Keratins as the main component for the mechanical integrity of keratinocytes

Cytoplasmic Intermediate Filaments in Cell Biology

Intermediate filaments (IFs) are one of the three major elements of the cytoskeleton. Their stability, intrinsic mechanical properties, and cell type-specific expression patterns distinguish them from actin and microtubules. By providing mechanical support, IFs protect cells from external forces and participate in cell adhesion and tissue integrity. IFs form an extensive and elaborate network that connects the cell cortex to intracellular organelles. They act as a molecular scaffold that controls intracellular organization. However, IFs have been revealed as much more than just rigid structures. Their dynamics is regulated by multiple signaling cascades and appears to contribute to signaling events in response to cell stress and to dynamic cellular functions such as mitosis, apoptosis, and migration.

Intermediate filaments in cardiomyopathy

Intermediate filament (IF) proteins are critical regulators in health and disease. The discovery of hundreds of mutations in IF genes and posttranslational modifications has been linked to a plethora of human diseases, including, among others, cardiomyopathies, muscular dystrophies, progeria, blistering diseases of the epidermis, and neurodegenerative diseases. The major IF proteins that have been linked to cardiomyopathies and heart failure are the muscle-specific cytoskeletal IF protein desmin and the nuclear IF protein lamin, as a subgroup of the known desminopathies and laminopathies, respectively. The studies so far, both with healthy and diseased heart, have demonstrated the importance of these IF protein networks in intracellular and intercellular integration of structure and function, mechanotransduction and gene activation, cardiomyocyte differentiation and survival, mitochondrial homeostasis, and regulation of metabolism. The high coordination of all these processes is obviously of great importance for the maintenance of proper, life-lasting, and continuous contraction of this highly organized cardiac striated muscle and consequently a healthy heart. In this review, we will cover most known information on the role of IFs in the above processes and how their deficiency or disruption leads to cardiomyopathy and heart failure.

A rim-and-spoke hypothesis to explain the biomechanical roles for cytoplasmic intermediate filament networks

Textbook images of keratin intermediate filament (IF) networks in epithelial cells and the functional compromization of the epidermis by keratin mutations promulgate a mechanical role for this important cytoskeletal component. In stratified epithelia, keratin filaments form prominent radial spokes that are focused onto cell-cell contact sites, i.e. the desmosomes. In this Hypothesis, we draw attention to a subset of keratin filaments that are apposed to the plasma membrane. They form a rim of filaments interconnecting the desmosomes in a circumferential network. We hypothesize that they are part of a rim-and-spoke arrangement of IFs in epithelia. From our review of the literature, we extend this functional role for the subplasmalemmal rim of IFs to any cell, in which plasma membrane support is required, provided these filaments connect directly or indirectly to the plasma membrane. Furthermore, cytoplasmic IF networks physically link the outer nuclear and plasma membranes, but their participation in mechanotransduction processes remain largely unconsidered. Therefore, we also discuss the potential biomechanical and mechanosensory role(s) of the cytoplasmic IF network in terms of such a rim (i.e. subplasmalemmal)-and-spoke arrangement for cytoplasmic IF networks.

Viscoelastic properties of vimentin originate from nonequilibrium conformational changes

Structure and dynamics of living matter rely on design principles fundamentally different from concepts of traditional material science. Specialized intracellular filaments in the cytoskeleton permit living systems to divide, migrate, and grow with a high degree of variability and durability. Among the three filament systems, microfilaments, microtubules, and intermediate filaments (IFs), the physical properties of IFs and their role in cellular mechanics are the least well understood. We use optical trapping of individual vimentin filaments to investigate energy dissipation, strain history dependence, and creep behavior of stretched filaments. By stochastic and numerical modeling, we link our experimental observations to the peculiar molecular architecture of IFs. We find that individual vimentin filaments display tensile memory and are able to dissipate more than 70% of the input energy. We attribute these phenomena to distinct nonequilibrium folding and unfolding of α helices in the vimentin monomers constituting the filaments.

Mouse Keratinocytes Without Keratin Intermediate Filaments Demonstrate Substrate Stiffness Dependent Behaviors

INTRODUCTION

Traditionally thought to serve active vs. passive mechanical functions, respectively, a growing body of evidence suggests that actin microfilament and keratin intermediate filament (IF) networks, together with their associated cell-cell and cell-matrix anchoring junctions, may have a large degree of functional interdependence. Therefore, we hypothesized that the loss of keratin IFs in a knockout mouse keratinocyte model would affect the kinematics of colony formation, i.e., the spatiotemporal process by which individual cells join to form colonies and eventually a nascent epithelial sheet.

METHODS

Time-lapse imaging and deformation tracking microscopy was used to observe colony formation for both wild type (WT) and keratin-deficient knockout (KO) mouse keratinocytes over 24 h. Cells were cultured under high calcium conditions on collagen-coated substrates with nominal stiffnesses of ~ 1.2 kPa (soft) and 24 kPa (stiff). Immunofluorescent staining of actin and selected adhesion proteins was also performed.

RESULTS

The absence of keratin IFs markedly affected cell morphology, spread area, and cytoskeleton and adhesion protein organization on both soft and stiff substrates. Strikingly, an absence of keratin IFs also significantly reduced the ability of mouse keratinocytes to mechanically deform the soft substrate. Furthermore, KO cells formed colonies more efficiently on stiff vs. soft substrates, a behavior opposite to that observed for WT keratinocytes.

CONCLUSIONS

Collectively, these data are strongly supportive of the idea that an interdependence between actin microfilaments and keratin IFs does exist, while further suggesting that keratin IFs may represent an important and under-recognized component of keratinocyte mechanosensation and the force generation apparatus.

Fossilized skin reveals coevolution with feathers and metabolism in feathered dinosaurs and early birds

Feathers are remarkable evolutionary innovations that are associated with complex adaptations of the skin in modern birds. Fossilised feathers in non-avian dinosaurs and basal birds provide insights into feather evolution, but how associated integumentary adaptations evolved is unclear. Here we report the discovery of fossil skin, preserved with remarkable nanoscale fidelity, in three non-avian maniraptoran dinosaurs and a basal bird from the Cretaceous Jehol biota (China). The skin comprises patches of desquamating epidermal corneocytes that preserve a cytoskeletal array of helically coiled α-keratin tonofibrils. This structure confirms that basal birds and non-avian dinosaurs shed small epidermal flakes as in modern mammals and birds, but structural differences imply that these Cretaceous taxa had lower body heat production than modern birds. Feathered epidermis acquired many, but not all, anatomically modern attributes close to the base of the Maniraptora by the Middle Jurassic.

