Disease-associated keratin mutations reduce traction forces and compromise adhesion and collective migration

Volumetric analysis of the terminal ductal lobular unit architecture and cell phenotypes in the human breast

The major lactiferous ducts of the human breast branch out and end at terminal ductal lobular units (TDLUs). Despite their functional and clinical importance, the three-dimensional (3D) architecture of TDLUs has remained undetermined. Our quantitative and volumetric imaging of healthy human breast tissue demonstrates that highly branched TDLUs, which exhibit increased proliferation, are uncommon in the resting tissue regardless of donor age, parity, or hormonal contraception. Overall, TDLUs have a consistent shape and branch parameters, and they contain a main subtree that dominates in bifurcation events and exhibits a more duct-like keratin expression pattern. Simulation of TDLU branching morphogenesis in three dimensions suggests that evolutionarily conserved mechanisms regulate mammary gland branching in humans and mice despite their anatomical differences. In all, our data provide structural insight into 3D anatomy and branching of the human breast and exemplify the power of volumetric imaging in gaining a deeper understanding of breast biology.

Leader Cells: Invade and Evade-The Frontline of Cancer Progression

Metastasis is the leading cause of cancer-related mortality; however, a complete understanding of the molecular programs driving the metastatic cascade is lacking. Metastasis is dependent on collective invasion-a developmental process exploited by many epithelial cancers to establish secondary tumours and promote widespread disease. The key drivers of collective invasion are "Leader Cells", a functionally distinct subpopulation of cells that direct migration, cellular contractility, and lead trailing or follower cells. While a significant body of research has focused on leader cell biology in the traditional context of collective invasion, the influence of metastasis-promoting leader cells is an emerging area of study. This review provides insights into the expanded role of leader cells, detailing emerging evidence on the hybrid epithelial-mesenchymal transition (EMT) state and the phenotypical plasticity exhibited by leader cells. Additionally, we explore the role of leader cells in chemotherapeutic resistance and immune evasion, highlighting their potential as effective and diverse targets for novel cancer therapies.

Identifying the effectiveness of 3D culture systems to recapitulate breast tumor tissue in situ

PURPOSE

Breast cancer heterogeneity contributes to chemotherapy resistance and decreased patient survival. To improve patient outcomes it is essential to develop a technology that is able to rapidly select the most efficacious therapy that targets the diverse phenotypes present within the tumor. Breast cancer organoid technologies are proposed as an attractive approach for evaluating drug responses prior to patient therapy. However, there remain challenges in evaluating the effectiveness of organoid cultures to recapitulate the heterogeneity present in the patient tumor in situ.

METHOD

Organoids were generated from seven normal breast and nineteen breast cancer tissues diagnosed as estrogen receptor positive or triple negative. The Jensen-Shannon divergence index, a measure of the similarity between distributions, was used to compare and evaluate heterogeneity in starting tissue and their resultant organoids. Heterogeneity was analyzed using cytokeratin 8 and cytokeratin 14, which provided an easily scored readout.

RESULTS

In the in vitro culture system HER1 and FGFR were able to drive intra-tumor heterogeneity to generate divergent phenotypes that have different sensitivities to chemotherapies.

CONCLUSION

Our methodology, which focuses on quantifiable cellular phenotypes, provides a tractable system that complements omics approaches to provide an unprecedented view of heterogeneity and will enhance the identification of novel therapies and facilitate personalized medicine.

Posttranslational modifications of keratins and their associated proteins as therapeutic targets in keratin diseases

The keratin cytoskeleton protects epithelia against mechanical, nonmechanical, and physical stresses, and participates in multiple signaling pathways that regulate cell integrity and resilience. Keratin gene mutations cause multiple rare monoallelic epithelial diseases termed keratinopathies, including the skin diseases Epidermolysis Bullosa Simplex (EBS) and Pachyonychia Congenita (PC), with limited available therapies. The disease-related keratin mutations trigger posttranslational modifications (PTMs) in keratins and their associated proteins that can aggravate the disease. Recent findings of drug high-throughput screening have led to the identification of compounds that may be repurposed, since they are used for other human diseases, to treat keratinopathies. These drugs target unique PTM pathways and sites, including phosphorylation and acetylation of keratins and their associated proteins, and have shed insights into keratin regulation and interactions. They also offer the prospect of testing the use of drug mixtures, with the long view of possible beneficial human use coupled with increased efficacy and lower side effects.

