Plectin contributes to mechanical properties of living cells

High matrix stiffness accelerates migration of hepatocellular carcinoma cells through the integrin β1-Plectin-F-actin axis

BACKGROUND

Abundant research indicates that increased extracellular matrix (ECM) stiffness significantly enhances the malignant characteristics of hepatocellular carcinoma (HCC) cells. Plectin, an essential cytoskeletal linker protein, has recently emerged as a promoter of cancer progression, particularly in the context of cancer cell invasion and metastasis. However, the responsiveness of plectin to changes in ECM stiffness and its impact on HCC progression remain unclear. In this study, we aimed to investigate whether plectin responds to variations in ECM stiffness and to explore its involved molecular mechanisms in regulating HCC cell migration.

RESULTS

Our results showed that, when compared with control group (7 kPa), high ECM stiffness (53 kPa) boosts HCC cell migration by upregulating plectin and integrin β1 expression and increasing F-actin polymerization. Knockdown of integrin β1 negated the high stiffness-upregulated plectin expression. Furthermore, reducing either plectin or integrin β1 levels, or using latrunculin A, effectively prevented the high ECM stiffness-induced F-actin polymerization and HCC cell migration.

CONCLUSIONS

These findings demonstrate that integrin β1-plectin-F-actin axis is necessary for high matrix stiffness-driven migration of HCC cells, and provide evidence for the critical role of plectin in mechanotransduction in HCC cells.

Prolonged depletion of profilin 1 or F-actin causes an adaptive response in microtubules

In addition to its well-established role in actin assembly, profilin 1 (PFN1) has been shown to bind to tubulin and alter microtubule growth. However, whether PFN1's predominant control over microtubules in cells occurs through direct regulation of tubulin or indirectly through the polymerization of actin has yet to be determined. Here, we manipulated PFN1 expression, actin filament assembly, and actomyosin contractility and showed that reducing any of these parameters for extended periods of time caused an adaptive response in the microtubule cytoskeleton, with the effect being significantly more pronounced in neuronal processes. All the observed changes to microtubules were reversible if actomyosin was restored, arguing that PFN1's regulation of microtubules occurs principally through actin. Moreover, the cytoskeletal modifications resulting from PFN1 depletion in neuronal processes affected microtubule-based transport and mimicked phenotypes that are linked to neurodegenerative disease. This demonstrates how defects in actin can cause compensatory responses in other cytoskeleton components, which in turn significantly alter cellular function.

The Role of Biophysical Factors in Organ Development: Insights from Current Organoid Models

Biophysical factors play a fundamental role in human embryonic development. Traditional in vitro models of organogenesis focused on the biochemical environment and did not consider the effects of mechanical forces on developing tissue. While most human tissue has a Young's modulus in the low kilopascal range, the standard cell culture substrate, plasma-treated polystyrene, has a Young's modulus of 3 gigapascals, making it 10,000-100,000 times stiffer than native tissues. Modern in vitro approaches attempt to recapitulate the biophysical niche of native organs and have yielded more clinically relevant models of human tissues. Since Clevers' conception of intestinal organoids in 2009, the field has expanded rapidly, generating stem-cell derived structures, which are transcriptionally similar to fetal tissues, for nearly every organ system in the human body. For this reason, we conjecture that organoids will make their first clinical impact in fetal regenerative medicine as the structures generated ex vivo will better match native fetal tissues. Moreover, autologously sourced transplanted tissues would be able to grow with the developing embryo in a dynamic, fetal environment. As organoid technologies evolve, the resultant tissues will approach the structure and function of adult human organs and may help bridge the gap between preclinical drug candidates and clinically approved therapeutics. In this review, we discuss roles of tissue stiffness, viscoelasticity, and shear forces in organ formation and disease development, suggesting that these physical parameters should be further integrated into organoid models to improve their physiological relevance and therapeutic applicability. It also points to the mechanotransductive Hippo-YAP/TAZ signaling pathway as a key player in the interplay between extracellular matrix stiffness, cellular mechanics, and biochemical pathways. We conclude by highlighting how frontiers in physics can be applied to biology, for example, how quantum entanglement may be applied to better predict spontaneous DNA mutations. In the future, contemporary physical theories may be leveraged to better understand seemingly stochastic events during organogenesis.

Reconstitution of cytolinker-mediated crosstalk between actin and vimentin

Cell shape and motility are determined by the cytoskeleton, an interpenetrating network of actin filaments, microtubules, and intermediate filaments. The biophysical properties of each filament type individually have been studied extensively by cell-free reconstitution. By contrast, the interactions between the three cytoskeletal networks are relatively unexplored. They are coupled via crosslinkers of the plakin family such as plectin. These are challenging proteins for reconstitution because of their giant size and multidomain structure. Here we engineer a recombinant actin-vimentin crosslinker protein called 'ACTIF' that provides a minimal model system for plectin, recapitulating its modular design with actin-binding and intermediate filament-binding domains separated by a coiled-coil linker for dimerisation. We show by fluorescence and electron microscopy that ACTIF has a high binding affinity for vimentin and actin and creates mixed actin-vimentin bundles. Rheology measurements show that ACTIF-mediated crosslinking strongly stiffens actin-vimentin composites. Finally, we demonstrate the modularity of this approach by creating an ACTIF variant with the intermediate filament binding domain of Adenomatous Polyposis Coli. Our protein engineering approach provides a new cell-free system for the biophysical characterization of intermediate filament-binding crosslinkers and for understanding the mechanical synergy between actin and vimentin in mesenchymal cells.

