitro motility assays of beads coated with formins, our model allowed us to characterize conditions for bead movement and bundle buckling. From the deformation profiles, we determined key biophysical properties of tethered actin bundles such as their rigidity and filament density.

Mkl1-dependent gene activation is sufficient to induce actin cap assembly

Actin-dependent forces mechanically control both the position and shape of the nucleus. While the mechanisms that establish nuclear position are well defined, less understood is how actin filaments determine nuclear shape. We recently showed that nuclear envelope-spanning LINC complexes promote stress fiber assembly by activating the small GTPase RhoA and Mkl1-dependent gene activation. We now report that a subset of these stress fibers associate with the apical face of the nuclear envelope through LINC complexes that contain the inner nuclear membrane protein Sun2. Apical stress fibers have previously been shown to specifically couple cell and nuclear morphology, suggesting that LINC complexes influence nuclear shape in part by regulating the small GTPase RhoA.

Cellular sensing of micron-scale curvature: a frontier in understanding the microenvironment

The vast majority of cell biological studies examine function and molecular mechanisms using cells on flat surfaces: glass, plastic and more recently elastomeric polymers. While these studies have provided a wealth of valuable insight, they fail to consider that most biologically occurring surfaces are curved, with a radius of curvature roughly corresponding to the length scale of cells themselves. Here, we review recent studies showing that cells detect and respond to these curvature cues by adjusting and re-orienting their cell bodies, actin fibres and nuclei as well as by changing their transcriptional programme. Modelling substratum curvature has the potential to provide fundamental new insight into cell behaviour and function in vivo.

Nuclear envelope deformation controls cell cycle progression in response to mechanical force

The shape of the cell nucleus can vary considerably during developmental and pathological processes; however, the impact of nuclear morphology on cell behavior is not known. Here, we observed that the nuclear envelope flattens as cells transit from G1 to S phase and inhibition of myosin II prevents nuclear flattening and impedes progression to S phase. Strikingly, we show that applying compressive force on the nucleus in the absence of myosin II-mediated tension is sufficient to restore G1 to S transition. Using a combination of tools to manipulate nuclear morphology, we observed that nuclear flattening activates a subset of transcription factors, including TEAD and AP1, leading to transcriptional induction of target genes that promote G1 to S transition. In addition, we found that nuclear flattening mediates TEAD and AP1 activation in response to ROCK-generated contractility or cell spreading. Our results reveal that the nuclear envelope can operate as a mechanical sensor whose deformation controls cell growth in response to tension.

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.

Ablation of SUN2-containing LINC complexes drives cardiac hypertrophy without interstitial fibrosis

The cardiomyocyte cytoskeleton, including the sarcomeric contractile apparatus, forms a cohesive network with cellular adhesions at the plasma membrane and nuclear--cytoskeletal linkages (LINC complexes) at the nuclear envelope. Human cardiomyopathies are genetically linked to the LINC complex and A-type lamins, but a full understanding of disease etiology in these patients is lacking. Here we show that SUN2-null mice display cardiac hypertrophy coincident with enhanced AKT/MAPK signaling, as has been described previously for mice lacking A-type lamins. Surprisingly, in contrast to lamin A/C-null mice, SUN2-null mice fail to show coincident fibrosis or upregulation of pathological hypertrophy markers. Thus, cardiac hypertrophy is uncoupled from profibrotic signaling in this mouse model, which we tie to a requirement for the LINC complex in productive TGFβ signaling. In the absence of SUN2, we detect elevated levels of the integral inner nuclear membrane protein MAN1, an established negative regulator of TGFβ signaling, at the nuclear envelope. We suggest that A-type lamins and SUN2 play antagonistic roles in the modulation of profibrotic signaling through opposite effects on MAN1 levels at the nuclear lamina, suggesting a new perspective on disease etiology.

Anisotropy vs isotropy in living cell indentation with AFM

The measurement of local mechanical properties of living cells by nano/micro indentation relies on the foundational assumption of locally isotropic cellular deformation. As a consequence of assumed isotropy, the cell membrane and underlying cytoskeleton are expected to locally deform axisymmetrically when indented by a spherical tip. Here, we directly observe the local geometry of deformation of membrane and cytoskeleton of different living adherent cells during nanoindentation with the integrated Atomic Force (AFM) and spinning disk confocal (SDC) microscope. We show that the presence of the perinuclear actin cap (apical stress fibers), such as those encountered in cells subject to physiological forces, causes a strongly non-axisymmetric membrane deformation during indentation reflecting local mechanical anisotropy. In contrast, axisymmetric membrane deformation reflecting mechanical isotropy was found in cells without actin cap: cancerous cells MDA-MB-231, which naturally lack the actin cap, and NIH 3T3 cells in which the actin cap is disrupted by latrunculin A. Careful studies were undertaken to quantify the effect of the live cell fluorescent stains on the measured mechanical properties. Using finite element computations and the numerical analysis, we explored the capability of one of the simplest anisotropic models – transverse isotropy model with three local mechanical parameters (longitudinal and transverse modulus and planar shear modulus) – to capture the observed non-axisymmetric deformation. These results help identifying which cell types are likely to exhibit non-isotropic properties, how to measure and quantify cellular deformation during AFM indentation using live cell stains and SDC, and suggest modelling guidelines to recover quantitative estimates of the mechanical properties of living cells.