In addition to the evolutionary innovation of feathers, bird skin has complex adaptations. Here, McNamara and colleagues examine exceptionally preserved skin from feathered dinosaurs and ancient birds from the Cretaceous and show the early acquisition of many skin attributes seen in modern species.

Keratin Retraction and Desmoglein3 Internalization Independently Contribute to Autoantibody-Induced Cell Dissociation in Pemphigus Vulgaris

Pemphigus vulgaris (PV) is a potentially lethal autoimmune disease characterized by blister formation of the skin and mucous membranes and is caused by autoantibodies against desmoglein (Dsg) 1 and Dsg3. Dsg1 and Dsg3 are linked to keratin filaments in desmosomes, adhering junctions abundant in tissues exposed to high levels of mechanical stress. The binding of the autoantibodies leads to internalization of Dsg3 and a collapse of the keratin cytoskeleton-yet, the relevance and interdependence of these changes for loss of cell-cell adhesion and blistering is poorly understood. In live-cell imaging studies, loss of the keratin network at the cell periphery was detectable starting after 60 min of incubation with immunoglobulin G fractions of PV patients (PV-IgG). These rapid changes correlated with loss of cell-cell adhesion detected by dispase-based dissociation assays and were followed by a condensation of keratin filaments into thick bundles after several hours. Dsg3 internalization started at 90 min of PV-IgG treatment, thus following the early keratin changes. By inhibiting casein kinase 1 (CK-1), we provoked keratin alterations resembling the effects of PV-IgG. Although CK-1-induced loss of peripheral keratin network correlated with loss of cell cohesion and Dsg3 clustering in the membrane, it was not sufficient to trigger the internalization of Dsg3. However, additional incubation with PV-IgG was effective to promote Dsg3 loss at the membrane, indicating that Dsg3 internalization is independent from keratin alterations. Vice versa, inhibiting Dsg3 internalization did not prevent PV-IgG-induced keratin retraction and only partially rescued cell cohesion. Together, keratin changes appear very early after autoantibody binding and temporally overlap with loss of cell cohesion. These early alterations appear to be distinct from Dsg3 internalization, suggesting a crucial role for initial loss of cell cohesion in PV.

Cell-Cell Junctions Organize Structural and Signaling Networks

Cell-cell junctions link cells to each other in tissues, and regulate tissue homeostasis in critical cell processes that include tissue barrier function, cell proliferation, and migration. Defects in cell-cell junctions give rise to a wide range of tissue abnormalities that disrupt homeostasis and are common in genetic abnormalities and cancers. Here, we discuss the organization and function of cell-cell junctions primarily involved in adhesion (tight junction, adherens junction, and desmosomes) in two different epithelial tissues: a simple epithelium (intestine) and a stratified epithelium (epidermis). Studies in these tissues reveal similarities and differences in the organization and functions of different cell-cell junctions that meet the requirements for the specialized functions of each tissue. We discuss cell-cell junction responses to genetic and environmental perturbations that provide further insights into their roles in maintaining tissue homeostasis.

Atomic Force Microscopy Provides New Mechanistic Insights into the Pathogenesis of Pemphigus

Autoantibodies binding to the extracellular domains of desmoglein (Dsg) 3 and 1 are critical in the pathogenesis of pemphigus by mechanisms leading to impaired function of desmosomes and blister formation in the epidermis and mucous membranes. Desmosomes are highly organized protein complexes which provide strong intercellular adhesion. Desmosomal cadherins such as Dsgs, proteins of the cadherin superfamily which interact via their extracellular domains in Ca2+-dependent manner, are the transmembrane adhesion molecules clustered within desmosomes. Investigations on pemphigus cover a wide range of experimental approaches including biophysical methods. Especially atomic force microscopy (AFM) has recently been applied increasingly because it allows the analysis of native materials such as cultured cells and tissues under near-physiological conditions. AFM provides information about the mechanical properties of the sample together with detailed interaction analyses of adhesion molecules. With AFM, it was recently demonstrated that autoantibodies directly inhibit Dsg interactions on the surface of living keratinocytes, a phenomenon which has long been considered the main mechanism causing loss of cell cohesion in pemphigus. In addition, AFM allows to study how signaling pathways altered in pemphigus control binding properties of Dsgs. More general, AFM and other biophysical studies recently revealed the importance of keratin filaments for regulation of Dsg binding and keratinocyte mechanical properties. In this mini-review, we reevaluate AFM studies in pemphigus and keratinocyte research, recapitulate what is known about the interaction mechanisms of desmosomal cadherins and discuss the advantages and limitations of AFM in these regards.

Keratins Regulate p38MAPK-Dependent Desmoglein Binding Properties in Pemphigus

Keratins are crucial for the anchorage of desmosomes. Severe alterations of keratin organization and detachment of filaments from the desmosomal plaque occur in the autoimmune dermatoses pemphigus vulgaris and pemphigus foliaceus (PF), which are mainly caused by autoantibodies against desmoglein (Dsg) 1 and 3. Keratin alterations are a structural hallmark in pemphigus pathogenesis and correlate with loss of intercellular adhesion. However, the significance for autoantibody-induced loss of intercellular adhesion is largely unknown. In wild-type (wt) murine keratinocytes, pemphigus autoantibodies induced keratin filament retraction. Under the same conditions, we used murine keratinocytes lacking all keratin filaments (KtyII k.o.) as a model system to dissect the role of keratins in pemphigus. KtyII k.o. cells show compromised intercellular adhesion without antibody (Ab) treatment, which was not impaired further by pathogenic pemphigus autoantibodies. Nevertheless, direct activation of p38MAPK via anisomycin further decreased intercellular adhesion indicating that cell cohesion was not completely abrogated in the absence of keratins. Direct inhibition of Dsg3, but not of Dsg1, interaction via pathogenic autoantibodies as revealed by atomic force microscopy was detectable in both cell lines demonstrating that keratins are not required for this phenomenon. However, PF-IgG shifted Dsg1-binding events from cell borders toward the free cell surface in wt cells. This led to a distribution pattern of Dsg1-binding events similar to KtyII k.o. cells under resting conditions. In keratin-deficient keratinocytes, PF-IgG impaired Dsg1-binding strength, which was not different from wt cells under resting conditions. In addition, pathogenic autoantibodies were capable of activating p38MAPK in both KtyII wt and k.o. cells, the latter of which already displayed robust p38MAPK activation under resting conditions. Since inhibition of p38MAPK blocked autoantibody-induced loss of intercellular adhesion in wt cells and restored baseline cell cohesion in keratin-deficient cells, we conclude that p38MAPK signaling is (i) critical for regulation of cell adhesion, (ii) regulated by keratins, and (iii) targets both keratin-dependent and -independent mechanisms.