Vimentin is a key regulator of cell mechanosensing through opposite actions on actomyosin and microtubule networks

The cytoskeleton is a complex network of interconnected biopolymers consisting of actin filaments, microtubules, and intermediate filaments. These biopolymers work in concert to transmit cell-generated forces to the extracellular matrix required for cell motility, wound healing, and tissue maintenance. While we know cell-generated forces are driven by actomyosin contractility and balanced by microtubule network resistance, the effect of intermediate filaments on cellular forces is unclear. Using a combination of theoretical modeling and experiments, we show that vimentin intermediate filaments tune cell stress by assisting in both actomyosin-based force transmission and reinforcement of microtubule networks under compression. We show that the competition between these two opposing effects of vimentin is regulated by the microenvironment stiffness. These results reconcile seemingly contradictory results in the literature and provide a unified description of vimentin's effects on the transmission of cell contractile forces to the extracellular matrix.

Cytoskeletal adaptation following long-term dysregulation of actomyosin in neuronal processes

Microtubules, intermediate filaments, and actin are cytoskeletal polymer networks found within the cell. While each has unique functions, all the cytoskeletal elements must work together for cellular mechanics to be fully operative. This is achieved through crosstalk mechanisms whereby the different networks influence each other through signaling pathways and direct interactions. Because crosstalk can be complex, it is possible for perturbations in one cytoskeletal element to affect the others in ways that are difficult to predict. Here we investigated how long-term changes to the actin cytoskeleton affect microtubules and intermediate filaments. Reducing F-actin or actomyosin contractility increased acetylated microtubules and intermediate filament expression, with the effect being significantly more pronounced in neuronal processes. Changes to microtubules were completely reversible if F-actin and myosin activity is restored. Moreover, the altered microtubules in neuronal processes resulting from F-actin depletion caused significant changes to microtubule-based transport, mimicking phenotypes that are linked to neurodegenerative disease. Thus, defects in actin dynamics cause a compensatory response in other cytoskeleton components which profoundly alters cellular function.

A Cdh3-β-catenin-laminin signaling axis in a subset of breast tumor leader cells control leader cell polarization and directional collective migration

Carcinoma dissemination can occur when heterogeneous tumor and tumor-stromal cell clusters migrate together via collective migration. Cells at the front lead and direct collective migration, yet how these leader cells form and direct migration are not fully appreciated. From live videos of primary mouse and human breast tumor organoids in a 3D microfluidic system mimicking native breast tumor microenvironment, we developed 3D computational models, which hypothesize that leader cells need to generate high protrusive forces and overcome extracellular matrix (ECM) resistance at the leading edge. From single-cell sequencing analyses, we find that leader cells are heterogeneous and identify and isolate a keratin 14- and cadherin-3-positive subpopulation sufficient to lead collective migration. Cdh3 controls leader cell protrusion dynamics through local production of laminin, which is required for integrin/focal adhesion function. Our findings highlight how a subset of leader cells interact with the microenvironment to direct collective migration.

Plectin Downregulation Inhibits Migration and Suppresses Epithelial Mesenchymal Transformation of Hepatocellular Carcinoma Cells via ERK1/2 Signaling

Plectin, as a cytoskeleton-related protein, is involved in various physiological and pathological processes of many cell types. Studies have found that plectin affects cancer cell invasion and metastasis, but the exact mechanism is not fully understood. In this study, we aim to investigate the role of plectin in the migration of hepatocellular carcinoma (HCC) cells and explore its relevant molecular mechanism. Herein, we found that the expression of plectin in HCC tissue and cells was significantly increased compared with normal liver tissue and cells. After downregulation of plectin, the migration ability of HCC cells was significantly lower than that of the control group. Moreover, the expression of E-cadherin was upregulated and the expression of N-cadherin and vimentin was downregulated, suggesting that plectin downregulation suppresses epithelial mesenchymal transformation (EMT) of HCC cells. Mechanically, we found that plectin downregulation repressed the extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation. Activation of ERK1/2 recovered the plectin downregulation-inhibited migration and EMT of HCC cells. Taken together, our results demonstrate that downregulation of plectin inhibits HCC cell migration and EMT through ERK1/2 signaling, which provides a novel prognostic biomarker and potential therapeutic target for HCC.

Novel insight into the role of clusterin on intraocular pressure regulation by modifying actin polymerization and extracellular matrix remodeling in the trabecular meshwork

This study provides comprehensive mechanistic evidence for the role of clusterin, a stress-response secretory chaperone protein, in the modulation of intraocular pressure (IOP) by regulating the trabecular meshwork (TM) actin cytoskeleton and the extracellular matrix (ECM). The pathological stressors on TM known to elevate IOP significantly lowered clusterin protein levels indicating stress-related clusterin function loss. Small interfering RNA-mediated clusterin loss in human TM cells in vitro induced actin polymerization and stabilization via protein kinase D1, serine/threonine-protein kinase N2 (PRK2), and LIM kinase 1 (LIMK1), and the recruitment and activation of adhesome proteins including paxillin, vinculin, and integrin αV and β5. A complete loss of clusterin as seen in clusterin knockout mice (Clu-/- ) led to significant IOP elevation at postnatal Day 70. Contrarily, constitutive clusterin expression using adenovirus (AdCLU) in HTM cells resulted in the loss of actin polymerization via decreased PRK2, and LIMK1 and negative regulation of integrin αV and β5. Furthermore, we found that AdCLU treatment in HTM cells significantly decreased the ECM protein expression and distribution by significantly increasing matrix metalloprotease 2 (MMP2) activity and lowering the levels of pro-fibrotic proteins such as transforming growth factor-β2 (TGFβ2), thrombospondin-1 (TSP-1), and plasminogen activator inhibitor-1 (PAI-1). Finally, we found that HTM cells supplemented with recombinant human clusterin attenuated the pro-fibrotic effects of TGFβ2. For the first time this study demonstrates the importance of clusterin in the regulation of TM actin cytoskeleton - ECM interactions and the maintenance of IOP, thus making clusterin an interesting target to reverse elevated IOP.