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.

Unchain My Heart: Integrins at the Basis of iPSC Cardiomyocyte Differentiation

The cellular response to the extracellular matrix (ECM) microenvironment mediated by integrin adhesion is of fundamental importance, in both developmental and pathological processes. In particular, mechanotransduction is of growing importance in groundbreaking cellular models such as induced pluripotent stem cells (iPSC), since this process may strongly influence cell fate and, thus, augment the precision of differentiation into specific cell types, e.g., cardiomyocytes. The decryption of the cellular machinery starting from ECM sensing to iPSC differentiation calls for new in vitro methods. Conveniently, engineered biomaterials activating controlled integrin-mediated responses through chemical, physical, and geometrical designs are key to resolving this issue and could foster clinical translation of optimized iPSC-based technology. This review introduces the main integrin-dependent mechanisms and signalling pathways involved in mechanotransduction. Special consideration is given to the integrin-iPSC linkage signalling chain in the cardiovascular field, focusing on biomaterial-based in vitro models to evaluate the relevance of this process in iPSC differentiation into cardiomyocytes.

Signal Transduction across the Nuclear Envelope: Role of the LINC Complex in Bidirectional Signaling

The primary functions of the nuclear envelope are to isolate the nucleoplasm and its contents from the cytoplasm as well as maintain the spatial and structural integrity of the nucleus. The nuclear envelope also plays a role in the transfer of various molecules and signals to and from the nucleus. To reach the nucleus, an extracellular signal must be transmitted across three biological membranes: the plasma membrane, as well as the inner and outer nuclear membranes. While signal transduction across the plasma membrane is well characterized, signal transduction across the nuclear envelope, which is essential for cellular functions such as transcriptional regulation and cell cycle progression, remains poorly understood. As a physical entity, the nuclear envelope, which contains more than 100 proteins, functions as a binding scaffold for both the cytoskeleton and the nucleoskeleton, and acts in mechanotransduction by relaying extracellular signals to the nucleus. Recent results show that the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex, which is a conserved molecular bridge that spans the nuclear envelope and connects the nucleoskeleton and cytoskeleton, is also capable of transmitting information bidirectionally between the nucleus and the cytoplasm. This short review discusses bidirectional signal transduction across the nuclear envelope, with a particular focus on mechanotransduction.

The role of lamin A/C in mesenchymal stem cell differentiation

Lamin A/C is the major architectural protein of cell nucleus in charge of the nuclear mechanosensing. By integrating extracellular mechanical and biochemical signals, lamin A/C regulates multiple intracellular events including mesenchymal stem cell (MSC) fate determination. Herein, we review the recent findings about the effects and mechanisms of lamin A/C in governing MSC lineage commitment, with a special focus on osteogenesis and adipogenesis. Better understanding of MSC differentiation regulated by lamin A/C could provide insights into pathogenesis of age-related osteoporosis.

Functional regulation of YAP mechanosensitive transcriptional coactivator by Focused Low-Intensity Pulsed Ultrasound (FLIPUS) enhances proliferation of murine mesenchymal precursors

Yes-associated protein (YAP) acts as a mechanotransducer in determining the cell fate of murine C2C12 mesenchymal precursors as investigated after stimulation with ultrasound. We applied Focused Low-Intensity Pulsed Ultrasound (FLIPUS) at a sound frequency of 3.6 MHz, 100 Hz pulse repetition frequency (PRF), 27.8% duty cycle (DC), and 44.5 mW/cm² acoustic intensity I_SATA for 5 minutes and evaluated early cellular responses. FLIPUS decreased the level of phosphorylated YAP on Serine 127, leading to higher levels of active YAP in the nucleus. This in turn enhanced the expression of YAP-target genes associated with actin nucleation and stabilization, cytokinesis, and cell cycle progression. FLIPUS enhanced proliferation of C2C12 cells, whereas silencing of YAP expression abolished the beneficial effects of ultrasound. The expression of the transcription factor MyoD, defining cellular myogenic differentiation, was inhibited by mechanical stimulation. This study shows that ultrasound exposure regulates YAP functioning, which in turn improves the cell proliferative potential, critical for tissue regeneration process.