The desmin network is a determinant of the cytoplasmic stiffness of myoblasts

BACKGROUND INFORMATION

The mechanical properties of cells are essential to maintain their proper functions, and mainly rely on their cytoskeleton. A lot of attention has been paid to actin filaments, demonstrating their central role in the cells mechanical properties, but much less is known about the participation of intermediate filament (IF) networks. Indeed the contribution of IFs, such as vimentin, keratins and lamins, to cell mechanics has only been assessed recently. We study here the involvement of desmin, an IF specifically expressed in muscle cells, in the rheology of immature muscle cells. Desmin can carry mutations responsible for a class of muscle pathologies named desminopathies.

RESULTS

In this study, using three types of cell rheometers, we assess the consequences of expressing wild-type (WT) or mutated desmin on the rheological properties of single myoblasts. We find that the mechanical properties of the cell cortex are not correlated to the quantity, nor the quality of desmin expressed. On the contrary, the overall cell stiffness increases when the amount of WT or mutated desmin polymerised in cytoplasmic networks increases. However, myoblasts become softer when the desmin network is partially depleted by the formation of aggregates induced by the expression of a desmin mutant.

CONCLUSIONS

We demonstrate that desmin plays a negligible role in the mechanical properties of the cell cortex but is a determinant of the overall cell stiffness. More particularly, desmin participates to the cytoplasm viscoelasticity.

SIGNIFICANCE

Desminopathies are associated with muscular weaknesses attributed to a disorganisation of the structure of striated muscle that impairs the active force generation. The present study evidences for the first time the key role of desmin in the rheological properties of myoblasts, raising the hypothesis that desmin mutations could also alter the passive mechanical properties of muscles, thus participating to the lack of force build up in muscle tissue.

Cellular biomechanics impairment in keratinocytes is associated with a C-terminal truncated desmoplakin: An atomic force microscopy investigation

In a tissue continuously challenged by mechanical stresses, such as the skin or the heart, cells perceive information about their microenvironment through several adhesive protein complexes and activate cell-signaling events to maintain tissue cohesion. Consequently, alteration of cell adhesion components leads to aberrant assembly of the associated cytoplasmic scaffolding and signaling pathways, which may reflect changes to the tissue physiology and mechanical resistance. Desmoplakin is an essential component of the cell-cell junction, anchoring the desmosomal protein complex to the intermediate filaments (IFs). Inherited mutations in desmoplakin are associated with both heart and skin disease (cardiocutaneous syndrome). In this study, we investigated the mechanical properties of human keratinocytes harboring a cardiocutaneous-associated homozygous C-terminal truncation in desmoplakin (JD-1) compared to a control keratinocyte line (K1). Using Single Cell Force Spectroscopy (SCFS) AFM-based measurements, JD-1 keratinocytes displayed an overall alteration in morphology, elasticity, adhesion capabilities and viscoelastic properties, highlighting the profound interconnection between the adhesome pathways and the IF scaffold.

Mechanical and signaling roles for keratin intermediate filaments in the assembly and morphogenesis of Xenopus mesendoderm tissue at gastrulation

The coordination of individual cell behaviors is a crucial step in the assembly and morphogenesis of tissues. Xenopus mesendoderm cells migrate collectively along a fibronectin (FN) substrate at gastrulation, but how the adhesive and mechanical forces required for these movements are generated and transmitted is unclear. Traction force microscopy (TFM) was used to establish that traction stresses are limited primarily to leading edge cells in mesendoderm explants, and that these forces are balanced by intercellular stresses in follower rows. This is further reflected in the morphology of these cells, with broad lamellipodial protrusions, mature focal adhesions and a gradient of activated Rac1 evident at the leading edge, while small protrusions, rapid turnover of immature focal adhesions and lack of a Rac1 activity gradient characterize cells in following rows. Depletion of keratin (krt8) with antisense morpholinos results in high traction stresses in follower row cells, misdirected protrusions and the formation of actin stress fibers anchored in streak-like focal adhesions. We propose that maintenance of mechanical integrity in the mesendoderm by keratin intermediate filaments is required to balance stresses within the tissue to regulate collective cell movements.

Deciphering psoriasis. A bioinformatic approach

BACKGROUND

Psoriasis is an immune-mediated, inflammatory and hyperproliferative disease of the skin and joints. The cause of psoriasis is still unknown. The fundamental feature of the disease is the hyperproliferation of keratinocytes and the recruitment of cells from the immune system in the region of the affected skin, which leads to deregulation of many well-known gene expressions.

OBJECTIVE

Based on data mining and bioinformatic scripting, here we show a new dimension of the effect of psoriasis at the genomic level.

METHODS

Using our own pipeline of scripts in Perl and MySql and based on the freely available NCBI Gene Expression Omnibus (GEO) database: DataSet Record GDS4602 (Series GSE13355), we explore the extent of the effect of psoriasis on gene expression in the affected tissue.

RESULTS

We give greater insight into the effects of psoriasis on the up-regulation of some genes in the cell cycle (CCNB1, CCNA2, CCNE2, CDK1) or the dynamin system (GBPs, MXs, MFN1), as well as the down-regulation of typical antioxidant genes (catalase, CAT; superoxide dismutases, SOD1-3; and glutathione reductase, GSR). We also provide a complete list of the human genes and how they respond in a state of psoriasis.

CONCLUSION

Our results show that psoriasis affects all chromosomes and many biological functions. If we further consider the stable and mitotically inheritable character of the psoriasis phenotype, and the influence of environmental factors, then it seems that psoriasis has an epigenetic origin. This fit well with the strong hereditary character of the disease as well as its complex genetic background.

Intermediate Filaments at the Junction of Mechanotransduction, Migration, and Development

Mechanically induced signal transduction has an essential role in development. Cells actively transduce and respond to mechanical signals and their internal architecture must manage the associated forces while also being dynamically responsive. With unique assembly-disassembly dynamics and physical properties, cytoplasmic intermediate filaments play an important role in regulating cell shape and mechanical integrity. While this function has been recognized and appreciated for more than 30 years, continually emerging data also demonstrate important roles of intermediate filaments in cell signal transduction. In this review, with a particular focus on keratins and vimentin, the relationship between the physical state of intermediate filaments and their role in mechanotransduction signaling is illustrated through a survey of current literature. Association with adhesion receptors such as cadherins and integrins provides a critical interface through which intermediate filaments are exposed to forces from a cell's environment. As a consequence, these cytoskeletal networks are posttranslationally modified, remodeled and reorganized with direct impacts on local signal transduction events and cell migratory behaviors important to development. We propose that intermediate filaments provide an opportune platform for cells to both cope with mechanical forces and modulate signal transduction.