Mechanobiology: Border crosstalk protects the nucleus from stress

A new study reports that the extracellular matrix component laminin-111 shields the nucleus from actin-mediated forces by engaging the keratin cytoskeleton. Thus, matrix composition represents a means by which tissues can protect cell nuclei from mechanical damage.

Myc-dependent dedifferentiation of Gata6+ epidermal cells resembles reversal of terminal differentiation

Dedifferentiation is the process by which terminally differentiated cells acquire the properties of stem cells. During mouse skin wound healing, the differentiated Gata6-lineage positive cells of the sebaceous duct are able to dedifferentiate. Here we have integrated lineage tracing and single-cell mRNA sequencing to uncover the underlying mechanism. Gata6-lineage positive and negative epidermal stem cells in wounds are transcriptionally indistinguishable. Furthermore, in contrast to reprogramming of induced pluripotent stem cells, the same genes are expressed in the epidermal dedifferentiation and differentiation trajectories, indicating that dedifferentiation does not involve adoption of a new cell state. We demonstrate that dedifferentiation is not only induced by wounding, but also by retinoic acid treatment or mechanical expansion of the epidermis. In all three cases, dedifferentiation is dependent on the master transcription factor c-Myc. Mechanotransduction and actin-cytoskeleton remodelling are key features of dedifferentiation. Our study elucidates the molecular basis of epidermal dedifferentiation, which may be generally applicable to adult tissues.

A Kaleidoscope of Keratin Gene Expression and the Mosaic of Its Regulatory Mechanisms

Keratins are a family of intermediate filament-forming proteins highly specific to epithelial cells. A combination of expressed keratin genes is a defining property of the epithelium belonging to a certain type, organ/tissue, cell differentiation potential, and at normal or pathological conditions. In a variety of processes such as differentiation and maturation, as well as during acute or chronic injury and malignant transformation, keratin expression undergoes switching: an initial keratin profile changes accordingly to changed cell functions and location within a tissue as well as other parameters of cellular phenotype and physiology. Tight control of keratin expression implies the presence of complex regulatory landscapes within the keratin gene loci. Here, we highlight patterns of keratin expression in different biological conditions and summarize disparate data on mechanisms controlling keratin expression at the level of genomic regulatory elements, transcription factors (TFs), and chromatin spatial structure.

Establishment of a system evaluating the contractile force of electrically stimulated myotubes from wrinkles formed on elastic substrate

Muscle weakness is detrimental not only to quality of life but also life expectancy. However, effective drugs have still not been developed to improve and prevent muscle weakness associated with aging or diseases. One reason for the delay in drug discovery is that no suitable in vitro screening system has been established to test whether drugs improve muscle strength. Here, we used a specific deformable silicone gel substrate to effectively and sensitively evaluate the contractile force generated by myotubes from wrinkles formed on the substrate. Using this system, it was found that the contractile force generated by an atrophic phenotype of myotubes induced by dexamethasone or cancer cell-conditioned medium treatment significantly decreased while that generated by hypertrophic myotubes induced by insulin-like growth factor-1 significantly increased. Notably, it was found that changes in the index related to contractile force can detect atrophic or hypertrophic phenotypes more sensitively than changes in myotube diameter or myosin heavy chain expression, both commonly used to evaluate myotube function. These results suggest that our proposed system will be an effective tool for assessing the contractile force-related state of myotubes, which are available for the development of drugs to prevent and/or treat muscle weakness.

Mechanotransduction in Skin Inflammation

In the process of mechanotransduction, the cells in the body perceive and interpret mechanical stimuli to maintain tissue homeostasis and respond to the environmental changes. Increasing evidence points towards dysregulated mechanotransduction as a pathologically relevant factor in human diseases, including inflammatory conditions. Skin is the organ that constantly undergoes considerable mechanical stresses, and the ability of mechanical factors to provoke inflammatory processes in the skin has long been known, with the Koebner phenomenon being an example. However, the molecular mechanisms and key factors linking mechanotransduction and cutaneous inflammation remain understudied. In this review, we outline the key players in the tissue's mechanical homeostasis, the available data, and the gaps in our current understanding of their aberrant regulation in chronic cutaneous inflammation. We mainly focus on psoriasis as one of the most studied skin inflammatory diseases; we also discuss mechanotransduction in the context of skin fibrosis as a result of chronic inflammation. Even though the role of mechanotransduction in inflammation of the simple epithelia of internal organs is being actively studied, we conclude that the mechanoregulation in the stratified epidermis of the skin requires more attention in future translational research.