Clinicopathological-genetic features of congenital myasthenic syndrome from a Chinese neuromuscular centre

Congenital myasthenic syndrome (CMS) encompasses a heterogeneous group of inherited disorders affecting nerve transmission across the neuromuscular junction. The aim of this study was to characterize the clinical, physiological, pathohistological and genetic features of nine unrelated Chinese patients with CMS from a single neuromuscular centre. A total of nine patients aged from neonates to 34 years were enrolled who exhibited initial symptoms. Physical examinations revealed that all patients exhibited muscle weakness. Muscle biopsies demonstrated multiple myopathological changes, including increased fibre size variation, myofibrillar network disarray, necrosis, myofiber grouping, regeneration, fibre atrophy and angular fibres. Genetic testing revealed six different mutated genes, including AGRN (2/9), CHRNE (1/9), GFPT1 (1/9), GMPPB (1/9), PLEC (3/9) and SCN4A (1/9). In addition, patients exhibited differential responses to pharmacological treatment. Prompt utilization of genetic testing will identify novel variants and expand our understanding of the phenotype of this rare syndrome. Our findings contribute to the clinical, pathohistological and genetic spectrum of congenital myasthenic syndrome in China.

Plectin-mediated cytoskeletal crosstalk controls cell tension and cohesion in epithelial sheets

The coordinated interplay of cytoskeletal networks critically determines tissue biomechanics and structural integrity. Here, we show that plectin, a major intermediate filament-based cytolinker protein, orchestrates cortical cytoskeletal networks in epithelial sheets to support intercellular junctions. By combining CRISPR/Cas9-based gene editing and pharmacological inhibition, we demonstrate that in an F-actin-dependent context, plectin is essential for the formation of the circumferential keratin rim, organization of radial keratin spokes, and desmosomal patterning. In the absence of plectin-mediated cytoskeletal cross-linking, the aberrant keratin-desmosome (DSM)-network feeds back to the actin cytoskeleton, which results in elevated actomyosin contractility. Also, by complementing a predictive mechanical model with Förster resonance energy transfer-based tension sensors, we provide evidence that in the absence of cytoskeletal cross-linking, major intercellular junctions (adherens junctions and DSMs) are under intrinsically generated tensile stress. Defective cytoarchitecture and tensional disequilibrium result in reduced intercellular cohesion, associated with general destabilization of plectin-deficient sheets upon mechanical stress.

Plectin in Skin Fragility Disorders

Plectin is a multi-faceted, 500 kDa-large protein, which due to its expression in different isoforms and distinct organs acts diversely as a cytoskeletal crosslinker and signaling scaffold. It functions as a mediator of keratinocyte mechanical stability in the skin, primarily through linking intermediate filaments to hemidesmosomes. Skin fragility may occur through the presence of mutations in the gene encoding for plectin, PLEC, or through the presence of autoantibodies against the molecule. Below, we review the cutaneous manifestations of plectinopathies as well as their systemic involvement in specific disease subtypes. We summarize the known roles of plectin in keratinocytes and fibroblasts and provide an outlook on future perspectives for plectin-associated skin disorders.

Intermediate Filaments from Tissue Integrity to Single Molecule Mechanics

Cytoplasmic intermediate filaments (IFs), which together with actin and microtubules form the cytoskeleton, are composed of a large and diverse family of proteins. Efforts to elucidate the molecular mechanisms responsible for IF-associated diseases increasingly point towards a major contribution of IFs to the cell’s ability to adapt, resist and respond to mechanical challenges. From these observations, which echo the impressive resilience of IFs in vitro, we here discuss the role of IFs as master integrators of cell and tissue mechanics. In this review, we summarize our current understanding of the contribution of IFs to cell and tissue mechanics and explain these results in light of recent in vitro studies that have investigated physical properties of single IFs and IF networks. Finally, we highlight how changes in IF gene expression, network assembly dynamics, and post-translational modifications can tune IF properties to adapt cell and tissue mechanics to changing environments.

Cytoskeletal prestress: The cellular hallmark in mechanobiology and mechanomedicine

Increasing evidence demonstrates that mechanical forces, in addition to soluble molecules, impact cell and tissue functions in physiology and diseases. How living cells integrate mechanical signals to perform appropriate biological functions is an area of intense investigation. Here, we review the evidence of the central role of cytoskeletal prestress in mechanotransduction and mechanobiology. Elevating cytoskeletal prestress increases cell stiffness and reinforces cell stiffening, facilitates long-range cytoplasmic mechanotransduction via integrins, enables direct chromatin stretching and rapid gene expression, spurs embryonic development and stem cell differentiation, and boosts immune cell activation and killing of tumor cells whereas lowering cytoskeletal prestress maintains embryonic stem cell pluripotency, promotes tumorigenesis and metastasis of stem cell-like malignant tumor-repopulating cells, and elevates drug delivery efficiency of soft-tumor-cell-derived microparticles. The overwhelming evidence suggests that the cytoskeletal prestress is the governing principle and the cellular hallmark in mechanobiology. The application of mechanobiology to medicine (mechanomedicine) is rapidly emerging and may help advance human health and improve diagnostics, treatment, and therapeutics of diseases.

Vimentin intermediate filaments modulate cell traction force but not cell sensitivity to substrate stiffness

The ability of cells to sense and respond to the mechanical stiffness of the surrounding matrix is important to support normal cell function, wound healing, and development. Central to the process of durosensing is the cytoskeleton composed of three classes of filaments: F-actin, microtubules, and intermediate filaments (IFs). Vimentin is an IF protein that contributes significantly to cell mechanics and cell traction force, which is required to probe extracellular matrix. The role of vimentin in how cells sense and respond to the mechanical rigidity of extracellular matrix is largely unclear. To investigate the role of vimentin in durosensing, we knocked down the vimentin expression level in 3T3 fibroblasts using shRNA transfection and measured cellular responses as functions of substrate stiffness. We quantified durosensitivity by the rates at which cell area and traction force change with substrate stiffness. Our results show that that vimentin plays a role in durosensing by modulating traction force and knocking out vimentin did not significantly affect durosensitivity. These results indicate that vimentin may be a redundant component of the machinery that cells use to sense substrate stiffness.