Coculture of mesenchymal stem cells and endothelial cells enhances host tissue integration and epidermis maturation through AKT activation in gelatin methacryloyl hydrogel-based skin model

A major challenge for clinical use of skin substitutes is insufficient host tissue integration leading to loosening and partial necrosis of the implant. In this present study, a three-dimensional (3D) coculture system constructed using human umbilical cord mesenchymal stem cells (uc-MSCs) and umbilical vein endothelial cells (HUVECs) encapsulated in gelatin methacryloyl (GelMA) hydrogels was evaluated to determine the outcomes of cell-cell interactions in vitro and in vivo. The results revealed that GelMA hydrogels displayed minor cytotoxicity on both cell types. An uc-MSC:HUVEC ratio of 50:50 demonstrated the highest cell proliferation and expression of angiogenic markers. The supplement of basic fibroblast growth factors (bFGF) in coculture system further induced cell proliferation and gene expression in vitro. In vivo transplantation of this cocultured constructs efficiently enhanced the implant and host tissue integration. Additionally, the proliferation of keratinocytes was well maintained on GelMA hydrogels and the gene expression related to cell proliferation and differentiation was significantly increased in coculture system comparing to monoculture. Mechanistically, AKT signaling pathways were activated in cocultures. Our findings suggest that coculturing MSC and EC in GelMA hydrogels could be a promising approach to substantially improve the integration of exogenous skin substitutes and host tissues.

STATEMENT OF SIGNIFICANCE

In this study, the co-culture of uc-MSCs and HUVECs in photocrosslinkable GelMA hydrogels significantly enhanced host tissue integration. Cell proliferation, ECM deposition and angiogenic genes expression were all substantially improved in vitro and the excellent host tissue integration into the implanted tissue was observed in vivo. When served as a dermal layer, the scaffold with co-cultured cells enhanced the proliferation and differentiation of keratinocytes. AKT signaling was proved to be involved in the regulation of cell survival and fate determination. This work demonstrated the importance of 3D cell co-culture to facilitate host tissue integration that can be a promising approach for long-term survival of skin substitutes.

Mutation in the Core Structure of Desmin Intermediate Filaments Affects Myoblast Elasticity

Elastic properties of cells are mainly derived from the actin cytoskeleton. However, intermediate filaments are emerging as major contributors to the mechanical properties of cells. Using atomic force microscopy, we studied the elasticity of mouse myoblasts expressing a mutant form of the gene encoding for desmin intermediate filaments, p.D399Y. This variant produces desmin aggregates, the main pathological symptom of myofibrillar myopathies. Here we show that desmin-mutated cells display a 39% increased median elastic modulus compared to wild-type cells. Desmin-mutated cells required higher forces than wild-type cells to reach high indentation depths, where desmin intermediate filaments are typically located. In addition, heat-shock treatment increased the proportion of cells with aggregates and induced a secondary peak in the distribution of Young's moduli. By performing atomic force microscopy mechanical mapping combined with fluorescence microscopy, we show that higher Young's moduli were measured where desmin aggregates were located, indicating that desmin aggregates are rigid. Therefore, we provide evidence that p.D399Y stiffens mouse myoblasts. Based on these results, we suggest that p.D399Y-related myofibrillar myopathy is at least partly due to altered mechanical properties at the single-cell scale, which are propagated to the tissue scale.

Desmosomes and Intermediate Filaments: Their Consequences for Tissue Mechanics

Adherens junctions (AJs) and desmosomes connect the actin and keratin filament networks of adjacent cells into a mechanical unit. Whereas AJs function in mechanosensing and in transducing mechanical forces between the plasma membrane and the actomyosin cytoskeleton, desmosomes and intermediate filaments (IFs) provide mechanical stability required to maintain tissue architecture and integrity when the tissues are exposed to mechanical stress. Desmosomes are essential for stable intercellular cohesion, whereas keratins determine cell mechanics but are not involved in generating tension. Here, we summarize the current knowledge of the role of IFs and desmosomes in tissue mechanics and discuss whether the desmosome-keratin scaffold might be actively involved in mechanosensing and in the conversion of chemical signals into mechanical strength.

Intracellular Motility of Intermediate Filaments

SUMMARYThe establishment and continuous cell type-specific adaptation of cytoplasmic intermediate filament (IF) networks are linked to various types of IF motility. Motor protein-driven active transport, linkage to other cellular structures, diffusion of small soluble subunits, and intrinsic network elasticity all contribute to the motile behavior of IFs. These processes are subject to regulation by multiple signaling pathways. IF motility is thereby connected to and involved in many basic cellular processes guarding the maintenance of cell and tissue integrity. Disturbances of IF motility are linked to diseases that are characterized by cytoplasmic aggregates containing IF proteins together with other cellular components.

Keratin 8 reduces colonic permeability and maintains gut microbiota homeostasis, protecting against colitis and colitis-associated tumorigenesis

Keratin 8 (CK8) is the major component of the intermediate filaments of simple or single-layered epithelia. Gene targeting mice model suggest that CK8 is involved in colonic active ion transport, colorectal hyperplasia and inflammation. In the present study, we found that CK8 is downregulated in the colon during DSS-induced colitis and AOM/DSS-induced colitis-associated colorectal cancer (CAC) development. In human patients with colon cancer, CK8 is downregulated. Using CK8 heterozygous knockout mice (CK8^+/-), we found that CK8^+/- mice are highly susceptible to DSS-induced colitis and more prone to AOM/DSS-induced CAC than wild type (WT) mice. The colonic permeability is increased with DSS or AOM/DSS treatment, leading to alteration of gut microbiota in CK8^+/- mice with CAC. Metagenomic analysis of fecal microbiota suggests Firmicutes and Proteobacteria are increased in CK8^+/- mice with CAC, while Bacteroidetes and Verrucomicrobia are decreased. Antibiotic treatment decreases the incidence of colorectal cancer tumorigenesis and TLR4 inhibitor attenuates the susceptibility of CK8^+/- mice to DSS-induced colitis. These data suggest CK8 protects mice from colitis and colitis-associated colorectal cancer by modulating colonic permeability and gut microbiota composition homeostasis.