Dystrophin missense mutations alter focal adhesion tension and mechanotransduction

Skeletal muscle is a mechanical organ that not only produces force but also uses mechanical stimuli as a signal to regulate cellular responses. Duchenne and Becker muscular dystrophy are lethal muscle wasting diseases that affect 1 in 3,500 boys and is caused by the absence or malfunction of dystrophin protein, respectively. There is a lack of understanding on how the integration of these mechanical signals is dysregulated in muscular dystrophy and how they may contribute to disease progression. In this study, we show that patient-relevant dystrophin mutations alter the mechanical signaling axis in muscle cells, leading to impaired migration. This work proposes dystrophin as a component of the cellular force-sensing machinery, furthering our knowledge in the pathomechanism of muscular dystrophy.

Dystrophin is an essential muscle protein that contributes to cell membrane stability by mechanically linking the actin cytoskeleton to the extracellular matrix via an adhesion complex called the dystrophin–glycoprotein complex. The absence or impaired function of dystrophin causes muscular dystrophy. Focal adhesions (FAs) are also mechanosensitive adhesion complexes that connect the cytoskeleton to the extracellular matrix. However, the interplay between dystrophin and FA force transmission has not been investigated. Using a vinculin-based bioluminescent tension sensor, we measured FA tension in transgenic C2C12 myoblasts expressing wild-type (WT) dystrophin, a nonpathogenic single nucleotide polymorphism (SNP) (I232M), or two missense mutations associated with Duchenne (L54R), or Becker muscular dystrophy (L172H). Our data revealed cross talk between dystrophin and FAs, as the expression of WT or I232M dystrophin increased FA tension compared to dystrophin-less nontransgenic myoblasts. In contrast, the expression of L54R or L172H did not increase FA tension, indicating that these disease-causing mutations compromise the mechanical function of dystrophin as an FA allosteric regulator. Decreased FA tension caused by these mutations manifests as defective migration, as well as decreased Yes-associated protein 1 (YAP) activation, possibly by the disruption of the ability of FAs to transmit forces between the extracellular matrix and cytoskeleton. Our results indicate that dystrophin influences FA tension and suggest that dystrophin disease-causing missense mutations may disrupt a cellular tension-sensing pathway in dystrophic skeletal muscle.

Combining Image Restoration and Traction Force Microscopy to Study Extracellular Matrix-Dependent Keratin Filament Network Plasticity

Keratin intermediate filaments are dynamic cytoskeletal components that are responsible for tuning the mechanical properties of epithelial tissues. Although it is known that keratin filaments (KFs) are able to sense and respond to changes in the physicochemical properties of the local niche, a direct correlation of the dynamic three-dimensional network structure at the single filament level with the microenvironment has not been possible. Using conventional approaches, we find that keratin flow rates are dependent on extracellular matrix (ECM) composition but are unable to resolve KF network organization at the single filament level in relation to force patterns. We therefore developed a novel method that combines a machine learning-based image restoration technique and traction force microscopy to decipher the fine details of KF network properties in living cells grown on defined ECM patterns. Our approach utilizes Content-Aware Image Restoration (CARE) to enhance the temporal resolution of confocal fluorescence microscopy by at least five fold while preserving the spatial resolution required for accurate extraction of KF network structure at the single KF/KF bundle level. The restored images are used to segment the KF network, allowing numerical analyses of its local properties. We show that these tools can be used to study the impact of ECM composition and local mechanical perturbations on KF network properties and corresponding traction force patterns in size-controlled keratinocyte assemblies. We were thus able to detect increased curvature but not length of KFs on laminin-322 versus fibronectin. Photoablation of single cells in microprinted circular quadruplets revealed surprisingly little but still significant changes in KF segment length and curvature that were paralleled by an overall reduction in traction forces without affecting global network orientation in the modified cell groups irrespective of the ECM coating. Single cell analyses furthermore revealed differential responses to the photoablation that were less pronounced on laminin-332 than on fibronectin. The obtained results illustrate the feasibility of combining multiple techniques for multimodal monitoring and thereby provide, for the first time, a direct comparison between the changes in KF network organization at the single filament level and local force distribution in defined paradigms.