Cellular Mechanotransduction: From Tension to Function

Living cells are constantly exposed to mechanical stimuli arising from the surrounding extracellular matrix (ECM) or from neighboring cells. The intracellular molecular processes through which such physical cues are transformed into a biological response are collectively dubbed as mechanotransduction and are of fundamental importance to help the cell timely adapt to the continuous dynamic modifications of the microenvironment. Local changes in ECM composition and mechanics are driven by a feed forward interplay between the cell and the matrix itself, with the first depositing ECM proteins that in turn will impact on the surrounding cells. As such, these changes occur regularly during tissue development and are a hallmark of the pathologies of aging. Only lately, though, the importance of mechanical cues in controlling cell function (e.g., proliferation, differentiation, migration) has been acknowledged. Here we provide a critical review of the recent insights into the molecular basis of cellular mechanotransduction, by analyzing how mechanical stimuli get transformed into a given biological response through the activation of a peculiar genetic program. Specifically, by recapitulating the processes involved in the interpretation of ECM remodeling by Focal Adhesions at cell-matrix interphase, we revise the role of cytoskeleton tension as the second messenger of the mechanotransduction process and the action of mechano-responsive shuttling proteins converging on stage and cell-specific transcription factors. Finally, we give few paradigmatic examples highlighting the emerging role of malfunctions in cell mechanosensing apparatus in the onset and progression of pathologies.

Focal Adhesions Undergo Longitudinal Splitting into Fixed-Width Units

Focal adhesions (FAs) and stress fibers (SFs) act in concert during cell motility and in response to the extracellular environment. Although the structures of mature FAs and SFs are well studied, less is known about how they assemble and mature de novo during initial cell spreading. In this study using live-cell Airyscan microscopy, we find that FAs undergo "splitting" during their assembly, in which the FA divides along its longitudinal axis. Before splitting, FAs initially appear as assemblies of multiple linear units (FAUs) of 0.3-μm width. Splitting occurs between FAUs, resulting in mature FAs of either a single FAU or of a small number of FAUs that remain attached at their distal tips. Variations in splitting occur based on cell type and extracellular matrix. Depletion of adenomatous polyposis coli (APC) or vasodilator-stimulated phosphoprotein (VASP) results in reduced splitting. FA-associated tension increases progressively during splitting. Early in cell spreading, ventral SFs are detected first, with other SF sub-types (transverse arcs and dorsal SFs) being detected later. Our findings suggest that the fundamental unit of FAs is the fixed-width FAU, and that dynamic interactions between FAUs control adhesion morphology.

STEF/TIAM2-mediated Rac1 activity at the nuclear envelope regulates the perinuclear actin cap

The perinuclear actin cap is an important cytoskeletal structure that regulates nuclear morphology and re-orientation during front-rear polarisation. The mechanisms regulating the actin cap are currently poorly understood. Here, we demonstrate that STEF/TIAM2, a Rac1 selective guanine nucleotide exchange factor, localises at the nuclear envelope, co-localising with the key perinuclear proteins Nesprin-2G and Non-muscle myosin IIB (NMMIIB), where it regulates perinuclear Rac1 activity. We show that STEF depletion reduces apical perinuclear actin cables (a phenotype rescued by targeting active Rac1 to the nuclear envelope), increases nuclear height and impairs nuclear re-orientation. STEF down-regulation also reduces perinuclear pMLC and decreases myosin-generated tension at the nuclear envelope, suggesting that STEF-mediated Rac1 activity regulates NMMIIB activity to promote stabilisation of the perinuclear actin cap. Finally, STEF depletion decreases nuclear stiffness and reduces expression of TAZ-regulated genes, indicating an alteration in mechanosensing pathways as a consequence of disruption of the actin cap.

The Biophysics of 3D Cell Migration

Three-dimensional (3D) cell culture systems have gained increasing interest not only for 3D migration studies but also for their use in drug screening, tissue engineering, and ex vivo modeling of metastatic behavior in the field of cancer biology and morphogenesis in the field of developmental biology. The goal of studying cells in a 3D context is to attempt to more faithfully recapitulate the physiological microenvironment of tissues, including mechanical and structural parameters that we envision will reveal more predictive data for development programs and disease states. In this review, we discuss the pros and cons of several well-characterized 3D cell culture systems for performing 3D migration studies. We discuss the intracellular and extracellular signaling mechanisms that govern cell migration. We also describe the mathematical models and relevant assumptions that can be used to describe 3D cell movement.

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.

Enucleated cells reveal differential roles of the nucleus in cell migration, polarity, and mechanotransduction

The nucleus has long been postulated to play a critical physical role during cell polarization and migration, but that role has not been defined or rigorously tested. Here, we enucleated cells to test the physical necessity of the nucleus during cell polarization and directed migration. Using enucleated mammalian cells (cytoplasts), we found that polarity establishment and cell migration in one dimension (1D) and two dimensions (2D) occur without the nucleus. Cytoplasts directionally migrate toward soluble (chemotaxis) and surface-bound (haptotaxis) extracellular cues and migrate collectively in scratch-wound assays. Consistent with previous studies, migration in 3D environments was dependent on the nucleus. In part, this likely reflects the decreased force exerted by cytoplasts on mechanically compliant substrates. This response is mimicked both in cells with nucleocytoskeletal defects and upon inhibition of actomyosin-based contractility. Together, our observations reveal that the nucleus is dispensable for polarization and migration in 1D and 2D but critical for proper cell mechanical responses.