Crosstalk between Cell Adhesion Complexes in Regulation of Mechanotransduction

Physical forces regulate numerous biological processes during development, physiology, and pathology. Forces between the external environment and intracellular actin cytoskeleton are primarily transmitted through integrin-containing focal adhesions and cadherin-containing adherens junctions. Crosstalk between these complexes is well established and modulates the mechanical landscape of the cell. However, integrins and cadherins constitute large families of adhesion receptors and form multiple complexes by interacting with different ligands, adaptor proteins, and cytoskeletal filaments. Recent findings indicate that integrin-containing hemidesmosomes oppose force transduction and traction force generation by focal adhesions. The cytolinker plectin mediates this crosstalk by coupling intermediate filaments to the actin cytoskeleton. Similarly, cadherins in desmosomes might modulate force generation by adherens junctions. Moreover, mechanotransduction can be influenced by podosomes, clathrin lattices, and tetraspanin-enriched microdomains. This review discusses mechanotransduction by multiple integrin- and cadherin-based cell adhesion complexes, which together with the associated cytoskeleton form an integrated network that allows cells to sense, process, and respond to their physical environment.

Scaling up single-cell mechanics to multicellular tissues - the role of the intermediate filament-desmosome network

Cells and tissues sense, respond to and translate mechanical forces into biochemical signals through mechanotransduction, which governs individual cell responses that drive gene expression, metabolic pathways and cell motility, and determines how cells work together in tissues. Mechanotransduction often depends on cytoskeletal networks and their attachment sites that physically couple cells to each other and to the extracellular matrix. One way that cells associate with each other is through Ca2+-dependent adhesion molecules called cadherins, which mediate cell-cell interactions through adherens junctions, thereby anchoring and organizing the cortical actin cytoskeleton. This actin-based network confers dynamic properties to cell sheets and developing organisms. However, these contractile networks do not work alone but in concert with other cytoarchitectural elements, including a diverse network of intermediate filaments. This Review takes a close look at the intermediate filament network and its associated intercellular junctions, desmosomes. We provide evidence that this system not only ensures tissue integrity, but also cooperates with other networks to create more complex tissues with emerging properties in sensing and responding to increasingly stressful environments. We will also draw attention to how defects in intermediate filament and desmosome networks result in both chronic and acquired diseases.

Hemidesmosomes modulate force generation via focal adhesions

Hemidesmosomes are specialized cell-matrix adhesion structures that are associated with the keratin cytoskeleton. Although the adhesion function of hemidesmosomes has been extensively studied, their role in mechanosignaling and transduction remains largely unexplored. Here, we show that keratinocytes lacking hemidesmosomal integrin α6β4 exhibit increased focal adhesion formation, cell spreading, and traction-force generation. Moreover, disruption of the interaction between α6β4 and intermediate filaments or laminin-332 results in similar phenotypical changes. We further demonstrate that integrin α6β4 regulates the activity of the mechanosensitive transcriptional regulator YAP through inhibition of Rho-ROCK-MLC- and FAK-PI3K-dependent signaling pathways. Additionally, increased tension caused by impaired hemidesmosome assembly leads to a redistribution of integrin αVβ5 from clathrin lattices to focal adhesions. Our results reveal a novel role for hemidesmosomes as regulators of cellular mechanical forces and establish the existence of a mechanical coupling between adhesion complexes.

Identification of genetic markers associated with milk production traits in Chinese Holstein cattle based on post genome-wide association studies

With the rapid development of dairy industry, the breeding process of dairy cows has been accelerated. In previous genome-wide association studies (GWAS), a large number of genetic markers have been reported which may contribute to the selection of Holstein populations with superior milk-producing traits, but they remain to be further verified before practical application. In this study, 90 single nucleotide polymorphisms (SNPs) were selected, which were reported to be significantly associated with five milk production traits, including 305-day milk yield (305MY), 305-day milk fat percent (305FC), 305-day milk protein percent (305PC), 305-day milk fat yield (305FY) and 305-day milk protein yield (305PY). Effective 305-day data and fresh DNA samples were obtained from 295 healthy cows with gestational age of 1-4. Matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) was used to perform precise genotyping of these loci, followed by site association and haplotype analysis. Results showed that 36 out of 90 loci were supported to be used as genetic markers. In particular, several novel and effective haplotypes were also presented. Overall, our results verified tens of useful markers and provided a basis for further development of breeding strategies.

The Structural Function of Nestin in Cell Body Softening is Correlated with Cancer Cell Metastasis

Intermediate filaments play significant roles in governing cell stiffness and invasive ability. Nestin is a type VI intermediate filament protein that is highly expressed in several high-metastatic cancer cells. Although inhibition of nestin expression was shown to reduce the metastatic capacity of tumor cells, the relationship between this protein and the mechanism of cancer cell metastasis remains unclear. Here, we show that nestin softens the cell body of the highly metastatic mouse breast cancer cell line FP10SC2, thereby enhancing the metastasis capacity. Proximity ligation assay demonstrated increased binding between actin and vimentin in nestin knockout cells. Because nestin copolymerizes with vimentin and nestin has an extremely long tail domain in its C-terminal region, we hypothesized that the tail domain functions as a steric inhibitor of the vimentin-actin interaction and suppresses association of vimentin filaments with the cortical actin cytoskeleton, leading to reduced cell stiffness. To demonstrate this function, we mechanically pulled vimentin filaments in living cells using a nanoneedle modified with vimentin-specific antibodies under manipulation by atomic force microscopy (AFM). The tensile test revealed that mobility of vimentin filaments was increased by nestin expression in FP10SC2 cells.