Dissociation rates from single-molecule pulling experiments under large thermal fluctuations or large applied force

Theories that are used to extract energy-landscape information from single-molecule pulling experiments in biophysics are all invariably based on Kramers' theory of the thermally activated escape rate from a potential well. As is well known, this theory recovers the Arrhenius dependence of the rate on the barrier energy and crucially relies on the assumption that the barrier energy is much larger than k_{B}T (limit of comparatively low thermal fluctuations). As was shown already in Dudko et al. [Phys. Rev. Lett. 96, 108101 (2006)PRLTAO0031-900710.1103/PhysRevLett.96.108101], this approach leads to the unphysical prediction of dissociation time increasing with decreasing binding energy when the latter is lowered to values comparable to k_{B}T (limit of large thermal fluctuations). We propose a theoretical framework (fully supported by numerical simulations) which amends Kramers' theory in this limit and use it to extract the dissociation rate from single-molecule experiments where now predictions are physically meaningful and in agreement with simulations over the whole range of applied forces (binding energies). These results are expected to be relevant for a large number of experimental settings in single-molecule biophysics.

The desmoplakin-intermediate filament linkage regulates cell mechanics

Desmoplakin connects desmosomal core components to intermediate filaments at sites of cell–cell adhesion. Modulating the strength of this linkage using desmoplakin mutants led to alterations in cell–substrate and cell–cell forces and cell stiffness as assessed by micropillar arrays and atomic force microscopy. Perturbation of the actin cytoskeleton leads to abrogation of these effects.

The translation of mechanical forces into biochemical signals plays a central role in guiding normal physiological processes during tissue development and homeostasis. Interfering with this process contributes to cardiovascular disease, cancer progression, and inherited disorders. The actin-based cytoskeleton and its associated adherens junctions are well-established contributors to mechanosensing and transduction machinery; however, the role of the desmosome–intermediate filament (DSM–IF) network is poorly understood in this context. Because a force balance among different cytoskeletal systems is important to maintain normal tissue function, knowing the relative contributions of these structurally integrated systems to cell mechanics is critical. Here we modulated the interaction between DSMs and IFs using mutant forms of desmoplakin, the protein bridging these structures. Using micropillar arrays and atomic force microscopy, we demonstrate that strengthening the DSM–IF interaction increases cell–substrate and cell–cell forces and cell stiffness both in cell pairs and sheets of cells. In contrast, disrupting the interaction leads to a decrease in these forces. These alterations in cell mechanics are abrogated when the actin cytoskeleton is dismantled. These data suggest that the tissue-specific variability in DSM–IF network composition provides an opportunity to differentially regulate tissue mechanics by balancing and tuning forces among cytoskeletal systems.

Intermediate filament reorganization dynamically influences cancer cell alignment and migration

The interactions between a cancer cell and its extracellular matrix (ECM) have been the focus of an increasing amount of investigation. The role of the intermediate filament keratin in cancer has also been coming into focus of late, but more research is needed to understand how this piece fits in the puzzle of cytoskeleton-mediated invasion and metastasis. In Panc-1 invasive pancreatic cancer cells, keratin phosphorylation in conjunction with actin inhibition was found to be sufficient to reduce cell area below either treatment alone. We then analyzed intersecting keratin and actin fibers in the cytoskeleton of cyclically stretched cells and found no directional correlation. The role of keratin organization in Panc-1 cellular morphological adaptation and directed migration was then analyzed by culturing cells on cyclically stretched polydimethylsiloxane (PDMS) substrates, nanoscale grates, and rigid pillars. In general, the reorganization of the keratin cytoskeleton allows the cell to become more ‘mobile’- exhibiting faster and more directed migration and orientation in response to external stimuli. By combining keratin network perturbation with a variety of physical ECM signals, we demonstrate the interconnected nature of the architecture inside the cell and the scaffolding outside of it, and highlight the key elements facilitating cancer cell-ECM interactions.

Nonlinear Loading-Rate-Dependent Force Response of Individual Vimentin Intermediate Filaments to Applied Strain

The mechanical properties of eukaryotic cells are to a great extent determined by the cytoskeleton, a composite network of different filamentous proteins. Among these, intermediate filaments (IFs) are exceptional in their molecular architecture and mechanical properties. Here we directly record stress-strain curves of individual vimentin IFs using optical traps and atomic force microscopy. We find a strong loading rate dependence of the mechanical response, supporting the hypothesis that IFs could serve to protect eukaryotic cells from fast, large deformations. Our experimental results show different unfolding regimes, which we can quantitatively reproduce by an elastically coupled system of multiple two-state elements.

Intermediate Filaments Supporting Cell Shape and Growth in Bacteria

For years intermediate filaments (IF), belonging to the third class of filamentous cytoskeletal proteins alongside microtubules and actin filaments, were thought to be exclusive to metazoan cells. Structurally these eukaryote IFs are very well defined, consisting of globular head and tail domains, which flank the central rod-domain. This central domain is dominated by an α-helical secondary structure predisposed to form the characteristic coiled-coil, parallel homo-dimer. These elementary dimers can further associate, both laterally and longitudinally, generating a variety of filament-networks built from filaments in the range of 10 nm in diameter. The general role of these filaments with their characteristic mechano-elastic properties both in the cytoplasm and in the nucleus of eukaryote cells is to provide mechanical strength and a scaffold supporting diverse shapes and cellular functions.Since 2003, after the first bacterial IF-like protein, crescentin was identified, it has been evident that bacteria also employ filamentous networks, other than those built from bacterial tubulin or actin homologues, in order to support their cell shape, growth and, in some cases, division. Intriguingly, compared to their eukaryote counterparts, the group of bacterial IF-like proteins shows much wider structural diversity. The sizes of both the head and tail domains are markedly reduced and there is great variation in the length of the central rod-domain. Furthermore, bacterial rod-domains often lack the sub-domain organisation of eukaryote IFs that is the defining feature of the IF-family. However, the fascinating display of filamentous assemblies, including rope, striated cables and hexagonal laces together with the conditions required for their formation both in vitro and in vivo strongly resemble that of eukaryote IFs suggesting that these bacterial proteins are deservedly classified as part of the IF-family and that the current definition should be relaxed slightly to allow their inclusion. The lack of extensive head and tail domains may well make the bacterial proteins more amenable for structural characterisation, which will be essential for establishing the mechanism for their association into filaments. What is more, the well-developed tools for bacterial manipulations provide an excellent opportunity of studying the bacterial systems with the prospect of making significant progress in our understanding of the general underlying principles of intermediate filament assemblies.