Proliferation of human aortic endothelial cells on Nitinol thin films with varying hole sizes

In this paper, we present the effect of micron size holes on proliferation and growth of human aortic endothelial cells (HAECs). Square shaped micron size holes (5, 10, 15, 20 and 25 μm) separated by 10 μm wide struts are fabricated on 5 μm thick sputter deposited Nitinol films. HAECs are seeded onto these micropatterned films and analyzed after 30 days with fluorescence microscopy. Captured images are used to quantify the nucleus packing density, size, and aspect ratio. The films with holes ranging from 10 to 20 μm produce the highest cell packing densities with cell nucleus contained within the hole. This produces a geometrically regular grid like cellular distribution pattern. The cell nucleus aspect ratio on the 10-20 μm holes is more circular in shape when compared to aspect ratio on the continuous film or larger size holes. Finally, the 25 μm size holes prevented the formation of a continuous cell monolayer, suggesting the critical length that cells cannot bridge is between 20 to 25 μm.

Nuclear lamin A/C harnesses the perinuclear apical actin cables to protect nuclear morphology

The distinct spatial architecture of the apical actin cables (or actin cap) facilitates rapid biophysical signaling between extracellular mechanical stimuli and intracellular responses, including nuclear shaping, cytoskeletal remodeling, and the mechanotransduction of external forces into biochemical signals. These functions are abrogated in lamin A/C-deficient mouse embryonic fibroblasts that recapitulate the defective nuclear organization of laminopathies, featuring disruption of the actin cap. However, how nuclear lamin A/C mediates the ability of the actin cap to regulate nuclear morphology remains unclear. Here, we show that lamin A/C expressing cells can form an actin cap to resist nuclear deformation in response to physiological mechanical stresses. This study reveals how the nuclear lamin A/C-mediated formation of the perinuclear apical actin cables protects the nuclear structural integrity from extracellular physical disturbances. Our findings highlight the role of the physical interactions between the cytoskeletal network and the nucleus in cellular mechanical homeostasis.

Heterochromatin protein (HP)1γ is not only in the nucleus but also in the cytoplasm interacting with actin in both cell compartments

Confocal and electron microscopy images, and WB analysis of cellular fractions revealed that HP1γ is in the nucleus but also in the cytoplasm of C2C12 myoblasts, myotubes, skeletal and cardiac muscles, N2a, HeLa and HEK293T cells. Signal specificity was tested with different antibodies and by HP1γ knockdown. Leptomycin B treatment of myoblasts increased nuclear HP1γ, suggesting that its nuclear export is Crm-1-dependent. HP1γ exhibited a filamentous pattern of staining partially co-localizing with actin in the cytoplasm of myotubes and myofibrils. Immunoelectron microscopic analysis showed high-density immunogold particles that correspond to HP1γ localized to the Z-disk and A-band of the sarcomere of skeletal muscle. HP1γ partially co-localized with actin in C2C12 myotubes and murine myofibrils. Importantly, actin co-immunoprecipitated with HP1γ in the nuclear and cytosolic fractions of myoblasts. Actin co-immunoprecipitated with HP1γ in myoblasts incubated in the absence or presence of the actin depolymerizing agent cytochalasin D, suggesting that HP1γ may interact with G-and F-actin. In the cytoplasm, HP1γ was associated to the perinuclear actin cap that controls nuclear shape and position. In the nucleus, re-ChIP assays showed that HP1γ-actin associates to the promoter and transcribed regions of the house keeping gene GAPDH, suggesting that HP1γ may function as a scaffold protein for the recruitment of actin to control gene expression. When HP1γ was knocked-down, myoblasts were unable to differentiate or originated thin myotubes. In summary, HP1γ is present in the nucleus and the cytoplasm interacting with actin, a protein complex that may exert different functions depending on its subcellular localization.

The Nuclear Option: Evidence Implicating the Cell Nucleus in Mechanotransduction

Biophysical stimuli presented to cells via microenvironmental properties (e.g., alignment and stiffness) or external forces have a significant impact on cell function and behavior. Recently, the cell nucleus has been identified as a mechanosensitive organelle that contributes to the perception and response to mechanical stimuli. However, the specific mechanotransduction mechanisms that mediate these effects have not been clearly established. Here, we offer a comprehensive review of the evidence supporting (and refuting) three hypothetical nuclear mechanotransduction mechanisms: physical reorganization of chromatin, signaling at the nuclear envelope, and altered cytoskeletal structure/tension due to nuclear remodeling. Our goal is to provide a reference detailing the progress that has been made and the areas that still require investigation regarding the role of nuclear mechanotransduction in cell biology. Additionally, we will briefly discuss the role that mathematical models of cell mechanics can play in testing these hypotheses and in elucidating how biophysical stimulation of the nucleus drives changes in cell behavior. While force-induced alterations in signaling pathways involving lamina-associated polypeptides (LAPs) (e.g., emerin and histone deacetylase 3 (HDAC3)) and transcription factors (TFs) located at the nuclear envelope currently appear to be the most clearly supported mechanism of nuclear mechanotransduction, additional work is required to examine this process in detail and to more fully test alternative mechanisms. The combination of sophisticated experimental techniques and advanced mathematical models is necessary to enhance our understanding of the role of the nucleus in the mechanotransduction processes driving numerous critical cell functions.