Tissue cell differentiation and multicellular evolution via cytoskeletal stiffening in mechanically stressed microenvironments

Evolution of eukaryotes from simple cells to complex multicellular organisms remains a mystery. Our postulate is that cytoskeletal stiffening is a necessary condition for evolution of complex multicellular organisms from early simple eukaryotes. Recent findings show that embryonic stem cells are as soft as primitive eukaryotes-amoebae and that differentiated tissue cells can be two orders of magnitude stiffer than embryonic stem cells. Soft embryonic stem cells become stiff as they differentiate into tissue cells of the complex multicellular organisms to match their microenvironment stiffness. We perhaps see in differentiation of embryonic stem cells (derived from inner cell mass cells) the echo of those early evolutionary events. Early soft unicellular organisms might have evolved to stiffen their cytoskeleton to protect their structural integrity from external mechanical stresses while being able to maintain form, to change shape, and to move.

Plectin is a novel regulator for apical extrusion of RasV12-transformed cells

Several lines of evidence have revealed that newly emerging transformed cells are often eliminated from the epithelium, though the underlying molecular mechanisms of this cancer preventive phenomenon still remain elusive. In this study, using mammalian cell culture systems we have identified plectin, a versatile cytoskeletal linker protein, as a novel regulator for apical extrusion of RasV12-transformed cells. Plectin is accumulated in RasV12 cells when they are surrounded by normal epithelial cells. Similarly, cytoskeletal proteins tubulin, keratin, and Epithelial Protein Lost In Neoplasm (EPLIN) are also accumulated in the transformed cells surrounded by normal cells. Knockdown or functional disruption of one of these molecules diminishes the accumulation of the others, indicating that the accumulation process of the individual protein mutually depends on each other. Furthermore, plectin-knockdown attenuates caveolin-1 (Cav-1) enrichment and PKA activity in RasV12 cells and profoundly suppresses the apical extrusion. These results indicate that the plectin-microtubules-EPLIN complex positively regulates apical elimination of RasV12-transformed cells from the epithelium in a coordinated fashion. Further development of this study would open a new avenue for cancer preventive medicine.

Experimental and computational assessment of F-actin influence in regulating cellular stiffness and relaxation behaviour of fibroblasts

In biomechanics, a complete understanding of the structures and mechanisms that regulate cellular stiffness at a molecular level remain elusive. In this paper, we have elucidated the role of filamentous actin (F-actin) in regulating elastic and viscous properties of the cytoplasm and the nucleus. Specifically, we performed colloidal-probe atomic force microscopy (AFM) on BjhTERT fibroblast cells incubated with Latrunculin B (LatB), which results in depolymerisation of F-actin, or DMSO control. We found that the treatment with LatB not only reduced cellular stiffness, but also greatly increased the relaxation rate for the cytoplasm in the peripheral region and in the vicinity of the nucleus. We thus conclude that F-actin is a major determinant in not only providing elastic stiffness to the cell, but also in regulating its viscous behaviour. To further investigate the interdependence of different cytoskeletal networks and cell shape, we provided a computational model in a finite element framework. The computational model is based on a split strain energy function of separate cellular constituents, here assumed to be cytoskeletal components, for which a composite strain energy function was defined. We found a significant influence of cell geometry on the predicted mechanical response. Importantly, the relaxation behaviour of the cell can be characterised by a material model with two time constants that have previously been found to predict mechanical behaviour of actin and intermediate filament networks. By merely tuning two effective stiffness parameters, the model predicts experimental results in cells with a partly depolymerised actin cytoskeleton as well as in untreated control. This indicates that actin and intermediate filament networks are instrumental in providing elastic stiffness in response to applied forces, as well as governing the relaxation behaviour over shorter and longer time-scales, respectively.

The cytolinker plectin regulates nuclear mechanotransduction in keratinocytes

The transmission of mechanical forces to the nucleus is important for intracellular positioning, mitosis and cell motility, yet the contribution of specific components of the cytoskeleton to nuclear mechanotransduction remains unclear. In this study, we examine how crosstalk between the cytolinker plectin and F-actin controls keratin network organisation and the 3D nuclear morphology of keratinocytes. Using micro-patterned surfaces to precisely manipulate cell shape, we find that cell adhesion and spreading regulate the size and shape of the nucleus. Disruption of the keratin cytoskeleton through loss of plectin facilitated greater nuclear deformation, which depended on acto-myosin contractility. Nuclear morphology did not depend on direct linkage of the keratin cytoskeleton with the nuclear membrane, rather loss of plectin reduced keratin filament density around the nucleus. We further demonstrate that keratinocytes have abnormal nuclear morphologies in the epidermis of plectin-deficient, epidermolysis bullosa simplex patients. Taken together, our data demonstrate that plectin is an essential regulator of nuclear morphology in vitro and in vivo and protects the nucleus from mechanical deformation.

Networking and anchoring through plectin: a key to IF functionality and mechanotransduction

Intermediate filaments (IFs) are involved in multiple cellular processes that are essential for the maintenance of cell and tissue integrity as well as response and adaption to stress. Mainly through pathological manifestations in patients and the analysis of genetic mouse models, it became evident that cytolinker proteins of the plakin protein family are essential for many of the functions ascribed to IFs. As discussed in this review, one of them, plectin, affects the assembly properties, interaction potential, compartmentalization, and linkage properties of IFs, making it to a key player for IF functionality. The far reaching consequences of IFs not being well-connected for skin and muscular integrity, migration, and mechanotransduction are highlighted.