Wrinkle force microscopy: a machine learning based approach to predict cell mechanics from images

Combining experiments with artificial intelligence algorithms, we propose a machine learning based approach called wrinkle force microscopy (WFM) to extract the cellular force distributions from the microscope images. The full process can be divided into three steps. First, we culture the cells on a special substrate allowing to measure both the cellular traction force on the substrate and the corresponding substrate wrinkles simultaneously. The cellular forces are obtained using the traction force microscopy (TFM), at the same time that cell-generated contractile forces wrinkle their underlying substrate. Second, the wrinkle positions are extracted from the microscope images. Third, we train the machine learning system with GAN (generative adversarial network) by using sets of corresponding two images, the traction field and the input images (raw microscope images or extracted wrinkle images), as the training data. The network understands the way to convert the input images of the substrate wrinkles to the traction distribution from the training. After sufficient training, the network is utilized to predict the cellular forces just from the input images. Our system provides a powerful tool to evaluate the cellular forces efficiently because the forces can be predicted just by observing the cells under the microscope, which is much simpler method compared to the TFM experiment. Additionally, the machine learning based approach presented here has the profound potential for being applied to diverse cellular assays for studying mechanobiology of cells.

A deep learning approach, termed wrinkle force microscopy, allows for conducting traction force microscopy by observing bright-field cell images but without using its conventional requirements such as fluorescent microbeads and confocal microscopy.

Keratins as an Inflammation Trigger Point in Epidermolysis Bullosa Simplex

Epidermolysis bullosa simplex (EBS) is a group of inherited keratinopathies that, in most cases, arise due to mutations in keratins and lead to intraepidermal ruptures. The cellular pathology of most EBS subtypes is associated with the fragility of the intermediate filament network, cytolysis of the basal layer of the epidermis, or attenuation of hemidesmosomal/desmosomal components. Mutations in keratins 5/14 or in other genes that encode associated proteins induce structural disarrangements of different strengths depending on their locations in the genes. Keratin aggregates display impaired dynamics of assembly and diminished solubility and appear to be the trigger for endoplasmic reticulum (ER) stress upon being phosphorylated by MAPKs. Global changes in cellular signaling mainly occur in cases of severe dominant EBS mutations. The spectrum of changes initiated by phosphorylation includes the inhibition of proteasome degradation, TNF-α signaling activation, deregulated proliferation, abnormal cell migration, and impaired adherence of keratinocytes. ER stress also leads to the release of proinflammatory danger-associated molecular pattern (DAMP) molecules, which enhance avalanche-like inflammation. Many instances of positive feedback in the course of cellular stress and the development of sterile inflammation led to systemic chronic inflammation in EBS. This highlights the role of keratin in the maintenance of epidermal and immune homeostasis.

Culturing Keratinocytes on Biomimetic Substrates Facilitates Improved Epidermal Assembly In Vitro

Mechanotransduction is defined as the ability of cells to sense mechanical stimuli from their surroundings and translate them into biochemical signals. Epidermal keratinocytes respond to mechanical cues by altering their proliferation, migration, and differentiation. In vitro cell culture, however, utilises tissue culture plastic, which is significantly stiffer than the in vivo environment. Current epidermal models fail to consider the effects of culturing keratinocytes on plastic prior to setting up three-dimensional cultures, so the impact of this non-physiological exposure on epidermal assembly is largely overlooked. In this study, primary keratinocytes cultured on plastic were compared with those grown on 4, 8, and 50 kPa stiff biomimetic hydrogels that have similar mechanical properties to skin. Our data show that keratinocytes cultured on biomimetic hydrogels exhibited major changes in cellular architecture, cell density, nuclear biomechanics, and mechanoprotein expression, such as specific Linker of Nucleoskeleton and Cytoskeleton (LINC) complex constituents. Mechanical conditioning of keratinocytes on 50 kPa biomimetic hydrogels improved the thickness and organisation of 3D epidermal models. In summary, the current study demonstrates that the effects of extracellular mechanics on keratinocyte cell biology are significant and therefore should be harnessed in skin research to ensure the successful production of physiologically relevant skin models.