Model of Murine Ventricular Cardiac Tissue for In Vitro Kinematic-Dynamic Studies of Electromagnetic and β-Adrenergic Stimulation

In a model of murine ventricular cardiac tissue in vitro, we have studied the inotropic effects of electromagnetic stimulation (frequency, 75 Hz), isoproterenol administration (10 μM), and their combination. In particular, we have performed an image processing analysis to evaluate the kinematics and the dynamics of beating cardiac syncytia starting from the video registration of their contraction movement. We have found that the electromagnetic stimulation is able to counteract the β-adrenergic effect of isoproterenol and to elicit an antihypertrophic response.

Re-evaluating the roles of myosin 18Aα and F-actin in determining Golgi morphology

The peri-centrosomal localization and morphology of the Golgi apparatus depends largely on the microtubule cytoskeleton and the microtubule motor protein dynein. Recent studies proposed that myosin 18Aα (M18Aα) also contributes to Golgi morphology by binding the Golgi protein GOLPH3 and walking along adjacent actin filaments to stretch the Golgi into its classic ribbon structure. Biochemical analyses have shown, however, that M18A is not an actin-activated ATPase and lacks motor activity. Our goal, therefore, was to define the precise molecular mechanism by which M18Aα determines Golgi morphology. We show that purified M18Aα remains inactive in the presence of GOLPH3, arguing against the Golgi-specific activation of the myosin. Using M18A-specific antibodies and expression of GFP-tagged M18Aα, we find no evidence that it localizes to the Golgi. Moreover, several cell lines with reduced or eliminated M18Aα expression exhibited normal Golgi morphology. Interestingly, actin filament disassembly resulted in a marked reduction in lateral stretching of the Golgi in both control and M18Aα-deficient cells. Importantly, this reduction was accompanied by an expansion of the Golgi in the vertical direction, vertical movement of the centrosome, and increases in the height of both the nucleus and the cell. Collectively, our data indicate that M18Aα does not localize to the Golgi or play a significant role in determining its morphology, and suggest that global F-actin disassembly alters Golgi morphology indirectly by altering cell shape.

The assembly and function of perinuclear actin cap in migrating cells

Stress fibers are actin bundles encompassing actin filaments, actin-crosslinking, and actin-associated proteins that represent the major contractile system in the cell. Different types of stress fibers assemble in adherent cells, and they are central to diverse cellular processes including establishment of the cell shape, morphogenesis, cell polarization, and migration. Stress fibers display specific cellular organization and localization, with ventral fibers present at the basal side, and dorsal fibers and transverse actin arcs rising at the cell front from the ventral to the dorsal side and toward the nucleus. Perinuclear actin cap fibers are a specific subtype of stress fibers that rise from the leading edge above the nucleus and terminate at the cell rear forming a dome-like structure. Perinuclear actin cap fibers are fixed at three points: both ends are anchored in focal adhesions, while the central part is physically attached to the nucleus and nuclear lamina through the linker of nucleoskeleton and cytoskeleton (LINC) complex. Here, we discuss recent work that provides new insights into the mechanism of assembly and the function of these actin stress fibers that directly link extracellular matrix and focal adhesions with the nuclear envelope.

Lamins and nesprin-1 mediate inside-out mechanical coupling in muscle cell precursors through FHOD1

LINC complexes are crucial for the response of muscle cell precursors to the rigidity of their environment, but the mechanisms explaining this behaviour are not known. Here we show that pathogenic mutations in LMNA or SYNE-1 responsible for severe muscle dystrophies reduced the ability of human muscle cell precursors to adapt to substrates of different stiffness. Plated on muscle-like stiffness matrix, mutant cells exhibited contractile stress fibre accumulation, increased focal adhesions, and higher traction force than controls. Inhibition of Rho-associated kinase (ROCK) prevented cytoskeletal defects, while inhibiting myosin light chain kinase or phosphorylation of focal adhesion kinase was ineffective. Depletion or inactivation of a ROCK-dependent regulator of actin remodelling, the formin FHOD1, largely rescued morphology in mutant cells. The functional integrity of lamin and nesprin-1 is thus required to modulate the FHOD1 activity and the inside-out mechanical coupling that tunes the cell internal stiffness to match that of its soft, physiological-like environment.

A minimal actomyosin-based model predicts the dynamics of filopodia on neuronal dendrites

A combination of computational and experimental approaches is used to show that the complex dynamics of dendritic filopodia, which is essential for synaptogenesis, is explained by a conceptually simple interplay among actin retrograde flow, myosin contractility, and substrate adhesion.