Determining the mechanical properties of plectin in mouse myoblasts and keratinocytes

Plectin is the prototype of an intermediate filament (IF)-based cytolinker protein. It affects cells mechanically by interlinking and anchoring cytoskeletal filaments and acts as scaffolding and docking platform for signaling proteins to control cytoskeleton dynamics. The most common disease caused by mutations in the human plectin gene, epidermolysis bullosa simplex with muscular dystrophy (EBS-MD), is characterized by severe skin blistering and progressive muscular dystrophy. Therefore, we compared the biomechanical properties and the response to mechanical stress of murine plectin-deficient myoblasts and keratinocytes with wild-type cells. Using a cell stretching device, plectin-deficient myoblasts exhibited lower mechanical vulnerability upon external stress compared to wild-type cells, which we attributed to lower cellular pre-stress. Contrary to myoblasts, wild-type and plectin-deficient keratinocytes showed no significant differences. In magnetic tweezer measurements using fibronectin-coated paramagnetic beads, the stiffness of keratinocytes was higher than of myoblasts. Interestingly, cell stiffness, adhesion strength, and cytoskeletal dynamics were strikingly altered in plectin-deficient compared to wild-type myoblasts, whereas smaller differences were observed between plectin-deficient and wild-type keratinocytes, indicating that plectin might be more important for stabilizing cytoskeletal structures in myoblasts than in keratinocytes. Traction forces strongly correlated with the stiffness of plectin-deficient and wild-type myoblasts and keratinocytes. Contrary to that cell motility was comparable in plectin-deficient and wild-type myoblasts, but was significantly increased in plectin-deficient compared to wild-type keratinocytes. Thus, we postulate that the lack of plectin has divergent implications on biomechanical properties depending on the respective cell type.

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The intermediate filament-associated protein plectin has divergent biomechanical implications depending on cell/tissue type.

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In plectin^−/− myoblasts, cell vulnerability, stiffness, strain and binding strength are lower than in wild-type cells.

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Plectin^−/− keratinocytes exhibit higher cell stiffness, binding strength, strain and velocity than wild-type cells.

Mechanosensing through focal adhesion-anchored intermediate filaments

Integrin-based mechanotransduction involves a complex focal adhesion (FA)-associated machinery that is able to detect and respond to forces exerted either through components of the extracellular matrix or the intracellular contractile actomyosin network. Here, we show a hitherto unrecognized regulatory role of vimentin intermediate filaments (IFs) in this process. By studying fibroblasts in which vimentin IFs were decoupled from FAs, either because of vimentin deficiency (V0) or loss of vimentin network anchorage due to deficiency in the cytolinker protein plectin (P0), we demonstrate attenuated activation of the major mechanosensor molecule FAK and its downstream targets Src, ERK1/2, and p38, as well as an up-regulation of the compensatory feedback loop acting on RhoA and myosin light chain. In line with these findings, we show strongly reduced FA turnover rates in P0 fibroblasts combined with impaired directional migration, formation of protrusions, and up-regulation of "stretched" high-affinity integrin complexes. By exploiting tension-independent conditions, we were able to mechanistically link these defects to diminished cytoskeletal tension in both P0 and V0 cells. Our data provide important new insights into molecular mechanisms underlying cytoskeleton-regulated mechanosensing, a feature that is fundamental for controlled cell movement and tumor progression.

RhoA GTPase interacts with beta-catenin signaling in clinorotated osteoblasts

Bone is a dynamic tissue under constant remodeling in response to various signals including mechanical loading. A lack of proper mechanical loading induces disuse osteoporosis that reduces bone mass and structural integrity. The β-catenin signaling together with a network of GTPases is known to play a primary role in load-driven bone formation, but little is known about potential interactions of β-catenin signaling and GTPases in bone loss. In this study, we addressed a question: Does unloading suppress an activation level of RhoA GTPase and β-catenin signaling in osteoblasts? If yes, what is the role of RhoA GTPase and actin filaments in osteoblasts in regulating β-catenin signaling? Using a fluorescence resonance energy transfer (FRET) technique with a biosensor for RhoA together with a fluorescent T cell factor/lymphoid enhancer factor (TCF/LEF) reporter, we examined the effects of clinostat-driven simulated unloading. The results revealed that both RhoA activity and TCF/LEF activity were downregulated by unloading. Reduction in RhoA activity was correlated to a decrease in cytoskeletal organization of actin filaments. Inhibition of β-catenin signaling blocked unloading-induced RhoA suppression, and dominant negative RhoA inhibited TCF/LEF suppression. On the other hand, a constitutively active RhoA enhanced unloading-induced reduction of TCF/LEF activity. The TCF/LEF suppression by unloading was enhanced by co-culture with osteocytes, but it was independent on the organization of actin filaments, myosin II activity, or a myosin light chain kinase. Collectively, the results suggest that β-catenin signaling is required for unloading-driven regulation of RhoA, and RhoA, but not actin cytoskeleton or intracellular tension, mediates the responsiveness of β-catenin signaling to unloading.

Plectin-containing, centrally localized focal adhesions exert traction forces in primary lung epithelial cells

Receptor clustering upon cell attachment to the substrate induces assembly of cytoplasmic protein complexes termed focal adhesions (FAs), which connect, albeit indirectly, the extracellular matrix to the cytoskeleton. A subset of cultured primary alveolar epithelial cells (AEC) display a unique pattern of vinculin/paxillin/talin-rich FAs in two concentric circles when cultured on glass and micropatterned substrates: one ring of FAs located at the cell periphery (pFAs), and another FA ring located centrally in the cell (cFAs). Unusually, cFAs associate with an aster-like actin array as well as keratin bundles. Moreover, cFAs show rapid paxillin turnover rates following fluorescence recovery after photobleaching and exert traction forces similar to those generated by FAs at the cell periphery. The plakin protein plectin localizes to cFAs and is normally absent from pFAs, whereas tensin, a marker of mature/fibrillar adhesions, is found in both cFAs and pFAs. In primary AEC in which plectin expression is depleted, cFAs are largely absent, with an attendant reorganization of both the keratin and actin cytoskeletons. We suggest that the mechanical environment in the lung gives rise to the assembly of unconventional FAs in AEC. These FAs not only show a distinctive arrangement, but also possess unique compositional and functional properties.