A novel function for the MAP kinase SMA-5 in intestinal tube stability

In vivo evidence links SMA-5 to the maintenance of the apical domain in the Caenorhabditis elegans intestine. sma-5 mutations induce morphological and biochemical changes of the intermediate filament system, demonstrating the close relationship between posttranslational modification and structural integrity of the evolutionarily conserved intestinal cytoskeleton.

Intermediate filaments are major cytoskeletal components whose assembly into complex networks and isotype-specific functions are still largely unknown. Caenorhabditis elegans provides an excellent model system to study intermediate filament organization and function in vivo. Its intestinal intermediate filaments localize exclusively to the endotube, a circumferential sheet just below the actin-based terminal web. A genetic screen for defects in the organization of intermediate filaments identified a mutation in the catalytic domain of the MAP kinase 7 orthologue sma-5(kc1). In sma-5(kc1) mutants, pockets of lumen penetrate the cytoplasm of the intestinal cells. These membrane hernias increase over time without affecting epithelial integrity and polarity. A more pronounced phenotype was observed in the deletion allele sma-5(n678) and in intestine-specific sma-5(RNAi). Besides reduced body length, an increased time of development, reduced brood size, and reduced life span were observed in the mutants, indicating compromised food uptake. Ultrastructural analyses revealed that the luminal pockets include the subapical cytoskeleton and coincide with local thinning and gaps in the endotube that are often enlarged in other regions. Increased intermediate filament phosphorylation was detected by two-dimensional immunoblotting, suggesting that loss of SMA-5 function leads to reduced intestinal tube stability due to altered intermediate filament network phosphorylation.

Physical view on migration modes

Cellular motility is essential for many processes such as embryonic development, wound healing processes, tissue assembly and regeneration, immune cell trafficing and diseases such as cancer. The migration efficiency and the migratory potential depend on the type of migration mode. The previously established migration modes such as epithelial (non-migratory) and mesenchymal (migratory) as well as amoeboid (squeezing motility) relay mainly on phenomenological criteria such as cell morphology and molecular biological criteria such as gene expression. However, the physical view on the migration modes is still not well understood. As the process of malignant cancer progression such as metastasis depends on the migration of single cancer cells and their migration mode, this review focuses on the different migration strategies and discusses which mechanical prerequisites are necessary to perform a special migration mode through a 3-dimensional microenvironment. In particular, this review discusses how cells can distinguish and finally switch between the migration modes and what impact do the physical properties of cells and their microenvironment have on the transition between the novel migration modes such as blebbing and protrusive motility.

Light and electron microscopic analyses of the high deformability of adhesive toe pads in White's tree frog, Litoria caerulea

White's tree frog (Litoria caerulea) has large, adhesive toe pads that are among the softest of all known biological structures. To explore the morphological basis for the physical properties of the toe pads, the internal microstructure of the toe pads in L. caerulea was examined using both light and transmission electron microscopy. Three design elements that are distinct from other areas of skin were observed. First, the keratinocytes comprising the adhesive surface of the toe pad all contained keratin filament bundles (tonofibrils) exhibiting structural anisotropy. Specifically, the curved conformation of the hierarchical (branching) tonofibrils was characterized by the formation of anastomoses consisting of tonofibrils beneath the adhesive cell surface and stem keratin filament bundles concentrated in the lower-middle part of the dorsal-side of adhesive cells. Second, the cytoplasm of keratinocytes in the most superficial cell layer contained glycoproteins (stained by periodic acid/Schiff reagent) that are considered to confer high viscoelasticity. Third, the dermis contained large lymph spaces interspersed with elastic fibers and collagen fibers, which were relatively sparsely distributed compared to the dorsal skin of the toe pads. The profiles of these structures were easily deformed by the slight application of pressure. These findings reaffirmed that the unique internal architecture of the toe pads in L. caerulea contributed to their remarkable softness and high deformability, which in turn increased the contact area and provided improved adaptability to the local topography of natural surfaces. J. Morphol. 277:1509-1516, 2016. © 2016 Wiley Periodicals, Inc.

Investigating the mechanical properties of zona pellucida of whole human oocytes by atomic force spectroscopy

The role of mechanics in numerous biological processes is nowadays recognized, while in others, such as the fertilization process, it is still neglected. In the case of oocytes the description of their mechanical properties could improve the comprehension of the oocyte-spermatozoon interaction and be helpful for application in in vitro fertilization (IVF) clinics. Herein the mechanical properties of whole human oocytes (HOs) immediately after retrieval are investigated by indentation measurements with atomic force spectroscopy under physiological conditions. Measurements are performed on immature (metaphase I - MI) and mature (metaphase II - MII) HOs. According to their morphological characteristics MII-HOs are classified as "suitable" and "rejected"; these latter would be usually rejected for intracytoplasmic sperm injection (ICSI). For all maturation stages we observe that the elastic response of the zona pellucida (ZP) outer layer was different and distinguishable from the rest of the ZP-HO. The elasticity of this ZP outer layer varies with maturation and quality: stiffness decreases from immature MI to good quality MII, up to poor-quality rejected MII. An indirect analysis with IVF outcome indicates that the ZP outer layer of analysed HOs donated by women who achieved pregnancy is stiffer than that of HOs from women with negative outcome. Our findings suggest that mechanical properties can represent important oocyte quality indicators that may be exploited for the design of innovative ICSI dedicated cell sorters.

Keratin-associated protein 5-5 controls cytoskeletal function and cancer cell vascular invasion

Cancer cell vascular invasion is a crucial step in the malignant progression towards metastasis. Here we used a genome-wide RNAi screen with E0771 mammary cancer cells to uncover drivers of endothelial monolayer invasion. We identified keratin-associated protein 5-5 (Krtap5-5) as a candidate. Krtap5-5 belongs to a large protein family that is implicated in crosslinking keratin intermediate filaments during hair formation, yet these keratin-associated proteins have no reported role in cancer. Depletion of Krtap5-5 from cancer cells led to cell blebbing and a loss of keratins 14 and 18, in addition to the upregulation of vimentin intermediate filaments. This intermediate filament subtype switching induced dysregulation of the actin cytoskeleton and reduced the expression of hemidesmosomal α6/β4-integrins. We further demonstrate that knockdown of keratin 18 phenocopies the loss of Krtap5-5, suggesting that Krtap5-5 crosstalks with keratin 18 in E0771 cells. Disruption of the keratin cytoskeleton by perturbing Krtap5-5 function broadly altered the expression of cytoskeleton regulators and the localization of cell surface markers. Krtap5-5 depletion did not impact cell viability but reduced cell motility and extracellular matrix invasion, as well as extravasation of cancer cells into tissues in zebrafish and mice. We conclude that Krtap5-5 is a previously unknown regulator of cytoskeletal function in cancer cells that modulates motility and vascular invasion. Thus, in addition to its physiologic function, a keratin-associated protein can serve as a switch towards malignant progression.