Mechanosensitive myosin II but not cofilin primarily contributes to cyclic cell stretch-induced selective disassembly of actin stress fibers

Cells adapt to applied cyclic stretch (CS) to circumvent chronic activation of proinflammatory signaling. Currently, the molecular mechanism of the selective disassembly of actin stress fibers (SFs) in the stretch direction, which occurs at the early stage of the cellular response to CS, remains controversial. Here, we suggest that the mechanosensitive behavior of myosin II, a major cross-linker of SFs, primarily contributes to the directional disassembly of the actomyosin complex SFs in bovine vascular smooth muscle cells and human U2OS osteosarcoma cells. First, we identified that CS with a shortening phase that exceeds in speed the inherent contractile rate of individual SFs leads to the disassembly. To understand the biological basis, we investigated the effect of expressing myosin regulatory light-chain mutants and found that SFs with less actomyosin activities disassemble more promptly upon CS. We consequently created a minimal mathematical model that recapitulates the salient features of the direction-selective and threshold-triggered disassembly of SFs to show that disassembly or, more specifically, unbundling of the actomyosin bundle SFs is enhanced with sufficiently fast cell shortening. We further demonstrated that similar disassembly of SFs is inducible in the presence of an active LIM-kinase-1 mutant that deactivates cofilin, suggesting that cofilin is dispensable as opposed to a previously proposed mechanism.

Growth, lifetime, directional movement and myosin-dependent motility of mutant keratin granules in cultured cells

Intermediate filament polypeptides (IFPs) are prominent components of cytoplasmic aggregates, which are pathognomonic for multiple diseases. Recent observations in cultured cells suggest that they are dynamic and subject to regulated turnover. The emerging concept is that multiple factors contribute to motility and turnover of IFP-containing aggregates. To understand their relative contribution, quantitative tools are needed. The current study addresses this need using epithelial cells producing mutant keratin IFPs that have been identified as the cause of the hereditary blister-forming skin disease epidermolysis bullosa simplex. Digital image analysis of individual granules allowed mapping of their complete life cycle, with information on multiple characteristics at any given time-point. The deduced signet features revealed rapid granule fusion and directed transport from the periphery towards the cell centre, and a limited, ~ 30 min lifetime with a slow, continuous growth phase followed by fast disassembly. As paradigmatic proof-of-principle, we demonstrate that inhibition of myosin II selectively reduces granule movement, linking keratin granule motility to retrograde cortical acto-myosin flow. The newly developed methods and established parameters will help in the characterization of known and the identification of novel regulators of IFP-containing aggregates.

The keratin network of intermediate filaments regulates keratinocyte rigidity sensing and nuclear mechanotransduction

The keratin network of intermediate filaments provides keratinocytes with essential mechanical strength and resilience, but the contribution to mechanosensing remains poorly understood. Here, we investigated the role of the keratin cytoskeleton in the response to altered matrix rigidity. We found that keratinocytes adapted to increasing matrix stiffness by forming a rigid, interconnected network of keratin bundles, in conjunction with F-actin stress fiber formation and increased cell stiffness. Disruption of keratin stability by overexpression of the dominant keratin 14 mutation R416P inhibited the normal mechanical response to substrate rigidity, reducing F-actin stress fibers and cell stiffness. The R416P mutation also impaired mechanotransduction to the nuclear lamina, which mediated stiffness-dependent chromatin remodeling. By contrast, depletion of the cytolinker plectin had the opposite effect and promoted increased mechanoresponsiveness and up-regulation of lamin A/C. Together, these results demonstrate that the keratin cytoskeleton plays a key role in matrix rigidity sensing and downstream signal transduction.

First person – Sachiko Fujiwara

First Person is a series of interviews with the first authors of a selection of papers published in Journal of Cell Science, helping early-career researchers promote themselves alongside their papers. Sachiko Fujiwara is first author on ‘Disease-associated keratin mutations reduce traction forces and compromise adhesion and collective migration’, published in JCS. Sachiko conducted the research described in this article while a Postdoctoral fellow in Thomas M. Magin's lab at Institute of Biology, Division of Cell & Developmental Biology, Leipzig University, Germany. She is now an assistant professor in the lab of Kazunori Imaizumi at the Graduate School of Biomedical & Health Sciences, Hiroshima University, Japan, investigating the physiological roles of cytoskeletons.