Effect of age and cytoskeletal elements on the indentation-dependent mechanical properties of chondrocytes

Articular cartilage chondrocytes are responsible for the synthesis, maintenance, and turnover of the extracellular matrix, metabolic processes that contribute to the mechanical properties of these cells. Here, we systematically evaluated the effect of age and cytoskeletal disruptors on the mechanical properties of chondrocytes as a function of deformation. We quantified the indentation-dependent mechanical properties of chondrocytes isolated from neonatal (1-day), adult (5-year) and geriatric (12-year) bovine knees using atomic force microscopy (AFM). We also measured the contribution of the actin and intermediate filaments to the indentation-dependent mechanical properties of chondrocytes. By integrating AFM with confocal fluorescent microscopy, we monitored cytoskeletal and biomechanical deformation in transgenic cells (GFP-vimentin and mCherry-actin) under compression. We found that the elastic modulus of chondrocytes in all age groups decreased with increased indentation (15-2000 nm). The elastic modulus of adult chondrocytes was significantly greater than neonatal cells at indentations greater than 500 nm. Viscoelastic moduli (instantaneous and equilibrium) were comparable in all age groups examined; however, the intrinsic viscosity was lower in geriatric chondrocytes than neonatal. Disrupting the actin or the intermediate filament structures altered the mechanical properties of chondrocytes by decreasing the elastic modulus and viscoelastic properties, resulting in a dramatic loss of indentation-dependent response with treatment. Actin and vimentin cytoskeletal structures were monitored using confocal fluorescent microscopy in transgenic cells treated with disruptors, and both treatments had a profound disruptive effect on the actin filaments. Here we show that disrupting the structure of intermediate filaments indirectly altered the configuration of the actin cytoskeleton. These findings underscore the importance of the cytoskeletal elements in the overall mechanical response of chondrocytes, indicating that intermediate filament integrity is key to the non-linear elastic properties of chondrocytes. This study improves our understanding of the mechanical properties of articular cartilage at the single cell level.

Fused in sarcoma (FUS) interacts with the cytolinker protein plectin: implications for FUS subcellular localization and function

Fused in sarcoma (FUS) is a multifunctional protein involved in transcriptional control, pre-mRNA processing, RNA transport and translation. The domain structure of FUS reflects its functions in gene regulation and its ability to interact with other proteins, RNA and DNA. By use of a recombinant fragment of FUS in pull-down experiments followed by mass spectrometry analysis we have identified a novel interaction between the FUS N-terminal and the cytolinker plectin. An in situ proximity ligation assay confirmed that FUS-plectin interactions take place in the cytoplasm of cells. Furthermore, plectin deficient cells showed an altered subcellular localization of FUS and a deregulated expression of mRNAs bound to FUS. Our results show that plectin is important for normal FUS localization and function. Mutations involving FUS are causative factors in sarcomas and leukemias and also hereditary forms of amyotrophic lateral sclerosis and frontotemporal lobar degeneration. Plectin deficiency causes epidermolysis bullosa, a disease involving the skin and neuromuscular system. The novel FUS-plectin interaction offers new perspectives for understanding the role of FUS and plectin mutations in the pathogenesis of these diseases.

Genetic heterogeneity and pathophysiological mechanisms in congenital myasthenic syndromes

Congenital myasthenic syndromes (CMS) are a rare heterogeneous group of inherited neuromuscular disorders associated with distinctive clinical, electrophysiological, ultrastructural and genetic abnormalities. These genetic defects either impair neuromuscular transmission directly or result in secondary impairments, which eventually compromise the safety margin of neuromuscular transmission. In this report we will explore the significant progress made in understanding the molecular pathogenesis of CMS, which is important for both patients and clinicians in terms of reaching a definite diagnosis and selecting the most appropriate treatment.

Biomechanics of the sarcolemma and costameres in single skeletal muscle fibers from normal and dystrophin-null mice

We studied the biomechanical properties of the sarcolemma and its links through costameres to the contractile apparatus in single mammalian myofibers of Extensor digitorum longus muscles isolated from wild (WT) and dystrophin-null (mdx) mice. Suction pressures (P) applied through a pipette to the sarcolemma generated a bleb, the height of which increased with increasing P. Larger increases in P broke the connections between the sarcolemma and myofibrils and eventually caused the sarcolemma to burst. We used the values of P at which these changes occurred to estimate the tensions and stiffness of the system and its individual elements. Tensions of the whole system and the sarcolemma, as well as the maximal tension sustained by the costameres, were all significantly lower (1.8-3.3 fold) in muscles of mdx mice compared to WT. Values of P at which separation and bursting occurred, as well as the stiffness of the whole system and of the isolated sarcolemma, were ~2-fold lower in mdx than in WT. Our results indicate that the absence of dystrophin reduces muscle stiffness, increases sarcolemmal deformability, and compromises the mechanical stability of costameres and their connections to nearby myofibrils.

Stress transmission within the cell

An outstanding problem in cell biology is how cells sense mechanical forces and how those forces affect cellular functions. During past decades, it has become evident that the deformable cytoskeleton (CSK), an intracellular network of various filamentous biopolymers, provides a physical basis for transducing mechanical signals into biochemical responses. To understand how mechanical forces regulate cellular functions, it is necessary to first understand how the CSK develops mechanical stresses in response to applied forces, and how those stresses are propagated through the CSK where various signaling molecules are immobilized. New experimental techniques have been developed to quantify cytoskeletal mechanics, which together with new computational approaches have given rise to new theories and models for describing mechanics of living cells. In this article, we discuss current understanding of cell biomechanics by focusing on the biophysical mechanisms that are responsible for the development and transmission of mechanical stresses in the cell and their effect on cellular functions. We compare and contrast various theories and models of cytoskeletal mechanics, emphasizing common mechanisms that those theories are built upon, while not ignoring irreconcilable differences. We highlight most recent advances in the understanding of mechanotransduction in the cytoplasm of living cells and the central role of the cytoskeletal prestress in propagating mechanical forces along the cytoskeletal filaments to activate cytoplasmic enzymes. It is anticipated that advances in cell mechanics will help developing novel therapeutics to treat pulmonary diseases like asthma, pulmonary fibrosis, and chronic obstructive pulmonary disease.