The dynamic mechanical properties of cellularised aggregates

Cellularised materials are composed of cells interfaced through specialised intercellular junctions that link the cytoskeleton of one cell to that of its neighbours allowing for transmission of forces. Cellularised materials are common in early development and adult tissues where they can be found in the form of cell sheets, cysts, or amorphous aggregates and in pathophysiological conditions such as cancerous tumours. Given the growing realisation that forces can regulate cell physiology and developmental processes, understanding how cellularised materials deform under mechanical stress or dissipate stress appear as key biological questions. In this review, we will discuss the dynamic mechanical properties of cellularised materials devoid of extracellular matrix.

Epithelial Intermediate Filaments: Guardians against Microbial Infection?

Intermediate filaments are abundant cytoskeletal components of epithelial tissues. They have been implicated in overall stress protection. A hitherto poorly investigated area of research is the function of intermediate filaments as a barrier to microbial infection. This review summarizes the accumulating knowledge about this interaction. It first emphasizes the unique spatial organization of the keratin intermediate filament cytoskeleton in different epithelial tissues to protect the organism against microbial insults. We then present examples of direct interaction between viral, bacterial, and parasitic proteins and the intermediate filament system and describe how this affects the microbe-host interaction by modulating the epithelial cytoskeleton, the progression of infection, and host response. These observations not only provide novel insights into the dynamics and function of intermediate filaments but also indicate future avenues to combat microbial infection.

Cross talk between the cytoplasm and nucleus during development and disease

Mechanotransduction is a process whereby mechanical stimuli outside the cell are sensed by components of the plasma membrane and transmitted as signals through the cytoplasm that terminate in the nucleus. The nucleus responds to these signals by altering gene expression. During mechanotransduction, complex networks of proteins are responsible for cross talk between the cytoplasm and the nucleus. These proteins include cell membrane receptors, cytoplasmic filaments, LINC complex members that bridge the nucleus and cytoplasm, and nuclear envelope proteins that connect to the chromatin. Mechanotransduction also plays a critical role in development. Furthermore, it is possible that disrupted mechanotransduction leads to changes in gene expression that underlie the pathogenic mechanisms of disease.

Effects of Plectin Depletion on Keratin Network Dynamics and Organization

The keratin intermediate filament cytoskeleton protects epithelial cells against various types of stress and is involved in fundamental cellular processes such as signaling, differentiation and organelle trafficking. These functions rely on the cell type-specific arrangement and plasticity of the keratin system. It has been suggested that these properties are regulated by a complex cycle of assembly and disassembly. The exact mechanisms responsible for the underlying molecular processes, however, have not been clarified. Accumulating evidence implicates the cytolinker plectin in various aspects of the keratin cycle, i.e., by acting as a stabilizing anchor at hemidesmosomal adhesion sites and the nucleus, by affecting keratin bundling and branching and by linkage of keratins to actin filament and microtubule dynamics. In the present study we tested these hypotheses. To this end, plectin was downregulated by shRNA in vulvar carcinoma-derived A431 cells. As expected, integrin β4- and BPAG-1-positive hemidesmosomal structures were strongly reduced and cytosolic actin stress fibers were increased. In addition, integrins α3 and β1 were reduced. The experiments furthermore showed that loss of plectin led to a reduction in keratin filament branch length but did not alter overall mechanical properties as assessed by indentation analyses using atomic force microscopy and by displacement analyses of cytoplasmic superparamagnetic beads using magnetic tweezers. An increase in keratin movement was observed in plectin-depleted cells as was the case in control cells lacking hemidesmosome-like structures. Yet, keratin turnover was not significantly affected. We conclude that plectin alone is not needed for keratin assembly and disassembly and that other mechanisms exist to guarantee proper keratin cycling under steady state conditions in cultured single cells.

X-rays Reveal the Internal Structure of Keratin Bundles in Whole Cells

In recent years, X-ray imaging of biological cells has emerged as a complementary alternative to fluorescence and electron microscopy. Different techniques were established and successfully applied to macromolecular assemblies and structures in cells. However, while the resolution is reaching the nanometer scale, the dose is increasing. It is essential to develop strategies to overcome or reduce radiation damage. Here we approach this intrinsic problem by combing two different X-ray techniques, namely ptychography and nanodiffraction, in one experiment and on the same sample. We acquire low dose ptychography overview images of whole cells at a resolution of 65 nm. We subsequently record high-resolution nanodiffraction data from regions of interest. By comparing images from the two modalities, we can exclude strong effects of radiation damage on the specimen. From the diffraction data we retrieve quantitative structural information from intracellular bundles of keratin intermediate filaments such as a filament radius of 5 nm, hexagonal geometric arrangement with an interfilament distance of 14 nm and bundle diameters on the order of 70 nm. Thus, we present an appealing combined approach to answer a broad range of questions in soft-matter physics, biophysics and biology.

Mechanical Properties of Intermediate Filament Proteins

Purified intermediate filament (IF) proteins can be reassembled in vitro to produce polymers closely resembling those found in cells, and these filaments form viscoelastic gels. The cross-links holding IFs together in the network include specific bonds between polypeptides extending from the filament surface and ionic interactions mediated by divalent cations. IF networks exhibit striking nonlinear elasticity with stiffness, as quantified by shear modulus, increasing an order of magnitude as the networks are deformed to large strains resembling those that soft tissues undergo in vivo. Individual IFs can be stretched to more than two or three times their resting length without breaking. At least 10 different rheometric methods have been used to quantify the viscoelasticity of IF networks over a wide range of timescales and strain magnitudes. The mechanical roles of different classes of cytoplasmic IFs on mesenchymal and epithelial cells in culture have also been studied by an even wider range of microrheological methods. These studies have documented the effects on cell mechanics when IFs are genetically or pharmacologically disrupted or when normal or mutant IF proteins are exogenously expressed in cells. Consistent with in vitro rheology, the mechanical role of IFs is more apparent as cells are subjected to larger and more frequent deformations.