Dendritic filopodia are actin-filled dynamic subcellular structures that sprout on neuronal dendrites during neurogenesis. The exploratory motion of the filopodia is crucial for synaptogenesis, but the underlying mechanisms are poorly understood. To study filopodial motility, we collected and analyzed image data on filopodia in cultured rat hippocampal neurons. We hypothesized that mechanical feedback among the actin retrograde flow, myosin activity, and substrate adhesion gives rise to various filopodial behaviors. We formulated a minimal one-dimensional partial differential equation model that reproduced the range of observed motility. To validate our model, we systematically manipulated experimental correlates of parameters in the model: substrate adhesion strength, actin polymerization rate, myosin contractility, and the integrity of the putative microtubule-based barrier at the filopodium base. The model predicts the response of the system to each of these experimental perturbations, supporting the hypothesis that our actomyosin-driven mechanism controls dendritic filopodia dynamics.

Cellular strain avoidance is mediated by a functional actin cap - observations in an Lmna-deficient cell model

In adherent cells, the relevance of a physical mechanotransduction pathway provided by the perinuclear actin cap stress fibers has recently emerged. Here, we investigate the impact of a functional actin cap on the cellular adaptive response to topographical cues and uniaxial cyclic strain. Lmna-deficient fibroblasts are used as a model system because they do not develop an intact actin cap, but predominantly form a basal layer of actin stress fibers underneath the nucleus. We observe that topographical cues induce alignment in both normal and Lmna-deficient fibroblasts, suggesting that the topographical signal transmission occurs independently of the integrity of the actin cap. By contrast, in response to cyclic uniaxial strain, Lmna-deficient cells show a compromised strain avoidance response, which is completely abolished when topographical cues and uniaxial strain are applied along the same direction. These findings point to the importance of an intact and functional actin cap in mediating cellular strain avoidance.

Rapid bladder cancer cell detection from clinical urine samples using an ultra-thin silicone membrane

Early detection of initial onset, as well as recurrence, of cancer is paramount for improved patient prognosis and human health. Cancer screening is enhanced by rapid differentiation of cancerous from non-cancerous cells which employs the inherent differences in biophysical properties. Our preliminary testing demonstrates that cell-line derived bladder cancer cells deform our <30 nm silicone membrane within an hour and induce visually distinct wrinkle patterns while cell-line derived non-cancerous cells fail to induce these wrinkle patterns. Herein, we report a platform for the rapid detection of cancerous cells from human clinical urine samples. We performed a blinded study with cells extracted from the urine of human patients suspected to have bladder cancer alongside healthy controls. Wrinkle patterns were induced specifically by the five cancer patient samples within 12 hours and not by the healthy controls. These results were independently validated by the standard diagnostic techniques cystoscopy and cytology. Thus, our ultra-thin membrane approach for cancer diagnosis appears as accurate as standard diagnostic methods while vastly more rapid, less invasive, and requiring limited expertise.

The inner workings of stress fibers - from contractile machinery to focal adhesions and back

Ventral stress fibers and focal adhesions are physically coupled structures that play key roles in cellular mechanics and force sensing. The tight functional interdependence between the two is manifested not only by their apparent proximity but also by the fact that ventral stress fibers and focal adhesions are simultaneously diminished upon actomyosin relaxation, and grow when subjected to external stretching. However, whereas the apparent co-regulation of the two structures is well-documented, the underlying mechanisms remains poorly understood. In this Commentary, we discuss some of the fundamental, yet still open questions regarding ventral stress fiber structure, its force-dependent assembly, as well as its capacity to generate force. We also challenge the common approach - i.e. ventral stress fibers are variants of the well-studied striated or smooth muscle machinery - by presenting and critically discussing alternative venues. By highlighting some of the less-explored aspects of the interplay between stress fibers and focal adhesions, we hope that this Commentary will encourage further investigation in this field.

The mammalian LINC complex regulates genome transcriptional responses to substrate rigidity

Mechanical integration of the nucleus with the extracellular matrix (ECM) is established by linkage between the cytoskeleton and the nucleus. This integration is hypothesized to mediate sensing of ECM rigidity, but parsing the function of nucleus-cytoskeleton linkage from other mechanisms has remained a central challenge. Here we took advantage of the fact that the LINC (linker of nucleoskeleton and cytoskeleton) complex is a known molecular linker of the nucleus to the cytoskeleton, and asked how it regulates the sensitivity of genome-wide transcription to substratum rigidity. We show that gene mechanosensitivity is preserved after LINC disruption, but reversed in direction. Combined with myosin inhibition studies, we identify genes that depend on nuclear tension for their regulation. We also show that LINC disruption does not attenuate nuclear shape sensitivity to substrate rigidity. Our results show for the first time that the LINC complex facilitates mechano-regulation of expression across the genome.