Plectin isoforms as organizers of intermediate filament cytoarchitecture

Intermediate filaments (IFs) form cytoplamic and nuclear networks that provide cells with mechanical strength. Perturbation of this structural support causes cell and tissue fragility and accounts for a number of human genetic diseases. In recent years, important additional roles, nonmechanical in nature, were ascribed to IFs, including regulation of signaling pathways that control survival and growth of the cells, and vectorial processes such as protein targeting in polarized cellular settings. The cytolinker protein plectin anchors IF networks to junctional complexes, the nuclear envelope and cytoplasmic organelles and it mediates their cross talk with the actin and tubulin cytoskeleton. These functions empower plectin to wield significant influence over IF network cytoarchitecture. Moreover, the unusual diversity of plectin isoforms with different N termini and a common IF-binding (C-terminal) domain enables these isoforms to specifically associate with and thereby bridge IF networks to distinct cellular structures. Here we review the evidence for IF cytoarchitecture being controlled by specific plectin isoforms in different cell systems, including fibroblasts, endothelial cells, lens fibers, lymphocytes, myocytes, keratinocytes, neurons and astrocytes, and discuss what impact the absence of these isoforms has on IF cytoarchitecture-dependent cellular functions.

A dystroglycan/plectin scaffold mediates mechanical pathway bifurcation in lung epithelial cells

In alveolar epithelial cells (AECs), the membrane-anchored proteoglycan dystroglycan (DG) is a mechanoreceptor that transmits mechanical stretch forces to activate independently the ERK1/2 and the adenosine 5'-monophosphate-activated protein kinase (AMPK) signaling cascades in a process called pathway bifurcation. We tested the hypothesis that the cytoskeleton cross-linker plectin, known to bind both DG and AMPK in muscle cells, acts as a scaffold to regulate DG-mediated mechanical stimulation and pathway bifurcation. We demonstrate that plectin and DG form a complex in AECs and that this complex interacts with ERK1/2 and AMPK. Plectin knockdown reduces DG interaction with AMPK but not with ERK1/2. Despite this, mechanoactivation of both signaling pathways is significantly attenuated in AECs deficient in plectin. Thus, DG has the dual role of mechanical receptor and scaffold for ERK1/2, whereas plectin acts as a scaffold for AMPK signaling but is also required for DG-mediated ERK1/2 activation. We conclude that the DG-plectin complex plays a central role in transmitting mechanical stress from the extracellular matrix to the cytoplasm.

ATP enhances neuronal differentiation of PC12 cells by activating PKCα interactions with cytoskeletal proteins

PKCα is a key mediator of the neuronal differentiation controlled by NGF and ATP. However, its downstream signaling pathways remain to be elucidated. To identify the signaling partners of PKCα, we analyzed proteins coimmunoprecipitated with this enzyme in PC12 cells differentiated with NGF and ATP and compared them with those obtained with NGF alone or growing media. Mass spectrometry analysis (LC-MS/MS) identified plectin, peripherin, filamin A, fascin, and β-actin as potential interacting proteins. The colocalization of PKCα and its interacting proteins increased when PC12 cells were differentiated with NGF and ATP. Peripherin and plectin organization and the cortical remodeling of β-actin were dramatically affected when PKCα was down-regulated, suggesting that all three proteins might be functional targets of ATP-dependent PKCα signaling. Taken together, these data demonstrate that PKCα is essential for controlling the neuronal development induced by NGF and ATP and interacts with the cytoskeletal components at two levels: assembly of the intermediate filament peripherin and organization of cortical actin.

Keeping the vimentin network under control: cell-matrix adhesion-associated plectin 1f affects cell shape and polarity of fibroblasts

Mature focal adhesions and fibrillar adhesions act as anchorage sites for vimentin filaments, with plectin isoform 1f being the crucial linker protein. Plectin serves as a nucleation and assembly center for the de novo formation of vimentin networks. Anchored vimentin creates a resilient cage-like core structure that affects cell shape.

Focal adhesions (FAs) located at the ends of actin/myosin-containing contractile stress fibers form tight connections between fibroblasts and their underlying extracellular matrix. We show here that mature FAs and their derivative fibronectin fibril-aligned fibrillar adhesions (FbAs) serve as docking sites for vimentin intermediate filaments (IFs) in a plectin isoform 1f (P1f)-dependent manner. Time-lapse video microscopy revealed that FA-associated P1f captures mobile vimentin filament precursors, which then serve as seeds for de novo IF network formation via end-to-end fusion with other mobile precursors. As a consequence of IF association, the turnover of FAs is reduced. P1f-mediated IF network formation at FbAs creates a resilient cage-like core structure that encases and positions the nucleus while being stably connected to the exterior of the cell. We show that the formation of this structure affects cell shape with consequences for cell polarization.

Congenital muscular dystrophy, myasthenic symptoms and epidermolysis bullosa simplex (EBS) associated with mutations in the PLEC1 gene encoding plectin

Mutations in the PLEC1 gene encoding plectin have been reported in neonatal epidermolysis bullosa simplex with muscular dystrophy of later-onset (EBS-MD). A neuromuscular transmission defect has been reported in one previous patient. We report a boy presenting from birth with features of a congenital muscular dystrophy and late-onset myasthenic symptoms. Repetitive nerve stimulation showed significant decrement, and strength improved with pyridostigmine. Subtle blistering noticed only retrospectively prompted further genetic testing, revealing recessive PLEC1 mutations. We conclude that PLEC1 should be considered in the differential diagnosis of congenital muscular dystrophies and myasthenic syndromes, even in the absence of prominent skin involvement.