Mechanical Probing of the Intermediate Filament-Rich Caenorhabditis Elegans Intestine

It is commonly accepted that intermediate filaments have an important mechanical function. This function relies not only on intrinsic material properties but is also determined by dynamic interactions with other cytoskeletal filament systems, distinct cell adhesion sites, and cellular organelles which are fine-tuned by multiple signaling pathways. While aspects of these properties and processes can be studied in vitro, their full complexity can only be understood in a viable tissue context. Yet, suitable and easily accessible model systems for monitoring tissue mechanics at high precision are rare. We show that the dissected intestine of the genetic model organism Caenorhabditis elegans fulfills this requirement. The 20 intestinal cells, which are arranged in an invariant fashion, are characterized by a dense subapical mesh of intermediate filaments that are attached to the C. elegans apical junction. We present procedures to visualize details of the characteristic intermediate filament-junctional complex arrangement in living animals. We then report on methods to prepare intestines with a fully intact intermediate filament cytoskeleton and detail procedures to assess their viability. A dual micropipette assay is described to measure mechanical properties of the dissected intestine while monitoring the spatial arrangement of the intermediate filament system. Advantages of this approach are (i) the high reproducibility of measurements because of the uniform architecture of the intestine and (ii) the high degree of accessibility allowing not only mechanical manipulation of an intact tissue but also control of culture medium composition and addition of drugs as well as visualization of cell structures. With this method, examination of worms carrying mutations in the intermediate filament system, its interacting partners and its regulators will become feasible.

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.

Keratins K2 and K10 are essential for the epidermal integrity of plantar skin

BACKGROUND

K1 and K2 are the main type II keratins in the suprabasal epidermis where each of them heterodimerizes with the type I keratin K10 to form intermediate filaments. In regions of the ears, tail, and soles of the mouse, only K2 is co-expressed with K10, suggesting that these keratins suffice to form a mechanically resilient cytoskeleton.

OBJECTIVE

To determine the effects of the suppression of both main keratins, K2 and K10, in the suprabasal plantar epidermis of the mouse.

METHODS

Krt2(-/-) Krt10(-/-) mice were generated by crossing Krt2(-/-) and Krt10(-/-) mice. Epidermal morphology of soles of hind-paws was examined macroscopically and histologically. Immunofluorescence analysis and quantitative PCR analysis were performed to analyze the expression of keratins in sole skin of wildtype and Krt2(-/-) Krt10(-/-) mice. Highly abundant proteins of the sole stratum corneum were determined by electrophoretic and chromatographic separation and subsequent mass spectrometry.

RESULTS

K2 and K10 are the most prominent suprabasal keratins in normal mouse soles with the exception of the footpads where K1, K9 and K10 predominate. Mice lacking both K2 and K10 were viable and developed epidermal acanthosis and hyperkeratosis in inter-footpad epidermis of the soles. The expression of keratins K1, K9 and K16 was massively increased at the RNA and protein levels in the soles of Krt2(-/-) Krt10(-/-) mice.

CONCLUSIONS

This study demonstrates that the loss of the main cytoskeletal components of plantar epidermis, i.e. K2 and K10, can be only partly compensated by the upregulation of other keratins. The thickening of the epidermis in the soles of Krt2(-/-) Krt10(-/-) mice may serve as a model for pathomechanistic aspects of palmoplantar keratoderma.

In vitro model for studying esophageal epithelial differentiation and allergic inflammatory responses identifies keratin involvement in eosinophilic esophagitis

Epithelial differentiation is an essential physiological process that imparts mechanical strength and barrier function to squamous epithelia. Perturbation of this process can give rise to numerous human diseases, such as atopic dermatitis, in which antigenic stimuli can penetrate the weakened epithelial barrier to initiate the allergic inflammatory cascade. We recently described a simplified air-liquid interface (ALI) culture system that facilitates the study of differentiated squamous epithelia in vitro. Herein, we use RNA sequencing to define the genome-wide transcriptional changes that occur within the ALI system during epithelial differentiation and in response to allergic inflammation. We identified 2,191 and 781 genes that were significantly altered upon epithelial differentiation or dysregulated in the presence of interleukin 13 (IL-13), respectively. Notably, 286 genes that were modified by IL-13 in the ALI system overlapped with the gene signature present within the inflamed esophageal tissue from patients with eosinophilic esophagitis (EoE), an allergic inflammatory disorder of the esophagus that is characterized by elevated IL-13 levels, altered epithelial differentiation, and pro-inflammatory gene expression. Pathway analysis of these overlapping genes indicated enrichment in keratin genes; for example, the gene encoding keratin 78, an uncharacterized type II keratin, was upregulated during epithelial differentiation (45-fold) yet downregulated in response to IL-13 and in inflamed esophageal tissue from patients. Thus, our findings delineate an in vitro experimental system that models epithelial differentiation that is dynamically regulated by IL-13. Using this system and analyses of patient tissues, we identify an altered expression profile of novel keratin differentiation markers in response to IL-13 and disease activity, substantiating the potential of this combined approach to identify relevant molecular processes that contribute to human allergic inflammatory disease.

Cell elasticity is regulated by the tropomyosin isoform composition of the actin cytoskeleton

The actin cytoskeleton is the primary polymer system within cells responsible for regulating cellular stiffness. While various actin binding proteins regulate the organization and dynamics of the actin cytoskeleton, the proteins responsible for regulating the mechanical properties of cells are still not fully understood. In the present study, we have addressed the significance of the actin associated protein, tropomyosin (Tpm), in influencing the mechanical properties of cells. Tpms belong to a multi-gene family that form a co-polymer with actin filaments and differentially regulate actin filament stability, function and organization. Tpm isoform expression is highly regulated and together with the ability to sort to specific intracellular sites, result in the generation of distinct Tpm isoform-containing actin filament populations. Nanomechanical measurements conducted with an Atomic Force Microscope using indentation in Peak Force Tapping in indentation/ramping mode, demonstrated that Tpm impacts on cell stiffness and the observed effect occurred in a Tpm isoform-specific manner. Quantitative analysis of the cellular filamentous actin (F-actin) pool conducted both biochemically and with the use of a linear detection algorithm to evaluate actin structures revealed that an altered F-actin pool does not absolutely predict changes in cell stiffness. Inhibition of non-muscle myosin II revealed that intracellular tension generated by myosin II is required for the observed increase in cell stiffness. Lastly, we show that the observed increase in cell stiffness is partially recapitulated in vivo as detected in epididymal fat pads isolated from a Tpm3.1 transgenic mouse line. Together these data are consistent with a role for Tpm in regulating cell stiffness via the generation of specific populations of Tpm isoform-containing actin filaments.