Opposing roles for distinct LINC complexes in regulation of the small GTPase RhoA

Different forms of nuclear envelope–spanning LINC complexes have opposing roles in the transcription-independent control of the small GTPase RhoA. Competition between LINC complexes in the nuclear envelope may therefore dictate the outcome of signaling to cytoskeletal networks.

Linker of Nucleoskeleton and Cytoskeleton (LINC) complexes span the nuclear envelope and transduce force from dynamic cytoskeletal networks to the nuclear lamina. Here we show that LINC complexes also signal from the nuclear envelope to critical regulators of the actin cytoskeleton. Specifically, we find that LINC complexes that contain the inner nuclear membrane protein Sun2 promote focal adhesion assembly by activating the small GTPase RhoA. A key effector in this process is the transcription factor/coactivator complex composed of SRF/Mkl1. A constitutively active form of SRF/Mkl1 was not sufficient to induce focal adhesion assembly in cells lacking Sun2, however, suggesting that LINC complexes support RhoA activity through a transcription-independent mechanism. Strikingly, we also find that the inner nuclear membrane protein Sun1 antagonizes Sun2 LINC complexes and inhibits RhoA activation and focal adhesion assembly. Thus different LINC complexes have opposing roles in the transcription-independent control of the actin cytoskeleton through the small GTPase RhoA.

Modeling of the mechano-chemical behaviour of the nuclear pore complex: current research and perspectives

Recent evidence suggests that mechanical deformation of the cell nucleus regulates the nuclear import of the transcriptional activators of genes involved in primary physiological cell responses such as stem cell differentiation. In addition, this nuclear mechanosensing response is de-regulated in pathological states, such as cancer and neurodegeneration. One hypothesis that could greatly advance the field is that the deformation of the nuclear envelope activates nuclear pore complexes through a direct mechanical link. The understanding of this possible mechanism for nuclear pore complex stretch-activation entails studying the mechanical connection of this complex to the nuclear envelope at the nanoscale. The nanomechanics of the nuclear pore complex is thus emerging as a novel research field, bridging nanoscience with nanotechnology. This review examines the frontier of research methodologies that are potentially useful for building a computational model of this interaction. This includes, for example, electron tomography to assess the geometrical features of the nuclear pore complex and nanoindentation to estimate its mechanical properties and that of the nuclear envelope. In order to summarize the state-of-the-art and perspectives in the field of NPC nanomechanics, this review covers highly interdisciplinary experimental and theoretical research methodologies pertaining to the fields of physics, chemistry, biology, materials and mechanics.

Physical role for the nucleus in cell migration

Cell migration is important for the function of many eukaryotic cells. Recently the nucleus has been shown to play an important role in cell motility. After giving an overview of cell motility mechanisms we review what is currently known about the mechanical properties of the nucleus and the connections between it and the cytoskeleton. We also discuss connections to the extracellular matrix and mechanotransduction. We identify key physical roles of the nucleus in cell migration.

Dorsal stress fibers, transverse actin arcs, and perinuclear actin fibers form an interconnected network that induces nuclear movement in polarizing fibroblasts

In polarized motile cells, stress fibers display specific three-dimensional organization. Ventral stress fibers, attached to focal adhesions at both ends, are restricted to the basal side of the cell and nonprotruding cell sides. Dorsal fibers, transverse actin arcs, and perinuclear actin fibers emanate from protruding cell front toward the nucleus and toward apical side of the cell. Perinuclear cap fibers further extend above the nucleus, associate with nuclear envelope through LINC (linker of nucleoskeleton and cytoskeleton) complex and terminate in focal adhesions at cell rear. How are perinuclear actin fibers formed is poorly understood. We show that the formation of perinuclear actin fibers requires dorsal stress fibers that polymerize from focal adhesions at leading edge, and transverse actin arcs that are interconnected with dorsal fibers in spots rich in α-actinin-1. During cell polarization, the interconnected dorsal fibers and transverse arcs move from leading edge toward dorsal side of the cell. As they move, transverse arcs associate with one end of stress fibers present at nonprotruding cell sides, move them above the nucleus thus forming perinuclear actin fibers. Furthermore, the formation of perinuclear actin fibers induces temporal rotational movement of the nucleus resulting in nuclear reorientation to the direction of migration. These results suggest that the network of dorsal fibers, transverse arcs, and perinuclear fibers transfers mechanical signal between the focal adhesions and nuclear envelope that regulates the nuclear reorientation in polarizing cells.

Cellular functions of the ADF/cofilin family at a glance

The actin depolymerizing factor (ADF)/cofilin family comprises small actin-binding proteins with crucial roles in development, tissue homeostasis and disease. They are best known for their roles in regulating actin dynamics by promoting actin treadmilling and thereby driving membrane protrusion and cell motility. However, recent discoveries have increased our understanding of the functions of these proteins beyond their well-characterized roles. This Cell Science at a Glance article and the accompanying poster serve as an introduction to the diverse roles of the ADF/cofilin family in cells. The first part of the article summarizes their actions in actin treadmilling and the main mechanisms for their intracellular regulation; the second part aims to provide an outline of the emerging cellular roles attributed to the ADF/cofilin family, besides their actions in actin turnover. The latter part discusses an array of diverse processes, which include regulation of intracellular contractility, maintenance of nuclear integrity, transcriptional regulation, nuclear actin monomer transfer, apoptosis and lipid metabolism. Some of these could, of course, be indirect consequences of actin treadmilling functions, and this is discussed.

A Robust Actin Filaments Image Analysis Framework

The cytoskeleton is a highly dynamical protein network that plays a central role in numerous cellular physiological processes, and is traditionally divided into three components according to its chemical composition, i.e. actin, tubulin and intermediate filament cytoskeletons. Understanding the cytoskeleton dynamics is of prime importance to unveil mechanisms involved in cell adaptation to any stress type. Fluorescence imaging of cytoskeleton structures allows analyzing the impact of mechanical stimulation in the cytoskeleton, but it also imposes additional challenges in the image processing stage, such as the presence of imaging-related artifacts and heavy blurring introduced by (high-throughput) automated scans. However, although there exists a considerable number of image-based analytical tools to address the image processing and analysis, most of them are unfit to cope with the aforementioned challenges. Filamentous structures in images can be considered as a piecewise composition of quasi-straight segments (at least in some finer or coarser scale). Based on this observation, we propose a three-steps actin filaments extraction methodology: (i) first the input image is decomposed into a ‘cartoon’ part corresponding to the filament structures in the image, and a noise/texture part, (ii) on the ‘cartoon’ image, we apply a multi-scale line detector coupled with a (iii) quasi-straight filaments merging algorithm for fiber extraction. The proposed robust actin filaments image analysis framework allows extracting individual filaments in the presence of noise, artifacts and heavy blurring. Moreover, it provides numerous parameters such as filaments orientation, position and length, useful for further analysis. Cell image decomposition is relatively under-exploited in biological images processing, and our study shows the benefits it provides when addressing such tasks. Experimental validation was conducted using publicly available datasets, and in osteoblasts grown in two different conditions: static (control) and fluid shear stress. The proposed methodology exhibited higher sensitivity values and similar accuracy compared to state-of-the-art methods.

We propose a novel actin filaments cytoskeleton analysis framework that allows extracting quasi-straight individual fibers in a robust manner, and provides their respective position, orientation, and length as output. The proposed framework is defined as a three-steps processing sequence, that can explicitly cope with high-throughput imaging related issues, such as noise/artifacts presence and heavy blurring, and can similarly process artifacts-free and well-focused images.

Mechanical phenotype of cancer cells: cell softening and loss of stiffness sensing

The stiffness sensing ability is required to respond to the stiffness of the matrix. Here we determined whether normal cells and cancer cells display distinct mechanical phenotypes. Cancer cells were softer than their normal counterparts, regardless of the type of cancer (breast, bladder, cervix, pancreas, or Ha-Ras^V12-transformed cells). When cultured on matrices of varying stiffness, low stiffness decreased proliferation in normal cells, while cancer cells and transformed cells lost this response. Thus, cancer cells undergo a change in their mechanical phenotype that includes cell softening and loss of stiffness sensing. Caveolin-1, which is suppressed in many tumor cells and in oncogene-transformed cells, regulates the mechanical phenotype. Caveolin-1-upregulated RhoA activity and ^Y397FAK phosphorylation directed actin cap formation, which was positively correlated with cell elasticity and stiffness sensing in fibroblasts. Ha-Ras^V12-induced transformation and changes in the mechanical phenotypes were reversed by re-expression of caveolin-1 and mimicked by the suppression of caveolin-1 in normal fibroblasts. This is the first study to describe this novel role for caveolin-1, linking mechanical phenotype to cell transformation. Furthermore, mechanical characteristics may serve as biomarkers for cell transformation.

Talin tension sensor reveals novel features of focal adhesion force transmission and mechanosensitivity

Integrin-dependent adhesions are mechanosensitive structures in which talin mediates a linkage to actin filaments either directly or indirectly by recruiting vinculin. Here, we report the development and validation of a talin tension sensor. We find that talin in focal adhesions is under tension, which is higher in peripheral than central adhesions. Tension on talin is increased by vinculin and depends mainly on actin-binding site 2 (ABS2) within the middle of the rod domain, rather than ABS3 at the far C terminus. Unlike vinculin, talin is under lower tension on soft substrates. The difference between central and peripheral adhesions requires ABS3 but not vinculin or ABS2. However, differential stiffness sensing by talin requires ABS2 but not vinculin or ABS3. These results indicate that central versus peripheral adhesions must be organized and regulated differently, and that ABS2 and ABS3 have distinct functions in spatial variations and stiffness sensing. Overall, these results shed new light on talin function and constrain models for cellular mechanosensing.